Radiologic and Clinical Outcomes of Unilateral Biportal Endoscopic Decompression for Thoracic Ossification of the Ligamentum Flavum

Radiologic and Clinical Outcomes of Unilateral Biportal Endoscopic Decompression for Thoracic Ossification of the Ligamentum Flavum | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Radiologic and Clinical Outcomes of Unilateral Biportal Endoscopic Decompression for Thoracic Ossification of the Ligamentum Flavum Sang-Woo Lee, JinWoo Jung, Sang-Kyu Son, Man-Kyu Park This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9194607/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 7 You are reading this latest preprint version Abstract Purpose Thoracic ossification of the ligamentum flavum (OLF) is a major cause of thoracic myelopathy requiring surgical decompression. While unilateral biportal endoscopy (UBE) has been increasingly applied to thoracic decompression, quantitative radiologic evidence regarding decompression adequacy and posterior structure preservation remains limited. Methods This retrospective single-center study included consecutive patients who underwent thoracic UBE decompression for OLF. Radiologic outcomes were assessed at the segment level using magnetic resonance imaging and computed tomography. Dural sac cross-sectional area (DSCA), facet joint length and area, and segmental kyphosis were quantitatively measured preoperatively and postoperatively. Clinical outcomes were evaluated using the modified Japanese Orthopaedic Association (mJOA) score, Nurick grade, and visual analog scale (VAS). Postoperative residual OLF and complications were recorded. Results A total of 45 patients (55 segments) were analyzed with a mean follow-up of 16.0 months. Mean DSCA increased significantly from 70.4 ± 20.4 mm² preoperatively to 119.8 ± 31.0 mm² postoperatively, corresponding to a mean expansion ratio of 75.0%. Facet preservation was asymmetric, with significantly greater preservation on the contralateral side compared with the ipsilateral approach side (facet area preservation: 79.4% vs. 64.2%, p < 0.001). Segmental kyphosis increased modestly at 1-year follow-up (mean change, 1.5°), while clinically significant kyphotic progression requiring surgical stabilization was rare. Clinical outcomes improved significantly, with a mean mJOA recovery rate of 71.2%. Residual OLF was observed in a subset of segments and primarily reflected an intentional floating strategy rather than incomplete decompression. Conclusions Thoracic UBE decompression for OLF achieves substantial dural sac expansion and favorable clinical outcomes while preserving posterior stabilizing structures. Quantitative radiologic analysis demonstrates that adequate decompression can be accomplished without routine fusion or extensive posterior element disruption. Thoracic UBE represents a viable, tissue-preserving alternative for the surgical management of thoracic OLF. Unilateral Biportal Endoscopy Thoracic decompression Minimally invasive Thoracic myelopathy Thoracic spinal stenosis Figures Figure 1 Figure 2 Figure 3 INTRODUCTION Thoracic ossification of the ligamentum flavum (T-OLF) represents a common structural cause of thoracic myelopathy and results in progressive compression of the spinal cord.[ 1 , 2 ] Once myelopathic symptoms appear, neurological function rarely improves spontaneously and, in many cases, continues to deteriorate over time.[ 3 ] Patients often present with gait disturbance and sensory impairment, and as the disease progresses, bladder dysfunction and axial back pain may also develop.[ 3 , 4 ] Conservative treatment has limited efficacy in established thoracic myelopathy, and surgical decompression is therefore required to prevent further neurological decline. Conventional posterior decompression remains the most widely used surgical strategy; however, it typically requires wide exposure with removal of multiple posterior stabilizing structures, including the lamina, facet joints, and ligamentous complex.[ 5 – 8 ] In elderly patients, this extent of tissue disruption may translate into increased perioperative burden and raises concern regarding postoperative alignment and segmental stability. In an effort to reduce approach-related morbidity, minimally invasive techniques have been increasingly adopted for spinal decompression. Among these, unilateral biportal endoscopy (UBE) has gained attention as an endoscopic platform that separates the viewing and working portals, allowing bimanual instrument handling and a wide operative field under high magnification.[ 9 ] This configuration enables precise bone and soft tissue resection while maintaining continuous visualization of neural structures, thereby facilitating decompression comparable to conventional microscopic surgery through a substantially smaller surgical corridor. Originally developed and refined in lumbar decompression and fusion procedures, the technical principles of UBE have gradually been extended to the thoracic spine, where preservation of posterior stabilizing structures is of particular importance.[ 9 , 10 ] Thoracic OLF presents unique technical challenges compared with degenerative conditions of the lumbar spine. The thoracic spinal canal is inherently narrower, the thoracic cord is less tolerant of manipulation, and ossification frequently extends to or involves the dura mater.[ 3 , 5 , 9 ] Preoperative computed tomography findings that shows dural ossifications(DO), such as tram-track, bridge, and comma signs, have been used to anticipate the presence of DO and to guide surgical strategy.[ 6 , 11 – 13 ] Nevertheless, most published thoracic UBE studies have focused primarily on technical feasibility or short-term clinical outcomes. Quantitative evaluation of decompression efficacy and preservation of posterior stabilizing structures has been limited. As a result, objective radiologic data on dural sac expansion, facet joint preservation, and postoperative alignment changes in thoracic UBE remain scarce. Accordingly, the purpose of this study was to evaluate the radiologic and clinical outcomes of thoracic UBE decompression in a consecutive cohort of patients with OLF. We aimed to quantify decompression efficacy using pre- and postoperative measurements of dural sac cross-sectional area and to objectively assess facet joint preservation with side-specific analysis reflecting the unilateral approach. Postoperative alignment changes and spinal cord signal evolution were also examined, and radiologic findings were correlated with clinical outcomes. We hypothesized that thoracic UBE would achieve substantial dural sac expansion with high preservation of posterior stabilizing structures, accompanied by meaningful clinical improvement, while postoperative alignment changes would predominantly represent mild kyphosis progression rather than true instability. MATERIALS AND METHODS Study design and Ethics This was a single-center, retrospective observational study including consecutive patients who underwent thoracic posterior decompression using UBE for symptomatic thoracic myelopathy. The study was approved by Public Institutional Review Board (IRB No. P01-202511-01-073), and the requirement for individual informed consent was waived owing to the use of de-identified data and minimal risk to participants. Patient Population Consecutive adult patients who underwent thoracic UBE decompression between July 2022 and October 2024 were included. Inclusion criteria: Symptomatic thoracic myelopathy or radiculopathy attributed to thoracic OLF with or without concomitant degenerative thoracic spinal stenosis. Underwent UBE decompression alone at one or two contiguous thoracic levels. (No additional lumbar decompression or fusion performed on the same day) Availability of preoperative thoracic MRI and CT, and early and late postoperative MRI (early: within 2 months; late: ≥12 months). Minimum clinical follow-up more than 12 months. Exclusion criteria: Infection, neoplasm, or acute traumatic instability at the index level. Concomitant lumbar decompression and/or fusion performed on the same day, because of postoperative symptom improvement could not be clearly attributed to thoracic decompression. Planned or intraoperative conversion to open decompression or instrumented fusion at the index level. Isolated intradural tumor or non-OLF compressive lesion. Incomplete imaging (missing preoperative MRI/CT or postoperative MRI) or insufficient clinical documentation. Loss to follow-up before 12 months. Imaging Evaluation All patients underwent pre- and post-operative thoracic MRI (1.5T or 3T) and thin-slice CT (≤ 1.0 mm) with multiplanar reconstruction. Standing thoracic radiographs (neutral, and flexion–extension when available) were also obtained. Preoperative classification OLF morphology was categorized on CT using the Sato classification (lateral, extended, enlarged, fused, tuberous) (Fig. 1 ).[ 14 ] Dural ossification (DO) was assessed on CT using established signs (tram-track, bridge, comma) (Fig. 2 )[ 12 , 13 ]. For patients with multilevel surgery, the index segment for radiologic analysis was defined as the level with the smallest preoperative dural sac cross-sectional area (DSCA) or the most severe OLF morphology. Postoperative imaging Early MRI (2 weeks–2 months) was used to evaluate decompression completeness, residual OLF, epidural hematoma, and early cord signal change. Late MRI (≥ 12 months) assessed maintenance of decompression and evolution of cord signal changes. Post operative CT was used to quantify facet preservation and confirm bony decompression. standing lateral and dynamic radiographs at 1, 3, 6, and 12 months were used to assess segmental kyphosis and its progression. Radiographic kyphosis progression was defined as an increase in segmental Cobb angle of ≥ 5°. Radiological Measurements All imaging measurements were performed independently by a fellowship-trained radiologist and a board-certified spine neurosurgeon, both blinded to clinical data and each other’s readings. A calibration session with pilot cases(N = 5) was performed prior to formal measurements. A random subset of cases(N = 15) was re-measured after a 2-week interval to assess intra-observer reliability. Inter- and intra-observer agreement for continuous variables were reported as intraclass correlation coefficients (ICC[ 2 , 1 ]); categorical labels (Sato class, DO signs, residual OLF) was assessed using Cohen’s κ. Measurements were performed using the INFINITT PACS system (INFINITT Healthcare, Seoul, Korea). Radiologic outcomes were assessed on a segment-by-segment basis. The primary radiologic parameters included DSCA, facet joint morphology (length and area), segmental kyphosis, spinal cord signal change, and residual OLF. DSCA was measured on axial T2-weighted MRI at the index segment using a standardized axial plane at the mid-facet level (Fig. 3 A, B). The inner contour of the dural sac was traced using a freehand region-of-interest method. Absolute change (ΔDSCA) and percentage gain relative to preoperative values were calculated. Facet joint morphology was evaluated on axial computed tomography images. Facet length was defined as the linear distance from the medial articular margin to the lateral edge of the articular surface, and facet area was calculated by manually outlining the cortical boundary of the facet joint (Fig. 3 C, D). Measurements were obtained separately for the ipsilateral (surgical approach side) and contralateral facets. Facet preservation ratio (%) was calculated as the postoperative value divided by the preoperative value. Segmental kyphosis was measured on standing lateral radiographs using the Cobb method between the upper endplate of the cranial vertebra and the lower endplate of the caudal vertebra at the index segment (Fig. 3 E). Spinal cord signal intensity was evaluated on sagittal and axial T2-weighted MRI and recorded as a binary variable (presence or absence of high signal intensity). Residual OLF was defined as persistent posterior compression of the dural sac on early postoperative MRI or CT. Surgical Technique All procedures were performed under general anesthesia with the patient in the prone position on a radiolucent frame. A standardized left-sided unilateral biportal endoscopic approach was used in all cases. After fluoroscopic confirmation of the index level, two paramedian portals were created along the left medial pedicle line for endoscopic visualization and instrument manipulation. Soft tissue over the lamina–facet junction was cleared using radiofrequency ablation and a shaver, followed by unilateral laminotomy using a high-speed diamond burr. The ligamentum flavum was preserved as a protective layer during bony decompression and exposed after adequate working space was obtained. Because all cases involved OLF, decompression strategy was determined according to the presence and severity of DO. In cases without DO, the OLF was circumferentially thinned to a paper-thin layer. After confirming clear margins, the interface between the OLF and dura was carefully explored using a blunt dissector or freer elevator, particularly when epidural fat was identified, and the OLF was gently removed. In cases with DO, the OLF was maximally thinned and separated from surrounding margins. When separation between the OLF and DO was achievable, the OLF was cautiously removed en-bloc. Residual DO was further thinned to allow floating of the ossified dura. If the thinned DO became semi-lucent and the inner dural layer or spinal cord pulsation was clearly visible, further removal was avoided to prevent dural injury, and the area was covered with a sealant material. When DO and OLF were inseparable, both were thinned together while carefully avoiding direct compression of the dural sac, circumferential margins were established, and a floating technique was employed. When complete removal of DO was considered safe, it was removed and immediately reinforced with sealant material. Hemostasis was achieved using low-power bipolar coagulation and topical hemostatic agents under low-pressure continuous irrigation. Decompression was considered complete when bilateral pedicle medial borders were visualized, free dural pulsation was observed, and no residual compressive elements were identified on circumferential endoscopic inspection. A closed-suction drain was placed at the surgeon’s discretion, and patients were mobilized beginning on postoperative day 1 unless contraindicated. Statistical analysis Statistical analyses were performed by an independent biostatistician using R software (Version 4.5.0, R Foundation for Statistical Computing, Vienna, Austria). To evaluate the inter-observer reliability of the radiographic measurements, the intraclass correlation coefficient [ICC(2, 1)] and Cohen’s kappa coefficient (κ) were calculated. Continuous variables are presented as mean ± standard deviation, and categorical variables as number (%). Pre- and postoperative comparisons were analyzed using paired t-tests or Wilcoxon signed-rank tests, as appropriate. Changes in spinal cord T2 high signal intensity were evaluated using the McNemar test. All statistical tests were two-sided, and a p value < 0.05 was considered statistically significant. RESULTS Patient Characteristics and Operative Data The inter-observer reliability for the radiographic measurements was excellent, demonstrating an overall ICC of 0.89 for continuous variables and a Cohen’s kappa of 1.00 for categorical variables. The study cohort consisted of 45 patients (19 males and 26 females) with a mean age of 69.5 ± 10.7 years. A total of 55 thoracic segments were decompressed using UBE. The mean symptom duration was 5.0 months, and the mean follow-up period was 16.0 months. The average operative time was 56.4 ± 15.1 minutes per segment, and the mean length of hospital stay was 8.8 ± 4.4 days (Table 1 ). Table 1 Patient Demographics and baseline characteristics (Patient-level, N = 45) Characteristic Value Age(years) 69.5 ± 10.7 (range 43–83) Sex (M:F) 19:26 Operative segments (N) 55 Operative time (min, per segment) 56.4 ± 15.1 Symptom duration (Months) 5.0 ± 2.6 (range 1–14) Follow-up (Months) 16.0 ± 3.9 (range 12–26) Length of hospital stay (days) 8.8 ± 4.4 (range 4–25) Preoperative mJOA score 7.7 ± 1.8 Preoperative Nurick score 2.3 ± 1.2 Preoperative back pain (VAS) 6.2 ± 1.6 Preoperative leg pain (VAS) 4.7 ± 1.7 Values are shown as number or mean ± standard deviation mJOA, modified Japanese Orthopaedic Association; VAS, Visual Analog Scale The most frequently treated levels were T11–12 (38.2%) and T10–11 (30.9%). According to the Sato classification, lateral-type OLF (Type A) was the most common morphology (49.1%), followed by the extended type (Type B, 27.3%). Preoperative CT evaluation demonstrated no DO signs in 72.7% of segments. Among segments with DO (27.3%), the tram-track sign was most prevalent (20.0%), followed by the comma (5.5%) and bridge signs (1.8%) (Table 2 ). Table 2 Operative characteristics (Segment-level, N = 55) Variable Value Level distribution Total = 55 T3-4 1 (1.8%) T6-7 2 (3.6%) T8-9 3 (5.5%) T9-10 8 (14.5%) T10-11 17 (30.9%) T11-12 21 (38.2%) T12-L1 3 (5.5%) OLF type † Lateral 27 (49.1%) Extended 15 (27.3%) Enlarged 6 (10.9%) Fused 5 (9.1%) Tuberous 2 (3.6%) DO type ‡ No 40 (72.7%) Tram track 11 (20.0%) Bridge 1 (1.8%) Comma 3 (5.5%) Values are presented as number (%) OLF, ossification of the ligamentum flavum; DO, dural ossification. †OLF type was classified according to the Sato classification as follows: A, lateral type;B, extended type; C, enlarged type; D, fused type; E, tuberous type. ‡DO type was determined based on preoperative computed tomography findings and categorized as: A, no dural ossification; B, tram-track sign; C, bridge sign; D, comma sign. Radiologic Outcomes Postoperative imaging confirmed adequate decompression in all treated segments (Table 3 ). The mean DSCA increased significantly from 70.4 ± 20.4 mm² preoperatively to 119.8 ± 31.0 mm² postoperatively (p < 0.001), representing a mean expansion ratio of 75.0 ± 36.7%. Table 3 Radiologic outcomes (Segment-level, N = 55) Variable Value p-value DSCA (mm 2 ) Preoperative Postoperative 70.4 ± 20.4 119.8 ± 31.0 < 0.001 Δ DSCA (mm 2 ) 49.4 ± 22.1 DSCA gain (%) 75.0 ± 36.7 Facet length (mm) Left (Ipsilateral) Right (Contralateral) Preoperative 12.1 ± 1.8 12.2 ± 1.7 0.642 Postoperative 8.0 ± 2.1 9.6 ± 1.7 < 0.001 p-value < 0.001 < 0.001 Preservation ratio (%) 65.1 ± 11.7 79.1 ± 10.5 < 0.001 Facet area (mm 2 ) Left (Ipsilateral) Right (Contralateral) Preoperative 123.3 ± 31.3 120.4 ± 26.7 0.458 Postoperative 79.7 ± 27.5 94.8 ± 21.4 < 0.001 p-value < 0.001 < 0.001 Preservation ratio (%) 64.2 ± 13.2 79.4 ± 11.2 < 0.001 Segmental Kyphosis (°) Preoperative Postoperative (1 year) 5.4 ± 7.8 6.9 ± 8.9 0.011 Cord T2 high SI, n (%) Preoperative Postoperative (1 year) 27 (49.1) 13 (23.6) 0.002 Residual OLF, n (%) 10 (18.2) Values are presented as mean ± standard deviation or number (%). Ipsilateral facet refers to the surgical approach side. P-values for continuous variables were calculated using paired t-test. Changes in cord T2 high signal intensity were analyzed using the McNemar test. DSCA, dural sac cross-sectional area; OLF, ossification of the ligamentum flavum; SI, signal intensity. Preoperative facet length and area did not differ significantly between the ipsilateral and contralateral sides (length: p = 0.642; area: p = 0.458). Postoperatively, ipsilateral facet length (8.0 ± 2.1 mm) and area (79.7 ± 27.5 mm²) were significantly smaller than those on the contralateral side (9.6 ± 1.7 mm and 94.8 ± 21.4 mm², respectively; both p < 0.001). Correspondingly, facet preservation ratios were significantly lower on the ipsilateral side compared with the contralateral side for both length (65.1 ± 11.7% vs. 79.1 ± 10.5%, p < 0.001) and area (64.2 ± 13.2% vs. 79.4 ± 11.2%, p < 0.001), consistent with the unilateral decompression approach. The segmental kyphotic angle increased modestly from 5.4 ± 7.8° preoperatively to 6.9 ± 8.9° at 1-year follow-up (p = 0.011). Although this change was statistically significant, the mean increase was limited to 1.5°, and radiographic instability (> 5° kyphotic progression) was observed in 3 of 55 segments (5.5%). Preoperative spinal cord high signal intensity on T2-weighted MRI was present in 27 segments (49.1%). At 1-year follow-up, persistent high signal intensity remained in 13 segments (23.6%), representing a significant reduction compared with baseline (p = 0.002). Residual OLF was identified in 10 segments (18.2%) on early postoperative imaging; however, no patient required reoperation for residual stenosis. Clinical Outcomes Significant improvements were observed across all clinical outcome measures following UBE decompression (Table 4 ). The mean mJOA score improved from 7.7 ± 1.8 preoperatively to 9.7 ± 1.5 at final follow-up (p < 0.001), corresponding to a mean recovery rate of 71.2 ± 28.1%. Functional gait impairment, assessed by the Nurick grade, significantly improved from 2.3 ± 1.2 to 0.6 ± 1.0 (p < 0.001). Table 4 Clinical outcomes (Patient-level, N = 45) Variable Preoperative Post-1 month Post- 1 year p-value † mJOA score 7.7 ± 1.8 9.1 ± 1.6 9.7 ± 1.5 < 0.001 mJOA RR ‡ (%), 1 year 71.2 ± 28.1 Nurick grade 2.3 ± 1.2 1.1 ± 1.1 0.6 ± 1.0 < 0.001 Back pain (VAS) 6.2 ± 1.6 3.3 ± 1.1 2.4 ± 1.4 < 0.001 Leg pain (VAS) 4.7 ± 1.7 2.5 ± 1.3 2.1 ± 1.0 < 0.001 Values are presented as mean ± standard deviation. mJOA, modified Japanese Orthopaedic Association score for thoracic myelopathy; RR, recovery rate; VAS, visual analog scale. †P-values were calculated using paired t-test and indicate overall preoperative vs final follow-up comparisons. ‡Recovery rate calculated as: (Postoperative mJOA - Preoperative mJOA) / (11 - Preoperative mJOA) × 100. Nurick grade is an ordinal scale and is presented as mean ± standard deviation for descriptive purposes. Pain outcomes also demonstrated substantial improvement. The mean VAS score for back pain decreased from 6.2 ± 1.6 to 2.4 ± 1.4, and leg pain improved from 4.7 ± 1.7 to 2.1 ± 1.0 at 1-year follow-up (p < 0.001 for both). Complications Overall complication rates were low. Incidental durotomy occurred in three segments, all associated with severe OLF morphology (Sato classification: fused and tuberous type; Fig. 1 D, 1 E) or advanced dural ossification (Bridge and comma sign; Fig. 2 B, 2 C). These cases were managed intraoperatively using sealant or patch reinforcement, and no persistent cerebrospinal fluid leakage was observed. One patient developed a postoperative epidural hematoma with acute neurological deterioration, which was successfully treated with immediate endoscopic hematoma removal surgery. Delayed segmental instability requiring posterior instrumented fusion occurred in one patient at 1-year follow-up. No surgical site infections were observed. DISCUSSION Thoracic OLF represents a challenging cause of thoracic myelopathy, in which the goals of surgery must balance effective neural decompression against preservation of spinal stability.[ 3 , 5 , 15 ] While conventional posterior decompression has long been the standard treatment, concerns regarding extensive posterior element disruption and the frequent need for additional stabilization have prompted interest in less invasive alternatives.[ 6 , 8 , 16 , 17 ] The present study provides a systematic radiologic evaluation of thoracic UBE decompression and demonstrates that this approach can achieve sufficient decompression while preserving posterior stabilizing structures, with acceptable postoperative alignment and favorable clinical outcomes. Adequacy of Decompression in Thoracic UBE A fundamental question in applying minimally invasive techniques to thoracic myelopathy is whether decompression through a unilateral corridor can be sufficient for the spinal cord. Using quantitative radiologic metrics, we demonstrated a substantial increase in DSCA, with a mean expansion of approximately 75%. This degree of dural sac enlargement indicates that thoracic UBE can provide meaningful canal expansion despite limited bony exposure. Importantly, adequate decompression was achieved across a wide spectrum of OLF morphologies and dural ossification patterns. Thoracic OLF frequently presents with bulky ossification, midline fusion, or dural involvement, all of which may limit the working space and increase the risk of incomplete decompression. Nevertheless, the observed DSCA gains, and consistent neurological improvement suggest that thoracic UBE, when applied with appropriate surgical judgment, can provide functionally sufficient decompression even in complex OLF cases. These findings support the applicability of UBE beyond selected “simple” lesions and underscore its feasibility in routine thoracic OLF surgery. Unlike conventional thoracic OLF series, which typically describe decompression adequacy in qualitative terms, this study provides objective, quantitative radiologic data. To our knowledge, standardized reporting of DSCA expansion ratios has been rarely performed in thoracic OLF surgery, particularly in open decompression series. The present analysis therefore fills an important gap by offering a reproducible metric for assessing decompression efficacy. Posterior Element Preservation and Structural Implications Preservation of posterior stabilizing structures is a defining characteristic of thoracic UBE. Quantitative CT-based analysis revealed a clear asymmetry in facet joint preservation, with significantly greater preservation on the contralateral side compared with the ipsilateral approach side. This pattern reflects the technical advantage(undercutting technique) of unilateral laminotomy with bilateral decompression, in which the contralateral lamina and facet can be undercut without violating their dorsal cortex. Conventional open thoracic decompression often involves wide bilateral exposure, including total laminectomy and, in some cases, substantial facetectomy.[ 2 , 5 , 18 ] Such approaches may necessitate posterior fixation, particularly in multilevel disease or in patients with pre-existing deformity. In contrast, thoracic UBE limits bone resection to what is necessary for neural decompression and avoids routine disruption of the posterior ligamentous complex.[ 7 , 9 , 16 ] Although the thoracic rib cage provides baseline stability, preservation of posterior elements may still be relevant in elderly patients or those with compromised bone quality. It is important to emphasize that this study does not claim that facet preservation directly prevents postoperative instability. Rather, facet preservation is presented as a radiologic indicator of structural restraint inherent to the unilateral approach. By quantifying facet length and area preservation, this study provides objective evidence that thoracic UBE minimizes unnecessary posterior element sacrifice while maintaining decompression adequacy. Kyphosis Progression Versus Instability Postoperative alignment change remains a key concern after thoracic decompression. In this cohort, a statistically significant increase in segmental kyphosis was observed at 1-year follow-up; however, the mean angular change was modest, averaging approximately 1.5°. Such changes are more appropriately interpreted as kyphosis progression rather than definitive postoperative instability. The distinction between kyphosis progression and instability is clinically important. Instability implies abnormal motion or mechanical failure, typically assessed through dynamic imaging or progressive symptomatic deformity. In contrast, kyphosis progression reflects a static alignment change over time and does not necessarily indicate pathological motion. In the present study, dynamic instability was not systematically evaluated, and therefore angular changes should be interpreted cautiously. Only a small number of segments met the predefined radiographic threshold for kyphosis progression, and surgical stabilization was required in a single patient. These findings suggest that thoracic UBE does not commonly result in clinically significant instability when performed without fusion. Preservation of contralateral facets and posterior ligamentous structures, combined with the inherent stabilizing effect of the rib cage, may contribute to this favorable alignment profile. Nonetheless, isolated cases of delayed kyphotic progression highlight the need for careful patient selection and preoperative assessment, particularly in patients with extensive disease, severe facet degeneration, or pre-existing sagittal imbalance. Intentional Floating and Interpretation of Residual OLF Residual OLF identified on postoperative imaging warrants careful interpretation. In thoracic OLF surgery, particularly in the presence of dural ossification, aggressive attempts at complete excision may substantially increase the risk of dural defect, cerebrospinal fluid leakage, and neurological injury. The UBE platform provides a magnified and well-illuminated surgical field that allows meticulous thinning of ossified tissue and direct assessment of dural pulsation. In this context, leaving a thinned ossified remnant as part of an intentional floating strategy represents a deliberate surgical choice rather than incomplete decompression. The substantial DSCA increase and favorable clinical outcomes observed in this series suggest that functional decompression can be achieved without complete radiologic removal of all ossified tissue. This concept is particularly relevant in thoracic surgery, where the margin for dural injury is narrow and complication avoidance is a critical determinant of success. Radiologic–Clinical Relationship and Contextual Factors Although significant dural sac expansion was achieved in all cases, the magnitude of DSCA increase did not demonstrate a linear association with neurological recovery. This finding highlights the multifactorial nature of clinical outcomes in thoracic myelopathy. Radiologic decompression appears to be a necessary but not sufficient condition for neurological improvement. Baseline neurological status, chronicity of compression, and intrinsic spinal cord changes likely influence postoperative recovery.[ 2 , 18 ] Accordingly, radiologic outcomes should be interpreted as indicators of decompression adequacy rather than direct predictors of clinical gain. The present findings support a conceptual framework in which thoracic UBE provides sufficient mechanical decompression while clinical recovery remains constrained by biological factors beyond the surgeon’s control. Limitations This study has several limitations. This study is retrospective and single center in design, which may limit generalizability. A direct comparison with conventional open decompression was not performed, precluding definitive conclusions regarding relative superiority. Radiologic outcomes were assessed at the segment level, whereas clinical outcomes were patient-based, complicating causal inference. In addition, follow-up duration was limited to early alignment assessment, and longer-term studies are required to evaluate late kyphotic progression. Finally, all procedures were performed using a standardized left-sided approach, and the findings may not be directly applicable to alternative surgical strategies. CONCLUSIONS Thoracic unilateral biportal endoscopic decompression for ossification of the ligamentum flavum achieves effective neural decompression while preserving posterior stabilizing structures. Quantitative radiologic analysis demonstrated substantial dural sac expansion, asymmetric but meaningful facet preservation, and only limited kyphosis progression at 1-year follow-up, despite the absence of routine fusion or fixation. Residual OLF on postoperative imaging often reflects an intentional floating strategy aimed at minimizing dural injury rather than incomplete decompression. These findings support thoracic UBE as a viable, tissue-preserving alternative to conventional posterior surgery in the management of thoracic OLF. Declarations NOTES Funding/Support None. Conflict of Interest: The authors have nothing to disclose. Author Contribution Conceptualization: MK ParkData curation: SW LeeFormal analysis: SW LeeFunding acquisition: N/AMethodology: SW LeeProject administration: MK ParkVisualization: JW JungWriting - original draft: SW LeeWriting - review & editing: MK Park, SW Lee , JW Jung, SK Son References Aizawa T, Sato T, Sasaki H et al (2006) Thoracic myelopathy caused by ossification of the ligamentum flavum: clinical features and surgical results in the Japanese population. J Neurosurg Spine 5:514–519. https://doi.org/10.3171/spi.2006.5.6.514 Miyakoshi N, Shimada Y, Suzuki T et al (2003) Factors related to long-term outcome after decompressive surgery for ossification of the ligamentum flavum of the thoracic spine. 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Neurospine 22:819–828. https://doi.org/10.14245/ns.2550338.169 Kim JY, Ha JS, Lee CK et al (2023) Biportal Endoscopic Posterior Thoracic Laminectomy for Thoracic Spondylotic Myelopathy Caused by Ossification of the Ligamentum Flavum: Technical Developments and Outcomes. Neurospine 20:129–140. https://doi.org/10.14245/ns.2346060.030 Zhao Y, Xiang Q, Jiang S et al (2023) Incidence and risk factors of dural ossification in patients with thoracic ossification of the ligamentum flavum. J Neurosurg Spine 38:131–138. https://doi.org/10.3171/2022.7.SPINE22645 Park M-K, Park D, Son S-K (2023) Unilateral Biportal Endoscopic Decompression for Thoracic Spinal Stenosis. J Minim Invasive Spine Surg Tech 8:82–88. https://doi.org/10.21182/jmisst.2023.00696 Espinoza XAS, Pérez EG, Choi D-J (2025) The unilateral biportal endoscopy journey: proposing a 10-tier difficulty progression framework for unilateral biportal endoscopy. Asian Spine J 19:311–323. https://doi.org/10.31616/asj.2025.0064 Li B, Qiu G, Guo S et al (2016) Dural ossification associated with ossification of ligamentum flavum in the thoracic spine: a retrospective analysis. BMJ Open 6:e013887. https://doi.org/10.1136/bmjopen-2016-013887 Muthukumar N (2009) Dural Ossification in Ossification of the Ligamentum Flavum: A Preliminary Report. Spine 34:2654–2661. https://doi.org/10.1097/BRS.0b013e3181b541c9 Chen G, Tao L, Chen Z et al (2024) Imaging signs for preoperative diagnosis of dural ossification in patients with thoracic ossification of the ligamentum flavum: a blind, randomized diagnostic accuracy study. Quant Imaging Med Surg 14:1466–1476. https://doi.org/10.21037/qims-23-634 Sato T, Kokubun S, Tanaka Y, Ishii Y (1998) Thoracic Myelopathy in the Japanese: Epidemiological and Clinical Observations on the Cases in Miyagi Prefecture. Tohoku J Exp Med 184:1–11. https://doi.org/10.1620/tjem.184.1 Moon BJ, Kuh SU, Kim S et al (2015) Prevalence, Distribution, and Significance of Incidental Thoracic Ossification of the Ligamentum Flavum in Korean Patients with Back or Leg Pain: MR-Based Cross Sectional Study. J Korean Neurosurg Soc 58:112. https://doi.org/10.3340/jkns.2015.58.2.112 Deng Y, Yang M, Xia C et al (2022) Unilateral biportal endoscopic decompression for symptomatic thoracic ossification of the ligamentum flavum: a case control study. Int Orthop 46:2071–2080. https://doi.org/10.1007/s00264-022-05484-0 Zhong Z, Ying J, Wu H et al (2025) Biportal Endoscopic Spinal Surgery for Posterior Decompression of Thoracic Myelopathy Caused by Single-level Thoracic Ossification of the Ligamentum Flavum. Spine Surg Relat Res 9:321–330. https://doi.org/10.22603/ssrr.2024-0094 Zhang J, Wang L, Li J et al (2016) Predictors of surgical outcome in thoracic ossification of the ligamentum flavum: focusing on the quantitative signal intensity. Sci Rep 6:23019. https://doi.org/10.1038/srep23019 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Under Review Version 1 posted Reviewers agreed at journal 11 May, 2026 Reviews received at journal 10 May, 2026 Reviewers agreed at journal 28 Apr, 2026 Reviewers invited by journal 28 Apr, 2026 Editor assigned by journal 24 Mar, 2026 Submission checks completed at journal 24 Mar, 2026 First submitted to journal 22 Mar, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9194607","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":635036697,"identity":"2581ff44-a8aa-4f42-ac1f-20fc199563be","order_by":0,"name":"Sang-Woo Lee","email":"","orcid":"","institution":"Hu hospital","correspondingAuthor":false,"prefix":"","firstName":"Sang-Woo","middleName":"","lastName":"Lee","suffix":""},{"id":635036698,"identity":"6d330692-9375-4080-ac53-07f217836628","order_by":1,"name":"JinWoo Jung","email":"","orcid":"","institution":"Hu hospital","correspondingAuthor":false,"prefix":"","firstName":"JinWoo","middleName":"","lastName":"Jung","suffix":""},{"id":635036699,"identity":"4c576e1d-e9ce-46c3-92b7-d021b8580a8b","order_by":2,"name":"Sang-Kyu Son","email":"","orcid":"","institution":"Good Moonhwa Hospital","correspondingAuthor":false,"prefix":"","firstName":"Sang-Kyu","middleName":"","lastName":"Son","suffix":""},{"id":635036701,"identity":"967564a2-a79d-4d86-8bcf-42554ce5dd76","order_by":3,"name":"Man-Kyu Park","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA1ElEQVRIiWNgGAWjYBACxmYeBsaGHzYJYF5CAbFaGnvSoFoMgAQbQU0gW9gOQ7QwEKOFuZ334McZPOfz+Gd3J354YMCQxy/fQMhhfMmSGyxuF0vcObtZAuiwYsk2ArYA/WIg+YDndmLDjdwNIC2JG44R1mL88wHbucT5N3I3/wBp2U+EFjPJDWwHEjfcyN0GsYWQ90FaLGf2JCduBGqxSDCQSJxxLAG/FsP+M8Y3e37YJc4DOuzmjwqbxP7mAwS0NKDyJQi4CgjkCSsZBaNgFIyCEQ8AprdHGbZeqocAAAAASUVORK5CYII=","orcid":"","institution":"Hu hospital","correspondingAuthor":true,"prefix":"","firstName":"Man-Kyu","middleName":"","lastName":"Park","suffix":""}],"badges":[],"createdAt":"2026-03-23 02:23:29","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9194607/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9194607/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":108955231,"identity":"f05ac4b6-ec32-4f26-8475-d274367f8295","added_by":"auto","created_at":"2026-05-11 08:06:18","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1140524,"visible":true,"origin":"","legend":"\u003cp\u003eRepresentative computed tomography images illustrating the Sato classification of thoracic ossification of the ligamentum flavum.\u003c/p\u003e\n\u003cp\u003eRepresentative axial CT images demonstrate the five morphological subtypes of thoracic ossification of the ligamentum flavum as described by Sato et al.\u003c/p\u003e\n\u003cp\u003e(A) \u003cstrong\u003eLateral type\u003c/strong\u003e, in which ossification is confined to the lateral portion of the ligamentum flavum without significant central canal compromise.\u003cbr\u003e\n(B) \u003cstrong\u003eExtended type\u003c/strong\u003e, characterized by medial extension of ossification toward the central canal while remaining relatively thin.\u003cbr\u003e\n(C) \u003cstrong\u003eEnlarged type\u003c/strong\u003e, showing bulky ossification with increased thickness and substantial encroachment on the spinal canal.\u003cbr\u003e\n(D) \u003cstrong\u003eFused type\u003c/strong\u003e, in which bilateral ossified ligamenta flava are connected across the midline, forming a continuous posterior compressive mass.\u003cbr\u003e\n(E) \u003cstrong\u003eTuberous type\u003c/strong\u003e, demonstrating nodular or beak-like ossification with irregular margins and severe focal compression of the dural sac.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-9194607/v1/40ffbdbe6bbff8a9fab095bf.png"},{"id":108977437,"identity":"914d4bf3-e1a1-4490-b571-866c3d6dc6b4","added_by":"auto","created_at":"2026-05-11 11:31:46","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":4504566,"visible":true,"origin":"","legend":"\u003cp\u003eRadiologic classification of dural ossification (DO) types.\u003c/p\u003e\n\u003cp\u003eAxial computed tomography (CT) images demonstrate three representative patterns of dural ossification associated with thoracic ossification of the ligamentum flavum (OLF).\u003cbr\u003e\n(A) \u003cstrong\u003eTram-track sign\u003c/strong\u003e, A thin, linear hyperdense structure lining the dorsal aspect of the dural sac (white arrows) is separated from the ossified ligamentum flavum mass (red arrowheads), forming a characteristic “tram-track” appearance.\u003c/p\u003e\n\u003cp\u003e(B) \u003cstrong\u003eBridge sign\u003c/strong\u003e, The ossified dura mater (white arrows) is fused with the OLF mass (red arrowheads) and forms a bony bridge across the midline, connecting the bilateral OLF lesions.\u003cbr\u003e\n(C) \u003cstrong\u003eComma sign\u003c/strong\u003e, The ossified dura mater (white arrows) extends ventrolaterally and is continuously fused with the OLF mass (red arrowheads), creating an asymmetric, comma-shaped configuration along the dural margin.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-9194607/v1/5f526c22ff2d8bf6b8369caf.png"},{"id":108978155,"identity":"1ec2ff7c-1b2c-4a68-a35b-21fd770c832f","added_by":"auto","created_at":"2026-05-11 11:34:23","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":7712041,"visible":true,"origin":"","legend":"\u003cp\u003eRadiologic measurement methods and imaging assessment.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e(A, B) Measurement of dural sac cross-sectional area (DSCA).\u003c/strong\u003e\u003cbr\u003e\nDSCA was measured on axial T2-weighted MRI at the identical slice level before (A) and after surgery (B). The inner margin of the dural sac was manually traced as a closed region of interest (ROI) (yellow closed line), and the enclosed area was automatically calculated using the PACS measurement tool and recorded in square millimeters (mm²).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e(C, D) Measurement of facet joint length and cross-sectional area.\u003c/strong\u003e\u003cbr\u003e\nFacet joint measurements were performed on axial CT images at the identical slice level before (C) and after surgery (D). The medial and lateral margins of each facet joint were defined by vertical reference lines (white lines). Facet joint length was measured as the maximal mediolateral distance between the medial and lateral margins (red arrow). Facet joint cross-sectional area was calculated by manually outlining the facet joint as a closed ROI (yellow-shaded area).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e(E) Measurement of segmental kyphosis angle.\u003c/strong\u003e\u003cbr\u003e\nSegmental kyphosis at the index level was measured on lateral radiographs using the Cobb method, defined as the angle between the superior endplate of the cranial vertebra and the inferior endplate of the caudal vertebra.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e(F) Assessment of spinal cord signal change.\u003c/strong\u003e\u003cbr\u003e\nSpinal cord signal change was evaluated on axial T2-weighted MRI. Intramedullary high signal intensity was identified and recorded when present (red arrow).\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-9194607/v1/5ec0c4b48981fb3ed6c739e2.png"},{"id":108979850,"identity":"219f291f-58be-42d0-84e6-fa13427ee9fe","added_by":"auto","created_at":"2026-05-11 12:01:57","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":23044882,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9194607/v1/2f558de0-1e00-4791-95c7-58e42959a429.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Radiologic and Clinical Outcomes of Unilateral Biportal Endoscopic Decompression for Thoracic Ossification of the Ligamentum Flavum","fulltext":[{"header":"INTRODUCTION","content":"\u003cp\u003eThoracic ossification of the ligamentum flavum (T-OLF) represents a common structural cause of thoracic myelopathy and results in progressive compression of the spinal cord.[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e] Once myelopathic symptoms appear, neurological function rarely improves spontaneously and, in many cases, continues to deteriorate over time.[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e] Patients often present with gait disturbance and sensory impairment, and as the disease progresses, bladder dysfunction and axial back pain may also develop.[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e] Conservative treatment has limited efficacy in established thoracic myelopathy, and surgical decompression is therefore required to prevent further neurological decline. Conventional posterior decompression remains the most widely used surgical strategy; however, it typically requires wide exposure with removal of multiple posterior stabilizing structures, including the lamina, facet joints, and ligamentous complex.[\u003cspan additionalcitationids=\"CR6 CR7\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e] In elderly patients, this extent of tissue disruption may translate into increased perioperative burden and raises concern regarding postoperative alignment and segmental stability.\u003c/p\u003e \u003cp\u003eIn an effort to reduce approach-related morbidity, minimally invasive techniques have been increasingly adopted for spinal decompression. Among these, unilateral biportal endoscopy (UBE) has gained attention as an endoscopic platform that separates the viewing and working portals, allowing bimanual instrument handling and a wide operative field under high magnification.[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e] This configuration enables precise bone and soft tissue resection while maintaining continuous visualization of neural structures, thereby facilitating decompression comparable to conventional microscopic surgery through a substantially smaller surgical corridor. Originally developed and refined in lumbar decompression and fusion procedures, the technical principles of UBE have gradually been extended to the thoracic spine, where preservation of posterior stabilizing structures is of particular importance.[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/p\u003e \u003cp\u003eThoracic OLF presents unique technical challenges compared with degenerative conditions of the lumbar spine. The thoracic spinal canal is inherently narrower, the thoracic cord is less tolerant of manipulation, and ossification frequently extends to or involves the dura mater.[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e] Preoperative computed tomography findings that shows dural ossifications(DO), such as tram-track, bridge, and comma signs, have been used to anticipate the presence of DO and to guide surgical strategy.[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan additionalcitationids=\"CR12\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] Nevertheless, most published thoracic UBE studies have focused primarily on technical feasibility or short-term clinical outcomes. Quantitative evaluation of decompression efficacy and preservation of posterior stabilizing structures has been limited. As a result, objective radiologic data on dural sac expansion, facet joint preservation, and postoperative alignment changes in thoracic UBE remain scarce.\u003c/p\u003e \u003cp\u003eAccordingly, the purpose of this study was to evaluate the radiologic and clinical outcomes of thoracic UBE decompression in a consecutive cohort of patients with OLF. We aimed to quantify decompression efficacy using pre- and postoperative measurements of dural sac cross-sectional area and to objectively assess facet joint preservation with side-specific analysis reflecting the unilateral approach. Postoperative alignment changes and spinal cord signal evolution were also examined, and radiologic findings were correlated with clinical outcomes. We hypothesized that thoracic UBE would achieve substantial dural sac expansion with high preservation of posterior stabilizing structures, accompanied by meaningful clinical improvement, while postoperative alignment changes would predominantly represent mild kyphosis progression rather than true instability.\u003c/p\u003e"},{"header":"MATERIALS AND METHODS","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eStudy design and Ethics\u003c/h2\u003e \u003cp\u003eThis was a single-center, retrospective observational study including consecutive patients who underwent thoracic posterior decompression using UBE for symptomatic thoracic myelopathy. The study was approved by Public Institutional Review Board (IRB No. P01-202511-01-073), and the requirement for individual informed consent was waived owing to the use of de-identified data and minimal risk to participants.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003ePatient Population\u003c/h3\u003e\n\u003cp\u003eConsecutive adult patients who underwent thoracic UBE decompression between July 2022 and October 2024 were included.\u003c/p\u003e \u003cp\u003eInclusion criteria:\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eSymptomatic thoracic myelopathy or radiculopathy attributed to thoracic OLF with or without concomitant degenerative thoracic spinal stenosis.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eUnderwent UBE decompression alone at one or two contiguous thoracic levels. (No additional lumbar decompression or fusion performed on the same day)\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eAvailability of preoperative thoracic MRI and CT, and early and late postoperative MRI (early: within 2 months; late: \u0026ge;12 months).\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eMinimum clinical follow-up more than 12 months.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e \u003cp\u003eExclusion criteria:\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eInfection, neoplasm, or acute traumatic instability at the index level.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eConcomitant lumbar decompression and/or fusion performed on the same day, because of postoperative symptom improvement could not be clearly attributed to thoracic decompression.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003ePlanned or intraoperative conversion to open decompression or instrumented fusion at the index level.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eIsolated intradural tumor or non-OLF compressive lesion.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eIncomplete imaging (missing preoperative MRI/CT or postoperative MRI) or insufficient clinical documentation.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eLoss to follow-up before 12 months.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e\n\u003ch3\u003eImaging Evaluation\u003c/h3\u003e\n\u003cp\u003eAll patients underwent pre- and post-operative thoracic MRI (1.5T or 3T) and thin-slice CT (\u0026le;\u0026thinsp;1.0 mm) with multiplanar reconstruction. Standing thoracic radiographs (neutral, and flexion\u0026ndash;extension when available) were also obtained.\u003c/p\u003e \u003cp\u003ePreoperative classification\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eOLF morphology was categorized on CT using the Sato classification (lateral, extended, enlarged, fused, tuberous) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eDural ossification (DO) was assessed on CT using established signs (tram-track, bridge, comma) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e)[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFor patients with multilevel surgery, the index segment for radiologic analysis was defined as the level with the smallest preoperative dural sac cross-sectional area (DSCA) or the most severe OLF morphology.\u003c/p\u003e \u003cp\u003ePostoperative imaging\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eEarly MRI (2 weeks\u0026ndash;2 months) was used to evaluate decompression completeness, residual OLF, epidural hematoma, and early cord signal change.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eLate MRI (\u0026ge;\u0026thinsp;12 months) assessed maintenance of decompression and evolution of cord signal changes.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003ePost operative CT was used to quantify facet preservation and confirm bony decompression.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003estanding lateral and dynamic radiographs at 1, 3, 6, and 12 months were used to assess segmental kyphosis and its progression. Radiographic kyphosis progression was defined as an increase in segmental Cobb angle of \u0026ge;\u0026thinsp;5\u0026deg;.\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e\n\u003ch3\u003eRadiological Measurements\u003c/h3\u003e\n\u003cp\u003eAll imaging measurements were performed independently by a fellowship-trained radiologist and a board-certified spine neurosurgeon, both blinded to clinical data and each other\u0026rsquo;s readings. A calibration session with pilot cases(N\u0026thinsp;=\u0026thinsp;5) was performed prior to formal measurements. A random subset of cases(N\u0026thinsp;=\u0026thinsp;15) was re-measured after a 2-week interval to assess intra-observer reliability. Inter- and intra-observer agreement for continuous variables were reported as intraclass correlation coefficients (ICC[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]); categorical labels (Sato class, DO signs, residual OLF) was assessed using Cohen\u0026rsquo;s κ. Measurements were performed using the INFINITT PACS system (INFINITT Healthcare, Seoul, Korea).\u003c/p\u003e \u003cp\u003eRadiologic outcomes were assessed on a segment-by-segment basis. The primary radiologic parameters included DSCA, facet joint morphology (length and area), segmental kyphosis, spinal cord signal change, and residual OLF.\u003c/p\u003e \u003cp\u003eDSCA was measured on axial T2-weighted MRI at the index segment using a standardized axial plane at the mid-facet level (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eA, B). The inner contour of the dural sac was traced using a freehand region-of-interest method. Absolute change (ΔDSCA) and percentage gain relative to preoperative values were calculated.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFacet joint morphology was evaluated on axial computed tomography images. Facet length was defined as the linear distance from the medial articular margin to the lateral edge of the articular surface, and facet area was calculated by manually outlining the cortical boundary of the facet joint (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eC, D). Measurements were obtained separately for the ipsilateral (surgical approach side) and contralateral facets. Facet preservation ratio (%) was calculated as the postoperative value divided by the preoperative value.\u003c/p\u003e \u003cp\u003eSegmental kyphosis was measured on standing lateral radiographs using the Cobb method between the upper endplate of the cranial vertebra and the lower endplate of the caudal vertebra at the index segment (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eE).\u003c/p\u003e \u003cp\u003eSpinal cord signal intensity was evaluated on sagittal and axial T2-weighted MRI and recorded as a binary variable (presence or absence of high signal intensity). Residual OLF was defined as persistent posterior compression of the dural sac on early postoperative MRI or CT.\u003c/p\u003e\n\u003ch3\u003eSurgical Technique\u003c/h3\u003e\n\u003cp\u003eAll procedures were performed under general anesthesia with the patient in the prone position on a radiolucent frame. A standardized left-sided unilateral biportal endoscopic approach was used in all cases. After fluoroscopic confirmation of the index level, two paramedian portals were created along the left medial pedicle line for endoscopic visualization and instrument manipulation. Soft tissue over the lamina\u0026ndash;facet junction was cleared using radiofrequency ablation and a shaver, followed by unilateral laminotomy using a high-speed diamond burr. The ligamentum flavum was preserved as a protective layer during bony decompression and exposed after adequate working space was obtained.\u003c/p\u003e \u003cp\u003eBecause all cases involved OLF, decompression strategy was determined according to the presence and severity of DO. In cases without DO, the OLF was circumferentially thinned to a paper-thin layer. After confirming clear margins, the interface between the OLF and dura was carefully explored using a blunt dissector or freer elevator, particularly when epidural fat was identified, and the OLF was gently removed.\u003c/p\u003e \u003cp\u003eIn cases with DO, the OLF was maximally thinned and separated from surrounding margins. When separation between the OLF and DO was achievable, the OLF was cautiously removed en-bloc. Residual DO was further thinned to allow floating of the ossified dura. If the thinned DO became semi-lucent and the inner dural layer or spinal cord pulsation was clearly visible, further removal was avoided to prevent dural injury, and the area was covered with a sealant material. When DO and OLF were inseparable, both were thinned together while carefully avoiding direct compression of the dural sac, circumferential margins were established, and a floating technique was employed. When complete removal of DO was considered safe, it was removed and immediately reinforced with sealant material.\u003c/p\u003e \u003cp\u003eHemostasis was achieved using low-power bipolar coagulation and topical hemostatic agents under low-pressure continuous irrigation. Decompression was considered complete when bilateral pedicle medial borders were visualized, free dural pulsation was observed, and no residual compressive elements were identified on circumferential endoscopic inspection. A closed-suction drain was placed at the surgeon\u0026rsquo;s discretion, and patients were mobilized beginning on postoperative day 1 unless contraindicated.\u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eStatistical analysis\u003c/h2\u003e \u003cp\u003eStatistical analyses were performed by an independent biostatistician using R software (Version 4.5.0, R Foundation for Statistical Computing, Vienna, Austria). To evaluate the inter-observer reliability of the radiographic measurements, the intraclass correlation coefficient [ICC(2, 1)] and Cohen\u0026rsquo;s kappa coefficient (κ) were calculated.\u003c/p\u003e \u003cp\u003eContinuous variables are presented as mean\u0026thinsp;\u0026plusmn;\u0026thinsp;standard deviation, and categorical variables as number (%). Pre- and postoperative comparisons were analyzed using paired t-tests or Wilcoxon signed-rank tests, as appropriate. Changes in spinal cord T2 high signal intensity were evaluated using the McNemar test. All statistical tests were two-sided, and a p value\u0026thinsp;\u0026lt;\u0026thinsp;0.05 was considered statistically significant.\u003c/p\u003e \u003c/div\u003e"},{"header":"RESULTS","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003ePatient Characteristics and Operative Data\u003c/h2\u003e \u003cp\u003eThe inter-observer reliability for the radiographic measurements was excellent, demonstrating an overall ICC of 0.89 for continuous variables and a Cohen\u0026rsquo;s kappa of 1.00 for categorical variables.\u003c/p\u003e \u003cp\u003eThe study cohort consisted of 45 patients (19 males and 26 females) with a mean age of 69.5\u0026thinsp;\u0026plusmn;\u0026thinsp;10.7 years. A total of 55 thoracic segments were decompressed using UBE. The mean symptom duration was 5.0 months, and the mean follow-up period was 16.0 months. The average operative time was 56.4\u0026thinsp;\u0026plusmn;\u0026thinsp;15.1 minutes per segment, and the mean length of hospital stay was 8.8\u0026thinsp;\u0026plusmn;\u0026thinsp;4.4 days (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003ePatient Demographics and baseline characteristics (Patient-level, N\u0026thinsp;=\u0026thinsp;45)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"2\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCharacteristic\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eValue\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAge(years)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e69.5\u0026thinsp;\u0026plusmn;\u0026thinsp;10.7 (range 43\u0026ndash;83)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSex (M:F)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e19:26\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eOperative segments (N)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e55\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eOperative time (min, per segment)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e56.4\u0026thinsp;\u0026plusmn;\u0026thinsp;15.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSymptom duration (Months)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5.0\u0026thinsp;\u0026plusmn;\u0026thinsp;2.6 (range 1\u0026ndash;14)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFollow-up (Months)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e16.0\u0026thinsp;\u0026plusmn;\u0026thinsp;3.9 (range 12\u0026ndash;26)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLength of hospital stay (days)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e8.8\u0026thinsp;\u0026plusmn;\u0026thinsp;4.4 (range 4\u0026ndash;25)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePreoperative mJOA score\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e7.7\u0026thinsp;\u0026plusmn;\u0026thinsp;1.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePreoperative Nurick score\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2.3\u0026thinsp;\u0026plusmn;\u0026thinsp;1.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePreoperative back pain (VAS)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e6.2\u0026thinsp;\u0026plusmn;\u0026thinsp;1.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePreoperative leg pain (VAS)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e4.7\u0026thinsp;\u0026plusmn;\u0026thinsp;1.7\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"2\"\u003eValues are shown as number or mean\u0026thinsp;\u0026plusmn;\u0026thinsp;standard deviation\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"2\"\u003emJOA, modified Japanese Orthopaedic Association; VAS, Visual Analog Scale\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe most frequently treated levels were T11\u0026ndash;12 (38.2%) and T10\u0026ndash;11 (30.9%). According to the Sato classification, lateral-type OLF (Type A) was the most common morphology (49.1%), followed by the extended type (Type B, 27.3%). Preoperative CT evaluation demonstrated no DO signs in 72.7% of segments. Among segments with DO (27.3%), the tram-track sign was most prevalent (20.0%), followed by the comma (5.5%) and bridge signs (1.8%) (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eOperative characteristics (Segment-level, N\u0026thinsp;=\u0026thinsp;55)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"2\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eVariable\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eValue\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLevel distribution\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTotal\u0026thinsp;=\u0026thinsp;55\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eT3-4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1 (1.8%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eT6-7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2 (3.6%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eT8-9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3 (5.5%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eT9-10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e8 (14.5%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eT10-11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e17 (30.9%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eT11-12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e21 (38.2%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eT12-L1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3 (5.5%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eOLF type\u003csup\u003e\u0026dagger;\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLateral\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e27 (49.1%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eExtended\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e15 (27.3%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eEnlarged\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e6 (10.9%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFused\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5 (9.1%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTuberous\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2 (3.6%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDO type\u003csup\u003e\u0026Dagger;\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e40 (72.7%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTram track\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e11 (20.0%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBridge\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1 (1.8%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eComma\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3 (5.5%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"2\"\u003eValues are presented as number (%)\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"2\"\u003eOLF, ossification of the ligamentum flavum; DO, dural ossification.\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"2\"\u003e\u0026dagger;OLF type was classified according to the Sato classification as follows:\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"2\"\u003eA, lateral type;B, extended type; C, enlarged type; D, fused type; E, tuberous type.\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"2\"\u003e\u0026Dagger;DO type was determined based on preoperative computed tomography findings and categorized as:\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"2\"\u003eA, no dural ossification; B, tram-track sign; C, bridge sign; D, comma sign.\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eRadiologic Outcomes\u003c/h2\u003e \u003cp\u003ePostoperative imaging confirmed adequate decompression in all treated segments (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The mean DSCA increased significantly from 70.4\u0026thinsp;\u0026plusmn;\u0026thinsp;20.4 mm\u0026sup2; preoperatively to 119.8\u0026thinsp;\u0026plusmn;\u0026thinsp;31.0 mm\u0026sup2; postoperatively (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001), representing a mean expansion ratio of 75.0\u0026thinsp;\u0026plusmn;\u0026thinsp;36.7%.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eRadiologic outcomes (Segment-level, N\u0026thinsp;=\u0026thinsp;55)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eVariable\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003eValue\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ep-value\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eDSCA (mm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePreoperative\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePostoperative\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e70.4\u0026thinsp;\u0026plusmn;\u0026thinsp;20.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e119.8\u0026thinsp;\u0026plusmn;\u0026thinsp;31.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eΔ DSCA (mm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003e49.4\u0026thinsp;\u0026plusmn;\u0026thinsp;22.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDSCA gain (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003e75.0\u0026thinsp;\u0026plusmn;\u0026thinsp;36.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFacet length (mm)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLeft (Ipsilateral)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eRight (Contralateral)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePreoperative\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e12.1\u0026thinsp;\u0026plusmn;\u0026thinsp;1.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e12.2\u0026thinsp;\u0026plusmn;\u0026thinsp;1.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.642\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePostoperative\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e8.0\u0026thinsp;\u0026plusmn;\u0026thinsp;2.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e9.6\u0026thinsp;\u0026plusmn;\u0026thinsp;1.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ep-value\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePreservation ratio (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e65.1\u0026thinsp;\u0026plusmn;\u0026thinsp;11.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e79.1\u0026thinsp;\u0026plusmn;\u0026thinsp;10.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFacet area (mm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLeft (Ipsilateral)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eRight (Contralateral)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePreoperative\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e123.3\u0026thinsp;\u0026plusmn;\u0026thinsp;31.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e120.4\u0026thinsp;\u0026plusmn;\u0026thinsp;26.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.458\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePostoperative\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e79.7\u0026thinsp;\u0026plusmn;\u0026thinsp;27.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e94.8\u0026thinsp;\u0026plusmn;\u0026thinsp;21.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ep-value\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePreservation ratio (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e64.2\u0026thinsp;\u0026plusmn;\u0026thinsp;13.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e79.4\u0026thinsp;\u0026plusmn;\u0026thinsp;11.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSegmental Kyphosis (\u0026deg;)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePreoperative\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePostoperative (1 year)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5.4\u0026thinsp;\u0026plusmn;\u0026thinsp;7.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e6.9\u0026thinsp;\u0026plusmn;\u0026thinsp;8.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.011\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCord T2 high SI, n (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePreoperative\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePostoperative (1 year)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e27 (49.1)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e13 (23.6)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.002\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eResidual OLF, n (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003e10 (18.2)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"4\"\u003eValues are presented as mean\u0026thinsp;\u0026plusmn;\u0026thinsp;standard deviation or number (%).\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"4\"\u003eIpsilateral facet refers to the surgical approach side.\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"4\"\u003eP-values for continuous variables were calculated using paired t-test.\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"4\"\u003eChanges in cord T2 high signal intensity were analyzed using the McNemar test.\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"4\"\u003eDSCA, dural sac cross-sectional area; OLF, ossification of the ligamentum flavum; SI, signal intensity.\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003ePreoperative facet length and area did not differ significantly between the ipsilateral and contralateral sides (length: p\u0026thinsp;=\u0026thinsp;0.642; area: p\u0026thinsp;=\u0026thinsp;0.458). Postoperatively, ipsilateral facet length (8.0\u0026thinsp;\u0026plusmn;\u0026thinsp;2.1 mm) and area (79.7\u0026thinsp;\u0026plusmn;\u0026thinsp;27.5 mm\u0026sup2;) were significantly smaller than those on the contralateral side (9.6\u0026thinsp;\u0026plusmn;\u0026thinsp;1.7 mm and 94.8\u0026thinsp;\u0026plusmn;\u0026thinsp;21.4 mm\u0026sup2;, respectively; both p\u0026thinsp;\u0026lt;\u0026thinsp;0.001). Correspondingly, facet preservation ratios were significantly lower on the ipsilateral side compared with the contralateral side for both length (65.1\u0026thinsp;\u0026plusmn;\u0026thinsp;11.7% vs. 79.1\u0026thinsp;\u0026plusmn;\u0026thinsp;10.5%, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001) and area (64.2\u0026thinsp;\u0026plusmn;\u0026thinsp;13.2% vs. 79.4\u0026thinsp;\u0026plusmn;\u0026thinsp;11.2%, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001), consistent with the unilateral decompression approach.\u003c/p\u003e \u003cp\u003eThe segmental kyphotic angle increased modestly from 5.4\u0026thinsp;\u0026plusmn;\u0026thinsp;7.8\u0026deg; preoperatively to 6.9\u0026thinsp;\u0026plusmn;\u0026thinsp;8.9\u0026deg; at 1-year follow-up (p\u0026thinsp;=\u0026thinsp;0.011). Although this change was statistically significant, the mean increase was limited to 1.5\u0026deg;, and radiographic instability (\u0026gt;\u0026thinsp;5\u0026deg; kyphotic progression) was observed in 3 of 55 segments (5.5%).\u003c/p\u003e \u003cp\u003ePreoperative spinal cord high signal intensity on T2-weighted MRI was present in 27 segments (49.1%). At 1-year follow-up, persistent high signal intensity remained in 13 segments (23.6%), representing a significant reduction compared with baseline (p\u0026thinsp;=\u0026thinsp;0.002). Residual OLF was identified in 10 segments (18.2%) on early postoperative imaging; however, no patient required reoperation for residual stenosis.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eClinical Outcomes\u003c/h2\u003e \u003cp\u003eSignificant improvements were observed across all clinical outcome measures following UBE decompression (Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). The mean mJOA score improved from 7.7\u0026thinsp;\u0026plusmn;\u0026thinsp;1.8 preoperatively to 9.7\u0026thinsp;\u0026plusmn;\u0026thinsp;1.5 at final follow-up (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001), corresponding to a mean recovery rate of 71.2\u0026thinsp;\u0026plusmn;\u0026thinsp;28.1%. Functional gait impairment, assessed by the Nurick grade, significantly improved from 2.3\u0026thinsp;\u0026plusmn;\u0026thinsp;1.2 to 0.6\u0026thinsp;\u0026plusmn;\u0026thinsp;1.0 (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eClinical outcomes (Patient-level, N\u0026thinsp;=\u0026thinsp;45)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eVariable\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePreoperative\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePost-1 month\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ePost- 1 year\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003ep-value\u003csup\u003e\u0026dagger;\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003emJOA score\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e7.7\u0026thinsp;\u0026plusmn;\u0026thinsp;1.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e9.1\u0026thinsp;\u0026plusmn;\u0026thinsp;1.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e9.7\u0026thinsp;\u0026plusmn;\u0026thinsp;1.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003emJOA RR\u003csup\u003e\u0026Dagger;\u003c/sup\u003e (%), 1 year\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e \u003cp\u003e71.2\u0026thinsp;\u0026plusmn;\u0026thinsp;28.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNurick grade\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2.3\u0026thinsp;\u0026plusmn;\u0026thinsp;1.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.1\u0026thinsp;\u0026plusmn;\u0026thinsp;1.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.6\u0026thinsp;\u0026plusmn;\u0026thinsp;1.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBack pain (VAS)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e6.2\u0026thinsp;\u0026plusmn;\u0026thinsp;1.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3.3\u0026thinsp;\u0026plusmn;\u0026thinsp;1.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.4\u0026thinsp;\u0026plusmn;\u0026thinsp;1.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLeg pain (VAS)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e4.7\u0026thinsp;\u0026plusmn;\u0026thinsp;1.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.5\u0026thinsp;\u0026plusmn;\u0026thinsp;1.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.1\u0026thinsp;\u0026plusmn;\u0026thinsp;1.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"5\"\u003eValues are presented as mean\u0026thinsp;\u0026plusmn;\u0026thinsp;standard deviation.\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"5\"\u003emJOA, modified Japanese Orthopaedic Association score for thoracic myelopathy; RR, recovery rate; VAS, visual analog scale.\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"5\"\u003e\u0026dagger;P-values were calculated using paired t-test and indicate overall preoperative vs final follow-up comparisons.\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"5\"\u003e\u0026Dagger;Recovery rate calculated as: (Postoperative mJOA - Preoperative mJOA) / (11 - Preoperative mJOA) \u0026times; 100.\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"5\"\u003eNurick grade is an ordinal scale and is presented as mean\u0026thinsp;\u0026plusmn;\u0026thinsp;standard deviation for descriptive purposes.\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003ePain outcomes also demonstrated substantial improvement. The mean VAS score for back pain decreased from 6.2\u0026thinsp;\u0026plusmn;\u0026thinsp;1.6 to 2.4\u0026thinsp;\u0026plusmn;\u0026thinsp;1.4, and leg pain improved from 4.7\u0026thinsp;\u0026plusmn;\u0026thinsp;1.7 to 2.1\u0026thinsp;\u0026plusmn;\u0026thinsp;1.0 at 1-year follow-up (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001 for both).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003eComplications\u003c/h2\u003e \u003cp\u003eOverall complication rates were low. Incidental durotomy occurred in three segments, all associated with severe OLF morphology (Sato classification: fused and tuberous type; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eD, \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eE) or advanced dural ossification (Bridge and comma sign; Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eB, \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eC). These cases were managed intraoperatively using sealant or patch reinforcement, and no persistent cerebrospinal fluid leakage was observed. One patient developed a postoperative epidural hematoma with acute neurological deterioration, which was successfully treated with immediate endoscopic hematoma removal surgery. Delayed segmental instability requiring posterior instrumented fusion occurred in one patient at 1-year follow-up. No surgical site infections were observed.\u003c/p\u003e \u003c/div\u003e"},{"header":"DISCUSSION","content":"\u003cp\u003eThoracic OLF represents a challenging cause of thoracic myelopathy, in which the goals of surgery must balance effective neural decompression against preservation of spinal stability.[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] While conventional posterior decompression has long been the standard treatment, concerns regarding extensive posterior element disruption and the frequent need for additional stabilization have prompted interest in less invasive alternatives.[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e] The present study provides a systematic radiologic evaluation of thoracic UBE decompression and demonstrates that this approach can achieve sufficient decompression while preserving posterior stabilizing structures, with acceptable postoperative alignment and favorable clinical outcomes.\u003c/p\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003eAdequacy of Decompression in Thoracic UBE\u003c/h2\u003e \u003cp\u003eA fundamental question in applying minimally invasive techniques to thoracic myelopathy is whether decompression through a unilateral corridor can be sufficient for the spinal cord. Using quantitative radiologic metrics, we demonstrated a substantial increase in DSCA, with a mean expansion of approximately 75%. This degree of dural sac enlargement indicates that thoracic UBE can provide meaningful canal expansion despite limited bony exposure.\u003c/p\u003e \u003cp\u003eImportantly, adequate decompression was achieved across a wide spectrum of OLF morphologies and dural ossification patterns. Thoracic OLF frequently presents with bulky ossification, midline fusion, or dural involvement, all of which may limit the working space and increase the risk of incomplete decompression. Nevertheless, the observed DSCA gains, and consistent neurological improvement suggest that thoracic UBE, when applied with appropriate surgical judgment, can provide functionally sufficient decompression even in complex OLF cases. These findings support the applicability of UBE beyond selected \u0026ldquo;simple\u0026rdquo; lesions and underscore its feasibility in routine thoracic OLF surgery.\u003c/p\u003e \u003cp\u003eUnlike conventional thoracic OLF series, which typically describe decompression adequacy in qualitative terms, this study provides objective, quantitative radiologic data. To our knowledge, standardized reporting of DSCA expansion ratios has been rarely performed in thoracic OLF surgery, particularly in open decompression series. The present analysis therefore fills an important gap by offering a reproducible metric for assessing decompression efficacy.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003ePosterior Element Preservation and Structural Implications\u003c/h2\u003e \u003cp\u003ePreservation of posterior stabilizing structures is a defining characteristic of thoracic UBE. Quantitative CT-based analysis revealed a clear asymmetry in facet joint preservation, with significantly greater preservation on the contralateral side compared with the ipsilateral approach side. This pattern reflects the technical advantage(undercutting technique) of unilateral laminotomy with bilateral decompression, in which the contralateral lamina and facet can be undercut without violating their dorsal cortex.\u003c/p\u003e \u003cp\u003eConventional open thoracic decompression often involves wide bilateral exposure, including total laminectomy and, in some cases, substantial facetectomy.[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] Such approaches may necessitate posterior fixation, particularly in multilevel disease or in patients with pre-existing deformity. In contrast, thoracic UBE limits bone resection to what is necessary for neural decompression and avoids routine disruption of the posterior ligamentous complex.[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] Although the thoracic rib cage provides baseline stability, preservation of posterior elements may still be relevant in elderly patients or those with compromised bone quality.\u003c/p\u003e \u003cp\u003eIt is important to emphasize that this study does not claim that facet preservation directly prevents postoperative instability. Rather, facet preservation is presented as a radiologic indicator of structural restraint inherent to the unilateral approach. By quantifying facet length and area preservation, this study provides objective evidence that thoracic UBE minimizes unnecessary posterior element sacrifice while maintaining decompression adequacy.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section2\"\u003e \u003ch2\u003eKyphosis Progression Versus Instability\u003c/h2\u003e \u003cp\u003ePostoperative alignment change remains a key concern after thoracic decompression. In this cohort, a statistically significant increase in segmental kyphosis was observed at 1-year follow-up; however, the mean angular change was modest, averaging approximately 1.5\u0026deg;. Such changes are more appropriately interpreted as kyphosis progression rather than definitive postoperative instability.\u003c/p\u003e \u003cp\u003eThe distinction between kyphosis progression and instability is clinically important. Instability implies abnormal motion or mechanical failure, typically assessed through dynamic imaging or progressive symptomatic deformity. In contrast, kyphosis progression reflects a static alignment change over time and does not necessarily indicate pathological motion. In the present study, dynamic instability was not systematically evaluated, and therefore angular changes should be interpreted cautiously. Only a small number of segments met the predefined radiographic threshold for kyphosis progression, and surgical stabilization was required in a single patient.\u003c/p\u003e \u003cp\u003eThese findings suggest that thoracic UBE does not commonly result in clinically significant instability when performed without fusion. Preservation of contralateral facets and posterior ligamentous structures, combined with the inherent stabilizing effect of the rib cage, may contribute to this favorable alignment profile. Nonetheless, isolated cases of delayed kyphotic progression highlight the need for careful patient selection and preoperative assessment, particularly in patients with extensive disease, severe facet degeneration, or pre-existing sagittal imbalance.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec18\" class=\"Section2\"\u003e \u003ch2\u003eIntentional Floating and Interpretation of Residual OLF\u003c/h2\u003e \u003cp\u003eResidual OLF identified on postoperative imaging warrants careful interpretation. In thoracic OLF surgery, particularly in the presence of dural ossification, aggressive attempts at complete excision may substantially increase the risk of dural defect, cerebrospinal fluid leakage, and neurological injury. The UBE platform provides a magnified and well-illuminated surgical field that allows meticulous thinning of ossified tissue and direct assessment of dural pulsation.\u003c/p\u003e \u003cp\u003eIn this context, leaving a thinned ossified remnant as part of an intentional floating strategy represents a deliberate surgical choice rather than incomplete decompression. The substantial DSCA increase and favorable clinical outcomes observed in this series suggest that functional decompression can be achieved without complete radiologic removal of all ossified tissue. This concept is particularly relevant in thoracic surgery, where the margin for dural injury is narrow and complication avoidance is a critical determinant of success.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec19\" class=\"Section2\"\u003e \u003ch2\u003eRadiologic\u0026ndash;Clinical Relationship and Contextual Factors\u003c/h2\u003e \u003cp\u003eAlthough significant dural sac expansion was achieved in all cases, the magnitude of DSCA increase did not demonstrate a linear association with neurological recovery. This finding highlights the multifactorial nature of clinical outcomes in thoracic myelopathy. Radiologic decompression appears to be a necessary but not sufficient condition for neurological improvement. Baseline neurological status, chronicity of compression, and intrinsic spinal cord changes likely influence postoperative recovery.[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/p\u003e \u003cp\u003eAccordingly, radiologic outcomes should be interpreted as indicators of decompression adequacy rather than direct predictors of clinical gain. The present findings support a conceptual framework in which thoracic UBE provides sufficient mechanical decompression while clinical recovery remains constrained by biological factors beyond the surgeon\u0026rsquo;s control.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec20\" class=\"Section2\"\u003e \u003ch2\u003eLimitations\u003c/h2\u003e \u003cp\u003eThis study has several limitations. This study is retrospective and single center in design, which may limit generalizability. A direct comparison with conventional open decompression was not performed, precluding definitive conclusions regarding relative superiority. Radiologic outcomes were assessed at the segment level, whereas clinical outcomes were patient-based, complicating causal inference. In addition, follow-up duration was limited to early alignment assessment, and longer-term studies are required to evaluate late kyphotic progression. Finally, all procedures were performed using a standardized left-sided approach, and the findings may not be directly applicable to alternative surgical strategies.\u003c/p\u003e \u003c/div\u003e"},{"header":"CONCLUSIONS","content":"\u003cp\u003eThoracic unilateral biportal endoscopic decompression for ossification of the ligamentum flavum achieves effective neural decompression while preserving posterior stabilizing structures. Quantitative radiologic analysis demonstrated substantial dural sac expansion, asymmetric but meaningful facet preservation, and only limited kyphosis progression at 1-year follow-up, despite the absence of routine fusion or fixation. Residual OLF on postoperative imaging often reflects an intentional floating strategy aimed at minimizing dural injury rather than incomplete decompression. These findings support thoracic UBE as a viable, tissue-preserving alternative to conventional posterior surgery in the management of thoracic OLF.\u003c/p\u003e "},{"header":"Declarations","content":"\u003cdiv id=\"Sec22\" class=\"Section2\"\u003e\n\u003cp\u003e\u003cstrong\u003eNOTES\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding/Support\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNone.\u003c/p\u003e\n\u003c/div\u003e\n\u003ch2\u003eConflict of Interest:\u003c/h2\u003e\n\u003cp\u003eThe authors have nothing to disclose.\u003c/p\u003e\n\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\n\u003cp\u003eConceptualization: MK ParkData curation: SW LeeFormal analysis: SW LeeFunding acquisition: N/AMethodology: SW LeeProject administration: MK ParkVisualization: JW JungWriting - original draft: SW LeeWriting - review \u0026amp; editing: MK Park, SW Lee , JW Jung, SK Son\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAizawa T, Sato T, Sasaki H et al (2006) Thoracic myelopathy caused by ossification of the ligamentum flavum: clinical features and surgical results in the Japanese population. J Neurosurg Spine 5:514\u0026ndash;519. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3171/spi.2006.5.6.514\u003c/span\u003e\u003cspan address=\"10.3171/spi.2006.5.6.514\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMiyakoshi N, Shimada Y, Suzuki T et al (2003) Factors related to long-term outcome after decompressive surgery for ossification of the ligamentum flavum of the thoracic spine. J Neurosurg Spine 99:251\u0026ndash;256. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3171/spi.2003.99.3.0251\u003c/span\u003e\u003cspan address=\"10.3171/spi.2003.99.3.0251\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAhn DK, Lee S, Moon SH et al (2014) Ossification of the Ligamentum Flavum. 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Neurospine 22:819\u0026ndash;828. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.14245/ns.2550338.169\u003c/span\u003e\u003cspan address=\"10.14245/ns.2550338.169\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKim JY, Ha JS, Lee CK et al (2023) Biportal Endoscopic Posterior Thoracic Laminectomy for Thoracic Spondylotic Myelopathy Caused by Ossification of the Ligamentum Flavum: Technical Developments and Outcomes. 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Sci Rep 6:23019. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1038/srep23019\u003c/span\u003e\u003cspan address=\"10.1038/srep23019\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"european-spine-journal","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"esjo","sideBox":"Learn more about [European Spine Journal](http://link.springer.com/journal/586)","snPcode":"586","submissionUrl":"https://submission.springernature.com/new-submission/586/3","title":"European Spine Journal","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Unilateral Biportal Endoscopy, Thoracic decompression, Minimally invasive, Thoracic myelopathy, Thoracic spinal stenosis","lastPublishedDoi":"10.21203/rs.3.rs-9194607/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9194607/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003ePurpose\u003c/h2\u003e \u003cp\u003eThoracic ossification of the ligamentum flavum (OLF) is a major cause of thoracic myelopathy requiring surgical decompression. While unilateral biportal endoscopy (UBE) has been increasingly applied to thoracic decompression, quantitative radiologic evidence regarding decompression adequacy and posterior structure preservation remains limited.\u003c/p\u003e\u003ch2\u003eMethods\u003c/h2\u003e \u003cp\u003eThis retrospective single-center study included consecutive patients who underwent thoracic UBE decompression for OLF. Radiologic outcomes were assessed at the segment level using magnetic resonance imaging and computed tomography. Dural sac cross-sectional area (DSCA), facet joint length and area, and segmental kyphosis were quantitatively measured preoperatively and postoperatively. Clinical outcomes were evaluated using the modified Japanese Orthopaedic Association (mJOA) score, Nurick grade, and visual analog scale (VAS). Postoperative residual OLF and complications were recorded.\u003c/p\u003e\u003ch2\u003eResults\u003c/h2\u003e \u003cp\u003eA total of 45 patients (55 segments) were analyzed with a mean follow-up of 16.0 months. Mean DSCA increased significantly from 70.4\u0026thinsp;\u0026plusmn;\u0026thinsp;20.4 mm\u0026sup2; preoperatively to 119.8\u0026thinsp;\u0026plusmn;\u0026thinsp;31.0 mm\u0026sup2; postoperatively, corresponding to a mean expansion ratio of 75.0%. Facet preservation was asymmetric, with significantly greater preservation on the contralateral side compared with the ipsilateral approach side (facet area preservation: 79.4% vs. 64.2%, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001). Segmental kyphosis increased modestly at 1-year follow-up (mean change, 1.5\u0026deg;), while clinically significant kyphotic progression requiring surgical stabilization was rare. Clinical outcomes improved significantly, with a mean mJOA recovery rate of 71.2%. Residual OLF was observed in a subset of segments and primarily reflected an intentional floating strategy rather than incomplete decompression.\u003c/p\u003e\u003ch2\u003eConclusions\u003c/h2\u003e \u003cp\u003eThoracic UBE decompression for OLF achieves substantial dural sac expansion and favorable clinical outcomes while preserving posterior stabilizing structures. Quantitative radiologic analysis demonstrates that adequate decompression can be accomplished without routine fusion or extensive posterior element disruption. Thoracic UBE represents a viable, tissue-preserving alternative for the surgical management of thoracic OLF.\u003c/p\u003e","manuscriptTitle":"Radiologic and Clinical Outcomes of Unilateral Biportal Endoscopic Decompression for Thoracic Ossification of the Ligamentum Flavum","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-05-11 08:06:13","doi":"10.21203/rs.3.rs-9194607/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"270052452409299242490849508295091727316","date":"2026-05-11T10:19:25+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-05-10T12:30:00+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"209755811326175828038799959108110075083","date":"2026-04-28T22:10:03+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-04-28T18:11:21+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-03-24T04:24:17+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-03-24T04:23:29+00:00","index":"","fulltext":""},{"type":"submitted","content":"European Spine Journal","date":"2026-03-23T02:08:28+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"european-spine-journal","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"esjo","sideBox":"Learn more about [European Spine Journal](http://link.springer.com/journal/586)","snPcode":"586","submissionUrl":"https://submission.springernature.com/new-submission/586/3","title":"European Spine Journal","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"e6b1bfa8-dd5a-40b0-be2e-938b73352b89","owner":[],"postedDate":"May 11th, 2026","published":true,"recentEditorialEvents":[{"type":"reviewerAgreed","content":"270052452409299242490849508295091727316","date":"2026-05-11T10:19:25+00:00","index":14,"fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-05-10T12:30:00+00:00","index":13,"fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-11T08:06:14+00:00","versionOfRecord":[],"versionCreatedAt":"2026-05-11 08:06:13","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9194607","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9194607","identity":"rs-9194607","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}