Stability of multiple-rod constructs and dual-rod constructs in cadaveric thoracic spines

Stability of multiple-rod constructs and dual-rod constructs in cadaveric thoracic spines | 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 Stability of multiple-rod constructs and dual-rod constructs in cadaveric thoracic spines Brandon J Herrington, Andrew P Peluso, Pawel Brzozowski, Chloe N Cadieux, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8124399/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Purpose Rod fracture and pseudoarthrosis are common complications in spinal fusion surgery. Multiple-rod constructs have been shown to mitigate the risks of rod fracture and pseudarthrosis in the lumbar spine, but their effect on the thoracic spine is less studied. This work aims to compare the stability of two-rod (dual-rod) constructs (DRCs) to four-rod (multiple-rod) constructs (MRCs) in cadaveric thoracic spines. Methods Nine intact human cadaveric thoracic spines (T1-T12) were instrumented in a randomized manner with either DRC or MRC. Specimens were then statically loaded in six planes of motion (flexion/extension, rotation, and lateral bend). A 1-hour bodyweight simulation fatigue test (FT) was then conducted on each specimen, followed by another static loading test. Constructs were then alternated, and this same sequence of testing was performed again. Range of motion (ROM) during the static loading tests was recorded using a digital imaging correlation (DIC) system. Results Total ROM pre-FT and post-FT was similar between DRCs and MRCs. However, when comparing pre-FT to post-FT within each construct type, DRCs exhibited an increase in flexion/extension ROM (15.3° ± 4.4° vs. 12.8° ± 5.3°, p = 0.009) while MRCs did not (16.5° ± 4.9° vs. 14.1° ± 3.7°, p = 0.076). Conclusions In this biomechanical analysis of intact cadaveric thoracic spines, MRCs exhibited more resistance to cyclical loading than DRCs did after FT. This work supports the current literature in the spine surgery field of reduced nonunion and rod breakage with MRCs. Cadaver Spinal Fusion Thoracic Spine Adult Spinal Deformity Dual-Rod Construct Multiple-Rod Construct Figures Figure 1 INTRODUCTION Spinal fusion remains a mainstay in the treatment of traumatic and degenerative spinal conditions [ 1 ]. As fusion volumes have risen, so too have revision procedures. Pseudarthrosis, or nonunion, is typically diagnosed ≥ 1 year after the index operation [ 2 ] and accounts for a substantial proportion of these revisions [ 3 ]. Rod fracture is among the most common implant-related failures after fusion and is closely linked to pseudarthrosis [ 4 – 7 ], with the odds of rod fracture reported to be 29 times higher in the presence of pseudarthrosis than in solid fusion [ 8 ]. To mitigate rod fracture, strategies that increase overall construct stiffness have been adopted [ 9 ]. A widely used approach is the addition of accessory rods to create multiple-rod constructs (MRCs), rather than standard dual-rod constructs (DRCs). Several clinical series suggest MRCs reduce rod fracture rates compared with DRCs [ 7 , 10 – 16 ], and biomechanical work—predominantly in lumbar models and often in the setting of osteotomy for adult spinal deformity—has emphasized better outcomes in construct stiffness [ 17 – 22 ]. However, most prior work has relied on static testing methods, such as applying only a small number of load cycles or gradually increasing the load until the construct fails [ 18 , 20 , 23 – 25 ]. Additionally, despite being the most common site of spinal metastasis and a frequent target for long constructs in deformity correction [ 26 ], the thoracic spine has been relatively understudied with respect to MRCs. To the best of our knowledge, no cadaveric thoracic investigation has directly compared the stability of MRCs to DRCs. We aimed to compare the stability and fatigue behavior of MRCs versus DRCs in cadaveric thoracic spines using a cyclic bodyweight loading protocol designed to assess for time-dependent construct degradation. We hypothesized that, compared to DRCs, MRCs would demonstrate increased stability during cyclic loading and exhibit reduced fatigue-related compromise after a 1-hour cyclical test. METHODS Cadaver Preparation Prior to specimen collection and preparation, approval was obtained from the institutional ethics board (ID#118078 and ID#125155). Full details around the initial collection and preparation of these specimens are detailed elsewhere [ 27 ]. Briefly, 11 full cadaveric specimens were procured and included non-identifiable information such as medical history, cause of death, age, and sex. All specimens were kept in a -20 o C freezer prior to use. Computed tomography (CT) scan was then performed to rule out internal bony abnormalities. Once the CT scan was complete, specimens were sectioned into cervical, thoracic, and lumbosacral segments. The thoracic spine (T1-T12) segment was used for this study. These specimens were then thawed to room temperature and prepared further. The majority of musculature was dissected off the specimen, leaving the facet capsules, posterior ligamentous complex, and costovertebral joints intact. The ribcage was removed, leaving approximately 2cm of posterior rib attached at the costovertebral joints. The most cranial (T1) and most caudal (T12) vertebral segments were then potted in cement with their endplates parallel to the ground, leaving the adjacent intervertebral discs free and able to allow range of motion. This setup can be seen in Online Resource 1. From here, all spines underwent native range of motion testing without instrumentation and were subsequently instrumented for non-destructive testing as part of a previous study [ 27 ]. Following this testing, all spines were stored back in a freezer at -20 o C. After reviewing the specimens’ imaging and medical history, preliminary testing of cadaveric spines for the present study was then conducted to develop the testing protocol. During this preliminary testing, 2 of the initial 11 cadaveric specimens sustained bony or ligamentous damage and were subsequently excluded from the present study. As such, 9 thoracic spines were non-damaged and available for further biomechanical testing. Demographics of cadaveric specimens used in this study are detailed in Online Resource 2. Instrumentation Prior to instrumentation and testing, specimens were thawed overnight at room temperature. Multi-level pedicle screw fixation was performed starting from T3 and extending down to include T11, leaving T2 as the first uninstrumented level, while T1 and T12 were both fixed in potted cement. Pedicle screw start point and insertion technique were done according to a previously described technique [ 28 ]. Using a pedicle probe, screw tracts were examined for breaches. The depth of each screw track was then measured, and 6.0mm polyaxial pedicle screws were inserted, ranging from 40mm to 55mm in length. Pedicle screws were inserted by hand to ensure a constant angle and speed until each screw was well seated. From here, specimens were randomized to undergo either initial testing with a MRC or with a DRC. To create a DRC, one 5.5mm diameter titanium rod was bent to the appropriate shape and attached to the left-sided pedicle screws spanning from T3-T11, while a second rod was attached to the right-sided pedicle screws. Both rods were then secured in place to the pedicle screws with locking set screws. To create an MRC, the same process was done that was used for the DRC set-up, followed by placement of one additional 5.5mm titanium rod just lateral to the existing 2 rods, with each new rod secured in place with one 5.5mm-5.5mm rod-to-rod connector cranially between T3 and T4 pedicle screws, and one 5.5mm-5.5mm rod-to-rod connector caudally between T10 and T11 pedicle screws. This resulted in a total of 4 rods in the MRC set-up. An example of the DRC and MRC setups can be seen in Fig. 1 . After completing instrumentation, markers were rigidly applied to the specimens to create landmarks to be captured with the digital imaging correlation system. These markers included twelve 2.0mm K-wires with attached 3D-printed hemispherical plastic components covered with multiple black and white contrasting stickers. One of these markers was inserted into each vertebral body spanning from T1-T12. An example of this construct and marker setup is seen in Online Resource 3. Specimen Testing Upon completion of instrumentation and application of markers to allow proper image capture, the potted specimens were ready for testing. Each specimen was tested individually, on separate days, one at a time. Potted specimens were mounted on a testing machine with custom design modifications (Instron® 5967, Norwood, MA, USA). The potted T12 was rigidly attached to a sliding x-y table, allowing unrestricted translation motion in the transverse plane. The potted T1 was rigidly attached to a custom testing jig capable of testing in six degrees of freedom. An example of this setup is seen in Online Resource 4. First, static range of motion (ROM) tests were performed with the intact spines prior to fusion. Then ROM tests were done following initial instrumentation (either DRC or MRC). The specimens were preloaded with 15N of force and set to maintain 15N during the test to remove any mechanical slack and aid in mounting the specimen through its neutral axis. ROM testing involved applying a pure rotational or bending load to the most cranial (T1) end up to a limit of ± 5Nm, at 1 o per second, and cycled three times with the first two cycles for preconditioning and the last for data analysis. This method was based on previous protocols [ 29 ]. ARAMIS Adjustable 12M system (GOM Metrology, Braunschweig, Germany) was used as a digital imaging correlation (DIC) system to track the motion in six degrees of freedom of the twelve previously inserted 3D printed markers rigidly attached to each vertebra. The setup consisted of two 24mm focal length and 4096-by-3600 pixels (pixel size of 3.45µm) resolution cameras. The two cameras were set at a 25 o angle, 4 Hz, and illuminated with two polarized LED light sources. The DIC setup was calibrated prior to each biomechanical test by a standard calibration protocol and calibration plate provided by GOM Metrology for a measuring volume of 570-by-430-by-430mm. The images were processed in ARAMIS Professional 2019 (GOM Metrology, Braunschweig, Germany). Torsion was applied and tracked by a universal testing device (Instron® 5967, Norwood, MA, USA) using a 50kN load cell, recording data at 50Hz. Load cells were able to measure both forces as well as moments. Bending was applied and tracked by a custom testing jig consisting of a stepper motor (NEMA 23, OMC) and a 2.2kN load cell (MC3A-6-1k, AMTI) recording data at 20Hz. Each specimen was tested in flexion/extension, lateral bend, and internal/external rotation in a random order. Next, the specimens were cyclically loaded during a 1-hour bodyweight simulation fatigue test (FT) to simulate a scenario of day-to-day wear and tear of the construct before bony fusion. Again, the spines were preloaded to 15N to remove mechanical slack and to aid in mounting through the neutral axis. The specimens were tested for one hour and were cyclically loaded in internal rotation and external rotation to ± 5Nm at 5 o per second and a cyclically applied axial load from 200N to 350N (representing one-quarter and one-half body weight respectively) at 150N/s for one hour. Specimens were periodically sprayed with a saline solution to prevent drying. Cyclical axial loads and cyclic torsional loads were independently applied. The torque data was captured by the same load cell as above (30kN load cell at 50Hz), while the positional/rotational data was collected in a predefined pulsed setup by the DIC system. The pulsed sequence was set to record the first 20 seconds at 10Hz, then to reduce file size, this was set to record data at 1/3Hz for the next 3560 seconds before recapturing the data at 10Hz for the last 20 seconds of the one-hour test. The DIC system recorded the position of the 12 vertebrae throughout the entire test. Immediately following FT, specimens were examined for any evidence of bony or ligamentous failure. Additionally, all screw-bone interfaces and screw-rod interfaces were examined for signs of loosening. After thorough inspection, if no damage was visualized, the specimens were statically tested once again in flexion/extension, lateral bend, and rotation. This was done to determine if any change in motion was observed following FT. The testing sequence of static ROM to FT to static ROM was then repeated for the other construct with the same specimen. ROM was calculated as the difference between the maximum and minimum ROM at each vertebra: 𝑅𝑂𝑀 𝑥 = 𝑅𝑂𝑀 𝑥,𝑚𝑎𝑥 − 𝑅𝑂𝑀 𝑥,𝑚𝑖𝑛 . The subscripts max and min are the maximum and minimum recorded values, respectively, and ROM x is the range of motion at the targeted vertebra. Statistical Analysis SPSS statistical software (version 29) was used for the statistical analysis of captured data. Each cadaveric specimen acted as its own control. The Wilcoxon signed-rank test was used to compare median differences in total range of motion between construct types (i.e., DRC vs. MRC) and between status of fatigue test (i.e. pre vs. post). Statistical significance was set at < 0.05. Individual segment range of motion median differences were compared using the Wilcoxon signed-rank test with statistical significance set at < 0.05. T1-T2 and T11-T12 were excluded from the analysis as the T1 and T12 vertebral bodies were fixed in cement. RESULTS Total Range of Motion Mean rotational ROM was significantly higher after 1-hour testing for DRC (25.4 +/- 4.6 o vs. 22.8 +/- 4.8 o , p = 0.009) and for MRC (25.0 +/- 5.1 o vs. 22.3 +/- 4.1 o , p = 0.009). (Table 1 ). ROM for rotation were similar between DRC and MRC regardless of fatigue status (Table 2 ). Mean lateral bend ROM was significantly higher after 1-hour testing for DRC (19.1 +/- 4.1 o vs. 16.9 +/- 5.3 o , p = 0.009) and for MRC (20.3 +/- 6.1 o vs. 16.6 +/- 4.2 o , p = 0.009). (Table 1 ). ROM for lateral bend were similar between DRC and MRC regardless of fatigue status (Table 2 ). Mean flexion/extension ROM was significantly higher after 1-hour testing for DRC (15.3 +/- 4.4 o vs. 12.8 +/- 5.3 o , p = 0.009) but not for MRC (16.5 +/- 4.9 o vs. 14.1 +/- 3.7 o , p = 0.076). (Table 1 ). ROM for flexion/extension were similar between DRC and MRC regardless of fatigue status (Table 2 ). Range of Motion by Vertebral Segment Overall, the range of motion at the first uninstrumented level (T2-T3) was significantly higher in all planes of motion after 1-hour fatigue testing than before, but there were no differences between DRC and MRC. As testing moved caudally, these differences were less pronounced. A detailed analysis of ROM for all measured vertebral levels is found in Table 3 . All range of motion was < 1 o at T5-T6 and distal, and there were no significant differences in motion between construct type or status of fatigue testing. As such, ROM values for T5-T6 were omitted from Table 3 . Axial Rotation by Vertebral Segment At T2-T3, mean rotation was significantly higher post 1-hour fatigue test for DRC (8.61 +/- 2.54 o vs. 7.37 +/- 2.60 o , p = 0.009) and for MRC (8.73 +/- 2.89 o vs. 7.75 +/- 2.47 o , p = 0.033). Rotation between MRC and DRC at this level was similar regardless of fatigue test status. At T3-T4, mean rotation was significantly higher post-1 hour fatigue test for DRC (1.82 +/- 1.08 o vs. 1.39 +/- 0.71 o , p = 0.018) and for MRC (1.80 +/- 0.95 o vs. 1.28 +/- 0.70 o , p = 0.013). Rotation between MRC and DRC at this level was similar regardless of fatigue test status. At T4-T5, mean rotation was similar post 1-hour fatigue test for DRC (0.76 +/- 0.38 o vs. 0.67 +/- 0.28 o , p = 0.343) but greater for MRC (0.75 +/- 0.38 o vs. 0.64 +/- 0.28 o , p = 0.044). Rotation between MRC and DRC at this level was similar regardless of fatigue test status. Lateral Bend by Vertebral Segment At T2-T3, mean lateral bend was significantly higher post 1-hour fatigue test for DRC (7.34 +/- 2.81 o vs. 6.19 +/- 2.44 o , p = 0.018) and for MRC (8.04 +/- 2.69 o vs. 6.30 +/- 2.46 o , p = 0.009). Lateral bend between MRC and DRC at this level was similar regardless of fatigue test status. At T3-T4, mean lateral bend was significantly higher post 1-hour fatigue test for DRC (1.20 +/- 0.81 o vs. 0.89 +/- 0.59 o , p = 0.042) but not for MRC (1.18 +/- 1.41 o vs. 1.00 +/- 0.91 o , p = 0.343). Lateral bend between MRC and DRC at this level was similar regardless of fatigue test status. At T4-T5, the mean lateral bend was similar pre- and post-1-hour fatigue test for both DRC and MRC. No differences in ROM were observed between construct types regardless of fatigue test status. Flexion/Extension by Vertebral Segment At T2-T3, mean flexion/extension was significantly higher post 1-hour fatigue test for DRC (4.25 +/- 2.36 o vs. 4.91 +/- 2.21 o , p = 0.015) and for MRC (4.48 +/- 2.07 o vs. 4.87 +/- 1.90 o , p = 0.028). No significant differences were noted in flexion/extension ROM at this level between construct types. At T3-T4, mean flexion/extension was significantly higher post 1-hour fatigue test for DRC (0.74 +/- 0.70 o vs. 0.38 +/- 0.36 o , p = 0.024) but not for MRC (0.94 +/- 1.06 o vs. 0.59 +/- 0.47 o , p = 0.076). No significant differences were noted in flexion/extension ROM at this level between construct types. At T4-T5, mean flexion/extension was similar between MRC and DRC regardless of construct type and status of fatigue test. Table 1 Total range of motion, comparison by fatigue status Total ROM ( o ) Test Scenario Mean (SD) P-Value Pre-FT Post-FT Dual-Rod Construct Rotation 22.8 (4.8) 25.4 (4.6) 0.009 Lateral Bend 16.9 (5.3) 19.1 (4.1) 0.009 Flexion/Extension 12.8 (5.3) 15.3 (4.4) 0.009 Multiple-Rod Construct Rotation 22.3 (4.1) 25.0 (5.1) 0.009 Lateral bend 16.6 (4.2) 20.3 (6.1) 0.009 Flexion/Extension 14.1 (3.7) 16.5 (4.9) 0.076 P-values obtained from two-sided Wilcoxon test Bolded values are p < 0.05 ROM, range of motion; SD, standard deviation; FT, fatigue test Table 2 Total range of motion, comparison by construct type Total ROM ( o ) Test Scenario Mean (SD) P-Value DRC MRC Pre-Fatigue Test Rotation 22.8 (4.8) 22.3 (4.1) 0.722 Lateral Bend 16.9 (5.3) 16.6 (4.2) 0.477 Flexion/Extension 12.8 (5.3) 14.1 (3.7) 0.076 Post-Fatigue Test Rotation 25.4 (4.6) 25.0 (5.1) 0.343 Lateral bend 19.1 (4.1) 20.3 (6.1) 0.554 Flexion/Extension 15.3 (4.4) 16.5 (4.9) 0.286 P-values obtained from two-sided Wilcoxon test Bolded values are p < 0.05 ROM, range of motion; SD, standard deviation; FT, fatigue test; DRC, dual-rod construct; MRC, multiple-rod construct Table 3 Range of motion by vertebral segment Dual-Rod Construct Multiple-Rod Construct P-Value Vertebral Level Direction of Motion Fatigue Test Pre vs Post ROM ( o ) Mean (SD) P-Value Pre vs Post ROM ( o ) Mean (SD) P-Value Pre vs Post DRC vs MRC T2-T3 Rotation Pre 7.37 (2.60) 0.009 7.75 (2.47) 0.033 0.554 Post 8.61 (2.54) 8.73 (2.89) 0.906 Lateral Bend Pre 6.19 (2.44) 0.018 6.30 (2.46) 0.009 0.813 Post 7.34 (2.81) 8.04 (2.69) 0.236 Flexion/ Extension Pre 4.25 (2.36) 0.015 4.48 (2.07) 0.028 0.314 Post 4.91 (2.21) 4.87 (1.90) 0.515 T3-T4 Rotation Pre 1.39 (0.71) 0.018 1.28 (0.70) 0.013 0.906 Post 1.82 (1.08) 1.80 (0.95) 1.000 Lateral Bend Pre 0.89 (0.59) 0.042 1.00 (0.91) 0.343 0.477 Post 1.20 (0.81) 1.18 (1.41) 0.234 Flexion/ Extension Pre 0.38 (0.36) 0.024 0.59 (0.47) 0.076 0.554 Post 0.74 (0.70) 0.94 (1.06) 0.636 T4-T5 Rotation Pre 0.67 (0.28) 0.343 0.64 (0.28) 0.044 0.813 Post 0.76 (0.38) 0.75 (0.38) 0.906 Lateral Bend Pre 0.10 (0.11) 0.624 0.07 (0.09) 0.813 0.155 Post 0.08 (0.12) 0.09 (0.14) 0.441 Flexion/ Extension Pre 0.09 (0.10) 0.097 0.04 (0.09) 0.477 1.000 Post 0.12 (0.08) 0.07 (0.14) 0.097 Note: T2-T3 is uninstrumented, while T3-T4 and T4-T5 are first and second instrumented levels, respectively. From T5-T11, ROM is negligible with all values < 1 o P-values obtained from two-sided Wilcoxon test Bolded values are p < 0.05 ROM, range of motion; SD, standard deviation; DRC, dual-rod construct; MRC, multiple-rod construct DISCUSSION In this study, 9 cadaveric thoracic spines were instrumented with two different construct types—dual-rod constructs (DRCs) and multiple-rod constructs (MRCs). Overall range of motion was then analyzed before and after a 1-hour bodyweight simulation fatigue test (FT). No significant differences were observed in ROM between DRCs and MRCs. However, after a 1-hour fatigue test, MRCs maintained a similar ROM to their pre-fatigue state during flexion/extension, while DRCs did not. This suggests that MRCs may provide additional stability to daily wear and tear following initial instrumentation, thus allowing further time for a bony fusion to take place. This is supported by other biomechanical literature in the lumbar spine showing lower stress forces with MRCs [ 21 , 22 , 24 , 30 , 31 ], as well as by a recent meta-analysis comparing MRCs to DRCs in adult spinal deformity that showed lower rates of nonunion with MRCs when compared to DRCs [ 13 ]. An important caveat to this finding is seen when the analysis is broken down into individual segments. Although DRCs and MRCs exhibited similar absolute ROM at each individual vertebral segment, they behaved differently pre- and post-fatigue testing. At the highest instrumented level (T3-T4), only DRCs demonstrated a significantly higher ROM with lateral bend and flexion/extension post-fatigue testing. Conversely, at the T4-T5 level (second highest instrumented level), MRCs exhibited higher flexion/extension ROM while the ROM of DRCs was similar across all three movement planes pre- and post-fatigue testing. It’s difficult to say what these differences translate to clinically. One hypothesis around the use of MRCs is that although they may increase overall construct stiffness and benefit fusion rates, this is achieved at the expense of increasing forces at the screw-bone interface. In a 2018 retrospective review of 106 patients with adult spinal deformity, MRCs were associated with a 1.9-fold increased risk of screw loosening at the cranial level, and a 4.1-fold increased risk of screw loosening at the caudal level[ 10 ]. This finding was echoed in a recent meta-analysis by Zhao et al [ 13 ]. None of the cadaveric spines tested in this study exhibited qualitative signs of screw-bone or screw-rod interface loosening. However, it is possible that the differences in ROM at the two cranial instrumented levels post-fatigue testing were early warning signs of loosening, and with further duration of testing, these differences may have become more pronounced. Aside from nonunion, rod breakage, and screw pullout, another serious complication in long instrumented spinal surgery is proximal junctional kyphosis (PJK). PJK is defined as a sagittal Cobb angle of 10-20 o between the uppermost instrumented vertebra and the vertebra 2 levels above, or when the Cobb angle between these two vertebrae is 10-15 o greater than the preoperative measurement [ 32 , 33 ]. This can result in new neurological deficits, fractures, severe pain, or worsening quality of life leading to revision surgery [ 34 ]. The reported incidence of PJK is 5–46% [ 35 ], and its development has been linked to mechanical, tissue, and demographic risk factors, with 76% of the research focusing on mechanical contributions to PJK [ 33 ]. Among these mechanical contributions to PJK, increased rigidity of the construct at the transitional zone between instrumented and non-instrumented vertebrae is thought to play the most significant role in pathogenesis [ 36 – 38 ], with some authors reporting a lower incidence of PJK by deploying a more gradual transition in construct stiffness at the transitional level [ 39 ]. Previous cadaveric work has examined semi-rigid constructs at the uppermost instrumented level and found a more linear and less abrupt change in biomechanics at the transitional zone with transverse process hooks when compared to all pedicle screws [ 40 ]. This softer transition in biomechanics is hypothesized to be protective against PJK. However, in this same analysis, semi-rigid constructs and all pedicle screw constructs were not compared to all pedicle screw constructs with multiple rods. Although increasing construct stiffness with MRCs has been linked to increased PJK [ 41 ], a recent meta-analysis explored the rates of rod fracture, pseudoarthrosis, and PJK between DRCs and MRCs. Six studies [ 10 , 42 – 46 ] were included in the meta-analysis, and no association was found between PJK and the type of construct (DRC vs MRC) [ 47 ]. In this study, no difference in absolute ROM was noted between DRCs and MRCs at the first uninstrumented or uppermost instrumented vertebral levels. This similarity in absolute construct stability supports the findings of this meta-analysis [ 47 ]. However, DRCs showed increased ROM at the first instrument level (T3-T4) following 1-hour fatigue testing, while MRCs did not, and at the second highest instrument level (T4-T5), MRCs showed increased ROM following 1-hour fatigue testing while DRCs did not. Although the differences in absolute ROM observed were low (< 1 o ), this may suggest these constructs have initial overall comparable stiffness but then react differently to daily wear-and-tear after initial instrumentation, which could ultimately affect rates of PJK. Although the clinical literature does not seem to support the notion that MRCs increase rates of PJK [ 48 ], longer duration fatigue testing is necessary to further explore this association. Limitations There were certain limitations to consider when drawing conclusions from the present analysis. The first is with the type of metal used in this experiment. Titanium rods were used in both constructs. While titanium is often the rod of choice in spine surgery, cobalt chromium is another frequently used option. It’s possible that a cobalt chromium DRC and MRC may behave differently from what was observed here. Indeed, cobalt chromium constructs have been demonstrated to be more rigid than titanium ones [ 49 , 50 ], but the differences between MRC rigidity when it comes to the two materials may be overstated, as rates of PJK are not different between the two materials when MRCs are utilized [ 48 ]. Similarly, only a single type of MRC (4-rod MRC) was investigated in this study. Different configurations of MRCs are used in spinal fusion surgery, including 3-rod MRCs, linked rod configurations, satellite rod configurations, accessory rod configurations, and intercalary rod configurations [ 51 ]. Most of the existing clinical research has focused on comparing all configurations of MRCs to DRCs [ 11 , 13 , 42 , 52 ], as subgroup analyses between the different MRC configurations have proved challenging as a result of small sample size. Due to the variable biomechanical properties of different MRC configurations [ 22 , 53 , 54 ], further studies are needed to compare the different MRC configurations both biomechanically and clinically. Further, the quality of the cadaveric soft tissues should be considered within this biomechanical analysis. In this study, paraspinal musculature and ribcages were dissected off specimens to facilitate instrumentation and testing, but this state likely does not adequately reflect in-vivo conditions, as these structures play a significant role in overall thoracic spine stability [ 55 , 56 ]. Additionally, the cadaveric spines in this study were overall inherently stable, as no osteotomies were performed [ 57 ]. This likely contributed to the overall similarities in ROM between MRCs and DRCs, as the degree of stability to gain was minimal with additional rods. Lastly, due to time and instrument constraints, FT was restricted to a 1-hour duration, and we could not cyclically load specimens to failure. Future studies should examine more construct configurations with different osteotomy options while performing cyclical loading of a greater duration, or until failure. CONCLUSION Multiple-rod constructs have been proposed as a way to increase stability in long spinal fusion surgery compared to dual-rod constructs. In this biomechanical analysis of intact cadaveric thoracic spines, multiple-rod constructs exhibited more resistance to cyclical loading than dual-rod constructs did after a 1-hour bodyweight simulation fatigue test. This work supports the current literature in the spine surgery field of reduced nonunion and rod breakage with multiple-rod constructs. Statements and Declarations The authors would like to acknowledge that the implants used in this study were supplied by Johnson & Johnson MedTech (Canada). However, no comparisons were made between these implants and those from other suppliers. The authors have no additional funding sources to disclose. Acknowledgements The authors would like to extend our gratitude to Johnson & Johnson MedTech (Canada) who provided the necessary materials and implants to complete this research. Author Contribution Conception and Design: B.J.H., R.J.R.F.Administrative Support: P.B.Provision of Study Materials: P.B., C.N.C., R.J.R.F.Collection and Assembly of Data: B.J.H., P.B.Data Analysis and Interpretation: B.J.H., A.P.P., P.B.Manuscript Writing: B.J.H., A.P.P., R.J.R.F.Final Approval of Manuscript: All authors Data Availability No publicly available datasets were generated during the creation of this article. However, primary data is available on request. Compliance with Ethical Standards Ethics approval was obtained from the institutional ethics board prior to commencing any work (ID#118078 and ID#125155). Competing Interests The authors would like to acknowledge that the implants used in this study were supplied by Johnson & Johnson MedTech (Canada). However, no comparisons were made between these implants and those from other suppliers. The authors have no additional funding sources to disclose. References Martin BI, Mirza SK, Spina N, et al (2019) Trends in Lumbar Fusion Procedure Rates and Associated Hospital Costs for Degenerative Spinal Diseases in the United States, 2004 to 2015. Spine Phila Pa 1976 44:369–376. https://doi.org/10.1097/BRS.0000000000002822 Raizman NM, O’Brien JR, Poehling-Monaghan KL, Yu WD (2009) Pseudarthrosis of the Spine. J Am Acad Orthop Surg 17:494–503. https://doi.org/10.5435/00124635-200908000-00003 Martin BI, Mirza SK, Comstock BA, et al (2007) Reoperation rates following lumbar spine surgery and the influence of spinal fusion procedures. 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Spine Phila Pa 1976 41 Suppl 7:S24–S24. https://doi.org/10.1097/BRS.0000000000001431 Shen FH, Woods D, Miller M, et al (2021) Use of the dual construct lowers rod strains in flexion-extension and lateral bending compared to two-rod and two-rod satellite constructs in a cadaveric spine corpectomy model. Spine J 21:2104–2111. https://doi.org/10.1016/j.spinee.2021.05.022 Tan Q-C, Huang J-F, Bai H, et al (2022) Effects of Revision Rod Position on Spinal Construct Stability in Lumbar Revision Surgery: A Finite Element Study. Front Bioeng Biotechnol 9:799727–799727. https://doi.org/10.3389/fbioe.2021.799727 Scheer JK, Tang JA, Deviren V, et al (2011) Biomechanical Analysis of Revision Strategies for Rod Fracture in Pedicle Subtraction Osteotomy. Neurosurgery 69: Luca A, Ottardi C, Sasso M, et al (2017) Instrumentation failure following pedicle subtraction osteotomy: the role of rod material, diameter, and multi-rod constructs. Eur Spine J 26:764–770. https://doi.org/10.1007/s00586-016-4859-8 Seyed Vosoughi A, Joukar A, Kiapour A, et al (2019) Optimal satellite rod constructs to mitigate rod failure following pedicle subtraction osteotomy (PSO): a finite element study. Spine J 19:931–941. https://doi.org/10.1016/j.spinee.2018.11.003 Hallager DW, Gehrchen M, Dahl B, et al (2016) Use of Supplemental Short Pre-Contoured Accessory Rods and Cobalt Chrome Alloy Posterior Rods Reduces Primary Rod Strain and Range of Motion Across the Pedicle Subtraction Osteotomy Level: An In Vitro Biomechanical Study. Spine 41:E388-395. https://doi.org/10.1097/BRS.0000000000001282 Pereira B de A, Godzik J, Lehrman JN, et al (2022) Pedicle Subtraction Osteotomy Construct Optimization: A Cadaveric Study of Various Multirod and Interbody Configurations. Spine Phila Pa 1976 47:640–647. https://doi.org/10.1097/BRS.0000000000004328 Kelly BP, Shen FH, Schwab JS, et al (2008) Biomechanical Testing of a Novel Four-Rod Technique For Lumbo-Pelvic Reconstruction: Spine 33:E400–E406. https://doi.org/10.1097/BRS.0b013e31817615c5 Park S-J, Lee C-S, Chang B-S, et al (2019) Rod fracture and related factors after total en bloc spondylectomy. Spine J 19:1613–1619. https://doi.org/10.1016/j.spinee.2019.04.018 Cadieux C (2022) Biomechanical Characterization of Semi-Rigid Constructs and the Potential Effect on Proximal Junctional Kyphosis in the Thoracic Spine. The University of Western Ontario (Canada) Kim YJ, Lenke LG (2005) Thoracic pedicle screw placement: free-hand technique. Neurol India 53:512–519. https://doi.org/10.4103/0028-3886.22622 Wilke HJ, Wenger K, Claes L (1998) Testing criteria for spinal implants: recommendations for the standardization of in vitro stability testing of spinal implants. Eur Spine J 7:148–154. https://doi.org/10.1007/s005860050045 Shekouhi N, Vosoughi AS, Goel VK, Theologis AA (2023) Does number of rods matter? 4-, 5-, and 6-rods across a lumbar pedicle subtraction osteotomy: a finite element analysis. Spine Deform 11:535–543. https://doi.org/10.1007/s43390-022-00627-0 Hartmann S, Thomé C, Abramovic A, et al (2020) The Effect of Rod Pattern, Outrigger, and Multiple Screw-Rod Constructs for Surgical Stabilization of the 3-Column Destabilized Cervical Spine - A Biomechanical Analysis and Introduction of a Novel Technique. Neurospine 17:610–629. https://doi.org/10.14245/ns.2040436.218 Bridwell KH, Lenke LG, Cho SK, et al (2013) Proximal junctional kyphosis in primary adult deformity surgery: evaluation of 20 degrees as a critical angle. Neurosurgery 72:899–906. https://doi.org/10.1227/NEU.0b013e31828bacd8 Haldeman PB, Ward SR, Osorio J, Shahidi B (2024) An evidence based conceptual framework for the multifactorial understanding of proximal junctional kyphosis. Brain Spine 4:102807–102807. https://doi.org/10.1016/j.bas.2024.102807 Kim HJ, Iyer S (2016) Proximal Junctional Kyphosis. J Am Acad Orthop Surg 24:318–326. https://doi.org/10.5435/JAAOS-D-14-00393 Lau D, Clark AJ, Scheer JK, et al (2014) Proximal Junctional Kyphosis and Failure After Spinal Deformity Surgery: A Systematic Review of the Literature as a Background to Classification Development. Spine Phila Pa 1976 39:2093–2102. https://doi.org/10.1097/BRS.0000000000000627 Kim YJ, Lenke LG, Bridwell KH, et al (2007) Proximal junctional kyphosis in adolescent idiopathic scoliosis after 3 different types of posterior segmental spinal instrumentation and fusions : Incidence and risk factor analysis of 410 cases. Spine Phila Pa 1976 32:2731–2738. https://doi.org/10.1097/BRS.0b013e31815a7ead Watanabe K, Lenke LG, Bridwell KH, et al (2010) Proximal Junctional Vertebral Fracture in Adults After Spinal Deformity Surgery Using Pedicle Screw Constructs: Analysis of Morphological Features. Spine Phila Pa 1976 35:138–145. https://doi.org/10.1097/BRS.0b013e3181c8f35d Lopez Poncelas M, La Barbera L, Rawlinson J, et al (2023) Proximal junctional failure after surgical instrumentation in adult spinal deformity: biomechanical assessment of proximal instrumentation stiffness. Spine Deform 11:59–69. https://doi.org/10.1007/s43390-022-00574-w Cazzulino A, Gandhi R, Woodard T, et al (2021) Soft Landing technique as a possible prevention strategy for proximal junctional failure following adult spinal deformity surgery. J Spine Surg Hong Kong 7:26–36. https://doi.org/10.21037/jss-20-622 Cadieux C, Brzozowski P, Fernandes RJR, et al (2024) Topping-Off a Long Thoracic Stabilization With Semi-Rigid Constructs May Have Favorable Biomechanical Effects to Prevent Proximal Junctional Kyphosis: A Biomechanical Comparison. Glob Spine J 21925682241259695–21925682241259695. https://doi.org/10.1177/21925682241259695 Han SH MD, Hyun S-J MD, PhD, Kim K-J MD, PhD, et al (2017) Rod stiffness as a risk factor for proximal junctional kyphosis after adult spinal deformity surgery: Comparative study between cobalt chrome multiple-rod constructs and titanium alloy two-rod constructs. Spine J 17:962–968. https://doi.org/10.1016/j.spinee.2017.02.005 Hyun S-J, Lenke LG, Kim Y-C, et al (2014) Comparison of Standard 2-Rod Constructs to Multiple-Rod Constructs for Fixation Across 3-Column Spinal Osteotomies. Spine Phila Pa 1976 39:1899–1904. https://doi.org/10.1097/BRS.0000000000000556 Yamato Y, Hasegawa T, Togawa D, et al (2020) Long additional rod constructs can reduce the incidence of rod fractures following 3-column osteotomy with pelvic fixation in short term. Spine Deform 8:481–490. https://doi.org/10.1007/s43390-020-00071-y Guevara-Villazón F, Boissiere L, Hayashi K, et al (2020) Multiple-rod constructs in adult spinal deformity surgery for pelvic-fixated long instrumentations: an integral matched cohort analysis. Eur Spine J 29:886–895. https://doi.org/10.1007/s00586-020-06311-z Gupta S, Eksi MS, Ames CP, et al (2018) A Novel 4-Rod Technique Offers Potential to Reduce Rod Breakage and Pseudarthrosis in Pedicle Subtraction Osteotomies for Adult Spinal Deformity Correction. Oper Neurosurg Hagerstown Md 14:449–456. https://doi.org/10.1093/ons/opx151 Dinizo M, Passias P, Kebaish K, et al (2023) The Approach to Pseudarthrosis After Adult Spinal Deformity Surgery: Is a Multiple-Rod Construct Necessary? Glob Spine J 13:636–642. https://doi.org/10.1177/21925682211001880 Moniz-Garcia D, Stoloff D, Akinduro O, et al (2023) Two- versus multi-rod constructs for adult spinal deformity: A systematic review and Random-effects and Bayesian meta-analysis. J Clin Neurosci 107:9–15. https://doi.org/10.1016/j.jocn.2022.11.011 Ye J, Gupta S, Farooqi AS, et al (2023) Use of multiple rods and proximal junctional kyphosis in adult spinal deformity surgery. J Neurosurg Spine 39:1–9. https://doi.org/10.3171/2023.4.SPINE23209 Shah KN, Walker G, Koruprolu SC, Daniels AH (2018) Biomechanical comparison between titanium and cobalt chromium rods used in a pedicle subtraction osteotomy model. Orthop Rev 10:7541–7541. https://doi.org/10.4081/or.2018.7541 Scheer JK, Tang JA, Deviren V, et al (2011) Biomechanical analysis of cervicothoracic junction osteotomy in cadaveric model of ankylosing spondylitis: effect of rod material and diameter: Laboratory investigation. J Neurosurg Spine 14:330–335. https://doi.org/10.3171/2010.10.SPINE1059 El Dafrawy MH, Adogwa O, Wegner AM, et al (2021) Comprehensive classification system for multirod constructs across three-column osteotomies: a reliability study. J Neurosurg Spine 34:103–109. https://doi.org/10.3171/2020.6.SPINE20678 Merrill RK, Kim JS, Leven DM, et al (2017) Multi-Rod Constructs Can Prevent Rod Breakage and Pseudarthrosis at the Lumbosacral Junction in Adult Spinal Deformity. Glob Spine J 7:514–520. https://doi.org/10.1177/2192568217699392 Jager ZS MD, İnceoğlu S PhD, Palmer D BS, et al (2016) Preventing Instrumentation Failure in Three-Column Spinal Osteotomy: Biomechanical Analysis of Rod Configuration. Spine Deform 4:3–9. https://doi.org/10.1016/j.jspd.2015.06.005 Berjano P, Xu M, Damilano M, et al (2019) Supplementary delta-rod configurations provide superior stiffness and reduced rod stress compared to traditional multiple-rod configurations after pedicle subtraction osteotomy: a finite element study. Eur Spine J Off Publ Eur Spine Soc Eur Spinal Deform Soc Eur Sect Cerv Spine Res Soc 28:2198–2207. https://doi.org/10.1007/s00586-019-06012-2 Sham ML, Zander T, Rohlmann A, Bergmann G (2005) Effects of the Rib Cage on Thoracic Spine Flexibility / Einfluss des Brustkorbs auf die Flexibilität der Brustwirbelsäule. 50:361–365. https://doi.org/10.1515/BMT.2005.051 Pennington Z, Cottrill E, Ahmed AK, et al (2019) Paraspinal muscle size as an independent risk factor for proximal junctional kyphosis in patients undergoing thoracolumbar fusion. J Neurosurg Spine 31:380–388. https://doi.org/10.3171/2019.3.spine19108 Borkowski SL, Sangiorgio SN, Bowen RE, et al (2017) Flexibility of thoracic spines under simultaneous multi-planar loading. Eur Spine J Off Publ Eur Spine Soc Eur Spinal Deform Soc Eur Sect Cerv Spine Res Soc 26:173–180. https://doi.org/10.1007/s00586-014-3499-0 Additional Declarations Competing interest reported. The authors would like to acknowledge that the implants used in this study were supplied by Johnson & Johnson MedTech (Canada). However, no comparisons were made between these implants and those from other suppliers. The authors have no additional funding sources to disclose. Supplementary Files ESJSupplementaryFilesNov15.pdf Cite Share Download PDF Status: Posted Version 1 posted 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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16:31:21","extension":"xml","order_by":7,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":137244,"visible":true,"origin":"","legend":"","description":"","filename":"ef3bccf327b04c1aa69a56dd5cc3775e1structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8124399/v1/e2f197af063988d7ef6a7910.xml"},{"id":98423408,"identity":"3723dcc0-8309-4dfe-bdf1-4fb61f016bf4","added_by":"auto","created_at":"2025-12-17 16:32:12","extension":"html","order_by":8,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":145399,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8124399/v1/071a5d285a738294f9ff7859.html"},{"id":97966836,"identity":"4c5ab797-08ce-416f-98cb-735dfa5f9539","added_by":"auto","created_at":"2025-12-11 10:00:31","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":503186,"visible":true,"origin":"","legend":"\u003cp\u003eCadaveric thoracic specimens were instrumented from T3 to T11 with either dual-rod or multi-rod constructs. Rods were applied bilaterally and fixed into place with locking set screws to create a dual-rod construct (\u003cstrong\u003eA\u003c/strong\u003e). To create the multiple-rod construct, an additional rod was placed bilaterally spanning the fusion site and held in place by 2 rod-to-rod connectors, one between T3-T4 and one between T10-T11 (\u003cstrong\u003eB\u003c/strong\u003e)\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8124399/v1/f45d6ba2b4820bec8ead8149.png"},{"id":100371900,"identity":"c57f3fb8-99bf-45ef-8519-8ddc3c4eeba5","added_by":"auto","created_at":"2026-01-16 08:11:11","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1404934,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8124399/v1/65dd329f-449f-40b4-ba9a-08f93a2a49a1.pdf"},{"id":98423458,"identity":"32bc8634-d7db-4d43-ad10-a4de28a04464","added_by":"auto","created_at":"2025-12-17 16:32:15","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":5584713,"visible":true,"origin":"","legend":"","description":"","filename":"ESJSupplementaryFilesNov15.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8124399/v1/1b92e4ccf011558f71794189.pdf"}],"financialInterests":"Competing interest reported. The authors would like to acknowledge that the implants used in this study were supplied by Johnson \u0026 Johnson MedTech (Canada). However, no comparisons were made between these implants and those from other suppliers. The authors have no additional funding sources to disclose.","formattedTitle":"Stability of multiple-rod constructs and dual-rod constructs in cadaveric thoracic spines","fulltext":[{"header":"INTRODUCTION","content":"\u003cp\u003eSpinal fusion remains a mainstay in the treatment of traumatic and degenerative spinal conditions [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. As fusion volumes have risen, so too have revision procedures. Pseudarthrosis, or nonunion, is typically diagnosed\u0026thinsp;\u0026ge;\u0026thinsp;1 year after the index operation [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e] and accounts for a substantial proportion of these revisions [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Rod fracture is among the most common implant-related failures after fusion and is closely linked to pseudarthrosis [\u003cspan additionalcitationids=\"CR5 CR6\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], with the odds of rod fracture reported to be 29 times higher in the presence of pseudarthrosis than in solid fusion [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eTo mitigate rod fracture, strategies that increase overall construct stiffness have been adopted [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. A widely used approach is the addition of accessory rods to create multiple-rod constructs (MRCs), rather than standard dual-rod constructs (DRCs). Several clinical series suggest MRCs reduce rod fracture rates compared with DRCs [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan additionalcitationids=\"CR11 CR12 CR13 CR14 CR15\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e], and biomechanical work\u0026mdash;predominantly in lumbar models and often in the setting of osteotomy for adult spinal deformity\u0026mdash;has emphasized better outcomes in construct stiffness [\u003cspan additionalcitationids=\"CR18 CR19 CR20 CR21\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. However, most prior work has relied on static testing methods, such as applying only a small number of load cycles or gradually increasing the load until the construct fails [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan additionalcitationids=\"CR24\" citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eAdditionally, despite being the most common site of spinal metastasis and a frequent target for long constructs in deformity correction [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e], the thoracic spine has been relatively understudied with respect to MRCs. To the best of our knowledge, no cadaveric thoracic investigation has directly compared the stability of MRCs to DRCs.\u003c/p\u003e\u003cp\u003eWe aimed to compare the stability and fatigue behavior of MRCs versus DRCs in cadaveric thoracic spines using a cyclic bodyweight loading protocol designed to assess for time-dependent construct degradation. We hypothesized that, compared to DRCs, MRCs would demonstrate increased stability during cyclic loading and exhibit reduced fatigue-related compromise after a 1-hour cyclical test.\u003c/p\u003e"},{"header":"METHODS","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eCadaver Preparation\u003c/h2\u003e\u003cp\u003ePrior to specimen collection and preparation, approval was obtained from the institutional ethics board (ID#118078 and ID#125155). Full details around the initial collection and preparation of these specimens are detailed elsewhere [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. Briefly, 11 full cadaveric specimens were procured and included non-identifiable information such as medical history, cause of death, age, and sex. All specimens were kept in a -20\u003csup\u003eo\u003c/sup\u003eC freezer prior to use. Computed tomography (CT) scan was then performed to rule out internal bony abnormalities.\u003c/p\u003e\u003cp\u003eOnce the CT scan was complete, specimens were sectioned into cervical, thoracic, and\u003c/p\u003e\u003cp\u003elumbosacral segments. The thoracic spine (T1-T12) segment was used for this study. These specimens were then thawed to room temperature and prepared further. The majority of musculature was dissected off the specimen, leaving the facet capsules, posterior ligamentous complex, and costovertebral joints intact. The ribcage was removed, leaving approximately 2cm of posterior rib attached at the costovertebral joints. The most cranial (T1) and most caudal (T12) vertebral segments were then potted in cement with their endplates parallel to the ground, leaving the adjacent intervertebral discs free and able to allow range of motion. This setup can be seen in Online Resource 1. From here, all spines underwent native range of motion testing without instrumentation and were subsequently instrumented for non-destructive testing as part of a previous study [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. Following this testing, all spines were stored back in a freezer at -20\u003csup\u003eo\u003c/sup\u003eC. After reviewing the specimens\u0026rsquo; imaging and medical history, preliminary testing of cadaveric spines for the present study was then conducted to develop the testing protocol. During this preliminary testing, 2 of the initial 11 cadaveric specimens sustained bony or ligamentous damage and were subsequently excluded from the present study. As such, 9 thoracic spines were non-damaged and available for further biomechanical testing. Demographics of cadaveric specimens used in this study are detailed in Online Resource 2.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eInstrumentation\u003c/h3\u003e\n\u003cp\u003ePrior to instrumentation and testing, specimens were thawed overnight at room temperature. Multi-level pedicle screw fixation was performed starting from T3 and extending down to include T11, leaving T2 as the first uninstrumented level, while T1 and T12 were both fixed in potted cement. Pedicle screw start point and insertion technique were done according to a previously described technique [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. Using a pedicle probe, screw tracts were examined for breaches. The depth of each screw track was then measured, and 6.0mm polyaxial pedicle screws were inserted, ranging from 40mm to 55mm in length. Pedicle screws were inserted by hand to ensure a constant angle and speed until each screw was well seated.\u003c/p\u003e\u003cp\u003eFrom here, specimens were randomized to undergo either initial testing with a MRC or with a DRC. To create a DRC, one 5.5mm diameter titanium rod was bent to the appropriate shape and attached to the left-sided pedicle screws spanning from T3-T11, while a second rod was attached to the right-sided pedicle screws. Both rods were then secured in place to the pedicle screws with locking set screws. To create an MRC, the same process was done that was used for the DRC set-up, followed by placement of one additional 5.5mm titanium rod just lateral to the existing 2 rods, with each new rod secured in place with one 5.5mm-5.5mm rod-to-rod connector cranially between T3 and T4 pedicle screws, and one 5.5mm-5.5mm rod-to-rod connector caudally between T10 and T11 pedicle screws. This resulted in a total of 4 rods in the MRC set-up. An example of the DRC and MRC setups can be seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\u003cp\u003eAfter completing instrumentation, markers were rigidly applied to the specimens to create landmarks to be captured with the digital imaging correlation system. These markers included twelve 2.0mm K-wires with attached 3D-printed hemispherical plastic components covered with multiple black and white contrasting stickers. One of these markers was inserted into each vertebral body spanning from T1-T12. An example of this construct and marker setup is seen in Online Resource 3.\u003c/p\u003e\n\u003ch3\u003eSpecimen Testing\u003c/h3\u003e\n\u003cp\u003eUpon completion of instrumentation and application of markers to allow proper image capture, the potted specimens were ready for testing. Each specimen was tested individually, on separate days, one at a time. Potted specimens were mounted on a testing machine with custom design modifications (Instron\u0026reg; 5967, Norwood, MA, USA). The potted T12 was rigidly attached to a sliding x-y table, allowing unrestricted translation motion in the transverse plane. The potted T1 was rigidly attached to a custom testing jig capable of testing in six degrees of freedom. An example of this setup is seen in Online Resource 4.\u003c/p\u003e\u003cp\u003eFirst, static range of motion (ROM) tests were performed with the intact spines prior to fusion. Then ROM tests were done following initial instrumentation (either DRC or MRC). The specimens were preloaded with 15N of force and set to maintain 15N during the test to remove any mechanical slack and aid in mounting the specimen through its neutral axis. ROM testing involved applying a pure rotational or bending load to the most cranial (T1) end up to a limit of \u0026plusmn;\u0026thinsp;5Nm, at 1\u003csup\u003eo\u003c/sup\u003e per second, and cycled three times with the first two cycles for preconditioning and the last for data analysis. This method was based on previous protocols [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eARAMIS Adjustable 12M system (GOM Metrology, Braunschweig, Germany) was used as a digital imaging correlation (DIC) system to track the motion in six degrees of freedom of the twelve previously inserted 3D printed markers rigidly attached to each vertebra. The setup consisted of two 24mm focal length and 4096-by-3600 pixels (pixel size of 3.45\u0026micro;m) resolution cameras. The two cameras were set at a 25\u003csup\u003eo\u003c/sup\u003e angle, 4 Hz, and illuminated with two polarized LED light sources. The DIC setup was calibrated prior to each biomechanical test by a standard calibration protocol and calibration plate provided by GOM Metrology for a measuring volume of 570-by-430-by-430mm. The images were processed in ARAMIS Professional 2019 (GOM Metrology, Braunschweig, Germany). Torsion was applied and tracked by a universal testing device (Instron\u0026reg; 5967, Norwood, MA, USA) using a 50kN load cell, recording data at 50Hz. Load cells were able to measure both forces as well as moments. Bending was applied and tracked by a custom testing jig consisting of a stepper motor (NEMA 23, OMC) and a 2.2kN load cell (MC3A-6-1k, AMTI) recording data at 20Hz. Each specimen was tested in flexion/extension, lateral bend, and internal/external rotation in a random order.\u003c/p\u003e\u003cp\u003eNext, the specimens were cyclically loaded during a 1-hour bodyweight simulation fatigue test (FT) to simulate a scenario of day-to-day wear and tear of the construct before bony fusion. Again, the spines were preloaded to 15N to remove mechanical slack and to aid in mounting through the neutral axis. The specimens were tested for one hour and were cyclically loaded in internal rotation and external rotation to \u0026plusmn;\u0026thinsp;5Nm at 5\u003csup\u003eo\u003c/sup\u003e per second and a cyclically applied axial load from 200N to 350N (representing one-quarter and one-half body weight respectively) at 150N/s for one hour. Specimens were periodically sprayed with a saline solution to prevent drying. Cyclical axial loads and cyclic torsional loads were independently applied. The torque data was captured by the same load cell as above (30kN load cell at 50Hz), while the positional/rotational data was collected in a predefined pulsed setup by the DIC system. The pulsed sequence was set to record the first 20 seconds at 10Hz, then to reduce file size, this was set to record data at 1/3Hz for the next 3560 seconds before recapturing the data at 10Hz for the last 20 seconds of the one-hour test. The DIC system recorded the position of the 12 vertebrae throughout the entire test.\u003c/p\u003e\u003cp\u003eImmediately following FT, specimens were examined for any evidence of bony or ligamentous failure. Additionally, all screw-bone interfaces and screw-rod interfaces were examined for signs of loosening. After thorough inspection, if no damage was visualized, the specimens were statically tested once again in flexion/extension, lateral bend, and rotation. This was done to determine if any change in motion was observed following FT. The testing sequence of static ROM to FT to static ROM was then repeated for the other construct with the same specimen.\u003c/p\u003e\u003cp\u003eROM was calculated as the difference between the maximum and minimum ROM at each vertebra: \u0026#119877;\u0026#119874;\u0026#119872;\u003csub\u003e\u0026#119909;\u003c/sub\u003e = \u0026#119877;\u0026#119874;\u0026#119872;\u003csub\u003e\u0026#119909;,\u0026#119898;\u0026#119886;\u0026#119909;\u003c/sub\u003e\u0026minus; \u0026#119877;\u0026#119874;\u0026#119872;\u003csub\u003e\u0026#119909;,\u0026#119898;\u0026#119894;\u0026#119899;\u003c/sub\u003e. The subscripts \u003cem\u003emax\u003c/em\u003e and \u003cem\u003emin\u003c/em\u003e are the maximum and minimum recorded values, respectively, and \u003cem\u003eROM\u003c/em\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e is the range of motion at the targeted vertebra.\u003c/p\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003eStatistical Analysis\u003c/h2\u003e\u003cp\u003eSPSS statistical software (version 29) was used for the statistical analysis of captured data. Each cadaveric specimen acted as its own control. The Wilcoxon signed-rank test was used to compare median differences in total range of motion between construct types (i.e., DRC vs. MRC) and between status of fatigue test (i.e. pre vs. post). Statistical significance was set at \u0026lt;\u0026thinsp;0.05. Individual segment range of motion median differences were compared using the Wilcoxon signed-rank test with statistical significance set at \u0026lt;\u0026thinsp;0.05. T1-T2 and T11-T12 were excluded from the analysis as the T1 and T12 vertebral bodies were fixed in cement.\u003c/p\u003e\u003c/div\u003e"},{"header":"RESULTS","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003eTotal Range of Motion\u003c/h2\u003e\u003cp\u003eMean rotational ROM was significantly higher after 1-hour testing for DRC (25.4 +/- 4.6\u003csup\u003eo\u003c/sup\u003e vs. 22.8 +/- 4.8\u003csup\u003eo\u003c/sup\u003e, p\u0026thinsp;=\u0026thinsp;0.009) and for MRC (25.0 +/- 5.1\u003csup\u003eo\u003c/sup\u003e vs. 22.3 +/- 4.1\u003csup\u003eo\u003c/sup\u003e, p\u0026thinsp;=\u0026thinsp;0.009). (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). ROM for rotation were similar between DRC and MRC regardless of fatigue status (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eMean lateral bend ROM was significantly higher after 1-hour testing for DRC (19.1 +/- 4.1\u003csup\u003eo\u003c/sup\u003e vs. 16.9 +/- 5.3\u003csup\u003eo\u003c/sup\u003e, p\u0026thinsp;=\u0026thinsp;0.009) and for MRC (20.3 +/- 6.1\u003csup\u003eo\u003c/sup\u003e vs. 16.6 +/- 4.2\u003csup\u003eo\u003c/sup\u003e, p\u0026thinsp;=\u0026thinsp;0.009). (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). ROM for lateral bend were similar between DRC and MRC regardless of fatigue status (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eMean flexion/extension ROM was significantly higher after 1-hour testing for DRC (15.3 +/- 4.4\u003csup\u003eo\u003c/sup\u003e vs. 12.8 +/- 5.3\u003csup\u003eo\u003c/sup\u003e, p\u0026thinsp;=\u0026thinsp;0.009) but not for MRC (16.5 +/- 4.9\u003csup\u003eo\u003c/sup\u003e vs. 14.1 +/- 3.7\u003csup\u003eo\u003c/sup\u003e, p\u0026thinsp;=\u0026thinsp;0.076). (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). ROM for flexion/extension were similar between DRC and MRC regardless of fatigue status (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eRange of Motion by Vertebral Segment\u003c/h3\u003e\n\u003cp\u003eOverall, the range of motion at the first uninstrumented level (T2-T3) was significantly higher in all planes of motion after 1-hour fatigue testing than before, but there were no differences between DRC and MRC. As testing moved caudally, these differences were less pronounced. A detailed analysis of ROM for all measured vertebral levels is found in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. All range of motion was \u0026lt;\u0026thinsp;1\u003csup\u003eo\u003c/sup\u003e at T5-T6 and distal, and there were no significant differences in motion between construct type or status of fatigue testing. As such, ROM values for T5-T6 were omitted from Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e\n\u003ch3\u003eAxial Rotation by Vertebral Segment\u003c/h3\u003e\n\u003cp\u003eAt T2-T3, mean rotation was significantly higher post 1-hour fatigue test for DRC (8.61 +/- 2.54\u003csup\u003eo\u003c/sup\u003e vs. 7.37 +/- 2.60\u003csup\u003eo\u003c/sup\u003e, p\u0026thinsp;=\u0026thinsp;0.009) and for MRC (8.73 +/- 2.89\u003csup\u003eo\u003c/sup\u003e vs. 7.75 +/- 2.47\u003csup\u003eo\u003c/sup\u003e, p\u0026thinsp;=\u0026thinsp;0.033). Rotation between MRC and DRC at this level was similar regardless of fatigue test status.\u003c/p\u003e\u003cp\u003eAt T3-T4, mean rotation was significantly higher post-1 hour fatigue test for DRC (1.82 +/- 1.08\u003csup\u003eo\u003c/sup\u003e vs. 1.39 +/- 0.71\u003csup\u003eo\u003c/sup\u003e, p\u0026thinsp;=\u0026thinsp;0.018) and for MRC (1.80 +/- 0.95\u003csup\u003eo\u003c/sup\u003e vs. 1.28 +/- 0.70\u003csup\u003eo\u003c/sup\u003e, p\u0026thinsp;=\u0026thinsp;0.013). Rotation between MRC and DRC at this level was similar regardless of fatigue test status.\u003c/p\u003e\u003cp\u003eAt T4-T5, mean rotation was similar post 1-hour fatigue test for DRC (0.76 +/- 0.38\u003csup\u003eo\u003c/sup\u003e vs. 0.67 +/- 0.28\u003csup\u003eo\u003c/sup\u003e, p\u0026thinsp;=\u0026thinsp;0.343) but greater for MRC (0.75 +/- 0.38\u003csup\u003eo\u003c/sup\u003e vs. 0.64 +/- 0.28\u003csup\u003eo\u003c/sup\u003e, p\u0026thinsp;=\u0026thinsp;0.044). Rotation between MRC and DRC at this level was similar regardless of fatigue test status.\u003c/p\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003eLateral Bend by Vertebral Segment\u003c/h2\u003e\u003cp\u003eAt T2-T3, mean lateral bend was significantly higher post 1-hour fatigue test for DRC (7.34 +/- 2.81\u003csup\u003eo\u003c/sup\u003e vs. 6.19 +/- 2.44\u003csup\u003eo\u003c/sup\u003e, p\u0026thinsp;=\u0026thinsp;0.018) and for MRC (8.04 +/- 2.69\u003csup\u003eo\u003c/sup\u003e vs. 6.30 +/- 2.46\u003csup\u003eo\u003c/sup\u003e, p\u0026thinsp;=\u0026thinsp;0.009). Lateral bend between MRC and DRC at this level was similar regardless of fatigue test status.\u003c/p\u003e\u003cp\u003eAt T3-T4, mean lateral bend was significantly higher post 1-hour fatigue test for DRC (1.20 +/- 0.81\u003csup\u003eo\u003c/sup\u003e vs. 0.89 +/- 0.59\u003csup\u003eo\u003c/sup\u003e, p\u0026thinsp;=\u0026thinsp;0.042) but not for MRC (1.18 +/- 1.41\u003csup\u003eo\u003c/sup\u003e vs. 1.00 +/- 0.91\u003csup\u003eo\u003c/sup\u003e, p\u0026thinsp;=\u0026thinsp;0.343). Lateral bend between MRC and DRC at this level was similar regardless of fatigue test status.\u003c/p\u003e\u003cp\u003eAt T4-T5, the mean lateral bend was similar pre- and post-1-hour fatigue test for both DRC and MRC. No differences in ROM were observed between construct types regardless of fatigue test status.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003eFlexion/Extension by Vertebral Segment\u003c/h2\u003e\u003cp\u003eAt T2-T3, mean flexion/extension was significantly higher post 1-hour fatigue test for DRC (4.25 +/- 2.36\u003csup\u003eo\u003c/sup\u003e vs. 4.91 +/- 2.21\u003csup\u003eo\u003c/sup\u003e, p\u0026thinsp;=\u0026thinsp;0.015) and for MRC (4.48 +/- 2.07\u003csup\u003eo\u003c/sup\u003e vs. 4.87 +/- 1.90\u003csup\u003eo\u003c/sup\u003e, p\u0026thinsp;=\u0026thinsp;0.028). No significant differences were noted in flexion/extension ROM at this level between construct types.\u003c/p\u003e\u003cp\u003eAt T3-T4, mean flexion/extension was significantly higher post 1-hour fatigue test for DRC (0.74 +/- 0.70\u003csup\u003eo\u003c/sup\u003e vs. 0.38 +/- 0.36\u003csup\u003eo\u003c/sup\u003e, p\u0026thinsp;=\u0026thinsp;0.024) but not for MRC (0.94 +/- 1.06\u003csup\u003eo\u003c/sup\u003e vs. 0.59 +/- 0.47\u003csup\u003eo\u003c/sup\u003e, p\u0026thinsp;=\u0026thinsp;0.076). No significant differences were noted in flexion/extension ROM at this level between construct types.\u003c/p\u003e\u003cp\u003eAt T4-T5, mean flexion/extension was similar between MRC and DRC regardless of construct type and status of fatigue test.\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\u003eTotal range of motion, comparison by fatigue status\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\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e\u003cp\u003eTotal ROM (\u003csup\u003eo\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTest Scenario\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e\u003cp\u003eMean (SD)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eP-Value\u003c/p\u003e\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\u003ePre-FT\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePost-FT\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\u003eDual-Rod Construct\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\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\u003eRotation\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e22.8 (4.8)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e25.4 (4.6)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u003cb\u003e0.009\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLateral Bend\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e16.9 (5.3)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e19.1 (4.1)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u003cb\u003e0.009\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eFlexion/Extension\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e12.8 (5.3)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e15.3 (4.4)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u003cb\u003e0.009\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMultiple-Rod Construct\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\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\u003eRotation\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e22.3 (4.1)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e25.0 (5.1)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u003cb\u003e0.009\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLateral bend\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e16.6 (4.2)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e20.3 (6.1)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u003cb\u003e0.009\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eFlexion/Extension\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e14.1 (3.7)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e16.5 (4.9)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.076\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colspan=\"4\" nameend=\"c4\" namest=\"c1\"\u003e\u003cp\u003eP-values obtained from two-sided Wilcoxon test\u003c/p\u003e\u003cp\u003eBolded values are p\u0026thinsp;\u0026lt;\u0026thinsp;0.05\u003c/p\u003e\u003cp\u003eROM, range of motion; SD, standard deviation; FT, fatigue test\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\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\u003eTotal range of motion, comparison by construct type\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\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e\u003cp\u003eTotal ROM (\u003csup\u003eo\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTest Scenario\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e\u003cp\u003eMean (SD)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eP-Value\u003c/p\u003e\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\u003eDRC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eMRC\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\u003ePre-Fatigue Test\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\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\u003eRotation\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e22.8 (4.8)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e22.3 (4.1)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.722\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLateral Bend\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e16.9 (5.3)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e16.6 (4.2)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.477\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eFlexion/Extension\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e12.8 (5.3)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e14.1 (3.7)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.076\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePost-Fatigue Test\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\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\u003eRotation\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e25.4 (4.6)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e25.0 (5.1)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.343\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLateral bend\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e19.1 (4.1)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e20.3 (6.1)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.554\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eFlexion/Extension\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e15.3 (4.4)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e16.5 (4.9)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.286\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colspan=\"4\" nameend=\"c4\" namest=\"c1\"\u003e\u003cp\u003eP-values obtained from two-sided Wilcoxon test\u003c/p\u003e\u003cp\u003eBolded values are p\u0026thinsp;\u0026lt;\u0026thinsp;0.05\u003c/p\u003e\u003cp\u003eROM, range of motion; SD, standard deviation; FT, fatigue test; DRC, dual-rod construct; MRC, multiple-rod construct\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\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\u003eRange of motion by vertebral segment\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"8\"\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\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e\u003cp\u003eDual-Rod Construct\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c7\" namest=\"c6\"\u003e\u003cp\u003eMultiple-Rod Construct\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c8\"\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\"\u003e\u003cp\u003eVertebral Level\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eDirection of Motion\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eFatigue Test Pre vs Post\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eROM (\u003csup\u003eo\u003c/sup\u003e)\u003c/p\u003e\u003cp\u003eMean (SD)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eP-Value\u003c/p\u003e\u003cp\u003ePre vs Post\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003eROM (\u003csup\u003eo\u003c/sup\u003e)\u003c/p\u003e\u003cp\u003eMean (SD)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003eP-Value\u003c/p\u003e\u003cp\u003ePre vs Post\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003eDRC vs MRC\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"5\" rowspan=\"6\"\u003e\u003cp\u003eT2-T3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eRotation\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePre\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e7.37 (2.60)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e\u003cb\u003e0.009\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e7.75 (2.47)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e\u003cb\u003e0.033\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.554\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePost\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e8.61 (2.54)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e8.73 (2.89)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.906\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eLateral Bend\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePre\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e6.19 (2.44)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e\u003cb\u003e0.018\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e6.30 (2.46)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e\u003cb\u003e0.009\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.813\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePost\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e7.34 (2.81)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e8.04 (2.69)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.236\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eFlexion/\u003c/p\u003e\u003cp\u003eExtension\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePre\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e4.25 (2.36)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e\u003cb\u003e0.015\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e4.48 (2.07)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e\u003cb\u003e0.028\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.314\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePost\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e4.91 (2.21)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e4.87 (1.90)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.515\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"5\" rowspan=\"6\"\u003e\u003cp\u003eT3-T4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eRotation\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePre\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1.39 (0.71)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e\u003cb\u003e0.018\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e1.28 (0.70)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e\u003cb\u003e0.013\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.906\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePost\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1.82 (1.08)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e1.80 (0.95)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e1.000\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eLateral Bend\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePre\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.89 (0.59)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e\u003cb\u003e0.042\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e1.00 (0.91)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e0.343\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.477\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePost\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1.20 (0.81)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e1.18 (1.41)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.234\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eFlexion/\u003c/p\u003e\u003cp\u003eExtension\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePre\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.38 (0.36)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e\u003cb\u003e0.024\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.59 (0.47)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e0.076\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.554\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePost\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.74 (0.70)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.94 (1.06)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.636\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"5\" rowspan=\"6\"\u003e\u003cp\u003eT4-T5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eRotation\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePre\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.67 (0.28)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e0.343\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.64 (0.28)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e\u003cb\u003e0.044\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.813\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePost\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.76 (0.38)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.75 (0.38)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.906\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eLateral Bend\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePre\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.10 (0.11)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e0.624\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.07 (0.09)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e0.813\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.155\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePost\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.08 (0.12)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.09 (0.14)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.441\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eFlexion/\u003c/p\u003e\u003cp\u003eExtension\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePre\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.09 (0.10)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e0.097\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.04 (0.09)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e0.477\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e1.000\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePost\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.12 (0.08)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.07 (0.14)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.097\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colspan=\"8\" nameend=\"c8\" namest=\"c1\"\u003e\u003cp\u003eNote: T2-T3 is uninstrumented, while T3-T4 and T4-T5 are first and second instrumented levels, respectively. From T5-T11, ROM is negligible with all values\u0026thinsp;\u0026lt;\u0026thinsp;1\u003csup\u003eo\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eP-values obtained from two-sided Wilcoxon test\u003c/p\u003e\u003cp\u003eBolded values are p\u0026thinsp;\u0026lt;\u0026thinsp;0.05\u003c/p\u003e\u003cp\u003eROM, range of motion; SD, standard deviation; DRC, dual-rod construct; MRC, multiple-rod construct\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"DISCUSSION","content":"\u003cp\u003eIn this study, 9 cadaveric thoracic spines were instrumented with two different construct types\u0026mdash;dual-rod constructs (DRCs) and multiple-rod constructs (MRCs). Overall range of motion was then analyzed before and after a 1-hour bodyweight simulation fatigue test (FT). No significant differences were observed in ROM between DRCs and MRCs. However, after a 1-hour fatigue test, MRCs maintained a similar ROM to their pre-fatigue state during flexion/extension, while DRCs did not. This suggests that MRCs may provide additional stability to daily wear and tear following initial instrumentation, thus allowing further time for a bony fusion to take place. This is supported by other biomechanical literature in the lumbar spine showing lower stress forces with MRCs [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e], as well as by a recent meta-analysis comparing MRCs to DRCs in adult spinal deformity that showed lower rates of nonunion with MRCs when compared to DRCs [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eAn important caveat to this finding is seen when the analysis is broken down into individual segments. Although DRCs and MRCs exhibited similar absolute ROM at each individual vertebral segment, they behaved differently pre- and post-fatigue testing. At the highest instrumented level (T3-T4), only DRCs demonstrated a significantly higher ROM with lateral bend and flexion/extension post-fatigue testing. Conversely, at the T4-T5 level (second highest instrumented level), MRCs exhibited higher flexion/extension ROM while the ROM of DRCs was similar across all three movement planes pre- and post-fatigue testing. It\u0026rsquo;s difficult to say what these differences translate to clinically. One hypothesis around the use of MRCs is that although they may increase overall construct stiffness and benefit fusion rates, this is achieved at the expense of increasing forces at the screw-bone interface. In a 2018 retrospective review of 106 patients with adult spinal deformity, MRCs were associated with a 1.9-fold increased risk of screw loosening at the cranial level, and a 4.1-fold increased risk of screw loosening at the caudal level[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. This finding was echoed in a recent meta-analysis by Zhao et al [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. None of the cadaveric spines tested in this study exhibited qualitative signs of screw-bone or screw-rod interface loosening. However, it is possible that the differences in ROM at the two cranial instrumented levels post-fatigue testing were early warning signs of loosening, and with further duration of testing, these differences may have become more pronounced.\u003c/p\u003e\u003cp\u003eAside from nonunion, rod breakage, and screw pullout, another serious complication in long instrumented spinal surgery is proximal junctional kyphosis (PJK). PJK is defined as a sagittal Cobb angle of 10-20\u003csup\u003eo\u003c/sup\u003e between the uppermost instrumented vertebra and the vertebra 2 levels above, or when the Cobb angle between these two vertebrae is 10-15\u003csup\u003eo\u003c/sup\u003e greater than the preoperative measurement [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. This can result in new neurological deficits, fractures, severe pain, or worsening quality of life leading to revision surgery [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. The reported incidence of PJK is 5\u0026ndash;46% [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e], and its development has been linked to mechanical, tissue, and demographic risk factors, with 76% of the research focusing on mechanical contributions to PJK [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. Among these mechanical contributions to PJK, increased rigidity of the construct at the transitional zone between instrumented and non-instrumented vertebrae is thought to play the most significant role in pathogenesis [\u003cspan additionalcitationids=\"CR37\" citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e], with some authors reporting a lower incidence of PJK by deploying a more gradual transition in construct stiffness at the transitional level [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e].\u003c/p\u003e\u003cp\u003ePrevious cadaveric work has examined semi-rigid constructs at the uppermost instrumented level and found a more linear and less abrupt change in biomechanics at the transitional zone with transverse process hooks when compared to all pedicle screws [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]. This softer transition in biomechanics is hypothesized to be protective against PJK. However, in this same analysis, semi-rigid constructs and all pedicle screw constructs were not compared to all pedicle screw constructs with multiple rods. Although increasing construct stiffness with MRCs has been linked to increased PJK [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e], a recent meta-analysis explored the rates of rod fracture, pseudoarthrosis, and PJK between DRCs and MRCs. Six studies [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan additionalcitationids=\"CR43 CR44 CR45\" citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e] were included in the meta-analysis, and no association was found between PJK and the type of construct (DRC vs MRC) [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]. In this study, no difference in absolute ROM was noted between DRCs and MRCs at the first uninstrumented or uppermost instrumented vertebral levels. This similarity in absolute construct stability supports the findings of this meta-analysis [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]. However, DRCs showed increased ROM at the first instrument level (T3-T4) following 1-hour fatigue testing, while MRCs did not, and at the second highest instrument level (T4-T5), MRCs showed increased ROM following 1-hour fatigue testing while DRCs did not. Although the differences in absolute ROM observed were low (\u0026lt;\u0026thinsp;1\u003csup\u003eo\u003c/sup\u003e), this may suggest these constructs have initial overall comparable stiffness but then react differently to daily wear-and-tear after initial instrumentation, which could ultimately affect rates of PJK. Although the clinical literature does not seem to support the notion that MRCs increase rates of PJK [\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e], longer duration fatigue testing is necessary to further explore this association.\u003c/p\u003e\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\u003ch2\u003eLimitations\u003c/h2\u003e\u003cp\u003eThere were certain limitations to consider when drawing conclusions from the present analysis. The first is with the type of metal used in this experiment. Titanium rods were used in both constructs. While titanium is often the rod of choice in spine surgery, cobalt chromium is another frequently used option. It\u0026rsquo;s possible that a cobalt chromium DRC and MRC may behave differently from what was observed here. Indeed, cobalt chromium constructs have been demonstrated to be more rigid than titanium ones [\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e, \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e], but the differences between MRC rigidity when it comes to the two materials may be overstated, as rates of PJK are not different between the two materials when MRCs are utilized [\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eSimilarly, only a single type of MRC (4-rod MRC) was investigated in this study. Different configurations of MRCs are used in spinal fusion surgery, including 3-rod MRCs, linked rod configurations, satellite rod configurations, accessory rod configurations, and intercalary rod configurations [\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e]. Most of the existing clinical research has focused on comparing all configurations of MRCs to DRCs [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e, \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e], as subgroup analyses between the different MRC configurations have proved challenging as a result of small sample size. Due to the variable biomechanical properties of different MRC configurations [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e, \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e], further studies are needed to compare the different MRC configurations both biomechanically and clinically.\u003c/p\u003e\u003cp\u003eFurther, the quality of the cadaveric soft tissues should be considered within this biomechanical analysis. In this study, paraspinal musculature and ribcages were dissected off specimens to facilitate instrumentation and testing, but this state likely does not adequately reflect in-vivo conditions, as these structures play a significant role in overall thoracic spine stability [\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e, \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e]. Additionally, the cadaveric spines in this study were overall inherently stable, as no osteotomies were performed [\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e]. This likely contributed to the overall similarities in ROM between MRCs and DRCs, as the degree of stability to gain was minimal with additional rods.\u003c/p\u003e\u003cp\u003eLastly, due to time and instrument constraints, FT was restricted to a 1-hour duration, and we could not cyclically load specimens to failure. Future studies should examine more construct configurations with different osteotomy options while performing cyclical loading of a greater duration, or until failure.\u003c/p\u003e\u003c/div\u003e"},{"header":"CONCLUSION","content":"\u003cp\u003eMultiple-rod constructs have been proposed as a way to increase stability in long spinal fusion surgery compared to dual-rod constructs. In this biomechanical analysis of intact cadaveric thoracic spines, multiple-rod constructs exhibited more resistance to cyclical loading than dual-rod constructs did after a 1-hour bodyweight simulation fatigue test. This work supports the current literature in the spine surgery field of reduced nonunion and rod breakage with multiple-rod constructs.\u003c/p\u003e"},{"header":"Statements and Declarations","content":"\u003cp\u003eThe authors would like to acknowledge that the implants used in this study were supplied by Johnson \u0026amp; Johnson MedTech (Canada). However, no comparisons were made between these implants and those from other suppliers. The authors have no additional funding sources to disclose.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors would like to extend our gratitude to Johnson \u0026amp; Johnson MedTech (Canada) who provided the necessary materials and implants to complete this research.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contribution\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eConception and Design: B.J.H., R.J.R.F.Administrative Support: P.B.Provision of Study Materials: P.B., C.N.C., R.J.R.F.Collection and Assembly of Data: B.J.H., P.B.Data Analysis and Interpretation: B.J.H., A.P.P., P.B.Manuscript Writing: B.J.H., A.P.P., R.J.R.F.Final Approval of Manuscript: All authors\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eNo publicly available datasets were generated during the creation of this article. However, primary data is available on request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompliance with Ethical Standards\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eEthics approval was obtained from the institutional ethics board prior to commencing any work (ID#118078 and ID#125155).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors would like to acknowledge that the implants used in this study were supplied by Johnson \u0026amp; Johnson MedTech (Canada). However, no comparisons were made between these implants and those from other suppliers. The authors have no additional funding sources to disclose.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eMartin BI, Mirza SK, Spina N, et al (2019) Trends in Lumbar Fusion Procedure Rates and Associated Hospital Costs for Degenerative Spinal Diseases in the United States, 2004 to 2015. 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Spine Deform 8:481\u0026ndash;490. https://doi.org/10.1007/s43390-020-00071-y\u003c/li\u003e\n \u003cli\u003eGuevara-Villaz\u0026oacute;n F, Boissiere L, Hayashi K, et al (2020) Multiple-rod constructs in adult spinal deformity surgery for pelvic-fixated long instrumentations: an integral matched cohort analysis. Eur Spine J 29:886\u0026ndash;895. https://doi.org/10.1007/s00586-020-06311-z\u003c/li\u003e\n \u003cli\u003eGupta S, Eksi MS, Ames CP, et al (2018) A Novel 4-Rod Technique Offers Potential to Reduce Rod Breakage and Pseudarthrosis in Pedicle Subtraction Osteotomies for Adult Spinal Deformity Correction. Oper Neurosurg Hagerstown Md 14:449\u0026ndash;456. https://doi.org/10.1093/ons/opx151\u003c/li\u003e\n \u003cli\u003eDinizo M, Passias P, Kebaish K, et al (2023) The Approach to Pseudarthrosis After Adult Spinal Deformity Surgery: Is a Multiple-Rod Construct Necessary? Glob Spine J 13:636\u0026ndash;642. https://doi.org/10.1177/21925682211001880\u003c/li\u003e\n \u003cli\u003eMoniz-Garcia D, Stoloff D, Akinduro O, et al (2023) Two- versus multi-rod constructs for adult spinal deformity: A systematic review and Random-effects and Bayesian meta-analysis. J Clin Neurosci 107:9\u0026ndash;15. https://doi.org/10.1016/j.jocn.2022.11.011\u003c/li\u003e\n \u003cli\u003eYe J, Gupta S, Farooqi AS, et al (2023) Use of multiple rods and proximal junctional kyphosis in adult spinal deformity surgery. J Neurosurg Spine 39:1\u0026ndash;9. https://doi.org/10.3171/2023.4.SPINE23209\u003c/li\u003e\n \u003cli\u003eShah KN, Walker G, Koruprolu SC, Daniels AH (2018) Biomechanical comparison between titanium and cobalt chromium rods used in a pedicle subtraction osteotomy model. Orthop Rev 10:7541\u0026ndash;7541. https://doi.org/10.4081/or.2018.7541\u003c/li\u003e\n \u003cli\u003eScheer JK, Tang JA, Deviren V, et al (2011) Biomechanical analysis of cervicothoracic junction osteotomy in cadaveric model of ankylosing spondylitis: effect of rod material and diameter: Laboratory investigation. J Neurosurg Spine 14:330\u0026ndash;335. https://doi.org/10.3171/2010.10.SPINE1059\u003c/li\u003e\n \u003cli\u003eEl Dafrawy MH, Adogwa O, Wegner AM, et al (2021) Comprehensive classification system for multirod constructs across three-column osteotomies: a reliability study. J Neurosurg Spine 34:103\u0026ndash;109. https://doi.org/10.3171/2020.6.SPINE20678\u003c/li\u003e\n \u003cli\u003eMerrill RK, Kim JS, Leven DM, et al (2017) Multi-Rod Constructs Can Prevent Rod Breakage and Pseudarthrosis at the Lumbosacral Junction in Adult Spinal Deformity. Glob Spine J 7:514\u0026ndash;520. https://doi.org/10.1177/2192568217699392\u003c/li\u003e\n \u003cli\u003eJager ZS MD, İnceoğlu S PhD, Palmer D BS, et al (2016) Preventing Instrumentation Failure in Three-Column Spinal Osteotomy: Biomechanical Analysis of Rod Configuration. Spine Deform 4:3\u0026ndash;9. https://doi.org/10.1016/j.jspd.2015.06.005\u003c/li\u003e\n \u003cli\u003eBerjano P, Xu M, Damilano M, et al (2019) Supplementary delta-rod configurations provide superior stiffness and reduced rod stress compared to traditional multiple-rod configurations after pedicle subtraction osteotomy: a finite element study. Eur Spine J Off Publ Eur Spine Soc Eur Spinal Deform Soc Eur Sect Cerv Spine Res Soc 28:2198\u0026ndash;2207. https://doi.org/10.1007/s00586-019-06012-2\u003c/li\u003e\n \u003cli\u003eSham ML, Zander T, Rohlmann A, Bergmann G (2005) Effects of the Rib Cage on Thoracic Spine Flexibility / Einfluss des Brustkorbs auf die Flexibilit\u0026auml;t der Brustwirbels\u0026auml;ule. 50:361\u0026ndash;365. https://doi.org/10.1515/BMT.2005.051\u003c/li\u003e\n \u003cli\u003ePennington Z, Cottrill E, Ahmed AK, et al (2019) Paraspinal muscle size as an independent risk factor for proximal junctional kyphosis in patients undergoing thoracolumbar fusion. J Neurosurg Spine 31:380\u0026ndash;388. https://doi.org/10.3171/2019.3.spine19108\u003c/li\u003e\n \u003cli\u003eBorkowski SL, Sangiorgio SN, Bowen RE, et al (2017) Flexibility of thoracic spines under simultaneous multi-planar loading. Eur Spine J Off Publ Eur Spine Soc Eur Spinal Deform Soc Eur Sect Cerv Spine Res Soc 26:173\u0026ndash;180. https://doi.org/10.1007/s00586-014-3499-0\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"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":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Cadaver, Spinal Fusion, Thoracic Spine, Adult Spinal Deformity, Dual-Rod Construct, Multiple-Rod Construct","lastPublishedDoi":"10.21203/rs.3.rs-8124399/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8124399/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003ePurpose\u003c/h2\u003e\u003cp\u003eRod fracture and pseudoarthrosis are common complications in spinal fusion surgery. Multiple-rod constructs have been shown to mitigate the risks of rod fracture and pseudarthrosis in the lumbar spine, but their effect on the thoracic spine is less studied. This work aims to compare the stability of two-rod (dual-rod) constructs (DRCs) to four-rod (multiple-rod) constructs (MRCs) in cadaveric thoracic spines.\u003c/p\u003e\u003ch2\u003eMethods\u003c/h2\u003e\u003cp\u003eNine intact human cadaveric thoracic spines (T1-T12) were instrumented in a randomized manner with either DRC or MRC. Specimens were then statically loaded in six planes of motion (flexion/extension, rotation, and lateral bend). A 1-hour bodyweight simulation fatigue test (FT) was then conducted on each specimen, followed by another static loading test. Constructs were then alternated, and this same sequence of testing was performed again. Range of motion (ROM) during the static loading tests was recorded using a digital imaging correlation (DIC) system.\u003c/p\u003e\u003ch2\u003eResults\u003c/h2\u003e\u003cp\u003eTotal ROM pre-FT and post-FT was similar between DRCs and MRCs. However, when comparing pre-FT to post-FT within each construct type, DRCs exhibited an increase in flexion/extension ROM (15.3\u0026deg; \u0026plusmn; 4.4\u0026deg; vs. 12.8\u0026deg; \u0026plusmn; 5.3\u0026deg;, p\u0026thinsp;=\u0026thinsp;0.009) while MRCs did not (16.5\u0026deg; \u0026plusmn; 4.9\u0026deg; vs. 14.1\u0026deg; \u0026plusmn; 3.7\u0026deg;, p\u0026thinsp;=\u0026thinsp;0.076).\u003c/p\u003e\u003ch2\u003eConclusions\u003c/h2\u003e\u003cp\u003eIn this biomechanical analysis of intact cadaveric thoracic spines, MRCs exhibited more resistance to cyclical loading than DRCs did after FT. This work supports the current literature in the spine surgery field of reduced nonunion and rod breakage with MRCs.\u003c/p\u003e","manuscriptTitle":"Stability of multiple-rod constructs and dual-rod constructs in cadaveric thoracic spines","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-11 10:00:26","doi":"10.21203/rs.3.rs-8124399/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"74a60407-f8dc-4a8b-ad7b-b4b296127651","owner":[],"postedDate":"December 11th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-01-14T16:23:53+00:00","versionOfRecord":[],"versionCreatedAt":"2025-12-11 10:00:26","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8124399","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8124399","identity":"rs-8124399","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}