Suspensory ligament branch desmopathy is associated with proximal sesamoid osteopathy in Thoroughbred racehorses on low-field magnetic resonance imaging

Suspensory ligament branch desmopathy is associated with proximal sesamoid osteopathy in Thoroughbred racehorses on low-field magnetic resonance imaging | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 31 December 2025 V1 Latest version Share on Suspensory ligament branch desmopathy is associated with proximal sesamoid osteopathy in Thoroughbred racehorses on low-field magnetic resonance imaging Authors : Kimberly V. Zullo 0009-0007-9222-1108 , Aimee Colbath [email protected] , John H. Pigott , and Amy B. Todd-Donato 0000-0002-2396-1694 Authors Info & Affiliations https://doi.org/10.22541/au.176717210.09344788/v1 162 views 84 downloads Contents Abstract Supplementary Material Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Background : Associations between suspensory ligament branch (SLB) desmopathy and proximal sesamoid bone (PSB) osteopathy have been investigated using ultrasonography and radiography but not standing magnetic resonance imaging (MRI). Objectives : To investigate the distribution of and relationship between SLB desmopathy and PSB osteopathy using standing MRI in racing Thoroughbreds. Study design : Retrospective analytical case control study. Methods : Standing MRI studies of the metacarpophalangeal region from sixty-eight racing Thoroughbreds (132 limbs) were blindly and independently evaluated by two board-certified equine specialists (surgeon and radiologist). SLB cross-sectional area (CSA) was recorded and SLB desmopathy and PSB osteopathy were graded. Data were analyzed using simple logistic regression, Wilcoxon matched-pairs signed-rank tests, and intraclass correlation coefficient calculations. Results : SLB desmopathy was identified in 76.5% of the population. Right forelimbs had significantly higher medial SLB grades (median:1.5, IQR:0-6.5) compared to left forelimbs (median:0, IQR:0-4), while left forelimbs had significantly higher lateral SLB grades (median:0, IQR:0-10.5) versus right forelimbs (median:0, IQR:0-1.5). Increased SLB CSA was associated with SLB desmopathy for both medial and lateral SLB when evaluated by a board-certified radiologist (medial distal odds ratio[OR]:1.62, CI:1.02-1.75, P=0.03; medial mid-length OR:1.31, CI:1.20-2.00, P=0.009; lateral distal OR:1.46, CI:1.17-1.85, P=0.0006; lateral mid-length OR:1.48, CI:1.08-2.09, P=0.01). Independent of laterality, the medial SLB (mid-length median:106.8, IQR:99.06-116.9; distal:114.6 IQR:105.4-125.9) had larger CSA than the lateral SLB (mid-length median:100.5, IQR:92.50-112.2; distal median:97.80, IQR:81.73-109). There was no difference in PSB grades between medial versus lateral or between limbs. Increasing PSB scores had increased odds of SLB desmopathy both medially (OR=1.73, CI: 1.17-2.67, P=0.04) and laterally (OR=1.24, CI: 1.03-2.37, P=0.04). Main limitations : Retrospective study only applicable to racing Thoroughbreds and limitations in imaging quality inherent to standing MRI. Conclusions : This study establishes a strong association between PSB osteopathy and SLB desmopathy on low-field MRI in Thoroughbred racehorses and documents distribution of forelimb SLB and PSB pathology. Suspensory ligament branch desmopathy is associated with proximal sesamoid osteopathy in Thoroughbred racehorses on low-field magnetic resonance imaging SUMMARY Background : Associations between suspensory ligament branch (SLB) desmopathy and proximal sesamoid bone (PSB) osteopathy have been investigated using ultrasonography and radiography but not standing magnetic resonance imaging (MRI). Objectives : To investigate the distribution of and relationship between SLB desmopathy and PSB osteopathy using standing MRI in racing Thoroughbreds. Study design : Retrospective analytical case control study. Methods : Standing MRI studies of the metacarpophalangeal region from sixty-eight racing Thoroughbreds (132 limbs) were blindly and independently evaluated by two board-certified equine specialists (surgeon and radiologist). SLB cross-sectional area (CSA) was recorded and SLB desmopathy and PSB osteopathy were graded. Data were analyzed using simple logistic regression, Wilcoxon matched-pairs signed-rank tests, and intraclass correlation coefficient calculations. Results : SLB desmopathy was identified in 76.5% of the population. Right forelimbs had significantly higher medial SLB grades (median:1.5, IQR:0-6.5) compared to left forelimbs (median:0, IQR:0-4), while left forelimbs had significantly higher lateral SLB grades (median:0, IQR:0-10.5) versus right forelimbs (median:0, IQR:0-1.5). Increased SLB CSA was associated with SLB desmopathy for both medial and lateral SLB when evaluated by a board-certified radiologist (medial distal odds ratio[OR]:1.62, CI:1.02-1.75, P=0.03; medial mid-length OR:1.31, CI:1.20-2.00, P=0.009; lateral distal OR:1.46, CI:1.17-1.85, P=0.0006; lateral mid-length OR:1.48, CI:1.08-2.09, P=0.01). Independent of laterality, the medial SLB (mid-length median:106.8, IQR:99.06-116.9; distal:114.6 IQR:105.4-125.9) had larger CSA than the lateral SLB (mid-length median:100.5, IQR:92.50-112.2; distal median:97.80, IQR:81.73-109). There was no difference in PSB grades between medial versus lateral or between limbs. Increasing PSB scores had increased odds of SLB desmopathy both medially (OR=1.73, CI: 1.17-2.67, P=0.04) and laterally (OR=1.24, CI: 1.03-2.37, P=0.04). Main limitations : Retrospective study only applicable to racing Thoroughbreds and limitations in imaging quality inherent to standing MRI. Conclusions : This study establishes a strong association between PSB osteopathy and SLB desmopathy on low-field MRI in Thoroughbred racehorses and documents distribution of forelimb SLB and PSB pathology. INTRODUCTION Suspensory ligament branch (SLB) disease and proximal sesamoid bone (PSB) pathology are common causes of lameness in Thoroughbred racehorses. 1-5 Insertional SLB desmopathy has been shown to affect racing potential, including earnings per start, number of races, and age at first start. 1 Increased PSB density has been associated with a higher risk of bi-axial sesamoid fracture resulting in euthanasia. 6 Prior publications have investigated the relationship between radiographic changes of the PSB and ultrasonographic abnormalities of the SLB insertion sites, 7,8 with a recent publication concluding that Thoroughbreds with enlarged vascular channels in the PSB are 4.6 times more likely to develop injury to the adjacent SLB. 3 While mild SLB abnormalities on ultrasound may be insignificant and can resolve by 2 years old, moderate to severe SLB abnormalities may decrease odds of racing and class achieved. 4 Increasing availability of advanced imaging modalities, such as MRI and nuclear scintigraphy, allow for the detection of more subtle pathology in the PSB and SLB in comparison to radiographic and ultrasonographic assessment. 9-13 Although high-field MRI provides better imaging quality and superior resolution of small structures in the equine fetlock region, 14,15 standing low-field MRI is a more accessible imaging modality for racehorses in training and negates the risks associated with general anesthesia. Compared to standing CT, MRI is considered superior for evaluation of soft tissue injury, as well as for differentiating adaptive versus maladaptive bone injury by the identification of osseous fluid-like signal. 16-18 The relationship between SLB disease and PSB disease has not been previously investigated on standing low-field MRI. Therefore, the aim of this study was to investigate the relationship between PSB osteopathy and SLB desmopathy in racing Thoroughbreds using standing low-field MRI. Specifically, we sought to identify the location and severity of SLB pathology and PSB osteopathy in racing Thoroughbreds. We hypothesized that horses with PSB osteopathy would have an increased incidence of SLB desmopathy on standing low-field MRI. MATERIALS AND METHODS STUDY DESIGN This study is a retrospective analytical case-controlled study. Medical records of all horses with a standing MRI (MRI) exam performed of the metacarpophalangeal joint (MCPJ) or metatarsophalangeal joint (MTPJ) from January 1, 2019, to April 30, 2024, at XXXX were reviewed. Inclusion criteria for this study were: racing Thoroughbred horses of any gender or age, and available medical records describing presence of lameness (including the limb and severity, if present). Imaging exams considered of poor quality for accurate assessment to meet study objectives were excluded, as determined by an ACVR-certified radiologist (XX). MEDICAL RECORDS REVIEW A review of the medical records for the horses that met inclusion criteria was performed by one of the authors (XX). The following data were recorded: patient demographic information, clinical presentation, lameness grade, blocking pattern (if performed/ reported), and imaging diagnoses. MAGNETIC RESONANCE IMAGING All MRI exams were performed at XXXX using the same low-field standing MRI unit [0.27 Tesla, Hallmarq sMRI, Hallmarq Veterinary Imaging Inc, Illinois, USA] and utilizing similar protocols. The sequences obtained were: T1-weighted GRE (transverse, sagittal, dorsal planes); STIR FSE (transverse, sagittal, dorsal planes); T2 FSE (transverse and dorsal planes); T2* GRE (transverse and dorsal planes). Technical parameters for the MRI sequences are included in Table 1 . For the purposes of this study, the MRI exams were independently reviewed; original imaging diagnoses were solely utilized by the medical records review author (XX) to assess sample size during the study design phase. MRI evaluators (XX, XX) were blinded to previous radiology reports. IMAGE SCORING Scoring of SLB from the MRI exams was adapted from prior studies. 19,20 The medial and lateral SLB were scored separately for each limb. For each SLB, the following data was recorded separately for the mid-length and distal portion of the branch (see Figure 1 ): cross-sectional area (CSA, cm 2 ); signal intensity on T1, T2 and STIR sequences (0-3; 0 = uniform, low signal intensity, smooth ligament/bone interface; 1 = focal increased signal intensity within region; well-defined margins; 2 = diffuse increased signal in region; well-defined margins; 3 = diffuse increase signal in region with poor distinction of margins); presence of avulsion fragments (0-1; 0 = none; 1 = present); surrounding soft tissue thickness (0-3; 0 = none, 1 = mild, 2 = moderate, 3 = severe); and STIR hyperintensity in surrounding soft tissues (0-3; 0 = none, 1 = mild, 2 = moderate, 3 = severe). The total score per region (mid-length and distal portions) was calculated (maximum per region, 17), as well as the total score per branch (maximum, 34). The lateral ‘mid’ suspensory branch could not be graded in 23/132 limbs, and the medial ‘mid’ suspensory branch could not be graded in 20/132 limbs due to the field of view. Scoring of PSB was adapted from prior studies. 6,16,21,22 The medial and lateral PSB were scored separately for each limb. Each PSB was divided into 8 regions, as shown in Figure 2 , labeled as: dorsoproximal-abaxial, palmaroproximal-abaxial, palmaroproximal-axial, dorsoproximal-axial, dorsodistal-abaxial, palmarodistal-abaxial, palmarodistal-axial, and dorsodistal-axial. For each of these regions, the following data was recorded: sclerosis, defined as reduced subchondral bone marrow intensity on T2 and T1 images (0-4; 0=none; 1 = 50% of region); presence of fluid-like signal, as evaluated on STIR and/or T2* sequences (0-4; 0= none; 1 = 50% of region); presence of a subchondral defect (0-4; 0 = normal, smooth chondro-osseous margin; 1= mild subchondral plate irregularity; 2 = marked subchondral plate irregularity with intact bone marrow; 3 = bone loss extending to medullary bone with preservation of some trabecular pattern; 4 = cyst-like formation); presence of a PSB fracture (0-1; 0=no; 1=yes); presence of osteophytes on the PSB margins (0-7; 0=none; 1=equivocal; 2=small; 3=small-moderate; 4=moderate; 5=moderate-large; 6=large; 7=very large); presence of enthesophytes on PSB [at insertion of SLB or intersesamoidean ligament] (0-3; 0=none, 1=proliferation, 2=resorption, 3=both). The total score per region was calculated (maximum, 23), as well as the total score per sesamoid (maximum, 184) for both the medial and lateral PSB. Scoring was performed independently by two of the authors, an ACVR-certified radiologist (XX) and an ACVS-certified surgeon (XX), both of whom were blinded to the medical records (lameness and disease status). The two observers’ scores were then assessed for significant score discrepancies (total score difference greater than 4 for SLB and 16 for PSB). For those cases identified with significant score discrepancies, both observers independently repeated their scoring. In the rare occasion (<10 scores of 132 scores for each PSB and SLB) that scores remained discordant, a final score was determined by consensus. The average total scores (between the two observers) for the medial PSB, lateral PSB, mid-length SLB, and distal SLB were utilized for data analysis. STATISTICAL ANALYSES A priori sample size estimation was performed using a power analysis program [G*Power, version 3.1.9.7, Heinrich Heini Universität Düsseldorf] for the primary study hypothesis, based on use of a two-tailed Wilcoxon signed-rank test. 23 A minimum sample size of 57 limbs was required to declare a standardized mean difference of 0.5 (medium effect size), statistically significant with 95% power at a significance level of 0.05. SLB grade, SLB CSA, and PSB grade were determined to be non-normal by a Shapiro-Wilk test. The relationship between SLB grade and PSB grade, and SLB CSA and PSB grade were evaluated using simple logistic regression, with results reported as an odds ratio (OR) with 95% confidence interval (CI). When comparing the left forelimb SLB grade and size versus the right forelimb, data was evaluated using a Wilcoxon matched-pairs signed rank test. All logistic regression and Wilcoxon matched-pairs signed rank tests were performed using GraphPad Prism [version 10.6.0; GraphPad Software]; significance was set at P0.9 are excellent. Inter-rater reliability analysis was performed using R Statistical Software [version 4.5.1; R Core Team]. 25 RESULTS Of the 224 MRI exams performed during this 5-year period, 147 of these were racing Thoroughbreds, consisting of 140 MCPJ and 7 MTPJ. Due to low sample numbers, MTPJ exams were excluded from this study. Eight MCPJ MRI exams were excluded due to unavailable medical record information or poor exam quality, leaving 132 MCPJ MRI exams for inclusion in this study (Supp. Figure 1) . The included exams were from 68 horses, consisting of 4 unilateral thoracic fetlock exams (2 left, 2 right) and 64 bilateral thoracic fetlock exams. There were 28 intact males, 15 geldings, and 25 female racing Thoroughbreds, with a median age of 42-months-old (range: 26-101). Ninety-three percent (63 of 68) of horses were reported to have forelimb lameness at the time of MRI, with 11 horses reported to be bilaterally forelimb lame. When forelimb lameness was reported, scores ranged from 1 to 4 (out of 5 on the American Association of Equine Practitioners [AAEP] scale, 26 with an average reported lameness score of 1. One horse was reported to have bilateral forelimb lameness, without a lameness score assigned in the medical record. INTRACLASS CORRELATION Cohen’s Kappa Intraclass correlation coefficients were calculated between observers for all outcome measures ( Table 2 ). Due to low intraclass correlation in SLB CSA, the relationship between CSA and SLB desmopathy was analyzed for individual scorers independently. All other analyses were run using average scores between observers due to moderate to high ICC. SUSPENSORY LIGAMENT DESMOPATHY SLB pathology was identified on MRI in 52 of 68 (76.5%) horses ( Figure 3 ). Of those horses, 40 (58.8%) were in the left forelimb and 44 (64.7%) were in the right forelimb. Of the 64 horses with bilateral forelimb MRI, 33 (48.5%) of these had bilateral SLB desmopathy. Medial SLB desmopathy was recorded in 69.1% (47/68) of enrolled horses, with grades ranging from 1.5 to 19 (mean: 6, median:1, IQR:0-6) in affected horses. Lateral SLB desmopathy was recorded in 60.2% (41/68) horses, with grades ranging from 1 to 18.5 (mean: 7.5; median:0, IQR:0-6). Desmopathy of both the medial and lateral SLB was recorded in 50% (34/68) horses. Of the 40 left forelimbs with SLB desmopathy, 10 (25%) had only medial SLB desmopathy, 10 (25%) had only lateral SLB desmopathy, and 20 (50%) had desmopathy of both the medial and lateral SLB. Of the 44 right forelimbs with SLB desmopathy, 17 (39%) had only medial SLB desmopathy, 4 (9%) had only lateral SLB desmopathy, and 23 (52%) had desmopathy of both the medial and lateral SLB. Right forelimbs had significantly higher medial SLB grades (range: 0-19, mean 4.0; median: 1.5, IQR: 0-6.5) when compared to left forelimbs (range: 0-13, mean: 2.18; media: 0, IQR: 0-4) (P=0.01). Whereas, left forelimbs had significantly higher lateral SLB grades (range: 0-18.5, mean: 4.17; median: 0, IQR: 0-10.5) versus right forelimbs (range: 0-16.5, mean: 2.41; median:0, IQR:0-1.5) (P=0.02) ( Figure 4 ). SUSPENSORY LIGAMENT CROSS-SECTIONAL AREA Regardless of limb, the mid-length of the medial SLB (range: 0.81-1.83, mean: 1.104; median: 1.07, IQR:1-1.78) had a larger average CSA (cm 2 ) than that of the lateral SLB (range: 0.55-1.62, mean: 1.02; median: 1.005, IQR: 0.925-1.122) (P<0.001), and the distal portion of the medial SLB (range: 0.86 – 1.895, mean: 1.17; median: 1.148, IQR: 1.054-1.269) had a larger average CSA (cm 2 ) than the distal portion of the lateral SLB (range: 0.72 -1.73, mean: 1.12; median: 1.116, IQR: 1.017-1.206) (P=0.01). When comparing CSA (cm 2 ) measurements between the board-certified radiologist (XX) and large animal surgeon (XX), measurements by the board-certified radiologist were larger for all locations (mid-medial suspensory, P<0.0001; distal-medial suspensory, P<0.0001; mid-lateral suspensory, P<0.0001; distal-lateral suspensory, P<0.0001) ( Figure 5 ). Due to the low ICC for CSA, the relationship between CSA and SLB desmopathy was analyzed for each observer. When CSA measured by a boarded-surgeon (XX) was compared to presence of SLB desmopathy, there was no significant relationship for any of the measurements. However, when a board-certified radiologist (XX) measured CSA of the SLB a significant relationship was found between CSA and desmopathy for medial and lateral SLB at both the mid-length and distal SLB location. For the medial distal SLB, with a 10mm 2 increase in CSA horses were 1.62 times more likely to have SLB desmopathy (OR: 1.62, CI: 1.02-1.75) (P=0.03). For the lateral distal SLB, with a 10mm 2 increase in CSA horses were 1.46 times more likely to have SLB desmopathy (OR: 1.46, CI: 1.17 – 1.85) (P=0.0006). For the lateral mid-length SLB, for every 10mm 2 change in CSA, horses were 1.48 times more likely to have SLB desmopathy (OR: 1.48, CI: 1.088-2.09, P=0.01). Finally, for the medial mid-length SLB, for every 10mm 2 change in CSA, horses were 1.3 times more likely to have SLB desmopathy (OR: 1.3, CI: 1.20-2.00, P=0.03). PROXIMAL SESAMOID BONE PATHOLOGY Total scores for the medial PSB ranged from 7 to 45 (mean: 20.7, median: 19, IQR: 14.5-25.5) in the left forelimb, and 6.5 to 57 (mean: 22.7, median: 21, IQR: 14-27.5) in the right forelimb, with all horses in the study having a non-zero score. Total scores for the lateral PSB ranged from 2.5 to 51 (mean: 19.4, median: 17.5, IQR: 11.5-24.5) in the left forelimb and 4 to 44.5 (mean: 19.95, median: 18.5, IQR: 14-25) in the right forelimb ( Figure 6 ). PSB sclerosis was reported in 68/68 horses (100%). Fluid signal was recorded in the medial PSB bone in 120/131 (92%) of limbs, with a range from 0 to 28 (mean: 5, median: 4, IQR: 1.5-6.87). Likewise, fluid signal was recorded in the lateral PSB in 120/131 (92%) of limbs, with a range from 0 to 24 (mean: 5, median: 4, IQR: 1.5-7) ( Supp. Figure 2 ). Sesamoid grades did not differ between left and right limbs for either medial (P=0.1) or lateral (P=0.4) PSB scores. Sesamoid pathology was greater in the distal half of the medial (P<0.0001) and lateral (P<0.0001) PSB compared to the proximal half of the PSB. Logistic regression showed no significant relationship between the medial and lateral distal PSB grade and SLB desmopathy. However, when the proximal medial PSB grade increased by 10 points, horses were 1.37 times more likely to have medial SLB desmopathy (OR: 1.37, CI: 1.39-4.89, P=0.002). When the proximal lateral PSB grade increased by 10 points, horses were 1.41 times more likely to have lateral SLB desmopathy (OR: 1.41, CI: 1.11-4.35, P=0.02). When the total medial SLB score was compared to the medial PSB, with every 10-point change in PSB score a horse was 1.73 times more likely to have SLB desmopathy (OR: 1.73, CI: 1.17-2.67) (P=0.0412). Similarly, when the lateral SLB score was compared to the lateral PSB score, with every 10-point change in PSB score a horse was 1.24 times more likely to have SLB desmopathy (OR: 1.24, CI: 1.03 – 2.37) (P=0.04). DISCUSSION This report establishes a relationship between SLB desmopathy and PSB osteopathy on standing low-field MRI. SLB desmopathy, defined as any abnormality in the SLB on MRI, was identified in 76.5% of horses. Interestingly, right forelimb medial SLB had greater pathology than left forelimb medial SLB, and left forelimb lateral branches had significantly higher scores than right forelimb lateral branches. PSB sclerosis was identified in 100% of horses, with no significant difference in score for either left, right, medial, or lateral sesamoid bones. However, the severity of PSB disease increased the likelihood of SLB desmopathy. Evaluation of the CSA confirmed a relationship between larger CSA and SLB desmopathy in both the mid-length and distal length of the SLB. The SLB and the PSB are intimately associated, with the SLB inserting on the abaxial surface of the corresponding PSB. 27 In the forelimb these structures play a large role in the suspensory apparatus, which is responsible for preventing over-extension of the metacarpophalangeal joint during load bearing and balancing forces applied on the third metacarpal bone to protect it from fractures. 27 Repeated load bearing at high speeds can put excessive load on these structures and result in injury. This study demonstrates that with increasing PSB osteopathy, SLB desmopathy is more likely. This relationship has not been previously established using standing low-field MRI. Previous studies using radiography and ultrasonography have shown that horses with more pronounced radiographic sesamoiditis were 5 times more likely to develop suspensory ligament branch (SLB) desmopathy, 8 and the severity of ultrasonographic SLB desmopathy is significantly related to ultrasonographic sesamoid osteopathy. 1 The current study further characterizes the distribution of PSB osteopathy and establishes that proximal PSB osteopathy is correlated with SLB desmopathy. Although a subset of changes on MRI can be a normal response to repetitive stress, continued changes to the subchondral bone in the PSB can indicate a source of pain that is not identifiable on radiographs. 9 Other imaging modalities such as positron emission tomography (PET) have also differentiated adaptive and maladaptive bone remodeling in subchondral bone. 28 On MRI, the presence of fluid-like signal can be suggestive of maladaptive remodeling characterized by active inflammation, edema, or contusion. 9 In the current study, all horses had PSB sclerosis on MRI consistent with the repetitive stress of racing, with 92.6% of horses exhibiting fluid-like signal. This study identified a higher incidence (76.5%) of SLB desmopathy in racing thoroughbreds than previous reports. A previous study by Olive et. al (2017) identified soft tissue abnormalities in only 20.8% with 47% of soft tissue injuries involving the SLB. 21 Likewise, Powell and colleagues identified SLB desmopathy in only 3.8% of racehorses. 29 A lower incidence of disease in previous studies of Thoroughbred racehorses with lameness localized to the fetlock may be partially explained by significantly improved low-field MRI technology, including substantial advances in motion correction software which provide improved image quality and recognition of more subtle lesions. Furthermore, prior studies based in Europe or the United Kingdom may not translate directly to the United States due to variations in training practices, racing practices, and track conditions. 21,29 Further, prior studies have categorized injuries by the most significant MRI finding, 21,29 which could lead to a decrease in the reporting of soft tissue disease which may be accompanied by more significant bone disease. In this study, desmopathy was defined as any abnormality (grade >1) in the suspensory ligament branch. As such, the study sought to identify any desmopathy including mild suspensory desmopathy that may not be immediately clinically relevant or the primary cause of lameness. The clinical significance of SLB disease has been previously established in racing Thoroughbreds. Thoroughbred racehorses with SLB disease have been reported with fewer starts in their 2- and 3-year-old seasons. 1 Further, horses with moderate to severe SLB desmopathy have higher reinjury rates with implications for overall racing performance, career longevity, and future sale value. 1 Our study had a similar distribution of SLB desmopathy between left forelimbs and right forelimbs. This is in agreement with prior studies in both flat racing and National Hunt Thoroughbreds. 30,31 The current study also reports significantly greater medial SLB desmopathy in right forelimbs and increasing severity of lateral SLB desmopathy in left forelimbs, which is in accordance with a previous study that found increased lateral SLB desmopathy in the left forelimb. 8 The pattern of SLB desmopathy (lateral branch of left forelimb, medial branch of right forelimb) may be associated with the direction of high-speed work or banking of the track surface. Additional track factors could result in increased strain on the lateral side of the left forelimb and the medial side of the right forelimb. However, future studies investigating differences in track surface, direction of training, and incidence of SLB pathology are needed to better understand the underlying etiology for this distribution of SLB disease. The medial SLB in the current study all had a consistently larger CSA at both the mid-length and distal regions in comparison to lateral suspensory branches. A prior study which assessed National Hunt horses without clinical disease also found the distal medial SLB CSA to be larger than the lateral in the thoracic limbs. 31 Other studies have reported increased SLB size as a feature of desmopathy on both ultrasonography and MRI. 10,32 Our study confirms that, with increasing CSA, horses are more likely to have SLB desmopathy when the CSA was measured by a board-certified radiologist. Limitations of this study include its retrospective nature, absence of further categorization of suspensory desmopathy, the inclusion of a single specific population, and limitations associated with low-field MRI. Specifically, the retrospective nature of the study did not allow for access to a consistent, complete diagnostic workup. Further, the study included Thoroughbred racehorses at a single referral practice. There are multiple extrinsic factors including geographic location, track surface, direction of high-speed work, and training schedules which may have influenced the results. 7,31,33 In addition, the presence or absence of SLB desmopathy was assessed and graded in this study, but desmopathy was not further categorized as mild, moderate, or severe. This aligned with the goal of the study to identify a relationship between PSB osteopathy and SLB desmopathy; establishing the clinical significance of suspensory desmopathy was beyond the scope of this study and has been previously investigated. 1 Although low-field standing MRI technology has advanced significantly in the past decade, image quality is still affected by slice thickness, available sequences, and motion artifact compared to high-field MRI. Inter-observer agreement in this study was moderate to high for SLB and PSB total scoring, supporting the common practice of veterinary surgeons providing the initial imaging evaluation. However, inter-observer agreement was low for CSA. The observers in this study were presented with the entire study rather than pre-selected single slice transverse images; as such, low CSA agreement may be due to the measurement of CSA at different locations or lack of experience measuring CSA by the board-certified surgeon compared to the board-certified radiologist. CONCLUSION In summary, this study establishes a strong association between PSB osteopathy and SLB desmopathy on standing low-field MRI in a large cohort of Thoroughbred racehorses. Additionally, this study documents distribution of thoracic limb pathology in suspensory ligament branches and proximal sesamoid bones. This data contributes to our knowledge of fetlock region pathology in racing Thoroughbreds. 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Longitudinal monitoring of fetlock lesions in Thoroughbred racehorses using standing 18F-sodium fluoride positron emission tomography. Am J Vet Res . Aug 18 2022;83(10)doi:10.2460/ajvr.22.03.006229. Powell SE. Low-field standing magnetic resonance imaging findings of the metacarpo/metatarsophalangeal joint of racing Thoroughbreds with lameness localised to the region: a retrospective study of 131 horses. Equine Vet J . Mar 2012;44(2):169–77. doi:10.1111/j.2042-3306.2011.00389.x30. Ramzan PH, Palmer L, Dallas RS, Shepherd MC. Subclinical ultrasonographic abnormalities of the suspensory ligament branch of the athletic horse: A survey of 60 Thoroughbred racehorses. Equine Vet J . Mar 2013;45(2):159–63. doi:10.1111/j.2042-3306.2012.00588.x31. Fairburn AJ, Busschers E, Barr ARS. Subclinical ultrasonographic abnormalities of the suspensory ligament branches in National Hunt racehorses. Equine Vet J . Jul 2017;49(4):475–479. doi:10.1111/evj.1263932. Dyson DMaSJ. Clinical features, diagnostic imaging findings and concurrent injuries in 71 sports horses with suspensory branch injuries Equine Vet J . 2014;26(6):312–321. doi:10.1111/eve.1217533. Colon JL, Bramlage LR, Hance SR, Embertson RM. Qualitative and quantitative documentation of the racing performance of 461 Thoroughbred racehorses after arthroscopic removal of dorsoproximal first phalanx osteochondral fractures (1986-1995). Equine Vet J . Nov 2000;32(6):475–81. doi:10.2746/042516400777584640 TABLES TABLE 1. Pulse sequence parameters used for evaluating the relationship between proximal sesamoid bone disease and suspensory ligament branch desmopathy in racing Thoroughbred horses on standing low-field MRI exams. T1 GRE 52 8 n/a 50 37.0 x 17.1 256 x 256 5 1 1 6 T2 FSE 1930 88 n/a 90 37.0 x 17.1 256 x 256 5 1 1 6 STIR FSE 2343 95 22 90 37.0 x 17.1 256 x 256 5 1 1 6 T2* GRE 68 13 n/a 28 37.0 x 17.1 256 x 256 5 1 1 6 Key: TR=repetition time; TE=echo time; TI=inversion time; FOV=field-of-view; ETL=echo train length; NEX=number of excitations. TABLE 2. Cohen’s Kappa Intraclass correlation coefficients were calculated between observers for all outcome measures for evaluating the relationship between proximal sesamoid bone disease and suspensory ligament branch desmopathy in racing Thoroughbred horses. Medial Suspensory Grade 0.94 (0.91, 0.95) Lateral Suspensory Grade 0.95 (0.94, 0.97) Medial Sesamoid Grade 0.74 (0.65, 0.81) Lateral Sesamoid Grade 0.68 (0.58, 0.76) Medial Mid Suspensory CSA 0.54 (0.39, 0.66) Medial Distal Suspensory CSA 0.34 (0.18, 0.48) Lateral Mid Suspensory CSA 0.42 (0.25, 0.56) Lateral Distal Suspensory CSA 0.24 (0.07, 0.39) Key: ICC= intraclass correlation coefficients; CI=confidence interval. FIGURES FIGURE 1. Sagittal and transverse standing low-field magnetic resonance imaging (MRI) of the metacarpophalangeal joint demonstrating the imaging planes at which suspensory branch ligament scoring was performed in 132 racing Thoroughbred thoracic limb MRI exams. (A) Sagittal T1 image (left) with a white line depicting the plan of image acquisition for the corresponding transverse T1 image (right) for performing mid-length SLB scoring. (B) Sagittal T1 image (left) with a white line depicting the plan of image acquisition for the corresponding transverse image (right) for performing distal SLB scoring. FIGURE 2. Metacarpophalangeal standing low-field magnetic resonance imaging (MRI) and proximal sesamoid bone (PSB) volume-rendered three-dimensional (3D) computed tomography (CT) reconstruction images from the same study as described in Figure 1. (A) Sagittal T1 image (left) with a white line depicting the plan of image acquisition for the corresponding transverse T1 image (right) for performing proximal region PSB scoring. The white lines on the transverse image demonstrate divisional lines for scoring the dorsal abaxial, dorsal axial, palmar axial, and palmar abaxial regions of each bone. (B) Sagittal T1 image (left) with a white line depicting the plan of image acquisition for the corresponding transverse T1 image (right) for performing distal region PSB scoring. The white lines on the transverse image demonstrate divisional lines for scoring the dorsal abaxial, dorsal axial, palmar axial, and palmar abaxial regions of each bone. (C) Volume-rendered 3D CT images of a proximal sesamoid bone in dorsal, sagittal and transverse planes demonstrating division of the PSB into the eight scoring regions. FIGURE 3. Standing low-field magnetic resonance imaging (MRI) of suspensory ligament branch desmopathy from two horses enrolled in a study evaluating the relationship between proximal sesamoid bone disease and suspensory ligament branch desmopathy in racing Thoroughbred horses on standing low-field MRI exams. T1 (images A and D), T2 (images B and E), and short-tau inversion recovery (STIR; images C and F) sequences are shown, with medial on the right side of each image. The top row of images (A, B, C), all from the same study horse, demonstrate enlargement of the medial suspensory ligament branch (white arrow) on all sequences with T1, T2, and STIR hyperintensity within the central and axial portions of the ligament. The bottom row of images (D, E, F), all from a second study horse, demonstrate bilateral suspensory ligament branch disease, characterized by enlargement and reduced marginal distinction on the T1 sequence and hyperintensity on T1, T2, and STIR sequences. FIGURE 4. Suspensory Ligament Branch (SLB) Grade. Medial and lateral SLBs were independently graded (/34) by two specialists. Medial SLB had greater pathology, defined by a higher grade, in the right forelimbs. While lateral SLB had greater pathology, defined by a higher grade in the left forelimbs. Significance was defined as P<0.05 (*: P<0.05, **:P<0.01). FIGURE 5. Cross-sectional area (cm 2 ) measurements between observers. The board-certified radiologist (XX) reported larger cross sectional area measurement for all locations compared to the board-certified surgeon (XX). Significance was set at P<0.05 (**** is <0.0001). FIGURE 6. Sesamoid Grade. A grade was assigned to each sesamoid bone by two independent observers. No difference was seen in grade severity between the medial and lateral sesamoid bone. SUPPLEMENTAL FIGURE Supplementary Figure 1. Flow chart of study enrollment Supplementary Figure 2. Standing low-field magnetic resonance imaging (MRI) of proximal sesamoid osteopathy from two horses enrolled in a study evaluating the relationship between proximal sesamoid bone disease and suspensory ligament branch desmopathy in racing Thoroughbred horses on standing low-field MRI exams. T1 (images A and E), T2 (images B and F), short-tau inversion recovery (STIR; images C and G), and T2* (images D and H) are shown, with medial on the right side of each image. The top row of images (A, B, C, D), all from the same study horse, demonstrate prominent T1 and T2 hypointensity of bilateral proximal sesamoid bones (medial > lateral) without associated STIR hyperintensity and without T2* cancellation artifact, consistent with sclerosis. Palmar third metacarpal sclerosis is also present. In the bottom row of images (E, F, G, H), all from a second study horse, the lateral proximal sesamoid bone (white arrow) has multifocal (dorsal and palmar) prominent T1 hypointense regions. The dorsal region is hypointense on T2 and STIR, without T2* cancellation artifact, consistent with sclerosis; the palmar region is hyperintense on T2 and STIR sequences with T2* cancellation artifact, consistent with fluid-like signal (e.g. inflammation, necrosis, hemorrhage). Also on the bottom set of images, the medial proximal sesamoid bone is predominantly T1 hypointense with mixed regions of T2 hyperintensity (axial) and hypointensity, axial STIR hyperintensity, consistent with axial fluid-like signal (e.g. inflammation, necrosis, hemorrhage) and palmar sclerosis. Supplementary Material File (figure 1 slb planes_fixed contrast.tif) Download 8.58 MB File (figure 2 final_combined psb mri planes and 3d images (002).tif) Download 25.97 MB Information & Authors Information Version history V1 Version 1 31 December 2025 Copyright This work is licensed under a Non Exclusive No Reuse License. Authors Affiliations Kimberly V. Zullo 0009-0007-9222-1108 Cummings School of Veterinary Medicine at Tufts University View all articles by this author Aimee Colbath [email protected] Cornell University College of Veterinary Medicine View all articles by this author John H. Pigott Cornell University College of Veterinary Medicine View all articles by this author Amy B. Todd-Donato 0000-0002-2396-1694 Cornell University College of Veterinary Medicine View all articles by this author Metrics & Citations Metrics Article Usage 162 views 84 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Kimberly V. Zullo, Aimee Colbath, John H. Pigott, et al. Suspensory ligament branch desmopathy is associated with proximal sesamoid osteopathy in Thoroughbred racehorses on low-field magnetic resonance imaging. Authorea . 31 December 2025. DOI: https://doi.org/10.22541/au.176717210.09344788/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. For more information or tips please see 'Downloading to a citation manager' in the Help menu . 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