178 AJVR • Vol 80 • No. 2 • February 2019 T he metacarpophalangeal joint is the most com- mon site of musculoskeletal disease that leads to reduced performance, premature retirement, cata- strophic failure, and euthanasia of Thoroughbred racehorses throughout the world. 1–5 Condylar frac- tures of the MC3 are the most common reason for Association of catastrophic condylar fracture with bony changes of the third metacarpal bone identified by use of standing magnetic resonance imaging in forelimbs from cadavers of Thoroughbred racehorses in the United States John G. Peloso DVM, MS Noah D. Cohen VMD, MPH, PhD James B. Vogler MD Patricia A. Marquis DVM Lynn Hilt Received March 1, 2018. Accepted June 11, 2018. From the Equine Medical Center of Ocala, 7107 W Hwy 326, Ocala, FL 34482 (Peloso, Hilt); the Depart- ment of Large Animal Clinical Sciences, College of Vet- erinary Medicine and Biomedical Sciences, Texas A&M University, College Station, TX 77845 (Cohen); The Orthopaedic Institute, 4500 Newberry Rd, Gaines- ville, FL 32607 (Vogler); and Gulfstream Racecourse, 901 S Federal Hwy, Hallandale Beach, FL 33009 (Marquis). Address correspondence to Dr. Peloso (jpeloso@ emcocala.com). OBJECTIVE To compare bony changes of the third metacarpal bone (MC3) of Thorough- bred racehorse cadavers with (cases) or without (controls) catastrophic con- dylar fracture by use of standing MRI. SAMPLE 140 forelimbs from 26 case horses (both forelimbs) and 88 control horses (single forelimb). PROCEDURES Bone marrow lesions (BMLs), identified as a decrease in T1-weighted (T1W) signal and increases in T2*-weighted (T2*W) and short tau inver- sion recovery (STIR) signals, and dense bone volume percentage (DBVP), identified as decreases in T1W, T2*W, and STIR signals, in the distopalmar aspect of MC3 were recorded. Logistic regression was used to compare fractured and nonfractured limbs of cases and fractured limbs of cases with randomly selected limbs of controls. RESULTS Among cases, fractured limbs were significantly more likely to have BMLs (26/26 [100%]) than were nonfractured limbs (7/26 [27%]). Fractured limbs of cases were significantly more likely to have BMLs (26/26 [100%]) than were limbs of controls (6/88 [7%]). Among cases, there was no significant difference in DBVP between fractured and nonfractured limbs in lateral (26% vs 21%, respectively) or medial (25% vs 20%, respectively) condyles. However, DBVP was significantly greater in fractured limbs of cases than in limbs of controls for lateral (26% vs 16%, respectively) and medial (25% vs 18%, respectively) condyles. CONCLUSIONS AND CLINICAL RELEVANCE Standing MRI revealed a significantly greater degree of bone change in race- horses with condylar fracture when comparing fractured and nonfractured limbs of case horses and fractured limbs of case horses with randomly se- lected limbs of control horses. (Am J Vet Res 2019;80:178–188) euthanasia of Thoroughbred racehorses in the United Kingdom 3 and the second most common reason for euthanasia in North America, after biaxial fracture of the proximal sesamoid bones. 2,4 A review of jockey injuries and fatalities associ- ated with Thoroughbred racing identified that 54% of horse falls injure a human. 6 The 2017 fatal injury rate for horses in the United States was 1.61 horses/1,000 starts. a If 10 horses/race were to compete in 10 races/d, there would be 1,000 starts in 10 days and, on average, 2 horses would be euthanized and 1 human injured every 10 days. This hypothetical example underscores the ur- gent need to develop additional surveillance strategies to prevent fractures in racehorses. Bone is a solid material that can change its internal architecture when stress is imposed (Wolff law). 5,7–29 By adapting to increased loads through remodeling, an increase in bone mineral density enables greater tol- ABBREVIATIONS BML Bone marrow lesion BSI Bone stress injury CI Confidence interval DBVP Dense bone volume percentage MC3 Third metacarpal bone MT3 Third metatarsal bone POD Palmar osteochondral disease SCB Subchondral bone sMRI Standing MRI STIR Short tau inversion recovery T1W T1-weighted T2*W T2*-weighted Unauthenticated | Downloaded 10/06/22 08:33 AM UTC