Identification of a previously unreported site of subchondral bone injury in the dorsodistolateral calcaneus in Thoroughbred racehorses.

Background: Increasing accessibility and advancements in computed tomographic (CT) imaging improve understanding of the contributors to poor performance in the Thoroughbred racehorse.
Objectives: To characterise an unreported site of tarsal subchondral bone injury (SBI) in Thoroughbred racehorses.
Study Design: Retrospective, cross-sectional analytical study.
Methods: Tarsal CT scans of 108 horses were reviewed for evidence of SBI in the dorsodistolateral calcaneus (DDLC). Signalment, including age, breed, sex, and discipline of horses, was recorded. The association of DDLC SBI with Thoroughbred racehorse compared to other breeds/disciplines was calculated. Nuclear scintigraphic scans of the hindlimbs of Thoroughbreds between 2007 and 2022 were also reviewed for increased radiopharmaceutical uptake (IRU) suggestive of DDLC SBI.
Results: Tarsal CT scans of 108 horses were analysed and lesion location and type were recorded. DDLC SBI was identified in 8/108 (7.3%) horses. All lesions were found in racing Thoroughbreds (Fischer's exact test p = 0.002) and in 20% (8/40) of racing Thoroughbreds that underwent tarsal CT. DDLC SBI was determined to be the primary cause of lameness in 3/8. Third tarsal bone fracture was considered the primary cause of lameness in 3/8 horses. A total of 1663 nuclear scintigraphic scans of 1603 Thoroughbred horses were reviewed. Increased IRU in the DDLC was present in 13 horses (0.9%).
Main Limitations: Study limitations include the retrospective nature of the study, incomplete medical records, lack of sensitivity/specificity data for lesion identification on various imaging modalities or comparison to histopathology, and limited substantive follow-up information.
Conclusions: The DDLC is a previously unreported site of SBI in the Thoroughbred racehorse. The lesion can be the primary source of lameness but may also be found in horses with other clinically relevant osseous injuries. This lesion may be underestimated in the Thoroughbred population. Further investigation is required to determine the true prevalence, clinical relevance, and prognosis in Thoroughbred racehorses with this injury.
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