Impact of ununited plantar eminence of the first phalanx on early racing performance in North American Standardbreds

OBJECTIVE: To determine the prevalence of ununited plantar eminence (UPE) of the first phalanx (P1) in North American Standardbreds and evaluate its association with early racing performance. STUDY DESIGN: Retrospective cohort study with matched sibling controls. SAMPLE POPULATION: Imaging records from 1897 yearling Standardbred horses. METHODS: Screening images were reviewed for the presence of UPE and other orthopedic abnormalities. Sex, age, sire, gait, and racing performance data were obtained from the United States Trotting Association. Affected horses were compared with unaffected siblings using non-parametric tests for performance outcomes. Within affected horses, regression analyses were used to assess the influence of sex, gait, and concurrent developmental orthopedic disease (DOD) lesions on performance. RESULTS: UPE was identified in 74 of 1898 horses (3.9%). Most affected horses (67.6%) had concurrent DOD lesions, most commonly plantar osteochondral fragments. At two years of age, there were no differences in performance outcomes between groups. As 3-year-olds, affected horses were less likely to start, and had fewer starts and lower earnings than unaffected siblings. CONCLUSION: UPE of P1 was an uncommon finding frequently associated with other DOD lesions. While 2-year-old racing performance was similar between groups, affected horses showed reduced performance during the three-year-old season. CLINICAL SIGNIFICANCE: UPE could have meaningful implications for long-term athleticism in Standardbred racehorses, given the reduced racing performance at 3 years of age in the affected study cohort. However, as UPE is frequently found in combination with other DOD lesions, the clinical relevance of UPE as an isolated lesion is not clear.