The Effect of Stress Fracture Occurring Within the First 12 Months of Training on Subsequent Race Performance in Thoroughbreds in Hong Kong

BACKGROUND Racehorses are at an increased risk of stress fracture within the first 12 months of racing and when resuming training after a break. Research in these high-risk periods and on the effect of performance post-recovery is limited. OBJECTIVES To describe the occurrence of stress fractures, diagnosed by nuclear scintigraphy (NS), in racehorses' first 12 months training in Hong Kong, and their impact on racing performance and career length. STUDY DESIGN Retrospective 1:2 matched case-control study. METHODS Clinical records of horses with NS-diagnosed stress fractures within 365 days of import between 2006 and 2018 were collated. Cases and controls were matched on import date. Univariable conditional logistic regression compared signalment, pre-fracture training and post-recovery racing performance between cases and matched controls. Shared Frailty Cox regression analysed time from import to fracture and total career length. RESULTS Eighty-seven horses sustained a NS-diagnosed fracture within their first year in Hong Kong (incidence risk 1.7% (95% confidence interval (CI) 1.4-2.1%; N=5,180)). The humerus (42.0%; 95% CI 31.8-52.6%; n=39) and tibia (28.0%; 95% CI 19.1-38.2%; n=26) were most common stress fracture sites. Cases missed a median of 63 days (Interquartile range (IQR) 49-82) of training because of fracture. Within the 12 months following diagnosis, case horses had a median of four (IQR 2-4, p<0.0001) fewer race starts and were down HK$206,188 (IQR HK$0-436,800, p=0.007) in race earnings compared to controls. Career length did not significantly differ between cases and controls (median 2 years and 3 months; IQR 15.3-39.1 months; p=0.2). MAIN LIMITATIONS Only stress fractures diagnosed by NS were included, hence, the study is not representative of all stress fractures occurring in racehorses in Hong Kong. CONCLUSIONS Racehorses sustaining a stress fracture within one year of entering Hong Kong lost significant time in training, earnings and race starts. However, overall career length was unaffected.