The effect of stress fracture occurring within the first 12 months of training on subsequent race performance in Thoroughbreds in Hong Kong.

Abstract

BACKGROUND: Racehorses are at an increased risk of stress fracture within the first 12&#xa0;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&#xa0;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&#xa0;days of import between 2006 and 2018 were collated. Cases and controls were matched on import date. Univariable conditional logistic regression compared signallment, 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&#xa0;=&#xa0;5180]). The humerus (42.0%; 95% CI 31.8%-52.6%; n&#xa0;=&#xa0;39) and tibia (28.0%; 95% CI 19.1%-38.2%; n&#xa0;=&#xa0;26) were the most common stress fracture sites. Cases missed a median of 63&#xa0;days (Interquartile range (IQR) 49-82) of training because of fracture. Within the 12&#xa0;months following diagnosis, case horses had a median of four (IQR 2-4, P&#xa0;<&#xa0;.0001) fewer race starts and were down HK$ 206&#xa0;188 (IQR HK$ 0-436&#xa0;800, P&#xa0;=&#xa0;.007) in race earnings compared to controls. Career length did not significantly differ between cases and controls (median 2&#xa0;years and 3&#xa0;months; IQR 15.3-39.1&#xa0;months; P&#xa0;=&#xa0;.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 1&#xa0;year of entering Hong Kong lost significant time in training, earnings and race starts. However, overall career length was unaffected.