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 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 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 = 5180]). 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 the 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 < .0001) fewer race starts and were down HK$ 206 188 (IQR HK$ 0‐436 800, P = .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 = .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 year of entering Hong Kong lost significant time in training, earnings and race starts. However, overall career length was unaffected.

A review of equine tibial fractures

“…When compared with matched control horses, California racehorses with tibial fractures had fewer official timed workouts, logged fewer active days, and accumulated less overall distance in workouts and events 8 . These findings were supported by analysis of records maintained by the Hong Kong Jockey Club, and a significant association between number of rest or layup days and incidence of tibial stress fracture was identified 7,12 . Another widely accepted risk factor for complete tibial fracture in a racing population is previous injury in the form of tibia stress remodelling.…”

“…A similar age distribution was noted in a retrospective analysis of tibial fracture cases diagnosed at Randwick Equine Centre between 1992 and 2000, with 71% of tibial fracture cases occurring in 2‐year‐old racehorses. This association remains true in recent epidemiological analyses based on racehorse data from the Hong Kong Jockey Club 7,9,12,34 …”

“…8 This represents a fracture rate of 0.106% for complete tibial in the humerus (42.0%) and tibia (26.9%) most often. 12…”

“…Of the large-scale retrospective epidemiologic studies, only Donati et al found a significant association between female gender and trauma-related tibial fracture, and this was outside of a racing population. 7,9,12,16 Two risk factors of racing surface and trainer effect have been proposed but are extensively affected by confounders such as geographic region, racing regulations, reporting discrepancies, socioeconomic status and other factors. In an analysis of racing surface and stress fracture incidence, MacKinnon et al noted that hindlimb stress fractures were more commonly diagnosed after a switch from dirt to synthetic track surface.…”

“…This association remains true in recent epidemiological analyses based on racehorse data from the Hong Kong Jockey Club. 7,9,12,34 Analysis of risk factors is important to development of prevention strategies to minimise or eliminate these injuries from occurring. A striking case report of bilateral tibial stress fracture in a racing Quarter Horse poses the question of association between race training and supra-physiological stresses on the equine tibia.…”