Association between long periods without high-speed workouts and risk of complete humeral or pelvic fracture in thoroughbred racehorses: 54 cases (1991-1994).

OBJECTIVE: To determine whether a two-month or longer period without official high-speed workouts (lay-up) is associated with humeral or pelvic fracture in Thoroughbred racehorses.
DESIGN: Reprospective study. ANIMALS: Thoroughbred racehorses in California that were euthanatized because of a complete humeral or pelvic fracture. PROCEDURE: Age, sex, activity, number of lay-ups, number of days from a race or official timed workout to fracture, number of days from end of last lay-up to fracture, mean duration of lay-ups, and total number of days in race training were compared between horses with humeral fractures and horses with pelvic fractures. A case-crossover study was used to estimate relative risk for fracture of the humerus or pelvis occurring within hazard periods of 10 and 21 days following lay-up, compared with periods following more regular participation in official racing or timed workout events.
RESULTS: Horses with pelvic fractures were more often female, older, and had 0 or > or = 2 lay-ups. Horses with humeral fractures were typically 3-year-old males that had 1 lay-up. Horses with pelvic fractures had more total days in race training, fewer days from last exercise event to fracture, and a greater number of days from end of last lay-up to fracture than horses with humeral fractures. Return from lay-up was strongly associated with risk for humeral fracture during hazard periods of 10 and 21 days (relative risk = 71 and 45, respectively). CLINICAL IMPLICATIONS: Risk of humeral fracture may be reduced if horses are cautiously reintroduced into race training after lay-up.