A longitudinal study of fractures in 1488 Thoroughbred racehorses receiving intrasynovial medication: 2006-2011.

BACKGROUND: There is a need for investigation of associations between intrasynovial medication and fracture risk in racehorses. OBJECTIVES: To document the proportion and type of fracture occurring following intrasynovial medication. STUDY DESIGN: Retrospective longitudinal study. METHODS: Review of clinical records from all Thoroughbred flat racehorses receiving intrasynovial medication under the care of a first-opinion veterinary practice between 2006 and 2011. Fractures pre- and post-medication were categorised by location, type, and severity; analysis of fractures sustained within 56-days of medication was undertaken. Survival analyses using Cox proportional hazards models were conducted for associations between medication used (corticosteroid vs. non-corticosteroid), age at first medication, sex and total number of medications and fracture. RESULTS: A total of 1488 horses received intrasynovial medication during the period of study; 8692 synovial spaces were medicated in 3925 sessions. There were 96 fractures that occurred within 56-days of medication, of which 44 were classified as 'serious' (44/1488; 3.0% total population, 95% CI 2.1, 3.8%) and 11 (11/1488, 0.7%, 95% CI 0.3, 1.2%) were euthanased due to severity of injury. Fifty-four of 96 cases (56.4%, 95% CI 46.3, 66.2%) returned to racing. Targeted imaging of the subsequent injury site was undertaken prior to injury in only 7 (7/96, 7.3%) horses that injured post-medication. Horses that had received ≥3 previous intrasynovial medication sessions had an increased hazard of sustaining a fracture within 56 days (HR 2.31, 95% CI 1.51, 3.54, P<0.001) compared with those receiving ≤3 medication sessions, adjusted for each increasing year of age (HR 0.67, 95% CI 0.53, 0.86, P = 0.001). MAIN LIMITATIONS: Absence of an unexposed cohort of nonmedicated racehorses. CONCLUSIONS: Serious musculoskeletal injury following intrasynovial medication occurred in 3% (44/1488) horses. Reduction in injury rates may be possible through greater use of premedication diagnostic imaging.