Stress fracture of the palmar, distal cortex of the third metacarpal bone: A diagnostic challenge with a good prognosis.

BACKGROUND: Transverse stress fracture of the palmar cortex of the distal aspect of the third metacarpal bone (TSF PCD McIII) is poorly documented. OBJECTIVES: To describe the typical signalment of this injury, the common clinical and radiological signs and prognosis. STUDY DESIGN: A retrospective hospital-based case series with follow-up of racing records. METHODS: All cases of TSF PCD McIII diagnosed among racehorses in training at the Hong Kong Jockey Club between 2011 and 2019 were identified. Clinical records were reviewed and documented signs were recorded for each case. Findings from a predefined list of radiological features were listed. Descriptive statistics were used to describe the age and training profile of horses, the clinical and radiological findings and time to return to ridden exercise. RESULTS: Twenty-three cases (n = 23) were identified. The majority (57%) had recently started or returned to training from a break and 35% (n = 8) of cases had not yet undertaken timed gallops at the time of injury. Most cases (20/23; 87%) were lame; in nine (39%), lameness was severe. Localised swelling of superficial soft tissues was reported in 15/23 (65%) cases. Pain on palpation of the distal McIII was present in 15/23 (65%) cases and in response to fetlock flexion in 12/23 (52%). Diffuse, localised increase in radiopacity, disruption to the outline of the bone periosteal surface and outward displacement of the button of the splint bone were common radiological features in early cases (70%, 67% and 67% of all cases). Sixty-one per cent of cases (14/23) were initially misdiagnosed. Most horses (n = 18/23) resumed training and racing after a median of 83 and 246 days. MAIN LIMITATIONS: Clinical notes were not consistent between cases. Radiographs were taken at different times. CONCLUSIONS: Horses commencing training are at risk of TSF PCD McIII, which presents with confusing clinical signs and subtle radiological findings. The long-term outlook is favourable.

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 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.

Using time-series analysis techniques to enhance the understanding of musculoskeletal injury in Thoroughbred racehorses.

BACKGROUND: Many racing jurisdictions monitor race-day musculoskeletal injury (MSI) but fail to evaluate injuries occurring during training. Additionally, previous risk factor analyses have failed to explore temporal trends in injury occurrence. OBJECTIVES: To use time-series analysis techniques to identify trends, cyclicity and peaks in MSI incidence, in racehorses training and racing at the Hong Kong Jockey Club (HKJC) from July 2010 to June 2018. STUDY DESIGN: Retrospective longitudinal study. METHODS: The monthly incidence of all MSI, superficial digital flexor tendon (SDFT) injury, suspensory ligament injury and appendicular skeletal fracture occurring in training and racing were collected from veterinary records. The number of horses in training was collated monthly from trainer records. Time-series analysis techniques were used to describe trends and cyclical patterns for injury types. For each injury, incidence risks above the 90th percentile were identified as peaks in incidence. RESULTS: A total of 1471 injuries were recorded over eight racing seasons; 605 fractures (41.1%), 550 SDFT injuries (37.4%), and 316 suspensory ligament injuries (21.5%). Evidence of seasonality was detected in fracture incidence risk; increasing from October (median 0.25 per 1000 horses) until May (median 0.71 per 1000 horses), coinciding with the racing season (ending mid-July). Elevated incidence of MSI occurred throughout 2012; however, the greatest incidence risks of SDFT (14.8 per 1000 horses) and fracture (1.3 per 1000 horses) occurred since 2017. MAIN LIMITATIONS: Monthly (opposed to daily) incidence risk of injury reduced the resolution of the data. Additionally, fracture was not described according to bone or fracture type, which may have confounded overall trends. CONCLUSIONS: Evidence for seasonal variation in the incidence of fracture occurrence has been demonstrated. Based on using time-series techniques, further epidemiological studies, retrospectively targeting periods of high peaks in injury incidence risk could be used to aid identification of risk factors for injury.