34th Annual Scientific Meeting proceedings

Introduction
Most fractures that occur in Thoroughbred horses while training and racing on the flat are a manifestation of material fatigue and can be considered “Stress Fractures” (Stover et al, 1993; Riggs, 2002). These fractures arise in specific anatomical locations (figure 1), exhibit consistent configurations and arise in the absence of obvious external trauma. Stress fractures are rarely recognised in horses that do not race.

Figure 1: Common locations of stress fractures in racehorses

The incidence of all fractures among Thoroughbred racehorses racing in Hong Kong was 2.2 per 1000 race starts (Sun et al, 2019). Most of these injuries involve chip fractures of the dorsal margins of the bones forming the carpus and metacarpophalangeal joint and incomplete fractures of the ilium, humerus, tibia and distal third metacarpal bone. Chip fractures and incomplete fractures of the distal condyles of the third metacarpal bone are treated surgically, by arthroscopic surgery, with or without osteosynthesis, while incomplete fractures of bones of the proximal limb are treated conservatively with stable rest followed by a staged return to work over three to four months. The prognosis for return to racing for stress fractures of the pelvis and proximal limb bones is good (McGlinchey et al, 2017). Conversely, complete fractures of these bones are often associated with a fatal outcome; over 80% of incidents in racing that result in the death of the horse are due to a fracture. This emphasises the importance of early, accurate diagnosis of stress fractures in horses.

Severe fractures, which lead to death of the horse, occur on average one in every one to two thousand race starters (Wright et al, 2024). Falls associated with fractures are a leading cause of injuries to jockeys (Hitchens et al, 2013). In addition to the obvious implications for horse and human welfare these incidents, which are highly visible to the public, undermine society’s acceptance of racing as a sport. Consequently, there has been a sustained research effort to understand the reasons behind, pathogenesis of and ways to prevent stress fractures in racehorses.

Pathogenesis and Epidemiology
Stress fractures in racehorses can be differentiated into two broad groups, which reflect variation in the underlying pathophysiological process: (i) those that develop soon after a horse enters training or returns to training following a rest period, and (ii) those that occur following a sustained period of work without rest (Hitchens et al, 2019). In both scenarios, fracture results from accumulated bone damage. In the first, the damage accumulates rapidly when the horse is exposed to loads for which the bone has not yet adapted, whilst the latter is the result of prolonged, high intensity exercise exceeding the capacity of adapted bone without opportunity for bone repair. Recent work elegantly demonstrates subtle variations in the pathogenesis of one of the most common racing fractures (parasagittal fractures of the distal condyles of the third metacarpal or metatarsal bone), which can be linked to these different scenarios (Bergstrom et al, 2024).

Prevention
Preventing severe fractures that arise secondary to fatigue damage is important to the racing industry. While some stress fractures are associated with severe lameness, this is often transitory in nature and many cases with focal bone lesions caused by fatigue show subtle if any signs. Pre-race clinical examination is largely ineffective at identifying animals with pathology that places them at high risk of fracture (Stewart & Lam, 2013). There has been significant investment in applied clinical research to develop tools with which to screen horses for the presence of fatigue pathology of bone. These include epidemiological modelling, use of molecular markers in blood or urine, subtle changes in the horse’s gait, identified through wearable technology, and diagnostic imaging, especially cross-sectional modalities (Colgate et al, 2025).

An alternative approach to decrease the incidence of these injuries is to modify the pattern of training and racing horses to keep the rate of accumulation of fatigue below the threshold likely to cause fracture (Wong et al, 2023). Use of training programmes that stimulate a bone adaptive response suited to racing speeds and which can attain and maintain racing fitness with minimal cycles of load are likely to be key to this process.

References

1. Bergstrom TC, Spriet M, Carpenter RS, Jacques KL and Stover SM. (2024) Condylar fracture location is correlated to exercise history in Thoroughbred racehorses. Equine Vet J. https://doi.org/10.1111/evj.14091
2. Colgate VA, Riggs CM, FRAT Group II (2025) IFHA Global Summit of Equine Safety and Technology: Fracture prediction and prevention. Equine Vet J., https://doi.org/10.1111/evj.14458
3. Hitchens PL,  Hill AE and Stover SM (2013) Jockey Falls, Injuries, and Fatalities Associated With Thoroughbred and Quarter Horse Racing in California, 2007-2011 Orthop J Sports Med 1 https://doi.org/10.1177/2325967113492625
4. Hitchens PL, Morrice-West AV, Stevenson MA, Whitton RC. (2019) Meta-analysis of risk factors for racehorse catastrophic musculoskeletal injury in flat racing. Vet J.; 245: 29-40  https://doi.org/10.1016/j.tvjl.2018.11.014
5. Lee S, Baker ME, Clinton M and Taylor SE. (2021) Use of omics data in fracture prediction; a scoping and systematic review in horses and humans. Animals (Basel). 11(4):959. https://doi.org/10.3390/ani11040959
6. McGlinchey L., Hurley M.J., Riggs C.M. and Rosanowski S.M. (2017) Description of the incidence, clinical presentation and outcome of proximal limb and pelvic fracture in Hong Kong racehorses (2003-2014). Equine Vet J. 49, 789-794.
7. Riggs CM. (2002) Fractures- A preventable hazard of racing thoroughbreds? Vet J. 163(1):19-29. https://doi.org/10.1053/tvjl.2001.0610
8. Stewart BD, Lam KH. (2012) Racing injury risk assessment at official pre-race veterinary inspections in Hong Kong Jockey Club. Proc International Conference of Racing Analysts and Veterinarians, New Zealand. 18:296-297
9. Stover SM, Ardans AA, Read DH, Johnson BJ, Barr BC, Daft BM, Kinde H, Anderson ML, Woods LW, Moore J, Stoltz J, Pool RR. (1993) Patterns of stress fractures associated with complete bone fractures in racehorses. Am Assoc Equine Prac.; 39: 131-132
10. Sun TC, Riggs CM, Cogger N, Wright J, Al-Alawneh JI. (2019) Noncatastrophic and catastrophic fractures in racing Thoroughbreds at the Hong Kong Jockey Club. Equine Vet J.;51: 77-82
11. Wright I, Minshall G, Young N and Riggs C. (2024) Fractures in Thoroughbred racing and the potential for pre-race identification of horses at risk. Equine Vet J. 56(3):424-436. https://doi.org/10.1111/evj.14046
12. Wong, A. S., Morrice-West, A. V., Hitchens, P. L., & Whitton, R. C. (2023). The association between Thoroughbred racehorse training practices and musculoskeletal injuries in Victoria, Australia. Front Vet Sci 10:1260554. https://doi:10.3389/fvets.2023.1260554.