Subchondral bone pathology occurs with a high prevalence in Thoroughbred racehorses presented for postmortem examination. The accumulation of subchondral bone damage with longer career duration is consistent with bone fatigue.

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Distal limb injuries are common in racing horses and track surface properties have been associated with injury risk. To better understand how track surfaces may contribute to equine limb injury, we developed the first 3D computational model of the equine hoof interacting with a racetrack and simulated interactions with model representations of 1) a dirt surface and 2) an all-weather synthetic track. First, a computational track model using the Smoothed Particle Hydrodynamics (SPH) method with a Drucker-Prager (D-P) elastoplastic material model was developed. It was validated against analytical models and published data and then calibrated using results of a custom track testing device applied to the two racetrack types. Second, a sensitivity analysis was performed to determine which model parameters contribute most significantly to the mechanical response of the track under impact-type loading. Third, the SPH track model was coupled to a biomechanical model of the horse forelimb and applied to hoof-track impact for a horse galloping on each track surface. We found that 1) the SPH track model was well validated and it could be calibrated to accurately represent impact loading of racetrack surfaces at two angles of impact; 2) the amount of harrowing applied to the track had the largest effect on impact loading, followed by elastic modulus and cohesion; 3) the model is able to accurately simulate hoof-ground interaction and enables study of the relationship between track surface parameters and the loading on horses’ distal forelimbs.

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Aims To evaluate the long‐term racing prognosis for Thoroughbred racehorses with displaced vs. nondisplaced fractures of the pelvis, identified scintigraphically. Methods Medical records of 31 Thoroughbred racehorses presenting to the University of Melbourne Equine Centre, that had fractures of the pelvis identified scintigraphically were reviewed. Pelvic fracture site was determined and defined as displaced or nondisplaced based on ultrasound and/or radiographic findings. Race records were analysed for each horse, with a minimum of 24 months follow‐up, and correlated to fracture type to determine long‐term prognosis for racing. Results are expressed as median and range. Results Fractures at a single site were more common (n = 22) than fractures involving 2 sites (n = 9) and the ilial wing was the most commonly affected (n = 12). Thoroughbred racehorses with displaced pelvic fractures at any site (n = 12) raced fewer times within 24 months of diagnosis than horses with nondisplaced fractures (n = 19) (0.5, 0–13, vs. 7, 0–24, P = 0.037) but there was no clear statistical difference in race earnings between the 2 groups (0, 0–123,250, vs. 14,440, 0–325,500, P = 0.080). Four horses with displaced fractures (33%) were subjected to euthanasia on humane grounds due to persistent severe pain. When these were excluded from the analysis there were no differences in performance variables between horses with displaced and nondisplaced fractures. Conclusions and practical significance Thoroughbred racehorses with displaced and nondisplaced pelvic fractures that survive the initial post injury period have a good prognosis for racing. Ethical animal research All owners consented to use of horse details within this retrospective analysis. Sources of funding: University of Melbourne Equine Centre. Conflicts of interest: None.

Fatigue-induced subchondral bone (SCB) injury is common in racehorses. Understanding how subchondral microstructure and microdamage influence mechanical properties is important for developing injury prevention strategies. Mechanical properties of the disto-palmar third metacarpal condyle (MCIII) correlate poorly with microstructure, and it is unknown whether the properties of other sites within the metacarpophalangeal (fetlock) joint are similarly complex. We aimed to investigate the mechanical and structural properties of equine SCB from specimens with minimal evidence of macroscopic disease. Three sites within the metacarpophalangeal joint were examined: the disto-palmar MCIII, disto-dorsal MCIII, and proximal sesamoid bone. Two regions of interest within the SCB were compared, a 2 mm superficial and an underlying 2 mm deep layer. Cartilage-bone specimens underwent micro-computed tomography, then cyclic compression for 100 cycles at 2 Hz. Disto-dorsal MCIII specimens were loaded to 30 MPa (n = 10), while disto-palmar MCIII (n = 10) and proximal sesamoid (n = 10) specimens were loaded to 40 MPa. Digital image correlation determined local strains. Specimens were stained with lead-uranyl acetate for volumetric microdamage quantification. The dorsal MCIII SCB had lower bone volume fraction (BVTV), bone mineral density (BMD), and stiffness compared to the palmar MCIII and sesamoid bone (p < 0.05). Superficial SCB had higher BVTV and lower BMD than deeper SCB (p < 0.05), except at the palmar MCIII site where there was no difference in BVTV between depths (p = 0.419). At all sites, the deep bone was stiffer (p < 0.001), although the superficial to deep gradient was smaller in the dorsal MCIII. Hysteresis (energy loss) was greater superficially in palmar MCIII and sesamoid (p < 0.001), but not dorsal MCIII specimens (p = 0.118). The stiffness increased with cyclic loading in total cartilage-bone specimens (p < 0.001), but not in superficial and deep layers of the bone, whereas hysteresis decreased with the cycle for all sites and layers (p < 0.001). Superficial equine SCB is uniformly less stiff than deeper bone despite non-uniform differences in bone density and damage levels. The more compliant superficial layer has an important role in energy dissipation, but whether this is a specific adaptation or a result of microdamage accumulation is not clear.