A catastrophic fracture of the radial carpal bone experienced by a racehorse during a Palio race was analyzed. Computational modelling of the carpal joint at the point of failure informed by live data was generated using a multibody code for dynamics simulation. The circuit design in a turn, the speed of the animal and the surface characteristics were considered in the model. A macroscopic examination of the cartilage, micro-CT and histology were performed on the radio-carpal joint of the limb that sustained the fracture. The model predicted the points of contact forces generated at the level of the radio-carpal joint where the fracture occurred. Articular surfaces of the distal radius, together with the proximal articular surface of small carpal bones, exhibited diffuse wear lines, erosions of the articular cartilage and subchondral bone exposure. Even though the data in this study originated from a single fracture and further work will be required to validate this approach, this study highlights the potential correlation between elevated impact forces generated at the level of contact surfaces of the carpal joint during a turn and cartilage breakdown in the absence of pre-existing pathology. Computer modelling resulted in a useful tool to inversely calculate internal forces generated during specific conditions that cannot be reproduced in-vivo because of ethical concerns.

Background: City circuit competitions (Palio) in Italy are traditional horse race held in the heart of some cities centre once a year. The prevalence of accidents during these competitions is only anecdotally reported; however there is a diffuse perception that these events adversely affect racehorse industry and impact on equine welfare. The aim of the study is to understand the basic physiopathology of catastrophic fracture of the carpus in a 7-y old thoroughbred euthanized following catastrophic fracture of the left carpus during a traditional horse race, comparing the localization and the magnitude of the contact forces at the level of the affected joint, obtained in simulated conditions with the macro- and microscopic structural damages.Methods: A retrospective analysis of a thoroughbred racehorse galloping at high speeds on an urban racetrack conditions was set up. Computational modelling of the carpal joint was generated using a multibody code for dynamics simulation, considering the circuit design, the speed of the animal, and the surface characteristic. The results were compared to the findings observed during computed tomography, micro-computed tomography and histological evaluation following hematoxylin and eosin and safranin-O fast green staining.Results: The articular surfaces of the radial, intermediate and ulnar facet of the radius together with the proximal articular surface of small carpal bones exhibited diffuse wear lines, erosions of the articular cartilage and subchondral bone exposure. The fracture line along the radial carpal bone initiating the injury does not travel in a region with bone sclerosis. In the computational model, the peak force for the contact between the radius and the radial carpal bone has a value of 6880 N. Conclusions: This study highlight how during traditional racing circuits it develops elevate impact forces at the level of contact surfaces of the carpal joint, due to effect of speed and curve, causing catastrophic bone fracture and hyaline cartilage breakdown in the absence of pre-existing pathology.