Strain as an Indicator for Stress Fractures in Racehorses

Abstract The horse racing industry, despite its financial significance, grapples with significant welfare issues, particularly stress fractures in racehorses due to rigorous training and competition. Third metacarpal (MC3) bone fractures, constituting about one-third of all limb fractures, severely impact horse health, performance, and welfare. The frequent occurrence of condylar fractures in the MC3, often resulting in bone comminution and complex fragmentation, poses high risks—sometimes necessitating euthanasia, and complicates the management and recovery processes for the rest. There is a lack of comprehensive methods for quantitatively determining fracture risks in horses, with strain measurements on the MC3 bone during racing conditions showing promise as an indicator of high loads and potential fracture risks. The primary long-term objective of this research is to develop implantable sensing technologies that can identify early indicators of potential failure, allowing for proactive management of horse health and minimizing fracture risks. This study investigated the mechanical properties of MC3 bones under high compressive loads using a fixture that secured the bone and mimicked natural conditions. Six MC3 bones from euthanized horses were instrumented with 350 Ω strain gauge rosettes at four key locations: Lateral Condyle (CondL), Medial Condyle (CondM), Dorsal DistoLateral (DisL), and Dorsal DistoMedial (DisM). Each bone underwent a preloading phase and five cycles of 12 kN sinusoidal loading to replicate galloping forces. Strain patterns were observed before and after introducing an artificial slot in the lateral condylar groove to simulate a fatigue crack. A strong correlation was found between load and minimum principal strain in both intact and fatigue crack-induced (FCI) bones, with R-squared values over 0.99. Post-FCI, 83% of bones exhibited an increase in the minimum principal strain-load response slope, indicating uniform changes in material behavior. Additionally, a rise in post-FCI y-intercept values across MC3 locations indicated higher initial strain and possible changes in bone properties.