The relationship between microstructure, stiffness and compressive fatigue life of equine subchondral bone.

This research examines the link between the microstructure of subchondral bone (bone just underneath the cartilage in a joint), its stiffness, and its ability to withstand repetitive stress or ‘fatigue’ in racehorses. The researchers found that denser, stiffer subchondral bone is better at resisting fatigue, which has implications for equine joint health and the development of osteoarthritis.

Study Design

• The research used racehorses as a model to explore the role of subchondral bone in joint disease. This choice is motivated by the fact that racehorses regularly experience subchondral bone injuries due to the intense strain of high-speed racing and training.
• The investigators examined the correlation of horse and racing-related factors, along with the micromorphology of the three different layers of subchondral bone to the initial stiffness and compressive fatigue life of bone plugs.
• They also evaluated three different ways to define fatigue failure of the subchondral bone during compressive fatigue testing.

Key Findings

• The research discovered a direct association between the compressive fatigue life of subchondral bone and bone volume fraction in the deeper layers of the bone, its maximal stiffness, and the number of cycles it took to achieve this maximal stiffness.
• Higher initial stiffness was linked with increased tissue mineral density in the deeper layers of the bone and a higher bone volume fraction in the superficial layer.
• Much of the samples presenting a fatigue life of less than 5,500 cycles fractured visibly before hitting a 30% decrease in stiffness.
• The cycles to a 10% decrease in maximal stiffness correlated strongly with the cycles to the lowest recorded stiffness upon visible fracture. This makes it a valid alternative measure of fatigue failure in compressive fatigue testing of subchondral bone.

Implications and Future Research

• The results suggest that subchondral bone that has increased density (known as sclerosis) due to high-speed exercise and measured as bone volume fraction tends to have a longer fatigue life. This suggests a protective role in safeguarding the subchondral bone from stress and strain.
• These findings challenge the commonly accepted notion that fatigue damage is often located in the sclerotic subchondral bone of racehorses. More research is needed to harmonize these seemingly contradictory observations.