Effect of exercise on bone density in distal regions of the equine third metacarpal bone in 2-year-old thoroughbreds.

The research examines whether high-intensity exercise can cause changes in bone density in young racehorses’ third metacarpal bones (McIII) and how this might be related to common fractures. It found that exercise does increase bone density, and the way it does so might lead to increased fracture risk.

Overview of the Study

• The study set out to investigate if and how high-intensity exercise might influence bone density in young racehorses, particularly in areas commonly associated with sagittal condylar fractures.
• Third metacarpal bones were studied from 12 thoroughbred fillies (young female horses), aged between 18.1 to 21.0 months.
• These were divided into an experimental group that underwent 19 weeks of high-intensity treadmill exercise at a 3% incline (Group E) and a control group that walked for 40 minutes each day (Group C).

Techniques Used

• Computed Tomography (CT) – to study overall clinical bone density of the right McIII.
• Digital radiography – to examine bone density at a 1:1 aspect, providing more focused, detailed measurements.
• Backscattered electron (BSE) imaging – to examine microscopic aspects of bone density, specifically how bone was deposited within the marrow space of cancellous (spongy) tissue.

Findings

• A clear pattern of regional variation in bone density was observed throughout the distal condyles (the rounded articular surfaces at the end of the bone) of the McIII.
• The exercised horses (Group E) had greater bone density than the control group (Group C), as observed from CT scans, radiographic density data, and bone volume fraction from BSE imaging.
• Evidence from BSE imaging also showed increased bone deposition within the marrow space in the cancellous tissue of the exercised horses compared to the control group, particularly in highly loaded aspects of the medial and lateral condyles.
• The newly formed, space-filling bone fraction had less mineralization than the original bone tissue, indicating that CT and radiography might underestimate the overall increased bone volume fraction in exercised horses.

Conclusions and Implications

• McIII distal condyle stress fractures are prevalent in Thoroughbred racehorses. This study proposes that excess stiffening of these condyles due to extensive new bone formation, as seen in the exercised fillies, might lead to strain concentration at the condylar grooves, causing cracking through the calcified cartilage and into the subchondral bone.
• Not only does this research help illuminate the biomechanical impact of race training on horse’s bone health, but it also suggests potential areas for medical and training protocols to consider in mitigating fracture risk in racing horses.