Analysis of the subchondral microarchitecture of the distopalmar aspect of the third metacarpal bone in racing Thoroughbreds.

This study aims to determine the different characteristics of the microarchitecture of subchondral bone found in the lower rear part of the third metacarpal bones in racing Thoroughbred horses at different stages of subchondral bone disease. It found that both the orientation and density of the bone varied depending on the disease severity, and these changes could indicate potential origins of some fractures.

Research Design and Methods

• The researchers studied twelve pairs of third metacarpal bones collected from racing Thoroughbred horses that were euthanized for various reasons.
• The horses were grouped into two categories based on the level of their subchondral bone (SCB) disease: mild (bone hardening and localized radiolucencies) and severe (multiple radiolucencies and defects on joint surface).
• Sample blocks of subchondral bone plate and trabecular bone (TBB) were taken from the lower rear part of the metacarpal condyles and the sagittal ridge (the raised section along the middle) for examination.
• The samples were examined using three-dimensional micro-computed tomography scans at a 45-micrometer isotropic voxel resolution to reveal their internal structure.
• The researchers calculated the angle between the main direction of the trabeculae (the structural elements of the TBB) and the sagittal plane, as well as the degree of anisotropy (DA), a measure of the variation in physical properties with direction.

Results of the Study

• The study found that the condylar samples had a significantly lower angle (mean, 8.9°) than the samples from the sagittal ridge (mean, 40.7°).
• The trabecular bone had a considerably higher degree of anisotropy than the subchondral bone plate, indicating that its structure is more uneven.
• Trabecular bone from horses with mild SCB disease showed a higher degree of anisotropy than that from horses with severe disease, suggesting a relationship between disease severity and bone microarchitecture.

Conclusions and Implications

• The consistent appearance of the trabeculae within the condyles, as opposed to the sagittal ridge, suggests that the majority of joint loading (the forces exerted on a joint during movement) is carried by the condyles.
• The sudden changes in the structural patterns of the subchondral bone trabeculae at the grooves of the condyles potentially indicate the presence of hypothetical tensile forces. These forces may contribute to fractures in this area, hinting at a potential mechanism for the formation of certain types of fractures.