Forces loading the tarsal joint in the hind limb of the horse, determined from in vivo strain measurements of the third metatarsal bone.

This research investigates the distribution of forces in the hind limb tarsal joint of a horse, using live strain measurements of the third metatarsal bone in six ponies.

Methodology

• The study used strain gauge rosettes, which were secured onto four separate locations on the third metatarsal bone in the hind limbs of 6 ponies. These locations included the dorsal (back), lateral (side), medial (middle), and plantar (bottom) aspects of the bone.

Findings

• The maximum compressive strain was about -600 X 10(-6) m/m, which was larger than the tensile strains at all locations on the bone. This indicates that the bone is subjected to greater forces in compression as opposed to tension.
• The first bending strain peak was higher in the dorsal and lateral aspects, while the second peak was larger in medial and plantar aspects. This suggests that there could be more bending forces acting on specific areas of the bone depending on the phase of the stride.
• The Young modulus of elasticity, which measures stiffiness of an elastic material, averaged 19.5 GPa in both tensile and compressive loads. This was calculated through a 4-point bending test on the dorsal and plantar sides of the bone.

Conclusions

• Applying linear bending theory, the researchers calculated the eccentricity of an axial force parallel or a perpendicular bending force on the bone. These forces follow an eccentric path, meaning they do not pass through the direct center of the bone’s cross section.
• The total force mainly entered the joint surface of the tarsometatarsal joint, which indicates that the joint is primarily loaded in eccentric compression. This means there are high forces acting on the joint but they are not evenly distributed, potentially leading to bone or joint stress over time.