How does bone strain vary between the third metacarpal and the proximal phalangeal bones of the equine distal limb?

The research article investigates the difference and pattern of strain, or force-induced deformation, in injury-prone sites of two types of bones in a horse’s lower limb: the third metacarpal (MC3) and the proximal phalangeal (P1) bones.

Research Methodology

• To study the force strain dynamics, the researchers collected and used limbs from nine equine cadavers (dead horses).
• The limbs were rigged with rosette strain gauges, instruments specifically designed to measure strain, and put under a load of up to 10,500N (Newtons).
• The gauges were attached at seven specific dorsal sites: middle and distal (lower) points of the MC3 (with subsections for medial, middle, and lateral), and the upper part of P1 (also with medial, middle, and lateral).
• The outcomes were then analyzed using a technique called repeated measures analysis of variance, a statistical method for comparing means in a data set to assess if any significant differences exist.

Findings

• The results showed that distal MC3 experienced the greatest strain, while proximal P1 underwent the smallest magnitude of force-induced deformation.
• The directions of maximum strain were found to be similar at the middle and lower parts of MC3, but differed noticeably by 20-40° compared to P1.
• The magnitude and direction of strain remained similar from medial-to-lateral (across each bone) on the lower MC3, but varied in pattern and magnitude amongst different sites of P1.
• As the load reached its maximum, the orientation of minimum principal strain (the smallest strain in any direction) became more axial (aligned along the axis) in both types of bones. The strains in the MC3 and P1 converged from opposite directions, thereby potentially enhancing the stability of the lower limb under maximum load.
• The researchers observed significant differences in strain parameters and the strain ratio for adjacent anatomic sites on lower MC3 and upper P1, a finding which was not anticipated due to the expected anatomic congruity of the metacarpophalangeal joint, the joint connecting these bones.
• High shear strain (perpendicular force) across upper P1 points towards shear forces as the primary biomechanical contributor to sagittal fractures of P1, a common type of bone fracture in horses.

Conclusion

• This research provides crucial insights into strain patterns in the equine distal limb and can enhance understanding of how injuries occur.
• The variations in different types of strain underloading could help improve preventive measures and treatments for injuries in horses, particularly around the MC3 and P1 bones in the lower limb.