Three-dimensional analysis of patterns of skin displacement over the equine radius

Reasons for performing study: Surface markers are usually used to track bone movement. However, skin movement related to the bone has a large effect on the analysis of skeletal kinematics. A 2-dimensional (2D) skin displacement correction model has been successfully developed, but no 3D skin displacement model exists. Objectives: To develop a 3-dimensional (3D) skin displacement model for the equine radial segment during trot. Methods: The 3D trajectories of 6 skin-based markers and a bone-fixed triad were captured at trot in 4 horses. Skin displacements in the bone-based coordinate system were calculated using a singular-value decomposition method. The truncated Fourier series models were developed for the skin displacements using a generalised cross-validatory spline. Results: Mean +/- s.d. of peak skin displacement of the 3 markers on the proximal radius as percentage of radial length was 10.7 +/- 0.5, 4.6 +/- 1.5 and 14.5 +/- 2.9 % in x, y and z direction, respectively. For the 3 markers on the distal radius, the equivalent displacements were 4.7 +/- 0.6, 1.7 +/- 0.8 and 73 +/- 1.8% in x, y and z direction, respectively. Conclusions: The 3D skin displacement model for correction of skin marker motion over the equine radius relative to the bone can be established using a truncated Fourier series, which has previously been used successfully to develop 2D models. Potential relevance: This method of determining 3D skin displacement correction needs to be extended to the entire fore- and hindlimbs to provide a more sensitive measure of kinematic analysis. Accurate descriptions of the 3D motions of the limb segments and interactions between adjacent segments at the joints are necessary for understanding of the mechanics of different gaits and the gait aberrations that manifest as lameness.