Evaluation of skin displacement in the equine neck

Abstract

Kinematic studies, using reflective skin markers, are commonly used to investigate equine joint motion in equitation science and for rehabilitation purposes. In order to interpret the registrations accurately, the degree of skin displacement has been described for the limbs and back, but not yet for the neck. The aim of the present study was to measure sagittal plane skin displacement in the equine neck. Radiopaque skin markers were applied to the skin over the first six cervical vertebrae of six healthy horses. Latero-lateral radiographs were taken in three standardised neck positions in the sagittal plane: control (horizontal neck), ‘on the bit’ and ‘nose to carpus’. The scales of the images were normalised and calculation of skin displacement was done by use of a coordinate system, dividing the displacement along an x-axis parallel to the vertebra's longitudinal axis and a y-axis perpendicular to the x-axis. Mixed models analysis was employed to study the differences in distances in x- and y-directions, and statistical significance was set to PÃ0.05. Between control and ‘nose to carpus’ positions, there were significant differences in skin marker locations, relative to the underlying vertebrae, in the x-direction for C1-6, and in y-direction for C3-6. Between normal and ‘on the bit’ positions, there were significant difference in both x- and y-directions for C6. Differences in marker locations along x- and y-axes, respectively, were 3±9 mm and 44±14 mm. The outcome of this study indicates that skin displacement should be considered when investigating equine neck motion with skin marker methodology.

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