Abstract
Introduction: Radial facet alignment of the third carpal bone (Rf-C3) may affect the way it loads and the carpus’ susceptibility to damage. To compare Rf-C3 angle in racehorses with early carpal damage to horses with long racing careers. Methods: Rf-C3 angle was measured in dorsopalmar (DP) radiographs from 21 Thoroughbreds radiographed after at least 11 races, compared with 27 horses that showed carpal pathology within their first few races. Rf-C3 angle was measured between two lines, one line perpendicular to the surface of Rf-C3 (Landmarks of the surface of Rf-C3 were at its proximodorsal articulation with the second carpal bone medially and at the sagittal ridge of C3 laterally). The second line was perpendicular to the proximal surface of the third metacarpal bone (Mc3) which was drawn between the proximodorsal end of the articulation between Mc3 and the second metacarpal medially and Mc3 and the fourth metacarpal laterally. A paired t-test, with two-sample assuming equal variances compared the two groups of horses. Twenty cadaver forelimbs transected at the radius distal third were fixed vertically in a custom-designed apparatus and radiographed six times in each of the vertical and horizontal axes with 5˚ intervals. The percentage change for a 5˚ increase in rotation was calculated as (slope x 5/absolute predicted value at zero degrees) x 100. Results: Rf-C3 angle changed less than 1% in each 5˚ of vertical or horizontal rotation. Rf-C3 angle was significantly less in racehorses with long careers (p=0.024) than in horses with early carpal damage [171.84˚ (SE ± 0.25) versus 173.01˚ (SE ± 0.39)]. Conclusions: Using a parameter with less than 1% change for a 5˚ rotation provides precision and reliability of Rf-C3 angle measurements. Thoroughbreds with a radial facet of 171.84˚ or less might have a longer racing career.