Mechanical and morphometric analysis of the third carpal bone of Thoroughbreds

SUMMARY The third carpal bone (C 3 ) was collected from both forelimbs of 27 Thoroughbreds. On the basis of age, training, and history, specimens were assigned to 1 of 5 groups: yearling, untrained horses (group 1, n = 4); 2- to 3-year-old, untrained horses (group 2, n = 7); trained 2-year-old horses (group 3, n = 6); trained 3-year-old horses (group 4, n = 6); and 3-year-old, trained horses with carpal pathologic features (group 5, n = 4). A transverse section of subchondral bone 5-mm thick was cut in a precise fashion 10 mm below the proximal articular surface of all specimens. After high-detail radiography was done, indentation testing was performed on the proximal surface of the section at points 5 mm apart. The stiffness of the subchondral cancellous bone was determined from the slope of the load vs displacement curve. Topographic plots of stiffness measurements were compared with radiographs of each specimen. Point determinations were averaged to derive measures for the radial and intermediate facets, and for regions 5, 10, 15, and 20 mm from the dorsal margin of C 3 . Area fraction (1-p; p = porosity) was measured for the radial and intermediate facets, using an automated image analysis system. Significant ( P < 0.05) increases in stiffness and area fraction were found in the C 3 from trained horses (groups 3 to 5), compared with untrained horses (groups 1 to 2). Stiffness and area fraction of the radial facet of pathologic C 3 were significantly higher than the same variables measured in C 3 from any other group. A typical profile of regional subchondral stiffness was identified in C 3 from normal horses, with maximal stiffness measured 10 mm from the dorsal articular margin. A different pattern was found in pathologic C 3 , with significantly greater stiffness 15 and 20 mm from the dorsal articular margin when compared with normal horses. A highly significant ( P < 0.0001) direct linear correlation between stiffness and area fraction at the radial facet was found. Topographic and radiographic analysis demonstrated good correlation between stiffness and radiographic density of the bone sections. The observed patterns of normal and pathologic C 3 were contrasted. In particular, a large gradient in subchondral stiffness was identified in pathologic C 3 at the dorsomedial aspect of the bone.