In vitro fatigue behavior of the equine third metacarpus: remodeling and microcrack damage analysis.

We studied remodeling and microcrack damage in specimens of Thoroughbred racehorse third metacarpal bone that had been subjected to monotonic or fatigue failure. We asked three questions. What effects does mechanical loading have on histologically observable microcrack damage? Are there regional variations in remodeling of the equine cannon bone, and do these variations correlate with mechanical properties? To what extent are remodeling and microcrack damage age-dependent? Machined beams from the medial, lateral, and dorsal cortices were loaded to fracture in four-point bending monotonically, or cyclically at a load initially producing 10,000 microstrain. Specimens were then bulk-stained in basic fuchsin, and cross sections were prepared from loaded and load-free regions of each beam. Current and past remodeling, porosity, and microcrack density and length were determined histomorphometrically. Strained and unstained microcracks were observed. Unstained cracks were associated with regions of woven bone and appeared to be damaged Sharpey's fibers. Their density (approximately 30/mm2) did not increase after failure, but their length (approximately 25 microns) did, especially near the surfaces of the beam. Stained cracks were wider and longer than unstained cracks and were located primarily near the fracture surface and on the compressed side of the beam. Stained cracks after failure were more numerous in those beams having a higher elastic modulus, a shorter fatigue life, or greater deformation at failure. The extent of past remodeling increased with age, especially in the medial region; the rate of current remodeling generally declined with age, but not in the dorsal region, which has the best fatigue resistance. In summary, while remodeling varied with age and region, its effects on bone structure did not appear to influence microdamage. Basic fuchsin staining of damage in fractured equine bone was independent of age and region and confined to near the fracture surfaces. Distributed microdamage consisted only of what appeared to be subtle disruptions of Sharpey's fibers.