SUBCHONDRAL BONE FAILURE IN AN EQUINE MODEL OF OVERLOAD ARTHROSIS

Gross examination of metacarpo-/metatarsophalangeal (fetlock) joints f rom racehorses revealed defects on the condylar surface that ranged fr om cartilage fibrillation and erosion to focal cartilage indentations and cavitation in subchondral bone characteristic of traumatic osteoch ondrosis. Because these lesions represented a spectrum of mechanically induced arthrosis in which microdamage is thought to play a role, a h istologic study of sagittal sections was made to study the morphogenes is, Subchondral bone failure developed beneath a flattened section of the condyle where the margin of the sesamoid bone produces compression as well as shear on impact of the foot with the ground, Milder lesion s had thickening of subchondral bone and underlying trabeculae, With a dvancing sclerosis an increased amount of osteocyte necrosis was prese nt, Occasional vascular channels with plugs of matrix debris and cells were present just beneath the cartilage, There was increased prominen ce of subchondral vessels, and osteoclastic remodeling was seen in and around the sclerotic zone, Apparent fragmentation lines in the subcho ndral bone suggested increased matrix fragility, Irregular trabecular microfractures developed at a depth of a few millimeters, Increased va scularity with hemorrhage, fibrin, and fibroplasia could be seen in en larged marrow spaces at this more advanced stage, The overlying articu lar cartilage was variably indented but remained largely viable with d egeneration and erosion limited to the superficial layers, Focally, br eaks in the calcified layer appeared to lead to collapse and cartilage infolding, In metacarpal condyles from experimental horses run on a t readmill, there were milder changes at the site. The subchondral bone was increased in volume and there was increased diffuse staining with basic fuchsin, but no increase in the number of microcracks was seen, The findings in the racehorses indicate that the equine fetlock condyl e is a consistent site of overload arthrosis in which microfracture an d failure in subchondral bone may occur, Controlled exercise in treadm ill horses may provide a model in which to study the pathogenesis. (C) 1998 by Elsevier Science Inc. All rights reserved.