Overload arthrosis: strain patterns in the equine metacarpal condyle

An overload arthrosis occurs consistently in the palmar region of the metacarpal condyle of the equine fetlock (metacarpophalangeal) joint characterized by subchondral bone sclerosis, devitalization and mechanical failure leading to collapse of the overlying articular cartilage. Samples were selected of joints with mild, moderate, and severe subchondral sclerosis, in which cartilage collapse had not yet occurred. An additional group that had severe sclerosis with focal rarefaction suggesting impending collapse was also studied (n=5/group). Parasagittal slices were milled to 2.0 mm thickness and subjected to palmar forces 50 to 200% of those applied by the sesamoid bone at angles corresponding to early, mid and late stance support phases of the gait cycle. From contact radiographs in the loaded and unloaded samples, strains were determined by recognizing displacements in the trabecular patterns using texture correlation analysis. Failure did not occur in any of the samples. Strains were generally proportional to the forces applied and greatest at midstance. Strain patterns varied between samples and with the different loading positions. With increased subchondral bone sclerosis there was greater shear strain in overlying trabeculae. Strain patterns were not consistently different within the sclerotic bone at the site of failure. Focally higher strains at the surface were sometimes related to the edge of the platen which was molded to mimic the sesamoid bone in vivo. These results indicate that sclerotic thickening of subchondral bone transmits stresses to overlying trabeculae. No consistent strain pattern was recognized where devitalization and mechanical failure occurs. Focally higher strains related to the edge of the opposing sesamoid bone may play a role.