Articular cartilage possesses only limited capacity for repair. Localized disruptions of the natural congruency of an articular surface generally heal poorly and are commonly linked with the development of osteoarthrosis. The extent to which repair does occur following injury appears to be dependent upon the size of the lesion. Clinically, smaller defects are associated with better outcomes than are larger defects. However, the local biomechanical factors responsible for altered cartilage repair have not yet been clearly elucidated.

A series of experimental and computational investigations were conducted to study the biomechanics of local articular incongruities. Contact stress distributions surrounding full-thickness circular osteochondral defects were recorded acutely (in vitro) and following ten months of healing using pressure sensitive film and quantified by digital image analysis. The ability of this technique to reliably transduce the applied contact stress distribution was verified by bench-top testing. The integrity of the repair tissue for the ten month specimens was characterized mechanically by indentation testing and histomorphometrically by standardized grading indices. Determination of contact stress distribution and repair tissue properties as a function of defect size served as input for computational investigations. A linear biphasic finite element formulation was used to assess the effects of defect diameter, cartilage thickness, subchondral plate regeneration, and progression of healing.

There was only a loose relationship between defect diameter and the degree of contact stress increase in the adjacent cartilage. After ten months of repair, this relation was even less distinct than its surprisingly weak acute counterpart. Severely elevated contact stress gradients observed acutely were also diminished by healing. Nonetheless, relative differences in the measured contact stress distribution and resistance to indentation corresponded to histologically apparent differences in the repair response. In general, the integrity of the repair tissue was mechanically and histologically inferior to that of the surrounding cartilage. Smaller defects, however, tended to repair more completely than larger defects. Computational results exhibited size-dependent behavior similar to that observed experimentally. Surprisingly small differences were associated with variations in cartilage thickness and subchondral plate regeneration.