Mechanical Wear of Degraded Articular Cartilage

To evaluate the mechanical wear of cartilage with different types of degradation.

Bovine osteochondral explants were treated with interleukin-1β (IL-1β) to mimic inflammatory conditions, with chondroitinase ABC (ChABC) to specifically remove glycosaminoglycans (GAGs), or with collagenase to degrade the collagen network during 5 days of culture. Viscoelastic properties of cartilage were characterized via indentation. Biochemical assays were performed to quantify the cartilage matrix loss to the media during culture and from an accelerated, ex vivo wear test. The coefficient of friction during the wear test was measured. Distribution of GAGs in the tissue was assessed histologically.

All three degradative treatments decreased the cartilage modulus values and depleted GAGs in histological sections. However, wear was not uniform among the different treatments. Collagen loss from the tissue due to mechanical wear was only higher with IL-1β and collagenase treatment, while collagen loss due to wear with ChABC treatment was similar to untreated controls. In addition, less GAG was released due to mechanical wear in all degraded groups than the controls, likely because GAGs had already been depleted from these tissues during culture. As no significant differences in the coefficient of friction were observed between groups, changes in wear were attributed to altered tissue composition and structure rather than to changes in frictional forces.

Results suggest that cartilage with a degraded collagen network is more susceptible to mechanical wear, but that cartilage wear may be relatively unaffected by the loss of GAGs. Furthermore, exacerbated mechanical wear could be an additional mechanism by which inflammatory cytokines induce cartilage breakdown.