Development of a Cartilage Shear-Damage Model to Investigate the Impact of Surface Injury on Chondrocytes and Extracellular Matrix Wear

Background: Many in vitro damage models investigate progression of cartilage degradation after a supraphysiologic, compressive impact at the surface and do not model shear-induced damage processes. Models also neglect the response to uninterrupted tribological stress after damage. It was hypothesized that shear-induced removal of the superficial zone would accelerate matrix degradation when damage was followed by continued load and articulation. Methods: Bovine cartilage underwent a 5-day test. Shear-damaged samples experienced 2 days of damage induction with articulation against polyethylene and then continued articulation against cartilage (CoC), articulation against metal (MoC), or rest as free-swelling control (FSC). Surface-intact samples were randomized to CoC, MoC, or FSC for the entire 5-day test. Samples were evaluated for chondrocyte viability, GAG (glycosaminoglycan) release (matrix wear surrogate), and histological integrity. Results: Shear induction wore away the superficial zone. Damaged samples began continued articulation with collagen matrix disruption and increased cell death compared to intact samples. In spite of the damaged surface, these samples did not exhibit higher GAG release than intact samples articulating against the same counterface (P = 0.782), contrary to our hypothesis. Differences in GAG release were found to be due to tribological testing against metal (P = 0.003). Conclusion: Shear-induced damage lowers chondrocyte viability and affects extracellular matrix integrity. Continued motion of either cartilage or metal against damaged surfaces did not increase wear compared with intact samples. We conjecture that favorable reorganization of the surface collagen fibers during articulation protected the underlying matrix. This finding suggests a potential window for clinical interventions to slow matrix degradation after traumatic incidents.