Synovial fluid delays delamination wear in immature bovine cartilage under pure cyclical compressive loading without...

Previous cyclical compressive fatigue studies in cartilage either incorporated concurrent sliding or, when sliding was absent, did not directly compare synovial fluid (SF) and phosphate-buffered saline (PBS) as bathing solutions. Here, we isolate the effect of SF by eliminating the putative influence of sliding. We hypothesize that SF delays the onset of delamination wear by reducing fatigue failure under pure cyclical compressive loading.

This study compared the fatigue failure rate of cartilage plugs under four bath conditions: PBS versus SF (Experiment 1), and 25% SF/PBS versus 50% SF/PBS (Experiment 2). Immature bovine cartilage contralateral femoral condyle samples were tested in a custom-built solenoid-driven cyclical compression device that continuously recorded load. A compressive load of 54.2 ± 0.7 N was applied at 2 Hz with a plano-convex glass lens (∅12.5 mm) for up to 36,000 cycles. Experiments 1 and 2 each included 8 cylindrical plugs per group (∅12 mm). Tests were terminated at 12,000 cycles if gross damage was observed; otherwise, tests proceeded to 36,000 cycles (5 h). Damage was evaluated by photography, contact area measurement, and surface roughness at baseline, 12,000, and 36,000 cycles.

In Experiment 1, all PBS samples failed by 12,000 cycles, whereas all SF samples remained intact through 36,000 cycles. In Experiment 2, 5 of 8 plugs failed in the 25% SF/PBS group, and 2 of 8 plugs failed in the 50% SF/PBS group. The average surface roughness (R_q) increased significantly in PBS: from 0.025 ± 0.003 mm at baseline to 0.068 ± 0.023 mm at test completion (p < 0.0001). In 25% SF/PBS, R_q rose from 0.024 ± 0.002 mm to 0.039 ± 0.013 mm (p < 0.01). In contrast, no significant change was observed in SF (0.024 ± 0.003 mm to 0.023 ± 0.001 mm; p > 0.99) or in 50% SF/PBS (0.023 ± 0.004 mm to 0.027 ± 0.005 mm; p = 0.45). PLM and histology confirmed cartilage failure by delamination, in all PBS samples and in damaged diluted-SF samples, while no damage was detected in SF. Delamination always occurred in the middle zone of the articular layer.

SF protects cartilage from fatigue failure under cyclical compressive loading, independent of any putative role in reducing friction between articular surfaces. Higher SF concentrations were associated with progressively lower fatigue failure rates, underscoring a critical protective function against fatigue failure, rather than boundary lubrication alone.