A major functional role of synovial fluid is to reduce the rate of cartilage fatigue failure under cyclical compressive loading.

Objective: Based on our recent study, which showed that cartilage fatigue failure in reciprocating sliding contact results from cyclical compressive forces, not from cyclical frictional forces, we hypothesize that a major functional role for synovial fluid (SF) is to reduce the rate of articular cartilage fatigue failure from cyclical compressive loading.

Design: The rate of cartilage fatigue failure due to repetitive compressive loading was measured by sliding a glass lens against an immature bovine cartilage tibial plateau strip immersed in mature bovine SF, phosphate-buffered saline (PBS), or SF/PBS dilutions (50% SF and 25% SF; n = 8 for all four bath conditions). After 24 h of reciprocating sliding (5400 cycles), samples were visually assessed, and if damage was observed, the test was terminated; otherwise, testing was continued for 72 h (16,200 cycles), with solution refreshed daily.

Results: All eight samples in the PBS group exhibited physical damage after 24 h, with an average final surface roughness of R_q= 0.210 ± 0.067 mm. The SF group showed no damage after 24 h; however, two of eight samples became damaged after 72 h, producing a significantly lower average surface roughness than the PBS group (R_q=0.059 ± 0.030 mm; p < 10^-4). For the remaining groups, at 72 h, one of eight samples was damaged in the 50% SF group, and five of eight samples were damaged in the 25% SF group.

Conclusions: The results strongly support our hypothesis, showing that decreased amounts of SF in the testing bath produce increased rates of fatigue failure in cartilage that was subjected to reciprocating sliding contact.