Synovial fluid does not retard fluid exudation during stress-relaxation of immature bovine cartilage

Interstitial fluid load support (FLS) is a dominant mechanism of lubrication in cartilage, producing a low friction coefficient while enhancing the tissue’s load bearing capabilities. Due to its viscosity, synovial fluid (SF) may retard loss of FLS by slowing the exudation of interstitial fluid from the cartilage. This study tested this hypothesis by comparing the stress-relaxation (SRL) response of immature bovine articular cartilage immersed either in phosphate buffered saline (PBS) or in healthy mature bovine SF, under unconfined compression (fluid exudation across cut lateral tissue boundary) and indentation testing (fluid exudation across articular surface). To investigate the influence of diffusion of SF molecular constituents into cartilage, the effect of incubation time in SF on SRL was also investigated. The SRL response in unconfined compression was not significantly different in PBS versus SF when compared directly (p = 0.98) and had a slope of$m$ = 1.00 ± 0.04 ($R^2$ = 0.989 ± 0.007). Samples tested in PBS exhibited characteristic relaxation times, ${\tau }^{\mathrm{PBS}}$=42.6 ± 5.3 s and${\tau }^{\mathrm{SF}}$ = 40.8 ± 4.7 s, that were not significantly different (p = 0.40). Incubation time of 24 h in SF resulted in no significant difference in the SRL response (p = 0.39, $m$=1.03 ± 0.12; $R^2$=0.983 ± 0.011, and${\tau }^{\mathrm{PBS}}$ = 43.4 ± 10.7 s versus${\tau }^{\mathrm{SF}}$ = 41.5 ± 4.8 s, p = 0.59). Indentation testing showed some statistically significant, but functionally insignificant, difference in SRL responses in PBS versus SF with a slope of$m$ = 0.958 ± 0.060 ($R^2$ = 0.957 ± 0.020, p = 0.029, and${\tau }^{\mathrm{PBS}}$ = 16.9 ± 2.6 s versus${\tau }^{\mathrm{SF}}$ = 19.4 ± 3.3 s, p = 0.073). Based on these results, we reject the hypothesis that healthy SF can retard the loss of FLS in cartilage due to its viscosity.