Experimental verification of the role of interstitial fluid pressurization in cartilage lubrication

Abstract

The objective of the current study was to measure the friction coefficient simultaneously with the interstitial fluid load support in bovine articular cartilage, while sliding against glass under a constant load. Ten visually normal 6-mm-diameter cartilage plugs harvested from the humeral head of four bovine shoulder joints (ages 2 4 months) were tested in a custom friction device under reciprocating linear motion (range of translation +2 mm; sliding velocity 1 mm/s), subjected to a 4.5 N constant load. The frictional coefficient was found to increase with time from a minimum value of pmln = 0.010 f 0.007 (mean f SD) to a maximum value of 0.243 k 0.044 over a duration ranging from 920 to 19,870 s (median: 4,560 s). The corresponding interstitial fluid load support decreased from a maximum of 88.8 k 3.8% to 8.7 5 8.6%. A linear correlation was observed between the frictional coefficient and interstitial fluid load support (r2 = 0.96 * 0.03). These results support the hypothesis that the temporal variation of the frictional coefficient correlates negatively with the interstitial fluid load support and that consequently interstitial fluid load support is a primary mechanism regulating the frictional response in articular cartilage. Fitting the experimental data to a previously proposed biphasic boundary lubrication model for cartilage yielded an equilibrium friction coefficient of peq = 0.284 f 0.044. The fraction of the apparent contact area over which the solid cartilage matrix was in contact with the glass slide was predicted at cp = I .7 i 6.30/,, significantly smaller than the solid volume fraction of the tissue, cp' = 13.8 i 1.8%. The model predictions suggest that mixed lubrication prevailed at the contact interface under the loading conditions employed in this study.

Alteration of articular cartilage frictional properties by transforming growth factor β, interleukin‐1β, and oncostatin M

“…All explants treated with cytokines started at a minimum and monotonically increased to an equilibrium (Figure 2), similar to controls and consistent with a biphasic lubrication phenomenon observed in other experiments using cartilage on glass (24,25,54).…”

“…Removal of proteoglycans is known to increase tissue hydraulic permeability (54), resulting in a decreased ability to pressurize fluid within the tissue. The hydrostatic pressurization of cartilage endows the tissue with the very low 0 observed in this study and others (24,25,43). Changes in 0 due to treatments previously shown to alter lubricin localization at the tissue surface (27,28,32) were not observed and, in fact, were not expected, since there are no known mechanisms for lubricin to alter water movement or tissue pressurization, factors that change 0 .…”

“…As such, differences observed between the time constants of the monotonic (t) curve in this study ( Figure 2) and others (24,25,54) are the result of testing cartilage samples of various diameters. Boundary mode lubrication was achieved upon equilibrium, as evidenced by invariance in eq over a range of applied normal strains (20-40%) (44,46).…”

“…A number of systems exist for evaluation of the frictional behavior of cartilage, from in vivo studies using whole joints (26,(56)(57)(58) to systems such as those used in the present study using cartilage explants on artificial surfaces (24,25,59). This in vitro cartilage-on-glass friction test has the advantage of the ability to reproduce boundary mode lubrication (44).…”

“…Lubricin has been observed to lubricate a range of bearings, from latex-glass to cartilage-cartilage, in the boundary mode regime (18,(21)(22)(23), although the mechanism of lubrication is not fully understood. The investigation of the role of lubricin in cartilage lubrication is further complicated by a biphasic or pressure-mediated lubrication mecha-nism, which occurs when cartilage tissue is pressurized (24,25).…”