Association between friction and wear in diarthrodial joints lacking lubricin

Abstract

Objective. The glycoprotein lubricin (encoded by the gene Prg4) is secreted by surface chondrocytes and synovial cells, and has been shown to reduce friction in vitro. In contrast to man-made bearings, mammalian diarthrodial joints must endogenously produce frictionreducing agents. This study was undertaken to investigate whether friction is associated with wear.Methods. The lubricating ability of synovial fluid (SF) samples from humans with genetic lubricin deficiency was tested in vitro. The coefficient of friction in the knee joints of normal and lubricin-null mice was measured ex vivo; these joints were also studied by light and electron microscopy. Atomic force microscopy was used to image and measure how lubricin reduces friction in vitro.Results. SF lacking lubricin failed to reduce friction in the boundary mode. Joints of lubricin-null mice showed early wear and higher friction than joints from their wild-type counterparts. Lubricin self-organized and reduced the work of adhesion between apposing asperities.Conclusion. These data show that friction is coupled with wear at the cartilage surface in vivo. They imply that acquired lubricin degradation occurring in inflammatory joint diseases predisposes the cartilage to damage. Lastly, they suggest that lubricin, or similar biomolecules, will have applications in man-made devices in which reducing friction is essential.

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

“…However, the in vitro test environment only approximates in vivo conditions, due to changes in cartilage from resection and articulation against a rigid nonpermeable glass surface. It is important to note that despite these differences, changes in due to loss of lubricin in this system testing a cartilage-glass interface are similar to those in systems testing cartilage-cartilage interfaces (18,60). Further, the present study was conducted with immature bovine tissue, which may differ from adult tissue in the response of lubricin metabolism to exposure to cytokines.…”

Role of lubricin and boundary lubrication in the prevention of chondrocyte apoptosis

“…In this study, the axial load is held constant across samples, testing the ability of each test solution to provide boundary lubrication, prevent shear stress, resultant strain, and consequential chondrocyte apoptosis. Lack of boundary lubrication causes stick-slip to occur, which results in subsurface strain (32) and causes surface asperities to deform destructively (24,33). In contrast to the inability of CACP-SF to reduce static COF, CACP-SF had the same ability to lower kinetic COF as purified human lubricin, yet this reduction in kinetic COF did not prevent apoptosis.…”

“…We lubricated this bearing system with five different test solutions: PBS, wild-type HSF, purified human synoviocyte lubricin in distilled water (HSL), HSF from patients with CACP (CACP-SF), and SF from patients with CACP to which purified human lubricin has been added (CACP-SF+HSL). CACP-SF is void of lubricin but otherwise contains normal SF components (24). The effects of these different test solutions on static and kinetic COF are depicted in Fig.…”

Decreased lubricin concentrations and markers of joint inflammation in the synovial fluid of patients with anterior cruciate ligament injury

“…Similarly, neutrophil elastase was detected in SF from the injured joints but not the contralateral joints. Neutrophil elastase is a serine protease that was shown to completely degrade lubricin (4,12,20) and increase cartilage friction, and hence wear, in vivo (5).…”

“…Lubricin is partly responsible for the lubrication of apposed and pressurized cartilage surfaces (4). It provides essential chondroprotective properties to articular cartilage, as evidenced by lack of cartilage surface integrity and surface disruption in PRG4-knockout mice (5). Previously, we have demonstrated decreased synovial fluid (SF) lubricating properties, indicative of decreased SF lubricin concentrations, in patients diagnosed as having traumatic synovitis (6).…”