Mixed and boundary lubrication in synovial joints

Human synovial joints have to withstand complex, varied and often harsh loading regimes. For approximately 90% of the time synovial joints and articular cartilage undergo very little motion, and these periods are commonly associated with significant amounts of prolonged loading. During these periods the lubricating films are too thin to separate the opposing articular cartilage surfaces and direct contact occurs within a mixed or boundary regime. Under these conditions friction must remain low to prevent damage to the articular cartilage. The lubrication mechanisms which continue to provide low friction and minimum wear in synovial joints are far from clearly understood and few adequate studies have been undertaken to address this important issue. The specific objective of this study was to enable an enhanced comprehension of how synovial joints perform under conditions of breakdown of the fluid film by investigating and clarifying the lubrication mechanism(s) which occur while mixed and boundary regimes predominate for articular cartilage specimens.

Friction testing of cartilage/metal , cartilage/cartilage and cartilage/hydrogel contacts were conducted when boundary and mixed lubrication was predicted. Both Ringer's solution and synovial fluid were used throughout as the lubricants in order to assess whether or not the constituents of the synovial fluid, not present in the Ringer's solution, served to reduce friction levels. The key variables for the friction tests were load duration and loading history of the cartilage specimens, choice of lubricant, contact configuration and loading regime (i.e. stationary, reciprocating or cyclic).

Loading time, and subesquent load removal, has throughout the course of this project shown to be a major determinant of friction for articular cartilage. Articular cartilage consists of a solid matrix phase and interstitial fluid phase. The interaction of these two phases is largely responisble for the biomechanical behaviour of the 'biphasic' cartilage. It has been reasonably argued that there exists a strong link between the fluid phase and friction coefficient of articular cartilage. The biphasic lubrication mechanism has been proposed as a rational explanation of the relationship between the fluid content and tribological behaviour of the cartilage. This mechanism is in keeping with the broadly held opinion that fluid flows occurs away from the contact zone and loaded areas of the tissue. In a mixed regime, it is believed that the proportion of the load carried by the fluid phase contributes little to the total or aggregate friction force. As the loading period is increased the load carried by the fluid phase decreases and that carried by the solid phase increases. Hence, the overall friction and friction coefficient increases. In respect of these conclusions, the coventional view of mixed lubrication in diarthrodial joints has been reappraised.

Well hydrated cartilage surfaces are considered to be of prime importance in maintaining low friction coefficients within a mixed lubrication regime. The fluid phase load carriage mechanism in reducing friction could be specific to a certain region of the cartilage, e.g. the superficial tangenital zone. It is also quite possible that the cartilage's surface or boundary layer acts independently from the cartilage matrix in affording biphasic lubrication. In view of normal physiological kinematics of healthy diarthrodial joints, during day-to-day activities, which permits the cartilage layers to remain well hydrated, the occurence of high, sustainable coefficients of friction, leading to cartilage wear and the potential onset of arthritis is thought to be unlikely.

Synovial fluid produced systematically reduced friction coefficients, in comparison to Ringer's solution, for the cartilage/metal contacts under constant load reciprocating motion and the cartilage/cartilage stationary loaded contacts; which were statistically significant (p<0.05). The synovial fluid is thought to replenish the cartilage's phosopholipid/glycoprotein boundary layer and so aid boundary lubrication, and also provide a boosted lubrication mechanism wherby the large spherical hyaluronic acid-protein macromolecular complexes (0.2-0.5 um in diameter, in their unloaded state) minimise further asperity contact between opposing surfaces in a mixed lubrication regime.

In addition, surface analysis of articular cartilage was undertaken. The tecnhiques utilised for this investigation were stylus and laser profilometry and scanning, environmental scanning and transmission electron microscopy. The main aim of this particular work was to reasonably quantify the surface roughness of articular cartilage, and to identify and characterise the features of any boundary layer that might exist. It was concluded that for general analyses of synovial joint lubrication, e.g. for the prediction of full fluid film or mixed regime lubrication, it was appropriate to ascribe a quantitative value of roughness of 1-2 um. This however was largely associated with the waviness or form of the cartilage surface . A more realistic value of intrinsic cartilage surface roughness of 0.1-0.3 um, due to surface features such as collagen fibres, was put forward. Cartilage lubrication theories involving microscopic analysis of surface roughness within a mixed lubrication regime would be better suited to adpot this range of values. A distinct acellular, noncollagenous surface/boundary layer was observed (<1 um thick) and, in consideration of the friction test results, was deemed to be an inherant feature of the articular cartilage surface . This layer has potential implications for cartilage nutrition and permeability, aside from its role in synovial joint lubrication.