Synovial fluid protects cartilage against fatigue failure in cyclical compression

This article reviews our current state of knowledge about friction, lubrication and wear of articular cartilage in diarthrodial joints, in relation to osteoarthritis. The main conclusion from this review is that the primary functional role of synovial fluid is to reduce the propensity of articular cartilage against wear from cyclical compression, not cyclical friction, though they are concurrent during normal joint motion. Contrary to widespread concepts about the role of synovial fluid, its primary function is not to reduce the friction coefficient of cartilage against cartilage, which is already very low when tested in physiological buffered saline. Instead, evidence presented from our recent studies demonstrates that synovial fluid delays the onset of delamination damage under reciprocating compressive contact, and that this mechanism depends on the concentration of synovial fluid. Therefore, it appears that some molecular constituent(s) of synovial fluid is (are) responsible for this protective effect. Identifying this (these) constituent(s), which must be able to rapidly transport (in a matter of minutes) into the top few hundreds of microns from the articular surface to impart their protective effect to the middle zone of cartilage, may become an important objective of future investigations. These findings may alter our understanding of the mechanical factors that might lead to the onset of osteoarthritis, by placing a greater emphasis on the synthesis and concentration of these molecular constituents in situ.