The joint is a very well engineered structure. Frictionless motion is provided by the combination of a smooth articular cartilage surface as well as lubrication of both the articular cartilage and the synovial membrane together which make up the entire surface area of the inside of the joint. Because of its resilient nature and ability to compress, articular cartilage in itself is a good shock absorber but its thickness and overall volume is far less than bone or soft tissues. Resilience of the soft tissue is important for normal motion as well as shock absorption. It has been alluded to previously that friction comes from both articular cartilage and synovial membrane.
Hyaluronic acid provides lubrication to the synovial membrane surface.
Until recently it has been felt that it does not provide any lubrication to the articular cartilage but more recently with some new research, it has been shown that hyaluronic acid, in addition to another protein structure called lubricin, is involved in the lubrication of articular cartilage. This substance moving over the surface of the joints is called boundary lubrication. A second mechanism of lubrication of the cartilage is effected by fluid being squeezed out of the cartilage onto the surface when weightbearing occurs. When weightbearing ceases, the fluid is absorbed back into the cartilage, ready for a next cycle of weightbearing. Diagram of adult articular cartilate showing four layers and arrangement of chondrocytes and collagenous fibers.