Joints are a relatively simple structures composed of articular surfaces (moving surfaces) that come together within a capsule called a joint capsule.
It should be noted that there are no blood vessels, lymphatic channels, or nerves that enter or pass through joints. The two articular surfaces of the bone are covered in a very tough and sheer resistant tissue called hyaline articular cartilage (HAC). In order for joints to move, the two opposing surfaces need to glide over each other with minimal effort, and lubrication is central to this process. The cartilage covering the bone has a thin layer of hyper-hydrated proteinaceous material adherening to the surface of the cartilage. This proteinaceous material is made of proteoglycans, which is essentially a combination of proteins and a special type of sugar molecule called glycosaminoglycan (GAG). It is the proteoglycan within the proteinaceous layer that provides hydration and lubrication to the cartilage surface. What this means is that when the joint is ‘functionally normal’, cartilage does not touch cartilage. Instead the layer of hyper-hydrated proteinaceous molecules interfaces with similar material on the opposing surface. Under load bearing conditions this boundary “weeps” water into the synovial fluid thereby dissipating the pressure. When load bearing conditions cease, the water is reabsorbed and the proteoglycans are again hyper-hydrated.
The other aspect to lubrication and protection of the joint is synovial fluid.The synovial membrane is the inner layer of the joint capsule. This membrane secretes a thick fluid called synovia that serves to lubricate moving parts and nourish the cartilage surface. Healthy synovial fluid is essentially a thick viscous substance made up of plasma filtrate (blood) along with synovia, and is the major source of nutrients for the cartilage tissue and its repair.