Anatomy, Pathology, and Treatments:
The knee is composed of thee bones, the femur (thigh bone), the tibia (shin bone) and the patella (knee cap). The ends of these bones that move about each other have a layer of cartilage, which forms the surface of the joint.
In arthritis the cartilage layer is progressively destroyed by mechanical factors and inflammation. Some treatment is aimed at preserving remaining cartilage by unloading this through osteotomy (surgery to realign bone) or bracing. There are also cartilage surgeries, which may involve moving a piece of good cartilage to an unhealthy area; however, this is usually reserved for an isolated traumatic lesion.
The Subchondral Bone:
The bone immediately behind the cartilage is called the subchondral bone. There is emerging evidence that suggests this plays a very important role in the progression of and pain associated with arthritis. The bone will routinely undergo changes that increase its density, making it harder and less able to absorb the shock and stress that is transmitted through the knee. There are procedures being developed and in use that attempt to disrupt this cycle. Osteophytes, or bone spurs, will also develop at the margins of the knee and are thought to be the body’s attempt to improve ligament balance in the knee and help unload pressure.
The Meniscus:
In between the femur and tibia lies the meniscus, which acts as a shock absorber. The meniscus will routinely undergo degenerative changes in the setting of knee arthritis and can develop a meniscus tear. A minimally invasive meniscus surgery may be indicated in select cases. The knee joint is surrounded by a capsule and within this capsule is a layer of synovium, which produces joint fluid that normally has a high content of hyaluronic acid. This fluid acts to improve the mechanics of the knee and to provide nutrition to cartilage, which does not have a good blood supply. This fluid is similar to some injections patients will routinely get. Some know these as “rooster comb shots” or “chicken shots”. The body’s production of this fluid is altered in the arthritic state. The synovium has also bee implicated in the development of inflammation as it produces chemicals (TNF-alpha, IL-1) which act to attract cartilage damaging cells to the joint and also signal for the cartilage and subchondral bone to self-destruct by undergoing changes that further progress osteoarthritis.