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.
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.
Discomfort and pain are described by patients as a dull, aching throb that is deep in the knee. Many will also describe a sharp, stabbing type of pain that is equilibrated to being “stabbed by an icepick”. Pain can also elicit a feeling of weakness in the muscles around the knee, which can result in the knee buckling or giving way. Stiffness is a common finding in patients with knee arthritis. This is described as stiffness in the morning, when they first get out of bed or when they rise from a seated position where they have been stationary for a long time. I commonly hear “it takes me a minute or two to get my joints warmed up before I start walking”. Swelling can be recurrent in people with arthritis. This is likely due to inflammation of the synovium, or joint lining, that produces fluid. This is described as “having too much fluid inside the knee.” A person’s range of motion, or movement, of the knee will also be affected. This effectively decreases ones ability to fully bend or straighten the knee In severe cases, a person may be able to feel or hear the knee joint moving and grinding. This may be a result of damaged cartilage and bone moving about each other or perhaps from loose fragments floating in the knee or a flap from a concomitant meniscus tear catching on the overlying bone.
As with many other forms of arthritis, there may be a genetic predisposition that increases ones risk for developing arthritis in the knee. Obesity with mechanical overloading has also been correlated with knee arthritis. Research also indicates an increased risk with metabolic abnormalities such as high cholesterol. Previous injury to the knee such as fractures that extend into the joint or ACL ruptures and meniscus tears may increase one’s risk of developing post-traumatic arthritis. Misalignment will also increase a person’s risk of developing arthritis as “knock-knees” or “bowed-legs” can cause an increase in pressure on the outer or inner aspects of the knee, respectively.