Anatomy of the Knee

The knee is a sophisticated hinge, held together by strong ligaments and stabilised by powerful muscles on the front of the thigh (quadriceps) and the back of the thigh (the hamstrings). The kneecap sits on the front of the joint and is a large sesamoid bone, lying within the tendon of the quadriceps muscles. The purpose of the kneecap (patella) is to increase the mechanical advantage of the quadriceps muscles, which are responsible for straightening the knee and holding the knee straight when we walk. In many ways, the quadriceps muscles act very much like a shock absorber in a car. As we walk with our knees in a slightly bent position, the quadriceps muscles act to absorb the shock ever time our foot strikes the ground.

For descriptive purposes, the knee joint is divided into three compartments, medial, lateral and patello-femoral compartments. Arthritis can affect a single compartment (unicompartmental arthritis), or all three compartments (tricompartmental) arthritis. The most common combination that is seen is arthritis of the medial compartment, with a degree of arthritis also in the patello-femoral compartment of the knee. This is because most of us have a tendency to be slightly more bow-legged than knocked-kneed and results in a greater load in the medial compartment of the knee, resulting in more rapid deterioration in this compartment.