The knee, like the elbow to some extent, is a hinge joint but not a simple one. The lower part of the thigh bone expands into the femoral condyles and the upper shin bone likewise expands into the tibial condyles, the junction of these two enlarged areas forming the knee joint. The small bone on the outside of the knee, known as the fibula, is not involved in weight bearing nor is actually part of the knee joint, serving only as an attachment area for the muscles controlling the ankle, foot and toes. The knee functions as a hinge dividing the leg, allowing the exertion of high levels of muscular propulsive force, the folding of the leg in bending and the shortening of the leg in stepping to allow efficient gait.
As our knees come towards straight in order to weight bear the quadriceps comes into action to straighten the leg towards full extension, engaging the locking position of the knee. One of the characteristics of the human knee is that it ensures efficient and safe weight bearing when standing along with very low energy requirements, unlike the knees of apes. As the knee approaches fully straight the inside part of the quadriceps muscle helps the joint swivel inwards into the locked position. When we stand with our knees straight, we can remain in that position with no activity in the quadriceps and so with very low energy output.
Inside the joint are two crescent-shaped structures made of cartilage, looking a little like banked tracks, accommodating the large rounded femoral condyles. Their exact function is not clear but they may contribute to guiding the knee towards locking, stabilise the knee by centring the condyles during bending and straightening and evening out any potential unwanted small movements during joint motion. The kneecap is the other part of the knee joint and is a small bone with an inner lining of articular cartilage which is suspended in front of the knee joint.
The kneecap or patella is placed within the tendon the main thigh muscle or quadriceps, the muscle which enables us to move our body weight up and down stairs and up from a chair. The patella is shaped on its inner surface with two facets, fitting into the groove formed between the condyles of the femur, sliding along the groove as the knee moves. The kneecap is present to allow the muscular forces developed by the quadriceps to be amplified across the knee and so enable application of high levels of power.
The flexion and extension plane is the natural plane of knee movement as this normal alignment makes knee pain problems occur less commonly. A bow-legged or knock-kneed posture allows abnormal sideways stresses to be applied to the knee, forcing pressure onto one side of the joint and increasing wear stresses which with time can cause pain symptoms or arthritic changes. Patellar misalignment can also occur, forcing one of its facets against the side of the femoral condyle groove and causing impingement pain due to increase in the friction forces.
The patella and the internal cartilages (also known as menisci) take a lot of force during knee movements and many problem conditions are related to these structures. The knee joint's range of movement is typically from zero degrees (straight) to around 140 degrees, although this does vary with general joint mobility and with body weight. During flexion and extension of the knee the femoral condyles slide and glide on the tibial condyles, which themselves slide backwards and forwards.
The forward and backward gliding of the condyles of the femur ensure that they will not slide off the back of the shin bone during motion. In motion one bone does not move in isolation, rather one bone moves in a complicated manner on a complementary bone which is also moving to complete the overall function. This permits a much larger range of motion than would be achievable without such a technique. The femur exhibits an amount of rotation also at the knee which is most discernible as the knee approaches fully straight and the thigh rotates inwards to lock the knee safely.