The knee joint is a hinge joint as is the elbow but has a complex hinging action. At the end of the thigh bone is the expanded area known as the femoral condyles, with a similar area on the upper part of the shin bone (the condyles of the tibia), making up the knee. The fibula is a smaller bone on the outside of the shin but not part of the knee joint and does not bear significant weight, functioning as an area of attachment for the ankle, foot and toe muscles. The knee divides the leg by its hinge, functioning to allow the delivery of high levels of muscular propulsive force, to enable shortening of the limb during gait and to bend the knee in sitting and other resting postures.
The quadriceps muscles, the main thigh group, act to bring the knee into its locking position as the joint get closer towards full straightening or extension. The apes and monkeys are not capable of extending their knees in standing so this posture takes high levels of energy, while full extension plus locking of the knee joint allows standing without much effort. The knee joint is rotated inwards by the main quadriceps muscle as it gets close to fully straight and begins to lock. Human upright posture has evolved by this process to permit prolonged standing with very low energy expenditure due to the inactivity in the knee and hip muscles.
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 high mechanical forces transmitted through the knee are responsible for a variety of knee conditions, often affecting the kneecap or the knee cartilages (more properly called menisci). The knee typically has a range of motion from full extension (stated as zero) to full flexion at around one hundred and forty degrees, depending to some extent on body size and bodily mobility. There is an important degree of internal glide and slide of the femoral condyles as they move on the reciprocally moving tibial surfaces.
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.
Jonathan Blood Smyth, editor of the Physiotherapy Site, writes articles about Physiotherapists, physiotherapy, physiotherapists in Birmingham, back pain, orthopaedic conditions, neck pain and injury management. Jonathan is a superintendant physiotherapist at an NHS hospital in the South-West of the UK.