Explanation Of The Human Knee

                                                                                         

Like the elbow, the knee joint is an example of a hinge joint with added complexity. The condyles of the femur are enlarged rounded areas at the base of the femur and they make up the knee joint with the enlarged flattened area of the upper end of the tibia. On the outer side of the shin lies the fibula, a thin, long bone which does not make up part of the knee nor bear much weight, mainly acting as an area of muscle origin for the muscles which move the foot, ankle and toes. The hinge joint of the knee splits the leg in two, allowing tidy folding in resting or active positions, the necessary shortening of the leg to allow effective walking and the large levels of of propulsive power required.

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.

During motion of the knee the menisci (the knee cartilages) and the kneecap are subject to high mechanical forces and are the cause of a number of common knee problems. The movement range of the knee is usually from straight (zero degrees) to full bend at about 140 degrees, varying with the body size and joint mobility. As the knee goes through the movements of extension and flexion the large rounded condyles glide and slide on the top of the tibia, with the tibial surface moving underneath them also.

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.

About the author:
Jonathan Blood Smyth, editor of the Physiotherapy Site, writes articles about Physiotherapists, physiotherapy, physiotherapists in Solihull, 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.

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