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.
The cartilages, structures known as menisci, are shaped like a banked track in a crescent configuration and fill in to some extend between the rounded condyles of the femur and the flat tibial surface. Their function has not exactly been demonstrated, however they may contribute to easing the knee into lock, minimising inappropriate movement of the condyles and bringing the condyles towards the joint centre in motion. The third component of the knee joint is the patella or kneecap, this small bone being lined with cartilage and placed anterior to the knee.
The tendon of the quadriceps muscle holds the patella or kneecap within it, the muscle’s main function being knee extension to raise the body from a chair or negotiate steps. The inner surface of the patella, divided into two facets, is lined with joint cartilage and articulates with the major groove between the condyles of the femur, sliding back and forth. The power of the quadriceps muscle to extend the knee against the body weight is significantly increased by the presence of the kneecap.
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.
It is in order to stop the femoral condyles slipping off the back of the tibial plateau that they perform the internal glide in a back and front direction. One bone does not move on its own as the joint changes its position, but rather both move in a complicated fashion on each other to allow the functionally desired movement to be achieved. A much more extensive range of motion is achievable with this design. Rotation also occurs at the knee joint but is less obvious, becoming more apparent as the knee extends more closely to the locking position and the internal rotation of the femur occurs.
Jonathan Blood Smyth, editor of the Physiotherapy Site, writes articles about Physiotherapy, back pain, orthopaedic conditions, neck pain, injury management and physiotherapists in Sheffield. Jonathan is a superintendant physiotherapist at an NHS hospital in the South-West of the UK.
Tags: Alternative medicine, back injury, Back pain, back pain relief, Frozen Shoulder, health, Health and fitness, injury management, pain management, physical fitness, physiotherapists, physiotherapy, Piriformis Syndrome, sciatica
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