The trunk and the legs are connected mechanically at the large ball and socket joints of the hips. The acetabulum, the large rounded and deep hip socket with a cartilage rim, neatly covers the similarly shaped femoral head, the ball at the top of the thigh bone. This is a stable joint with deep structure and strong stabilisers. The surfaces of the hip joint are covered by an almost friction-free material called articular cartilage which allows movement to occur easily under stress, a vital ability in this joint. The greatest weight is taken in the roof of the socket and the uppermost part of the femoral head and this is where cartilage is thickest.
The capsule of the hip joint, a fibrous bag significantly strengthened all around by the large ligaments of the hip, surrounds the hip from the base of the neck up to the margins of the socket. It is lined by synovial membrane which is responsible for secretion of synovial fluid which feeds and lubricates the joint. For the thigh bone to fit into the pelvic socket effectively to transfer the required loads the upper femur is designed to take an inward angle of 125 degrees from the upright so the ball can enter the socket. This allows the appropriate transmission of locomotive and weight bearing forces through the joint.
The internal anatomy of the hip is also designed to support the hip's function of weight bearing and moving the body weight. Inside the bone there are denser areas in the bone trabeculae (internal struts) which lie along the lines of most force and reinforce the bone where it most needs it. So the hip has strong areas to cope with high forces but also weaker areas which can become important later in life when falls commonly cause a fractured neck of femur across a weaker area. This is a very important issue as many elderly people die after such an event.
Standing, the control of locomotion and coping with moving the body are the main functions of the hip joints. The muscles of the hip area are the strongest in the body and are capable of keeping the body stable or of moving it with speed and power. The main muscle groups are the gluteal muscles, the abductors and the adductors. The side to side stability of the pelvis is performed mostly by the hip abductor muscles and the most powerful muscle, the gluteus maximus, functions to move the body weight.
When performing activities such as climbing stairs, jumping or running, the joint surfaces of the hip are exposed to much higher mechanical stresses than just the body weight. To allow for this the most exposed areas to the weight are covered by the thickest cartilage. The underlying bone and the synovial membrane fluid are responsible for nutrition to the cartilage but this is not a very strong process. Cartilage turnover is slow with some forming at the base as the upper layers get damaged and are lost.
The hip capsule is lined by the synovial membrane which secretes small quantities of fluid into the joint. Even though the amount of fluid is not great it is thought to act mechanically to disperse loads within the joint, to lubricate the joints movements and to aid absorption of wear particles so that a kind of "sandpaper" does not collect in the joint and contribute to wear. If the joints have to work hard, the cyclical pressure and release in the joint stimulates the membrane to secrete more fluid to meet the requirements.
Gait is the most likely function to be compromised if there is a reduction in the movements available at the hip, as this is our most important activity. A balanced pattern of gait with an even length of stride is required. Individuals typically have a fairly restricted range of gait motions but if hip extension is reduced this becomes rapidly noticeable. Taking the leg behind the body as the other leg steps forward is hip extension, which typically measures around twenty degrees. Hip flexion however is much greater at around one hundred and thirty degrees and any loss is much less easily noticed.
Author Resource:
Jonathan Blood Smyth, editor of the Physiotherapy Site, writes articles about Physiotherapists , physiotherapy, Physiotherapists in Exeter , 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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