Surrounding the head, socket and neck is the capsule of the hip joint which runs from the lip of the socket down over the head and neck to the base of the neck. It is a fibrous bag lined by synovial membrane which secretes the lubricating and nutritional fluid for the cartilage, and is strengthened by denser bands of tissue, the major hip ligaments. The hip is designed so that the femur can fit into the socket in the pelvis at an appropriate angle for weight bearing and exerting mechanical forces. The femur comes up to the hip area and then the neck turns inwards at an angle of 125 degrees to access the pelvic socket.
The bone’s internal structure in the upper femur reflects the need of the hip to bear weight and to move the weight of the body. Denser areas of bone struts or trabeculae have developed which reinforce the bone at the points of greatest mechanical need. These strengthened areas function well in ensuring we manage vigorous activities but this has left much weaker areas which show themselves, particularly in elderly people, by the frequency of hip fractures. This is a vital matter as many fractures in elderly people are not survived well by this group.
The main activities the hip is required to perform are locomotion, moving the weight of the body and standing. The strongest bodily muscles are located around the hip and can allow powerful and fast body movement or stabilise the area strongly. The hip adductors, abductors and gluteal muscles are the main groups. The hip abductors are primarily responsible for the stability of the pelvis in a sideways direction and the main movement activity of the body is performed by the biggest and strongest gluteus maximus muscle.
The articular surfaces of the hip are subject to very high mechanical forces, much greater than the body weight in activities such as running, jumping and climbing stairs. To cope with this the articular cartilage is thick on the most exposed areas, although the nutritional supply to cartilage mostly relies on synovial fluid and some from the underlying bone. New cartilage is slowly formed from below as some is worn off at the top by activity, and the balance between these two actions is critical to the health of the joint.
The synovial membrane lines the capsule of the hip and performs the secretion of small amounts of synovial fluid. This fluid is not present in large amounts but is thought to lubricate the movements of the joint, help particles of wear be absorbed to prevent a grinding paste being developed and spread mechanical loads throughout the joint. If the joint is under great load, the synovial membrane can respond to the cyclical stresses by increasing its secretion rate to protect the joint and lubricate it during periods of high activity.
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.
Jonathan Blood Smyth, editor of the Physiotherapy Site, writes articles about Physiotherapy, back pain, orthopaedic conditions, neck pain, injury management and physiotherapists in Harpenden. Jonathan is a superintendant physiotherapist at an NHS hospital in the South-West of the UK.
