The mechanical connection between the legs and the mass of the trunk occurs at the hip joints with their large socket and ball. The hip socket or acetabulum is a deep rounded structure with a rim of cartilage which closely holds the head of the femur, the large ball structure at the top of the thigh bone. As befits a joint where large forces are applied, the hip is stable, deep and has strong stabilising features. To permit fluid movement under considerable bodily loads the hip surfaces are covered with articular cartilage which confers almost friction-free motion under these loads. This is essential to the hip and the thickest cartilage is where the greatest forces are applied.
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 hip is designed to perform locomotion of the body and to stand and manage the body weight through the legs. To perform these actions the hip joints have very strong muscles to stabilise them and move them and the body in some cases quickly and with force. The hip abductors, adductors and gluteal muscles are all major stabilisers and movers of the joint. The abductors play a large part in the sideways stability of the pelvis and the gluteals, the body's most powerful muscles, move the body weight around.
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 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.