The Hip Explained

                                                                                         

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.

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.

Inside the hip the bony anatomy of the supporting struts or trabeculae are also mechanically aligned along the lines of most force, responding to the requirements of bodily motion and bearing weight. The densest areas reinforce the parts suffering the highest stresses. If the hip copes with these stresses by developing strengthened areas it also has areas of less strength which can become relevant in older people as they fall and suffer fractured necks of femur across these areas. As the number of older people dying after this fracture is relatively high, this matter is of concern.

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.

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 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.

Any disturbance in the available movements of the hip will have consequences on its most important function, gait. An even stride length and balanced gait pattern is essential for normal daily function and to the health of the hip joints themselves. We have a typical and restricted pattern of joint movement when we walk but the movement which often becomes limited first is extension, the ability to take the leg behind as the other leg strides forward. At around 20 degrees there is much less hip extension that hip flexion, the ability to take the thigh towards the chest, which is about 130 degrees.

About the author:
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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