The Human Hip - Part Two

By Jonathan Blood Smyth

Accessory movements are the small gliding and sliding movements which occur inside a joint during normal movement but which cannot be performed in isolation by a person. They are essential to the function of a joint and if lost can interfere with the ability to perform movements or can contribute to painful conditions. In the hip the accessory movements are not obvious with few gliding and sliding movements due to its deep and stable nature. One accessory movement which does occur is upwards and downwards, i.e. compression into the socket and distraction of the femoral head out of the socket.

Walking and bearing weight involve reciprocal cycles of relaxation and compression, which promotes the nutrition and health of the articular cartilage. On compression the cartilage will be indented to a degree and once the force is released it will move back towards its normal shape. In this way fluid is forced out of the tissue under compression and sucked back in again on relaxation of the pressure. This pumps fluid from inside the articular cartilage and from the bone under it, setting up a system of fluid replacement.

When the joints experience normal mechanical stresses they respond with the synthesis of new cartilage, with the cyclical stresses with rest periods important to cope with the levels of force involved just in heel strike in gait. Encouraging growth of cartilage may be possible by using the joints in big movements through their ranges but it may reduce cartilage growth if stresses are removed or it is subjected to static loads over time. These static loads when kept up, using a stick to reduce hip forces and having high bodyweight may all contribute.

When a joint is painful it may not always be the best idea to rest it although pain will be reduced at least initially. Without normal forces the cartilage regrowth stimulation does not occur and there may be a tightening of the joint capsule and a loss of the full movement of the joint. The joint may then become more painful as the tightness increases compressive forces. Respecting a painful joint is important but overall it is better to keep an arthritic joint moving than to keep it static. The normal cyclical rhythm of gait is very important in maintaining movement and good blood supply to the upper hip area.

In the inside of the hip, leading from the acetabulum to the femoral head, is the band like ligamentum teres which carries blood vessels that can be stimulated by the typical cycle of gait and allow a fluid pump. The may give an improved blood supply to the femoral head and maintain bone health. The normal forces which walking places on the upper femoral region are vital to keeping the bone composition and density within normal ranges. Resting in bed or using a walking aid can both contribute to a reduction in mineralisation and density in bone, making it less flexible and so less able to counter strains and jars.

In western societies we typically use little of the relatively large available ranges of movement of the hip joint. We walk in a limited, repetitive range and when we sit we typically do so at a mostly high level so our hips don't go beyond 90 degrees flexion. We seldom push our hips to the extremes of movement of which they are capable and this tendency increases greatly with age. Overall the hip will benefit from maintaining a variety of its movements and from placing it at the ends of its ranges at times. Eastern peoples typically squat or sit cross legged, even to iron, and seem to have lower levels of hip arthritis.

If the ranges of movement are not routinely used the capsule of the joint will tend to tighten to a degree, increasing the forces which compress the head into the socket. A discrepancy in the length of leg can have a subtle effect on the hip range of movement, particularly of extension. When standing the longer leg will tend to shorten itself slightly to compensate for its length and keep the head and eyes at a normal angle. This means it is held forward by a slight flexion of the hip and knee and so some of the range of hip extension is lost as a hip flexion contracture develops. - 32188

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