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The Human Hip - Part Two

A normal joint exhibits accessory movements which are small sliding and gliding motions inside the joint, occurring during movement but a person is incapable of performing them in isolation. They are very important in joint function and should they be reduced or lost this will impact on the ability to use the joint and could cause pain. Because the hip joint is stable and very deep it is not obvious there is much in the way of accessory movement. The main one is a downwards and upward movement of the head in and out of the socket.

The health and well-being of the articular cartilage are related to the repeated cycles of compression and relaxation applied to the joint such as with weight bearing and walking. The cartilage is pressed downwards to a degree when pressure is applied and this indentation returns to normal once the pressure is relieved. This engages a fluid pumping system where it is squashed out of the cartilage under compression and pulled back in on relief of the pressure. This pumping of fluid from the deeper cartilage and underlying bone keeps the surface healthy.

The typical mechanical stresses through the joints stimulate the production of new cartilage cells, with the on-off stresses being important to counter the high forces involved when we put our heels down in gait. Bigger movements in larger amplitudes may encourage cartilage growth whilst absence of stresses or sustained loads may impede synthesis of cartilage. Cartilage breakdown may be encouraged by high bodyweight, static loading for long periods or by reducing loads such as by use of a stick.

While resting a joint may reduce the pain at first it may not be the most successful treatment for a painful joint. The normal processes of cartilage regeneration do not occur when insufficient force is applied to the joint and the joint capsule may shrink to some degree, tightening the joint. This tightness may increase the typical joint compression and so the pain. It is important to take into account any pain in a joint but on average it is better to keep arthritic joints mobile rather than still. A good blood circulation to the areas around the hip is kept up by the rhythms of cyclical gait.

The ligamentum teres, a band like structure running from the head of the femur to the socket, has blood vessels which may be affected by the cycle of gait which produces a effect of pumping fluid through. This may allow better blood supply to the head of the femur and keep the bone healthy. To maintain the density and normal composition of the bone in the upper femur it is important for this area to be subject to normal forces such as walking. Use of a walking aid or resting in bed can cause loss of bone density and mineralisation, with the bone becoming less flexible and less resistant to jars and strains.

Typically in western countries people utilise only a small part of the rather large range of movements available to the hip. Walking is a repetitive movement in only a small range and on sitting we don't often go over ninety degrees flexion. As we get older we are less and less inclined to make our hips go to the ends of the ranges they are capable of. Hips seem to benefit particularly from keeping a variety in their movement available and stressing them to their extremes now and again. In eastern societies squatting, even to iron, is common, and our levels of hip arthritis may be higher than theirs.

A lack of use in the end ranges of a joint can mean the joint capsule will exhibit some tightening and in this way increase the compressive forces suffered by the head. Extension of the hip can be particularly affected by a difference in leg length. The longer leg in standing will tend to bend slightly at the hip and knee to keep the head level for the eyes to function best. This compensation can lead to stiffness developing with some loss of hip and knee extension as the hip develops a fixed flexion deformity.

 

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