The Hip Joint

The hip is the largest joint in the human body. From the time infants learn to walk, their hips have to support much of their weight. By the time people reach old age, their hip joints have been exposed to a tremendous amount of wear and tear. If the wear is excessive, it can lead to severe pain and stiffness, which make walking difficult. At this point, many people will question whether their quality of life would improve if they were to have hip replacement. This is a major surgical procedure in which a deteriorated natural hip joint is replaced with an artificial hip joint in order to relieve pain and improve leg movement. So what exactly is an artificial hip?

An artificial hip is a kind of prosthesis (synthetic body part) that typically consists of two or more components. It usually has a stem which fits into the femur, a ball fitted to the top of the stem, and a cup which is fitted into the acetabulum and which provides a smooth gliding surface for the ball (Fig. 3a).

The materials from which prostheses are constructed have changed over the years and so have surface treatments of these materials. The earliest artificial hips, constructed almost 100 years ago, were made of glass; however, most of them broke within a few months. Pyrex, ivory, Bakelite and other plastics were also tried but discarded for various reasons.

In the early 1960s, Sir John Charnley significantly improved hip replacement by developing a low-friction prosthesis; this remains the standard against which newer variations are measured.

Prosthesis design has continued to evolve and diversify so that a number of artificial hips are now on the market. Some types have only the stem/head component and an acetabular cup while others are more modular. Stems are typically made of titanium alloys or chrome cobalt which are considered to be the strongest and most biocompatible implant materials. Heads are usually ceramic or a cobalt-chromium alloy. Most acetabular cups consist of a metal shell securely mated to a polyethylene liner.

The correct choice of prosthesis depends upon the individual. For example, an 82 year old lady with mild cardiac problems weighing 54 kg will have different prosthesis requirements from a 40 year old manual laborer of 95 kg.

Various kinds of cement have been used for anchoring the prosthesis components to bone. Cementless versions have a textured surface to allow for bone cell ingrowth. Some cementless acetabular cups are reinforced with screws.

The success of total hip replacement is essentially based on creating a stable, artificial weight-bearing surface that is fixed securely in bone and which has low friction between the components.