Total prosthetic replacement of the hip is performed for arthritis of this joint and is one of the most successful of Orthopaedic surgical procedures. This operation serves well many of those people who undergo it for the duration of their lives. Unfortunately a proportion of inserted hip prostheses will, for a variety of reasons, fail and need to be replaced.
The artificial hip joint is constituted of three major sections. An acetabular component fits into the acetabular cavity of the pelvis. A femoral component is inserted into the upper shaft of the femur. A femoral ‘head’ undertakes the articulation between these two components. The artificial joint can fail in one or more of these areas and each has to be addressed during the revision procedure.
Acetabular failure may simply be due to mechanical wear of its articular surface. In many instances, however, the acetabular cup becomes loose within the bony socket in the pelvis. This loosening may be associated with loss of the surrounding bone which allows the prosthetic cup to move out of position. Revision of the acetabular component then requires two actions.
Firstly it requires the removal of the cup itself. This necessitates breaking any remaining bond between it and the surrounding bone. Bone cement can be removed using chisels and gouges. A recent innovation is the removal of ‘cement’ by an instrument vibrating at ultrasonic frequencies. Where an ‘uncemented’ prosthesis has been used it is necessary to cut the cup away from the bone.
- Secondly it is necessary to make good any deficiency of the surrounding bone. This can be achieved in a number of ways. It is sometimes expedient just to use a larger cup. In some cases, however, pressure on the loosening acetabular component has forced it upward causing an oval shaped bony cavity. Special ‘oval shaped’ acetabular components can be used to meet this contingency. In some cases the loosening of the acetabulum is associated with perforation of the inner wall into the pelvis. In these cases this inner wall needs to be built up again, which is done by using a variety of supports and large quantities of bone graft material.
It is unusual for modern femoral components to break. This component is more likely to fail if the bone around it is resorbed causing the stem to loosen and to subside within the femoral shaft. Sometimes, because of the weakness of the bone, the femur breaks in relation to the prosthesis, usually just at its tip.. Like the acetabulum, revision of the femoral component requires the same two actions.
- The stem component must be removed from the shaft of the femur. This can sometimes be more difficult than removing the acetabulum as the stem is usually surrounded by a layer of cement for a long way down the shaft of the bone. Again a selection of chisels, gouges and ultrasonic instruments is helpful.
- If the bone is eroded it is then usually necessary to make good its deficiency in the upper femur. This can be difficult and a couple of techniques are available to overcome this difficulty. One technique is simply to use a long stemmed prosthesis. This inserts into the unaffected bone lower down the femur and supplements, or bypasses, the weakened area. This technique is particularly useful if there has been a fracture of the bone which occasionally occurs in association with loosening of the prosthesis. The other method is to use what is termed the ‘bone impaction’ technique. This method builds up the weakened area of the femur by filling the upper part of the bone with morscellated bone which is then impacted using a series of reamers. The cavity thus formed within this bone aggregate accommodates the new prosthetic stem. The finely chopped, aggregate bone is gradually incorporated by the femur and serves to thicken and strengthen its upper part.
As stated, these types of surgical procedures require much consideration of the ‘pros and cons’ beforehand and need to be approached with realistic expectations. Revision of a total hip in an elderly person is a major operation and intense rehabilitation is usually needed afterwards. On the other hand, such procedures relieve the discomfort of a loose prosthesis and allow for a very much greater quality of life for the affected individual