When is a prosthesis placed?
A hip prosthesis (= an artificial hip) is applied if there is severe damage to the hip joint. The cause of this damage is usually osteoarthritis or rheumatism. This causes pain and the hip joint malfunctions. The damage is visible on an x-ray of the hip.
The hip joint is composed of the head of the femur that smoothly turns in the cup of the pelvis. This smooth movement is made possible by a layer of cartilage on the head of the femur and in the cup of the pelvis. Cartilage is a smooth and elastic tissue.
The wearing-out of the cartilage is called osteoarthritis. Such wear is a normal aging process that occurs to everybody. Some people’s hip joints wear out faster than others. Causes are usually carrying abnormal weight, accidents or deformities of the hip joint. There is also the possibility that this ailment may be hereditary. Wear is not always the cause of this problem.
Such wear causes pain. When the pain can not be remedied by other means a hip replacement has to be done. In cases of severe hip damage both the femoral head and the hip cup needs to be replaced.
This is referred to as a complete/full hip replacement. We often place a prosthesis if the femoral head breaks off. It is not always necessary for the entire joint to be replaced. Usually, only the broken head of the femur is replaced. We call this a “half-prosthesis” or “hemiprothesis.”
How is a hip replacement done?
A hip prosthesis is an imitation of a normal hip. During surgery the femoral head is removed (See Figure A). The remaining cartilage in the hip cup is then removed (See Figure B)
The artificial ball and cup joint is then installed in the place of the hip joint. A new bowl made of artificial cartilage and metal is then placed on the hip bone in the acetabular cup (see Figure C). Then a metal pin with a small ball (that fits into the artificial plastic cup) is placed in the upper part of the femur. (Figure D). The ball fits into the plastic cup. This pin is also called the “stem.”
There are several ways to secure the parts. When we do this procedure the metal cup is usually clipped to the original hip bowl. The bone grows on to the prosthesis and in approximately six weeks this growth is complete. Sometimes we use one or two screws to ensure extra stability.
The ball part of the joint is usually made from high grade plastic. It is placed in the new cup.
The metallic rod is composed of chromium-cobalt or titanium, and can be placed in two ways.
The stem of the prosthesis may be cemented to the top of the femur with special cement. Here the bone also grows on to the stem of the prosthesis. This natural process also needs approximately six weeks to be completed. The other method of securing this part of the prosthesis is to clamp it to the femur without using cement. The method selected in this case is determined by the patient’s age and the quality of the patient’s bones. A ball is attached to the top of the stem of the prosthesis. This ball fits into the new cup attached to the pelvis. The combination of the artificial ball and cup is called “the coupling.” In the first hip replacements we used a plastic ball and a metal cup. The plastic ball was the weak link in these cases because it wore out quite quickly. Such wear occurs in every one of us, but the fact that the plastic ball wore out so quickly caused a loosening of the prosthesis. The life expectancy of the plastic/metal prostheses was approximately 10 to 15 years. So we used a ceramic ball and sometimes also a ceramic cup and we found that with these new materials the rate of wear was much slower. Therefore we believe that this new material may increase the effectiveness of the new prostheses to more than 15 years.