The hip is a ball and socket joint where the spherical head of the femur (thigh bone) fits smoothly into the cup-shaped socket of the pelvis. The surfaces of the bones are covered in a smooth cartilage to allow them to slide over each other. This cartilage can become damaged and worn meaning that the bones rub directly against each other causing the hip to become stiff and painful. Often the only treatment is to replace the joint with an artificial one.
During the operation, the worn femoral head (the ball) and the acetabulum (socket) will be removed and replaced with artificial ones.
The new femoral head is attached to a piece of metal called the femoral stem and inserted into the femur. It can either be covered in a special material that allows the bone to grow around it or it can be cemented in. There are hundreds of different stems available and the patient will decide the best option with their surgeon.
Many countries, including England and Wales, have a ‘Joint Registry’ – a database of all joint replacement operations performed. It is possible to look at how each prosthesis (artificial joint replacement) performs. This information is freely available to the public at http://www.njrcentre.org.uk/njrcentre/default.aspx
The reports generated by the English and Welsh joint registry show that the Exeter V40 has one of the best survivor records with one of the lowest rates of complications of any of the prosthesis used. This same result is found in all of the other major joint registries, including those of Sweden, Norway, Finland, Australia and New Zealand.
As with the stem, there are hundreds of different cups (acetabular components) available. Again, these can be cemented or uncemented. The Trident cementless cup is covered in a material that the body treats as bone, allowing the bone to grow onto the cup. It also allows the use of modern high performance bearing surfaces.
The bearing surfaces are the materials that interact with each other. Traditionally, a hip replacement has had a metal head, which moves within a plastic cup made from ultra-high density polyethylene. The plastic is softer than the metal and wears out, usually in 10-15 years.
This action produces wear particles which stay around the hip joint and cause inflammation. This inflammation will cause small holes in the bone, eventually leading to either the prosthesis loosening, or the bone fracturing.
There has been vast progress in plastics technology in the last decade. Plastics have been made which wear out at less than a tenth of the rate of the older plastics. This means that the bearing surface should have a life expectancy of at least 15-20 years.
Unfortunately, these new plastics have only been used for seven years and so the actual lifetime of the plastic is not known. Early results have shown that the plastic is so far behaving as expected, with a very low wear rate. These plastics have also allowed the use of larger femoral head diameters which reduces the chance of dislocation.
Metal on Metal Joints
Before modern plastics were invented, surgeons used joints with a metal head within a metal cup to get around the wear issue. Although these metal cups took a long time to wear out, eventually they would releasing small particles of chromium and cobalt into the tissues.
A healthy human can remove these particles via the bloodstream and the kidneys but in some people these metals can damage the surrounding tissue leading to dead muscle and bone cells and producing large collections of inflammatory fluid.
Ceramics are very hard, brittle material and have an incredibly low wear rate. One study showed that the bearing used in a ceramic on ceramic hip joint will last for 10,000 years but they do have disadvantages.
Ceramics can shatter on impact. In the past, some ceramic joints had weaknesses, and broke with minimal trauma, although, the manufacturing process has now improved.
Ceramics are also difficult and unforgiving to insert. The positioning of the prostheses must be exact, otherwise dislocation, wear or fracture will occur.
Occasionally a ceramic hip replacement can squeak in female patients. The squeak is usually only audible when the patient rises from sitting.
Ceramics are also three times the cost of a modern plastic bearing surface although insurance companies will cover the difference in cost.