Hip replacement is one of the most successful operations in the history of medicine. It has recently been the 50th anniversary of the first hip replacement performed by Sir John Charnley at Wrightington Hospital. Things have changed a lot over the last 50 years but the principles are still the same.
A total hip replacement replaces the “ball and socket” of the native hip joint. There are many different brands of hip replacement but my preference is the Corail / Pinnacle hip manufactured by DePuy (Johnson and Johnson). This hip has excellent long-term results and is suitable for almost all patients. My preference is to use a posterior surgical approach to insert the hip replacement. This is much less likely to damage the hip abductor muscles that can lead to limping after the surgery.
The Corail / Pinnacle hip comes as four pieces which fit together to form the total joint replacement. The first piece is the Pinnacle acetabular component or shell. This fits into the hip socket of the pelvis. To make it fit, the pelvis is “reamed” up and the arthritic bone and cartilage is removed. The shell has a very rough surface that initially holds it into the bone by friction. With time (usually several months), the skeleton grows onto the shell and bonds it to the patient’s bone. This is a very strong fix and means that the shell should last for many years. Sometimes if the initial fixation is not perfect, screws are used to supplement the grip of the shell to the pelvis. This means that the patient can still get up and about without restriction straight after the surgery.
The second piece of the hip replacement is the acetabular liner, which fits into the shell. There are a number of choices for this bearing surface. The most commonly used liner is made of high-density polyethylene (plastic). Plastic liners have been around for many years and have an excellent track record. On average they last for around 15 years and are usually the first thing to fail in a hip replacement.
The second most commonly used liner is made of ceramic. This has a number of benefits over a plastic liner. It is much harder wearing than plastic and in theory should last longer. Because it is much harder wearing, it can be made much thinner than a plastic liner. This is very important as it means a larger head hip replacement can be used.
When a hip replacement is inserted its size is obviously determined by the size of the patient. Someone with a small pelvis will only fit a small acetabular shell. If a plastic liner is then used it needs to be thicker than an equivalent ceramic bearing. The thicker the liner, the less room is left to fit in the ball (head) part of the hip replacement. Therefore a ceramic liner can accommodate a larger head size than a plastic one. Why is this important? This is because the larger the head size, the more stable the hip joint. In the past, hip replacements used small head sizes (22mm) and they had problems with dislocation. The ball would come out of the socket if the patient bent or twisted the hip excessively. It is painful and distressing when it occurs and usually requires admission to hospital to put the hip back in joint. Using a larger head size (32 or 36mm) reduces the risk of dislocation significantly. With a larger head, most patients “forget” they have had a hip replacement as it feels normal to them. Whilst it is possible for it to dislocate, I have never had a dislocation in a primary hip replacement for arthritis with a 36mm head. There is no benefit in using a head size larger than 36mm. If fact, there is some evidence that they do not function as well.
The final choice is a metal liner (metal on metal bearing). This has fallen out of favour recently and I do not use them.
The third part of the hip replacement is the Corail stem (femoral component). This is inserted into the canal of the femur (thigh bone). The femur is a hollow tube that is prepared with broaches to take the hip replacement. The bone inside the femur is compacted to form a solid bed for the stem to sit in. Other types of hip replacement remove this bone. The Corail stem is therefore bone preserving. The stem has an artificial bone coating called Hydroxyapatite. It gives the stem a rough, white appearance like coral (which it is named after). The bone in the femur bonds to this coating making the stem very stable. After a few months the bone has bonded completely and the stem should last for many years. There is usually no need to restrict the amount of weight put on the leg after surgery and most patients are up and about the next day.
The final part of the hip replacement is the head. This is fixed to the stem with a simple taper. It can therefore be removed and changed in the future if necessary. The choices for the head are similar to that of the acetabular liner. They are either made of metal or ceramic (no plastic head is manufactured). If possible, a 36mm head is used to give the best stability and reduce the risk of dislocation.
Once all the components are in place, the muscles and tendons are stitched back onto the bone. These take several weeks to heal and most patients like to use crutches initially. Most patients are doing well by six weeks after the surgery. I would hope they have discarded their crutches and are walking well. I would also expect pain levels to have significantly improved. Some patients take longer than others to recover however and hip replacements keep improving up to two years after surgery.