1. Total hip replacements take out the ball and socket.
Whereas a knee replacement is more of a resurfacing, a total hip replacement does take out the entire joint. In a traditional hip replacement, the cup (acetabulum) is reamed out. A metal cup is inserted into the pelvis at the proper angle. A piece of plastic (polyethylene) is locked into the cup. The head and neck of the femur are sawed off. A metal stem is inserted into the femoral (thigh bone) canal. A metal ball is placed on top of the stem. The metal ball moves within the plastic liner in the metal cup. Voila, your new total hip. Partial hip replacements (hemiarthroplasties) take out the ball only and leave the native socket. These are used mainly for trauma cases.
2. A little history of hip replacements.
Sir John Charnley implanted the first total hip near Manchester, England, during the 1960s. It was a metal ball articulating with a plastic cup, which has proved successful for decades. However, the metal eventually wears away the plastic. The main drawback to a hip replacement is the body's intense inflammatory reaction to the plastic debris. It causes the body's immune system to eat away the bone (osteolysis) and eventually loosen the prosthesis.
3. Newer materials are available.
Newer technology has created alternative bearing surfaces, as well as improved the plastic. Radiation is used to cross-link the plastic molecules and improve the wear characteristics. This creates less plastic debris and osteolysis in the metal-on-polyethylene hips.
Another design has been to eliminate the plastic and allow a metal head to move within a metal liner in a metal cup. The metal-on-metal design has the advantage of no plastic debris. However, as it wears, there are increased cobalt and chromium ions in the body. This is potentially carcinogenic, but as of yet, no study has found an increased incidence of cancer with this type of prosthesis. Even so, it should not be used in women of childbearing age.
Another advance has been a ceramic head with a ceramic liner in a metal cup. The advantages of ceramic-on-ceramic are no plastic debris or metal ions. It is a very hard material and has minimal wear. However, it is brittle and can crack. Studies about the newer cross-linked polyethylene, metal-on-metal and ceramic-on-ceramic hip replacements are encouraging.
4. These are not "bionic" hips.
Hip replacements are meant to allow you to walk, sleep and do your daily activities more comfortably. They are not meant to withstand high-impact activities or sports. In the end, they are still made of metal (cobalt-chrome or titanium alloys) or ceramic (alumina or zirconia) and plastic (polyethylene). In the lab, they have been proven to withstand millions of cycles during testing. So if you go out and run marathons, you will use up your cycles sooner. Then, you might need a replacement of a replacement, which is always more difficult to perform. Under low demand, hip replacements can last for 15 years and more.
5. There are hip resurfacings available.
For some younger patients, hip resurfacing may be an option. It is a bone-conserving surgery. The cup is still reamed and replaced with a metal cup. However, instead of removing the head and neck, the head is resurfaced. A metal cap is placed over the end of the femoral head. The metal cap moves within the metal cup. This technology was introduced in the 1960s and 1970s, but had high failure rates. It was largely abandoned in favor of total hip replacements. Lately, there has been a resurgence in interest in hip resurfacing, with newer implants and designs that are showing promise.
