Dr David A Fisher MD :: Joint Replacement ::

Joint Replacement

Joint Replacement

Joint replacement surgery is a procedure where the arthritic areas of the joint are removed and the bone ends capped with man made implants.

Other joints which can be replaced include the shoulder, elbow, wrist, fingers, toes, and ankles. However, the most common joints replaced are the hip and knee, with over 230,000 combined procedures performed annually in the United States alone.

Hip Implant Technologies
In the hip these joints consist of a metal stem which is implanted into the thigh bone (femur), and an artificial socket which is placed in the pelvis (acetabulum). A metal ball is on top of the stem and it articulates with an artificial socket, which is often contained in a metal shell.

Hip replacements are ball and socket implants made of metal, ceramic and or plastic. The metals used today include primarily Titanium or Chrome Cobalt alloys. Tantalum and Oxinium are now used in some implants. The implants are modular and composed of an acetabular cup, a cup liner, a femoral stem and a femoral head. The acetabular cup fits into the socket of the pelvis. The cup is then fitted with a liner that can be polyethylene, metal, or ceramic. The femoral stem goes down into the canal of the femur (thigh bone) and is fitted with a metal or ceramic ball at the top. The ball is then seated into the acetabular cup liner. The ball sizes vary in diameter and depend on the liner type and patient size.

Traditional hip implants have a plastic (polyethylene) liner between the cup of the implant, which is in the pelvis or socket side of the hip, and the metal ball. The plastic liner is essentially the cushion between the metal cup and metal ball, and its presence ensures the smooth and painless range of motion of the artificial joint. However, as time passes, the plastic liner is the part of the hip replacement that is most likely to wear and may cause the implant to require additional surgery. This problem has primarily occurred in younger patients with high activity levels. Improvements in the manufacturing and sterilization of polyethylene have led to much more wear resistant material in todays total hips. An operation on a hip that has been previously replaced and now requires another surgery to replace the implant is called a revision.

The actual number of patients requiring revision surgery (called a bearing exchange) for a worn plastic cup liner has been on the order of 5 % at 10 to 15 years. This means that 95% of hip replacements continue to function well 15 years after surgery. The risk of wearing out a plastic liner is higher in younger (less than 60 years old) and more active patients. Because of this potential for wear, doctors and implant manufacturers have looked for different types of bearings beyond polyethylene which may be more resistant to wear. The search for a longer wearing bearing has led to the development of metal on metal and ceramic on ceramic hip implants. These devices are commonly used today in patients less than 60 years of age.

Total Knee Implant Technology
Total knee implants consist of three components which resurface the ends of the femur, the tibia (leg bone), and the undersurface of the kneecap (patella).

They are made of metal and polyethylene materials. There are 3 parts of the knee that can be resurfaced with these implants including the femur, tibia, and often times the patella. The femoral component is usually metal (although ceramic parts are being investigated) and made of Chrome Cobalt. When used, the patellar implant is polyethylene. The tibial implant can be all polyethylene or modular with a metal base and polyethylene liner. The modular tibial implants can have either fixed or mobile polyethylene bearings. The difference between the two is that in a fixed bearing the polyethylene is fixed to the metal base plate and motion only occurs between the bearing and the femoral implant. When properly aligned, this construct can be very durable and long lasting in either all polyethylene or metal backed designs.

The mobile bearing implants allow rotation between the polyethylene bearing and the tibial base plate, as well as motion between the bearing and the femoral implant. In laboratory studies, mobile bearings have been shown to have theoretical advantages over the fixed bearings with regard to wear over a long period of time. However, patients with one of each have been unable to determine any difference in function and long term studies so far have been inconclusive in showing any clear advantage. Therefore, this type of knee has been reserved for patients with greater than 30 years of life expectancy in the hopes that it may last longer than traditional knee implants.