Why Do Artificial Joints Wear Out?

Posted on November 2, 2015 by NHOC Tech under Dr. Hogan, Featured Articles, InMotion Newsletter

By Kathleen Hogan, M.D.

Nothing lasts forever. Articular cartilage allows our joints to move freely without pain. As that cartilage wears out or becomes damaged, arthritis develops. If the resulting pain and deformity from arthritis becomes severe, many joints can be replaced, usually with metal and plastic implants. However, these artificial joints wear out with time and use.

How Long Will Hip or Knee Replacements Last?

Surgeons expect that the hip and knee replacements that we are implanting today may last twenty years or longer. There are many factors that impact longevity. Patient weight and activity level are important factors. Excessive body weight is a risk factor in the development of arthritis and can contribute to early wear and failure of orthopedic implants. High impact activities such as running and jumping place excessive loads on the components.

What Are Artificial Joints Made Of?

When joint replacements were first being developed, one of the biggest problems was to find a material that was durable with a low frictional coefficient, with a low wear rate that did not cause an adverse reaction in the body. Teflon was tried, but very quickly failed. It was slippery, but not durable, and the resulting wear particles resulted in excessive inflammation. Metal on metal implants were also used but also had problems with implant design and manufacturing problems. Eventually, polyethylene, which has many industrial uses was discovered to be an excellent bearing surface. According to legend, Sir John Charnley, the inventor of the modern hip replacement, first implanted polyethylene in his own leg to ensure that it would not cause an adverse reaction in the body!

Since polyethylene was first used in hip replacements in the 1960’s, it has been adapted for use as a bearing surface in knees, shoulders, ankles, and fingers. Modifications have been made to further increase its durability and reduce wear rates. An ultra high molecular weight polyethylene can be used to increase the strength of the material. Most polyethylene used in joint replacement is now highly cross linked by treating the plastic to promote the formation of bonds between carbon fibers. This process increases the wear resistance of the plastic but can diminish strength. The increased durability from cross linking can be diminished, however, if the polyethylene becomes oxidized. The overall wear rate of polyethylene depends on many different patient and implant factors.

What Happens When An Implant Wears Out?

When an implant begins to wear out, the wear particles from the polyethylene can cause inflammation. This inflammation can loosen the bond between the joint and the bone. It can also cause pain and instability. If significant wear occurs, a revision surgery may be required.

How long will a joint replacement last? The estimated annual failure rate of hip and knee replacements is 0.5%. Approximately 80% of hip and knee replacements performed 20 years ago are still functioning well. It is expected that joints replaced today will last at least that long and improvements in implant design may further increase that longevity. Patient weight and activity levels have increased, however, and people are having joints replaced at younger ages. For this reason, it is still recommended to get follow up radiographs of joint replacements every few years to ensure that the joint is continuing to function well.

What Can I Do To Make My Artificial Joint Last Longer?

What can you do to increase the longevity of your hip or knee replacement? Although moderate activity is encouraged, it is recommended to avoid high impact activities such as running. Keep your weight within recommended limits. Avoid lifting heavy weights (over 50 lbs) with your job or at the gym. Talk to your surgeon if you have questions about activity after surgery or if you have concerns about your joint replacement.