In many ways, a total knee replacement is similar to a set of automobile tires. How long the knee replacement lasts is related to the type and amount of use, and not simply how long it has been implanted. A set of automobile tires can last for many years if the car is not driven very much. Alternatively, a set of automobile tires can wear out in less than one year if they are driven many miles over rough roads. This principle is applicable to total knee replacement.
Although it is anticipated that a total knee replacement will last for many years, some fail sooner than expected. The main causes of failure are loosening, wear, osteolysis and component breakage. Fortunately, these occurrences are rare. Unfortunately, they can occur and generally necessitate additional surgery. A prosthetic knee component can loosen from the bone due to relative motion between the component and the bone. The intended use of a total knee replacement results in wear of the polyethylene tibial and patellar components. Just as small pieces of rubber wear off an automobile tire when it rolls, the intended motion of the knee replacement generates very small particles of polyethylene. These particles are released into the tissue around the joint. If enough particles are generated, they can cause inflammation. This type of inflammation can result in resorption of the bone around the total knee replacement. This type of bone resorption is called osteolysis and can necessitate additional surgery.
Although uncommon, it is possible to wear-out these plastic parts. Very rarely, a total knee replacement component actually breaks. This usually occurs because the component is no longer supported by bone because of bone resorption. Fracture of the bone around a total knee replacement can also occur and surgery may be necessary to stabilize the fracture.
Loosening, wear and osteolysis generally occur slowly over time. The patient may not initially experience any pain or other symptom indicating a problem. For this reason, it is generally recommended that all total knee replacements be evaluated with x-rays on a yearly basis. This way, problems can be detected and treated early and, therefore, more easily.