Evaluation of the Failed Total Knee
Patients who require revision surgery for the most part experience pain. Other symptoms may include functional limitation, leg length shortening, mechanical popping or grinding, limb deformity, and in the case of infection, fever. After a careful history and physical, multiple diagnostic modalities are used to evaluate the painful total knee arthroplasty. The best diagnostic tool is the plain x-ray. The x-ray can delineate modes of failure such as wear and loosening.
If infection is suspected, multiple tests can be used to help make the diagnosis. This includes aspirating the knee for cultures, bone scan, indium scan and blood tests. No one test is completely diagnostic of an infection. Even a positive or negative aspirate (fluid sample from the joint) is not diagnostic. A positive culture from an aspirate can be a contaminant. A negative culture occurs in approximately 30% of infected cases. Therefore, a diagnosis of infection is made using all information together.
An aspiration of the knee is typically performed in the office. The knee is cleaned with a solution and the skin is anesthetized. A needle is placed into the knee. Fluid drawn from the joint is sent to the laboratory where it is cultured. Bacteria that might grow in this culture medium are identified.
A bone scan can be used to determine if a joint replacement is loose or infected. This requires the intravenous injection of a radioisotope call technetium. This very safe dye is picked up by bone relatively rapidly. It is also excreted by the kidneys and cleared from the body through urination. A scanning device is used to determine the distribution of the dye. If there are areas of “increased activity” in the bone, the dye will accumulate there more than normal bone and this will be seen by the scanning device. The bone scan is very sensitive, meaning it can pick up an abnormality easily. However, the bone scan is not specific. That is, the bone scan will not make the specific diagnosis for us. Also, bone scans are positive for up to two years after the initial joint replacement surgery.
Sometimes, an indium scan is used. This is similar to the bone scan with regard to the scanning procedure. However, in the indium scan, white blood cells are “tagged” with indium. Since, white blood cells tend to accumulate where there is an infection, a “positive” indium scan can indicate an infected joint. White blood cells accumulate in any area of inflammation which can occur without an infection. Thus, the indium scan by itself is not always diagnostic of an infection.
Also, to help in the diagnosis of infection, blood laboratory may be helpful but not diagnostic. These tests are used with the other tests discussed to increase diagnostic accuracy.