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Researchers Predict Likelihood of Infection in Patients with Knee Arthroplasty

Total knee arthroplasties (replacements) are becoming much more common. This is due to a number of reasons, such as advances in the prosthesis and the increasing number of baby boomers who are advancing in age and who are experiencing knee problems. Once a surgery for people mainly over the age of 70, knee replacements are now becoming more common in younger adults, less than 50 years old in some cases. Unfortunately, one of the complications of knee replacements is the occurrence of infections and the number of infections remains high, from between 1 percent to 4 percent of replacements, so this issue needs to be addressed to avoid patient pain and discomfort.

It isn't always easy to diagnose an infection in a prosthetic knee and there isn't one particular test that is better than another to make this diagnosis. However, pre-operatively (before surgery), the synovial fluid (fluid that lubricates the joint, allowing it to move smoothly) can be tested. The results may give the surgeons an idea of who will develop an infection. The authors of this study wanted to examine the value of testing the synovial fluid in diagnosing infections.

To perform the study, researchers reviewed records of patients at three hospitals who had received a total knee replacement and then a revision, and who also had the synovial fluid of the knee examined at the time of the surgery. These patients at two of the hospitals had the fluid analyzed if here was a strong reason to suspect there was an infection and in the third, the analysis was done routinely on all patients undergoing knee replacement surgery. When examining the records, the researchers did not include any patient who had gout, pseudogout, rheumatoid arthritis, or systemic lupus erythematosus, or if a patient had a history of previous infections in the joint replacement.

The researchers declared that an infection was present if the patient met one of the following three criteria:

1- there was an abscess or sinus tract (space or channel made by the infection)
2- the culture (test) of the synovial fluid was found to have infection
3- the patient had two or more positive cultures taken during surgery

After the records of 429 knee surgeries were examined, the researchers found that 161 knees did have an infection and 268 did not. The patients were, on average 67 years old, and there was an average time of 441 days from the initial knee replacement to the revision surgery. Some of the patients, 33 percent of the group, were having a second revision done on the knee. The rest were having the first revision.

When examining the fluid, it was felt that the cut-off for determining if there was an infection was if was a certain amount (more than 1100 cells in a certain amount of fluid) of leukocytes (cells that help fight infection) and more than 64 percent of neutrophils (cells that fight foreign cells) in that same amount. A protein found in the blood that increases in the body when there is infection, c-reactive protein, was also measured. Another blood test, erythrocyte sedimentation rate, was also done.

Comparing the records showed that if the analyzed fluids fell below for leukocytes and neutrophils, it was easier to predict that an infection would not happen. But, if they were both above the cut-off, an infection was likely and most often confirmed. If the researchers looked at the neutrophil and the c-reactive protein, and they were less than the cut-offs, an infection was unlikely.

The authors of the study pointed out that there were a few drawbacks to their study. For example, the study was retrospective (looking back) so information may not be as accurate as if it was obtained by the researchers themselves. The researchers also only looked at the infection in a total knee replacement so they cannot say if these findings would work for other replacements, such as for hips. Nonetheless, the authors stated that their findings do show that it may be easier to detect an infection by combining the different cell counts and the results of the blood tests.

Elie Ghanem, MD et al. Cell count and Differential of Aspirated Fluid in the Diagnosis of Infection at the Site of Total Knee Arthroplasty. In Journal of Bone and Joint Surgery. August 2008. Vol. 90. Pp. 1637-1643.


*Disclaimer:* The information contained herein is compiled from a variety of sources. It may not be complete or timely. It does not cover all diseases, physical conditions, ailments or treatments. The information should NOT be used in place of visit with your healthcare provider, nor should you disregard the advice of your health care provider because of any information you read in this topic.
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