Infection test for false joints

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US scientists have perfected a more accurate way to detect infection in prosthetic joints.

The new method, which samples bacteria which stick to surface of the joint, was tested on 331 patients with a problematic prosthetic hip or knee.

It accurately detected 78.5% of cases of infection, compared with 60.8% using the conventional method.

The study, by the Mayo Clinic in Rochester, features in the New England Journal of Medicine.

The effectiveness of treatment is critically dependent on the identification of the infecting organism Mr John NolanBritish Hip Society

Most people who have their hips or knees replaced experience dramatic benefit.

However, a small percentage of patients suffer from pain caused by an infection or loosening of the prosthetic joint, and require additional surgery.

The conventional method for detecting infection samples tissue around the prosthetic joint at the time of surgery.

But surgeons need to test multiple tissue specimens, because the sensitivity of a single specimen is not good.

Another potential problem is that bacteria normally found on the skin can be picked up on the tissue specimen as it is extracted, yielding a false-positive result.

However, these same bacteria may actually be responsible for infection, so it can be difficult for doctors to come up with a definitive diagnosis.

Sticky colonies

Bacteria associated with prosthetic joint infection are often found in slimy colonies known as biofilms that cling to its surface, and can be difficult to isolate and treat.

The new test involves surgeons removing the prosthetic joint as they normally would, placing it in a special container, and sending it to the laboratory.

In the laboratory, a solution is added to the container and then the container is subjected to a combination of vortexing (shaking) and sonication (exposure to ultrasound) which has been shown to remove biofilm bacteria.

The bacteria, which are in the solution, are then cultured.

Using the technique, researchers found a wide variety of different types of bacteria.

Researcher Dr Robin Patel said: "This is important to recognise, because it is ideal for the doctors to know what type of infection they are dealing with in order to treat it properly."

Mr John Nolan is a consultant orthopaedic surgeon from the Norfolk and Norwich University Hospital and treasurer of the British Hip Society.

He said infection in hip and knee replacements was rare, probably occurring in no more than 1% of cases.

However, he said when it did happen the implant usually had to be replaced in a process often requiring two or more revision procedures.

He said: "The ultimate outcomes are not always terribly satisfactory for the patient.

"The effectiveness of treatment is critically dependent on the identification of the infecting organism, so any procedure which aids in the diagnosis is to be welcomed."