One of the biggest reasons joint replacements fail is due to joint infection. Bacteria (sometimes referred to as “bugs”) and fungi can travel through the bloodstream. These pathogens can be carried by the blood to anywhere in the body including the joints. Once in the joint, they can form a biofilm on the surface of the implant.
Periprosthetic infection (in or around the joint) can develop anytime from early on (within the first six weeks of surgery) up to months or years later. Diagnosis can be a challenge. For example, this effect doesn’t show up on ordinary imaging studies such as X-rays. And it isn’t usually until the patient develops serious symptoms such as fever, nausea, and fatigue that there is even any awareness of the problem.
In some cases, a channel from the joint out through the skin (called a sinus tract) is the first sign of a problem. The patient develops pain and oozing (infectious) drainage that sends him or her to the physician or clinic for help.
It isn’t always clear from the results of tests to diagnose periprosthetic joint infection that there is a problem. A suspected (but not confirmed) infection must be evaluated more carefully because if treatment is delayed or not given at all, the joint can (and often does) loosen. The result can be chronic pain and disability. Sorting out loosening implants from septic (infectious) causes and aseptic (without infection) causes is important in planning the most appropriate treatment.
When present, fluid from the sinus tract or directly from the joint is aspirated (collected) and tested for bugs (bacteria) or fungi. Whenever joint infection is suspected, blood tests are ordered to look for inflammation. If two specific blood tests (ESR and CRP) are elevated, then aspiration is required. Tissue biopsy may also be ordered.
The presence of potential risk factors for periprosthetic joint infection raises a red flag. These can include older age, low socioeconomic status (poor nutrition and self-care), obesity, male gender, and knee implant. Poor general health due to comorbidities (other diseases) such as diabetes, cancer, or rheuatoid arthritis are additional risk factors.
Additionally, anyone who has had a previous joint replacement in the same joint is at increased risk of infection. Patients receiving a joint replacement who were in surgery for more than three hours or who received a blood transfusion from a donor (rather than using their own blood) face an increased risk of periprosthetic infection.
Even when diagnosed in a timely fashion, treatment isn’t always so straightforward or easy. Because of the biofilm that forms around the joint, the pathogens stick tight. They don’t always show up on blood tests. In the meantime, they become resistant to antibiotics.
The surgeon may be able to irrigate (wash out) the joint to remove these pathogens and keep them from spreading. But sometimes nothing short of removing the implant works. All the more reason why early recognition of a developing infection and quick intervention are important (to save the joint if possible).
In summary, early identification of infected joints after joint replacement surgery requires awareness of many things. Patients must keep in mind what symptoms might be the first sign of an infection. Physicians should review risk factors and target patients at high risk for joint periprosthetic infection. Closer follow-up for a longer post-operative period of time may be warranted for anyone with red flag risk factors.
Diagnostic testing and evaluation is not a simple process. There is hope that more sensitive and reliable diagnostic tests can be developed. Researchers are currently looking for telltale biomarkers in synovial fluid. Simple, inexpensive strips have been designed that can use one drop of synovial fluid to detect an enzyme present with bacterial infections. Other molecular techniques to detect fungi, viruses, bacteria, and other pathogens are also under investigation.