Radiographic signs are neither sensitive nor specific for infection. E.g., radiographic evidence of implant loosening may be present because of instability, infection, or both.
It is helpful to examine serial x-rays for progressive changes that could suggest infection. Obtaining old x-rays that document postoperative progress is useful.
It is difficult to differentiate injury swelling and inflammation after fracturing from infection, which limits the value of MRI.
Blood samples to measure CRP and ESR are not diagnostic because both infection and acute injury can cause an elevation.
Therefore, if the infection is suspected, diagnostic workup should include deep tissue samples from the prosthesis interface taken during surgery. Sterile aspiration of joint fluid is also used. At least 5 specimens should be taken for microbiology testing. One sample should be sent for histopathology. Both aerobic and anaerobic cultures should be undertaken.
If the hardware is removed, this can be cultured as well, preferably after sonication. Histological investigation can reveal a bacterial etiology, even if the microbiological tests are negative. Superficial wound swabs should be avoided because of low sensitivity and frequent contamination by surface organisms. Ideally, before tissue sampling for culture, it is important to discontinue any antibiotic therapy for at least a week.
The presence of bacteria in these samples is proof of infection, but it can be difficult to identify if bacteria are present. Microscopic examination (Gram’s stain) and appropriate microbiological cultures may provide evidence of bacterial presence. It must be borne in mind, however, that most bacteria may be trapped in biofilms. Furthermore, preceding antibiotic therapy may confuse the situation by reducing the diagnostic yield.
3. Treatment strategy
When the workup suggests a low risk for underlying infection, a single-stage repair is appropriate.
If the workup suggests deep infection, and in high-risk patients with poor immune response and where the virulent infection is more likely, bacteriological sampling should be done as a separate procedure before the surgery for the periprosthetic fracture, especially if revision arthroplasty is planned. In such cases, a two-stage revision surgery consisting of liberal biopsy samples and cultures and possible debridement and component explantation as a first stage can be appropriate. Splinting or temporary spanning external fixation may be necessary until the soft tissues are appropriate for reconstruction.
4. Antibiotic therapy
Empiric antibiotics before deep tissue sampling should not be used. Eradication of infection is far less likely when antibiotic therapy is not directed by microbiological assessment of pathogens and their sensitivity. Therefore, antibiotic therapy should be commenced after tissue sampling and in collaboration with colleagues in microbiology.
Microbiologists can advise on appropriate antibiotic selection to combat the septic biofilm that is present in the prosthetic joint infection.
Long courses of therapy are usually required. The switch from intravenous to the oral route of administration should be made after discussion with microbiologists and consideration of therapeutic response. Serial CRP measurements are useful.