Summary:
Prosthetic joint infection (PJI) is a significant complication following knee replacement surgery, with an incidence ranging from 1% to 12%. Diagnosis of PJI can be challenging and relies on a combination of clinical suspicion, radiographic findings, and biochemical/microbiological investigations.
Pre-operative joint aspiration is the mainstay microbiological tool for diagnosing PJI. It has shown a pooled sensitivity of 0.78 and specificity of 0.96 in total knee arthroplasty patients. Blood tests, such as erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP), are essential in diagnosing PJI, along with radiological assessment using X-rays. The synovial fluid obtained through aspiration should be sent for analysis, including culture, cell count, and point-of-care testing.
Identification of the causative microorganism is crucial for targeted antibiotic treatment and long-term eradication of PJI. Joint biopsy, obtained through blind or formal arthroscopic techniques, is necessary to obtain tissue samples for culture and histological assessment. During biopsy, 5 independent samples using separate sterilized instruments is currently recommended. The biopsy samples should be incubated for at least 7 days, with a longer incubation period for atypical organisms. Consider repeat aspiration and/or biopsy if negative first culture but suspicion remains high. Antibiotics should be stopped for a minimum of 2 weeks prior to pre-operative sampling.
Pre-operative joint biopsy has a reported sensitivity of 100% and specificity of 94.7% in diagnosing PJI, while alternative techniques like interface biopsy have shown a sensitivity of 88.2% and specificity of 100%. These techniques can be used when aspiration fails to identify the causative organism or when there is significant doubt. Duration of culture, including pathogen and host factors, should be discussed with a local microbiology/ID department