Concomitant Septic Arthritis of the Hip in Patients with Osteonecrosis of the Femoral Head.

Septic hip is a rare condition and is known to occur in immune-compromised patients. In general, surgeons are not concerned about the superimposed septic hip when they operate on patients with osteonecrosis of the femoral head (ONFH) if the patient is not immune compromised. We evaluated 1) the proportion of septic arthritis among patients with ONFH, 2) the clinical and laboratory features, and 3) the outcomes of two-stage THA in those patients.We identified patients who were diagnosed as having concomitant septic arthritis of the hip among 1,226 patients who underwent THA due to ONFH from 2011 to 2018 at our institution. A diagnosis of septic arthritis was made by aspirated joint fluid; white blood cell (WBC) count >15,000/ml and neutrophils >75%, microbiological culture, and/or the findings of septic arthritis on magnetic resonance imaging (MRI) scan. Osteonecrotic patients with infection were treated with two-stage THA.Among the 1,226 osteonecrotic patients, 14 (1.1%) had concomitant septic arthritis of the hip. There were nine men and five women. None of them were immune compromised or had a remote septic focus. In the preoperative evaluation, all 14 patients had elevated serum erythrocyte sedimentation rate (ESR) (>20mm/hr) and/or C-reactive protein (CRP) (>0.5mg/dL), and three patients had a fever (>37.5°C). Findings of septic hip were seen in all 12 patients who had preoperative MRI. The neutrophil count in the high-power field was >5 in all 12 patients who had intraoperative frozen section histology. The 14 patients were followed for one to seven years after the arthroplasty, and no patient had evidence of infection at the final follow up.When a patient with ONFH has an unexplained elevation of ESR and/or CRP, concomitant septic arthritis of the hip should be suspected.