False-negative joint aspiration of septic arthritis of the hip in neonates.

In neonates, timely detection of septic arthritis of the hip can be challenging. Joint aspiration can be a useful diagnostic procedure, but scanty fluid in a septic joint has been reported, leading false-negative results. This study aimed to investigate clinical course of neonates with septic arthritis of the hip despite initial negative findings on joint aspiration. The neonates who surgically treated for septic arthritis of the hip between 2003 and 2013 for septic arthritis of the hip despite initial negative joint aspiration were retrospectively reviewed. Clinical presentations, MRI, intraoperative findings, functional and radiographic outcomes were evaluated. Six neonates were included with a mean follow-up of 12 years (range 5-15 years). All patients showed negative results on joint aspirations performed with ultrasound guidance or fluoroscopy. The mean duration between the onset of symptoms and initial surgery was 15.2 days (range 4-25 days). Four patients (67%) had extracapsular abscesses that were connected to perforated joint capsules on MRI. Intraoperatively, all patients were found to have hip joint instability with a ruptured capsule. Five (83%) patients experienced a complicated recovery. Four patients required reconstructive hip surgery, or further procedures to correct leg length discrepancy. Neonates with false-negative aspiration typically had a delay in appropriate surgical treatment. These cases suggest that the absence of aspirable fluid contents in the hip joint does not rule out septic arthritis in neonates. Our findings highlight the importance of considering spontaneous hip joint capsular perforation as the cause of extra-articular drainage of pus and instability.