Accumulation of Fatty Marrow in the Osteonecrotic Hip Mimicking Joint Infection

Factors such as lipids or cholesterol crystals in body fluids can cause a spurious elevation of leukocytes (WBC) in an automated cell count. This artifact can occur in automated WBC counts of hip fluid from femoral head osteonecrosis (ON), and an erroneous diagnosis of septic arthritis can be made.

We describe the frequency of this artifactual leukocytosis in femoral head ON and how to differentiate it from septic arthritis.

From September 1997 to June 2004, 414 patients (486 hips) with femoral head ON were scheduled to undergo THAs. Although we had no preoperative suspicion of infection, we intraoperatively suspected concomitant pyogenic arthritis in eight patients (eight hips) because pus-like fluid gushed from the joint during the operation. To confirm the presence or absence of infection we intraoperatively evaluated the joint fluid with automated blood cell count and microscopy. The automated WBC count was elevated in all eight patients. In seven of the eight patients, WBCs were not identified under microscopic observation and the leukocytosis of automated cell counting was considered to be erroneous. These seven patients underwent THAs. The minimum followup was 3 years (average, 45 months; range, 36–60 months).

These seven patients were considered to have a spurious elevation of WBC (range, 4500–18,400/mm³; mean, 8970/mm³) in an automated cell count. The prevalence of the spurious leukocytosis was 1.4% (7/414) in osteonecrotic hips. In all seven patients, we observed numerous fat cells and globules instead of WBCs under microscopic observation. No bacteria were seen in the smear and culture of the joint fluid. The seven patients underwent THAs, and no patient had a subsequent deep wound infection as of the last followup.

Fatty joint effusion in an osteonecrotic hip may appear like pus and erroneously can be interpreted as leukocytes in an automated cell count. In this situation, a microscopic evaluation of joint fluid smear should be performed to determine the presence of infection and the treatment method.

Level III, prognostic study. See Guidelines for Authors for a complete description of levels of evidence.