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Accumulation of fatty marrow in the osteonecrotic hip mimicking joint infection
Tae-Young Kim1, Sun-Joo Kim, Young-Kyun Lee
1Department of Orthopaedic Surgery, Hallym University Sacred Heart Hospital, Anyang, South Korea.
Background:
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.
Question/Purposes:
We describe the frequency of this artifactual leukocytosis in femoral head ON and how to differentiate it from septic arthritis.
Methods:
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).
Results:
These seven patients were considered to have a spurious elevation of WBC (range, 4500-18,400/mm(3); mean, 8970/mm(3)) 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.
Conclusions:
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 Of Evidence:
Level III, prognostic study. See Guidelines for Authors for a complete description of levels of evidence.
Insights
Fatty joint effusions in osteonecrosis of the femoral head can mimic infection by causing falsely elevated white blood cell (WBC) counts in automated analysis. Microscopic examination of joint fluid is crucial to differentiate this artifact from true septic arthritis.
Area of Science:
- Orthopedics
- Rheumatology
- Pathology
Background:
- Lipids and cholesterol crystals in body fluids can cause spurious elevations in automated leukocyte (WBC) counts.
- This artifactual leukocytosis in hip fluid from femoral head osteonecrosis (ON) can lead to misdiagnosis of septic arthritis.
Purpose of the Study:
- To determine the frequency of artifactual leukocytosis in femoral head ON.
- To describe methods for differentiating this artifact from septic arthritis.
Main Methods:
- Analysis of 414 patients (486 hips) with femoral head ON undergoing total hip arthroplasty (THA) between 1997 and 2004.
- Intraoperative evaluation of joint fluid using automated cell count and microscopy in eight patients with suspected pyogenic arthritis.
- Microscopic examination of joint fluid smears to identify fat cells and globules instead of WBCs.
Main Results:
- Spurious leukocytosis occurred in 7 of 8 suspected cases (1.4% prevalence in osteonecrotic hips), with automated WBC counts ranging from 4500-18,400/mm(3).
- Microscopic observation revealed numerous fat cells and globules, not WBCs, in these cases.
- No bacteria were detected, and none of the seven patients developed deep wound infections post-THA.
Conclusions:
- Fatty joint effusions in osteonecrosis can be misinterpreted as leukocytes in automated counts, mimicking infection.
- Microscopic evaluation of joint fluid smears is essential for accurate diagnosis and appropriate treatment planning.
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