Fatty Marrow Necrosis in Osteonecrosis of the Femoral Head Mimicking a Pyogenic Hip Arthritis
YC Ha¹, KH Koo², HJ Kim², JJ Yoo², YM Kim^2
1Department of Orthopaedic Surgery, Gyeong-Sang National University School of Medicine, Chinju, Korea, and ²Department of Orthopaedic Surgery, Seoul National University College of Medicine, Seoul, Korea

Purpose: Necrotic fatty marrow is yellow, thick and turbid like pus and fat cell is counted as white blood cell in automated cell counting. When necrotic fatty marrow leaks into the hip joint through a crack in the cartilage of osteonecrotic femoral head, a misdiagnosis of pyogenic infection can be made. The authors report cases of osteonecrosis of the femoral head, in which a misdiagnosis of pyogenic infection was made during the operation.

Materials and Methods: Between September 1997 and December 2001, pyogenic arthritis was suspected during the operation in seven patients who were operated due to advanced osteonecrosis of the femoral head. The markers of infection including white blood cell count, erythrocyte sedimentation rate, and C reactive protein in preoperative laboratory examination were normal in all of the seven patients. Total hip arthroplasty was scheduled for all patients. When hip joint capsule was incised, joint fluid gushed out in all patients. The appearance, white blood cell count in automated cell counting, microscopic findings, and the results of culture of the joint fluid were evaluated.

Results: The joint fluid was yellow, thick and turbid like pus. A pyogenic arthritis was suspected and the joint fluid was sent to the laboratory for automated blood cell count, smear and culture. The count of white blood cell ranged from 5800 to 18000 in automated cell counting. No microorganism was identified on joint fluid smear. On microscopic cell counting using hemocytometer, white blood cell was scarcely seen and majority of cells, which were counted as white blood cell was necrotic fat cell. Total hip arthroplasty was performed immediately after microscopic examination of the joint fluid. No microorganism was identified in culture of the joint fluid. There was no evidence of infection after total hip arthroplasty at a minimum of two-year followup.

Conclusion: When necrotic fatty marrow leaks into the hip joint the joint fluid looks like pus and white blood cell count of the joint fluid is increased in automated cell counting because fat cell is counted as white blood cell. In this situation, microscopic examination of the joint fluid is necessary. If the white blood cell count is not increased in microscopic cell counting, replacement arthroplasty can be performed without risk of infection after hip arthroplasty.