[Influencing factors of disease progression for asymptomatic osteonecrosis of the femoral head].

Objective: To analyze the progression of asymptomatic osteonecrosis of the femoral head by evaluating its lesion size and location.

Methods: From January 2008 to June 2009, 24 cases or hips were available for outcome analysis. There were 16 males and 8 females with a mean age of 36.4 (22-46) years and a mean body mass index of 25.4 (17.5-35). The stages were Ia (n = 2), b (n = 4), IIa (n = 6), IIb (n = 8) and IIc (n = 4) according to ARCO staging system. The etiologies included corticosteroids (n = 16), alcohol abuse (n = 4) and idiopathic disease (n = 4). Magnetic resonance imaging (MRI) was used to measure lesion sizes with necrotic index method. And lesion locations were classified as medial, central and lateral.

Results: The mean follow-up period was 40.3 (32-60) months. Among 7 painful hips, there were corticosteroids (n = 5) and alcohol abuse (n = 2). And 5 hips (20.8%) collapsed. The mean interval between diagnosis and symptom was 40 (36-48) months. The lesion locations were lateral-central (n = 4) lateral-central-medial (n = 3). Significant difference existed between the average necrotic index of symptomatic and asymptomatic hips (45.4% vs 32.7%).

Conclusion: There is a high risk of progression into symptomatic disease when the necrotic lesion occurs on the lateral and lateral-central or lateral-central-medial zone of the femoral head. Asymptomatic small and medially located lesions may be managed conservatively with close medical imaging observations.