BACKGROUND: Although numerous studies describe the clinical characteristics of idiopathic osteonecrosis of the femoral head (ONFH) in specific study populations, these have not been confirmed in countrywide studies. QUESTIONS/PURPOSES: We therefore determined: (1) the annual number of patients seeking medical care and number of patients newly diagnosed; and (2) the distribution of the age and gender of the patients, potential causative factors, severity of the disease, and operative procedures performed. PATIENTS AND METHODS: We conducted a nationwide epidemiologic survey in 2005. The survey included all orthopaedic departments in Japan by stratified random sampling according to the number of beds. RESULTS: The number of patients who sought medical care for idiopathic ONFH during 2004 was estimated to be 11,400 (95% confidence interval, 10,100-12,800). We obtained clinical information from 1502 of these patients. The peak in age distribution occurred in the 40s. Potential causative factors were systemic steroid administration (51%) and habitual alcohol use (31%). Hip replacement was the most frequently performed procedure (65%). Among patients with a history of systemic steroid administration, systemic lupus erythematosus was reported most frequently (31%) as the underlying disease. Among patients younger than 40 years, steroid use was the most prominent potential causative factor (60%), and hip replacement frequently was performed (45%). A greater proportion of patients with no history of steroid or alcohol use was observed among patients 65 years or older (41%). CONCLUSIONS: In addition to the disease burden of idiopathic ONFH in Japan, our results confirmed the importance of developing preventive and treatment strategies, especially among the younger population. LEVEL OF EVIDENCE: Level IV, prognostic study. See Guidelines for Authors for a complete description of levels of evidence.
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