**1. Epidemiology of osteonecrosis of the femoral head**

Femoral head necrosis (FHN) is a painful disorder of the hip joint [1, 2]. Without treatment, more than 70% of clinically diagnosed cases of FHN proceed to collapse of the femoral head, requiring prosthetic joint replacement within 3–4 years after diagnosis [3, 4]. FHN typically occurs in adults, 30–40 years old, and is more prevalent in males than females, with evidence of bilater‐ al involvement identified in 75% of cases [2]. The exact pathomechanism of FHN is not well understood. However, obstruction of blood supply caused by steroid use, alcoholism, sickle cell anemia, and femoral neck fracture are predisposing factors due to loss of osteogenic cells in the greater trochanteric region [1, 2].

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In the natural history of FHN, 59% of cases progress to symptomatic disease and collapse of the femoral head [5]. The prognosis is different depending on the etiology. FHN resulting from sickle cell disease has the highest risk for progressing to collapse (73%), while 47% of cases due to excessive alcohol consumption are at risk of collapse and 46% of cases resulting from renal failure. The risk for collapse associated with corticosteroid use (26%) and for idiopathic FHN (38%) is comparable to the overall prevalence of collapse (38%). Cases of FHN associated with human immunodeficiency virus infection (15%) or systemic lupus erythematosus (7%) have a relatively lower risk for collapse, compared to the overall prevalence [5]. Therefore, under‐ standing the etiologic factor of FHN is important for treatment planning.

Nonoperative treatment of FHN has been shown to have limited success in preventing disease progression [2]. Consequently, the use of joint preserving procedures has decreased in the United States, from being the treatment of choice in 25% of cases of FHN in 1992 compared to 12% of cases in 2008. Over the same period, total hip replacement for the management of FHN has increased from 75% in 1992 to 88% in 2008 [6].
