Introduction Osteonecrosis of the femoral head

Introduction Osteonecrosis of the femoral head (ONFH) is a pathologic condition of the hip joint that was previously referred to as avascular necrosis (AVN). Disruption of the blood supply to the femoral head is commonly believed to cause bone necrosis and further hip joint destruction. As early as 1738, Munro first described a case of ONFH. Over 200 years later, Mankin reported on a series of 27 cases and named the disease AVN. This brought the disease to the attention of orthopedic specialists and the literature on ONFH has been increasing since. The socioeconomic impact of ONFH on society cannot be overemphasized. In the USA, ONFH accounts for 5–18% of total hip replacement (THR) surgeries annually. Among the Asian population, ONFH plays a much more important role. In Taiwan, 46.3% of all THR surgeries are for ONFH. The literature shows a high prevalence of the disease in Japan and Korea compared to Caucasian populations. A case series of 647 hips in Hong Kong revealed that ONFH accounted for 45.6% of all hip replacements. These data indicate that the disease is more prevalent in Asia and hence has been studied more thoroughly by Asian scientists and surgeons.
Etiology Despite the general belief that ONFH stems from insufficient blood supply to the femoral head, the reason for such circulation disruption remains to be explored. ONFH can be categorized into three groups: trauma-associated ONFH, in which vessels supplying the orphan receptor head are torn during a traumatic event; ONFH associated with known risk factors; and idiopathic ONFH. Known risk factors for ONFH include corticosteroid use, alcohol abuse, and coagulopathies such as Gaucher’s disease, renal failure, and protein C/protein S deficiency. Corticosteroid use is ranked top for all possible etiologies. Approximately 5–25% of those who received intensive corticosteroid therapy (over 1 month) could develop ONFH. Numerous articles in the literature have revealed that many cases previously thought to be idiopathic ONFH were in fact related to genetic traits. Miyamoto et al reported that a recurrent mutation in the type II collagen gene caused pediatric ONFH (Legg–Calve–Perthes disease) in a Japanese family. Liu et al discovered the gene responsible for hereditary primary ONFH in Taiwan. Glueck et al and Jones et al also reported that heritable thrombophilia or hypofibrinolysis is often present in Western countries. Regardless of the proposed etiology, current evidence tends to support the hypothesis that the common prelude to ONFH is microvascular thrombosis (Figure 1). Long-term intensive corticosteroid therapy or alcohol abuse can lead to the formation of fat emboli. Mutations in various genes, such as hypoxia-inducible factor 1-alpha, NO synthase, and factor V Leiden, lead to microvascular disturbance. According to these new findings, it is now more likely that ONFH is a common consequence of various disease entities.
Disease staging and diagnosis The most popular staging system for ONFH is the Ficat system. In Stage I disease, the patient has ONFH but plain radiographs show no abnormality. Stage II disease involves an osteonecrotic lesion with either higher radio-opaque density or a cystic appearance. The shape of the femoral head remains intact without collapse. When the disease progresses further to Stage III, there is osteochondral fracture of the femoral head which leads to the typical crescent appearance on plain radiographs. In Stage IV disease, there are secondary osteoarthritic changes in both the femoral head and the acetabulum. Many other staging or classification systems exist, but most of those are modifications of the original Ficat system. The strategy for ONFH diagnosis is much the same as for other orthopedic pathologies. These include taking the patient’s medical and family history, a physical examination, and subsequent imaging studies as indicated. A differential diagnosis is warranted if the patient has a medical history of corticosteroid therapy, alcohol abuse, or a family history of ONFH. As the disease progresses, the patient may present with a limping gait due to severe pain in the hip joint. A Patrick test should reveal positive signs (i.e., pain in the groin area). Plain radiographs remain the first-line screening tool. If these show no bone abnormality but ONFH is highly suspected, magnetic resonance imaging (MRI) is indicated. It has been demonstrated that MRI is the most sensitive and specific imaging modality for diagnosis of Stage I ONFH. Scintigraphy is less sensitive and specific in detecting early-stage disease. Currently, scintigraphy can be performed on high-risk patients to delineate multi-focal lesions in addition to femoral head lesions.