Understanding avascular necrosis of the hip

Avascular Necrosis of the Hip: a painful hip condition.

Avascular necrosis (AVN) of the hip is a debilitating condition that usually affects people in their thirties and forties. The incidence of avascular necrosis of the hippp has been estimated to be between 10,000 and 20,000 cases per year in the United States, with an average age of presentation of 38 years. Unfortunately, most cases are recognized in the advanced stages when treatment options are limited. If this entity is promptly recognized, early treatment and hip saving surgical treatment may be initiated. 

Although still not entirely understood, there are various causes and risk factors that have been closely linked with this disease process. Initially, the blood circulation to a portion of the femoral head is interrupted. This interrupted blood flow leads to cell death and results in an area of bone necrosis (death) that eventually collapses and leads to progressive arthritis of the joint. The disruption of blood flow could result from a single traumatic event or from multiple instances of minor damage from various non-traumatic events. 

Femoral neck fractures and hip dislocations are the most common traumatic events that can disrupt the blood flow to the femoral head. In 1991, a case was reported detailing the incidence of AVN in the hip of a professional football player who sustained a traumatic hip subluxation. Thus, an adequate history must include an accurate assessment of the severity and mechanism of any trauma to the hip. 

Among non-traumatic cause of AVN, up to 90 percent of new cases have been associated with either systemic corticosteroid use or heavy alcohol use. Corticosteroids, such as prednisone, are hormone-related substances used in several autoimmune diseases, including asthma, have been implicated the most. In contrast, anabolic steroids commonly used by male and female athletes, especially weight lifters, to increase muscle bulk and strength have not been reported to cause any incidence of avascular necrosis. 

Most risk factors and pathologic conditions associated with a traumatic avascular necrosis have systemic effects. Some of these risk factors and conditions include smoking, sickle cell anemia, bleeding disorders, lupus, hypercholesterolemia, organ transplatation, and hypertriglyceridemia. Despite a large number of risk factors that can be directly linked with the incidence of AVN of the hip, approximately ten to 20 percent of cases have no clearly identifiable risk factors and are classified as idiopathic (unknown cause) avascular necrosis.

A high index of suspicion and awareness of the predisposing factors (i.e. systemic illnesses that require corticosteroid use or a history of trauma) that may lead to avascular necrosis of the hip is critical. Athletes with avascular necrosis of the femoral head will typically complain of groin pain. Occasionally, nonspecific hip pain, that may affect the buttock or knee region, may be described as well. On physical examination, hip range of motion and gait will be normal in the early stages of the disease process. In the advanced stages, internal rotation is severely restricted and passive rotation of the hip joint will generally reproduce the pain. The pain is typically worse with attempted internal rotation of the hip.

X-rays of both hips should be performed if AVN is suspected. Early radiographic changes may not appear until three months after symptoms begin. Therefore, if radiographic findings are normal but the disease is still suspected based on history or associated risk factors, an MRI of the hip should be obtained. An MRI (magnetic resonance image) is the most accurate imaging modality to identify early AVN. In addition to working up a hip suspected of having AVN, a complete exam of the contralateral hip should be performed, due to a 40-80 percent incidence of bilateralism reported.

No single surgical or medical method has been demonstrated to universally prevent disease progression. For early disease, symptomatic treatment and restricted weight bearing has been advocated in the past, but this is currently thought to be ineffective in altering the disease process. If caught early, a minimally invasive surgical decompression of the femoral head can provide pressure relief and increase the blood flow to the femoral head. With blood flow restored, the degenerative destruction of the hip can be prevented. In advanced stages, the range of motion and gait are severely affected and surgical correction typically involves performing a total hip replacement.

Prompt recognition of this disease is critical for the best clinical outcome. If diagnosed early with a good history, an individual with avascular necrosis of the hip can be treated and have the best chance of avoiding the catastrophic collapse and deformity of the femoral head which can result in significant disability. 

Author
William Sherman MD Board Certified Orthopedic Surgeon Fellowship Trained Knee/Hip Replacement Surgery

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