Osteonecrosis of the hip, commonly known as avascular necrosis (AVN) of the hip, is the death of the femoral head as a result of the vascular disruption. AVN of the hip results in pain around the hip which is insidious in onset. The cause is generally multifactorial and more commonly seen in males compared to females. Furthermore, the age of presentation from symptomatic AVN of the hip is younger than that of osteoarthritis. The treatment of AVN of the hip is controversial, and as such, there are many different treatment options for AVN. The ideal treatment option depends on the severity and stage of the disease.
AVN of the femoral head results from disruption of the vascular supply to the bone. Potential causes of AVN of the femoral head include causes that are clearly known such as when direct trauma has occurred, and in instances where the inciting event is unknown and only can be associated with risk factors that predispose to the disease process. Below are risk factors associated with the development of AVN of the femoral head.
Previous associations have linked AVN of the femoral head to inherited coagulopathies and genetic polymorphisms that are potentially treatable pathophysiologic conditions. It is believed that AVN of the femoral head is a multifactorial disease associated in some cases with a genetic predisposition and an exposure to one or more risk factors.
AVN of the femoral head is diagnosed at a rate of 10,000 to 20,000 new cases per year in the United States. The typical age ranges from 35 to 50 years old with the average age of presentation being 36 years old. The prevalence is higher in men than women. There is also a high prevalence of bilateral disease.
AVN of the femoral head associated with trauma results from damage to the medial femoral circumflex artery which is the main blood supply to the femoral head. This leads to the death of cells within the bony matrix. The reparative process begins which weakens the subchondral bone, leading to bony collapse and eventually arthritic changes of the joint.
AVN in which the direct cause is unknown but likely from the previously mentioned risk factors results from intravascular coagulation. Coagulation of the intraosseous microcirculation results in venous thrombosis. This leads to arterial occlusion due to the lack of venous outflow. The arterial occlusion causes an increase in pressure within the bone which ultimately leads to decreased blood flow to the femoral head and eventual death of the cells. Much like with direct trauma, once the cells die, the reparative process begins and results in bony collapse and arthritic changes.
It is essential to obtain a full, detailed history pertaining to the symptomatology including onset, location of the pain, duration of symptoms, the characteristics of the pain, alleviating and aggravating symptoms, radiation of symptoms, and timing of symptoms. It is equally important to obtain a detailed medical and surgical history to identify any of the risk factors associated with AVN.
Patients with AVN of the femoral head will present with symptoms of hip pain with insidious onset. Typically, the pain will be associated with standing from a seated position, stairs, inclines, and impact loading. The pain will tend to be more noticeable in the anterior hip or groin as opposed to the buttock or greater trochanter.
There is no single diagnostic test that is 100% reliable. Proper evaluation of a patient with suspected AVN of the femoral head is similar to the patient that presents with hip pain. The physical exam includes inspection of the hip, including the skin, the position of the lower extremity, and gait evaluation as well as palpation of the surrounding bony structures such as the greater trochanter. The range of motion testing includes flexion, extension, abduction, adduction, and internal and external rotation. A thorough neurovascular exam in needed to evaluate for sensory and motor function. There are helpful special hip tests including the log roll and the Stinchfield test to help determine intra-articular pathology.
Standard imaging includes radiographs of the pelvis and hip, including an anteroposterior view of the pelvis, an anteroposterior, and a frog-leg lateral view of the affected hip. The frog-leg lateral view is the radiographic test to confirm subchondral collapse and make decisions regarding patient management.
If there is a high index of suspicion for AVN of the femoral head despite negative radiographs, an MRI of the hip is indicated. MRI has been shown to be approximately 98% sensitive and specific. Findings consistent with AVN of the femoral head include a “double density” appearance which appears dark on the T1 sequence and bright on the T2 sequence. The presence of a bright signal on the T2 sequence indicates bone marrow edema and is predictive of worsening pain and progression of the disease.
Treatment of AVN of the femoral head is dependent on symptomatology as well as radiographic findings such as evidence of the collapse of the femoral head, size of the necrotic lesion in the femoral head, and involvement of the acetabulum. Treatment options can be broken down into nonoperative and operative categories.
Nonoperative Treatment (Stages 0-2)
This is largely limited to small lesions, less than 10% involvement of the femoral head, without evidence of collapse. The evidence is inconclusive on whether or not bisphosphonates prevent femoral head collapse.
Core decompression has traditionally been the treatment of choice during precollapse stages, although the evidence is mixed. The idea behind core decompression is to relieve the increase in pressure within the femoral head thus allowing a return in blood flow and healing. There has been some success demonstrated with the injection of stem cells into the necrotic area. Vascularized free-fibula grafting can be performed in patients with or without collapse in stages 0 to 3. The goal of this particular surgery is to remove the necrotic area and use a free-fibular graft to act as a strut to prevent collapse or to tamp up the collapsed area. The long-term results of this surgery have shown consistent and successful results in regards to functional recovery and pain relief. Total hip resurfacing is indicated in select patients with AVN with careful evaluation of the size of the lesion, its location, and the quality of the bone. The resurfacing procedure requires the adequate bone stock to support the resurfacing component. The component design of total hip resurfacing is a metal-on-metal articulation, and its use in young patients, female patients, or patients with a history of the renal disease is not suggested. Furthermore, studies have shown that the outcomes for total hip resurfacing in AVN are worse than in patients with osteoarthritis. Total hip arthroplasty provides the most reliable pain relief and good early clinic results for patients with advanced stages of AVN. It is indicated in younger patients with a crescent sign or more advanced femoral head collapse and patients older than 40 years old with large lesions.
Steinberg Classification for staging AVN of the femoral head (modification of the Ficat classification)
The risk of femoral head collapse with AVN can be stratified into three groups based on the modified Kerboul combined necrotic angle. This is calculated by the summation of the arc of femoral head necrosis on mid-sagittal and midcoronal MR image.
Mnemonic to help remember the most common causes of AVN