AVN avascular necrosis hip cover

Avascular Necrosis of the Femoral Head: Background and Diagnosis

Avascular necrosis (AVN), also known as osteonecrosis, atraumatic necrosis or ischemic necrosis, is a debilitating disease that affects a wide range of individuals including children, but is most common in the middle aged population. AVN of the femoral head (AVNFH) is associated with impaired blood supply to the femoral head resulting in bone necrosis and collapse. 

It can result from direct damage to the bone (e.g. femoral neck fracture), direct injury of the bone or marrow elements (e.g. radiation). However, in many patients, the mechanisms in which the disorder develops are not fully understood. There are 20,000 new cases of AVN of the femoral head per year in the United States and accounts for 10% of total hip arthroplasties performed. We will be covering atraumatic avascular necrosis of the femoral head.

AVN of the femoral head is characterized by apoptosis of bone cells (bone marrow, bone forming, and bone destroying cells) that results in bone collapse with a subsequent involvement of the overlying cartilage, provoking flattening of the head surface with, eventually, development of secondary osteoarthritis [1]. The denomination “avascular” is not entirely appropriate as the femoral head blood supplying vessels do not disappear; rather they suffer a pathological process which results in blood flow interruption. 

The most common denomination for atraumatic ANFH has been idiopathic, a metabolic disorder background is the most common keystone in every patient. The hepatic metabolism is usually altered either by a primary syndrome, by alcohol intake or by the administration of steroid medication for an immunogenic or autoimmune disease. Other conditions increase the tendency of intravascular coagulation that can be intraosseous or extraosseous. In cases of trauma, the resulting osteonecrosis is better understood as the vascularity around the femoral head is severely disturbed (mainly medial femoral circumflex artery).

The most common etiology for altered metabolism in adults is alcohol intake. This is dose dependent with the relative risk increasing as intake increased, with 9.8 relative risk with 1 liter per week and 17.9 relative risk if more than 1 liter per week [2]. Alcohol disturbs the phospholipid and cytokine metabolism and this interrupts blood flow. Steroid administration has also been shown to be dose dependent with prolonged treatment more than 20-40 mg per day for more than 3 months having a greater chance of developing AVN of the femoral head [3]. Alcohol intake and steroid usage accounts for more than 80 percent of atraumatic cases [5]. 

Sickle cell hemoglobinopathies have bone necrosis as a complication and has been shown to increase chances of having AVNFH. There are also other conditions that are associated with AVNFH or bone necrosis including systemic lupus erythematosus (SLE), Gaucher Disease, antiphospholipid antibodies, hypercortisonism, decompression disease, acute lymphoblastic leukemia (ALL) and Human Immunodeficiency Virus (HIV) [6]. Most of these conditions increase the tendency of intravascular coagulation. It is also worth noting cultural, geographical and genetic circumstances have also been related to idiopathic cases, with Legg Calvé Perthes’ Disease (LCPD) being fairly well known.
Bone necrosis physiopathology is very similar in children and adults. What differentiates both age groups is cartilage maturity, with children having growth potential and better prognosis. There is an ischemic phase that occurs and the timing on this is unknown. Some can have a slower and more dormant course, while others have a rapid course. Once ischemia occurs, monocytes and mesenchymal stem cells migrate to the area. This eventually weakens the subchondral bone and the femoral head articular surface collapses. Adults will develop secondary osteoarthritis and children’s course depends greatly on the potential capability of the epiphyseal cartilage to synthesize new cartilage matrix. For reference, most children under 4 years old have a very good prognosis and most children older than 9 have a poor prognosis [7].
Early diagnosis and treatment can provide more opportunities for hip preservation. Patients will typically present late in the course of the disease and a high suspicion and knowledge of risk factors (mainly alcohol and corticosteroid use) may help aid in earlier diagnosis. The most common presenting symptoms of AVN is pain. Pain tends to mimic joint pain and many complain of anterior hip pain radiating to the groin. Others have thigh and buttock pain. Pain is usually exacerbated by weight bearing, though up to two-third will complain of pain at rest. Up to one third of patients also complain of night pain [5]. Physical examination is fairly nonspecific. There may be limitations in motion and pain with FADIR maneuver. Others may have tenderness along the anterior hip or posterior hip. Antalgic gait may be present. There is a portion of patients that will also be asymptomatic and not respond to any provocative maneuvers.
AVN avascular necrosis hip crescent sign xray

Figure 1. Crescent sign (adopted from [13])

The initial imaging modality should be plain film radiography and include AP views and frog leg lateral views. Close attention should be paid to the superior portion of the femoral head for subchondral abnormalities. It is possible that these can be normal for months and the earliest findings are mild density changes. The crescent sign refers to a linear area of subchondral lucency seen most frequently in the anterolateral aspect of the proximal femoral head and is a sign of imminent collapse. Later stages reveal loss of sphericity or collapse of femoral head. 

Radionuclide testing (Technetium-99m) bone scanning can be used in patients with suspected disease with negative radiographs and has been shown to be slightly more sensitive in early stages. Increased burn turnover increases uptake in the femoral head and surrounds a cold area. This is sometimes called the doughnut sign [8]. 

AVN avascular necrosis hip doughnut sign

Image 2. “Doughnut” sign on the left hip, adopted from [8])

However, multiple studies have shown this to be less sensitive than magnetic resonance imaging, or MRI [9]. MRI is the gold standard with sensitivity up to 100 percent [9,10]. T1 images appear dark with a low intensity band and T2 images have focal brightness or marrow edema. Presence of marrow edema has been shown to predict worsening pain and future progression of disease [11]. There is also a “double line” sign that can be seen on T2 Spin echo or turbo spin echo sequences. This consists of a low signal intensity outer and a high signal intensity inner rim [15]. This sign or scheme was not ever used widely, as it did not correlate with radiographic staging, clinical presentation and prognosis [16].
AVN avascular necrosis hip double line sign MRI

Figure 3. “Double Line” sign, adopted from [16])

MRI can also be used to predict prognosis or risk of femoral head collapse. By this method (Kerboul combined necrotic angle), an angle is calculated by adding the arc of the femoral head necrosis on a midsagittal and mid-coronal MR image. Less than 190 degrees can be characterized as low risk (0/4 patients had collapse). The moderate-risk group was between 190 degrees and 240 degrees with 4/8 patients in the original study having collapse of femoral head. High-risk is greater than 240 degrees with 25/25 patients developed femoral head collapse during the 3 year trial period [12].
AVN avascular necrosis hip necrotic angle MRI

Figure 4. Calculation of necrotic angle, adopted from [12])

The Association Research Circulation Osseous (ARCO) system is the most common staging system used, with many features encompassing the older systems. Most systems are based on the presence or absence of several factors, including (1) head collapse, (2) lesion size, (3) presence and/or degree of head depression, and (4) acetabular involvement. Older staging systems are still used by some and include the Ficat-Arlet staging system, the Hungerford-Lennox system, the Steinberg classification system and the Ohzono system [13].
ARCO classification of avascular necrosis hip AVN

Table 1. ARCO classification system (adopted from [13])

AVN avascular necrosis hip illustration and staging

Figure 5. Progression of AVNFH (adopted from [13])

In summary, avascular necrosis of the femoral head is a debilitating condition that is caused by apoptosis of bone cells that lead to collapse. It accounts for more than 10% of total hip arthroplasties in the United States and affects mostly middle aged individuals. Most cases are linked to abundant alcohol or steroid use, but many other associations and risk factors exist. Most patients describe non specific pain that is typically out of proportion with their x-ray images in the early stages. Plain x-ray is the first step to aid in diagnosis, but MRI should be performed if AVNFH is suspected by history and examination. Many classification systems exist, but the ARCO and Steinberg classifications are most common in the United States.
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