Introduction

Avascular necrosis of the femoral head is a type of osteonecrosis due to disruption of blood supply to the proximal femur. There are approximately 10000 to 20000 new cases reported each year in the United States alone.[1] It can occur due to a variety of causes, either traumatic or atraumatic in origin. These causes include fractures, dislocations, chronic steroid use, chronic alcohol use, coagulopathy, congenital causes; among many others. Avascular necrosis of the femoral head is a debilitating disease and is an important condition requiring healthcare professionals to be vigilant for its presentation. This activity will provide an overview of the etiology and management, along with pertinent clinical pearls.

The majority of the blood supply to the head of the femur comes from the medial and lateral circumflex branches of the profunda femoris, which itself is a branch of the femoral artery (the profunda femoris is the deep penetrating branch of the upper thigh). The medial and lateral circumflex femoral arteries anastomose to form a ring around the neck of the femur, from which many small arteries branch off to perfuse the femoral head. Another direct source of blood supply is from the foveal artery, aka the artery of the ligamentum teres. The ligamentum teres is the ligament connecting the head of the femur to the acetabulum. The foveal artery runs within the ligament, though its contribution is only significant in pediatric populations.[2] Two important anastomoses provide collateral blood flow (though limited) to support the femoral head. One is the cruciate anastomosis, which is between the inferior gluteal artery and the medial circumflex femoral artery. The other is the trochanteric anastomosis, which is between the superior gluteal artery and medial/lateral circumflex femoral arteries. Both the superior and inferior gluteal arteries are branches off of the internal iliac artery (also known as the hypogastric artery) which is the main artery of the pelvis and supplies part of the buttock and posterior thigh.[2] The internal iliac artery comes from the common iliac artery, which in turn comes from the aorta. The femoral head articulates with the acetabulum, which is the “socket” of the hip. The acetabulum’s blood supply comes mainly from the acetabular branch of the obturator artery, along with contributions from pubic branches of the obturator artery, and deep branches of the superior gluteal artery.[3] Because of limited collateral circulation, disruption of the blood supply to the head of the femur can lead to ischemia and subsequent necrosis. If restoration of blood supply does not occur promptly, this will lead to the progressive death of osteocytes followed by the collapse of the articular surface, and eventually by degenerative arthritis.[4]

Etiology

Many different etiologies can precipitate this condition.

One of the most common traumatic causes is femoral neck fracture or dislocation of the femoral head from acetabulum. When these types of trauma occur, the blood supply to the head of the femur can be easily disrupted, leading to avascular necrosis. Osteonecrosis is the result in 15% to 50% of fractures of the neck of the femur and 10% to 25% of hip dislocations.[5] 

Chronic steroid use and excessive alcohol consumption represent the bulk of non-traumatic etiologies, contributing to more than 80% of them. Steroid-associated osteonecrosis represents the second most common cause of osteonecrosis overall, after trauma. Despite evidence demonstrating the correlation between steroid use and osteonecrosis, the exact pathophysiology is not clear and probably multifactorial. The cause is most likely an aggregate of factors such as fat emboli, fat cell hypertrophy leading to increased intraosseous pressure, endothelial dysfunction, hyperlipidemia, and abnormality of the stem cell pool of the bone marrow; all of which contribute to ischemia and subsequent necrosis.[6] Alcohol-induced osteonecrosis is similarly not well understood but most likely stems from bone marrow fat cell hypertrophy and proliferation, serum lipid level changes, blood vessel occlusion, increased intraosseous pressure, and subsequent lack of perfusion.[7] 

Sickle cell disease can often precipitate osteonecrosis. The misshapen and rigid red blood cells impede blood flow leading to ischemia and bony infarction, with the femoral head being the most common site of osteonecrosis in these patients.[8] Autoimmune and chronic inflammatory disorders, e.g., systemic lupus erythematosus (SLE), are well-known to be associated with osteonecrosis of the femoral head. The risk of developing the condition in these patients is usually attributed to long-term steroid treatment, though there are reports of cases in those who are steroid naive.[9] 

There are other less common, but still clinically important causes, one of which is Legg-Calve-Perthes. Legg-Calve-Perthes is idiopathic avascular necrosis of the femoral head that affects pediatric populations. Lack of blood supply causes necrosis of the femoral head leading to a deformity that puts the patient at high risk of developing osteoarthritis and/or losing range of motion (ROM). This disease progresses through four stages[10][11]:

1. Initial/necrosis – when blood supply gets disrupted, and necrosis begins
2. Fragmentation – when the body resorbs the necrotic bone and replaces it with woven bone that is weak and prone to breaking and collapse
3. Reossification – when stronger bone develops
4. Healed/Remodeling – when bone regrowth is complete, and shape becomes finalized (whether normal or abnormal, depending on the damage done during the fragmentation phase)

Vascular disease secondary to diabetes, as well as direct damage from cytotoxic agents have also been implicated in the development of femoral head osteonecrosis.[12] 

Epidemiology

The incidence of avascular necrosis of the femoral head within the United States is estimated to occur at a rate between 20000 to 30000 new cases each year, contributing to 10% of the approximately 250000 total hip arthroplasties performed annually.[13] This percentage is comparable to the incidence of 0.01% found in German-speaking countries and the incidence of 1.9 per 100000 found in Japan.[14] There is no association with race, except regarding cases associated with sickle cell disease, which is more prevalent in patients of African descent. Overall, this condition is more prevalent in men than women, with studies estimating ratios from 3 to 1 to 5 to 1.[15][16] The average age at treatment is 33 to 38 years old.[1] 

Pathophysiology

The exact pathophysiologic mechanisms behind avascular necrosis of the femoral head are not always clear and generally regarded as being multifactorial.[17] Regardless of the precipitating factor, the outcome is essentially the death of osteocytes and bone marrow that results from insufficient blood flow to the subchondral bone of the proximal femur.[5] This cell death will inevitably lead to a collapse of the femoral head and subsequent osteoarthritis, if not treated effectively in the early stages. 

History and Physical

Patients may be asymptomatic early on in the disease process. When they become symptomatic, however, the stated history is usually of hip pain that may radiate to the groin and/or thigh. The pain is typically aggravated by activities such as walking and climbing stairs and alleviated by rest. The pain will often still be present, even in the absence of movement.[17] Some examples of physical exam findings indicative of osteonecrosis of the femoral head are restricted range of motion, pain upon abduction and internal rotation, and tenderness to palpation of the hip region.[18] 

Evaluation

Early identification can significantly affect outcomes. Diagnosis is made by pairing the clinical presentation with appropriate imaging. Imaging can include x-rays, radionuclide bone scanning, and magnetic resonance imaging (MRI). The use of imaging in the context of the patient’s symptoms can help guide appropriate treatment.

Plain-film radiography is typically obtained in two planes, utilizing both anterior-posterior and frog-leg lateral films. Radiographs may show subchondral radiolucency, which is the pathognomonic “crescent sign,” indicating subchondral collapse. Uptake of Technetium-99m can show a “donut sign,” which is a ring of increased uptake around a cold center. This sign represents accelerated bone turnover at the demarcation, where reactive bone meets the dead bone.[13] MRI is the gold standard of diagnosis for osteonecrosis. Though both X-rays and radionuclide scans can aid in the diagnosis, neither is as sensitive as MRI; nor as reliable at showing radiographic evidence early in disease progression. MRI can visualize bone marrow changes, size/location of the necrotic area, the effect on acetabular cartilage, depth of collapse, etc.; which are incredibly helpful when ascertaining a patient’s prognosis and formulating a plan of care.[17] 

Once obtaining adequate imaging, the extent of necrosis can be classified. While multiple staging systems exist, the most commonly used is the Steinberg staging system. It identifies seven stages as follows:

Steinberg staging system[13]

Stage   Features

0          Normal radiograph, bone scan, and MRI

I          Normal radiograph, abnormal bone scan and or magnetic resonance imaging

           IA Mild (involves less than 15% of the femoral head).

           IB Moderate (involves 15% to 30% of the femoral head)

           IC Severe (involves over 30% of the femoral head)

II         Cystic and sclerotic change of the femoral head

           IIA Mild (involves less than 15% of the femoral head)

           IIB Moderate (involves 15% to 30% of the femoral head)

           IIC Severe (involves more than than 30% of the femoral head)

III       Subchondral collapse (crescent sign) without flattening of the femoral head

           IIIA Mild (involves under 15% of the femoral head)

           IIIB Moderate (involves 15% to 30% of the femoral head) 

           IIIC Severe (involves over 30% of the femoral head)

IV        Flattening of the femoral head/femoral head collapse

           IVA Mild (involves under 15% of the femoral head)

           IVB Moderate (involves 15% to 30% of the femoral head)

           IVC Severe (involves greater than 30% of the femoral head)

V         Joint space narrowing and/or acetabular changes

           VA Mild

           VB Moderate

           VC Severe

VI        Advanced degenerative joint disease

A laboratory workup should be done to help rule out other causes of hip pain as well as to assess for comorbid factors in patients with suspected osteonecrosis. A workup may include a complete blood count (CBC), lipid panel, erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), rheumatoid factor (RF), anti-nuclear antibody (ANA), anti-cyclic citrullinated peptide (anti-CCP), and hemoglobin electrophoresis. Elevated ANA and/or RF would indicate an active autoimmune process but are non-specific. Both ESR and CRP are elevated by inflammatory processes but are also non-specific. Elevated anti-CCP antibodies are specific for rheumatoid arthritis, while hemoglobin electrophoresis showing HbS with a low concentration of HbF would indicate sickle cell disease.[19][20] A CBC showing evidence of normocytic or microcytic anemia with an elevated reticulocyte count would also be consistent with a diagnosis of sickle cell disease. Rheumatoid arthritis and sickle cell disease are two conditions that can precipitate the development of osteonecrosis in the femoral head and can cause hip pain even without osteonecrosis. 

A biopsy is not often necessary, as the diagnosis can be made accurately based off imaging and clinical presentation. If a biopsy is performed, however, the characteristic histological findings will be trabecular necrosis (more than 50% of empty osteocytic lacunae) and necrotic hematopoietic marrow without specific evidence of inflammation, tumor cells, or sepsis.[21] Similarly, angiography studies are not routinely performed, though they do provide good visualization of the vasculature and may contribute to research and understanding of disease pathology.[22][19] 

Differential Diagnosis

Symptoms of avascular necrosis of the femoral head overlap with other etiologies that are important to include in a differential diagnosis. One is bone marrow edema syndrome (BMES), aka transient osteoporosis, that usually occurs as a reactive response to an injury, increases in physical activity, or to osteoarthritis. This condition presents as rapid-onset, sometimes atraumatic, hip pain that is self-limited and typically resolves within one year. MRI is usually significant for widespread bone marrow edema, which gives the disease its eponymous title.[35] Another condition that can become confused for osteonecrosis of the proximal head of the femur is a subchondral fracture, which usually occurs as a fracture after minor trauma in the elderly, in the context of osteoporosis leading to subchondral insufficiency.

Problems to consider in the differential include the following:

• Complex regional pain syndrome
• Inflammatory synovitis
• Neoplastic bone conditions
• Osteomyelitis
• Osteoarthritis
• Osteoporosis
• Soft tissue trauma

Prognosis

The prognosis of femoral head osteonecrosis is dependent upon many factors. One is the point in which it gets diagnosed. The earlier in the disease process the diagnosis occurs, the more effective the preemptive measures, and the better the prognosis. In addition to advanced disease at the time of diagnosis, some other indicators of a poor prognosis include lateral head involvement (as opposed to medial), the involvement of greater than one-third of the weight-bearing portion of the femoral head (as determined by MRI), and age greater than 50-years-old[36]. Even with the presence of one or more of these factors, a patient’s prognosis can vary and should be determined by a qualified physician after appropriate evaluation.[37] 

Complications

Complications include progressively worsening joint pain, restricted range of motion, and osteoarthritis.[38] These complications can cause significant disability in patients.

Consultations

When a clinician suspects a patient of having osteonecrosis of the femoral head, consultation from a qualified orthopedic surgeon is for expert evaluation.

Deterrence and Patient Education

Patients should seek medical care if they are experiencing pain in the hip, thighs, and/or buttocks. Also, certain patient populations should be advised of the risk factors and screened as needed; since early avascular necrosis of the femoral head can be asymptomatic. Examples of higher-risk patients include those on long-term steroid treatment, long-term bisphosphonates, excessive alcohol use, hemoglobinopathies, chemotherapy or radiation patients, and those who suffer trauma to the hip and surrounding area.[13] 

Enhancing Healthcare Team Outcomes

Healthcare teams are often made up of many interprofessional members and can include (but are not limited to) a physician, nurse practitioner or physician assistant, pharmacist, and nurse. All such team members treating patients with avascular necrosis of the femoral head must be able to not only recognize the condition but also to participate in a competent management plan and obtain expert consultation when indicated. The pharmacist can help with assessing medication regimens, both leading up to AVN as well as attempts to treat AVN pharmaceutically, reporting any concerns to the physician.

If the patient requires surgery, an orthopedic specialist will lead the treatment, and nursing will assist in preparing the patient for surgery, monitor the patient during and after surgery, and check on treatment efficacy as well as administer post-operative medications, with an eye towards watching for adverse effects, which should be reported promptly.

The primary care clinicians, including nurses, should educate patients about the harms of alcohol. Also, clinicians should prescribe the lowest effective dose of corticosteroids. Nursing and physicians should ask patients on long term corticosteroids should be asked about hip symptoms at every clinic visit. The pharmacist should provide patient education on the necessity to quit smoking and can work with the physician on pharmaceutical aids to assist with smoking cessation. Patients should receive training about the symptoms of AVN so that they can seek help early.

Improved awareness and knowledge of avascular necrosis of the femoral head for each interprofessional healthcare team member will improve patient care and prognostic outcomes. [Level V]