Causes / typical injury mechanism: Osteonecrosis (also called avascular necrosis [AVN] or aseptic necrosis) of the femoral head results from disrupted blood supply to the bone in this region. Femoral neck fractures, hip dislocations, and bone marrow injuries are common causes of traumatic osteonecrosis. The etiology of atraumatic cases is poorly understood, but glucocorticoids and excessive alcohol use are associated with AVN onset and progression.

Classic history and presentation: Patients describe achy, throbbing groin pain that may be referred to the knee or buttock. In more advanced disease stages, patients experience limited range of motion and difficulty bearing weight on the affected leg.

Prevalence:

• Osteonecrosis of the femoral head typically presents between the ages of 35 and 50 years and is more common in males than in females.
• Bilateral hips are affected in 80% of cases.
• Three percent of patients with atraumatic AVN have multifocal involvement (3 or more sites).

Risk factors:

• Trauma, especially femoral neck fracture
• Glucocorticoids
• Marrow-replacing diseases (eg, Gaucher disease)
• Hematologic diseases (eg, sickle cell disease)
• Caisson disease
• Irradiation
• Systemic lupus erythematosus
• Alcohol use disorder
• Hypercoagulable states
• Organ transplant
• Viruses
• Protease inhibitors

Pathophysiology: The femoral head receives its blood supply from the obturator and femoral circumflex arteries. This configuration leaves a "watershed" region with limited vascular supply that is particularly susceptible to ischemia. Traumatic cases are typically caused by damage to the medial femoral circumflex artery. Atraumatic cases result from arterial occlusion due to coagulation and thrombosis. Reduced blood flow to the femoral head leads to osteocyte death, bone resorption and disordered formation, loss of structural integrity, trabecular fracture, subchondral bone collapse, joint incongruity, and ultimately degenerative arthritis.

Grade / classification system: The Ficat classification system is based on plain x-rays and the patient's symptoms.

• Stage 0: normal x-rays, asymptomatic; "silent hip" if patient has known contralateral AVN
• Stage I: normal x-rays or subtle trabecular blurring or osteopenia; pain and limited hip range of motion
• Stage II: normal contour of the femoral head but with cystic or sclerotic evidence of bone remodeling; worsening clinical signs
• Stage III: "crescent sign," abnormal contour or flattening of the femoral head from subchondral collapse with joint space preserved
• Stage IV: degenerative osteoarthritis with narrowed joint space and osteophytes; progressively diminished hip motion