Avascular necrosis ( AVN, also known as aseptic necrosis, osteonecrosis, or ischemic necrosis ) is bone death due to compromised blood supply. Necrosis can occur when capillaries are occluded by sickled erythrocytes at distal portions of a bone near a joint where hypoxia is maximal and collateral circulation is inadequate. The hip joint is the most common site of avascular necrosis. Involvement of the shoulder and other joints is less common.
Risk factors for avascular necrosis of the femoral head include sickle cell disease ( SCD ) genotype, age, frequency of painful episodes, hemoglobin level, and alpha-gene deletion. The overall prevalence of avascular necrosis in sickle cell disease is about 10%, whereas in people with hemoglobin SS, it is about 50% by age 33.
People with HbSS and concomitant alpha-thalassemia are at particular risk. The SCD genotypes that are associated with relatively mild anemia, such as HbSS-alpha-thalassemia and HbSbeta0-thalassemia, are at a particularly high risk to develop avascular necrosis at a younger age.
Avascular necrosis of the femoral head causes chronic severe pain and disability. The pain is generally worse on walking, relieved by rest, and may be accompanied by a moderate or severe limitation of motion when the patient bears weight on the affected extremity.
About 40–80% of cases of avascular necrosis of the hips are bilateral and, hence, evaluation of patients with avascular necrosis should focus on both hips.
The therapeutic approach to avascular necrosis depends on the stage of the disease. Ficat proposed a four-stage radiographic classification of avascular necrosis of the hip based on plain radiography. MRI was not available at the time.
Steinberg et al. expanded the Ficat staging system into six stages using MRI data.
A report from the Comprehensive Sickle Cell Centers ( CSCC ) investigators defined an adaptation from the Ficat and Steinberg systems that combines radiography, MRI, and bone scans.
Most orthopedists consider core decompression to be most beneficial for Ficat stage I and II of avascular necrosis of the hip.
Stages of avascular necrosis: EARLY: Stage 0. Preclinical; Radiographic signs: none; marrow necrosis may be present histologically; EARLY: Stage I. Preradiographic; Radiographic signs: none; abnormal MRI with marrow and bone necrosis; EARLY: Stage II. Before flattening of head or sequestrum formation; Radiographic signs: diffuse porosis, sclerosis, or cysts; TRANSITION: Radiographic signs: femoral head flattening; crescent sign; LATE: Stage III. Collapse; Radiographic signs: broken contour of head, sequestrum, joint space normal; LATE: Stage IV. Osteoarthritis; Radiographic signs: flattened contour, decreased joint space, collapse of head. ( Xagena )
NIH - Evidence-Based Management of Sickle Cell Disease - Expert Panel Report, 2014