Avascular Necrosis (AVN, Osteonecrosis) of the Femoral Head
Avascular necrosis, or bone death, of the femoral head occurs as a result of loss of blood supply to the area. Without adequate blood supply, the bone is deprived of nutrients. This inhibits the normal process of bone breakdown and remodeling. Causes of blood supply loss to the femoral head include a displaced femoral neck fracture, traumatic hip dislocation, or the use of corticosteroids. Eventually, the dead bone will attempt to heal itself but often heals irregularly or collapses. This results in an uneven joint surface that leads to painful wearing of the hip joint (arthritis).
Some cases of femoral head avascular necrosis present with little or no symptoms because blood supply regrows into the femoral head before it softens or collapses. Treatment of symptomatic avascular necrosis is mainly surgical. In less advanced forms of the disease (prior to collapse of the femoral head), surgical removal of the dead tissue (decompression) may be performed. The space created by removal of dead tissue is sometimes refilled with a bone graft. In more advanced cases, a total hip replacement is performed in order to ensure the removal of all dead tissue. Total hip replacement also serves to resolve any symptoms related to the development of hip osteoarthritis secondary to avascular necrosis.
Avascular necrosis, also called osteonecrosis, simply means “bone death”. Bone death can result from a toxin that kills the cells, but most often in the case of osteonecrosis of the femoral head, is due to loss of blood supply to the bone (ischemia). The femoral blood supply can be interrupted if the arteries leading to the femoral head are damaged, or if the small channels leading to the bone cells themselves are blocked. Blood supply can also be interrupted by conditions that block outflow or increase pressure within the bone, known as intra-osseous hypertension.
The interruption of blood supply leads to a failure of bone remodeling, the process by which cells remove worn bone and deposit new bone. This metabolic process requires energy and oxygen, and thus a healthy blood supply. When remodeling is interrupted, damage accumulates, leading to collapse or softening of the femoral head. This in turn leads to asymmetry within the hip joint -akin to a squarish peg trying to fit in a round hole. This asymmetry within the hip joint leads to uneven wear of the joint overtime with loss of joint cartilage and eventually early onset osteoarthritis.
Some cases of femoral head avascular necrosis are asymptomatic if the femoral head does not lose its shape or collapse before the blood supply is restored. However, symptomatic avascular necrosis of the hip usually causes a limp with walking and later progresses to limitations in range of motion of the hip. The level of pain present is dependent on the stage of the disease. In later stages, the head of the femur collapses, resulting in damage to the overlying cartilage. This leads to osteoarthritis. As such, the clinical presentation of hip arthritis applies here as well.
Males are more likely to develop avascular necrosis than females, and those affected are typically between 40-65 years of age. Traumatic dislocation of the hip and displaced fractures of the neck of the femur are common causes of avascular necrosis. Other risk factors include a history of sickle cell anemia, alcohol abuse, corticosteroid steroid use, and radiation therapy to treat cancer.
Individuals presenting with avascular necrosis may or may not have findings on physical examination. If there is end stage osteoarthritis caused by avascular necrosis, the physical examination will reflect that. There may be pain with motion of the hip joint, referred pain to the knee, and a limp with walking.
X-rays of the hip are a useful tool for the assessment of avascular necrosis and secondary osteoarthritic changes. Before there is collapse of the bone, x-rays may reveal an area of subchondral sclerosis, or hardening of the bone, within the femoral head. It is also quite common for x-rays to appear normal in the early stages of avascular necrosis. Patients in whom the diagnosis of avascular necrosis is suspected may also need an MRI to reveal the extent of the disease.
A barrier in the management of avascular necrosis is that the primary insult, death of bone cells, is largely undetectable. All that can be seen on imaging with certainty are the osteoarthritic changes associated with bone death, which occur at later stages of the disease. Considering this, treatment of symptomatic femoral head avascular necrosis is largely surgical. However, in patients who have been diagnosed with this condition, and in whom collapse of the femoral head has not yet occurred, it can be beneficial to limit weight-bearing on the affected hip until the bone revascularizes or surgery is indicated. Additionally, given that avascular necrosis leads to osteoarthritis, non-operative treatment that is normally applied to cases of osteoarthritis can be therapeutic. This includes pain medication such as NSAIDs, and activity modification to avoid activities that cause pain.
Symptomatic femoral head avascular necrosis that has not yet resulted in collapse of the head of the femur can be treated with a surgical decompression procedure, or removal of dead tissue. This can be taken a step further by filling the area with a bone graft that may or may not have its own blood supply (a vascularized graft).
The more commonly performed surgical procedure for avascular necrosis is a total hip replacement. Total hip replacement to treat avascular necrosis differs from other reasons for total hip replacement because these individuals are often younger than the typical patient suffering from hip osteoarthritis. As such, the muscles and ligaments surrounding the hip joint are usually more robust, and the individual may be at lower risk for certain surgical complications. On the other hand, young people tend to “beat up” their joints by using them more. Regardless, because the age of onset of osteoarthritis due to avascular necrosis is younger than that for osteoarthritis of other causes, even a long-lived prosthesis may not last for the individual’s entire lifespan. This means a revision total hip replacement may be required down the line.