Bones are in a continual process of breaking down, absorbing the old bone and rebuilding new bone. Bones rebuild themselves after an injury, as well as during normal growth. In AVN, bones break down faster than the body can rebuild them. If untreated, avascular necrosis leads to the bones and joint surfaces breaking down and collapsing, this in turn leads to pain and arthritis. Although people can get AVN at any age, individuals between 30 and 50 are at the greatest risk for AVN.
At first, patients with AVN experience no symptoms. The time it takes for the disease to progress from first symptoms to loss of joint function varies by patient, and may range from months to more than a year. Over time as the disease progresses, most patients have stiffness and then pain in the affected joint. The condition also may lead to muscle spasms in the affected area. At first, the pain only occurs when weight is put on the affected joint, but over time the joint hurts all the time. The pain may be mild or severe, and usually develops gradually. If the joint surface collapses, pain may increase suddenly. The range of motion in the affected joint is sometimes limited. If AVN strikes the hip joint, disabling osteoarthritis may develop.
Cause and Risk Factors
AVN may be caused by an injury, such as a dislocated joint, called trauma-related avascular necrosis, or by various risk factors, called non-traumatic avascular necrosis. When a joint is fractured or dislocated, damage to the blood vessels may occur, compromising blood flow. This type of trauma-related avascular necrosis affects about 20 percent of people who dislocate a hip joint.
When used for a long time, steroid medications, which are used as anti-inflammatories to combat rheumatoid arthritis, irritable bowel disease and vasculitis, can lead to non-traumatic avascular necrosis. Such long-term use of steroids appears to be related to about 35 percent of cases of non-traumatic AVN. Although the cause is unknown, it may be that steroid use decreases the body's ability to break down fatty substances in blood vessels, which then block blood flow to bones.
Alcohol abuse is a common cause of non-traumatic AVN. Although the exact mechanism by which this occurs is not well understood, it is thought that people who drink alcohol excessively may harm their body's ability to break down fatty substances, which can build up in blood vessels, hence reducing the flow of blood to bones.
A number of other conditions also carry the risk of getting AVN, including Gaucher's disease, pancreatitis, chemotherapy and radiation treatments, decompression diseases, such as the bends as a result of scuba diving, and blood disorders, such as sickle cell disease.