Bones are living tissue relying on blood vessels to bring blood to keep them alive. Most living tissues have blood vessels that come from many directions into the tissue. If one blood vessel is damaged, it may not cause problems. There may be a backup blood supply coming in from a different direction. Certain joints of the body including the hip have only a few blood vessels that bring in blood. When this blood supply is damaged, it usually results in what is called avascular necrosis of the hip.
The hip joint comprises the hip socket (acetabulum) that surrounds the ball of the upper thighbone. The thighbone itself is called the femur, and the ball on the end is the femoral head. The hip joint is surrounded by the thick muscles of the buttock at the back and the thick muscles of the thigh in the front.
The surface of the femoral head and the inside of the acetabulum are covered with articular cartilage. The articular cartilage surface is a tough, slick material that allows the bones to slide against one another without damage.
The blood supply enters the femoral head through the neck of the femur bone, a thinner area of bone that connects the ball to the shaft. If this blood supply is damaged, there is no backup. Damage to the blood supply can lead to the death of the femoral head. This is referred to as avascular necrosis of the hip or AVN. Once this occurs, the bone is no longer able to maintain itself.
Living bone is always changing. To maintain a bone's strength, bone cells are constantly repairing the wear and tear damage to bone tissue. If this process stops, the bone can begin to weaken. Eventually, the bone structure begins to collapse.
When avascular necrosis occurs in the hip joint, the top of the femoral head collapses and begins to flatten. This is where most of the weight is concentrated. The flattening creates a situation where the femoral head no longer fits perfectly inside the socket. The joint begins to wear itself out. This leads to osteoarthritis of the hip joint and arthritis pain.
Many things can cause avascular necrosis. Anything that damages the blood supply to the hip can cause avascular necrosis.
Injury to the hip itself can damage the blood vessels. Fractures of the femoral neck (the area connecting the ball of the hip joint) can damage the blood vessels. Hip dislocation can tear the blood vessels. It usually takes several months for avascular necrosis to show up. It can even show up and become a problem up to two years following this type of injury.
Some medications are known to cause avascular necrosis. Cortisone is the most common drug known to lead to avascular necrosis. This is usually only a problem in patients who must take cortisone every day due to other disease processes, such as advanced arthritis, or to prevent rejection of an organ transplant. Avascular necrosis has not been proven to be caused by short courses of treatment with cortisone, such as one or two injections into joints to treat arthritis or bursitis.
A clear link exists between avascular necrosis and alcoholism. Excessive alcohol intake somehow damages the blood vessels and leads to avascular necrosis. Deep-sea divers and miners who work under great atmospheric pressures also are at risk for damage to the blood vessels. The pressure causes tiny bubbles to form in the blood stream, which can block the blood vessels to the hip, damaging the blood supply.
The first symptom of avascular necrosis is pain when weight is placed on the hip. The pain can be felt in the groin area, the buttock area, and down the front of the thigh. As the problem progresses, the symptoms include development of a limp when walking and stiffness in the hip joint. Eventually, the pain will also be present at rest and may even interfere with sleep.
The diagnosis of avascular necrosis begins with a history and physical examination. Your doctor will want to know about your occupation, what other medical problems you have, and your medication use. You'll be asked whether you drink alcohol. A physical examination will be done to determine how much stiffness you have in the hip and whether you have a limp. Once this is done, X-rays or an MRI will most likely be ordered. Magnetic resonance imaging (MRI) is probably the most common test used to look for AVN of the hip. The MRI scan is very sensitive and can show even small areas of damage to the blood supply of the hip within hours after the damage has occurred.
Once avascular necrosis has occurred, treatment choices depend on how far along the problem is and your symptoms. While your symptoms may be reduced with pain medications and anti-inflammatory medications, no medical treatments will completely restore the blood supply to the femoral head and reverse the avascular necrosis. If the femoral head has not begun to collapse, your doctor may suggest an operation to try to increase the blood supply to the femoral head.
Decompressing the Femoral Head
Decompressing the femoral head involves drilling one or several holes through the femoral neck and into the femoral head to reach the area lacking blood supply. The purpose of this surgery is to create a channel for new blood vessels to quickly form into the area lacking blood supply, and to relieve pressure inside the bone of the femoral head. Relieving pressure may help decrease the pain you experience from avascular necrosis.
Fibular Bone Graft
Another procedure for increasing the blood supply to the femoral head is a vascularized fibular bone graft procedure. This operation is actually a transplant.
The surgeon removes a piece of bone from your lower leg (fibula) along with the blood vessels to the bone. He or she then drills a hole through the side of the femur and into the femoral head. The surgeon attaches the blood vessels from the bone graft to one of the blood vessels around the hip. This creates instant blood flow into the bone graft and into the head of the femur. This operation brings blood flow to the femoral head through the bone graft. Also, the fibular bone graft helps keep the femoral head from collapsing as the bone heals itself.
Artificial Hip Replacement
When avascular necrosis is in the advanced stages, the condition is no different from osteoarthritis of the hip joint. Your doctor will probably recommend replacing the hip with an artificial hip joint.
After decompression of the femoral head, you will probably use crutches for six or more weeks. The drill holes weaken the hipbone, making it possible to fracture your hip. Using crutches allows your bone to heal safely and reduce the risk of fracturing your hip. Patients who have had bone and blood vessels grafted are required to limit how much weight they place on the hip for up to six months.
Your doctor will let you know when you can put full weight on your leg. Your doctor may also have you work with a physical therapist to help regain hip range of motion and strength.
Patients who require artificial hip joint replacement will follow a structured program of physical therapy beginning shortly after surgery.