Osteonecrosis

What Your Doctor May Not Tell You About Hip and Knee Replacement Surgery
by Ronald P. Grelsamer, M.D.

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Osteonecrosis, Aka Avascular Necrosis (AVN)

It's easy to think of bone as a piece of wood. That's because the only bone you've ever seen (presumably) is on your dinner plate. But bone is a very living tissue. It is richly supplied with blood vessels and it is constantly being broken down and built back up.

When blood is unable to reach part of a bone, that part literally dies. That is because bone is made of cells like any other human tissue, and when cells are deprived of oxygen, they die. This is akin to the bone having suffered a stroke. The term osteonecrosis literally means “bone death.” The term avascular necrosis is not as helpful. It is actually a redundancy because the word necrosis already implies death due to a lack of blood supply.

Nevertheless, its abbreviation AVN is commonly used, probably because it is less of a mouthful than osteonecrosis or avascular necrosis. AVN, avascular necrosis, and osteonecrosis all denote the same condition.

For reasons that are not always completely understood, there are bones in the human body that are subject to seeing their blood supply interrupted. In the case of the thighbone (femur), it's not the whole bone that is at risk, just the two ends: the femoral head at the hip and the femoral condyles at the knee.

Sometimes there exists a clear-cut risk factor for a person to develop AVN: At particular risk are patients with sickle cell anemia and Gaucher's disease (a congenital “lipid storage” liver condition that allows large, vessel-clogging molecules to form), patients who have taken steroid medications (not including injections into tendons and joints), patients who ingest more than moderate amounts of alcohol, deep-sea divers who come up too quickly, and patients who have sustained specific types of trauma.

Some of these risk factors are relatively easy to understand: In sickle cell anemia the red cells clump together and clog small arteries called arterioles. If these arterioles represent the only source of blood for a particular section of bone, it's going to be a bad day for that piece of bone when those arterioles become clogged. The use of steroids is thought to increase the amount of tiny fat globules in the bloodstream, and these tiny globules can block an arteriole just like a clump of red cells.

It is important here to distinguish between different types of steroids. The steroids that can cause AVN are steroids taken by mouth or by injections into muscles (IM injections).

Such steroids are given for a multitude of medical conditions including asthma, inflammatory arthritis (see above), organ transplants, head trauma, and chronic pain. Prednisone is typical of this category. Prednisone and related steroids are very different from the steroid preparations injected directly into a joint, such as Kenalog, Depo-Medrol, and Celestone. The latter are designed to work specifically on the joint that has been injected. They are not supposed to have so-called systemic effects, i.e., effects throughout the entire body. Sure, a small amount is certainly absorbed from the joint into the rest of the body, but by and large this small systemic dissemination has little effect. So with this second type of steroid you don't see the common side effects of prednisone such as swelling of the face, raising of blood sugar, and osteoporosis.

Most significantly, you don't develop AVN from injections into a joint, assuming the injections are given at a reasonable frequency (for most patients three injections per year in a given joint would be considered reasonable).

Alcohol can affect the liver's ability to metabolize fats and it has been postulated that this can also lead to clogging of certain bone arterioles. Fractures can occasionally damage the arteries feeding a certain section of bone. This is particularly true of certain hip fractures. Divers who surface too quickly suffer from a condition called the bends. Nitrogen comes out of solution and forms bubbles. These bubbles, like clumped red cells and fat globules, can also clog arterioles.

But for half the patients suffering from AVN, there is no discernible cause. It just happens. When doctors cannot explain where a condition comes from, they term the condition idiopathic. If the doctor says you have idiopathic AVN, it means he or she can't identify any risk factor that would have caused your bone necrosis. Note that as more risk factors are identified, fewer people with AVN will be said to have the idiopathic variety. For example, I'm willing to bet that in the not too distant future we'll identify a certain gene that predisposes someone to the condition.

Investigators have noted that a bone with AVN builds up abnormal amounts of pressure. It remains to be determined whether this is the result or partly the cause of AVN. How old are patients with AVN? Patients with AVN of the hip tend to be on the younger side, the average age being close to forty. Patients developing AVN of the knee are in their sixties or early seventies. Interestingly, though AVN can take months or years to develop, the pain often comes on precipitously, especially about the knee. Patients report pain coming out of the blue, a lightning bolt out of the sky. Hip AVN is much more common in men, but knee AVN is more common in women.

The natural progression of AVN. Sometimes the part of the bone that is affected by AVN is tiny. Bone being living tissue, it gobbles up the tiny dead part and replaces it with healthy, living bone. You may never have had a day of pain. If the part of the bone that is affected is bigger, you may develop pain, but the body may still be able to heal itself. Once it reaches a certain size, though, the condition progresses: Parts of the bone become rock hard, other parts become soft as the body tries to introduce new bone, and certain areas literally collapse. The collapse usually takes place near the surface-in other words, near the articular cartilage. This is referred to as subchondral collapse. In the hip joint, the bone, instead of being convex, appears concave over the affected area.

This is called the crescent sign. Over time the cartilage deteriorates, and the bones on both sides of the joint rub together, producing a frankly arthritic picture.

Copyright © 2004 by Ronald Grelsamer, M.D.

About the Author

RONALD P. GRELSAMER, M.D., is the chief of hip and knee reconstruction at Maimonides Medical Center, and a staff orthopaedic surgeon at the NYU Hospital for Joint Diseases/Orthopaedic Institute.