What is AVN?
AVN – avascular necrosis – is a condition in which the ball – or femoral head – in a child’s hip joint loses its circulation. This can occur at any age, but it has different effects on infants, children, and adults.
What causes AVN?
AVN from developmental hip dysplasia (DDH) is a result of treatment but it doesn’t mean that your doctor did anything wrong. The circulation to an infant’s hip is naturally weak because the blood comes in from blood vessels below the hip because the blood supply can’t come in through the joint surface at the top of the ball . If the hip is dislocated, the blood vessels are kinked from the dislocation. Sometimes the blood vessels stop working when the hip is pulled down from the dislocated position and put back into the hip joint. AVN is more common when a lot of force is required to put the hip back into the joint or when an extreme position of the hip is used to hold the hip in the joint. Rarely, the blood supply can be damaged during surgery, but the usual cause is simply putting the hip back into the joint. The International Hip Dysplasia Institute (IHDI) is studying the frequency of AVN to see if there are ways to find more risk factors so we can find better ways to avoid AVN.
What happens after AVN occurs?
When an adult hip loses its blood supply, the hip usually deteriorates rapidly and causes severe arthritis. In children between the ages of 5 and 12 years, the condition is called Legg-Calvé-Perthes disease and the results are variable depending on severity and treatment. Loss of circulation in this 5-12 year age group can cause collapse of the ball so that it’s flat instead of round. Flattening of the ball in this age group limits activities and leads to early arthritis.
AVN in infants and very young children doesn’t act anything like AVN after the age of six years, so there may be some confusion when you look up AVN online and see what happens to adults or slightly older children. When AVN occurs in babies or young children, the hip stays round and eventually recovers its blood supply, but the temporary loss of circulation can damage the growth plate of the upper end of the thigh bone. As the child grows, the leg on the affected side may be shorter, or the neck of the femur may be short and cause a limp.
This is an 8 year and 3 month old girl who developed AVN after closed reduction and spica cast for a dislocated right hip at age 16 months. You can see the difference between the normal left hip and the shortened right hip from growth disturbance. The good news is that the ball is round and she is not likely to develop arthritis at a young age. Surgery around the age of 12 years can restore her leg lengths to equal and the hip to a more normal shape, but it’s usually best to wait until about that age to do anything for AVN that develops from DDH treatment.
A bigger problem that often goes along with AVN is incomplete reduction of the hip. There’s no need to treat the AVN at a young age, but incomplete reductions need to be corrected whether there’s AVN or not.
How do you identify AVN?
AVN is identified after treatment with a Pavlik harness, a cast, or surgery when the bone in the ball of the hip fails to appear on x-ray or fails to grow for a period of one year. Once it does start to appear, it looks different from the other hip. The x-rays are all that’s needed to diagnose AVN after DDH. Tests like MRI, bone scans, or CT scans are used to diagnose AVN in adults of in older children with Legg-Calvé-Perthes disease, but these tests are rarely helpful before the age of five years. There’s no need to diagnose AVN right away in babies or young children because there is no treatment. The focus of treatment in this age group needs to be on making sure the hip stays in the socket because the growth disturbance from AVN takes time to show up and more time before treatment.
This is a 19 month old girl who was treated with a Pavlik harness at age 3 months. She is 16 months after treatment and the bone that should be in the ball of both hips has not appeared, although there is a faint appearance of some bone on the right side. Both hips are in the sockets and there is no need for more treatment at this age. Eventually, she may need surgery if the shape of the upper thigh becomes abnormal with growth. Compare this to the hips of the child below.
This is a 19 month old boy who was treated with a Pavlik harness at the age of eight months. You can see that both hips have formed normally and the bone in the ball of each hip is large and round. This boy did not develop AVN.
AVN is a serious problem because it means that the child is not finished with treatment. Every doctor who treats DDH occasionally has a child with AVN and this is worrisome to the doctors and the parents because we cannot always look back and see why it happened. However, there is no need for early treatment except to make sure the hip is in the socket. There are four grades of AVN and grade 1 rarely causes trouble, but grade 4 always needs more surgery around the age of 8-12 years. The only good way to tell the grades is to follow the child and see what happens. The IHDI is doing all it can to study and teach methods that may decrease the risk of AVN.