Risk Factors for AVN
Avascular Necrosis (AVN) is a worrisome complication from the treatment of hip dysplasia (learn more about AVN here). As the name suggests, this occurs when there is loss of the blood supply to the ball of the femur (femoral head) when the ball is put back into the socket during treatment. Many times the baby recovers without any long-term consequences, but severe loss of blood supply can cause growth disturbances and lead to extra surgery.
Known risk factors include excessive pressure on the femoral head when a severe dislocation is popped back into the hip socket. This occurs because the muscles become tight when the hip is pulled down to the level of the socket. Extreme positions in a cast or brace may increase the risk of AVN, especially for more severe dislocations.
One question that has puzzled doctors now has an answer according to a paper published in May, 2017 in the Journal of Bone and Joint Surgery and co-authored by a member of the IHDI Medical Advisory Board (view the complete paper here). That question is whether it helps to wait for the hips to show early signs of growth before beginning vigorous attempts at putting the hip back into the socket. Before the age of approximately six to twelve months, the femoral head is totally cartilage without any bone showing on x-ray. As the hip grows, some bone forms in the center of the ball to give added support. This bone center is called the “ossific nucleus”, but the appearance of the ossific nucleus may be delayed when the hip is dislocated.
Some doctors have the opinion that the cartilage ball is softer and more likely to be damaged before the bone center has appeared on x-ray. This has led them to postpone closed or open reduction until the child is older after the hip has developed the bone center. However, the recent report in the Journal of Bone and Joint Surgery has conclusively determined that the presence of the bone center does not have an effect on the risk of AVN.
The authors of the study counted the results of twenty-one published papers that reported reduction of hip dislocations. There were a total of 608 cases that were reduced before the bone center was present, and 969 cases that were reduced after the appearance of the bone center. There were no differences in the risk of AVN between these two groups of patients. A previous study published in 2009 had similar findings but this recent report includes four times more patients than the 2009 report. This makes the current study much more reliable because of the larger number of patients that were reviewed.
This means that closed reduction can proceed when it is needed instead of waiting for the bone center to appear on x-ray. This is good news because earlier reduction generally leads to better hip development. One worrisome part of the report is that although the ossific nucleus didn’t play a role in the risk of AVN, the amount of AVN is still too high. The IHDI and other researchers are trying to find ways to decrease the risk of AVN, but now we can eliminate the bone center as a factor that has consumed a lot of research time and expense.