A little understanding of the anatomy of the child’s hip will help explain this phenomenon.
In the growing child, there are special structures at the end of most bones called growth plates. The growth plate is sandwiched between two areas of the bone called the epiphysis and the metaphysis. The growth plate is made of a special type of cartilage that builds bone on top of the end of the metaphysis and lengthens the bone as we grow. In the hip joint, the femoral head is one of the epiphyses of the femur.
The capital femoral epiphysis is somewhat unique. It is one of the few epiphyses in the body that is inside the joint capsule. The joint capsule is the tissue that surrounds the joint.
Here’s the key to osteonecrosis developing: the blood vessels that go to the epiphysis run along the side of the femoral neck and are in danger of being torn or pinched off if something happens to the growth plate. This can result in a loss of the blood supply to the epiphysis and then death of the bone (osteonecrosis) that your surgeon told you about.
The question about what can be done to prevent osteonecrosis has received a great deal of attention and study. There are many proposed theories: length of time between diagnosis and surgery, amount of slippage and pressure inside the joint capsule, age at the time of diagnosis, body weight, and so on.
In a recent study (probably one of the largest studies done on this topic), pediatric orthopedic surgeons from The Children’s Hospital of Philadelphia (CHOP) report finding three factors that might be significant.
The first risk factor for osteonecrosis after surgery to stabilize SCFE was age. Younger children with very little warning symptoms before the problem was diagnosed had a higher incidence of osteonecrosis after surgery. The second was severity of slippage at the time of diagnosis. It’s likely that the more severe the problem and the greater the instability, the shorter the time before symptoms develop.
The third significant factor was based on the type of surgery that was done. The children in the study from Children’s Hospital of Philadelphia fell into one of three groups based on the type of surgical treatment. Group one consisted of children who had the slipped epiphysis held in place with a screw. This procedure is called in situ screw fixation.
Group two had a closed reduction. Reduction means the slipped epiphysis went back into place. Closed tells us this happened without open surgery with an incision. Sometimes just positioning the hip in a certain way will reduce or realign the hip. This can happen while moving the child or placing him or her on the operating table.
And group three had open surgery to put the slipped epiphysis back in place and hold it there with a long pin (screw) placed through the bone. This procedure is called an open reduction and internal fixation (ORIF).
Group two (closed reduction) had the largest number of cases of osteonecrosis (26 per cent). Group one (in-situ fixation) had the second highest incidence (19 per cent).
Group three (open reduction and fixation) had only one patient develop osteonecrosis. For all the children in the study, the more severe the slip was before surgery, the greater the risk of developing osteonecrosis after surgery. The overall incidence of osteonecrosis for the entire group (all 70 children) was around 20 per cent.
So, that’s a lot of information to say, we really don’t know all the reasons why children develop osteonecrosis after surgery to correct slipped capital femoral ephiphysis. The factors found to be significant so far (age, severity at diagnosis, and type of surgery) aren’t really under your control.