It’s decision time for the pediatric orthopedic surgeon. The problem? Hip subluxation (partial dislocation) in children with cerebral palsy. Spasticity (increased muscle tone) is common with cerebral palsy and can cause the muscles to pull unevenly on the hip.
Over time, as the child grows and the muscles remain short and tight, the head of the femur (thigh bone) is forced out of the hip socket. The direction the round head shifts is clear on X-rays (usually posterolateral or back and to the side). But where is the weak point in the hip socket that allowed the femoral head to pop out?
This question must be answered when planning surgery to reconstruct the hip. If it’s not addressed, then the same problem can happen again after surgery. There could be instability at the top of the socket (superior direction). This superior deficiency could be more toward the front (anterior) or more toward the back (posterior) part of the hip.
Then again the instability or deficiency could be multidirectional (present in more than one direction). The problem is really more complicated than that. In many cases, normal growth and development of the bones is altered in these children by the change in muscle pull and biodynamics.
For example, the femur may twist or tilt thus placing the head of the femur in the socket at an angle. Likewise, any change in the shape or orientation of the pelvic bones that form the upper part of the hip socket can have an impact on alignment.
The surgeon must take both the direction of the hip subluxation and the location of the acetabular (hip socket) deficiency into consideration when planning corrective surgery. How is this type of evaluation done?
There are three-dimensional CT scans that allow orthopaedic surgeons to see the entire acetabulum (hip socket). The surgeon needs information on direction, depth, and degree of hip dysplasia (shallow socket). The CT scan provides depth and direction but not degree of dysplasia.
Pelvic X-rays may offer a better view to measure something called the acetabular index. The acetabular index is a measure already in use to look at the angle of the acetabular roof. Combining these two tests together (CT scan and pelvic X-ray) makes it possible to get a three-dimensional view of the angle and curve of the roof (top) of the acetabulum (socket).
And thanks to the recent research of orthopedic surgeons in Taiwan, the use of this technique has been validated as a reliable way to define acetabular development in all directions. In that study, they compared a group of children with hip subluxation caused by spastic cerebral palsy to a group of children without any medical problems (the control group).
Using these two groups for comparison, they were able to see that acetabular dysplasia (shallow hip socket) of children with spastic cerebral palsy does affect the whole socket, not just one side or another. But the anterior (front) portion of the socket had the greatest area of deficiency.
How will surgeons use this information? Using the acetabular index will give surgeons a more accurate measure of all the planes of the acetabulum. Taking this measurement in consideration along with other factors such as child’s age, function, and other deformities will help direct surgical choices when reconstructing the dysplastic hip in children with spastic cerebral palsy.