At the shoulder, flexibility is key. We depend on that flexibility to be able to scratch an itch on our back or toss a Frisbee to a buddy. At the hip, it’s stability that counts. We depend on our hip joints to support our weight, especially while standing. As a result, the human hip joint has evolved into a ball-and-socket joint. Its needed stability comes from the tight interface between the head of the femur and the socket of the pelvis.
Bone grows based on the forces placed on it. When a child stands, her weight causes the head of the femur to push into the socket. As a result, the socket grows deeper and deeper, and the hip joint becomes more and more stable. Of course, the opposite happens when there are not enough forces pushing the head of the femur into the socket.
The other factor is the direction of forces, which needs to be in the direction of the socket. When there is muscle imbalance around the hip joint, it becomes more difficult for the hip joint to stay secure. For individuals with Cerebral Palsy, Traumatic Brain Injury and other similar conditions that cause muscle tightness, the muscles that pull the legs in towards each other (adduction) tend to be tighter than the ones pulling the legs apart (abduction). Similarly, the muscles that twist the legs inward (internal rotation) so the knees are facing each other are tighter than the ones that twist the legs outward (external rotation). These forces torque the head of the femur out of the socket.
These two factors combined can be a recipe for disaster. The lack of adequate pressure pushing into the socket results in a socket that is more saucer-like than cup-like.
Over time, the shallow socket and the twisting/pulling forces result in the head of the femur moving out of the socket (dislocation).
Why is dislocation bad? Sometimes it is not. Some individuals can sit comfortably and stand on dislocated hips. Many have no pain at all. But, many lose the ability to walk, and do have pain. If only one hip dislocates, that person may not be able to sit evenly. This uneven sitting can cause pain, skin breakdown, and even cause scoliosis to develop or worsen. These, in my opinion, are very important reasons to keep hips as healthy and stable as possible.
Coming Soon…How Do We Keep the Hips Healthy?