Computer modeling of the pathomechanics of spastic hip dislocation in children

Spastic muscles about the hip cause subluxation, dislocation, and lead to a cetabular dysplasia. Spastic hip disease occurs when the muscles about the hip exert forces that are too high or in the wrong direction or both. To de termine the role of the hip forces in the progression of spastic hip diseas e and the effect of both muscle-lengthening and bony reconstructive surgeri es, a computerized mathematical model of a spastic hip joint was created. T he magnitude and direction of the forces of spastic hips undergoing surgery were analyzed preoperatively and postoperatively to determine which proced ure is best suited for the treatment of spastic hip disease. The muscle-len gthening procedures included (a) the adductor longus, (b) the psoas, iliacu s, gracilis, adductor brevis, and adductor longus, and (3) the psoas, iliac us, gracilis, adductor brevis, adductor lon,longus, semimembranosus, and se mitendinosus. The bony reconstructive and muscle-lengthening procedures inc luded (a) lengthening the psoas, iliacus, gracilis, adductor brevis, adduct or longus, semimembranosus, and semitendinosus combined with changing femor al neck anteversion from 45 to 10 degrees, (b) lengthening of the psoas, il iacus, gracilis, adductor brevis, adductor longus, semimembranosus, and sem itendinosus combined with changing neck-shaft angle from 165 to 135 degrees , and (c) lengthening of the psoas, iliacus, gracilis, adductor brevis, add uctor longus, semimembranosus, and semitendinosus combined with changing fe moral neck anteversion from 45 to 10 degrees and neck-shaft angle from 165 to 135 degrees. Results show that a child with spastic hip disease has a hi p-force magnitude 3 times that of the a child with a normal hip in the norm al physiologic position. Based on this mathematical model the best to norma lize the magnitude of the hip-joint reaction force, the muscles to be lengt hened should include the psoas, iliacus, gracilis, adductor brevis, and the adductor longus, To normalize the direction of the hip force, the extremit y should be positioned in the normal physiologic position. The impact of de creasing the femoral anteversion or femoral neck-shaft angle or both had li ttle additional effect on the direction or magnitude of hip forces.