Computer modeling of the pathomechanics of spastic hip dislocation in children.

Spastic muscles about the hip cause subluxation, dislocation, and lead to acetabular 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 determine the role of the hip forces in the progression of spastic hip disease and the effect of both muscle-lengthening and bony reconstructive surgeries, a computerized mathematical model of a spastic hip joint was created. The magnitude and direction of the forces of spastic hips undergoing surgery were analyzed preoperatively and postoperatively to determine which procedure is best suited for the treatment of spastic hip disease. The muscle-lengthening procedures included (a) the adductor longus, (b) the psoas, iliacus, gracilis, adductor brevis, and adductor longus, and (3) the psoas, iliacus, gracilis, adductor brevis, adductor longus, semimembranosus, and semitendinosus. The bony reconstructive and muscle-lengthening procedures included (a) lengthening the psoas, iliacus, gracilis, adductor brevis, adductor longus, semimembranosus, and semitendinosus combined with changing femoral neck anteversion from 45 to 10 degrees , (b) lengthening of the psoas, iliacus, gracilis, adductor brevis, adductor longus, semimembranosus, and semitendinosus combined with changing neck-shaft angle from 165 to 135 degrees , and (c) lengthening of the psoas, iliacus, gracilis, adductor brevis, adductor longus, semimembranosus, and semitendinosus combined with changing femoral 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 hip-force magnitude 3 times that of the a child with a normal hip in the normal physiologic position. Based on this mathematical model the best to normalize the magnitude of the hip-joint reaction force, the muscles to be lengthened should include the psoas, iliacus, gracilis, adductor brevis, and the adductor longus. To normalize the direction of the hip force, the extremity should be positioned in the normal physiologic position. The impact of decreasing the femoral anteversion or femoral neck-shaft angle or both had little additional effect on the direction or magnitude of hip forces.