Mechanics of Hip Dysplasia Reduction in Infants Using the Pavlik Harness: A Physics Based Computational Model

The biomechanical factors influencing reduction of dislocated hips by the Pavlik harness in patients of Developmental Dysplasia of the Hip (DDH) were studied using a three-dimensional dynamic computer model capable of simulating hip reductions in (1) subluxated and (2) fully dislocated hip joints. Eight hip adductor muscles were identified as key mediators in the prognosis of hip dysplasia and the non-dimensional force contribution of each in the direction necessary to achieve concentric hip reductions was determined. Results point to the adductor muscles as the mediators of the reduction of 2 subluxated hips, as their mechanical action is a function of the degree of hip dislocation. For subluxated hips in abduction and flexion, the Pectineus, Adductor Brevis, Adductor Longus, and proximal Adductor Magnus contribute positively to reduction, while the rest of the Adductor Magnus contributes negatively. Furthermore, in full dislocations all muscles contribute detrimentally to reduction, elucidating the need for traction to reduce Graf IV type of dislocations. Reduction of dysplastic hips was found to occur in two distinct phases in which the muscles act distinctively: (I) release phase and (II) reduction phase. This is the first study that uses dynamic computer simulations to understand hip dysplasia reductions by the Pavlik harness, bringing to light conclusive findings that may be used to predict successful vs. unsuccessful treatments, allowing physicians to make better treatment decisions.