A physics-based computational model of neonatal Developmental Dysplasia of the Hip (DDH) following treatment with the Pavlik Harness was developed to obtain muscle force contribution in order to elucidate biomechanical factors influencing the reduction of dislocated hips. Clinical observation indicates that reduction occurs in deep sleep and involves passive muscle action. Consequently, a set of five (5) adductor muscles, namely, the Adductor Brevis, Adductor Longus, Adductor Magnus, Pectineus, and Gracilis were identified as mediators of reduction using the Pavlik Harness. A Fung-type model was used to characterize the hyperelastic stress-strain muscle response. Four grades (1–4) of dislocation as specified by the International Hip Dysplasia Institute (IHDI) were considered. A three-dimensional model of the pelvis-femur-lower limb assembly of a representative 10 week-old female was generated based on CT scans of a 6-month and 14-year old female as well as the visible human project with the aid of anthropomorphic scaling of anatomical landmarks.
The muscle model was calibrated to achieve equilibrium at 90° flexion and 80° abduction. The hip was computationally dislocated according to the grade under investigation, the femur was restrained to move in an envelope consistent with Pavlik Harness restraints, and the dynamic response under passive muscle action and under the effect of gravity was resolved using the ADAMS solver in Solidworks.
Results of the current model with an anteversion angle of 50° show successful reduction IHDI Grades 1–3, while IHDI Grade 4 failed to reduce with the Pavlik Harness. These results are consistent with a previous study based on a simplified anatomically-consistent synthetic model and clinical reports of very low success of the Pavlik Harness for Grade 4. However, our model indicates that it is possible to achieve reduction of Grade 4 dislocation by hyperflexion. This finding is consistent with clinical procedures that utilize hyperflexion to help achieve reduction for patients with severe levels of DDH for whom the Pavlik Harness fails.