Mechanobiological predictions of growth front morphology in developmental hip dysplasia

Developmental dysplasia of the hip (DDH) is the most common orthopedic problem of newborn children. Most clinicians and researchers agree that the primary cause of DDH is abnormal mechanical forces on the head of the femur due to limb position, pressure from the womb, or ligament laxity. The abnormal mechanical forces result in altered growth and bony deformities, in particular large neck-shaft and anteversion angles in the proximal femur and a shallow acetabulum. Previous studies have suggested that intermittent octahedral shear stress promotes growth and ossification, while intermittent hydrostatic compressive stress inhibits growth and ossification. We implemented these mechanobiological principles into a finite element model to predict the rate of progression of the growth front and the formation of coxa valga (large neck-shaft angle) in DDH. Under the assumed normal fetal loading conditions the hydrostatic stress was even across the growth front, but the octahedral shear stress was higher in the center than at the edges. This stress profile promoted growth in the center and a produced a convex growth front shape. Under loading conditions of the dysplastic hip, the octahedral shear stress was much larger on the medial side than on the lateral side, which promoted growth on the medial side and resulted in coxa valga. These results indicate that abnormal forces on the prenatal hip might influence total bone morphology and the development of DDH. These findings might help in understanding the etiology and pathology of other developmental bone deformities.