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Finite element models of the proximal femur at birth, 2 years of age, and at 8 years of age were constructed to investigate stress patterns under different loading conditions. These loading conditions represent typical activities of a normal developing child and abnormal activity associated with muscle spasticity. The hypothesis is that the shear stresses in the growth plate correlate with the neckshaft angle as associated with valgus and normal development. Loads for the finite element models were derived from a separate muscle model used to calculate the forces across the hip joint for an arbitrary subject and activity. Results show there is an inverse relationship between the relative magnitude of the shear stress in the growth plate and the developing neck-shaft angle. The relatively high shear stresses generated by normal activity in the 2-year-old’s growth plate correlate with the decrease in neck-shaft angle that accompanies normal development. Alternatively, lower shear stresses are generated in the growth plate by loading conditions representing spasticity. These lower magnitude shear stresses correlate with a valgus deformity, which is often observed clinically.
T.G. Ribble is with Automotive Operations, Product Development, Ford Motors, Edison, NJ 08817; M.H. Santare is with the Dept. of Mechanical Engineering, University of Delaware, Newark, DE 19716; F. Miller is with the Dept. of Orthopaedics, A.I. duPont Institute, Wilmington, DE 19899.