Three-dimensional finite element analysis of Legg-Calve-Perthes disease.

We studied the theoretical stress distribution in the proximal femoral epiphysis by means of a three-dimensional finite element model that allowed simulation of the central epiphyseal necrosis found in Legg-Calve-Perthes disease. The model was altered to study effects of age, extent of necrosis, portion of gait cycle, and effects of osteotomy on containment. It was found that small infarcts were less likely to exhibit collapse because of stress shielding, which decreased when the infarct was extensive or the area was not contained. There was little difference in stresses regardless of age or portion of the gait cycle. Femoral and pelvic osteotomies did little to modify mechanical stresses, and specifically failed to stimulate stress shielding in the case of extensive necrosis. This study suggests that basic mechanical behavior of the femoral epiphysis is similar in younger and older children, and good results in the former may be due to remodelling and low body weight. Although containment may have a place in less extensive lesions, there is no obvious mechanical support for the practice of performing osteotomies in the face of extensive epiphyseal necrosis.

Mechanical behavior of the femoral head in Legg-Perthes disease.

The research described herein is concerned with the development of a "restricted" three-dimensional finite element model of the femoral head that permits the mechanical behavior of the hip joint in Legg-Perthes' disease to be simulated and studied. This "restricted" finite element model allows the femoral head to be treated as a non-axisymmetrically loaded axisymmetric solid of revolution. In order to minimize computer costs, and in recognition of the several uncertainties present in the mechanical description of the system, a number of simplifying assumptions are introduced. These assumptions include an axisymmetric geometry, axisymmetric and linear-elastic material properties, small deformation and displacement, and an approximate load distribution on the femoral head. The computer analyses reveal the relation between the magnitude and the distribution of the stresses in the femoral head and such parameters as (1) the position in the gait cycle, (2) the thickness of the articular cartilage, (3) the extent of the necrotic bone, and (4) the type of corrective osteotomy. A sampling of the preliminary results is presented and some conclusions are drawn with respect to the implications of the study.