HP15: FINITE ELEMENT ANALYSIS AND SURGICAL CONSIDERATIONS OF THE IMPACT OF OSTEOPOROSIS AND OSTEOARTHRITIS ON HIP RESURFACING

Abstract

The aim of the study was to examine the stress and strain relationships in proximal femurs, using finite element analysis techniques. We looked at normal, osteoporotic and osteoarthritic models, to detect any differences, and specifically, in relation to neutral or valgus alignments of the femoral components in a cemented prosthetic femoral head resurfacing situation. A CAD model of a third-generation composite femur was virtually operated upon to implant the femoral component. The femoral component, geometry was of a 54 mm Birmingham hip resurfacing. A 1 mm cement mantle was allowed for. Finite element model is were generated with 10 node tetrahedral elements. The material properties of both cortical and cancellous bone were assigned according to standard parameters.

Our analysis of the stress and strain in the resurfaced femoral head under the implant showed significant reductions in the stress and strain compared to the intact femur and this was the case for all stem-bone interface conditions. This region of high stress and strain was not seen in the model with the stem was overreamed and there was no bone contact with the stem. The stress and strain levels were generally higher when osteoporotic bone was modelled. The peak maximum tensile stress and strain in the cortical bone at the superolateral femoral neck was 4% to 24% greater in the resurfaced femur for all by the conditions with valgus implant positioning experiencing high at peak stresses and strain then neutral alignment. Maximum tensile stress in the cement at the had- implant rim junction was not greatly different for the different bone conditions except for osteoporosis where the stress was almost 50% greater than the other bone conditions. Generally the highest tensile stresses occurred anteroinferiorly and were greater in the neutral alignment than in the valgus alignment. The superolateral offset associated with a valgus orientation, rather than the valgus orientation itself maybe what reduces the stress and strain in the neck leading to a lower incidence of fracture. Stresses were lower than 8 MPa, the fatigue strength of cement, for all the valgus models except osteoporosis. All neutral models contained some locations where the tensile stress exceeded 8 MPa.

The postoperative stress and strain in the femoral head and neck maybe increased in comparison to the intact femur. Under the component there may be significant reduction in stress and strain, causing resorbtion. The biomechanical reason why a more valgus orientation protects against femoral neck fracture is more complex, sends in some critical locations stress and strain has reduced but in others it is increased. Further study is being planned.