ABSTRACT
The long term success of total joint replacement can be limited by fatigue failure of the acrylic cement and the resulting disruption of the bone-cement interface. The incidence of such problems may be diminished by reduction of the fatigue notch factor in the cement, so that stress concentrations are avoided and the fatigue crack initiation time maximized. This study describes a method for numerical shape optimization whereby the finite element method is used to determine an optimal shape for the femoral stem of a hip prosthesis in order to minimize the fatigue notch factor in the cement layer and at interfaces with the bone and stem. A two-dimensional model of the proximal end of a femur fitted with a total hip prosthesis was used which was equivalent to a simplified three-dimensional axisymmetric model. Software was developed to calculate the fatigue notch factor in the cement along the cement/stem and cement/bone interfaces and in the proximal bone. The fatigue notch factor in the cement at the cement/stem interface was then minimized using the ANSYS finite element program while constraining the fatigue notch factor at the cement/bone interface at or below its initial level and maintaining levels of stress in the proximal bone to prevent stress shielding. The results were compared with those from other optimization studies.
Subject(s)
Bone Cements , Hip Prosthesis , Algorithms , Computer Simulation , Femur/anatomy & histology , Femur/physiology , Models, Biological , Prosthesis Design , Stress, Mechanical , Weight-Bearing/physiologyABSTRACT
The load transferred through the hip joint is one of the major forces occurring in the human body. After the replacement of this joint in THR arthroplasty, the load is transferred through the implant to the femoral bone. Loosening of the fixation of the implant and the fatigue failure of prosthetic stems create problems for both patient and surgeon. Both problems can be reduced by the use of Finite Element (FE) analysis to predict stresses and fatigue lifes but the results are sensitive to assumptions regarding the loading conditions and the idealisation of the components. Consequently the stress distributions and resulting fatigue notch factors in the human femur with an endoprosthesis have been determined for different assumptions regarding the form of the idealisation, the load conditions, and the interface conditions. The FE results show that a realistic loading condition without a tension banding force always produces the highest fatigue notch factor and von Mises stresses. An equivalent 2D plane stress model obtained by varying the thickness is likely to give more realistic stresses because it predicts more realistic strains than other 2D approximations. The full bonded interface is a satisfactory approximation for the real interface conditions because it predicts stress distributions of the correct form without excessive stress concentration.
