Strategic Debulking of the Femoral Stem Promotes Load Sharing Through Controlled Flexural Rigidity of the Implant Wall: Optimization of Design by Finite Element Analysis

Abstract

Hip arthroplasty prostheses are often constructed of metal alloys, and the inherent disparity in the modulus of elasticity between the implant and the femur is attributed to the altered stress-strain pattern in adjacent bone. Rigid implants shield surrounding bone from mechanical loading, and the reduction in skeletal stress required to maintain bone mass and density results in accelerated bone loss, the forerunner to implant loosening and implant failure. Femoral stems of various geometric profiles and surface modifications, materials and material distributions for graded functionality, and porous stem structures have been investigated to achieve mechanical properties of stems that are closer to bone to mitigate stress shielding. For improved load transfer from implant to femur, the proposed study investigated a strategic debulking effort to impart controlled flexibility while retaining sufficient strength and endurance properties of the femoral stem. Using an iterative design process, debulked configurations based on an internal skeletal truss framework were evaluated using finite element analysis as outlined in ISO 7206 standards, with implants offset in natural femur or potted in testing cylinders. The commonality across the debulked designs was the minimization of proximal stress shielding compared to conventional solid implants. Stem topography can influence performance, and the truss implants with and without the calcar collar were evaluated. Load sharing was equally effective irrespective of the collar however, the collar was critical to reducing the stresses in the implant. When bonded directly to bone or cemented in the femur, the truss stem was effective at limiting stress shielding. Nevertheless, a localized increase in principal stress at the lateral proximal junction could negatively affect cement integrity and the bonding of cemented implants. The study determined that superior biomechanical performance of the truss implant is realized with a collared stem that is placed in an interference fit. Mechanistically, the controlled accommodation of deformation of the implant wall provides contextual flexibility and load sharing characteristics to the truss implant.