Methodology for Bone–Implant Stiffness Evaluation

Abstract

It has been difficult to improve the intramedullary nail technique because of the lack of consistency in the procedures used to evaluate the bone-implant stiffness. The goal of this study was to develop a simple methodology for determining the stiffness of a bone implant that considers the physiological loads and bone orientation, and allows a finite element analysis and its validation using mechanical experimentation. Finite element models for a composite tibia before and after an intramedullary nail was implanted were created and validated using the results of a set of mechanical experiments, in which the stiffness values of the model were measured and compared under axial compression, 4-point bending, shear, and torsional loads considering the patient’s condition in the early healing phase. Grips with personalized bone interfaces were developed to guarantee the physiological loads and bone orientation. In the 4-point bending, torsional, and shear loading modes, the developed bone-implant finite element model showed a satisfactory level of predictive potential in relation to the experimental observations, with a percentage variation of less than 35%. This study also demonstrated that despite the high stiffness of the bone-implant construct, motion was always generated at the interfragmentary site during the early healing phase. In addition, during this stage, the nail supported most of the load applied to the lower limb (up to 85%). This strategy could contribute to the future determination of the ideal mechanical environment at a fracture site and how this environment evolves throughout the healing process.