A numerical investigation of the mechanics of swelling-type intramedullary hip implants

The novel concept of swelling-type intramedullary hip implants that attain self-fixation by an expansion-fit mechanism resulting from controlled swelling of the implant (by absorption of body fluids) was examined in detail using a finite element model of the implant-femur system. Some of the potential advantages of this technique over traditional techniques include enhanced fixation, lower relative micromotions, improved bony ingrowth, and elimination of acrylic cement. The finite element model created in this study incorporated: (i) the major aspects of the three-dimensional geometry of the implant and femur, (ii) the anisotropic elastic properties of bone and implant materials and the changes in orientation of the principal axes of anisotropy along the length of the implant-femur system, (iii) a layer of cancellous bone between the implant and cortical bone in the proximal femoral region, and (iv) frictional sliding between the bone and implant. The model was used to study quantitatively the parametric influence of various material design variables on the micromotions and stress fields in the bone-swelling-type implant system. The results of the finite element analyses were used to establish material behavior goals and provide targets for a material development study.