A NUMERICAL INVESTIGATION OF THE MECHANICS OF SWELLING-TYPE INTRAMEDULLARY HIP IMPLANTS

The novel concept of swelling-type intramedullary hip implants that at tain self-fixation by an expansion-fit mechanism resulting from contro lled swelling of the implant (by absorption of body fluids) was examin ed in detail using a finite element model of the implant-femur system. Some of the potential advantages of this technique over traditional t echniques include enhanced fixation, lower relative micromotions, impr oved bony ingrowth, and elimination of acrylic cement. The finite elem ent model created in this study incorporated: (i) the major aspects of the three-dimensional geometry of the implant and femur, (ii) the ani sotropic elastic properties of bone and implant materials and the chan ges in orientation of the principal axes of anisotropy along the lengt h of the implant-femur system, (iii) a layer of cancellous bone betwee n the implant and cortical bone in the proximal femoral region, and (i v) 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-swe lling-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.