Simulated bone remodeling around two types of osseointegrated implants for direct fixation of upper-leg prostheses.

Direct attachment of an upper leg prosthesis to the skeletal system by a percutaneous implant is an alternative solution to the traditional socket fixation. In this study, we investigated long-term periprosthetic bone changes around two types of fixation implants using two different initial conditions, namely immediate post-amputation implantation and the conventional implantation after considerable time of socket prosthesis use. We questioned the difference in bone modeling response the implants provoked and if it could lead to premature bone fracture. Generic CT-based finite element models of an intact femoral bone and amputated bone implanted with models of two existing direct-fixation implants, the OPRA system (Integrum AB) and the ISP Endo/Exo prosthesis (ESKA Implants AG) were created for this study. Adaptive bone-remodeling simulations used the heel-strike and toe-off loads from a normal walking cycle. The bone loss caused by prolonged use of socket prosthesis had more severe effects on the ultimate bone quality than adaptation induced by the direct-fixation implants. Both implants showed considerable bone remodeling; the titanium screw implant (OPRA system) provoked more bone loss than the porous coated CoCrMo stem (ISP implant). The chance of the peri-prosthetic bone fracture remained higher for the post-socket case as compared to the direct amputation cases. In conclusion, both direct-fixation implants lead to considerable bone loss and bone loss is more severe after a prolonged period of post-socket use. Hence, from a biomechanical perspective it is better to limit the post-socket time and to re-design direct fixation devices to reduce bone loss and the probability of peri-prosthetic bone fractures.