Experimental assessment of a new direct fixation implant for artificial limbs

An alternative to prosthetic socket rehabilitation of patients with transfemoral amputation is realized by means of direct skeletal fixation devices. offering significant improvements in mobility and comfort. However, strain shielding due to high stiffness of these metal-based implants causes considerable and progressive bone loss. To overcome this phenomenon a new concept of a direct fixation implant, in form of a collared metallic pin articulating inside a polymer intramedullary part, was developed. In this study we used experimental and finite element techniques to assess whether the novel concept produced a more physiological strain distribution in the bone as compared to a standard titanium implant. Cortical strains were measured experimentally on seven human cadaver femora, both intact and implanted with a generic standard implant and the new implant. Three load configurations were considered, simulating: heel strike, toe off and one leg stance. A finite element model derived from computed tomography data was used to calculate strains in intact bone and bone with generic models of the two implant types. Significant strain shielding occurred around both implant types, albeit that for the novel design strain shielding was generally less (p<0.04). Significant differences in strain shielding between both implant types were obtained for heel strike at the distal (p<0.04) and the middle level (p<0.03), as well as for the one leg stance at the middle level (p<0.03) showing 21-29% less strain shielding for the new implant in these cases. Finite element results were in agreement with the experimental findings: more strain shielding for the standard implant as compared to the novel design. In fact, the benefit of the new design was bigger in the simulations as compared to the experimental measurements, which was attributed to the idealized collar-cortex fit in the FE model of the new design which was not obtained in the experiments. In conclusion, the study showed that the new implant has a potential to increase distal load transfer to the femur and reduce strain shielding as compared with the standard implant. Collar-cortex contact is an important aspect and requires further attention when developing the surgical technique. The encouraging results obtained in this study justify further development of this concept in order to improve the quality and applicability of direct skeletal fixation devices for patients requiring a transfemoral amputation.