Proposed frequencies of a vibrator used for implant retrieval at the time of hip joint revision surgery.

The number and the rate of success of hip implantation surgeries have increased significantly during last thirty years, not only in the USA, but also throughout the world. It has been reported that the failure rates of implanted hip joints are less than 8% after 10 years, and less than 20% after twenty years. Failures occur directly or indirectly due to wear, stress shielding and infection. Revision surgery is needed for those failed implant replacements. In the future, as the elderly population increases, the frequency of this type of revision surgery will also increase. At the time of revision surgery, removal of the existing cemented femoral implant can be a problem for the surgeon. Use of a vibrator for loosening of the existing cement layer between the bone and the implant may be a helpful solution. In this study, we investigated the optimum resonance frequencies of such a vibrator that might be used to loosen the cement layer easily and efficiently. Natural frequencies of different-sized implants and of different materials were determined. For harmonic analysis, CT scan data of a femur was processed in the image processing software MIMICS. Then the outline of the total hip was modeled and was analyzed by the finite element software ANSYS. The required portion of the femoral part was edited, implant and cement layer were introduced in that model, and elements were generated in that FEA software. Then elements of the femoral part, except the cement layer and the implant, were sent to MIMICS software again for assignment of different Youngs modulus of each element, which are proportionate to their densities. Then the elements were brought back to the FEA software. The harmonic analysis was performed for the total model in the FEA software ANSYS. For that particular boundary condition, the first three natural frequencies of the three types of implant sizes and materials varied by a maximum of 7-8%. Results of the numerical harmonic analysis showed that at the bone-cement interface, the resonance frequencies were at the ranges of 4 to 6 Hz, 26 to 29 Hz, and 43 to 49 Hz. The vibration response was similar for three cement-bone interface locations examined. This suggests that a vibrator that will produce a resonance frequency response may cause cracks in the bone-cement mantle and thus may facilitate the removal of the failed femoral component. Retrieval of hip implant may be easier using a vibrator in that band of frequencies with a moderate amplitude. The magnitude of those frequencies may not differ significantly from implant to implant as the natural frequencies of different types of implant, for that particular boundary conditions, are within a close range.