[The bending rod prosthesis: an experimental approach to metaphyseal hip arthroplasty]

The aim of our study was to develop a femoral component for total hip arthroplasty that would exclusively anchor in the metaphysis of the femoral neck. To forego trochanteric fixation, the load needs to be transferred to the metaphysis at as many points as possible. A computer simulation model suggested that an implant with a central cylinder and 16 rods aligned along a thread would be the preferable solution. To evaluate primary implantation stability, 14 fresh frozen cadaver femora were used. A special instrument set was developed to allow for centered implantation of the prosthesis without the need to dissect the greater trochanter. For our tests, we used two prototype implants: one made from titanium and the other from a CoCrMo alloy. For the measurement of micromotions at the medial proximal femur, sinusoid dynamic loading with a force between 300 N and 1700 N and a frequency of 1 Hz was employed. In a neutral position of 16 degrees adduction and 9 degrees antetorsion, the average micromotions measured were 119 microm. Despite these convincing in vitro results with regards to primary stability, circular cut-out of the implant, followed by aseptic osteonecrosis, loosening might still occur in a clinical situation. Animal experiments are therefore required to further evaluate this new implant design.