Properly designed femoral stem impactors help to avoid overstuffing and make a second trial reduction unnecessary

For cementless implantation of hip stems, it is very important that the original stem fits exactly into the femoral prosthesis bed previously shaped using a stem rasp. If the geometry of the stem rasp does not match the prosthesis geometry, this can result in either overstuffing or post-sintering of the original stem in relation to trial reduction with the stem rasp. Overstuffing results in leg lengthening, while subsidence of the stem results in shortening with the risk of dislocation and impingement. Trial reduction with the original stem and trial head can prevent this, but is associated with additional soft tissue trauma and a longer operating time. Three groups were prospectively randomized. Group 0 was treated with the Fitmore B stem and the conventional rasp system, Group 1 with the Optimys stem and Group 2 with the Fitmore B stem with a new, optimized rasp system via a minimally invasive posterolateral approach. Intraoperatively, the differences in leg length and offset between trial reduction with the stem rasp and the original stem were recorded using a hip navigation system. The conventional rasp system led to significant overstuffing (1.2 ± 0.5 mm, p = 0.024) with the Fitmore B stem, compared with the instruments of the Optimys stem. In contrast, the optimized rasp system with the Fitmore B stem resulted in an equally precise fit of the endoprosthesis to that achieved with Optimys. There are relevant differences in the fit of rasp and original stem between different manufacturers and system evolutions. In order to ensure reproducible comparability of the fit of the original stem to the trial rasp, it is imperative that trial rasps are optimally adapted to the stems. This is an important way to avoid trial reduction after implantation of the original stem. This study was registered in the German Clinical Trials Register (Deutsches Register Klinischer Studien) with the registration number DRKS00026749.