Comparison of Forces Applied During Inserting A Cementless Femoral Stem Versus A Test To Failure: A Pilot Trial

Background The increasing number of total hip arthroplasties (THA) and the more common use of uncemented stems are leading to an increasing incidence of intraoperative proximal femoral fractures (IPFF), making them a clinically relevant complication. When inserting the stem, the surgeon has to find a compromise between sufficient anchorage and the risk of fracture. Only a few biomechanical studies investigated forces that are necessary, or already risky, when inserting a femoral stem. Therefore, the aim of the study was to determine the forces occurring during the insertion of the stem and the forces that lead to IPFF. Methods Two female chemically untreated fresh-frozen human cadavers, aged 71 and 83 years and weighed 54 kg and 65 kg respectively, underwent left and right implantation of a THA stem by an experienced orthopaedic surgeon. The insertion of the stem was followed by a failure test, in which the surgeon hit the stem so hard that a fracture occurred. The forces, occurred while insertion and test to failure, were measured using an impulse hammer. Results The observed forces varied greatly. Over the four performed implantations, the forces ranged from 1317.7 N to 4347.5 N during insertion, and from 526.5 N to 7383.9 N during the failure test. The comparison between insertion versus failure test showed significantly higher peak forces (P < 0.001, P < 0.001, P = 0.002) in the failure test for implantations 1, 3 and 4. The comparison was not possible for implantation 2 because of a sensor error. Conclusions The biomechanical experiments presented in this study have shown that even with the small number of implantations, significantly higher hammering forces were required to cause a visually or acoustically perceptible IPFF than were necessary for adequate seating of the femoral stem. However, as fractures often cannot be perceived intraoperatively, it is important to advance the development of supporting systems for intraoperative use.