Design of Surgical Impaction Instruments Matters.

Background: Femoral stem impaction in total hip arthroplasty is commonly performed by mallet blows on a metal impactor attached to the stem. Factors including the surgeon, the impactor, and the patient can influence the impaction. A wide range of impactors, varying in design and thus in mass and stiffness, are available. However, little is known about their influence on the force transmission and, consequently, about the proportion of the mallet force that ultimately reaches the implant. This study aimed to investigate the force transmission through the impactor for different impactor designs, while investigating different patient-specific femur-tissue systems in situ and in silico. Methods: The mallet and impactor forces of 9 consecutive blows on seated femoral stems were measured for 2 different approaches on each of 4 cadavers. The mallet-implant force transmission was calculated using a phenomenological model for 2 different impactor designs. Results: The attenuated force in the impactor achieved approximately 65% to 75% of the corresponding mallet peak force, regardless of cadaver or surgical approach. Measuring the force distant from the tip resulted in an overestimation of the transferred forces. Depending on impactor design just 24% to 47% of the applied mallet peak force reached the implant itself. Conclusions: The force transmission for overcritical mallet blows can be regarded as independent from patient- and approach-specific boundary conditions and primarily dependent on the impactor design. Surgeons must be aware of this relationship and exercise caution when using other or novel instruments to prevent intraoperative complications.