A parametric numerical analysis of femoral stem impaction.

Press-fitted implants are implanted by impaction to ensure adequate seating, but without overloading the components, the surgeon, or the patient. To understand this interrelationship a uniaxial discretised model of the hammer/introducer/implant/bone/soft-tissues was developed. A parametric analysis of applied energy, component materials and geometry, and interactions between implant and bone and between bone and soft-tissues was performed, with implant seating and component stresses as outcome variables. To reduce the impaction effort (energy) required by the surgeon for implant seating and also reduce stresses in the hardware the following outcomes were observed: Reduce energy per hit with more hits / Increase hammer mass / Decrease introducer mass / Increase implant-bone resistance (eg stem roughness). Hardware stiffness and patient mechanics were found to be less important and soft tissue forces, due to inertial protection by the bone mass, were so low that their damage would be unlikely. This simple model provides a basic understanding of how stress waves travel through the impacted system, and an understanding of their relevance to implantation technique and component design.

Quantitative characterisation of impaction events during femoral broaching in total hip arthroplasty.

Total hip arthroplasty (THA) broaching involves impacting a broach handle with a mallet, facilitating force transmission to progress broaches into the femoral medullary canal. Limited surgical access during direct anterior THAs increases off-axis forces, potentially contributing to tissue damage. The aim was to characterise impactions during broaching. In a cadaver lab, movement of handle, femur and instrumented mallet was tracked during direct-anterior approach (DAA) and posterior approach (PA). Mallet velocity, broach displacement and input energy were calculated. Following the cadaver lab, different impaction strategies were investigated using bone simulants for a simulated DAA. The effects of reduced mallet velocity inputs and the influence of different impaction locations on the broach handle strike plate were investigated. To seat the broach in cadaveric bone, lower impaction energy (-59%) and number of strikes (-53%) were observed during PA compared to DAA. Suboptimal broach progression was reached in bone simulant at low mallet velocities (3.1 ± 0.3 m/s). Impacting the strike plate's upper part caused larger deflections (p < 0.001) of the Sawbones femur which suggested that higher off axis forces occurred. Awareness of reduced load transmission during DAA broaching using off-axis broach handles, compared to the traditional PA, could help promoting a more efficient and careful impaction strategy in surgeries.