Appropriate seating of acetabular and femoral components during total hip arthroplasty (THA) surgery is essential for implant longevity. Additionally, the appropriate assembly of components is essential for proper function, for example to prevent taper corrosion or acetabular component disassembly. However the current understanding of the forces and energies imparted during surgery is sparse. Perhaps more importantly, there exists a risk that much of the preclinical testing performed to develop implants and surgical techniques do not apply the appropriate boundary conditions to surgical impaction and component assembly, leading to the possibility of huge overestimations in impaction force. This in-vitro study examines the influence of mechanical boundary condition parameters that affect the forces imparted to implant and patient during THA surgery; including the attenuation of two common types of acetabular cup introducer and the hard tissue (pelvic) boundary conditions. A drop tower test-rig that allows full customisation of impaction and implantation parameters was built, with pelvis boundary conditions simulated with silicone cylinders using adjustable geometry to vary stiffness and damping. The least stiff setup represented a large, unbolstered patient on the operating table. A medium stiffness setup represented a slim, well bolstered patient. An extremely stiff, metal boundary was selected to replicate the pre-clinical testing conditions usually employed in implant or instrument testing, where impact testing takes place in a vice, or metal test frame. For each of these stiffness scenarios, piezo-load cells and LVDTs were used to measure forces and displacement of the pelvis model. We also investigated the use of two common implant introducers; a straight and a bent introducer. The latter is often used for large patients or for specific approaches (e.g. direct anterior). In total, 180 drop weight tests and 120 strikes by an orthopaedic surgeon were measured. For the drop weight testing the peak force measured varied between 7.6kN and 0.4kN for stiffest and softest support conditions respectively. When the surgeon applied the impact strike manually, the range was between 13.2kN and 0.8kN for the stiffest and softest support conditions respectively (Figure 1). Using the bent introducer attenuated the load by between 13.0% and 115% compared to the straight introducer (Figure 1). Pelvic boundary conditions are overlooked in much of the literature on implant seating or assembly in THA surgery. In laboratory settings with impaction performed on a workbench or frame of a materials testing machine, high forces may be sufficient to seat or assemble implants. However our data show that these high forces will not be replicated in vivo, and this could be a causative factor in poor assembly of acetabular components or femoral head/stem tapers, which can lead to clinical problems like disassembly or crevice corrosion. We found the geometry of the introducer and the stiffness of the pelvis support had significant attenuating influence. We also found that the surgeon does not compensate for these differences, resulting in vast differences in the delivered strike force. It is recommended these factors are carefully considered when designing surgical tools and in particular conducting pre-clinical testing.
