A model of uniaxial implant seating by impaction

In-silico modelling of multi-strike insertion and torsional resistance of tapered revision hip stems: Insight into spline design philosophy

Despite the clinical success of tapered splined titanium stems, a knowledge gap still exists between spline design and its primary mechanical stability, which is critical to the long-term success of revision hip arthroplasty. Additionally, almost all published pre-clinical studies relied on resource-intensive benchtop and cadaveric testing. Hence, the present study developed a novel computational model to investigate effects of spline geometry and configuration on axial and torsional stability of tapered stem. Dynamic explicit Finite Element Analysis coupled with a state-of-the-art adaptive meshing technique was used to simulate the highly non-linear contacts and large bony material deformations. Hybridising primary straight splines with secondary angled splines results in 41% and 10% increases of peak insertion force and post-seating moment than the predicate device for the same seating position. The primary straight splines cut at multiple circumferential bony locations, enhancing torsional stability; while the alternatively placed secondary angled splines form wedges with the bone, providing reliable seating and additional torsional resistance. To the best knowledge of the author, this is the first in-silico investigation of its kind to simulate multi-strike seating and torsional resistance of revision hip stems, offering an effective and efficient platform for future multi-factorial parametric study and uncertainty quantification.

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.