Abstract
Introduction
Modular junctions in total hip replacement (THR) have been a primary source of fretting and corrosion which can lead to implant failure. Fretting is a result of unintended micromotion between the femoral head and stem tapers and is suspected to result after improper taper seating during assembly. Two design factors known to influence in-vitro taper assembly mechanics are relative taper alignment—mismatch angle—and the surface finish—micro-grooves. However, these factors have not been systematically evaluated together.
Objective
The objective of this study was to employ a novel, micro-grooved finite element (FEA) model of the hip taper interface and assess the role of taper mismatch angle and taper surface finish—smooth and rough—on the modular junction mechanics during assembly.
Methods
A two-dimensional, axisymmetric model of a CoCrMo femoral head taper and Ti6Al4V stem taper was created using median measurements taken from over 100 retrieved implants. Micro-grooves on the stem and head taper were modeled using a sinusoidal function with amplitude and period corresponding to median retrieval measurements. To evaluate effects of a “smooth” head taper surface finish, additional models were run with a head taper having a flat edge (no micro-grooves). Lastly, mismatch between the stem and head taper was varied between distal-locked, no mismatch, and proximal-locked.
To simulate assembly during surgery, boundary conditions were applied to move the femoral head taper at a constant velocity onto the stem taper until a 4kN reaction load was achieved. Models were assembled and meshed in ABAQUS Standard (v 6.17) using four-node linear hexahedral, reduced integration elements. Contact was modeled between the stem and head taper using surface-to-surface formulation with penalty contact and a coefficient of friction of 0.2. Forty simulations (5 mismatch angles x 2 head taper surface types x 4 stem taper surface finishes) were run. Outcome variables included contact area, contact pressure, equivalent plastic strain, and number of micro-grooves undergoing plasticity.
Results
As expected, taper mismatch angle drove the location of contact to the distal or proximal ends. Increasing taper mismatch led to significant decreases in contact area for both micro-grooved and flat head taper models (Figure 1A). Taper mismatch had minimal effects on contact pressure (∼2.15 GPa) with the “rough” head taper surface finish but influenced the range of contact pressures (1.30 – 1.91 GPa) in the “smooth” head taper models (Figure 1B). Stress at the micro-grooves varied depending on the stem taper surface finish (Figure 2). Significant plastic deformation of the micro-grooves was only found in models with the “rough” head taper surface finish.
Conclusion
Regardless of the taper surface finish, contact area decreased by 30% – 58% when going from a 3’ – 12’ mismatch. Reduced contact area may significantly influence the long-term stability of the implant. Modeling the taper micro-grooves led to plastic deformation consistent with those found from retrieved implants—indicating the importance of modeling the surface finish of tapers. These models will be used to identify the optimal design factors to maximize stability of the modular taper junctions.
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