INTRODUCTION:  Modular junctions in total hip replacement (THR) have been a primary source of fretting-based corrosion, leading to adverse tissue reactions and implant failure [1]. One attributed cause of onset of fretting-based corrosion is micromotion between the femoral head and stem taper due to improper seating and assembly. The taper mechanics upon assembly can be influenced by numerous design factors, including relative taper alignment—mismatch angle—and the surface finish—micro-grooves—of the stem and head taper. Systematic evaluation of mismatch angle has been conducted using finite element analysis (FEA) models [2, 3]; however, a limitation is existing taper models do not account for the micro-grooves of both stem and head tapers, limiting clinical relevance. The objective of this study was to employ a novel, micro-grooved FEA model of the hip taper interface and assess the role of taper mismatch angle and head taper finish—smooth and rough—on the modular junction contact mechanics.
METHODS:  Finite element models were created to simulate the effect of varying taper mismatch angle on contact characteristics at the hip modular junction. A two-dimensional, axisymmetric model representative of a CoCrMo femoral head taper (E=210 GPa; v=0.3) and Ti6Al4V stem taper (E=119 GPa; v=0.3) geometry was created using median geometrical measurements taken from over 100 retrieved implants [4]. A sinusoidal function was used to model the micro-grooves of the stem and head taper interfaces with the amplitude and period of the function corresponding to median retrieval measurements (stem taper: height=11µm, spacing=200µm; head taper: height=2µm, spacing=25µm) [5]. Material properties for both tapers were modelled as elastic-perfectly plastic (CoCrMo head taper: Yield Stress=827 MPa / Ti6Al4V stem taper: Yield Stress=795 MPa). Each model was assembled and meshed in ABAQUS Standard (v 6.17) using four-node linear hexahedral, reduced integration elements. Contact was modeled between the head and stem taper (surface-to-surface formulation, penalty contact with a 0.2 coefficient of friction).
	To simulate modular assembly during surgery, boundary conditions were applied to move the femoral head taper at a constant velocity onto the stem taper until a 4kN assembly reaction load was achieved. Mismatch between the stem and head taper were modeled as 0 (no mismatch), 3’ (0.05°), and 12’ (0.2°) in both positive (distal-locked) and negative (proximal-locked) directions. To evaluate effects of a “smooth” head taper surface finish on contact mechanics, additional models were run with a head taper having a flat edge with no topography. A total of 10 simulations (5 mismatch angles x 2 head taper surface types) were run. Outcome variables included contact area, contact pressure, equivalent plastic strain, and number of micro-grooves undergoing plasticity.
RESULTS: Simulation times averaged 7.4 hours for “smooth” head taper models and 30.9 hours for “rough” head taper models. As expected, taper mismatch angle drove the location of contact to the distal or proximal ends. Regardless of the head taper surface finish, the displacement required to reach 4kN assembly loads ranged from 17 – 109 µm depending on the taper mismatch. Increasing taper mismatch led to significant decreases in contact area for both head taper models (Figure 1A). Taper mismatch had minimal effects on contact pressure (~2.15 GPa) with the “rough” head taper surface finish (Figure 1B). However, taper mismatch angle affected the range of contact pressures in the “smooth” head taper models (1.30 – 1.91 GPa). Significant plastic deformation of the micro-grooves was only found in models with the “rough” head taper surface finish. Interestingly, as the mismatch angle became more pronounced (either positive or negative), the equivalent plastic strain increased significantly in the micro-grooves undergoing contact, reaching values as high as 65%. In contrast, modeling the head taper as a “smooth,” idealized surface led to no plastic deformation of the stem taper micro-grooves, with only a minimal amount of plasticity (0.75%) at 2 micro-grooves in the most severely mismatch angle cases (Figure 1C&D).
DISCUSSION:  The aims of this study were to evaluate the effect of head and stem taper mismatch angle on modular junction contact mechanics as well as assess the effect of head taper surface finish. Regardless of the head taper surface finish, contact area decreased by 30% - 58% when going from a 3’ – 12’ mismatch. This reduced contact area may have significant influence on the long-term stability of the implant. This finding is consistent with other FEA studies [2, 3], although these prior studies did not consider taper surface finish. When employing computational models to identify the contact mechanics of the head/stem taper junction, it is paramount to employ techniques that simulate realistic conditions. Taper topography, especially on the head taper, is largely ignored. In the current study, modeling the head taper as an idealized “flat” surface led to a 19% reduction in the contact pressure at the interfaces and no plastic deformation of the stem taper micro-grooves—unlike the “rough” head taper model. Retrieved implants clearly show that plastic deformation occurs in-vivo [6]; therefore, failing to consider the head taper surface finish may lead to incorrect conclusions. One limitation of the current study was the use of one taper geometry with idealized assembly conditions. The effect of geometry and off-axis loads needs to be considered, particularly as it may have an interaction with the taper mismatch angle. The current study has demonstrated the effects of altered taper angles and head taper topography on contact mechanics of the modular junction, which we believe can impact micromotion. Ultimately, we aim to identify the optimal topography and design factors by designing large parametric studies varying material couple and geometry of modular taper junctions.
SIGNIFICANCE:  A novel, micro-grooved FEA model of the hip taper identified effects of taper mismatch angle on contact mechanics at the modular junction. Realistic taper surface topography is necessary to simulate taper junction contact mechanics representative of surface damage seen in-vivo.
ACKNOWLEDGEMENT: Consultation by Dr. Steven P. Mell and funding by NIH/NIAMS grants R03 AR066829 and R01 AR070181 and is appreciated.
REFERENCES: [1] Langton et al., JBJS Br, 2011. [2] Ashkanafar et al., J Mech Beh Biomed Mater, 2017. [3] Fallahnezhad et al., J Mech Beh Biomed Mater, 2017. [4] Lundberg et al., ASTM STP 1591, 2016. [5] Pourzal et al., CORR, 2016. [6] Hall et al., J Biomed Mater Res B Appl Biomater, 2018.

2131 - Effects of Taper Mismatch Angle and Head Topography on Modular Hip Taper Contact Mechanics