Abstract
Introduction
Relative motion at the modular head-neck junction of hip prostheses can lead to severe surface damage through mechanically-assisted corrosion. One factor affecting the mechanical performance of modular junctions is the frictional resistance of the mating surfaces to relative motion. Low friction increasing forces normal to the head-neck interface, leading to a lower threshold for slipping during weight-bearing. Conversely, a high friction coefficient is expected to limit interface stresses but may also allow uncoupling of the interface in service. This study was performed to examine this trade-off using finite element models of the modular head-neck junction
Methods
A finite element model (FEM) of the trunnion/ head assembly of a total hip prosthesis was initially created and experimentally validated. CAD models of a stem trunnion (taper size: 12/14mm) and a prosthetic femoral head (diameter: 28mm) were discretized into elements for finite element analysis (FEA). The trunnion (Ti6Al4V) was modelled with a hexahedral mesh (33,648 elements) and the femoral head (CoCrMo) with a tetrahedral mesh (51,182 elements). A friction-based sliding contact interface was defined between the mating surfaces. The model was loaded in 2 stages: (i) an assembly load of 4000N applied along the trunnion axis, and (ii) 500N applied along the trunnion axis in combination with a torque of 10Nm. A linear static solution was set up using Siemens NX-Nastran solver. Multiple simulations were executed by modulating the frictional coefficient at the taper-bore interface from 0.05 to 0.15 in increments of 0.01, the coefficient of 0.1 serving as the control case (Swaminathan and Gilbert, 2012).
Results
The vertical and tangential displacements of the nodes on the taper of the trunnion relative to the femoral head demonstrated a strong inverse dependence upon the coefficient of friction at the interface (Fig. 1). A similar trend was observed with respect to the peak interface pressure (Fig. 2). The peak von Mises stress, however, increases with increasing coefficient of friction (Fig. 2). A Fisher's R to Z correlation test was performed on each output variable to determine its correlation with coefficient of friction. The coefficient of friction correlated significantly (p<0.0001) with both tangential displacement (r = −0.990) and vertical displacement (r = −0.974). Peak von Mises stress (r = 0.995) and peak contact pressure (r = −0.984) were also found to be significantly (p<0.0001) correlated to the coefficient of friction.
Discussion
A higher coefficient of friction at the taper-bore interface led to lower contact pressure and sliding at the modular junction. However, higher coefficients of friction also led to increased von Mises stresses within the bore and the trunnion increasing the risk of yielding and fatigue failure. The current results strongly indicate that factors affecting the frictional coefficient at the interface likely influence the occurrence of and severity of mechanically-assisted corrosion in THA.
Significance
The results from this study will help us set tolerances for the interlocking mechanism, identifying the minimum frictional coefficient required to obtain stable implant mechanics.
