Abstract
Introduction
Angular mismatch between the head and trunnion is recognized as a contributing factor to mechanically-assisted corrosion of modular hip prostheses. Although manufacturing standards have been adopted to define acceptable tolerances for taper angles of mating components, the relationship between the head and trunnion taper angles (positive or negative) differs between manufacturers. In this study, we investigated the effect of positive and negative angular mismatch on the interface mechanics of a standard design of taper junction using finite element analysis (FEA).
Methods
Computer simulations were executed using an FE model which had been previously verified through direct comparison with experimental studies. The neck and trunnion of a Ti6Al4V femoral component (taper size: 12/14mm) were modelled using a stable hexahedral mesh (33,648 elements), while the femoral head (CoCrMo, size: 32mm) was modelled using a tetrahedral mesh (51,182 elements). Assembly of the head on the trunnion was simulated through the application of a load of 4000N along the trunnion axis. This was followed by the application of a gait load of 1638N (2.34×700N BW) at an angle of 30o to the trunnion axis. A friction-based sliding interface (mu=0.12) was simulated at the trunnion-head junction. A linear static solution was set up using Siemens NX Nastran. In addition to a perfect match, 7 positive and negative mismatch angles were simulated ranging from −0.100 to 0.100 degrees. Head taper interface motion, contact pressure and internal stresses (von Mises) were calculated for each mating condition.
Results
During gait loading, the maximum tangential displacement at the head-trunnion interface was 34µm, virtually identical for positive and negative mismatches in taper angle. There was minimal change in interface motion (5 µm) with variations in taper mismatch. The maximum contact pressure between the trunnion and the head increased dramatically with the magnitude of the mismatch from 144Mpa for a per4fect fit to 924MPa and 555MPa for taper clearances of 0.1 degrees for −0.1degrees, respectively. Smaller changes were seen in the maximum internal stresses developed within the trunion, which rose by a factor of approximately X2.5 over the same range of taper mismatch (perfect fit: 85MPa vs 220Mpa for 0.1deg. and 228MPa for −0.1 deg.). One difference between cases examined was the magnitude of the dilation of the bore within the femoral head during gait loading which increased from 2.19µm to 4.23µm with a change in mismatch from +0.015o to −0.015o.
Discussion
The results support the conclusion that, provided that manufacturing tolerances are maintained to within ±0.015o, both a positive and negative angular mismatch results in similar levels of contact stress and interface micromotion in modular junctions. As expected, peak stresses were located at the proximal edge of the contact zone with a negative taper mismatch and at the distal edge with a positive mismatch. This may have important implications in vivo, due to differences in the chemical environment (pH and oxygen tension) at each of these locations.
