Abstract
Introduction
Total hip replacement is one of the most successful orthopaedic surgeries, not least because of the introduction of modular systems giving surgeons the flexibility to intraoperatively adapt the geometry of the artificial joint to the patient's anatomy. However, taper junctions of modular implants are at risk of fretting-induced postoperative complications such as corrosion, which can lead to adverse tissue reactions. Interface micro-motions are suspected to be a causal factor for mechanical loading-induced corrosion, which can require implant revision.
The aim of this study was to determine the micro-motions at the stem-head taper interface during daily activities and the influence of specific material combinations.
Materials & Methods
The ball heads (ø 32mm, 12/14, size L, CoCr or Al2O3) were quasi-statically assembled to the stems (Ti or CoCr, Metha, Aesculap AG, Germany, v=0.5 kN/s, F=6 kN, n=3 each, 10° adduction/ 9° flexion according to ISO 7206-4) and then loaded sinusoidally using a material testing machine (Mini Bionix II, MTS, USA, Figure 1). The peak forces represented different daily activities [Bergmann, 2010]: walking (2.3 kN), stair climbing (4.3 kN) and stumbling (5.3 kN). 2,000 loading cycles (f=1 Hz) were applied for each load level. Six eddy-current sensors, placed between stem and head, were used to determine the displacement (interface micro-motion and elastic deformation) between head and stem (Figure 1). A finite element model (FEM) based on CAD data was used to determine the elastic deformation of the prostheses for the experimentally tested activities (Abaqus, Simulia, USA). Tie-junctions at all interfaces prevented relative movements of the adjacent surfaces. The resultant translations at the centre of the ball head were determined using a coordinate transformation and a subsequent subtraction of the elastic deformation.
Results
FEM simulations exhibited a negligibly small elastic deformation for all material configurations indicating that the taper axis was located close to the direction of force application. The resultant micro-motions ranged between 3.3 and 33.4 µm and increased with rising peak forces for all material couplings (p<0.001, Figure 2). Ti–stems exhibited significantly larger micro-motions especially when combined with Al2O3-heads (Figure 2). The differences between the head materials on a similar stem material were not significant (p>0.857, Figure 2).
Discussion
The observed differences between the two stem materials are unexpected. They might be due to the differences in stiffness between head and stem or to different taper surface morphologies. The same factors might also explain the different trends for the Ti-stems (larger for Al2O3-heads) compared to the CoCr-stems (larger for CoCr-heads). The magnitude of the observed micro-motions probably increases with contamination [Jauch, 2011] or insufficient assembly forces, facilitating fretting and crevice corrosion, which has been described for all of the combinations tested. Whether the measured magnitudes of micro-motion do already comprise a problem is presently unclear and requires further clarification.
