Abstract
Introduction
Clinical symptoms arising from corrosion within taper junctions of modular total hip prostheses are of increasing concern [1]. In particular, bi-modular implant designs showed increased failure rates due to wear originating from the neck-stem junction [2]. In-vivo corrosion-related failure is less frequently observed for head-stem junctions [3]. It is hypothesized that fretting and crevice corrosion are associated with micromotions between the mating surfaces of a taper junction [4]. The aim of this study was to measure micromotion occurring within a head-stem junction of a conventional prosthesis and clarify by how much it is exceeded in a neck-stem junction of a bi-modular prosthesis that exhibited severe corrosion and early implant failure.
Material & Methods
The micromotions within two taper articulations were investigated: a head-stem taper (Corail, DePuy Synthes, Leeds, UK, Figure 1) and a neck-stem taper of a bi-modular THA prosthesis (Rejuvenate, Stryker, Kalamazoo, MI, USA). Both tapers were assembled with 2000 N. Loading at an angle of 50° to the taper axes (identical for both) in direction of the stem axis was incrementally increased from 0 N to 1900 N (n=3). Small windows (< 2.5 mm2) were cut through the female tapers by electric discharge machining, exposing the male taper surface for direct micromotion measurements by microscopic topographic measurements (Infinite Focus Microscope, Alicona Imaging GmbH, Austria). Subsequently, feature matching of the images from the differently loaded implants was applied (Matlab 2016b, The MathWorks Inc., Natick, MA, USA) to determine the local relative motion between the mating surfaces.
Results
Loading with 1900 N resulted in micromotions of 1.0 µm ± 0.1 µm at the head-stem taper (Figure 2). The stepwise loading showed the motion trajectory, suggesting toggling with the dominant displacement in axial direction and small transversal movements. Neck-stem micromotion was significantly higher (14.2 µm ± 1.7 µm, p < 0.001). The trajectory revealed a tilt of the neck in direction of the force. The male taper returned into its initial position after the load was removed, indicating a repetitive rocking motion within every load sequence.
Discussion
The higher micromotion at the neck-stem taper junction is likely caused by the larger lever arm (20-fold) between load application and taper engagement. This can serve to explain the susceptibility of bi-modular prostheses to an elevated rate of problems due to fretting corrosion. Similar findings are speculated to apply for large-diameter heads, which showed high failure rates in clinical practice [5].
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