Abstract
Introduction
Failure of the neck-stem taper in one particular bi-modular primary hip stem due to corrosion and wear of the neck piece has been reported frequently1, and stems were recalled. A specific pattern of material loss on the CoCr neck-piece taper in the areas of highest stresses on the proximal medial male taper was observed in a retrieval study of 27 revised Rejuvenate implants revised after 3 to 38 month time in situ (Stryker, Kalamazoo, MI, USA) (Figure 1). One neck piece exhibited additionally wear marks at the distal end of the flat male neck taper indicating contact with the female taper of the stem. The purpose of this study was to understand the observed failure scenario of bottoming-out by investigating the stem taper morphologies.
Materials and Methods
The geometry of taper contact surfaces was determined using a Coordinate Measurement Machine (BHN 805, Mitutoyo, Japan). An algorithm based on the individual unworn areas of the respective taper surfaces was applied to all retrievals. One retrieval is additionally investigated by infinite focus microscopy (G4, Alicona, Austria) in the main wear areas on the neck piece taper, and the bottom, facing each other inside the junction (surfaces of the distal end of the male and the bottom of the female taper).
Results
The bottom of the male neck piece taper showed a prominent patch surrounded by a corrosion area (Figure 2), similar to the pattern observed in the proximal taper articulation medial- proximally and lateral-distally (Figure 1). The bottom of the female titanium stem taper did not show material loss to this extent, but only slight scratches. The proximal male taper of the neck piece showed the usual corrosion pattern, with an elevated patch protruding inside the corrosion cavity (Figure 3, left). Contrarily to observations on other retrievals (Figure 3, right), the height of this patch did not reach the original surface level. The wear depth at the patch of the bottomed out taper was 93µm with wear reaching as deep as 170µm.
Discussion
Only one of 27 retrieved neck pieces showed bottoming out. It can be speculated that the other retrievals were revised before this situation could occur. In the assembled situation, the initial space between the bottom faces of the stem taper is expectedly in the order of a few 100µm. During the observed wear process, permanent contact at patches on the neck piece prevents its reseating, until these localized structures give in to mechanical and electrochemical exposure. This end stage of failure only occurring in one explant indicates high clinical risks of the implants during earlier stages of intact patches, that are usually observed in retrievals. The formation of such patches has not been reported before, and might be characteristic for the wear and corrosion mechanism of the employed TMZF-CoCr combination, possibly due to the reduced stiffness of the TMZF Titanium.
Acknowledgements
The research received funding from the European Union's Seventh Framework Program (FP7/2007-2013) under grant agreement GA-310477 (LifelongJoints).
