Analysis of carbon fiber reinforced PEEK hinge mechanism articulation components in a Rotating Hinge Knee Design: a comparison of in vitro and retrieval findings

Due to early clinical failures of conventional polyethylene bushings in total knee replacements, carbon fiber reinforced poly-ether-ether-ketone (CFR-PEEK) represents a promising alternative material for this application, having already shown satisfying wear performance in high-congruency contact situations. The objective of the present study was to evaluate the damage modes and the extent of damage observed on CFR-PEEK hinge mechanism articulation (HMA) components in articulation with CoCr29Mo6 and a zirconium nitride (ZrN) multilayer surface after in vivo service in a rotating hinge knee (RHK) system and to compare the results with corresponding components subjected to in vitro wear tests. Key question was, if there were any similarities or differences between in vivo and in vitro damage characteristics. Twelve retrieved EnduRo® RHK systems (Aesculap AG Tuttlingen, Germany) after an average of 34.9months in vivo underwent wear damage analysis with focus on the four integrated CFR-PEEK components and distinction between different damage modes and classification with a scoring system. The analysis included visual examination (documentation with digital single lens reflex camera (DSLR)), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) of representative specimens, as well as surface roughness and profile measurements. The main wear damage modes were comparable between retrievals and in vitro specimens (n=3), whereby the size of affected area on the retrieved components showed a higher variation. Overall, the retrieved specimens seemed to be slightly heavier damaged. This was probably attributable to the more complex loading conditions in vivo and the fact that simulation studies aim to reproduce the wear damage occurring in an optimally aligned knee prosthesis without taking into account patient and surgery specific deviations from the ideal.