Abstract
INTRODUCTION
The role and importance of fretting and corrosion in modular hip endoprostheses has become of more and more interest within the last years. Especially bearing couples with large diameters may experience high friction moments leading to an increase of relative micro movements between the surfaces of the taper connections. Recently published studies show that the risk of fretting and corrosion is significantly reduced by using ceramic ball heads compared to metal ball heads.
OBJECTIVES
Goal of this study was to investigate the risk of fretting and corrosion as well as possible loosening of large ceramic ball heads with metal sleeves.
METHODS
BIOLOX®OPTION systems have been investigated, consisting of ceramic ball heads (BIOLOX®delta) with sleeves (Ti6Al4V) using generic tapers made of different metals (Ti6Al4V, FeCrNiMo, CoCrMo):
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Fretting test according to ASTM F1875-98 (IIb)
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Corrosion test under in-vivo-like fatigue loading
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Simulator test creating large frictional moments
Ad B): If corrosion occurs under in-vivo-like conditions and compromises the connection between the modular parts, was investigated using three different consecutive scenarios. At first, a fatigue test was performed in a corrosive medium with 4.5Mio. cycles under a load of 4.5kN. Afterwards, a very high axial load of 10kN was applied for 0.5Mio. cycles. Finally, pull-off tests were performed and compared to reference samples.
RESULTS
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In the ASTM test, for all taper materials under investigation, decreasing corrosion currents on a comparatively low level have been measured.
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The interface strength between both the taper/sleeve surface and the sleeve/ball head surface did not changed significantly after fatigue loading and storage in a corrosive medium.
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Even though large bending moments and torques were applied in the simulator test, no loosening between the components was observed. The analysis of the involved surfaces exhibited only plastic deformation of the microstructure and tribochemical reaction layers as expected. No signs of excessive corrosion was found.
CONCLUSIONS
The results of the fretting and corrosion tests of ceramic ball heads with adapter sleeves exhibited no signs of noticeable corrosion even under adverse conditions (large ball head diameter, high inclination, large bending moments and torques). No loosening events have been observed. The proper assembling of the involved modular components (dry, no debris between the surfaces) according to the manufacturer´s Instructions For Use is an essential requirement hereby.
