Abstract
Introduction:
There has been widespread concern regarding the adverse tissue reactions after metal-on-metal (MoM) total hip replacements (THR). Concerns have also been expressed with mechanical wear from micromotion and fretting corrosion at the head/stem taper junction in total hip replacements. In order to understand the interface mechanism a study was undertaken in order to investigate the effect of surface finish and contact area associated with modular tapers in total hip replacements with a single combination of materials of modular tapers.
Methods:
An inverted hip replacement setup was used (ASTM F1875-98). 28 mm Cobalt Chrome (CoCr) femoral heads were coupled with either full length (standard) or reduced length (mini) 12/14 Titanium (Ti) stem tapers. These Ti stem tapers had either a rough or smooth surface finish whilst all the head tapers had a smooth surface finish. Wear and corrosion of taper surfaces were compared after samples were sinusoidally loaded between 0.1 kN and 3.1 kN for 10 million cycles at 4 Hz. In test 1 rough mini stem tapers were compared with rough standard stem tapers whilst in test 2 rough mini stem tapers were compared with smooth mini stem tapers. Surface parameters and profiles were measured before and after testing. Electrochemical static and dynamic corrosion tests were performed between rough mini stem tapers and smooth mini stem tapers under loaded and non-loaded conditions.
Results:
In test 1 following the mechanical loading test the surface roughness parameters on the head taper were significantly increased when they were coupled with the mini stem tapers compared to the standard stem tapers (p = 0.046). Similarly in test 2 the surface roughness parameters on the head tapers were significantly increased when mini rough stem tapers were used compared to smooth mini stem tapers (p = 0.04). Corrosion testing showed breaching of the passive film on the rough but not the smooth neck tapers.
Conclusion:
This study has identified enhanced fretting corrosion at the modular taper junction associated with roughened surface finish and small neck tapers and points to the overall concern associated with the use of modular taper connections in orthopaedic implants. Crevice corrosion is identified as the predominant mechanism, with evidence of pitting in all rough mini neck tapers. The greatest wear and corrosion was in the plane where the greatest bending moments were generated, implicating fretting as a mechanism. The rough mini neck tapers have a reduced surface area at the interface and ultimately bending forces are concentrated here.
