Abstract
Introduction
The National Joint Registry has recently identified failure of large head metal on metal hip replacements. This failure is associated with the high torque at the interface of standard modular taper junction leading to fretting and corrosion. A number of manufacturers produce mini spigots, which in theory, provide a greater range of motion as the neck head junction is reduced. However, the relative torque to interface ratio at this junction is also increased. In this study we investigated hypothesis that the use of small spigots (minispigots) will increase wear and corrosion on modular tapers.
Methods
Wear and corrosion of spigots were compared in-vitro when loaded with a force representative of the resultant force passing through the hip. The heads (female tapers) were made of cobalt-chrome-molybdenum (CoCrMo) and the stems (male tapers) of titanium alloy (Ti). Commercially available tapers and heads were used. The surface parameters & profiles were measured before & after testing. Electrochemical static and dynamic corrosion (pitting) tests were performed on minispigots under loaded and non-loaded conditions.
Results
Post-testing the surface parameters Ra, Ry & Rz on the head taper associated with the minispigots had become greater compared with standard spigots. In all instances the profile of the titanium male tapers was unchanged. SEM showed the corroded region of the head was similar to the profile on the Ti male taper, with evidence of pitting in the cobalt chrome. In the CoCrMo/ Ti combinations, wear and corrosion were increased in minispigots compared with standard spigots. On minispigots the rough surface finishes were affected more severely than those with a smoother surface. Static corrosion tests showed evidence of fretting in the rough but not the smooth minispigots. Pitting scans showed a greater hysteresis with the rough surface finishes on the minispogot indicating potentially greater corrosion in the former.
Conclusion
The relative size of the taper in comparison to the head combined with the surface finish was crucial. As the relative torque to interface ratio at this junction increased corrosion of the cobalt chrome head increases and is further enhanced if the surface finish on the tapers is rough.
