Abstract
Summary
Corrosion and fretting damage at the head-neck interface of artificial hip joints is more severe with larger head sizes. This is a concern, as the release of metal particles and ions can cause adverse tissue reactions, similar to those observed high wear metal-on-metal articulations.
Introduction
In the last few years corrosion was increasingly observed at head-neck interfaces of artificial hip joints, especially in joints with larger heads. There has always been evidence of some corrosion at modular junctions of artificial joints, but except for few designs, it was not seen as a real problem. It is important to better understand the factors contributing to corrosion at modular interfaces, so that necessary improvements can be made to minimise or completely avoid corrosion, in order to avoid possible adverse tissue reactions.
Methods
Over 100 retrieved stems and heads of 28, 32, 36, 40 and larger heads with metal-on-polyethylene (MoP) and metal-on-metal (MoM) articulations were scored for corrosion and fretting damage, in order to get a better picture of the magnitude of the problem. For some of the head sizes it was possible to assess the fretting and corrosion damage separately from implants from two different manufacturers. The tapers of the stem and head were subdivided into eight regions each, and scored for the severity of fretting and corrosion damage, as well as of the affected area within each sub-section. The scoring was undertaken by three persons with a fair intraclass correlation. The fretting and corrosion scores were also assessed based on the location of the center of the head with respect to the center of the taper. The distance between these two centers influences the toggling motion between the head and neck, as the main load is about 30 degrees out of axis during walking and other activities of daily living.
Results
It was found that head-neck interfaces of two manufacturers of 36mm heads had significantly more corrosion than 28mm heads. There is a significant relationship between head and neck fretting damage, and between corrosion and fretting damage. There is also more corrosion damage in 32, 40 and larger heads, but these groups were from different manufacturers, so that it was not possible to perform statistical tests. More corrosion was observed when the centre of the head was at a larger distance from the centre of the head, leading to an increased toggling moment due to the out-of-axis loading.
Discussion
It is of some concern that more corrosion is being observed with larger heads. Corrosion generally gets worse over time, which could negatively impact on the long-term behavior of these hip joints. Furthermore, it is possible that the metal particle and ion release due to corrosion and fretting could have adverse soft tissue reactions, similar to those observed at some MoM articulations. The fact that there are significant differences in the observed corrosion and fretting damage between the head-neck interfaces of two companies, indicates that even subtle changes in the geometry and the machined taper surface are important. A better understanding of these factors is required to make sure that the corrosion and fretting damage is minimised, or even better eliminated for all heads of artificial joints.
