Abstract
INTRODUCTION
Systemic levels of metal ions are surrogate markers of in-vivo wear of metal-on-metal hip resurfacings (MoMHRA). The wear-related generation of metal ions is associated with component size and positioning but also with design specific features such as coverage angle, clearance, metallurgy and surface technology.
OBJECTIVES
The objective of the study was to investigate whether a hip resurfacing design (ACCIS) with TiNb engineered bearing surfaces would generate less chromium (Cr) and cobalt (Co) ions during and after the run-in phase of wear and whether Ti ions could be detected indicating wear of the coating.
METHODS
Whole blood and serum Cr, Co and Ti levels were measured at 3, 6, 12, 24 and 60 months (ICP-MS).
RESULTS
The Cr and Co levels were virtually untraceable during the running-in phase till 1 year. After that, there was a significant increase at the 2 years interval and further at the 5 years interval for both Cr and Co. Overall mean 5 years concentrations of Cr 4.8 μg/l (range 0.5–10.5 µg/L) and Co 4.3 μg/l (range 0.7–12.1µg/l)) had evolved above the established acceptable upper limits (Cr < 4.6 μg/l - Co < 4.0μg/l)1. Elevated whole blood Ti levels were demonstrated in all patients (mean levels of 9.16 μg/l, 12.54 μg/l and 9.17 μg/l at 3,6 and 12 months respectively) with a peak level at 6 months although there was no statistical difference between the measurements at 3, 6, 12 and 24 months.
DISCUSSION
These findings correspond with a release of Ti ions from the TiNb surface during the running-in phase. The whole blood Ti levels were higher compared to whole blood Ti levels in the literature for uncemented grit-blasted acetabular and femoral MoMTHA components and Ti plasma spray coated MoMHRA acetabular components. In those reports, the Ti release is due to passive corrosion from non-articulating surfaces (acetabular and/or femoral fixation surfaces) and is thus likely to be less elevated compared to release due to articulating surface wear as with the ACCIS HRA. A remarkable finding was the continuous elevation of Cr and Co ion levels after the TiNb coating seems to have been worn off. This may be due to a change in clearance as the head is slightly migrating into the cup because of the worn-off coating at the cup-head contact area.
CONCLUSION
The ACCIS design has no traceable Cr and Co ions in the running in phase because of its TiNb ceramic surface coating that prevents Cr and Co release. Once the coating is worn off, which only happens at the patch area of friction, a release of Co and Cr ions starts. Although TiNb coating allegedly protects the CoCrMo surface from corrosion by acting as a seal, Cr and Co ions continued to increase at 5 years indicating further wear possibly as a result of change in clearance.
