Abstract
Introduction
The use of bone cement as a fixation agent has ensured the long-term functionality of THA implants 1. However, some studies have shown the undesirable effect of wear of stem-cement interface, due to the release of metals and polymeric debris lead to implant failure 2,3. Debris is generated by the micromotion together with a severely corrosive medium present in the crevice of stem-cement interface 3,4. FEA studies showed that micromotion can affect osseointegration and fretting wear 5,6. The aim of this research is to investigate if the micromotions measures from in silico analysis of the stem-cement correlate with the fretting-corrosion damage observed on in vitro testing.
Methods
The in vitro fretting-corrosion testing was made with positioning and loading based on ISO 7206-4 and ISO 7206-6. It was used Exeter stems embedded in bone cement (PMMA) and immersed in a saline solution (9.0 g/L of NaCl). A fatigue testing system (Instron 8872, USA) was used to conduct the test, applying a sinusoidal cyclic load at 5.0 Hz. The tests were finished after 10 million cycles and images of stem surfaces were taken with a photographic camera (Canon EOS Rebel T6i, Japan) and a stereoscope (Leica M165C, Germany).
For the computational analysis, the same testing configurations were modeled on software ANSYS. The analysis was performed using linear isotropic elasticity for both stem (E=193GPa; ⱱ=0.27; σy=400MPa) and PMMA cement (E=2.7GPa; ⱱ=0.35; σu=76MPa)7,8.
A second-order tetrahedral element was used to mesh all components with a size of 0.5 mm in the stem-cement contact area, increasing until 1.0 mm outside from them. A frictional contact (µ=0.25) with an augmented Lagrange formulation was used. The third cycle of loading was evaluated and a variation of sliding distance less than 10% was set as convergence criteria. The micromotion was measured as the sliding distance on the stem-cement interface.
Results and Discussion
The in silico analysis showed the presence of areas almost without micromotion in the proximal lateral and distal medial regions. In these regions, there is no evidence of fretting-corrosion after the in vitro testing. The lack of micromotion is caused by the debonding due to testing configurations and implant design. The absence of contact doesn't allow wear by abrasion or third body, avoiding the fretting-corrosion damage. For the regions distal lateral and proximal medial, it is possible to observe fretting-corrosion due to micromotions, which is supported by the in silico analysis results. The region proximal medial had the highest micromotion on computational analysis and the fretting-corrosion was more severe on laboratory testing, reinforcing the relevance of micromotion in the fretting-corrosion damage on the stem-cement interface.
Conclusion
The results indicate a correlation of micromotion calculated by in silico analysis and fretting-corrosion damage observed on in vitro testing. The developed FEA model may be a useful tool to predict the fretting-corrosion damage on the THA implants on pre-clinical testing. Additional efforts are needed to apply this tool on bone-implant systems to predict fretting-corrosion damage observed in vivo.
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