Abstract
Introduction
Due to the commercial launch of newly developed ceramic-on-metal (COM) bearings, we compared the deformation and stresses in the liner with ceramic-on-ceramic (COC), metal-on-metal (MOM) as well as ceramic-on-polyethylene (COP) bearings using a finite-element (FE)-model, analyzing a variety of head size and implant position. Liner deformation in terms of change in inner diameter as well as peak stresses were evaluated.
Methods
The FE-model consisting of a commercial THR, the proximal femur and a section of the hemipelvis was created based on our previously published approach. Static load and muscle forces were applied according to the maximum load during gait. Polyethylene was modelled using a nonlinear definition with isotropic hardening, cobalt-chromium was modelled elastic-plastic and ceramic was modelled linear-elastic. Validity of the model was checked using an experimental setup with artificial bone and strain gauges located at the rim of the liner.
Implant material (COM vs. COC vs. MOM vs. COP), head size (28 mm vs. 36 mm) and cup position (45° inclination/15° anteversion vs. 60° incl./0° antev.) were varied.
Results
The experimental validation showed high correlation between strain measurements and FE-results. Liner deformation was evaluated by change in diameter at different levels. Change in head size had a high influence on cup deformation in COM, COC and MOM bearings, most possibly due to decreased liner thickness using bigger heads. Differences in MOM, COC and COM liner deformation were only in sub-micrometer range and not further evaluated.
Evaluation of von Mises stress and minimum principal stress showed high differences between the bearing couples, implant positions and head sizes. COM liner stress was less sensitive to the steep cup position, but principal stress amounts were about ten times higher than in polyethylene liners. Thereby, MOM liners developed about 13 % less peak stress than COM. COC liners showed 11 % to 16 % higher stresses than COM. In accordance with published results, bigger head size correlated with lower principal stresses in the liner. Also, bigger heads were less sensitive to steep cup positions.
Discussion
Deformation of the liner in total hip replacement has an important influence on lubrication, wear and clinical long-term success. The deformation occurring during intraoperative impaction and press-fit of the metal shell was not included in this study, hence the results are only valid considering the late postoperative phase when the implant is fully integrated in the bone. The FE-analysis showed no significant difference in liner deformation between COM, COC and MOM bearings. However, principal stresses were slightly higher in COM under the same conditions, but lower than COC.
