Abstract
Introduction
For a safe design of ceramic components in total hip arthroplasty it is important to know the stress state within each part of the system under in vivo loading scenarios. Besides several design parameters, e.g. diametrical clearance between ball head and liner or angular mismatch in the taper region of metal shell and liner, also physiological factors, like patients' weight or bone quality, influence the stresses within the components. Therefore, the aim of the current study was to experimentally determine the stresses in a ceramic liner varying two of the factors: clearance and inclination angle of the liner.
Materials and Methods
Two ceramic liners were instrumented at the outer contour with five strain gauge (SG) rosettes (measuring grid length: 1.5 mm) on each liner (Fig.1). Metal shells were seated in an asymmetric press-fit Sawbones® model using a 0.5 mm under-reaming, and liners were afterwards axially assembled with a 2 kN load. SG5 was placed at the flat area of the liner, the other four were placed circumferentially in 90 degrees offset on the rear side of the liner. SG2 and SG4 were mounted opposite to each other in press-fit direction (contact of metal shell to the Sawbones® block) whereas SG1 and SG3 were placed in the non-supported direction (no contact of metal shell to the Sawbones® block). Four different inclination angles (0°, 30°, 45°, 60°) were tested under in vivo relevant loads of 4.5 and 11 kN. Two ceramic ball heads were used to examine a mid tolerance clearance and a clearance at the lower tolerance limit. Strain data was converted to stresses and compared using a paired two-sided Wilcoxon Rank Sum Test at an α-level of 0.05.
Results
Generally, similar stress distributions under the two in vivo relevant loads were found (Fig.1, Fig.2). Highest stresses were found for the 0° inclination and decreased with increasing inclination angle. Due to the asymmetric press-fit distinctively higher stresses for SG2 and SG4 were found. For these stresses no significant difference was found throughout the whole testing. Larger clearance in combination with different inclination angles have had a significant effect on the stresses where SG1 and SG5 were affected the most. At 0° inclination SG5 showed significantly higher stresses for the larger clearance. With increasing inclination angle this effect switched towards SG1. Stresses of SG3 stayed constant with increasing inclination angle.
Discussion
The results show that the ceramic liner examined here is well adapted to in vivo loading conditions showing decreasing stresses with increasing inclination angle. Nevertheless, 0° inclination was identified to be the most critical load case. As expected, the stresses resulting from different clearances are attributed to the contact area where higher stresses were found for a smaller contact area. Due to the dimension of strain gauges the strain measurements are limited to a defined region and thus no complete strain map of the whole component could be determined. Therefore, the current data will be used to calibrate finite element models to perform parametrical studies.
