Abstract
Introduction
Modular acetabular liners are fixed in metal shells by a taper locking mechanism. Male tapers of the liner and female tapers of the metal shell have different taper angles resulting in an angular gap. Depending on the specific manufacturing tolerances varying angular gaps may result and, thus, different contact mechanics may be generated that could alter the stresses within the acetabular liner. Therefore, the aim of the current study was to experimentally determine stresses in a ceramic liner depending on different angular gaps under in vivo like loading conditions.
Materials and Methods
Two ceramic liners were instrumented at the outer contour with five strain gauge (SG) rosettes each (Fig.1). First, metal shells were axially seated in an asymmetric press-fit model with 0.5 mm under-reaming, then liners were assembled with a 2 kN axial 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. SG2 and SG4 were mounted opposite to each other in press-fit direction while SG1 and SG3 were placed in the non-supported direction. Three inclination angles (0°, 30°, 45°) were tested under in vivo relevant loads of 4.5 and 11 kN. Four positive angular gaps (A1=0.162°±0.007°, A2=0.084°±0.002°, A3=0.054°±0.004°, A4=0.012°±0.005°) and one negative angular gap (A5=−0.069°±0.006°) were examined. For all tests a mid-tolerance clearance between liner and ball head of 70 µm was chosen. Strain data were converted to stresses and compared using a paired 2-sided Wilcoxon Signed Rank Test at an α-level of 0.05.
Results
Generally, similar stress distributions under the two loads were found (Fig.2, Fig.3). Stresses for SG1 increased with increasing inclination angle while stresses for the other strain gauges decreased. Highest stresses were found for 0° inclination in press-fit direction (SG2, SG4) and at the bottom of the liner (SG5). Almost no effect of the different angular gaps on the stresses was found for SG5 while for SG1, SG2, SG3 and SG4 significantly lower stresses with decreasing angular gap were found.
Discussion
This study showed that the in vivo stress state of acetabular liners strongly depends on acetabular component orientation. Higher stresses for larger angular gaps are attributed to the contact zone of the liner with respect to the metal shell resulting in larger lever arms and, thus, leading to higher bending moments acting on the liner. The contact zone shifts downward with decreasing angular gap. 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. Nevertheless, current results are very useful to calibrate numerical studies concerning ceramic components of a total hip arthroplasty.
