Abstract
Introduction
Acetabular cup deformation is an important topic in today's THA and was investigated for a variety of metal cup designs (e.g. 1,2,3). Cup deformation caused by press-fit forces can have negative effects on the performance of such systems (e.g. high friction, metal ion release). When considering new materials for monolithic acetabular cups - such as ceramics - detailed knowledge about the deformation behaviour is essential to ensure successful performance. Therefore, the deformation behaviour of monolithic ceramic cups was investigated.
Materials and Methods
Testing was conducted with monolithic ceramic cups (under development, not approved) of size 46mm and 64mm. One cup design of each size had a constant wall thickness of 3.0mm and an offset of 0.0mm (center of rotation on front face level), the other design was lateralized with an offset of 3.5mm (46mm) or 5.0mm (64mm), leading to an increased wall thickness. First, 3 cups of each design were impacted into 1.0mm underreamed Sawbones® blocks (pcf 30, geometry: see (2)). Second, all cups were quasi-statically assembled into the Sawbones® blocks of the same design using a material testing machine. Third, the cups were placed in a two-point-loading frame (acc. to ISO/DIS 7206–12:2014(E)) and a load of up to 1kN was applied. The inner diameter of all cups was measured under unloaded and loaded conditions for all scenarios using a coordinate measurement machine at 9 locations of each cup, 1.5mm below the front face (Fig.1). As the diametrical deformation (unloaded inner diameter – loaded inner diameter) was not normally distributed a Wilcoxon test was performed to statistically analyse the deformation differences of the different cup designs (p<0.05).
Results
Impaction or quasi-static assembly of the cups into Sawbones® showed similar deformation behaviour (Fig.2). With increasing cup size the deformation increased by up to 81% (p<0.001) and lateralization reduced the deformation by up to 85% (p<0.001). Two-point-loading showed the same deformation behaviour. Deformations of up to 215 µm were measured (Fig.3). Increasing the cup size increased the deformation by up to 105% (p<0.001) and lateralization reduced the deformation by up to 77% (p<0.001).
Discussion
Stiffening the cups by lateralization fairly reduced cup deformation and can help avoiding negative effects resulting from cup deformation. In comparison to metal cup designs the maximum deformation values for the lateralized ceramic cups are by up to 70 % smaller (3). Due to the lower deformation the clearance range could be narrowed in comparison to metal cups. Therefore, it might be possible to more precisely predict the friction and wear behaviour of ceramic on ceramic resurfacing systems.
