Abstract
Introduction
It is well accepted that larger heads provide more stability in total hip arthroplasty. This is due to an increase in jump height providing increased resistance to subluxation. However, other implant parameters also contribute to the bearing's stability. Specifically, the liner's rim design and the centre of rotation relative to the liner's face. Both these features contribute to define the Cup Articular Arc Angle (CAAA). The CAAA describes the degree of dysplasia of the acetabular liner, and plays an important role in defining the jump height.
The aim of this study was to determine the difference in jump height between bearing materials with a commonly used acetabular implant system.
Methods
From 3D models of the Trinity acetabular implant system (Corin, UK), the CAAA was measured in CAD software (SolidWorks, Dassault Systems, France) for the ceramic, poly and modular dual mobility (DM) liners, for cup sizes 46mm to 64mm. The most commonly used bearing size was used in the analysis of each cup size. For the ceramic and poly liners, a 36mm bearing was used for cups 50mm and above. For the 46mm and 48mm cups, a 32mm bearing was used. The DM liners were modelled with the largest head size possible. Using a published equation, the jump height was calculated for each of the three bearing materials and each cup size. Cup inclination and anteversion were kept constant.
Results
CAAA varied substantially between cup sizes and bearing materials. The mean CAAA for the ceramic, poly and DM bearings were 166°, 175° and 186°, respectively. Consequently, over the entire size range, the ceramic liners had the lowest mean jump height of 12.9mm. In comparison to the ceramic liner, there was a mean 10% increase in jump height when transitioning to a poly (14.2mm), and a further 30% increase when transitioning from a poly to the dual mobility bearing (18.5mm) [Fig.1]. However, the difference in jump heights between bearings was variable, and dependent on cup size.
Discussion
It is well understood that increasing head size increases stability in THA. However, other implant design parameters contribute to stability. With this particular implant system, the poly bearing had a greater jump height than the ceramic for cup sizes 50mm and above. The DM bearing improved jump height over the ceramic and poly by a mean of 41% and 30%, respectively.
In conclusion, different liners have different design features that affect jump height. Consequently, not all bearings of identical head size are the same. We encourage a dialogue with your implant provider to understand the differences in CAAA between cup sizes and bearing materials.
For any figures or tables, please contact the authors directly.
