Impaction of a typical press-fit modular acetabular cup using a dynamic finite element method: The effect of cup oversizing and

INTRODUCTION In cementless hip arthroplasty fixation of acetabular cups is often achieved using a press-fit between the cup and the bone, where the acetabulum is typically under-reamed by 1-4mm [1-4] with respect to the size of the cup. Deformation of the cups during insertion remains a concern as it could lead to changes in the clearance and sphericity of the bearing, which can adversely affect fluid film lubrication and lead to increased wear in hard-on-hard bearings. In extreme cases equatorial or edge contact can occur, which may cause the joint to seize [3,5]. Jin et al [5] and Yew et al [6] together showed that cup deformation increased as (1) cup wall thickness decreased, (2) the amount of oversizing increased, and (3) the size of the cup increased. These results suggest that many current hip implant designs, which have adopted a large diameter femoral head and thin walled acetabular cup, may experience significant deformation. Jin et al reported deformations of a monoblock cup with 100 micron nominal clearance (ASR, Depuy) between 25 and 103 microns, or 25% and 103% of the diametral clearance, for 1mm under-reaming of the acetabulum. The deformed shape of the cup equator was roughly elliptical or “fish-eyed”, caused by squeezing between the ischeal and ilial columns of the pelvis. In modular cups with shell-liner taper junctions, deformation of the shell caused by the insertion process may compromise the effectiveness of the taper. Squire et al [3] introperatively observed “toggle” of the liner following impaction of the shell before the liner was firmly implanted. They reported in vivo diametral deformations of the shell from a modular cup (Pinnacle 100, Depuy) to be in the range from 0.0 to 0.57mm with an average of 0.16 0.16mm. Although this deformation of the shell was corrected to a large extent following impaction of the liner into the shell [3], the residual effects of deformation on the shellliner taper junction are unknown. It is proposed that impaction of the undeformed liner into the deformed elliptically shaped shell may compromise the taper because contact at the taper occurs predominantly along the minor axis of the shell, not evenly all the way around the taper. Consequences of a compromised taper may include an increased risk of liner dislocation and increased wear between the shell and liner. Additionally, Walter et al [7] suggested that uncoupling of the shell and liner may lower the natural frequency of the assembled cup to within the audible range such that squeaking noises may be emitted under a driving frictional force. As uncoupling may be increased due to a compromised taper, this suggests that cup deformation may play a role in causing hip squeaking. This study is part of a long term project to investigate press-fit fixation and its effects. This part of the project used a dynamic finite element model to simulate the impaction of a modular hip implant into a under-reamed acetabulum. A „typical‟ implant geometry was used to investigate the effects of varying the amount of cup oversizing and shell wall thickness on cup deformation and effects of cup deformation on the effectiveness of the modular taper junction.