The effect of geometry and abduction angle on the stresses in cemented UHMWPE acetabular cups – finite element simulations and experimental tests

Rami K Korhonen,Arto Koistinen,Seppo S Santavirta,Yrjö T Konttinen,Reijo Lappalainen

doi:10.1186/1475-925x-4-32

Abstract

BackgroundContact pressure of UHMWPE acetabular cup has been shown to correlate with wear in total hip replacement (THR). The aim of the present study was to test the hypotheses that the cup geometry, abduction angle, thickness and clearance can modify the stresses in cemented polyethylene cups.MethodsAcetabular cups with different geometries (Link®: IP and Lubinus eccentric) were tested cyclically in a simulator at 45° and 60° abduction angles. Finite element (FE) meshes were generated and two additional designs were reconstructed to test the effects of the cup clearance and thickness. Contact pressures at cup-head and cup-cement interfaces were calculated as a function of loading force at 45°, 60° and 80° abduction angles.ResultsAt the cup-head interface, IP experienced lower contact pressures than the Lubinus eccentric at low loading forces. However, at higher loading forces, much higher contact pressures were produced on the surface of IP cup. An increase in the abduction angle increased contact pressure in the IP model, but this did not occur to any major extent with the Lubinus eccentric model. At the cup-cement interface, IP experienced lower contact pressures. Increased clearance between cup and head increased contact pressure both at cup-head and cup-cement interfaces, whereas a decreased thickness of polyethylene layer increased contact pressure only at the cup-cement interface. FE results were consistent with experimental tests and acetabular cup deformations.ConclusionFE analyses showed that geometrical design, thickness and abduction angle of the acetabular cup, as well as the clearance between the cup and head do change significantly the mechanical stresses experienced by a cemented UHMWPE acetabular cup. These factors should be taken into account in future development of THR prostheses. FE technique is a useful tool with which to address these issues.

Highlights

Contact pressure of UHMWPE acetabular cup has been shown to correlate with wear in total hip replacement (THR)
3-D finite element analyses and experimental cyclic tests were used for the estimation of the combined effects of acetabular cup geometry and orientation on the stresses in cemented UHMWPE
Finite element (FE) analyses of the present study showed that the geometrical design, thickness and abduction angle of acetabular cup, as well as the clearance between cup and head modify the mechanical stresses experienced by UHMWPE

Summary

Introduction

Contact pressure of UHMWPE acetabular cup has been shown to correlate with wear in total hip replacement (THR). The aim of the present study was to test the hypotheses that the cup geometry, abduction angle, thickness and clearance can modify the stresses in cemented polyethylene cups. Aseptic loosening is the most common cause for longterm failure of total hip replacement (THR). Wear, which contributes to aseptic loosening [1,2,3,4], has been shown to correlate with the contact pressure of the UHMWPE acetabular cup [5,6]. An increase of the abduction angle of the acetabular cup has been shown to increase contact pressure and wear [7,8]. Parameters reducing contact pressure reduce plastic deformation and wear, and may diminish the risk for aseptic loosening

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