Comparison between two different concepts of lumba

Authors: A Templier L Denninger C Mazel F Lavaste W Skalli

Publish Date: 1998/02/01

Volume: 8, Issue: 1, Pages: 27-36

PDF Link

Abstract

The present study is a numerical comparison using finiteelement analysis FEA of two different concepts of spinal fixation devices when implanted These implants are 1 the Easy® “rigid” Screw/Rod ø 6mm system 2 the Twinflex® “dynamic” system ø 2 X 25 mm ELF A parameterised 3D FEA model of an L3sacrum segment developed by Lavaste Skalli Robin was used Geometric and mechanical models of each implant were then constructed before being inserted in the spinal segment model Then for model validation these two L3S2 instrumented segmental models were submitted to similar boundary conditions as used in a previous in vitro comparison of the same implants Flexion loaddisplacement curves were then controlled using experimental results Loads acting on screws and longitudinal elements were calculated and analysed for a better understanding of the intrinsic differences between both constructs Loaddisplacement responses of both constructs were quite similar L3 sagittal rotation at 10 Nm = ~15° while loads in the implant were not For example the axial pushin forces at the S1 screws were equal to 30 N for the Twinflex® and 150 N for the Easy® Screw/Rod system The pullout forces at the S2 screws were respectively 100N and 200 N for the Twinflex and Screw/Rod Concept At other levels axial forces were all lower than 60 N the Twinflex® values being higher than the Easy® ones Bending moments along screws were respectively 07 Nm and 14 Nm at the L3 level for the Twinflex® and the Easy® systems At lower levels values were all below 06 Nm again with a reversed proportion Bending moments calculated along longitudinal elements were always lower than 03 Nm for the Twinflex® and up to 2 Nm for the Easy® system Axial forces in the Twinflex® longitudinal elements were about 160 N and about 100 N in the Easy® rods Although the numerical approach mainly provides tendencies it clearly seems that reducing flexural stiffness of lumbar fixation induces more homogeneous load transmission along the construct and greatly reduces axial pushin/pullout forces at the S1/S2 levels and all this without reducing the rigidity of the whole construct Conversely it has been shown that “rigid” longitudinal elements may concentrate stresses at the construct extremities relieving loads at intermediate levels at the same time which may be the sign of a stressshieldinglike phenomenon These differences arise from a fundamental difference between both kinds of longitudinal elements in the way they transmit loads The ø 6 mm rods mainly oppose a bending reaction torque to the applied flexion moment whereas the Twinflex® construct mainly balances the applied flexion torque by an anterior compression of the anterior column combined with posterior traction on its longitudinal elements ELF

Keywords: