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Clinical Biomechanics
Article in Press, Corrected Proof.
Received 2 July 2008; accepted 25 November 2008. Available online 3 January 2009.

Influence of different artificial disc kinematics on spine biomechanics

Thomas Zandera, Antonius Rohlmann, a, and Georg Bergmanna

aJulius Wolff Institut, Charité – Universitätsmedizin Berlin, Campus Virchow-Klinikum, Augustenburger Platz 1, 13353 Berlin, Germany

Abstract

Background

There are several different artificial discs for the lumbar spine in clinical use. Though clinically established, little is known about the biomechanical advantages of different disc kinematics.

Methods

A validated finite element model of the lumbosacral spine was used to compare the results of total disc arthroplasty at level L4/L5 performed by simulating the kinematics of three established artificial disc prostheses (Charité, ProDisc, Activ L). For flexion, extension, lateral bending, and axial torsion, the intervertebral rotations, the locations of the helical axes of rotation, the intradiscal pressures, and the facet joint forces were evaluated at the operated and adjacent levels.

Findings

After insertion of an artificial disc, intervertebral rotation is reduced for flexion and increased for extension, lateral bending, and axial torsion for all studied discs at implant level. The positions of the helical axes are altered especially for lateral bending and axial torsion. Increased facet joint contact forces are predicted for the Charité disc during extension – influenced by the existence of anterior scar tissue – and for the ProDisc and the Activ L during lateral bending and axial torsion. The studied artificial discs have only a minor effect on the adjacent levels.

Interpretations

For some load cases, total disc arthroplasty leads to considerably altered kinematics and increased facet joint contact forces at implant level. The spinal kinematic alterations due to an artificial disc exceed by far the inter-implant differences, while facet joint contact force alterations are strongly implant and load case dependent. The importance of implant kinematics is often overestimated.

Keywords: Artificial disc; Finite element method; Helical axes; Charité disc; ProDisc; Activ L; Lumbar spine

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