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. 2013 Oct;22(10):2271-8.
doi: 10.1007/s00586-013-2909-z. Epub 2013 Jul 20.

Parameters Influencing the Outcome After Total Disc Replacement at the Lumbosacral Junction. Part 1: Misalignment of the Vertebrae Adjacent to a Total Disc Replacement Affects the Facet Joint and Facet Capsule Forces in a Probabilistic Finite Element Analysis

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Free PMC article

Parameters Influencing the Outcome After Total Disc Replacement at the Lumbosacral Junction. Part 1: Misalignment of the Vertebrae Adjacent to a Total Disc Replacement Affects the Facet Joint and Facet Capsule Forces in a Probabilistic Finite Element Analysis

A Rohlmann et al. Eur Spine J. .
Free PMC article

Abstract

Purpose: After total disc replacement with a ball-and-socket joint, reduced range of motion and progression of facet joint degeneration at the index level have been described. The aim of the study was to test the hypothesis that misalignment of the vertebrae adjacent to the implant reduces range of motion and increases facet joint or capsule tensile forces.

Methods: A probabilistic finite element analysis was performed using a lumbosacral spine model with an artificial disc at level L5/S1. Misalignment of the L5 vertebra, the gap size of the facet joints, the transection of the posterior longitudinal ligament, and the spinal shape were varied. The model was loaded with pure moments.

Results: Misalignment of the L5 vertebra reduced the range of motion up to 2°. A 2-mm displacement of the L5 vertebra in the anterior direction already led to facet joint forces of approximately 240 N. Extension, lateral bending, and axial rotation caused maximum facet joint forces between 280 and 380 N, while flexion caused maximum forces of approximately 200 N. A 2-mm displacement in the posterior direction led to capsule forces of approximately 80 N. Additional moments increased the maximum facet capsule forces to values between 120 and 230 N.

Conclusions: Misalignment of the vertebrae adjacent to an artificial disc strongly increases facet joint or capsule forces. It might, therefore, be an important reason for unsatisfactory clinical results. In an associated clinical study (Part 2), these findings are validated.

Figures

Fig. 1
Fig. 1
Finite element model of the lumbosacral spine. a Intact model, b exponential curve describing the relationship between contact facet joint force and gap size, c segment L5/S1 with a kinematic model of total disc replacement
Fig. 2
Fig. 2
Finite element models representing the original model (left) and the different types of spinal shapes, after Roussouly et al. [17]. The models vary regarding sacral slope, lordosis tilt angle, angle of global lordosis, number of lordotic vertebrae, inflection point, position of the apex, and the angle and number of vertebrae in the upper and lower arc of lordosis
Fig. 3
Fig. 3
Effect of an a.p. displacement of the L5 vertebra on intervertebral rotation
Fig. 4
Fig. 4
Effect of an a.p. displacement of the L5 vertebra on intervertebral range of motion in the main anatomical planes. The trendlines have their maximum value for small a.p. displacements
Fig. 5
Fig. 5
Influence of an a.p. displacement on contact facet joint forces for the load cases without external load, flexion, and extension
Fig. 6
Fig. 6
Effect of an a.p. displacement on facet capsule tensile forces for the load cases without external load, flexion, and extension
Fig. 7
Fig. 7
Influence of an a.p. displacement on contact facet joint forces for lateral bending and axial rotation
Fig. 8
Fig. 8
Effect of an a.p. displacement on facet capsule tensile forces for lateral bending and axial rotation

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