Elsevier

The Spine Journal

Volume 6, Issue 3, May–June 2006, Pages 258-266
The Spine Journal

Technical Reports
Disc arthroplasty design influences intervertebral kinematics and facet forces

https://doi.org/10.1016/j.spinee.2005.07.004Get rights and content

Abstract

Background context

Total disc replacement is a novel approach for dynamically stabilizing a painful intervertebral segment. While this approach is gaining popularity, and several types of implants are used, the effect of disc arthroplasty on lumbar biomechanics has not been widely reported. Consequently, beneficial or adverse effects of this procedure may not be fully realized, and data for kinematic optimization are unavailable.

Purpose

To characterize kinematic and load transfer modifications at L5/S1 secondary to joint replacement.

Study design

A human cadaveric biomechanical study in which the facet forces and instant axes of rotation (IAR) were measured for different spinal positions under simulated weightbearing conditions before and after total disc replacement at L5/S1 using semiconstrained (3 degrees of freedom [DOF]; Prodisc) and unconstrained (5 DOF; Charité) articulated implants.

Methods

Twelve radiographically normal human cadaveric L5/S1 joints (age range 45–64 years) were tested before and after disc replacement using Prodisc II implants (Spine Solutions, Paoli, PA) in six specimens and SB Charité III (Johnson & Johnson, New Brunswick, NJ) in six other specimens. Semiconstrained fixtures in combination with a servo-hydraulic materials testing system subjected the test specimens to a physiologic combination of compression and anterior shear. Multiple intervertebral positions were studied and included up to 6 degrees of flexion, extension, and lateral bending. The IAR was calculated for every 3-degree intervals, and the force through the facet joints was simultaneously measured using flexible intra-articular sensors. Data were analyzed using repeated-measures analysis of variance.

Results

During flexion/extension, the average IAR positions and directions were not significantly modified by implantation with the exception that the IAR was higher relative to S1 end plate with the Charité (p=.028). The IAR had a vertically oriented centrode throughout flexion/extension with the Prodisc (p<.001) and the Charité (p<.016). The centrode tended to be greater with the Prodisc. There was a trend that the facet force was decreased throughout flexion/extension for the Prodisc; however, this was statistically significant only at 6° extension (27%, p=.013). In lateral bending, the IAR was significantly modified by Prodisc replacement, with a decreased inclination relative to S1 end plate, (ie, increased coupled axial rotation). While the IAR moved in the horizontal plane toward the side of bending, this effect was more pronounced with the Prodisc. The ipsilateral facet force was significantly increased in 6° lateral bending with the Charité (85%; p=.001).

Conclusions

The degree of constraint affects post-implantation kinematics and load transfer. With the Prodisc (3 DOF), the facets were partially unloaded, though the IAR did not match the fixed geometrical center of the UHMWPE. The latter observation suggests joint surface incongruence is developed during movement. With the Charité (5 DOF), the IAR was less variable, yet the facet forces tended to increase, particularly during lateral bending. These results highlight the important role the facets play in guiding movement, and that implant constraint influences facet/implant synergy. The long-term consequences of the differing kinematics on clinically important outcomes such as wear and facet arthritis have yet to be determined.

Introduction

Chronic low back pain is a common problem in the general population, leading to an estimated 13 million doctor visits and 50 million chiropractor visits per year in the US [1]. When nonoperative management is unsuccessful, selected patients may undergo intervertebral fusion. Recent advances in surgical technique and instrumentation have created interest in disc arthroplasty as a novel approach for treating degenerative disc disease [2]. Several intervertebral implant designs have been proposed for restoring disc height and painless motion [3] including articulated and cushion-like discs. Among articulating discs, ProDisc II (Spine Solutions, Paoli, PA) theoretically allows 3 degrees of freedom (DOF) (three unconstrained planes of rotation with three axes of translation constrained) using a bearing with a ball-and-socket design, whereas the SB Charité III (Johnson & Johnson, New Brunswick, NJ) allows 5 DOF (three unconstrained rotations and two unconstrained translations) with a mobile central inlay. Both prostheses use metal end plates and polyethylene bearing surfaces. Multicenter clinical trials evaluating these implants (total disc replacement vs. fusion) are currently under way in the US [4], [5], [6], [7], and short- and medium-term results in patient series have been reported in Europe [2], [8], [9], [10], [11], [12], [13], [14], [15]. However, few studies quantify the biomechanical effect of the currently used implants on spinal kinematics and load distribution [6], [10], [16]. Although the concept of dynamic stabilization is attractive and early clinical results with total disc arthroplasty are promising in terms of patient satisfaction and function [4], [7], the lack of data regarding biomechanical response of treated spinal segments limits thorough evaluation and optimization of this procedure.

Because intervertebral movement is complex and constrained by both the disc and facet joints, disc replacement will necessarily alter facet forces. Theoretical effects of 3 and 5 DOF designs on the facet joint have been hypothesized, suggesting facet overload with the more constrained, ball-and-socket design [15], [17]. In contrast, a less-constrained design may not participate in supporting intervertebral shear, leading to increases in facet loading, as recent clinical data from patients implanted with the Charité suggest that these patients may be at risk for developing facet arthrosis [18]. Whether this is a natural consequence of their disease, or induced by implant design features is unclear.

The goal of this study was to quantify intervertebral kinematic modifications of the L5/S1 level induced by arthroplasty using 3 and 5 DOF disc replacement. Specifically, we were interested in correlating kinematic changes to facet force alterations. We chose the L5/S1 functional spinal unit because this level is the most common site of disc replacement, and because sagittal obliquity of the sacral end plate induces significant intervertebral shear [19], [20], [21] that places demands on the facet joints. Our data reinforce the notion that the facet joints are significant participants in defining intervertebral motion, and that implant design can influence kinematics and facet forces.

Section snippets

Specimen selection and preparation

The lumbosacral spine was harvested from 12 donors aged 45 to 64 at the time of death (9 male and 3 female). Only specimens with no radiographic evidence of bone disease, disc degeneration, or facet arthritis were used in this study. Specimen preparation consisted in meticulous removal of muscular tissue so as to retain the integrity of the capsular and ligamentous elements. For each specimen, the superior half of the L5 vertebrae and inferior half of S1 vertebrae were potted in

Data analysis

In the ProDisc and the Charité groups separately, paired t tests were used to investigate the effect of disc replacement on the measured IAR parameters and facet forces (SPSS 11.5; SPSS Inc., Chicago, IL). Analyses were performed separately for flexion/extension and lateral bending.

In each group after replacement, variations of IAR parameters and facet force during motion were also investigated using repeated-measures analysis of variance. In lateral bending, left and right facet forces were

Intact specimens

The IAR parameters for the intact specimens assigned to the ProDisc group were not statistically different from those in the Charité group (p>.05). During flexion and extension, the IAR was directed laterally. The IAR intersection with the mid-sagittal plane moved upwards relative to S1 end plate during flexion, and backwards during extension.

The facet forces were not different between the two groups, except during lateral bending where the ipsilateral facet loads were less in the ProDisc group

Discussion

We sought to quantify the effect of total disc replacement on intervertebral kinematics and facet forces during physiologic loading at the L5/S1 level with 3 and 5 DOF implants. We observed that after implantation the IAR remained normally located in the posterior part of the intervertebral disc, moving vertically upwards during flexion and ipsilaterally during lateral bending, demonstrating the prominent role of the facet joints in guiding motion independently from the articulated implant

Acknowledgments

The authors thank Tamer Hadi and Kirk Perdersen for their participation.

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    FDA device/drug status: approved for this indication (SB Charité III device); not approved (ProDisc II).

    Authors DSB and RB acknowledge a financial relationship with Spine Solutions which may indirectly relate to the subject of this research. The study was funded by a grant from Spine Solutions.

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