Technical ReportsDisc arthroplasty design influences intervertebral kinematics and facet forces
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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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.