Biomechanical comparison between fusion of two vertebrae and implantation of an artificial intervertebral disc

Abstract

Surgical treatments for lower back pain can be distributed into two main groups: fusion (arthrodesis) and disc replacement (arthroplasty). The objective of this study was to compare, under severe loading conditions, the biomechanics of the lumbar spine treated either by fusion or total disc replacement (TDR). A three-dimensional model of a two-level ligamentous lumbar segment was created and simulated through static analyses with the finite-element method (FEM) software ABAQUS. The model was validated by comparing mobility, pressure on the facets, force in the ligaments, maximum stresses, disc bulge, and endplate deflection with measured data given in the literature. The FEM analysis predicted that the mobility of the model after arthrodesis on the upper level was reduced in all rotational degrees of freedom by an average of approximately 44%, relative to healthy normal discs. Conversely, the mobility of the model after TDR on the upper level was increased in all rotational degrees of freedom by an average of approximately 52%. The level implanted with the artificial disc showed excessive ligament tensions (greater than 500 N), high facet pressures (greater than 3 MPa), and a high risk of instability. The mobility and the stresses in the level adjacent to the arthroplasty were also increased. In conclusion, the model for an implanted movable artificial disc illustrated complications common to spinal arthroplasty and showed greater risk of instability and further degeneration than predicted for the fused model. This modeling technique provides an accurate means for assessing potential biomechanical risks and can be used to improve the design of future artificial intervertebral discs.

Introduction

Eight out of ten people in the United States will have problems with lower back pain at some point in their life (Vallfors, 1985). A significant proportion of lower back pain is related to the degeneration of the spinal intervertebral discs (IVD), which is a natural process that may start as early as the second decade of life (Vernon-Robert, 1988). While initial treatments for disc degeneration are generally conservative, it is necessary to consider surgical treatments in some cases. The most significant surgical operations can be distributed into two main groups of solutions: arthrodesis and arthroplasty. Arthrodesis, or fusion, consists of distraction and surgical immobilization of a joint, in this case of a functional spine unit (FSU), to alleviate pain and prevent mechanical instability. Arthroplasty consists of the implantation of an artificial disc in order to alleviate pain by restoring relevant functionalities of the degenerated IVD.

The clinical outcomes for both groups of surgical treatments are comparable. For fusion, good outcomes are reported in 49–88% of the cases (Fritzell et al., 2003; Hanley and David, 1999; Madan et al., 2003; McAfee, 1999). For implantation of an artificial intervertebral device, good outcomes are reported in 50–99% of the cases (Guyer and Ohnmeiss, 2003; Jin et al., 2003; Kleuver et al., 2003).

Nevertheless, multiple and diverse complications are observed. For fusion, the most common complications at the treated level are failure to achieve solid bony union, broken screws, device loosening, and migration of the implant (Fritzell et al., 2003; Marchesi, 2000). Regarding levels adjacent to the treated level, the literature reports mixed results. Some studies found that fusion leads to degeneration of the adjacent FSUs (Bastian et al., 2001; Kumar et al., 2001a), whereas other studies had other conclusions (Kumar et al., 2001b; Penta et al., 1995). For artificial discs, it has been reported that the main complications are: degeneration of the facet joints at the same level, degeneration of the disc and the facet joints at adjacent levels, and subsidence or migration of the artificial disc (van Ooij et al., 2003).

The biomechanical changes associated with these surgical treatments play a significant role in the complications listed above. The aim of this study is to compare the biomechanical alterations of the lumbar spine subject to either fusion or implantation of a movable artificial disc under severe loading conditions.