Influence of single-level lumbar degenerative disc disease on the behavior of the adjacent segments—A finite element model study
Introduction
Degenerative disc disease (DDD) is a progressive condition that alters the biochemistry and morphology of the intervertebral disc (IVD) eventually affecting its ability to support and transfer loads. Observation of the macroscopic patterns of the disease suggests that further deterioration of the disc is significantly related to the number of degenerated discs and ageing (Elfering et al., 2002; Waris et al., 2007). Why IVDs adjacent to a degenerated level have a higher prevalence of developing the disease remains unknown. Abnormal mechanical loads and/or motion patterns have been traditionally linked to degenerative changes of the spine and a risk for injury (Adams et al., 2000; Stokes and Iatridis, 2004). Understanding how DDD alters the biomechanics of the spine is of clinical significance, due to its association with chronic lower back pain, sciatica, and adult onset scoliosis (Kjaer et al., 2006).
With degeneration, the nucleus dehydrates and transitions from a fluid-like behavior to a solid-like behavior making it harder to distinguish it from the annulus fibrosus (Iatridis et al., 1997). The progressive loss of hydration leads to a decrease in the disc height and high localized stress peaks in the annulus (Adams et al., 1996; Frobin et al., 2001). Abnormal mechanics in terms of magnitude and asymmetry of motion, transverse or sagittal translation, location of the instantaneous axis of rotation, patterns of coupled motion and load bearing at the neural arch and facet joints have been reported with DDD (Seligman et al., 1984; Mimura et al., 1994; Fujiwara et al., 2000; Lund et al., 2002; Pollintine et al., 2004). Despite numerous studies on the mechanics of the degenerated disc, limited data exists on how the condition affects the adjacent caudal and cephalic segments, thus contributing to the progression of disc degeneration.
The goal of this study was to quantitatively describe how DDD at a single level alters the loading and motion patterns of the adjacent discs by means of a three-dimensional finite element model (FEM) of the lumbar spine subjected to physiological loading conditions. We hypothesized that progressive degeneration at a single-level would cause the adjacent cephalic and caudal segments to undergo,
- (1)
an increase in rotational motion in the three principal axes of motion,
- (2)
an increase in the maximum shear stress and von Mises stress of the annulus ground substance,
- (3)
an increase in facet contact forces and,
- (4)
an increase in nucleus pressure.
Section snippets
Non-linear three-dimensional FEM of a healthy lumbar spine
A FEM of the lumbar spine was previously developed by our research team (Renner et al., 2007). The vertebral body cancellous bone, cortical bone and posterior elements were modeled with a linear isotropic elastic law (Table 1). The annulus was modeled as a fiber-reinforced composite material. The annulus ground substance was modeled by Mooney–Rivlin hyper-elastic elements. Three layers of fibers were embedded in the annulus at an angle of inclination of ±30° with the horizontal plane. The
Results
The predicted response of the FEM of the healthy spine compared favorably with values obtained in-vitro. The segmental range of motion during FLEX/EXT bending loads with 800 N of follower preload and without preload, fell within one standard deviation of the in-vitro results at all levels except L3–L4 and L4–L5 (Fig. 1). In-vitro values of segmental motion during LB and AR were only available under no preload conditions. The predicted segmental range of motion during LB loads for all levels fell
Discussion
A three-dimensional non-linear FEM of the lumbar spine with degeneration at the L4–L5 disc level has been developed. Two different grades of disc degeneration were modeled by changing both the geometry and associated material properties. Adjacent level effects were tested using a hybrid protocol as per the recommendations of Panjabi (2007). Models were subjected to compressive preloads of physiological magnitude using the follower load technique (Patwardhan et al., 1999).
Some of the FEM results
Conflict of interest statement
The authors do not have any financial and personal relationships with other people or organizations to disclose that could inappropriately influence and/or bias the work.
Acknowledgements
The authors would like to thank Susan M. Renner, Ph.D., for providing the in-vitro data. This study was funded in part by the National Institute of Health (NIH AR48152-02).
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