Boston Barham and Anton Bowden, Mechanical Engineering
INTRODUCTION
Schmorl’s nodes are intrusions of the disc into the vertebral endplate in the lumbar spine and are common in spinal imaging (figure 1). They are significantly associated with the presence and severity of spinal disc degeneration [1], which is a major cause of work disability and a costly health care issue. However, the clinical significance of Schmorl’s nodes has remained unclear. They are associated with disc degeneration, but the correlation has been inconsistent. It is hypothesized that the Schmorl’s node’s wide variety of topology and morphology is a leading factor in this inconsistency. They are most commonly round and located in the center of the vertebra [2], however, they can be rectangular or pointed and located on the front, back, or side of the vertebral endplate. It is unclear whether the presence of Schmorl’s nodes lead to disc degeneration, or vice-versa.
METHODS
This study utilized nonlinear finite element method to investigate the consequences of a common (round) Schmorl’s node on the intervertebral disc pressure, and the angle of deflection in both the disc and lumbar spine. Research of this kind provides valuable insight into how Schmorl’s nodes effect the intervertebral disc, and leads to a better understanding of the relationship between Schmorl’s nodes and disc degeneration.
The present work utilized a previously validated and published non-linear finite element model of the lumbar spine [3] (figure 2). The model was previously validated against experimental and published data for material properties [table 1], quality of motion, range of motion, cortical strains, and disc pressure.
A second model was simulated from the control mentioned above using Altair Hyperworks. Utilizing the morphing function, a Schmorl’s node was simulated at the top of the mesh elements of the L2 vertebra (figure 2), patterned after common round Schmorl’s nodes, in MRI imaging [2]. This approach is optimal because the geometric and material properties are perfectly identical in both models, with the exeption of the Schmorl’s node geometry.
Using LS DYNA, the S1 vertebra was fixed and a 444 N compressive follower load was applied to simulate gravity loading of the upper torso as well as the muscular compressive loading [4]. Each model was then subjected to an applied moment of 7.5 Nm extension, which is based on previously published work [3].
RESULTS
Disc pressures were calculated by averaging the pressures from elements found at the center of the intervertebral disc. Figure 3 shows a distribution of all pressure throughout the control and Schmorl’s node L1-L2 vertebra and disc, which range from -1.04 to 1.42 Mpa. Disc pressures where computed twice, once after the 444 N follower load and again after the 7.5 Nm moment was applied (Table 2). Pressure were measured to be an average of 38% higher in the Schmorl’s node model then in the control.
Angle of deflection was measured for two regions, first for the lumbar spine and second for the L1-L2 disc. This was done by drawing two lines on top and bottom of the spine and disc, and measuring the angle between them before and after the moment was applied (Table 3).
DISCUSSION
Based on our results, presence of a Schmorl’s node significantly increases disc pressure, resulting in a 38% increase with respect to the otherwise identical control model. These results are significant in that they identify that geometrical effects of Schmorl’s node presence may be a precipitating factor in disc degeneration for some patients
The present work shows that the presence of Schmorl’s nodes does not significantly impact the movement of the spine. Angles of deflection showed less than .5% change in deflection when loaded with an applied moment.
CONCLUSION
It can be concluded from the present work that the biomechanical presence of Schmorl’s nodes has a significant impact on vertebral disc pressure and may contribute to disc degeneration.
Figure 1: MRI scan showing a variety of Schmorl’s node morphology and topology. [2]
Figure 2: Non-linear finite element model of the lumbar spine. [3]
Figure 3: Simulation of a common Schmorl’s node, morphed from the control model.
Figure 4: Vertical cut of control and Schmorl’s node models showing distribution of pressure.
REFERENCES
- Florence P.S. Mok, et al., Spine (Phila Pa 1976). 35(21):1944-952, 2010.
- Florence P.S. Mok, et al., Classification of Schmorl’s Nodes of the Lumbar Spine and Association with Disc Degeneration: a large-scale population-based MRI study.(Under Revision)
- Von Forell, G. A., Comput. Methods Biomech. Biomed, 2013, (in press).
- Patwardhan, A. G., J. Orthop. Res., 21(3): 540–546, 2003.