Skip to main page content
Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
, 8 (5), 460-470

The Role of Sacral Slope in the Progression of a Bilateral Spondylolytic Defect at L5 to Spondylolisthesis: A Biomechanical Investigation Using Finite Element Analysis

Affiliations

The Role of Sacral Slope in the Progression of a Bilateral Spondylolytic Defect at L5 to Spondylolisthesis: A Biomechanical Investigation Using Finite Element Analysis

Vivek A S Ramakrishna et al. Global Spine J.

Abstract

Study design: A biomechanical study using finite element analysis.

Objectives: The main objective of this study was to investigate the role of sacral slope in the progression of a L5 bilateral spondylolytic defect to spondylolisthesis.

Methods: A 3-dimensional model of lumbosacral spine was built using computed tomography (CT) data procured from an anonymized healthy male subject. The segmented CT data was manipulated to generate 3 more models representing L5 bilateral spondylolytic defect with normal sacral slope (SS), sacral slope increased by 10° (SS+10), and sacral slope decreased by 10° (SS-10). The 3D models were imported into finite element modelling software Strand7 for preprocessing, running nonlinear static solves, and postprocessing of the results.

Results: Directional biomechanical instabilities were induced in the lumbosacral spine as a result of changes in the L5-S1 disc shape secondary to the changes in sacral slope. Compared with the normal L5 lytic model, wedging of the L5-S1 disc (SS+10) resulted in a significantly greater range of motion in flexion (18% ↑) but extension motion characteristics were similar. Conversely, flattening of the L5-S1 disc (SS-10) resulted in a significantly greater range of motion in extension (16% ↑) but flexion motion characteristics were similar to that of the normal L5 lytic model.

Conclusions: Variations in sacral slope while preserving the L5-S1 mid-disc height and orientation of the L5 vertebra resulted in variations in the L5-S1 disc shape. The results suggest that for such extremities in the L5-S1 disc shape different pathomechanisms exist for the progression of the L5 lytic defect to spondylolisthesis.

Keywords: L5-S1 disc shape; biomechanical instability; finite element analysis; sacral slope; spondylolisthesis; spondylolysis; wedging.

Conflict of interest statement

Declaration of Conflicting Interests: The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Figures

Figure 1.
Figure 1.
(a) A solid model of the lumbosacral spine comprising only the bony elements and the intervertebral discs. (b) Ligaments were modelled using nonlinear beam elements. (c) The concentric layers (n = 4) of crisscross collagen fibers modelled in the annulus fibrosus. (d) A fully preprocessed finite element model of the lumbosacral spine.
Figure 2.
Figure 2.
L5-S1 range of motion (RoM) in flexion (Fx) and extension (Ex) bending modes at peak loading (10 N m). The presence of a bilateral spondylolytic defect in the L5 vertebra increased RoM in both Fx and Ex. Compared with the normal LYTIC model, RoM increased in the SS+10 model during Fx, and in the SS-10 model during Ex.
Figure 3.
Figure 3.
(a) L5-S1 interpedicular travel (IPT) in flexion (Fx) and extension (Ex) bending modes at peak loading (10 N m). The IPT metric captures the translational movement of a vertebra by measuring the movement of a pedicle with respect to the caudal pedicle. The bilateral L5 lytic defect increased IPT in both Fx and Ex. Compared with the normal LYTIC model, IPT increased in the SS+10 model during Fx, and in the SS-10 model during Ex. (b) L5-S1 ID in Fx and Ex bending modes at peak loading (10 N m). The ID metric captures translational deflection between adjacent segment pedicles in going from neutral to extreme position. The bilateral L5 spondylolytic defect increased ID in both Fx and Ex bending modes. Compared with the normal LYTIC model, the SS-10 model experienced a significant increase in ID during Ex.
Figure 4.
Figure 4.
Axial strain in capsular ligaments under peak loading (10 N m) in flexion (Fx) and extension (Ex). The presence of a bilateral L5 spondylolytic defect significantly decreased capsular ligament strain in Fx and to a larger extent in Ex. Compared with the normal LYTIC model, changes in SS did not produce any significant change in capsular ligament strain. Error bars represent standard deviation.
Figure 5.
Figure 5.
Normal stresses in the mid-discal plane of the L5-S1 disc in flexion (Fx) and extension (Ex) bending modes at peak loading (10 N m). Compared with the normal LYTIC model, the anterior annulus was abnormally loaded in compression during Fx in the SS-10 model, and the posterior annulus was abnormally loaded in compression during Ex in the SS+10 model.
Figure 6.
Figure 6.
Posteroanterior (PA) directed shear stresses in the mid-discal plane of the L5-S1 disc in flexion (Fx) and extension (Ex) bending modes at peak loading (10 N m). Compared with the INTACT model, the loss in posterior tension band in the normal LYTIC model resulted in a decrease in area under positive PA shear stress in Fx (69% to 51%) and Ex (78% to 48%).
Figure 7.
Figure 7.
Changes in the L5-S1 disc shape secondary to the changes in sacral slope (SS). In all the models, the mid-disc height and the orientation of the L5 vertebra were kept the same.

Similar articles

See all similar articles

References

    1. Fredrickson BE, Baker D, McHolick WJ, Yuan HA, Lubicky JP. The natural history of spondylolysis and spondylolisthesis. J Bone Joint Surg Am. 1984;66:699–707. - PubMed
    1. Leone A, Cianfoni A, Cerase A, Magarelli N, Bonomo L. Lumbar spondylolysis: a review. Skeletal Radiol. 2011;40:683–700. - PubMed
    1. Roussouly P, Gollogly S, Berthonnaud E, Labelle H, Weidenbaum M. Sagittal alignment of the spine and pelvis in the presence of L5-s1 isthmic lysis and low-grade spondylolisthesis. Spine (Phila Pa 1976). 2006;31:2484–2490. - PubMed
    1. Labelle H, Mac-Thiong JM, Roussouly P. Spino-pelvic sagittal balance of spondylolisthesis: a review and classification. Eur Spine J. 2011;20(suppl 5):641–646. - PMC - PubMed
    1. Hresko MT, Labelle H, Roussouly P, Berthonnaud E. Classification of high-grade spondylolistheses based on pelvic version and spine balance: possible rationale for reduction. Spine (Phila Pa 1976). 2007;32:2208–2213. - PubMed

LinkOut - more resources

Feedback