Abstract
BackgroundSpondylolytic (or lytic) spondylolisthesis is often associated with disc degeneration at the index-level; however, it is not clear if disc degeneration is the cause or the consequence of lytic spondylolisthesis. The main objective of this computed tomography based finite element modelling study was to examine the role of different grades of disc degeneration in the progression of a bilateral L5-lytic defect to spondylolisthesis.MethodsHigh-resolution computed tomography data of the lumbosacral spine from an anonymised healthy male subject (26 years old) were segmented to build a 3D-computational model of an INTACT L1-S1 spine. The INTACT model was manipulated to generate four more models representing a bilateral L5-lytic defect and the following states of the L5-S1 disc: nil degeneration (NOR LYTIC), mild degeneration (M-DEG LYTIC), mild degeneration with 50% disc height collapse (M-DEG-COL LYTIC), and severe degeneration with 50% disc height collapse(S-COL LYTIC). The models were imported into a finite element modelling software for pre-processing, running nonlinear-static solves, and post-processing of the results.ResultsCompared with the baseline INTACT model, M-DEG LYTIC model experienced the greatest increase in kinematics (Fx range of motion: 73% ↑, Fx intervertebral translation: 53%↑), shear stresses in the annulus (Fx anteroposterior: 163%↑, Fx posteroanterior: 31%↑), and strain in the iliolumbar ligament (Fx: 90%↑). The S-COL LYTIC model experienced a decrease in mobility (Fx range of motion: 48%↓, Fx intervertebral translation: 69%↓) and an increase in normal stresses in the annulus (Fx Tensile: 170%↑; Fx Compressive: 397%↑). No significant difference in results was noted between M-DEG-COL LYTIC and S-COL LYTIC models.ConclusionsIn the presence of a bilateral L5 spondylolytic defect, a mildly degenerate index-level disc experienced greater intervertebral motions and shear stresses compared with a severely degenerate index-level disc in flexion and extension bending motions. Disc height collapse, with or without degenerative changes in the stiffness properties of the disc, is one of the plausible re-stabilisation mechanisms available to the L5-S1 motion segment to mitigate increased intervertebral motions and shear stresses due to a bilateral L5 lytic defect.
Highlights
Spondylolytic spondylolisthesis is often associated with disc degeneration at the index-level; it is not clear if disc degeneration is the cause or the consequence of lytic spondylolisthesis
From the baseline INTACT state, the greatest increase in Fx and Ex range of motion (RoM) was observed in the M-DEG LYTIC model (Fx: 7.2° (INTACT) to 12.4° (M-DEG LYTIC); Ex: 7.0° (INTACT) to 9.5° (M-DEG LYTIC))
INTACT model, M-DEG LYTIC model experienced the greatest increase in kinematics (Fx ROM: 73% ↑; Fx interpedicular travel (IPT) mid-discal projection: 53%↑), shear stresses in the annulus (Fx anteroposterior: 163%↑; Fx posteroanterior: 31%↑), and average strain in the iliolumbar ligament (ILL) fibres (Fx: 90%↑)
Summary
Spondylolytic (or lytic) spondylolisthesis is often associated with disc degeneration at the index-level; it is not clear if disc degeneration is the cause or the consequence of lytic spondylolisthesis. The main objective of this computed tomography based finite element modelling study was to examine the role of different grades of disc degeneration in the progression of a bilateral L5-lytic defect to spondylolisthesis. Lytic spondylolisthesis is often associated with disc degeneration at the olisthetic segment; it is not clear whether disc degeneration is the cause or consequence of lytic spondylolisthesis [11]. Case studies have cited the rare progression of pars defect to vertebral slippage in adults without any associated disc degeneration, adding to the ambiguity around the role of disc degeneration in the pathomechanism for slippage progression [14, 15]
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