Abstract
BackgroundPelvic incidence, sacral slope and slip percentage have been shown to be important predicting factors for assessing the risk of progression of low- and high-grade spondylolisthesis. Biomechanical factors, which affect the stress distribution and the mechanisms involved in the vertebral slippage, may also influence the risk of progression, but they are still not well known. The objective was to biomechanically evaluate how geometric sacral parameters influence shear and normal stress at the lumbosacral junction in spondylolisthesis.MethodsA finite element model of a low-grade L5-S1 spondylolisthesis was constructed, including the morphology of the spine, pelvis and rib cage based on measurements from biplanar radiographs of a patient. Variations provided on this model aimed to study the effects on low grade spondylolisthesis as well as reproduce high grade spondylolisthesis. Normal and shear stresses at the lumbosacral junction were analyzed under various pelvic incidences, sacral slopes and slip percentages. Their influence on progression risk was statistically analyzed using a one-way analysis of variance.ResultsStresses were mainly concentrated on the growth plate of S1, on the intervertebral disc of L5-S1, and ahead the sacral dome for low grade spondylolisthesis. For high grade spondylolisthesis, more important compression and shear stresses were seen in the anterior part of the growth plate and disc as compared to the lateral and posterior areas. Stress magnitudes over this area increased with slip percentage, sacral slope and pelvic incidence. Strong correlations were found between pelvic incidence and the resulting compression and shear stresses in the growth plate and intervertebral disc at the L5-S1 junction.ConclusionsProgression of the slippage is mostly affected by a movement and an increase of stresses at the lumbosacral junction in accordance with spino-pelvic parameters. The statistical results provide evidence that pelvic incidence is a predictive parameter to determine progression in isthmic spondylolisthesis.
Highlights
Pelvic incidence, sacral slope and slip percentage have been shown to be important predicting factors for assessing the risk of progression of low- and high-grade spondylolisthesis
The study of low grade spondylolisthesis revealed significant differences in terms of stress distribution within the growth plate and the intervertebral disc in a spine defined by a lumbosacral junction between the nutcracker- (PI = 52°, sacral slope (SS) = 45°) and shear-type (PI = 75°, SS = 60°) cases (Additional file 4: cases 1 and 4)
The elevated stresses in low grade cases were mostly located around the domeshaped area of the growth plate and intervertebral disc (Figures 4, 5 and 6)
Summary
Sacral slope and slip percentage have been shown to be important predicting factors for assessing the risk of progression of low- and high-grade spondylolisthesis. The objective was to biomechanically evaluate how geometric sacral parameters influence shear and normal stress at the lumbosacral junction in spondylolisthesis. Pelvic incidence (PI) has been shown to be an important predicting factor for assessing the risk of progression of low-grade spondylolisthesis [1,2]. Essential to maintain global sagittal balance (such as slip angle, the lumbar index or the sacral contour), are thought to be secondary changes as their role is related to the degree of dysplasia rather than the cause of slippage [1,14,15]. Since spondylolisthesis is mainly assessed using radiographic measurements, forces responsible for the progression of the deformity remain unclear
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