Background: Pelvic incidence, sacral slope and slip percentage have been shown to be important predicting\r\nfactors for assessing the risk of progression of low- and high-grade spondylolisthesis. Biomechanical factors, which\r\naffect the stress distribution and the mechanisms involved in the vertebral slippage, may also influence the risk of\r\nprogression, but they are still not well known. The objective was to biomechanically evaluate how geometric sacral\r\nparameters influence shear and normal stress at the lumbosacral junction in spondylolisthesis.\r\nMethods: A finite element model of a low-grade L5-S1 spondylolisthesis was constructed, including the\r\nmorphology of the spine, pelvis and rib cage based on measurements from biplanar radiographs of a patient.\r\nVariations provided on this model aimed to study the effects on low grade spondylolisthesis as well as reproduce\r\nhigh grade spondylolisthesis. Normal and shear stresses at the lumbosacral junction were analyzed under various\r\npelvic incidences, sacral slopes and slip percentages. Their influence on progression risk was statistically analyzed\r\nusing a one-way analysis of variance.\r\nResults: Stresses were mainly concentrated on the growth plate of S1, on the intervertebral disc of L5-S1, and\r\nahead the sacral dome for low grade spondylolisthesis. For high grade spondylolisthesis, more important\r\ncompression and shear stresses were seen in the anterior part of the growth plate and disc as compared to the\r\nlateral and posterior areas. Stress magnitudes over this area increased with slip percentage, sacral slope and pelvic\r\nincidence. Strong correlations were found between pelvic incidence and the resulting compression and shear\r\nstresses in the growth plate and intervertebral disc at the L5-S1 junction.\r\nConclusions: Progression of the slippage is mostly affected by a movement and an increase of stresses at the\r\nlumbosacral junction in accordance with spino-pelvic parameters. The statistical results provide evidence that pelvic\r\nincidence is a predictive parameter to determine progression in isthmic spondylolisthesis
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