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Abstract
The aim of this project was to develop a parametric finite element model (FEM) of a paediatric spine to analyze the conditions involved in the risk of progression of the pathology and to test the hypothesis that pelvic incidence is a predictive parameter of the deformity. This model includes the spondylolysis and sacral dome, and can be personalized depending on the type of spondylolisthesis. A detailed biomechanical analysis of the intervertebral disc and growth plate of the lumbosacral junction was done with the FEM in order to determine predictable parameters conducive to the progression.
The geometry of the spine, pelvis and rib cage was reconstructed in 3D using biplanar (postero-anterior and lateral) radiographs of a low-grade patient with a shear-type. Seventeen anatomical landmarks were used for the 3D reconstruction of the thoracic and lumbar vertebrae and 23 were used for the pelvis. The personalized FEM was adapted using a kriging technique, with enhanced details of the L4 to pelvis segment. Taking into consideration the modeling of relevant inter and para-vertebral connective tissues (collagen fibers, anterior and posterior longitudinal ligaments, intertransverse, flavum and capsular ligaments), a personalized FEM was established. Mechanical properties of anatomical structures were taken from the literature.
A design of experiments was undertaken to study the ideal conditions of some spino-pelvic parameters in spondylolisthesis progression. The selected parameters were the PI, which is a fundamental pelvic anatomic parameter, the SS, which characterizes the spatial orientation of the pelvis, and the slip percentage, which represents the grade of slippage. Each parameter had two modalities to include most of low- and high-grade spondylolistheses. In order to satisfy our clinical hypothesis (i.e. it is impossible to have a high grade with a low pelvic incidence and a low sacral slope), a constraint equation was inserted in the design to avoid some non realistic cases. Ten cases of spondylolisthesis have been simulated from one patient thanks to a modeling that included the three independent parameters. A personalized bi-lateral lysis in the pars interarticularis was created by removing posterior elements of L5. The gap of the lysis was adjustable. A sacral dome was also simulated. The combined effect of muscles and gravity was based on a mathematical model, “the follower load”. This method consists in applying a resultant force to the gravity and muscles forces on each vertebra in the rotation center. The forces had the particularity to be tangential to the spine curve. These simulations have been done to assess potential stress differences during spondylolisthesis progression and correlate parameters which can promote the risk of progression.
Compression and shear stresses were analyzed in the lumbosacral joint, especially in the anterior area of the growth plate of S1 and in the anterior area of intervertebral disc L5-S1. In high grade spondylolisthesis with a balanced pelvis, the shear and the compression stresses were higher than in high grade spondylolisthesis with a retroverted pelvis. The same observation was done for the low grade spondylolisthesis, the stresses of the shear type being higher than the nutcracker type. In the intervertebral disc of L5-S1, the zone of shear was larger for high grade spondylolisthesis whereas shear stress for low grade was uniform except for the dome shaped area, which was less stressed. Statistical results revealed a correlation between PI and stresses, and with slip percentage and shear of the L5-S1 intervertebral disc.
The parameters (PI, SS and slip percentage) had an impact on the increase of shear and normal stresses and thus, on the potential risk of progression of the pathology. The anterior area of the growth plate appears important for the risk of progression, where the stress is increased. The statistical study provided evidence that pelvic incidence is the most predictive parameter which can influence significantly the risk of progression in isthmic spondylolisthesis. (Abstract shortened by UMI.)
