Etude biomecanique du traitement de la scoliose idiopathique par orthese: Effets des parametres de conception des corsets sur les corrections geometriques et sur les contraintes internes du rachis

This study was divided into 5 parts. A simulation process was firstly developed to represent the gravity forces in a finite element model (FEM) of the trunk of a scoliotic patient. An optimization process computed the forces to be substracted from the FEM, based on the 3D reconstruction of biplanar x-rays of the patient, in order to obtain after the inclusion of the gravity forces a model corresponding to the actual geometry of the patient. The difference in the vertebral positions from the geometry acquired from radiographs and the computed geometry of the model including the gravity forces was inferior to 3 mm. The forces and compressive stresses in the scoliotic spine were then computed.The brace model was in a third part adapted to simulate the Charleston brace, which is worn over the night and imposes a supine side-bending to the patient in the direction of its major scoliotic curve. Braces were designed for two scoliotic patients and their installation was simulated. The efficiency of the simulated Charleston braces was studied by computing the geometrical corrections and the effect on the internal stresses of the spine. The reduction of the major scoliotic curve varied between 58% and 97% and was in the range of published clinical data. Internal compressive stresses of up to 1 MPa were generated on the convex side of the major scoliotic curve and tensile stresses up to 1 MPa on its concavity. However, increased compressive stresses were exerted on the concavity of the secondary curves and added tensile stresses in their convexity. The study confirmed the working principle of the brace assumed by its designers, which consists in inverting the asymmetrical compressive loading at the level of the major scoliotic curve.In the fourth part, for three patients presenting different types of scoliotic curves, custom-fit braces following the Boston brace system principles were modeled and their installations simulated. Two sets of mechanical properties of the spine (stiff and flexible) were tested. The influences of 15 design factors on the 3D correction generated by the brace were evaluated following a design of experiments simulation protocol allowing computing the main and two-way interaction effects of the design factors. Results showed a great variability of the braces effectiveness. The most influential design factors were the position of the brace opening (posterior vs anterior), the strap tension, the trochanter extension side, the lordosis design and the rigid shell shape. The position of the brace opening modified the correction mechanism. The trochanter extension position influenced the efficiency of the thoracic and lumbar pads by modifying their lever arm. Increasing the strap tension improved corrections of coronal curves. The lordosis design had an influence in the sagittal plane but not in the coronal plane.In the fifth part, for the same three patients of the precedent study, 1024 different virtual braces were tested and, for each brace, immediate in-brace correction of the coronal Cobb angles and the bending moment acting on the apical vertebrae were computed and their correlation was studied. Two sets of mechanical properties of the spine (stiff and flexible) were tested. Immediate correction of coronal curves and corresponding impact on the apical vertebrae bending moments were linearly correlated (mean R2 = 0.88). The amount of immediate correction necessary to nullify the bending moment ranged between 19% and 61% with average 48% (flexible spine model) and 27% (stiff spine model). This study was then extended to a total of 30 patients in order to reinforce its conclusions. The correlation between immediate correction of coronal curves and corresponding impact on the apical vertebrae bending moments was confirmed (mean R2 = 0.86). 10% to 99% of immediate correction was necessary to nullify the asymmetrical loads, with an average of 49% (flexible spine model) and 35% (stiff spine model). (Abstract shortened by UMI.)In a second part, a method to simulate brace treatment including the representation of gravity forces previously described was developed. To show the feasibility of the approach, custom-fit braces following the Boston brace system principles were designed for five scoliotic patients and their installations were simulated. Immediate geometrical corrections and pressures generated by the brace were computed. The brace's effect on the asymmetrical compressive loading of the vertebral endplates in the coronal plane was analyzed. The influence of the strap tension, of the spine stiffness and of the presence of the gravity forces was evaluated. Results showed that the presence of the gravity forces is essential to adequately simulate brace treatment.