Role biomecanique de la cage thoracique dans le traitement de la scoliose par orthes

This Ph.D. thesis addresses the overall hypothesis that biomechanical relationships and coupling mechanisms exist between scoliotic trunk anatomical structures which may explain incomplete and unexpected results obtained by orthotic and surgical treatments.;A biomechanical finite element model of the scoliotic spine and rib cage was developed in order to investigate this hypothesis. The input geometry to the model was obtained from a personalized geometric model built using an hybrid method which combines multi-view radiographic 3-D reconstructions of the spine and rib cage of given scoliotic patients with serial CT-scan 3-D reconstructions of typical human vertebrae. The biomechanical model was then used to simulate orthotic and surgical loads in order to address the hypothesis of this thesis. In parallel, a clinical retrospective study was conducted on a cohort of 36 adolescent idiopathic subjects treated by Boston brace in order to confirm biomechanical studies and verify the existence of relationships and coupled corrections between the spine and rib cage initiated by brace wear.;The retrospective clinical study done on a cohort of 36 adolescent scoliotic subjects showed that the Boston brace treatment produces complex trunk motions. Rib cage offsets were statistically correlated to corresponding apical spine offsets (r = 0.88 in frontal plane and r = 0.65 in sagittal plane). However, rib hump rotation and axial rotation at apical thoracic level were not correlated. Substantial forward displacements of the thorax were observed and were found to be coupled with lateral exteriorly oriented displacements of the spine (r = $-$0.41).;The biomechanical simulations confirmed the clinical observations and coupled movements were also found. Anteriorly oriented forces applied on the posterior rib hump pushed the trunk forward and was associated with an increase of lateral offset and curvatures of the spine. Lateral loads on the rib cage appear to provide better correction, although not optimal. Inverse coupling mechanisms (generated by the rib cage when loads are applied on the spine) were also investigated and were found to be quite similar to the ones found when the rib cage was loaded, even if the influence of CVCT joints was negligible in this case. The role played by the CVCT joints on the trunk overall response to orthopaedic treatments was investigated by varying mechanical and geometrical parameters associated to these joints. The influence of costal neck orientation was negligible while the orientation of CV joints and CVCT stiffness properties had a more substantial influence on trunk translational motions and rib hump rotations. It demonstrated that physical and mechanical alterations of scoliotic CV ligaments may play a role in the coupled movements observed between the spine and rib cage. However, these joints were not found to be the "missing link" that can explain all coupling phenomena. Results found in this thesis suggest that orthotic and surgical treatments of scoliotic deformities should not only be administered in terms of main corrections, but also in terms of coupled motions in order to obtain more optimal corrections of scoliotic deformities. (Abstract shortened by UMI.).