3D Deformation Patterns of S Shaped Elastic Rods as a Pathogenesis Model for Spinal Deformity in Adolescent Idiopathic Scoliosis

Abstract

Adolescent idiopathic scoliosis (AIS) is a three-dimensional (3D) deformity of the spinal column in pediatric population. The primary cause of scoliosis remains unknown. The lack of such understanding has hampered development of effective preventive methods for management of this disease. A long-held assumption in pathogenesis of AIS is that the upright spine in human plays an important role in induction of scoliosis. Here, the variations in the sagittal curve of the scoliotic and non-scoliotic pediatric spines were used to study whether specific sagittal curves, under physiological loadings, are prone to 3D deformation leading to scoliosis. To this end, finite element models of the S shaped elastic rods, which their curves were derived from the radiographs of 129 sagittal spinal curves of adolescents with and without scoliosis, were generated. Using the mechanics of deformation in elastic rods, this study showed that the 3D deformation patterns of the two-dimensional S shaped slender elastic rods mimics the 3D patterns of the spinal deformity in AIS patients with the same S shaped sagittal spinal curve. On the other hand, the rods representing the non-scoliotic sagittal spinal curves, under the same mechanical loading, did not twist thus did not lead to a 3D deformation. This study provided strong evidence that the shape of the sagittal profile in individuals can be a leading cause of the 3D spinal deformity as is observed in the AIS population.

What causes different coronal curve patterns in idiopathic scoliosis?

“…It was previously showed that two axial subgroups exist in right thoracic AIS: loops shaped and lemniscate shaped [3,19]. The sagittal curvature of the spine relates to the pattern of the 3D deformity development, particularly the aforementioned axial subgroups of right thoracic AIS [19].…”

“…As shown previously, the axial projection of the curve carries important information for surgical and non-surgical management of the right thoracic AIS [6,[8][9][10]. It was also shown that the sagittal profile strongly relate to the patterns of axial deformation using mechanics of deformation in an elastic rod [3,11,12]. Here, it is studied if the axial projection of the curve varies within the Lenke types and whether that variation relates to the sagittal curvature of the spines within these subtypes.…”

“…It was previously showed that two axial subgroups exist in right thoracic AIS: loops shaped and lemniscate shaped [3,19]. The sagittal curvature of the spine relates to the pattern of the 3D deformity development, particularly the aforementioned axial subgroups of right thoracic AIS [19]. When the spine was considered as a curved elastic rod, it was shown quantitatively that the distribution of the moments along the spine are altered by the initial geometry of the sagittal spinal curvature which in turn results in different 3D deformation patterns among scoliotic patients [11,19].…”

“…The sagittal curvature of the spine, without considering the frontal deformity, was used to develop a 2D reduced order finite element model (FEM) of the spine [12]. The S shaped (sagittal profile) 2D model of a homogeneous, isotropic, slender elastic rod (Young modulus=1000Pa and Poisson ratio=0.3) with a circular cross section, was loaded by vertical loading (gravity) at each vertebral level and a torsional moment representing the trunk mass asymmetry as shown in [18].…”

“…The S shaped (sagittal profile) 2D model of a homogeneous, isotropic, slender elastic rod (Young modulus=1000Pa and Poisson ratio=0.3) with a circular cross section, was loaded by vertical loading (gravity) at each vertebral level and a torsional moment representing the trunk mass asymmetry as shown in [18]. This model previously generated the 3D deformation patterns of the scoliotic spine [12]. The reduced-order FEM was created for all the axial clusters to calculate the 3D deformation patterns.…”