Design, optimization, and evaluation of a fusionless device to induce growth modulation and correct spinal curvatures in adolescent idiopathic scoliosis

The global objective of this thesis was to design, optimize, and evaluated experimentally fusionless device concepts to induce growth modulation and correct spinal curvatures in adolescent idiopathic scoliosis (AIS). The central theme addressed in this thesis is that: improved fusionless treatments for AIS may be developed subsequently to understanding biomechanical factors in its pathomechanism, identifying shortcomings of previous fusionless devices, and utilizing a comprehensive design platform that include in silico, in situ, and in vivo analyses.

The foremost undertaking of this thesis was the FEM platform development. This self-adjusting computer model was integrated with an iterative control system that simulated physiological growth as a function of stress variation respecting in vivo correlations. First, the FEM explored biomechanical factors (physiological stress shielding in the form of concave-convex mechanical biases: migration of nucleus to convexity and increased bone mineral density and local disc degeneration on concavity) in the pathomechanism of AIS. This interpretation suggests that concave-convex mechanical biases increased apical asymmetrical stress distribution by 37% and effectively augmented vertebral wedging by up to 1„a (10-20%). Deductions and experimental methods were then extended towards the biomechanical analysis of current fusionless methods. Second, the FEM was utilized to critically explore current fusionless growth modulation devices tailored to AIS. Results from this analysis demonstrated the biomechanical ability of these devices to reduce asymmetrical growth plate loading by up to 50% (flexible tether) and decrease scoliotic progression to 11% (stainless steel staple and flexible tether). Conversely, this analysis highlighted several limitations that could be improved. The explored concepts simply reduce convex growth, span the disc space, and neglect sagittal and axial implications of the scoliotic deformity.

Two devices were proposed for development: an improved intravertebral epiphyseal device (rigid device that locally halts growth without spanning the disc space – feasibility reported in a previous in-house study using a rat tail) and a 3D tether (tethered configuration that targets axial correction in addition to coronal and sagittal control).

The intravertebral epiphyseal device successfully demonstrated its ability to provide fusionless growth modulation without the need to cross the disc space. Moreover, its influence on intervertebral disc health is insignificant pending accurate insertion of the device. Experimental pigs achieved vertebral wedging of 4.1°±3.6°, resulting in a cumulative vertebral deformity of up to 25° over only 4 instrumented levels. Adjacent to device, disc height increased 0.8mm±0.2 and growth plate hypertrophic zone and cell height reduced by a factor of two. Positive disc viability was confirmed by radiographic and histological grading and the lack of collagen type X. This device is the first of its kind to achieve growth modulation in an animal model with vertebral dimensions similar to adolescents without spanning the disc space.

The 3D tether also achieved local growth modulation resulting in vertebral wedging of up to 4° and coronal manipulation of up to 10° in porcine models. Axial and sagittal manipulations were confirmed using in silico and in situ platforms but experimental limitations restricted their objective in vivo confirmation. This is the first fusionless device to seek and demonstrate 3D correction of AIS.

Several notable advances have been achieved in the context of this thesis. The developed finite element platform provides an innovative way to explore biomechanical factors involved in the progression of AIS. In addition, in the context of device design, this FEM platform allows preliminary analyses and optimization to be performed prior to moving forth with expensive in situ and in vivo testing. Two novel fusionless growth modulating devices (intravertebral epiphyseal device and 3D tether) were refined and developed using a complete engineering design approach making use of in silico, in situ, and in vivo analyses. The developed FEM, the identified biomechanical factor in AIS pathomechanism, and the surgical devices conceived over the course of this thesis provide a hopeful step towards the improved management of adolescents with idiopathic scoliosis. (Abstract shortened by UMI.)