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Adolescent Idiopathic Scoliosis (AIS) is a 3D deformity of the spine, with a progressive lateral curvature and axial rotation. It affects 1-3% of the 10-16-year-old population in the United States. Anterior Vertebral body tethering (AVBT) is an emerging surgical technique that leverages growth modulation to gradually correct the spine deformity until the patient reaches skeletal maturity. Since the post-growth multi-planar scoliotic correction cannot be consistently predicted based on viewing the acute post-op spine orientation alone, the potential for under-correction or over-correction exists after continued growth occurs. Finite Element (FE) models of the pediatric spine have been utilized previously to simulate AIS treatment, however, with limited consideration of variability in anatomical structures as well as growth patterns within and across vertebral levels. Therefore, the objective of this study is to create anatomically realistic osteo-ligamentous patient-specific FE model of AIS patients to predict the growth-dependent outcomes of AVBT. On the path to prediction of AVBT outcomes with a model that fills the current research gaps, the first task was to develop a FE model of the growth of the normative pediatric spine. This involved the partitioning of model geometry such that an orthotropic thermal expansion method could be applied to represent the granular morphology changes in vertebrae over time. Vertebral morphology was represented over two years of growth from ages 10 to 12 years to 2.85±1.73% strain error, while T1-L5 spine growth velocity was 1.55 cm/year, within the normal range of 1.2-1.6 cm/year. Second was representation of the asymmetrical growth that occurs in those with AIS, as well as of the differences in growth rates that occur due to AVBT. A 12-year-old female AIS patient geometry was used to interpolate, or morph, the now validated normative model geometry to that of the AIS patient, after which growth was simulated with and without AVBT instrumentation. The uninstrumented model showed signs of curve progression and increased vertebral wedging. Conversely, the AVBT instrumented model exhibited reversal of asymmetrical growth rates. Demonstration of this principle served as an empirical validation of the modelling techniques used to predict AVBT outcomes. The third and final step in predicting AVBT outcomes was to perform three case studies for detailed comparison between FE model predictions and AVBT case results. Patient-specific osteo-ligamentous FE models were created for each of the three patients who averaged age 12 years and presented with a Cobb angle of approximately 50 degrees. Each model implemented age- and vertebral level- specific material properties. AVBT instrumentation was installed according to clinical notes, and surgical correction was performed level-by-level to match the existing surgical technique. Based on the stress levied by the AVBT instrumentation, asymmetric growth was simulated for one year after surgery, during which clinical index, spine height and length, apical vertebral wedging, and AVBT instrumentation parameters were measured and compared to the corresponding patient X-Ray values. After one year of growth, clinical indices were predicted within an average of 2.9±1.9 degrees. Spine length-to-height ratios indicated correction, moving closer to one-to-one, and vertebral wedging decreased by approximately 2% per year. Tether segment tensions increased with time, though further analysis is necessary to assess the accuracy of tether material properties and behavior under tension. The model creation and analysis herein filled the research gaps of representing the granularity of vertebral growth, and including the entirety of the bony structures, thus allowing the analysis of age-based stress and range-of-motion analysis. Furthermore, such models may find use in simulation of both interactions with other soft structures such as the lungs and heart, and with other corrective devices.
DOI: 10.17918/00000027