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Abstract
The general objective of this project was to study the multi-axial biomechanics of scoliosis progression. The specific objective was to model the deformation process, including the spinal growth and mechanobiological growth modulation due to multi-axial loads, and analyze how these loads are involved in the resulting characteristic scoliotic deformities.
In the first part, the analytical formulation of mechanobiological growth developed by Stokes et al. (1990) and Carter et al. (1988) was compared using a finite element model representing a thoracic vertebra as solid elements. Stokes’s model only concerned axial stress, while Carter’s model involved multi-axial stresses. The epiphyseal growth plates were represented using three layers similar to those found in the vertebral bodies: a loading sensitive area, a growth area, and a mineralized area. The two mechanobiological growth models were numerically integrated into the growth plate model. The two models were further used to simulate vertebral growth modulation resulting from different physiological loading conditions applied on the vertebra (tension, compression, shear, as well as combined tension/shear and combined compression/shear).
Mechanosensing carried out the transformation of mechanical loading into biological response by energy. Mechanoregulation followed the mechanosensing and induced the biological modification. The energy-based mechanobiological growth model was finally developed from those two analytical procedures. It was implemented in the growth plates of the previously developed vertebra finite element model. The model was tested with different loading conditions (tension, compression, shear, combined tension/shear, and combined compression/shear), and the validation was based on comparisons with published experimental studies on growth response to axial and shear loading in animals, and numerical simulation of growth modulation in humans. Simulation results under axial loading conditions agreed with the Hueter-Volkmann law, the Stokes’ model and animal experiments. The shear stress increased the mechanobiological growth (20%-40%) in the combined axial/shear loading condition, which agreed with the Carter’s mechanobiological theory.
The energy-based growth model involved multi-axial stresses and made it possible to reproduce the modification of vertebral morphology similarly as what is seen in adolescent idiopathic scoliosis. The morphological modification process was simulated by using finite element modeling technique. Energy-based model was integrated into a pediatric FEM model of a thoracic functional unit T7-T8 personalized to an eleven-year-old healthy male child. The spinal loads were designed as axial loading, shear, torsion, and combined axial/shear or torsion. The measurement included the wedging angle of T7, which was an essential characteristic to measure a vertebral deformity, and intervetebral axial rotation between T7 and T8. Simulation results indicated that both axial and non-axial loading (shear) were able to induce the wedging of the vertebrae in the coronal plane (1.4°∼4.8°) and the intervertebral rotation (0.7°∼3.7°). The wedging angle in the sagittal plane was little modified (0.1°∼1.0°). The asymmetric axial loading induced a 4.8° wedging angle that approached published measurements (5.2°) of Parent et al. (2003).
The comparative study found the strengths and limits of two modeling techniques. The simulation study indicated that this model agreed with most experimental studies and Carter’s theoretical studies in mechanobiology. This study confirmed the mechanobiological contribution of both axial and non-axial loading to the development of scoliotic vertebrae. This study found that scoliotic wedging occurs only in the coronal plane. This study confirmed the primary role of axial loading on inducing scoliotic vertebrae and coupling secondary role of shear and torsion. The energy concept can also explain coupling mechanisms existing in multi-axial loads. Multi-axial loads resulted in axial and non-axial stresses, which non-linearly physically integrated into energy. This non-linearity led to coupling mechanobiological impact generated from those loads. (Abstract shortened by UMI.)
