Effect of posterior inclination and facet joint orientation on the annulus fibrosus stiffness and rotational stability of the thoracolumbar spine

The etiology of adolescent idiopathic scoliosis (AIS) remains unresolved. The human upright posture results in vertebral posterior inclination. It has been hypothesized that this can lead to increased posterior shear in the thoracolumbar spine depending on the actual inclination angle and facet joint orientation which in turn could lead to unlocking of facet joints. This would result in increased axial rotation and thereby the likelihood of overstraining the fibers of the anterior annulus fibrosus (AF). Potentially, these aspects may enhance the risk of AIS development and progression. In this population-based in silico study, we use novel computational techniques to examine how posterior vertebral inclination and facet joint orientation affect range of motion and AF fiber strain in a cohort of children with increased AIS risk. Finite element subject-specific models of the T11-T12 motion segment were created from MR images of 18 prepubescent girls. An axial compressive force representing the combined action of gravity and muscle forces together with axial rotation moment was applied at three posterior inclination angles (5°, 15°, 25°). Facet joint orientation was modelled as subject-specific, lumbar, or thoracic. Posterior inclination had little impact on the stiffness of the neutral zone. However, the fraction of fibers exceeding 15 % strain increased from 14.5 ± 9.3 % at 5° to 18.7 ± 12.4 % at 25°. Transverse facet joint orientation angle highly correlated with the range of motion, but poorly correlated with fiber overstraining. Comparing the lumbar-oriented to the thoracic-oriented facet joints, fiber overstraining increased across all inclination degrees. This study showed that posterior inclination and increasing thoracic-like facet joint orientation increases AF fiber strains, providing further biomechanical evidence that helps understanding AIS development.