The effect of dorsal versus ventral shear loads on the rotational stability of motion segments; a finite element study
Scoliosis is a complex three-dimensional deformity of the spine, mainly characterized by a lateral deviation of the spinal column in combination with axial rotations of the vertebrae. It affects 2-3% of the population (mainly women) and more than 80% of the cases are idiopathic. Despite years of extended research, the aetiology and pathogenesis of idiopathic scoliosis has still not been resolved. A number of researchers and physicians postulate that backwardly inclined vertebrae, as prominent in the low-thoracic and high-lumbar regions of the spine, are subjected to dorsally directed shear loads. These shear loads render the joints between adjacent vertebrae inoperative which in turn leads to rotational imbalance. The internal forces required to restore balance induce asymmetric loading of motion segments, thereby enhancing slight pre-existing asymmetries according to Hueter-Volkmann’s law or Wolff’s law. It is assumed that the spine will naturally follow a built-in rotational pattern, rather than revert to the opposite direction. Whether a spinal deformity actually occurs and progresses depends on the disturbance of balance between spinal loads and compensating mechanisms such as bone and soft tissue stiffness, muscle strength and propriosepsis.
This hypothesis regarding the role of dorsal shear forces in the pathogenesis of idiopathic scoliosis is already supported by a number of studies. The present study contributes to this research by means of a finite element model. The main goal is to investigate the axial rotational stability of the thoracic and lumbar spine under dorsal versus ventral shear loads for a number of geometric and mechanical variations. More specific, the focus is on the critical (sub)structures of the spine.
The finite element model used for this study is developed at the University of Twente and includes individual motion segments of three spinal levels; mid-thoracic (T6-T7), low-thoracic (T11-T12) and lumbar (L2-L3). First of all, the models are validated for different loading conditions. Subsequently, a number of simulations are run to (1) analyse the effect of regional spine characteristics, (2) disc degeneration and (3) increasing shear load magnitude on rotational stability.
This research is in collaboration with the University Medical Centre Utrecht.