The role of dorsal shear forces in the pathogenesis of adolescent idiopathic scoliosis--a hypothesis.

The pathogenesis of adolescent idiopathic scoliosis (AIS), a condition exclusive to man, has been the subject of many studies, but remains little understood. Previous work has shown that backward inclination of vertebrae in the sagittal plane has prognostic significance in the progression of AIS, and that certain regions of the human spine may be subject to dorsal shear forces, depending on posture, unlike what is found in other vertebrates. We postulate that these dorsal shear forces, acting exclusively upon specific regions of the human spine, may contribute to rotational instability of the spine. Asymmetric loading of the posterior parts of the vertebrae then would lead to asymmetrical growth in all three planes of specific parts of vertebrae, according to Hueter-Volkmann's law. Asymmetrical growth of the neurocentral cartilage of the vertebra, for instance, has been shown to lead to AIS-like deformities in growing pigs, and could explain the development and progression of the deformity in humans.