External measurement techniques first locate the spinous processes starting from the skin surface and then estimate the distance from the spinous processes to the centers of the vertebral bodies. Literature, however, only partially describes thickness values of the intermediate layer (consisting of muscles, ligaments, and other soft tissues) between the processes and the skin. Furthermore, only limited information about the distance to the centers of the vertebral bodies is available, and the position of this center is additionally disturbed by a poor correlation between the axial vertebral rotation and the rotation
visible at the surface. Therefore, measurement techniques that visualize the internal shape of the vertebral column generally yield more reliable results than external techniques.