Abstract
Introduction Vertebral rotation is an important aspect of spinal deformity in idiopathic scoliosis, associated with ribcage asymmetry. Although both lateral curvature and rotation appear to increase together in progressive scoliosis, the mechanisms driving vertebral rotation are not clearly established and it is not known whether lateral curvature precedes rotation, or vice versa. This paper investigates the hypothesis that intravertebral (within the bone) rotation in idiopathic scoliosis is caused by growth in the presence of gravity-induced torsions, the twisting moments generated by body weight forces acting on the scoliotic spine.
Methods Three-dimensional spinal curvature was measured for a small group of idiopathic scoliosis patients using standing radiographs and equations of static equilibrium were used to calculate gravity-induced torsion profiles along the length of each spine due to head, neck and torso weight. Intravertebral rotations were then measured for the same patients using Aaro & Dahlborn’s technique (Aaro S, et al; Spine 1982) with reformatted computed tomography images. The gravity-induced torsion curves were compared with rotation measurements to see whether gravity-induced torsion is a likely contributor to intravertebral rotation in scoliosis.
Results Gravity-induced torques as high as 7.5Nm act on the spines of idiopathic scoliosis patients due to static body weight in the standing position, and maximum intravertebral rotations (for a single vertebra) are approximately 7°. There appears to be general agreement between the measured intravertebral rotations and profiles of gravity-induced torsion along the length of the spine.
Discussion Gravity-induced torsion is a potential cause of vertebral rotation in idiopathic scoliosis. Since the spine must be curved in three-dimensions (out of plane) to produce such torques, vertebral rotation would be expected to occur subsequent to an initial lateral deviation, suggesting that lateral curvature precedes vertebral rotation in idiopathic scoliosis.
The abstracts were prepared by Professor Bruce McPhee. Correspondence should be addressed to him at Orthopaedics Division, The University of Queensland, Clinical Sciences Building, Royal Brisbane & Women’s Hospital, Herston, Qld, Australia
