The aim of 1 his study Is to quantity cliffs run ces between three dimensional rotations in space and their calculated values, either on two-dimensional projections (radiographs or computed tomographic scans) or three-ctimensicnal calculations using various mathematical procedures.
To use a vertebral model to quantify differences between three-dimensional rotations and their calculated values, using two-dimensional projections or various three-dimensional mathematical procedures.
A specific program allowed us to move a geometric vertebral model in space using given values and sequences of lateral, sagittal, and axial rotations. Differences in positions due to differen t sequences were visualized and quantified. Differences due to rotation around global or vertebral axes were considered.
For rotations of about 10º, differences are about 2º between three-dimensional and projected angles. Differences increase when combined rotations are large, as generally occurs in a scoliotic spine. They reach 16º for lateral and sagittal rotations of 30º.
Axial rotation measured on transverse projection is misleading for vertebrae rotated in space. Moreover, dealing with large three-dimensional rotations is meaningful only if the used mathematical convention is given.
From Laboratoire de Biomécanique, Ecole Nationale Supérieure d'Arts et Métiers (ENSAM), Paris, and Réseau de Recherche Clinique INSERM: convention n°490011, Paris, France