Objectives: To quantify the projection errors caused by the combining of selected x-axis translations with selected y-axis rotations as seen on plain film anteroposterior (AP) radiographs.
Study design: A computer was used to model the projection of the lamina junction and points on the lateral body margins of a 2-dimensional vertebral model onto an AP radiograph. This simulation was done in the neutral position and in a number of combinations of x-axis translations and y-axis rotations.
Results: Some combinations of x-axis translation and y-axis rotation can increase or decrease the projected horizontal lamina offset as compared with what would be found with rotation alone. The projection of some combinations may lead the viewer to believe that rotation of the vertebral model is in the opposite direction of its true rotation.
Conclusion: The projections of combined movements of x-axis translations and y-axis rotations can lead to confusion when the clinician attempts to discern quantity and direction of y-axis vertebral rotation from the AP radiograph.