The effects of combined x-axis translations and y-axis rotations on projected lamina junction offset

Objectives: To quantify the projection errors caused by the combining of se lected 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 jun ction and points on the lateral body margins of a 2-dimensional vertebral m odel onto an AP radiograph. This simulation was done in the neutral positio n and in a number of combinations of x-axis translations and y-axis rotatio ns. Results: Some combinations of x-axis translation and y-axis rotation can in crease or decrease the projected horizontal lamina offset as compared with what would be found with rotation alone. The projection of some combination s 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 an d Y-axis rotations can lead to confusion when the clinician attempts to dis cern quantity and direction of y-axis vertebral rotation from the AP radiog raph.