Background: One use of the anteroposterior lumbar radiograph is to determine axial (y-axis) rotation of the lumbar vertebrae. Rotation might be an element of interest to clinicians seeking to evaluate vertebral positioning.
Objectives: Correlate and quantify movements of the projected pedicles relative to the projected vertebral body during axial rotation and determine if vertebral asymmetry and changes in object film distance affect these movements.
Design: A three-dimensional computer model of the fourth and fifth lumbar vertebrae, a modeled radiograph source, and a modeled film were produced. The vertebral model was placed in various degrees of axial rotation at a number of different object film distances. Lines from the source were passed through the pedicles of the fourth lumbar vertebral model and additional lines erected tangent to the lateral body margins. These lines were extended to points of contact with the modeled film.
Results: The projected pedicles move relative to the projected vertebral body during y-axis rotation. Vertebral asymmetry and object film distances can also affect the distance of the projected pedicle relative to the projected lateral body margin.
Conclusion: Axial rotation produces movement of the projected pedicles relative to the projected vertebral body. However, vertebral asymmetry and changes in object film distance also affect the position of the projected pedicles relative to the projected lateral body margin and might serve as confounders to the clinician seeking to analyze vertebral rotation through the use of the projected pedicles.