Correlation and quantification of relative 2-dimensional projected vertebral endplate z-axis rotations with 3-dimensional y-axis vertebral rotations and focal spot elevations.
BACKGROUND: The use of lines erected on the vertebral endplates of the anterior-to-posterior radiograph to assess z-axis vertebral rotation is a common clinical practice. OBJECTIVE: To quantify the projection/distortion error of lateral flexion (z-axis rotation) measurement, which results from actual axial (y-axis) rotation and changes in focal spot elevation, on AP radiographs. STUDY DESIGN: A 3-dimensional model of a 4th and 5th lumbar vertebrae was constructed with a computer. The angle between the projected inferior vertebral endplate of the 4th lumbar vertebra, and the projected superior vertebral endplate of the 5th lumbar vertebra was measured. This was done for combinations of 0, 7, 14, and 21 degrees of axial (-y-axis) rotation with 0, 15, and 30 cm of elevation of a modeled focal spot. RESULTS: An angle was produced between the projected inferior 4th lumbar vertebral endplate and the projected superior 5th lumbar vertebral endplate as a result of y-axis rotation of the 3-dimensional model. Increasing magnitudes of y-axis rotation and increasing focal spot elevation produced a lack of confidence in this measurement. CONCLUSION: In a clinical setting, limited ranges of y-axis rotation have little significant effect on the accuracy of this measurement. Increases in y-axis rotation and focal spot elevation can affect measurement accuracy.