Correlation and quantification of projected 2-dimensional radiographic images with actual 3-dimensional y-axis vertebral rotations

Background: Historically, measurement of 2- dimensional (2-D) radiographic images on the anteroposterior radiograph has been made to assess 3-dimensional (3-D) y-axis vertebral rotations. Objectives: To correlate and quantify measurements of the projected 2-D radiographic image with the degree of 3-D y-axis rotation. Study design: A computer model was positioned in a simulated x-ray beam. Points of model contact with the simulated beam were projected onto a line in the neutral position and the first 7 degrees of both positive and negative y-axis rotation using two different axes of rotation. A larger model, a shape-altered model, and a decreased source-object-distance model were also studied. Results: 3-D y-axis rotation of vertebrae causes an off-center displacement of the 2-D projected lamina junction in relation tothe projected vertebral body. The magnitude of displacement increases with increasing degrees of rotation. Tn our model, no clinically significant difference was found in the amount of the projected off-center displacement of the lamina junction between either of our two chosen axes of rotation. However, significant differences in the projected offset were found between vertebrae with the same degree of rotation as a result of changes in vertebral shape, size, and positioning. The projected lamina junction off-centering at a given rotation is quantified for our model. Conclusion: Use of millimetric measurement of the projected lamina offset on the anteroposterior radiograph is an inaccurate method for the assessment of the degree of 3-D y-axis vertebral rotation.