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 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.

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 to the projected vertebral body. The magnitude of displacement increases with increasing degrees of rotation. In 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.

The effects of x-axis vertebral translation on projected y-axis vertebral rotation.

BACKGROUND: Few studies have quantified the projection/distortion errors that occur in anteroposterior (AP) radiographs. OBJECTIVES: To quantify the projection/distortion errors on AP radiographs caused by lateral translation of the vertebrae. To demonstrate the effect of vertebral shape on projection errors. STUDY DESIGN: Three models of increasing complexity were constructed to document the distortion. Model 1 consisted of three metal pins embedded in a piece of metal tubing that was X-rayed in three positions. Model 2, a simple model of the X-ray beam mounted on a wooden platform, allowed for translation of a vertebral model in a simulated X-ray beam and the measurement of projected points of contact between the model and rays of the simulated beam. Model 3 is a computer simulation of the X-ray beam in which a vertebral model was translated laterally to varying locations. The projected points of contact between the simulated rays and the vertebral body margins and lamina junction were measured. Model 3 also showed that vertebral body shape has a large effect on the projected axial rotation. Two other simple models were created and discussed in relation to shape-dependent projection errors. RESULTS: X-axis translation results in projected y-axis rotation. Increasing magnitudes of x-axis vertebral translation results in increasing magnitudes of projected y-axis vertebral rotation. The projected rotation is also influenced by vertebral shape. CONCLUSION: We have shown that three-dimensional lateral translation projects as axial rotation on the AP radiograph. Projection error is largely influenced by the shape of the object and by the increasing obliquity of the rays of the X-ray beam. This would seemingly create confusion and invalidate spinal listings of vertebral position obtained from the AP radiographic image.