Projectional distortion in an X-ray image can produce artifacts that cause error in the measurement of vertebral rotation. By means of a stereotaxic positioning device, the effect of varying object-film distance, vertical and horizontal off-centering, rotation, and lateral flexion on the amount of distortion in the image of a third lumbar vertebra was determined. Mathematical analysis of the results revealed that projectional distortion due to vertical off-centering does not affect the apparent rotation of the vertebra measurably; that there is a linear relationship between apparent vertebral rotation and horizontal off-centering; for the object-film distances of 35.64 + 0.12 cm and 19.48 + 0.08 cm, the vertebra displayed 1 mm of apparent rotation for every 2 to 3 cm of lateral off-centering regardless of whether the vertebra is rotated or laterally flexed; and that as long as a given osseous segment is compared to its adjacent segment (as in analysis for subluxation), the apparent vertebral rotation may be regarded as a sufficiently accurate representation of the actual rotation of the vertebra.
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