Projectional distortion is known to produce artifactual dysrelationships between images of osseous segments on X-ray films. According to Gonstead, lines constructed parallel to the adjacent vertebral endplates will converge if a lateral flexion malposition is present and will be parallel if no such subluxation exists. To test the accuracy of these lines, we mounted two vertebrae on supports so that the superior vertebra could be laterally flexed by a known amount upon the fixed inferior vertebra. The specimen was X rayed with no flexion or with various degrees of right or left lateral flexion of the superior vertebra. Lateral and inferior off-centering, rotation about the z axis, and object-film distance were also varied systematically. In every instance, off-centering produced no measurable effect on the position of the constructed Gonstead lines. We therefore conclude that these lines can be confidentially used to indicate structural dysrelationship during lateral flexion. No correction for projectional distortion appears to be necessary.
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