Measurement error of spondylolisthesis as a function of radiographic beam angle.

Slip progression in patients with spondylolisthesis is most commonly identified by comparing serial lateral radiographs of the lumbosacral spine. Although the demonstration of a progression in slip in a symptomatic individual can be an indication for surgical intervention, accurate measurements remain somewhat elusive. This is because of factors that introduce error into the measurement process itself. Foremost among these is the comparison of lateral radiographs that are not "true lateral" radiographs but rather have an unknown variable obliquity to the radiologic beam. To define the relationship between the measured slip and the obliquity of the x-ray beam angle from the true lateral, an experimental model was constructed with a fixed degree of slip at the L5-S1 level. Three different fixed degrees of slip (approximately 25, 50, and 75%) were studied. The standard deviation and hence the inherent measurement error of the measured slip increased as the obliquity of the beam away from the true lateral increased. In addition to this, the standard deviation as a function of beam angle from the true lateral also appeared to increase for larger slips.