Insights into the measurement of vertebral translation in the sagittal plane.

In this discussion paper, vertebral translation in the sagittal plane is considered. A number of different measurement protocols are investigated and it is shown that, in general, the protocols lack a clear notion of the ideal. This can lead to difficulties when interpreting clinical data. One possible ideal is encapsulated in the concept of George's line, the posterior vertebral body alignment line. It is shown that translation can be measured such that deviations from George's line are quantified. Furthermore, an intrinsic centre of rotation is implied, and by considering translation in conjunction with intervertebral angle, inferences can be made regarding the position of the instantaneous centre of rotation of the joint. The relationship between static values of translation viewed on a single radiograph and changes in translation viewed across a number of images is also investigated. Results from some previous clinical studies are discussed in the light of the findings.