Principles of measurement of vertebral rotation from frontal projections of the pedicles.

Measurement of vertebral rotation from frontal X-ray projections of the pedicles was introduced by Nash and Moe. By the introduction of a vertebral model, their method and different modifications can be described and characterized easily. A geometrical analysis shows, that two parameters are sufficient for this model. When applying an appropriate interpretation of vertebral rotation, rotation measurement can be performed independent of lateral tilting and forward-backward inclination. As a test of the Nash-Moe method 65 vertebrae are each investigated from 15 directions. The measurements are analyzed for each pedicle separately and also compared to the opposite pedicle of the particular vertebra. The results indicate figures for the accuracy of the Nash-Moe method and its modifications for absolute and relative measurements. A simple correction to the Nash-Moe method is proposed.