The nasion-sella reference line in cephalometry: a methodologic study.

This investigation was undertaken to examine the effects of the registration error of the nasion-sella reference line (NSL) upon the values of angular and linear head film measurements. A comparison was made between two methods of constructing the NSL: (1) using the anatomically defined landmarks nasion and sella (L method) and (2) using the stable skull structures in the anterior cranial base (S method). Double registrations of sixteen pairs of lateral head films, from before and after 3 years of activator treatment, were performed by two observers. The results of the investigation revealed the following: (1) The registration error of the NSL affected angular and linear head film measures to a considerable degree. (2) The registration error was larger for the S method than for the L method. (3) The two observers differed in locating the NSL. The interobserver difference was especially apparent for the S method. The clinician should be aware of the NSL-registration error in the analysis of both single and serial radiographs. A misjudgment of the position of the reference line may result in diagnostic misinterpretations, thus affecting treatment planning or leading to false conclusions when the outcome of a particular treatment procedure is evaluated. In longitudinal studies covering a short period of growth, the L method would be the method of choice.