Saddle angle and maxillary prognathism: a radiological analysis of the association between the NSAr and SNA angles.

Abstract

In cephalometric radiography, two angles have been commonly used, the saddle angle (NSAr) and the SNA angle. The former has been regarded as the cranial base angle, the latter as an indicator of the antero-posterior relation of the maxilla to the cranial base. In this study, a sample of 138 lateral skull radiographs of untreated orthodontics patients aged 7-15 years and analysed. The sample consisted of Angle's Class I, Class II, and Class III malocclusion cases. Means and standard deviations of the NSAr and SNA angles and the S-N and S-Ar lines for different malocclusion groups were estimated, and for each malocclusion group, the model of linear regression (y = bx + a) between the SNA and NSAr angles was computed. The mean of the NSAr angle was significantly larger in the Class II malocclusion group than in the Class III malocclusion group (p less than 0.05). The computed regressions indicated that about 14-40 per cent of the variation of the SNA angle in different malocclusion groups and about 24 per cent in the whole sample could be explained by the variation of the NSAr angle. Because at least a part of the correlation between the NSAr and SNA angles had a topographical causation, the main cause of this topographical correlation probably being the rotation of the S-N plane, the SNA angle was regarded as an unreliable indicator, the use of which can lead to considerable errors.