Does the Eastman correction over- or under-adjust ANB for positional changes of N?

Cephalometric analyses are useful for planning and provision of orthodontic treatment. A and B points are used to resemble the anterior part of maxilla and mandible and N represents the anterior part of the cranial base. The position of N may influence measurements of SNA, SNB, and ANB and changes of N in the antero-posterior plane are thought to have a larger influence on the above measurements than vertical ones. Several methods have been proposed to address this issue, such as the Wits appraisal, the 'Individualized ANB', and the 'Eastman correction'; the latter is mainly used in the UK. This study used a geometrical model of a lateral cephalogram with standard Caucasian average values for SN length, position of A and B points for male and female patients. Linear and angular measurements were digitized using Dolphin™ Imaging software. N was moved in antero-posterior and vertical planes and subsequent changes of SNA, SNB, and ANB were measured and the Eastman correction applied. The correction overestimated the SNA and ANB values in the opposite direction to the directly measured SNA and ANB when N moved posteriorly towards S: The directly measured values tended to show a Class II relationship but the correction indicated a Class III skeletal base. As N moved anteriorly, the Eastman correction overestimated the measured Class III skeletal relationship; SNA did not fall below 81 for correction in the opposite direction. Vertical positional changes had little impact on the underlying ANB and correction was not indicated clinically.