Analysis of factors affecting angle ANB

Abstract

Cephalometric analyses based on angular and linear measurements have obvious fallacies, which have been discussed in detail by Moyers and Bookstein. 11 11 Moyers RE, Bookstein FL, Guire KE: The concept of pattern in craniofacial growth. Am J Orthod 76: 136–148, 1979. However, the clinical application of such an analysis by the orthodontic profession in treatment planning is widely accepted. Variations of angle ANB are commonly used to determine relative jaw relationships in most of the cephalometric evaluations. Several authors, including Jacobson, 2 2 Jacobson A: Application of “Wits” appraisal. Am J Orthod 70: 179–189, 1976. showed that the anteroposterior position of point N relative to points A and B influences angle ANB, as does rotational growth of the upper and lower jaws. In addition, the authors point out that growth in a vertical direction (distance N to B) and an increase of the dental height (distance A to B) may contribute to changes in angle ANB. For a Class I relation (Wits = 0 mm), a mathematical formula has been developed which enables the authors to study the geometric influence of angle ANB caused by the following four effects: (1) rotation of the jaws and/or occlusal plane relative to the anterior cranial base; (2) anteroposterior position of N relative to point B, (3) vertical growth (distance N to B); (4) increase in dental height (distance A to B). It was observed that, contrary to the common belief that an ANB angle of 2 ± 3.0 ° is considered normal for a skeletal Class I relation, 6 6 Holdaway RA: Changes in relationship of points A and B during orthodontic treatment. Am J Orthod 42: 176–193, 1956. the calculated values of angle ANB will vary widely with changes in these four controlling factors under the same skeletal Class I conditions (Wits = 0 mm). Therefore, in a case under consideration, angle ANB must be corrected for these geometric effects in order to get a proper perspective of the skeletal discrepancy. This is facilitated by comparing the measured ANB angle with the corresponding ANB angle calculated by a formula for a Class I relationship. The corresponding calculated angle ANB can be taken from the tables which are based upon the formula using the same values for SNB, ω (angle between occlusal plane and anterior cranial base), b (which is distance N to B) and a (dental height measured as perpendicular distance A to occlusal plane plus perpendicular distance occlusal plane to B). The difference between actual and calculated angle ANB is a measurement of the severity of the skeletal discrepancy. This leads to a new definition of what denotes skeletal Class II and III relationships, since an angle ANB calculated for a skeletal Class I (Wits = 0 mm) can vary widely and can be either negative or positive. Therefore, when a Class II skeletal relation exists, the actual ANB is larger than the calculated measurement, whether or not the calculated value of angle ANB is positioned in the positive or negative part of the scale. The reverse is true for a Class III relation; the actual ANB is smaller than the calculated ANB. The clinical application of this method in the analysis of jaw disharmonies must be cautious, as the premise of this analysis is based on the occlusal plane and Wits values being equal to zero. It has been found that the cant of the occlusal plane is subject to growth changes independent of the forward or backward jaw rotations. The article highlights the complexity of the problem and recommends consideration of all the variables that may affect angle ANB.