Marcello M. | January 30, 2025
The anteroposterior relationships of the maxilla and mandible are evaluated in relation to the anterior cranial base by measuring the SNA and SNB angles. A significant increase in these values compared to population norms may indicate maxillary or mandibular protrusion (prognathism), while a reduction indicates retrusion (retrognathism).
The difference between the SNA and SNB angles (the ANB angle) determines the anteroposterior relationship between the maxilla and the mandible:
Measuring the jaw position relative to the anterior cranial base can be problematic due to alterations in the SN plane, which may be caused by incorrect positioning of the sella or nasion. Faulty nasion positioning can affect the ANB angle.
The Eastman correction helps adjust the ANB angle when there are variations in nasion positioning:
This correction is valid only when the SN-maxillary plane is within the normal range of 8 ± 3 degrees. If the SN-maxillary plane is outside this range, it suggests a faulty sella position, and no correction to the ANB value is necessary.
However, this method has limitations, as the nasion moves both forward and backward relative to the sella, potentially leading to an overestimation of actual values (Kamaluddin et al., 2011).
To avoid errors related to cranial base divergence, a complementary analysis independent of this region is recommended. The Wits appraisal evaluates jaw disharmony by measuring the relationship of the jaws to the functional occlusal plane (FOP). Perpendicular lines are drawn from point A and point B (denoted as AO and BO, respectively) to the FOP.
It is important to note that errors in locating the functional occlusal plane can introduce some inaccuracy in evaluating jaw relationships using this method.
