Early in Class 111 treatment planning, it is necessary to. decide whether the case has a prognathic mandible or a retrognathic maxilla, or a combination of these two possibilities. In many cases, this will be clear from visual examination of the patient and the radiographs.
There are several methods of conventional cephalometric analysis to assess A/P skeletal discrepancy. For example, SNA, SNB, and ANB can be compared with Michigan normals' for a male or female of the same age. This method has accepted shortcomings, but has been a mainstay indicator of skeletal disproportion for many years. McNamara2 suggested dropping a line from nasion perpendicular to Frankfort horizontal to evaluate the position of 'A' point and 'B' point, as a useful indicator of skeletal discrepancy (Fig. 8.3).
The Arnett analysis (p. 163) uses a true vertical line (TVL) as a facial reference and it is recommended (Fig. 8.4) as a more sophisticated and accurate method of deciding the needs of the case (Fig. 8.5).
Fig. 8.3 McNamara suggested analysis based on nasion perpendicular.
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