SNA angle

Definition

The angle at Nasion between the anterior cranial base (S–N) and the line from Nasion to A point — the anteroposterior position of the MAXILLA relative to the cranial base.

How

Draw S→N (anterior cranial base) and N→A (A point = deepest concavity of the anterior maxilla). SNA is the angle between them, measured at Nasion.

Purpose

Tells you whether the maxilla itself sits forward or back under the cranial base — so you can localize a Class II/III problem to the upper jaw before blaming the mandible. Steiner's starting point for the skeletal diagnosis.

High

Above the norm reads as a protrusive (prognathic) maxilla — the midface is positioned forward.

Low

Below the norm reads as a retrusive (retrognathic) maxilla — the midface is deficient, common in Class III and some airway-compromised patterns.