Measurement · SNB
SNB angle
Loading the reference ceph.
Definition
The angle at Nasion between the anterior cranial base (S–N) and the line from Nasion to B point — the anteroposterior position of the MANDIBLE relative to the cranial base.
How
Draw S→N and N→B (B point = deepest concavity of the anterior mandible, above Pogonion). SNB is the angle between them at Nasion.
Purpose
Localizes the lower jaw's sagittal position. Read together with SNA: a normal SNA with a low SNB means the Class II is mandibular (the common case, and the one a mandibular-advancement approach targets).
High
Above the norm reads as a protrusive (prognathic) mandible — skeletal Class III contribution from the lower jaw.
Low
Below the norm reads as a retrusive (retrognathic) mandible — skeletal Class II contribution; also a structural risk factor for a constricted airway.
This is a reviewed tracing on one real cephalogram — the same ceph on every page in Learn — so SNB angle here is a real reading and not a diagram.