Measurement · CONVEXMM
Convexity of A point (mm)
Loading the reference ceph.
Definition
The perpendicular distance of A point from the facial plane (N–Pog) — Ricketts' linear convexity.
How
Draw the facial plane N–Pog; drop a perpendicular from A point; read in mm, positive when A is anterior to the line (norm ≈ 2 mm, decreasing with age).
Purpose
Ricketts' primary Class II/III screen: it captures the same 'profile bend' as Downs' angle of convexity but in millimeters, which makes treatment change (e.g. restraining the maxilla) directly readable.
High
Above the norm reads as a convex skeletal midface — Class II pattern (maxillary excess and/or mandibular retrusion).
Low
Below the norm reads as a straight-to-concave midface — Class III pattern.
This is a reviewed tracing on one real cephalogram — the same ceph on every page in Learn — so Convexity of A point (mm) here is a real reading and not a diagram.