Measurement · FMA
FMA — Frankfort-mandibular plane angle
Loading the reference ceph.
Definition
The angle between the Frankfort horizontal (Porion–Orbitale) and the mandibular plane (Gonion–Gnathion). The vertical cornerstone of the Tweed triangle.
How
Draw Frankfort horizontal through Porion (Po) and Orbitale (Or), and the mandibular plane through Gonion (Go) and Gnathion (Gn); FMA is the angle where they meet (posteriorly).
Purpose
The classic read of vertical facial growth. It tells you whether the mandible grows on a horizontal (brachyfacial) or vertical (dolichofacial) pattern — which drives anchorage planning, bite-opening tolerance, and extraction decisions in the Tweed philosophy.
High
Above the norm reads as a steep mandibular plane — hyperdivergent, vertical (dolichofacial) growth with a long lower face; bites are easy to open and hard to close.
Low
Below the norm reads as a flat mandibular plane — hypodivergent, horizontal (brachyfacial) growth with a short lower face and often a deep bite.
This is a reviewed tracing on one real cephalogram — the same ceph on every page in Learn — so FMA — Frankfort-mandibular plane angle here is a real reading and not a diagram.