Measurement · FMIA
FMIA — Frankfort-mandibular incisor angle
Loading the reference ceph.
Definition
The angle between the Frankfort horizontal and the lower incisor's long axis — Tweed's esthetic target angle (his 'Z' concept ancestor).
How
Draw Frankfort (Po–Or) and the L1 long axis (L1A→L1T); FMIA is the angle between them. With FMA and IMPA it closes the Tweed triangle to 180°.
Purpose
Tweed's proxy for a balanced profile: he aimed for FMIA ≈ 65–68° (harder to reach in high-FMA faces). Low FMIA means the lower incisor leans too far forward relative to the whole face — the classic indicator he used to justify premolar extraction.
High
Above the norm reads as upright-to-retroclined lower incisors relative to the face — profile esthetics favored.
Low
Below the norm reads as forward-leaning lower incisors relative to the face — protrusive profile; Tweed's extraction flag.
This is a reviewed tracing on one real cephalogram — the same ceph on every page in Learn — so FMIA — Frankfort-mandibular incisor angle here is a real reading and not a diagram.