Measurement · IMPA
IMPA — lower incisor to mandibular plane (Tweed)
Loading the reference ceph.
Definition
The lower central incisor's long-axis angle to the mandibular plane (Go–Me); the third angle of the Tweed diagnostic triangle (FMA + FMIA + IMPA = 180°).
How
Draw Go–Me and the L1 long axis (L1A→L1T); IMPA is the posterior angle between them. In the Tweed triangle it closes the 180° identity with FMA and FMIA.
Purpose
Tweed built his whole extraction philosophy on standing this incisor upright (~87–90°) over basal bone: IMPA tells you how far the lower incisor is from that goal, and therefore how much space (extraction/IPR/expansion) achieving it would consume.
High
Above the norm reads as proclined lower incisors — camouflage already 'spent'; uprighting them consumes arch length.
Low
Below the norm reads as retroclined lower incisors — room exists to advance/procline within Tweed's target.
This is a reviewed tracing on one real cephalogram — the same ceph on every page in Learn — so IMPA — lower incisor to mandibular plane (Tweed) here is a real reading and not a diagram.