Measurement · L1
IMPA — lower incisor to mandibular plane
Loading the reference ceph.
Definition
The Incisor-Mandibular Plane Angle: the inclination of the lower central incisor's long axis to the mandibular plane (Go–Me).
How
Draw the mandibular plane Go–Me and the lower incisor long axis (apex L1A → tip L1T); IMPA is the posterior-superior angle between them (~90–95°: the incisor stands roughly upright on its base).
Purpose
The reference for lower-incisor position over basal bone. Proclined lower incisors (high IMPA) often mean the arch is already compensating for a Class II or crowding — limiting how much non-extraction expansion/advancement the periodontium will tolerate.
High
Above the norm reads as proclined lower incisors — dentoalveolar compensation; further advancement risks bone/gingival support.
Low
Below the norm reads as retroclined lower incisors — retroclination over basal bone (Class III compensation or a deep-bite trap).
This is a reviewed tracing on one real cephalogram — the same ceph on every page in Learn — so IMPA — lower incisor to mandibular plane here is a real reading and not a diagram.