Measurement · MMDIFF
Maxillomandibular differential
Loading the reference ceph.
Definition
Effective mandibular length (Co–Gn) minus effective midfacial length (Co–A) — how much longer the mandible is than the midface.
How
Measure Co→A and Co→Gn and subtract. The expected difference grows with the face: ~20 mm in the early mixed dentition up to ~30+ mm in large adults — read it against face size, not just the adult mean.
Purpose
McNamara's core harmony check: the jaws can each be 'normal' yet mismatched. A small differential = mandible short for its own midface (true Class II); a large one = mandibular excess (Class III), independent of where either jaw sits in space.
High
Above the norm reads as mandible long relative to the midface — skeletal Class III proportionality.
Low
Below the norm reads as mandible short relative to the midface — skeletal Class II proportionality (functional-appliance territory in a grower).
This is a reviewed tracing on one real cephalogram — the same ceph on every page in Learn — so Maxillomandibular differential here is a real reading and not a diagram.