Measurement · MIDFACE
Effective midfacial length (Co–A)
Loading the reference ceph.
Definition
The distance from Condylion to A point — McNamara's 'effective length' of the midface.
How
Measure Co→A in mm. Its role is as the comparator for effective mandibular length (Co–Gn) in the maxillomandibular differential.
Purpose
Sizes the maxilla along the same axis as the mandible so the two can be compared directly. An age-appropriate Co–A with a short Co–Gn pins the discrepancy on the mandible (and vice versa).
High
Above the norm reads as a long midface (contributes convexity / Class II).
Low
Below the norm reads as a short midface (midfacial deficiency — Class III contribution).
This is a reviewed tracing on one real cephalogram — the same ceph on every page in Learn — so Effective midfacial length (Co–A) here is a real reading and not a diagram.