Measurement · SUMANG
Björk's sum of angles
Loading the reference ceph.
Definition
Saddle (N-S-Ar) + articular (S-Ar-Go) + gonial (Ar-Go-Gn) — the total of the Björk polygon's three posterior angles. 396° is the neutral value.
How
Measure the three angles of the N–S–Ar–Go polygon chain and add them. Geometrically, the sum equals 360° plus the angle the mandibular plane makes with S–N — which is why 396° (i.e. SN-MP ≈ 36°) is neutral.
Purpose
Jarabak's single growth-direction verdict: above 396° predicts clockwise (vertical, backward) rotation of the mandible during growth; below predicts counterclockwise (horizontal, forward) rotation — steering appliance choice and timing in a growing patient.
High
Above the norm reads as sum > 396° — clockwise/vertical growth prediction (face lengthens, chin rotates back).
Low
Below the norm reads as sum < 396° — counterclockwise/horizontal growth prediction (strong chin, deep-bite trend).
This is a reviewed tracing on one real cephalogram — the same ceph on every page in Learn — so Björk's sum of angles here is a real reading and not a diagram.