Cheat Sheet - Cephalometric Measurements and Norms
| Measurement | What it tells you | Norm (White / Black) | Limits of normal | ↑ High value | ↓ Low value |
|---|---|---|---|---|---|
| SNA (SN to N–A) | A-P position of maxilla vs cranial base | 81° / 84.7° | Orthognathic 77–85° | >85° = maxilla prognathic (further forward) | <77° = maxilla retrognathic (further back) |
| SNB (SN to N–B) | A-P position of mandible vs cranial base | 78.2° / 79.2° | Orthognathic 75–85° | >85° = mandible prognathic | <75° = mandible retrognathic |
| ANB (N–A to N–B) | Maxilla vs mandible, sagittal | 2.8° / 5.5° | Class I 1–5° (straight profile) | >5° = Class II (mandible behind maxilla, convex) | <1° = Class III (mandible ahead, concave) |
| SN-GoGn (mandibular plane to SN) | Vertical jaw relationship / growth pattern | 32° / 38.2° | — | Steep MP → long-face / skeletal open-bite tendency | Flat MP → short-face / skeletal deep-bite tendency |
| Maxillary incisor to SN (U1-SN) | Upper incisor inclination to cranial base | 103.8° / 108.9° | — | Proclined upper incisors | Retroclined upper incisors |
| Mandibular incisor to mandibular plane (L1-MP, Go-Gn) | Lower incisor inclination to mandible | 97.3° / 100° | — | Proclined lower incisors | Retroclined lower incisors |
| Interincisal angle (upper × lower long axes) | Combined incisor inclination | 126.8° / 113.8° (lecture quotes ≈131°) | — | High = upright/retroclined incisors (often Class II div 2 / deep bite) | Low = both incisors proclined (bimaxillary protrusion) |
| Maxillary incisor to NA | A-P position (mm) / angulation (°) vs maxilla | 3 mm / 22–23° (W); 7.4 mm / 24.1° (B) | — | Protruded / proclined upper incisor | Retruded / retroclined upper incisor |
| Mandibular incisor to NB | A-P position (mm) / angulation (°) vs mandible | 5.4 mm / 27.3° (W); 11.4 mm / 36.7° (B) | Protruded if linear >4 mm; proclined if angle >25° | Protruded / proclined lower incisor | Retruded / retroclined lower incisor |
| Pogonion to N-perpendicular | Bony chin prominence | −4 to −2 mm | — | More forward/positive = prominent chin | More negative = retrusive / deficient chin |
Quick A-P screen: N-perpendicular
- Point A should be on or slightly in front of the N-perpendicular line: ~2 mm in front (range 0 to +4 mm).
- Point B should be slightly behind it: ~2 mm behind (range −4 to 0 mm).
- The difference between the lines through A and B should not exceed 4 mm — otherwise the teeth can’t occlude properly.
Vertical proportions (Sassouni)
- Five horizontal reference planes: SN, true horizontal (perpendicular to true vertical through Or), palatal plane (ANS–PNS), functional occlusal plane, mandibular plane (Go–Gn).
- In a well-proportioned face these planes converge toward a single point.
- If they intersect close to / in front of the face and diverge quickly anteriorly → long anteriorly, short posteriorly → skeletal open bite.
- If they are nearly parallel (converge far behind the face) → predisposition to skeletal deep bite.
Warning
SNA/SNB reference standards assume a normal inclination of SN — which is not always correct. Cross-check A-P relationships with ANB and the N-perpendicular rather than relying on SNA/SNB alone.
Source: Level II Unit B Summary—integrated, L4 Level II Unit B, Workbook, 10 6 Orthodontic Diagnosis