Every line, plane and measurement is built from a small set of landmarks. Locate them before measuring anything: an error in a landmark carries through to every value built on it.
| Landmark | Definition |
|---|---|
| Sella (S) | Centre of the sella turcica. Posterior limit of the anterior cranial base. |
| Nasion (N) | Most anterior point of the frontonasal suture. Anterior limit of the anterior cranial base. |
| A point (A) | Deepest point of the anterior concavity of the maxilla, between the anterior nasal spine and the alveolar crest. Anterior limit of the maxillary base; upper end of the A–Pog line. |
| B point (B) | Deepest point of the anterior concavity of the mandible, between the alveolar crest and the chin. Anterior limit of the mandibular base. |
| Porion (Po) | Uppermost, outermost point of the external auditory meatus. Posterior point of the Frankfort plane. |
| Orbitale (Or) | Lowest, most anterior point of the orbital margin. Anterior point of the Frankfort plane. |
| Pogonion (Pog) | Most anterior point of the bony chin. Lower end of the A–Pog line. |
| Gnathion (Gn) | Most anterior and inferior point of the chin. |
| Menton (Me) | Lowest point of the mandibular symphysis in the midline. Anterior point of the mandibular plane. |
| Gonion (Go) | Most posterior and inferior point on the angle of the mandible. Posterior point of the mandibular plane. |
| Posterior nasal spine (PNS) | Tip of the bony posterior nasal spine in the midline. Posterior point of the maxillary plane. |
| Anterior nasal spine (ANS) | Tip of the bony anterior nasal spine in the midline. Anterior point of the maxillary plane. |
| Incisor superius (Is) and upper incisor apex (UIA) | Incisal tip and root apex of the most prominent upper central incisor. Together they define its long axis. |
| Incisor inferius (Ii) and lower incisor apex (LIA) | Incisal tip and root apex of the most prominent lower central incisor. Together they define its long axis. |
| Subnasale (Sn) | Soft tissue: where the nasal septum meets the upper lip in the midsagittal plane. |
| Soft-tissue pogonion | Soft tissue: most anterior point of the soft-tissue chin. |
The lines and planes they build
- SN (S–N): the anterior cranial base.
- Frankfort plane (Po–Or).
- Maxillary plane (ANS–PNS).
- Mandibular plane (Go–Me).
- Functional occlusal plane: through the occlusion of the first permanent molars and premolars.
- A–Pog line: reference for the lower incisor.
- Incisor long axes: tip to apex, upper and lower.
- E-line: from the tip of the nose to soft-tissue pogonion.
Main source: Cobourne MT, Fleming PS, DiBiase AT, Ahmad S. Clinical Cases in Orthodontics. Wiley-Blackwell, 2012, chapter 1.