Cephalometric landmarks explained: the main points and reference lines
Key points
- A cephalometric landmark is a point placed on a lateral cephalogram at a defined anatomical site. Every angle and distance in an analysis is measured from these points.
- Most analyses share a core set: S, N, Po, Or, ANS, PNS, A, B, Pog, Gn, Me and Go.
- Lines through these points, such as SN, FH, Go-Gn and the occlusal plane, are the references the readings are measured against.
- Moving one point by 1 mm can change an angle by about 1°, and every reading that uses the point changes with it.
- The same name does not always mean the same definition. A reading should be compared with norms that used the same points.
What a cephalometric landmark is
A cephalometric analysis turns a lateral cephalogram into numbers. It does this through landmarks: points placed on the image at defined sites [1]. Angles and distances are then measured between the points, or between lines drawn through them. A tracing is only as good as its points.
Landmarks fall into three broad groups, and the group tells you how a point can go wrong:
- Points on a structure, such as the tip of the anterior nasal spine. They are found where the structure is.
- Points defined by a direction, such as the deepest or the most anterior point of a curve. Where they fall depends on the direction they are judged from, so the orientation of the image matters.
- Constructed points, built from other points or lines, such as gonion found from two tangents. They carry the errors of the lines they are built from.
Cranial base and orientation points
- S, sella: the centre of the pituitary fossa (sella turcica). It is judged by eye, as the middle of the rounded outline of the fossa.
- N, nasion: the most anterior point of the frontonasal suture in the midline, where the frontal and nasal bones meet.
- Po, porion: the highest point on the upper edge of the external auditory meatus. Anatomical porion is on the bony ear canal. Machine porion is the top of the ear rod of the cephalostat; it is a different point and should not be swapped for the anatomical one.
- Or, orbitale: the lowest point on the lower rim of the orbit.
Maxillary points
- ANS, anterior nasal spine: the tip of the bony spine at the front of the nasal floor.
- PNS, posterior nasal spine: the tip of the posterior nasal spine of the palatine bone, at the back of the hard palate. Other structures can overlap it, which makes the tip hard to see.
- A, point A (subspinale): the deepest point on the front curve of the maxilla, between ANS and the alveolar bone over the upper incisors.
Mandibular points
- B, point B (supramentale): the deepest point on the front curve of the mandible, between the alveolar bone over the lower incisors and the chin.
- Pog, pogonion: the most anterior point of the bony chin. It is also written Pg.
- Gn, gnathion: the most anterior and inferior point on the outline of the chin, between pogonion and menton. Authors define it in slightly different ways, so check which definition an analysis uses.
- Me, menton: the lowest point on the outline of the symphysis.
- Go, gonion: the point on the angle of the mandible. It is often constructed: draw a tangent to the back of the ramus and a tangent to the lower border of the mandible, and take the point on the angle that lies midway between them.
Steiner noted that gonion and gnathion are hard to locate [2]. Both sit on curved outlines with no sharp feature to aim at.
| Landmark | Region | Kind of point |
|---|---|---|
| S | Cranial base | Centre of a structure, by eye |
| N | Cranial base | On a structure (the suture) |
| Po | Cranial base | By direction (highest point); paired |
| Or | Orbit | By direction (lowest point); paired |
| ANS, PNS | Maxilla | On a structure (spine tips) |
| A | Maxilla | By direction (deepest point) |
| B | Mandible | By direction (deepest point) |
| Pog | Mandible | By direction (most anterior point) |
| Gn | Mandible | By direction or constructed, depending on the definition |
| Me | Mandible | By direction (lowest point) |
| Go | Mandible | Often constructed from two tangents; paired |
“Paired” means the structure exists on the left and the right side, so the radiograph can show two outlines.
Reference lines
- SN: from sella to nasion. It stands for the anterior cranial base. Riedel used this line as a reference plane [3]. The Steiner analysis takes most of its readings from it, such as SNA and SNB.
- FH, Frankfort horizontal: from porion to orbitale. Several analyses use it as their horizontal; in Simplified Ceph these include Downs [4], Ricketts and Tweed-Merrifield.
- Palatal plane: from ANS to PNS.
- Go-Gn: from gonion to gnathion. It is Steiner’s mandibular plane [2]. Other analyses draw the mandibular plane through other points, such as gonion and menton, or as a tangent to the lower border. Mandibular plane angles are therefore comparable only with norms that used the same line.
- Occlusal plane: a line through the teeth in occlusion. One version runs through the incisor overlap and the molar contact; the functional occlusal plane runs through the premolar and molar contacts only. Jacobson’s Wits appraisal projects A and B onto the occlusal plane [5]; Simplified Ceph reads it on the functional occlusal plane.
Worked example: what 1 mm does
The tracing in Figure 1 gives SNA 83.1°, SNB 80.1° and ANB 3.0°, Go-Gn at 27.9° to SN and Go-Me at 24.5° to FH. The table shows what happens when one point is moved by 1 mm and every other point stays where it is. The positions are schematic; they are not from a patient.
| Point moved 1 mm | SNA | SNB | ANB | Go-Gn to SN | Go-Me to FH |
|---|---|---|---|---|---|
| None (Figure 1) | 83.1° | 80.1° | 3.0° | 27.9° | 24.5° |
| N forward | 82.2° | 79.6° | 2.6° | 27.8° | 24.5° |
| A forward | 84.1° | 80.1° | 4.0° | 27.9° | 24.5° |
| Go up | 83.1° | 80.1° | 3.0° | 28.6° | 25.2° |
| Or up | 83.1° | 80.1° | 3.0° | 27.9° | 25.2° |
| Po up | 83.1° | 80.1° | 3.0° | 27.9° | 23.8° |
- Moving nasion forward lowers SNA, SNB and ANB, although neither jaw has moved. Hussels and Nanda showed that ANB changes with the front-to-back position of nasion [6].
- Moving A point forward changes SNA and ANB by a full degree each. SNB does not change.
- Moving gonion up tilts the mandibular plane. Both mandibular plane angles change, but none of the SN angles to the jaws.
- Moving orbitale or porion changes FH, and with it every reading measured from FH, in opposite directions for the two points.
So a small error in one point does not stay small or local. It moves every reading that uses that point or a line through it.
Common sources of identification error
- Two sides, two outlines. Porion, orbitale and gonion belong to paired structures. The left and right sides lie at different distances from the film, so they are magnified differently and may not overlap. In a study of the structures used to superimpose mandibular tracings, on fifty lateral films, midline structures were traced more consistently than bilateral ones [7]. Use one rule every time; Simplified Ceph’s definition of orbitale, for example, says to mark the midpoint when there are two shadows.
- Faint or crowded outlines. Some points lie where several outlines overlap, or on thin bone where the edge is faint. Porion, orbitale and PNS are examples. A point and B point lie on thin bone over the incisor roots.
- Points on a gentle curve. The deepest or most anterior point of a broad curve can slide along it. Two careful people can place it in different spots on the same outline [8].
- Different definitions under one name. Gonion can be anatomical or constructed, porion anatomical or machine, and the mandibular plane can use several pairs of points. Mixing definitions makes a reading look abnormal when it is not.
- Constructed points inherit errors. A point built from tangents moves when either tangent moves.
- The image itself. Blur, low contrast and head position all make points harder to place. A uniform magnification does not change angles, but distances are only as accurate as the calibration on the film ruler.
Placing each point at a comfortable zoom, using the same definitions every time, and tracing a few films again after a while to see how far your own points move, all help to keep this error visible [9].
Do this in Simplified Ceph
Simplified Ceph lets you trace a lateral cephalogram on screen. Calibrate it on the film ruler, pick an analysis, and the landmark list shows only the points it uses, each with a short definition. A loupe and arrow keys help you place each point, and the readings update as you move it. There is a Free plan for lateral cephalometric analysis.
Educational content for clinicians and students. Landmark definitions vary between analyses. This page does not replace clinical judgement.
References
- Jacobson A, Jacobson RL, eds. Radiographic Cephalometry: From Basics to 3-D Imaging. 2nd ed. Chicago: Quintessence; 2006. ISBN 978-0-86715-461-0.
- Steiner CC. Cephalometrics in clinical practice. Angle Orthod. 1959;29(1):8-29. Publisher’s page
- Riedel RA. The relation of maxillary structures to cranium in malocclusion and in normal occlusion. Angle Orthod. 1952;22(3):142-145. doi:10.1043/0003-3219(1952)022<0142:TROMST>2.0.CO;2.
- Downs WB. Variations in facial relationships: their significance in treatment and prognosis. Am J Orthod. 1948;34(10):812-840.
- Jacobson A. The “Wits” appraisal of jaw disharmony. Am J Orthod. 1975;67(2):125-38. doi: 10.1016/0002-9416(75)90065-2. PMID: 1054214
- Hussels W, Nanda RS. Analysis of factors affecting angle ANB. Am J Orthod. 1984;85(5):411-23. doi: 10.1016/0002-9416(84)90162-3. PMID: 6586080
- Cook PA, Gravely JF. Tracing error with Björk’s mandibular structures. Angle Orthod. 1988;58(2):169-178. doi:10.1043/0003-3219(1988)058<0169:TEWBMS>2.0.CO;2.
- Baumrind S, Frantz RC. The reliability of head film measurements. 1. Landmark identification. Am J Orthod. 1971;60(2):111-127. PMID: 5283996.
- Houston WJB. The analysis of errors in orthodontic measurements. Am J Orthod. 1983;83(5):382-390. doi:10.1016/0002-9416(83)90322-6.