Mandibular Plane Angle: FMA, SN-MP and High-Angle Cases
The mandibular plane angle is the simplest way to put a number on the vertical dimension of the face. A steep mandibular plane points to a long, "high-angle" face; a flat one to a short, "low-angle" face. That angle then shapes the incisor target, the extraction discussion, the anchorage and the mechanics you can afford. This guide covers how the plane is drawn, FMA and SN-MP with the norms our free tool applies, what high-angle and low-angle patterns imply, how to cross-check the reading and the tracing pitfalls. It gives criteria to reason with, not treatment recipes.
What is the mandibular plane?
The mandibular plane is a line representing the lower border of the mandible on the lateral cephalogram. The problem is that there is no single way to draw it. Each analysis has its own construction:
| Construction | Points | Used by |
|---|---|---|
| Go-Gn | Gonion to Gnathion | Steiner (SN-GoGn) |
| Tangent to the lower border | Line touching the lower border of the mandible, through Menton | Tweed (FMA) |
| Go-Me | Gonion to Menton | Our free tool, as an approximation of Tweed’s plane |
Two landmarks add to the variation. Gonion is usually constructed in course materials: the bisector of the angle formed by the tangent to the posterior border of the ramus and the mandibular plane cuts the mandibular angle. Some tools, ours included, define it more simply as the most posterior and inferior point of the angle. Gnathion is either constructed (the bisector of the angle between N-Pog and the mandibular plane cuts the symphysis) or placed midway between Pogonion and Menton.
The practical consequence: values measured on different mandibular planes are not interchangeable. An older table reproduced by Bassigny gives an average MP/SN of about 40° for a "normal" face, against 32° in the Steiner norms taught today. Different era, different sample, and not necessarily the same plane. Always compare a measurement with the norm of the analysis it comes from.
FMA: the Frankfort–mandibular plane angle
The FMA is the angle between the Frankfort horizontal (Porion–Orbitale) and the mandibular plane. It is one side of the Tweed triangle, with FMIA (lower incisor to Frankfort) and IMPA (lower incisor to the mandibular plane). The three angles add up to 180°, which gives a useful check: if the sum is wrong, one measurement is wrong.
The norm used in D.U. course handouts, and in our free cephalometric analysis tool, is:
| Measurement | Norm | Hypodivergent (low angle) | Normodivergent | Hyperdivergent (high angle) |
|---|---|---|---|---|
| FMA | 25° ± 3° | below 22° | 22° to 28° | above 28° |
The cut-offs vary slightly between sources. Some course slides classify hypodivergence below 20°, which leaves 20–22° unclassified; Schudy’s grid as reported by Boileau uses 22° and 28° with no gap. The 22/28° grid is the one our tool applies. In the tool, FMA is measured from Po-Or to the Go-Me line.
SN-MP and SN-GoGn
SN-MP is the generic name for the angle between the anterior cranial base (Sella–Nasion) and the mandibular plane. In the Steiner analysis, the mandibular plane is Go-Gn, so the measurement is called SN-GoGn (or GoGn-SN).
| Measurement | Norm | Hypodivergent | Mesodivergent | Hyperdivergent |
|---|---|---|---|---|
| SN-GoGn | 32° ± 5° | below 27° | 27° to 37° | above 37° |
The Steiner course adds a confirming sign: an occlusal plane steeper than about 14° to S-N supports a hyperdivergent reading when SN-GoGn is above 37°, and a flatter one supports hypodivergence when SN-GoGn is below 27°.
FMA or SN-MP? They describe the same thing from two references. The inclination of S-N varies between patients: a steep cranial base raises SN-MP without any real excess of facial height, a "pseudo-hyperdivergence" listed in the differential diagnosis of the courses. Frankfort depends on Porion and Orbitale, two fragile landmarks. Neither is "better": measure both and check that they agree. Our Steiner analysis guide places SN-GoGn among the other Steiner measurements.
High-angle vs low-angle: what the numbers describe
The vocabulary differs between schools: hyperdivergent / hypodivergent (Schudy), long face / short face, dolichofacial / brachyfacial (Ricketts), posterior / anterior rotation (Björk). One distinction matters: hyper- and hypodivergence describe a state, while anterior or posterior rotation describes a change between two dates. The two are not synonyms.
| Feature | High-angle (hyperdivergent) | Low-angle (hypodivergent) |
|---|---|---|
| Face | Long, increased lower facial height, lip incompetence with mentalis strain, gingival smile | Wide, prominent masseters, square mandible, reduced lower facial height |
| Profile | Convex, effaced labiomental fold, short ramus, open gonial angle, antegonial notch | Marked labiomental fold, closed gonial angle, prominent chin, long ramus |
| Symphysis | Thin, with thin cortices | Thick, with thick cortices |
| Occlusion | Tendency toward open bite, flat curve of Spee | Tendency toward deep bite, deep curve of Spee |
| Muscles and function | Weak musculature, mouth breathing and low tongue frequent | Strong elevator muscles, sometimes bruxism |
The high-angle patient
A steep mandibular plane goes with a long lower face and, often, an anterior open bite. But no open bite does not mean no problem: a mandibular dentoalveolar compensation (long alveolar process, thin bone and periodontium) can mask the vertical excess. The thin symphysis also limits lower incisor movement. See our article on anterior open bite (in French).
The low-angle patient
A flat mandibular plane goes with a short lower face, strong muscles and, very often, a deep bite. In a child, the deep bite can lock mandibular growth, which argues for early management. The low-angle pattern is generally favorable in a Class II but unfavorable in a Class III. The deep bite article (in French) details the options.
What the mandibular plane angle changes in treatment planning
The core idea of the Tweed analysis is that divergence, which changes little, dictates the lower incisor target. The rule taught most often, and used by our tool:
| FMA | Lower incisor guideline |
|---|---|
| 22° to 28° | Aim for FMIA 67° |
| Above 28° | Aim for FMIA 65°: the incisor is kept more upright in a high-angle face |
| Below 22° | Do not exceed an IMPA of about 92°: a more proclined incisor is tolerated |
Bounds and targets vary between courses (21° instead of 22°, or 20/30° with a target of 68°): use the version of your program. The gap between the target and the measured FMIA is converted into arch length at 0.8 mm per degree, the cephalometric correction detailed in our Tweed analysis article (in French).
Beyond the incisor, the courses list criteria that weigh on the plan:
- High-angle: no orthopedic appliance corrects a hyperdivergence; the realistic goal is not to make it worse. Extrusion is the easiest tooth movement and it opens the bite, so every extrusive mechanic is suspect: cervical headgear, Class II elastics without vertical control, distalization, tip-back bends anchored on molars. High-pull headgear and, in adults, miniscrews are the usual tools for vertical control. Trying to close the FMA mechanically is described as illusory, which leads to a strategy more open to extractions (no distalization, incisor uprighting), even though a "wedge effect" of extractions on the vertical is not demonstrated.
- Maxillary expansion tends to increase divergence, but the effect is small and partly transient (Bassigny quotes 1 to 2° of mandibular rotation, half of which relapses): caution in high-angle patients, not a formal contraindication.
- Low-angle: correct the deep bite, preferably by extruding the buccal segments, and avoid extractions that flatten the profile. The courses call premolar extractions "contraindicated" in the low-angle patient; Tweed and Boileau treat them as an unfavorable factor rather than an absolute ban. Molar extrusion tends to relapse with strong muscles, so it is more stable done during growth.
- Severe patterns, in either direction, call for an orthodontic-surgical approach.
One nuance from the literature: a 2024 systematic review of 18 studies (2 randomized, 1,094 Class II patients) found that cervical headgear was not, on average, consistently associated with an increase in the mandibular plane angle compared with untreated growth, with low to very low certainty of evidence (Hussain et al., Eur J Orthod 2024). An average effect says little about the individual high-angle patient, which is where the courses advise caution.
Cross-checking: SN-GoGn, facial height index and Björk signs
The rule is to never conclude on one vertical number. FMA is cross-checked with SN-GoGn and with the facial height index (FHI) of the Tweed-Merrifield analysis: posterior facial height (Ar to the mandibular plane, about 45 mm) divided by anterior facial height (palatal plane to Menton, about 65 mm). As a ratio, it can be read even on an uncalibrated tracing.
| FHI | Reading |
|---|---|
| 0.65 to 0.75 (mean 0.69) | Normal range |
| Above 0.75 | Tendency toward closing, deep bite |
| Below 0.65 | Tendency toward opening, open bite |
| Outside 0.60–0.80 | Vertical harder to control orthodontically: surgery to be discussed |
The Tweed course considers the FHI more sensitive than FMA: it separates "normodivergent" patients who behave as low- or high-angle ones, and at the same FMA a rising FHI is a better sign than a falling one. Two teaching cases show why the cross-check matters: FMA 26° (normal) with an FHI of 0.81; SN-GoGn 41° (high) with an FHI of 0.80. When indicators disagree, none decides alone. Go back to the face and to the structural signs of Björk:
| Sign | Anterior rotation | Posterior rotation |
|---|---|---|
| Condylar neck | Thick, directed forward | Thin, directed backward |
| Mandibular canal | Curved | Straight |
| Lower border | Convex, no notch | Concave, antegonial notch |
| Symphysis | Inclined down and forward | Inclined down and backward |
| Interincisal / intermolar angle | Open / above 180° | Closed / below 180° |
| Lower facial height | Reduced | Increased |
These signs are reliable only when the rotation is marked and the patient is well advanced in growth; they are hard to read in young children. Some published lists invert an item of this table: check before copying one.
Measurement pitfalls
- Double lower border. The two halves of the mandible often project as two contours. If they cross at the angle, trace the most posterior border of the ramus and the highest border of the body; if they are offset without crossing, trace the average and note a possible sign of asymmetry.
- Gonion construction. Bisector or "most posterior-inferior point": the two definitions do not give the same point. Compare tracings made with the same convention. Gonion and Porion are among the landmarks with the largest identification errors on lateral cephalograms, because they lie on structures without sharp borders (Lagravère et al., Am J Orthod Dentofacial Orthop 2010, a small study of 10 patients).
- Porion and Orbitale. Both are bilateral and low in contrast; an error on either tilts Frankfort and changes FMA directly.
- Cranial base inclination. A steep S-N inflates SN-MP: check it against FMA before labeling the patient high-angle.
- Mixing references. Do not read a Go-Gn value against a tangent-plane norm, and do not carry an occlusal plane measured on S-N into a Tweed sheet that expects it on Frankfort.
- Vertical excess without a steep plane. In Bassigny’s table, the long face without open bite has the same average MP/SN as the average face; only the facial heights and the clinical signs (gingival smile, lip incompetence) reveal it.
Our cephalometric tracing guide covers the other landmarks and the order of the tracing.
Can treatment change the mandibular plane angle?
Within limits. Normal growth tends to reduce divergence; a posterior rotation (condylar problem, weak muscles, mouth breathing) worsens it. Orthodontics mainly avoids opening the angle; molar intrusion on miniscrews can produce some counterclockwise rotation in moderate or borderline cases. Record FMA at the start and end of treatment and watch the FHI, which should rise with good vertical control. Large changes belong to orthognathic surgery, where posterior maxillary impaction is the reference for closing a skeletal open bite. Even then, a systematic review of 9 retrospective studies found long-term vertical relapse in a share of patients whatever the surgery, more marked for the mandibular plane angle after bimaxillary surgery than after Le Fort I alone (Solano-Hernández et al., J Oral Maxillofac Surg 2012). See our article on orthognathic surgery indications (in French).
In practice: a checklist
- Note which mandibular plane you used (Go-Gn, Go-Me or tangent) and compare it only with its own norm.
- Check Porion, Orbitale, Gonion and Menton; handle a double lower border explicitly.
- Read FMA (25° ± 3°) and SN-GoGn (32° ± 5°) together.
- Confirm with the facial height index and the occlusal plane inclination.
- If indicators disagree, look at the Björk signs and the face: lower facial height, lip competence, smile, overbite.
- Derive the lower incisor guideline from FMA (FMIA 67°, 65° above 28°, IMPA up to about 92° below 22°).
- List the mechanics to avoid or to use with care for this vertical pattern before choosing them.
Frequently asked questions
What is a normal FMA?
In the D.U. course norms used by our tool, FMA is 25° ± 3°: between 22° and 28° the face is normodivergent, above 28° hyperdivergent, below 22° hypodivergent. Some sources place the lower bound at 20° or 21°. The corresponding Steiner norm, SN-GoGn, is 32° ± 5°.
What is a high-angle patient?
A patient with a steep mandibular plane (FMA above 28°, SN-GoGn above 37°), usually with a long lower face, lip strain on closure, a thin symphysis and a tendency toward open bite. It calls for a more upright lower incisor target (FMIA 65°) and strict vertical control.
What is the difference between SN-MP and FMA?
Both measure the inclination of the mandibular plane, but against different references: S-N (anterior cranial base) for SN-MP, Frankfort (Porion–Orbitale) for FMA. They also often use different mandibular planes (Go-Gn in Steiner, a lower-border plane in Tweed). Their norms differ, so read each against its own norm and check that they agree.
Can orthodontic treatment change the mandibular plane angle?
Mostly, orthodontics keeps it from opening; no orthopedic appliance corrects a hyperdivergence. Larger changes require orthognathic surgery, with a risk of partial vertical relapse (see the section above).
With GS Ortho
In GS Ortho, AI places 29 landmarks on the lateral cephalogram and the Steiner and Tweed analyses are calculated straight away. You check each landmark, Gonion and Porion included, correct it if needed, then validate: the decision stays yours. In the diagnostic studio, FMA, SN-GoGn and the facial height index are read side by side, and a note flags when they disagree instead of forcing a single label. An orthodontist’s opinion can be requested in one click, straight from the case. The GS Ortho interface is currently available in French.