ANB Angle, SNA and SNB: Norms, Skeletal Classes, Wits

ANB is the first number most clinicians look for on a lateral cephalogram: one angle that sums up the sagittal relationship between the jaws. It is also one of the easiest to over-read. This article covers SNA, SNB and ANB, their norms and the skeletal classes as taught in university orthodontic programs, the situations where ANB misleads, the Wits appraisal, and the way ANB feeds Steiner’s chevrons. For the rest of the analysis, see our Steiner analysis guide.

The landmarks: S, N, A and B

All three angles share the same four landmarks and the same reference line, S-N, which represents the anterior cranial base.

A and B sit in low-contrast alveolar bone and are among the hardest points to place. N matters just as much: a Nasion placed a little too high or too far forward changes SNA, SNB and ANB in one stroke. Our cephalometric tracing guide covers the pitfalls of each landmark.

The SNA angle

SNA is the angle formed at Nasion between S-N and N-A. It locates the maxilla, or more exactly point A, relative to the anterior cranial base.

• Norm: 82° ± 2°.
• Above 84°: maxillary prognathism (a forward maxilla).
• Below 80°: maxillary retrognathism (a retruded maxilla).

Two cautions. Point A is a periosteal, alveolar point: it remodels, and incisor movements can shift it. And SNA describes A relative to S-N, not the size of the maxilla. The French school of Delaire, for example, considered SNA too sensitive to the remodeling of A and judged maxillary orientation with other measurements.

The SNB angle

SNB is the angle between S-N and N-B. It locates the mandible, through point B, relative to the cranial base.

• Norm: 80° ± 2°.
• Above 82°: mandibular prognathism.
• Below 78°: mandibular retrognathism.

Point B lies on the alveolar process, so it is influenced by the lower incisor. This is why some Steiner sheets add SND (norm 76° ± 2°), where D is the center of the symphysis and is not affected by the incisor; a low SNB confirmed by a low SND points more firmly to a retrognathic mandible. The course materials also note that SNB changes very little during treatment and increases mainly when the vertical dimension decreases.

The ANB angle: definition and normal value

ANB is the angle between N-A and N-B. In practice it is calculated as SNA − SNB. It expresses the sagittal discrepancy between the two jaws:

• Norm: 2° ± 2°.
• A positive value means that A is ahead of B as seen from Nasion; a negative value means that B is ahead of A.

Ballard’s skeletal classes

The usual skeletal classification by ANB, attributed to Ballard, is the one used in the D.U. course handouts and by our free cephalometric analysis tool:

The cut-offs are guidelines. Within the Tweed tradition, for instance, the cephalometric table uses 2° ± 2° (0 to 4°) for diagnosis, while the craniofacial difficulty index uses a range of 1 to 5°: two different uses, two different ranges. A value close to a boundary always calls for a second look.

Reading ANB with SNA and SNB

ANB tells you how much; SNA and SNB tell you where. The same ANB of 7° can come from a forward maxilla, a retruded mandible or both, and the treatment reasoning is not the same.

A teaching example from the course: ANB 4°, Wits −1 mm, SNA 79° and SNB 75°. The reading is a skeletal Class I with bimaxillary retrusion, not a Class II.

The location matters most in Class III, where the courses list the site of the discrepancy among the decision criteria, a maxillary origin being the one that responds better to orthopedics. In Class II and Class III alike, the incisor measurements then show the dentoalveolar compensations: upper incisors retroclined and lower incisors proclined in Class II, the reverse in Class III.

Why ANB can mislead

ANB is convenient, but it depends on structures that have nothing to do with the relationship between the jaws:

• the position of Nasion: Bassigny, an older French reference, already pointed out that the same discrepancy between the jaws gives different ANB values depending on where Nasion sits and on the facial type;
• the inclination of S-N: a steep or flat cranial base shifts SNA and SNB together;
• jaw rotation: a clockwise rotation of the mandible tends to increase ANB, a counterclockwise rotation to decrease it;
• the vertical dimension: the course handouts insist that the vertical pattern distorts ANB. In a hyperdivergent Class III, the posterior rotation moves B backward and can falsely "normalize" ANB (Boileau).

The literature has tried to correct for this. One approach is "floating norms": regression equations that individualize the expected ANB from other skeletal variables. In 71 Caucasian subjects with normal occlusion, the prediction of the expected ANB improved when the palatal plane inclination, the cranial base angle, Ricketts’ facial axis and Hasund’s index were added (Paddenberg et al., 2021, J Orofac Orthop, PubMed). The sample is small, but the message is consistent with the courses: a "normal" ANB depends on the face it belongs to.

Three teaching examples show what is at stake:

1. ANB 4° with a Wits of −1 mm: the upper limit of ANB, but a borderline Class I, not a Class II.
2. ANB 2° with a Wits of −3 mm and SND 80°: a normal ANB hiding a Class III tendency.
3. ANB 6° with a Class I occlusion: a skeletal Class II masked by proclined lower incisors (IMPA 103°, L1-NB 35°). The occlusion must always be compared with the skeletal diagnosis.

The Wits appraisal

The Wits appraisal, described by Jacobson, measures the same discrepancy without using Nasion. Points A and B are projected perpendicularly onto the occlusal plane, giving AO and BO, and the distance between the two projections is measured in millimeters. It is positive when AO lies ahead of BO. The Wits is not part of the Steiner analysis, but it is read alongside it in most programs.

In the D.U. sheets, the occlusal plane runs from the middle of the first molar overlap to the middle of the incisor overlap; Jacobson described his measurement on the functional occlusal plane. Whichever plane you use, write it down.

Wits norms: two traditions

With the 0 ± 2 mm norm, the Class II and Class III courses read a Wits above 2 mm as a skeletal Class II (2 to 5 mm being a mild one) and below −2 mm as a skeletal Class III. The two traditions coexist, and one Tweed sheet even writes the range in an ambiguous way. A Wits value means nothing without the norm it is compared with: always state which one you use.

The strengths and weaknesses of the Wits

Its strength is that it is little influenced by the vertical pattern. According to Boileau, a negative Wits is more reliable than ANB in Class III, especially in hyperdivergent patients.

Its weaknesses are its own:

• it depends directly on how the occlusal plane is traced, which is difficult in the mixed dentition and whenever the occlusion is irregular;
• the occlusal plane is a dental reference, so tooth movements and the eruption pattern affect it;
• it is a linear measurement: on an uncalibrated software tracing left in pixels, it cannot be compared with any norm, and radiographic magnification (about 7% at 1.5 m) must be the same between two films you compare.

ANB or Wits: which one to trust?

Neither, alone. The courses teach them as a pair: ANB is confirmed by the Wits. The literature does not dethrone ANB either. In 146 adults, ANB showed the best agreement with the final group diagnosis (kappa 0.802) and the highest sensitivity and positive predictive value in Class II, while the Wits, the Beta angle and Downs’ angle of convexity performed well in Class III (Ahmed et al., 2018, Dental Press J Orthod, PubMed). The limits are real: a single center, an adult sample, and a reference standard built from the majority of the same measurements.

When ANB and the Wits disagree, no number settles it. Look at the profile, the vertical pattern, SNA and SNB, and if the patient has a forward functional shift, compare tracings in centric relation and in maximum intercuspation.

ANB in treatment planning: Steiner’s chevrons

ANB is also the starting point of Steiner’s prognosis. In the second chevron, you estimate the end-of-treatment ANB, then read the incisor position Steiner considers acceptable for that ANB.

• Growing patient, favorable conditions (remaining growth, anterior rotation, possible orthopedic treatment, cooperation): final ANB ≈ initial ANB / 2 + 1. If conditions are less favorable, the estimate is weighted downward.
• Adult, no growth: orthodontics alone does not change ANB, so the initial ANB is kept.

Example: a growing boy with an ANB of 6° gives an estimated final ANB of 6 / 2 + 1 = 4°. Steiner’s compromise for 4° places the upper incisor at 20° and 2 mm from N-A and the lower incisor at 27° and 4.5 mm from N-B.

The compromise is defined only for a final ANB between −1° and 8°. Beyond that range, compensation through incisor position alone is not enough and correction of the skeletal discrepancy should be discussed. Our Steiner’s chevrons calculator applies these rules step by step, and the cephalometric analysis tool can send the ANB it measures straight to it.

Surgical thresholds: variants, not rules

The courses do not agree on a single ANB threshold for orthognathic surgery, and the differences are worth knowing:

• Class II: one course considers camouflage possible after growth when ANB is below 8° and orthodontic-surgical treatment above 8°. Within the Tweed approach, an ANB of 4 to 7° is listed as a mild Class II compatible with non-extraction treatment, Horn’s favorable pattern includes an ANB of 6° or less, and the risk of failure is said to increase from 7°. Boileau, on the other hand, gives no ANB threshold and relies on the periodontium, the profile, the vertical dimension and asymmetry.
• Class III: one course treats an ANB below −2° as a severe Class III and a surgical indication after growth, while the diagnosis of Class III starts below 0°; another course gives no figure. Boileau again uses several criteria (severity, esthetics, associated vertical or transverse problems, compensations) and favors the Wits over ANB in hyperdivergent patients.

A single ANB threshold is a guideline at best; the decision remains the clinician’s.

Can ANB change during treatment?

Yes, for several reasons that have to be told apart:

• growth: the Tweed growth types compare ANB on two cephalograms taken 6 months to 1 year apart. Type A (about 25%) keeps a constant ANB, type B (about 15%) sees it increase, type C (about 60%) sees it decrease through mandibular catch-up;
• mechanics: in Class II, side effects such as molar extrusion can rotate the mandible down and back and increase ANB;
• function: in a Class III with a forward functional shift, correcting the anterior crossbite changed one course case from ANB −3° to −1°, Wits −8 to −4 mm and SNB 81° to 80°. Part of the "Class III" was functional, not skeletal;
• compensation: in a course example of Class II correction, ANB went from 5° to 3° while IMPA rose from 84° to 97°, so the occlusion was corrected partly through the lower incisors.

In adults, the rule used for the chevrons applies: orthodontics alone is not expected to change ANB.

In practice: a checklist

1. Place S, N, A and B carefully, and check N and A first: they change every angle at once.
2. Read SNA and SNB before ANB, to know where the discrepancy comes from.
3. Classify with Ballard’s cut-offs (0 to 4°, above 4°, below 0°), then question any value near a boundary.
4. Confirm ANB with the Wits, and state the Wits norm and the occlusal plane you used.
5. Check the vertical pattern: in a hyperdivergent patient, ANB can underestimate a Class III.
6. Compare the skeletal class with the occlusion and the profile.
7. For planning, estimate the final ANB (initial / 2 + 1 in growing patients, unchanged in adults) and stay within −1° to 8° for Steiner’s compromise.

Frequently asked questions

What is a normal ANB angle?

The normal ANB is 2° ± 2°. In Ballard’s classification, 0 to 4° is a skeletal Class I, above 4° a Class II and below 0° a Class III. Because ANB depends on Nasion, the cranial base and the vertical pattern, confirm it with the Wits appraisal and the clinical profile.

What is the SNA angle?

SNA is the angle between S-N and N-A; it locates the maxilla relative to the anterior cranial base. Its norm is 82° ± 2°: above 84° suggests maxillary prognathism, below 80° maxillary retrognathism. SNB (80° ± 2°) locates the mandible, and ANB is the difference between the two.

What does a high ANB mean?

An ANB above 4° suggests a skeletal Class II. SNA and SNB tell you whether it comes from a forward maxilla, a retruded mandible or both. A clockwise mandibular rotation or an unusual Nasion position can also inflate it, so check the Wits and the vertical pattern before concluding.

Should I use ANB or the Wits appraisal?

Use both. The Wits does not depend on Nasion and is little influenced by the vertical pattern, which makes it more reliable in hyperdivergent Class III patients, but it depends on the occlusal plane and has two competing norms (0 ± 2 mm and 2 ± 2 mm). When they disagree, the clinical examination decides.

Can ANB change with treatment?

In growing patients, yes: growth, orthopedics, mechanics and the correction of a functional shift can all change it. In adults, orthodontics alone is not expected to change ANB, and the initial value is kept for planning.

With GS Ortho

In GS Ortho, AI places 29 landmarks on the lateral cephalogram, and SNA, SNB and ANB are calculated straight away with the rest of the Steiner and Tweed analyses. The Wits appraisal is calculated once you draw the occlusal plane yourself. You check each landmark, correct it if needed, then validate: the decision stays yours. The measurements feed into the guided diagnostic studio, and an orthodontist’s opinion can be requested in one click, straight from the case. The GS Ortho interface is currently available in French.

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