Class III Malocclusion: Underbite, ANB Below 0°, Diagnosis

Lower incisors in front of the upper ones can be a simple forward functional shift, a dentoalveolar Class III, or a hereditary skeletal Class III that growth will make worse. The diagnosis decides everything else. This guide, written for dentists, residents and students, follows the D.U. courses behind GS Ortho and Boileau, a French orthodontic textbook used in those courses, and adds what systematic reviews and trials report on treatment. These are criteria: the decision stays with the clinician, and a personal case belongs with an orthodontist.

Class III at a glance

What "Class III" means

The word covers three different things that should be named separately.

• Skeletal Class III (Ballard’s classification): an anteroposterior discrepancy between the jaws relative to the cranial base. It can come from maxillary retrognathia, mandibular prognathism or both.
• Occlusal Class III (Angle): mesiocclusion of the lower first molar and canine. Depending on compensation, the incisors are in crossbite, edge to edge or even in normal overlap. A unilateral form is a right or left subdivision.
• Functional Class III (forward shift): the mandible moves forward because of an obstacle, often dental (interferences, habit). In centric relation (CR), the incisors come back to edge to edge: CR differs from maximum intercuspation (MI).

Skeleton and occlusion are usually associated but dissociable: a skeletal Class III can be compensated into a Class I occlusion, and the reverse.

Mandibular prognathism or maxillary retrognathia?

"Mandibular prognathism" is often used as a synonym for skeletal Class III ("Class 3 underbite"). That is inaccurate. According to Delaire, cited in Boileau, the most frequent form is maxillary retrognathia, in about 65% of cases. The EMC, a French medical encyclopedia used in the courses, states that the share of mandibular protrusion varies from 30% to 55% depending on the series, and that mandibular length is often not increased: a forward tilt of the ramus with an open gonial angle frequently brings the chin forward. The location of the discrepancy weighs heavily in the decision.

How common is Class III?

In a systematic review of 53 prevalence studies, Class III accounted for about 5.9% of the permanent dentition and 4% of the mixed dentition worldwide, with a range from 1% to 20%, and was most frequent in the mixed dentition of the group the review calls Mongoloid (East Asian populations) (Alhammadi et al., 2018, Dental Press J Orthod, PubMed). Another systematic review opens with a figure of 5% to 15% of the population (Woon and Thiruvenkatachari, 2017, Am J Orthod Dentofacial Orthop, PubMed). The figures depend on the population and on the definition used.

Causes: look for heredity first

Class III has a very strong hereditary component, especially in mandibular prognathism. Boileau describes a polygenic threshold model, and heredity darkens the prognosis. The history therefore looks for a family resemblance before the examination even starts.

The courses also list congenital causes (Binder and Crouzon syndromes, cleft lip and palate, whose scars restrain maxillary growth, acromegaly), functional causes (low, large, protrusive tongue, mouth breathing, enlarged tonsils) and local causes (upper incisors erupting in crossbite, lateral incisor agenesis with a small premaxilla). The idea that early loss of primary anterior teeth underdevelops the midface is taught, but its evidence remains limited.

The clinical examination

The signs that establish the diagnosis and point toward a form: concave profile, midface deficiency, reversed lip relationship, prominent chin.

• Face: long lower third, flat cheekbones, full lower lip. Look for asymmetry: it complicates the prognosis.
• Profile: flat in high-angle patients, concave in low-angle patients. The nasolabial angle is "often open" for one course and the EMC, but "closed" for Boileau: measure it, do not assume it.
• Intraoral: Class III molars and canines, open bite or deep bite, often a narrow maxilla or upper arch "inscribed" in a large lower arch. Crowding is more frequent in the upper arch (retained canines) than in the lower.
• A subtle sign noted by the EMC: upper lateral incisors alone in palatal position can be the only occlusal sign of a Class III.
• TMJ, tongue, breathing: examine them from the start.

Our free clinical exam form lists the findings outside the norm.

Pseudo-Class III: the centric relation test

This is the most important step of the workup. In any anterior crossbite, compare MI and CR.

The De Nevrezé maneuver guides the mandible into centric relation:

• the incisors reach edge to edge: it is a forward functional shift, a pseudo-Class III (CR ≠ MI);
• the mandible does not move back: it is a true Class III (CR = MI).

The test is done before any orthopedic decision; Boileau recommends a lateral cephalogram in CR and in MI to quantify the shift.

Why it matters: a functional shift distorts the measurements. In a course case, correcting the anterior crossbite changed SNB from 81° to 80°, ANB from −3° to −1° and the Wits from −8 to −4 mm: part of the discrepancy measured in MI was functional. The course considers the shift a priority, because without correction it can evolve into a structural Class III.

Cephalometric criteria

Cephalometrics confirms the discrepancy, locates it and quantifies the compensations. The course reference values:

Three reading rules:

1. The Wits as much as ANB. In high-angle patients, point B moves back and brings ANB falsely closer to the norm: Boileau considers the Wits more reliable in Class III. The Wits norm differs between sources (0 ± 2 mm in the Steiner course materials, 2 ± 2 mm on the Tweed sheets): state the one you use. In 146 adults, ANB, the Wits and the Beta angle showed the highest sensitivity (0.902) for the Class III group (Ahmed et al., 2018, Dental Press J Orthod, PubMed); the study is single-center, in adults, with a reference standard built from the same measurements. Our ANB angle guide covers the limits of ANB.
2. The location is not just SNA and SNB. Also look at maxillary and mandibular lengths.
3. Think vertical and transverse: Björk’s structural signs for rotation, a frontal cephalogram for asymmetry.

One point is disputed: Bassigny, an older French reference, describes an open cranial base angle in true prognathism with a long face, whereas Boileau (after Delaire) and the growth course describe a closed angle. Do not use it alone as a diagnostic argument.

To review landmarks and norms, see our cephalometric tracing guide and Steiner analysis guide; the free cephalometric analysis tool calculates SNA, SNB, ANB and incisor position on your own image.

Growth prognosis and its limits

Growth works against Class III

Björk’s average figures place the pubertal peak at about 12 years in girls and 14 in boys. The mandible grows more than the maxilla and finishes about 2 years later: a Class III can worsen late, and relapse is common. According to the EMC, a Class III merely stabilized during growth is already a result. Boileau adds that no growth direction is favorable: horizontal growth worsens the sagittal discrepancy, vertical growth creates a vertical problem.

Poor prognostic factors

Strong heredity; mandibular rather than maxillary origin; anterior growth rotation with a concave profile; a mild syndromic form; an asymmetry that increases with age; a thin periodontium with lower incisor recession that progresses while the crossbite persists.

The limits of prediction

Skeletal age (cervical vertebrae on the existing cephalogram, or a hand-wrist film) places the patient on the growth curve better than chronological age. Even so, the end of mandibular growth varies by source: 16 years in girls and 18 in boys in the course, 17 and 19 in Bassigny, about 18 and 22 from Björk’s curves as cited by the EMC. Practical consequence: an interceptive Class III treatment cannot be declared successful before adulthood.

Treatment options: criteria, not recipes

The eight decision criteria

The course, following Boileau, lists eight criteria:

1. severity, esthetic impact, hereditary or endocrine origin;
2. location: Class III from maxillary deficiency responds better to orthopedics;
3. age and remaining growth;
4. vertical pattern: mandibular growth can only be redirected into posterior rotation, contraindicated in markedly high-angle patients;
5. alveolar compensations: large initial compensations contraindicate orthodontics alone;
6. transverse plane: a true maxillary constriction is treated first; an asymmetry points toward surgery;
7. anterior guidance: to restore or maintain at every phase;
8. functional risks: a fragile TMJ, and reduced tongue space that exposes to relapse and sleep apnea.

Orthopedics in growing children

Favorable signs: growing child, maxillary origin, a functional component, low-angle or normal-angle pattern, moderate compensations. Unlike Class II, there is no waiting for the peak: the course places Class III orthopedics as early as possible, from 4 to 5 years, with a facemask before 10 years as a guideline. Our French guide to the facemask (in French) covers the appliance.

What the evidence adds:

• In a systematic review of 15 studies, three randomized trials (141 participants) compared a protraction facemask with untreated controls in children aged 7 to 12: reverse overjet improved by 2.5 mm and ANB by 3.9° in favor of the facemask. The authors found moderate evidence of short-term skeletal and dental benefit and a lack of evidence on long-term benefit (Woon and Thiruvenkatachari, 2017, PubMed).
• In a multicenter randomized trial of 73 children aged 7 to 9, followed for 6 years, 36% of the facemask group needed orthognathic surgery against 66% of the untreated group, and 68% of the treated children kept a positive overjet. There was no significant difference in skeletal or occlusal improvement, self-esteem or esthetic impact, and the authors note that the reduced need for surgery could not be explained by maintained skeletal change (Mandall et al., 2016, J Orthod, PubMed).
• A network meta-analysis of 18 studies (667 patients) found that bone-anchored maxillary protraction produced more SNA increase, less clockwise mandibular rotation and less upper incisor proclination than tooth-anchored protraction, but did not restrain mandibular growth better (Wang et al., 2022, Korean J Orthod, PubMed). Skeletal anchorage is discussed in our guide to TADs.

For severe hereditary forms, the sources describe two schools: abstain, or attempt orthopedics as a therapeutic test, stopped after about 6 months if it fails. Either way, the family is told that surgery may be needed later.

Orthodontic camouflage

Camouflage proclines the upper incisors and retroclines the lower ones. The course indicates it in adults with at most a mild skeletal Class III, without pre-existing compensations, with mild transverse and vertical anomalies and no major esthetic deficit; Boileau adds compensations compatible with the periodontium.

A systematic review of six studies of borderline Class III patients reported more upper incisor proclination and lower incisor retroclination with camouflage, and a better jaw relationship with surgery; a meta-analysis was not possible because the studies differed too much (Alhammadi et al., 2022, Clin Oral Investig, PubMed).

In growing patients, postpone any irreversible decision: a borderline Class III can worsen late.

Orthognathic surgery

Signs pointing toward surgery: a severe skeletal Class III, a major esthetic impact, an associated vertical or transverse anomaly, marked compensations, failed orthopedics. Surgery is performed after growth, but the indication can be raised at the initial workup when the discrepancy is severe, hereditary or endocrine.

Numeric thresholds vary between sources:

• one course cites an ANB below −2° as a surgical indication after growth;
• Boileau gives no ANB threshold and reasons on several criteria, preferring the Wits in high-angle patients;
• Bassigny lists among unfavorable factors a negative overjet without incisor contact with CR = MI, a long face, macroglossia and very strong heredity;
• in a comparison of 20 severe Class III patients treated orthodontically with 20 offered surgery, ANB, the M/M ratio, lower incisor inclination and the Holdaway angle differed most; below an ANB of −4° and a lower incisor angulation of 83°, surgery was almost always carried out (Kerr et al., 1992, Br J Orthod, PubMed).

These figures come from different populations and methods: they are guidelines, not boundaries. Our French guide to orthognathic surgery indications (in French) goes further.

A last, frequent trap: do not extract before choosing between camouflage and surgery. The extraction patterns of the two options are opposite, and an extraction made for one closes the door to the other.

When a general dentist should refer

• any anterior crossbite or edge-to-edge bite in a growing child, whatever the age;
• a family history of Class III with early signs (concave profile, palatal lateral incisors);
• lower incisor recession on a tooth in crossbite;
• an adult with a Class III before any prosthetic or restorative plan on the anterior teeth.

In practice: a checklist

1. Ask about heredity and habits.
2. Compare MI and CR with the De Nevrezé maneuver; if a forward shift exists, quantify it before interpreting SNB, ANB and the Wits.
3. Locate the discrepancy (SNA, SNB, lengths) and compare ANB with the Wits, stating the Wits norm.
4. Record the vertical pattern, the incisor compensations, the transverse plane and asymmetry.
5. Place the patient on the growth curve with skeletal age.
6. Go through the eight criteria and note what is missing.
7. Postpone extractions until the direction (camouflage or surgery) is set.

Frequently asked questions

What is a Class 3 bite?

A Class III bite is one in which the lower first molars and canines sit mesial to the upper ones. The lower incisors are often in front of the upper ones ("underbite"), but they can also meet edge to edge or overlap normally if the teeth compensate. It can be functional, dental or skeletal.

What is the difference between Class III and pseudo-Class III?

A pseudo-Class III is a forward functional shift: the mandible slides forward because of an occlusal obstacle. Guided into centric relation, it moves back to an edge-to-edge incisor contact. In a true Class III, centric relation and maximum intercuspation coincide.

Can a Class III malocclusion be corrected without surgery?

Sometimes. In growing children with a maxillary component, early orthopedics can reduce the discrepancy, and one trial found a lower need for surgery at 6 years after facemask treatment. In adults, camouflage is discussed for mild skeletal discrepancies; severe cases usually call for surgery after growth.

At what age should a Class III be assessed?

As soon as it is noticed. The courses place Class III orthopedics as early as possible, from 4 to 5 years, and any anterior crossbite in a child deserves an orthodontic opinion. Because the mandible grows late, follow-up continues until the end of growth.

With GS Ortho

In GS Ortho, AI places 29 landmarks on the lateral cephalogram; you check and correct them before SNA, SNB, ANB and incisor position feed the Steiner and Tweed analyses. The 12-panel guided diagnostic studio brings the clinical examination, the photos, the cephalometrics and the treatment plan of the same patient together, so a borderline Class III is reasoned case by case. From a panel, "Request an opinion" sends your question, panel attached, to a colleague or supervisor you choose, and the answer stays in the record. Data are hosted at your practice. The GS Ortho interface is currently available in French.

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