Orthodontic Treatment of Anterior Open Bite: Differential Diagnosis and Intrusion Mechanics

Anterior open bite (AOB) — defined as the absence of vertical contact between the upper and lower anterior teeth at maximum intercuspation (ICP), with negative overbite measured in millimeters — is one of the most therapeutically challenging malocclusions in orthodontics. Its complexity stems from its frequently multifactorial nature: the same clinical presentation can involve a dentoalveolar component (extrusion of posterior molars, failure of incisor eruption), a skeletal component (hyperdivergent pattern with an increased mandibular plane angle, excessive vertical development of the lower third of the face), and a neuromuscular component (persistent harmful habits, atypical swallowing with tongue interposition, mouth breathing). Correct differential diagnosis between these components — not possible without structured cephalometric analysis and functional clinical examination — is the prerequisite for planning rational treatment with a predictable prognosis.

Cephalometric analysis of AOB must quantify the vertical skeletal component through specific parameters: the mandibular plane angle (SN-GoGn, normal 32° ± 5°; values >37° indicate skeletal hyperdivergence), lower anterior facial height (LAFH, ANS-Me distance, normal 65-70 mm in adults — increased values indicate maxillary vertical excess or reduced vertical condylar growth), and the ratio of posterior to anterior facial height (Jarabak's index, normal 62-65%; values <60% indicate hyperdivergent mandibular rotation). The distinction between dentoalveolar AOB (tilted occlusal plane with molar extrusion, normal or mildly increased mandibular plane angle) and skeletal AOB (structural hyperdivergence with excessive vertical maxillary growth and/or deficient condylar growth) radically affects treatment choice: dentoalveolar AOB responds to orthodontic intrusion mechanics, while moderate-severe skeletal AOB in adult patients requires orthognathic surgery.

Orthodontic mechanics for correcting dentoalveolar AOB rely on the principle of posterior segment intrusion — upper and lower molars contribute 60-70% of lower anterior facial height through their crown height, and intruding them by 2-3 mm produces counterclockwise mandibular rotation that closes the open bite both anteriorly and in the lower third of the face. Before TADs were introduced, molar intrusion mechanics using only teeth as anchorage (MEAW multiloop wires, intrusive bite-blocks) were technically demanding and produced dental tipping side effects. TADs — mini-screws placed in the palate (for maxillary intrusion) or in the buccal interdental region (for mandibular intrusion) — have turned intrusion mechanics into a clinically reliable procedure, allowing intrusive forces applied directly to the wire or molar via mesh elastics that bypass the anterior teeth.

The TAD-assisted protocol for maxillary molar intrusion has been standardized by several research groups. One of the most documented techniques involves placing two paramedian palatal mini-screws between the second premolars and first maxillary molars, connected via a transpalatal bar (TPA) or a modified palatal arch acting as rigid anchorage for the intrusive forces. Applied forces of 150-200 gf per side produce molar intrusion of 0.5-1 mm/month in the first 6-8 weeks, progressively slowing due to alveolar bone resorption around the intruding molar and periodontal ligament remodeling. Average results documented by Sherwood's meta-analysis (Am J Orthod Dentofacial Orthop, 2020) on 18 studies using TADs for AOB show average vertical closure of 3.8 mm with 73% correction success at 1 year; relapse — AOB's main problem — is 25-35% at 3 years in cases treated with orthodontics alone.

Early interception of AOB in mixed dentition is indicated when an identifiable dental cause is present (digit-sucking habit prolonged beyond age 4, persistent atypical swallowing) which, if eliminated, allows spontaneous open bite closure through the eruptive forces of the incisors. The optimal timing for eliminating the harmful habit is between ages 4 and 6, before the habit consolidates a structured swallowing pattern and before muscular forces stably shape the maxilla. Reinforcing the cingulum with an anti-tongue-thrust crib — a fixed device physically preventing tongue interposition during swallowing — is effective at eliminating the habit but requires concurrent collaboration with a speech therapist for swallowing retraining. Spontaneous improvement after eliminating the habit is documented in 60-75% of mild AOB cases in mixed dentition, while severe AOB (>5 mm) or with a skeletal component tends to persist even in mixed dentition.

Indications for orthognathic surgery for skeletal AOB in adults follow combined criteria: the extent of hyperdivergence (mandibular plane angle >42°, LAFH >75 mm), the extent of AOB (>4-5 mm), the patient's aesthetic expectations, and response to preoperative orthodontic decompensation treatment. The surgery of choice is typically maxillary setback with posterior impaction (Le Fort I with closure of the posterior segment) — which rotates the mandible anteriorly, reducing the occlusal plane and closing the open bite — possibly combined with mandibular osteotomy (BSSO) for correcting concurrent sagittal discrepancies. Relapse rates for orthognathic surgery for AOB are 15-25% at 5 years depending on the osteosynthesis system (rigid titanium plates vs. wire) and preoperative cephalometric characteristics: a severe hyperdivergent pattern with low posterior facial height is the most important relapse risk factor.