4.2 Orthodontics

Classification of Malocclusion

Angle Classification

The Angle classification is based on the mesiodistal relationship of the mesiobuccal cusp of the upper first permanent molar to the buccal groove of the lower first permanent molar:

Incisor Classification

The British (incisor) classification is based on the lower incisor edge to the cingulum plateau of the upper incisors:

• Class I — lower incisor edge occludes with or below the cingulum plateau (overjet 2-4 mm).
• Class II Division 1 — lower incisor edge lies behind the cingulum plateau; upper incisors proclined, increased overjet.
• Class II Division 2 — lower incisor edge lies behind the cingulum plateau; upper central incisors retroclined, deep overbite.
• Class III — lower incisor edge lies in front of the cingulum plateau; reduced or reverse overjet.

Skeletal Pattern

The skeletal pattern is assessed in three planes:

1. Anteroposterior (AP) — Class I, II, or III; determined by the relationship of the maxilla to the mandible (often reflected in the ANB angle on cephalometric analysis).
2. Vertical — average, increased (high angle, anterior open bite tendency) or decreased (low angle, deep overbite). The Frankfort-mandibular plane angle (FMPA) is the standard measure.
3. Transverse — symmetry or asymmetry; posterior crossbite may indicate a transverse skeletal discrepancy.

Skeletal pattern guides treatment modality: a marked Class II skeletal base with a retrusive mandible is best treated with a functional appliance during active growth; an adult with the same pattern may require orthognathic surgery.

Overjet, Overbite, and Crossbite

• Overjet (OJ) — horizontal distance from the labial surface of the lower incisor to the labial incisal edge of the upper incisor. Normal 2-4 mm. Increased OJ carries a higher risk of traumatic dental injury.
• Overbite (OB) — vertical overlap of the upper and lower incisors. Normal 2-4 mm (30-50% overlap). Deep overbite >50%; anterior open bite = no vertical overlap.
• Anterior crossbite — one or more upper incisors occlude lingual to the lower incisors; may cause tooth wear and gingival recession.
• Posterior crossbite — upper buccal cusps occlude inside the lower buccal cusps; unilateral crossbite often indicates a mandibular shift and may warrant early maxillary expansion.

Index of Orthodontic Treatment Need (IOTN)

IOTN (Brook and Shaw, 1989) assesses treatment need and is used for NHS funding decisions in the UK. It has two components:

Dental Health Component (DHC) — Grades 1-5

The DHC records the worst feature of the malocclusion using the MOCDO hierarchy (Missing teeth, Overjet, Crossbite, Displacement, Overbite):

Aesthetic Component (AC) — Grades 1-10

The AC uses a 10-photograph scale (1 most attractive, 10 least attractive). NHS eligibility normally requires DHC 4 or 5, or DHC 3 with AC 6 or above.

Aetiology of Malocclusion

Malocclusion arises from the interaction of genetic and environmental factors:

• Skeletal — inherited jaw discrepancy (e.g. Class III mandibular prognathism).
• Dental — crowding, spacing, anomalous tooth number (hypodontia, supernumeraries).
• Local — early loss of primary teeth, retained primary teeth, ectopic eruption, supernumeraries, habits (digit sucking → proclined uppers, anterior open bite).
• Environmental — mouth breathing, airway obstruction.

Growth Modification and Timing

Growth modification is only effective during active growth:

• Functional appliances (e.g. Twin Block) for Class II with retrusive mandible — peak effect at the pubertal growth spurt (peak height velocity; clinically, before menarche in girls and at the start of the voice break in boys).
• Maxillary expansion (rapid maxillary expansion, RME) for posterior crossbite — most effective in the early mixed dentition before the midpalatal suture fuses.
• Once growth is complete, skeletal discrepancies require orthognathic surgery combined with fixed appliances.

Interceptive Orthodontics

Interceptive treatment in the mixed dentition can simplify or eliminate later comprehensive treatment:

• Crossbite correction with an upper removable appliance before permanent canine eruption.
• Space maintenance after early loss of a primary molar (band and loop, Nance appliance).
• Extraction of retained primary teeth preventing ectopic permanent eruption.
• Digit-sucking habit cessation to permit spontaneous overjet reduction.
• Serial extractions for severe crowding in Class I cases.

Appliances and Retention

Removable vs Fixed Appliances

Retention and Relapse

Retention is the phase after active treatment that maintains tooth position. Teeth have a lifelong tendency to relapse due to gingival and periodontal fibres, occlusal forces, and continued growth. Protocols include:

• Bonded retainer — wire bonded lingual to lower incisors (often lifelong).
• Removable retainer — Hawley or vacuum-formed; full-time for 6-12 months, then nights only, ideally indefinitely.

Relapse is expected without indefinite retention; patients must be warned at the consent stage.