3.3 Classifying Occlusion: Angle and BSI Incisor Relationships

Classifications of Occlusion & Malocclusion

Angle's Classification (Molar Relationship)

Introduced by Edward Angle, this classification is based on the sagittal anteroposterior relationship of the permanent first molars:

                  Angle Class I (Normal Molar Relation)
                 ┌──────────────────────────────────────┐
Maxillary M1:    │   MB Cusp aligns with MB Groove      │
Mandibular M1:   │   of the Mandibular First Molar      │
                 └──────────────────────────────────────┘

   Angle Class II (Distoclusion)           Angle Class III (Mesioclusion)
┌──────────────────────────────────┐    ┌──────────────────────────────────┐
│ Mandibular MB groove is DISTAL   │    │ Mandibular MB groove is MESIAL   │
│ to the Maxillary MB cusp.        │    │ to the Maxillary MB cusp.        │
└──────────────────────────────────┘    └──────────────────────────────────┘
  ├── Division 1: Proclined U1           └── Associated with anterior
  │               Increased overjet          crossbite / reverse overjet
  └── Division 2: Retroclined U1
                  Deep overbite

• Class I (Neutroclusion): The mesiobuccal (MB) cusp of the permanent maxillary first molar occludes in the mesiobuccal groove of the permanent mandibular first molar.
• Class II (Distoclusion): The mesiobuccal groove of the mandibular first molar is positioned distal to the mesiobuccal cusp of the maxillary first molar.
  • Class II Division 1: Maxillary central incisors are proclined, resulting in an increased overjet (>3.5 mm) and often an incomplete or deep overbite.
  • Class II Division 2: Maxillary central incisors are retroclined, while maxillary lateral incisors are typically proclined or overlapped labially. The overjet is normal or slightly decreased, and the overbite is severely increased (deep traumatic bite).
• Class III (Mesioclusion): The mesiobuccal groove of the mandibular first molar is positioned mesial to the mesiobuccal cusp of the maxillary first molar; characteristically associated with a reversed overjet (anterior crossbite) and mandibular prognathism.

British Standards Institute Incisor Classification (BSI 4492)

Because Angle's classification relies on first molars that may be absent, drifted, or restored, the British Standards Institute (BSI 4492) incisor classification is the standard orthodontic and restorative diagnostic framework used in the UK and on the GDC ORE examination. It classifies malocclusions based strictly on the relationship of the mandibular incisal edges to the cingulum plateau (middle third of the palatal surface) of the maxillary central incisors.

Overjet, Overbite and the Vocabulary Examiners Use

Classification is only half the diagnostic language. Overjet is the horizontal distance from the labial surface of the lower incisor to the labial surface of the upper incisor, measured parallel to the occlusal plane; a normal value is 2 to 4 mm. Overbite is the vertical overlap of the lower incisor by the upper incisor, conventionally expressed as a fraction or percentage of the lower incisor crown height; one-third overlap is average. An overbite is complete when the lower incisor edge contacts either the palatal surface of the upper incisor or the palatal mucosa, and incomplete when there is no contact at all. An anterior open bite is a negative vertical overlap with no incisor contact in the intercuspal position.

These are separate measurements and can vary independently, which is where candidates lose marks. A Class II division 1 case can have a deep, average or open bite; the division tells you about incisor inclination, not about the vertical dimension. Conversely, a deep overbite is not diagnostic of Class II division 2 unless the upper central incisors are retroclined.

Crossbites and Displacement

A crossbite exists when the normal buccolingual relationship is reversed, and it is described by location and extent: anterior or posterior, unilateral or bilateral, and involving one tooth or a segment. The critical clinical question is whether there is an associated mandibular displacement — a shift of the mandible on closure from the retruded contact position into the intercuspal position because of a premature contact. A unilateral posterior crossbite with displacement produces an apparent facial asymmetry that disappears when the mandible is guided into retruded contact, and it is the presence of displacement, not the crossbite itself, that usually justifies interceptive treatment. A scissor bite (Brodie bite) is the opposite deviation, with the palatal cusps of the upper posterior teeth occluding buccal to the buccal cusps of the lower teeth.

How the Two Systems Are Combined in a Diagnosis

A complete UK occlusal diagnosis states the incisor classification first, then the skeletal pattern, then the molar and canine relationships, then the vertical and transverse features. For example: "Class II division 1 incisor relationship on a mild Class II skeletal base with average vertical proportions; molars half a unit Class II bilaterally; overjet 7 mm; overbite increased and complete to tooth; upper centreline 2 mm to the right with a lower centreline shift secondary to a unilateral crossbite with displacement." Examiners construct SBAs by giving you these descriptive elements and asking for the classification, or by giving the classification and asking which finding is inconsistent with it. A frequent distractor is an item that describes a molar relationship and invites you to state the incisor class — the two do not have to agree, because tooth size discrepancies, early loss of deciduous teeth and mesial drift all uncouple molar from incisor relationships.

Why the BSI Classification Is the UK Standard

Angle's system assumes that both first permanent molars are present, unrestored and in their correct anteroposterior position. In an adult population with extraction histories, mesially drifted molars and restored contact points, that assumption frequently fails, and the molar relationship then reflects a history of tooth loss rather than the underlying skeletal and dental pattern. The incisor relationship remains interpretable in those patients, which is why BSI 4492 is the classification used in UK undergraduate teaching, in orthodontic referral letters and in the index of orthodontic treatment need. Learn both systems, but answer in BSI terms unless the stem explicitly asks for the Angle class.