the canine.
Class I: the upper canine occludes and lies in the embrasure between the lower canine and first premolar (Figure 3.8).
Class II: the upper canine occludes anteriorly and lies in the embrasure between the lower lateral incisors and the canine (Figure 3.9).
Class III: the upper canine occludes posteriorly and lies in the embrasure between the lower first and second premolars (Figure 3.10).
Class II25: the upper canine occludes one‐quarter anteriorly from being in embrasure between the lower lateral incisors and the canine.
Class II50: the upper canine occludes half a unit anteriorly and lies directly over the lower canine (cusp to cusp with the lower canine).
Class II75: the upper canine occludes three‐quarters anteriorly from being in embrasure between the lower lateral incisors and the canine (not a full unit class II).
Class III25: the upper canine occludes one‐quarter posteriorly from being in embrasure between the lower first and second premolars.
Class III50: the upper canine occludes half a unit posteriorly and lies directly between the lower first and second premolars.
Class III75: the upper canine occludes three‐quarters posteriorly from being in embrasure between the lower first and second premolars (not a full unit class III).
Figure 3.5 Incisor relationship class II div I.
Figure 3.6 Incisor relationship class II div II.
Figure 3.7 Incisor relationship class III.
Figure 3.8 Class I canine relationship.
Figure 3.9 Class II canine relationship.
Figure 3.10 Class III canine relationship.
3.4 Andrew’s Six Keys
This classification was developed by Lawrence Andrews, the same orthodontist who developed the preadjusted edgewise appliance. Over time Andrews decided that it was not always possible to achieve a good class I occlusion, therefore the six keys were developed to identify each feature to evaluate why this was not possible. A seventh key was added subsequently because correct tooth size can have an effect on achieving good class I occlusion.
1 Correct molar relationship:The mesiobuccal cusp of the upper first molar occludes in the buccal groove of the lower first molar.The distobuccal cusp of the upper first molar occludes on the mesiobuccal cusp of the lower second molar.
2 Correct crown angulation:All tooth crowns are angulated mesially.
3 Correct crown inclination:Incisors are inclined towards the buccal or labial surface.Buccal segment teeth are inclined lingually; in the lower buccal segments this is progressive.
4 Flat curve of Spee.
5 No rotations.
6 No spaces.
7 Correct tooth size.
4 Aetiology of Malocclusion
This chapter will look at all the different types of causes for the types of malocclusions we see in orthodontics.
4.1 Skeletal Factors
Skeletal factors are categorised into all three planes of space.
4.1.1 Anteroposterior Plane (AP)
Dento‐alveolar compensation is a term used when teeth compensate for the skeletal pattern. For example, a patient may have class I incisors but be presenting with a class III skeletal pattern.
4.1.2 Vertical Plane
Growth rotations occur in the mandible. This is extra growth which can be seen once the patient has stopped growing. There are two types:
Forward growth rotation – decreased lower anterior facial height (LAFH).
Backward growth rotation – increased LAFH.
4.1.3 Transverse Plane
Absolute transverse maxillary deficiency:
This term relates to crossbites and is most commonly seen in class III cases.
Commonly seen in maxillary retrusion, this is where the maxilla is reduced in all three dimensions, which can result in crossbites anteriorly and posteriorly.
Dento‐alveolar compensation:
This term is used when teeth compensate for the skeletal pattern.
In this case transversely, teeth can compensate to result
