During the 1970’s, andrews six keys to normal occlusion came forward after studying models of 120 patients with ideal occlusion.
Andrews considered the presence of these features essential to achieve an optimal occlusion. Andrews six keys to normal occlusion are considered under the following headings:
1. Molar inter-arch relationship
2. Mesio-distal crown angulation
3. Labio-lingual crown inclination
4. Absence of rotation
5. Tight contacts
6. Curve of spee
Molar inter-arch relationship
The mesio-buccal cusp of the upper first molar should occlude in the groove between the mesial and medial buccal cup of the lower first molar. The mesio-lingual cusp of the upper first molar should occlude in the central fossa of lower first molar. The crown of the upper first molar must be angulated so that the distal marginal ridge occludes with the mesial marginal ridge of lower second molar. This is first key out of andrews six keys.
Mesio-distal crown angulation
The second key makes use of a line that passes along the long axis of the crown through the most prominent part in the center of the labial or buccal surface. this line is called the long axis of the clinical crown. This is second key out of andrews six keys.
For the occlusion to be considered normal, the gingival part of the long axis of the crown must be distal to the occlusal part of the line. Different teeth exhibit different crown angulation.
Labio-Iingual crown inclination
The crown inclination is determined from a mesial or distal view. If the gingival area of the crown is more lingually placed than the occlusal area, it is referred to as positive crown inclination. In case the gingival area of the crown is more labially or buccally placed than the occlusal area it is referred to as negative crown inclination.
The maxillary incisors exhibit a positive crown inclination while the mandibular incisors show a very mild negative crown inclination. The maxillary and mandibular posteriors have a negative crown inclination. This is third key out of andrews six keys.
Absence of rotation
Normal occlusion is charecterized by absence of rotation. Rotated posterior teeth occupy more space in the dental arch while rotated incisors occupy less space in the arch. This is fourth key out of andrews six keys.
To consider an occlusion as normal, there should be tight contact between adjacent teeth.
Curve of Spee
A normal occlusal plane according to Andrews should be flat, with the curve of Spee not exceeding 1.5mm. This is sixth key out of andrews six keys.