Andrews' Six Keys To Occlusion

Andrews' Six Keys (1972) to normal (or optimal) are a widely quoted set of static occlusal goals for tooth relationships in the intercuspal position:

Correct interarch relationships

Crown angulation(tip)

Crown inclination(torque)

No rotations

Tight contact points

Flat curve of Spee

The distal surface of the disto-buccal cusp of the upper first permanent molar occludes with the mesial surface of the mesio-buccal cusp of the lower second molar.

The mesio-buccal cusp of the upper first permanent molar falls within the groove between the mesial and middle cusps of the lower first permanent molar. The mesio-lingual cusp of the upper first molar seats in the central fossa of the lower first.
The premolars enjoy a cusp-embrasure relationship buccally, and a cusp fossa relationship lingually.
Maxillary Canine has a cusp-embrasure relationship with Mandibular Canine & 1st Premolar. The cusp tip is slightly mesial to embrasure Maxillary Incisors overlap Mandibular Incisors & midlines of arches match.

The gingival part of the long axis of the crown is distal to the incisal part of the axis. The extent of angulation varies according to tooth type.
Long axis of crown measured from line 90° to occlusal plane.
The term crown angulation refers to angulation (or tip) of the long axis of the crown, not to angulation of the long axis of the entire tooth.
The gingival portion of the long axis of each crown was distal to the incisal portion, varying with the individual tooth type.
The long axis of the crown for all teeth, except molars, is judged to be the mid- developmental ridge, which is the most prominent and innermost vertical portion of the labial or buccal surface of the crown. The long axis of the molar crown is identified by the dominant vertical groove on the buccal surface of the crown.

Long axis of crown measured from line 90° to occlusal plane.
Crown tip is expressed in degrees, plus or minus. The degree of crown tip is the angle between the long axis of the crown (as viewed from the labial or buccal surface) and a line bearing 90° from the occlusal plane.
A ‘plus reading’ is assigned when the gingival portion of the long axis of the crown is distal to the incisal portion and a ‘minus reading’ when the gingival portion of the long axis of the crown is mesial to the incisal portion.

Labiolingual or Buccolingual inclination. crown inclination is determined from the mesial or distal perspective. Angle formed by a line 90 degree to occlusal plane & a line tangent to bracket site.
A + reading is given if the gingival portion of the tangent line (or of the crown) is lingual to the incisal portion, A - reading is recorded when the gingival portion of the tangent line (or of the crown) is labial to the incisal portion.
ANTERIOR CROWN INCLINATION. In upper incisors + crown inclination. In lower incisors -crown inclination The average inter-incisal crown angle - 174°.
Properly inclined anterior crowns contribute to normal overbite and posterior occlusion, when too straight-up and -down they lose their functional harmony and overeruption results.
If the inclination of the anterior crowns is not sufficient, space, in treated cases, is often incorrectly blamed on tooth size discrepancy.

POSTERIOR CROWN INCLINATION— UPPER. A minus crown inclination for each crown from the Upper canine through the Upper-2nd PM. A slightly more negative crown inclination existed in the Upper-1st & 2nd molars.
POSTERIOR CROWN INCLINATION— LOWER. A progressively greater "minus" crown inclination existed from the lower canines through the lower second molars.
As the anterior portion of an upper rectangular arch wire is lingually torqued, a proportional amount of mesial tip of the anterior crowns occurs. The ratio is approximately 4:1. For every 4° of lingual crown torque, there is 1 ° of mesial convergence of the gingival portion of the central and lateral crowns.

The fourth key to normal occlusion is that the teeth should be free of rotations, Because if molars rotated, would occupy more space than normal. So it will create a situation unreceptive to normal occlusion. If incisors rotated less space.

Contact points should abut unless there is a tooth size discrepancy in mesiodistal crown diameter.

The planes of occlusion found in the non orthodontic models ranged from flat to slight curve of spee. Andrew believes that a flat plane should be treatment goal as a form of over treatment since there is natural tendency for the curve of spee to deepen with time.
The mandibular curve of spee should not be deeper than 1.5 mm.