A definition of the optimal occlusion in the permanent dentition as defined by Lawrence F. Andrews in 1972.They have significant clinical implications for routine orthodontic therapy.
Andrews L. F. The six keys to normal occlusion. Am J Orthod 1972;62:296–309.
Andrews' six keys to occlusion
Molar relationship: 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.
Crown angulation (mesio-distal tip): 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.
• Crown inclination (labio-lingual, bucco-lingual): the incisors are at a sufficient angulation to prevent overeruption.
• Upper posterior teeth: the lingual tip is constant and similar from canine to second premolar and increased in the molars.
• Lower posterior teeth: the lingual tip increases progressively from the canines to the molar.
Rotations are not present.
There are no interdental spaces.
There is a flat plane of occlusion.