Understanding, planning, and managing tooth movement: orthodontic force system theory.

A simple force may cause translation and/or rotation of the tooth upon which it acts. The closer the line of force to the tooth's center of resistance, the greater is the proportion of translation relative to rotation, and vice versa. When a tooth is subjected to a tipping moment, strain is concentrated in the areas of the alveolar crest and root apex. This may explain how a light force can tip a tooth readily, while translation, involving a more even distribution of strain throughout the length of the root, requires more force with little or no moment. It is difficult to sustain a desired combination of rotational and translational movement over any significant distance. Couples induce rotation only. Couples and simple forces can be used in combinations to effect movements that are not possible with either one alone. Molars provide anchorage for incisor intrusion. This "vertical anchorage" can be reinforced by elastics producing moments opposite those produced by arch wire anchor bends. Calculation of the elastics' contribution provides an indication of the intrusive force that may be applied to incisors without reciprocal molar tipping. The common use of the term center of rotation in orthodontics may be inappropriate and/or confusing. It may be more accurate and straightforward to view most tooth movements as combinations of linear translation with rotation about the center of resistance.