A longitudinal study of arch size and form in untreated adults.

Adulthood-the lengthy phase following attainment of biologic maturity-often is perceived as a period of "no change" or one of slow deterioration. Recent skeletodental studies discount this stereotype. Changes in arch size and shape were studied here in a longitudinal series of 60 adults with intact dentitions. Full-mouth study models were taken at about 20 years of age and again at about 55 years. Some variables-particularly those between arches (incisor overbite and overjet, molar relationship) and mandibular intercanine width-remained age-invariant. In contrast, all other measures of arch width and length changed significantly (P < 0.01): Arch widths increased over time, especially in the distal segments, whereas arch lengths decreased. These changes significantly altered arch shape toward shorter-broader arches. The data suggest that changes during adulthood occur most rapidly during the second and third decades of life, but do not stop thereafter. Possible mechanisms driving these changes in tooth position are discussed.