Investigation of premolar rotation in a group of 15-year-old Norwegian children.

The first aim of this study was to devise a meaningful, objective, and reproducible method of measuring premolar rotation on study casts. A new method of measurement was devised, using standardized arch forms as the reference line superimposed on the study casts. The method error of the difference in two determinations was calculated as +/- 2.95 degrees. Using this method, premolar rotations were measured on study casts at 15 years of 203 children enrolled in the Nittedal Growth Study. Histograms of the frequency distribution for each premolar were drawn and the mean rotations compared. Correlation coefficients were then calculated to determine if any intra-quadrant, intra-arch, or inter-arch relationships existed. Significant correlations were found between contra-lateral teeth in the same arch (P less than 0.001) and to a lesser degree between premolars in the same quadrant. Significant inter-arch correlation was found between left upper and lower second premolars (P less than 0.001). Surprisingly, some cross-arch relationships were found between the upper left second premolar and lower right second premolar (P less than 0.001), and between the upper right second premolar and lower left second premolar (P less than 0.01). It is suggested that aetiological factors in premolar rotation are genetic and, to a lesser extent, local factors. The significant correlation found between the upper and lower left second premolars suggests that occlusal factors may have a role to play.