Effect of molar rotation on arch length

The aim of this study was to evaluate the effect of maxillary first molar rotation on arch length in patients whose Class II, division 1 malocclusion was treated with a straight-pull headgear. The sample included 19 children who represent a subset of a population enrolled for 2 years in a prospective clinical trial of Class II treatment. The inner bow was adjusted at monthly visits and the average distalizing force was 14-16 oz per side. Measurements were made with digital calipers on casts obtained at baseline and at a time point averaging 16 months (SD 3 months) into treatment. Molar movement was related to a coordinate system referenced to the midpalatal raphae and the occlusal plane through a palatal plug that could be transferred to successive casts of the same patient. Distances from the cusp tips were measured to the reference system. The distal and lateral displacement, and the rotation of the molars over time were then computed. High correlations were observed between molar rotation and tooth displacement (that is the net differential movement between the mesiobuccal - MB - and mesiopalatal - MP - cusps): r=0.9, p=0.0001 (left side); r=0.8, p=0.0001 (right side). The average rotation of the molar was 15.4 degrees (SE+/-3.80) on the left side and 10.8 degrees (SE+/-2.20) on the right side. The average differential displacements between the MB and MP cusps were 1.04 mm (SE+/-0.29) (left) and 0.73 mm (SE+/-0.23) (right). The differences between the two sides were not statistically significant. The molar rotation did not correlate with any of the following parameters: initial tooth position; initial distance between the MB and MP cusps as a correlate of molar anatomy; transverse (mostly buccal) movement of the MB and MP cusps; distal movement of the tooth, as measured by the total displacement of the MB and MP cusps. This study qualifies the relationship between molar rotation and increase in arch length. The mean total distal movement of the molar was larger (nearly 3 mm) than the average contribution of molar rotation to space gain (about 1 mm), suggesting that the distal movement of the tooth was a combination of rotation and translation (or tipping). The contribution of the molar rotation is more significant with a smaller total displacement of the tooth.