THE EFFECT OF A TRANSPALATAL ARCH FOR THE CORRECTION OF FIRST MOLAR ROTATION

The effects of a transpalatal arch for the symmetrical derotation of r otated first molars were evaluated in 50 children, 8-13 years of age. The positions of the molars were compared with those in 34 individuals , aged 12-18 years, with normal occlusion. Prefabricated (GAG) stainle ss steel arches were used for 60-198 days (median time 122 days). The effect was recorded with a measuring microscope on dental casts from b efore and after the treatment. Molar positions were determined from th e tips of the four cusps of the tooth in relation to a coordinate syst em based on palatal reference points. The centre of rotation of the mo lars during derotation were calculated from the movement of their cusp s.Before derotation the first molars were significantly mesiopalatally rotated compared with the normal occlusion group. The derotation over compensated the initial rotation. In about two-thirds of the cases the mesiobuccal cusp of the molar moved distally during the derotation. I n the remaining cases it moved mesially or remained unchanged. The med ian distal movement was 0.3 mm on the right and 0.5 mm on the left sid e. Because many molars moved mesially, on average there was no gain in space in the dental arch from the derotation. The location of the cen tre of derotation varied widely but it was on average located midway b etween the distobuccal and distopalatal cusps. In most cases the derot ation resulted in a small, unintended, expansion. The study showed tha t mesiopalatally rotated first molars can effectively be derotated wit h a transpalatal arch. The effect on the mesiodistal position of the m esiobuccal cusp, and particularly with regard to space gain, is, howev er, unpredictable.