MOMENTS AND FORCES DELIVERED BY TRANSPALATAL ARCHES FOR SYMMETRICAL FIRST MOLAR ROTATION

The moments and forces delivered by round transpalatal arches of steel and of beta-titanium (TMA) for sym metrical derotation of molars were studied in laboratory experiments. Three sizes of arches were tested in two series. In the first series, the degree of activation was check ed for symmetry in a computer-based strain-gauge measuring system, In the second series, the activation was carried out in a way simulating clinical use, The mesio-distal and transverse forces and the derotatin g moments at full activation and during derotation in steps of 5 degre es were measured. At full activation, the steel arches delivered relat ively large moments which, however, decreased rapidly during deactivat ion, The TMA arches had a larger working range, It was not possible to achieve full symmetry of the moments at the two ends of the arch. The difference of the two moments resulted in forces acting on the two an chorage teeth in a mesio-distal direction. These forces were generally small but could reach clinically relevant magnitude, The derotation r esulted in a contractive force of up to 2.7 N which has to be compensa ted for by expansion, The mode of activation simulating clinical use r esulted in reasonably constant forces and moments. The use of a vice t o hold the arch during activation was found to be of great help and is recommended in the clinical setting, Because of the larger working ra nge, TMA arches are recommended if substantial derotation is needed.