An improved transpalatal bar design. Part I. Comparison of moments and forces delivered by two bar designs for symmetrical molar derotation.

Moments and forces delivered during symmetrical derotation of upper molars by 10 Goshgarian-type (GTPB) and 10 Zachrisson-type transpalatal bars (ZTPB) were measured in laboratory experiments using a computer-based strain gauge. The bar passivity in sagittal, transverse, and vertical planes was first assessed at the measurement apparatus. Then each end of the 20 passive bars was symmetrically activated by 10 mm in the sagittal plane using a template. The activated bars were placed into lingual attachments of the measuring apparatus, and three consecutive measurement steps were done for each bar. Measurements were made when the attachments were at 0 degrees, 5 degrees, and 10 degrees of deactivation. The mesiodistal (sagittal) forces, the horizontal forces, and the moments of rotation at the right and left attachments were measured at each step. The horizontal forces and the moments of rotation of the two designs had statistically significant differences. Greater moments of rotation were produced by the GTPB. The ZTPB produced significantly lower contractive horizontal forces than did the GTPB at 5 degrees and 10 degrees of deactivation.