Three-dimensional change of molar position during alignment and leveling with PASS and MBT™: a randomized controlled trial

Introduction: Molar tip back bends added on steel wire is a classical anchorage control method used in Tweed Edgewise and Begg technique. With the development of straight wire appliance (SWA), especially the wide use of Nickel-titanium (NiTi) wire makes bending up impractical in the first stage of the treatment. However, mesial movement of molars occurs when fine NiTi wire was engaged into the molar buccal tube with forward tipping prescription. This single-center 2-arm parallel randomized clinical trial aimed to compare the physiologic anchorage spee-wire system (PASS) vs MBT™ straight-wire system with regards to their capacity of anchorage control during alignment and leveling.Method: The sample consisted of 64 CL I/ CL II orthodontic patients who underwent maxillary first premolar extraction. Minimal randomization was used to select the sample. Two bracket systems were used—PASS and MBT™. The digital study models included three time points: pre-treatment(T0), the first 3 months during treatment(T1), and the first 6 months during treatment(T2). Study models of different time point were superimposed and movements of maxillary first molar, canine, and central incisor were evaluated. Results: In the first 6 months of the treatment, the maxillary first molars were tipped distally (2.67±4.59°) and moved distally(0.03±1.02mm) in the PASS group; They were tipped mesially(1.95±3.04°) and moved mesially (0.67±0.79mm) in the MBT™ group. Vertically, the maxillary first molars were slightly extruded in the PASS group – the mesial cusp 0.50±0.73mm and the distal cusp 0.18±0.67mm; however, they were intruded in the MBT™ group – the mesial cusp 0.19±0.52mm and the distal cusp 0.05±0.56mm. Besides molars, the maxillary canines also exhibited treatment difference– the inter-canine width was increased by 1.31±2.00mm in the PASS group in contrast to 2.37±1.72mm in the MBT™ group. No other difference was found. Conclusion: Compared with MBT™, PASS was better at preserving maxillary anchorage during alignment and leveling.