The effects of a newly designed twin-slot bracket on severely malpositioned teeth--a typodont experimental study

The aims of this study were to design a twin-slot bracket featuring two horizontal slots and to examine its efficiency in tooth displacement. Based on the structure of a traditional edgewise bracket, an additional slot was added to a twin-slot bracket and the prototype products were fabricated for the typodont experiments. The orthodontic correction of malpositioned canines was conducted on a typodont to examine the efficiency of the twin-slot bracket in tooth displacement compared with a single-slot edgewise bracket. Three modalities of tooth movement requiring a heavy force moment, namely, axial rotation, mesiodistal tipping, and bodily translation, were conducted. The canine positions before and after simulation were measured and the changes identified. Statistical analysis was undertaken using a t-test to determine the significance of the differences in canine repositioning between the two bracket types. The results showed that in the twin-slot bracket group, the treatment changes in the canine position by derotation and uprighting were 40 +/- 3 and 25 +/- 2 degrees, respectively, compared with 20 +/- 5 and 10 +/- 2 degrees in the edgewise group (P < 0.01). When retracted into an extraction space with an initial 10 degrees of mesial tip, the mesiodistal angulation of the canines in the twin-slot bracket group remained unchanged while in the edgewise group the canines became distally tipped by 5 +/- 2 degrees (P < 0.01). The twin-slot bracket significantly increased the bracket width without reducing the interbracket span and therefore can generate increased force moments within the bracket, leading to an improved manipulation in tooth repositioning.