Methods: The prototype of the twin-slot brackets were fabricated featuring two parallel slots on the facial surface of the bracket. The typodont plastic canine bonded with either twin-slot or single-slot bracket is placed within the silicon bath simulating alveolar bone tissue. The canine bonded with single slot bracket was engaged with a single .016 round, and a single .016 x .022 rectangular wire respectively; The canine bonded with twin slot bracket was engaged with two round wires (.016 and .018ss). The canine was retracted by pulling forces of 10 CN, 15 CN and 20 CN respectively. A laser sensor for displacement (LK-030, Keyence, Japan) was placed facing the moving canine to measure the changes of canine position, and a computerized system connected to the Silicon/Sensor calculated the velocity to indicate FR during canine movement .
Results: With pulling force of 10 CN, 15 CN and 20 CN, the average velocity of twin slot bracket group with round wires (1.07±0.2mm/sec) was lower than single slot bracket group with rectangular wires (1.21±0.3 mm/sec) and the single slot bracket group with round wires (1.26±0.3 mm/sec) (P <0.001). With increment of pulling forces, the average velocity of twin slot bracket with round wires increased significantly (P <0.01), whilst the velocity increase of single slot bracket with rectangular wires was insignificant (P >0.05).
Conclusion: Indicated by movement velocity, twin slot bracket with two round wires confronted with less frictional resistance than conventional single slot bracket with rectangular wire when retracted with sliding mechanism.