The ability of a specific bracket system to express torque is dependent of the play of the wire interface and the bracket slot, the wire stiffness, and the ligation system.
This video show that passive self ligating bracket and most active self ligating brackets has more play at the wire bracket interface than one can expect. The test is done on a .020 x .025 SS speedwire and a .021 x .021 SS Dwire.
SPEED™ bracket with its active Niti spring clip is the only bracket that is maintained stable on the wire. Therefore, the prescribed torque of the bracket will be expressed by the spring clip homing action on the archwire. A study (AJDO 2007, v132) has shown that the Speed spring clip deliver 1,2 N on the archwire. No difference were found between "as received" and used Speed spring clip. The same study has shown that aged CrCo spring clip (InOvation R,InOvation C) have lost up to 50% of its force after use. The clinical rellevance is the inability of the relaxed clip to apply forces forces on the wire due to aging and inadequate engagement of the wire into the bracket slot may arise in the last stages of treatment when rectangular wires are inserted.
Passive self ligating bracket can not express the prescribed torque efficiently because of the torque play at the wire interface. This is quite obvious on this video.
QuicKlear and Quick 2.0 from Forestadent, which are the same bracket as Nexus from Ormco, is free to rotate on the square .021 x .021 SS Dwire and has excessive torque play on a .020 x .025 SS Speedwire.
Clarity SL (smart clip) show up to ± 20° of play. Empower has surprisingly less torque play than other SLB except SPEED. This may be related that despite the same design of InOvation R, the spring clip engage the wire more than the clip of InOvation R does. However, since the clip material is the same, we may expect a similar clip relaxation when ti it ages.