A large number of different self-ligating bracket (SLB) systems are available on the market today. The properties of an individual system depend on the materials used for the bracket, as well as on the ligation mechanism. Usually, SLBs are divided into two main categories: active and passive. Theoretically, no force is exerted by the clip itself when the archwire is ligated in passive systems ( Fig. 3.1 ). However, the clip in active systems is designed to actively “press” the wire into the bracket slot upon locking of the mechanism ( Fig. 3.2 ). Some manufacturers additionally subdivide the systems into semiactive or interactive locking mechanisms. In these, archwires are not actively forced into the slot until the wire has reached a certain dimension. The proposed advantage of passive systems lies in the reduction of friction.12 However, to date, an actual reduction of friction between the archwire and bracket has only been confirmed in in-vitro experiments.3 The alleged advantage of reduced friction in passive systems comes at the expense of inferior biomechanical properties; the lack of active ligation often reduces rotational and torque control in passive SLBs. The manufacturers therefore offer different archwire sequences and dimensions for the systems: rectangular archwires with small dimensions and high elasticity, such as a 0.014 × 0.025 superelastic wire, are introduced early on in the treatment to fill the slot as much as possible.4 The aim is to rapidly progress to thicker archwires, ideally completely filling the slot early on in the treatment so that all of the preprogrammed values are transferred to the teeth as soon as possible.
Using either traditional tie-wing brackets in combination with elastomeric or steel ligatures or SLBs with an active locking mechanism leads to active engagement of the wire into the slot. Given appropriate slot and bracket dimensions, this allows outstanding rotational and torque control. However, wires with very rounded edges provide less torque control, regardless of how well they are engaged ( Fig. 3.3 ).