ORTHO TRIBUNE | NOv./DEC. 2009 f OT page 1 Trends Fig. 3 3 Slot position If we consider de-rotation as an isolated step, then we know the power applied works on the horizontal plane. In principle, by all brackets with horizontal slots, the arch wire can slip off (Fig. 5). Two factors can avoid this problem. The ligature holds the arch wire into the slot. This effect can support the force direction.6 But this effect can be eliminated if the force direction pulls the arch wire out of the slot. This can happen very frequently with the lingual technique. Using light forces and also small diameter arches make it more difficult and almost impossible to derotate a rotated tooth at the moment of leveling. Contingent on the various force g OT page 4 Leveling mechanics is due to the application of: • adequate inter bracket distance, • light and elastic force, • preformed memory arch wire. One of the advantages of nickel titanium (NiTi) and super elastic arch wire is that we can fill the bracket slot earlier during the course of the treatment plan. In lingual orthodontics compared to labial orthodontics, rotational movement to level a single rotated tooth is not easy to achieve.3 There are several points to consider. Fig. 4a Fig. 3 (above): Rotated teeth move lingually into the shorter length of the arch, with less space for movement. Figs. 4a, 4b (right): The interbracket distance is short with the lingual technique, especially in the lower jaw (b). The only force system that can produce pure rotation (a moment with no net force) is a couple, which is two equal and opposite and parallel forces, but non-collinear.4 The rotational movement depends on the moment of the forces. The moment of the force is equal to the magnitude of the force applied, multiplied by the perpendicular distance of the line of action to the center of resistance. These forces applied to the tooth should produce efficient rotation. However, in buccal orthodontics, rotation movement of rotated teeth can be accomplished even without an exact application of this force system. Memory-shaped pre-formed arches in large cross-sections, filling the slot of the bracket, have good control of the tooth movement and can perform this task within a short amount of time. In labial orthodontics, leveling seems to be easier and can be resolved in less time. Reference the clinical case (Figs. 1 and 2). In the lingual technique, the arch wire could move the teeth in the lingual direction.5 That is the reason why some movements are difficult to achieve, as they are in the labial technique. The problems are: • During the rotational movement, teeth are moved lingually into a shorter length of the arch, with less space for movement (Fig. 3). • The small size of the arch and subsequent short inter-bracket distance (Fig. 4). • Less control of the arch in the bracket slot. The short inter-bracket distance necessarily means that any moment produced across a given bracket will be decreased due to the short lever-arm to the center of rotation. This is more significant in the mandible dental arch because it is more constricted than the maxillary and the incisor mesial-distal width, which is less than the maxillary incisors (Fig. 4b).5 Depending on the available space for de-rotation, it can be necessary to open space as the first step. The second step is the de-rotation. Movement mechanics of a rotated tooth Fig. 4b AD