Providing adequate incisor torque

Torque control is the weakness of the preadjusted appliance system, and any system which is based on the edgewise bracket. There are three factors, and because of these, there does not seem to be a single set of torque values that will solve the needs of all patients:

1. Approximately a 1-mm segment of rectangular steel wire is placed in a bracket of about the same dimension. This is required to carry out a rather difficult tooth movement, which involves moving an entire portion of the root through alveolar bone (Fig. 10.8).

2. A full-size wire is normally not used because such wires do not slide efficiently through the posterior bracket slots. In order for sliding mechanics to be effective, the authors use a .019 x .025 wire in the .022 slot. This reduces the effectiveness of the rectangular wire, relative to torque control (Fig. 10.8).

3. The upper and lower anterior torque needs of patients vary greatly.

With most Class I and Class II patients, there is a tendency for upper incisors to be reiroclined and lower incisors to be proclined. In Class III cases, the opposite tendency occurs. Because the majority of patients are either Class I or Class II in many practices, the general tendency in orthodontic appliances is to place additional palatal root torque in the upper incisor brackets and additional labial root torque in the lower incisor brackets.

When designing the MBTIM system, the authors chose to add an additional 10° of palatal root torque to the upper central incisor brackets, an additional 7° of palatal root torque to the upper lateral incisor brackets, and an additional 5° of labial root torque to the lower incisor brackets. This additional torque, relative to the research findings, is helpful. However, it is frequently necessary to also add torque bends to the rectangular steel wire in the incisor regions. In most Class II cases and many Class 1 bimaxillary protrusive cases, it is beneficial to add an additional amount of palatal root torque to the upper archwire (up to 20°) and an additional 10-15° of labial root torque to the lower archwire (Fig. 10.9). In general, the torque features in the MBT™ brackets, and appropriate torque bends in the archwires, where necessary, help to make the upper anterior segment bigger and the lower anterior segment smaller, thus improving tooth fit within the 60% group (CaseTC, p. 194).

A .019/.025 rectangular wire with 2 mil radii will have 10" of slot play in the .022 slot

Fig. 10.8 The preadjusted appliance is not efficient at controlling torque owing to the small segment of rectangular steel wire and also owing to bracket 'slop'. It is frequently necessary to add torque bends to the rectangular steel wires in the incisor regions.

Fig. 10.8 The preadjusted appliance is not efficient at controlling torque owing to the small segment of rectangular steel wire and also owing to bracket 'slop'. It is frequently necessary to add torque bends to the rectangular steel wires in the incisor regions.

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