Progressive buccal crown torque has been designed into the appliance system in the lower posterior segments, as described in Chapter 2 (p. 33), and this has been a significant improvement. When the rectangular stainless steel wire is placed in the brackets, the lower posterior segments move to an upright position, providing a relatively flat curve of Wilson (Fig. 10.14). This does have a slight widening tendency in the lower arch. However, if arch form is maintained relative to the basal bone of the mandible, this effect is minimal. Typically, the lower posterior roots move lingually, away from the cortical plate, rather than the crowns moving buccally.
The upper cuspid and bicuspid brackets have -7° of buccal root torque, and this positions them ideally relative to the lower arch. Upper molars have been provided with additional buccal root torque, relative to the research findings, which helps to place them into the proper position. However, there are many cases where additional buccal root torque needs to be added to the posterior segments of the upper archwire. Due to the anatomy of the upper molar roots, it is important to have adequate width in the maxillary bone, so that the buccal roots are not compressed against the cortical plate (Fig. 10.15). If this occurs, it may be impossible to establish correct buccal root torque in the upper posterior segments.
Finally, the placement of the upper .045 'jockey wire' in the headgear tube (p. 82) is beneficial in many cases. Following the widening effect of this wire, it can be removed, and additional buccal root torque can be placed in the rectangular stainless steel wire. This allows the posterior teeth to move into their proper positions. Then, in the final stage of settling of teeth at the end of finishing, the upper posterior segments normally settle properly with the lower posterior segments.
Fig. 10.14 During correction of torque, lower posterior roots move lingually away from the cortical plate, and the crowns move slightly buccally, if arch form is maintained relative to the basal bone of the mandible.
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