Stage III: setting the torque

CHAPTER 16 Stage III: setting the torque

Torque in increased overbite cases

A zero torque archwire setting throughout will still be the aim. However, where overbite reduction has been required in Stage I (and therefore maintained with bite sweeps throughout Stage II) it follows that vertical bite sweeps will be needed to maintain the reduced overbite during the third stage also. This introduces complications, since sweeping a vertical curvature along the buccal segments of any rectangular wire will automatically alter the torque setting at the front, relative to the occlusal plane, producing unwanted incisor proclination. This is shown in a lower archwire which has zero torque when flat (Fig. 16.3A), then after placement of a vertical bite sweep (Fig. 16.3B). It will be seen from Fig. 16.3B that the distal archwire ends no longer bisect the plier handles, but are now below them, denoting labial crown torque, which will cause proclination of the lower incisors.

What is therefore required is some lingual crown torque in the anterior segment of the archwire, to counteract the labial crown torque produced by the bite sweep, thereby restoring zero torque to the lower incisors. Zero torque should also be set along the buccal segments. In former times, this combination required intricate torque manipulations. Fortunately, now that we have pretorqued archwires, none of this is any longer necessary.

Pretorqued archwires

These are identical dimensionally to the plain rectangular archwires already illustrated, but come with 5 degrees of lingual crown torque in the maxillary (Fig. 16.4) and 8 degrees of lingual crown torque in the mandibular arch (Fig. 16.5). Used in a case with an initially increased overbite, the purpose of the inbuilt retroclination will be to cancel out the proclination produced by the bite sweep (Fig. 16.6). Because of the continuity of the wire, the pretorque in each archwire extends overall, including the buccal segments (Fig. 16.7) from which it must later be removed. However, this avoids the necessity for the orthodontist to stock an inventory of different arch sizes; tailoring each archwire, and placing the hooks appropriately, allows one size to be adapted to all.

Pretorqued archwires are identified by their centreline markings, which appear on the upper surfaces only. The maxillary archwire therefore carries its single mark on the gingival surface, while the mandibular has double markings, both on the occlusal surface (Fig. 16.8). By this means, both upper and lower centreline markings will be visible to the operator when the archwires are in the mouth (Fig. 16.9) thereby avoiding accidental misfitting of an archwire upside down, which would, of course, reverse the torque.

Dec 31, 2014 | Posted by in Orthodontics | Comments Off on Stage III: setting the torque
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