Читать книгу Orthodontic Treatment of Impacted Teeth - Adrian Becker - Страница 58

Tying a ‘light’ NiTi wire to an ectopic canine may produce adverse effects. The arbitrary levelling of a high canine, without the simultaneous use of a stabilizing rigid base arch, can produce significant side effects. Although the moments on the canine and the horizontal forces produced cancel each other out, the extrusive forces will be doubled and the desired space‐opening effect will be accompanied by intrusion and tipping on the adjacent teeth (Figure 3.9a–c).

Fig. 3.9 (a) When aligning a high canine using a continuous and fully engaged NiTi wire, forces and moments would normally be generated. However, a deflection of more than 3 mm will produce binding in the brackets and the vertical forces on the canine will be nullified. (b, c) If the deflection of the NiTi wire is more than 3 mm, the angulation may exceed the clearance of the bracket slot. Under such circumstances the frictional effects are rapidly exceeded by the binding effects, and motion may cease altogether. (d) For the super‐elastic wire to perform to best advantage, it should only be tied to a more distant tooth three or four teeth along in the arch. When the ectopic tooth is gradually brought into the arch, the tying position can be moved up incrementally until the ectopic tooth is fully aligned.

The average forces and moments produced by super‐elastic NiTi archwires are reported to be high. Large deflections will generate maximum force levels, which are greater than the recommended values found in the literature and are generally accepted as being excessive. At an interbracket distance of 7 mm, wire deflections of more than 3 mm in the vertical or horizontal plane will create maximum binding. Disturbances to the system such as mastication may cause sporadic release of the binding of the wire in the brackets of the adjacent teeth as well as the release of traction to the canine.

Below the critical angle for binding, the wire can slide freely through the bracket slots. It has been reported that in cases where the impacted canines are high in the maxilla that are treated with continuous NiTi wires ligated in the slots, there is a rapid initial reduction in vertical forces. This occurs at the same time that sagittal forces are rapidly increasing from zero, due to binding. The presence of binding on the adjacent teeth reduces the magnitude of vertical forces on the canine. With sufficiently high binding forces on the adjacent teeth, the vertical force on the displaced canine may be reduced to zero, thereby creating a total lock. It has been shown that vertical forces on the canine are greatest at 3 mm wire deflection [25]. Binding in adjacent brackets will cause excess wire length to build up between adjacent teeth as the canine descends, thereby generating mesio‐distal forces acting on the adjacent teeth [25]. Because the critical angle for binding is difficult to measure in the clinical setting, it may be assumed, as a rule of thumb, that binding in adjacent teeth in the canine area reaches its maximum at about >+3 mm of wire deflection [25, 26].

It should be stressed that levelling and aligning mechanics performed recklessly and irresponsibly, involving large deflections of the NiTi wire in the vertical and/or horizontal planes of space, may generate forces that are far too high and unphysiological [25, 27–28]. When binding occurs, the applied forces become pathogenic and it must then be assumed that the danger of root resorption of the adjacent teeth increases. An additional adverse effect of binding is a resultant decrease in the extrusive force on the canine down to values close to zero, which will be reflected in a much increased treatment time.

Super‐elastic wires should only be applied when overlaid on the main archwire and tied directly to the attachment of the canine with a single‐point contact (Figure 3.9d). A deflection of the piggyback wire of more than 3 mm should be avoided, as emphasized above, and the piggyback wire should not be attached to all the brackets along the way on the base arch. Ligation of the super‐elastic spring should only be tied closer to the ectopic canine when the tooth is near to its final place.