than 3 mm, the angulation may exceed the clearance of the bracket slot. (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."/>
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.
Creative wire bending using V bends between anchorage unit and ectopic tooth
The V bend delivers a force system that is highly dependent on its position [2, 3]. Placed exactly in the middle of the interbracket distance, it will always deliver two equal and opposite couples and no forces. This is independent of whether it is placed as a second‐ or a first‐order bend [1–3]. This situation simulates a Burstone geometry VI. The two teeth will be subject to a pure rotation with neither extrusive nor intrusive forces. It is essential that the wire first be checked outside the mouth for total passivity, before the bend is placed midway between the two bracket units [2].
The force system produced by an alpha/beta spring or by a V bend varies during the tooth movement and is defined as ‘fluctuating’ (Figure 3.10a–c). The configuration changes between Burstone geometry VI and geometry IV, which only delivers an extrusive force to the canine. Friction and the resistance to movement of the reactive unit are factors that may influence the force system and the clinical effects. Thus, uprighting a canine by pure rotation is not easy to achieve unless distally uprighting the molar is needed as the reactive force vector. The uprighting of the canine can therefore be very slow compared to the action of a cantilever [2].
Fig. 3.10 (a–c) Changing the position of the V bend will create totally different force systems. To show the influence of the interbracket position of a V bend, the force systems (forces and moments) generated are indicated by arrows. Note how a displacement can alter the distribution and direction of forces and moments completely.
The activation of a statically indeterminate system includes two angles, with two brackets. The measurement of the angular values in the clinic is difficult to assess, and has little significance. The wire activation with respect to the two brackets, however, is important information and may be assessed by other means [2].
The angular activation corresponds to a linear activation, namely the distance between the wire end and one bracket when the wire has been inserted in the other. This can be measured by means of a caliper [2].
Root springs (alpha–beta springs)
Root springs (also called alpha–beta springs) can be made from 0.017 in. × 0.025 in. TMA wire. In most cases a truncated V design corresponding to a centred V bend is used.
If the interbracket distance between the displaced tooth and the adjacent tooth is sufficient (>10 mm), intra‐segmental mechanics may also be incorporated in a continuous arch. In order to reduce the load deflection rate, the continuous arch should be made of a rectangular Connecticut New Archwire or NiTi wire. (Note: bends have to be made with a hammerhead plier or the Sander Memory Maker.) A V bend activated in geometry VI will produce molar uprighting by pure rotation. If both units are displaced at the same time, no vertical forces will be generated on either side.
Many other configurations of the root spring/centred V bend are possible. It delivers two equal and opposite moments and no forces. When using a V bend, it is important that the wire has been initially adjusted so that it is completely passive before the V bend is placed (Figure 3.11a, b).
Fig. 3.11 (a) The passive configuration of the alpha–beta spring has to be made and first tested in the mouth. (b) It is recommended to make the V bend activation bend outside the mouth to ensure the geometry corresponds to a geometry VI. The root
