(torsion) that creates the light extrusive force of 25–35 cN on the canine, with a low deflection rate.
The force delivered to the impacted tooth by this mechanism is derived from the horizontal and upwards deflection of the vertical loop as it deforms the circumferential archform. The force may be reduced by using a finer‐gauge archwire or a lesser deflection. It may be increased by including an offset mesial to the molar band, inserted into an auxiliary tube. Alternatively, an elongated end of the wire, exiting the distal end of the molar tube, may be bent occlusally and in contact with the buccal surface of the molar, prior to engagement of the loop with the canine. Engaging the loop in the canine attachment will then activate the extrusive force. It should be noted here that force measurement of the loaded spring is very simple to adjust and regulate.
This method may also be used for a labial canine by constructing the loop to lie horizontally in its passive state and turned upwards in the vestibulum to be activated by ensnaring its terminal helix in the twisted ligature from the canine.
Elastics
As impacted teeth require to be moved in two directions, an eruptive force is needed to bring the tooth to the level of the occlusal plane and a horizontal (buccal or distal) force to bring the tooth into alignment in the arch. Palatally impacted teeth are generally moved into the arch using elastic chains or elastic threads extending from the canine to a main continuous buccal archwire, provided there is a free direct path, without the interference with a lateral incisor root (Figure 3.6a, b).
The conventional use of elastics to archwires for the purpose of completing this task successfully can still be accompanied by the appearance of undesirable movements of the adjacent teeth, such as displacement or rotations, indicating that anchorage is not sufficient. The need for a rigid base arch in this context is elementary.
Unwanted side effects, produced by intramaxillary elastics to the continuous archwire, are common. Intramaxillary elastic traction should not be used with non‐rigid archwires, except when applying palatal elastic traction to a transpalatal arch or in the presence of intermaxillary traction.
Fig. 3.6 (a, b) Using an elastomeric chain is relatively simple and cost‐effective in terms of time and materials. In order to achieve a good treatment outcome, it is crucial to control the direction of force application in the interests of avoiding unwanted side effects.
Fig. 3.7 (a) Short vertical elastics exhibit a greater vertical component of force compared to a horizontal force. (b, c) Long class II elastics to the lower first or second molars may rotate the mandibular arch in a clockwise direction, with extrusion of the mandibular posterior teeth. The occlusal plane of the mandibular arch will rotate clockwise (steepen), which will influence the degree of vertical overlap. The equivalent moments, operating at the centre of rotation of the mandibular arch, are determined by the points of force application of the elastic and the lines of action of the forces.
Vertical elastics (Figure 3.7a) may be very helpful in these cases, but these must be used carefully since they may unintentionally cant an occlusal plane. Their rotational effect should be monitored in all three dimensions at each appointment.
When using triangular elastics from the maxillary canine to the mandibular first premolar and canine, vertical forces will be acting approximately through the CR of the mandibular dentition and, therefore, no tipping will occur in the sagittal plane.
When using unilateral triangular or long class II elastics to extrude and distalize an ectopic buccal canine, only light forces should be exerted (80 cN), using larger or thinner‐gauge elastics.
Long class II elastics can produce a large moment at the CR of the mandibular arch. This may steepen the mandibular occlusal plane (Figure 3.7b, c).
When using higher forces, a rotation of the entire mandibular arch can be produced in the sagittal and frontal planes of space.
NiTi closed‐coil springs
NiTi springs generate approximately the same force system as elastics. They have a favourable load deflection rate and do not require cooperation of the patient. NiTi springs can be recommended in order to attain adequate traction and the force level will be adjusted to the required low forces [22–24]. They may also be used with patients whose compliance may be suspect.
NiTi open‐coil springs
Most impacted lower second molars are tipped mesially and therefore need to be tipped distally in order to clear the distal aspect of the first molar. Only the occlusal surface of the second molar need be exposed; a button can be bonded in the second molar’s central groove and a sectional equipped with a compressed NiTi super‐elastic open‐coil spring (Figure 3.8a, b). This is hooked onto the button and placed in the slots of the self‐ligating SnaplinkTM tube on the first molar and the premolar brackets. As the spring expands, the sectional wire will slide distally taking the second molar with it, to clear the first molar. If there is not sufficient wire remaining mesial to the first premolar to enable complete alignment of the second molar, the first premolar bracket can be debonded.
Fig. 3.8 (a) Sliding mechanics with a NiTi open‐coil spring threaded over a 0.016 in. stainless steel sectional for freeing a mesially tipped lower second molar. (b) The hook on the distal end of the sectional is fixed to the button. A 360° helix is used as a stop for the NiTi spring.
Using continuous NiTi wires
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).
