Orthodontic Treatment of Impacted Teeth. Adrian Becker
to the CR, the molar is tipped lingually during uprighting. With a long cantilever from the molar to the incisors, the point of force application is lingual to the CR, and the molar is tipped buccally (Figure 3.4d).
If the second molar needs to be intruded, a second cantilever may be inserted into a vertical tube and welded to the continuous arch between the first and second premolar teeth. It must be tied in a one‐point contact to a short distal extension from the auxiliary tube of the second molar [16].
An increase in cantilever length reduces the load deflection ratio [16]. However, long cantilevers will generate high moments and a permanent deformation of the wire may occur. For this reason, if small forces are needed (50 cN and below), 0.017 in. × 0.025 in. TMA wires have been recommended. Even lower forces may be generated by using cantilevers made of Connecticut New Archwire or NiTi wire. When permanent bends are required to be introduced into NiTi wires, these should be made using hammerhead pliers or the Sander Memory Maker.
If larger forces are needed, wires of the same gauge should be of stainless steel. When using stainless steel cantilevers, an unacceptably high load deflection rate may often be generated. This may be reduced by adding loops to the stainless steel appliance. Moreover, a permanent deformation of the incorporated tip‐back bend may be caused when the cantilever is activated. This may be prevented using a ‘safety pin configuration’ with a 360° helix/coil placed in front of the molar tube, which will be closed when activated [2].
Fig. 3.5 (a, b) Ballista spring. The active configuration may differ in vertical loop length for applying different force values and vectors. The ballista spring may be tied to the eyelet with suture material to prevent soft tissue irritation caused by the cut end of the ligature wire. Patient comfort may also be enhanced by masking the cut end with a light‐cured droplet of ‘flow’ composite material. (c) In some self‐ligating brackets, the light auxiliary labial arch may have stepped ends for better torque generation, but in general the curvature of the arch dictates the extrusive force, whether there is a stepped end or not. (d) Passive shape. In (c) and (d) a continuous archwire is serving as anchorage. Using self‐ligating brackets, the circumferential light auxiliary labial arch is stepped on both ends and engaged in the utility tubes. Using conventional brackets, the arch is ligated ‘piggyback’ to the brackets on the other teeth, over the passive base arch. This is impossible with Damon or any other self‐ligating bracket, which cannot accept two archwires. In anticipation of the arrival of the displaced tooth, the space may be maintained by threading a closed‐coil spring onto the archwire. An additional tie mesial to the molar bands may sometimes be necessary. (e, f) Force direction: vertical force with an oblique transverse force direction in (e) and mainly vertical force in (f).
Ballista springs/torsion springs
The ballista spring (Figure 3.5a–f) was introduced by Jacoby in 1978 [17]. A cantilever made of round stainless steel wire of varied gauges (0.014–0.018 in.) engages both the headgear and the buccal tube to prevent rotation in the slot. The cantilever arm is extended to the canine area incorporating a 90° bend towards the lower arch. Attaching this arm to the palatally displaced canine by turning it upwards produces the torsion required to provide extrusive force on the canine.
A modification of this concept was introduced by Caprioglio [18, 19]. There are, however, some reservations about this concept, since the resultant extrusive force of 3–4 oz is too high to be physiologically appropriate. In a similar fashion, there are doubts regarding the auxiliaries using reversed mousetrap spring mechanisms introduced by Bowman (Kilroy Spring I for palatally impacted and Kilroy Spring II for buccally impacted canines), both of which develop excessive forces, thereby causing undesired intrusive and transverse side effects on the adjacent teeth [20].
By contrast, more appropriate physiological forces will be applied when using the ballista spring modification of Kornhauser et al., known as the light auxiliary labial arch [21]. A vertical loop pointing downwards in the canine area is fashioned into a preformed circumferential arch of 0.016 in. stainless steel. This full arch auxiliary should always be used as a piggyback wire on a stiff continuous base arch, with both ends being inserted into the auxiliary tubes on the first molars or, occasionally, into the second premolar brackets. Displacing the loop upwards towards a palatally displaced canine produces a twist (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.