Читать книгу Orthodontic Treatment of Impacted Teeth - Adrian Becker - Страница 54

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.