Читать книгу Orthodontic Treatment of Impacted Teeth - Adrian Becker - Страница 55

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.