Orthodontic Treatment of Impacted Teeth. Adrian Becker

would be preferred for stabilization of the buccal segments.

      All buccal cantilever systems should be used with a bypassing but rigid base arch.

      Composite TPA TMA cantilever

TMA cantilevers (0.018 in. TMA) can be welded to a TMA TPA (0.032 in. × 0.032 in. or 0.032 in. round) and used for erupting palatally displaced canines (Figure 3.3a, b). Possible side effects resemble those created by buccally engaged cantilevers and should be countered with a stiff rectangular continuous bypass archwire.

Fig. 3.2 (a) The passive cantilever, made of rectangular wire, extends from the molar auxiliary tube and crosses to the lingual through the space in the canine site. The anterior end is occlusal to the canine. N.B. A stepped bypass is introduced into the continuous main arch. (b, c) The base arch is stepped upwards to permit unobstructed extrusion and to allow the cantilever to cross to the lingual. (d) Activation of the cantilever creates two moments at the molar to rotate it mesio‐lingually and in a crown mesial and root distal direction (in the sagittal plane).

      This is arguably the best method for extrusion and distalization of palatally impacted canines that can be accomplished.

      Stainless steel TPA cantilever combination

      A stainless steel 0.016 in. torsion spring can be welded to a stainless steel TPA (Figure 3.3c–e). These cantilevers can easily produce force vectors, which may be difficult to generate by other means [2].

      It is emphasized that the cantilever should not be ligated directly into a canine bracket, but tied to the eyelet/bracket/ligature wire on the canine to create a one‐point contact. A rigid continuous canine bypass archwire will minimize undesirable side effects by distributing them to a larger number of teeth. Nevertheless, a flattening of the posterior occlusal plane resulting from the forward tip moment on the first molar should be monitored at every appointment and adjusted as necessary.

      When using light forces of the order of 25–35 cN, adverse effects should not occur.

      Cantilevers used as uprighting springs

Ectopic lower second molars often need uprighting before they can be aligned. The magnitude of the moment necessary for a molar uprighting has been suggested – on an empirical basis – to be at least 1000 cN‐mm [2]. Depending on the cant of the occlusal plane, uprighting may be combined with an antero‐posterior or vertical displacement, i.e. intrusion or extrusion (Figure 3.4a).

Fig. 3.3 (a) For illustration purposes only, a combination of the beta‐titanium (TMA) transpalatal arch (TPA) with a rectangular TMA cantilever has been used, which may generate excessive forces. The recommendation is to weld a 0.018 in. round TMA cantilever to the TMA TPA for clinical applications. The activated spring may be welded directly to the TPA, with or without including the 360° helix. (b) The canine will be extruded and moved posteriorly away from the roots of the incisors. (c) A 0.016 in. stainless steel torsion spring is here welded to a stainless steel TPA in its passive mode. (d, e) Using the same configuration with a 0.016 in. stainless steel spring and stainless steel TPA. The helical torsion spring should be welded halfway towards the midline and four or five loops wound clockwise (viewed from above) around the TPA close to the palatal mucosa. The number of loops is crucial for force reduction. The spring should be kept in the horizontal part of the TPA to ensure a linear vertical line of action. A stiff continuous base arch made of 0.017 in. × 0.025 in. or 0.019 in. × 0.025 in. stainless steel serves admirably as an anchorage.

For the adjustment of the appropriate cantilever length in the sagittal plane, the necessary case‐based combination of vertical movement and uprighting has to be determined. If a significant extrusion of the molar is needed, the cantilever should be short in order to produce a high force delivered to the molar tube/bracket. If little or no extrusion is desired, a low force should be combined with a large moment. In these cases the cantilever has to be as long as possible. The moment is calculated as the product of the length of the cantilever and the force, which can be measured by a digital push–pull force gauge or a Correx tension gauge: moment = force × distance or M = F × d.

Fig. 3.4 (a) The biomechanical force system generated by a cantilever is a combination of a moment and a force generated at the unit into which the cantilever is inserted. With a one‐point ligation on the other end, only a single force is developed. (b) Base arch extension with V bend placed mesial off centre. The extension and the cantilever have a one‐point ligation using chain links. (c) If extrusion of the molar is contraindicated, the cantilever for uprighting should be counteracted by a second cantilever; alternatively, the base arch should be extended to the second molar with a one‐point ligation at the second molar and configured with an mesial off‐centre V bend as shown above. (d) The force system using a long cantilever.

      Molar uprighting, as a pure rotation, where the centre of rotation coincides with the tooth CR, can be produced with two cantilevers by means of a statically determinate force system [2, 14, 15]. Alternatively it may be achieved with a root spring (alpha–beta spring) activated in Burstone geometry VI using a statically non‐determinate force system [2]. As a third option, the base arch may be extended to the second molar with an off‐centre V bend placed mesially (Figure 3.4b). Because the vertical forces generated cancel each other out, the base arch can be made using 0.019 in. × 0.025 in. NiTi wire. The V bend has to be placed outside the mouth with a hammerhead plier or Sander Memory Maker.

      In a statically determinate force system the two cantilevers generate equal and opposite moments and forces to the molar and the anchorage unit. The developed forces thereby become neutralized (Figure 3.4c) [2].

      In brachycephalic patients with good musculature, a long cantilever to the front teeth can be used as an alternative [2, 16]. Instead of 50 cN, which is produced by a medium‐length cantilever, this approach will produce only 30–35 cN of vertical force, which may be controlled by occlusal forces [2, 16]. The force acts lingually, as opposed to both the root spring and the V bend, which act parallel to the dental arch and in close proximity to the CR. Accordingly the points of application of the cantilevers are not necessarily parallel to the alveolar process. Instead, they will be on either side of the CR. This will generate an additional tipping in either the buccal or the lingual directions to the CR of the molar. Using short or medium cantilevers