is more disappointing than might initially be thought.
The manufacturer’s spool of elastic thread usually comes in the form of fine hollow tubing, which is easier to tie than a solid elastic thread. Most orthodontists tie the thread with a simple knot that, when used to tie ordinary string, will not unravel. The stretch factor is set by trial and error, as there can be no accurate control on the amount of force applied. Unlike ordinary string, however, when tying elastomeric thread the knot tends to loosen and much of the original force of the connection will be lost. When subjected to tension, any of the materials that are used to make this elastic thread suffer very rapid and very significant force decay. Force levels of chains of various lengths are known to decay to below the force required for tooth movement. This takes place in a period of between one and three weeks, depending upon the amount of tension initially applied [17, 18].
A shorter piece of stretched elastic (Figure 2.5) will have a very short range and run the risk of applying an initial excessive amount of force if the tie is good – or none at all, if the tie loosens. An excessive initial force could severely test the immature periodontal ligament (PDL) of the recently exposed tooth and the bond strength. In the case of an unerupted tooth close to the line of the arch, traction applied directly from its attachment to the archwire will generally be very inefficient, requiring frequent changes and producing a minimal response at each change. Moreover, for all practical purposes, it is impossible to measure or control such a force.
For these reasons, it is prudent to use more distant sites from which to apply traction to the unerupted tooth, where the greater length of stretched elastic thread serves to increase the range of the traction force and its effectiveness in moving the tooth over a longer period of time. To achieve this, the elastic thread needs to be drawn to the target area on the archwire, through the agency of a loop bent into the archwire at that point. The thread may then be tied back to the hook on the molar tube of the same side, with care being taken to insert a stop in the archwire, mesial to the tube, or indeed any other means that will prevent mesial movement of the molar.
The ‘slingshot’ elastic is an excellent alternative for the situation, in which space has been opened for an unerupted or partially erupted and buccally, palatally or superiorly displaced tooth. A cut piece of steel tubing or a closed coil spring may be fed onto the archwire between the brackets in order to maintain the space and an elastic chain or thread is stretched across the gap between the brackets on the two adjacent teeth. The middle of the chain is then gripped with an artery forceps or Howe plier and stretched over the attachment on the ectopic tooth (Figure 2.6a, b) or on the twisted steel ligature, coming through the sutured edge of a replaced surgical flap.
As a general rule, elastic thread should only be used as a link, connecting the non‐elastic steel pigtail to a similarly non‐elastic and heavy archwire. If a lighter flexible archwire is used, the tie should be made with a steel ligature – the archwire will then provide the elastic displacement. Nickel–titanium alloy wires may be used with great effect in this context, but the distortion of the archwire will bring about alteration in both the horizontal and vertical planes, to produce unwanted change in the form of the dental arch and an uneven occlusal plane, i.e. loss of anchorage. Therefore, if a single super‐elastic archwire is tied into the brackets on each of the teeth in the levelled and aligned arch and then into an attachment bonded to a severely displaced impacted tooth, control of overall archform will be lost. This will be evident in the three planes of space, causing tipping movements of individual teeth, alterations in the occlusal plane, asymmetrical skewing of the shape of the arch and loss of occlusal contacts. The adjacent teeth will be relatively intruded and will be displaced buccally or palatally and tipped towards the space reserved for the impacted tooth. Super‐elastic wires should not be used in circumstances of displacement without a heavy base arch in place capable of resisting these unwanted movements of the immediately adjacent anchor teeth. However, it should be clearly understood that for a nickel–titanium archwire to develop adequate vertically directed eruptive force, it must be free to slide in the bracket slots of the other teeth to which it is ligated (Figure 2.7a, b). The presence of a heavy base arch tied in with elastomeric ligatures will, however, considerably increase the friction and binding of the super‐elastic archwire in the brackets. This may not be evident when the elastomeric is first placed and it is difficult to check. As a consequence, the pressure from the deflection, which was applied in order to fully engage the super‐elastic wire in the slots, may be nullified by the inability of the wire to slide freely through the brackets.
The combined use of a flexible archwire and an elastic thread tie [7] will be counterproductive, since the elasticity of the one that exerts the stronger force will be effectively neutralized and offer no physical advantage over a steel ligature. At the same time, the displacement of the weaker element will be the only factor that will be active in moving the teeth.
Fig. 2.6 (a) The slingshot elastic. A palatally impacted canine has erupted into the palate (see Chapter 7). The elastic chain module, placed between the bracket of lateral incisor and first premolar, is stretched towards the canine and ligated to the buccal eyelet. The steel tube on the archwire maintains the space. (b) The slingshot used on a buccal canine.
Orthodontists generally use elastic ligatures and chains to move teeth by tying the material in its stretched, elongated form, thereby drawing the dental elements towards one another. However, the range of elasticity in this longitudinal direction will be limited and, as pointed out above, will decay rapidly. Nevertheless, a lateral displacement of an elongated elastic thread produces a potentially greater range of movement, within suitable orthodontic force levels, than does a longitudinal displacement. This ‘slingshot’ principle may be more efficiently applied to move teeth that lie at a distance from the main arch, giving more controlled and measurable forces (Figure 2.7a, b).
Fig. 2.7 (a, b) The use of nickel–titanium auxiliary wire as the active element in applying eruptive force to the unerupted canines, by being thread through the ‘rolled‐up’ stainless steel pigtails that are ligated to the eyelets. There is a heavy 0.020 in. gauge base arch. The low‐profile eyelets, which were bonded at exposure in a closed surgery procedure, can be seen through the translucency of the healthy and uninflamed gingiva.
Notwithstanding these comments and with the required careful consideration in the planning of their use, elastic ties, nickel–titanium auxiliary archwires, chains and modules are extremely helpful in many situations created by the presence of impacted teeth. However, properly designed springs, auxiliary to a heavy base arch, are usually more efficient: they are able to deliver a measured and controlled force, the force decay is lower, a variety of metallic alloys are available for spring fabrication, their range of action may be very broad and their direction is accurate. These will be illustrated in the succeeding chapters in the consideration of cases as they pertain to the individual groups of teeth.
Thus far, our discussion has centred on maintenance of a steady force through as wide a range as possible. Now we must address the force values that are appropriate for application to an impacted tooth.
When planning traction to a single‐rooted tooth through its long axis, pure
