CHAPTER 8 Setting up Stage I
The description that follows shows how to set up a case along the lines illustrated in the previous chapter. The malocclusion features a marked increase in overjet, an increased overbite, plus severe upper and lower anterior crowding. The reader is invited to assume that the typodont does not have a greatly raised maxillary–mandibular planes angle or a severe increase in lower facial height, and is therefore suitable for Class II intermaxillary elastic traction. Four first premolars are extracted (Fig. 8.1).
A question commonly asked is why such a case cannot be started more simply by means of nickel–titanium aligning archwires, before proceeding to stainless arches. The answer is that, while such wires do an excellent job aligning teeth, they are unable to control the vertical dimension, and particularly the molars, in the way that stainless steel can; neither can they support the use of intermaxillary elastics. The outcome would therefore be some loss of anchorage, by mesial molar migration, and a delay in implementing overbite and overjet reduction. Both are telling disadvantages in the more severe cases, particularly in the child patient. Besides, with experience, the following method becomes easy to carry out and, once set up, is easy to adjust and maintain without a change of archwires, over the 6 months or so necessary to obtain all first stage objectives.
.016 inch round high tensile stainless steel is the ideal archwire for Stage I, in both arches. This has the necessary resilience to withstand forces of occlusion, combined with the flexibility required to align mild tooth-to-tooth irregularities. It is particularly suitable for bite opening, offering a good range of action without excessive forces. The long established Special Plus grade Wilcock Australian wire continues to perform well, as does the more recent Bow-Flex® wire from TP Orthodontics (La Porte, Indiana, USA) (Fig. 8.2).
The ‘horseshoe shaped’ archform, as used in straight-wire techniques, is not appropriate for the early stages of Tip-Edge, since use of an anchorage bend requires a straight posterior leg. Combining a buccal archwire curvature with a vertical anchor bend will cause the wire to ‘wriggle’ and rotate within the buccal tube, which will invite some molar rotation. Even if an initial molar rotation is present, a straight leg is the rule. No toe-in, in any shape or form, should ever be combined with an anchor bend.
The anterior curvature is interrupted by the ‘cuspid circles’, each of which should be placed mesial to its respective canine bracket. How close to the bracket will vary, according to alignment and available space. If the labial segment is crowded, the canines will need to drift distally to accommodate instanding teeth, so that the circles may be immediately mesial to the canine brackets at the start of treatment. This will minimize the need for subsequent repositioning of the circles distally along the arch. Conversely, if the incisors are spaced, it makes better sense to site the circles further mesially, to allow for the mesial movement of the canines, relative to the archwire, as the anterior space is gathered up.
Once the anteriors are aligned and in contact, the correct position for the circles will be just mesial to the canine brackets. However they should not be more than 2 mm in front of the brackets, bearing in mind that in the aligned segment the circles will be ligated to the canines by elastomeric modules (as described later in this chapter) which, if too stretched, could provoke mesial canine rotations.
If bending an archwire up by hand, begin with the anterior curvature. Each circle should then be formed so that the posterior section loops to the labial of the anterior segment and not vice versa. It should also be noted that the anterior archform curvature extends distal to the circles, across the face of the canines (Fig. 8.3).
When using Class II elastics and anchor bends, it is good practice to incorporate some overall expansion in the lower arch, as both components exert some elevating effect on the molar tube, which may cause some lingual crown deflection. Five millimetres of expansion each side, measured across the molars, is generally adequate (Fig. 8.4), but this may be increased up to twice this amount in difficult deep bite cases. Even this does not amount to a strong expanding force in so light a wire. Since the upper molars will not be carrying intermaxillary elastics, only a marginal degree of archwire expansion will be required here (Fig. 8.5), simply to counteract the effect of the anchor bend.
A considerable amount of clinical time can be saved by using preformed archwires, which are fabricated from .016 inch Bow-Flex wire (Fig. 8.6). These are size graded according to the distance in millimetres between the cuspid circles, which can be ascertained in the mouth with a flexible plastic ruler, between the mesial surfaces of both canine brackets (Fig. 8.7). As a check, the circles can be tried across the front of the mouth without inserting the posterior archwire legs, before removing the sticky identification label. Arch length and arch width can then be tailored accordingly.
Correct positioning of the anchor bends should be approximately 2 mm in front of the molar tubes in both arches. The most accurate way of finding this is to mark the archwire at the mesial of the buccal tubes, then remove the arch from the mouth and make each bend 2 mm mesial to the marks. Placing the anchor bends further forward than this will cause them to obtrude occlusally and will also slightly lessen the amount of overbite reduction imparted to the anterior segment.
The anchor bends not only boost the anchorage available from the first molars, but also exert vertical control. The degree of the bends is therefore significant to both. There is no hard and fast rule as to the angle of the tip backs, but each needs to be assessed by the amount of intrusive deflection that the archwire produces at the midline. In a case where there is little or no overbite reduction required, the bends can be minimal: just enough to prevent the lower molars tipping mesially. This may amount to only a couple of millimetres of active gingival deflection at the midline. However, in a low mandibular angle deep bite case, the anchor bends can be made to work harder and will correspondingly generate stronger molar anchorage. The maximum vertical deflection permitted in such cases can be to the depth of the labial s/>