CHAPTER 14 Stage III: how it works
To conventional orthodontic thinking, it may be difficult to conceive that such a finish will be readily possible, either mechanically or physiologically, from an appliance that accommodates angles of torque and tip discrepancy that lie far outside the range of recovery normally possible with straight-wire brackets. Perhaps even more difficult to imagine is that such a recovery, in the hands of an experienced operator, is largely maintenance free, accomplished by a single upper and lower rectangular archwire only, frequently requiring no removal for adjustment.
Indeed, having exploited all the advantages of free tipping early in treatment, it is imperative that Tip-Edge should have a reliable uprighting and torquing mechanism, well beyond the capability of conventional brackets. Without this, it would be unable to match the finishing precision for which the straight-wire appliance has set the standard. That the Tip-Edge appliance has this capability is due to the use of rectangular wire in an entirely new way, which is unique in orthodontics.1 For reasons which will be fully explained, it has the intrinsic capability to produce a more accurate finish than existing bracket systems. To aid understanding, this and the hitherto established method of torquing with rectangular archwires will be compared.
A conventional edgewise or straight-wire bracket has a fixed vertical bracket slot dimension, into which freeplay can only be introduced by using undersized archwires. As is well known, torque is achieved by the use of an active archwire of rectangular cross section, in which a third order torque deflection of the archwire will occur when inserted into the archwire slot. The torque force imparted will depend on the torque discrepancy between the archwire and the bracket, the elastic properties of the wire, and the degree to which the size of archwire fills the slot.
Although such a simple method may have served well over many generations, within its self-imposed limits, it is fundamentally flawed in one major area. Primarily, this is because the active component in torque production is the archwire itself, which is effectively acting as a spring. The rectangular wire is therefore required to provide the two-fold function of actively torquing those teeth requiring correction, while at the same time offering three-dimensional stability to the remainder. Combining these two conflicting functions in a single archwire is a physical impossibility. Many world respected researchers have identified and tried to address the problems of the unwanted reciprocals, whereby active torque imparted to a single unit, or quadrant, will inevitably impart unwanted secondary torque reactions in adjacent units, or groups of teeth, resulting in some ‘round tripping’.2–13
Provided torque discrepancies are only minor, many orthodontists successfully ride through such problems, which are inherent in full rectangular archwire mechanics, rather than resorting to complicated segmental arch configurations, designed to eliminate such secondary reactions. However, there are other compromises in conventional torquing mechanics, which can less easily be ignored.
Returning, for a moment, to the conflicting requirements asked of a single rectangular wire in stabilizing some teeth while torquing others, there is a further dilemma concerning the ideal physical properties of the wire itself. For example, the relative flexibility of a nickel–titanium archwire may be preferable for progressive torquing with light forces. On the other hand, stainless steel might be the natural choice elsewhere in the arch for maintaining stability, particularly in the lateral and vertical dimensions, while being robust enough to support intra- or intermaxillary traction concurrently.
Lastly, there is the question of archwire size. It is customary, in the majority of straight-wire techniques, not to exceed archwires of .019 × .025 inch, within a standard .022 × .028 inch bracket slot. This is partly to limit the active forces fed to a tooth by too heavy an archwire, but also to allow ease of insertion and removal. This degree of tolerance will equate to nearly 10 degrees of ‘torque slop’ between archwire and bracket slot.14 Although evidently regarded as a physiological safety feature, it may nonetheless result in up to 10 degrees of undertreatment, unless compensated by individualized torque adjustment in the archwire, or a modified torque prescription in the bracket itself.15
It is essential first to understand that a Tip-Edge bracket cannot be torqued by an active archwire in the conventional manner, even with a fully closed bracket slot. Examination of the design of the bracket will explain why (Fig. 14.1). The intact upper and lower finishing surfaces in each bracket (see Fig. 3.2) are offset from one another, and are therefore never directly opposed. Insertion of an actively torqued rectangular archwire will therefore elevate one finishing surface and depress the other. In effect, the torquing effort in the archwire will have dispersed, by increasing the vertical dimension within the Tip-Edge slot. The net result will be a relapse of root uprighting in the mesio-distal direction, rather than any third order torque imparted to the root (Fig. 14.2). This is known as ‘torque escape’.
Paradoxically, the selfsame bracket properties have opened new doors of possibility, allowing a new and entirely original method of torque delivery, simultaneous with tip correction. It represents a big step forward for orthodontics by relieving the rectangular wire of its double function: now the heavy base archwire becomes a passive platform, preserving three-dimensional rectangular control where necessary, while light auxiliary forces provide the flexibility to torque and tip each bracket into conformity with the archwire.
Originally, with the Rx-1 bracket, the Side-Winder spring provided the auxiliary force and this was the first time in orthodontics that an auxiliary spring, acting in the tip plane, had been shown to generate third order torque (those with an interest in the underlying mathematics are referred to Parkhouse & Parkhouse16). While the method might have appeared cumbersome, since it required a Side-Winder for each individual bracket, the Plus bracket has revolutionized Stage III by eliminating the springs altogether. Now, all torquing and tipping can be carried out more simply and less obtrusively with a nickel–titanium auxiliary archwire, threaded through the deep tunnels, beneath the overall security of a stainless rectangular main archwire. This contains t/>