The Concept of the Straight Wire Appliance in Orthodontics

 

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The occlusion research (study of the non-orthodontic normals) had been launched to get a better understanding of occlusion and how it was related to the buccolabial surfaces of the crowns. There was no intention originally to produce a new appliance, but the need for one became evident.

     The arising concept was to Pre-program suitable treatment into the appliance and therefore the SWA was developed.

The built-in features of the SWA guide the teeth along direct vector lines, eliminating jiggling, round-tripping and other excessive movements.

Andrews’s philosophy of treatment

 

 A bracket type for each tooth type:

 

   Since within any dentition, teeth differ from each other, therefore, each tooth would require its own bracket.

   Measurements of the non-orthodontic normal models had provided data about the angular positions of crowns in excellent occlusion. Such data are useless unless known to be taken from specified, unchanged crown sites.

If an appliance is not located properly, all that has been programmed into it will be altered.

Where the bracket should be located on the tooth?

 

The LACC

    Crown morphology includes many developmental landmarks. Among these are the ridges and grooves on the crown's labial or buccal surfaces. These ridges and grooves can be easily recognized, and are not significantly subject to environmental alterations such as chipping, wear and fracture.

The long axis of the clinical crown (LACC), (as viewed from the labial or buccal perspective), for all teeth except molars, is located at the mid-developmental ridge that runs vertically and is the most prominent portion in the central area of the labial or buccal surface.

The long axis of molar crowns is identified by the dominant vertical groove on the buccal surface.

The LA-Point

 

   That vertical line, the LACC, is easy to find. But how far "up" on it should the bracket be placed?

At its mid-point. The LA-point is the mid-point on the Long Axis of the crowns. The crown has no obvious horizontal axis but the accuracy of this procedure has been measured and found satisfactory.

 

The LACC is practical for measuring bracket position. No x-rays needed, no guessing; this axis can be directly seen, touched, and marked with a pencil.

Keying to the long axis of the crown makes accurate bracket placement much easier. One simply places the tie-wings of the bracket parallel to the LACC, and the base point of the bracket at the LA-point.

Torque in Bracket Base

 

     "Torque" refers to buccolingual or labiolingual inclination.

 

How is torquing accomplished by the SWA?

 

    The bracket base is inclined in relation to the stem, allowing the stem to be parallel to the Andrews plane, and the LA-point, base point and slot point to be included in the Andrews plane. Making the torquing function a product of the base design allows all slots (at the completion of active treatment) to be aligned with each other and thus receive a flat, unbent rectangular archwire. A separate bracket for each tooth type is necessary, with torque built into the bracket base. 

Compound-Contoured Bases

 

    Most conventional appliances have simple horizontal curvature in the bracket base. The Straight-Wire Appliance added vertical curvature. It is the combination of the two that we refer to as "compound contour" or "compound curvature."

Installing a bracket with a vertically flat base against the vertically curved surface of a tooth allows a variety of slot-to-tooth positions. Not only is torque affected by any rolling motion during bracket placement but also is the height of the slot in relation to the occlusal plane or Andrews plane.

Angulated Slots

   The slots are angulated for tip, but each bracket is squarely aligned with the LACC. If any existing bracket with a horizontally curved base is angulated on the tooth to achieve tip, this creates a two-point contact between tooth and two diagonally-opposite corners of the bracket base, resulting in a potential for rocking.

 

   To obtain the full effects of the Straight-Wire approach, tip must be built into the slot of the bracket, torque must be in the base, and the base of each bracket must be contoured to fit firmly and unchangingly when the bracket is installed "squarely" (not angulated) at the chosen site.

 

In/Out and Molar Offset

 

  As Teeth types differ in buccolingual thickness, the thickness of the brackets was varied according.    Elimination of first order archwire bends has several advantages, including time-saving and the enabling of en masse space closures without interference from wirebends.

 

Other Aspects

 

Designing an individual bracket for each tooth type also permits innovations not involving the slot or the base. SWA tie-wings, instead of being bilaterally symmetrical, step-out farther on the gingival sides of posterior brackets, resulting in easier ligation and less frequent gingival impingement.

Identification

 

    One further special design element in the Straight-Wire Appliance is that every bracket can be identified as to its tooth type.

 

Extraction Brackets Series

    Bodily tooth movement usually is necessary in extraction cases, presenting a requirement for anti-tip and anti-rotation forces (in addition to torque, tip and in/out). The amount required is proportionate to the distance a tooth must be moved.

There are three brackets designed for each posterior tooth type. The three are designated Minimum, Medium and Maximum.

   SWA extraction brackets, like Standard SWA brackets, have built-in tip, torque and in/out. In addition, they provide two functions not needed in the Standard brackets: anti-tip and anti-rotation. 

Incisor Bracket Sets

 

    As shown by Steiner, the inclination (torque) requirement of incisors is directly related to the ANB angle. SWA design accommodates these requirements with three sets of incisor brackets, designated A, S and C, with S representing Standard.

 

Classification of Treatment

 

    Angle's classification of malocclusion does not indicate the degree of difficulty of a case. Another approach proposed by Andrews can be beneficial in selection of brackets sets.

This approach divides cases into three categories: Class I, Class II and Class II-Classic. Each of these is then subdivided into more types.

Class I cases are those that have an Angle Class I molar relationship. Class II cases are those with an Angle Class II molar relationship and lower arch length problems. Class II-Classic also deals with Angle Class II molars, but only those cases with no lower arch length problems and the lower anteriors in good relationship to the face. 

Brackets types are determined by the amount of crowding and/or protrusion found in the lower dental arch.

The brackets are organized into sets and series. These are referred to with numerals and letters that match each such combination of brackets with the Classification for which it is designed. This permits immediate selection of the brackets most suitable for resolving a particular problem.

Roth Modifications

 

The extraction brackets had counter-tip and counter-rotation built in to offset relapse, and to keep teeth from rotating and tipping into the extraction sites as they were being moved along a rectangular wire. In other words, the brackets were constructed to allow translation of teeth as much as possible and to offset any relapse tendency by overcorrection.

If teeth tend to relapse back whence they started, and if counter-tip, counter-rotation, counter-torque, and leveling of the curve of Spee were applied to the SWA in every possible direction, then it should be possible to use primarily one prescription for most cases, and to finish to an "end of appliance therapy" goal in which all tooth positions are slightly overcorrected and from which the teeth will most likely settle into non-orthodontic normal positions.

 Also, Roth introduced changes in the configuration of the appliance. Power arms were cast as integral parts of the brackets designed by Andrews to place the force near the CR of the teeth.

The Advantages of the Straight-Wire Appliance

 

1. Ease of arch wire construction, since there is no need for bends of the arch wire.

 2. No need for inter-bracket span, since there are no bends in the arch wire.

 3. Ease of arch wire placement. When the teeth are properly aligned into ideal positions, all the bracket slots lie parallel to the occlusal plane.

 4. Less "round-tripping." Since the teeth move in more direct vector lines from their maloccluded positions to their individualized positions.

 5. Better control of tooth positions. Since all requirements for idealized tooth positions are built into the brackets rather than the arch wires.

 6. Better and more consistent results with shorter treatment time. The movement of teeth in direct vector lines to their final positions, limited by what is engineered into the appliance, results in shorter treatment time for the patient and less chair time for the orthodontist .

 7. Patient comfort. Placing of large rectangular arch wires is easier for both the patient and the orthodontist with less jiggling of the teeth and therefore much less discomfort.

 8. Complete space closure in both extraction and non-extraction cases can be accomplished with one set of arch wires since there are no bends in the arch wires to interfere with the brackets themselves during space closure.

 9. Ease of Ligation. The gingival wings of the brackets are constructed so that they stand away from the gingiva labially and buccally.

 10. Bracket Identification. Easy bracket identification due to the identifying markings that are cast into the brackets.

11. Easier, more accurate bracket placement. Specific bracket base compound curvature insures consistent, reliable fit.

 12. The SWA also allows one to take full advantage of the new nickel titanium wires. These wires are difficult to bend, and bends do not stay in nickel titanium wires.

At last

 

    The SWA has proven itself under the test of time. It has been upgraded with technical advances, and perhaps its more recent "facelifts" have made it look like a different appliance. The tooth positioning ingredients, however, are the same as ever.

The SWA key is to get the brackets properly placed and if they are not properly placed, they must be reset.

    The SWA will not diagnose cases, it will not set up the treatment plan, and it will not figure out the mechanics needed to correct the malocclusion but it will allow us to reach our treatment goals….which are the six key of normal occlusion.