School of Dentistry

ORTHODONTICS LECTURE 6THE STRAIGHTWIRE APPLIANCE

Introduction

The term Straight-Wire appliance (SWA) was originally applied to a patented appliance developed by Dr Andrews (of the Six Keys). The SWA has the ability to exert 3D control by virtue of the close fit of rectangular archwires in accurately made brackets. The particular feature of the SWA is a bracket design which permits ease of slot line up to facilitate sliding mechanics. The brackets also incorporate tip and torque, individualised for each tooth so that wire bending is simplified. The SWA is therefore sometimes referred to as a "preadjusted appliance".

Later appliances copied the original concept of the Straight-Wire appliance and the term “straightwire” is now applied generally to any fixed appliance in which the brackets and other attachments incorporate preadjusted tip and torque values.

The concept for the SWA began with a paper written by Andrews entitled “The Six Keys of Normal Occlusion” (AJO 62, September 1972). From the study of 120 ideal untreated occlusions Andrews proposed the following norms:

1. Class I molar occlusion with 6 tipped mesially to provide sufficient space for 5 to bite correctly between 56 and the distal cusp in contact with marginal ridge of second molar.

2. Correct mesio-distal crown tip, since the angulation of each tooth affects the space that it occupies.

3. Correct bucco-palatal crown angulation (torque).

4. No rotations, except that 6 must be slightly disto-palatally rotated for a correct fit.

5. No spaces.

6. Flat occlusal plane, since an increased curve of the Spee produces overbite increase.

The SWA was developed to facilitate attainment of these norms using archwires without the need to bend loops and coils into and archwire, and without the need for in/out bends i.e. “Straight” archwires.

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Features of the SWA

Brackets

The first straightwire brackets were cast rather than being of the more usual milled type. Casting provided a more intricate shape than could be achieved by milling and this was thought to be the key to slot alignment. More recently milled straightwire brackets have become available which tend to cost less than cast ones and are harder so that they resist deformation better.

Generally brackets are placed on the LA point of the tooth: the ‘long axis’ point. This is the point at which the long axis of the tooth (looking down the root), and the horizontal axis of the crown intersect. Accurate bracket placement is crucial since it affects torque, in/out values, vertical alignment and rotation. It may sometimes be necessary to reposition one or more brackets after initial alignment of the teeth is completed if pre-treatment crowding has prevented proper placement.

Features of bracket design are:

1. In Birmingham the 0.022” slot is used to permit use of thicker archwires. Brackets with an 0.018” slot are also available.

2. Torque is incorporated into the bracket base so that the slot is always on the same plane as the centre of the base, i.e. the bracket base is angulated to ensure when the tooth aligns it too is angulated in a bucco-lingual plane. Torque is individualised to each tooth.

3. Tip is incorporated into the bracket slot: the slot is placed on an angle to allow the crown to tip mesially as the tooth aligns. Again this is individualised for each tooth. Values vary, the largest tip being 8° on the maxillary lateral and canine brackets. The lower incisors and upper premolars have no tip, allowing the brackets to be placed on either side of the arch.

4. In-out adjustment is incorporated into bracket bases. The canine brackets have a very thin base so that they stand labially placed once alignment is complete, whereas the upper lateral incisor brackets have a much thicker base so that the laterals will be slightly in-standing once alignment is complete.

5. Bracket bases are contoured to permit ease of placement on the LA point and to give a good fit against the tooth surface. In practice the cleverly contoured bracket bases permit easy identification of the LA point and give a feel to the correct position of a bracket.

6. Gingival tie wings stand away to allow ease of ligation.

7. An identification system is incorporated into each bracket: each has a dot on the disto-gingival tie wing. The exceptions are the upper premolars, where there is a dot on each of the gingival tie-wings, and the lower incisors, here the brackets are the same shape as the crown (i.e. wider at the top and narrower at the bottom). This is due to the fact that

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these brackets have no tip, and so it does not matter which side they are placed on. Each quadrant has its own colour, to further aid bracket identification.

Tubes

Each molar has its own tube: this is, in effect, a rectangular bracket with a slot that is covered, rather than open. The wire slides through the tube and so does not require ligation. Each tube has a hook welded on to it: this always faces disto-gingivally and is used for placement of inter-arch and intra-arch traction, as well as ligatures. Second molars may be bonded to provide overbite control.

Bands

Bands are pre-welded and contoured to fit around the molar teeth. Bands are placed rather than tubes when there are concerns that a tube may debond (e.g. if there is a heavy occlusal interference), or where they are being used to support auxiliary parts of the appliance (e.g. lingual or palatal arches, or a quad helix). The size should be chosen carefully in order to provide a friction fit, but are then also cemented in with glass-ionomer luting cement. The band should be burnished to fit the tooth tightly, ensuring there are no spaces.

Archwires

A variety of archwire materials are available, but generally in a conventional, labially placed fixed appliance the 2 materials used are nickel-titanium (Niti) and stainless steel (SS). Initial alignment is completed with a round, thin, NiTi wire (typically 0.012” or 0.014”). NiTi is 3-5x more flexible than the same diameter of stainless steel, and also possess shape-memory (if distorted it will return to its original archform). This makes it an ideal aligning wire as it is flexible enough to engage displaced teeth, but applies a light force that is optimal for tooth movement. However because it is only 0.014” it does not fill the bracket slot well, and so can only give rough alignment, therefore a gradual increase in wire size is necessary to fill the bracket slot (for example 0.016”x0.022” NiTi, followed by 0.018”x0.025” NiTi). This furthers alignment, but still keeps forces light. If you tried to engage a thick NiTi in straight away at bond-up you would put far too much force on the teeth.

As treatment progresses the bracket slots align: this first phase of treatment is called levelling and alignment, and by the end of it you should have the bracket slots aligned in a straight line.Once this has occurred the second phase of treatment is undertaken:

Space closure and overjet reduction is the next phase and it cannot be undertaken on NiTi wires: in order to close spaces it is necessary to be able to slide the teeth (sliding mechanics) and move them together. This involves the placement of auxiliaries that have inherent elastic recoil across the space, but if placed over a NiTi wire they will buckle the wire, this ‘dumps’ the teeth in and alignment is lost. Therefore a rigid wire must first be placed: typically a 0.019”x0.025” SS wire (also called the working archwire). This is rigid enough to provide resistance to deformation, allowing you to push or pull the teeth around the wire (like curtains on a curtain rail), but also it provides significantly less friction that NiTi wire in a steel slot, thus further enabling sliding mechanics.

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It is important to remember that, whilst this is a typical wire sequence there is a vast array of wires. It is important to select a wire based on the properties you require at that stage of treatment, and not because it is the next wire in the only sequence you know!

Important tips (supplement this reading with notes from your typodont course)

There are certain things that must be considered when carrying out different phases of treatment:

1. Levelling and Aligning

o Leave very displaced teeth off starting archwires: even a 0.012” NiTi could place excessive force on these teeth, and you can also run into anchorage problems.

o Consider lacing back canines to prevent mesial crown tip as bracket slot tip is expressed.

o Turn up the annealed archwire ends tightly behind the molar tubes if needed, to prevent incisor proclination.

o Never place figure of 8 modules at the first visit: this again may place excessive force on the teeth, and your patients will find it painful!

2. Anchorage Control

Lower extraction spaces may provide anchorage for overjet reduction by IMT.

Lingual and palatal arches, Nance buttons and temporary anchorage devices (orthodontic mini-implants) can all be used to supplement anchorage control. They should be used when space is tight to prevent space loss from the molars moving into your extraction space.

Headgear is still used occasionally, when used for anchorage you need 250gm per side, and it is worn for sleeping hours

If you do not plan your anchorage carefully the case is sure to fail! It must be planned from the start, with a clear management strategy throughout.

3. Overbite Control

Achieved by a combination of anterior intrusion and posterior extrusion. Overbite control can be a problem with the SWA. Good control depends upon:

0.022 slotInclusion of second molars to improve vertical anchorageClass II elastics2-3 mm reverse Spee in lower arch or use of “Counterforce” NiTi arches with preformed Reverse Curve of Spee.In addition you can use:

A “clip-over” bite plane Bonded bite turbos

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Using a removable appliance with FABP before the fixed appliance is fittedFitting low pull headgear to the molars

You may also consider altering your bracket position: bonding the incisor brackets a little more incisal than normal to aid intrusion.

4. Overjet correction and Space Closure

This is the last stage of treatment, and should not be undertaken until levelling and alignment is complete. It is typically done on rectangular SS wire e.g. 0.018x0.025” or 0.019 x 0.025”. It can be achieved through

i. Intermaxillary traction using latex elastics. These are run in a class II or class III configuration, dependent upon the malocclusion

ii. Intra-arch traction. This is achieved by running powerchain, ligamods, NiTi coil springs or elastics from the archwire hooks to the molar tubes.

5. Seating

This is the final detailing of the occlusion and is undertaken with seating elastics. The working archwire is swapped for a round SS (e.g. 0.018” SS) in the arch which requires correction. At this stage small bends may be placed into the archwire if necessary to further detail the occlusion.

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5. Finishing

i. Tooth positions over-corrected to allow for relapse - e.g. Bite reduced to slightly less than normal.

ii. Arches co-ordinated.

iii. *Correct incisor tip and torque.

iv. *Correct posterior torque to prevent interference and allow centric seating of cusps.

v. Midlines coincident.

vi. *Roots parallel.

vii. *Spaces closed - Residual spaces invite tooth movement and relapse.

viii. *Flat occlusal plane.

viiii. *No rotations.

* Six Keys requirements.

If the case is worked up to 19 x 25 rectangular arches this finishing will be induced automatically, the great advantage of a pre adjusted appliance.

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