Lingual Orthodontics using the Light Wire Bracket, Part 2

E. Mizrahi. BDS, DORTH RCS, FDS RCS, MSc, PhD.

www.lingualcourse.com Vol 2, No. 1 May 2004

Lingual Orthodontics is an exciting and comparatively new form of orthodontic appliance therapy. Undoubtedly the demand for lingual orthodontics will increase particularly as it becomes more patient driven and due course we will see the evolution of many more different brackets, designs and techniques. In this series of two articles I will present my rationale for using the Light Wire bracket and discuss some of the laboratory and clinical procedures involved. This first article will discuss the pre-clinical procedures and the second article will relate more to the clinical aspects of the technique.

The light wire bracket
The light wire bracket was designed by Raymond Begg and is based on a modified ribbon arch bracket originally designed by Edward Angle in the early part of the last century. It is essentially a one point contact bracket, which allows free tipping of the tooth in the mesiodistal and labiolingual planes. This bracket offers the lowest resistance to tooth movement when compared to any other bracket. The archwire is held in the slot with brass pins or ligature wires, the slot size as well as the friction generated between the wire and the bracket can be altered by using different pins.
Dimensions
The bracket slot is 0.5 mm in depth and 1 mm in height. The width of the functional part of the bracket, i.e. the component that actually holds the archwire is 1.3 mm. The thickness of the base is 0.7 mm that is the distance the archwire is away from the tooth surface.
Compared to any other commercially available edgewise bracket, this bracket allows the closest approximation of the archwire to the tooth surface and probably more important for lingual orthodontics, it provides the largest interbracket span.
Using a mounted wheel stone, the width and height of the curved (for canines and premolars) or flat (for incisors) bracket base may be reduced without affecting the functionality of the bracket. This is an important benefit when accommodating the bracket in a crowded dentition and adapting the base to teeth with reduced crown height such as lower premolars.

Orientation of the bracket on the tooth
In conventional labial light wire orthodontics, the bracket is orientated with the archwire slot facing gingivally. To allow the archwire to be inserted from the incisal aspect when using the lingual technique, the bracket is placed with the archwire slot facing incisally.
Rationale for using the light wire bracket
1. Acceptance of the Begg philosophy of treatment, which allows for crown tipping, followed by the correction of root angulations and root torque with intra oral auxiliary springs.
2. The small dimension of the bracket reduces the encroachment on the lingual tongue space and provides the greatest inter-bracket distance when compared to any other commercially available bracket system. This is an extremely important feature and critical in lingual orthodontics particularly in relation to the mandibular incisor teeth.
Molar attachments.
Conventional Begg single round buccal tubes are bonded to the palatal surface of the first molars. These tubes have an internal diameter of 0.9 mm inches with a gingivally and distally directed molar hook. The same tube can be bonded to the second molars.

Stainless steel mesh spot welded to band.
When obliged to use conventional bands particularly on mandibular molars, where in some cases, it is difficult to achieve a dry field, the buccal surface of the band can be masked and made aesthetically acceptable by covering the surface with a flowable composite. After fitting the band and welding a lingual tube, weld a strip of 100 gauge stainless steel mesh to the buccal surface. (Figure 1) Cement the band in the mouth, then dry the mesh and flow a composite material over the mesh and light cure. Once cured, the material can be smoothed, shaped and contoured to suit the case. (Figure 2)

Fig 1: Stainless steel mesh spot welded to the buccal surface of a band.
Flowable composite to be applied after band cementation.

                   Fig 2: Buccal surface of mandibular stainless steel bands covered with flowable composite

Laboratory procedures  
Before discussing the laboratory procedures, a few words about the problems associated with the palatal surface morphology of incisor teeth. The palatal surface is concave to varying depths, and the marginal ridges may be excessively pronounced. This presents a problem if the bracket is bonded directly to the palatal enamel in the depth of the palatal concavity.
In the horizontal plane, access to the bracket is limited and difficult and more significant, where the archwire crosses the contact area it may impinge or lie very close to the marginal ridges. This interferes with the placement of auxiliary springs.
In the vertical plane, because of the palatal concavity and the variation in thickness of the tooth at the cervical and incisal regions, small vertical variations in the placement of the bracket along the palatal concavity, significantly influences the distance of the archwire to the labial surface of the tooth, and consequently the labio-lingual relationship of adjacent teeth.
In view of these problems, it is essential to eliminate the palatal concavity so that the bracket can be bonded to a smooth palatal surface.
The laboratory procedures can be carried out in-house or by specialised dental laboratories. I will briefly describe the in-house laboratory procedure.
On a duplicated working model, using plaster, fill in the palatal concavity and flatten the palatal surfaces of the lateral and central incisors. (Figure 3)

Fig 3: Plaster applied to the palatal surface of maxillary incisors to flatten the palatal concavity. (Photo by courtesy of Ortholine Laboratory)

Mark the required vertical heights of the brackets on the model and using a thickness gauge, measure and adjust the labio-lingual thickness of the central and lateral incisors to be equal or slightly thinner for the laterals as required.
Place a drop of metal primer on the mesh base of the bracket, then apply a small amount of light curing light bodied composite. Firmly place the bracket on the palatal surface of the tooth which has been pre treated with a separating medium. The base of the bracket will become modified and conform accurately to the modified palatal surface. Make sure the level of the archwire slot lines up with the horizontal markings on the plaster tooth. (Figure 4)

Fig 4: Brackets placed in position on incisor teeth after surface modification.

Once all the brackets and tubes have been tacked onto the model, then using a clear silicone material such as Memosil, create the transfer tray.

Vacuum formed template.
In order to copy and transfer the modified incisor palatal surface morphology from the model to the mouth, it is necessary to construct a plastic transfer tray. Using 1 mm clear Essix type material, create a pressure or vacuum formed tray over the modified six anterior teeth overlapping the incisal edges and covering 2mm of the labial surfaces. (Figure 5)

Fig 5: Applying flowable composite to plastic transfer tray for modification of palatal surface morphology.

 If the case is sent out to a laboratory, a prescription for bracket placement and palatal surface morphology alteration should be given to the technician.

Preparation of palatal surfaces.
Follow standard procedures regarding isolation, pumicing, washing, etching, washing and drying of the palatal surfaces of the six anterior teeth. To enhance bonding to the palatal surfaces two features need to be emphasised.
1. First, gently run over the enamel surface with a small round stone or diamond bur. As opposed to the smooth labial surface, the palatal surface is more irregular, convoluted and often has a layer or plaque or calculus which is not always visible to the naked eye. Pumice on its own will not remove this layer and you will end up bonding to a thin layer of calculus which will in turn invite bond failure.
2. When using pumice and brush, do not use a bristle cup brush or a rubber cup, these will not adequately clean a concave surface, rather use a round or pointed tuft brush, this cleans the concave surface more efficiently.1 For the palatal surfaces of premolars and molars where the surfaces are convex, use a rubber cup rather than a bristle brush cup. The rubber cup allows you to get closer to the gingival margin without causing gingival bleeding.

Modification of the palatal surface of maxillary incisors.
To prevent composite adhering in the interdental areas, Vaseline is liberally applied to the labial surface of the incisor teeth with a cotton wool roll, encouraging the Vaseline into the interdental areas. Paint sealant on the relevant dried and etched palatal surfaces, place sufficient easy flowing composite on the palatal surface of the vacuum formed template, seat the template carefully and firmly then light cure each tooth. Remove the template, if there is excess resin in the interdental areas or gingival margin areas, remove carefully. If there is a risk of gingival bleeding then postpone the removal of excess composite till after bonding the brackets.
Brackets may be bonded to the modified incisors and the remaining teeth either directly or indirectly using the previously created silicone transfer tray. 

Instruments
The most useful pliers are the curved Weingart, these come in different degrees of curvature and fineness of the beaks. 
A double ended notched flat plastic hand instrument is extremely useful for moving archwires into and out of the bracket slot; it can also be used to uncurl the pin tails prior to cutting.
A lingual pin cutter was specifically made by Plydentco, (325 Philmont Ave. Unit A, Feasterville, PA 19053. USA) has the cutting blades at 450 to the body of the pliers, but more important, the cutting edge is on the inside angle of the blades.
With the brackets bonded in their correct positions, the case is now ready for the placement of archwires. Clinical aspects of treatment using the light wire bracket technique will be described in a subsequent article.

Reference
1. Mizrahi E. Orthodontic Pearls. A selection of practical tips and clinical expertise. Published by Taylor and Francis. ISBN 1-84184-252-4  2004) London and New York. Chapter 13 Page 155.

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Adult and Lingual Orthodontics
EDITORS:
Dr. Silvia Geron D.M.D., M.Sc
Dr. Rafi Romano D.M.D., M.Sc
Dr. Pablo Echarri D.M.D., M.Sc