Orthodontics

Light-curing acrylic resin as an orthodonticbaseplate materiaiJohn Brown, MSc, LOTA, LBIST*AV. John S. KerT, DDS, MDS, DOrth, FDS, FFD*

Heat-curing autopolymerizing (self-cure or cold-cure), thermoplastic, and light-curing acrylic resinare the most commonly used orthodontic baseplate materiais. While cured acrylic resins present fewproblems to the patient, in the laboratory acrylic resin has to be sprayed, mixed, or packed in a fume-extraction unit because of the harmful fumes emitted by the raw inflammable chemicals. Light-curing ma-terial, on the other hand, is virluatly nonflammable and has virtually no aroma. A light-cure techniquefor the construction of orthodontic baseplates is described. While buildup of the baseplate is slightlyslower ¡han for .•.-elf-cured material, the shorter time involved in trimming and polishing means that over-ait construction isfasier. It is easier to obtain a uniform thickness with light-cured materiai. and it pro-vides superior fit. These results, however, are subject to more extensive ciinical trials. The only apparentdisadvantage is the fine powder produced during trimming. Even with a bench equipped with an extrac-tion unit, it is advisable to use a face mask to prevent the inhalation of dust. (Quirttessence Int1998:29:508-512)

Key words: acrylic resin, light curing, orthodontic baseplate, self ctiriug

Clinical relevance

Light-curing materials are less hazardous to thetechnician than seif-curing materials, and the fabri-cation time for orthodontic baseplates is shorter.

Despite the problems and hazards associated withthe tnanipuiation of acryiic resins, they have re-

mained the materials of choice for orthodontic haseplateconstruction for more than 40 years. Information sheetssupplied with methyl methacrylate products list the ma-terial as an "irritant and highly inflamtnabie liquid" thatcan, with inhaiation, be an "irritant to the respiratorysystem and excessive exposure to it may cause dizzinessand narcosis." The monomer, "when mixed with the ap-propriate powder, forms a cross-linked mass which isused intraorally and the product is inert and nontoxic."The finished product, when cured, presents few prob-lems.

Initially, heat-curing denture base resins were usedfor orthodontic appliances. With advances in the rnethod

•Unit of Orthodontics. University of Glasgow. Dental School, Glasgow,Scotland.

ReprinI requests; Mr John Brown, Onit of Orthodontic!, University ofGlasgow. Denial School, 378 Sauchiehall Street. Glasgow G2 3JZ,Scotland. E-mail: j.brown@dentai.gla.ac.uk

of mixing and curing, these resins were modified tomake it possible to apply and process the resin directlyon the cast with a spray technique. The spray technique,also described as the "salt and pepper technique" by theBritish Standards Institution,' is when monomer andpolymer are dispensed onto the surface of the ca.st alter-nately as the buiidup progresses, until a sufficient coat-ing of resin is present.

Self-curing acrylic baseplate materials for orthodon-tic use may be prone to problems such as a weaker base-plate, poorer fit hecause of contraction, and a potentiallygreater risk of residual monomer compared to heat-cured tesin. The main problem with all acrylic resins isthat they have to be used withiti a fume-extracfion unitbecause the material is highly inflammahle and irritatingto the skin. Acryiic resins are well accepted by patientsbecause the cured material is relatively kind to the tis-sues.

The extraoral acrylic resin iight-curing system Triad(DeTrey/Dentsply) for dental lahoratories was intro-duced in 19S3 and described hy Lewis et al.- This wassuperseded in the 1990s by a second-generation systemTriad 2000 (DeTrey/Dentsply). The premixed TriadSystem materials, according to the manufacturer, are pli-ahle and cure on command with no waste, mess, ormonomer odor. The material used in the present studywas Thixotec (David Burdess Orthodontics), a lioht-curing gel supplied in toothpaste-sty le tubes. Thixotecgel is similar to the Triad gel.

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Fig 1 Gel being squeezed around wire tags during buiidup. Fig 2 Smali painibrush being used to draw the gei between theteeth.

Laboratory technique with light-curing gel materials

Wire work anil prepciration of ihe cas!

Wire components are constructed using the same tech-nique, regardless of whether the apphance is constructedof self-cured or light-cured resin. All the tags of thewire components must have a space of 0.5 to 1.0 mm toaliow the baseplate material to flow under them duringthe adaptation of the material. Failure of the baseplatematerial to flow completely under the wire componentscould lead to the wire tags becoming detached from thebaseplate, or the entire component becoming detachedduring treatment, leading to appliance failure and traumato the tissues.

The baseplate material must not damage the cast sur-face and must he easily removed after processing. Asealer is used to prevent the baseplate material from ad-hering to the surface of the cast. Sodium alginate di-luted with 50% water was used to increase the flow andprovide a very thin layer on the surface of the cast.Sodium alginate, because of its viscosity, can form a gelwhen setting between the wires and the cast. Loweringthe viscosity with water allows the thinner sealer tosoak more quickly into the cast, preventing contamina-tion of the wires. Separation of baseplate materials andthe cast is not affected by this modification.

With the light-cure technique, while it is not neces-sary to soak the cast, it is preferable if the cast is veryslightly damp when the material is adapted because thiswill prevent the separating medium from dryingout. The gel tube nozzle is positioned approximately0.5 tnm from the cast surface and gently squeezed, start-ing with the most posterior wire tags. The gel materialis squeezed around the wire tags, forcing the materialunder the wires (Fig I). The procedure is continuedaround the tags up to the gingival margins and then

along to the next wire tag. and the procedure is repeatedagain. When the opposite side of the cast at the pos-terior border is reached, application of the tnaterial isstopped. A small paintbrush' is used to pull the light-curing material between the teeth (Fig 2). Finally, thevault of the palate is filled, and the brush is used againto shape the extension of the baseplate. The uncuredlight-curing material surface will, if left for a few sec-onds, settle and form a .smooth surface. The materialwill not slump, even if left longer, but has the conve-nient characteristic of flowing or settling into an evensurface.

The thickness of the light-cured material should notexceed 2 to 3 mm at any part of the baseplate; otherwi,seit will cure on the top surface only and on the surfacenext to the cast, leaving a void between the layers.

Curing procedure

The correct polymerization of light-curing materials de-pends on the wavelength of the light .source. Dependingon their composition, materials may use different lightsources and times. The recommended rating forThixotec is between 400 and 500 nm and uses a ZeonStrobe light unit. There are two settings on this machine(I = 30 seconds and II = 3 minutes); the first setting isused for tacking the material, and the second setting isused for the final curing. The appliance is placed intothe light-curing unit, consisting of a box large enough toaccommodate a plane line articulator or an occluder,complete with cast, and set at cycle I. The strohe unitwill operate for 30 seconds; then the appliance is re-moved and inspected for thickness. If the appliance hasa sufficient thickness of material, the final curing cyclemay proceed.

If the thickness is not .sufficient, the surface shouldnot be touched because the oxide, or dispersion, layer

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Fig 3 Buiiding up posterior bite pianes, with support (rom sheetwax.

Fig 4 Basepiate prior to finishing prccedures.

will become contaminated. The dispersion layer allowsmore light-curing gel to be added to the existing curedmaterial. A further layer of gel is added to thin areas,but, to achieve an extremely smooth finish, the brush isused to coat the whole of the appliance. This techniquecan prevent any demarcation areas between original andnewly adapted material- Any baseplate additions, eg, an-terior, posterior, and inclined bite planes, should bemade by adding the gel and curing in layers until thedesired thickness is achieved (Fig 3).

Problems can he encountered when appliances withexpansion screws are cured. becau,se the gel under theexpansion screw remains uncured because the light failsto penetrate. This problem can be solved by using aportable curing light and tacking the light-curing gelwhen the screw is positioned or by tilting the appliancein the light box to allow the light waves to reach the gelunder the screw. The rest of the buildup can continue aspreviously described.

To seal the surface of the dispersion layer and obtaina complete cure, an air-barrier coating should be used.Petroleum jelly is an alternative material. These materi-als seal the surface and prevent the air from coming intocontact with the surface, thus ensuring that the surfaceis cured completely. The surface is coated with a thinlayer of air-harrier coating, and the appliance is re-placed in the curing unit for a further cycle at .setting II.The appliance is removed from the cast, and the fittingor contact surface is cured in the same manner as theprevious side. The appliance is boiled out to remove thewax in the same manner as other appliances.

Finishing the appliance

The reproduction of the fitting surface hy light-curedgel material is extremely good. The light-curing gel ma-terial flows under the wires, and all the wires have a

complete covering of baseplate material after curing. AllLhe detail of the cast surface is also recorded, withoutvoids on the haseplate material surface, in contrast tothe findings of a previous study using the light-curing sheet materials.-* The gel light-curing materialflows extremely well onto the cast surface and gives anexcellent reproduction when the baseplate material iscured.

The fit against the cast surface is also better than thatohtained with self-cured resin. A light-cured and a self-cured appliance was sectioned and checked visually forcloseness of adaptation to the cast. The light-cured ma-terial gave a much closer adaptation to the cast surfacethan did the self-cured material.

Because the natural finish of the material is ex-tremely good (Fig 4). the only trimming required is atthe back edge and around the collets. Light-cured appli-ances can be trimmed quickly hecause of the accurateadaptation process. Standard steel rotary trimmers andtungsten carbide burs can be used. When light-curedmaterials are trimmed, a fine dust is given off duringabrasion. Even with a dust-extraction unit, it is advis-able to wear a dust-filtering face mask because of thefineness of the dust.

The standard procedure for polishing acrylic resinappliances is used for light-cured materials. After pol-ishing, the appliance is washed thoroughly with soapand water, rinsed, dried, checked, and sealed in a plas-tic hag.

Additions or repairs

The dispersion layer has to be reintroduced on com-pletely cured or used appliances so that new materialcan he added. A thin layer of gel is painted on the sur-face after trimming and cured at cycle I. The huildup iscontinued, and the standard curing cycles are used.

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I Light-cured

HOrihnrBSir

Buildup Trimming Polishing

Fig 5 Production limes for buildup, trimming, and polishing ofiight-cured and self-cured resins.

I Light-curedlOrthoreEin

Overall production

Fig 6 Overaii baseplate production limes for hght-cured andself-cured resins.

Comparison of adaptation times for light-cured andself-cured materials

Fourteen light-cured and 11 self-cured orthodontic ap-pliances were constructed under standardized condi-tions by the same technician, and the times for comple-tion of each stage of baseplate construction wererecorded (Figs 5 and 6).

Buildup of baseplate imirerials

The self-cured appliances were recorded from the start ofthe buildup of the monomer and polymer onto the modeland stopped when the buildup was completed. The tim-ing of the light-cured appliances started when the capwas removed from the tube of gel and stopped wheneach stage of the buildup was completed. Light-curingmaterial took longer to build up the appliance to the de-sired shape. The average light-cured apphance took ap-proximately 2.5 minutes longer to complete the adapta-tion of die baseplate material (curing times not included).

Trimming

No time was recorded for boiling off the wax becausethere is no difference in the procedure for light-curingand self-curing materials. The light-cured appliancesrecorded the shorter time for trimming because theycould be adapted more accurately during the buildupstages. These appliances only required trimming aroundthe collets and the back edges. Anterior and posteriorhite platforms might require slightly more trimming. Theself-cured material required slightly more trimming timebecause the material had to he slightly overbuilt and alsoresulted in a rough granular surface. This surface had tobe trimmed all over to ensure that a smooth finish wasachieved before proceeding to the polishing stage.

Pali.filing

The light-cured material was easier to trim and polishbecause the material adapted to a smooth surface duringtbe initial buildup of the appliance. The material took ona reasonable pohsh using conventional polishing materi-als. The .self-cured appliances recorded a slightly longertime because the whole of the appliance had to be thor-oughly polished, but the average time difference wasonly about 1 minute.

Statistical analysis

A two-sample f test was performed to determine thedifferences in time required to adapt the baseplatematerial, trim the baseplate material, and polisb thebaseplate material, A P value of .000 indicated thatthere was a statistically significant difference in thematerials for huildup (greater time for light-curedappliances) and trimming (less time for light-curedappliances). There was no statistically significant dif-ference in tbe time needed for polisbing. A two-sample t test for the combined times of buildup, trim-ming, and polisbing indicated that there was astatistically significant difference in production timesof the materials {P ^ .008), being shorter for light-cured appliances.

Conclusion

Light-curing material was easier to adapt and took aslightly shorter time overall to complete baseplate con-struction. Although the time difference was small, thematerial was being used for the first time, and famil-iarity might further reduce this time. Clinical studiesare required to assess the durability of the materialunder normal usage.

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The main disadvantage of light-cured tnaterials wasthe Fttie dust produced during Irimmitig, The dust was avery fine powder as opposed to the shavings producedwith self-cured resin. This fine powder could be a dis-tinct disadvantage to the widespread nse of light-curingmaterials. When these light-curing materials are used, itis advisable to use a protective face dust mask as wellas an efficietit dust-extraction unit at the laboratorybench.

References

1. British Standard Specification for Orthodontic Resins. BS6747.London: British Standards Institution, 1987,

2. Lewis EA, Ogle RE, Sorenson SE. Zysik DA. Clinical and labo-ratory evaluation of visible light-cured denture base resins andtheir application iti orthodonlics. Am J Orthod 19SS;94:207-215.

3. Brown J. The Evalualion of Light Cure Baseplate Material [the-sis]. University of Glasgow, 1996.

4. Tan H-K, Brudvik JS, Nicholls Jl. Adaptation of visible light-cured denture base material. J Prosthet Dent I989;6y:326-Î31.

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