7/29/2019 Ceramics Bracket Ideal

1/6

Trends Biomater. Artif. Organs, Vol 20 (2), pp 000-000 (2007) http://www.sbaoi.org

Ceramic Brackets: In search of an ideal!

Pawan Gautam and Ashima Valiathan*

Dept. of OrthodonticsManipal College of Dental Sciences, Manipal.Karnataka. 576104

* Corresponding Author

Best Paper Award: National Conference on Ceramics for Medical Applications CMA2005, IIT Chennai, September2005

Since their introduction to orthodontics in 1986, the Ceramic Brackets have become popular as esthetic appliances.The newer designs of ceramic brackets offer excellent optical properties and the promise of additional esthetic appealwithout significant functional compromises. Ceramic brackets are durable, allow adequate force control over longtreatment periods, and their risk for discoloration is minimal. The introduction of Ceramic brackets was a much-heralded development in the orthodontic treatment of adult patients. Their acceptance by these patients has beenunprecedented in the practice of orthodontics and contributed significantly in the expansion and development ofcontemporary orthodontic therapeutic modalities. Apart from offering esthetics, ceramic brackets exhibit excellentbiocompatibility. There has been much concern regarding the allergic and cytotoxic effects induced by constituents andthe corrosion products of the stainless steel brackets. Nickel and Chromium are the most common causes of metal-induced allergic contact dermatitis in man. Nickel has recently been reported to be moderately cytotoxic. Hence theadvantages offered by the Ceramic brackets namely excellent esthetics, biocompatibility, corrosion resistance, stabilityin the oral environment and non toxic nature has made them an integral part of the orthodontists armamentarium.

Introduction

Characteristic of an ideal orthodonticappliance include good esthetics andoptimum technical performance. An increasein the number of adult patients led to thedevelopment of various esthetically superiorappliances. Ceramic brackets wereintroduced to orthodontic specialty in 1986and since then have become an integral partof the orthodontists armamentarium. Sincetheir introduction, product design and clinicalperformance has greatly improved. Thesuperior aesthetics of ceramic bracketscompared to conventional stainless steel

brackets is not only well accepted by thepatient, particularly adults, but are positivelysought for

1.

Ceramics - material science

Ceramic brackets are composed ofaluminum oxide .Polycrystalline alumina &

monocrystalline alumina are the two mostcommon varieties. Another category that isbeing developed is theZirconium brackets.

Monocrystalline brackets manufacturingprocess

It is manufactured by heating aluminumoxide to temperatures in excess of 2100 C.The molten mass is cooled slowly, and thebracket is machined from the resultingcrystal.

Polycrystalline brackets Manufacturingprocess

Manufactured by blending aluminum oxideparticles with a binder, the mixture can beformed into a shape from which a bracketcan be machined (sintering process).Temperatures above 1800 C are used toburn out the binder and fuse together theparticles of the molded mixture. Then it is

7/29/2019 Ceramics Bracket Ideal

2/6

heat treated to remove surface imperfectionsand relieve stresses created by the cuttingoperation. The presence of pores, machining

interferences, and propagation linescontribute to compromises of bracket useanytime during clinical use

2.

Optical properties and strength areincompatible for polycrystalline ceramics.The larger the ceramic grains, the greaterthe clarity or translucency. However, thematerial tends to become weaker when thegrains reach a size of about 30 microns.Polycrystalline brackets begin as aluminumoxide particles of about .3 microns, which arefused to produce ceramic grains of 20-30microns. Heat treatments after machiningmust be carefully controlled to preventfurther grain fusion, which could detract fromphysical properties.

The main advantage of the polycrystallinemanufacturing process is its ability to moldbrackets a relatively inexpensive operationthat yields large quantities. Thedisadvantages of this molding process arethe presence of structural imperfections atgrain boundaries or of trace amounts ofimpurities. Impurities in quantities as minuteas .001 percent or slight imperfections canserve as foci for crack propagation understress.

An alternative method of makingpolycrystalline brackets is injection molding.This process does not require the brackets tobe machined and thus eliminates structuralimperfections created by the cutting process.

The molding process and heat treatmentproduce fused aluminum oxide with grainboundaries that refract light, resulting in adegree of opacity. The most apparentdifference between polycrystalline andsingle-crystal brackets is in their opticalclarity

2.

Single-crystal ceramic brackets are

manufactured by an entirely differentprocess. Orthodontic manufacturerspurchase these large single crystals and millthem into the shapes and dimensions ofvarious brackets, using ultrasonic cuttingtechniques, diamond cutting, or a

combination of the two. After milling, thesapphire crystals are heat-treated to removesurface imperfections and to relieve stresses

induced by the milling operations.

The primary advantage of single-crystalmanufacturing is the elimination of possiblestress-inducing impurities or imperfections.The disadvantage is the difficulty and addedexpense of milling the third-hardest knownmaterial. This involves the development ofnew technology with its inherent newproblems. Production of polycrystallinebrackets is less difficult, and hence thesebrackets are more readily available atpresent.

Single-crystal brackets have noticeably more

optical clarity than polycrystalline brackets.Whether the difference is significant is ajudgment to be made by each clinician

Zirconia brackets

Zirconia is a mineral extracted from beachsands of Australia. The PSZ (Partiallystabilized Zirconium) developed by theCommonwealth Scientific and IndustrialResearch Organization (CSIRO) as a reliablehighly stress-resistant material.

A remarkable quality of zirconia -basedadvanced ceramics is that wear actuallymakes the material stronger. In theory, thelow frictional coefficients achievable withyttria-stabilized zirconia should make it asuitable alternative to alumina for bracketconstruction. However, zirconia bracketshave problems related to color and opacity,which detract from the esthetics, and caninhibit composite photopolymerization.Studies have shown that zirconia bracketsoffer no significant improvement overalumina brackets with regard to theirfrictional characteristics.

Comparison of properties

Production of polycrystalline brackets is lesscomplicated, these brackets are more readilyavailable at present. Single crystal bracketsare noticeably clearer than polycrystallinebrackets, which tend to be translucent. Bothresist staining and discoloration. They comein a variety of edgewise structures including

7/29/2019 Ceramics Bracket Ideal

3/6

true Siamese, semi-Siamese, solid, andLewis/Lang designs.

BiocompatibilityNickel(Ni) has been implicated as a toxicmaterial in several dental and non-dentalapplications. Airborne exposure to Ni in dusthas been related to its potential to inducecarcinogenesis in nasal and lung tissues. Inaddition Ni is a common sensitizer. Contactwith Ni containing jewelry, button, zippersand clasps on clothing induce sensitization.The prevalence is estimated to be 10% inwomen and less than 1% in men. Allergicreactions to Ni can be manifested bothlocally and systemically from contact withskin. Reaction to Ni by the mucous

membrane has also been reported. Certainforms of Chromium have been associatedwith lung cancer in industrial exposures.However, carcinogenesis related to dentaland medical applications has not beenreported. Chromium has been reported tosensitize individuals and produce a chronicdermatitis. The sensitivity is the result ofcontact with chrome salts, which result fromcorrosion of such alloys. Apart fromexhibiting excellent esthetics, ceramicbrackets are chemically stable and inert inthe oral environment, and exhibit excellentbiocompatibility with the oral tissues.

Ceramic brackets- hardness

Due to extremely high hardness ofaluminium oxide, both monocrystalline andpolycrystalline alumina have a significantadvantage over stainless steel.

Because ceramic brackets are nine timesharder than stainless steel brackets orenamel, severe enamel abrasion fromceramic brackets might rapidly occur, ifcontacts between teeth and ceramicbrackets exist.

Ceramic brackets- tensile strength

The tensile strength of monocrystallinealumina >polycrystalline alumina, >>stainless steel.

This property also depends on the conditionof the surface of the ceramic. A shallowscratch on the surface of a ceramic bracket

will drastically reduce the load required forfracture. The elongation for ceramic at failureis less than 1% in contrast with

approximately 20% of stainless steel, thusmaking ceramic brackets more brittle.Ceramics have highly localized, directionalatomic lattice that does not permit shifting ofbonds and redistribution of stress. So whenstresses reach critical levels, inter-atomicbonds break, and brittle failure occurs5.

Ceramic brackets- fracture toughness

Fracture toughness in ceramics is 20 to 40times less than in stainless steel, making itmuch easier to fracture a ceramic bracketthan a metallic one. Among ceramicmaterials, polycrystalline alumina presents

higher fracture toughness than single-crystalalumina

5. Semi-twin brackets,( Fascination,

Mystique, & Virage) have significantly highertensile fracture strength than true-twinbrackets,( Clarity, lnVu, & Luxi) . Mono-crystalline brackets (Inspire) are notfractured easily

6.

Bond strength

Ceramic material does not bond chemicallywith adhesives

Chemical bonding : glass is added to thealuminum oxide base and is treated with asilane coupling agent. Silane bonds withglass and leaves a free end of its moleculesthat react with any of the acrylic bondingmaterials. Shiny surfaces of ceramicbrackets bonded chemically allow greaterdistribution of stress over the whole adhesiveinterface without the presence of anylocalized stress areas. Significantly greatershear bond is needed to cause debondingand pure adhesive failure with chemicalbonding.

Mechanical bonding : brackets haveretentive grooves in which edge angles are90. There are also crosscuts to prevent the

brackets from sliding along the undercutgrooves that have sharp edge angles, thusleading to high localized stressconcentrations around the sharp edges andresulting in brittle failure of the adhesive. Onapplication of shear debonding force, part ofthe adhesive is left on the tooth and part on

7/29/2019 Ceramics Bracket Ideal

4/6

the grooved bracket. Mean shear bondstrength of the polycrystalline ceramicbrackets is significantly greater than that

obtained when stainless steel brackets areused. Single crystal ceramic bracketsproduce the lowest mean shear bondstrength values

7. Gwinnet

8(reported that the

mean values for the different bracket typesare not statistically significant, but thisconflicts with the results of many otherstudies.

Bond strengths are greater with chemicalbonding than with mechanical retentionwhich shows bond strengths comparable tometal bracket).Decreasing etching time, (with37% phosphoric acid) from 30 seconds to 10seconds maintains a clinically useful-bondstrength. Light-cured GICs provide sufficientstrength for bonding ceramic brackets, but interms of bond failure site and bracketfracture, they provide no advantage overcomposite adhesives. Mean bond strengthsof Clarity brackets (polycrystalline) andInspire brackets (monocrystalline) are foundto be comparable. No enamel damage wasevident in any specimen when the bracketsare removed with the appropriate pliersrecommended by the manufacturers

9.

Frictional Resistance

Stainless steel brackets generate lowerfrictional forces than ceramic brackets,because of their lower surface roughness,which is clearly visible when comparingscanning electron micrographs. Ceramicbrackets produce significantly greaterfriction

10. Beta-titanium and nickel-titanium

wires are associated with higher frictionalforces than stainless steel or cobalt-chromium wires. Progressively increasingfrictional values: stainless steel bracket