Esthetic Dentistry

Case selection for porcelain veneersChing Chiat Lim*

The options for correction of unesthetic teeth are numerotis: among these treatments,porcelain veneers have received substantial attention from the profession and patients.The increase in dental health awareness and advances in dental materials and technology,such as the acid-etching technique, improved physical properties of composite luting resin,and the use of silane coupling agents, have contributed to the success ceramic veneersenjoy today. However, no single material or technique is ivlthout its limitations. Thisarticle therefore aims to review the criteria for case selection and the limitations ofporcelain veneers in restorative dentistry. (Quintessence Int 1995:26:311-3¡5.)

Introduction

Patients often request cosmetic improvements in theirteeth. Frequently, such an improvement also improvesthe patients' sense of well being and self-esteem. Howwell an individual looks is often a contributory factorto success or failure in life in today's highly competitivesocieties.

A porcelain veneer is a thin facing, about 0.5 to0.7 mm thick,'- covering the labial aspects of anteriorteeth and the buccal aspects of some premoiar teeth.Veneers can be fabricated from traditional feldspathicporcelain or other ceramic materials, such as castableor machinable glass ceramics. The veneer is thenbonded to the enamel surface by mechanical andchemical means. The mechanical attachment is achievedby etching the enamel and the porcelain. The resincomposite is then allowed to form a strong micro-mechanical bond similar to that achieved with enamel.̂An additional chemical attachment to the porcelaininterface is accomplished by silane bonding.* Studieshave shown that etching the porcelain surface is ofgreater importance to overall bond strength than isusing a silane coupling agent alone.^

• Lecturer. DepartmenlofRestorative Dentistry. National University ofSingapore, Faculty of Denlistij. Singapore.

Reprint requests: Dr Ching Chiat Lim. Lecturer, Department ofRestorative Dentistry, National University of Singapore, Faculty ofDentistry. 5 Lower Kent Ridge Road. Singapore 0511, Singapore.

Uses of porcelain veneersManagement of noncarious surface defects

Porcelain veneers can be used for the management ofnoncarious defects, such as localized enamel malfor-mations and hypoplasia. Porcelain veneers have beensuggested for the treatment of eroded teeth^ andenamel hypocaicification.' However, bonding ofveneers to poor-quality enamel might lead to marginalleakage and the eventual failure ofthe restorations.

Masking of discoloration

Discoloration arising from fluorosis, tetracycline stain-ing, or necrosis can be treated with porcelain veneers,provided that the stains are not too dark. Limitationsshould always be recognized. Staining arising fromfluorosis can usually be treated successfully withceramic veneers when the surface enamel is removedduring tooth preparation (Figs I to 3).

Repair of structural defeets

Horn* suggested the use of porcelain veneers to repairfractured incisai edges, to correct mild malalignment ofteeth, to achieve closure of diastemas, and to restorepeg-shaped lateral incisors. It is the author's opinionthat resin composite may be a better materiai for casesthat require only minimal replacement of tooth struc-ture. This is because a tooth preparation of some kindis required for a porceiain veneer, no matter how smallthe tooth preparation, to allow enough bulk ofporcelain at the margin for strength. On the other

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Esthetic Dentistry

Ftg 1 Severe tluorosis ¡s evident on the maxillary anteriorteeth.

Fig 2 The dark discoloration has been removed alterintraenamei preparation for porcelain veneers was com-pleted.

Fig 3 Six maxillary anterior porcelain veneers are cemented.

hand, replacement of tooth structure with direct resincomposite can easily be achieved without any toothpreparation, making the procedure more conservative.Tooth preparation is necessary for porcelain veneersbecause it provides the ceramist with a definite marginfor finishing. However, when a few teeth are siightlytnalaligned, involving full labial buildup, porcelainveneers involving some tooth preparation may be moreesthetic and durable than direct resin compositeveneers, which consume more clinical time.

Porcelain veneers can also be used for the correc-tion of cosmetic or minor functional needs in adoles-cent's teeth. Crown preparations on adolescent teethare difficult because ofthe risk of infringement on thelarge, immature pulp by too deep a shoulder prepara-tion,^ Porcelain veneers are also indicated in endo-dontieally treated teeth in which esthetic improvement

is required but placement of a post core is contra-indicated.

Repair offractured porcelain facings on fixed prosthesesMillar' suggested the use of porcelain vetieers to repairfractured porcelain facings on fixed partial dentures.This is only possible if the other aspects of theprosthesis are satisfactory and otily the porcelainfacing requires replacement. Otherwise, it is better toreplace the whole prosthesis.

Replacement of old resin composite veneersPorcelain veneers are frequently indicated for thereplacement of old resin composite veneers, withstaining and wear being the most common complaintsfrom patients.

Orthodontic retainers

Porcelain veneers have also been used as orthodondicretainers to close postorthodontic diastemas."*" Thisworks by maintaining tooth contact with the adjacenttooth, thus preventing the reappearance of spaces. Thelong-term stability of this form of retention is un-known. Applications of this new concept are exten-sive, but care must be taken to evaluate the hmits of thistechnique with adequate long-term chnical studies.

Clinical observations have also confirmed thatetched porcelain veneers are ideally suitable fortnandibular incisors if adequate interincisal spaceexists and the correct tooth preparation design ischosen for the required occlusion.'^

Criteria for case .selection

Patients who are selected to receive porcelain veneersor any other restorative work, should have good oral

312 Quintessence '"'"mntinnnl Vnlr itmhnr.i'ijor.

Esthetic Dentistry

hygiene, healthy gingival tissue, and no occlusaldisharmony.'-' The placement of porcelain veneers isan irreversible procedure because ofthe neccessity fortooth preparation. Therefore, the criteria for porcelainveneers must be carefully reviewed before the proce-dure is undertaken.

Síi7í/f and dynamic occlusal relationshipLong-term studies have indicated that porcelain ve-neers seldom delaminate.'^''' The usual mode of failureis fracture of comers; this frequently happens at theincisai edges. It is therefore important to analyze thestatic and dynamic occlusion before tooth preparation.The occlusion will dictate the choice of tooth prepara-tion design (not discussed in this article). The incisaiporcelain finishing line is determined by the contactrelationships between the incisors and canines incentric and eccentric positions."" The margins shouldbe placed so they do not contact the opposing dentitionduring the rest position.' Ideally, occlusal contactsduring centric occlusion and lateral excursion should beon porcelain or tooth strucmre completely. Occlusalinterferences and parafunctional habits are contraindi-cations for porcelain veneers, because they will result incrack formation and eventual fracture ofthe restoration,

Periodontal and oral health stattisA healthy periodontium fonns a strong foundation onwhich all restorative work will rest. It also facilitatesease of impression taking, cementation, and futuremaintenance. It is therefore important to assess thepatient's overall periodontal and oral health statusbefore the procedure is begun, Mouthbreathers whohave poor gingival health are poor candidates forporcelain veneers.

Condition of tooth

Degree of discoloration

If the tooth is grossly discolored, it maybe necessary tobleach the tooth before the veneer is placed. However,the unpredictable stability of bleaching poses theproblem of the return of discoloration beneath theporcelain veneers once they have been cemented.Usually, the discolorarion in tetracycline stainingbecomes more severe as the enamel is reduced. Suchcases may require masking of areas of marked disco-loration with opaque glass-ionomer cement or opaqueporcelain. In either case, the parient should be warnedofthe compromised esthetics whereby the veneers mayappear opaque. Because the bond strength betweenglass-ionomer cement and resin composite is not aspredictable as that between etched enamel and resin

composite, it is preferable to mask areas of markeddiscoloration with opaque or intensive porcelain (VitaAlpha porcelain. Vita Zahnfabrik). Precise instruc-tions to the laboratory technician regarding areas ofmarked discoloration are therefore crucial. A photo-graph of the prepared tooth is usually helpfiil if anin-house technician is not available. The use of resincomposite to mask marked discoloration is usuallyinadequate, because the resin is usually too translucentto have any masking effect.

Extent of caries

Caries, if present, should be minimal. If little or noenamel is left after caries removal, placement of aporcelain veneer is usually contraindicated, Tiiis isbecause tbe thin veneer is strengthened by bondingwith the underlying etched enamel. The veneer-toothcomplex is weakened when the surface area of enamelavailable for bonding is decreased by 50%,'^

Extent of restoration

A restoration, if present, should be smail enough thatthe area for bonding with enamel is not compromised.The cavity design of the restoration should incindemargins that end in enamel, where possible. The oldrestorations should be replaced if the underlying cariesstatus is questionable.

Quality' of tooth

If the amount of enamel for bending is compromiseddue to any ofthe factors above, a veneer shell crown orconventional crown may be indicated instead of aveneer facing.

Structural defects, such as amelogenesis imperfectaand dentinogenesis imperfecta, that leave insufHicientenamel and tooth structure for bonding are a definitecontraindication for veneer.

Teeth with iarge areas of exposed dentin areunsuitable for porcelain veneers. However, small areasof exposed dentin may be treated with dentinalbonding agents or glass-ionomer cements.

Patients motivation to tnaintain

The patient's attitude toward dental health care shouldbe assessed before porcelain veneers are attempted. Avigilant home care program and regular recalls areimportant to maintain the longevity ofthe restoration.

Patient's expectations

The patient's expectations should be realistic. Adiagnostic waxup may be required to demonstrateareas of limitations when porcelain veneers are to beused for correction of mild malalignment.

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Esthetic Dentistry

Orai habits

Tlie presence of oral habits, such as nail or pencilbiting, is a contraindication for veneers because theshearing stress may be too great for the ceramics towithstand.'*

Advantages of porcelain veneers

The porcelain veneer is usually fabricated in tht;laboratory. It therefore utilizes the ceramist's expertisein creating a lifelike restoration, and relies less on theartistic skill ofthe dentist to individualize and charac-terize the veneers, as is required with direct resincomposite veneers." Compared with the resin laminateveneer, the porcelain laminate veneer also has thefollowing advantages:*

1. Color stability and lifelike appearance2. Maintenance of luster3. Higher resistance to abrasion, staining, and the

deleterious effects of alcohol, medications, andother solvents

4. Biocompatibility with gingival tissue5. Increased chemica! and mechanical bonding to

resin composite6. Less clinical chairside time

When two or more veneers are indicated, directresin composite veneer may be more lime consumingto fabricate. It would be more eificient to indicateporcelain veneers for such cases, provided that theother criteria are met.

Limitations of porcelain veneers

1. Changes in the color, length, contour, and contactareas of porcelain veneers fabricated by therefractory die or platinum foii technique arelimited,* because no reglazing or firing is possiblewith either technique. Tlie veneers have to beremade or modified with ctalored luting cementswhen the shade is incorrect, and the success ofshade modification with luting cement is highlyquestionable. However, when the veneers arefabricated with the castable DICOR (Dentsply)leucite-reinforced ceramic press system (IPS Em-press. Ivoclar} or the copy milling system (Celay,Mikronat), postadjustment staining and reglazingare possible.

2. Stabilization at try-in stage is difficuh becausethere is no tensofrictional grip." However, thisdifficulty can be overcome by making a defhiiteincisai step on the labial surface of the veneer

preparation (Ironside JG. personal communica-tion, 1992) or dimples in the cenicr of thepreparation.'*

3. The dental laboratory costs involved may makethe restoration more expensive to produce andthus less appealing to the patients."

4. Margina! adaptation of most veneers is about 1Ü0|im or more. '̂ ''̂ Very lew with a marginal accuracyof less than 20 \xm have been achieved.'

5. Thin sections of porcelain tend to be monochro-matic and lack the break up of color present innatural teeth. Ifthe veneer is too translucent, areasof marked discoloration may show through;however, a more opaque porcelain buiidup mayproduce a high-Vaiue, monochromatic veneer,lacking in transiucency and dentinal contrast.^

6. It is ven,' difficult to match the shade of a singleporcelain veneer to the rest of the dentition. Insuch a case, a direct resin composite veneer maybe preferred to the porcelain veneer.

7. The bonding procedure is very technique sensitiveand time consuming.

8. Diagnosis of caries underneath cemented porce-lain veneers is difficult (Wlson NHF, personalcommunication, 1994).

9. It is difficuh to replace and repair porcelainveneers when they fail (Wilson NHF. personalcommunication, 1994).

10. Acidulated phosphate fluoride and stannoiisfluoride should be avoided on porcelain veneersbecause both the porcelain and resin compositesilica may be affected.-'

Conclusions

The success of porcelain veneers depends very muchon the method of fabrication and, most importantly,case selection. Because some of the research in thisfield has been based on personal preference andanecdotal information, more objective research isrequired so that porcelain veneers will become fullyaccepted by both the profession and the public as analternative treatment in the field of conservativedentistry.

References

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4. Quinn F, McConnell RJ, Byrne D. Porceiain laminate; A review. BrDentJ I985;i6l:61-6.í.

5. CalaniLa JR. Simonsen RJ. EfTect of coupling agents on bondstrength oietciied porcelain | abstract 79]. J Dent Res Í984;o.l: 179.

6. Reid JS, Simpson MS. Taylor GS. Using porcelain veneers to treateroded teeth. Dent Abstr Í99I:36:255-2S6.

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1\. Essig ME. IsenbergBP. Leinfelder KF. Liu PR In vitro evalution ofCerec veneers utilizing standardized preparation template. In:Mörmann WH (cd). Proceedings of an International Symposium onComputer Restorations: State ofthe An ofthe CEREC Method.Chicago: Quintessence, 1991:547-552.

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