Effect Of Porcelain And Enamel Thickness On Porcelain .
Effect of porcelain and enamel thicknesson porcelain veneer failure loads in vitroChunling Ge, DDS, PhD,a Chad C. Green, BSE, DDS,bDalene Sederstrom, CDT,c Edward A. McLaren, DDS,d andShane N. White, BDentSc, PhDeUCLA School of Dentistry, Los Angeles, Calif; Peking University Schoolof Stomatology, Peking ChinaStatement of problem. Bonded porcelain veneers are widely used esthetic restorations. Although high success and survivalrates have been reported, failures occur. Fracture is the most common failure mode. Fractures range from incomplete cracksto the catastrophic. Minimally invasive or thin partial veneers have gained popularity.Purpose. The aim of this study was to measure the influences of porcelain veneer thickness and enamel substrate thickness onthe loads needed to cause the initial fracture and catastrophic failure of porcelain veneers.Material and methods. Model discoid porcelain veneer specimens of varying thickness were bonded to the flattened facialsurfaces of incisors, artificially aged, and loaded to failure with a small sphere. Individual fracture events were identified andanalyzed statistically and fractographically.Results. Fracture events included initial Hertzian cracks, intermediate radial cracks, and catastrophic gross failure. Increasedporcelain, enamel, and their combined thickness had like effects in substantially raising resistance to catastrophic failure butalso slightly decreased resistance to initial Hertzian cracking. Fractographic and numerical data demonstrated that porcelainand tooth enamel behaved in a remarkably similar manner. As porcelain thickness, enamel thickness, and their combinedthickness increased, the loads needed to produce initial fracture and catastrophic failure rose substantially. Porcelain veneerswithstood considerable damage before catastrophic failure.Conclusions. Increased enamel thickness, increased porcelain thickness, and increased combined enamel and porcelainthickness all profoundly raised the failure loads necessary to cause catastrophic failure. Enamel and feldspathic porcelainbehaved in a like manner. Surface contact damage occurred initially. Final catastrophic failure followed flexural radialcracking. Bonded porcelain veneers were highly damage tolerant. (J Prosthet Dent 2014;111:380-387)Clinical ImplicationsIncreased enamel and porcelain thickness both substantially raised theloads needed to cause the catastrophic failure of model porcelainveneers. Porcelain and enamel thickness should be maximized, withenamel preservation being prioritized. Porcelain veneers are highlydamage-tolerant restorations. Wherever possible, occlusal contactshould be avoided on porcelain veneers.aAssistant Professor, Peking University School of Stomatology; and Visiting Scholar, UCLA School of Dentistry.Student, UCLA School of Dentistry.cLaboratory technician, UCLA School of Dentistry.dAdjunct Associate Professor, UCLA School of Dentistry.eProfessor, UCLA School of Dentistry.bThe Journal of Prosthetic DentistryGe et al
May 2014Bonded porcelain veneers have beenwidely used to address esthetic dentalproblems for more than 3 decades.1,2Porcelain veneers have many advantages, including pleasing esthetics,abrasion resistance, and stability. Toothpreparation for veneers preserves muchprecious tooth structure, especiallyenamel and the important dentinoenamel junction.3-5 The success of porcelainveneers has been attributed to a durablebond between 2 materials of similarelastic moduli, porcelain and enamel.6-9Initial reports on porcelain veneersdescribed a nonpreparation technique.2,6,10 However, bulky gingivalcontours can limit cleansing and provideunnatural gingival profiles, and gingivalrecession is a common problem.11Subsequently, techniques with toothpreparation became widely accepted.Conservative tooth preparation facilitates optimizing the emergence profileand overall contour and provides adefinite finishing line. As well as facialreduction, preparation designs ofteninclude incisal, proximal, and evenlingual reduction.8,12-18Recently, minimally invasive veneerpreparation designs have become popular. These involve less tooth reduction,partial coverage, and minimal porcelainthickness.19-23 Minimally invasive veneers have also been described as beingmini, minimal, minimal thickness, ultraconservative, ultrathin, partial, orsectional veneers.19-25 Thicknesses of0.3 mm have been reported for minimally invasive veneers,19-22 whereasconventional porcelain veneers generally range from 0.3 to 1.0 mm inthickness.14,16,17,26-29 Clinical outcomedata for minimally invasive veneers haveyet to be published.Excellent clinical outcomes, goodsatisfaction ratings, and high survivalrates have often been reported forconventional porcelain veneers.10,12,1416,28-37However, more rigorousstudies with a wider variety of outcomemetrics, Kaplan-Meier or lifetime survival curves, and entailing moredemanding analyses have generally reported lower long-term success andsurvival rates.7,11,17,18,38-45 AlthoughGe et al381leakage, marginal discrepancies, debonding, esthetic problems, caries, periodontal problems, and pulpal diseasemay all occur, fracture is the mostcommon failure mode.10,11,14-18,39,42,45Clinical fracture modes for conventional bonded porcelain veneers includelongitudinal or radial cracking; chipping or fracturing in incisal areas, areasof occlusal contact, and areas close tothe veneer margins, and in marginalareas, semicircular half-moon fractures,10,14,15,17,18,28-30,39,42,46 reminiscent of those found in porcelain jacketcrowns.47 Fractures have been ascribedto the application of flexural tensilestresses to porcelain veneers by functional loading,15,17,31,39 cement polymerization shrinkage, and thermalcycling.46,48Fractographic analyses of clinicallyfractured ceramic crowns have indicatedthat failures initiate on their inner, ortensile, surfaces.49,50 For ex vivo crownmodels, flexural radial cracking isdependent on ceramic thickness.51These ex vivo crown models underwent2 distinct fracture events: first, Hertziancone cracking at the surface contactarea, and second, radial cracking starting from flaws in the inner ceramic surface from flexural tensile stresses.Although veneers differ from crowns inthat they are thinner and more flexible,as well as durably bonded to an intactlayer of supporting enamel, similarfracture events may occur.52 A finiteelement analysis indicated that thinnerporcelain veneers were prone to highstresses on both their internal andexternal surfaces after bonding withresinous cements.48 Furthermore, anex vivo compressive test reported thatincreasing the thickness of a porcelainveneer from 0.5 to 1 mm increased thefracture strength.53The effects of both porcelain andenamel substrate thickness on fractureresistance warrant investigation, particularly given the increasing use of minimally invasive porcelain veneers and thelimited thickness of tooth enamel.54 Thepurpose of this study was to measure theinfluence of porcelain veneer thicknessand enamel substrate thickness on theloads needed to cause initial and catastrophic porcelain veneer failure. Thenull hypothesis was that porcelainthickness and enamel thickness do notinfluence the loads needed to producecatastrophic failure.MATERIAL AND METHODSTeethMaxillary central and lateral incisorswith completely intact crowns androots and free of caries or restorationswere selected, cleaned, and stored in0.01% thymol solution at room temperature. The teeth were embedded inepoxy resin (Fast Cure Epoxy; Extec)within phenolic rings (11/400 RingMolds; Extec). The labial surfaces weregently ground under water with 240,400, and 600 grit silicon carbide (Carbimet Paper Strips; Buehler) until flatareas of enamel more than 6 mm indiameter were obtained. All experimental procedures were performed inan ambient atmosphere of 70% to 75%humidity and 20 C to 23 C.Porcelain veneersModel discoid porcelain veneerspecimens were sectioned from cylinders made of feldspathic porcelain.Feldspathic dental porcelain powder(Vita VM13; Vita Zahnfabrik) wasplaced in a 6.2-mm-diameter cylindricalpolyvinyl siloxane mold (Aquasil UltraXLV Regular Set; Dentsply Intl). Thepowder was packed into the moldagainst a glass slab with an acrylic resinplunger, and sufficient water was addedto wet the material. The wet powderwas then compressed by using theplunger with light tapping forces from asmall mallet. Excess moisture wasremoved with an absorbent tissue. Theporcelain specimens were removed, andthe remaining moisture was extractedby drying in front of a heated furnace(Vacumat 40T; Vita Zahnfabrik). Specimens were preheated to 450 C, heldfor 2 minutes, and fired with a heat riseof 25 C/min to 890 C under vacuumfor 17.3 minutes, held for 2 minutes, let
382Volume 111 Issue 5cool to 350 C with the muffle 70%open, then held under the muffle for 10minutes. The porcelain cylinders werethen trued to a diameter between 5.4and 5.5 mm and sectioned with a slowspeed diamond saw (Isomet; Buehler)to make specimens with a thicknessranging from 0.2 to 1.4 mm. Thethicknesses of the individual specimenswere measured with a dial caliper(Mitutoyo) to 0.01 mm.another 50 minutes, then stored in waterfor 10 days. Specimens were artificiallyaged by thermal cycling 1000 times between 5 C and 55 C, with dwell times of120 seconds and a transfer time of 15seconds. Thermal cycling is known todecrease the strength of bonded veneers.52,55 Extended dwell times wereused to ensure adequate heat transferfor the specimens and their largemounts.56Bonding procedureTesting and analysisNomarski, and polarization to identifyand study fracture initiation sites andmodes. Fracture load was plottedagainst enamel thickness, porcelainthickness, and combined enamel andporcelain thickness. Regression analysiswas used to identify the simple linearequations relating fracture load tothickness, and correlation coefficients,R2, were calculated.RESULTSThirty veneers of different thicknesswere assigned to teeth. The porcelainveneers were etched with 9.5% bufferedhydrofluoric acid gel (Porcelain Etchant;Bisco) for 90 seconds, rinsed with water,and thoroughly air dried. Two coats of a2-part silane coupling agent (BISSILANE; Bisco) were applied and dried30 seconds later. The teeth were cleanedwith a slurry of pumice, rinsed, and dried.The enamel was etched with 32% phosphoric acid with benzalkonium chloride(UNI-Etch; Bisco) for 15 seconds, rinsedthoroughly, and dried lightly, leaving theenamel visibly moist. Two coats of a2-part dual-polymerized bonding agent(ALL-BOND3; Bisco) were applied, airdried for 12 seconds to evaporate solvents, and light polymerized for 10 seconds. Thin layers of a porcelain bondingresin (Hema-free unfilled resin; Bisco)were applied to the veneers, which werelined with a light-polymerized veneercement (Veneer cement, Choice 2;Bisco). The veneers were gently seated,and static vertical loads of 2.83 N wereapplied to standardize seating load andcement layer thickness.48 The seated veneers were light polymerized (Optilux500; Kerr) for 4 seconds to tack them inplace before excess cement was removed.The veneers were then polymerizedcircumferentially from their peripheriesfor 40 seconds before being polymerizedfrom their facial aspects for a further40 seconds.Artificial agingBonded specimens were stored in airfor 10 minutes, in 100 % humidity forSpecimens were radiographed digitally (XDR; Cyber Medical Imaging) tomeasure the enamel thickness beneaththe center of each veneer. Moist specimens were placed onto the platen of auniversal testing machine (5966; Instron) with the porcelain uppermost. Atungsten carbide sphere, 1.59 mm inradius, was placed on the center of eachmodel veneer. The radius of the spherewas somewhat larger than the radius ofan incisal edge but smaller than that of alarge cusp. This method was known toproduce elastic deformation beforeHertzian cracking and plastic deformation in the outer surface contact areawithout initially causing bulk or catastrophic fracture.57 Radial cracking wasexpected to start from the inner bondedceramic surface after surface damage51;clinical cracking and chipping in theareas of occlusal and incisal contactshas been widely reported for veneers10,14-17,30,32,33,39,42,44,45 and forglass-ceramic onlays.58 The specimenswere loaded at a crosshead speed of 0.01mm/min, and load-time data wererecorded until catastrophic failureoccurred. Individual fracture events wereidentified by post hoc analysis of theuniversal testing machine load-time datafiles with a spreadsheet (Excel; Microsoft). During pilot testing on additionalspecimens, individual fracture eventswere studied and sequenced by stoppingthe test after individual events had beenidentified on the load-time display andon performing qualitative fractographicexamination. Fractographic analysis wasperformed with a variety of light microscopic techniques, brightfield, darkfield,The Journal of Prosthetic DentistryInfluence of porcelain veneerthickness on porcelain veneerfracture eventsIncreasing porcelain thickness tended to slightly decrease the load neededto form initial cone cracks (Fig. 1); thelinear equation produced by regressionanalysis for the influence of porcelainveneer thickness on the initial conecrack fracture event was y¼ 130xþ402(R2¼0.04). Increasing porcelain thickness markedly increased the loadneeded to produce terminal catastrophic fracture (Fig. 1); the linearequation produced by regression analysis for the influence of porcelain veneerthickness on the terminal catastrophicfracture event was y¼723xþ517(R2¼0.5).Influence of enamel thickness onporcelain veneer fracture eventsThe influence of enamel thickness onporcelain veneer fracture events wasremarkably similar to that of porcelainthickness (Figs. 1, 2). Increasing enamelthickness tended to slightly decrease theloads needed to form initial cone cracks(Fig. 2); the linear equation produced byregression analysis for the influence ofenamel thickness on the initial conecrack was y¼ 118xþ367 (R2¼0.01).Increasing enamel thickness markedlyincreased the load needed to produceterminal catastrophic fracture (Fig. 2);the linear equation produced by regression analysis for the influence of enamelthickness on the terminal catastrophicfracture event was y¼804xþ590(R2¼0.3).Ge et al
May 201438316001600Catastrophic Failurey 723x 5171200120010001000800600400800600400Initial Failurey –130x 40220000.00Catastrophic Failurey 804x 5901400Load (N)Load (N)14000.501.001.502.00Initial Failurey –118x 36720000.002.500.50Porcelain Thickness (mm)1 Plot of fracture load against porcelain veneer thickness.Initial Hertzian surface cracks are plotted as light blue diamonds; intermediate radial cracks as small black dots; andfinal catastrophic failures as large blue circles. Regressionlines for initial Hertzian surface cracks and final catastrophicfailures are plotted.Influence of combined porcelainveneer and enamel thickness onporcelain veneer fracture eventsGe et al1.502.002.50Enamel Thickness (mm)2 Plot of fracture load against supporting enamel thickness.Initial Hertzian surface cracks are plotted as light blue diamonds; intermediate radial cracks as small black dots; andfinal catastrophic failures as large blue circles. Regressionlines for initial Hertzian surface cracks and final catastrophicfailures are plotted.multiple intermediate events oftenoccurred at different loads (Figs. 1-3).Qualitative fractographyThe influence of combined porcelainveneer and enamel thickness on porcelain veneer fracture events was consistent with those of separately plottedporcelain veneer and enamel thicknesses(Figs. 1-3). Increasing porcelain thickness tended to slightly decrease the loadneeded to form initial cone cracks(Fig. 3); the linear equation produced byregression analysis for the influence ofporcelain veneer thickness on initialcone crack fracture events wasy¼ 83xþ413 (R2¼0.04). Increasingporcelain thickness markedly increasedthe load needed to produce the terminalcatastrophic fracture (Fig. 3); the linearequation produced by regression analysis for the influence of porcelain veneerthickness on the terminal catastrophicfracture event was y¼405xþ506(R2¼0.4).Intermediate radial crack loads werenot subjected to regression analysisbecause, for individual specimens,1.00No porcelain fractures or debondingoccurred during cementation, thermalcycling, or storage. Upon loading, theinitial fracture event was the formation of a Hertzian cone crack in theporcelain veneers of all thicknesses(Figs. 1-4). In half of the specimens, 15of 30, complete cone cracks continuedthrough the cement and extended intoenamel (Fig. 5). In another 3 specimens, partial cone cracks extended intoenamel. Some of the extensions intoenamel were shallow; others penetratedto the dentinoenamel junction (DEJ),but none crossed the DEJ into dentin.Intermediate fracture events involvedthe formation of radial cracks withinthe veneer before final catastrophicfailure (Figs. 1-4). In these bondedveneer specimens, most radial cracksappeared to originate from sites on theinternal intaglio surface involved in theHertzian cracks rather than fromnatural flaws directly under the bluntloading point. Some radial cracksextended from porcelain into enamel,or vice versa, but did not cross the DEJ(Fig. 5). Often, several distinct intermediate radial cracking events wereidentified after initial Hertzian crackingand before gross catastrophic failure(Figs. 1-3).DISCUSSIONThe null hypothesis was rejected;porcelain thickness, enamel thickness,and their combined thickness all influenced the loads needed to producecatastrophic failure.The results of this study showed thatthe effects of porcelain and enamelthickness were almost identical, bothfor the initial fracture events and for thefinal catastrophic events (Figs. 1, 2).Consistent with this finding, the effectsof porcelain and enamel thickness weresummative (Figs. 1-3). Qualitativefractographic findings also indicatedthat porcelain and enamel behaved inmechanically similar ways (Figs. 4, 5).
384Volume 111 Issue 5160014001200Catastrophic Failurey 405x 506Load (N)100080060040020000.00Initial Failurey –83x 4130.501.001.502.002.50Porcelain Enamel Thickness (mm)3 Plot of fracture load against combined porcelain veneerand supporting enamel thickness. Initial Hertzian surfacecracks are plotted as light blue diamonds; intermediate radialcracks as small black dots; and final catastrophic failures aslarge blue circles. Regression lines for initial Hertzian surfacecracks and final catastrophic failures are plotted.In many instances, the Hertzian conecracks penetrated all the way throughthe porcelain, through the thin cementlayer, and continued into bulk enamel(Fig. 5). The Hertzian cracks did notcause the veneers to debond, nor didthey directly cause catastrophic failure.A similar occurrence has been reportedin vivo.39Qualitative fractographic analysisrevealed 3 major fracture events in thisexperimental model; initial Hertziancracking extending from the outer surface downward; intermediate radialcracking extending from the inner surface outward and laterally; and finallygross catastrophic failure (Figs. 1-4).Hertzian cone cracks are formed whenspherical indenters produce symmetricaltensile stress around the periphery oftheir contact area to cause crack propagation of conical form extendingdownward and outward from the surface contact area, analogous to the4 Facial view of transilluminated porcelain veneer withinitial central Hertzian cone crack (HCC) and intermediateradial cracks (white arrows) before catastrophic failure.The Journal of Prosthetic Dentistrydamage pattern produced by a bulletimpacting a glass pane (Figs. 4, 5).57,59In contrast, the intermediate radialcracks were believed to originate fromflexural tensile stresses applied to theinner intaglio surface of ceramic restorations and to travel upward to therestoration surface; they are consideredto be the dominant failure mechanismof ceramic crowns (Fig. 5).49-51 Catastrophic failure produced destruction ofthe specimen and disintegration intomany small fragments. Clinical failuresin areas of occlusal or incisal contacthave been frequently reported, consistent with damage induced by directcontact.10,14-17,30,32,33,39,42,44,45 However, this model produced “ideal” Hertzian cracks, rather than the irregularasymmetric damage typically producedby uncontrolled clinical surface contactdamage. Again, it is important to stressthat this in vitro model used a small hardball, rather than the natural anatomyand material of opposing tooth structure, to load the test specimens. Radialcracking, often without gross failure, hasbeen reported for porcelain veneersin vivo12,15-18,30,33,35,39,42,44,46 and forenamel in vitro.60 Clinical radialcracking probably initiates at naturalflaws on the intaglio surface of theveneer in areas of stress concentrationor in areas of bond failure. Gross failureof porcelain veneers has also beendescribed.17,31,42 Thus, the failuremodes produced in this study may beconsidered to have clinical relevance.5 Facial view of transilluminated tooth after catastrophicfailure and total loss of veneer; Hertzian cone crack andradial cracks (white arrows) extend through enamel.Ge et al
May 2014The wide range in loads betweeninitial fracture events, intermediateradial cracking events, and final catastrophic failures indicated that porcelain veneers bonded to enamel formhighlydamage-tolerantstructures(Figs. 1-3). This success can now beexplicitly attributed to a strong durablebond between substrates that areclosely matched in mechanical properties, including elastic modulus, thePoisson ratio, and toughness.61-64Failure modes and loads could profoundly differ if veneers were made ofmaterials differing from the feldspathicporcelain used in this study, or if theinterface between the veneer and toothwas less durable.63 Glass ceramics andceramics tend to have higher elasticmoduli than feldspathic porcelain orenamel; therefore, their failure modesand damage tolerance could differ fromthose of bonded feldspathic porcelainveneers.Which event, initial, intermediate, orfinal, matters to a patient? Althoughthe initial event is a key step in theevolution of failure, it is unlikely to beof consequence to the patient; surfacecracks and radial cracks can only beseen under certain lighting conditions,whereas catastrophic failure is obviously important. Pertinently, the loadsneeded to cause final catastrophic failure tended to be considerably largerthan those needed to cause initialfracture, especially as enamel, porcelain, or their combined thicknessincreased (Figs. 1-3). Therefore, aveneer even with initial damage couldsurvive, possibly indefinitely, until asubstantially higher load was eventuallyapplied. These data suggest that damage tolerance could be increased byretaining enamel thickness duringpreparation and by maximizing porcelain veneer thickness during restoration.Initial Hertzian cone cracks occurredat relatively low loads (Figs. 1-3). Thesedata could be interpreted to suggestthat areas of occlusal contact should bemaintained on intact enamel, becauseincreased porcelain thickness does notincrease the loads needed for Hertziancrack initiation. Also, accidentlyGe et al385occluding on something hard like apiece of bone could easily initiatedamage. These data may also apply toporcelain onlays. These risks of contactdamage may be inevitable58 becauseincreased thicknesses of enamel andporcelain could also be protective ofcatastrophic failure.The findings that increased porcelainthickness, enamel thickness, and combined porcelain and enamel thicknessslightly decreased the load needed tocause initial cone crack fractures may atfirst appear counterintuitive. However,cone cracking is more likely to occurwhen flexure of the substrate is constrained.59,65,66 Therefore, increasingthe thicknesses of relatively stiff porcelain and enamel decreases overall specimen flexure, slightly favoring the initialsurface fracture event but greatly helpingto prevent final catastrophic fracture.These results demonstrated thatmaximizing both remaining enamelthickness and porcelain thickness wereimportant in preventing catastrophicfailure. However, esthetics and hygienelimit practical clinical applications.Doubling the thickness of enamel,porcelain, or their combined thicknessincreased the loads needed to causecatastrophic failure by approximately1.5 times (Figs. 1-3). These data suggest that nonpreparation veneers mayhave significant advantages in preventing catastrophic failure and avoidingdentin exposure, along with the concomitant risks of microleakage, sensitivity, and debonding. Enamel isgenerally thin in the gingival thirds ofthe facial surfaces of incisors, 0.3 to0.5 mm, so preparation must beextremely conservative in that area.54 Byvirtue of limited enamel and porcelainthickness, cervical areas will be moresusceptible to catastrophic failure whenloaded. Likewise, thin porcelain shouldbe avoided in areas of high stress.Preservation of enamel thicknessshould be prioritized whenever possible.Unlike porcelain, enamel is not replaceable. Furthermore, the integrity of thedentinoenamel junction zone must bepreserved.3-5 Moreover, retaining sufficient enamel is prudent in the event thatthe veneer needs to be replaced duringthe patient’s lifetime.The trends identified in this studywere clear and consistent; however,considerable scatter in the data wasevident (Figs. 1-3). Brittle fracture has ahigh inherent variance; feldspathicporcelain and enamel are both brittle.Furthermore, real incisors were used tosupport the porcelain veneers and hadinherent differences in form and overallsize; their histories before extractionwere unknown.Although this experimental modelused a blunt spherical indenter to loadthe specimens, flexural tensile radialfracture of the veneers was produced(Figs. 1-4), as has often been reportedclinically.12,15-18,30,33,35,39,42,44,46 Theresults were broadly consistent with thefew studies reporting on the effect ofporcelain thickness on veneer fracture.52,53,67 As in other layered systemswhere the elastic modulus mismatch wassmall, the first fracture event occurred onthe top outer surface, whereas in layeredsystems where the mismatch was largeand the underlying layer flexible, the firstfracture event tended to occur at thelower internal surface.62,65,66,68 Therefore, porcelain veneers (or other restorations) that are bonded, or bonded inpart, to dentin may behave in a differentmanner from those bonded to enamel;additional study is needed.11CONCLUSIONSFor a bonded feldspathic porcelainveneer model system:1. Increased enamel thickness, porcelain thickness, and increased combined enamel and porcelain thicknessall profoundly raised the loads tocatastrophic failure.2. Enamel and feldspathic porcelainbehaved in a like manner.3. Initial damage was from surfacecontact; intermediate radial cracks originating from the inner intaglio surfacefollowed; lastly, catastrophic failureoccurred.4. Bonded porcelain veneers werehighly damage tolerant.
386Volume 111 Issue 5REFERENCES1. Calamia JR. Etched porcelain facial veneers: anew treatment modality based on scientificand clinical evidence. N Y J Dent 1983;53:255-9.2. Horn HR. Porcelain laminate veneers bondedto etched enamel. Dent Clin North Am1983;27:671-84.3. White SN, Paine ML, Luo W, Sarikaya M,Fong H, Yu Z, et al. The dentino enameljunction is a broad transitional zone unitingdissimilar bioceramic composites. J AmCeram Soc 2000;83:238-40.4. White SN, Miklus VG, Chang PP, Caputo AA,Fong H, Sarikaya M, et al. Controlled failuremechanisms toughen the dentino-enameljunction zone. J Prosthet Dent 2005;94:330-5.5. Imbeni V, Kruzic JJ, Marshall GW,Marshall SJ, Ritchie RO. The dentin-enameljunction and the fracture of human teeth.Nat Mater 2005;4:229-32.6. Calamia JR. Etched porcelain veneers: thecurrent state of the art. Quintessence Int1985;16:5-12.7. Shaini FJ, Shortall AC, Marquis PM.Clinical performance of porcelain laminateveneers. A retrospective evaluation over aperiod of 6.5 years. J Oral Rehabil 1997;24:553-9.8. Calamia JR, Calamia CS. Porcelain laminateveneers: reasons for 25 years of success. DentClin North Am 2007;51:399-417.9. Matson MR, Lewgoy HR, Barros Filho DA,Amore R, Anido-Anido A, Alonso RC, et al.Finite element analysis of stress distributionin intact and porcelain veneer restored teeth.Comput Methods Biomech Biomed Engin2011;1:1-6.10. Jordan RE, Suzuki M, Senda A. Clinicalevaluation of porcelain laminate veneers: afour-year recall report. J Esthet Dent 1989;1:126-37.11. Burke FJT. Survival rates for porcelainlaminate veneers with special reference tothe effect of preparation in dentin: a literature review. J Esthet Restor Dent 2012;24:257-65.12. Karlsson S, Landahl I. Stegersjӧ G, MilledingP. A clinical evaluation of ceramic laminateveneers. Int J Prosthodont 1992;5:447-51.13. Crispin BJ. Expanding the application offacial ceramic veneers. J Calif Dent Assoc1993;21:43-8.14. Nordbø H, Rygh Thoresen N, Henaug T.Clinical performance of porcelain laminateveneers without incisal overlapping: 3-yearresults. J Dent 1994;22:342-5.15. Friedman MJ. A 15-year review of porcelainveneer failure-a clinician’s observations.Compend Contin Educ Dent 1998;19:625-36.16. Peumans M, Van Meerbeek B, Lambrechts P,Vuylsteke-Wauters M, Vanherle G. Five-yearclinical performance of porcelain veneers.Quintessence Int 1998;29:211-21.17. Peumans M, De Munck J, Fieuws S,Lambrechts P, Vanherle G, Van Meerbeek B.Prospective ten-year clinical trial of porcelainveneers. J Adhes Dent 2004;6:65-76.18. Granell-Ruiz M, Fons-Font A, Labaig-Rueda C,Martínez-González A, Román-Rodríguez JL,Solá-Ruiz MF. A clinical longitudinal study 323porcelain laminate veneers. Period of studyfrom 3 to 11 years. Med Oral Patol Oral CirBucal 2010;15:e531-7.19. Rouse J, McGowan S. Restoration of theanterior maxilla with ultraconservative veneers: clinical and laboratory considerations.Pract Periodontics
Recently, minimally invasive veneer preparation designs have become pop-ular. These involve less tooth reduction, partial coverage, and minimal porcelain thickness.19-23 Minimally invasive ve-neers have also been described as being mini, minimal, minimal thickness, ul-traconservative, ultrathin, pa
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