delaminacion zirconia, variables

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Influence of substructure design, veneer applicationtechnique, and firing regime on the in vitro performanceof molar zirconia crowns

Verena Preis , Christoph Letsch, Gerhard Handel, Michael Behr, Sibylle Schneider-Feyrer,Martin RosentrittDepartment of Prosthetic Dentistry, University Medical Center Regensburg, 93042 Regensburg, Germany

a r t i c l e i n f o

Article history:Received 20 September 2012Received in revised form23 April 2013Accepted 25 April 2013

Keywords:ZirconiaCeramicsVeneeringSubstructure designChippingFracture resistance

a b s t r a c t

Objectives. The aim of this in vitro study was to evaluate the influence of substructure design,veneer application technique, and firing regime on the failure and fracture resistance ofmolar zirconia crowns.Methods. Six groups (n = 8/group) of zirconia crowns were fabricated in simple core (SC) oranatomically reduced (AR) design, veneered with different feldspathic or glass ceramicmaterials, and defined according to the application technique and firing regime (LT: lay-ering technique; LT L: LT with long-term cooling; PT: press technique; DV: digital veneeringtechnique). The following groups were investigated: SCLT, ARLT, SCLT L, SCPT, ARPT, ARDV.Crowns were adhesively bonded to polymethylmethacrylate abutment teeth and subjectedto thermal cycling (TC: 2 3000 5/55) and mechanical loading (ML: 1.2 106; 50 N; 1.6 Hz)in a chewing simulator with metal-ceramic molar crowns as antagonists. Failures weremonitored and fracture resistance determined after aging. Data were statistically analyzed(one-way analysis of variance, ANOVA; post hoc Bonferroni, = 0.05). Crowns were subjectedto scanning electron microscopy for fractographic failure analysis.Results. Failures (chipping, cracks) during TCML were observed in groups SCLT (2), ARDV(2) and SCLT L (1). Defect sizes varied between 3.5 mm (SCLT: crack) and 30.0 mm2 (SCLT L:chipping). Mean (SD) fracture forces ranged between 1529.0 (405.2) N for SCPT and 2372.3(351.8) N for ARDV.Significance. The failure frequency of veneered zirconia crowns could be reduced by usinganatomically reduced substructures, the press veneering technique, and an adapted coolingprotocol. Fracture resistance increased with use of anatomically reduced substructures andthe digital veneering technique.

2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

1. Introduction

For restoring molar teeth with single crowns, yttria-stabilizedzirconia cores veneered with dental porcelains are highly

Corresponding author. Tel.: +49 941 944 6055; fax: +49 941 944 6171.E-mail address: [email protected] (V. Preis).

esthetic alternatives to conventional metal-ceramics. Zirconiaceramics can be processed with CAD/CAM (computer aideddesign/computer aided manufacturing) or CAM technologies,and their suitability as high-strength substructure materialshas been proven under in vitro and in vivo conditions over

0109-5641/$ see front matter 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.dental.2013.04.011

e114 d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) e113e121

the past years [15]. Furthermore, zirconia ceramics providehigher fracture toughness, a smaller range of strength varia-tion, and higher structural reliability than glass ceramics [6].The number and sequence of fabrication steps (for example,wax modeling versus CAD) depend on the chosen system andmaterial, but all zirconia substructures finally undergo par-ticular CAM processing. After milling, these frameworks haveto be veneered with feldspathic or glass ceramics by meansof the layering, the press, or the digital veneering technique.Although the application of full-contour zirconia restorationsis currently discussed as an alternative to commonly veneeredrestorations [5,7,8], esthetically superior results can only beachieved by applying veneering materials with mechanicalproperties inferior to those of the frameworks. Because theveneering glass ceramic is the weakest part in this system,clinically observed failures are mainly restricted to the veneerlayer [1,2]. As the veneering porcelain is directly exposed tochewing, clenching, and moisture, fatigue mechanisms andstress corrosion further weaken the veneer and finally resultin cracks or chippings [9,10].

These brittle breakdowns are typical failures of ceramicmaterials. Although chippings may also present a problemwith porcelain-fused-to-metal (PFM) restorations, particularlythe chipping failures with all-ceramic zirconia restorationsare discussed [2,3,11]. Various influencing factors have beenreported, such as the support and thickness of the veneer[12,13], the morphology of the circular finishing line [14], theadhesive forces between substructure and veneering [15], themismatch of coefficients of thermal expansion (CTE) [16], orthe firing protocol during the veneering process [17]. Opti-mization of the zirconia substructure design has been provenas a considerable factor in reducing chipping failures [13],and coping modifications are still a topic of current investi-gations [18,19]. The main issues of Y-TZP-based restorationsseem to be the structural integrity of the veneering porcelainon the one hand and its support by the zirconia substructureon the other hand; therefore, the influence of the applica-tion technique and firing regime of the veneering material incombination with the zirconia substructure design should befurther investigated to decrease failure rates.

Laboratory tests, such as the finite element analysis, mayhelp to predict the fracture behavior of specific material com-binations [18,20]. But failure types and patterns are notablyinfluenced by clinical variables, such as an individual crowndesign with its occlusal variations, a patients chewing behav-ior, and functioning in an oral environment. These variablesmay have different effects on loading, force distribution, andaging. Chewing simulations that imitate the clinical situa-tion with dynamic loading and thermal cycling may helpexamine specimen behavior under clinically approximatedconditions [4,21,22]. Material breakdowns during simulationcan be compared with clinically observed failures, and frac-tographic methods can be applied for further investigationsof failed ceramic restorations [2325]. Chippings and frac-tures are mostly initiated by flaws inside the material ordefects in the marginal areas or on the occlusal surface[23,24]. Even in cases without any catastrophic failures (frac-ture) during oral application, aging and deterioration effectsmight occur, which weaken the ceramic structure, thusreducing strength and fracture resistance. In these cases a

subsequent static fracture test may help locate initiated weakpoints.

The hypothesis tested in this study was that differentsubstructure designs (simple core or anatomically reduceddesign), veneer application techniques (layering, press, ordigital veneering technique), and firing regimes (normal orslow cooling) influence the number and dimension of failuresin zirconia-based all-ceramic crowns during simulated oralservice and affect the fracture resistance after fatigue testing.

2. Materials and methods

The tooth 46 (Morita, Dietzenbach, Germany) was preparedfor a single crown according to the directives for zirconiaall-ceramic restorations. A circular and occlusal anatomicalreduction of 1.52.0 mm was carried out with a preparationangle of 4. The finishing line resulted in a 1 mm deep circularshoulder with rounded inner angles at an isogingival heightof the tooth cervix. Sharp inner edges and undercuts wereeliminated. This prepared tooth was then multiplied result-ing in 48 identical polymethylmethacrylate (Palapress Vario,Heraeus-Kulzer, Hanau, Germany) teeth. Their roots were cov-ered with a 1 mm thick layer of polyether material (Impregum,3M Espe, Seefeld, Germany) to simulate periodontal toothmobility [22]. For achieving a constant layer, the roots weredipped in a wax bath, which was replaced by polyether ina second fabrication process, as described in previous stud-ies [22,26]. We positioned the teeth in resin blocks (PalapressVario, Heraeus-Kulzer) and made polyether impressions (Per-madyne, 3M Espe) and working dyes of class IV dental stone(Fuji Rock, GC-Corporation, Tokyo, Japan). 48 substructures forthe molar crowns were fabricated with yttria-stabilized zirco-nia (Lava, 3M Espe) using the CAD/CAM technique accordingto the manufacturers instructions. Six groups were defined(n = 8/group) that finally showed the same crown shape butdiffered in substructure design, veneer application technique,and firing regime:

SCLT, simple core; veneered in layering technique (referencegroup)ARLT, anatomically reduced core; veneered in layering tech-niqueSCLT L, simple core; veneered in layering technique (long-term cooling)SCPT, simple core; veneered in press techniqueARPT, anatomically reduced core; veneered in press tech-niqueARDV, anatomically reduced core; veneered with digitalveneering technique.

The substructures were divided into two groups:

- simple core (SC) with an overall thickness of 0.5 mm, result-ing in a varying veneer thickness

- anatomically reduced (AR) core, in which the crown dimen-sion was reduced all around by 1 mm, resulting in asubstructure thickness between 0.5 and 1 mm to ensureoptimal support and constant thickness of the veneer layer.

Massiel Narvez

d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) e113e121 e115

Table 1 Materials (data provided by manufacturer).

Material Manufacturer Type Application Fracturestrength(MPa)

Lava Ceram 3M Espe, Germany Feldspathic porcelain Veneer, layering technique 100IPS e.max ZirPress Ivoclar Vivadent, Liechtenstein Fluorapatite glass ceramic Veneer, press technique 110Experimental material 3M Espe, Germany Glass ceramic Veneer, digital veneering technique 100Lava 3M Espe, Germany Y-TZP Substructure, CAD/CAM >1100

The maximal thickness of the entire restoration was 2 mm.Thus, the veneering thickness depended on the substruc-ture design. Veneering was done using the layering technique(Lava Ceram, 3M Espe), the press technique (IPS e.max Zir-Press, Ivoclar Vivadent, Schaan, Liechtenstein), or the digitalveneering technique (exp. material, 3M Espe) according to themanufacturers instructions. Details on the materials usedare provided in Table 1. For the digital veneering technique,the CAD/CAM-manufactured veneer and the Y-TZP core wereconnected with fusion porcelain. Long-term cooling (SCLT L)was performed according to standard regimes, only the cool-ing phase was prolonged to 6 min. All crowns were cementedusing a self-adhesive resin-based cement (RelyX Unicem Apl-icap, 3M Espe; 4 20 s, Bluephase C8, 800 mW/cm2, IvoclarVivadent).

48 identical crowns were fabricated based on tooth 16(Morita) of CoCr-alloy (Wirobond LFC, Bego, Bremen, Germany)and a veneering porcelain (VM 15, Vita, Bad Sckingen,Germany), which ought to serve as antagonists during theentire chewing simulation. The all-ceramic crowns and theirantagonists were adjusted in a three point contact in a dentalarticulator (Artex CN, Amann Girrbach, Pforzheim, Germany)and transferred to the chewing simulator (EGO, Regensburg,Germany) using a bite registration. According to an ideal-ized occlusion concept, the distobuccal and the two lingualcusps were loaded by the opposing mesiopalatinal cusp ofthe antagonistic crown via tripodized contacts in the centralfossa. Further antagonistic contact areas were located at themesiobuccal and distobuccal cusp tips.

Fig. 1 Scanning electron micrograph (90) of a wear facetof series SCLT.

Thermal cycling (TC: 2 3000 5/55; 2 min each cycle)and mechanical loading (ML: 1.2 106; 50 N; 1.6 Hz) was per-formed. Parameters are based on literature data on zirconiaand ceramic restorations expressing that chewing simulationusing these parameters might simulate a maximum of fiveyears of oral service [4,21]. During the simulation time, allcrowns and their antagonists were monitored, appearing fail-ures of the specimens were documented (type, number ofmechanical cycles) and failed crowns were excluded from thefurther simulation process. Location (mesial, distal, buccal orlingual direction) and extension area or length of the occurringfailure mode were determined by means of a light micro-scope (M420, Wild, Heerbrugg, Switzerland). Scanning electronmicroscopy (SEM; magnification: 20600; working distance:20.4 mm; voltage: 510 keV; low vacuum; Quanta FEG 400, FEICompany, Hillsboro, USA) was used for fractographic failureanalyzing. This way, overview and detailed micrographs wereproduced.

Molar crowns without any failure during TCML were sub-sequently loaded with a testing machine (Zwick 1446, Ulm,Germany; v = 1 mm/min) until failure. The force was appliedusing a steel ball (d = 12 mm) and a folded tin foil (4 0.25 mm,Dentaurum, Ispringen, Germany), between crown and antag-onist prevented force peaks. Crowns were optically examinedafter fracture testing, and failure modes were divided intochipping of the veneer or combined fracture of the veneer andcore.

Calculations and statistical analysis were carried out usingSPSS 19.0 for Windows (SPSS Inc., Chicago, IL, USA). Meanvalues and standard deviations (SD) were calculated and ana-lyzed by means of one-way analysis of variance (ANOVA) andthe Bonferroni multiple comparison test for post hoc analysis.The level of significance was set to = 0.05.

3. Results

During the simulation process, failures occurred in the exam-ination groups SCLT, ARDV (2 each) and SCLT L (1), whereasthe two groups with the press technique as well as the ARLTgroup remained undamaged. Failures mostly consisted ofchipping of the veneering ceramic, and only in one case dida crack occur (SCLT). Failure descriptions are summarized inTable 2. Defect sizes varied between 3.5 mm (SCLT: crack) and30.0 mm2 (SCLT L: chipping), but neither the area (p = 0.578)nor the number (p = 0.359) of failures showed any significantdifferences. Antagonistic metal-ceramic crowns did not revealany chipping failures but only wear facets.

The SEM evaluation illustrated that the occlusal contactareas of all molar zirconia crowns were roughened by theirantagonist. The microcracked and ploughed surface of these


Fig. 2 (a) Overview (27) and (b) detail scanning electron micrographs (80200) of a small-area chipping of series ARDV: O,point of origin at wear facet (W); C, crack propagation; H, hackles and wake hackles; A, arrest lines.

wear facets (Fig. 1) measuring between 2 and 4 mm2 onaverage was similar in all examination groups. As the fracto-graphic analysis showed, these load-bearing areas were theorigin of cracks running through the veneering porcelain,which resulted in the described chipping failures. ExemplarySEM pictures are provided as overview and detailed figures(Figs. 24). Fractographic features were identified for interpre-ting failure patterns, showing their occlusal origin, fracturemirrors, hackles, wake hackles, and arrest lines as indicatorsof the direction of the crack propagation. Whereas both defectmodes of SCLT were located at the mesiobuccal cusp, all otherchippings in ARDV and SCLT L started at the disto- or mesi-olingual cusps and were directed toward the crown equator ormargin. In addition to the major chipping in distolingual direc-tion, one failed crown of ARDV (Fig. 3) showed a secondary

smaller chipping, which also started at the distolingual cuspbut proceeded in mesial direction. The large-area chipping inSCLT L (Fig. 4) reached the finishing line but without exposingthe zirconia substructure. A thin layer of veneering porcelainremained in the marginal area of the zirconia core. In eachinvestigated crown, the failure occurred inside the veneer, andwe did not find any interfacial fractures between the zirconiasubstructure and the veneering porcelain.

Mean (SD) fracture forces (Table 2) varied between 1529.0(405.2) N for SCPT and 2372.3 (351.8) N for ARDV. The resultsfor SCPT were significantly lower than for ARLT (p = 0.036) andARDV (p = 0.012). The most frequent fracture pattern was afracture of both veneer and core. Only ARDV showed a con-gruent number of sole veneer chipping and combined veneerand core fractures (3 each).

Fig. 3 (a) Overview (31) and (b) detail scanning electron micrographs (80300) of a small-area chipping (twin) of seriesARDV: O, point of origin with wear facet (W); C, crack propagation; H + A, wake hackles and arrest lines.


Fig. 4 (a) Overview (23) and (b) detail scanning electron micrographs (37150) of a large-area chipping of series SCLT L:O, point of origin; C, crack propagation; H + A, wake hackles and arrest lines.

4. Discussion

Both parts of the hypothesis of this study that differentsubstructure designs, veneer application techniques, andfiring regimes influence the number and dimension of fail-ures in zirconia-based all-ceramic crowns during simulatedoral service and affect the fracture resistance after fatiguetesting, were confirmed. The failure frequency during theendurance test could be decreased with an anatomicallyreduced substructure design and the press veneering tech-nique. The adapted cooling protocol of the layering techniqueshowed a lower number of failures for the simple substructuredesign, but also an increased chipping area, however withoutany statistically significant difference. The highest fractureforces during subsequent single load fracture testing wereobserved for the digital veneering technique and for the layer-ing technique in combination with the anatomically reduceddesign.

Standardization of the examined restorations and the test-ing conditions was required in this in vitro study becausethe evaluation of crown performance and underlying rea-sons for clinically observed chipping failures is complicatedby individual variables, such as tooth structure, periodontalmobility, occlusal loads, chewing behavior, the oral environ-ment, as well as differences in the preparation design of theabutment teeth and the material selection of both crown andluting materials [23,27]. Furthermore, the fabrication of dentalrestorations in the laboratory is a process that highly dependson the skills and preferences of the individual dental tech-nician [28]. Therefore, in this in vitro study, the multiplyingof one prepared tooth as well as the application of CAD/CAMtechnologies allowed the fabrication of molar crowns iden-tical in shape and size. Such uniformity is important for areliable comparison of different groups and may allow foridentifying single risk factors. In return, the comparabilityof in vitro and in vivo loading situations is limited because

artificial abutment teeth differ from human teeth in termsof modulus of elasticity and bonding capacity to the cement[29]. The influence of resilient support of the abutment teethon the aging process and fracture strength of molar crownsshould be considered by a polyether interface, which natu-rally does not stay abreast of the complex human periodontalligament, but might avoid an overestimation of the strengthof ceramic restorations [26]. TCML parameters have been cho-sen congruent to numerous other in vitro studies [13,28,30].They are supposed to simulate restoration stress according toa maximum of five years of intraoral use [4,21]. Different stresssimulation parameters might have affected the appearance offailures and the fracture load. Especially an additional lateralmovement is considered to be essential for simulating fatiguewear of ceramics [22], results in shear loading produced byside shift chewing forces and may have fostered cracks andthe chance of chipping. Although no lateral movements dur-ing the chewing simulation were included in the present study,minor sliding contacts occurred as a result of deflection bythe polyether layer ligament. Metal-ceramic antagonists wereused, because they reflect a high percentage of clinical situa-tions and allow the standardization of antagonistic situations.Though, the tribological and mechanical properties of humanenamel, the natural antagonist in the oral cavity, differ fromglass ceramics [31], which might have caused different wear[7,32], damage, and finally chipping behavior.

Supposed crucial factors for chipping failure during chew-ing simulations are occlusal overloading [23], stress corrosion,and fatigue [1012], particularly in the case of impropersubstructure design [13], which does not provide sufficientocclusal support for the veneering porcelain. Occlusal over-loading may be restricted to individual cases of bruxism orforce peaks when people bite on a hard object [23,33,34] butcan be excluded for average chewing forces as applied duringthe chewing simulation. A more common reason for chippingis fatigue as a result of antagonist wear and selective load-ing via chewing impact and clenching [4,28]. Wear results in


Table

2

Numbe

r

of

faile

d

crow

ns

and

description

of

failu

res

during

thermal

cycling

and

mechan

ical

load

ing

(TCML)

and

subseq

uen

t

fracture

testing.

Group

TCML

Fracture

test

Number

offaile

dcrow

ns

Number

ofmechan

ical

cycles: typ

eof

failu

re

Chipping

area

(mm

2) o

r

crack

length

(mm);

location

Fracture

force

(N)

Mean

(SD)

Type

offailu

re/number

offaile

d

crow

ns

Ven

eer

chipping

Ven

eer

and

core

fracture

SCLT

263,006: crack

3.5

mm; m

esiobu

ccal

1841.8

(257.1)

06

915,079:

chipping

6.3

mm

2; m

esiobu

ccal

ARLT

0

2252.6

(331.6)

1 7

SCLT

L

1

1,200,000:

chipping

30.0

mm

2; m

esiolin

gual

2025.6

(385.4)

1

6SC

PT

0

1529.0

(405.2)

1

7ARPT

0

1690.1

(541.7)

0

8

ARDV

2375,152:

chipping

5.3

mm

2; d

istolin

gual

2372.3

(351.8)

33

375,152:

chipping

6.0

mm

2; d

istolin

gual

roughening of the ceramic surface by microploughing, micro-cracking, and microcutting and causes the formation andpropagation of subsurface cracks [35]. In simple substructures,such flaws can propagate through the veneering ceramic with-out being stopped or deflected. This theory is supported by thepresent observations of failures (chipping, cracks) that origi-nated from occlusal wear facets and were primarily detectedin crowns with a simple core design; such failures are mainlydue to insufficient support and the varying thickness of theveneer layer.

According to the present in vitro results that did not includeany interfacial cracks between substructure and veneer, chip-ping failures within the veneering material represent the mostcommon type of failure observed in clinical studies [1,2,23].These cohesive failures indicate a reliable bond between theveneering ceramics and the high-strength zirconia frame-works but also reveal the weakness of the veneering porcelain.By cyclic loading, a hydraulically assisted steady propagationof inner and partial cone cracks occurs in the mid-layer of theveneering ceramic that may finally lead to failure by chipping[9,10,20]. The occlusal load is divided into two componentsthat are either directed at the fossa or at the equator [13];therefore a supporting substructure is necessary that allowsan efficient shift of the stress distribution from the veneer tothe core layer. In this case, a thinner layer of the weak veneer-ing porcelain is supported by the underlying zirconia cusps,which may serve as an explanation for the reduction of failuresfor molar crowns with anatomically reduced core design.

The results showed that not only the substructure designbut also the application technique and type of veneeringmaterial influenced the chipping behavior of zirconia molarcrowns. In contrast to the layering and digital veneering tech-nique, no failures were observed for cores (SC, AR) veneeredby the press technique. It has been suggested earlier thatlower amounts of voids are introduced to the veneer layerwhen applying a controlled process such as the press tech-nique [3,36], which contributes to a more homogenous anddamage-resistant structure because any porosity may act asa stress-raiser for crack initiation [37]. Accordingly, a prospec-tive 3-year clinical trial on Y-TZP fixed partial dentures hasshown promising results with the over-pressing technique,i.e. no veneer chipping and a success rate of 95.2% with zir-conia frameworks [3]. A further reason for the outstandingresistance to fatigue of pressed material may be an improvedceramic microstructure, because IPS e.max ZirPress is a fluora-patite glass ceramic that might exceed the strength (100 MPa)and fracture toughness (1 MPa m1/2) values of common felds-pathic layering porcelains such as Lava Ceram. Although adigital veneering technique is supposed to achieve indus-trial quality standards and therefore reduce failure rates [38],chipping behavior of group ARDV was similar to the groupswith simple cores and layered veneers, indicating that evenalmost flawless materials are not able to withstand fatigue, ifmechanical properties (strength, fracture toughness) are infe-rior. When comparing the present results with in vitro studiesreporting improved resistance to failure and higher fractureloads for CAD/CAM-fabricated veneers compared to the layer-ing technique [38,39], it has to be considered that substantiallystronger veneering materials of lithiumdisilicate may havecontributed to low failure rates.


Because of the low thermal conductivity of the Y-TZP core,the development of thermal gradients during cooling couldlead to the entrapment of tempering stresses in the veneerand therefore increase the risk of pre-damage during the man-ufacturing process [17]. The assumed favorable effects of aprolonged cooling rate reduced the failure rate of the layeredveneer for the simple core design. However, although only onecrown of SCLT L showed chipping, the damage was larger thanthat observed in all other groups. Therefore, the combina-tion of a prolonged cooling rate with an anatomically reduceddesign should be most effective in reducing the number anddimension of chipping failures.

However, because of the unexpectedly low differences inthe number and dimension of failures in the various groups,it should be kept in mind that fatigue failures might be ran-dom events associated with material or fabrication defects. Ahigher sample size might have allowed a better differentiationbetween the investigated groups.

The fact that the majority of chippings in this studyoccurred in lingual direction can be explained by interocclusaltooth references. The main chewing forces of the mandibularmolars tested occur in the central lingual areas. The non-loadbearing lingual cusps have to withstand the side shift chewingforces. Thus, shearing forces produced by clenching mainlyoccur in distal and lingual directions, causing chipping fail-ures [13]. A finite element analysis of the force distributionin molars during clenching and mastication was conductedby Dejak [40], who showed that maximal stress occurs at theocclusal surface and the cervical region of the lingual wallof the first mandibular molar. It has been further suggestedthat the higher fracture potential of nonfunctional cusps inmandibular teeth is related to their anatomical shape, becausethey are narrower, and the angular inclinations of these cuspsare smaller than the functional cusps [18]. Therefore, theymight be more susceptible to the horizontal loading compo-nent. Further studies investigating the influence of the crowndesign (cusp inclination and curvature) on chipping behaviorseem to be necessary.

The present SEM observations with chipping failures origi-nating from occlusal wear facets show the necessity of lookingat the antagonist teeth and the condition of the crown sur-face. On a smooth surface, the antagonist slides easily intothe final situation with an optimal cuspfossa relation. Withincreasing roughness (e.g. due to wear) and destruction of theloading point, biting forces are distributed over a larger areaand the loading toward the equator of the crown is reducedwith the flattening of the crown [13]. As the antagonisticcrowns were not changed during the entire fatigue testing,it might be argued that different stress scenarios and con-tact pressures that are associated with ongoing wear of thesurface limit the significance of this study. However, this nat-ural phenomenon also takes place in clinical service. Thefriction between crown and antagonist gradually increases,and progressively ploughed and flawed surfaces [7] further thedevelopment of cracks and the chance of chipping. Therefore,polishing wear facets on ceramics every now and then may berecommendable.

The number of mechanical cycles at failure may giveimportant clues about the underlying failure reason. Cracksor chippings at the beginning of the endurance test might

indicate the presence of processing defects, for example,during the layering process or residual stresses from fir-ing during the veneering process, as may be assumed forthe early crack in group SCLT (63,006 mechanical cycles). Inreturn, failures at higher mechanical cycles may be more typ-ical for fatigue mechanisms of the porcelain. Zirconia-basedrestorations have shown strong TCML-dependent aging [4].Therefore, fatigue probably played a major role in chippingat mechanical cycles higher than 300,000. However, prema-ture cracks may accelerate further failures, such as chipping,and reduce fracture resistance. This theory is supported bya study on fatigue testing of zirconia-based crowns, whichmostly revealed crack initiation and propagation before chip-off fractures [41]. Though, reliable conclusions on this topicrequire further investigations.

In contrast to TCML, fracture testing, during which crownsare loaded to failure in one single stroke, shows no clinicalrelevance but may provide helpful data for comparing testedspecimens. During oral simulation, flaws, superficial wear, oraging effects contribute to the deterioration of the materialand reduce fracture strength [22,42], particularly in the caseof improper substructure design. Therefore, fracture testingafter simulation may allow the identification of initiated weakpoints and permit the differentiation of material and designvariations. The digital veneering technique was shown to beefficient in increasing fracture resistance compared to the lay-ering or press technique, which was also demonstrated in anin vitro study by Beuer [38]. Provided that the same veneeringmaterial was applied, crowns with anatomically reduced sub-structure design showed higher fracture forces than crownswith simple core design, yet without any significant differ-ences. Despite the absence of failures during TCML, the molarcrowns fabricated in press technique showed the lowest frac-ture forces. However, it has to be kept in mind that fractureresistance may have been overestimated in groups with fail-ures during TCML, because the failed crowns were excludedfrom subsequent single load fracture testing. Nevertheless,as the observed fracture loads considerably exceeded maxi-mum chewing forces, which are reported to be up to 900 N[43], all molar zirconia crowns have the potential to with-stand occlusal forces applied in the posterior region. Failuresfor fatigue testing were caused by chip and crack formation,whereas the most frequent failure type for single load fracturetesting was combined veneer and core fracturing.

These catastrophic failures, which have also beendescribed in other studies [10,38], underline the inability ofsevere fracture testing to replicate clinically observed fail-ure modes. Thus, only limited insight into clinically relevantmechanisms of damage initiation and propagation can be pro-vided.

The present results indicate that chipping seems to be aphenomenon, which is not limited to zirconia restorations butis strongly influenced by the design of the substructure andthe intactness of the veneer. Chipping definitely appears tobe a cohesive failure of the veneering porcelain and not ofthe interface between substructure and veneering ceramic asassumed earlier [15,23]. As different types of veneering mate-rials for zirconia or alloy-supported substructures show smalldifferences in composition, mechanical properties, and sinter-ing temperatures, which are only adapted for the veneering


of the respective core material, there should be no differ-ence between the chipping rates of alloy- or zirconia-basedcrowns. Unfortunately, the design of the substructure, partic-ularly with the launch of zirconia restorations, has providedabsolutely no support of the veneer and may have caused thereported high numbers of chippings [1,41]. Use of controlledveneer application techniques, such as the press technique, aswell as minimizing stress during the firing regime may consti-tute one possibility to reduce cracking and chipping failures.However, only in combination with an anatomically reducedsubstructure design and a constant layer of the veneeringporcelain, the number and dimension of failures (chippings,cracks) is likely to be effectively reduced.

5. Conclusion

The failure (chipping, cracks) frequency of veneered zirconiacrowns could be reduced by using an anatomically designedsubstructure, the press veneering technique, and an adaptedcooling protocol. Fracture resistance increased with use ofanatomically reduced substructures and the digital veneeringtechnique.

Acknowledgement

We would like to thank 3M Espe for providing the materials.

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Influence of substructure design, veneer application technique, and firing regime on the in vitro performance of molar zir...1 Introduction2 Materials and methods3 Results4 Discussion5 ConclusionAcknowledgementReferences

delaminacion zirconia, variables

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