RESEARCH AND EDUCATION

Supported byaAssociate PrbSenior EngincProfessor, D

630

Load at fracture of monolithic and bilayered zirconiacrowns with and without a cervical zirconia collar

Marit Øilo, DDS, PhD,a Ketil Kvam, MSc,b and Nils Roar Gjerdet, DDS, PhDc

ABSTRACTStatement of problem. The effect of anatomic contour design in all or parts of zirconia crowns isuncertain regarding clinical reliability and survival rates.

Purpose. The purpose of this in vitro study was to compare the load at fracture of monolithic,anatomic contour zirconia crowns with bilayered crowns with and without a cervical zirconia collar.

Material and methods. Thirty zirconia crowns were fabricated for a shallow chamfer molarpreparation, 10 with a normal core-veneer design, 10 with a core-veneer design with an additionalcervical collar of zirconia, and 10 with a monolithic, anatomic contour design. Veneering ceramicwas applied to the first 20 specimens to create an anatomic form. All crowns were cemented toepoxy abutments and loaded until complete fracture with a clinically relevant test method. Thefracture modes and load at fracture were recorded.

Results. Statistically significant differences were found in the load at fracture and fracture modesamong the test groups (P<.001). All fractures except one initiated in the crown margin, mostly in theproximal region. The mean load at fracture was 4091 N for the normal core-veneer design, 4712 Nfor the collar design, and 6517 N for the monolithic, anatomic contour design.

Conclusions. Monolithic, anatomical contour design gave higher loads at fracture than traditionalcore-veneer design. Crowns with a cervical zirconia collar had higher load at fracture than the core-veneer design, but lower than the monolithic crowns. (J Prosthet Dent 2016;115:630-636)

Fractures are still one of themain problems reported forceramic restorations in clinicalpractice.1-5 Reports of theclinical performance of zirco-nia dental crowns are limited,but the available data for par-tial fixed dental prosthesesindicate that both core frac-tures and adhesive fractures(chipping) are clinical prob-lems.6-8 Monolithic, anatomiccontour zirconia crowns havebeen introduced as alterna-tives to porcelain veneeredcrowns to increase fracturestrength and reduce the risk ofadhesive fractures.8-12 Recentstudies have shown that frac-

ture resistance can be improved with alternative coredesigns such as cervical core collars.13-15 Optimizing coredesign may contribute to increased clinical success andesthetically acceptable alternatives with the use of acolored zirconia. Furthermore, the single-layer designreduces the need for tooth removal, which may improvepulpal response. However, the optimal design for zirco-nia crowns is not certain. Many trials have attempted toevaluate the effect of the margin design, wall thickness,core-veneer ratio, test method, and materials used onfracture strength.16-29 The results are not all in agree-ment, and the clinical relevance is uncertain.30-32 Frac-tographic analyses and finite element analyses imply that

the Nordic Institute of Dental Materials and University of Bergen.ofessor, Department of Clinical Dentistry, Faculty of Medicine and Dentisteer, Nordic Institute of Dental Materials, Songsveien, Oslo, Norway.epartment of Clinical Dentistry, Faculty of Medicine and Dentistry, Univers

the stress in dental ceramic crowns during masticationmay be higher in the cervical margin than on the occlusalsurface and that thin margins may be the cause of frac-ture.18,28,33-47

The purpose of this study was to evaluate the effectsof 2 different crown margin designs for bilayered resto-rations and monolithic zirconia crowns on load at frac-ture. Experience has shown that margins are vulnerableand that margin flaws can initiate core fractures duringclinical use. An additional aim was to analyze the fracturemodes by using fractographic methods to identify thefracture features and assess the clinical relevancy of theresults. The hypothesis was that increased wall thickness

ry, University of Bergen, Bergen, Norway.

ity of Bergen, Bergen, Norway.

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Clinical ImplicationsA new marginal design for bilayered zirconia crownsimproves their strength and reliability. However,monolithic crowns made with zirconia are thestrongest crown type.

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at the crown margin and in the axial walls increases theload at fracture of ceramic dental crowns.

MATERIAL AND METHODS

One cylindrical model was prepared with a shallowchamfer design and a taper of 5 degrees and roundededges. For this model, 30 zirconia crowns (LAVA; 3MESPE)were produced in a computerized design andmillingunit (LAVA Scan ST; 3M ESPE). Ten were made with astandard coping for core-veneer crowns, 10 weremade in astandard core-veneer design but with an additional 1-mmcervical zirconia collar, and 10 monolithic, anatomic con-tour crownswere produced to fit themodel tooth (Fig. 1). Afinal adjustment of the crown margins was performed byhand to achieve a smooth boundary between the tooth andcrown margins. The cores for the bilayered structures wereveneered with veneering ceramics (e.max Ceram; IvoclarVivadent AG) to anatomic form. Images of all crown mar-gins were made at ×10 magnification to identify flaws.Crowns were luted to epoxy resin (Epofix Resin; Struers)replicas of the preparationwith zinc phosphate cement (DeTrey Zink; Dentsply DeTrey GmbH). Excess cement wasremoved, and crownswere placed in distilledwater at 37�Cfor 20 ±2 hours. The crowns were subsequently loadedcentrally at the occlusal surface with a steel ball 30 mm indiameter, cushioned with a 3-mm-thick ethylene propyl-ene diene rubber disk with a hardness of 90 Shore A(EPDM 90A) to avoid contact damage. The load wasapplied in a universal testing machine (Zwick Roell) with acrosshead speed of 0.5 mm/min until fracture occurred.Crowns were immersed in water at 37�C during testing,and the load at fracture was recorded. Fractured crownswere analyzed by fractographic methods to evaluate thefracture modes and determine the fracture origins.48,49

Statistical software (SPSS v19; IBM Corp) and nonpara-metric statistics were used to calculate differences amonggroups (Kruskal-Wallis) and between groups (Mann-Whitney U test). Spearman rho (r) was used to calculatecorrelations (a=.05).

RESULTS

Statistically significant differences were found in the loadat fracture among the test groups (P<.001) (Fig. 2).Fractographic analyses revealed that the fracture originswere mostly in the cervical margin in the proximal region

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and were similar to clinical fractures (Figs. 3-5, Table 1).One monolithic crown had an occlusal fracture origin.Fractographic features such as hackle, arrest lines, andcrack branching were observed in both core and veneer.Fracture origin was typically a flat, smooth region in thecore material (mirror), followed by a rougher region withflaking of the zirconia core (mist) region before thefracture surfaces smoothed out and the hackle and arrestlines could be seen (hackle region). The monolithiccrowns fractured in more pieces than the core and collargroups. Most of the crowns in the core group fractured in2 or 3 pieces, whereas crowns in the collar group dis-played more than 3 fracture lines. Twenty-six of the 30crowns had minor flaws or edge chips in the marginbefore testing (Fig. 6). In 10 crowns, the fracture originwas in a pre-existing margin flaw, whereas in theremaining 20 specimens, no flaws were detected in thatregion before testing. Cracks in the veneer of the bilay-ered crowns were both seen and heard at loads that were500 to 1000 N lower than the load at complete fracture,but the exact load at the first crack was not registered.The veneer did not separate from the core before com-plete fracture occurred in any crowns. The load wasinterrupted at 2500 N in the first trial for 2 crowns in thecore group. Both of these crowns exhibited cracks (in-fractions) in the veneer originating from the crownmargin but no core damage (Fig. 7). These crowns wereremounted and subjected to load until complete fracture(3960 and 5290 N, respectively). Because of the possibilityof damage to either crown or abutment during the firsttest, these were excluded from further analysis.

No statistically significant correlation (P=.5) wasfound between the location of fracture origin and load atfracture. No correlation (P=.8) was found between theobserved flaws at fracture origin and the load at fracturein any of the test groups.

DISCUSSION

The fact that the monolithic crowns fractured at a sta-tistically significant higher load and presented morecomplicated fracture modes than the core-veneer crownsindicates that the crown wall thickness affects the fracturestrength of zirconia crowns. Monolithic crowns are, thus,well-suited for clinical use in areas of high stress. The loadat fracture for the monolithic crowns, however, was notmuch higher than that for the core-veneer crowns, asmight have been expected based on the increase in ma-terial thickness alone and previous studies.17 This in-dicates that the crown margin design affects fracturestrength more than wall thickness. Differences in loadand fracture modes between the bilayered crowns withand without the cervical collar indicate that small modi-fications in marginal design can reduce the risk for bothcore fractures and adhesive fractures. The cervical collar

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Figure 1. A, Shallow chamfer preparation and 3 different crowns designed for this preparation. B, Core for bilayered core-veneer design. C, Collar forcore-veneer design with cervical collar of zirconia. D, Monolithic, anatomic contour zirconia design. Dotted line indicates preparation contour.

8000

6000

4000

Core* Collar

Test Group

Monolithic

2000

0

Loa

d (N

)

P=.01P=.001

P<.001

Figure 2. Boxplot diagram of load at fracture (in N). Boxes represent 25thand 75th percentiles of findings, horizontal bars represent medianvalues, and error bars represent minimum and maximum values. Statis-tically significant differences were found among groups (P�.001,Kruskal-Wallis). P values on horizontal bars indicate statistical differencesamong groups (Mann-Whitney U test). *n=8 in core-veneer group.

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seems to function as a reinforcing ferrule, reducing thestrain in the cervical region of the crown. A larger collarwould probably have a larger effect on load at fracture.

Previous studies have focused chiefly on the thicknessof the material at the finish line and not especially onthe design of the crown margin.20-23 Most in vitro studieshave been based on tests performed on standardizedcrowns with a uniform design along the circumference ofthe crown and the use of a deep chamfer design.24,25,30,31

One study tested a similar circumferential collar designon a shallow chamfer as in this study but found no ef-fect.20 Another study tested different wall thicknessesand margin designs on a deep chamfer and found aneffect of wall thickness only.17 Most fractures in thesestudies seemed to originate from contact damage at theocclusal loading point and may thus have limited clinicalsignificance.30,31,37 The fracture patterns found in thepresent study were similar to those found in zirconiacrowns fractured during clinical function, which oftenreveal a fracture pattern that indicates a fracture origin inthe cervical margin in the proximal area where the finishline curves toward the occlusal surface.18,34,36,38,39 Thisindicates that the results are highly relevant clinically.This area has limited space between the pulp and theneighboring tooth for adjustments of the wall thickness

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Figure 3. A, B, Fracture modes in core group typically displayed 1 major fracture line from mesial to distal side of crown. Cracks often branchedinto 1 or 2 partial or complete additional fracture lines. Typical fracture features such as hackle and arrest lines seen in both veneer and corematerial. Direction of crack propagation indicated by dashed arrows. C, D, and E Reassembled crown after fracture. Fracture origin on crownmargin and crack propagation across occlusal surface where it branches off into 2 fracture lines. One branch stopped on occlusal surface, whileother continued across to other surface. Some veneer chipped off during testing. Note: no contact damage from loading. Horizontal white linesindicate 1 mm.

Table 1. Location of fracture origin in different test groups and numberof fractures that originated from pre-existing defects

FractureMode

MesialMargin

DistalMargin

PalatalMargin Occlusal

Origin inDefect

Core 6 3 1 0 2

Collar 6 3 1 0 3

Monolithic 6 3 0 1 5

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or crown margin. The mesial and distal margins of aceramic crown preparation exhibit highest stress valuesduring compression.43 The current findings indicatethat the proximal region is the weakest part of thecrown. Previous findings of failure modes of both in vitroand in vivo crowns support this.18,33-40,49 The curvatureof the finish line was equal on the mesial and distal side,yet 18 fractures originated on the mesial side comparedwith 9 on the distal side. A small difference in thepreparation between the 2 sides may have contributedto this difference.

The finding that most crowns had flaws distributedalong the crown margins indicates that marginal flaws

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are an intrinsic part of this restoration, or at least animportant to fracture origin. One-third of the fracturesoriginated in flaws, but the lack of correlation betweenobserved margin quality and the load at fracture indicatesthat margin quality does not greatly affect the fracturestrength of dental crowns during static loading. However,margin flaws will likely be more vulnerable to aging andthus more detrimental to strength because of the stressaccumulations at the flaws. Previous studies of bothclinical failures and in vitro failures have shown bothflaw-induced fractures and fractures originating fromapparently flawless margins.18,33 The findings indicatethat more attention should be paid to reducing themargin flaws of ceramic restorations in order to improveclinical quality and reliability.43-47

An advantage of the collar and anatomic contourcrowns is the simplified application of veneer ceramics.Previous analyses of crowns fractured during clinicalfunction have revealed that veneering material on theinside of the core is a relatively common occurrence.18

This will affect the seating, the fit, and the stress

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Figure 4. Fracture modes in collar group similar to core group. Fatal cracks often branched off into more fracture lines than in core group. Fracturefeatures visible in both core and veneer. A, Compression curl evident where cracks branched off. B, C, Areas in boxes in image A at higher magnifi-cation, where wake hackle shows direction of crack propagation, dashed arrows. D, E, reassembled fractured crown. Fracture origin located in palatalcrown margin. In an occlusal view, main fracture line branched off into 4 fracture lines. Horizontal white lines indicate 1 mm.

Figure 5. A, Monolithic crowns usually fractured into several pieces, and reassembly was difficult. Images show piece where fracture features clearlyshow crack propagation (dashed arrows) from margin towards occlusal surface. B, Boxed area at higher magnification in connected image. Area oforigin flat and regular (mirror) followed by region with flaking and irregularities (mist), while remaining fracture surfaces remarkably smooth. Horizontalwhite lines indicate 1 mm.

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Figure 6. Quality of crown margins varied, but no statistically significant differences found among test groups. Only 4 crowns flawless. A, Core veneercrown with multiple flaws (arrows). B, Core veneer with collar crown without margin flaws. C, Anatomic contour crown with one margin flaw (arrow).D, Represents boxed region in image CA. E, Represents boxed region in image B. F, Represents boxed region in image C. Horizontal white lines indicate1 mm.

Figure 7. Cracking (infractions) in veneer observed in one of crownswhere test terminated before complete fracture (at 2500 N). Infractionsoriginated from crown margin but did not involve core material. Thesekinds of infractions occurred in most crowns short time before completefracture. Horizontal white line indicates 200 mm.

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distribution of the crown and may be a cause of earlyfailures. A cervical collar without the need for veneerwill probably reduce the risk for contamination insidethe core. Furthermore, smooth transitions between thetooth, core, and veneer facilitate biofilm removal.

Chipping is reported as one of the major clinicalproblems with zirconia restorations in clinical use forboth tooth- and implant-based restorations.3-6 Findingsindicate that anatomic contour crowns or crowns with a

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cervical collar are stronger and may reduce the risk ofchipping. Differences in thickness and thermal and re-sidual stress due to lack of compatibility between the coreand veneer may influence the chipping of zirconiacores.19,26-29,50,51 The findings that infractions occurred inthe veneering ceramic before complete fracture indicatethat differences in the flexural properties of the coreand the veneer may also affect the rate of chipping. Theloads at the first infraction crack were, however, aboveclinically relevant loads and indicate that all designs aresuited for clinical use. Previous studies of alternative coredesigns indicate that the rate of chipping is reduced forcores with cervical collars in the proximal and lingualregions.13-15 The collar or anatomic contour design mayreduce the need of invasive tooth preparation in certainareas of the mouth. The zirconia core materials can beproduced in several different degrees of translucency andin multiple tooth-colored shades. The esthetic needsshould therefore be achievable in most patients in boththe premolar and molar regions with either the anatomiccontour or strengthening collar design.

CONCLUSIONS

The crown wall thickness and the marginal crown designaffect the load at fracture of zirconia crowns. Monolithic,anatomical contour design gave higher loads at fracturethan traditional core-veneer design. Crowns with cervicalzirconia collar had higher load at fracture than the core-veneer design, but lower than the monolithic crowns.

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Corresponding author:Dr Marit ØiloDepartment of Clinical DentistryFaculty of Medicine and DentistryUniversity of BergenAarstadveien 19. NO-5009 BergenNORWAYEmail: marit.oilo@iko.uib.no

AcknowledgmentsThe authors thank Svetlana Mulyukova, dental technician at Proteket AS, Oslo,Norway, for helping with the design, and Anneli Skjold, for helping with pre-paring the specimens for the fracture test.

Copyright © 2016 by the Editorial Council for The Journal of Prosthetic Dentistry.

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