estudio ce.novation zirconio

VOLUME 40 • NUMBER 8 • SEPTEMBER 2009 655

QUINTESSENCE INTERNATIONAL

Long-term experience has shown that hot-

pressed glass-ceramics, which ensure good

esthetic results, as well as a good biologic

compatibility, are suitable as adhesively

bonded posterior crowns in clinical applica-

tion. Against the background of new and

faster technologies (computer-aided design/

manufacturing [CAD/CAM]), easier handling

(no adhesive cementation required), and

increased strength, new materials are being

introduced into the market. Standardized

computer-controlled ceramic processes such

as milling (Lava, 3M ESPE; Cercon,

DeguDent), ceramic buildup (ce.novation,

ce.novation), or computerized slip casting/

electrophoresis (alumina-ceramic Wolceram,

Wolceram) are discussed for the fabrication

of dental restorations.1,2 Both high-strength

hot isostatically pressed (HIP) or partially sta-

bilized zirconia ceramics show a high frac-

ture strength with a small range of strength

Fracture performance of computer-aided manufactured zirconia and alloy crownsMartin Rosentritt, PhD1/Michael Behr, PhD2/Christian Thaller, DDS3/

Heike Rudolph, DDS4/Albert Feilzer, DDS, PhD5

Objective: To compare the fracture resistance and fracture performance of CAD/CAM zir-

conia and alloy crowns. Method and Materials: One electrophoretic deposition alumina

ceramic (Wolceram, Wolceram) and 4 zirconia-based systems (ce.novation, ce.novation;

Cercon, DeguDent; Digizon, Amann Girrbach; and Lava, 3M ESPE) were investigated. A

porcelain-fused-to-metal method (Academy, Bego Medical) was used in either convention-

al casting technique or laser sintering. Sixteen crowns of each material were fabricated

and veneered with glass-ceramic as recommended by the manufacturers. Crown and root

dimensions were measured, and 8 crowns of each system were adhesively bonded or

conventionally cemented. After the crowns were artificially aged in a simulated oral envi-

ronment (1,200,000 mechanical loads with 50 N; 3,000 thermal cycles with distilled water

between 5°C and 55°C; 2 minutes per cycle), fracture resistance and fracture patterns

were determined and defect sizes investigated. Results: The fracture force varied

between 1,111 N and 2,038 N for conventional cementation and between 1,181 N and

2,295 N for adhesive bonding. No significant differences were found between adhesive

and conventional cementations. Fracture patterns presented mostly as a chipping of the

veneering, in single cases as a fracture of the core, and in 1 case as a fracture of the

tooth. Conclusions: Crown material and cementation do not have any significant influ-

ence on the fracture force and fracture performance of all-ceramic and metal-based

crowns. Therefore, it may be concluded that adhesive bonding is not necessary for the

application of high-strength ceramics. (Quintessence Int 2009;40:655–662)

Key words: adhesive bonding, CAD/CAM, cementation, dental crown, fracture, zirconia

1Engineer, Department of Prosthetic Dentistry, University

Medical Center Regensburg, Regensburg, Germany.

2Professor, Department of Prosthetic Dentistry, University

Medical Center Regensburg, Regensburg, Germany.

3Assistant, Prosthetic Dentistry, University Medical Center

Regensburg, Regensburg, Germany.

4Assistant Professor, Prosthetic Dentistry, University Medical

Center Regensburg, Regensburg, Germany.

5Professor and Chair, Department of Dental Materials Science,

Academic Centre for Dentistry, Universiteit van Amsterdam and

Vrije Universiteit, Amsterdam, The Netherlands.

Correspondence: Dr Martin Rosentritt, Regensburg University

Medical Center, Department of Prosthetic Dentistry, Franz-

Josef-Strauss Allee 11, Regensburg D-93042, Germany. Email:

[email protected]

© 2009 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART OF THIS ARTICLE MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.

656 VOLUME 40 • NUMBER 8 • SEPTEMBER 2009


Rosentr i t t e t a l

variation and a high structural reliability

compared to conventional dental glass-

ceramics.3,4 Metal-based restorations may be

fabricated conventionally by means of cast-

ing techniques or alternatively with laser-sin-

tering (Bego Medical). Both technologies

should show only small differences in the

composition and final structure of the alloys,

regardless of the mode of melting used, ie,

laser-sintered or conventionally melted.

The computer-based fabrication process

starts with digitizing the clinical situation with

a 3-dimensional scanner, followed by CAD

designing the cores of the restorations and

their fabrication in the particular CAM

process. One system allows the alternative

scanning of waxup models (Cercon). A weak

point in view of processing and strength may

be that CAD/CAM cores have to be veneered

with comparatively low-strength conventional

glass-ceramics in press or layering technique.

Chipping of the veneering ceramic has

already been reported for porcelain-fused-to-

metal (PFM) restorations,5 and particularly

chipping of the veneering ceramic for zirco-

nia has been widely discussed6–8 since the

launch of the current zirconia systems. The

basic effects of veneering on the core-veneer-

ing interface,9 as well as on the fracture per-

formance of 2- or 3-layer specimens, have

been reported, helping to understand failure

mechanisms.10–12 Laboratory results allow the

prediction of the combination of material lay-

ers, but failure type and pattern may vary for

clinically relevant restorations. The main rea-

son may be in the individual design and

dimension of a special restoration, in which,

for example, compliance with an optimal

veneering thickness is difficult to achieve. On

the other hand, in vivo conditions may differ

from loadings in the laboratory.

To investigate the performance of new

materials, fracture benchmark tests were

conducted. This static test on dental restora-

tions may reveal different failure patterns in

comparison to in vivo situations.13,14 Moreover,

the influence of improper alternative tooth

abutment material (for instance, steel)15–17

may falsify results. Simulation procedures

with dynamic loading and thermal cycling

using clinically relevant chewing forces and

bath temperatures are applied for aging

specimens and are supposed to result in a

performance approximated to the clinical sit-

uation of restorations.18,19 Failures during sim-

ulation can be compared to failures during

oral application and may help to estimate the

lifetime of new materials. If no failures occur

during simulation, a subsequent static frac-

ture test allows the locating of initiated weak

points or at least permits comparison of the

tested materials to clinically well-known sys-

tems. Basic fractographic information,20,21

which describes ceramic failures as initiated

by flaws or damages from the marginal side

or occlusal surface,14 contributes to the eval-

uation of results.

The null hypothesis of this investigation

was that no significant difference exists

between the fracture force and fracture per-

formance of all-ceramic and metal-based

crowns after simulation of oral service. The

influence that the dimensions of both tooth

and crown, as well as the type of cementa-

tion, have on a fracture should be consid-

ered.

METHOD AND MATERIALS

To simulate the human periodontium, the

roots of human molars (n = 96) were first

coated with a 1-mm layer of polyether materi-

al (Impregum, 3M ESPE) and then inserted

into polyethylenemethacrylate (PMMA) resin

(Palapress Vario, Heraeus Kulzer). This layer

allows the maximum tooth mobility of 0.1 mm

in axial and vertical directions at a load of

50 N. Human molars were used to ensure a

clinically relevant modulus of elasticity of the

abutments and simulate a relevant bonding

between crown and tooth. Each tooth was

prepared according to the directives for

ceramic restoration techniques using a 1-mm-

deep circular shoulder crown preparation.

Sixteen crowns of each material group listed

in Table 1 were fabricated according to

the manufacturers’ instructions. All frame-

works were veneered according to the man-

ufacturers’ instructions using glass-ceramic

materials, which were recommended by the

manufacturers of the core materials (see

Table 1).





To compare the type of cementation, 8

crowns of each group were luted adhesively

with dual-curing composite (Variolink 2 with

Syntac Classic for bonding, Ivoclar Vivadent),

whereas the other 8 crowns were cemented

with conventional zinc oxide–phosphate

cement (Harvard, Hoffman & Richter). The

dimensions of the investigated teeth and

crowns were determined for adhesive/conven-

tional cementation, respectively: mean height ±

standard deviation (SD) of the crown (mm): 6.8

± 1.1 / 6.9 ± 1.0; length ± SD of the root (mm):

11.5 ± 2.2 / 10.9 ± 2.0; distal-mesial length ±

SD (mm): 9.4 ± 1.4 / 9.0 ± 1.6; and palatal-buc-

cal length ± SD (mm): 9.9 ± 1.0 / 9.9 ± 1.1.

Artificial aging was performed to simulate

5 years of oral service using the following

loading parameters18: 1,200,000 mechanical

loads with 50 N and simultaneous thermal

cycling with distilled water between 5°C and

55°C (3,000 times for 2 minutes per cycle). A

human molar was adjusted as an antagonist

in a dental articulator (Artex CN, Amann

Girrbach), and tooth and crown were trans-

ferred to the simulator. Antagonist-tooth rela-

tion was controlled with an occlusal foil. Aging

was interrupted every 100,000 mechanical

loading cycles, and the crowns were checked

optically for failures (fracture, chipping).

After aging, each crown was loaded until

failure by means of a testing machine (Zwick

1446, Zwick; velocity = 1 mm/min). Force

was applied using a steel ball (diameter = 12

mm), and a tin foil (1 mm) between crown

and antagonist prevented force peaks. The

crowns were examined optically before and

after fracture testing. The failure mode was

divided into the following fracture patterns:

initial crack, chipping in the veneering

ceramic, chipping down to the framework,

and fracture of the core or the tooth (Fig 1).

Location and size of failure were analyzed in

mesial, distal, buccal, and lingual directions.

Material/ Conventional veneering (batch) Manufacturer cementation Adhesive bonding

Academy/Vita Omega Bego Medical/Vita Zahnfabrik Harvard, Syntac Classic/Variolink 2, Ivoclar Vivadent(22920/7427) Hoffman & Richter (G22359/21260/18808/14040/

(1104C09/B11/1116B02/ 24888/L16678/22838/L9241)2121000311)

Academy/Vita Omega Bego Medical/Vita ZahnfabrikLaser sintering (16020/15560)ce.novation/Cercon ce.novationCeram Kiss(50708/50739)Cercon/Cercon DeguDentCeram Kiss (25264/24917)Digizon/GC Initial Amann Girrbach/(8276/7811) GC EuropeLava/Lava Ceram(KW00400133/400116) 3M ESPEInceram (Wolceram)/Vita Alpha (2331/2029/ Wolceram/Vita Zahnfabrik11050601/5303)

Table 1 Materials and manufacturers

Fig 1 Type of failure.

Crack

Fracture in the veneering

Fracture between framework and veneering

Fracture in tooth/crown




The type of crown failure was analyzed in

detail by means of scanning electron micro-

scopy (SEM; Quanta, FEI-Phillips). Overview

and detailed photographs were made (mag-

nification: 10� to 1,000�; working distance:

20.4 mm; voltage: 5 kV; low vacuum).

Medians, as well as 25th and 75th per-

centiles of the fracture resistance (newtons),

were calculated. Statistical analysis was per-

formed using 1-way analysis of variance

(ANOVA) and the Kruskal-Wallis test to detect

statistically significant differences between

values by pairwise comparisons (� = .05).

Calculations were conducted using statistical

software (SPSS 11.5 for Windows, SPSS). For

power calculation, the relative effects of the

pairwise comparisons were calculated. The

power calculation for the Wilcoxon (Mann-

Whitney) rank-sum test was performed using

G*Power (Kiel University).22 Using 8 samples

for each material and accepting a 2-sided

type I error of 5% for each comparison, a

power of 80% was achieved (0.807).

RESULTS

The mean fracture resistance of the tested

systems varied between 1,111 N and 2,038 N

for conventional cementation and 1,181 N

and 2,295 N for adhesive bonding. Fracture

force with adhesive bonding was lower for

the systems Academy laser sintering (P =

.574), Digizon (P = .279), and Lava (P = .382)

compared to conventional cementation. The

other systems revealed a higher fracture

strength with adhesive bonding, but the

results were not statistically significant

(P = .382) (Fig 2). The main failure type was

chipping of the veneering ceramic. For

Cercon (P = .779) and Wolceram (P = .382),

1 fracture of the framework could be found

for both types of cementation. For ce.novation

(P = .574), 1 core fracture could be determined

for conventional cementation and 2 core frac-

tures for adhesive bonding. In the Cercon

group, only 1 case of conventional cementa-

tion showed a fractured tooth. The detailed

fracture patterns are shown in Table 2. The

portion of the failed veneering was related to

the surface of the whole crown.


VariolinkHarvard

Academy Academylaser

ce.novation

CerconKiss

Digizon Lava Wolceram

3,500

3,000

2,500

2,000

1,500

1,000

0

Frac

ture

forc

e (N

)

VariolinkHarvard

WolceramLavaCerconKiss

ce.novation

DigizonAcademylaser

Academy

50

40

30

20

10

0

Failu

re c

row

n a

rea

(mm

2 )

Fig 3 Failure crown area (mean, SD).

Fig 2 Fracture force after thermal cycling and loading (mean, SD).





Failures varied between 5% and 32% (Fig

3). PFM restorations and Cercon Kiss (adhe-

sive) showed the lowest values, of about 10%

and less. No significant differences (P = .723)

in the propagation of defects could be deter-

mined, neither for adhesive bonding nor con-

ventional cementation. Defects occurred

more frequently on mesial and distal tooth

sides compared to labial or palatal sides (Fig

4). Figure 5 provides an example of an SEM

image of a typical crown failure.

AcademyCercon laser

Digizon Lava ce.novation Kiss Wolceram Academy sintering

Ad Co Ad Co Ad Co Ad Co Ad Co Ad Co Ad Co

Tooth 1Framework 2 1 1 1 1 1Chipping 8 8 8 8 6 7 7 6 7 7 8 8 8 8

Crack 2 2Fracture in 3 4 8 8 2 5 5 2 1 2the veneeringFracture between 5 2 4 1 1 4 7 8 6 8 8 8framework and veneering

(Ad) Adhesive; (Co) conventional.*For defect type, see Fig 1.

Table 2 No. and type of failure*

PalatinalMesial

Wolceram Lava CerconKiss

ce.novation

Digizon Academylaser

Academy

1.0

0.6

0.4

0.2

0.0

–0.2

Failu

re s

ize

(%) 0.8

PalatinalMesial


ce.novation


Academy

1.0

0.6

0.4

0.2

0.0

–0.2

Failu

re s

ize

(%) 0.8

Harvard

VariolinkDistalMesial


ce.novation


Academy

1.0

0.6

0.4

0.2

0.0

–0.2

Failu

re s

ize

(%) 0.8

DistalMesial


ce.novation


Academy

1.0

0.6

0.4

0.2

0.0

–0.2

Failu

re s

ize

(%) 0.8

Harvard

Variolink

Figs 4a and 4b Failure crown area (mean, SD).





DISCUSSION

The null hypothesis of this investigation has

to be corroborated, ie, that no significant dif-

ference exists between the fracture force and

fracture performance of all-ceramic and

metal-based crowns after the simulation of

oral service. No significant influence of the

type of cementation on the fracture could be

determined. Assuming that the strength of

the ceramic crowns tested was reduced by

cyclic loading23,24—which was supposed to

simulate oral service of about 5 years19 —frac-

ture loading of the tested systems exceeded

the postulated requirements of 500 N25 and

was therefore high enough to resist in vivo

chewing forces in posterior applications.

Fracture testing as a single-load test

shows no clinical relevance but may provide

helpful data for comparing tested speci-

mens. During oral simulation, flaws or other

superficial wear or aging effects contribute to

the deterioration of the material and reduce

fracture strength. Therefore, fracture testing

after simulation allows for the differentiation

of materials. In comparison to well-known

systems, these data may help to estimate the

clinical performance of new materials.

The wide distribution of fracture results

restricts their significance, indicating the high

individuality of restorations. The characteris-

tics of materials (strength, Weibull modu-

lus),26 their fabrication (density, severity,

flaws, voids, or cracks),27 or improper super-

ficial polishing may contribute to the high

variation of results. More relevant factors are

the differences in the core thickness of

ceramic crowns and the resulting varying lay-

ers of veneering ceramics because of labo-

ratory work. The (manual) veneering of cores

of a uniformly low thickness will result in an

increased thickness of the veneering that will

be more prone to fracture, independent of

the buccolingual or mesiodistal crown

dimensions. This could be the reason the

measured crown dimensions did not have

any significant influence on fracture results,

in contrast to the results described by other

authors for crown material and thickness.16

The fracture pattern showed that the frac-

ture strength depends on the strength of the

weakest part of the crown, which seems to

be the veneering ceramic. In most cases, the

tested systems showed chipping of the com-

parable low-strength veneering ceramic but

seldom a fracture of the high-strength core or

the whole tooth. The fact that, in most cases,

only veneering is involved in the failure pat-

tern explains why no significant differences

exist among crowns made of differing core

materials. High-strength HIP zirconia (Digi-

zon) did not show any strength advantages

compared to zirconia systems, which were

milled in a partially sintered state or fabricat-

ed in a ceramic build-up process. Although

electrophoretically manufactured alumina

demonstrates even lower strength values, no

statistically different fracture values could be

detected between Wolceram and PFM

crowns. The fact that the fracture results

depend on the glass-ceramic veneering

would also explain why these results were

comparable to adhesively bonded glass- or

leucite-reinforced all-ceramic systems,17,28–30

since both have a comparable chemical

basis and strength (~100 to 200 GPa).16

Furthermore, the described fracture pattern

in the veneering shows that the type of

cement did not contribute to the fracture

resistance of crowns with high-strength

cores, but significantly influences the fracture

strength of glass-ceramic crowns.28

Fig 5 SEM image of a typical crown failure (Cercon).

Origin

Wake hackles

Arrest linesVeneeringCore





Fractographic analysis by SEM showed

that the fracture origin in the veneering was

mostly on the occlusal surface. Here, the

antagonist caused wear or superficial flaws

during thermal cycling and loading, which

was the origin of the fracture or chipping in

the following fracture test (see Fig 5). These

results are partly in agreement with investiga-

tions of in vivo failures of glass-ceramic sys-

tems, for which a failure of occlusal and mar-

ginal areas has been described.14 These

results may explain the clinically described

chipping of PFM- and zirconia-based restora-

tions,6–8 but no detailed failure analysis of zir-

conia restorations under clinical conditions

has been conducted so far.

In accordance with the literature,9,11,12 the

present SEM pictures also demonstrated

that chipping can be divided into 2 types:

Chipping occurred interfacially between

cores and veneering ceramic or cracks ran in

the veneering itself. In the case of interfacial

fractures, a thin layer of the veneering ceram-

ic remained on the core material. These

findings underline that fracture strength is

influenced by the properties (for instance,

strength, fracture toughness) of the veneer-

ing material itself and to some smaller extent

by the veneering-core bonding. Cracks with-

in the veneering at lower fracture forces sug-

gest a lower strength value of the applied

veneering ceramic material (Lava), whereas

lower fracture forces refer to either a combi-

nation of a lower fracture toughness of the

veneering and core material or one of these

2 aspects (Wolceram). These effects could

mask one another. Further investigations are

needed with regard to the extent the surface

structure of the framework (manufacturing

process: smooth as machined, roughened

microstructure, milling patterns; see Fig 5)

contributes to the bonding of veneering

ceramics to the zirconia core.

The fact that the fracture area of PFM

crowns was smaller in contrast to failure

areas of all-ceramic crowns (see Fig 3)

requires further detailed and systematic eval-

uation. A clinical consequence may be that

although these defects are smaller, they are

more obvious because of the exposure of the

metal framework. Chipping of the ceramic

may not be visible or could be easily

removed by polishing. The small increase of

failures in the distal and mesial directions

(see Fig 4) may be attributed to the design of

the crown and may be of interest when fabri-

cating restorations for patients with high

chewing forces or bruxism.

The achieved fracture values suggest a

sufficient strength of the crown systems for

clinical application, but the fracture patterns

underline the requirement for a core design,

which supports the occlusal veneering

ceramic.31,32 Wear of the veneering ceramic

may cause superficial defects, which may

cause chipping of an insufficiently supported

veneering in the long term.33 The results

show that the clinical survival of all crowns

supposedly depends on the surface quality

of the veneering (strength, fracture tough-

ness, surface roughness) and, to a lesser

extent, on the bond at the veneering and

core interface, but not on the strength of the

underlying core structures.

CONCLUSIONS

This study indicated that the fracture force

and fracture performance of types of all-

ceramic and metal-based crowns depend

neither on the crown material nor the type of

cementation. Thus, it may be concluded that

adhesive bonding, which is required for other

nonzirconia ceramic systems, is not neces-

sary for crowns based on high-strength

ceramics.

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