Bon

zirc

Carla Müller Ram

Supported by FAPESP 2011/18061-0

aDoctoral student, Department of DbProfessor, Department of BiomatercProfessor, Department of Dentistry,dProfessor, Department of Physics, FeGraduate student, Bauru School offProfessor, Department of Dentistry,

Ramos et al

d strength and Raman analysis of the

onia-feldspathic porcelain interface

os, DDS, MS,a

Paulo Francisco Cesar, DDS, MS, PhD,b

Rafael Francisco Lia Mondelli, DDS, MS, PhD,c

Americo Sheitiro Tabata, MS, PhD,d Juliete de Souza Santos, DDS,e

and Ana Flávia Sanches Borges, DDS, MS, PhDf

Bauru School of Dentistry, Faculty of Science, University of São Paulo(USP), Bauru, SP, Brazil

Statement of problem. Zirconia has the best mechanical properties of the available ceramic systems. However, the stability ofthe zirconiaefeldspathic porcelain interface may be jeopardized by the presence of the chipping and debonding of thefeldspathic porcelain.

Purpose. The purpose of this study is to evaluate the shear bond strength of 3 cold isostatic pressed zirconia materials and afeldspathic veneer by analyzing their interface with microeRaman spectroscopy.

Material and methods. The test groups were experimental zirconia, Zirkonzahn zirconia, and Schuetz zirconia. Blocks ofpartially sintered zirconia were cut into disks (n¼20) and then veneered with a feldspathic porcelain. Half of the specimensfrom each group (n¼10) were incubated in 37�C water for 24 hours, and the other half were thermocycled. All the specimenswere then subjected to shear testing. The fractured areas were analyzed with optical stereomicroscopy and classified asadhesive, cohesive, or an adhesive-cohesive failure. Spectral patterns were examined to detect bands related to the zirconiaand feldspathic porcelain phases. The shear strength data were submitted to 2-way ANOVA.

Results. No significant differences in shear bond strength were observed among the 3 groups, regardless of whether or not thespecimens were thermocycled. Adhesive failures were the most prevalent types of failure (70%). Raman spectra were clearlydistinguished for all the materials, which showed the presence of tetragonal and monoclinic phases.

Conclusions. The controlled production of the experimental zirconia did not influence the results of the bond strength.Raman analysis suggested a process of interdiffusion by the presence of peaks associated with the zirconia and feldspathicceramics. (J Prosthet Dent 2014;-:---)

Clinical Implications

The success of the zirconia-feldspathic veneer combination could belimited by chipping or debonding of the feldspathic porcelain. Anevaluation of the shear bond strength between a new zirconia and afeldspathic porcelain, and an analysis of the interface with Ramanspectroscopy can be used to improve the understanding of thezirconiaefeldspathic ceramic relationship.

.

entistry, Endodontics and Dental Materials, Bauru School of Dentistry.ials and Oral Biology, Bauru School of Dentistry.Endodontics and Dental Materials, Bauru School of Dentistry.aculty of Science, São Paulo State University.Dentistry.Endodontics and Dental Materials, Bauru School of Dentistry.

Table I. Materials used in study, Composition and Coefficient of thermalexpansion (CTE)

Material Code Manufacturer Composition CTE*

Zirconia

Experimental EP Marília Zr(hf)O2, 94.7%; Y2O3,5.25%; SiO2, 0.020%; Fe2O3,0.010%; Na2O, 0.010%; CL,

0.020%; TiO2, 0.010%

10

Zirkonzahn ZK ZirkonzahnGmbH

ZrO2, 91.5%; HfO2, 3.0%;Y2O3, 5%; SiO2, 0.020%;

Fe2O3, 0.0050%; Na2O, 0.01%

10.5

Shuetz SH SchuetzDentalGmbH

Zr (O2), <96%; HfO2, >1%;Y2O3, >4%; Al2O3 <1%

10

Feldspathic ceramic

CeramicDentin A3

P ZirkonzahnGmbH

SiO2, Al2O3, P2O5, K2O, Na2O,CaO, F, TiO2 > 97%

9.7

CeramicDentin Opaker

PO ZirkonzahnGmbH

SiO2, Al2O3, P2O5, K2O, Na2O,CaO, F, TiO2 > 97%

9.7

*Coefficient of thermal expansion (CTE) in �10-6K-1 between 25�C and 500�C.

2 Volume - Issue -

Yttria-stabilized tetragonal zirconiahas been increasingly used in dentistryto replace metal substructures becauseof its strength,1-3 fracture toughness,4,5

chemical stability, and biocompati-bility.6-8 Feldspathic porcelain veneersare applied to zirconia, not only foresthetic reasons but also because theyare stable in combination with yttria-stabilized tetragonal zirconia9-11 andare brittle and of limited tensilestrength when used alone. However,clinical failures, such as the chippingand debonding of the feldspathicveneer, have been reported as one ofthe most common concerns in thistype of restorative combination ofmaterials.12

The mechanical strength provided bythe zirconiaefeldspathic veneer combi-nation may be limited by the fragilefeldspathic component with its relativelylow fracture toughness.13-16 Feldspathicceramic chipping and fracture arecommon clinical problems.17 The ratesof clinical failure of the zirconiaefeldspathic veneer range from 13% to15% over a period of between 2 and 5years.9,10,18 When failures occur, theymay be either cohesive within the feld-spathic layer (chipping) or adhesive atthe zirconia-veneer interface, which re-sults in the delamination of the veneerlayer.19,20 According to Komine et al,21

fractures in the feldspathic veneer usu-ally progress from the area subjected tohigher tensile stress toward the interface.When this phenomenon occurs, thestresses in the feldspathic layer may ormay not reach the interface with thezirconia substructure.22 Cohesive fail-ures are associated with the use of linersor bonding agents, the presence of poreswithin the feldspathic porcelain layer, orthe location of the restoration inside theoral cavity.23-25 Moreover, adhesivefailures between the feldspathic veneerand the zirconia substructure may beexplained, in part, by the high contactangle between these 2 materials,26

differences in coefficients of thermalexpansion (CTE),2,27 poor thermal con-ductivity associated with the poor ther-mal diffusivity,28 or the surface state ofthe zirconia substructure.21

The Journal of Prosthetic Dentis

Delamination failures are morefrequent19 and may be associated withthe residual stress29,30 promoted bytemperature changes at the substructure-veneer interface.24,31 The strength of thebond at the substructure-veneer inter-face32,33 and the resistance of thebonding layer itself may be affected byextensive thermal residual stress,34 aproblem that may then develop duringthe cooling phase after sintering.22,35,36

Moreover, the residual stresses may begenerated by differences in the CTEbetween the 2 materials.22 The bondstrength of feldspathic-veneering ceramicand the zirconia interface has not beenextensively studied.37 The aim of thisstudy was to evaluate the shear bondstrength of 3 cold isostatic pressed zirco-nia materials and the feldspathic veneerby analyzing their interface with microeRaman spectroscopy.38-40 The null hy-potheses were that no difference wouldbe found in the shear bond strengthof 3 zirconia materials and that thermo-cycling would not influence zirconiaefeldspathic ceramic bond strength.

MATERIAL AND METHODS

Experimental zirconia (EP) blockswere obtained with isostatic pressing by

try

applying pressure uniformly in all di-rections. A flexible elastomeric mem-brane was used as a matrix. Theelastomeric matrix cavity was filled withEP powder (see Table I for composi-tion) and sealed with a metal plate. Theouter surfaces of the matrix werepressed by a fluid that transmits pres-sure toward the matrix, which resultedin powder compaction (Fig. 1). Thepressure used for each block was 120MPa for 5 seconds at 25�C and at 60%humidity.

Blocks of partially sintered zirconiafrom the 3 groups (dimensions 15.5 �19 � 39 mm) were milled in a panto-graph system (Zirkonzahn GmbH) toform disks (11 mm in diameter and2.7 mm in thickness). After sintering at1530�C for 5 hours and 52 minutes, asrecommended by the manufacturers,all the disks were polished with siliconcarbide and airborne-particle abradedwith 50 mm aluminum oxide at astandard distance of 10 mm.15 Thespecimens were cleaned by immersingthem in acetone in an ultrasound ma-chine (Ultrasonic Cleaner; Unique) for10 minutes. A thin layer of opaqueceramic dentin (Opaker; ZirkonzahnGmbH) was applied onto all the zir-conia disks, followed by firing (EDG

Ramos et al

1 Isostatic pressing. (A) Matrix cavity was filled with exper-imental zirconia (EP). (B) Isostatic pressure in all directions.(C) Compacted powder. (D) EP block.

- 2014 3

Equipamentos) at 900�C for 30 mi-nutes. A custom-designed metallic de-vice (5 mm in diameter and 5 mm inheight) was constructed for the appli-cation of the feldspathic-veneer cera-mic (Ceramic Dentin A3; ZirkonzahnGmbH) (Fig. 2). The feldspathic por-celain was mixed with modeling liquid(ICE; Zirkonzahn GmbH), placed in thedevice, and manually condensed with aWard condenser number 4 (SSWhite;Duflex). The device was removed, andthe specimens with the feldspathicveneer were sintered at 820�C for 30minutes, according to the manufac-turer’s instructions. After sintering, thespecimens were embedded with acrylicresin (JET; Classico) in a polyvinylchloride (PVC) cylinder of 10 mm indiameter.

For the shear bond strength test,the specimen-size calculation was

2 A, Custom-designed metal device. B,

Ramos et al

performed based on a pilot study whenconsidering an a of .05 and a powerof .80. The calculations revealed that10 specimens per group would beneeded to detect the postulated effectsize. Therefore, 10 specimens from eachgroup (n¼10) were stored for 24 hoursin water at 37�C, and 10 were ther-mocycled (500 cycles; 30 seconds at5�C and 30 seconds at 55�C, with a 10-second dwell time) for approximately10 hours.32 Thereafter, all specimenswere subjected to a shear test41-43 witha universal testing machine (EMICDL500; EMIC Test Equipment andSystems Solutions) with a load cell of50 Kgf and a special mechanical testingdevice (Fig. 3) with metallic ribbon at aspeed of 0.5 mm/min until fracture.This device was adapted from thatdeveloped by Sinhoreti et al44 to mini-mize bending stress. A mechanical

Feldspathic porcelain on zirconia disk.

support was placed opposite the spec-imen to avoid any movement at thezirconiaefeldspathic porcelain interface.

The analysis of the zirconiaefeldspathic porcelain interface was per-formed for all specimens with opticalmicroscopy (Discovery V8 Stereo; CarlZeiss Microimaging GmbH) at �32magnification. Each specimen was clas-sified according to the type of failureobserved as follows: adhesive failure atthe zirconia and feldspathic porcelaininterface, cohesive failure of the feld-spathic porcelain veneer, and when bothtypes of failure were present.

Specimens from each group thatshowed a remaining feldspathic veneerwere cut perpendicularly, and theinterface was examined with micro-Raman spectroscopy. The micro-Raman measurements were carried outat room temperature in backscatteringgeometry by means of a JobinYvonmicro-Raman system (model T64000;Horiba Group) at the Physics Instituteof the State University of São Paulo(USP). The 514.5 nm (2.41 eV) radia-tion of an argon ion laser (SpectraPhysic Inc) was used for excitation. Thebeam was focused with a microscopeat �500 magnification with a laser spotof approximately 5 mm in diameter. Nopolarization analyzer was used for thescattered beam. To avoid any thermaldamage, the laser power was kept aslow as 10 mW. The Raman spectrawere analyzed by means of a doublesubtractive monochromator, with afocal length of 0.64 m and equipped

3 Schematic illustration of device to measure shear bondstrength.

EP-2420.18 ±1.82

0.00

22.0020.0018.0016.0014.0012.0010.00

8.006.004.002.00

ZK-2417.75 ±1.52

SH-CT18.81 ±1.60

SH-2418.81 ±1.82

EP-CT17.75 ±1.52

ZK-CT15.33 ±1.52

MPa SD

4 Shear bond strength between feldspathic porcelain and ZrO2. EP, experimentalzirconia; ZK, Zirkonzahn zirconia; SH, Shuetz zirconia after storage for 24 hours inwater (-24); after thermocycling (-CT). Columns show mean values of 10 mea-surements (MPa) �standard deviation (SD).

100%

90%

80%

70%

60%

50%

40%

30%

20%

10%

0%EP-24 ZK-24 SH-24 SH-CTEP-CT ZK-CT

Mixed 20% 10% 10% 30% 10% 10%

10% 0% 10% 10% 30% 20%Cohesive

70% 90% 80% 60% 60% 70%Adhesive

5 Percentage of failure types. EP, experimental zirconia; ZK, Zirkonzahn zir-conia; SH, Schuetz zirconia after storage for 24 hours in water (-24) and afterthermocycling (-CT).

4 Volume - Issue -

The Journal of Prosthetic Dentistry

with a diffraction grating with 1800grooves/mm. The slit width was set to200 mm, which provided a spectralresolution of approximately 2 cm�1. Aline scan was recorded on the sectionedspecimen that crossed from the zirconiato the feldspathic ceramic with acharge-coupled device camera (SpectraOne; Horiba Group), which yielded 5spectra from each group with 8 mm ofdistance between them. Shear bondstrength data were analyzed by 2-wayANOVA followed by the pairwise mul-tiple comparison Tukey test (a¼.05).The failure types were classified withoptical microscopy and calculated as apercentage for each group. Differencesin the wavenumbers and the broad-ening bands of the Raman spectra werecompared among groups.

RESULTS

The results of the shear bondstrength tests for the specimens after 24hours of storage in water and afterthermocycling are shown in Figure 4.According to 2-way ANOVA, the inter-action between the material and ther-mocycling was not significant. Theshear bond strength values obtained forthe EP were not significantly differentfrom the Zirkonzahn zirconia (ZK) andShuetz zirconia, either before or afterthermocycling. On average (�SD), theEP showed a shear bond strength of20.18 �1.82 MPa compared with17.75 �1.52 MPa for the ZK and 18.81�1.82 MPa for the Shuetz zirconia afterstorage in water for 24 hours. Afterthermocycling for 24 hours, the EPshowed 17.75 �1.52 MPa comparedwith 15.33 �1.52 MPa for the ZK and18.81 �1.60 MPa for the Shuetz zir-conia. The effect of thermocycling onthe shear bond strength was not sta-tistically significant for any group. Themost common type of failure was ad-hesive (Fig. 5). Representative images ofthe failure types are shown in Figure 6.

The peaks used in this study corre-sponded to the main bands related tothe zirconia and feldspathic porcelaincomponents.3 Thewavelengths of bandsof 5 spectra from each group were

Ramos et al

6 Failure types by optical microscopy (�32 magnification). A, Adhesive. B, Cohesive. C, Mixed.

Ram

an s

igna

l (co

unts

/s)

Raman shifts (cm–1)

20

100 200 300

EP Zirconia

400 500 600 700

40

60

80

7 Experimental zirconia (EP) strong peaks.

- 2014 5

analyzed. In all images, the 2 upperspectra are related to the zirconia, themiddle spectrum (red) was recordedfrom the exact visible transition inter-face, and the 2 lower spectra are relatedto the feldspathic porcelain. In the EPgroup, strong peaks were found atapproximately 260, 320, 464, and 642cm�1 (Fig. 7). In the ZK group, strongpeaks were found at approximately 260,320, 465, and 640 cm-1 (Fig. 8). In theShuetz zirconia group, strong peaks alsowere found at approximately 260, 320,465, and 640 cm�1 (Fig. 9). Additionalweak features were observed for all zir-conia groups at approximately 145cm�1 (Figs. 7-9). The feldspathic por-celain showed strong peaks at approxi-mately 256, 320, 460, and 640 cm�1

(Figs. 7-9). The spectra of each groupwere composed of 924 data points in aspectral region between approximately100 and 700 cm�1. The mean spectra ofthe groups had the same baseline;however, they were dissociated to

Ramos et al

improve the visualization of each spec-trum (Figs. 7-9).

DISCUSSION

The 2 null hypotheses were accepted.No differences in shear bond strength

were found among the groups, regard-less of thermocycling. The bond strengthbetween the feldspathic porcelain andthe zirconia substructure can determinethe longevity of dental restorations inthe oral environment.13,14 According toYamamoto,45 an isolated analysis of

Ram

an in

tens

ity

(co

unts

/s)

Raman shift (cm–1)

20

100 200 300

ZK Zirconia

400 500 600 700

40

60

80

100

120

8 Zirkonzahn zirconia (ZK) strong peaks.

Ram

an s

igna

l (co

unts

/s)

Raman shifts (cm–1)

0

100 200 300

SH Zirconia

400 500 600 700

30

60

90

9 Schuetz zirconia (SH) strong peaks.

6 Volume - Issue -

each ceramic is critical to predict thesuccess of the bilayer. Therefore, abilayer that involves 2 ceramics is moreclinically realistic and behaves differentlyfrom each isolated ceramic.

The bond strength between thefeldspathic ceramic and zirconia can beinfluenced by certain factors, such asthe feldspathic porcelain sintering, thezirconia surface treatment, and thermalvariations, which in the present studywere identical and, therefore, did notinfluence the results. The feldspathicporcelain application was carried outcarefully, with the same condensationforce by a specific device. So, all spec-imens showed standardized volume

The Journal of Prosthetic Dentis

and shape of the feldspathic porcelain.A thin layer of the liner applied onto thefeldspathic porcelain surface (CeramikOpaker) at a temperature of 920�C isrecommended by the manufacturer andis thought to increase the molecularinteraction between the 2 materials.Aboushelib et al24 showed that the linerapplication significantly improves thebond strength of the zirconia coreefeldspathic porcelain veneer and re-duces interfacial failures.11,14

The zirconia surface treatment rec-ommended by the manufacturer wasfollowed by airborne-particle abrasionwith 50 mm aluminum oxide. This proce-dure has been controversial. The surface

try

treatment associated with liner reducechances of delamination of the feld-spathic ceramic as shownx-raydiffractionexperiments.24 The airborne-particleabrasion with 125 mm aluminum oxiderecommended by the manufacturerbefore applying the veneering ceramicchanged the structure of the tetragonalzirconia tomonoclinic (9.5% and a depthof 27 mm); the feldspathic porcelainapplication reversed this phase trans-formation.27 Despite the controversythat involves zirconia surface treatmentwith airborne-particle abrasion beforeapplying veneering ceramic, it may in-crease the wetting of the zirconia surfaceby increasing the free surface energy,which is measured by the contact anglebetween the liquid (feldspathic ceramic)and the surface to be bonded(zirconia).26

Polycrystalline ceramics under wetand cyclic loading conditions are mostsusceptible to subcritical crackgrowth.8,11,46 The current study showedthat the EP yielded bond strength re-sults comparable with other zirconiaproducts tested after thermocycling inanother study.37 With respect to thefeldspathic porcelain, its bond strengthis related to its crystal structure20 and isinfluenced by factors such as the CTE,thickness,18 and cooling rate.35,36 Themaximum stresses developed are acombination of different stresses in-duced by CTE and tempering residualstress in both ceramics.22 The poorthermal conductivity associated withthe poor thermal diffusivity of the ce-ramics results in a difference of tem-perature in the zirconiaefeldsphaticporcelain interface.28

The importance of the CTE of ce-ramics has been widely discussed in theliterature27 because the difference in thevalues between 2 materials with dif-ferent compositions, such as feld-spathic ceramic and zirconia, can leadto residual stresses and decrease thebond strength between them. Manu-facturers should provide similar CTEsfor ceramics to be used in multilayers.In this study, zirconia and feldspathicceramics had values considered similarto each other, but the small difference

Ramos et al

- 2014 7

between them may also have beenresponsible for the stress generatedat the interface. When the feldspathicporcelain is subjected to high temper-atures, a phase change occurs at thestructural level, counteracted by thetensile stress of the crystalline trans-formation of zirconia, which leads to anincreased volume expansion that, evenif as low as 4% in volume, couldsecondarily develop tensile stress in thefeldspathic porcelain.29,30 Altogether,this can cause dimensional differencesthat influence the bond between thefeldspathic porcelain and zirconia.32,34

Bond strength in terms of nominalstress values can be questionablebecause of the heterogeneous stressdistribution and also the occurrence ofcohesive failures.43 Results of the pre-sent study show that the failures weremostly of the adhesive type for allgroups, which indicate that the interfacebetween the feldspathic porcelain andzirconia is often compromised. Slowcooling during the final veneering ofdental restorations with zirconia frame-works reduces the temperature gradientsand residual stresses within the porce-lain layer, one possible cause forchipping.22

Shear and microtensile bondstrength tests are routinely used tomeasure the bond strength betweenfeldspathic porcelain and zirco-nia.15,41,42 The choice of a shear testwith stainless-steel tape is based on astudy by Sinhoreti et al44 that showedthat stainless-steel tape provided slidingbetween 2 tested surfaces. The use ofstainless-steel tape produced smallertensile and compression forces on theinterface than those obtained fromother tests when using a chisel and or-thodontic wires. The use of the chiselindicated by International Organizationfor Standardization (ISO) TR 1140547

promoted a cleavage stress that wasinitially concentrated on the sub-superficial layer of the covered material.The tests that used orthodontic wirecaused a flexion stress and conse-quently tensile and compressive stresseson the axial loading regions, which areconcentrated diametrically in opposed

Ramos et al

and perpendicular directions to theinterface evaluated. The supportadapted to the upper face of thestainless-steel tape to minimize anypossible bending stresses and cleavage(Fig. 3). This could cause higher adhe-sive failures because the tension distri-bution is more homogeneous along theinterface during the shear test.48 In thepresent study, the failures were mostlyadhesive for all groups (Fig. 5), which isin agreement with the expected resultsfor this test.44

The characterization of the interfacewas performed with microeRamanspectroscopy and the results are basedon spectra analyses. The bands are a setof wavenumbers that contain a mainwavenumber related to the peak of theband. The current study identified 5wavenumbers of the peaks (approxi-mately 260, 320, 464, 642, and 147cm�1) (Figs. 7-9), similar to the fre-quencies of the vibration that corre-sponds to the 6 Raman active bands ofthe tetragonal zirconia.3 According tosome studies,3,38,49 the phase trans-formations can be identified becausethe 3 main phases (monoclinic, tetrag-onal, and cubic) exhibit changes in thezirconium oxide bond angles andlengths.3,4,38,49 The main broad bandsshown in the present study were atapproximately 260, 464, and 642 cm�1

(Figs. 7-9). They are characteristic ofthe tetragonal or cubic phases.3

Different chemical stabilizers4,5 andcrystal sizes5 probably caused fewspectral variations at the tetragonal-cubic phases of this study (approxi-mately 260, 464, and 642 cm�1)(Figs. 6-8).

The feldspathic porcelain spectrashown in the present study are betweenapproximately 100 and 700 cm�1,similar to the frequencies of vibrationthat correspond to the feldspathic por-celain between approximately 300 and700 cm�1 previously described.3 In thefeldspathic porcelain, the presence ofmore symmetrical peaks was observed,which suggests localized vibrations froma well-ordered crystalline structure,which can be explained by the compo-sition of silicon oxide (SiO2).

39 The

strong peaks were found at approxi-mately 256, 320, 460, and 640 cm�1,and similar peaks were found in a studyof glass-based SiO2.

40 The 580 cm�1

vibration corresponds to either SieOstretching or to Si-O-Si bending.39 Thealuminum and potassium chemical ele-ments embedded in the matrix glass(SiO2) are responsible for changing theproperties of the crystalline grid,50 withstronger and more defined peaks.

The interface analysis illustrated bythe red line (Figs. 7-9) shows the tran-sition among the different materialswith characteristic bands. Analysis ofthis line suggests a process of interdif-fusion by the presence of peaks asso-ciated with zirconia and feldspathicceramics. Some chemical elements suchas silicon, aluminum, sodium, andpotassium can diffuse the zirconiumdioxide layer, gradually decreasing to a8- to 10-mm depth.24 The presence ofdefects in the crystalline solid canfacilitate the movement of atoms,which characterizes the interdiffusion.3

Although the bond mechanism be-tween the zirconia and feldspathicporcelain is still unknown,47 one hy-pothesis is that the interaction beginsby a compressive stress during feld-spathic porcelain firing.2 The differentCTE of the 2 materials induces interiorcompressive stresses near the substruc-ture and the periphery of the zirconiagrains, which could be the main site ofthe feldspathic porcelain interaction.3

CONCLUSIONS

Based on the results and limitationsof this study, the controlled productionof EP did not influence the results of thebond strength. Raman analysis suggestsa process of interdiffusion by the pres-ence of peaks associated with zirconiaand feldspathic ceramics.

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Ramos et al

Corresponding author:Dr Ana Flávia Sanches BorgesBauru School of Dentistry, University of São PauloAl. Octávio Pinheiro Brisola, 9-75Vila Universitária17012-901, Bauru, SPBRAZILE-mail: afborges@fob.usp.br

AcknowledgmentsThe authors thank Renato Murback, HeitorMarques Honório, and Adolfo Coelho for theircollaboration on this study.

Copyright ª 2014 by the Editorial Council forThe Journal of Prosthetic Dentistry.