Apparent fracture toughness of metal ceramic restorations with different manipulative variables

of dentin to produce a rough surface texture and expose numerous lateral canals.‘0 This surface irregularity allows for bonding to occur by mechanical interlocking. Despite the concern over the ’ ‘smear layer” that occupies the interface of most restorative materials and dentinal matrices, the results of this study showed that it may not be as detrimental as speculated when left in conjunction with the Scotchbond adhesive and the P-30 composite. The supplemental treatment of the prepared dentinal surface with EDTA and NaOCL exhibited no improvement in bond strength over the control, and additional treatment with ferric oxalate compromised the bond between the dentinal adhesive and the dentin. It was also shown that dentinal adhesive should be used to maximize bonding of the composite to dentin.

posite system may be obtained with an intact dentinal smear layer. REFERENCES 1. Cotton R. Introduction: smear layer on dentin. Oper Dent 1984;9:1-3. 2. Goldman M, DeVitre R, Pier M. Effect of the dentin smeared layer on tensile strength of cemented posts. J PRWTHET DENT 1984,52:485-8. 3. Pearlman S. The cutting edge: interfacial dynamics of cutting and grinding. Washington, DC: U.S. Department of Health, Education, and Welfare, 1976; DHEW publication No. 76-670; 170. 4. Dipper HW, Borggreven JMPM, Hoppenbromvers PMM. Morphology and permeability of the dentinal smear layer. J PROSTHET DENT 1984;52:657-62.

5. McInnes-Ledoux P, Austin JC, Cleaton-Jones PE. Effect of citric acid pretreatment on dentinal cavity walls. J PROSTHET DENT 1984;52:353-6. 6. Stangel I, Ostro E, Benko J. Effect of surface treatment on bond strength of dentin adhesives [Abstract]. J Dent Ras 1986; 65174. 7. Newman SM, Porter H. Dentin treatment effects on dentinal bonding [Abstract]. J Dent Res 1986;65:174. 8. Pashley DH. Smear layer: physiological considerations. Oper Dent 1984; Suppl3:13-29. 9. Eliades GC, Caputo AA, Vougiovklakis GJ. Composition, wetting properties and bond strength with dentin of six new dentin adhesives. Dent Mater 1985;1:170-6. 10. Gwinnett AJ. Smear layer: morphological considerations. Oper Dent 1984;Suppl 3:3-12.

CONCLUSIONS This investigation studied the interaction between dentinal smear layer removal, using various agents, a dentinal adhesive, and the shear bond strength of a posterior composite to the dentin. It was found that using Scotchbond dentinal adhesive in conjunction with the composite P-30 with the smear layer intact produced the highest shear bond strengths. Removal of the smear layer with EDTA produced similar bond strengths, but no improvement. Application of ferric oxalate resulted in diminished bond strengths. Consequently, optimal bond strengths with this adhesive-com-

Apparent fracture restorations with S. F. Rosenstiel, The

Ohio

State

Apparent

fracture

brittle the

tip

cally

failure

modulus-to-hardness

(2)

occurs.

with

13 different

variables.

firing

temperature,

(3)

significant

differences two

to altering

of the

Los

ANGEL=

in K, modeling

firing

and

Ohio

of the

of tensile

before

between

respect

level

elastic

made

Columbus,

is a measure

to the

K,

porcelain, and

(K,)

It relates and

to:

and S. S. Porter**

of Dentistry,

of a crack

measured crowns

College

toughness

materials.

requests

DR. ANGELO A. CAPIJ~, UNIVERSTTY OF CALIFORNIA, SCHOOL OF DENTISTRY Los ANGELES, CA 90024

toughness of metal ceramic different manipulative variables

B.D.S., M.S.D.,*

University,

Reprint

Using

E/H

temperature

(E/II)

variables

condensation,

liquids.

absorption

that

must

an indentation ratio

The

strain

stress

and

were

found

No significant or condensation.

technique,

this

of 65 metal

ceramic

studied (4)

modeling

in respect differences (J

capacity

be exceeded

were

(1) liquid.

study

brand

of

Statisti-

to porcelain were

of at

brand found

in

PROSTHET DENT

1989;61;185-91.)

T

echnique preferencesare inevitable in the fabrication of metal-ceramic restorations and ceramistscommonly

Supported by a grant Dentistry. *Assistant Professor, tistry. **Dental student. THE

JOURNAL

from Section

OF PROSTHETIC

The

Ohio

State

of Restorative

DENTISTRY

University,

College

and Prosthetic

of

Den-

modify standard methods of porcelain manipulation. Technique variations may have a negative effect on the physical properties of materials and decreasethe longevity of the restoration. Studies have evaluated the manipulative variables on firing shrinkage of dental porcelain and color of metalceramicrestorati0ns.l~2This study examined their effects on fracture properties. 185

ROSENSTIEL

Table

PORTER

Variables investigated

I.

Group (XI = 6 in each)

Porcelain

Supplier

Ceramco II 2

Crystar

3

Jelenko

4

Spectratone

5

Vita VMK 68

11 12 13

7 8 10 9 6

Vita VMK Vita VMK Vita VMK Vita VMK Vita VMK Vita VMK Vita VMK Will-Ceram

68 68 68 68 68 68 68

Johnson & Johnson Co. East Winsor, N.J. Unitek Corp. Monrovia, Calif. J.F. Jelenko Armonk, N.Y. DVA, Inc. Anaheim Hills, Calif. Vident Baldwin Pk., Calif. Vident Vident Vident Vident Vident Vident Vident Williams Gold Refining Co., Inc. Buffalo, N.Y.

Firing temperature

Condensation*

Sta-Wet?

975T%

Normal

Distilled water?

960°

Modeling

liquid

Body mediumt980”

C t:

The criterion for the study was fracture toughness(K,), which is considereda measureof the strain-absorbingproperties of brittle materials. It identifies the level of tensile stressexceeded at the tip of a crack before failure occurs. Fracture toughnesscan be determined by the indentation technique3,4 where K, is calculated from the length of four radial cracks emanating from the corners of a Vickers indentation usedon a brittle material. The method hasbeen usedto determine fracture toughnessfor commercialdental porcelains,6*’ and becauseof the small size of the indenter it is convenient for measurementsof dental material. This technique has also been successfulfor measuring residual stressesin toughenedglassthat demonstrated higher apparent fracture toughness values than annealed glass, attributed to residual compressive stresses.’ Residual compressive stressesare alsopresent in the metal-ceramic system. They are induced by slight differencesin the thermal expansionof metal and porcelain and provide strengthening. For this reasonthe values in this study are more accurately termed “apparent fracture toughness.” Toughness measurements wererecordedin preferencesto compressivefracture strength becausetoughnessexhibits lessvariability from specimen loading and surface flaws. The variables reviewed for the study were the brand of porcelain, firing temperature, degree of condensation, and various modeling liquids.

Normal

Cf

Normal

Distilled water?

9200 C%

Vita modeling liquid?

960'

(2%

Normal

Distilled water Carv-Eze$ Rainbow liquid1 Vita modeling liquidt Vita modeling liquidt Vita modeling liquid? Vita modeling liquid7 Buildup liquid?

960' 960° 960' 930" 990° 960° 960" 960

Cl C$ C$ c c Cl C$ Cl

Normal Normal Normal Normal Normal Maximum Minimum Normal

*Normal condensation; manufacturer’s recommendation; maximum condensation, built up in small increments with vibration allowed to air dry. tManufacturer’s recommended model liquids. $Manufacturer’s recommended firing temperature with porcelain furnaces used (Ney Mark III, J. M. Ney Co., Bloomfield, $George Taub Products and Fusion Co., Inc., Jersey City, N.J. I( H.D. Justi Co., Oxnard, Calif.

186

AND

MATERIAL

AND

and blotting;

minimum

condensation,

Conn.).

METHODS

A metal-ceramic tooth preparation was made on an Ivorine central incisor tooth model (Columbia Dentoform Corp., New York, N.Y.). Sixty-five metal-ceramic frameworks were made,using a standardized technique by adding wax to a thermoplastic coping (Austenal Dental Inc., Chicago, Ill.) and using a silicone mold. This process ensured a consistent thickness throughout the 65 restorations. The patterns weresprued and invested in Ceramigold(Whip Mix Corp., Louisville, Ky.) investment material by following the manufacturer’s recommendations.Sixty of the frameworks were cast with a gold-palladium alloy (Olympia, J. F. Jelenko, Armonk, N.Y.). The remaining five were cast in a palladium alloy (Pd 80+, Unitek Corp., Monrovia, Calif.) because of the incompatibility of Olympia gold and Crystar porcelain noted by Unitek. The frameworks were then recoveredfrom the investments, oxidized, air abraded, and ultrasonically cleanedin accordancewith the manufacturers’ instructions. The 60 Olympia alloy frameworks were then randomly assignedto one of 12 groups of five restorations. The remaining group was formed by the Pd 80+/Crystar material combination. The variables for the13 groupsare listed in Table I. Two coats of opaque porcelain were applied to each coping and fired by following manufacturer’s recommendations for the porcelain furnace (Ney Mark III, J. M. Ney Co.,

FEBRUARY

1989

VOLUME

61

NUMBER

2

APPARENT

FRACTURE

TOUGHNESS

OF RESTORATIONS

/ //

.-- -. bR ‘\ \L

I-

I \. \ \

R

\

0 B

1’ /

‘A

‘i

/

-’

1 A, Scanning electron micrograph of Vickers indentation-fracture system. B, Schematic of A: a, indentation half-diagonal; b, zone of permanent deformation; CR, radial crack R length; CL, lateral crack L length. Fifth median crack under indenter. Fig.

Bloomfield, Conn.). The furnace was calibrated with pure silver before eachfiring. The crownswere then normally developed freehand with body and incisal porcelains with the same thicknessesof unfired porcelain. Thesewere measured in four locations with dial calipers (Health Aids, Maywood, Calif.). Distribution of the body and incisal porcelain was controlled by reference to a shadedistribution chart usedin clinical practice. Groups 1 through 6 werecondensedand fired according to the manufacturer’s recommendations.The firing temperature was respectively increased and decreased30” C for groups 7 and 8. Group 9 received minimal condensationduring build-up. The body and incisal porcelain in this group was applied to the opaquedmetal frameworks without blotting or vibration and allowed to air dry. Group 10 received maximum condensation. Small increments of porcelain were applied and each was subjected to blotting and vibration to remove moisture during application of porcelain. Groups 11 through 13 had the Vita modeling liquid replaced with an alternative liquid. Rainbow (H. D. Justi Co., Oxnard, Calif.) modelingliquid wasdesignedto observe the color of the final restoration during porcelain application. The porcelain powder was preheated to burn off the pink and blue coloring and allowed to cool before the porce-

THE

JOURNAL

OF PROSTHETIC

DENTISTRY

Table II. Analysis of variance procedure (dependent variable K,) Source of variation

Total model Model Group Spec (group)* Error B. Spec (group) as error term Group

Mean square

F value

64

0.72

12

2.18

11.7 35.6 6.2

df

P

A.

52 520

12

0.38 0.06

2.18

5.7

<.ool <.OOl COO1

COO1

df = Degrees of freedom; P = statistical probability. *Replicate specimens nested with each group.

lain buildup. Carv-Eze (GeorgeTaub Products and Fusion Co., Inc., Jersey City, N.J.) modeling liquid improves the handling characteristics and reducesthe drying time of the porcelain. After firing, the restorations were embeddedin an epoxy resin (Epoxide Resin, Leco Corp., St. Joseph, Mich.). The resin was reduced to exposethe facial surface of the restorations and the surfacewaspolishedto an optical finish with polishing agentsculminating in 0.05Km alumina slurry. The

187

ROSENSTIEL

AND

PORTER

grand of Porcelain

25

2.0

:N

1.6

E

2 1.2 H 0.8

Ceramco

Crystar

Jelenko

Spectratone Vita VMK Will - Ceram

Minimum significant difference (p <0.05) K, = 0.45 MPa . ml12 E/H = 7.1 Fig. 2. Effect of brand of porcelain on apparent fracture toughness(K,) and elastic modulus-to-hardnessratio (E/H).

2.4

r

firing Temperature

- 30 - 25

1.80 5

1.6

- 20

E.

-15

a” 1.2 H

-10 -5

Recommended Temperature Minimum significant difference (p <0.05) Kc = 0.45 MPa. ml12 E/H = 7.1 Fig. 3. Effect of firing temperature on apparent fracture toughness(K,) and elastic modulus-to-hardnessratio (E/H).

188

FEBRUARY

1989

VOLUME

61

NUMBER

2

APPARENT

FRACTURE

TOUGHNESS

OF RESTORATIONS

2.4

r

Condensation 2.03

>N

1.6

E

$ 1.2 H 0.8

Under

Normal

Over

Minimum significant difference (p <0.05) KC = 0.45 MPa - ml/z E/H = 7.1 Fig. 4. Effect of condensation to-hardness

on apparent

polished surface was coated with approximately 40 nm of gold by using a vacuum vaporization process. The gold coating facilitates the visualization of the cracks from the Vickers diamond. A hardness tester (Model M-400, Leco Corp.) was used to determine the apparent fracture toughness and the modulus of elasticity-to-hardness ratio of the porcelains. The fracture toughness was calculated by using the length of the cracks emanating from the Vickers indentation (Fig. 1) according to the equation? K, = .016(E/H)1’2 (P/c,~‘“) where P is the peak load and c, is the crack length at equilibrium after the indenter was released. The indentations were performed under oil to prevent slow cracking caused by atmospheric moisture. The indenter load was 9.8 N. This was sufficiently high that the radial cracks were approximately equal to or greater than the diagonal of the indentation, without lateral chipping. Crack length was measured within 1 minute of indentation by using the traveling microscope on the microhardness tester. A total of nine indentations was completed for each restoration. The modulus of elasticity-to-hardness ratio (E/H) was calculated from the ratio of the major and minor diameters of Knoop indentations from the equations: b’la’ = b/a - aH/E where b’la’ is the ratio of the diagonals of the Knoop indentation, b/a the ratio of the diagonals of the indenter,

THE

JOURNAL

OF PROSTHETIC

fracture

toughness

(K,) and elastic modulus-

ratio (E/H).

DENTISTRY

and (Y = 0.45. Three Knoop indentations were completed for each specimen. The data were computed for statistical significance with an analysis of variance. Statistical significance of differences among groups was determined by using the error variance of the mean square for specimens nested within these groups. This was followed by Tukey’s standardized multiple range test also using this appropriate error term.g

RESULTS The analysis of variance for the fracture toughness is presented in Table II. There was an error variance of 0.06 with a variance of 0.38 between specimens within groups. There were significant differences among groups and between specimens (p < .OOl). The analysis of variance of the elastic modulus-to-hardness ratio is shown in Table III. The multiple range test revealed a minimum significant difference for fracture toughness of 0.45 MPa . m1’2 and for elastic modulus-to-hardness of 7.1 at the confidence level of 0.05. The effects of brand of porcelain on fracture toughness and E/H are shown in Fig. 2. Crystar porcelain recorded the highest fracture toughness of 2.36 MPa . m1’2. Will-Ceram porcelain had the next highest fracture toughness (2.05 MPa . m1’2). Crystar porcelain was statistically different from Spectratone porcelain with the lowest fracture toughness of 1.51 MPa . m1’2. Will-Ceram, Ceramco, Jelenko, and Vita porcelains had statistically similar values. Will-Ceram

189

ROSENSTIEL

AND

PORTER

Modeling liquid

2.4

1.78

Distilled Water

Body M

Carv - Eze

Rainbow

Minimum significant difference (p <0.05) Kc = 0.45 MPa - rn’h E/H = 7.1 Fig. 5. Effect of brand of modeling liquid on apparent fracture toughness (K,) and elastic modulus-to-hardness ratio (E/H).

III. Analysis of variance procedure (dependent variable E/H)

Table

Source of variation

A. Total model Model Group Spec (group)* Error

Mean square

F value

P

18.8 51.6 11.3


130

52.7 144.3 31.5 2.8

12

144.3

4.6


df

64 12

52

B. Spec (group) as error

Group

term

df = Degrees of freedom; P = statistical probability. *Replicate specimens nested with each group.

porcelain had the highest E/H value of 26.9. Will-Ceram porcelain’s value of 26.9 was statistically different from Jelenko (19.2) and Spectratone (15.6) porcelains. The effects of firing temperature on fracture toughness and E/H of Vita VMK 68 porcelain are shown in Fig. 3. No statistically significant differences were evident among these groups. If the observed values are typical, however, reducing the firing temperature resulted in reduced fracture toughness. The effects of porcelain condensation on fracture toughness and E/H are shown in Fig. 4. No statistical differences were found. All three samples yielded K, and E/H differences that were far below the minimum significant difference 190

value of 0.45 MPa . ml” and 7.1. The degree of condensation had no effect on fracture toughness of E/H. The effects of different modeling liquid with Vita VMK 68 porcelain are shown in Fig. 5. When Rainbow modeling liquid was used, the fracture toughness was 2.26 MPa . ml”. This was an increase from 1.91 MPa . ml” that was obtained with Vita Modeling liquid. When Carv-Eze liquid was used the value was 1.78 MPa . m1’2. This was the lowest value for the modeling liquids and was statistically significantly different compared with the Rainbow liquid. The E/H was also statistically significantly different between these two groups.

DISCUSSION This study demonstrated a significant difference in fracture toughness and E/I-I of the porcelains. These differences should be examined when porcelain restorations are exposed to elevated levels of stress. Porcelain/modeling liquid combinations can also effect the fracture toughness and should be considered when combining different brands of porcelain and modeling liquids. No statistically significant differences were revealed with different firing temperatures despite a noted trend. The influence of firing temperature on crack propagation may be diminished by specimen preparation that involves polishing and removing the glaze. However, polishing is inherent to the indentation technique and inconsistent temperatures in the porcelain furnace may have masked measurements. Significant differences were discovered between measureFEBRUARY

1989

VOLUME

61

NUMBER

2

APPARENTFRACTURETOUGHNESSOFRESTORATIONS

ments within the same sample. These differences (Table II) were represented by a 0.06 error variance during the tests for fracture toughness. The nested within-group variance also reflected a 0.38 fracture toughness variance between specimens within the same group. The indentation technique could be useful in evaluating the fracture toughness of dental materials.

REFERENCES 1. Rosenstiel SF. Linear firing shrinkage of metal-ceramic restorations. Br Dent J 1987;162:390-2. 2. Rosenstiel SF, Johnston WM. The effects of manipulative variables on the color of ceramic metal restorations. J Prosthet Dent 1988;60:297-303. 3. Anstis GR, Chantikul P, Lawn BR, Marshall DB. A critical evaluation of indentation techniques for measuring fracture toughness: I. direct crack measurements. J Am Ceram Sot 1981;64:533-8. 4. Rosenstiel SF, Porter SS. Apparent fracture toughness of dental porcelain with a metal substructure [Abstract]. J Dent Res 1987;66:270. 5. Anusavice KJ, Lee RB. Effect of firing temperature and water exposure on crack propagation in unglazed porcelain [Abstract]. J Dent Res 1985;64:296. 6. Morena R, Lockwood PE, Fairhurst CW. Fracture toughness of commercial dental porcelains. Dent Mater 1986258-62. 7. Marshall DB, Lawn BR. An indentation technique for measuring stresses in tempered glass surfaces. J Am Ceram Sot 1977;60:86-7. 8. Marshall DB, Noma T, Evans AG. A simple method for determining elastic-modulus-to-hardness ratios using Knoop indentation measurements. Comm Am Ceram Sot 1982;C175-C-176. 9. Montgomery DC. Design and analysis of experiments. 2nd ed. New York: John Wiley & Sons, 1984;379-84.

CONCLUSIONS Statistically significant differences were recorded in the fracture toughness and the elastic-to-hardness ratio of commercial metal-ceramic porcelains. The effect of firing temperature on the toughness of Vita porcelain was not statistically significant. There was also not a significant difference between groups subjected to varying degrees of condensation. However, modeling liquids did have an effect on fracture toughness.

We thank Drs. K. T. Faber assistance and the manufacturers in this study.

and W. M. for supplying

Scanning electron cavities involving

Johnston for the materials

Reprint requests tot DR.STEPHEN F.ROSENSTIEL THEOHIOSTATEUNIVFJW~Y COLLEGE OF DENTISTRY COLUMBUS,OH 43210-1241

their used

microscopic study of prepared enamel, dentin, and cementum

J. A. Hargreaves, L.D.S., M.Ch.D., A.M., M.R.C.D. (C),* E. S. Grossman, BSc., M.Sc. (Dent),** and J. M. Matejka,

B.D.S.***

The University of Alberta, Faculty of Dentistry, Edmonton, Alta., Canada, Council University of the Witwatersrand, Johannesburg, South Africa

and Medical

The

surface

dental

treatment

class

V restorations.

teeth

with

margins

class could

preparations Most

marginal

leakage.

This

cementum

in class

occurring,

depending

undertaken marginal

in relation

involved.

in regard preparation,

emphasizes restorative

to special

treatment

and

marginal

(J PROSTHET

*Professor and Director of Graduate Studies and Research, The University of Alberta, Faculty of Dentistry. **Medical Researcher, Dental Research Institute, University of the Witwatersrand. ***Medical Research Council Scholar, Dental Research Institute, University of the Witwatersrand. PROSTHETICDENTISTRY

and

requirements

s

junction and

whether margin related DENT

as

possible

inclusion

of marginal

cementum

urveys of the dental needs of the white elderly have shown a marked decrease in edentulousness in the segment of the population above 65 years of age in the past 50 years. I* ’ Sixty-three percent of the elderly subjects surveyed

THEJOURNALOF

of the

possibility used

on the

leakage.

process

and

cementum

dentinocemental

importance the

caries

of extracted

well-developed

sealing

material

studies

root

examination the

the and

Further

involve

that

regard

restorative

preparation

on the

people microscopic

showed studies

to the

study

V cavity

elderly

electron

V cavity

as dentin

are

of many

be produced.

behaving

techniques

needs

A scanning

Research

of leakage

acid-etch should

be

to bonding, 1989;61:191-7.)

had root caries and 73 % of these had teeth affected by severe attrition. Of the teeth that were present, 39% had gingival recession. Many of the carious lesions in the elderly were present in the anterior teeth. The restorative materials of choice for such lesions are composites. This material has the ability to blend into existing tooth structure and the physical and chemical

properties

storative

materials.3

are superior Marginal

to any leakage

previous is a problem

esthetic

re-

in den-

tin-bordered cavities.4 Because projected estimates for the year 2000 indicate a marked increase in the population over 191