Castability, opaque masking, and porcelain bonding of 17 porcelain-fused-to-metal alloys

Castability, opaque masking, and porcelain porcelain-fused-to-metal alloys Randolph Michael

P. O’Connor, DMD,a J. Rodway Mackert, Jr., DMD, L. Myers, DMD,” and Edward E. Parry, CDTd

Medical College of Georgia, School ADA Health Foundation, National

of Dentistry, Augusta, Institute of Standards

bonding

of 17

PhD,b

Ga., and P~fenbar~er Research and Technology, Gaithersburg,

Center, 1Md.

Seventeen porcelain-fused-to-metal alloys, which represented a cross section of the various alloy types available, were evaluated for castability, opaque masking, and porcelain bond strength. The base metal alloys generally cast more completely than the noble alloys, with the presence of beryllium as an important factor for greater eastability among the base metal alloys. Statistically significant differences were observed in the ability of an opaque porcelain to mask the different alloy substrates but no systematic effect of alloy type was observed. Porcelain bond testing revealed that nickel-chromium-beryllium alloys produced significantly better porcelain-metal bonds than nickel-chromium alloys without beryllium. In addition, it was found that palladium-copper alloys produced significantly better bonds with porcelain than paladin-cob~t alloys. (J PROSTHET DEW 1~~7~~67.74.)

I

n 1968, the U. S. federal government stopped maintaining the $35per-ounce price of gold that had been established by the Gold Reserve Act of 1934. The price of gold began to rise gradually at frrst and then steeply rose in 1972. Dental manufacturers sought to reduce materials costs to the dental laboratory and to the dentist by modifying the nickel-chromium and cobalt-chromium alloys that had been in use since the 1930s for removable partial denture (RPD) frameworks to make them suitable for poreel~n-bed-to-rne~ (PFM) restorations. These alternative alloys are still widely used, even though the price of gold has moderated considerably in recent years. In the late 1970s and early 198Os, additional alternative alloys such as the palladium-silver and “high-palladium” alloys were developed. Althougb the wide array of alloys available for PFM restorations offers the dentist and laboratory technician abundant choices, the selection of an alloy is often a confusing process. For a PFM alloy to be clinically successful, it must satisfy several criteria. At minimum, it must have adequate physical properties to provide strength and rigidity for a metal-ceramic restoration. These physical properties are relatively easy to measure for comparison of different al-

Supported by National

Institutes of Healt~ational Research grant DE 06374. Professor, Department of Oral Rehabilitation, College of Georgia. bProfessor, Division of Dental Materials, Department habilitation, Medical College of Georgia. cAssociate Professor and Vice Chairman, Department habilitation, Medical College of Georgia. dSenior Dental Laboratory Tech~cian, PaBenbarger

Dental %ssociate

Institute

of

Medical of Oral

Re-

of Oral

Re-

Research

Center, Copyright 0 1996 by The Editorial PROSTRETIC DENTISTRY. 0022-3913/96/$5.00 + 0. 1011/70015

APRJZ199f3

Council

of THE JOURNAL

OF

10~s. Other properties that are no less important but more difficult to measure are castability, opaque masking, and porcelain bond quality. The goal of this investigation was to evaluate these latter properties of 17 PFM alloys that represent a cross section of different alloy types. Numerous studies have evaluated the castability of metal-ceramic alloys.1-6 Several studies have concluded that Ni-&-Be alloys can be cast as well as noble al10ys.~ Beryllium has been identified as an important constituent for castability in Ni-Cr alloy systems5 The metallic substructure of a PFM restoration is veneered with porcelain to provide an esthetic result. The opaque layer of porcelain must completely mask the color of the underlying metal if a proper shade match is to be achieved. Studies have evaluated various metal and porcelain combinations to determine whether the metal substructure has an effect on the color of the porcelain.7-17 Brewer et al.1° described differences between the color of porcelain fired to a palladium-silver alloy compared with high-gold and nickel-chromium alloys. Jacobs et al.li found that, for one porcelain shade, the color was shifted toward yellow-red for gold-platinum-p~ladium alloys in comparison to nickel-chromium and high-palladium alloys. There was no difference in the val.ue or chroma. Crispin et al. I6 found no color difference between five alloys after opaque application but did find differences after body porcelain application. The bond of the veneering porcelain to the metal framework is one of the essential properties of a successful PFM alloy. Many different tests have been used to evaluate porcelain-metal bond strength. With a three-point flexure test and a shear test, Lorenzana et alis found that high-palladium alloys had similar porcelain-rne~l bond strengths compared with a high-noble alloy. Wu et ali found a higher bond strength in a Ni-Cr-Be alloy as compared with an Ni-Cr alloy without beryllium. A shear test was used in

THE JOURNAL

Table

I.

OF PROSTHETIC

Alloys Alloy

DENTISTRY

O’CONNOR

studied Code

name

Composition

ATH

74

Biobond II

BB2

80.7

Biobond Plus Cobond Eclipse

BB+ COB ECL

76.9 Ni, 13.25 Cr, MO 3.2 67 Co, 27 Cr, 6 MO 52 Au, 38 Pd, 3.8 Zn, 3 In, 3.2 Sn

Jelenko ‘“0”

J-O

87.5

Jelstar Litecast B Naturelle

JST LCD NAT

60 Pd, 28 Ag, 77.5 Ni, 12.5

6 Sn, 6 In Cr, 4 MO, 1.7 Be 79 Pd, 10 Cu, 9 Ga, 2 Au

Neydium gold ceramic Olympia Option PTM-88 Rexillium III Rx-91 Will-Ceram W-l Will-Ceram W-3

NGC OLY OPT PTM

49 Au, 32 Pd, 15 Ag, 4 Sn 51.5 Au, 38.5 Pd, 8.5 In, 1.5 Ga 79 Pd, 10 Cu, 9 Ga, 2 Au 88 Pd, 8 Ga, 4 Co 13.5 Cr, 5.5 MO, 2.5 Al, 1.8 Be 53.5 Pd, 37.5 Ag, 8.5 Sn, 0.5 In 53.5 Pd, 37.5 Ag, 8.5 Sn, 0.5 In 48.5 Au, 39.5 Pd, I1 In, 0.5 Sn, 0.5 Ga

Rx3

R91 W-l w-3

Pd, 14.5 Cu, 5 In, 1.5 Ga Ni, 13.5 Cr, 4 V, 1.8 Be

Au, 4.5 Pt, 6 Pd, 1 Ag

the study by Uusalo et a1.,2o who found greater bond strengths for gold alloys compared with Co-Cr and Ni-Cr alloys. In contrast, by means of a different shear test, Hammad and Steinzl reported a greater bond strength for

a base metal alloy compared with a high-gold alloy whe: one porcelain was used, but no difference was found when another porcelain was used. Nearly all of the extant tests for evaluation

of the qual-

ity of the porcelain-metal bond, which include the tests used in the previously mentioned studies, are attempts to measure the strength of the bond. Finite element analysis has shown that all of the major bond tests in the literature are plagued by significant stress concentration effects.z2, 23

The effects of stress concentration are, among other things, a function of the difference in modulus of elasticity between the metal and the porcelain. Because different types of PFM alloys have widely varying elastic moduli, comparisons of porcelain bonding among them is problematic when current bond tests are used. By use of the available porcelain-metal bond strength tests, it is doubtful whether comparisons

can be made between,

for exam-

ple, Ni-Cr alloys and gold alloys, because of the factor-oftwo difference in their moduli of elasticity. It is interesting to note that more than 20 years ago, the Porcelain Enamel Institute adopted a test for the adherence of porcelain enamel to sheet steel that did not depend on measurement of the strength of the bond, but instead measured

the cohesive site density

of enamel

that adhered

to steel after controlled deformation of the steel substrate.24 Ringle et aLz5 and Mackert et a1.26adapted this approach to the measurement of the quality of dental PFM

368

(wt %)

Athenium

meaningful

ET AL

Ivoclar-Williams Amherst, N.Y. Dentsply International, York, Pa. Dentsply Inte~ation~ Dentsply International J. M. Ney Co., Bloomfield, Corm. J. F. Jelenko & Co., Armonk, N. Y. J. F. Jelenko & Co. Ivoclar-Williams JenericlPentron, Wallingford, Conn. J. M. Ney Co. J. F. Jelenko & Co. J. M. Ney Co. J. F. Jelenko & Co.

Jeneric/Pentron Jeneric/Pentron Ivoclar-Williams Ivoclar-Williams

bonds. This porcelain bond test used a constant-strain flexure apparatus that allowed the deformation of the alloy specimens to a constant radius of curvature irrespective of alloy elastic modulus.26 This test was used in this study to to compare the dissimilar alloys with respect to porcelain bond quality.

MATERIAL Castability

AND METHODS

The datability of each alloy was dete~ned by the ability of the alloy to cast a mesh pattern made from polypropylene sieve cloth. w The specimen used in this study was a slightly smaller version of the National Institute of Standards and Technology (NIST), formerly the National Bureau of Standards (NBS) castability test piece. It measured 7 mm x 7 mm and contained 49 complete squares with 112 filament segments. The smaller test piece required substantially less metal than the NIST pattern, an advantage for casting the more expensive alloys. The filament diameter of the sieve cloth (Spectramesh. #146410, Spectrum Me~c~ Industries, Los Angeles, CalX) was 0.5 mm and the mesh spacing was 1.0 mm. Runner bars made from lo-gauge wax sprue were attached to two sides of the sieve cloth. The runner bars were then connectc?d to an S-gauge wax sprue in the same manner as in the NIST test. Five specimens were made for each of the 17 alloys (Table I). The pattern was invested in the m~ufacture~s recommended alloy investment. For alloys that had no specific investment recommendation, Ceramigold investment material

(Whip-Mix

Corp., Louisville,

KY.) was used. Burnout

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Fig. 1. Opaque masking specimens prepared for rater evaluation. Specimens were covered with electrical tape with 3 mm diameter hole for viewing opaque porcelain. Specimens were labeled on opposite side, but labels were not visible to raters during evaluation.

and casting procedures were performed according to the manufacturer’s recommendations in an induction casting machine (Model ACF-1, Lepel Corp., Maspeth, N.Y.) to minimize casting variability. The casting ring was positioned in the casting machine so that the sieve cloth portion of the mold was vertically oriented. The patterns were cast according to a randomized schedule to minimize the effects of systematic errors due to specimen preparation variables. After casting, the specimen was devested and airabraded with 60 pm aluminum oxide abrasive to remove any remaining investment. Each casting was then examined to determine the total number of filament segments that were successfully cast. All segments that were bound on both ends by a filament intersection were counted. It should be noted that this counting procedure is slightly different than for the NIST pattern test, which is based on counts of numbers of intersecting filaments. For each specimen, the number of segments cast out of a possible 112 (versus 220 for the NIST pattern test) was recorded. Differences in the number of percentages were subjected to statistical analysis with the Tukey honest significant difference (HSD) test to determine differences in alloy castability.

Opaque

masking

Five specimens of each alloy, which measured 13 mm x 13 mm x 1 mm thick, were cast according to the manufacturer’s recommendations. As before, induction casting was used. After the casting was removed from the investment, it was air-abraded with 60 pm alumina. The manufacturer’s recommended surface treatment and oxidation protocol was followed for each alloy. The specimens were designed to be used both for the opaque masking test and for the porcelain bonding test. Therefore they were prepared in the same manner as for the porcelain bonding

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test specimens described previouslyz6 and briefly herein. A paper tape mask with a 6 mm hole in the center was applied to the alloy coupon such that the hole was centered on the prepared surface. A layer of opaque porcelain (Ceramco Vacuum Porcelain B, POP 51, Lot #6334, Ceramco, Inc., Burlington, N.J.) was painted into the masked area and allowed to dry. The paper tape mask was then removed and the porcelain was fired according to the manufacturer’s instructions. After the specimen had cooled, a paper tape mask with a 6 mm hole was again applied to the specimen with the hole positioned around the previous layer of opaque. A second layer of opaque porcelain was applied on top of the first such that the total thickness would be greater than 0.2 mm after firing. The paper tape mask was removed and the porcelain was fired according to the manufacturer’s instructions. The opaque porcelain layer was then ground to a 600-g& finish until the porcelain was 0.2 mm thick (iO.005 mm). The porcelain thickness was measured for each specimen to ascertain whether the slight deviations in porcelain thickness from the intended 0.2 mm would have an effect on the masking power of the layer. In a blinded experiment, five observers independently ranked the color of the 85 opaqued specimens from light to dark. Before the specimens were evaluated by the raters, each specimen was covered with black electrical tape in which a 3 mm diameter hole was punched and then positioned on the specimen such that only the porcelain was visible. The specimen was then labeled on the opposite side (Fig. 1). The specimens were placed label-side-down in random order on a laboratory bench. Each rater was asked to rank the specimens from lightest to darkest as to the degree of match of the applied opaque porcelain layer to a standard of opaque. The raters were instructed to place specimens in groups if the opaque color appeared indistin-

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Fig. 2. Three mesh pattern castings represent range of castability of various alloys in study. Castability percentages represented by these specimens are 100% (left), 51% (center), and 3% (right). guishable among the specimens in a group, but they were not told to use a specific number of groups. After each rater had finished ranking the specimens, the number of groups designated by the rater and the identities of the specimens in each group were recorded. Any statistically significant differences among the observed color of the opaqued specimens were assessed by nonparametric analysis. In addition to the rater evaluation of color differences among the 85 specimens, the specimens were also evaluated by a fiberoptic calorimeter (Chromascan, APM Sterngold, Attleboro, Mass.) to measure the red-green-blue intensity values according to the method of Ringle et a1.14 The specimens were analyzed with the calorimeter without the black electrical tape mask that was used in the rater evaluations. A drop of glycerin was placed on the surface of the opaque porcelain to provide optical coupling with the tip of the calorimeter’s fiber-optic wand. The values for red, green, and blue for the different specimens were subjected to discriminant analysis by the stepswise and direct methods.

Porcelain

bonding

After evaluation for opaque masking, each specimen was prepared for the porcelain bonding evaluation. Two layers of body porcelain (Ceramco II, Body D4, Lot #7532, Ceramco, Inc., East Windsor, N.J.) were applied to each specimen and fired under vacuum at the recommended temperature. The body porcelain was ground to obtain a body porcelain thickness of 0.8 mm (1-0.02 mm) for a total porcelain thickness of 1.0 mm. The specimen was then glazed in air at the recommended firing temperature. The porcelain on the specimen was fractured in a controlled fashion by use of an apparatus designed to deform the specimen to a constant radius of curvature. Details of the design, construction, and use of this constantstrain flexure apparatus are described in a previous article.26 This apparatus was designed to minimize the effects of physical property differences (particularly differences in modulus of elasticity) of different types of alloys on the assessment of porcelain bond quality. Each speci-

370

men was deformed in the apparatus in a universal testing machine (Model TTB, Instron Corp., Canton, Mass.) at a crosshead speed of 0.25 mm/minute until the specimen had reached the point of maximum deflection as described.26 After fracture, the specimen was cleaned with a nylon brush under running water and placed in an ultrasonic cleaner for 3 minutes to remove any loose porcelain particles. The porcelain remaining on the alloy surface was analyzed by energy dispersive X-ray spectroscopy. The specimen was placed in a scanning electron microscope to analyze a defined area of the surface. The instrument conditions used were as follows: specimen tilt of 25 degrees, working distance of 15 mm, beam current of 60 pA, and detector distance of 30 mm. Silicon X-ray counts were obtained from each specimen for 40 seconds. X-ray counts were also obtained from the 100% opaque porcelain slab and from bare alloy to provide the silicon X-ray counts for 100% and 0% porcelain, respectively, for each alloy. This allowed the calculation of an area fraction of the specimen that remained covered with porcelain by using the formula of Ringle et al.25: porcelain area fraction = s where: Si, = silicon X-ray counts from the fracture site, Si, = silicon X-ray counts from the 100% opaque porcelain slab, and Si, = silicon X-ray counts from the bare alloy. Three measurements were made for each specimen and averaged. Significant differences in areas of retained porcelain for the different alloys were assessed by the Tukey HSD test.

RESULTS Castability The castability part of the study revealed significant differences in degree of test-pattern duplication between the different alloys (Fig. 2). The alloy castability ranged from 112 segments cast (100%) to 3.2 segments cast (2.8%). The data were subjected to an analysis of variance (ANOVA) followed by Fisher’s modified least significant

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not significantly different(p ) 0.05)

Fig. 3. Castability of 17 PFM alloys as measured by modified version of NIST mesh pattern test. Test measured ability of alloys to cast possible 112 mesh segments. Inset key indicates alloy types. Error bars indicate one standard deviation. Alloys that were not significantly different in castability at the 0.05 level of confidence are joined by a horizontal bar.

difference (LSD) procedure atp c 0.05. The mean number of segments cast for each alloy type is shown graphically in Fig. 3. The nickel-chromium-beryllium alloys were significantly better than all other alloys.

Opaque

masking

Each rater’s scores were transformed with z-score transformations and were then averaged. ANOVA was followed by Fisher’s modified LSD procedure at p < 0.05 to determine differences in perceived lightness and darkness of the opaqued alloys. The results are listed in Table II with the lightest group at the top of the list. Intraclass correlation coefficient, computed as a measure of interrater reliability, was 0.65. The rater evaluations were capable of discriminating at the 0.05 probability level in 37 of the possible 136 paired comparisons between alloys. In contrast, the calorimeter readings were able to discriminate at the 0.05 probability level in 104 of the possible 136 paired comparisons between alloys (Fig. 4). Discriminant analysis revealed that a linear combination of the red, green, and blue (RGB) values measured by the calorimeter produced a significant predictor of group membership (p < 0.0005). The mean RGB values for each alloy are shown in Fig. 4, and the Euclidean distance, A, between each pair of points in three-dimensional RGB color space was calculated from the relation: A = VCR, -RI)’

+ (Gz - G,)’ + (Bz - B,)’

where R, Gi, and Bi are the mean a given alloy, i.

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1996

R, G,

and

B values

for

Table

II.

Opaque

masking

measured

by five human

observers Alloynanle

Alloy type

Meanz-scores

Litecast I3 Biobond Plus Will-Ceram W-3 Jelstar Biobond II Jelenko “0” Option Rx-91 Eclipse Rexillium III PTM-88 Neydium gold ceramic Olympia Athenium Naturelle Will-Ceram W-l Cobond

Ni-Cr-Be Ni-Cr Au-Pd Pd-Ag Ni-Cr-Be Au-Pt-Pd Pd-Cu-Ga Pd-Ag Au-Pd Ni-Cr-Be Pd-Ga-Co Au-Pd-Ag Au-Pd Pd-Cu Pd-Cu-Ga Pd-Ag Co-Cr

-0.7945 -0.5994

Porcelain The area men were comparisons cedure to alloys atp in Fig. 5.

-0.4121 -0.4037 -0.2438 -0.2301 -0.2196 -0.0762 0.0421 0.1097 0.1360 0.1629 0.2549 0.4209 0.5035 0.6503 0.6993

bonding fractions of retained porcelain for each specistatistically analyzed by ANOVA. Post hoc were made by Fisher’s modified LSD prodetermine significant differences among the < 0.05. The mean area fractions are illustrated

371

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R

G

0

ATH

221.6

BB+

228.6

201.6

167.6

BBZ

229.6

201.6

165.8

COB

226.8

179.0

162.6

ECL

228.6

200.2

165.0

J-O

232.2

202.4

167.0

JST

235.2

203.8

168.6

LCB

221 .o

NAT

218.4

NGC

231.4

201.0

OLY

224.4

196.4

162.8

OPT

217.2

199.2

164.8

PTM

235.0

204.0

167.8

1291

235.4

203.0

167.4

Rx3

224.2

197.0

162.6

W-l

240.8

208.2

174.6

W-3

230.6

203.2

169.2

DENTISTRY

BR+

BBZ

O’CONNOR

COB

ECL

J-0

JST

LCB

NAT

NGC

OLY

OPT

PTM

R91

RX3

W-l

ET AL

W-3

-

Fig. 4. Mean RGB values of alloy specimens masked with a 0.2 mm opaque layer and measured with a calorimeter. Euclidean distances between RGB points in three-dimensional color space are listed for each paired comparison. Significant differences were determined by discriminant analysis (N.S., not significant at the 0.05 level of confidence).

DISCUSSION Castability The mesh monitor test method used in this study produced a wide spread of castability values for the various alloys evaluated. The Ni-Cr-Be alloys cast almost twice the number of segments as the next grouping of alloys. These were followed by the other base metal alloys and several of the high-palladium alloys. These results are in agreement with a study by Okuno et a1.,5 who found that beryllium-containing Ni-Cr alloys cast better than Ni-Cr alloys without beryllium. Young et a1.3 used a mesh test and found that a Ni-Cr-Be (Rexillium III) alloy cast significantly better than an Au-Pd (Olympia) and a Pd-Ag (Jelstar) alloy. In contrast, a study of marginal accuracy of a high-gold alloy (Jelenko “O”), four high-palladium alloys, and an Ni-Cr-Be alloy (Rexillium III) by Byrne et a1.2 reported that for all alloys, marginal completeness was adequate and marginal openings were considerably less than 50 urn. This study demonstrated that the mesh casting method is a severe test of castability. The Pd-Ag and Au-Pd alloys, which are widely used for clinical castings, could not be successfully cast in the mesh pattern. It is noted that Nay-

372

lor et a1.28 criticized the NIST mesh pattern test as not yielding the same results as a coping casting test for evaluating alloy castability. However, part of their criticism was based on their expectation that the coping test and the mesh test should produce numerically equal casting percentages. Their coping test piece consisted of a standardized coping with a 1.0 mm beveled margin. The percentage score for this coping test piece was arbitrarily defined as the percentage length of this 1.0 mm bevel (as measured in four locations on the casting) that was cast by the alloy that was studied. If a chamfer or other margin design had been used, or if a different fiducial length of bevel had been established as 100% castability, the numerical percentage values would have been different for the coping test; thus the coping and mesh tests should not be expected to produce numerically equal percentage values. Because a coping casting accuracy test is a simulation of actual casting, whereas a mesh pattern test is an abstraction, it might be assumed that any lack of agreement between the two tests could be blamed only on shortcomings in the mesh pattern test. Lack of agreement between the two types of castability test does not necessarily mean that the coping-type test is a superior measure of casting per-

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0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20

0.10 0.00 $5

-

2

I; 0

$j z

2 m

g

a

$qkqmmm-7;+5 -,PQS!B:, w

not significantly different(p > 0.05)

Fig. 5. Quality

of porcelain bond attainable with alloys studied. Test measured area fraction of porcelain retained on alloy after mechanical removal of bulk porcelain in constant-strain flexure apparatus. Different hatch patterns represent various types of alloys studied, as indicated in key in Fig. 3. Error bars indicate one standard deviation. Alloys that did not exhibit significant differences in porcelain bonding are joined by a horizontal bar. formance, however. Coping-type casting accuracy tests are strongly influenced by the degree to which proper casting shrinkage compensation is obtained. If casting shrinkage compensation is inadequate, the undersized coping will fail to seat fully and a poor casting accuracy score will result. Casting shrinkage compensation is dependent on many factors that are unrelated to the alloy itself-investment type, amount of silica sol used in mixing the investment, burnout temperature, investment cooldown before casting, and so forth. Because the goal of this portion of the study was to measure the fundamental casting performance of an alloy with minimal interference of confounding variables, the mesh pattern test was deemed most suitable to evaluate alloy castability. It is worth noting that, although the coping and mesh tests did not produce identical rankings in the study by Naylor et a1.,2s the best and worst alloys were grouped similarly by both tests.

Opaque

masking

Alloy LCB, an Ni-Cr-Be alloy, was rated significantly better than 10 of the 16 other alloys in regard to alloy show-through in the opaque porcelain layer. In contrast, COB, a cobalt-chromium alloy, was rated significantly worse than eight of the 16 other alloys. Although individual alloys exhibited differences in opaque masking, no systematic effect of alloy type was observed. Jacobs et al.ll found no difference in value or chroma of 0.5 to 1.5 mm porcelain thicknesses fired to Au-Pt-Pd, high-palladium, and Ni-Cr alloys. Five ceramic alloys and a calorimeter

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1996

were used by Crispin et al. l6 to evaluate the color at the opaque and dentin porcelain steps and they found no significant differences at the opaque stage. However, the Pd-Ag and NiCr alloys were lower in value after dentin porcelain application. The samples fired from Au-Pd and high-palladium alloys were not significantly different from the control alloy (high Au). Brewer et allo also found that color differences were the least at the opaque layer but increased after dentin porcelain application. They found that a high-gold ceramic alloy (84% Au) was lighter than a Pd-Ag alloy (W-l).

Porcelain

bonding

With one exception, the base metal alloys tended to produce lower bond strength than the noble alloys. Daftary and Donovan2g reported a tendency for adhesive failure through the porcelain-metal interface for base metal alloys. When Uusalo et a120 compared base metal alloys with several gold ceramic alloys, they also found slightly lower bond strengths with more frequent adhesive fractures. The porcelain bond strength of BB+ (which contains no beryllium) was significantly lower than that of the three beryllium-containing Ni-Cr alloys. This result agrees with a study of base metal alloys that used a three-point bending test in which Wu et all9 reported that a Ni-Cr alloy with beryllium had higher bond strengths than one without beryllium. In this study, PTM had the lowest retained porcelain value. Lorenzana et at.ls found that PTM, which contains

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no gold, produced lower bond strengths than three goldcontaining high-palladium alloys. However, it should be noted that PTM is a palladium-cobalt alloy, whereas the other high~p~ladium aIloys in this study are palladiumcopper alloys (Table I). It is likely that this difference is at least as important to porcelain bonding as the presence or absence of gold. Jochen et at.30 used a four-point flexural bond test and found that W-l had slightly greater bond strengths than R91. However, despite the greater bond strengths recorded for W-l in the study of Jochen et al.,3o the authors observed that W-l retained the least opaque porcelain. They had no explanation for this discrepancy. DeHoff et aLz3 performed a finite element analysis of the four-point flexural test and concluded that because of the complexity of the stress dis~ibution in the specimen, “the four-point flexural test may give misleading information concerning the effects of experimental variables on interface failure.”

CONCLUSIONS Under the conditions of this study, the following conclusions were drawn. 1. There is .a wide range of castability of ceramic alloys as eval.uated by a mesh casting test. Base metal alloys generally cast better than noble alloys. Nickel-chromium alloys cont~ning be~llium cast better than ones without beryllium. 2. Although individual differences among alloys were noted, no systematic effect of alloy type was observed in the ability of the different alloys to be masked by a 0.2 mm layer of opaque porcelain. 3. ~e~l~urn-cont~ing Ni-Cr alloys produced better porcelain-metal bonds than Ni-Cr alloys without beryllium. 4. Of the high-palladium alloys, the palladium-copper alloys produced better porcelain-metal bonds than the palladium cobalt alloy. We thank P. Kenneth Morse, Harry C. Davis, and Carl M. Russell for assistance with the statistical analysis of the data, Robert D. Ringle for performing the calorimetry measurements, and William D. Vickers for assistance with specimen preparation.

~FE~NCES 1. Asgar K, Arfaei AH. Castability of crown and bridge alloys. J PROS~T DENT 1985;54:60-3. 2. Byrne G, Goodacre CJ, Dykema RW, Moore BK. Casting accuracy of high-palladium alloys. J PROSTHET DENT 1986;55:297-301. 3. Young HM, Coffey JP, Caswell CW. Sprue design and its effect on the castability of ceramometal alloys. J PROSTHET DENT 1987;57: 160-4. 4. Hirano S, Tesk JA, Hinman RW, Argentar H, Gregory TM. Casting of dental alloys: mold and alloy temperature effects. Rent Mater 1987; 3:307-14. 5. Okuno 0, Tesk JA, Penn R. Mesh monitor casting of Ni-Cr alloys: element effects. Dent Mater 1989;5:294-300. 6. Tjan AH, Li T, Logan GI, Baum L. Marginal accuracy of complete crowns made from alternative casting alloys. J PROSTHET DENT 1991; 66~157-64.

7. Bar&i N, Richardson ST. A study of various factors influencing of bonded porcelain. J PROSTHET DENT 1978,39:282-4.

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ET AL

8. Jorgenson

MW, Goodkind RJ. Spectrophotometric study of five porcelain shades relative to the dimensions of color, porcelain thickness, and repeated firings. J PROSTHET DENT 1979;42:96-105. 9. Obregon A, Gaodkind RJ, Schwabacher WB. Effects of opaque and porcelain surface texture on the color of ceramometal restorations. J F’ROSTHET DENT 1981;46:3.%40. 10. Brewer JD, Akers CK, Garlapo DA, Sorensen SE. Spectrometric analysis of the influence of metal substrates on the color of metal-ceramic restorations. J Dent Res 1985;64:74-7. 11. Jacobs SH, Goodacre CJ, Moore BK, Dykema RW. Effect of porcelain thickness and type of metal-ceramic alloy on color. J PROSTHET DEW 1987;67:138-45.

12. O’Neal SJ, Leinfelder KF, Lemons JE, Jam&on HC. Effect of metal surfacing on the color characteristics of porcelain veneer. Dent Mater 1987;3:97-101. 13. Terada Y, Maeyama S, Hirayasu R. The influence of different thicknesses of dentin porcelain on the color reflected from thin opaque porcelain fused to metal. Int J Prosthodont 1989;2:352-6. 14. Ringle RD, Mackert JR Jr, Fairhurst CW. Detecting silver-containing metal ceramic alloys that discolor porcelain. Int d ~o~hodont 1989; 2563-8. 15. Hammad IA, Stem RS. A qualitative study for the bond and color of ceramometals. Part II. J PROSTHET DENT 1991;65:169-79. 16. Crispin BJ, Seghi RR, Globe H. Effect of different metal ceramic alloys on the color of opaque and dentin porcelain. J PROSTHET DENT 1991; 65~351-6. 17. Brewer JD, Glennon JS, Garlapo DA. S~ctrophotome~ic anaIysis of a nongreening, metal-fusing porcelain. J FROSTHET DENT 1991;65:63441. 18. Lorenzana RE, Chambless LA, Marker VA, Staffanou RS. Bond strengths of high-palladium content alloys. J PROSTWET DENT 1990; 64677-80. 19. Wu Y, Moser JB, Jameson LM, Malone WF. The effect of oxidation heat treatment of porcelain bond strength in selected base metal alloys. J PRODENT 1991;~:439-~. 20. Uusalo EK, Lassila VP, Yli-Urpo AU. Bonding of dental porcelain to ceramic-metal alloys. J PROSTHET DENT 1987;57:26-9. 21. Hammad IA, Stein RS. A qualitative study for the bond and color of ceramometals. Part I. J PROSTHET DENT 1990;63:643-53. 22. Anusavlce KJ, DeHoff PH, Fairhurst CW. Comparative evaluation of ceramic-metal bond tests using finite element stress analysis. J Dent Res 1980;59:668-13. 23. DeHoff PH, Anusavice KJ, Hathcock PW. An evaluation of the four-point flexural test for metal-ceramic bond strength. J Dent Res 1982;61:1066-9. 24. Porcelain Enamel Institute. Test for the adherence of porcelain enamel cover coats direct-to-steel, PEI Bulletin T-29(721, Washington, DC, Porcelain Enamel Institute, Ine, 1972: 1-6. 25. Ringle RD, Mackerr JR Jr, Fairhurst CW. An X-ray spectrometric technique for measuring ~or~l~n-metal adherence. J Dent Res 1983;62:933-6. 26. Mackert JR Jr, Ringle RD, Parry EE, Evans AL, Fairhurst CW. The relationship between oxide adherence and porcelain-metal bonding. J Dent Res 1988;67:474-8. 27. Hinman RW, Tesk JA, W&lock RP, Parry EE, Durkowski JS. A technique for characterizing casting behavior of dental alloys. J Dent Res 1985;64:134-8. 28. Naylor WP, Moore BK, Phillips RW, Goodacre CJ, Munoz CA. Comparison of two tests to determine the castability of dental alloys (published erratum appears in Int J Prosthodont 1990;3:512). Int J Prosthodont 1990;3:413-24. 29. Daftary F, Donovan T. Effect of four pretreatment techniques on porcelain-to-metal bond strength. J PROSTHET DENT 1986;56:535-9. 30. Jochen DG, Caputo AA, Matyas AS. Effect of opaque porcelain application on strength of bond to sever-palladia alloys. J &OSTHET DENT 1990;63:414-8. Reprint

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DR. J. ROUWAY MIACJCERT, JR. DENTAL MATEXULS LABORATORY DEPARTMENT OF ORAL REHABILITATION SCHOOL OF ~~~IS~Y MEDICAL COLLEGE OF GEOIKZA AUGUSTA, GA 30912-1260

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75

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4