Porcelain-fused-to-metal surface oxidation effects on cemented casting retention

IMPROVED COLOR MATCHING: PART II

6. Tylman SD. Tylman’s theory and practice of fixed prosthodontics. 2nd ed. St Louis: The CV Mosby Co, 1978;711.

7. Eissman H. Visual perception and tooth contour. In Yamada HN, ed. Dental porcelain: the state of the art-1977. Los Angeles: University of Southern California, 1977;299.

8. Obregon A, Goodkind RJ, Schwabacher WB. Effects of opaque and porcelain surface texture on the color of ceramometal restorations. J PROSTHET DEN,T 1981;46:330-40.

9. Burk B. Color and esthetics, In Yamada HN, ed. Dental porcelain: the state of the art--1977. Los Angeles: University of Southern California, 1977;293-5.

10. McLean JW. The science and art of dental ceramics, vol II. Chicago: Quintessence Publishing Co Inc, 1980;117.

Reprint requests to: DR. JOHN A. SORENSEN UNIVERSITY OF CALIFORNIA SCHOOL OF DENTISTRY CHS 33-041 Los ANGELES, CA 90024

Porcelain-fused-to-metal surface oxidation effects on cemented casting retention

David A. Felton, D.D.S., M.S.,* B. Ed Kanoy, D.D.S., M.A.,* James T. White, D.D.S., M.S.,** and Stephen C. Bayne, M.S., Ph.D.*** University of North Carolina, School of Dentistry, and Veterans Administration Medical Center, Chapel Hill, N.C.

N umerous cavity preparation and restoration factors influence the retention of full veneer crowns cemented on abutment teeth. The cavity preparation factors include (1) the amount of taper of the prepared abutment tooth,‘,’ (2) the extent of the surface area of the prepared tooth,‘, 3 (3) the amount of surface roughness of the prepared tooth,4 and (5) the presence of auxiliary circumferential grooves in the casting and abutment tooth.‘s6 The restoration factors include (1) the type of cementing medium used,7,8 (2) the surface roughness of the casting:, ‘O and (3) the chemical condition of the internal surface of the casting. As abutment taper increases, cemented casting retention declines. Factors increasing the surface area for cement bonding tend to increase the retention values for cemented castings. Polycarboxylate and glass-ionomer cements can also bond to tooth structure’, * and to certain chemically active gold substitute alloys,” thereby providing additional retention.

The influence of the surface oxidation of ceramic alloys during porcelain application and glazing has been recognized as imperative for the proper bonding of dental porcelains to ceramic alloy substructures,‘,” but its effect on cement retention has not been evaluated as a

Supported by NIH Grant No. RR05333. *Assistant Professor, Department of Fixed Prosthodontics, University

of North Carolina, School of Dentistry. **Veterans Administration Medical Center, Durham, N.C. ***Associate Professor, Department of Operative Dentistry, Section

Head of Biomaterials, University of North Carolina, School of Dentistry.

THE JOURNAL OF PROSTHETIC DENTISTRY

Table I. Composition of alloys tested (wt. %)*

In, Fe, Sn Alloy Au Pt Pd Ag Zn, Ga

Jelenko “0” 87.5 4.5 6.0 1.0 1.0 Cameo 52.5 27.0 16.0 4.5 Olympia 51.5 38.5 11.0 Jelstar 60.0 28.0 12.0

Alloy Co Cr W Ru Fe, MO, Mn, Si, C, Al

Genesis II 33.0 27.0 10.0 3.0 27.0

*Per Physical Properties chart supplied by Penwalt/Jelenko, Armonk, NY.

variable. This study evaluated the effects of surface oxidation during porcelain firing of porcelain-fused- to-metal (PFM) alloys on their retention to prepared teeth when cemented with zinc phosphate cement.

MATERIAL AND METHODS

One hundred fifty extracted posterior teeth were cleaned and the root surfaces notched for anchorage. The roots were then embedded in individual blocks of acrylic resin to within 2 mm of the cementoenamel junction. A surveyor was used to ensure that the clinical crown of each tooth was parallel to the acrylic resin block. The samples were stored in a 100% humidity environment except during preparation, cementation, and crown removal procedures.

Thirty samples were randomly assigned to each of five different alloy groups. Each group involved prepared

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weight of each foil strip was then compared with the mean weight of foil strips of a known surface dimension. The surface area of each sample was calculated from this comparison (Fig. 4).3

Patterns 1 mm in thickness simulating full cast crowns were waxed directly to the prepared teeth. Acrylic resin templates were attached to the occlusal surface of the wax patterns by using the dental surveyor to orient the templates parallel to the axis of draw of each abutment tooth (Fig. 5). A hole in each template was used to secure the samples in the Instron Universal testing machine (Instron Corp., Canton, Mass.) for tensile testing. Pat- terns were invested in phosphate bonded investment (Ceramigold, Whip Mix Corp., Louisville, Ky.) and cast by using a gas-oxygen torch in one of five ceramic casting alloys. All castings were cleaned and air abraded with 25 pm aluminum oxide powder for 10 seconds (unfired condition). They were then numbered to correspond with their respective abutment teeth. Fifteen samples from each alloy group were heat-treated to simulate porcelain application in a programmable porcelain furnace. Each sample was subjected to one metal- conditioning firing, two opaque firings, two body porcelain firings, and one glaze bake (fired condition). The firing cycle parameters used are described in Table II. Each casting was cemented with zinc phosphate cement (Fleck’s Extraordinary, Mizzy, Inc., Clifton Forge, Va.) mixed according to manufacturer’s directions. All castings were cemented by using a 25 kg compressive load that was held for the first 10 minutes (Fig. 6). Excess cement was removed and the specimens were stored in 100% humidity for 24 hours before tensile testing.

All of the castings were removed parallel to the axis of draw by using the Instron Universal testing machine with a crosshead speed of 0.02 cm/minute until cement bond failure occurred (Fig. 7). The bond strengths were calculated by dividing the maximum load required to dislodge each casting from its abutment tooth by the total tapered surface area of the prepared abutment tooth.

The internal aspects of selected castings for each alloy type were examined in the scanning electron microscope @EM) (ETEC Autoscan, Monrovia, Calif.). Photomi- crographs were made of the unfired (as cast, sand- blasted) condition and in the fired (as cast, sand-blasted, and heat-cycled) condition at a standard magnification (x1400) (Figs. 8 through 12). Changes in the surface morphology were qualitatively rated between conditions and between alloys.

RESULTS

The mean retention values for all alloys tested are summarized in Fig. 13. A one-way ANOVA, followed

Fig. 1. H283K carbide bur used for crown preparation. (Magnification x40.)

abutment teeth with full veneer crowns of one of five different ceramic alloys (Jelenko “O”, Cameo, Olympia, Jelstar, and Genesis II, Penwalt/Jelenko Co., Armonk, N.Y.). The composition of the alloys is presented in Table I. The H282K carbide bur (Brasseler, U.S.A., Inc., Savannah, Ga.) was used for tooth preparation to standardize the amount of taper and marginal configu- ration (Fig. 1). Each bur was used for five abutment preparations before being discarded. All preparations were made by using water spray to prevent dessication of the specimens.

The occlusal surfaces of the teeth were reduced to the depth of the central groove and were prepared flat and perpendicular to the axis of the clinical crown. An aluminum jig was made and used to attach a high-speed handpiece to a dental surveyor. The movable table of the surveyor was adapted to secure the embedded specimens such that the long axis of each clinical crown was parallel to the bur mounted in the handpiece (Fig. 2). Each tooth was prepared with axial walls 4 mm in length and a total axial taper of 5 degrees, which was the total taper allowed by the bur selected for this study.

The actual geometric surface area of each preparation was calculated by adapting 4 mm wide foil strips to the axial surfaces of each prepared tooth (Fig. 3). The

678 DECEMBER 1987 VOLUME 58 NUMBER 6

SURFACE OXIDATION EFFECTS ON RETENTION

Fig. 2. Paralleling device used for crown preparations. Fig. 3. Prepared tooth with 4 mm foil strip.

Fig. 4. Foil strip adapted to prepared tooth. Fig. 5. Template used for crown removal procedures attached to wax pattern with surveyor.

by a Tukey’s post hoc evaluation, determined statistical- 4.97 k 1.07 MPa for the fired condition. The compari- ly significant differences (p < .Ol). For the comparison son of bond strengths for the five alloys for both of unfired and fired retention values for each alloy, the conditions showed a significant difference only between only significant difference was for the Jelenko “0” alloy. the three strongest and two weakest alloys. Specifically, The mean bond strengths for Jelenko “0” increased Jelenko “0” samples fired (mean 4.97 f 1.07 MPa) from 3.26 + 0.72 MPa for the unfired condition to and Olympia samples unfired and fired (mean 4.58 f

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Fig. 6. Casting cemented and excess cement removed. Fig. 7. Instron Universal testing machine with casting removal apparatus.

Table II. Firing cycle parameters

Stage Preheat time

(min/sec)

Low Heat temp rate rc) (“C/min)

Vat. level

(in Hg)

High temp.

(“a

Hold time

(min)

Vent temp.

(“C)

Cool time bed

Metal conditioning o/10 704 56 0 1038 0 0 10 Opaque 1 610 540 56 28 995 0 995 10 Opaque 2 6/O 540 56 28 980 0 980 10 Body 1 9/O 540 56 28 960 1 960 10 Body 2 9/O 540 56 28 950 1 950 10 Glaze 3/O 540 56 0 940 0 0 10

1.04 MPa and 4.54 f 1.14 MPa) demonstrated greater retention values than did the unfired and fired Genesis II samples (mean 2.87 + 1.10 MPa and 2.76 2 1.14 MPa).

The SEM photomicrographs of surfaces before cementation showed a roughened morphology for all alloy specimens, changed surface morphology from the unfired to the fired condition, and changes that were not uniform for all alloy types (Table III). After bond- strength testing, castings and tooth surfaces were examined to determine the mode of cement failure. In all tests, the mode of cement failure was both adhesive and cohesive in nature.

DISCUSSION

Absolute values of retentive bond strength depend on a host of retentive design factors that were not individually investigated during these experiments. These include the

degree of taper of the preparation, the presence of a dentinal smear layer, the dentin roughness created by the choice of bur, the casting roughness created by the sandblasting particle size, and the rate of debonding of the materials. These and other factors will affect the observed results. However, within the limited parameters of this investigation, it is still possible to rank the effect of porcelain firing cycles on oxidation that would alter the cemented bond strengths.

The retentive strength for cemented crowns will only be as great as the weakest link in the combination of materials and interfaces across the region of cementation. For an oxidized crown, the layers involve a metal casting, an oxide layer, a cement layer, a dentin smear layer, and dentin. In most systems the metal oxide will occur predominantly as an external oxide. However, if oxygen diffuses faster than metal ions in the oxide film, the film growth will penetrate inwardly and an internal



8. S surface morphology of Jelenko 0 alloy. A, Nonoxidized; B, oxidized. (A Lagnifi- caiion X 1400.)

oxide will be present (Fig. 14).” The strongest materials are the metal casting and the oxide films. The metal and metal oxide interfacial bond for Jelenko “0” has been reported as high as 40 MPa.14 For these particular experiments, all of the failures seemed to occur within the cement and between the cement and the casting surface. Therefore, the weakest link was assumed to be the cement interface with the casting surface. In most instances, that surface was covered with an oxide.

At the interface between the cement and casting, the key factors influencing adhesion are (1) wetting of the cement on the casting surface, (2) mechanical interlock- ing of the cement with the casting surface roughness, and (3) chemical bonding of the cement with the casting surface. The degree of surface oxidation may influence all three of these factors. An oxidized surface should be

wet more readily by the water-based ceramic composition of unset zinc phosphate cement.” A thick oxide layer may begin to eliminate some of the original surface roughness for mechanical retention. Finally, the presence of surface oxide would allow a chemical reaction with cements capable of chelation, such as polycarboxy- lates.7-‘0 With zinc phosphate cement, because there should be little or no chemical reaction, the first two factors would be expected to predominate.

To compare the oxide effects on wetting and roughness, the oxidation potential and degree of apparent surface roughness as demonstrated by the SEM photomicrographs are listed for each alloy in Table III. The retentive bond strength before and after surface oxidation is noted for reference. An estimate of the oxidation potential was based on the amount of oxidizable material


FELTON ET AL

Tat ,le III. stre mgth

Fig. 9. Surface morphology of Cameo alloy. A, Nonoxidized; B, oxidized. (Magi tion X1400.)

Oxidation potential and resultant alloy surface roughness effects on cemeni bation bond

nifica-

Alloy Oxidizable Change in surface

components (%I roughness due to oxidation Bond strength

pre/post oxidation (MPa)

Jelenko “0” 2 Olympia 11 Jelstar 40 Cameo 20.5 Genesis II 64

No change Smoother Smoother Smoother Smoother

3.20 J4.95 4.65/4.60 3.60/3.70 3.60/3.30 2.90/2.80

in the overall composition for each alloy (Table III). The actual thickness of any oxide would depend on the time and temperature conditions of the casting and porcelain application cycles. For alloys with only a small amount of oxidizable material, more extensive heat treatment would be required before any morphologic change that

might correspond to oxide appearance and film growth would be observed.

The major result of comparisons of the unfired and fired conditions of all of the alloy systems was that only the Jelenko “0” system showed a statistically significant difference in retentive bond strength. This alloy contain-



Fig. 10. Surface morphology of Olympia alloy. A, Nonoxidized; B, oxidized. ( cation X1400.) - --

ed only a small percentage of oxidizable material. After casting there was little or no surface oxide present on the alloy, and sandblasting would have removed any oxide that might have formed. However, the longer time and temperature conditions of the oxidizing cycles created a fully oxidized surface. The oxide seemed to improve the wetting of the metal surface with zinc phosphate cement. From the appearance of the SEM photomicrographs (Fig. S), there did not seem to be any appreciable oxide buildup that would compromise the surface roughness for mechanical retention.

Although the Jelenko “0” alloy showed a difference in cementation bond strength between the “as cast” and oxidized conditions, there were no statistically significant differences for any of the remaining alloys. The heat treatments that simulated porcelain application may have contributed to additional surface oxide growth and

;niIi-

decreased surface roughness. This corresponds to changes in surface roughness observed in all the other SEM photomicrographs.

From Fig. 13 it is obvious that the individual alloys fall into three different ranges of retentive bond strengths that seem to correspond to the compositional classes of the alloys. Jelenko “0” and Olympia are Au-Pt-Pd and Au-Pd alloys, respectively, whose oxides are produced by In and Sn additions to the composition. The oxide- forming elements are at low concentrations in Jelenko “0” and the time and temperature conditions of porcelain firing are required to generate the oxide. In both Jelenko “0” and Olympia, the oxides are the same. Therefore, once oxidation has occurred, the retentive bond strengths should be the same. It is logical that the oxidized Jelenko “0” and both “as cast” and oxidized Olympia alloys are statistically equal in strength and


FELTON ET AL

Fig. 11. Surface morphology of Jelstar alloy tion X1400.)

A, Nonoxidized; B, oxidized. (Magnifica-

stronger than the rest of the cemented alloys. In contrast, Jelstar and Cameo represent Ag-Pd and Au-Ag-Pd alloys. These alloys form a silver-rich oxide that seems to occur predominantly as an internal oxide below the casting surface. l4 This internal oxide is strongly bound to the metal casting. The alloys also contain In and Sn, which contribute an external oxide that may not be as adherent because of the underlying layer of internal oxide for these alloys. This could result in an adhesive failure of the cement at the cement/casting interface. Finally, Genesis II is a base metal alloy that instanta- neously passivates to form chromium oxide as an external oxide that completely covers the casting surface. The oxide strength to the underlying metal alloy has been reported as 1.7 to 40 MPa for many base metal alloys. I43 I6 A poor oxide-to-metal bond could result in an adhesive failure of the zinc phosphate cement at the

cement/casting interface. However, the oxide/metal bond strength for Genesis II was not determined in this investigation.

One of the challenges for studying the oxidation of these alloys is identifying the relatively thin and some- times irregular oxide films. From the SEM photomicrographs in Figs. 8 through 12 in these experiments, there was little evidence of a thick external oxide. Any oxide present was thinner than the greatest dimension of surface roughness of 5 pm. This is in agreement with cross-sectional views of oxides on Ag-Pd alloys observed by Mackert et al. l3 Their photomicrographs showed 1 to 2 pm zones of internal and/or external oxidation. Those parameters were not monitored in the present work. However, the occurrence of internal oxidation would explain the change in surface morphology without rapid elimination of the surface roughness.



12. Surface morphology of Genesis II alloy. A, Nonoxidized; B, oxidized. (M agni- fication X1400.)

CONCLUSIONS Within the experimental limitations of the design used

to simulate clinical conditions, the retentive bond strengths for different alloys with zinc phosphate cement were observed to vary with different conditions of oxidation. The following conclusions were drawn.

1. The oxidizing conditions of PFM firing cycles produced a statistically significant increase in retentive bond strength only for Jelenko “0” (high gold PFM) alloy.

2. The high gold alloy systems, Jelenko “0” and Olympia, produced statistically greater retentive bond strengths than the base metal alloy, Genesis II.

3. The degree to which dental cements wet the surfaces of dental casting alloys and the effect of surface oxidation and roughness in this wetting phenomenon warrants further investigation.

We express our sincere appreciation to Mr. Larry Strayhorn, C.D.T., for his technical expertise in fabricating the restorations, to the J.F. Jelenko Company for supplying the alloys studied, and to the Whip-Mix Corporation for supplying the investments.

REFERENCES

1. Nicholls JI. Crown retention. Part II. The effect of convergence angle variation on the computed stresses in the luting agent. J PROSTHET DENT 1974;31:651-7.

2. Shillingburg HT, Hobo S, Whitsett LD. Fundamentals of fixed prosthodontics. Chicago: Quintessence Publishing Co, 1978;67- 99.

3. Lorey RE, Myers GE. The retentive qualities of bridge retain- ers. J Am Dent Assoc 1968;76:568-72.

4. Felton DA, Kanoy BE, White JT. Surface roughness of crown preparation: effect on cemented casting retention [Abstract]. J Dent Res 1986;65:312.

5. Chan KC, Hormati AA, Boyer DB. Auxiliary retention for complete crowns provided by cement keys. J PROSTHET DENT 1981;45:152-5.


FELTON ET AL

CEMENTATION BOND STRENGTH: Unfired vs. Fired (MPa)

Fig. 13. Comparative bond strength of oxidized and nonoxidized alloys. (Error bar indicates + 1 standard deviation.)

CEMENTATION REGION INTERFACES

. . . . , . . . . ,

. . . . . . . . . ,

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . .

. . . . . l I= . . . . . . . . . . . . . . . . . . . . . x!z . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ls . - l-l

\ I niinlTll External

oxide Smear layer

III Alloy Castlng Dental Cement Dentin

Fig. 14. Cross-sectional representation of materials and interfaces across region of cementation.

686

6. Worley JL, Hamm RC, van Fraunhofer JA. Effects of cement on crown retention. J PROSTHET DENT 1982;48:289-91.

7. Arfaei AH, Asgar K. Bond strength of three cements determined by centrifugal testing. J PROSTHET DENT 1978;40:294-8.

8. McComb D. Retention of castings with glass ionomer cement. J PROSTHET DENT 1982;48:285-8.

9. Phillips RW. Science of dental materials. 8th ed. Philadelphia: WB Saunders Co, 1982;452-79.

10. Ady AB, Fairhurst CW. Bond strength of two types of cement to gold casting alloy. J PROSTHET DENT 1972;29:217-20.

11. Saito C, Sakai Y, Node H, Fusayama T. Adhesion of polycarboxylate cements to dental casting alloys. J PROSTHET DENT 1976;35:543-8.

12. McLean JW. The science and art of dental ceramics. Chicago: Quintessence Publishing Co, 1976, vol I.

13. Mackert JR, Ringle RD, Fairhurst CW. High-temperature behavior of a Pd-Ag alloy porcelain. J Dent Res 1983;62:1229- 35.

14. Baran G. Auger chemical analysis of oxides on Ni-Cr alloys. J Dent Res 1984;63:76-80.

15. Adamson AW. Physical chemistry of surfaces. 3rd ed. New York: John Wiley &. Sons, 1976;333-68.

16. Mackert JR, Parry EE, Hashinger DT, Fairhurst CW. Mea- surement of oxide adherence to PFM alloys. J Dent Res 1984;63:1335-40.

Reprint requests to: DR. DAVID A. FELTON UNIVERSITY OF NORTH CAROLINA SCHOLL OF DENTISTRY 211 H CHAPEL HILL, NC 27514

DECEMBER 1987 VOLUME 58 NUMBER 6

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Porcelain-fused-to-metal surface oxidation effects on cemented casting retention