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The effect of metal recasting on porcelain-metal bonding: A force-to-failure study
The effect of metal recasting on porcelain-metal bonding: A force-tofailure study Ruohong Liu, BDS, DDS, MS,a William M. Johnston, PhD,b Julie A. Holloway, DDS, MS,c William A. Brantley, PhD,d and Tridib Dasgupta, MSe College of Dentistry, The Ohio State University, Columbus, Ohio; University of Missouri-Kansas City School of Dentistry, Kansas City, Mo Statement of problem. Noble dental alloys are commonly remelted in the dental laboratory, but the effect of repeated casting on porcelain bond strength requires further documentation. Purpose. The purpose of this study was to determine if casting up to 3 times affected metal ceramic bond strength for 3 noble alloys using methodology in ANSI/ADA Specification No. 38. Material and methods. Representative high-gold (Brite Gold XH), gold-palladium (W-5), and palladium-silver (IPS d.SIGN 53) alloys were cast into metal strips (25 x 3 x 0.5 mm), using torch melting. IPS InLine porcelain with overall dimensions of 8 x 3 x 1.1 mm was centrally applied on each strip. Metal ceramic specimens were also prepared after each alloy was melted a second and third time. There were 12 specimens in each of the 9 groups. Force to failure and porcelain bond compatibility index (ob) were determined for each specimen, and statistical comparisons were made using the Ryan-Einot-Gabriel-Welsch multiple range test (experimental_=.05). Fractured specimens were examined with a scanning electron microscope. Results. Mean values of ob for specimen groups ranged from 40.6 to 48.2 MPa, and there were no significant differences among the 3 alloys after the first casting. For the high-gold alloy, ob was significantly different for the first and third castings. Increases in size and frequency of interfacial voids were observed with the SEM when all alloys were cast 2 additional times. Conclusions. All 3 alloys had adequate porcelain bond strength (>25 MPa). The bond strength for the high-gold alloy was significantly greater for the third casting than for the first casting. (J Prosthet Dent 2010;104:165-172)
$MJOJDBM*NQMJDBUJPOT Recasting of the 3 types of representative noble alloys evaluated does not adversely affect porcelain bond strength, but increased porosity resulting from recasting may affect other properties. This research was partially supported by NIH/NIDCR (grant DE10147) and by Ivoclar Vivadent, Inc, Amherst, NY. Some results from this study were previously presented at the 35th Annual Meeting of the American Association for Dental Research, Orlando, Fla, and the 2006 Carl O. Boucher Conference, Columbus, Ohio. Based upon a Master of Science thesis submitted by the first author to the Graduate School of The Ohio State University. a Former Resident, Advanced Prosthodontics Program, College of Dentistry, The Ohio State University; Assistant Professor, Department of Restorative Dentistry, University of Missouri-Kansas City School of Dentistry. b Professor, Division of Restorative and Prosthetic Dentistry, College of Dentistry, The Ohio State University. c Associate Professor, Division of Restorative and Prosthetic Dentistry; Associate Director of the Advanced Prosthodontics Program, College of Dentistry, The Ohio State University. d Professor and Director of the Graduate Program in Dental Materials Science, Division of Restorative and Prosthetic Dentistry, College of Dentistry, The Ohio State University. e Director of Materials Research and Development, Ivoclar Vivadent, Inc, Amherst, NY.
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166 Reuse of high-noble and noble alloys by recasting discarded sprues is common practice in the dental laboratory. Authors and manufacturers have recommended that 50% new alloy be used in each casting for noble metal ceramic restorations.1 Remelting noble alloys for metal restorations a small number of times does not cause much change in composition and mechanical properties.2-4 However, loss of trace base metals, such as Zn, In, Sn, and Fe, during recasting of noble alloys for porcelain might be detrimental to their castability and formation of the oxide layer during the initial oxidation step, which could adversely affect porcelain bonding.1 For a high-gold porcelain alloy, Tuccillo et al2 reported progressive and severe iron loss over the course of 5 torch meltings, and recommended the addition of new metal when remelting. For a gold-palladium alloy, Rasmussen and Doukoudakis5 found that, although the use of 85% or more recast metal increased the frequency and size of interfacial voids, there was no evidence that the bond with porcelain was degraded. Effects of remelting have also been reported for palladium-based porcelain alloys. Hong et al6 found that the thickness of the oxide layer and the silver oxide concentration increased after each remelting of a Pd-Ag alloy up to 4 times, with or without the addition of 50% new alloy. Jochen et al7 bonded porcelain to strips of a Pd-Ag alloy containing different percentages of new metal, tested the specimens in 4-point bending, and found that once-cast alloy with the addition of less than 50% new alloy produced significantly lower metal ceramic bond strength. Papazoglou et al8 compared the porcelain adherence to high-palladium alloys (3 Pd-Cu-Ga alloys and 1 Pd-Ga alloy) and a gold-palladium alloy after casting 3 times. While the Au-Pd alloy and 2 Pd-Cu-Ga alloys had no changes in the area fraction of adherent porcelain on the alloy surface of specimens fractured in biaxial flexure, the porcelain attachment to
the 2 other high-palladium alloys was adversely affected. Horasawa and Marek9 found that melting a silverpalladium alloy up to 4 times had little effect on its corrosion susceptibility in artificial saliva and attributed the severe degradation after the fifth melting to decreased gold content on the alloy surface. With the much less expensive Co-Cr,10 Ni-Cr-Be,11 and Ni-Cr12 base metal alloys, remelting a small number of times does not appear to greatly affect their properties, although further study of the effects on porcelain bonding to remelted base metal alloys is required. Because few previous studies6-8 have examined the effects of recasting the alloy on metal ceramic bonding, the rationale for the present study was to investigate this matter with more recently introduced, representative Au-Pd and Pd-Ag alloys, since these alloy types are in widespread use. The methodology in ANSI/ADA Specification No. 3813 was used; thus, a 3-point flexure test was used to evaluate the metal ceramic bonding. Previous investigations of noble metal ceramic bonding examined the importance of alloy composition,14 the use of a metal conditioner and appropriate surface roughness,15 the necessity of closely matching thermal expansion coefficients, and the effects of ceramic and specimen geometry.16,17 The appropriate test to evaluate metal ceramic bonding14,15,18-23 has been a matter of extensive discussion for several decades. The recommendation by O’Brien24 to focus on classification of failure modes for metal ceramic specimens and the optimum cohesive failure through porcelain, rather than to measure bond strength, has been replaced in ANSI/ADA Specification No. 3813 by a 3-point bending test with finite element analysis advocated by Lenz et al.23 Results from the porcelain adherence test, in which the area fraction of adherent porcelain is determined from metal ceramic specimens fractured in biaxial flexure, have been compared by Papazoglou and Brantley25 for 4 Pd-Ga alloys and a
The Journal of Prosthetic Dentistry
Pd-Ag alloy with the values of force to failure from the 3-point bending test in ANSI/ADA Specification No. 38, and no correlation was observed. The purposes of this study were to measure the bond strength of 3 representative noble alloys to a dental porcelain, using the protocol in ANSI/ ADA Specification No. 38,13 to determine if casting up to 3 times affected the metal ceramic bond strength of each alloy, and to investigate the fracture mode by using scanning electron microscopy (SEM). The null hypotheses were that there would be no difference in the porcelain bond strengths among the 3 originally cast alloys, and that recasting each of the alloys would have no effect on the bond strength.
MATERIAL AND METHODS Three representative noble metal alloys (Brite Gold XH, W-5, and IPS d.SIGN 53; Ivoclar Vivadent, Inc, Amherst, NY) were selected for this study. Brite Gold XH is a high-gold alloy, W-5 is a Au-Pd alloy, and IPS d.SIGN 53 is a Pd-Ag alloy. Their nominal compositions, obtained from the manufacturer, are presented in Table I. Metal strips, 25 x 3 x 0.5 mm, were cast from wax patterns cut from wax sheets of 0.5-mm thickness (Corning Wax, Ronkonkoma, NY), in accordance with ANSI/ADA Specification No. 38.13 Wax patterns were invested with a phosphate-bonded investment (Sure-Vest High Heat; Ivoclar Vivadent, Inc). Alloys were melted in individual ceramic crucibles with a multi-orifice propane-oxygen torch and cast with a standard broken-arm centrifugal casting machine (Centrifico; Kerr Corp, Orange, Calif ). All castings were bench-cooled to room temperature, followed by devesting and airborne-particle abrasion with 50-μm Al2O3 (Sterngold Dental, Attleboro, Mass). The buttons and sprues from the first and second castings were airborne-particle abraded and ultrasonically cleaned in distilled water, followed by visual inspection to
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5BCMF* Compositions of tested alloys provided by manufacturer (wt%) Alloy
Au
Brite Gold XH
88.9
W-5
52.2
IPS d.SIGN 53
Pd
Ag
Pt
Sn
In
Ir
Fe
Li
9.0
<1.0
<1.0
<1.0
<1.0
<1.0
2.7
<1.0
<1.0
7.7
1.7
26.0
17.1
<1.0
53.8
34.9
<1.0
Zn
1.2
Re
Ru
Mn <1.0
<1.0 <1.0
<1.0
<1.0
5BCMF** Test specimen groups (n=12) Specimen Group
Metal Type
Times Cast
B1
Brite Gold XH
1
B2
Brite Gold XH
2
B3
Brite Gold XH
3
W1
W-5
1
W2
W-5
2
W3
W-5
3
d1
IPS d.SIGN 53
1
d2
IPS d.SIGN 53
2
d3
IPS d.SIGN 53
3
Letter codes for alloys: B: Brite Gold XH; W: W-5; and d: IPS d.SIGN 53.
ensure that remaining investment materials were removed, before they were cast for the second and third times, respectively. No new metal was added for the second and third castings. Nine groups of test specimens (n=12) were prepared (Table II) after initial experiments with a power analysis indicated that this sample size would be a conservative number for obtaining statistically significant results. The power analysis26 used a clinically plausible change of 20% in shear bond strength and a power of 80% (`=0.20) to estimate the sample size. The cast alloy strips were finished with a laboratory handpiece (Z 500 Labo Motors; NSK, Kanuma, Japan), ultrasonically cleaned in distilled water, and oxidized following the recommendations of the manufacturer. Porcelain (IPS InLine; Ivoclar Vivadent, Inc), recommended by the manufacturer as compatible with the alloys selected,
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was used to prepare the metal ceramic specimens. Two layers of opaque porcelain with a total thickness of 0.1 mm were applied over a rectangular area of 8 x 3 mm in the center of one side of each cast metal specimen strip. Two layers of body porcelain were subsequently applied to achieve a rectangular porcelain shape having a thickness of 1.1 ±0.1 mm after firing, as measured by a Boley gauge (Benco no. 1162-994; Benco Dental, Pittston, Pa). Final glaze firing was then performed according to the recommendations of the manufacturer. All specimens were subjected to 3-point-flexure following the protocol in ANSI/ADA Specification No. 38. The metal ceramic specimens were supported by a jig (20-mm distance between supports) with the ceramic positioned symmetrically on the side opposite the applied load. The surface of the loading piston in contact with
the specimen was 1 mm wide. The apparatus for bond testing was attached to a screw-driven mechanical testing machine (model 4204; Instron Corp, Norwood, Mass). The crosshead speed was 1.5 mm/min, and the force on the test specimen was recorded up to failure. The fracture force (Ffail) for each specimen corresponded to the formation of a debonding crack that was visually observed at one end of the ceramic layer, and which was also noted on the load-deflection plot as a sudden drop in force. The metal ceramic bond strength was calculated as the bonding compatibility index (ob), following the methodology in ANSI/ADA Specification No. 38.13 The thickness of all specimens was measured at both ends, and the mean values for each specimen were subjected to ANOVA. The mean thickness values were used to calculateob, along with the value of
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168 Young’s modulus (E), for each alloy provided by the manufacturer (Brite Gold XH, 107 GPa; W-5, 118 GPa; IPS d.SIGN 53, 132 GPa). After ob was calculated for each specimen, 2-way ANOVA (factors: alloy and number of times cast), followed by the RyanEinot-Gabriel-Welsch (REGWQ) multiple range test, was performed to compare ob after the first casting for the 3 alloys, and ob for each alloy after casting 3 different times. Since the REGWQ test was applied 4 times to the experimental data and it is only appropriate to apply this test to the data once, a standard Bonferroni correction, that is, setting an _ value of .01 to each REGWQ test, allowed an experimental _ error of .05 to be safely maintained. Statistical software (SAS 9.2; SAS Institute, Inc, Cary, NC) was used to perform these analyses. One specimen from Group W1, 2 specimens from Group W2, and 2 specimens from Group d1 (Table II) cracked during handling and were excluded from testing. No additional new specimens were made. A post hoc sensitivity analysis
(G*Power, v. 3.1.2)27 was also performed on the final experimental data to validate the initial conservative power analysis, in which the objective was to maintain an overall or experimentwise _ error of .05. The resultant-effect size determined the difference that would be detected as statistically significant by the relationship: effect size = (detectable difference)/(root mean square error). One representative specimen from each group was selected for SEM examination (model JSM-820; JEOL, Ltd, Tokyo, Japan). The ceramic layer previously cracked during the bond strength testing was carefully removed from the underlying alloy, the metal ceramic interface was examined for mode of failure, and the frequency and size of voids were estimated visually.
RESULTS All specimens satisfied the ANSI/ ADA Specification No. 3813 requirement (metal thickness of 0.5 ±0.05 mm), and mean values are listed in Table III. There was a significant dif-
ference in metal thickness among each of the 9 groups shown in Table II (P<.001), attributed to the fact that each group was prepared using a different sheet of wax. Since ob was calculated for each specimen, this difference in specimen thickness was considered. All groups showed satisfactory bonding compatibility according to ANSI/ADA Specification No. 3813 (ob >25 MPa). After the first casting, the mean ob values (SD) for each alloy were: 41.4 (4.1) MPa (Brite Gold XH), 46.4 (4.7) MPa (W-5), and 44.4 (6.0) MPa (IPS d.SIGN 53). Statistical analysis failed to show a significant difference among the 3 alloys for the first casting, nor did it show a significant difference after the 3 castings for W-5 and IPS d.SIGN 53. For Brite Gold XH, there was a significant difference in ob (P<.01) between the first and third castings (Table III). Results from the 2-way ANOVA are shown in Table IV. The root mean square error (RMSE) from the final experimental results was 5.41 MPa. This indicated that a differ-
5BCMF*** Summary of bonding compatibility index (ob) results for tested alloy groups, with values for standard deviations (SD) and 95% confidence limits (CL)
Alloy
Times Cast
Mean Thickness (mm)
Mean τb (MPa)
SD τb (MPa)
95% CL (MPa)
B1
B
1
0.51
41.4 A*
4.1
38.8-44.0
B2
B
2
0.51
46.1 AB
5.2
42.8-49.4
B3
B
3
0.51
48.2 B
5.9
44.5-52.0
W1
W
1
0.51
46.4
4.7
43.2-49.6
W2
W
2
0.50
40.6
5.7
36.5-44.7
W3
W
3
0.48
45.1
4.7
42.2-48.1
d1
d
1
0.51
44.4
6.0
40.1-48.6
d2
d
2
0.51
42.9
6.3
38.9-46.9
d3
d
3
0.48
43.8
5.7
40.2-47.5
Group
Letter codes for alloys: B: Brite Gold XH; W: W-5; and d: IPS d.SIGN 53. *For Brite Gold XH, different uppercase letters correspond to significant differences among 3 groups (individual P<.01). For W-5 and IPS d.SIGN 53, there were no differences among 3 groups for each alloy at overall α=.05 level.
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5BCMF*7 Summary of ANOVA results df
Sum of Squares
Mean Square
F
P
Metal type
2
47
24
0.81
.448
Number of times melted
2
117
58
2.00
.141
Metal type x number of times melted
4
388
97
3.32
.014
Source
1 Interfacial fracture surface for Brite Gold XH after first casting (x500). Scale bar length = 100 μm.
2 Interfacial fracture surface for Brite Gold XH after third casting (x500). Scale bar length = 100 μm.
3 Interfacial fracture surface for W-5 after first casting (x500). Scale bar length = 100 μm.
4 Interfacial fracture surface for W-5 after third casting (x500). Scale bar length = 100 μm.
5 Interfacial fracture surface for IPS d.SIGN 53 after first casting (x500). Scale bar length = 100 μm.
6 Interfacial fracture surface for IPS d.SIGN 53 after third casting (x500). Scale bar length = 100 μm.
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170 ence of 2.6 MPa in ob would be detected as statistically significant, while still maintaining an overall_ error of .05 with a power of 0.95 (` error=0.05). At low magnification (x20), all alloy fracture surfaces at the metal ceramic interface were covered by thin layers of porcelain. At high magnification (x500), it was observed that the frequency and size of voids at the interfaces increased with the number of casting times. The fracture surfaces at the interfaces after the first and third casting, respectively, are shown in Figures 1 and 2 for Brite Gold XH, Figures 3 and 4 for W-5, and Figures 5 and 6 for IPS d.SIGN 53.
DISCUSSION The data supported acceptance of the null hypothesis that there would be no difference in bond strength among the 3 alloys after 1 casting. The data supported rejection of the null hypothesis that recasting would have no effect on the porcelain bond strength for the alloy group Brite Gold XH. The data supported acceptance of the null hypothesis that recasting would have no effect on porcelain bonding for the alloy groups W-5 and IPS d.SIGN 53. According to ANSI/ ADA Specification No. 38, the force that causes fracture (Ffail) of the metal ceramic specimen results in crack propagation from one end of the ceramic layer, and this behavior was observed with the test specimens. The precise value of the force that initiates failure of the metal ceramic specimen depends upon the mode of detection, such as visual observation, an auditory sound, or some more sensitive instrumentation technique. In this study, the highest value of force recorded by the mechanical testing machine before the sudden drop corresponding to bond failure was designated as Ffail, and was used to calculate the values of ob in Table III. The load-deflection plot for each test specimen was also manually marked when a crack was audible and visible with unaided eyes. It was found that
auditory detection agreed with the value of Ffail on the load-deflection plot about 80% of the time, while at other times the auditory signals were not detected. Visual detection of the macroscopic crack emanating from one side of the porcelain usually corresponded to a higher force than Ffail, as might be expected, even with the use of lowmagnification eyeglasses (x2.5). Anusavice et al20 and Lenz et al23 recognized that the ideal metal ceramic bond strength test does not exist. Currently used tests have significant stress concentrations at certain regions of the specimens, or the tensile stress induced in the specimens exceeds the interfacial shear stress. When the latter occurs, tensile failure of the metal ceramic bond is more probable than shear failure. Lenz and Schwarz26 noted that crack propagation perpendicular to the metal ceramic interface could occur in the center of the specimen if sufficiently high tensile stress exists. In the present study, only a crack at one end of the porcelain layer was observed. Calculation of ob based upon ANSI/ADA Specification No. 3813 is dependent on the dimensions of the metal ceramic specimen. The cast alloy strip to which the porcelain is bonded has a thickness of only 0.50 ±0.05 mm, and this thin metal strip can be deformed during dental laboratory manipulations, such as when preparing the wax pattern, grinding the alloy surface, and handling during heat treatment. If the metal strip is not completely flat, firing of porcelain to the convex or concave side may produce difference ob values. In the present study, all cast strips were observed visually to be flat before the application of porcelain. The conventional torch melting used in dental laboratories is technique sensitive, so the experience and skill of the operator are important for the casting to be properly accomplished. This technique does have the advantage of being less expensive and requiring less maintenance than the electric melting techniques. With
The Journal of Prosthetic Dentistry
some casting machines, the alloy is heated by electromagnetic induction or electrical resistance effects,1 but there can be concern about overheating with the former, and accuracy of the optical pyrometer used to determine the molten alloy temperature with the latter. Tuccillo et al2 emphasized that overheating the alloy may result in loss of trace base metal elements that are important for formation of the surface oxide layer needed to achieve optimum porcelain bonding. In the present study, the torchmelting technique was used because it is the most common procedure in dental laboratories. Bonding compatibility between the alloy and porcelain is related to the difference in their thermal expansion coefficients (6_), the test speci-
men geometry, and the porcelain/ metal (P/M) thickness ratio.16,17 Walton and O’Brien17 showed that metal ceramic specimens with greater dimensions, values of 6_, and P/M thickness ratios were more subject to porcelain failures. ANSI/ADA Specification No. 3813 does not include the P/M thickness ratio or the porcelain thickness for the calculation of ob, perhaps because the shear stress developed at the interface is more sensitive to the thickness of the metal than that of the porcelain.16 In the present study, the definitive porcelain thickness of each specimen was not recorded, since it is not used in the calculation of ob. However, during specimen fabrication before application of the glaze, the porcelain thickness was measured using a Boley gauge (Benco Dental) to ensure that it satisfied the specification requirement of 1.1 ±0.1 mm. The results from the present study, in which a decrease in ob after 100% recasting up to 2 times was not observed, are in agreement with previous conclusions by Rasmussen and Doukoudakis5 and some observations by Papazoglou et al.8 Brite Gold XH even showed a significant increase inob after 3 castings. It is hypothesized that recasting may have increased the oxi-
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September 2010 dation of this high-noble alloy (98% Au and Pt), which in turn increased the bonding compatibility index (ob). For the W-5 and IPS d.SIGN 53 alloys, it is hypothesized that the amount of oxidation after initial heat treatment of the first-cast metal was adequate for optimum metal ceramic bonding, and that an increase in the amount of oxidation with subsequent recasting did not further improve the interfacial bond. Alternatively, it is possible that the inadvertent addition of contaminants to the recast alloys may have positively affected the oxide layer and counteracted any loss of the trace base metal elements needed for porcelain adherence. Further studies are needed to determine which of these proposed mechanisms are most important with respect to the effect of recasting on metal ceramic bonding. The fracture surfaces of all test specimens contained adherent porcelain, which indicated the desirable occurrence of cohesive failure through the ceramic, according to the classification by O’Brien.24 A future SEM study using the energy-dispersive x-ray spectrometric (EDS) technique developed by Mackert et al21 is needed to determine the area fraction of adherent porcelain and whether this porcelain adherence is correlated with ob, which was not found by Papazoglou and Brantley.25 The increased frequency and size of voids in the metal at the fracture surface after an increasing number of remeltings (Figs. 1 through 6) is similar to previous results from studies by Reisbick and Brantley3 and Rasmussen and Doukoudakis.5 The value of ob did not change with an increased number of voids, which is also in agreement with results obtained by Rasmussen and Doukoudakis.5 Further SEM study at higher magnification is needed to examine the nature and location of these interfacial voids. Further studies are also needed to determine the changes in the oxide layer for all alloy groups with remelting, and to determine the area fraction of adherent porcelain for metal
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ceramic specimens loaded in biaxial flexure for possible correlation with the relative values of ob. Lastly, while the present study was designed to obtain unambiguous information about the effects of multiple meltings of the 3 selected alloys on metal ceramic bond strength, it is noted that common dental laboratory practice is to add new alloy of the same type to the previously melted alloy when casting the copings for porcelain bonding. The present results show that there is no problem with porcelain compatibility when these 3 noble alloys are remelted completely twice, even without the addition of new alloy. However, without further research, dental laboratories should be cautious about remelting the alloys more times with the addition of new alloy.
CONCLUSIONS Within the limitations of this study, the following conclusions were drawn: 1. All 3 noble alloys after all 3 castings showed satisfactory bonding compatibility with the selected porcelain, according to ANSI/ADA Specification No. 38 (ob>25 MPa). There was no significant difference in the bonding compatibility of all 3 alloys after casting a single time. 2. There were no significant differences in metal ceramic bonding (bonding compatibility (ob) with the selected porcelain) for the Au-Pd alloy W-5 and the Pd-Ag alloy IPS d.SIGN 53 with casting up to 3 times. 3. The high-gold alloy (Brite Gold XH) showed significantly higher bonding compatibility after the third casting, compared to the first casting. 4. SEM examination of the metal ceramic interface showed an increased size and frequency of interfacial voids in each alloy with casting up to 3 times. However, these interfacial defects did not affect the porcelain bonding compatibility (ob) of the W-5 and IPS d.SIGN 53 alloys.
REFERENCES 1. Anusavice KJ. Phillips’ science of dental materials. 11th ed. St. Louis: Elsevier; 2003. p. 330-5, 578, 580-7. 2. Tuccillo JJ, Lichtenberger H, Nielsen JP. Composition stability of gold base dental alloys for different melting techniques. J Dent Res 1974;53:1127-31. 3. Reisbick MH, Brantley WA. Mechanical property and microstructural variations for recast low-gold alloy. Int J Prosthodont 1995;8:346-50. 4. Ayad MF. Compositional stability and marginal accuracy of complete cast crowns made with as-received and recast type III gold alloy. J Prosthet Dent 2002;87:162-6. 5. Rasmussen ST, Doukoudakis AA. The effect of using recast metal on the bond between porcelain and a gold-palladium alloy. J Prosthet Dent 1986;55:447-53. 6. Hong JM, Razzoog ME, Lang BR. The effect of recasting on the oxidation layer of a palladium-silver porcelain alloy. J Prosthet Dent 1988;59:420-5. 7. Jochen DG, Caputo AA, Matyas J. Reuse of silver-palladium ceramic metal. J Prosthet Dent 1991;65:588-91. 8. Papazoglou E, Brantley WA, Johnston WM, Carr AB. Effects of dental laboratory processing variables and in vitro testing medium on the porcelain adherence of high-palladium casting alloys. J Prosthet Dent 1998;79:514-9. 9. Horasawa N, Marek M. The effect of recasting on corrosion of a silver–palladium alloy. Dent Mater 2004;20;352-7. 10.Hesby DA, Kobes P, Garver DG, Pelleu GB Jr. Physical properties of a repeatedly used nonprecious metal alloy. J Prosthet Dent 1980;44:291-3. 11.Presswood RG. Multiple recast of a nickelchromium-beryllium alloy. J Prosthet Dent 1983;50:198-9. 12.Ozdemir S, Arikan A. Effects of recasting on the amount of corrosion products released from two Ni–Cr base metal alloys. Eur J Prosthodont Restor Dent 1998;6:149-53. 13.American National Standard/American Dental Association. ANSI/ADA Specification No. 38-2000. Metal-ceramic systems: 2000. Chicago: American Dental Association; 2000. Available at: http://webstore. ansi.org 14.Shell JS, Nielsen JP. Study of the bond between gold alloys and porcelain. J Dent Res 1962;41:1424-37. 15.Lavine MH, Custer F. Variables affecting the strength of bond between porcelain and gold. J Dent Res 1966;45:32-6. 16.Nielsen JP, Tuccillo JJ. Calculation of interfacial stress in dental porcelain bonded to gold alloy substrate. J Dent Res 1972; 51:1043-7. 17.Walton TR, O’Brien WJ. Thermal stress failure of porcelain bonded to a palladiumsilver alloy. J Dent Res 1985;64:476-80. 18.Kelly M, Asgar K, O’Brien WJ. Tensile strength determination of the interface between porcelain fused to gold. J Biomed Mater Res 1969;3:403-8.
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172 19.Anthony DH, Burnett AP, Smith DL, Brooks MS. Shear test for measuring bonding in cast gold alloy-porcelain composites. J Dent Res 1970;49:27-33. 20.Anusavice KJ, Dehoff PH, Fairhurst CW. Comparative evaluation of ceramic-metal bond tests using finite element stress analysis. J Dent Res 1980;59:608-13. 21.Mackert JR Jr, Parry EE, Hashinger DT, Fairhust CW. Measurement of oxide adherence to PFM alloys. J Dent Res 1984;63:1335-40. 22.Lorenzana RE, Chambless LA, Marker VA, Staffanou RS. Bond strengths of highpalladium content alloys. J Prosthet Dent 1990;64:677-80. 23.Lenz J, Schwarz S, Schwickerath H, Sperner F, Schäfer A. Bond strength of metal-ceramic systems in three-point flexure bond test. J Appl Biomater 1995;6:55-64.
24.O’Brien WJ. Dental porcelains. In: Craig RG, editor. Dental materials review. Ann Arbor: University of Michigan Press; 1977. p. 123-35. 25.Papazoglou E, Brantley WA. Porcelain adherence vs force to failure for palladiumgallium alloys: a critique of metal-ceramic bond testing. Dent Mater 1998;14:112-9. 26.Cohen J. Statistical power analysis for the behavioral sciences. 2nd ed. London: Taylor & Francis; 1988. 27.Erdfelder E, Faul F, Buchner A. GPOWER: a general power analysis program. Behav Res Methods Instrum Comput 1996;28:1-11.
Corresponding author: Dr William A. Brantley Division of Restorative and Prosthetic Dentistry College of Dentistry, The Ohio State University Postle Hall, Room 3005-L 305 W 12th Ave Columbus, OH 43218-2357 Fax: 614-292-9422 E-mail: [email protected] or wbrantle@ columbus.rr.com Acknowledgments The authors thank several staff members at Ivoclar Vivadent, Inc, Amherst, NY, for their expert assistance in preparing and testing the specimens: Patrick McCabe (Manager, Materials Research and Development), Kelly Chlosta (Senior Technician), Ron Fachko (Technician), and Debra Costantino (Technician). Copyright © 2010 by the Editorial Council for The Journal of Prosthetic Dentistry.
Noteworthy Abstracts of the Current Literature Influence of loading frequency on implant failure under cyclic fatigue conditions Karl M, Kelly JR. Dent Mater 2009 Nov;25(11):1426-32. Epub 2009 Jul 29. Objectives. Preliminary studies on implant fatigue testing suggested that fractures were more likely to occur at 2 Hz than at 30 Hz (r2, p<0.05). This investigation explores frequency and base elastic modulus effects on strain, strain rate and failure. Methods. A total of 66 implants were mounted in different base materials (acrylic, glass-filled epoxy, aluminum) and loaded up to 106 cycles per ISO 14801 (20 N to 420-500 N) at frequencies of 2 Hz and 30 Hz (chosen to accelerate frequency as the stressor). Absolute strain magnitudes and strain rates under varying loading conditions and with different base materials were measured using one strain-gauged implant. Failure probability distributions were analyzed by both Weibull and life data analysis. Measured strain was used to validate an FEA model. Fracture surfaces were examined by SEM. Results. Number of failures and failure-rates-per-cycle differed significantly between implants tested at 2 Hz versus 30 Hz (p<0.05). Strain magnitude was independent of frequency. Strain rates were highly correlated with frequency (linear r2>0.99) and differed significantly under failure conditions (420 N): 2 Hz=8292 μstrain/s; (500 N): 30 Hz=80,840 μstrain/s. Measured and FEA-calculated strains were similar. Fracture surfaces were indistinguishable (2 Hz versus 30 Hz). Significance. Fatigue failure was significantly more likely at 2 Hz than 30 Hz whereas base material and loading magnitude seemed to have only minor influence. Absolute strain was identical at these frequencies suggesting strain rate sensitivity for this commercially pure titanium implant. Both the Weibull and SEM analyses support an identical failure mechanism with damage accumulation more severe at lower frequencies, an interpretation consistent with strain rate sensitivity. Reprinted with permission of the Academy of Dental Materials.
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$MJOJDBM*NQMJDBUJPOT Recasting of the 3 types of representative noble alloys evaluated does not adversely affect porcelain bond strength, but increased porosity resulting from recasting may affect other properties. This research was partially supported by NIH/NIDCR (grant DE10147) and by Ivoclar Vivadent, Inc, Amherst, NY. Some results from this study were previously presented at the 35th Annual Meeting of the American Association for Dental Research, Orlando, Fla, and the 2006 Carl O. Boucher Conference, Columbus, Ohio. Based upon a Master of Science thesis submitted by the first author to the Graduate School of The Ohio State University. a Former Resident, Advanced Prosthodontics Program, College of Dentistry, The Ohio State University; Assistant Professor, Department of Restorative Dentistry, University of Missouri-Kansas City School of Dentistry. b Professor, Division of Restorative and Prosthetic Dentistry, College of Dentistry, The Ohio State University. c Associate Professor, Division of Restorative and Prosthetic Dentistry; Associate Director of the Advanced Prosthodontics Program, College of Dentistry, The Ohio State University. d Professor and Director of the Graduate Program in Dental Materials Science, Division of Restorative and Prosthetic Dentistry, College of Dentistry, The Ohio State University. e Director of Materials Research and Development, Ivoclar Vivadent, Inc, Amherst, NY.
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166 Reuse of high-noble and noble alloys by recasting discarded sprues is common practice in the dental laboratory. Authors and manufacturers have recommended that 50% new alloy be used in each casting for noble metal ceramic restorations.1 Remelting noble alloys for metal restorations a small number of times does not cause much change in composition and mechanical properties.2-4 However, loss of trace base metals, such as Zn, In, Sn, and Fe, during recasting of noble alloys for porcelain might be detrimental to their castability and formation of the oxide layer during the initial oxidation step, which could adversely affect porcelain bonding.1 For a high-gold porcelain alloy, Tuccillo et al2 reported progressive and severe iron loss over the course of 5 torch meltings, and recommended the addition of new metal when remelting. For a gold-palladium alloy, Rasmussen and Doukoudakis5 found that, although the use of 85% or more recast metal increased the frequency and size of interfacial voids, there was no evidence that the bond with porcelain was degraded. Effects of remelting have also been reported for palladium-based porcelain alloys. Hong et al6 found that the thickness of the oxide layer and the silver oxide concentration increased after each remelting of a Pd-Ag alloy up to 4 times, with or without the addition of 50% new alloy. Jochen et al7 bonded porcelain to strips of a Pd-Ag alloy containing different percentages of new metal, tested the specimens in 4-point bending, and found that once-cast alloy with the addition of less than 50% new alloy produced significantly lower metal ceramic bond strength. Papazoglou et al8 compared the porcelain adherence to high-palladium alloys (3 Pd-Cu-Ga alloys and 1 Pd-Ga alloy) and a gold-palladium alloy after casting 3 times. While the Au-Pd alloy and 2 Pd-Cu-Ga alloys had no changes in the area fraction of adherent porcelain on the alloy surface of specimens fractured in biaxial flexure, the porcelain attachment to
the 2 other high-palladium alloys was adversely affected. Horasawa and Marek9 found that melting a silverpalladium alloy up to 4 times had little effect on its corrosion susceptibility in artificial saliva and attributed the severe degradation after the fifth melting to decreased gold content on the alloy surface. With the much less expensive Co-Cr,10 Ni-Cr-Be,11 and Ni-Cr12 base metal alloys, remelting a small number of times does not appear to greatly affect their properties, although further study of the effects on porcelain bonding to remelted base metal alloys is required. Because few previous studies6-8 have examined the effects of recasting the alloy on metal ceramic bonding, the rationale for the present study was to investigate this matter with more recently introduced, representative Au-Pd and Pd-Ag alloys, since these alloy types are in widespread use. The methodology in ANSI/ADA Specification No. 3813 was used; thus, a 3-point flexure test was used to evaluate the metal ceramic bonding. Previous investigations of noble metal ceramic bonding examined the importance of alloy composition,14 the use of a metal conditioner and appropriate surface roughness,15 the necessity of closely matching thermal expansion coefficients, and the effects of ceramic and specimen geometry.16,17 The appropriate test to evaluate metal ceramic bonding14,15,18-23 has been a matter of extensive discussion for several decades. The recommendation by O’Brien24 to focus on classification of failure modes for metal ceramic specimens and the optimum cohesive failure through porcelain, rather than to measure bond strength, has been replaced in ANSI/ADA Specification No. 3813 by a 3-point bending test with finite element analysis advocated by Lenz et al.23 Results from the porcelain adherence test, in which the area fraction of adherent porcelain is determined from metal ceramic specimens fractured in biaxial flexure, have been compared by Papazoglou and Brantley25 for 4 Pd-Ga alloys and a
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Pd-Ag alloy with the values of force to failure from the 3-point bending test in ANSI/ADA Specification No. 38, and no correlation was observed. The purposes of this study were to measure the bond strength of 3 representative noble alloys to a dental porcelain, using the protocol in ANSI/ ADA Specification No. 38,13 to determine if casting up to 3 times affected the metal ceramic bond strength of each alloy, and to investigate the fracture mode by using scanning electron microscopy (SEM). The null hypotheses were that there would be no difference in the porcelain bond strengths among the 3 originally cast alloys, and that recasting each of the alloys would have no effect on the bond strength.
MATERIAL AND METHODS Three representative noble metal alloys (Brite Gold XH, W-5, and IPS d.SIGN 53; Ivoclar Vivadent, Inc, Amherst, NY) were selected for this study. Brite Gold XH is a high-gold alloy, W-5 is a Au-Pd alloy, and IPS d.SIGN 53 is a Pd-Ag alloy. Their nominal compositions, obtained from the manufacturer, are presented in Table I. Metal strips, 25 x 3 x 0.5 mm, were cast from wax patterns cut from wax sheets of 0.5-mm thickness (Corning Wax, Ronkonkoma, NY), in accordance with ANSI/ADA Specification No. 38.13 Wax patterns were invested with a phosphate-bonded investment (Sure-Vest High Heat; Ivoclar Vivadent, Inc). Alloys were melted in individual ceramic crucibles with a multi-orifice propane-oxygen torch and cast with a standard broken-arm centrifugal casting machine (Centrifico; Kerr Corp, Orange, Calif ). All castings were bench-cooled to room temperature, followed by devesting and airborne-particle abrasion with 50-μm Al2O3 (Sterngold Dental, Attleboro, Mass). The buttons and sprues from the first and second castings were airborne-particle abraded and ultrasonically cleaned in distilled water, followed by visual inspection to
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5BCMF* Compositions of tested alloys provided by manufacturer (wt%) Alloy
Au
Brite Gold XH
88.9
W-5
52.2
IPS d.SIGN 53
Pd
Ag
Pt
Sn
In
Ir
Fe
Li
9.0
<1.0
<1.0
<1.0
<1.0
<1.0
2.7
<1.0
<1.0
7.7
1.7
26.0
17.1
<1.0
53.8
34.9
<1.0
Zn
1.2
Re
Ru
Mn <1.0
<1.0 <1.0
<1.0
<1.0
5BCMF** Test specimen groups (n=12) Specimen Group
Metal Type
Times Cast
B1
Brite Gold XH
1
B2
Brite Gold XH
2
B3
Brite Gold XH
3
W1
W-5
1
W2
W-5
2
W3
W-5
3
d1
IPS d.SIGN 53
1
d2
IPS d.SIGN 53
2
d3
IPS d.SIGN 53
3
Letter codes for alloys: B: Brite Gold XH; W: W-5; and d: IPS d.SIGN 53.
ensure that remaining investment materials were removed, before they were cast for the second and third times, respectively. No new metal was added for the second and third castings. Nine groups of test specimens (n=12) were prepared (Table II) after initial experiments with a power analysis indicated that this sample size would be a conservative number for obtaining statistically significant results. The power analysis26 used a clinically plausible change of 20% in shear bond strength and a power of 80% (`=0.20) to estimate the sample size. The cast alloy strips were finished with a laboratory handpiece (Z 500 Labo Motors; NSK, Kanuma, Japan), ultrasonically cleaned in distilled water, and oxidized following the recommendations of the manufacturer. Porcelain (IPS InLine; Ivoclar Vivadent, Inc), recommended by the manufacturer as compatible with the alloys selected,
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was used to prepare the metal ceramic specimens. Two layers of opaque porcelain with a total thickness of 0.1 mm were applied over a rectangular area of 8 x 3 mm in the center of one side of each cast metal specimen strip. Two layers of body porcelain were subsequently applied to achieve a rectangular porcelain shape having a thickness of 1.1 ±0.1 mm after firing, as measured by a Boley gauge (Benco no. 1162-994; Benco Dental, Pittston, Pa). Final glaze firing was then performed according to the recommendations of the manufacturer. All specimens were subjected to 3-point-flexure following the protocol in ANSI/ADA Specification No. 38. The metal ceramic specimens were supported by a jig (20-mm distance between supports) with the ceramic positioned symmetrically on the side opposite the applied load. The surface of the loading piston in contact with
the specimen was 1 mm wide. The apparatus for bond testing was attached to a screw-driven mechanical testing machine (model 4204; Instron Corp, Norwood, Mass). The crosshead speed was 1.5 mm/min, and the force on the test specimen was recorded up to failure. The fracture force (Ffail) for each specimen corresponded to the formation of a debonding crack that was visually observed at one end of the ceramic layer, and which was also noted on the load-deflection plot as a sudden drop in force. The metal ceramic bond strength was calculated as the bonding compatibility index (ob), following the methodology in ANSI/ADA Specification No. 38.13 The thickness of all specimens was measured at both ends, and the mean values for each specimen were subjected to ANOVA. The mean thickness values were used to calculateob, along with the value of
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168 Young’s modulus (E), for each alloy provided by the manufacturer (Brite Gold XH, 107 GPa; W-5, 118 GPa; IPS d.SIGN 53, 132 GPa). After ob was calculated for each specimen, 2-way ANOVA (factors: alloy and number of times cast), followed by the RyanEinot-Gabriel-Welsch (REGWQ) multiple range test, was performed to compare ob after the first casting for the 3 alloys, and ob for each alloy after casting 3 different times. Since the REGWQ test was applied 4 times to the experimental data and it is only appropriate to apply this test to the data once, a standard Bonferroni correction, that is, setting an _ value of .01 to each REGWQ test, allowed an experimental _ error of .05 to be safely maintained. Statistical software (SAS 9.2; SAS Institute, Inc, Cary, NC) was used to perform these analyses. One specimen from Group W1, 2 specimens from Group W2, and 2 specimens from Group d1 (Table II) cracked during handling and were excluded from testing. No additional new specimens were made. A post hoc sensitivity analysis
(G*Power, v. 3.1.2)27 was also performed on the final experimental data to validate the initial conservative power analysis, in which the objective was to maintain an overall or experimentwise _ error of .05. The resultant-effect size determined the difference that would be detected as statistically significant by the relationship: effect size = (detectable difference)/(root mean square error). One representative specimen from each group was selected for SEM examination (model JSM-820; JEOL, Ltd, Tokyo, Japan). The ceramic layer previously cracked during the bond strength testing was carefully removed from the underlying alloy, the metal ceramic interface was examined for mode of failure, and the frequency and size of voids were estimated visually.
RESULTS All specimens satisfied the ANSI/ ADA Specification No. 3813 requirement (metal thickness of 0.5 ±0.05 mm), and mean values are listed in Table III. There was a significant dif-
ference in metal thickness among each of the 9 groups shown in Table II (P<.001), attributed to the fact that each group was prepared using a different sheet of wax. Since ob was calculated for each specimen, this difference in specimen thickness was considered. All groups showed satisfactory bonding compatibility according to ANSI/ADA Specification No. 3813 (ob >25 MPa). After the first casting, the mean ob values (SD) for each alloy were: 41.4 (4.1) MPa (Brite Gold XH), 46.4 (4.7) MPa (W-5), and 44.4 (6.0) MPa (IPS d.SIGN 53). Statistical analysis failed to show a significant difference among the 3 alloys for the first casting, nor did it show a significant difference after the 3 castings for W-5 and IPS d.SIGN 53. For Brite Gold XH, there was a significant difference in ob (P<.01) between the first and third castings (Table III). Results from the 2-way ANOVA are shown in Table IV. The root mean square error (RMSE) from the final experimental results was 5.41 MPa. This indicated that a differ-
5BCMF*** Summary of bonding compatibility index (ob) results for tested alloy groups, with values for standard deviations (SD) and 95% confidence limits (CL)
Alloy
Times Cast
Mean Thickness (mm)
Mean τb (MPa)
SD τb (MPa)
95% CL (MPa)
B1
B
1
0.51
41.4 A*
4.1
38.8-44.0
B2
B
2
0.51
46.1 AB
5.2
42.8-49.4
B3
B
3
0.51
48.2 B
5.9
44.5-52.0
W1
W
1
0.51
46.4
4.7
43.2-49.6
W2
W
2
0.50
40.6
5.7
36.5-44.7
W3
W
3
0.48
45.1
4.7
42.2-48.1
d1
d
1
0.51
44.4
6.0
40.1-48.6
d2
d
2
0.51
42.9
6.3
38.9-46.9
d3
d
3
0.48
43.8
5.7
40.2-47.5
Group
Letter codes for alloys: B: Brite Gold XH; W: W-5; and d: IPS d.SIGN 53. *For Brite Gold XH, different uppercase letters correspond to significant differences among 3 groups (individual P<.01). For W-5 and IPS d.SIGN 53, there were no differences among 3 groups for each alloy at overall α=.05 level.
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5BCMF*7 Summary of ANOVA results df
Sum of Squares
Mean Square
F
P
Metal type
2
47
24
0.81
.448
Number of times melted
2
117
58
2.00
.141
Metal type x number of times melted
4
388
97
3.32
.014
Source
1 Interfacial fracture surface for Brite Gold XH after first casting (x500). Scale bar length = 100 μm.
2 Interfacial fracture surface for Brite Gold XH after third casting (x500). Scale bar length = 100 μm.
3 Interfacial fracture surface for W-5 after first casting (x500). Scale bar length = 100 μm.
4 Interfacial fracture surface for W-5 after third casting (x500). Scale bar length = 100 μm.
5 Interfacial fracture surface for IPS d.SIGN 53 after first casting (x500). Scale bar length = 100 μm.
6 Interfacial fracture surface for IPS d.SIGN 53 after third casting (x500). Scale bar length = 100 μm.
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170 ence of 2.6 MPa in ob would be detected as statistically significant, while still maintaining an overall_ error of .05 with a power of 0.95 (` error=0.05). At low magnification (x20), all alloy fracture surfaces at the metal ceramic interface were covered by thin layers of porcelain. At high magnification (x500), it was observed that the frequency and size of voids at the interfaces increased with the number of casting times. The fracture surfaces at the interfaces after the first and third casting, respectively, are shown in Figures 1 and 2 for Brite Gold XH, Figures 3 and 4 for W-5, and Figures 5 and 6 for IPS d.SIGN 53.
DISCUSSION The data supported acceptance of the null hypothesis that there would be no difference in bond strength among the 3 alloys after 1 casting. The data supported rejection of the null hypothesis that recasting would have no effect on the porcelain bond strength for the alloy group Brite Gold XH. The data supported acceptance of the null hypothesis that recasting would have no effect on porcelain bonding for the alloy groups W-5 and IPS d.SIGN 53. According to ANSI/ ADA Specification No. 38, the force that causes fracture (Ffail) of the metal ceramic specimen results in crack propagation from one end of the ceramic layer, and this behavior was observed with the test specimens. The precise value of the force that initiates failure of the metal ceramic specimen depends upon the mode of detection, such as visual observation, an auditory sound, or some more sensitive instrumentation technique. In this study, the highest value of force recorded by the mechanical testing machine before the sudden drop corresponding to bond failure was designated as Ffail, and was used to calculate the values of ob in Table III. The load-deflection plot for each test specimen was also manually marked when a crack was audible and visible with unaided eyes. It was found that
auditory detection agreed with the value of Ffail on the load-deflection plot about 80% of the time, while at other times the auditory signals were not detected. Visual detection of the macroscopic crack emanating from one side of the porcelain usually corresponded to a higher force than Ffail, as might be expected, even with the use of lowmagnification eyeglasses (x2.5). Anusavice et al20 and Lenz et al23 recognized that the ideal metal ceramic bond strength test does not exist. Currently used tests have significant stress concentrations at certain regions of the specimens, or the tensile stress induced in the specimens exceeds the interfacial shear stress. When the latter occurs, tensile failure of the metal ceramic bond is more probable than shear failure. Lenz and Schwarz26 noted that crack propagation perpendicular to the metal ceramic interface could occur in the center of the specimen if sufficiently high tensile stress exists. In the present study, only a crack at one end of the porcelain layer was observed. Calculation of ob based upon ANSI/ADA Specification No. 3813 is dependent on the dimensions of the metal ceramic specimen. The cast alloy strip to which the porcelain is bonded has a thickness of only 0.50 ±0.05 mm, and this thin metal strip can be deformed during dental laboratory manipulations, such as when preparing the wax pattern, grinding the alloy surface, and handling during heat treatment. If the metal strip is not completely flat, firing of porcelain to the convex or concave side may produce difference ob values. In the present study, all cast strips were observed visually to be flat before the application of porcelain. The conventional torch melting used in dental laboratories is technique sensitive, so the experience and skill of the operator are important for the casting to be properly accomplished. This technique does have the advantage of being less expensive and requiring less maintenance than the electric melting techniques. With
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some casting machines, the alloy is heated by electromagnetic induction or electrical resistance effects,1 but there can be concern about overheating with the former, and accuracy of the optical pyrometer used to determine the molten alloy temperature with the latter. Tuccillo et al2 emphasized that overheating the alloy may result in loss of trace base metal elements that are important for formation of the surface oxide layer needed to achieve optimum porcelain bonding. In the present study, the torchmelting technique was used because it is the most common procedure in dental laboratories. Bonding compatibility between the alloy and porcelain is related to the difference in their thermal expansion coefficients (6_), the test speci-
men geometry, and the porcelain/ metal (P/M) thickness ratio.16,17 Walton and O’Brien17 showed that metal ceramic specimens with greater dimensions, values of 6_, and P/M thickness ratios were more subject to porcelain failures. ANSI/ADA Specification No. 3813 does not include the P/M thickness ratio or the porcelain thickness for the calculation of ob, perhaps because the shear stress developed at the interface is more sensitive to the thickness of the metal than that of the porcelain.16 In the present study, the definitive porcelain thickness of each specimen was not recorded, since it is not used in the calculation of ob. However, during specimen fabrication before application of the glaze, the porcelain thickness was measured using a Boley gauge (Benco Dental) to ensure that it satisfied the specification requirement of 1.1 ±0.1 mm. The results from the present study, in which a decrease in ob after 100% recasting up to 2 times was not observed, are in agreement with previous conclusions by Rasmussen and Doukoudakis5 and some observations by Papazoglou et al.8 Brite Gold XH even showed a significant increase inob after 3 castings. It is hypothesized that recasting may have increased the oxi-
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September 2010 dation of this high-noble alloy (98% Au and Pt), which in turn increased the bonding compatibility index (ob). For the W-5 and IPS d.SIGN 53 alloys, it is hypothesized that the amount of oxidation after initial heat treatment of the first-cast metal was adequate for optimum metal ceramic bonding, and that an increase in the amount of oxidation with subsequent recasting did not further improve the interfacial bond. Alternatively, it is possible that the inadvertent addition of contaminants to the recast alloys may have positively affected the oxide layer and counteracted any loss of the trace base metal elements needed for porcelain adherence. Further studies are needed to determine which of these proposed mechanisms are most important with respect to the effect of recasting on metal ceramic bonding. The fracture surfaces of all test specimens contained adherent porcelain, which indicated the desirable occurrence of cohesive failure through the ceramic, according to the classification by O’Brien.24 A future SEM study using the energy-dispersive x-ray spectrometric (EDS) technique developed by Mackert et al21 is needed to determine the area fraction of adherent porcelain and whether this porcelain adherence is correlated with ob, which was not found by Papazoglou and Brantley.25 The increased frequency and size of voids in the metal at the fracture surface after an increasing number of remeltings (Figs. 1 through 6) is similar to previous results from studies by Reisbick and Brantley3 and Rasmussen and Doukoudakis.5 The value of ob did not change with an increased number of voids, which is also in agreement with results obtained by Rasmussen and Doukoudakis.5 Further SEM study at higher magnification is needed to examine the nature and location of these interfacial voids. Further studies are also needed to determine the changes in the oxide layer for all alloy groups with remelting, and to determine the area fraction of adherent porcelain for metal
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ceramic specimens loaded in biaxial flexure for possible correlation with the relative values of ob. Lastly, while the present study was designed to obtain unambiguous information about the effects of multiple meltings of the 3 selected alloys on metal ceramic bond strength, it is noted that common dental laboratory practice is to add new alloy of the same type to the previously melted alloy when casting the copings for porcelain bonding. The present results show that there is no problem with porcelain compatibility when these 3 noble alloys are remelted completely twice, even without the addition of new alloy. However, without further research, dental laboratories should be cautious about remelting the alloys more times with the addition of new alloy.
CONCLUSIONS Within the limitations of this study, the following conclusions were drawn: 1. All 3 noble alloys after all 3 castings showed satisfactory bonding compatibility with the selected porcelain, according to ANSI/ADA Specification No. 38 (ob>25 MPa). There was no significant difference in the bonding compatibility of all 3 alloys after casting a single time. 2. There were no significant differences in metal ceramic bonding (bonding compatibility (ob) with the selected porcelain) for the Au-Pd alloy W-5 and the Pd-Ag alloy IPS d.SIGN 53 with casting up to 3 times. 3. The high-gold alloy (Brite Gold XH) showed significantly higher bonding compatibility after the third casting, compared to the first casting. 4. SEM examination of the metal ceramic interface showed an increased size and frequency of interfacial voids in each alloy with casting up to 3 times. However, these interfacial defects did not affect the porcelain bonding compatibility (ob) of the W-5 and IPS d.SIGN 53 alloys.
REFERENCES 1. Anusavice KJ. Phillips’ science of dental materials. 11th ed. St. Louis: Elsevier; 2003. p. 330-5, 578, 580-7. 2. Tuccillo JJ, Lichtenberger H, Nielsen JP. Composition stability of gold base dental alloys for different melting techniques. J Dent Res 1974;53:1127-31. 3. Reisbick MH, Brantley WA. Mechanical property and microstructural variations for recast low-gold alloy. Int J Prosthodont 1995;8:346-50. 4. Ayad MF. Compositional stability and marginal accuracy of complete cast crowns made with as-received and recast type III gold alloy. J Prosthet Dent 2002;87:162-6. 5. Rasmussen ST, Doukoudakis AA. The effect of using recast metal on the bond between porcelain and a gold-palladium alloy. J Prosthet Dent 1986;55:447-53. 6. Hong JM, Razzoog ME, Lang BR. The effect of recasting on the oxidation layer of a palladium-silver porcelain alloy. J Prosthet Dent 1988;59:420-5. 7. Jochen DG, Caputo AA, Matyas J. Reuse of silver-palladium ceramic metal. J Prosthet Dent 1991;65:588-91. 8. Papazoglou E, Brantley WA, Johnston WM, Carr AB. Effects of dental laboratory processing variables and in vitro testing medium on the porcelain adherence of high-palladium casting alloys. J Prosthet Dent 1998;79:514-9. 9. Horasawa N, Marek M. The effect of recasting on corrosion of a silver–palladium alloy. Dent Mater 2004;20;352-7. 10.Hesby DA, Kobes P, Garver DG, Pelleu GB Jr. Physical properties of a repeatedly used nonprecious metal alloy. J Prosthet Dent 1980;44:291-3. 11.Presswood RG. Multiple recast of a nickelchromium-beryllium alloy. J Prosthet Dent 1983;50:198-9. 12.Ozdemir S, Arikan A. Effects of recasting on the amount of corrosion products released from two Ni–Cr base metal alloys. Eur J Prosthodont Restor Dent 1998;6:149-53. 13.American National Standard/American Dental Association. ANSI/ADA Specification No. 38-2000. Metal-ceramic systems: 2000. Chicago: American Dental Association; 2000. Available at: http://webstore. ansi.org 14.Shell JS, Nielsen JP. Study of the bond between gold alloys and porcelain. J Dent Res 1962;41:1424-37. 15.Lavine MH, Custer F. Variables affecting the strength of bond between porcelain and gold. J Dent Res 1966;45:32-6. 16.Nielsen JP, Tuccillo JJ. Calculation of interfacial stress in dental porcelain bonded to gold alloy substrate. J Dent Res 1972; 51:1043-7. 17.Walton TR, O’Brien WJ. Thermal stress failure of porcelain bonded to a palladiumsilver alloy. J Dent Res 1985;64:476-80. 18.Kelly M, Asgar K, O’Brien WJ. Tensile strength determination of the interface between porcelain fused to gold. J Biomed Mater Res 1969;3:403-8.
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24.O’Brien WJ. Dental porcelains. In: Craig RG, editor. Dental materials review. Ann Arbor: University of Michigan Press; 1977. p. 123-35. 25.Papazoglou E, Brantley WA. Porcelain adherence vs force to failure for palladiumgallium alloys: a critique of metal-ceramic bond testing. Dent Mater 1998;14:112-9. 26.Cohen J. Statistical power analysis for the behavioral sciences. 2nd ed. London: Taylor & Francis; 1988. 27.Erdfelder E, Faul F, Buchner A. GPOWER: a general power analysis program. Behav Res Methods Instrum Comput 1996;28:1-11.
Corresponding author: Dr William A. Brantley Division of Restorative and Prosthetic Dentistry College of Dentistry, The Ohio State University Postle Hall, Room 3005-L 305 W 12th Ave Columbus, OH 43218-2357 Fax: 614-292-9422 E-mail: [email protected] or wbrantle@ columbus.rr.com Acknowledgments The authors thank several staff members at Ivoclar Vivadent, Inc, Amherst, NY, for their expert assistance in preparing and testing the specimens: Patrick McCabe (Manager, Materials Research and Development), Kelly Chlosta (Senior Technician), Ron Fachko (Technician), and Debra Costantino (Technician). Copyright © 2010 by the Editorial Council for The Journal of Prosthetic Dentistry.
Noteworthy Abstracts of the Current Literature Influence of loading frequency on implant failure under cyclic fatigue conditions Karl M, Kelly JR. Dent Mater 2009 Nov;25(11):1426-32. Epub 2009 Jul 29. Objectives. Preliminary studies on implant fatigue testing suggested that fractures were more likely to occur at 2 Hz than at 30 Hz (r2, p<0.05). This investigation explores frequency and base elastic modulus effects on strain, strain rate and failure. Methods. A total of 66 implants were mounted in different base materials (acrylic, glass-filled epoxy, aluminum) and loaded up to 106 cycles per ISO 14801 (20 N to 420-500 N) at frequencies of 2 Hz and 30 Hz (chosen to accelerate frequency as the stressor). Absolute strain magnitudes and strain rates under varying loading conditions and with different base materials were measured using one strain-gauged implant. Failure probability distributions were analyzed by both Weibull and life data analysis. Measured strain was used to validate an FEA model. Fracture surfaces were examined by SEM. Results. Number of failures and failure-rates-per-cycle differed significantly between implants tested at 2 Hz versus 30 Hz (p<0.05). Strain magnitude was independent of frequency. Strain rates were highly correlated with frequency (linear r2>0.99) and differed significantly under failure conditions (420 N): 2 Hz=8292 μstrain/s; (500 N): 30 Hz=80,840 μstrain/s. Measured and FEA-calculated strains were similar. Fracture surfaces were indistinguishable (2 Hz versus 30 Hz). Significance. Fatigue failure was significantly more likely at 2 Hz than 30 Hz whereas base material and loading magnitude seemed to have only minor influence. Absolute strain was identical at these frequencies suggesting strain rate sensitivity for this commercially pure titanium implant. Both the Weibull and SEM analyses support an identical failure mechanism with damage accumulation more severe at lower frequencies, an interpretation consistent with strain rate sensitivity. Reprinted with permission of the Academy of Dental Materials.
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