- Email: [email protected]
Ag−Pd alloys for resin-bonded retainers
Ag-Pd alloys for resin-bonded retainers
J.S.Jensson, K. F. Leinfelder, W. R. Lacefield, J. E. Lemons University of Alabama School of Dentistry, Birmingham, USA.
Jensson JS, Leinfelder KF, Lacefield WR, Lemons JE. Ag-Pd alloys for resinbonded retainers. Dent Mater 1986: 220-224. Abstract - A method for electrolytically etching silver-palladium alloys has been investigated. All the alloys included in the study responded quite favorably to the process. Bond strength values were determined for a composite-resin luting agent as well as a laboratory-cured composite-resin veneer. Bond strengths for the various Ag-Pd alloys in conjunction with the resin-bonding agent ranged from approximately 8-15 MPa. Values for the light-cured composite-resin veneer ranged from 5.5-12.0 MPa.
Resin-bonded fixed partial dentures are a recent development in prosthodontics. As compared to conventional retainers, they offer a number of advantages. Most notably, these include conservation of tooth structure, esthetics and economics (1). Until recently only base metal alloys, principally those containing nickel and chromium, have been used for the acidetched retainers (2). This type of alloy has been the material of choice since some of the surface metal can be selectively removed with the appropriate acid etchants. This in turn creates irregular or tortuous surface features into which the luting agent can penetrate. Unfortunately, the nickel-chromium alloys exhibit a number of disadvantages. Allergenic responses to the nickel, for example, have been reported (3). Also, from a mechanical properties point of view, these alloys possess low elongation or ductility values (4). Furthermore, as compared to precious alloys they are considerably more technique sensitive. It was the purpose of this study, therefore, to investigate the possibility of using silver-palladium alloys as alternatives to nickel-chromium for resin bonded retainers.
Material and methods A series of proprietary alloys were selected for investigation. The alloys, their respective manufacturer and nominal compositions are given in Table 1. A base metal alloy containing nickel-chromium was also included for the purpose of comparison. A general summary of the physical and mechan-
ical properties of these alloys is given in Table 1. All alloys included in the study were cast in the form of disks 12 mm in diameter and 1.5 in thickness. Wax patterns were cut from a sheet of baseplate wax with a cork borer, painted with a wetting agent*, invested in a phosphatebonded investment~, sprued on their edges and invested in multiples of 5 for casting. Five replicate specimens were fabricated for each variable investigated. All castings were made using a multiple orifice, gas/oxygen torch and a centrifugal casting macbine. Casting procedures were carried out in accordance * Wip Mix Corporation, Louisville, Kentucky High Span, Jelenko Co., Inc. Armonk, NY
Key words: silver-palladium alloys, electrolytic etching, laboratory light-cured resin veneer, resin-bonded retainer luting agents Karl F. Leinfelder, Department of Biomaterials, Box 49 University Station, University of Alabama School of Dentistry, University of Alabama at Birmingham, Birmingham, Alabama 35294, USA.
Received December 5, 1985; accepted January 7, 1986
with the manufacturers' recommendations. The disks were cut from the sprues, sandblasted with alumina (50 V) and cleaned in ethanol, the disks were then connected to the end of an insulated copper wire by means of sticky wax. Silver paint was used at the connection site to insure good electrical conduction. The samples were again sandblasted (50 ~ alumina) and subsequently rinsed with tap water. The composition of the etching agents for the silverpalladium alloys as well as the base metal control are given in Table 2. The etching time for all the silver-palladium alloys ranged between 1.5 and 3 min. During the etching process the current was adjusted to 300 ma/cmL Ultrasonic stirring was used during etching of the silver-palladium alloys only. After etching, the samples were rinsed in tap water, and then
Table 1. Alloys and compositions. Alloy
W.L.W.
Manufacturer
Williams Gold Co. Buffalo, NY Albacst Jelenco Co. Armonk, NY No. 25 W.E. Mowrey Co. St. Paul, MN Baker C/B Engelhard Industries Carteret, NJ Litecast B Williams Gold Co. Buffalo, NY
Nominal Melting Hardness Elongation Yield Strength Composition Range (VHN) (%) 0.2% offset (%) (~ MPa (psi) Ag: 71 Pd: 25
10461132
S 150 H 170
S 5.0 H 6.5
S 321 (46,600) H 338 (48,400)
Ag: Pd: Ag: Pd:
70 25 71 25
10211099 10821135
S H S H
S H S H
S H S H
Ag: 74 Pd: 25
10821121
S 141 H 161
S 10 H 6.5
S 320 (46,500) H 333 (48,300)
Ni: Cr: Be: Mo:
11601271
S 335
S 12
S 772 H 806
77.5 12.5 4.9 1.7
130 140 131 176
10 8 5.5 5.2
262 (38,000) 324 (47,000) 327 (47,500) 338 (49,!00)
(112,ooo) (116,8oo)
Ag-Pd alloys for resin-bonded retainers Table 2. Etching and cleaning procedures silver-palladium alloys. Etching solution*
Etching times
Current density
NaNO3 NaF HNO 3 H20
3, 1.5, 1 min.
300 ma/cm2
Cleaning time
Mode of cleaning
15 min.
ultrasonic stirring
Litecast B Etching time
Mode of cleaning
6 min.
no stirring
Cleaning solution
Cleaning time
Mode of cleaning
HC1
10 min.
ultrasonic stirring
50 g 50 g 4 ml (conc.) to make a liter of solution
Cleaning solution H2SO 4 H3PO 4
(conc.) 50 ml (conc.) 50 ml
H/O
to make a liter of solution
Etching solution H2SO4 H20 MeOH
100 ml 1000 ml 120 ml
18%
*Etchant composition recommended by Englehard, Baker Division, Carteret, New Jersey.
placed in another beaker containing the cleaning solution. The composition of this solution is provided in Table 2. All samples were cleaned for 15 min using ultrasonic stirring followed by rinsing with tap water. The control alloy, Litecest B, was electrolytically etched using the method described by Simonsen, Thompson and Barreck (1). After evaluating the etched surfaced with the scanning electron microscope (SEM), the disks were prepared for bonding. A modified Kemper/Kilian device (5) was selected for all bonding determinations. This apparatus was chosen since it assured excellent alignment of the resin and metal surfaces during polymerization of the composite resin as well as during the tensile testing procedure. Certain modifications of the apparatus were made so that the disks could readily be positioned into place. A photograph of the mounting device used for the disks is illustrated in Fig. la. Shown in Fig. l b is the bond alignment apparatus with the disk and composite resin in place. A composite resin luting agent, Comspan~, was injected into the brass cylinder. The internal walls of the cylinder were tapered to prevent the composite sample from being extracted from the device during testing. The filled cylinder was inserted into the upper member of the aligning device. It was maintained in this position without * Caulk co., Milford, Delaware 15"
Fig. la. Metal disk and holding device.
contacting the electrolytically etched disk during preparation and application of the bonding agent to its surface. Next, the cylinder was lowered into contact with the disk itself. A t this point a weight of 400 grams was placed on top of the device to insure constant pressure for the duration of the luting procedure. After 15 min the joined samples were removed from the alignment apparatus. Each sample subsequently was thermally cycled 1000 times between temperatures of 5 and 55~ The dwell time at each temperature was one minute. The cross head speed for all bonding tests was 0.05 cm (0.02 in)/minute. In an effort to evaluate the early stages of etching of the silver-palladium, one of the compositions (Baker C/B) was ground and polished using standard metallographic procedures. The etching times used for this evaluation were 2, 4, 10 and 16 s. Scanning electron micrographs of these surfaces were taken at a magnification of 1000x. The next phase of the study dealt with the ability of the composite resin luting agent to penetrate the etched surface of the alloy. Also investigated under the same conditions was the ability of a laboratory photocured microfill, Dentacolor ~, to penetrate the etched metal surface. This material recently has been marketed as a porcelain substitute for metal veneering. After application of the materials to the etched surface, the specimens were embedded in acrylic. They were then sectioned so that the resin-metal interfaces could be evaluated. In addition to an evaluation of resin penetration into the etched surface, the morphologies of the resin tags were also investigated. After electrolytically etching the alloy and then bonding the resin to the surface, the alloy substrate was removed. This was accomplished first by grinding away a portion of the alloy using metallographic paper. The rest of the alloy was then electrolytically etched with the agents previously described. The final remnants of alloy were removed in a bath of 10% hydrochloric acid over a 3-day period. Finally, using energy dispersive spectroscopy (EDS) an attempt was made to identify elements selectively removed from the surface during the etching process.
Fig. lb. Bond alignment apparatus with disk and composite resin in place.
221
Kulzer, Inc., Irvine, CA
222
Jensson et al.
Fig. 2a. Baker C and W.
Fig. 3a. 2s.
Fig. 4a. Comspan.
Fig. 2b. WLW.
Fig. 3b. 4s.
Fig. 4b. Dentacolor. Fig. 4. Photomicrographs of resin-metal interface after etching for 3 min (200x).
Fig. 2c. Albacast.
Fig. 3c. 10 s.
Fig. 2d. Mowrey 25.
Fig. 3d. ,16 s.
Fig. 2. SEM's of silver-palladiumalloys after electrolytic etching for 3 min (250x).
Fig. 3. SEM's of silver-palladium alloy (Baker C and W) after electrolytic etching for various times (1000•
Results
veneer. In each case, a well defined irregular subsurface was generated in three minutes or less. The alloys may be identified as follows: 2a-Baker C&W, 2b, WLW, 2c-Albacast and 2dMowrey No. 25. The differences in microstructure associated with each alloy may have been due to minor differ-
All of the silver-palladium alloys included in this study responded quite favorably to electrolytic etching. As can be seen in Fig. 2a and 2d, the surface microstructures appeared most favorable for the retention of a resin
ences in composition. A n indication of the rate at which certain phases were extracted electrochemically from the surface is illustrated in Fig. 3a-3d. Note the volume of alloy removed from a polished surface in a matter of seconds. The alloy depicted in the last series of scanning electron micrographs is Baker C&W. Photomicrographs of the bonded surfaces after cross sectioning are illustrated in Fig. 4a and 4b. Fig. 4a demonstrates the penetration capability of the resin-bonded luting agent whereas Fig. 4b illustrates the penetration potential for the microfilled resin veneer. The morphology of the resin tags of the luting agent below the electrolytically etched surface is shown in Fig. 5a and 5b. The scanning electron photomicrographs represent the penetration of the resin into Baker C&B after 1.5 and 3.0 min, respectively. Differences in size and shape of the resin tags after the 2 time periods is apparent. Values for retention of the resin luting agent to the etched surface of the various alloys included in the study are illustrated in Fig. 6. Values are presented for all the Ag-Pd alloys when electrolytically etched for 1.5 and 3.0 min. The effect of etching one of the alloys (Baker C/B) for only 1.0 min is also included. Bond strength values for the control alloy (Litecast B) are repre-
Ag-Pd alloys for resin-bonded retainers
293
Table 3. Bond strength of Dentacolor to casting alloys. Alloy
Litecast B Baker C/B No. 25 W.L.W. Albacast
Bond strengths (Mean)
Std. Dev.
Lower limit of multiple contrast (Tukey's test)
(MPa)
(PSI)
(MPa)
(PSI)
(MPa)
(PSI)
17.88~ 12.00J 8.72[ 5.98~ 5.46J
2,590] 1,740J 1,265[ 865~ 790J
4.93 1.64 3.64 3.69 1.49
715 238 528 535 216
11.54 5.66 2.38 --0.36 0.88
1,673 821 345 52 126
Fig. 5a. 90 s etch.
Fig. 5b. 3 min etch. Fig. 5. SEM photomicrograph of resin pen etration into etched metal surfaces. sented by the bar graph located to the far right. The horizontal fines drawn across the illustration represent the range of values commonly reported for bonding of composite resin to etched enamel (6-8). Although the mean bond strength associated with Baker C&B was greater than the mean values of the other Ag-Pd alloys, these results were not different significantly. While 3-min etching appeared to enhance the bond strength, the differences in values obtained at one minute were not statistically significant. The tack of signifiComparison of )ban Bond Strengths of Comspan to Aq-Pd A l l o y s and Litecast B
6 Minutes Etching ) Minutes Etching 1,5 Minutes Etchinq ?n
1 rlinut~ Etching
ATbecast
~(o, ~5
W~L.~.
gak~r C/B
Litoea~( I~
Fig. 6. Bar Graft. Bond strength of resin luting agent to etched metal.
cance, of course, can be attributed to the relatively wide differences in bond strength values. The differences obtained between the control alloy (Litecast B) and the four Ag-Pd alloys however, were statistically significant (p _< 0.05). A n examination of the fracture or failure sites revealed that, although there were differences, a significant number of specimens fractured below the etched surface of the metal. Direct visual and SEM observations revealed a portion of the etched metal surface was retained in the resin luting agent. At the same time the bonding resin could also be detected as tags below the surface of the alloy. Analysis of elemental composition by energy dispersive spectroscopy (EDS) revealed that zinc was the major element depleted from the alloy as a result of etching. Analytical measurements were made at 2 locations on each sample. One reading was taken in the m i d d l e of the section, the other at the etched surface. In each case the area near the etched surface was significantly lower in zinc than that just below it. The bond strengths of Dentacolor to the various alloys included in the study are given in Table 3. As can be seen, values ranged from 5.46 MPa (792 psi) for Albacast to 12.00 MPa (1,740 psi) for Baker C/B. Tensile bond strengths for Liteeast B, the control alloy, averaged 17,80 MPa (2, 580 psi). The variance ratio from the one factor A N O V A test was 11.646 and the critical value at the five percent level of significance was 2.87. There were, therefore, statistically significant differences among the bond strengths attained. The Tukey's test of multiple contrasts is presented in Table 3. The lower limit of Litecast B is higher than the mean of all the silver-palladium alloys except Baker C/B. The bond strength for Liteeast B was, therefore, significantly higher than for No. 25, WLW, and Albacast. There were, on
the other hand, no statistically significant differnces between the bond strengths attained between Litecast B and Baker C/B. The brackets immediately adjacent to each set of values include all those in which there are no significant differences. Discussion All of the Ag-Pd alloys included in this study were effectively etched using appropriate combinations of NaNO3, NaF and HNO3. In each case the surfaces were effectively attacked by the electrolyte in a short period by sequential etching with significant amounts of material selectively removed within several seconds. However, so much undercutting occurred during etching that the cross section of typical Ag-Pd tags was reduced to a point that simultaneous fracture of numerous tags during testing perhaps lowered the measured bond strength considerably. The addition of several percent of zinc to the Ag-Pd base alloy by the manufacturer is primarily responsible for the microscopically rough, tortuous surface of the casting when subjected to electrolytic etching, Results of the EDS analysis which show a depletion of zinc at the etched surface is as expected since the Ag-Pd binary is a solid solution with no significant microstructural features except for grain boundaries and perhaps a cored structure. It should be pointed out, however, that the early etching patterns suggested that the etching process may have been initiated at the grain boundaries. Furthermore, as the process continued, individual, directionally oriented grains or dentrites became apparent. This dentritic structure was even more obvious when the electrolytically etched surfaces were sectioned and examined. The retention of both the composite resin luting agents as well as the laboratory cured microfilled resin veneer to the different metal surfaces varied con-
224
J e n s s o n et al.
siderably. Statistically, however, the differences were not significant. A n examination of the fracture surfaces revealed that the a portion of the etched metal was retained in the luting agent. Such a fracture pattern should represent the optimum condition since the tensile tests then relates to the strength of the resin as well as the projected metal tags. Since higher values were obtained for the nickel base alloy as well as one of the Ag-Pd alloys, it is possible that the metal protrusions into the resin cement are the weakest link in the system. Perhaps, also, the bond strengths of the other alloys could be improved considerably by generating electrolytic etching conditions that favor a more favorable microstructure on the alloy surface (i.e., thicker metal tag diameter but still sufficient mechanical interlocking). Another point to consider is that nickel chromium is considerably stronger than the silver-palladium alloys tested. Thus, for tags of the same diameter, measured bond strength would be lower for the Ag-Pd alloys, based on the assumption that the tags play a major role in determining bond fracture strength. A reasonable criterion for evaluating the success of an alloy used as a metal retainer is that the bond strength between the alloy surface and the luting composite be as high as the bond strength between the composite and a properly etched enamel surface (15-18 MPa). Using this criterion, none of the Ag-Pd alloys tested - etched under the stated conditions in this study - may be acceptable as substitutes for the Ni-Cr alloys currently used for this purpose. One observation discussed earlier was that the microscopic Ag-Pd tags have been observed to fracture during bond
strength testing, indicating that either the alloys were weakened during etching or that the average or minimum cross section of the tags was too small to provide the needed strength. Further research on etching to provide proper tag diameter and increased metal-composite bond strength is necessary if silver-palladium alloys are to be used successfully for metal retainers.
the etching behavior of these alloys is necessary before Ag-Pd alloys can be considered as acceptable substitutes for the Ni-Cr alloys currently used for resin retainers. Acknowledgement - The authors wish to
thank Engelhard, Baker Division for the basic formulation of the electrolytic etchant used in this study.
Conclusions 1. The tensile bond strengths measured for the etched Ag-Pd alloys bonded to composite resin materials ranged from 8-15 MPa. These values are significantly less than values obtained for the Ni-Cr etched alloy surfaces bonded to the same composite resins (17-20 MPa). 2. The composite-resin luting agent (Comspan) and the Dentacolor microfilled resin-veneering material penetrated the etched surfaces satisfactorily, although metal tags were often fractured from the alloy surface during bond testing. Although the bond strengths of the Ag-Pd alloys to the composite resins were relatively low, the results indicate that some degree of direct bonding of precious-metal alloys to tooth structure is possible so that beads and other types of conventional retention may not be necessary. 3. Under the etching conditions developed for this study, the silver-palladium alloys tested did not quite meet the objective of achieving a bond strength with composite equal to that for a typical etched enamelcomposite bond (15-18 MPa). Therefore, from a mechanical strength viewpoint more research on
References 1. Simonsen RY, Thompson V, Barrack G. Etched cast restorations: clinical and laboratory techniques. Chicago: Quintessence, 1983. 2. Thompson VP, Del Castillo ED, Livadiris GJ. Resin-bonded retainers. Part I: Resin bond to electrolytically etched nonprecious alloys. J Prosthet Dent 1983: 50: 771-779. 3. Council on Dental materials, Instruments, and Equipment. Biological effects on nickel-containing dental alloys. J A D A 1982: 104: 501-505. 4. Craig RG. In: Restorative dental materials. 6th ed, St. Louis: CV Mosby, 1980: 330. 5. Kemper RN, Kilian RJ. New system for bond strength testing. J Dent Res Abst 1976:308 55B: B 138. 6. Gottlieb EW, Retief DH, Jamison HC. An optimal concentration of phosphoric acid as an etching agent. Part I: Tensile strength studies. J Prosthet Dent 1982: 48: 48-51. 7. Retief DH, Woods E. Is a low viscosity bonding resin necessary? J Oral Rehab 1981: 8: 255-266. 8. Arends J, Keizer S. In vitro studies on enamel resin systems with special reference to improvement of resin adhesion. Proc Int Symp: The acid etch technique. St. Paul, Minn: North Central Publishing, 1975: 40-49.
J.S.Jensson, K. F. Leinfelder, W. R. Lacefield, J. E. Lemons University of Alabama School of Dentistry, Birmingham, USA.
Jensson JS, Leinfelder KF, Lacefield WR, Lemons JE. Ag-Pd alloys for resinbonded retainers. Dent Mater 1986: 220-224. Abstract - A method for electrolytically etching silver-palladium alloys has been investigated. All the alloys included in the study responded quite favorably to the process. Bond strength values were determined for a composite-resin luting agent as well as a laboratory-cured composite-resin veneer. Bond strengths for the various Ag-Pd alloys in conjunction with the resin-bonding agent ranged from approximately 8-15 MPa. Values for the light-cured composite-resin veneer ranged from 5.5-12.0 MPa.
Resin-bonded fixed partial dentures are a recent development in prosthodontics. As compared to conventional retainers, they offer a number of advantages. Most notably, these include conservation of tooth structure, esthetics and economics (1). Until recently only base metal alloys, principally those containing nickel and chromium, have been used for the acidetched retainers (2). This type of alloy has been the material of choice since some of the surface metal can be selectively removed with the appropriate acid etchants. This in turn creates irregular or tortuous surface features into which the luting agent can penetrate. Unfortunately, the nickel-chromium alloys exhibit a number of disadvantages. Allergenic responses to the nickel, for example, have been reported (3). Also, from a mechanical properties point of view, these alloys possess low elongation or ductility values (4). Furthermore, as compared to precious alloys they are considerably more technique sensitive. It was the purpose of this study, therefore, to investigate the possibility of using silver-palladium alloys as alternatives to nickel-chromium for resin bonded retainers.
Material and methods A series of proprietary alloys were selected for investigation. The alloys, their respective manufacturer and nominal compositions are given in Table 1. A base metal alloy containing nickel-chromium was also included for the purpose of comparison. A general summary of the physical and mechan-
ical properties of these alloys is given in Table 1. All alloys included in the study were cast in the form of disks 12 mm in diameter and 1.5 in thickness. Wax patterns were cut from a sheet of baseplate wax with a cork borer, painted with a wetting agent*, invested in a phosphatebonded investment~, sprued on their edges and invested in multiples of 5 for casting. Five replicate specimens were fabricated for each variable investigated. All castings were made using a multiple orifice, gas/oxygen torch and a centrifugal casting macbine. Casting procedures were carried out in accordance * Wip Mix Corporation, Louisville, Kentucky High Span, Jelenko Co., Inc. Armonk, NY
Key words: silver-palladium alloys, electrolytic etching, laboratory light-cured resin veneer, resin-bonded retainer luting agents Karl F. Leinfelder, Department of Biomaterials, Box 49 University Station, University of Alabama School of Dentistry, University of Alabama at Birmingham, Birmingham, Alabama 35294, USA.
Received December 5, 1985; accepted January 7, 1986
with the manufacturers' recommendations. The disks were cut from the sprues, sandblasted with alumina (50 V) and cleaned in ethanol, the disks were then connected to the end of an insulated copper wire by means of sticky wax. Silver paint was used at the connection site to insure good electrical conduction. The samples were again sandblasted (50 ~ alumina) and subsequently rinsed with tap water. The composition of the etching agents for the silverpalladium alloys as well as the base metal control are given in Table 2. The etching time for all the silver-palladium alloys ranged between 1.5 and 3 min. During the etching process the current was adjusted to 300 ma/cmL Ultrasonic stirring was used during etching of the silver-palladium alloys only. After etching, the samples were rinsed in tap water, and then
Table 1. Alloys and compositions. Alloy
W.L.W.
Manufacturer
Williams Gold Co. Buffalo, NY Albacst Jelenco Co. Armonk, NY No. 25 W.E. Mowrey Co. St. Paul, MN Baker C/B Engelhard Industries Carteret, NJ Litecast B Williams Gold Co. Buffalo, NY
Nominal Melting Hardness Elongation Yield Strength Composition Range (VHN) (%) 0.2% offset (%) (~ MPa (psi) Ag: 71 Pd: 25
10461132
S 150 H 170
S 5.0 H 6.5
S 321 (46,600) H 338 (48,400)
Ag: Pd: Ag: Pd:
70 25 71 25
10211099 10821135
S H S H
S H S H
S H S H
Ag: 74 Pd: 25
10821121
S 141 H 161
S 10 H 6.5
S 320 (46,500) H 333 (48,300)
Ni: Cr: Be: Mo:
11601271
S 335
S 12
S 772 H 806
77.5 12.5 4.9 1.7
130 140 131 176
10 8 5.5 5.2
262 (38,000) 324 (47,000) 327 (47,500) 338 (49,!00)
(112,ooo) (116,8oo)
Ag-Pd alloys for resin-bonded retainers Table 2. Etching and cleaning procedures silver-palladium alloys. Etching solution*
Etching times
Current density
NaNO3 NaF HNO 3 H20
3, 1.5, 1 min.
300 ma/cm2
Cleaning time
Mode of cleaning
15 min.
ultrasonic stirring
Litecast B Etching time
Mode of cleaning
6 min.
no stirring
Cleaning solution
Cleaning time
Mode of cleaning
HC1
10 min.
ultrasonic stirring
50 g 50 g 4 ml (conc.) to make a liter of solution
Cleaning solution H2SO 4 H3PO 4
(conc.) 50 ml (conc.) 50 ml
H/O
to make a liter of solution
Etching solution H2SO4 H20 MeOH
100 ml 1000 ml 120 ml
18%
*Etchant composition recommended by Englehard, Baker Division, Carteret, New Jersey.
placed in another beaker containing the cleaning solution. The composition of this solution is provided in Table 2. All samples were cleaned for 15 min using ultrasonic stirring followed by rinsing with tap water. The control alloy, Litecest B, was electrolytically etched using the method described by Simonsen, Thompson and Barreck (1). After evaluating the etched surfaced with the scanning electron microscope (SEM), the disks were prepared for bonding. A modified Kemper/Kilian device (5) was selected for all bonding determinations. This apparatus was chosen since it assured excellent alignment of the resin and metal surfaces during polymerization of the composite resin as well as during the tensile testing procedure. Certain modifications of the apparatus were made so that the disks could readily be positioned into place. A photograph of the mounting device used for the disks is illustrated in Fig. la. Shown in Fig. l b is the bond alignment apparatus with the disk and composite resin in place. A composite resin luting agent, Comspan~, was injected into the brass cylinder. The internal walls of the cylinder were tapered to prevent the composite sample from being extracted from the device during testing. The filled cylinder was inserted into the upper member of the aligning device. It was maintained in this position without * Caulk co., Milford, Delaware 15"
Fig. la. Metal disk and holding device.
contacting the electrolytically etched disk during preparation and application of the bonding agent to its surface. Next, the cylinder was lowered into contact with the disk itself. A t this point a weight of 400 grams was placed on top of the device to insure constant pressure for the duration of the luting procedure. After 15 min the joined samples were removed from the alignment apparatus. Each sample subsequently was thermally cycled 1000 times between temperatures of 5 and 55~ The dwell time at each temperature was one minute. The cross head speed for all bonding tests was 0.05 cm (0.02 in)/minute. In an effort to evaluate the early stages of etching of the silver-palladium, one of the compositions (Baker C/B) was ground and polished using standard metallographic procedures. The etching times used for this evaluation were 2, 4, 10 and 16 s. Scanning electron micrographs of these surfaces were taken at a magnification of 1000x. The next phase of the study dealt with the ability of the composite resin luting agent to penetrate the etched surface of the alloy. Also investigated under the same conditions was the ability of a laboratory photocured microfill, Dentacolor ~, to penetrate the etched metal surface. This material recently has been marketed as a porcelain substitute for metal veneering. After application of the materials to the etched surface, the specimens were embedded in acrylic. They were then sectioned so that the resin-metal interfaces could be evaluated. In addition to an evaluation of resin penetration into the etched surface, the morphologies of the resin tags were also investigated. After electrolytically etching the alloy and then bonding the resin to the surface, the alloy substrate was removed. This was accomplished first by grinding away a portion of the alloy using metallographic paper. The rest of the alloy was then electrolytically etched with the agents previously described. The final remnants of alloy were removed in a bath of 10% hydrochloric acid over a 3-day period. Finally, using energy dispersive spectroscopy (EDS) an attempt was made to identify elements selectively removed from the surface during the etching process.
Fig. lb. Bond alignment apparatus with disk and composite resin in place.
221
Kulzer, Inc., Irvine, CA
222
Jensson et al.
Fig. 2a. Baker C and W.
Fig. 3a. 2s.
Fig. 4a. Comspan.
Fig. 2b. WLW.
Fig. 3b. 4s.
Fig. 4b. Dentacolor. Fig. 4. Photomicrographs of resin-metal interface after etching for 3 min (200x).
Fig. 2c. Albacast.
Fig. 3c. 10 s.
Fig. 2d. Mowrey 25.
Fig. 3d. ,16 s.
Fig. 2. SEM's of silver-palladiumalloys after electrolytic etching for 3 min (250x).
Fig. 3. SEM's of silver-palladium alloy (Baker C and W) after electrolytic etching for various times (1000•
Results
veneer. In each case, a well defined irregular subsurface was generated in three minutes or less. The alloys may be identified as follows: 2a-Baker C&W, 2b, WLW, 2c-Albacast and 2dMowrey No. 25. The differences in microstructure associated with each alloy may have been due to minor differ-
All of the silver-palladium alloys included in this study responded quite favorably to electrolytic etching. As can be seen in Fig. 2a and 2d, the surface microstructures appeared most favorable for the retention of a resin
ences in composition. A n indication of the rate at which certain phases were extracted electrochemically from the surface is illustrated in Fig. 3a-3d. Note the volume of alloy removed from a polished surface in a matter of seconds. The alloy depicted in the last series of scanning electron micrographs is Baker C&W. Photomicrographs of the bonded surfaces after cross sectioning are illustrated in Fig. 4a and 4b. Fig. 4a demonstrates the penetration capability of the resin-bonded luting agent whereas Fig. 4b illustrates the penetration potential for the microfilled resin veneer. The morphology of the resin tags of the luting agent below the electrolytically etched surface is shown in Fig. 5a and 5b. The scanning electron photomicrographs represent the penetration of the resin into Baker C&B after 1.5 and 3.0 min, respectively. Differences in size and shape of the resin tags after the 2 time periods is apparent. Values for retention of the resin luting agent to the etched surface of the various alloys included in the study are illustrated in Fig. 6. Values are presented for all the Ag-Pd alloys when electrolytically etched for 1.5 and 3.0 min. The effect of etching one of the alloys (Baker C/B) for only 1.0 min is also included. Bond strength values for the control alloy (Litecast B) are repre-
Ag-Pd alloys for resin-bonded retainers
293
Table 3. Bond strength of Dentacolor to casting alloys. Alloy
Litecast B Baker C/B No. 25 W.L.W. Albacast
Bond strengths (Mean)
Std. Dev.
Lower limit of multiple contrast (Tukey's test)
(MPa)
(PSI)
(MPa)
(PSI)
(MPa)
(PSI)
17.88~ 12.00J 8.72[ 5.98~ 5.46J
2,590] 1,740J 1,265[ 865~ 790J
4.93 1.64 3.64 3.69 1.49
715 238 528 535 216
11.54 5.66 2.38 --0.36 0.88
1,673 821 345 52 126
Fig. 5a. 90 s etch.
Fig. 5b. 3 min etch. Fig. 5. SEM photomicrograph of resin pen etration into etched metal surfaces. sented by the bar graph located to the far right. The horizontal fines drawn across the illustration represent the range of values commonly reported for bonding of composite resin to etched enamel (6-8). Although the mean bond strength associated with Baker C&B was greater than the mean values of the other Ag-Pd alloys, these results were not different significantly. While 3-min etching appeared to enhance the bond strength, the differences in values obtained at one minute were not statistically significant. The tack of signifiComparison of )ban Bond Strengths of Comspan to Aq-Pd A l l o y s and Litecast B
6 Minutes Etching ) Minutes Etching 1,5 Minutes Etchinq ?n
1 rlinut~ Etching
ATbecast
~(o, ~5
W~L.~.
gak~r C/B
Litoea~( I~
Fig. 6. Bar Graft. Bond strength of resin luting agent to etched metal.
cance, of course, can be attributed to the relatively wide differences in bond strength values. The differences obtained between the control alloy (Litecast B) and the four Ag-Pd alloys however, were statistically significant (p _< 0.05). A n examination of the fracture or failure sites revealed that, although there were differences, a significant number of specimens fractured below the etched surface of the metal. Direct visual and SEM observations revealed a portion of the etched metal surface was retained in the resin luting agent. At the same time the bonding resin could also be detected as tags below the surface of the alloy. Analysis of elemental composition by energy dispersive spectroscopy (EDS) revealed that zinc was the major element depleted from the alloy as a result of etching. Analytical measurements were made at 2 locations on each sample. One reading was taken in the m i d d l e of the section, the other at the etched surface. In each case the area near the etched surface was significantly lower in zinc than that just below it. The bond strengths of Dentacolor to the various alloys included in the study are given in Table 3. As can be seen, values ranged from 5.46 MPa (792 psi) for Albacast to 12.00 MPa (1,740 psi) for Baker C/B. Tensile bond strengths for Liteeast B, the control alloy, averaged 17,80 MPa (2, 580 psi). The variance ratio from the one factor A N O V A test was 11.646 and the critical value at the five percent level of significance was 2.87. There were, therefore, statistically significant differences among the bond strengths attained. The Tukey's test of multiple contrasts is presented in Table 3. The lower limit of Litecast B is higher than the mean of all the silver-palladium alloys except Baker C/B. The bond strength for Liteeast B was, therefore, significantly higher than for No. 25, WLW, and Albacast. There were, on
the other hand, no statistically significant differnces between the bond strengths attained between Litecast B and Baker C/B. The brackets immediately adjacent to each set of values include all those in which there are no significant differences. Discussion All of the Ag-Pd alloys included in this study were effectively etched using appropriate combinations of NaNO3, NaF and HNO3. In each case the surfaces were effectively attacked by the electrolyte in a short period by sequential etching with significant amounts of material selectively removed within several seconds. However, so much undercutting occurred during etching that the cross section of typical Ag-Pd tags was reduced to a point that simultaneous fracture of numerous tags during testing perhaps lowered the measured bond strength considerably. The addition of several percent of zinc to the Ag-Pd base alloy by the manufacturer is primarily responsible for the microscopically rough, tortuous surface of the casting when subjected to electrolytic etching, Results of the EDS analysis which show a depletion of zinc at the etched surface is as expected since the Ag-Pd binary is a solid solution with no significant microstructural features except for grain boundaries and perhaps a cored structure. It should be pointed out, however, that the early etching patterns suggested that the etching process may have been initiated at the grain boundaries. Furthermore, as the process continued, individual, directionally oriented grains or dentrites became apparent. This dentritic structure was even more obvious when the electrolytically etched surfaces were sectioned and examined. The retention of both the composite resin luting agents as well as the laboratory cured microfilled resin veneer to the different metal surfaces varied con-
224
J e n s s o n et al.
siderably. Statistically, however, the differences were not significant. A n examination of the fracture surfaces revealed that the a portion of the etched metal was retained in the luting agent. Such a fracture pattern should represent the optimum condition since the tensile tests then relates to the strength of the resin as well as the projected metal tags. Since higher values were obtained for the nickel base alloy as well as one of the Ag-Pd alloys, it is possible that the metal protrusions into the resin cement are the weakest link in the system. Perhaps, also, the bond strengths of the other alloys could be improved considerably by generating electrolytic etching conditions that favor a more favorable microstructure on the alloy surface (i.e., thicker metal tag diameter but still sufficient mechanical interlocking). Another point to consider is that nickel chromium is considerably stronger than the silver-palladium alloys tested. Thus, for tags of the same diameter, measured bond strength would be lower for the Ag-Pd alloys, based on the assumption that the tags play a major role in determining bond fracture strength. A reasonable criterion for evaluating the success of an alloy used as a metal retainer is that the bond strength between the alloy surface and the luting composite be as high as the bond strength between the composite and a properly etched enamel surface (15-18 MPa). Using this criterion, none of the Ag-Pd alloys tested - etched under the stated conditions in this study - may be acceptable as substitutes for the Ni-Cr alloys currently used for this purpose. One observation discussed earlier was that the microscopic Ag-Pd tags have been observed to fracture during bond
strength testing, indicating that either the alloys were weakened during etching or that the average or minimum cross section of the tags was too small to provide the needed strength. Further research on etching to provide proper tag diameter and increased metal-composite bond strength is necessary if silver-palladium alloys are to be used successfully for metal retainers.
the etching behavior of these alloys is necessary before Ag-Pd alloys can be considered as acceptable substitutes for the Ni-Cr alloys currently used for resin retainers. Acknowledgement - The authors wish to
thank Engelhard, Baker Division for the basic formulation of the electrolytic etchant used in this study.
Conclusions 1. The tensile bond strengths measured for the etched Ag-Pd alloys bonded to composite resin materials ranged from 8-15 MPa. These values are significantly less than values obtained for the Ni-Cr etched alloy surfaces bonded to the same composite resins (17-20 MPa). 2. The composite-resin luting agent (Comspan) and the Dentacolor microfilled resin-veneering material penetrated the etched surfaces satisfactorily, although metal tags were often fractured from the alloy surface during bond testing. Although the bond strengths of the Ag-Pd alloys to the composite resins were relatively low, the results indicate that some degree of direct bonding of precious-metal alloys to tooth structure is possible so that beads and other types of conventional retention may not be necessary. 3. Under the etching conditions developed for this study, the silver-palladium alloys tested did not quite meet the objective of achieving a bond strength with composite equal to that for a typical etched enamelcomposite bond (15-18 MPa). Therefore, from a mechanical strength viewpoint more research on
References 1. Simonsen RY, Thompson V, Barrack G. Etched cast restorations: clinical and laboratory techniques. Chicago: Quintessence, 1983. 2. Thompson VP, Del Castillo ED, Livadiris GJ. Resin-bonded retainers. Part I: Resin bond to electrolytically etched nonprecious alloys. J Prosthet Dent 1983: 50: 771-779. 3. Council on Dental materials, Instruments, and Equipment. Biological effects on nickel-containing dental alloys. J A D A 1982: 104: 501-505. 4. Craig RG. In: Restorative dental materials. 6th ed, St. Louis: CV Mosby, 1980: 330. 5. Kemper RN, Kilian RJ. New system for bond strength testing. J Dent Res Abst 1976:308 55B: B 138. 6. Gottlieb EW, Retief DH, Jamison HC. An optimal concentration of phosphoric acid as an etching agent. Part I: Tensile strength studies. J Prosthet Dent 1982: 48: 48-51. 7. Retief DH, Woods E. Is a low viscosity bonding resin necessary? J Oral Rehab 1981: 8: 255-266. 8. Arends J, Keizer S. In vitro studies on enamel resin systems with special reference to improvement of resin adhesion. Proc Int Symp: The acid etch technique. St. Paul, Minn: North Central Publishing, 1975: 40-49.