a comparison of electrolytic and chemical etch systems on the resin-to-metal tensile bond strength

A comparison of electrolytic and chemical etch systems on the resin-to-metal tensile bond strength

G. E. Krueger, D.D.S., M.S.,* A. M. Diaz-Arnold, D.D.S., M.S.,** S. A. Aquilino, D.D.S., M.S.,*** and F. R. Scandrett, D.D.S., M.S.**** University of Iowa, College of Dentistry, Iowa City, Iowa

This investigation compared the tensile bond strengths of a nickel-cbromium- beryllium alloy etched electrolytically and etched with a commercially available chemical gel. The number of applications and the thermal conditions of the cbemi- cal etchant were varied to assess their influence on the composite-to-metal tensile bond strength. Etched metal cylinders were bonded end-to-end with a resin luting agent and were subsequently tested for tensile strength. Etch patterns, mean bond strengths, and mode of failure were recorded. Significant differences relating to the application number and the thermal conditions of the chemically etched specimens were noted. (J PROSTHET DENT 1990;64:610-7.)

R esin-bonded metal restorations have recently gained wide acceptance in dentistry as an alternative to conventional fixed partial dentures. Rochettel described a technique for splinting periodontally involved teeth together with a resin-retained perforated metal retainer. Shortly after, this concept was used for the replacement of missing anterior teeth, and then later for replacement of missing posterior teeth. 2, 3 Methods to improve the retention of these restorations led to the development of the electrolytic etch procedure. 4p5 Techniques were subsequently developed for electrolytically etching different alloys.6 High bond strengths obtained between electrolytically etched metal castings and resin luting agents have contributed to their acceptance as a permanent restoration.6-g

Recently, chemical etch systems have been developed as an alternative to the electrolytic procedure. Initially, chemical etching was accomplished by immersing the casting in a freshly mixed chemical solution that was considered un- stable and hazardous to store.‘O An etching time of 60 minutes was required to produce comparable results.lO* l1

Several chemical etching systems have been introduced to further simplify the etching procedure. Reported etching conditions included the application of a gel for 3 minutes at 150’ F and reapplication for 7 to 10 minutes, or application of the gel for 20 minutes at room temperature.12s l3 Additional information on the evaluation of different etching prescriptions has yet to be reported in the literature.

presented at the American College of Prosthodontista meeting, Tucson, Ariz; First place, John J. Sbarry Research Award com- petition.

*Graduate student, Department of Prostbodontics. **Assistant professor, Department of Family Dentistry, ***Associate professor, Department of Prosthodontics. ****professor and Head, Department of Prosthodontics. 10/l/21271

610

This investigation compared the tensile bond strength of a nickel-chromium-beryllium (Ni-Cr-Be) alloy etched electrolytically and etched with a commercially available gel (Etch-It, American Dental Supply, Easton, Pa.) under various conditions.

METHODS AND MATERIAL

One hundred acrylic resin dowels 6.4 mm in diameter and 12 mm in length (Cope Plastics, Cedar Rapids, Iowa) served as patterns. A retaining hole, perpendicular to the long axis of the cylinder, was drilled 2 mm away from the end to be tested (Fig. 1). The patterns were sprued on the end clos- est to the retaining hole, invested in a phosphate-bonded investment (Highspan, J.F. Jelenko Co., Armonk, N.Y.) and cast in a Ni-Cr-Be alloy (Rexillium III, Jeneric Gold Co., Wallingford, Conn.) with an induction casting machine (Jelenko Accu-Therm II 1000, J.F. Jelenko Co.). The castings were allowed to bench cool before divesting. The cast metal cylinders were machined on a lathe to assure a flat surface for tensile testing. The cast cylinders were air fired in a calibrated porcelain furnace (Ultramat CDF, United Co., Monrovia, Calif.) at 995’ C to simulate the oxidation cycle for porcelain application. The testing surface of each specimen was air abraded with 50 pm aluminum oxide and was ultrasonically cleaned for 10 minutes in double deionized water.

The cast specimens were then arbitrarily divided into five groups of 10 paired specimens and were etched in the following manner (Fig. 2).

Group 1: Electrolytic etch. Ten pairs of cylinders were etched in 10% sulfuric acid for 3 minutes at a current density of 300 mA/cm2. The specimens were then ultrasonically cleaned in 18% hydrochloric acid for 10 minutes, followed by sonication in double deionized water for 10 minutes6

Group 2: Chemical etch. Ten pairs of cylinders were chemically etched with one application of Etch-It gel at room temperature for 20 minutes. The specimens were

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ELECTROLYTIC VS A CHEMICAL ETCH SYSTEM

Fig. 3. Alignment apparatus.

Fig. 1. Acrylic resin pattern.

I , I

Fig. 2. Testing scheme.

then neutralized in a solution provided by the manufac- turer for 15 seconds, and were ultrasonically cleaned in double deionized water for 10 minutes.

Group 3: Chemical etch. Ten pairs of cylinders were chemically etched with one application of Etch-It gel at 110’ F for 10 minutes. The specimens were neutralized and cleaned in the same manner as those in group 2.

Group 4: Chemical etch. Ten pairs of cylinders were chemically etched with two applications of Etch-It gel at room temperature for 20 minutes. The specimens were neutralized and cleaned after each application of the etchant.

Group 5: Chemical etch. Ten pairs of cylinders were chemically etched with two applications of Etch-It gel at 110” F for 10 minutes. The specimens were neutralized and cleaned in the same manner as those in group 4.

Fig. 4. Tensile testing apparatus.

Manufacturers’ recommendations were followed for the etch procedures. Representative specimens from each test group were examined with a scanning electron microscope (SEM) to determine if differences existed in the etched surface pattern between the different etching procedures.

An alignment apparatus held the specimens in a rigid position during bonding (Fig. 3). A micrometer device positioned within the apparatus permitted a constant film thickness of 20 pm to be maintained. Before luting, two cylinders were positioned within the device and the com- bined length of the rods was recorded. The luting agent (Comspan Opaque Resin Bonded Cement, L.D. Caulk Co., Millford, Del.) was mixed and applied in excess to the ends

THE JOURNAL OF PROSTHETIC DENTISTRY 611

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.5 4070 40.59 4037 Il.W 12-J) 12.23)

Fig. 5. Mean tensile bond strengths. Standard error of mean in parentheses.

Table II. Analysis of variance, general linear model

Variable Degree of Sum of Mean (source) freedom square* square F value P>F

Model 4 1146.87 286.72 8.27 0.0001* Error 45 1560.91 34.69 Total 49 2707.79

*Significant at p < 0.05.

Table III. Analysis of variance, chemical etch

Variable Degree of Sum of Mean (source) freedom squares square F value P> F

Model Error Total Application Temperature Application,

temperature

3 1061.65 333.88 9.59 0.0001 36 1253.67 34.82 39 2255.33

1 654.04 18.78 0.0001’ 1 164.77 4.73 0.0363* 1 182.85 5.25 0.0279*

*Significant at p < 0.05.

Table I. Duncan’s multiple range test for different etching systems

Duncan Mean tensile multiple

Etch strength range procedure (MPa) group

Electrolytic

etch Etch-It

room temp 1x

Etch-It

110” F 1x

Etch-It room temp 2x

Etch-It

110’ F 2x

40.70 5.84 1.85 14.36 A

28.23 5.05 1.60 17.90 B

36.56 4.67 1.48 12.78 A

40.59 6.50 2.05 16.00 A

40.37 7.05 2.23 17.47 A

CV, Coefficient of variation.

of the rods. The rods were then repositioned within the alignment apparatus, and the micrometer was adjusted to read 20 pm more than the initial recorded measurement.

The specimens were stored for 85 hours in 37” C double deionized water and were then thermocycled for 5 hours between 5’ C and 60” C water baths. Each cycle was 80 seconds with a 30-second dwell time, for a total of 230 cy- cles over the 5-hour period.

The specimens were debonded in tension with an Instron testing machine (Instron Corp., Canton, Mass.) (Fig. 4). A 0.5 cm/min. crosshead speed was used. The force per unit

Table IV. Chemical etch, room temperature specimens, application number analysis

Number of Mean tensile strength Duncan multiple applications (MW rangegroups

1 28.23 B 2 40.59 A

Table V. Chemical etch, 110’ F specimens, application number analysis

Number of applications

Mean tensile strength Duncan multiple OfPa) range groups

1 36.56 A

2 40.37 A

area in MPa was calculated as the measure of the tensile bond strength. The debonded specimens were examined with light microscopy to determine the character of the bond failure.

RESULTS

The mean tensile bond strengths, standard deviations, standard error of the mean, and coefficients of variance for the electrolytically and chemically etched specimens are shown in Table I and Fig. 5. The results were subjected to analyses of variance and Duncan’s multiple range tests at the 95% confidence level (Tables I through VII).

The specimens chemically etched once at room temper-

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Fig. 6. Typical adhesive failure.

Fig. 7. SEM of electroetched surface; original magnification X1000.

Table VI. Chemical etch, one-application specimens, Table VII. Chemical etch, two-application specimens, temperature analysis temperature analysis

Temperature

Mean tensile strength

OfPa) Duncan multiple

range groups

Room temperature 110” F

28.23 B

36.56 A

Mean tensile strength Duncan multiple

Temperature OfPa) range groups

Room temperature 40.59 A

110” F 40.37 A

ature for 20 minutes demonstrated a significantly lower mean tensile bond strength than any of the other four etching procedures (Table II). The effects of the number of applications and the temperature in the chemically etched groups are shown in Tables III through VII. The bond

strength of the room temperature specimens was significantly increased by the second application of the chemical etchant (Table IV). At the higher temperature (110’ F), a second application of Etch-It gel did not significantly increase the bond strength of the specimens (Table V). The


KRUEGER ET AL

Fig. 8. SEM of chemically etched surface, one application at 110” F. Original magnification X1000.

Fig. 9. SEM of chemically etched surface, two applications at 110” F. Original magnification X1000.

bond strength of the specimens that received only one ap- resin luting agent and the etched metal surface (Fig. 6). A plication of Etch-It gel was significantly increased by an portion of the resin luting agent remained on each etched increase in temperature (Table VI). When two applications surface with apparently no resin on the corresponding sur- of the chemical etched were used, an increase in tempera- face of the paired specimen. ture did not significantly increase the bond strength (Ta- SEM analysis revealed differences in the etched surface ble VII). pattern between the chemically etched specimens etched

All specimens exhibited adhesive failures between the once at room temperature for 20 minutes and the other four

614 NOVEMBER 1999 VOLUME 64 NUMBER 5


Fig. 10. SEM of chemically etched surface, one application at room temperature. Orig- inal magnification X1000.

Fig. 11. SEM of chemically etched surface, two applications at room temperature. Orig- inal magnification X1000.

groups (Figs. 7 through 11). The etch pattern for this group appeared less in depth and lacked the larger pores present in the other groups.

DISCUSSION

The mean tensile bond strengh for the electrolytically etched specimens was comparable to that found in other

investigations,7-g but greater than that reported by Livadi- tis and Thompson.4 Differenes in these reported bond strengths may be attributed to differences in methodology. The methodology used in this investigation was similar to the methodology used in previous investigations reporting comparable bond strengths. The metal cylinders were bonded together end-to-end with a limited film thickness


KRUEGER ET AL

of cement. Livaditis and Thompson4 bonded a column of composite resin to an etched metal rod.

Altering the etch conditions of the chemical gel produced a significant difference in the bond strength of the group that was etched with one application of the gel at room temperature for 20 minutes compared with the bond strengths of the other groups. The mean tensile bond strength of this group was 28.23 MPa, whereas the mean bond strengths of the electrolytically etched samples and the two chemically etched samples that received two applications of the etchant were 40.70, 40.59, and 40.37 MPa, respectively.

The mean tensile bond strength of the chemically etched samples that received one application of the etchant at 110’ F was 36.56 MPa. At this temperature, an increase in the number of applications did not significantly increase the mean tensile bond strength.

Aquilino and Diaz-Arnold9 measured the tensile bond strengths of three chemical gel systems using similar methodology. The mean tensile bond strength of the three gels was comparable to the mean tensile bond strength reported in this study for one application of Etch-It gel at room temperature. Other investigations of tensile bond strength for chemical etching gels have reported mean tensile bonds from 11.8 to 36 MPa.131 l4 Differences in meth- odologies and availability of information on the etching technique make it difficult to compare results.

Subjective evaluation of SEMs at 1000 power magnification revealed differences that were representative of the results reported in this study. An attempt was made with the SEM to view the same location on the metal surface for the samples that received a second application of the chemical etch. The specimens chemically etched at an elevated temperature after one and two applications of the chemical etchant (groups 3 and 5) revealed a deeper etch pattern and more exposed surface area (Figs. 8 and 9). Dif- ferences noted in the room temperature specimens etched once and twice (groups 2 and 4) not only demonstrated more surface area but evidence of larger and deeper pores (Figs. 10 and 11). Thompson et al6 viewed fracture sites of the composite-to-metal bond under SEM. They reported that the limiting factor of the bond between the etched metal and the composite resin rod was the penetration depth of the unfilled resin into the projections of the alloy surface. Their conclusion might explain why mean tensile bond strengths peaked around 40 MPa, and why the mean tensile bond strength of the electrolytically etched samples, which had the most extensive etch pattern, was not significantly higher than that of the other etched groups (Fig. 7).

Light microscopy revealed typical adhesive failures between the Comspan opaque luting agent and the etched metal cylinders for all groups (Fig. 6). This observation was consistent for all samples tested, but the microscopic evaluation used in this study was unable to detect whether or

not there was a cohesive failure within the unfilled resin. The unfilled resin used was that supplied with the Com- span opaque resin luting agent.

The results of this in vitro study indicate good overall bond strengths between the resin luting agent and the etched Ni-Cr-Be alloy. Moreover, the alloy-composite resin bond strength of the chemically etched samples, one at room temperature, was approximately three times higher than the accepted enamel-composite resin bond strengths reported in the literature. 15* l6 The bond strengths of the other groups were approximately four times the enamel- composite resin bond strengths. Controlled, long-term clinical trials are needed to better predict the stability of the alloy-composite-enamel bond.

CLINICAL IMPLICATIONS

In acknowledging the difference between the resin- to-enamel bond strengths versus the resin-to-etched metal bond strengths, the use of the chemical etching system, Etch-It, becomes a viable alternative to electrolytic etching based on the technical advantages of the chemical gel. These advantages include: (1) no additional equipment is necessary, thus decreasing the overall time and allowing the etching to become a chairside procedure; (2) elimina- tion of the need to predict surface area, current density, and time, and therefore potentially yielding more predict- able results; (3) eliminating large quantities of chemical solutions and reducing the potential of acid exposure and spills; and (4) decreased cost. Maximum benefit of the chemical etchant would be obtained through two applications of 20 minutes each of the etchant at room temperature, or one application at a temperature of 110’ F.

CONCLUSIONS

1. The tensile bond strength of one application of Etch- It gel at room temperature was significantly lower than that of all other groups.

2. A second application of Etch-It gel at room temperature or a rise in temperature application to 110“ F significantly increased the mean bond strength of the chemically etched specimens.

3. Tensile bond strengths obtained with Etch-It gel from one or two applications at an elevated temperature (110’ F) or two applications at room temeprature were not aig- nificantly different from the bond strengths obtained by the electrolytically etched samples.

4. Optimal use of the chemical etchant produced resin- to-metal tensile bond strengths approximately four times greater than the resin-to-enamel bond strengths reported in the literature.

REFERENCES

1. Rochette AL. Attachment of a splint to enamel of lower anterior teeth. J PROSTHET DENT 1913;30:413-23.

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2. Howe DF, Denehy GE. Anterior fixed partial dentures utilizing the acid etched technique and a cast metal framework. J PROSTHET DENT 1977;37:28-31.

3. Livaditis GJ. Cast metal resin-bonded retainers for posterior teeth. J Am Dent Assoc 1980;101:926-9.

4. Livaditis GJ, Thompson VP. Etched castings: an improved retentive mechanism for resin-bonded retainers. J PROSTHET DENT 1982;47:52-8.

5. McLaughlin G. Composite bonding of etched metal anterior splints. Compend Contin Ed 1981;2:279-83.

6. Thompson VP, Castillo ED, Livaditis GJ. Resin-bonded retainers. Part 1: Resin bond to electrolytically etched nonprecious alloys. J PROSTHET DENT 1983;50:771-9.

7. Haywood VB, Lundeen TF, Taylor DF. Tensile bond strengths of Comspan to electrolytically etched metal with and without enamel bonding agent. Dent Mater 1987;3:29-32.

8. Diaz-Arnold AM, Williams VD, Aquilino SA. Tensile strengths of three luting agents for adhesion fixed partial dentures. Int J Prosthodont 1989;2:115-22.

9. Aquilino SA, Diaz-Arnold AM. Tensile bond strengths of electrolyt- tally and chemically etched base metals [Abstract]. J Dent Res 1989; 68350.

10. Love LD, Breitman JB. Resin retention by immersion-etched alloy. J PROSTHET DENT 1985;53:623-4.

11. Livaditis GJ. A chemical etching system for creating micromechanical retention in resin-bonded retainers. J PROSTHET DENT 1986;56:181-8.

12. Doukoudakis A, Cohen B, Tsoutsos A. A new chemical method for etching metal frameworks of the acid-etched prosthesis. J PROSTHET DENT 1987;58:421-3.

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Re GJ, Kaiser DA, Malone WFP, Garcia-Godoy F. Shear bond strengths and scanning electron microscope evaluation of three different retentive methods for resin-bonded retainers. J PROSTHET DENT 1988;59:568-73. Stevens A, Bradshaw B, Christensen R, Bamgerter V, Christensen G. Comparison of three surface treatments and eleven cements for resin retained prosthesis [Abstract]. J Dent Res 1987:66:291. Yedid SE, Chan KC. Bond strength of three esthetic restorative mate- rials to enamel and dentin. J PROSTHET DENT 1980;44:573-6. Young KC, Hussey M, Gillespie FC, Stephans K.W. In vitro studies of physical factors affecting adhesion of fissure sealant to enamel. In: Sil- vestone LM, Dagon IL, eds. Proceedings of the International Sympo- sium on the Acid-Etch Technique. St Paul, Minn: Central Publishing co, 1975:40-52.

Reprint requests to: DR. GORDON E. KRUEGER 6740 CROSSWINDS DR. N. ST. PETERSBURG, FL 33710

Contributing author

L. R. Huber, D.D.S., Assistant Professor, Department of Prosthodontics, University of Iowa, College of Dentistry, Iowa City, Iowa


a comparison of electrolytic and chemical etch systems on the resin-to-metal tensile bond strength

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