Thermal removal of composite resin: Effect on rebonding etched metal

Van B. Raywood, D.M.D.,* B. Ed Kanoy, Jr., M.A., D.DS,** Karen J. Bruggers, D.D.S.,*** and Steven B. Andreaus**** University of North Carolina, School of Dentistry, Chapel Hill, N.C.

f f a correctly etched-metal, resin-bonded iixed partial denture debonds, one recommendation for reuse is to clean the prosthesis by oven burnout, then to recement it without reetching. The purpose of this study was to determine whether the tensile strength of the bond of composite resin cement to either electrolytically or chemically etched metal was affected by earlier removal of residual resin with a burnout procedure. Pairs of rods made of nickel-chromium-beryllium were electro- lytically or chemically etched according to accepted techniques, then bonded end-to-end with an enamel bonding agent and composite resin cement in an alignment apparatus. The rods were stored for 24 hours in 37” C water, then debonded to determine the tensile bond strengths in megapascals. After debonding, the rods were placed in the burnout oven at 510° C for 30 minutes. The rods were then ultrasonically cleaned in ethyl alcohol for 6 minutes. The pairs were rinsed under running water and then rebonded and debonded nine more times under the same conditions. A linear regression analysis revealed that there was no statisti- cally significant di%‘erence @ < 0.05) in the tensile strength of the bonds after repeated thermal cleanings and bondings. Correctly etched metal, resin-bonded fixed partial dentures may be recemented without re-etching after thermal cleaning without a statistically significant loss in the tensile strength of the bonds. (J PBOSTAET DENT 1990;63:289-91,)

A &d-etched, resin-bonded fixed partial dentures have been in wide use in dentistry for more than 10 years.1”4 Although many of these prostheses function well, some problems are inherent in their use. One problem is that one retainer may debond while another remains attached. A method for removal of the prosthesis has been demon- strated, by means of an ultrasonic unit with modified ultrasonic scaler tips.5 This method of removal does not damage the retainer portion of the prosthesis and allows rebonding after appropriate cleaning and fif needed) reetching.

Once a prosthesis is removed with no damage to the re- tainers, the next step is to determine how the prosthesis should be treated to be rebonded successfully. If the orig- inal etch was inadequate, the prosthesis must be reetched before ~emen~tion. The adequacy of the etched-metal surface may be determined by examination of the location

This investigation was supported with funds from NIW grant No. Z-SO7-RRO5333.

Presented at the International Association for Dental Research, Montreal, Canada.

*Assis~t Professor, Dep~tment of Fixed Pr~~~onti~. **Clinical Associate Professor, Department of Fixed Prosthodon-

tics. ***Graduate Prosthodontics resident. ****Student Research Assistant. 10/l/15844

of the debonded comp~i~ resin cement. A lack of resin on the etched-metal surface indicates that the original etch was inadequate.4 The presence of composite resin cement on the etched-metal surface indicates that the original etch was adequate. The remaining composite resin must be re- moved from the retainers before the procedure is contin- ued. The prosthesis may then be re-etched. The previously etched retainers must be resurfaced to eliminate the orig- inal etch. Failure to remove the correctly etched surface will result in an overetch and a sign~cant loss of bond strength.2 An alternative to resufiacing and re-etching of the metal is to clean the residual resin in a manner that preserves the original etch. Simonsen et al.* have reported that this may be accomplished by placement of the pros- thesis in an oven at 700’ C for 10 to 15 minutes, followed by ultrasonic cleaning in a soap solution for 5 minutes. Evaluation of the adequacy of the cleaned etched surfaces was based on visual inspection.

This study determined whether the tensile bond strength of the composite resin cement to either electrolytically or chemically etched metal is affected by earlier removal of residual resin with a burnout procedure similar to that de- scribed by Simonsen.4

MATERIAL AND METHODS Matched pairs of rods 2 cm in length and 6.4 mm in di-

ameter were cast in a nickel”chromium-be~lli~ alloy (LiteCast B, Williams Gold Refining Co., Inc., Buffalo,

THE JOURNAL OF PROSTHETIC DENTISTRY 289

HAYWOOD ET AL.

40-

35-

30-

25-

20-

15-

. -y-e_.-.

Q ---___ ----

D 0 . I

‘j ( , ( , ( , 1 , , ( 0 1 2 3 4 5 6 7 6 9 10

Run Number

Fig. 1. Graph of linear regression analysis of tensile strength of bonds of properly elec- trolytically and chemically etched metal.

N.Y.). To ensure uniform casting temperatures, castings were made by induction (Ticomat casting machine, Tico- nium Company, Division of CMP Industries, Inc., Albany, N.Y.). The castings were bench cooled and, with a lathe, each cast-metal rod was then machined along its sides for removal of casting irregularities.6 The ends were machined flat and perpendicular to the long axis of the rod. A retain- ing hole was drilled 2mm from one end of the rod, perpen- dicular to and through the long axis. The rods were then air-fired twice in a porcelain furnace at 980” C to simulate degassing and initial porcelain application. The end oppo- site the retaining hole was air-abraded with 50 pm alumi- num oxide. Pairs of rods were matched and numbered to maintain consistent pairing. The retaining hole, which was used to grasp and align the sample during cementation and testing of the tensile bond, was marked on one side only to allow exact replacement in the bonding apparatus and in the alignment apparatus.

The pairs were then electrolytically or chemically etched according to accepted techniques.2* r With an accepted etching apparatus (Oxy-Etch, Oxy Dental Products, Inc., Hillside, N.J.) for 6 minutes at a current density of 200 mA/cmP (64 mA), the first pair was etched electrolytically in an 8 ~01% solution of HsS04 (sulfuric acid),8 (103 ml 96% sulfuric acid, 1000 ml distilled water, and 110 ml methanol). Then the rods were cleaned ultrasonically in 18% HCl (hydrochloric acid) for 10 minutes. The second and third pairs were etched chemically in a solution of HNOsHCl (nitric and hydrochloric acids, Assure-Etch, Williams Gold Refining Co.) for the recommended period of time (60 minutes). Because Assure-Etch material has a shelf-life of only 3 months, a fresh batch was mixed for the etching. To confirm that they were correctly etched, all samples were then rinsed with water and evaluated with scanning electron microscopy against known standards.

A bonding-alignment apparatus and technique were used as previously reported.6 The rods were aligned in the apparatus (according to the markings) in the same manner for each evaluation. With a calibrated syringe, equal amounts of the base and catalyst of the composite resin ce- ment (Comspan Opaque, L.D. Caulk Company, Division of Dentsply International, Inc., Milford, Del.) were dispensed. The enamel bonding agent was first mixed and placed on the etched surfaces of the sample pair, and the composite resin cement was applied. The matched rods were then bonded at a premeasured 25 pm film thickness. The bonded sample remained in the apparatus for 15 minutes and was then stored in 37’ C water for 24 hours before testing.

Evaluations of the strength of the bonds were completed with the use of a universal testing machine (Instron, Instron Corp., Canton, Mass.) at a crosshead speed of 1 mm/min. The position and orientation of the grasping holes in each rod and the repeatable bonding position re- sulting from the bonding alignment apparatus ensured a tensile failure. Tensile bond strengths were determined by dividing the recorded strength at failure (kg) by the calcu- lated surface area of the bond (cm2). A conversion was made to megapascals (MPa).

Residual composite resin cement on the debonded sam- ples was removed by means of thermal treatment. The debonded samples were placed in an oven at 510” C (950’ F) for 30 minutes, followed by ultrasonic cleaning in ethyl alcohol for 6 minutes. The pairs were then rinsed under running water and rebonded and debonded under the same conditions nine more times. The means and standard de- viations were determined for each of the 10 rebondingl debonding sequences, and an analysis of variance (ANOVA) was used to determine whether there was any significant difference among the three groups. Linear regression anal- ysis was performed for each group to test whether repeated

MARCH 1990 VOLUME 6.9 NUMBER 3

REBONDING OF ETCHED METAL

cementations resulted in a decrease in the strength of the Table I. Tensile strength of bonds of etched samples bond. (MPa)

RESULTS Bond PEE PEC PEC

The tensile strengths of the bonds and their means and standard deviations are listed in Table I. A linear regres- sion analysis (Fig. 1) revealed no statistically significant difference in the tensile strength of the bonds resulting from repeated thermal cleanings and rebondings for any of the groups (p < 0.05). ANOVA showed there was no statis- tically significant difference among the strength of the bonds for the three groups (p < 0.05).

DISCUSSION

The burnout technique chosen for this study used a lower temperature and took a longer time than that previously reported.4 This was done in an attempt to minimize the potential for metal warpage or for porcelain discoloration. The combination of temperature and time was determined by setting the oven at the desired temperature and then evaluating the sample over time to ascertain the period re- quired to achieve the cleanest surface.

1 24.74 37.29 17.21

2 37.96 31.97 28.38

3 29.89 15.96 17.65

4 22.17 15.17 15.30

5 25.36 13.88 23.72

6 35.39 21.64 33.26

7 27.3 24.9 11.2

8 17.2 16.2 17.7

9 40.0 14.1 19.3

10 15.9 20.0 27.2

x 27.60 21.11 21.09

SD 7.84 7.63 6.47

PEE: Properly etched electrolytically. PEC: Properly etched chemically.

the bonds of the electrolytically or chemically etched metal when etched correctly.

The results of this study are in contrast with the results of a recent study that found a decrease in the strength of a surface that has been rebonded.g In that study, however, the temperature/time for cleaning was very different, a light-cured composite resin was used, and only one rebond was possible. In addition, it was not clear how the castings were re-etched after being cleaned, possibly resulting in an overetch.

CONCLUSIONS

With no statistically significant loss .of tensile bond strength, correctly etched-metal, resin-bonded prostheses may be recemented without re-etching after thermal clean- ings alone.

REFERENCES

1.

Up to this point, the large standard deviation in most studies has been attributed to the variance in etching and bonding of different pieces of metal at different times. However, because this was a bonding of the same etched surface in the same orientation, other variables must be considered. The large standard deviation in the tensile strength of the bond in this study, which is consistent with other studies, may actually indicate a weakness in the lab- oratory design for this and other apparatus used for tensile debonding. In a future study, a universal joint attachment may be placed into the Y-fork debonding apparatus in an attempt to reduce the variance. This rebonding study may provide a useful prototype testing technique for evaluation of the dependability of any laboratory test mechanism be- fore other variables are introduced, as well as provide clin- ical information for the rebonding of an etched-metal, resin-bonded prosthesis.

2.

3.

Rochette AL. Attachment of a splint to enamel of lower anterior teeth. J PROSTHET DENT 1973;30:418-23. Livaditis GJ, Thompson VP. Etched casting: an improved retentive mechanism for resin-bonded retainers. J PROSTHET DENT 1982;47:52-8.

Thompson VP, Del Castillo E, Livaditis GJ. Resin-bonded retainers. Part I: resin bond to electrolytically etched nonprecious alloys. J PROS- THET DENT 1983;50:771-9.

4. Simonsen R, Thompson VP, Barrack G. Etched cast restorations: clin- ical and laboratory techniques. Chicago: Quintessence Publishing, 1983:140-2.

5. Jordan RD, KrelI KV, Aquilino SA, et al. Removal of acid-etched fixed partial dentures with modified ultrasonic scaler tips. J Am Dent Assoc 1986;112:505-7.

6.

I.

8.

9.

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. Livaditis GJ. A chemical etching system for creating micromechanical retention in resin-bonded retainers. J PROSTHET DENT 1986;56:181-8. Haywood VB, Taylor DF, Lundeen TF, Andreaus SB. Concentration of solutions for electrolytically etching Rexillium HI. Dent Mater 1987:3:150-2. Naifeh D, Wendt SL Jr, Dormois LD, McKnight JP. A laboratory eval- uation of rebond strengths of solid retainers of the acid-etched fixed partial denture. J PROSTHET DENT 1988;59:583-7.

In contrast to another study,7 the mean tensile bond strength of the electrolytically etched metal was slightly higher than the mean tensile bond strength of the chemi- cally etched metal. However, our study concurred with that study in demonstrating that there is no statistically signif- icant difference between the resulting tensile strength of

Reprint requests to: DR. VAN B. HAYWOOD SCHOOL OF DENTISTRY CB #7450 UNIVERSITY OF NORTH CAROLINA CHAPEL HILL, NC 27599-7450

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