Resin-bonded retainers. Part I: Resin bond to electrolytically etched nonprecious alloys

a OPERATIVE

FIXED PROSTHODONTICS SECTION

DENTISTRY

EDlTORS

SAMUEL E. GUYER J. CHRISTENSEN WILLIAM F. P. MALONE LEFKOWITZ

GORDON WILLIAM

Resin-bonded electrolytically College of Dental

Surgery,

Dental

Enrique

Del Castillo,

School, University

lhe resin bonding of castings to acid-etched tooth structure was pioneered by Rochette.’ He reported on its use for periodontal splints. The resin-to-alloy retention depended on holes placed in the alloy framework. The holes were flared from the tooth structure interface outward. Using this method, Howe and Denehy’ placed cast anterior fixed partial dentures in patients with an open bite. LivaditiG recently reported on its application in posterior fixed partial dentures in occlusion. These fixed partial dentures and splints are limited by two factors: (1) the level of retention is compromised by the number of retaining holes in the framework since too many holes result in a weakened framework and (2) the composite resin used for retention is subjected to occlusion at the holes and may wear, with loss of mechanical retention being the long-term result. This latter point is particularly cxent as the low film thickness of a composite resin (Comspan, L. D. Caulk Co., Milford Del.) developed for the bonding of posterior fixed partial dentures has a filler content of only 65% by weight and can be expected to wear more rapidly.) In 1979 Tanaka et al.’ published a method for retaining acrylic resin facings on castings with mechanical retention developed by pitting corrosion of the alloy. Based on this and earlier work of Dunn and Reisbick’ with eleclrolytic etching of a Co-Cr alloy for ceramic retention, it was apparent that electrolytic etching of Ni-Cr casting alloys could yield a surface suitable for micromechanical bonding of dental resins. This surface could remove the previous limitations on cast resin-bonded prostheses. Indeed, Livaditis and Thompson6 have reported on electrolytic etching of a Presented Cl&go,

al rhe Inrrrnakxxxl

As.w&rion of Dental Research. III. l ?\ssociate Professor of Fixed Restorative Dentistry; Dirrcwr of I)ental Materials. **Postqraduatr student, Department of Removable Prosthcdontics. ***Assistant Climcal Prnfessor, Departmrnr of Fixed Restorative

Dentistry.

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C. SPROULL

retainers. Part I: Resin bond to etched nonprecious alloys

Van P. Thompson, D.D.S., Ph.D.,* and Gus J. Liv.aditis, D.D.S.‘** Baltimore

ROBERT

DENTISTRY

D.D.S.,**

of Maryland,

Baltimore,

Electrode

Md.

Holder

Fig. 1. Diagram of etching apparatus. nonprecious alloy and clinical applications of this technique. The purpose of the following research was to discover a convenient method of electrolytically etching nonprecious alloys and to determine the strength of the bond of dental resins to the etched surface. MATERIAL

AND METHODS

Disks 6.5 mm in diameter and 1 mm thick were cast in groups of 10 to 15 with Biobond C&B (Dentsply International Inc., York, Pa.) or Rexillium III (Generic Industries, Wallingford, Conn.). The back of each disk was finished flat, while the face remained untouched to represent the as-cast surface. Unless otherwise indicated, all samples were subjected to the equivalent of porcelain firing by heating four times to 920” C. The disks were mounted three at a time with sticky wax on a stainless steel electrode %6inch in diameter with the face outward. Electrical contact with the electrode was assured with an application of colloidal silver paint. All parts of the electrode and the disks except the face were masked with sticky wax. A small area at the edge of the face of each sample was also masked to serve as an unetched control area. The sample faces were then air-abraded with 50 pm alumina and then waterwashed. The sample electrode was then mounted opposite a &inch stainless steel cathode at a distance

771

THOMPSON,

Fig. 2. Diagram

of bond-alignment

apparatus.

Fig. 3. Diagram

DEL CASTILLO,

of column

AND

LIVADITIS

tensile bond test.

Fig. 4. Biobond C&B alloy electrolytically etched in 0.5N nitric acid at 250 mA/cm* for 5 minutes and cleaned in 18% HCl. Note etched (E) and unetched (U) areas, with a transition zone between them. (Original magnification X1,000; full field is 115 Mm.) 772

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Fig. !i. Biobond C&B alloy; nitric acid etch. (Original magnification ~1,200; full field is 100 pm.) of approximately 1.5 cm. The etching apparatus is shown in Fig. 1 and comprises a low-voltage direct current power supply (Model 1601 Regulated D.C. Power Supply, Dyascan Corp., Chicago, Ill.), a magnetic stirrer, and a beaker with an appropriate acid solution. After etching, all samples were “cleaned” of corrosion products by immersion in 18% HCI solution with ultrasonic agitation.” Samples were unmounted under cold water, and care was taken to avoid embedment of any sticky wax in the etched surface. The following etching solutions were evaluated by various etching times and current densities based on the work of Lewis’ and Baran? 0.5N nitric acid, 5N nitric acid, 5% chromic acid, 5% chromic acid plus 0.5% acetic acid, and 10% sulfuric acid. All sample etching was evaluated by visual observation at magnifications of x20 to x80 with a stereomicroscope (Stereozoom 680, American Optical, Buffalo, N.Y.); and selected samples were mounted, goldTHE JOURNAL

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sputtered, and observed with the scanning electron microscope (SEM) (AMR 1000, Advanced Metals Research, Burlington, Miss.) at magnifications of x20 to x2,000. From visual and SEM observation, those etching conditions that give retentive-appearing surfaces were selected for tensile bond testing. Sample groups of 6 to 10 disk specimens were prepared and etched. After inspection each disk was washed twice with methyl isobutyl ketone, air-dried, and placed on the rubber alignment pad of the bondalignment apparatus shown in Fig. 2. A self-curing unfilled resin bonding agent (L.D. Caulk Co.) was applied to the etched surface by brush (spreading occurs immediately on the etched surface). Composite resin (Comspan, L. D. Caulk Co.) (mixed at the time of mixing the bonding agent) was packed into a knife-edged, beveled stainless steel tube (3.25 mm O.D., 2.25 1.D.). The tube was placed in the upper member of the alignment apparatus and the upper 773

THOMPSON,

DEL CASTILLO,

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Fig. 6. Rexillium III alloy surface after chromic acid etching (5%), 250 mA/cm2 for 5 minutes, and HCl cleaning. (Original magnification ~2,000; full field is 60 pm.)

Table I. Tensile bond strength (0.5N nitric acid etch for 5 minutes) Condition Alloy

Biobond C&B Rexillium 111

100 mA/cm’

200 mA/cm’

25.5 f 9.4(6)* -

27.3 f 3.7(6) 6.4 rt 1.7(6)

Control 11.0 f 2.4(5)

5.5 f 0.5(5)

*Number of samples is indicated in parentheses.

member located along the aligning rods until the beveled tube contacted and aligned the etched alloy disk on the rubber pad in the apparatus base. Contact was maintained with finger pressure and the plunger inserted through the hole in the upper member and pushed through to make contact with the setting resin. The weight on the plunger can be varied but was maintained at approximately 2 kg/cm.’ The above operation was completed within 90 seconds after the beginning of the mix. Five minutes after the mix was begun, the sample with the tube and 774

its internal resin column were removed from the alignment apparatus and placed in a 37” C water bath. All samples were then thermally cycled between 5” and 60” C in water baths for a minimum of 1,000 cycles. Tensile testing of the resin-to-alloy bond was performed in the apparatus shown in Fig. 3, which is based on the method of Standlee et aL9 A collet was used to hold the stainless steel tube in the upper member. The upper member connected through a DECEMBER

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Fig. 7. Rexillium III alloy surface after sulfuric acid etch and HCl cleaning. (Original magnification X1,000; full field is 115 pm.) universal joint to the load cell of the tensile tester (Model 65TM, Thwing-Albert Co., Philadelphia, Pa.). Tensile testing used a constant strain rate of 1 mm/min.

RESULTS AND DISCUSSION Biobond C&B, a non-beryllium-containing alloy, was found to yield the most consistent and visually retentive surfaces when 0.5N nitric acid was used with a current density of 250 mA/cm2 for 5 minutes. A typical junction between an etched and unetched surface is shown in Fig. 4 and the general etching pattern in Fig. 5. Both chromic acid and particularly sulfuric acid failed to etch the surface selectively. With both acids, even at low-current densities, electropolishing occurred. Rexillium III, an alloy containing beryllium (1.8%), was found to etch with nitric, chromic, and sulfuric acids. Nitric acid alt 0.5N and 5% chromic acid at current densities of 2100to 600 mA/cmZ for 5 minutes THE JOURNAL

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resulted in surfaces that were similar between magnifications of x20 and ~2,000. This etching removed the dendritic and interdendritic phases at slightly different rates. The interlamellar interdendritic phase was highly accented with chromic acid etching, as can be seen in Fig. 6. This etching pattern was observed by Baran” for other alloys but at a very different level of relief. Table I shows the comparison of the resin bond strength to Biobond C&‘B and Rexillium III etched with nitric acid. The control for each material was the 50 pm alumina air-abraded alloy surface. The bonding agent resin spread slowly on nitric acid-etched Rexilhum in contrast to the Biobond surface. In this table the Biobond samples were not subjected to porcelain firing cycles prior to etching, while all Rexillium samples were so conditioned. Etching of Rexillium III with 10% sulfuric acid at 300 mA/cm’ for 3 minutes resulted in massive removal of the interdendritic phase and selective removal of the minor intradendritic gamma prime phase (Fig. 7).” 775

THOMPSON,

DEL CASTILLO,

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LIVADITIS

Fig. 8. Alloy surface of Biobond C&B tensile bond specimen after debond test. Fracture is adhesive. (Original magnification x20; full field is 2.3 mm.)

Table II. Tensile bond strength to Rexillium

III

5% chromic acid and 250 mA/cm* for 5 min

10% sulfuric acid and 300 mA/cm’ for 3 min

13.8 + 1.5(7) -

24.3 k 7.9(9), 26.1 -+ X5(7)+

*Rubber pad for specimen alignment. tRubber pad and upper washer for specimen alignment.

This surface was found to give excellent bonding, as shown in Table II. In addition, the bonding agent was found to spread quite rapidly on the sulfuric acidetched surface. SEM observations of the fractured surface of the tensile test specimens were made. The surface of Biobond C&B after adhesive debond at 30 MPa* is *l MPa = 145 psi.

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shown in Figs. 8 and 9. The failure appears to be in the unfilled resin near the height of the alloy projections. Note the excellent resin adaptation to the surface. The Rexillium III alloy surface after debonding is shown in Fig. 10. This was an adhesive-cohesive bond failure at a stress of 39 MPa. The surface shows the unfilled resin adaptation to the coral-like dendritic arms of the sulfuric acid-etched surface. The small filler particle size of the Comspan resin, which the manufacturer states is less than 5 pm, is confirmed as seen in the cohesive failure surface in the upper right of the micrograph. Biobond C&B alloy after procelain firing was difficult to etch, and an etch-resistant “skin” could be observed on some areas of the surface. The tensile bond tests were repeated on a sample group of Biobond C&B alloy after four thermal cycles to 920” C before the nitric acid etching. The lower bond values resulting are

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Fig. 9. Biobond C&B alloy surface after resin debond (same surface as center field of Fig. 8). (Original magnification x2,000; full field is 60 Mm.) shown in Table III. The nature of the etch-resistant layer is being investigated. Based on this and other studieq3s6 a large clinical study has been undertaken to determine the long-term efficacy of the etched alloy resin-bonded retainer. The ultraconservative nature of the restoration and the minimal extent of the preparation are shown in Fig. 11.

Table III. Porcelain firings and tensile bond strength to Biobond C&B (0.5N nitric acid and 200 mA/cm’ for 5 minutes) Control’

Cycledt

27.3 f 3.7(6)

18.1 * 4.4(9)

*No porcelain firings (etched from as-cast condition). tFour cycles to 920’ C before etching.

SUMMARY Nonprecious Ni-Cr casting alloys can be electrolytitally etched to yield a highly retentive surface for micromechanical bonding of dental resins. The acid, current density, and etching time to achieve the retentive features are specific for each alloy. Conditions for etching one beryllium-containing and one non-berylliurn-containing alloy are described. The tensile strength of a resin system to these alloys has been determined to

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be over two times the accepted value of the resin bond to acid-etched enamel. CONCLUSIONS 1. Electrolytic etching conditions, which give resinto-alloy bonds that exceed by almost a factor of 2 the accepted average value for resin-to-etched tooth structure, have been found for the two alloys investigated.

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Fig. 10. Alloy surface of Rexillium III tensile bond specimen after debond from Caulk self-cure bonding agent and Comspan composite resin column. Fracture was adhesivecohesive. (Original magnification ~2,000; full field is 60 pm.)

Fig. 11. Clinical photographs of an etched casting resin-bonded- retainer. A, Enamel preparation for retainer. Objective is to create an occlusogingival path of insertion. Enamel will be etched with phosphoric acid. B and C, Restoration after enamel etching and resin-bonding procedures. Notes esthetics, open embrasures, and supragingival margins. 778

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The etching conditions are as follows: (1) Biobond C&B: (a) OSN nitric acid at 250 mA/cm2 for 5 minutes; (h) 18% HCl cleaning in ultrasonic bath for 10 to 12 minutes; (2) Rexillium III: (a) 10% sulfuric acid at 300 mA/cm* for 3 minutes; (b) 18% HCl cleaning in ultrasonic bath for 10 to 12 minutes. 2. BIS-GMA-based bonding agents spread well on solvent-cleaned etched alloy surfaces as prepared above. 3. Electrolytic etching of Ni-Cr alloy castings to be bonded to acid-etched tooth structure permits manifold uses in dentistry. Currently, over 350 etched casting resin-bonded fixed partial dentures have been placed at the Dental School, University of Maryland.

3. 4.

5. 6

7. 8. 9.

IO.

REFERENCES I. 2.

Lrvaditis, G.: Cast metal resin-bonded retainers for posterior teeth. J Am Dent Assoc lOt:926, 1980. Tanaka. T., Atsutz, M., Uchiyama, Y., and Kawashima, I.: Pitting corrosion for retaining acrylic resin facings. J PROSTIIET DENT 423282, 1979. Dunn B.. and Reishick, hl. f1.: Adherence of ceramic coatings on chromium-cobalt structures. J Dent Res 55:328. 1976. Livaditis, (;. J.. and Thompson, V. P.: Etched castings: An improved retentive mechanism for resin-bonded retainers. J PKO.I.II~~ I)F.x~ 4i:52, 1082. IX\VIS. A. J.: The mechanism of tensile failure in a nickel-base casting alloy. J I)rnt Res 56:631, 1977. Baran, (;. K.: Phase changes in base metal alloys along mrtal-porcelain interfaces. J Dent Rrs 58:2095, 1979. Standlee, J. P., Caputo, A. A., and Hanson, E. C.: Retention of cndodontic dowels: Ellccts of cement dowel length, diameter, and design. J PRWTIW DEM. 39:401, 1078. Sims, C:. ‘I‘., and Ifagel. W. C.: I‘hr Superalloys. New York, 1072. John Wiley & Sons. Inc.. p 79.

Rochette, A. L.: Attachment of a splint to enamel of lower anterior teeth. _I PR(~THB.I. DE.W 30~418, 1973. Howe, I). F., and Denehy, G. E.: Anterior fixed partial dentures utilizing the acid-etch technique and a cast metal framework. J PROSTIW DENT 37~28, 1977.

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D.D.S., and Sergio Spatakis, D.D.S.

A psychologic study of self-concept of patients in relation to artificial and natural teeth l-1. A. Alvi, B.Sc., B.D.S., N. K. Agrawal, B.D.S., M.S., Suresh Chandra, B.D.S., M.D.& and M. Rastogi, M.A., Ph.D.

Using the existing complete denture as a surgical template Ali Bolouri, D.M.D.,

D.D.S., and Craig E. Williams, D.D.S.

Effect of disclosing wax on bonding strength of cemented crowns C. E. Brukl, Ph.D., .J. W. Nicholson, D.D.S., and B. K. Norling, Ph.D.

In vivo forces on endosteal implants: A measurement system and biomechanical considerations John B. Brunski, Ph.D., and John A. Hipp

The development of an augmentation and a technique for its use F Blaise Curcio, D.M.D.,

An intraoral-extraoral

endodontic-endosseous

implant

M.A.

radiation carrier for tumors of the lip

S. M. Cameron, D.D.S., Madhava Baikadi, M.D., and LaLitha M. Janaki, M.D. THE JOURNAL

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