Chairside electrolytic etching of cast alloys for resin bonding

GREG0

Fig. 4. Anterior restoration. 12. Apply the dental adhesive (Scotchbond, 3M Co., St. Paul, Minn.) to the dentin and enamel. Evaporate the solvent gently with an air syringe. Repeat as necessary. 13. Mix equal volumes of paste A and B-of P-10 (3M Co.). 14. Place the mixed composite material in the tip of the syringe and insert the plunger (C. R. Syringe, Centrix Inc., Stratford, Conn.). 15. Inject the mixed composite material into the cavity, overfilling it from the inside out. 16. Test the setting of the material by passive pressure of a plastic-tipped instrument. The composite material should remain undisturbed for 5 minutes after the onset of hardening. 17. After setting, the restoration should be shaped to proper tooth form with suitable diamond burs. The brown line placed to indicate the outline form of the cavity can be seen through the composite resin; therefore, the contouring of the restoration is simplified.

Chairside bonding Thomas

electrolytic

R. Jackson, D.D.S.,*

Fig. 5. Finished posterior composite resin restoration. 18. Adjust the occlusion as indicated. 19. Finish the composite resin with appropriate finishing burs (Figs. 4 and 5).

DISCUSSION Outlining the cavity perimeter aids in determining the original contours of the tooth during the finishing process. This is especially important in the restoration of a molar tooth that exhibits complex contours. How the placement of a wax pencil line on the tooth affects the bonding of the composite resin to tooth structure has not been determined. Reprint requests lo: DR. ELIAS GREGO Av DE LAS FUENTES 41-A 1102 TECAMACHALCO C.P. 53950 MEXICO

etching of cast alloys for resin

and Kent W. Healey, D.D.S., M.S.**

Northern Hospital of Surry County, Mt. Airy, N.C., and University of North Carolina, Schoolof Dentistry, Chapel Hill, N.C.

C

ast metal resin-bonded fixed partial dentures and perioprosthodontic splints were originally bonded to *Chief, Department of Dentistry. **Associate Professor, Department

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of Removable

Prosthodontics.

teeth using acid etching of enamel and bonded on the metal side by retentive holes in the casting.’ The method was improved by electrolytic etching of the metal surfatzzA6Etching metal by corrosion for bonding of acrylic resin facings has been reported.’ Salt crystals have been DECEMBER

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Fig. 2. Spot etching rectifier with electrolytes. Fig. 1. Chemical reaction that takes place on submersion etching of NiCrBe nonprecious alloy in dilute sulfuric acid (H2S04). Electrolytic oxidation-reduction equations: Ni’ + 2 e- - Ni(s); SO,‘- + 4 H+ + 2 e- - SOz (g) + 2 HD.

and show highest bond strengths because the intradendritic berylium-rich phase is etched out preferentially, leaving a screen-lattice pattern of delep microscopic voids.8

SPOT ETCHING

1984.

A new electronic instrument, the Jackson Electrolytic Etcher (Solid State Innovations, Inc., Mt. Airy, N.C.), (Figs. 2 and 3) enables the dentist to complete the etching phase chairside. The technique is quick and produces a clean uniform etch without immersing the restoration in a beaker. It also saves one patient appointment compared with the traditional etching sequence using a commercial laboratory. The etching technique involves activating the electrostripping oxidation-reduction reaction by attaching the anode directly to the metal by means of a small clip. The cathode clip is attached to a cotton-tipped applicator (Fig. 4). Before attaching the cotton-tipped applicator to the cathode, the cotton is saturated with dilute nitric or sulfuric acid, depending on the alloy to be etched. The applicator is then touched to the specific area to be etched (Fig. 5). The rectifier is set for 6 to 12 V direct current at 300 to 450 mAmp, and the cotton tip is held in place for 2 minutes. This technique creates a miniature electrolytic bath at the cotton tip-alloy interface. The direct current passes through the cathode, into the cotton tip, to the framework, and to the anode. The same electrostripping oxidation-reduction reaction that takes place in a beaker takes place at the ,cotton-tip-alloy interface. The obvious advantage is that the appliance can be spot-etched chairside without complete immersion. In addition, the unetched surfaces do not require protection with sticky wax. The etching reaction takes place only directly beneath the cotton tipped applicator. Slightly higher milliamperage levels than those of immersion technique must be used to produce equivalent

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765

used to form positive retentive undercuts on the resin side of the casting.* Traditionally, cast metal frameworks have been etched in a laboratory and returned to the dentist for bonding. This article presents a method, and its associated instruments, that permits the dentist to etch the metal. The procedure is uncomplicated.

ELECTROLYTIC

ETCHING

Most nonprecious alloys can be etched by an electrochemical process called electrostripping. The chemical reaction is represented by an oxidation-reduction equation and involves the oxidation of the metal into its anionic form: from a solid into a solution. The cation is usually obtained from nitric acid (NO,-) when etching chromium-cobalt or sulfuric acid (SO,-) for nickelchromium-berylium alloys (Fig. 1). The cation is reduced to a gas (nitrogen dioxide [NO,] or sulfur dioxide [SO,]) and dissipates into the air.

ADVANTAGES

OF ELECTROLYTIC

ETCHING

Resin bonding of electrolytically etched metal to etched enamel presents advantages over the nonetched metal techniques. The bonding layer is thinner, better protected from oral fluids and abrasion, and is more retentive.8 Inexpensive, nonprecious alloys are well suited for resin bonded retainers because of their high strength at thin dimensions (0.3 to 0.5 mm). Nickelchromium-berylium alloys yield the best etched textures

*Holmes THE

JR:

JOURNAL

Personal

communication,

OF PROSTHETIC

JACKSON

Fig. 3. Chemical reaction that takes place on spot etching dilute sulfuric acid (H2S01).

etching because of the resistance of the cotton tip. After the alloy is spot-etched in the appropriate areas, the restoration is washed with water and lightly dried (Fig. 6). The retainer is examined with loops for uniformity of the etch. If adequately etched, the dark oxide or slight metal membrane is removed by changing the electrode output from direct current to alternating current. A clean cotton tip dipped in 10% hydrochloric acid is clipped to the cathode, and the etched part is clipped to the anode. The rectifier is set at 6 V alternating current and the cotton tip is applied to the dark etched surfaces for 30 to 60 seconds. This will remove the dark oxide metal residue and leave a clean, gray-white etched surface (Fig. 7). The gray-white color imparted to the alloy by alternating current is a vast improvement over the typical black-brown oxidized color produced by direct current submersion etching techniques for nickelchromium-berylium alloys. The etch may be further tested by applying a drop of dentin bonding solutions to the etched surface. An adequately etched surface will cause the drop to spread uniformly over the surface while it will not flow by this capillary-like action if the etch is inadequate.’ The drop is removed with acetone or 766

of nickel

nonprecious

AND

HEALEY

metal in

chloroform. The retainer is washed in water and alcohol and then dried. The fixed partial denture is ready for bonding to the etched enamel with conventional techniques.

SEM OF SPOT-ETCHED

SURFACES

Scanning electron micrographs (SEMs) indicate that the etched texture produced by this spot etching technique are equivalent to previous submersion etching techniques (Figs. 8 and 9). The alternating current cleans the dark oxide membrane from the etched surface without affecting the metal tags. Since alternating current knows no direction, the metal anions are not moved from the metal surface. The gray-white surface creates fewer problems with show-through on translucent abutments and reduces the need for gold electroplating or opaquing.

ETCHING INTERNAL RESTORATIONS

SURFACE OF

The internal metal surface of onlays or crowns may also be etched by this technique to enhance retention to enamel or dentin. If the casting is constructed with DECEMBER

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Fig. 4. Framework is attached to anode terminal clip. Cotton tipped applicator is attached to cathod terminal. Fig. 6. Retainers have been etched with direct current and washed. Note dark oxide surface.

Fig. 5. Cotton tippeP applicator is saturated and applied to area to be etched.

with

acid

NiCrBe alloy, the preparation side of the restoration may be etched by placing the acid-saturated cotton tipped applicator inside the onlay or crown. The anode clip is touched to the metal collar at the lingual side of the restoration. The same spot etch technique described for etching Maryland fixed partial dentures will also etch the internal surface of the crown. For onlay preparations with this technique, it is advantageous to preserve as much enamel as possible. The enamel can be etched and the restoration can be bonded with a microfilled luting composite. If the preparation is entirely limited to dentin, then the restoration can be bonded using a dentin bonding agent. Some of these bonding agents can accelerate setting times of luting composites, The light polymerized dentin bonding THE

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DENTISTRY

Fig. 7. Terminals are changed from direct to alternating current and clean cotton tip is saturated in dilute hydrochloric acid. Note clean white etched metal.

agents seem to be more compatible with luting composites in regard to setting time.

ETCHING OF PARTIAL FOR RESIN COATING

DENTURE

CLASPS

Another application of the spot etch technique is to bond composites to etched metal for esthetic reasons such as coating partial denture clasps. Since most partial denture frameworks are constructed from NiCrBe or CoCr nonprecious metals, it is possible to etch the clasps to create a micromechanical bond for resin coating. Ticonium (Ticonium Co., Div. CMP Industries, Alba767

JACKSON

AND

HEALEY

Fig. 10. Spot etching large amalgam restoration for esthetic reasons. Note tip of thermal probe on restoration.

Fig. 8. SEM of spot-etched Ni-Cr alloy (Biobond, C&B, Dentsply Int., York, Pa.). (Magnification x1200.)

Fig. 9. SEM of spot-etched Ni-Cr-Be alloy (Rexillium III, Generic Industries, Wallingford, Conn.). (Magnification X1200). ny, N.Y.), which is a NiCrBe partial denture alloy, works well for this technique. A light-polymerizing opaque yellow shade microfilled veneering composite resin is cured over the clasp. Of course, those areas of clasps that tend to require a greater degree of flexibility are not good candidates for this technique. On the other hand, any nonflexing part of the framework can be coated using this technique. The coatings will require additions or repairs with time. The composite resin coating usually will last 1 year on rigid clasp areas before it must be reapplied. 768

INTRAORAL

SPOT ETCHING

The spot-etch technique may also be used to etch metal within the oral cavity. Possible applications include intraoral bonding of resin to previously cemented fixed partial dentures, amalgam restorations, and, the bonding of orthodontic brackets or splints to preexisting metal restorations. The intraoral etching and repair of ceramic or acrylic-faced fixed partial dentures by acrylic resin coating of exposed metal is another possible use. The area to be etched must be carefully isolated with a rubber dam to prevent contact of the dilute acid with surrounding tissues. Dental amalgams etch best with 0.5 N nitric acid at 6 V and 300 to 400 mAmp for 2 minutes. After the deep etch with direct current, the area is cleaned with 10% hydrochloric acid with alternating current for 30 seconds. Finally, it is washed with water followed by alcohol, and dried. Amalgam or cast restorations may be etched in the mouth with the same technique as etching out of the mouth. Care must be taken when etching in the mouth not to overheat the restoration and thermally injure the pulp. A specially designed digital thermal probe, the Electra Therm 99 (Solid State Innovations, Inc.), can be attached to the anode that contacts the restoration (Fig. 10). The temperature can be monitored and should never exceed 140” F. If the heat reaches this limit, the circuit is momentarily broken by pulling the anode probe and allowing the heat to dissipate. Different alloys have different heating characteristics because of their internal resistance to electric current. Figs. 11 and 12 show an intraorally etched facial surface of an amalgam and the esthetic result of the applied composite veneer. SEMs of spot etched silver amalgam indicate a surface suitable for micromechanical bond of veneer composite resins (Fig. 13). To electrolytically etch amalgam or cast restorations in the mouth, special care must be taken. Although the DECEMBER

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Fig. 12. Amalgam restoration coated using Visoopaque (ESPE, Oberbay, West Germany) light-cured bonding composite resin.

Fig. 11. Cleaned etched facial surface of intraorally

etched onlay restoring part of facial surface. rectifier voltage is low (6 V), if both electrodes are inadvertently touched to wet soft tissue, a slight electric shock is felt. The voltage is not high enough to cause injury but can be about as uncomfortable as an electric pulp tester set on medium strength. If the electrode setup is properly applied to the restoration, no current will be felt. The direct current circuit travels not through the soft tissues but through the restoration, which is the path of least resistance. The acid concentrations are dilute and will not cause severe injury to soft tissues unless the contact exceeds 30 seconds or more. If some dilute acid inadvertently contacts the tissue, it should be washed with water immediately. Dilute acid must always be used with care in the facial area.

Fig. 13. SEM of silver amalgam electrolytically spot etched with dilute nitric acid (Titan, SSWhite, Philadelphia, Pa.). (Magnification x1200.)

5.

retentive mechanism DEHT 47~52, 1982. Council on Dental

6.

acid-etching McLaughlin

SUMMARY A new technique and the instruments for using it for chairside electrolytically etching metal have been described. The technique simplifies the process and permits the dentist to etch castings for resin retention in the operatory. The method also applies to spot etching techniques for fixed restorations and removable partial dentures. Intraoral use of the technique was described. Rochette AL: Attachment teeth. J PROSTHET DENT

7.

8.

REFERENCES 1.

splints. Dental

of a splint to enamel 30~418, 1973.

of lower

anterior 9.

Tanaka corrosion

Quintessence Publishing Thompson V: Continuing sity of North

Carolina,

Kepnnl reyut3/.rlo: DR. T. R. JACKSON 1007 R~CKFORD ST. MOLW~. AIRY, NC: 27030

THE JOURNAL

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DENTISTRY

castings:

An

improved

retainers. Devices.

J

Status

PROSTHET report

on

Assoc 97 505, 1978. of etched metal anterior

(University

cf Penn.

School

of

Y. Kawashima I: Pitting facings. ,J, PROSTHET DENT

42:282, 1979. Simonsen R, Thompson V, Barrack G: Etchtzd Cast Restorations: Clinical and Laboratory Techniques. Chicago, 1983,

Livaditis GJ: Cast metal resin-bonded teeth. J Am Dent Assoc 101:926, 1980. Livaditis GJ, Thompson VP: Etched

4.

and

T, Atsuta M, Uchiyama for retaining acrylic resin

3.

posterior

Materials

Compendium Cant Educ Medicine) 2:279, 1981.

Howe DF, Denehy GE: Anterior fixed partial denture utilizing the acid-etch technique and a cast metal framework. J PROSTHET DENY 37:2X, 1977. for

resin-bonded

procedures. J Am Dent G: Composite bonding

2.

retainers

for

Co, Inc. education Chapel

Hill,

course. NC,

Presenrrd

at Univer-

1984.

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