Restorative Dentistry

Surface treatment of gold alloys for resin adhesionAndrew Eder*/Judith Wicketis**

Abstract This in vitro study compared three methods of surface treating gold alloys toimprove resin adhesion. The tensile bond strengths and modes of failure betweenspecimen pairs cemented with a chemically adhesive resin were recorded. Heat-treated gold alloy specimens were significantly more resistant to bond failure undertensile loading than were either alumina-blasted or tin-plated gold specimens.There was no siatistically significant difference in bondfaHure between alumina-b!asted and tin-p!ated go!d specimens. The .mrface treatment a!tered t!ie modeof failure from adhesive and/or adhesive-cohesive for alumina-blasted and tin-plated goid specimens to cohesive (within the resin) for iieat-treated goldspecimens. Three case reports are presented to illustrate clinical applications ofheat-treated goid alloys. (Quintessence Int ¡996:27:35-40.)

Clinical relevance

A predictable and conservative clinical technique toprotect teeth and restore fi.inction with adhesivelyretained-cast gold alloy restorations for posteriorocclusal and anterior palatal surfaces is described.The interface between cast gold alloy restorationsand luting resins is a common site for failure. Thisresearch supports the application of alumina-blastedand heat-treated gold alloy castings in conjunctionwith resins to achieve a predictable and reliablebond.

Introduction

Twenty years of research into bonding techniques hasled to changes in all aspects of restoring and replacingteeth. Certain factors are now considered flindameiitalto achieving successful bonding; these recommenda-

* Clinical Lecturer in Conservative Dentistry, Depanmert of Con-serrative Dentistry. Eastman Dental Hospilal and Institute, London,England.

*- Consultant in Restorative Dentistry, Department or ConservativeDentistry, Eastman Dental Hospital and Institute, London, England,

Reprint requests^ Dt Andrew Eder, Senior Clinical Lecturer, Dspan-raent of Conservative Dentistry, Eastman Denial Hospitai and Institute,256 Gray's Inn Road, London WC IX 8LD, England,

tions are supported in reviews of tbe literature onresin-bonded restorations'"";

1, Occlusai assessment and careful planning are im-perative.

2, Tooth preparation, if indicated, should remainconservative.

3, Maximal tooth coverage by the retainer is essentialfor success,

4, Alloys no longer need to rely on perforations formacromechanical support, because both micro-mechanical and specific adhesion provide suffi-cient retention.

5, A resin that bonds to both tooth and metals viamicromechanical, physical, and chemical meansshould be selected,

6, Care should be given to the overall clinicalprocedure, with particular regard to techniquesensitivity and tooth isolation,

7, It is recommended that the number of pontics bekept to a minimum; double abutments are contra-indicated.

When adhesive procedures are carefully executed,associated caries, periodontal disease, and the need torebond or remake restorations have not been found tobe clinically significant. Appropriately designed resin-bonded restorations may now be considered anexcellent form of definitive treatment in the careftillyselected patient.*

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Table 1 Mean tensile values

GroupSurface

treatmentNo. of Tensile load at

specimens failure (mean ± SD)

1 Alumina blasting2 Tin plating3 Heat treatment

12.3 ± 3.4 MPa14.5 ±4 .4 MPa29.9 ± 4 . 7 MPa

The retention of resin-bonded restorations dependson ( I) adhesion between the cast alloy retainer and anintermediate resin; (2) cohesion ofthe resin itself, and(3) adhesion between the intermediate resin and thetooth surface. Although successllil bonding has beenachieved clinically at the latter two sites, adhesionbetween the alloy retainer and resin is relativelyinconsistent and a common site for failure.

in recent years, nickel-chrome base metal alloyshave been widely used for retainers in adhesivedentistry. However, a number of problems have beenidentified relating to their use. including nickel al-lergy,̂ abutment discoloration.^ beryllium carcinogen-esis,' and abrasion to opposing natural teeth.* Goldalloys have been successfijlly used in restorativedentistryfor many years and have been proposed as analternative adhesive alloy because of their biocompati-bility.' acceptable esthetics,* low potential for abrasionof opposing teeth.* and good marginal adaptation andaccuracy.'

Gold alloys, however, are expensive and unable toresist flexion when used in thin sections. These majordrawbacks may limit their application"' in adhesivedentistry to anterior palatal veneers and posterioronlays.

The aim of this in vitro study was to investigate theabilities of three different surface treatments of goldalloy to improve resin adhesion.

Method and materials

Test specimens were cast in a high-copper gold ailoywith a standard lost wax procedure. Milled plastic rods4 and 6 mm in diameter formed the patterns. Thisdesign ensured a controlled bonding area, equivalentto the working surface area ofthe smaller specimen.

Ten pairs of specimens were surface treated in eachof three groups. In group 1 (alumina biasting),specimens were blasted with 50-|im alumina at adistance of 1 cm from the specimen until a uniform

surface appearance was achieved. The specimens werethen steam cleaned and bench dried.

In group 2 (tin plating), specimens were blastedwith alumina as in group 1 and tin plated with theKura-Ace (Cavex) 100 tin-plating system. Specimenswere steam cleaned and bench dried.

In group 3 (heat treatment), specimens werealumina blasted as in group 1 and air fired at 400°C for4 minutes. Specimens were bench cooled, providingthat copper oxidation (blue-green surface) was ap-parent.

All specimens were aligned along their long axes ina jig {Fig 1) and cemented with Panavia Ex resin(Kuraray) within 1 hour of surface treatment. Thecement was left to set for 10 minutes under a seatingload of 1 kg. Cemented pairs were subsequently storedunder water at 34°C for 24 hours prior to tensileloading in an Instron universai testing machine. Axialpull on the bonded area was ensured via the use ofuniversal coupling joints between the testing machineand the test pair. The force at bond fracture and modeof failure were recorded in each case.

Results

The mean tensile loads at which bond failure occurredare presented in Table 1. Student's /test analysis wasapplied to determine whether statistically significantadvantages could be attributed to a particular methodof surface treatment. These results indicated that, with-in the limitations of this study, heat-treated gold alloypairs were significantly (P< .001) more resistant totensile bond failure than were pairs in either of theother two groups. There was, however, no statisticallysignificant difference between the alumina-blasted andtin-plated groups.

Photographs of working surfaces were recordedafter fracture (Figs 2a to 2c).

Clinical implications

Results of this study indicated that when high-coppergold alloys and Panavia Ex were used for resin-bondedrestorations there was no significant advantage intin-plating the alumina-blasted surfaces to improve thetensile bond strength of resin to alloy. However, heattreatment following alumina blasting of high-coppergold alloys appeared to provide a highly significantincrease in the resistance to bond failure under tensileloading over that achieved by alumina blasting alone orused in combination with tin plating.

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Fig 1 Gold alloy specimen pair aligned ¡n a cementation jigprior to removal of excess cement and application ofoxygen-inhJbiting gel.

Fig 2a Group 1 specimen pair following fensiie testing,indicating adhesive failure, primarily involving the smallerspecimen.

Fig 2b Group 2 specimen pair following tensile testing,indicating adhesive failure involving both specimens simi-larly.

Fig 2c Group 3 specimen pair tollowing tensile testing,indicating cohesive failure ot the resin itself.

The heat treatment method described has a numberof clinical advantages and, being a simple procedure,can be applied to most gold alloys. Laboratorypreparation is no longer mandatory because hot airbtimers may be used at the chairside, where thecontinuous color change of the gold alloy restorationmay be monitored while the surface treatment isachieved." The oxide layer can cause discolorationwhen gold veneers are applied to restore the palatalsurfaces of thin maxillary anterior teeth. Considerationin such cases should be given to the application ofopaque resins as an alternative to Panavia Ex.

Trials are necessary to evaluate the durability andperformance of alumina-blasted and heat-treated high-copper gold alloys cemented with Panavia Ex. If theoutcomes of such studies support the early successreported, this technique may become readily appli-

cable to the clinical restoration of posterior occiusaland anterior palatai surfaces, as described in thefollowing case reports. The application of gold alloysin resin-bonded prostheses, however, will remainlimited because of the inability of these alloys to resistflexion when cast in thin section.

Case reports

Case !

A 24-year-old woman presented with a generalizedtooth surface loss and a history of anorexia combinedwith bulimia. The palatal surfaces of the maxillarycentral incisors were the most severely affected; theyexhibited considerable loss of incisai length andexposed dentin except for a halo of enamel (Figs 3aand 3b), Caries involving the palatal surface of the

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Figs 3a and 3b Extensive tooth surface loss, caries, and enamel halo at the maxillary central incisors.

Fig 3a Fig 3b

Fig 3c Gold alloy veneers in situ following cementation. Fig 3d View followitig addition of resin composite to themaxillary central incisors to improve esthetics and function.

right central incisor and surrounding a resin compositerestoration in the ieft central incisor was removed andrestorations were placed. The treatment was plannedwith the aid of mounted study casts and a diagnosticwaxup, which allowed gold alloy backings to beconstructed to ideal esthetics and function. Heat-treated backings were bonded to the palatal surfaces ofthe maxillary central incisors with Panavja Opaqueresin, which veneered the entire bonding surface of thegold alloy (Fig 3c). Resin composite was then appliedconventionally to the iabial deficits, to restore ap-pearance and function (Fig 3d).

Case 2

A 45-year-old patient complained of a grinding habitand short anterior teeth (Fig 4a). The periodontal

tissues were healthy and caries experience was insignif-icant. Surgical crown lengthening was undertaken forthe maxillary and mandibular anterior teeth with a viewto providing crowns for some of these teeth. However,the patient was delighted with the re-created length ofher front teeth. In conjunction with space creation,heat-treated gold alloy veneers were bonded withPanavia Opaque resin to the palatal surfaces of the sixmaxillary atiterior teeth. The right lateral incisorwas lengthened in the same mantier as was describedin case 1 (Fig 4b). Incisai and canine guidance wasreestablished on the backings. A posttreatmentcomplete-coverage, heat-cured, maxillary acrylic resinsplint was provided for night wear to limit thepotentially harmfiil effects of future parafunctionalactivity.

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Fig 4a Advanced tooth surface loss. Fig 4b View toliowing crown lengthening and addition ofresin composite to the maxiiiary right lalerai incisor.

Fig 5a Prepared mandibular firsl molar prior to cemenla-tion of goid aiioy onlay.

Fig 5b Fit surface of gold aiioy oniay foiiowing heattreatinent exhibits biue surface coioration, indicating thepresence of copper oxide.

Case 3

A 21-year-old dental suigery assistant presented with amandibular right first molar that had symptomsdiagnostically characteristic of a cracked tooth. Thedentition was intact and unrestored except for anocclusal amalgam restoration in the symptomatictooth. This restoration was removed and the tooth wasinvestigated. A vertical fracture line was identifiedbetween the lingual cusps. The molar was restored witha calcium hydroxide lining and amalgam. A decisionwas taken to provide a cusp-covering gold onlayextending linguocervically to protect the crackedcusps. Minimal tooth preparation was undertaken toprovide occlusal clearance (Fig 5a), while remainingabove the mesial and distal interdental contact points,A beat-treated gold alloy onlay (Fig 5b) was cementedwith Panavia Ex (Fig 5c). Symptoms resolved iltllywithin a few days.

Fig 5c Goid aiioy oniay foiiowing cementation.

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Discussion

In ihis in vitro study, as many variables as possible werecontrolled to achieve reproducible and comparableresults. The methods used were reliable to assesssurface treatments of gold alioys for resin adhesionunder tensile testing. Within the limitations of thisinvestigation, heat-treated gold alioy specimens ex-hibited a significantly greater resistance to bond failureunder tensile loading than did altimina-blasted ortin-plated specimens.

Photographs of fractured surfaces after loadingrevealed failure of an adhesive and/or adhesive-cohesive nature for alumina-blasted and tin-platedspecimens. Heat-treated specimens, however, exhibit-ed cohesive failure within the Panavia Ex resin,suggesting that the tensile bond strength between resinand gold alloy may have been greater than the cohesivestrength of the resin itself These results also suggestthat the surface oxide film achieved in the heattreatment process was of optimal thickness. If the filmis too thin, failure tends to be adhesive because of areduced reaction between Panavia Ex and the oxides;this mode of failure would resemble the failureobserved in the other two test groups. Conversely, ifthe film grows beyond the optimal thickness, cohesivefailure of the oxide layer itself can occur. In either case,the adhesive strength and durability of the bondbetween the resin and gold alloy are weakened,'^

Although the provision of heat-treated gold alloyveneers is a predictable and simple treatment optionfor the restoration of potentially complex problems,careful treatment planning and occlusal assessment areessential for success. The manufacturer's testing ofPanavia Ex resin has indicated a tensile bond strengthof 16 MPa to acid-etched enamel and 8 MPa to dentin.after immersion of specimens in water at 34°C for 24

hours prior to testing,'^ Further in vitro studies andclinical trials are indicated to determine the durabilityofthc bond between a variety ofresins and heat-treatedgold alloys, especially when their provision is beingconsidered in conjunction with large areas of exposeddentin.

AcknowledgmentsThe authors would like to Ihank Dr G, Pearson and Mr E, Davies of theBiomaterials Unit, Institute of Dental Surgery, for their adviee andassistance in the design and execution of the investigation. We are aisograteful to Miss Louise Thompson for the preparation of the manuscriptand Davis ttealthcare for Supplying Panavia Ex resin.

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