Dental Research Use of self-curing composite resins to ... · Use of self-curing composite resins to facilitate amalgam repair Alton M. Lacy* / Robert Rupprecht* " I Larry Watanabe*

Dental Research

Use of self-curing composite resins to facilitate amalgam repairAlton M. Lacy* / Robert Rupprecht* " I Larry Watanabe* * *

Resin cements, which have been shown to adhere to various metal alloys, wereinvestigated as possible repair adhesives for dental amalgam. Te.st bars of repairedamalgam, formed by condensing new alloy against previously set alloy with or withoutthe use of "adhesive" resins, were subjected to three-point bending measurements oftransverse fracture strength. The results indicated that application of adhesive resin didnot improve the breaking strength of the repaired specimens from that of specimensprepared without the tise of such resins. The breaking strength of ail repaired specimenswas approximately 15 % of the transverse fracture strength of the original intactamalgam bars. Scanning electron microscopy revealed that there was an intermixing offresh amalgam and unset interfacial resin, which led to mechanical bonding of thesematerials, but there was no evidence of adhesion of the resin to the previously setamalgam. {Quintessence Inl 1992:23:53-59.)

Introduction

While defective amalgam restorations are ideallytreated by replacement with new amalgam or an alter-native restorative material, it is often clinicaUy desirableto simply repair a defect in or adjacent to an existingrestoration. Such a treatment is not to be thought of asideal dentistry, but may be dictated by circumstancesof hmited time, limited finances, or clinical difficultiesthat would matidate against total removal and replace-ment of tbe restoration. When such repairs are per-formed, it would be desirable for the union betweenthe amalgams to be as strong and intimate as possibleto mitiimize microleakage and maximize strengtb ofthe final repaired restoration.

' Associace Professor, Department of Restorative Dentistry.University of California, San Francisco, School of Dentistry,San Francisco, California 94143-0758.

*• 1600 Wedgewood Drive, No. 409, Gurnee, tttinois oOO.il.'" Research Technician, Department of Restorative Dentistry,

University of California, San Francisco.

Hibler et al' have reviewed many investigations onthe strength of bonds between "new" and "old" amal-gam and have pointed out that the data in the litera-ture vary considerably depending on the age of tbeamalgam, tbe treatment of tbe old amalgam surfacebefore new amalgam is condensed against it, and themethod of testing. A recent report by Erkes et al'suggests that the repaired bond strength of two con-temporary amalgams falls within 17% to 31% of thefracture strength of unrepaired control specimens.Although pubhshed data vary, in general all studiesindicate tbat the bond strength of repaired amalgam issignificantly less than the original breaking strength ofintact amalgam.

Omura et aP and Masubara'' have recently reportedthe development of resin dental adhesives, now mar-keted as Panavia (Kuraray Co) and Superbond C & B{Sun Medical), tbat bond strongly to a variety of dentalalloys, most notably nickel-cbromium, Patiavia is apowder-liquid composite resin system coniairiing aproprietary pbosphate monomer, and Superbond is anunfilled 4-metbacryloxyethyl trimellitate anhydride{4-META) resin. In addition, another 4-META resin.Cover-up (Parkell), has recently been marketed forthe purpose of camouflaging amalgam restorations

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Dental Research

Fig 1 Schematic diagram ofpreparation for repair sets 1 to 9.

beneath a veneer of composite resin that bonds to theCover-up resin, which is applied to prepared amalgamsurfaces. There appears to be no data available on thebond strength of these resins to set amalgam, but theiruse for effectively joining fresh amalgam to toothstructure has been reported by several investigators.^"'These studies indicate that the use of these self-curingresins at the interface of amalgam restorations placedin extracted teeth results in a significant reduetion ininterfacial microleakage, suggesting that some bond mayform between the resin and the freshly placed amalgam.Staninec and Holt** have reported that the tensile bondstrength between amalgam and both dentin and enamelis greatly increased by applying a layer of Panavia tothe prepared tooth surface before condensing amalgam.Simiiar results have also been reported by Varga et al.^Staninec"' further reported that an adhesive resinbonding technique is more effeetive than are mechanicalundercuts in retaining amalgam restorations, andEakle et al" have reported that mesio-occlusodistalamalgam restorations placed in cavities treated withadhesive resins show higher fraeture strength than dosimilar restorations placed without cavity liner treat-ment.

The purpose of this study was to investigate theeffect of interfacial preparation, including the use ofself-curing "adhesive" resins, on the bond strength of

new amalgam condensed against previously set amal-gam, and to determine the physical nature of thebonds that may form between the resin and freshlycondensed or previously set amalgam.

Method and materials

Row charts describing the speeimen preparations areshown in Eigs 1 (specimen sets 1 to 9) and 2 (specimenset 10). For all tests, two pre-encapsulated amalgamswere used: Tytin (SS White Co), a high-copper, spher-ical, single-phase alloy, and Dispersalloy (Johnson andJohnson Dental Care Co), a high-copper, ad-mixedalloy. All test specimens were fabricated as 2 x 4 x12-mni rectangular bars. First, the new amalgam wascondensed by hand into a stainless steel mold; then a14-MPa load was applied for 60 seconds with a hydrauhcplunger. After storage of ah specimens for 24 hours at37X and 100% humidity, each specimen was bro-ken by three-point bending in an Instron universaltesting machine (Instron Corp) at a crosshead speed ofI mrn/min. The tratisverse strength was calculated bythe following formula:

S, =3WL

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where Wwas the fracture load (kg); L was the lengthof the specimen between supports (10 mm); ß was thewidth of the specimen (4 mm); H was the thickness ofthe specimen (2 mm), (Appropriate conversions wereapplied to express the data in MPa),

Specimen set 1 was prepared by condensing the amal-gam into the enfire mold space to form homogenousbars of alloy. Fifteen bars of each amalgam were pro-duced and these were designated T-O (Tytin-Original)aud D-O (Dispersalloy-Original).

For specimen set 2. fifteen of the half-specimens ofeach amalgam, recovered after the testing of set 1,were finished on one end with a fine, high-speed dia-mond and were then "repaired" by replacing theminto a Teflon mold {of the same shape as the original)and recondensing new amalgam against the old speci-men without further surface treatment. These specimenswere designated T-f (Tytin-fine diamond) and D-f (Dis-pers alloy-fine diamond).

For specimen set 3. one end of each of the remaininghalf-specimens from the broken set 1 was similarlyresurfaced with the fine diamond, and the half-bar ofamalgam was placed in tbe split Teflon mold, Panavia-EX resin, mixed according to manufacturer's directions,was painted onto the abraded surface and new amalgamwas condensed against this specimen using hand andhydraulic pressure as previously described. These twosets of 15 specimens were designated T-f-PAN andD-f-PAN.

For specimen set 4, 15 each of the original amalgamhalf-specimens recovered after testing of set 2 wereagain resurfaced with a fine diamond and SuperboodC iS; B was applied to the sample face before the speci-mens were repaired with new amalgam. These speci-mens were designated T-f-SB and D-f-SB,

For specimen set 5, 15 each of the original half-speci-mens recovered from test set 3 were resurfaced with afine diamond, and Cover-up was applied to the face ofthe half-specimen before new amalgam was condensedto complete the test specimen. These were designatedTf-CU and D-f-CU,

Specimen sets 6, 7, and 8 were designed to investigatethe effect of resurfacing the old amalgam with coarse,instead of fine, diamonds. Fifteen remaining half-specimens from set 3 were divided into groups of five.Each set was prepared as in sets 2, 3, and 4, but acoarse diamond was used for surfacing. No resin.Panavia, and Superbond were applied to sets 6, 7. and8, respectively, prior to condensation of new amalgam.These sets were designated T-c, T-c-PAN, and T-c-SB(and D-c, D-c-PAN, and D-c-SB), respectively.

10 X (H-PAN-T, D]

J - Composite

Composite

Fig 2 Schematic diagram for preparation of compositeresin-amalgam specimens (set 10).

Specimen set 9 was prepared without interfacialresin, but with a large, macroscopic undercut in thesurface of the old half-specimens that were recoveredfrom the original halves of set 4, The broken face ofthe half-specimens was undercut with a No, 33'/̂ in-verted cone bur by drawing it along the width of theface. Then new amalgam was condensed against theundercut face to complete the repair.

Observation of the data and the fractured surfacesindicated that the bond strength of the resins to thepreviously set amaigam was very low and that thebond strength of freshly condensed amalgam to resinwas relatively higher, A final set of specimens wasmade to shed more light on the interaction of freshamalgam and the self-curing resin. Test specimen set 10was made by first fabricating 10 half-specimens (foreach alloy) of Hercuhte composite resin (Kerr/SybronCorp), then surfacing the face of the specimens with afine diamond and applying Panavia resin. Amalgamwas then condensed against the Panavia to completethe specimens (see Fig 2), and they were stored andbroken as indicated previously. These sets were desig-nated H-PAN-T and H-PAN-D (according to the amal-gam used).

Scanning electron micrographs were taken of thebroken surfaces of representative specimens to assessthe nature of the break and the adhesion of resin tothe specimens. The fracture strength data wasanalyzed with a two-way analysis of variance and theStudent-Newman-Keuls test to determine significantdifferences between sets of data.

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Table I Transverse breaking strength (MPa) of repaired amalgam and amalgam-composite resin specimens

Tytin Dispersalloy

Set Interfacial treatment Jo.

15

15

15

15

15

5

5

5

15

10

Code

T-O

T-f

T-f-PAN

T-f-SB

T-f-CU

T-c

T-c-PAN

T-c-SB

T-uc

H-PAN-T

Strength

149.0 d

24.8 d

20.7 d

19.4 d

12.8 d

24.8 d

21.0 d

19.9 Ú

19.3 H

41.8 d

b 18.6

: 6.0

: 3.8

: 11.1

: 7.2

r 15.0

: 2.2

: 4.0

1 6.8

: 12.0

Code

D-O

D-f

D-f-FAN

D-f-SB

D-f-CU

D-c

D-c-PAN

D-c-SB

D-uc

H-PAN-D

Strength

110.2 + 22.9

13.1 ± 5.6

12.6 + 2.1

11.8 ± 3.2

22.0 ± 3.0

20.7 ± 3.0

11.6 ± 1.6

11.6 + 4.4

31.1 ± 6.1

39.8 ± 9.8

1. Original amalgam sample

2. No resin (fine diamond)

3. Panavia (fine diamond)

4. SuperbondC & B (fine diamond)

5. Cover-Up (fine diamond)

6. No resin (coarse diamond)

7. Panavia (coarse diamond)

8. Superbond C&B (coarsediamond)

9. No resin (undercut interface)

10. Tytin condensed against Herculitewith Panavia

Results

The average breaking strengths of all sets of speci-mens are given in Table 1 and are shown graphically inFig 3. Among all amalgam specimens that were re-paired with new amalgam condensed against relativelyflat surfaces (sets 2 to 8), there were no statistieallysignificant differences in strength of any of the sets, ir-respective of alloy or interfacial treatment used.

The interfacial bond strength of specimens undercutprior to repair with new amalgam (set 9) was shghtly(and for Dispersalloy, significantly) greater than sim-ilar specimens repaired without undercuts (sets 2 and6). The overall average bond strength of all amalgamspecimens repaired with new amalgam was approxi-mately 15% of the transverse fracture strength ofintact, original (ie, unrepaired) samples. There was nostatistically significant difference between bondstrengths of specimens resurfaced with fine diamondsand those treated with coarse diamonds.

When the two alloys were compared, the averageStrengths of the Dispersalloy specimens were generallylower than those of Tytin, but the difterences were notstatistically significant. Exceptions to this trend werethe specimens joined first by undercutting (set 9) and

the specimens treated with Cover-up (set 5); in thesecases, repaired Dispersalloy was stronger than Tytin.

The transverse breaking strength of Hercnlite towhich amalgam was bonded (set 10) was significantlygreater than that of any of the amalgam specimensthat were repaired with amalgam. The average breakingstrength of these amalgam-composite resin specimenswas roughly 30% of the strength of the intaet amalgam.

Discussion

Scanning electron micrographs of the broken surfaceswere taken in the orientation illustrated in Fig 4. Figures5a and 5b show the typical broken interfaces of T-f,which was formed simply by condensing new amalgamagainst the abraded surface of previously set Tytin.The presence of abrasion grooves on the face of theold amalgam (Fig 5b) after fracture suggested thatthere was no substantial bonding of the new amalgamto the old. Examination of the freshly condensedamaigam surface of the sample showed relatively poorreproduction of the opposing cut surface by the newlycondensed amalgam (Fig 5a).

Figures 6a and 6b show the approximal surfaces of

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Fig 3 Flexural breaking strengthof original and repaired amalgambars.

180

160-

140-

130-

100 .

80 -

60

40

20

O

FLEXURAL BREAKING STRENGTH (MPa)

r- 0 - f -f-PAN -t-SB -f-CU - c -c-PAN -C-SB -uc -HPAN

Tytin I 1 Dispersalioy

SAMPLE ORIENTATiONFOR SEM VIEWING

Fig 4 Sample orientation for scanning electron micro-

scopy.

Fig 5b (nght) Broken interlace of "origina]" half of speci-men T-f. There is no evidence of any bonding cf the old andnew amalgams. {Original magnification x 500.)

Fig 5a Broken interface of freshly condensed Tytin amal-gam from broken specimen T-f (no adhesive resin applied),(Original magnification x 500,)

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(NEW) iííVMALGAW

i, ¡ FACE •' .

• I , '• •• ',

, r. • PANAVIA

^ • ; ; t .1

Fig 6a Broken interface of freshly condensed amalgam ofspecimen T-f-PAN (Panavia painted onto tbe surface of theoriginai amalgam prior to condensation of the new Tytin.]Tbe Panavia is joined to tbe Tytin. {Original magnificationX 500.)

Fig 7a Broken interface of freshiy condensed amalgam ofspecimen Herc-PAN-T (Panavia painted onto the surface ofHerculite composite resin prior to condensation of Tytin}.There is no evidence of attachment ot Panauia to tbe amal-gam. (Original magnification x 500.)

Fig 6b Broken interface of the original haif of specimenT-f-PAN. Tbere is no evidence of bonding of Panavia to thepreviously set amaigam. (Originai magnification x 500.¡

Fig 7b BnDken interface of the Herculite side of specimeniHerc-PAN-T. Tbere is complete joining of Panavia and Her-culite resin. Fracture at the amalgam-Panavia interface hasleft scattered particies of Tytin amalgam embedded in thePanavia. Tbis indicates a comparatively weak union betweenfresbly condensed amalgam and Panavia. (Original magnifi-cation X 500.)

T-f-PAN. All the interfacial resin adhered to the newlycondensed amalgam (Fig 6a) and bad broken cleanlyfrom the abraded surface of the old amalgam (Fig 6b).Similar results were seen in all specimens in whichSuperbond or Cover-up had been used as an interlacialtreatment. None of the resins showed any evidertce ofattachment to previously sel Tytin or Dispersalloy.

Figures 7a and 7b show the interfaces of a typicalspecimen of Herc-PAN-T (Tytin joined to Hercuhtecomposite resin through an interfacial layer ofPanavia), In tbis case, separation of materials occurredbetween the freshly condetised amalgam (Fig 7a) and

the Panavia resin; residual amalgam particles wereembedded in the Panavia {Fig 7b). The Panavia chem-ically bonded (copolymerized) with the Hercuhte, andthis polymer bond was evidently stronger than thehnkage of tbe amalgam alloy to the Panavia.

It is not known if the attachment of the newly con-densed amalgam to these resins involved only mechan-ical interlocking of tbe resin witb the alloy particles orif some measure of adhesive chemical bond also contri-buted. It is clear, however, that no strong bond formedbetween these adhesive resins and previously set amal-gam. The observed intimate attachment of resin to

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freshly condensed amalgam supports previous studiesshowing that these resins are effective in reducingmicroleakage between amalgam and tooth. Resinfilms may, therefore, permit bonding of fresh amalgamto etched enamel, glass-ionomer cement, and treateddentin, and this may have important chnical implica-tions in using amaigam to strengthen weakened toothstructure.

Conclusions

1. After surface abrasion, with and without the appli-cation of interfacial resins, the repaired transverseflexural strength of dental amalgam was approxi-mately 15% of that of unrepaired, intact amalgamwhen measured by three-point bending of rectangu-lar specimens.

2. The use of coarse versus fine diamonds for surfacepreparation had no significant effeet on bondstrength of repaired amalgam samples.

3. The use of adhesive resins (Panavia EX, Super-bond C & B, and Cover-up) did not improve thestrength of bond between old and new amalgam,primarily because there was no evident bonding ofthe resin to the prepared surface of previously setamalgam.

4. Stronger repairs were formed through the use ofinterfacial mechanical undercuts that locked thematerials together and increased the surface area ofcontact,

5. Adhesive resins bonded intimately to the surface offreshly condensed amalgam, presumably throughmechanieal interlocking with the irregular surfaceof the freshly condensed amalgam. This observa-tion validates the use of "adhesive" resin to bondamalgam to etehed dental surfaces or existing com-

posite resin in efforts to control microleakage andto support weakened tooth structure.

6. Since the bond of composite resin to previonsly setamalgam was weak, and appeared to be strictlymechanical, claims by manufacturers of uniqueadhesion of composite resin veneering products toamalgam for purposes of camouflaging the darkcolor of amalgam were not borne out by the resultsof this study.

References

t. Hibler JA. Foor JL, Miranda FH, et al: Bond strenglh com-parison of repaired dental amalgams. Quintessence lnt 1988;19:411^15,

2. Erkes EO, Burgess JO, Hornbeck DD: Amalgam repair: anin vitro evaluation of bond integrity. Gen Dent 1990;38:303-205.

3. Omura I, Yanjsuchi J, Harada I, et al: Adhesive and tnechan-ical properties ot a new dental adbesive. J Dent Res 1984;63:233 {abstr No. 561).

4. Masuhara E: A new adhesive resiti consisting ot 4-META.ShikaiTembo 1982 ;59:611-670.

5. Shintiiu A,Takashi U. Kawakami M, et al: Adhesive amalgamrestoration with resin cement lining. lapan J Comerv Deiut987;30(4):68-75.

6. Shimiiu A, Ui T. Kawakami M; Microleakage of amalgatnrestorations with adhesive resin cement lining, glass ionomercement base and fltioride treatment. Dent Mater I (lapan)1987 ;6:64-69.

7. Ben-Amar A, Nordenberg D. Liberman R, et al: Tbe controlof marginal microleakage in amalgam restorations using dentinadhesive: a pilot study. Dent Mater 1987;3:94-96.

S. Staninec M, Holt M: Tensile adhesion and microleakage inresin bonded amalgam restorations. J Prosthet Dem t988;59:397^02.

9, Varga J, Matsumura H. Masuhara E: Bonding of amalgamfilhng to tooth cavity with adhesive resin. Dem Mater.! (Japaiil19S6;5:158-164,

10. Staninec M: Retention of amalgatn restorations: undercutsversus bonding. Quintessence lnt 1989:26:347-351.

11. Eakle WS, Staninec M, Lacy AM, et al: Effect of bondedMOD amalgams on fracture resistance of teeth. J Dem Res1990;69:287 (absti No. 1429). D

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