effect of fluorides on the bonding of resins to phosphoric acid-etched bovine enamel

Archs oral Biol. Vol. 17, pp. 1037-1045, 1972. Pergamon Ress. Printed in Great Britain.

EFFECT OF FLUORIDES ON THE BONDING OF RESINS TO PHOSPHORIC ACID-ETCHED BOVINE ENAMEL

Z. SHEYKHOLESLAM, M. G. BUONOCORE and A. J. GWINNETT

Eastman Dental Center, 800 Main Street East, Rochester, New York 14603, U.S.A. and University of Western Ontario, London, Canada

Summary-Various fluorides, such as NaF, SnF,, acidulated NaF, TiF,, and ZrF, as well as casein and saliva, were applied to acid-conditioned enamel surfaces to study their effect on bonding, resin penetration and wettability of enamel. Topical treatment of ground-etched enamel with these agents, prior to application of Sevriton self-curing resin adhesive, generally resulted in significant reductions in bond strength.

It was shown by SEM that fluoride application produced reaction products, or pre- cipitates, which covered the surface and appeared partly to fill inter-prismatic spaces, interfering with resin penetration and tag formation which are considered responsible in large measure for the bonding obtained on enamel surfaces. The formation of fluoride reaction products also reduced the wettability of the surface, a situation also conducive to lower bonding strengths.

Titanium tetrafluoride produced an acid-resistant, coherent film which floated off the enamel surface after its decalcification in 5 per cent HCI.

The findings support the idea that, in the clinical use of adhesives for sealing pits and fissures against caries, topical fluoride treatments or contact with saliva should be avoided subsequent to acid-conditioning of the enamel surface. Fluoride treatments, however, may be given subsequent to adhesive application.

INTRODUCTION

SEVERAL studies support the idea that the bonding of resin adhesives to the labial surfaces of bovine and permanent human incisor teeth is associated with and en- hanced by the formation of “prism-like” tags (GWINNETT and MATSUI, 1967;

BUONOCORE, MATSUI and GWINNEIT, 1968; SHEYKHOLESLAM, 1970). Tags are also formed by an ultraviolet light polymerized adhesive used clinically for sealing pits and fissures for caries prevention (BUONOCORE, 1971a). These structures appear to be attached to and project from the under surface of the adhesive after dissolving away the enamel to which the adhesive had been bonded. Tags presumably result from the penetration of adhesive into spaces created in the enamel by acid etching employed to condition the enamel surface for bonding. The tags may be considered a mechanical combination of the organic matter of enamel and the adhesive.

The present research was undertaken to study the effect of various topical fluoride treatments on the bonding of a self-curing acrylic resin to enamel surfaces, tag formation and the wettability of enamel. Etched enamel surfaces were employed as they pick up considerably greater quantities of fluoride than unetched surfaces (MYERS, HAMILTON and BECK, 1952; BRUDEVOLD et al., 1957 ; BRUDEVOLD, 1968 ; CASLAVSKA et al., 1971; DE PAOLA, AASENDEN and BRUDEVOLD, 1971) and because the effect of this fluoride acquisition on factors affecting bonding have not been previously studied.

1037

1038 Z. SHEYKHOLESLAM, M. G. BUONOC~RE AND A. J. GWINNEIT

The information obtained could be applicable to the clinical technique of pit and fissure sealing, where acid etching of the enamel is employed prior to application of the resin adhesive.

MATERIALS AND METHODS

Fresh bovine incisors were employed. Following removal of the root, adherent soft tissue and pulp, a slab of approximately 6.5 x 9.8 mm of the flattest labial enamel was outlined and cut out. A round aluminum foil mold was prepared and the enamel slab was fixed to the base of the mold using double-faced adhesive tape. Bioplastic embedding medium (Ward’s Natural Science Establish- ment Rochester, N.Y.) was [poured into the mold until the dentine base of the enamel slab was covered with about 6.5 mm of the embedding medium. After hardening of the Bioplastic, the enamel surface was ground flat using sandpapers of 180 to 320 grades (3M Company, St. Paul, Minnesota) and finished by a few minutes rubbing on No. 600 sandpaper and on alumina powder of 0.5 pm particle size on a glass slab. The opposite side of the block was made approximately ;parallel to the enamel surface by hand-sanding on coarse grade sandpaper. The prepared blocks were examined carefuliy under a dissecting microscope and those showing dentine exposure through the enamel were discarded.

Groups of seven blocks containing the embedded and polished enamel slabs were selected for each test. The enamel surfaces were first etched for 60 set with 50 per cent phosphoric acid containing 7 per cent ZnO by weight, then treated with the following aqueous fluoride solutions for five minutes; 8 per cent freshly prepared stannous fluoride, 2 per cent zirconium tetrafluoride, 1 per cent titanium tetrafluoride, 2 per cent acidulated sodium fluoride or 2 per cent “neutral” sodium fluoride, and then washed under running water. In addition, 1 per cent casein solution was used for 5 and 30 min, and whole saliva for 5 and 24 hr. Ethylenediamine-treated blocks were prepared by boiling flat, ground enamel slabs in an 80 per cent aqueous ethylenediamine solution in a Soxhlet apparatus (WILLIAMS and IRVINE, 1954) for 3 hr. They were then subsequently embedded in partially hardened Bioplastic, face up, without double-faced adhesive tape, and the surface was not further ground after the Bio- plastic hardened. Natural and ground enamel surfaces, and surfaces which were ground and acid- etched, served as controls. Subsequent to washing, surfaces were dried with compressed air.

A 5-mm dia. hole was punched into a 12.5 mm square of waterproof plastic adhesive tape (Cat. no 190-A, 3M /Company, St. Paul Minnesota). The tape was then pressed onto the compressed air-dried, treated enamel surface to expose a 19.62 mm* window of enamel at approximately the centre of the enamel. To prepare a specimen for tensile testing, a U-shaped, steel-pin positioning device having two legs connected by a flat horizontal bar, the ‘surface of which is parallel to any flat surface on which the legs rest, was used (Fig. I). Two holes were precision-drilled through the top bar at right angles to the surface to sleeve-fit a threaded steel pin. A mixture of Sevriton (Amalgamated Dental, London, England) self-curing resin prepared according to the manufact- urer’s directions was placed as a thin layer over the exposed enamel window surface. The embedded enamel slab was then placed under the positioning device and the threaded end of the pin centered on the resin-covered enamel surface. Additional resin was applied over the original resin and the threaded end of the pin, to produce a cone-shaped retaining mass as shown in Fig. 1. After polymerization, the adhesive tape was removed, leaving the resin in contact with the enamel previously exposed by the window. The areas ‘of enamel outside the periphery of the adhesive cone were used as support against which the pin was pulled in tension. Tests were made 1 week after specimen preparation, and storage in water, on an Instron testing machine operated at a crosshead speed of 0.2 in./min.

For contact angle measurements (GLANTZ and NYQUIST, 1966; GLANTZ, 1969, SHEYKHOLESLAM, 1970) and SEM studies, enamel blocks were similarly prepared and tested after treatment with the various fluorides. The measurement of the contact angle was done through a horizontally positioned microscope optical system, the ocular of which contained a protractor scale. A micropipette fitted with a needle was employed to deliver the drop of water to the enamel surface. While observing through the microscope, the enamel surface was brought close to the needle by means of a jack and a droplet of water released approximately at the centre of the enamel surface. The jack was then lowered to remove the needle from the drop. The contact angles were measured when the liquid drop ceased to spread over the enamel surface and had reached an apparent state of equilibrium. The position of the drop was adjusted so that one edge of the drop was at the centre of the crosslines of the protractor and the other edge in line with the zero point of the protractor. By turning the ocular containing the protractor, a tangent passing through the intersection of the crosshairs could be made with the arc of the

BONDINGOFRESINSTOPHOSPHORICACID-ETCHEDENAMEL 1039

drop. The degree of turn to accomplish this is the contact angle which could be read directly from the protractor.

For tire SEM studies, specimens were shadowed with gold-palladium at 45” and examined with a Cambridge Stereoscan electron microscope (Cambridge Instrument Company, Glendon Instruments, Toronto, Canada) operated at 10 kV. The presence of resin tags was determined by the method o Gwm and MAT~UI (1967) after dissolving the enamel from the resin.

RESULTS

Bonding results

Figure 2 shows a plot of the means and the 95 per cent confidence intervals of

bonding strengths of Sevriton self-curing acrylic resin to enamel surfaces treated with various test materials. A bonding value was considered to have a statistically significant difference from another if its lower confidence limit was greater than the higher confidence limit of the other. Only the reduction obtained with acidulated sodium

fluoride was not significantly different from the control. Zirconium tetrafluoride and

1400 r

G ground

E etched

ED ethylenediamine

Fro. 2. Graph of means and 95 per cent confidence intervals of bonding strengths of Sevriton self-curing acrylic resin to acid conditioned, bovine enamel surfaces treated

with various fluorides, cesein and saliva.

1040 Z. SHEYKHOLESLAM, M. G. BIJONOCORE AND A. J. GWINNETT

stannous fluoride resulted in approximately 50 per cent reductions in bonding strength, with sodium fluoride showing a somewhat smaller reduction. Saliva and casein showed similar large reductions in bonding strengths while treatment with titanium tetrafluoride and ethylenediamine resulted in complete loss of bonding.

Surface effect of jluoride treatment

The effect of fluoride treatment of etched enamel surfaces is shown in Figs. 3b-3f. The enamel surface after the SnF, treatment was indistinguishable from the control etched surfaces (Fig. 3a). Sodium fluoride treatment produced what appeared to be a surface almost completely covered by small, ball-like structures (Fig. 3b). Enamel surfaces treated with acidulated sodium fluoride appeared covered with an amorphous type deposit (Fig. 3~). A mass of globular deposits was produced on the surface treated with titanium tetrafluoride (Fig. 3d). This gross covering of the enamel surface was a relatively durable, acid resistant, coherent film, which remained after decalcification of the enamel in 5 per cent HCl. Globular, hollow, pear-shaped deposits of varying size resulted from treatment with zirconium tetrafluoride (Fig. 3e).

Contact angle measurements

As revealed by contact angle measurements utilizing water, ail the etched, fluoride- treated enamel surfaces were less wettable than etched-only controls as indicated by distinct contact angles rather than spontaneous spreading (Table 1). The smallest contact angle was shown on acidulated sodium fluoride-treated enamel. The titanium tetrafluoride-treated surfaces showed the largest contact angle of all the fluorides and also showed complete absence of bonding. The low bonding values obtained with casein solution and whole saliva are also consistent with the rather high contact angles observed, which approximate those of natural enamel to which the resins do not bond.

SUhCe Treatment condition agent

Average Treatment contact time (min) angle* s. D.

i2%: Natural-etched Ground-etched Ground-etched Ground-etched Ground-etched Ground-etched Ground-etched Ground-etched Ground-etched Ground-etched Ground-etched Ground-etched

mt- ED - NaF NaF, H,POJ

sz:FF’ TiF: Casein ;$?‘g,

Saliva

200

-

2% 5

5 5

25.6 39.5 12.5 13.7

Spontaneous spreading Spontaneous spreading Spo;$%$eous sp$$ng

14.5 13.1 20.1 +3.5

5 is.5 ZI.2 : 27.2 2:::

30 :;:; C2.8 5 13.5 f2.2

24 hr 35.2 12.4

* Average of at least 10 tests. t Ethylene diamine.

BONDING OF RESINS TO PHo(pPHORIC ACID-ETCHED ENAMEL 1041

Tag formation

Resin penetration into enamel as evidence by tag formation was classified according to its amount and length, ranging from complete coverage of the undersurface of the resin (+ + + +) to questionable coverage (f) (SHEYKHOLESLAM, 1970).

Table 2 shows the amount and length of tags after fluoride treatment compared to control surfaces. It ranged from questionable presence of tags for TiF, and ZrF, treated surfaces, where gross reaction products were evident, to (+ + +) for SnF,, where there was little or no evidence of reaction products. It was interesting to note that relatively few tags were obtained from acidulated sodium fluoride-treated enamel surfaces although bonding strength was high.

Surface condition

Tw amount

Range of tag length

(a)

N-E G-E N-E SnFl G-E SnF, G-E NaF G-E NaF

Hap04 G-E ZrFa G-E TiFI G-E ED.

(i++)-(++++) 7-14 ((w]-(++++, IO-14

15-20 (+++) 15-20

#:[&+’ 6-8 -

(f) -

c’i: +H+ + + f) 7-10

Key: N-Natural; G-Ground; E-Etched; ED-Ethylenediamine.

DISCUSSION

The fact that various fluorides used in treating the enamel surface subsequent to etching reduced the bond strength of resins could be related to the changes they produced on the enamel surface. The relative absence of resin penetration (tags) into enamel surfaces after treatment with titanium and zirconium tetrafluorides may be in large part attributed to the formation of reaction products as shown by SEM. The presence of these products, presumably partly or completely filling interprismatic and other spaces, would prevent or interfere with tag formation, thus reducing mechanical retention and resulting in lower bonding values.

The strength of attachment of the reaction products to the enamel surface could be of considerable importance in determining the bonding strength of the adhesive- enamel assemblies. If the reaction products are strongly attached to the enamel surface and in turn the adhesive is capable of strongly bonding to them, then a strong resin bond would result. This could be the case with the acidulated sodium fluoride since, in spite of the fact that few or no tags were obtained, the bonding strength was not significantly different from controls. The relatively low contact angle produced on surfaces treated with this fluoride would also be conducive to bonding. The presence of phosphoric acid in the acidulated fluoride could account for its different behaviour compared to “neutral” sodium fluoride.

1042 Z. SHEYKHOLESLAM, M. G. BUON~~ORE AND A. J. GWINNETT

The formation of reaction products as a result of fluoride treatment, however, generally appears to interfere with the formation of tags, and/or also reduces the wettability of the surface, both situations conducive to lower bonding strengths and greater susceptibility of the bond to disruption by water. The latter could result from gradual dissolution of the intermediate layer or through separation of the phases by ingress of water at the resin-enamel interface.

The highest bonding values, which were obtained with etched surfaces, generally were more representative of the cohesive strength of the adhesive rather than the forces required to bring about interfacial separation. With the higher bonding values, large proportions of adhesive remained bonded to the enamel surface, with the tensile break occurring in the body of the resin at the threaded end of the pin which is an area of potential stress concentration. The breaks were of the cone-saucer type, indicative of a ductile fracture (REED-HILL, 1964). Thus, the strength of the bonding between resin and tooth structure is higher than that shown by the tensile values obtained. As bonding strength decreased, it was apparent that less resin remained attached to the enamel surface. The low bonding values obtained with etched, ethylenediamine- treated surfaces probably represented, in large measure, fracture of the enamel surface weakened as a result of the removal of the organic matter by the ethylenediamine. This idea was supported by the diffuse whitish appearance of the undersurface of the resin, suggesting that a thin layer of anorganic, brittle enamel was entrapped in the resin. The results reported herein with bovine teeth are similar to the findings obtained utilizing human teeth, particularly with regard to the fluoride reactions (GWINNETT, BUONOCORE and SHEYKHOLESLAM, 1971).

The low bonding values obtained with casein solution and whole saliva suggest that the absorbed casein has a low affinity for the resin as do the deposits from saliva and compares with the complete absence of bonding on unetched, natural enamel surfaces.

The use of fluoride treatments on etched surfaces, prior to the application of the self-curing resin, would appear to reduce initial bonding strengths and, because of the presence of the intermediate gross reaction products layer, the bonding assembly might be more susceptible to bonding disruption by prolonged water immersion. The initial loss of bonding strength associated with fluoride pre-treatment is probably the result of interference with tag formation and related to the poorer wettability of the surface, both of which are undesirable from a clinical aspect. Pre-treatment of etched enamel with the various fluorides tested would appear, therefore, to be contraindicated in the clinical application of adhesives for caries prevention in pits and fissures. Fluoride treatment, however, could be instituted subsequent to adhesive application to protect the surfaces of the tooth not protected by adhesive. The results also support previous recommendations of BUONOCORE (1971) that after etching of the enamel surface, exposure to saliva should be avoided to reduce surface adsorption of salivary constituents which could also result in lower bonding strengths.

Acknowledgements-This work was supported in part by Grant DE 2969 from National Institute of Health and was part of a thesis submitted by Dr. SHEYKHOLIZSLAM

BONDING OF RESINS TO PHOSPHORIC ACID EKHED ENAMEL 1043

in partial fulfillment of the requirements for the MS. Degree at the University of Rochester, School of Medicine and Dentistry, 1970.

R&m&Des fluorures divers comme les NaF, SnF*, NaF acidulk, TiF,, et ZrF, aussi bien que la caseine et la salive ont 6tB appliqu&s sur des surfaces d’&nail condition&s par l’acide, pour Ctudier leurs effets sur la cohesion, la p&&ration de la &sine et l’humiditb de l%mail. Le traitement local par dkapage de 1’Cmail avec ces agents, avant I’application de l’adhbif rksineux Sevriton-pour auto-gu&ison-r&ulta g&&alement dans une rkduction significative de la force de coh6sion.

On a montrk par SEM que l’application de fluorure provoquait des produits de r&action ou des pr&ipit& qui couvraient la surface et paraissaient remplir partiellement des espaces inter-prismatiques, g&ant la ptnttration de la &sine et la formation de ferrets (noyaux durs), qui sont consid&& responsables en large mesur pour la cohbion obtenue sur les surfaces d’kmail. La formation de produits de &action de fluorure, reduisit aussi l’humidite de la surface, une situation qui contribue aussi & l’abaissement de la force de cohtsion.

La t&rafluorure de titanium produisit une pellicule acidor&istante adherente, qui flottait &par& de la surface d’email, apr&s sa d&calcification dans I’HCl B 5 pour cent.

Les r&ultats de cette etude soutiennent l’id& que dans l’usage clinique d’adhtsifs pour sceller les cavitCs et les fissures contre les caries, les traitements locaux avec fluorure ou le contact avec la salive devraient &tre &it6 apr& le conditionnement de la surface de 1’Cmail. Cependant, des traitements avec fluorure peuvent etre don& apr& I’application d’adhesifs.

Zusammenfassung-Verschiedene Fluoride wie NaF, SnF2, saure NaF, TiF4 und ZrFL, wie such Kasein und Speichel wurden auf SBure-konditionierte Email. Ober- flkhen aufgetragen, urn ihre Wirkung auf Bindung, Harz-Durchdringung und die Netzbarkeit von Email zu priifen. Topische Behandlung von grundgeltztem Email mit diesen Mitteln, vor der Applikation von Sevriton ‘self curing resin adhesive’ (selbstkon- solidierenden Harzbindemitteln) endete im allgemeinen in beachtenswerten Minderungen der Bindestlrke.

SEM zeigte dass Fluoridzusatz Reaktionsprodukte erzeugte, die die Oberfllche bedeckten und teilweise die inter-prismatischen Liicken zu fiillen schien, indem er die Harzdurchdringung und die Spitzenformung stiirte, die in grossem Masse fiir die Bindung, die auf Emailoberfl&chen erzielt wird, verantwortlich sind. Die Formung von Fluorid Reaktionsprodukten verminderte ebenfalls die Netzbarkeit der OberflBche, was such zu geschwschter Bindkraft fiihrte.

Titan tetrafluorid erzeugte eine slurefeste, zusammenhlngende Schicht, die sich von der Email-OberflBche lBste, nach deren Entkalkung in 5-prozentigem HCI.

Die Resultate dieser Studie bestltigen die Feststellung, dass, in klinischer Behand- lung, als Bindemittel fiir das Verschliessen von HGhlen und Spalten gegen Caries, topische Fluoridbehandlungen oder Beriihrung mit Speichel, nach der Stiurebehandlung der Emailoberfliiche, vermieden werden sollten. Doch kiinnen Fluoridbehandlungen nach Bindemittel-Applikation, angewandt werden.

REFERENCES

AASENDEN, R., BRUDEVOLD, F. and MCCANN, H. 1968. The response of intact and experimentally altered human enamel to topical fluoride. Avchs oral Eiol. 13, 543-552.

BRUDEVOLD, F., HEIN, J., BONNER, J., NEVIN, R., BIBBY, B. and HODGE, H. C. 1957. Reaction of tooth surfaces with one ppm of fluoride as sodium fluoride. J. dent. Res. 36,771-779.

BUONOCQRE, M. G., MA~SUI, A. and GWINNET, A. J. 1968. Penetration of resin dental materials into enamel surfaces with reference to bonding. Archs orat Biol. 13, 61-70.

BUONOCORE, M. G. 1971. Caries prevention in pits and fissures sealed with an adhesive resin polymerized by ultraviolet light: a two-year study of a single adhesive application. J. Am. dent. Ass. 82, 1090-1093.

BUONOCORE, M. G. 1971a. Adhesives for caries prevention. Presented at Symposium on the Chemistry and Physiology of Enamel, 17-18 September, 1971, University of Michigan Press. In press,

1044 Z. SHEYKHOLESLAM, UONOCORE AND A. J. GWINNETT

CASLAVSKA, V., BRUDEVOLD, F., VRBIC, V. and MORENO, E. 1971. Response of human enamel to topical application of ammonium fluoride. Int. Ass. Dent. Res. Preprinted abstracts, 49th Gen. meeting. Abstract 64.

DE PAOLA, P., AASENDEN, R. and BRUDEVOLD, F. 1971. The application of acidulated phosphate- fluoride preceded mild enamel etching. Int. Ass. Dent. Res. Preprinted abstracts, 49th Gen. meeting. Abstract 149.

GLANTZ, P.-O. 1969. On wettability and adhesiveness. Odont. Revy 20, Suppl. 17. GLANTZ, P.-O. and NYQUIST, G. 1966. The effect of topical fluorine applications on the free surface

energy of dentine. Odont. Revy 17, 332-335. GWINNETT, A. J., BUONOCORE, M. G. and SHEYKHOLESLAM, Z. 1971. Effect of fluoride on etched

enamel as seen by scanning electron microscopy. Int. Ass. Dent. Res. Preprinted abstracts, 49th Gen. meeting. Abstract 122.

GWINNETT, A. J. and MATSUI, A. 1967. A study of enamel adhesives, the physical relationship between enamel and adhesive. Archs oral Biol. 12, 1615-1620.

MYERS, H., HAMILTON, J. G. and BECK, H. 1952. A tracer study of the transfer of F18 to teeth by topical application. J. dent. Res. 31, 743-750.

REED-HILL. 1964. Physical Metallurgy Principles, Van Nostrand, Princeton, New Jersey. SHEYKHOLESLAM, Z. 1970. Bonding of adhesive resins to enamel surfaces. M. SC. Thesis, University of

Rochester, New York. WILLIAMS, J. B. and IRVINE, J. W., Jr. 1954. Preparation of the inorganic matrix of bone. Science

119,771-2.

BONDING OF RESINS TO PHOSPHORIC ACID ETCHED ENAMEL

FIG. I. Paralleling device allows a threaded pin to be placed perpendicular and at the centre of an enamel area exposed through a window in a piece of plastic tape placed over the enamel block. Here a cone of adhesive has been built up by applying it around the pin and covering the enamel. The tape was removed after polymerization of the

adhesive.

PLATE 1

A.O.B. f.p. lC44

Z. SHEYKHOLFSLAM, M. G. I~UONOCOR~ AND A. J. GWINNFTT

BONDING OF RESINS TO PHOSPHORIC ACID ETCHED ENAMEL 1045

PLATE 2

FIG. 3a. Scanning electron micrograph of bovine enamel surface etched for 60 set with 50 per cent phosphoric acid. Note that prism periphery appears to be etched more than the prism core (PC). The following figures represent enamel etched in the same way and subsequently treated with various fluorides for 5 min. In all instances, bar equals 2 pm.

FIG. 3b. Treated with 8 per cent stannous fluoride solution. Note absence of visible gross alteration of this surface, which is similar to the untreated surface of Fig. 3a.

FIG. 3c. Treated with 2 per cent sodium fluoride. Note globular deposits, presumably calcium fluoride, on the surface. Interprismatic areas seem to be relatively free of such

deposits.

FIG. 3d. Treated with acidulated sodium fluoride. Note an amorphous type deposit on the surface. Interprismatic areas seem to be relatively free of such deposits.

FIG. 3e. Treated with 1 per cent titanium tetrafluoride. Note mass of globular deposits on the surface. Interprismatic regions (I) seem to be relatively free of such deposits.

FIG. 3f. Treated with 2 per cent zirconium tetrafluoride. Note heavy deposits on the surface including globular and mushroom-shaped forms.

effect of fluorides on the bonding of resins to phosphoric acid-etched bovine enamel

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