Australian Dental Journal, June, 1977
Volume 22, No. 3
Factors affecting the structure of a fissure sealant at the enamel/sealant interface::Robert J. Breakspere, B.Sc., Ph.D., C.Chem., M.R.I.C. AND
Allan Wilton, B.Sc.
ABsTRAcT-Scanning electron microscope study of a fissure sedant on enamel in vilro shows the effect of varying methods of application, viscosity, dryness and time of etching of enamel. The effective attachment of the sealant to enamel required that it be brushed into the clry etched enamel. Viscosity seemed unimportant. (Received for publication June, 1976)
Introduction The technique in which fissures are sealed with a polymeric material is now an accepted method of great potential in preventive dentistry. However, results of clinical trials, to date, are not always in agreement. For instance, Ripa and Cole1 using methyl-2-cyanoacrylate as a sealant, found that 91.2 per cent of treated teeth retained their sealant after three months, 68 per cent after six months but only 32 per cent after 12 months. An 84.3 per cent reduction in occlusal caries was observed. On the other hand, Parkhouse and Winterz, again using methyl-2-cyanoacrylate, found at recall after approximately six months, almost total loss of the fissure sealant. Furthermore the treated teeth showed no significant decrease in carious incidence compared with
the untreated control teeth. Many other disagreements of this type are reported in the literature. Such large discrepancies in the results of clinical trials using the same sealant, prompted us to investigate the effect of the technique of application of fissure sealants upon their adhesive bond strength. It is known that the degree of penetration of polymeric tags into the etched enamel surface influences the adhesive bond strength of the sealant to the enamel. In this communication we report the results of a scanning electron microscope study of the sealant and enamel interface on varying the:(a) method of application of the unpolymerized fissure sealant to the tooth surface; (b) viscosity of the unpolymerized sealant; (c) dryness of the tooth surface; (d) time of etching of the enamel surface.
Financial support by the Medical Research Council is gratefully acknowledged.
Materials and methods The sealant used throughout this study was Nuva-Seal a commercial fissure sealant based on the bisphenol-A-glycidyl methacrylate monomer.*
IRiDa. L. W.. and Cole. W. W . 4 c c l u s a l rcalini nnd * c i r i Z preienti&n:Iesdts i2 mbnthi-aft;; i-iiigTe Xplication of adhesive resin. J. D. Res.. 49:l. 1714b Jan -Feb.) 1970 zPar!kho&et R. C.. and Winter, G. B.-A fissure sealant containme methvl-2-cvanoacrvlate as a cariec nrevm-
*L. D. Caulk Co., Milford, Del.
Australian Dental Journal, June, 1977
Fig. 1.-Sealant Fig. 2.--Sealant Fig. 3.-Sealant polymerization
efter brushing into etched enamel surface for 30 seconds before polymerization (X 900). after application to the etched enamel surface as a drop and left for 30 minutes (X 940). of viscosity 1890 CP after brushing into the etched enamel surface for 30 seconds before (X 940). Cf. Fig. 1 . Fig. 4.-Viscous sealant a s in Fi 3, but left on etched enamel surface as il drop for 30 minutes before polymerization (?lOSO). Cf. Fig. 2.
It is polymerized on the tooth surface, after the addition of an initiator, by ultraviolet light. A selection of monomer solutions of different viscosities was prepared by mixing finely-ground polymerized Nuva-Seal with the monomer in varying proportions. The viscosities of these solutions (at 298K) were 459, 816, 1143 and 1890 CP compared to pure unpolymerized sealant of 256 cP. These viscosities were measured using the method described by Lidstones. Monomeric dipentaerythritol monohydroxy penta acrylate (viscosity of 4400 CP at 298K) was also used, and polymerized on the tooth surface by ultraviolet light. Ground occlusal surfaces of human molars were used throughout and were stored in 70 per cent ethanol prior to use. Before application of the monomer they were, if required, etched with 50 per cent phosphoric acid buffered with 7 per cent ZnO, washed with water and dried with a warm air blower. The presence of tags on the underside of the sealant in contact with the enamel was investigated using scanning electron microscopy. The sealant film was removed from the tooth by dissolving away the underlying enamel with 50 per cent HCI. The back of the sealant was then shadowed and observed in the scanning electron microscope. All experiments were performed three times using three different tooth samples. Lidstone, F. M.-A microviscometer of improved design. Chemistry and Industry, 36: 873-874 (Sept. 6 ) 1952.
Results and discussion
Effect of method of application A molar tooth was cut into four quadrants, the occlusal surfaces of which were etched and dried as previously described. One occlusal surface was used as a control to monitor the degree of etching for each tooth sample. The other three were each treated with unpolymerized material, previously mixed with the free radical initiator, applied by one of the following techniques:(i) by brushing onto the etched surface for 30 seconds using a small brush (supplied by the manufacturer); (ii) by dabbing onto the enamel surface for 30 seconds using n cotton wool swab; (iii) applied as a drop onto the etched surface and left for 30 minutes. Following these applications, the sealant was polymerized, the tooth enamel dissolved away in acid, and the underside of the polymer examined using a scanning electron microscope. Figure 1 shows the polymer tag structure obtained when the fissure sealant was brushed into the etched enamel surface before polymerization. In comparison, Fig. 2 shows the minimal tag formation produced when the sealant was left as a drop on the surface. When a cotton wool swab was used to apply the sealant, tags of an intermediate character were produced. No tags were produced when the drop of sealant was left on the surface for only 30 seconds.
Austrdian Dental Journal, June, 1977
Fia. 5.-Sealant after brushine into an etched but damD enamel surface for 30 seco6ds before polymerization (X 940). Cf. Fig. I . Fig. 6.-Sealant as in Fig. 5 but left as a drop on dampened etched enamel surface for 30 minutes before polymerization (X 990).
A comparison of Fig. 1 and 2 indicates that to produce a large number of well formed tags the sealant should, before polymerization, be brushed into the etched surface. This procedure is probably of clinical importance as an increase in tag structure has been shown43 to enhance the adhesive bond strength, and thus the lifetime of a polymer film adhering to an enamel surface. This improved adhesion, which is associated with a well developed tag structure, is probably due to an increase in the mechanical interlocking between the polymer and the enamel, although an enhanced chcmical bonding, caused by an increase in the overall area of contact between the polymer and the enamel, cannot be completely eliminated. Effect of viscosity The same procedures used to test the method of application was repeated using each of the viscous solutions. SEM photographs obtained when the most viscous solution (1890 CP 298K) was both brushed into the dry etched enemel surface and left for 30 minutes as a droplet on the surface of the enamel, are shown in Fig. 3 and 4 respectively.
‘Breakspere R J
Tranter T C. and Weldon L. H P prelir;linary”examinaiion’ of Borne of the f;ctors ing fissure sealant behavisur. J . Dent. 5:1, 57-66 (March) 1977. Buonocore, M. G.-Simple method of increasing the adhesion of acrylic filling materials to enamel surfaces. J. D. Res., 34:4, 849-853 (Dec.) 1955. A
In general, the results were similar to those obtained when testing the method of application, except that the differences are more marked. On brushing even the most viscous sealant mixture into an etched enamel surface, tags of the same length and number as those obtained with the standard material were produced (Fig. 1, 3). However, when viscous sealants were left as a drop on the surface, fewer, poorly formed tags were produced compared with the less viscous solutions (Fig. 2, 4). There was, however, a possibility that the ground polymer/monomer mixes were not true homogeneous viscous liquids. This would enable the monomer from the mixture to be brushed preferentially into the etched surface and produce a uniform tag density and length whatever the measured viscosity of the mix. To examine this possibility, pure dipentaerythritol monohydroxy penta acrylate, a monomer of high viscosity (4400 CP at 298K) and capable of similar polymerization was used. When brushed into the etched enamel surface in the standard way it produced a tag structure which was as well developed as that produced by the much less viscous materials (256 CP at 298K). Thus, the length and population of the tags is not dependent upon the viscosity of the unpolymerized sealant, provided that the sealant is brushed into the etched surface. Effect of moisture on enamel surface Four quadrants of an occlusal surface of the same tooth were etched for one minute. Three of the surfaces were wetted with distilled water and the surface moisture removed with absorbent paper, the remaining surface was dried in the usual way to use as a control specimen. Scanning electron micrographs of the underside of the polymer produced when it was brushed into the dampened enamel surface for 30 seconds, and also left as a drop on the surface for 30 minutes, are shown in Fig. 5 and 6 respectively. There is not a great difference in the number of tags or their length when sealant is brushed into, or simply left as a drop on the dampened etched surface. However, in all cases the large number of small tags usually detected on dry enamel surfaces were missing, and the depth of penetration into the enamel of the tags was less than that found for the corresponding application of sealant to dry etched enamel surfaces (Fig. 1, 5). Thus, the presence of moisture on the etched enamel surface drastically effects the development of the formation of tags and the adhesion of the s a l a n t to the t00th43. mis decrease in tag formation on surfaces con-
Australian Dental Journal, June, 1977
Fig 7.-Sealant applied to enamel surface etched for one minute (X 990) Fig. &-Sealant a plied to enamel suriace etched for two minutes (X 990). Fig. 9.4ealant applied to enamel surface etched three minutes (X 940).
taining moisture can be explained by assuming that the moisture occupies all, or part, of the etched enamel prism cores thus preventing penetration of the sealant. This conclusion is supported by contact angle measurement&. The presence of liquid in the etched surface would of course, minimize the effect of brushing the sealant into these etched areas. Effect of time of etching A further variant in the application of fissure sealants is the extent of surface etching. Sections of an occlusal surface of the same tooth were etched for one, two and three minutes at 298K and dried using a stream of warm air. For each section, initiated sealant was brushed into the etched enamel surface for 30 seconds and then polymerized. The underside of the sealants indicated an increase in the lengths of the tags with an increase in the time of etching (Fig. 7, 8, 9). It seems therefore, that the sealant can be pushed into the base of the deepest etched pit during this brushing procedure. Qualitatively it would be expected that a more stable adhesive bond would be produced after an acid etch of three minutes compared with that of a shorter time. However, in vivo, the time of etching must of course, be optimized; if too much enamel is removed the tooth sub-surface may be
OBreakspere R. J, and Wilton A.--The variation in contact anile of a fissure seaiant on enamel surface. J. Oral Rehab., 4:1, 77-81 (Jan.) 1977.
irreversibly weakened. It may also be difficult to remove all the moisture from very deeply etched pits which could lead to a decrease in the adhesive bond strength. Conclusions
Although the experiments described in this paper were conducted on extracted teeth in vitro, the general conclusions will be applicable to fissure sealant technology in vivo. Thus, for a well formed tag structure, which may be correlated with long term stability of a fissure sealant, the tooth surface must be dry, etched sufficiently, and the sealant must be applied to the tooth surface with care. For the ultraviolet initiated sealant used in this study brushing of the sealant into the etched surface seems imperative. An obvious advantage of ultraviolet initiated sealants is that they can be brushed into the etched .enamel surface before polymerization is initiated. Some physical properties, such as viscosity, do not seem to be of prime importance. The results presented in this paper indicate that great care must be taken to prepare the tooth surface and apply the sealant correctly. The wide variation in the results of clinical trials published to date, may perhaps be partly explained by variations in tooth preparation and methods of application of the sealant. Similar precautions may also be necessary in the use of polymeric restorative materials. Department of Chemistry, University College,
P.O. Box 78, Cardiff, U.K.