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An in vitro assessment of the retentive behaviour of fissure sealants
Journal of Dentistry,
4, 131-138
An in vitro assessment of the retentive behaviour of fissure sealants T. Low, BDS, MSc” J. A. von Fraunhofer,
MSc, PhD, MIM,
C Chem, FRIC
Institute of Dental Surgery (Eastman Dental Hospital), ABSTRACT The transverse strengths and dimensional changes upon water immersion of three polymeric fissure sealants have been investigated. All three sealant materials expanded following water absorption, with Epoxylite 9075 showing the greatest amount of expansion and ESPE 71730 the least. The transverse strengths of the three materials were significantly different from each other, with NuvaSeal having the highest transverse strength and Epoxylite 9075 the lowest. The transverse strength of both ultraviolet-light-cured materials, NuvaSeal and ESPE 71730, continued to increase after the initial set, and this may be related to their incomplete polymerization at the initial set stage. The study of these two physical properties has been used to account for the different clinical retention characteristics of the three fissure sealants as well as their in vitro tensile bond strengths to enamel.
INTRODUCTION CLINICALstudies in recent years have shown that the presence of a coating of sealant material over the pits and fissures of teeth can prevent the onset of dental caries in these sites (Buonocore, 1971 ; Rock, 1974; Williams et al., 1974a). Studies have also shown that over periods of up to 2 years the number of treated teeth with retained fissure sealant (i.e. the sealant retention rate) varied considerably with
*Present Dentistry, Malaya.
address : Department Faculty of Dentistry,
of Children’s University of
University of 1 ondon
different fissure sealants. A sealant retention rate of 80-90 per cent has been repeatedly reported for Nuva-Seal. More limited clinical studies have shown that Epoxylite 9075 and ESPE 71730 have much lower retention rates, namely 51.5 and 39.8 per cent respectively (Rock 1974; Williams et al., 1974a). The onset of dental caries in teeth sealed with Nuva-Seal or ESPE 71730 is usually associated with the absence of a clinically detectable coating of fissure sealant, which presumably was lost from the teeth during the period of the clinical trials. However, when teeth are sealed with Epoxylite 9075, it has been shown that the treated teeth may still be protected from carious attack despite the apparent loss of some or all of the applied sealant. Rock (1974) found that 5.4 per cent of the teeth sealed with Epoxylite 9075 had developed caries after 2 years (34.7 per cent in the control teeth), although only 51.5 per cent of the treated teeth were still fully sealed. The three fissure sealants are all polymeric materials based on the diacrylate resin. However, their ability to be retained on the etched enamel surface varies considerably. Numerous studies have been conducted in the past to determine the tensile bond strength of these polymeric fissure sealants to enamel (Williams et al., 1974b; Low et al., 1975a). The results of bond strength studies only provide an indication of the retentive potential of the sealant material when used clinically. They do not explain why Nuva-Seal is retained on a higher proportion of treated teeth than ESPE 71730. Also, the tensile bond strength of these sealant
132
Journal of Dentistry, Vol. ~/NO. 3
ST SB
Fig. SB,
I.-Diagram of a sealant coating. F, Fissure; sealant bulk; ST, sealant tag.
materials to enamel changes with water storage. The tensile bond strength of Nuva-Seal to enamel has been shown to increase whilst that of Epoxylite 9075 decreases following storage in water for periods of up to 6 months. No studies have yet been reported which attempt to account for these various aspects of the retentive behaviour of the polymeric fissure sealants. An understanding of the mechanism that controls the variable retentive behaviour of the three fissure sealants currently available should be helpful in the development of new materials and the modification of existing ones. This paper reports the results of an in vitro investigation of two physical properties of three fissure sealants designed to aid clarification of the following aspects of the use of fissure sealants: (a) the clinical retention; (b) the changes in tensile bond strength following water storage; (c) the mechanism contributing to the caries protection observed with Epoxylite-9075-treated teeth following apparent loss of all or part of the applied sealant.
DESIGN
OF THE STUDY
It is generally accepted that the retention of the polymeric fissure sealants currently available is largely dependent on mechanical interlocking between sealant tags and the system of pores on the etched enamel surface of the inclined planes of the tooth. Fig. 1 is a diagrammatic representation of a fissure sealant coating over a fissure of a tooth.
M
Fig.
2.-Experimental set-up for determination of dimensional changes in water. M, Mikrokator; C, Perspex container; W, water; S, sealant specimen; A, anvil.
Dimensional changes in the fissure sealant can theoretically affect the retention of the fissure sealant coating in two ways. The ability of the tags to retain the sealant bulk depends largely on the tags adapting closely to the walls of the pores or etch pits to provide the necessary frictional resistance as well as engaging into any available undercuts. Changes in dimension of the sealant bulk would tend to move the position of the tags in relation to the pores in the enamel surface to which it is anchored. However, since each tag is maintained in a constant position by its associated pore on the enamel surface, stress would develop at the point where the tag joins the sealant bulk when dimensional changes occur. The ability of the tags, which are only l-2 pm in diameter, to resist the displacing stresses depends largely on the strength and stiffness of the material. Based on this empirical analysis, the two parameters that are likely to influence the retention of fissure sealants are the dimensional changes following water absorption and the transverse strength of the fissure sealant. The transverse strength gives an indication of how much deflection or displacement the sealant tags are likely to be able to withstand before fracture occurs.
MATERIALS
AND
METHODS
Three polymeric fissure sealants currently used in clinical studies at the time that this study was initiated were investigated. The three
Low and von Fraunhofer:
Retention
of Fissure Sealants
fissure sealants were Nuva-Seal (L. D. Caulk Ltd, USA), Epoxylite 9075 (Lee Pharmaceuticals, USA) and ESPE 71730 (Espe GmbH, Germany). ESPE 71730 is substantially the same as ESPE 717 except that the polymerization time has been shortened by increasing the benzoin methyl ether catalyst content from 1 to 2 per cent. Nuva-Seal and Epoxylite 9075 are commercially available whilst ESPE 71730 at present is classed as a research material. The two ultraviolet-light-curing materials were cured by exposing each layer to 30 seconds of light from a suitable source* in accordance with the manufacturers’ recommendations.
Dimensional
changes in water
Cylindrical test specimens of diameter 4 mm and height approximately 10 mm were prepared in split ring dies lined with PTFE. The fissure sealant was polymerized in layers and was removed from the die 5 minutes after initial set. The two end faces of the cylindrical test specimens were ground on a waterlubricated lathe to render them flat and parallel to each other. The prepared test specimens were measured and then placed in a Perspex container filled with distilled water (Fig, 2). The specimen and container were then mounted on the measuring platform of a Johannson Mikrokator,? and the entire experimental set-up was kept in a constant temperature cabinet at 3751 “C. The Mikrokator was only zeroed 10 minutes after the test specimen was mounted on the platform (30 minutes after the fissure sealant had set) so that the specimen was conditioned to the test temperature (Jacobsen, 1975). Readings were taken at weekly intervals for 3 months. Six specimens were prepared and tested; 2 for each material.
Transverse strength Specimens 27 mm long, 6 mm wide and 1.5 mm thick were prepared in a stainless steel die lined *BLE Spectroline, Black Light Eastern Division, Spectronics Corporation, New York. TAktieboIaget
C. E. Johannson,
Sweden.
133
with PTFE. The specimens were removed 5 minutes after setting and trimmed on a waterlubricated lathe to the following dimensions: 25 x 44 x 1 mm. This procedure was found to be essential because in the as-set condition the surface in contact with air was often very irregular and the edges of the specimens were poorly defined. The specimens were tested after 1 hour, 1 month and 3 months. The l-month and 3-month test specimens were stored in distilled water at 37f 1 “C. The test specimens were tested on a transverse loading jig on an Instron Universal Testing Machine. The jig consisted of two rods, 3 mm in diameter, placed parallel to each other, and 16 mm apart at their centres, welded to a base plate. This half of the jig was placed on the compression load cell and supported the specimen. A third rod, 3 mm in diameter, parallel to and positioned midway between the other two rods was attached to the cross-head of the Instron. The specimen was subjected to transverse loading at a cross-head speed of 0.2 mm/min until fracture. Fifteen specimens were prepared for each fissure sealant and 5 were tested for each storage condition. The transverse strength T was calculated using the formula T = (3PL)/(2bd2), where P is the maximum force exerted on the specimen, L the distance between the supports, b the width of the specimen and d the thickness of the specimen.
RESULTS The average dimensional changes of the fissure sealant following immersion in water at 37* 1 “C for 3 months were calculated and are shown graphically in Fig. 3. All three fissure sealants expanded during the first 6 weeks in water. Nuva-Seal and Epoxylite 9075 continued to expand after the first 6 weeks and eventually achieved a stable dimension after about 8 weeks. ESPE 71730, however, began to shrink after about 6 weeks. The amounts of expansion at the end of 3 months were l-60, 0.60 and 0.25 per cent for Epoxylite 9075, Nuva-Seal and ESPE 71730 respectively.
134
Journal 18
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of Dentistry,
~/NO. 3
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Fig.
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I.5 2.0 2.5 Deflection (cmxlO-‘)
3.0
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4.-Transverse stress/strain curve of the fissure sealants at 1 hour.
Fig.
3.-Dimensional changes of the fissure sealants in water.
Fig.
I .2
I.0
Fracture
2.5 3.0 (cm x 10~‘)
9075 of specimens 3.5
4.0
stress/strain curve of the
fissure sealants at 1 month. Figs. 4 and 5 show typical transverse stress/ strain curves of the three fissure sealants at 1 hour and 1 month respectively The l-hour Nuva-Seal specimens were found to be very flexible and none of the 5 specimens fractured at a deflection of 5.5 mm, which was the limit permitted by the jig. The transverse strengths together with the standard deviations of the three fissure sealants were calculated using the formula given and are shown in Table I. Table ZZ gives the statistical analysis of the results using Student’s t test: Nuva-Seal was found to have the highest flexural strength at the three test periods and Epoxylite 9075 the lowest. The differences between the three
fissure sealants were statistically highly significant (P~O~OOl). Epoxylite 9075 appeared to exhibit its maximum mean transverse strength at 1 hour, with progressively decreasing strength over the next 3 months, although the differences in strength were statistically nonsignificant (P>O*O5). The transverse strength of Nuva-Seal at 1 hour was 76 per cent of its maximum value, which was reached at 1 month. It then showed an apparent but statistically non-significant decrease at 3 months. The transverse strength of ESPE 71730 at 1 hour was only 40 per cent of its maximum value which occurred at 3 months. The difference in the transverse strength of ESPE at 1 hour and 1 month was significant (PO.O5). The maximum deflections required to fracture the specimens are given in Table ZZZ.
DISCUSSION The determination of dimensional changes upon water immersion consists of measurement of linear changes. It therefore provides a method of assessment of the dimensional changes in the sealant bulk in relation to the position of the sealant tags. However, the results can also be used to indicate the volumetric changes taking place in the sealant tags and any sealant that has penetrated into pits and fissures. The expansion of the fissure sealants was due to water absorption, and previous studies (Williams et al., 1975) have
Low and von Fraunhofer : Retention of Fissure Sealants Table /.-Transverse Fissure sealant Nuva-Seal ESPE 71730 Epoxylite 9075
strength
At 7 h
Transverse strength (mean is.d. in kg cm-‘) At 3 mth At 1 mth V (%)
V (%)
714.0 +25.2 540.0 176.1 78.0 i18.1
4.9 14.1 23.2
V W)
HS, Highly significant: Table I//.-Deflection
Fissure sealant
Nuva-Seal ESPE 71730 Epoxylite 9075
3.3 14.3 7.9
fracture.
Table //.-Statistical analysis of the differences at 1 hour, 1 month, and 3 months
Nuva-Seal ESPE 71730 Epoxylite 9075
748.8 & 24.5 484.8 & 69.5 84.8 *6.7
9.1 11.2 7.0
565.2 &51.3* 218.4 *24.5 88.5 16.2
V, Coefficient of variation. *Specimens deflected to 5.5 mm without
Fissure sealant
135
in transverse strength
Comparison of differences in transverse strength 1 h : 1 mth 1 h:3rnth lrnth.3rnth HS HS NS
HS HS NS
NS NS NS
P
NS, Not significant:
P >0.05.
at fracture Deflection at fracture (mean *s.d. in cm x 10-l) At 1 rnth At 3 mth At 1 h 5.5* 3.22 ho.26 0.42 ho.1 2
3.42 kO.44 2.70 f0.35 0.40 1-0.08
2.85 I-to.1 1 2.62 ho.24 0.25 30.01
*All the specimens failed to fracture with a deflection
shown that all three fissure sealants absorbed the greatest amount of water during the first month of immersion. Furthermore, rates of water absorption during the first month paralleled the rates of expansion, that is, greatest in Epoxylite 9075 and least in ESPE 71730. The possibility of continuing polymerization of ultraviolet-light-cured fissure sealants after the initial set has been raised previously (Watkins, 1975; Williams et al., 1975), and this could account for the shrinkage observed with ESPE 71730 if the polymerization shrinkage approaches the expansion due to water absorption. The change with time of the transverse strength of the three fissure sealants (see below) also suggests that the polymerization process continues for some time after the initial set. For any given polymer system it is a general rule that strength and stiffness increase with
of 5.5 mm.
the degree of polymerization (Phillips, 1973), whilst the strength of the resin is reduced by absorption of water (Swaney et al., 1953). Of the three fissure sealants, only the two ultraviolet-light-activated materials, Nuva-Seal and ESPE 71730, showed an increase in strength after 1 hour. The increase in strength was therefore most probably due to continuing polymerization within the bulk material after the initial set. However, the chemically polymerized fissure sealant, Epoxylite 9075, exhibited its maximum strength at 1 hour. It therefore appears that 60 seconds’ exposure to ultraviolet light, as recommended by the manufacturers for the former materials, may be inadequate for full polymerization, owing to failure of the ultraviolet light to reach the interior of the sealant once the surface layer has polymerized. The polymerization process of Nuva-Seal would appear to be finally
136
completed some time between 1 and 3 months and that of ESPE 71730 after 3 months. The decrease in the strength of Epoxylite 9075 after 1 hour may be ascribed to water absorption (Swaney et al., 1953), and other workers (Watkins, 1975; Williams et al., 1975) have also shown that the microhardness of Epoxylite 9075 progressively decreases over a 3-month period whilst that of Nuva-Seal and ESPE 71730 shows an increase. These changes indicate that the materials interact with water, and with saliva in the clinical situation, so that the long term serviceability of these materials may be at doubt. The expansion following water absorption and the change in strength of the fissure sealants appear to correlate reasonably well with their reported in vitro and clinical retentive behaviour. Polymerization of Epoxylite 9075 has been shown to be accompanied by a large exotherm (von Fraunhofer and Williams, 1974). The evolution of heat would increase the amount of porosity within the fissure sealant, and hence weaken the material. In this respect the transverse stress/strain curves (Figs. 3, 4) show that the deflection at fracture of the Epoxylite 9075 specimens was very small compared with that of the other two materials (Table III). The large expansion that takes place with Epoxylite 9075 should ensure that the polymerization shrinkage occurring in the sealant tags is compensated. The additional expansion then wedges the tags tightly into surface irregularities so that they are firmly anchored into the enamel. Dimensional changes taking place in the sealant bulk may therefore result in a weakening or even fracture of some tags at their attachment points to the bulk material. This might render the sealant bulk susceptible to any dislodging forces during mastication and would account for the low rate of sealant retention observed clinically (Rock, 1974). In teeth with deep and narrow pits and fissures the unpolymerized fissure sealant, which has a very low viscosity, can be expected to flow adequately into these areas, provided that there is no physical obstruction by plaque, food debris or cleaning agent at the entrance. The mechanism of loss of Epoxylite 9075, as outlined above, suggests that the sealant
Journal of Dentistry, Vol. ~/NO. 3
materials within the pits and fissures and system of pores should be retained despite loss of the sealant bulk from the tooth. This would account for the caries protection observed despite the absence of clinically detectable sealant in Epoxylite 9075-treated teeth. After 2 years 5.4 per cent of treated teeth had developed occlusal caries (34.7 per cent in the control teeth), although only 51.5 per cent of the treated teeth were fully sealed (Rock, 1974). ESPE 71730 was found to be very flexible and of low strength immediately after the initial set. Following water absorption, ESPE 71730 only expands by O-25 per cent after 3 months. No data on the polymerization shrinkage of fissure sealants are available to date. Although it is not possible to assess the degree to which expansion due to water absorption compensates for polymerization shrinkage, the low expansion would appear to be inadequate to provide complete compensation. The absence of compensation for the polymerization shrinkage would lead to the establishment of a space at the interface between sealant tags and the wall of the pores in the enamel, thereby reducing its ability to engage any undercuts and provide the required mechanical interlocking effect. The same effect can be expected to occur within the fissure sealant that has penetrated into deep pits and fissures. These factors render the sealant bulk vulnerable to any shearing forces acting on it during the. period immediately after application. Such a force would tend to ‘peel’ away all or part of the fissure sealant from the pits and fissures and possibly from some or all of the pore system. Any ESPE 71730 sealant coating which survives the initial few weeks could be expected to have an improved opportunity to be retained successfully since the strength of the material increases by about 100 per cent during the first month. The clinical behaviour predicted from the physical properties is substantiated by a clinical study (Williams et al., 1974a). The fissure sealant was retained fully on only 39.8 per cent of the treated teeth after 7.4 months. However, after a further 8 months (15.4 months after application) 33.7 per cent of the teeth that were not re-treated were still fully
Low and van Fraunhofer
: Retention
of Fissure Sealants
sealed. The authors attributed the high loss during the first 7.4 months to faulty application procedures. However, no other clinical studies using an ultraviolet-light-activated sealant under comparable experimental conditions have shown such a high rate of sealant loss during the first 6 months to 1 year. This result would tend to support the contention that its initial failure rate was due to inherent factors within the material rather than to poor technique. After a mean period of 15.4 months, 8.6 per cent of the treated pits and fissures and 18.8 per cent of the control pits and fissures were found to be carious, although 67.9 per cent of the teeth (both those that had received one treatment and re-treated teeth) were still fully sealed. This showed that when the sealant bulk was lost, the tooth was no longer completely protected from caries attack. With Nuva-Seal the physical properties discussed above tend to contribute to an improvement of its retentive potential. The expansion following water absorption appears to compensate for the polymerization shrinkage and ensures that the tags are effectively retained to the etched enamel surface. The stress created in the tags by the expansion of the sealant bulk would not be expected to weaken the tags to any degree since the degree of expansion was less than that of Epoxylite 9075 and the l-hour specimens were able to tolerate a deflection of 5.5 mm without fracture. Also at the time of initial set, Nuva-Seal has already attained 76 per cent of its final transverse strength. The superior stiffness of Nuva-Seal compared with ESPE 71730 probably enables the applied fissure sealant to resist shearing forces with greater effectiveness. A sealant retention rate of 80-90 per cent and a caries protection of 7&90 per cent for periods of up to 2 years have been repeatedly reported for Nuva-Seal. The tensile bond strengths of the three fissure sealants to enamel reported previously (Rock, 1974; Williams et al., 1974b; Low et al., 1975b) have been very variable. In the results reported by Williams et al. (1974b), the tensile bond strengths at 1 week in decreasing order were: Epoxylite 9075, Nuva-Seal and ESPE 717. Based on these results, it was not possible
137
to correlate any of the findings in this study with the l-week tensile bond strengths of the three fissure sealants. In the same study it was reported that the tensile bond strengths of Epoxylite 9075 and ESPE 717 decreased whilst that of Nuva-Seal was unchanged over 6 months. However, other work (Low and von Fraunhofer, 1976) found that the tensile bond strengths of both Nuva-Seal and ESPE 71730 increased whilst that of Epoxylite 9075 decreased over 3 months. This difference in tensile bond strength arises from a modification and improvement in the test technique used in the later study. Changes in the tensile bond strength of the three fissure sealants to enamel after water storage may be accounted for in the manner discussed above. The results obtained in this study appear to explain most, but not all, of the observed in vitro and clinical retentive behaviour of the three fissure sealants. Numerous studies have been reported in the literature on the in vitro tensile bond strength of Nuva-Seal to .enamel and the retention rate in clinical trials. Unfortunately, similar studies with ESPE 71730 and Epoxylite 9075 are more limited. The discussion of the in vitro and clinical retentive behaviour of ESPE 71730 and Epoxylite 9075 in this paper is based on the results of the few available reports. Furthermore, there may have been modifications and changes to the fissure sealants over the years. Although some aspects of the retentive behaviour of the three fissure sealants have been successfully explained, a more comprehensive evaluation of the behaviour of ESPE 71730 and Epoxylite 9075 must await further data becoming available. Other parameters, notably polymerization shrinkage and the coefficient of thermal expansion, have to be investigated before a complete picture of the retentive behaviour of fissure sealants can be presented. An attempt to develop a clinical model for testing the explanation presented in this paper will be conducted shortly. Apart from the inherent physical properties of the fissure sealant, there are other factors that can contribute to the loss of fissure sealants in clinical use. These include inadequate isolation from moisture, incomplete polymerization of the fissure sealant (Rock,
138
1973; Williams et al., 1974a; Burt et al., 1975), topical application of fluoride (Low et al., 1975a), abrasion (Tranter and Douglas, 1975) and factors related to the enamel which have not yet been completely elucidated. Typically, in a tensile bond strength study involving a series of 50 extracted teeth the tensile bond strength of Nuva-Seal to enamel was found to vary from 16.4 to 57.1 kg cm-2 (Low et al., 1975b). This wide range of bond strengths suggests that the variability of enamel and its response to acid etching and other surface treatments may be a major factor in the successful use of fissure sealants. Additional research work must be performed to investigate this facet of fissure sealant studies.
CONCLUSION The transverse strength and dimensional changes in water can explain most of the in vitro and clinical retentive behaviour of fissure sealants. The expansion of the fissure sealant following water absorption must exceed the polymerization shrinkage in order that the sealant tags effectively contribute to the retention of the sealant bulk on the etched tooth surface. Nuva-Seal and ESPE 71730 continue to polymerize for some time after the initial set. This indicates that the polymerization time for the ultraviolet-light-activated fissure sealants as recommended by the manufacturers may be inadequate.
Acknowledgements The authors would like to thank Mr E. H. Davies for his advice and help on the testing methods. REFERENCES BUONOCORE M. G. (1971) Caries prevention in pits and fissures sealed with an adhesive resin polymerised with U.V.light : a 2-year study of a single adhesive application. J. Am. Dent. Assoc. 82, 1090-1093.
Journal
of Dentistry,
Vol. ~/NO. 3
BURT B. A., BERMAN D. S., GELBIER S. and SILVERSTONE L. M. (1975) Retention of a fissure sealant six months after application. Br. Dent. J. 138,98-100. VON FRAUNHOFERJ. A. and WILLIAMSB. (1974) Heat liberation during the setting of four fissure sealants. Br. Dent. J. 136, 4.98499. JACOBSENP. H. (1975) An assessment of the influence of the setting characteristics of polymeric restorative materials on their possible clinical behaviour. MDS Thesis, University of London. Low T. and VON FRAUNHOFERJ. A. (1976) In preparation. Low T., VONFRAUNHOFERJ. A. and WINTERG. B. (1975a) The bonding of a polymeric fissure sealant to topical fluoride-treated teeth. J. Oral Rehabil. 2, 303-307. Low T., DAVIESE. H. and VONFRAUNHOFERJ. A. (1975b) A method of determining the tensile bond strength of fissure sealant materials. J. Oral Rehabil. 2, 341-347. PHILLIPSR. W. (1973) Skinner’s Science of Dental Materials, 7th ed. Saunders, Philadelphia, pp. 217-242. ROCK W. P. (1973) Fissure sealants: results obtained with two different bis-GMA type sealants after one year. Br. Dent. J. 134, 193196. ROCK W. P. (1974) Fissure sealants: further results of clinical trials. Br. Dent. J. 136, 317-321. SWANEYA. C., PAFFENBANGER G. C., CAUL H. J. and SWEENEYW. T. (1953) American Dental Association Specification No. 12 for Denture Base Resins; Second Revision. J. Am. Dent. Assoc. 46, 54-66. TRANTERT. C. and DOUGLASW. H. (1975) Studies of fissure sealant behaviour in vivo. J. Dent. Res. 54, Abstr. No. L349, p. L87. WATKINSJ. J. (1975) The surface hardness of three bisphenol A and glycidyl methacrylate fissure sealants. J. Dent. 3,68-72. WILLIAMSB., CASSONM. H. and WINTER G. B. (1974a) A clinical study using a new ultraviolet light-polymerized fissure sealant. J. Dent. 2, lOl105. WILLIAMSB., VON FRAUNHOFER J. A. and WINTER G. B. (1974b) Tensile bond strength between fissure sealants and enamel. J. Dent. Res. 53, 23-27. WILLIAMS B., VON FRAUNHOFERJ. A. and WINTER G. B. (1975) A comparative evaluation of the water solubility and water microhardness, absorption of fissure sealants. J. Dent. 3, l-8.
4, 131-138
An in vitro assessment of the retentive behaviour of fissure sealants T. Low, BDS, MSc” J. A. von Fraunhofer,
MSc, PhD, MIM,
C Chem, FRIC
Institute of Dental Surgery (Eastman Dental Hospital), ABSTRACT The transverse strengths and dimensional changes upon water immersion of three polymeric fissure sealants have been investigated. All three sealant materials expanded following water absorption, with Epoxylite 9075 showing the greatest amount of expansion and ESPE 71730 the least. The transverse strengths of the three materials were significantly different from each other, with NuvaSeal having the highest transverse strength and Epoxylite 9075 the lowest. The transverse strength of both ultraviolet-light-cured materials, NuvaSeal and ESPE 71730, continued to increase after the initial set, and this may be related to their incomplete polymerization at the initial set stage. The study of these two physical properties has been used to account for the different clinical retention characteristics of the three fissure sealants as well as their in vitro tensile bond strengths to enamel.
INTRODUCTION CLINICALstudies in recent years have shown that the presence of a coating of sealant material over the pits and fissures of teeth can prevent the onset of dental caries in these sites (Buonocore, 1971 ; Rock, 1974; Williams et al., 1974a). Studies have also shown that over periods of up to 2 years the number of treated teeth with retained fissure sealant (i.e. the sealant retention rate) varied considerably with
*Present Dentistry, Malaya.
address : Department Faculty of Dentistry,
of Children’s University of
University of 1 ondon
different fissure sealants. A sealant retention rate of 80-90 per cent has been repeatedly reported for Nuva-Seal. More limited clinical studies have shown that Epoxylite 9075 and ESPE 71730 have much lower retention rates, namely 51.5 and 39.8 per cent respectively (Rock 1974; Williams et al., 1974a). The onset of dental caries in teeth sealed with Nuva-Seal or ESPE 71730 is usually associated with the absence of a clinically detectable coating of fissure sealant, which presumably was lost from the teeth during the period of the clinical trials. However, when teeth are sealed with Epoxylite 9075, it has been shown that the treated teeth may still be protected from carious attack despite the apparent loss of some or all of the applied sealant. Rock (1974) found that 5.4 per cent of the teeth sealed with Epoxylite 9075 had developed caries after 2 years (34.7 per cent in the control teeth), although only 51.5 per cent of the treated teeth were still fully sealed. The three fissure sealants are all polymeric materials based on the diacrylate resin. However, their ability to be retained on the etched enamel surface varies considerably. Numerous studies have been conducted in the past to determine the tensile bond strength of these polymeric fissure sealants to enamel (Williams et al., 1974b; Low et al., 1975a). The results of bond strength studies only provide an indication of the retentive potential of the sealant material when used clinically. They do not explain why Nuva-Seal is retained on a higher proportion of treated teeth than ESPE 71730. Also, the tensile bond strength of these sealant
132
Journal of Dentistry, Vol. ~/NO. 3
ST SB
Fig. SB,
I.-Diagram of a sealant coating. F, Fissure; sealant bulk; ST, sealant tag.
materials to enamel changes with water storage. The tensile bond strength of Nuva-Seal to enamel has been shown to increase whilst that of Epoxylite 9075 decreases following storage in water for periods of up to 6 months. No studies have yet been reported which attempt to account for these various aspects of the retentive behaviour of the polymeric fissure sealants. An understanding of the mechanism that controls the variable retentive behaviour of the three fissure sealants currently available should be helpful in the development of new materials and the modification of existing ones. This paper reports the results of an in vitro investigation of two physical properties of three fissure sealants designed to aid clarification of the following aspects of the use of fissure sealants: (a) the clinical retention; (b) the changes in tensile bond strength following water storage; (c) the mechanism contributing to the caries protection observed with Epoxylite-9075-treated teeth following apparent loss of all or part of the applied sealant.
DESIGN
OF THE STUDY
It is generally accepted that the retention of the polymeric fissure sealants currently available is largely dependent on mechanical interlocking between sealant tags and the system of pores on the etched enamel surface of the inclined planes of the tooth. Fig. 1 is a diagrammatic representation of a fissure sealant coating over a fissure of a tooth.
M
Fig.
2.-Experimental set-up for determination of dimensional changes in water. M, Mikrokator; C, Perspex container; W, water; S, sealant specimen; A, anvil.
Dimensional changes in the fissure sealant can theoretically affect the retention of the fissure sealant coating in two ways. The ability of the tags to retain the sealant bulk depends largely on the tags adapting closely to the walls of the pores or etch pits to provide the necessary frictional resistance as well as engaging into any available undercuts. Changes in dimension of the sealant bulk would tend to move the position of the tags in relation to the pores in the enamel surface to which it is anchored. However, since each tag is maintained in a constant position by its associated pore on the enamel surface, stress would develop at the point where the tag joins the sealant bulk when dimensional changes occur. The ability of the tags, which are only l-2 pm in diameter, to resist the displacing stresses depends largely on the strength and stiffness of the material. Based on this empirical analysis, the two parameters that are likely to influence the retention of fissure sealants are the dimensional changes following water absorption and the transverse strength of the fissure sealant. The transverse strength gives an indication of how much deflection or displacement the sealant tags are likely to be able to withstand before fracture occurs.
MATERIALS
AND
METHODS
Three polymeric fissure sealants currently used in clinical studies at the time that this study was initiated were investigated. The three
Low and von Fraunhofer:
Retention
of Fissure Sealants
fissure sealants were Nuva-Seal (L. D. Caulk Ltd, USA), Epoxylite 9075 (Lee Pharmaceuticals, USA) and ESPE 71730 (Espe GmbH, Germany). ESPE 71730 is substantially the same as ESPE 717 except that the polymerization time has been shortened by increasing the benzoin methyl ether catalyst content from 1 to 2 per cent. Nuva-Seal and Epoxylite 9075 are commercially available whilst ESPE 71730 at present is classed as a research material. The two ultraviolet-light-curing materials were cured by exposing each layer to 30 seconds of light from a suitable source* in accordance with the manufacturers’ recommendations.
Dimensional
changes in water
Cylindrical test specimens of diameter 4 mm and height approximately 10 mm were prepared in split ring dies lined with PTFE. The fissure sealant was polymerized in layers and was removed from the die 5 minutes after initial set. The two end faces of the cylindrical test specimens were ground on a waterlubricated lathe to render them flat and parallel to each other. The prepared test specimens were measured and then placed in a Perspex container filled with distilled water (Fig, 2). The specimen and container were then mounted on the measuring platform of a Johannson Mikrokator,? and the entire experimental set-up was kept in a constant temperature cabinet at 3751 “C. The Mikrokator was only zeroed 10 minutes after the test specimen was mounted on the platform (30 minutes after the fissure sealant had set) so that the specimen was conditioned to the test temperature (Jacobsen, 1975). Readings were taken at weekly intervals for 3 months. Six specimens were prepared and tested; 2 for each material.
Transverse strength Specimens 27 mm long, 6 mm wide and 1.5 mm thick were prepared in a stainless steel die lined *BLE Spectroline, Black Light Eastern Division, Spectronics Corporation, New York. TAktieboIaget
C. E. Johannson,
Sweden.
133
with PTFE. The specimens were removed 5 minutes after setting and trimmed on a waterlubricated lathe to the following dimensions: 25 x 44 x 1 mm. This procedure was found to be essential because in the as-set condition the surface in contact with air was often very irregular and the edges of the specimens were poorly defined. The specimens were tested after 1 hour, 1 month and 3 months. The l-month and 3-month test specimens were stored in distilled water at 37f 1 “C. The test specimens were tested on a transverse loading jig on an Instron Universal Testing Machine. The jig consisted of two rods, 3 mm in diameter, placed parallel to each other, and 16 mm apart at their centres, welded to a base plate. This half of the jig was placed on the compression load cell and supported the specimen. A third rod, 3 mm in diameter, parallel to and positioned midway between the other two rods was attached to the cross-head of the Instron. The specimen was subjected to transverse loading at a cross-head speed of 0.2 mm/min until fracture. Fifteen specimens were prepared for each fissure sealant and 5 were tested for each storage condition. The transverse strength T was calculated using the formula T = (3PL)/(2bd2), where P is the maximum force exerted on the specimen, L the distance between the supports, b the width of the specimen and d the thickness of the specimen.
RESULTS The average dimensional changes of the fissure sealant following immersion in water at 37* 1 “C for 3 months were calculated and are shown graphically in Fig. 3. All three fissure sealants expanded during the first 6 weeks in water. Nuva-Seal and Epoxylite 9075 continued to expand after the first 6 weeks and eventually achieved a stable dimension after about 8 weeks. ESPE 71730, however, began to shrink after about 6 weeks. The amounts of expansion at the end of 3 months were l-60, 0.60 and 0.25 per cent for Epoxylite 9075, Nuva-Seal and ESPE 71730 respectively.
134
Journal 18
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of Dentistry,
~/NO. 3
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.
.
.
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ESPE71730 Epoxylite
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Fig.
7’
,’ ,’ ,’ H ,+’ , * I ,,’ ’ I ,a’ ,*‘, / ‘ __
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,’
I.0 2
I.5 2.0 2.5 Deflection (cmxlO-‘)
3.0
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4.-Transverse stress/strain curve of the fissure sealants at 1 hour.
Fig.
3.-Dimensional changes of the fissure sealants in water.
Fig.
I .2
I.0
Fracture
2.5 3.0 (cm x 10~‘)
9075 of specimens 3.5
4.0
stress/strain curve of the
fissure sealants at 1 month. Figs. 4 and 5 show typical transverse stress/ strain curves of the three fissure sealants at 1 hour and 1 month respectively The l-hour Nuva-Seal specimens were found to be very flexible and none of the 5 specimens fractured at a deflection of 5.5 mm, which was the limit permitted by the jig. The transverse strengths together with the standard deviations of the three fissure sealants were calculated using the formula given and are shown in Table I. Table ZZ gives the statistical analysis of the results using Student’s t test: Nuva-Seal was found to have the highest flexural strength at the three test periods and Epoxylite 9075 the lowest. The differences between the three
fissure sealants were statistically highly significant (P~O~OOl). Epoxylite 9075 appeared to exhibit its maximum mean transverse strength at 1 hour, with progressively decreasing strength over the next 3 months, although the differences in strength were statistically nonsignificant (P>O*O5). The transverse strength of Nuva-Seal at 1 hour was 76 per cent of its maximum value, which was reached at 1 month. It then showed an apparent but statistically non-significant decrease at 3 months. The transverse strength of ESPE 71730 at 1 hour was only 40 per cent of its maximum value which occurred at 3 months. The difference in the transverse strength of ESPE at 1 hour and 1 month was significant (P
DISCUSSION The determination of dimensional changes upon water immersion consists of measurement of linear changes. It therefore provides a method of assessment of the dimensional changes in the sealant bulk in relation to the position of the sealant tags. However, the results can also be used to indicate the volumetric changes taking place in the sealant tags and any sealant that has penetrated into pits and fissures. The expansion of the fissure sealants was due to water absorption, and previous studies (Williams et al., 1975) have
Low and von Fraunhofer : Retention of Fissure Sealants Table /.-Transverse Fissure sealant Nuva-Seal ESPE 71730 Epoxylite 9075
strength
At 7 h
Transverse strength (mean is.d. in kg cm-‘) At 3 mth At 1 mth V (%)
V (%)
714.0 +25.2 540.0 176.1 78.0 i18.1
4.9 14.1 23.2
V W)
HS, Highly significant: Table I//.-Deflection
Fissure sealant
Nuva-Seal ESPE 71730 Epoxylite 9075
3.3 14.3 7.9
fracture.
Table //.-Statistical analysis of the differences at 1 hour, 1 month, and 3 months
Nuva-Seal ESPE 71730 Epoxylite 9075
748.8 & 24.5 484.8 & 69.5 84.8 *6.7
9.1 11.2 7.0
565.2 &51.3* 218.4 *24.5 88.5 16.2
V, Coefficient of variation. *Specimens deflected to 5.5 mm without
Fissure sealant
135
in transverse strength
Comparison of differences in transverse strength 1 h : 1 mth 1 h:3rnth lrnth.3rnth HS HS NS
HS HS NS
NS NS NS
P
NS, Not significant:
P >0.05.
at fracture Deflection at fracture (mean *s.d. in cm x 10-l) At 1 rnth At 3 mth At 1 h 5.5* 3.22 ho.26 0.42 ho.1 2
3.42 kO.44 2.70 f0.35 0.40 1-0.08
2.85 I-to.1 1 2.62 ho.24 0.25 30.01
*All the specimens failed to fracture with a deflection
shown that all three fissure sealants absorbed the greatest amount of water during the first month of immersion. Furthermore, rates of water absorption during the first month paralleled the rates of expansion, that is, greatest in Epoxylite 9075 and least in ESPE 71730. The possibility of continuing polymerization of ultraviolet-light-cured fissure sealants after the initial set has been raised previously (Watkins, 1975; Williams et al., 1975), and this could account for the shrinkage observed with ESPE 71730 if the polymerization shrinkage approaches the expansion due to water absorption. The change with time of the transverse strength of the three fissure sealants (see below) also suggests that the polymerization process continues for some time after the initial set. For any given polymer system it is a general rule that strength and stiffness increase with
of 5.5 mm.
the degree of polymerization (Phillips, 1973), whilst the strength of the resin is reduced by absorption of water (Swaney et al., 1953). Of the three fissure sealants, only the two ultraviolet-light-activated materials, Nuva-Seal and ESPE 71730, showed an increase in strength after 1 hour. The increase in strength was therefore most probably due to continuing polymerization within the bulk material after the initial set. However, the chemically polymerized fissure sealant, Epoxylite 9075, exhibited its maximum strength at 1 hour. It therefore appears that 60 seconds’ exposure to ultraviolet light, as recommended by the manufacturers for the former materials, may be inadequate for full polymerization, owing to failure of the ultraviolet light to reach the interior of the sealant once the surface layer has polymerized. The polymerization process of Nuva-Seal would appear to be finally
136
completed some time between 1 and 3 months and that of ESPE 71730 after 3 months. The decrease in the strength of Epoxylite 9075 after 1 hour may be ascribed to water absorption (Swaney et al., 1953), and other workers (Watkins, 1975; Williams et al., 1975) have also shown that the microhardness of Epoxylite 9075 progressively decreases over a 3-month period whilst that of Nuva-Seal and ESPE 71730 shows an increase. These changes indicate that the materials interact with water, and with saliva in the clinical situation, so that the long term serviceability of these materials may be at doubt. The expansion following water absorption and the change in strength of the fissure sealants appear to correlate reasonably well with their reported in vitro and clinical retentive behaviour. Polymerization of Epoxylite 9075 has been shown to be accompanied by a large exotherm (von Fraunhofer and Williams, 1974). The evolution of heat would increase the amount of porosity within the fissure sealant, and hence weaken the material. In this respect the transverse stress/strain curves (Figs. 3, 4) show that the deflection at fracture of the Epoxylite 9075 specimens was very small compared with that of the other two materials (Table III). The large expansion that takes place with Epoxylite 9075 should ensure that the polymerization shrinkage occurring in the sealant tags is compensated. The additional expansion then wedges the tags tightly into surface irregularities so that they are firmly anchored into the enamel. Dimensional changes taking place in the sealant bulk may therefore result in a weakening or even fracture of some tags at their attachment points to the bulk material. This might render the sealant bulk susceptible to any dislodging forces during mastication and would account for the low rate of sealant retention observed clinically (Rock, 1974). In teeth with deep and narrow pits and fissures the unpolymerized fissure sealant, which has a very low viscosity, can be expected to flow adequately into these areas, provided that there is no physical obstruction by plaque, food debris or cleaning agent at the entrance. The mechanism of loss of Epoxylite 9075, as outlined above, suggests that the sealant
Journal of Dentistry, Vol. ~/NO. 3
materials within the pits and fissures and system of pores should be retained despite loss of the sealant bulk from the tooth. This would account for the caries protection observed despite the absence of clinically detectable sealant in Epoxylite 9075-treated teeth. After 2 years 5.4 per cent of treated teeth had developed occlusal caries (34.7 per cent in the control teeth), although only 51.5 per cent of the treated teeth were fully sealed (Rock, 1974). ESPE 71730 was found to be very flexible and of low strength immediately after the initial set. Following water absorption, ESPE 71730 only expands by O-25 per cent after 3 months. No data on the polymerization shrinkage of fissure sealants are available to date. Although it is not possible to assess the degree to which expansion due to water absorption compensates for polymerization shrinkage, the low expansion would appear to be inadequate to provide complete compensation. The absence of compensation for the polymerization shrinkage would lead to the establishment of a space at the interface between sealant tags and the wall of the pores in the enamel, thereby reducing its ability to engage any undercuts and provide the required mechanical interlocking effect. The same effect can be expected to occur within the fissure sealant that has penetrated into deep pits and fissures. These factors render the sealant bulk vulnerable to any shearing forces acting on it during the. period immediately after application. Such a force would tend to ‘peel’ away all or part of the fissure sealant from the pits and fissures and possibly from some or all of the pore system. Any ESPE 71730 sealant coating which survives the initial few weeks could be expected to have an improved opportunity to be retained successfully since the strength of the material increases by about 100 per cent during the first month. The clinical behaviour predicted from the physical properties is substantiated by a clinical study (Williams et al., 1974a). The fissure sealant was retained fully on only 39.8 per cent of the treated teeth after 7.4 months. However, after a further 8 months (15.4 months after application) 33.7 per cent of the teeth that were not re-treated were still fully
Low and van Fraunhofer
: Retention
of Fissure Sealants
sealed. The authors attributed the high loss during the first 7.4 months to faulty application procedures. However, no other clinical studies using an ultraviolet-light-activated sealant under comparable experimental conditions have shown such a high rate of sealant loss during the first 6 months to 1 year. This result would tend to support the contention that its initial failure rate was due to inherent factors within the material rather than to poor technique. After a mean period of 15.4 months, 8.6 per cent of the treated pits and fissures and 18.8 per cent of the control pits and fissures were found to be carious, although 67.9 per cent of the teeth (both those that had received one treatment and re-treated teeth) were still fully sealed. This showed that when the sealant bulk was lost, the tooth was no longer completely protected from caries attack. With Nuva-Seal the physical properties discussed above tend to contribute to an improvement of its retentive potential. The expansion following water absorption appears to compensate for the polymerization shrinkage and ensures that the tags are effectively retained to the etched enamel surface. The stress created in the tags by the expansion of the sealant bulk would not be expected to weaken the tags to any degree since the degree of expansion was less than that of Epoxylite 9075 and the l-hour specimens were able to tolerate a deflection of 5.5 mm without fracture. Also at the time of initial set, Nuva-Seal has already attained 76 per cent of its final transverse strength. The superior stiffness of Nuva-Seal compared with ESPE 71730 probably enables the applied fissure sealant to resist shearing forces with greater effectiveness. A sealant retention rate of 80-90 per cent and a caries protection of 7&90 per cent for periods of up to 2 years have been repeatedly reported for Nuva-Seal. The tensile bond strengths of the three fissure sealants to enamel reported previously (Rock, 1974; Williams et al., 1974b; Low et al., 1975b) have been very variable. In the results reported by Williams et al. (1974b), the tensile bond strengths at 1 week in decreasing order were: Epoxylite 9075, Nuva-Seal and ESPE 717. Based on these results, it was not possible
137
to correlate any of the findings in this study with the l-week tensile bond strengths of the three fissure sealants. In the same study it was reported that the tensile bond strengths of Epoxylite 9075 and ESPE 717 decreased whilst that of Nuva-Seal was unchanged over 6 months. However, other work (Low and von Fraunhofer, 1976) found that the tensile bond strengths of both Nuva-Seal and ESPE 71730 increased whilst that of Epoxylite 9075 decreased over 3 months. This difference in tensile bond strength arises from a modification and improvement in the test technique used in the later study. Changes in the tensile bond strength of the three fissure sealants to enamel after water storage may be accounted for in the manner discussed above. The results obtained in this study appear to explain most, but not all, of the observed in vitro and clinical retentive behaviour of the three fissure sealants. Numerous studies have been reported in the literature on the in vitro tensile bond strength of Nuva-Seal to .enamel and the retention rate in clinical trials. Unfortunately, similar studies with ESPE 71730 and Epoxylite 9075 are more limited. The discussion of the in vitro and clinical retentive behaviour of ESPE 71730 and Epoxylite 9075 in this paper is based on the results of the few available reports. Furthermore, there may have been modifications and changes to the fissure sealants over the years. Although some aspects of the retentive behaviour of the three fissure sealants have been successfully explained, a more comprehensive evaluation of the behaviour of ESPE 71730 and Epoxylite 9075 must await further data becoming available. Other parameters, notably polymerization shrinkage and the coefficient of thermal expansion, have to be investigated before a complete picture of the retentive behaviour of fissure sealants can be presented. An attempt to develop a clinical model for testing the explanation presented in this paper will be conducted shortly. Apart from the inherent physical properties of the fissure sealant, there are other factors that can contribute to the loss of fissure sealants in clinical use. These include inadequate isolation from moisture, incomplete polymerization of the fissure sealant (Rock,
138
1973; Williams et al., 1974a; Burt et al., 1975), topical application of fluoride (Low et al., 1975a), abrasion (Tranter and Douglas, 1975) and factors related to the enamel which have not yet been completely elucidated. Typically, in a tensile bond strength study involving a series of 50 extracted teeth the tensile bond strength of Nuva-Seal to enamel was found to vary from 16.4 to 57.1 kg cm-2 (Low et al., 1975b). This wide range of bond strengths suggests that the variability of enamel and its response to acid etching and other surface treatments may be a major factor in the successful use of fissure sealants. Additional research work must be performed to investigate this facet of fissure sealant studies.
CONCLUSION The transverse strength and dimensional changes in water can explain most of the in vitro and clinical retentive behaviour of fissure sealants. The expansion of the fissure sealant following water absorption must exceed the polymerization shrinkage in order that the sealant tags effectively contribute to the retention of the sealant bulk on the etched tooth surface. Nuva-Seal and ESPE 71730 continue to polymerize for some time after the initial set. This indicates that the polymerization time for the ultraviolet-light-activated fissure sealants as recommended by the manufacturers may be inadequate.
Acknowledgements The authors would like to thank Mr E. H. Davies for his advice and help on the testing methods. REFERENCES BUONOCORE M. G. (1971) Caries prevention in pits and fissures sealed with an adhesive resin polymerised with U.V.light : a 2-year study of a single adhesive application. J. Am. Dent. Assoc. 82, 1090-1093.
Journal
of Dentistry,
Vol. ~/NO. 3
BURT B. A., BERMAN D. S., GELBIER S. and SILVERSTONE L. M. (1975) Retention of a fissure sealant six months after application. Br. Dent. J. 138,98-100. VON FRAUNHOFERJ. A. and WILLIAMSB. (1974) Heat liberation during the setting of four fissure sealants. Br. Dent. J. 136, 4.98499. JACOBSENP. H. (1975) An assessment of the influence of the setting characteristics of polymeric restorative materials on their possible clinical behaviour. MDS Thesis, University of London. Low T. and VON FRAUNHOFERJ. A. (1976) In preparation. Low T., VONFRAUNHOFERJ. A. and WINTERG. B. (1975a) The bonding of a polymeric fissure sealant to topical fluoride-treated teeth. J. Oral Rehabil. 2, 303-307. Low T., DAVIESE. H. and VONFRAUNHOFERJ. A. (1975b) A method of determining the tensile bond strength of fissure sealant materials. J. Oral Rehabil. 2, 341-347. PHILLIPSR. W. (1973) Skinner’s Science of Dental Materials, 7th ed. Saunders, Philadelphia, pp. 217-242. ROCK W. P. (1973) Fissure sealants: results obtained with two different bis-GMA type sealants after one year. Br. Dent. J. 134, 193196. ROCK W. P. (1974) Fissure sealants: further results of clinical trials. Br. Dent. J. 136, 317-321. SWANEYA. C., PAFFENBANGER G. C., CAUL H. J. and SWEENEYW. T. (1953) American Dental Association Specification No. 12 for Denture Base Resins; Second Revision. J. Am. Dent. Assoc. 46, 54-66. TRANTERT. C. and DOUGLASW. H. (1975) Studies of fissure sealant behaviour in vivo. J. Dent. Res. 54, Abstr. No. L349, p. L87. WATKINSJ. J. (1975) The surface hardness of three bisphenol A and glycidyl methacrylate fissure sealants. J. Dent. 3,68-72. WILLIAMSB., CASSONM. H. and WINTER G. B. (1974a) A clinical study using a new ultraviolet light-polymerized fissure sealant. J. Dent. 2, lOl105. WILLIAMSB., VON FRAUNHOFER J. A. and WINTER G. B. (1974b) Tensile bond strength between fissure sealants and enamel. J. Dent. Res. 53, 23-27. WILLIAMS B., VON FRAUNHOFERJ. A. and WINTER G. B. (1975) A comparative evaluation of the water solubility and water microhardness, absorption of fissure sealants. J. Dent. 3, l-8.