Effects of ‘resin-compatible’ resin microhardness
Kai Chiu Chan, DDS, MS,a and Edward J. Swift, University of Iowa, Collegeof Dentistry, Iowa City, Iowa
Although the use of cavity varnishes with composite resins has traditionally been discouraged, several “resin-compatible” varnishes are currently available. This in vitro study evaluated the effects of resin-compatible cavity varnishes on a hybrid composite resin. The results of the study indicate that these varnishes soften the composite resin in contact with varnished dentin. (J PROSTAETDENT 1992;67:791-3.)
avity varnishes are solutions of natural or synthetic resins in solvents such as chloroform, alcohol, or acetone. These volatile solvents evaporate rapidly after the varnish is applied, leaving a thin resin film on the surface.‘, 2 Varnishes can decrease dentin permeability and reduce initial microleakage at the margins of amalgam restorations.3-10 However, the use of cavity varnishes with composite resins has traditionally been discouraged because the varnish solvent can interfere with resin polymerization. Conversely, the resin monomers of the composite can disrupt the varnish film. Furthermore, the varnish prevents proper wetting of the surface by the composite resin.2 Several varnishes are now available that are said to be “resin-compatible,” meaning that they can be used with composite resin. The purpose of our study was to determine whether these cavity varnishes affect the microhardness of a hybrid composite resin.
The resin-compatible varnishes used in this study were Barrier (Teledyne-Getz, Elk Grove Village, Ill.), a polyamide varnish; Universal (Mission Dental, Inc., Tinton Falls, N.J.), a nitrocellulose varnish; and Contact (Harry J. Bosworth Co., Skokie, Ill.), unknown composition (proprietary information). Copalite (Harry J. Bosworth Co.), a copal resin, was used as a positive control because it causes softening of composite resins. To simulate clinical conditions, each varnish was applied to five dentin disks according to its manufacturer’s directions. The varnish was left undisturbed for 90 seconds, and then was blown gently with compressed air to evaporate any remaining solvent. A Plexiglas mold (5 mm diameter, 2 mm deep) was placed on the dentin and filled with a universal shade hybrid composite resin (Herculite XR, Sybron/ Kerr, Romulus, Mich.). The composite resin was cured for 40 seconds with a Optilux 400 visible light-curing unit (Demetron Research Corporation, Danbury, Conn.). Five
aProfessor,Department of Operative Dentistry. bAssistant
composite resin samples were also made on dentin disks without varnish to serve as negative controls. Approximately 5 minutes after curing, the top and bottom hardness of each composite resin specimen were measured with a Micromet II microhardness testing machine (Buehler, Ltd., Lake Bluff, Ill.) using a 50 gm load and 15 second load time. Three measurements were made on each surface, averaged together, and converted to Knoop hardness numbers (KHN). To test the effect of varnish application on the hardness of set composite resin, Herculite XR was condensed into Plexiglas molds on glass slabs and cured for 40 seconds with the Optilux light. Barrier, Universal, Contact, and Copalite varnishes were each applied to the top surfaces of five composite samples and blown dry. The hardness of each sample was measured three times with the microhardness tester. Five composite resin samples without varnish treatment were fabricated and measured as the control group. Data from both parts of the experiment were analyzed using the general linear models procedure of the SAS statistical software package (SAS Institute, Cary, N.C.).
RESULTS The top surface hardness values of each group in the clinical simulation portion were similar (Table I). The slight differences between groups were not statistically significant. Bottom hardness values were significantly (p < 0.05) less than top surface hardness, except in the control and Copalite groups. To account for the effect of the varnish and to reduce the effects of random variations in microhardness, we calculated the ratio of bottom-to-top hardness (B/T) for each samp1e.l’ The mean hardness ratios for each group were compared with Duncan’s multiple range test and are reported in Table II. The top and bottom hardness values of composite resin samples polymerized on unvarnished dentin disks were nearly identical (B/T ratio of 0.98.) The B/T ratios of all other groups were significantly lower than the B/T ratio of this control group. The “resin-compatible” varnishes all caused more softening (that is, lower B/T ratios) than Copalite, although the differences were not significantly different. 791
I. Mean microhardness values (Knoop hardness numbers) for composite resin applied to varnish-coated dentin disks Table
None Copalite Universal Contact
40.7 51.3 53.8 37.5
(4.1) (2.0) (3.4) (6.7)
38.6 23.4 24.7 10.1
(6.6) (8.3) (4.1) (4.1)
II. Ratio of bottom-to-top hardness (KHN) of composite resin samples made on varnished dentin disks
None Copalite Universal Contact Barrier
Mean hardness ratio*
0.98 0.60 0.48 0.35 0.33
(0.17) (0.34) (0.13) (0.17) (0.24)
*Standard deviations given in parentheses. tCroups with same letter are not significantly
A B B B B
(a = 0.05).
The hardness values of already-polymerized composite resin after application of varnish are shown in Table III. Copalite caused a significant decrease in surface hardness. All of the resin-compatible varnishes apparently caused significant decreases in microhardness as well. However, each of these materials, particularly Barrier, left a relatively thick film on the surface of the composite resin specimens. Microhardness measurements of surfaces with this residue cannot be considered totally accurate, so the effects of these varnishes on polymerized composite resin are not clear. DiSCUSSION Bottom hardness values in all groups were lower than those for the top surface hardness, in part because composite resin polymerization and hardness decrease with depth (or more accurately, with distance from the curing tip) in light-cured composite resins.12,l3 Bottom hardness values were further reduced by the presence of cavity varnish. All of the varnishes evaluated in this study, including those claimed to be “resin-compatible,” caused softening of a hybrid composite resin. Because Knoop hardness is a reliable measure of monomer-to-polymer conversion,14, l5 the lower KHN values indicate that the varnishes interfered with resin polymerization. In addition, solvents can cause chemical degradation of composite resins that are already polymerized.16 Softening of the composite resin in contact with the tooth surface could reduce the overall quality of the restoration. Inferior adhesion and defective marginal sealing would be expected. A rationale for using a resin-compatible varnish with 792
III. Mean microhardness (KHN) of composite resin after application of cavity varnish
None Copalite Universal Barrier Contact
47.3 (5.5) 16.5 (3.6) 14.9 (2.6)
A B C C C
2.09 (0.5) 1.47 (0.2)
*Standard deviations given in parentheses. tGroups with same letter are not significantly
are given in parentheses.
(a = 0.05).
composite resin restorations is that the varnish can prevent passage of chemical irritants from the composite resin to the pul~.~ At one time, chemical irritants were considered the primary cause of adverse pulpal responses associated with composite resins. However, most studies now indicate that microleakage and bacterial ingress at marginal gaps are the primary causes of pulpal irritation. 17,ls In fact, almost any restorative material in direct contact with the pulp will cause no harmful effects if the surface is well sealed-l9 Therefore the most effective method for reducing pulpal irritation is to ensure an adequate marginal seal. Bonding of composite resin to acid-etched enamel provides an excellent marginal seal with minimal microleakage.20 However, bonding to cavosurface margins in root surfaces is more difficult and unpredictable. Still, thirdgeneration dentin adhesives provide somewhat effective bonding and reduced microleakage.21, 22 Some studies23-25 indicate that the use of a glass ionomer liner in conjunction with the adhesive further reduces leakage. There is no evidence that varnishes can enhance the marginal seal of composite resin restorations in the same manner as enamel etching and dentin bonding. In fact, one varnish has been shown to decrease the sealing ability of a phosphonate ester bonding agent.26 The availability of acid-etch and dentin bonding techniques eliminate the need for a resin-compatible varnish. Furthermore, the results of our laboratory study indicate that these varnishes are in fact not “resin-compatible.” Rather, they cause significant softening of composite resin. CONCLUSIONS 1. Cavity varnishes, including those said to be “resincompatible,” actually cause softening of composite resin in contact with varnished dentin surfaces. 2. Softening probably indicates an inhibition of resin polymerization by the varnish solvent. REFERENCES 1. Going RE. Status report on cement bases, cavity liners, varnishes, primers, and cleansers. J Am Dent Assoc 1972;85:654-60. 2. Craig RG. Restorative dental materials. 8th ed. St. Louis: CV Mosby, 1989:2X. 3. Sneed WD, Hembree JH, Welsh EL. Effectiveness of three cavity varnishes in reducing leakage of a high-copper amalgam. Oper Dent 1964;9:32-4. 4. Newman SM. Microleakage of a copal rosin cavity varnish. J PROSTHET DENT 1984;51:499-502. JUNE
5. Pashley DH, O’Meara JA, Williams EC, Kepler EE. Dentin permeability: effects of cavity varnishes and bases. J PROSTHET DENT 1985;53: 511-6.
6. Pashley DH, Depew DD. Effects of the smear layer, Copalite, and oxalate on microleakage. Oper Dent 1986;11:95-102. 7. Tjan AHL, Grant BE, Nemetz H. The efficacy of resin-compatible cavity varnishes in reducing dentin permeability to free monomer. J PROSTHET DENT 1987;57:179-85.
8. Kelsey WP, Panneton MJ. A comparison of amalgam microleakage between a copal varnish and two resin-compatible cavity varnishes. Quintessence Int 1988,19:895-8. 9. Manders CA, Garcia-Godoy F, Barnwell GM. Effect of a copal varnish, ZOE or glass ionomer cement bases on microleakage of amalgam restorations. Am J Dent 1990;3:63-6. 10. Fitchie JG, Reeves GW, Scarbrough AR, Hembree JH. Microleakage of a new cavity varnish with a high-copper spherical amalgam alloy. Oper Dent 1990;15:136-40. 11. Johnston WM, Leung RL, Fan PL. A mathematical model for post-irradiation hardening of photoactivated composite resins. Dent Mater 1985;1:191-4. 12. Council on Dental Materials, Instruments, and Equipment. Visible light-cured composites and activating units. J Am Dent Assoc 1985; 110:100-3. 13. McCabe JF, Carrick TE. Output from visible-light activation units and depth of cure of light-activated composites. J Dent Res 1989;68:1534-9. 14. Rueggeberg FA, Craig RG. Correlation of parameters used to estimate monomer conversion in a light-cured composite. J Dent Res 1988;67: 932-7. 15. Chung KH. The relationship between composition and physical properties of posterior resin composites. J Dent Res 1990;69:852-6. 16. Wu W, McKinney JE. Influence of chemicals on wear of dental composites. J Dent Res 1982;61:1180-3.
17. Briinnstrijm M. The cause of postrestorative sensitivity and its prevention. J Endodont 1986;12:475-81. 18. Council on Dental Materials, Instruments, and Equipment. Biocompatibilityandpostoperativesensitivity. JAmDentAssoc 1988;116:767-8. 19. Cox CF, Keall CL, KeaIl HJ, Ostro E, Bergenholts G. Biocompatihility of surface-sealed dental materials against exposed pulps. J PROSTHET DENT 1987;57:1-8.
20. Shaffer SE, Barkmeier WW, Kelsey WP. Effects of reduced acid conditioning on enamel microleakage. Gen Dent 1987;35:278-80. 21. Pintado MR, Douglas WH. The comparison of microleakage between two different dentin bonding resin systems. Quintessence Int 198819: 905-7. 22. Barkmeier WW, Cooley RL. Shear bond strength, microleakage and SEM study of the XR Bond Adhesive system. Am J Dent 1989;2:111-5. 23. Hembree JH. Microleakage at the gingival margin of class II composite restorations with glass ionomer liners. J PROSTHET DENT 198;61:28-30. 24. Mathis RS, DeWaId JP, Moody CR, Ferracane JL. Marginal leakage in class V composite resin restorations with glass ionomer liners in vitro. J PROSTHET DENT 1990;63:522-5. 25. Kemp-Scholte CM, Davidson CL. Complete marginal seal of class V resin composite restorations effected by increased flexibility. J Dent Res 1990;69:1240-3. 26. Grim GA, Shay JS. Effect of pretreatment procedures on the microleakage of a dentin bonded composite resin material. Quintessence Int 198819365-7. Reprint
DR. EDWARD J. SWIFT, JR. DEPARTMENT OF OPERATIVE DENTISTRY COLLEGE OF DENTISTRY UNIVERSITY OF IOWA IOWA CITY, IA 52242-1068
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