Indications for self-cured and light-cured adhesive corn
Tokyo Medical and Dental University, Tokyo, Japan Self-cured and light-cured adhesive composite resins have biologic advantages and disadvantages. In contrast to the self-cured composite resin, the light-cured composite resin has a greater tendency to separate from a cavity wall and gingival margin. Alternative methods of insertion such as the incremental technique and cement liner have decreased the tendency to separate, but have failed to completely eliminate the problem. The light-cured composite resin may also fracture the marginal enamel rods. The light-cured composite resin is indicated for shallow facial restorations and veneers. The self-cured composite resin is indicated to restore deeper cavities, those with a dentinal gingival margin, and for occlusal restorations.(J PROSTHETDENT 1992;67:46-51.)
he introduction of the light-cured adhesive composite resin has been a practical adjunct to conservative dentistry. In contrast to the self-cured composite resin, the light-cured resin possessesremarkable advantages. It provides increased working time since the material is supplied as one paste and sets after radiation. The superficial layer has increased hardness and wear resistance of limited depth. Also, the multishaded light-cured composite resin can more accurately match shades and is more color-stable. In addition, porosity resulting from spatulation is avoided. The light-cured composite resin presents problems in adaptation to cavity walls and marginal seals, which are biologically important. Light-cured composite resins polymerize by irradiation from the surface, and the degree of polymerization decreases with depth. This report offers indications for the selective use of the chemically adhesive composite resins
FROM THE CAVITY
The self-cured chemically adhesive composite resin begins to polymerize at the cavity wall where it is warmed by body temperature. Accordingly, it contracts toward the cavity wall to which the composite resin also adheres. The light-cured composite resin begins to polymerize at the surface by irradiation from above and therefore contracts toward the fast-setting superficial layer, tending to separate from the cavity wall (Fig. 1). The tendency to contract became apparent in sections of self-cured and light-cured composite resin restorations (Fig. Z).l The self-cured composite resin Clearfil F (Kuraray Co., Osaka, Japan) showed no separation at the interface of the composite resin and dentinal cavity wall. Eight light-cured composite resins: Durafil (Kulzer & Co., GmbH, aEmeritusProfessor. 10/l/29661
Bad Hamburg, Germany), Heliosit (Vivadent, Inc., Schaan, Liechtenstein), L-l (3M Dental Products, St. Paul, Minn.), Superlux Daylight (Dental-material-gesellschaft, Hamburg, Germany), Visio Dispers (ESPE-Premier, Norristown, Pa.), Nuvafil (L. D. Caulk, Division of Dentsply International Inc., Milford, Del.), Photofil (Johnson & Johnson Dental Products Co., E. Windsor, NJ.), and Plurafil Super (LITEMA-Dentalvertrieb, Baden-Baden, Germany) showed a greater or lesser separation at the cavity wa11.2Of these, the first five included bonding agents. Such separation not only weakens retention but induces postoperative sensitivity because of the pressure or volume change under thermal or mechanical stress.2 Bonding agents can prevent such separation in cavities of limited depth, but the restrained polymerization contraction leaves residual stress in the cavity wall (Fig. 1, c). When it is irradiated from the surface, the polymerizing superficial layer tends to contract but the chemical adhesion to the etched enamel wall tends to stretch the layer. This causes a horizontal tensile stress accompanied by a resultant vertical compressive stress to increase vertical contraction, as determined by photoelastic analysis (Fig. 3).3 The vertical contraction sustained by the first-setting superficial layer tends to separate it from the cavity wall and the stress remains in the deeper portion of the composite resin, if the bonding agent does not permit separation. The bond strength of the adhesive light-cured composite resin is therefore weaker than that of the adhesive self-cured composite resin and it becomes weaker with time because of residual stress.*, 5 The effect of irradiation decreases as a function of the distance from the light source. In addition, as the light passes through the polymerized composite resin, the intensity lessens. Underpolymerization is thus possible in the deeper layer despite complete polymerization of the superficial layer. Underpolymerization presents another fac-
Fig. 1. a, Polymerization of self-cured composite resin is aided by warm cavity wall and contracts toward it. b, Light-cured composite resin polymerizes from above and contracts toward surface, separating from cavity wall. c, Restriction creates stress in cavity wall.
by bonding agent
Fig. 2. Microscopic examination
of Clearfil F self-cured composite resin showed no separation at cavity wall. Three light-cured composite resins-L-l, Durafil, and Superlux Daylight-separated at cavity wall. High power scanning electron microscopic view of replicas in top-left corners shows extent of separation. (Photographs courtesy of Kato et al.‘)
tor capable of weakening the bond of the light-cured composite resin to the cavity wall. This is particularly true at a deep gingival wall of a class II cavity where effective irradiation is difficult.
The first alternative approach to overcome the tendency of separation and underpolymerization at the cavity wall of the light-cured composite resin was the lamination or incremental insertion technique. This method can decrease the tendency toward separation but cannot completely eliminate it. The first layer can be withdrawn from a cavity wall by contraction of the second layer (Fig. 4).6 In ad-
dition, a layer of voids usually occurs at the interface of the first and second layers. A second alternative method is the application of a cement lining to the dentinal wall. A glass ionomer cement, which adheres to dentin by chelation and is pulp-compatible, is commonly used. The cement surface is etched to bond the composite resin to the cement. Nevertheless, postoperative sensitivity may persist in some instances. Examination of completed restorations showed contraction of the overlaid, bonded, light-cured composite resin, which detached the cement lining from the cavity wall (Fig. 5).6 Apparently, the cement/composite resin bond is stronger than the cement/dentin bond. Such separation from a
- - - - - - Compressive
Fig. 3. Principal
stress lines in light-cured composite resin restoration in which contraction was confined by strong bonding agent. (Courtesy of Kinoshita and Tsuchtani.)
Fig. 4. In incremental insertion, contraction of second layer of light-cured composite resin can separate first layer from cavity wall. Note voids at junction of two layers. (Photograph courtesy of Hisamitsu et a1.6)
separation from cavity
Fig. 5. Light-cured
composite resin can separate cement lining from cavity wall. (Photograph courtesy of Hisamitsu et a1.6)
cavity wall is considered responsible erative sensitivity.2
for occasional postop-
AT A DENTINAL
Itoh and Wakumoto7 compared the marginal separation of the self-cured and light-cured composite resins inserted into cylindrical dentinal cavities in extracted teeth using the wall-to-wall contraction test.8 They tested the self-
cured and light-cured composite resins marketed by two manufacturers (Kuraray Co. and 3M Dental Products). The light-cured composite resins resulted in greater separation than the self-cured composite resins (Fig. 6), because irradiation from above caused horizontal contraction of the superficial layer. In class III or V cavities with an occlusal (incisal) enamel margin and a gingival dentinal margin, the tendency
Photo Clearfil ”
Fig. 6. Facsimile of contraction gaps of two pairs of self-cured (upper) and light-cured (lower) adhesive composite resins inserted in cylindrical dentinal cavities. Gaps are recorded as percentages of diameter and are illustrated at 500 power magnification. (Diagrams courtesy of Itoh and Wakumoto.7)
Fig. 7. Light-cured composite resin may produce white enamel margin (left panel). Note separation of marginal enamel rods caused by polymerization contraction (right panel). (Photographs courtesy of Okamoto and Iwaku.) toward separation at the dentinal margin is further advanced. The polymerization contraction of the superficial layer, sustained by the stronger bond to the etched enamel margin, tends to enlarge the separation at the gingival dentinal margin. The light-cured composite resin is thus less capable of providing a satisfactory seal at a gingival dentinal margin. This tendency is further increased if the cavity wall is lined with cement.g SEPARATION RODS
The polymerization contraction of the superficial layer of the light-cured composite resin is apt to separate the mar-
ginal enamel rods from the neighboring rods if the chemical bond of the composite to the enamel wall is strong.iO This separation is clinically evident as a white margin (Fig. 7). It rarely occurs with the self-cured composite resin and can be prevented by preparing the enamel wall with a 60to 80-degree inclination. FACIAL
The multishaded, light-cured adhesive composite resin is indicated for extensive, shallow facial cavities because of accurate and stable color match and technical convenience in restoring the dental anatomy (Fig. 8). Small shallow cavities in the facial surface may also be restored with the
Sha I I cm caries
Wedge-shaped en-0s i on
Fig. 8. Selective use of light-cured (L) and self-cured (S) composite resins for facial res torations.
Fig. 9. Opposing teeth aid in establishing occlusal anatomy with self-cured adhesive composite resin in class I and class II cavities. light-cured composite resin. However, a deep cavity would be better restored with the self-cured composite resin in the deeper portion to ensure adhesion to the cavity wall. The outer portion is then restored with the light-cured composite resin for color match and stability. In cervical root cavities with margins in dentin or cementum, such as the wedge-shaped erosion or root caries, the self-cured composite resin is preferred. It provides a superior marginal seal and adaptation to the walls. The requirement of color match is not as critical on the root surface as on the crown. Currently available self-cured adhesive composite resins serve this purpose well. OCCLUSAL
The factor of most concern to investigators has been the wear resistance of composite resins. The superficial hardness is generally higher with the light-cured composite resin, but the underlying softer layer may be exposed by
correction of occlusal anatomy. On the other hand, the self-cured composite resin now has significantly improved wear resistance as a result of hybrid fillers.r1,r2 Shintani et a1.13longitudinally studied the clinical results of two brands of self-cured posterior adhesive composite resins, Clearfil Posterior (Kuraray Co.) and Scotch Bond P-10 (3&I Dental Products) in extensive occlusal cavities. After 4 years, only 5 of the 213 restorations (2.3%) were replaced for clinically significant wear. A worn occlusal surface can be readily repaired by slight reduction followed by the addition of an adhesive composite resin.14 This type of repair requires no additional sacrifice of dental tissue. Replacement of traditional nonadhesive composite resin restorations may result in a major loss of dental tissue if it is repeated several times. The adhesive composite resin offers increased conservation of dental tissue. Other factors are color match and stability, for which the
self-cured composite resin is inferior to the light-cured resin. Color is not a significant problem in posterior restorations. It is even conceivable that a slight color difference may be preferred for distinguishing the cavity outline. In addition, the light-cured composite resin may induce a white margin resulting from separation of the enamel rods. There seems to be little basis for selecting the light-cured composite resin for occlusal restorations. Complete elimination of postoperative sensitivity or pain on occlusion is difficult with the light-cured composite resin, which tends to separate from cavity walls. The self-cured composite resin with a selected chemical adhesive bonding agent does not separate when inserted into totally etched cavities. In addition, it protects the pulp by resin tags and by a resin-impregnated dentinal layer.2 The gingival marginal seal of extensive proximal cavities is also difficult with the light-cured composite resin because of irradiation from above and because of the upward polymerization contraction. The self-cured composite resin is therefore preferred to biologically protect the pulp and tooth. OCCLUSAL
It is difficult to establish the occlusal anatomy in large posterior restorations with composite resins. However, the technique is simplified with the self-cured chemically adhesive composite resin. The composite resin is inserted in an amount slightly in excess of what is needed on the occlusal surface. The patient is then requested to close in centric occlusion for 5 minutes (Fig. 9). The occlusal anatomy is thus partially formed by the opposing teeth. Upon polymerization, the occlusal surface becomes hard and wear-resistant. The composite resin polymerizes fully upon isolation from atmospheric oxygen, which inhibits polymerization. Once the cavity walls are covered with the chemically adhesive composite resin, the seal or bond cannot be affected by seepage of the oral fluid.15 Removal of surplus composite resin may be completed upon polymerization. The technique of forming anatomy by occlusion is possible only with the self-cured adhesive composite resin. The restoration of occlusion with the self-cured hybridfilled chemically adhesive composite resin takes full advantage of recent developments. Caries removal using the Caries Detector (Kuraray Co.) instrument and the simplified cavity prepartition possible with adhesive composite
resins has greatly aided operative dentistry.16 The method is thoroughly acceptable, since the self-cured chemically adhesive composite resin is better tolerated than amalgam.11 REFERENCES 1. Kate H, Watanabe A, Hisamitsu H, Wakumoto S. Observation of polymerizing contraction of light-cured resins. Jpn J Conserv Dent 1981;24:159-71. 2. Fusayama T. Factors and prevention of pulp irritation by adhesive composite resin restorations. Quintessence Int 1987;18:633-41. 3. Kinomoto Y, Torii M, Tsuchitani Y. Analysis of internal stress of lightcured composite resin restorations. Japanese Association of Dental Research. Thirty-seventh annual meeting. Tokyo: 1989, Proceeding No. 103. 4. Takemaru A, Inoue K, Endo S, et al. A study on visible light-cured composite resin. Part 1. The adhesive strength. Jpn J Conserv Dent 1985;28:1286-92. 5. Iga M, Kobayashi K, Takeshige F, et al. Effect of irradiation methods on the marginal seal of Class V light-cured composite resin restoration. Jpn J Conserv Dent 1987;30:1404-14. 6. Hisamitsu H, Kato H, Watanabe A, Wakumoto S. Polymerization contraction of visible light-cured composite resin. Shikai Tenbo 1981; 57:603-14. Itoh K, Wakumoto S. Wall-to-wall polymerization contraction of five composites. J Showa Dent Res Sot 1985;5:113-7. Assmussen E. Composite restorative resins; composite versus wall-to wall polymerization contraction. Acta Gdontol Stand 1975;23:33’7-44. Fusayama A, Kono A. Marginal closure of composite restorations with the gingival wall in cementum/dentin. J PROSTHET DENT 1989;61: 293-6.
10. Kan R, Okamoto A, Fukushima M, Iwaku M. Microcracks in the marginal enamel due to composite restoration. Jpn J Conserv Dent 1989; 32z(Autumn session No. A51). 11. Horie K. Studies on the occlusal wear of posterior composite resin restorations. Part 2. Clinical evaluation. J Jpn Stom Sot 1984;51:45-65. 12. Hendriks FHJ. Posterior composite restoration. Holland: Grafisk Bedrijf Outhuis BV, 1985:76. 13. Shintani H, Satou N, Satou J. Clinical evaluation of two posterior composite resins retained with bonding agents. J PROSTHET DENT 1989; 62627-32. 14. Chiba K, Hosoda H, Fusayama T. The addition of an adhesive composite resin to the same material: bond strength and clinical technique. J PROSTHET DENT 1989;61:669-‘75.
15. Endo A, Fujitani M, Hosoda H. Effect of immersion in water during polymerization on adaptation of adhesive resin restorations to cavity wails. Jpn J Conserv Dent 1988;31:506-13. 16. Fusayama T. New concepts in operative dentistry. Berlin, Chicago, and Tokyo: Quintessence Publishing Co, 1980. 17. Fusayama T. Posterior composite resin: a historic review. J PROSTHET DENT 1990;64:534-8.
DR. TAKAO FUSAYAMA 4-16-23 KAMIIGUSA, SUCINAMIKU TOKPO 167 JAPAN