Gradual Increases in Marginal Leakage of Resin Composite Restorations with Thermal Stress Y. MOMOI, H. IWASE, Y. NAKANO, A. KOHNO, A. ASANUMA', and K. YANAGISAWA' Department of Operative Dentistry and 'Department of Physiology, School of Dental Medicine, Tsurumi University, 2-1-3 Tsurumi, Tsurumi-ku, Yokohama 230, Japan
The effects of thermal stress on the marginal leakage of resin composite restorations in bovine teeth were investigated by a method that preserved the specimens. The changes in marginal leakage of specimens with increasing numbers of thermal cycles were measured by an electrical conductivity method. Four brands of posterior resin composites were used to fill cylindrical cavities (2.0 mm in diameter and 1.5 mm in depth) on the labial surfaces of bovine incisors, according to the manufacturer's instructions. Thermal cycling stress was applied to the specimens for up to about seven weeks (9000 cycles). During this time, the electrical conductance between the pulp and a drop of physiological saline solution covering the resin restoration was measured periodically by application of an electrical potential (60 Hz, 10 Vp p). Thermal stress increased the marginal leakage gradually, rather than step-wise. Even before application of any thermal stress, wide variations of marginal leakage were found among different specimens restored with the same brand of resin. Specimens with less initial leakage showed less increase in leakage, and vice versa, throughout the experimental period. J Dent Res 69(10):1659-1663, October, 1990
Introduction. In the mouth, teeth and restorations are unavoidably subjected to thermal stresses. There are several factors that affect the
marginal leakage of resin restorations (Ben-Amar, 1989). Among them, thermal stress is an important factor, especially in resin composite restorations, because of the high thermal expansion coefficient of resin, compared with that of the tooth (Souder and Paffenbarger, 1942; Kullmann and Potters, 1984). Destructive methods involving dye penetration (Crim and GarciaGodoy, 1987; Scherer et al., 1989) or silver nitrate staining (Darbyshire et al., 1988; Eakle, 1986) have commonly been used for evaluation of the marginal leakage of resin restorations. With these methods, specimens are destroyed at the time of evaluation, and the leakage evaluation can be performed only for certain sections. In addition to these defects, quantitative evaluation is very difficult. Therefore, no consideration has been given to the question of how the marginal leakage of resin composites occurs with thermal stress, i.e., whether the marginal leakage develops gradually or suddenly under thermal stress. Accordingly, in this study, we measured the change in marginal leakage with time by an electrical method (Nakano, 1985).
of electrodes into the tooth specimens, the pulp chambers were enlarged. Then, all teeth were stored in a 370C physiological saline solution for seven days. During this period, the solution was renewed, and the pulp chambers were cleaned several times. Four cylindrical cavities (2 mm in diameter, 1.5 mm in depth, with no bevel) were prepared on the labial surface of each tooth (Fig. 1) with a high-speed diamond bur (ISO #011, Shofu Dental Mfg. Co., Kyoto, Japan). An operational microscope was used to check for cracks around the cavity and the thickness of the enamel in the cavity wall. Nine teeth with no cracks and enamel 1.0 - 1.2 mm in thickness were selected for this experiment from about 50 teeth. Each cavity was filled with one of four brands of posterior resin composite: one selfcured resin, Clearfil Photo Posterior (Kuraray Co., Ltd., Kurashiki, Japan); and three light-cured resins-Clearfil Photo Posterior (Kuraray Co., Ltd.), P-30 (3M Co., St. Paul, MN), and P-50 (3M Co.). So that any influence of variations in individual tooth specimens could be avoided, each of the four cavities in a single tooth was filled with a different brand of resin, following the manufacturer's instructions, with etching and bonding. After the margins of the restorations were finished with a carbide finishing bur (#7902, Shofu Dental Mfg. Co., Kyoto, Japan), the absence of cracks or defects in the restored portion, particularly around the marginal enamel area, was confirmed by use of an operational microscope. This confirmation was also done at every electrical measurement of the marginal leakage. Consequently, 36 resin restorations in nine teeth (nine restorations with each of the four brands of resin) were produced for this experiment. We recognized an enamel crack adjacent to one filling with P-30 after 2000 thermal cycles. All results in this restoration were eliminated from this study. Thernal stress and electrical measurements.- Immediately after the restorations were finished, all specimens were stored in a 370C physiological saline solution. After one h of storage, the first electrical measurements were made before thermal
P-30
Clearfil Posterior
P-50
Clearfil Photo Posterior
Materials and methods. Specimens. -The roots of freshly extracted bovine incisors were removed, and their coronal parts were used. For insertion Received for publication February 2, 1990 Accepted for publication May 25, 1990
Fig. 1-The positions of the resin restorations on the extracted bovine teeth. Four cavities on the labial surface of each incisor were restored with four brands of posterior resin composite. The position of each one of the four brands of resin was changed tooth by tooth so that locational bias would be avoided.
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stress was applied. Successively,- eight specimens were thermocycled in temperature-controlled physiological saline baths using a thermal cycling machine (custom-made, Sunaka Co., Tokyo, Japan), for up to 47 days (9000 cycles). During this time, 21 measurements were performed electrically, as the number of thermal cycles was increased. One thermal cycling consisted of 370C (one min) -- 40C (two min) -* 370C (1 min) -- 60'C (2 min) -. The fluctuation of the temperature in each of the four baths was less than + 20C. To avoid any change in the conductivity of the physiological saline, due to evaporation of water, the solution level in the baths was kept constant by the automatic supply of distilled water. The conductivity of the saline in each of the baths was checked periodically during this experiment. The solution in each of the four baths was constantly stirred by a small propeller for maintenance of uniform thermal distribution within each bath. The ninth specimen was kept in the 370C physiological saline solution without thermal stress. During the same period, electrical measurements were also performed on this control specimen at the same time and in the same manner as for the thermally stressed teeth. Initially, measurements were performed after one, 25, 50, 75, and 100 thermal cycle(s). The set-up for the performance of the electrical measurements is shown in Fig. 2. After the tooth surface was wiped with a paper towel, the pulp chamber was refilled with physiological saline, and the restoration was covered with a drop of physiological saline containing toluidine blue. Then, silver electrodes were inserted into the pulp chamber and the drop of solution covering the restoration, and a sinusoidal voltage (60 Hz, 10 Vpp) was applied to the experimental circuit from a function generator (FS-2201, TOA Electronics Ltd., Tokyo, Japan). The voltage drop caused by the resistor (10, 15, or 20 Kfl, tolerance; ± 1%) in the circuit was measured with a cathode-ray oscilloscope (CRO, DM 1562A, Toshiba, Tokyo, Japan; input impedance, lMfl, 35 pF). From this voltage drop, the conductance between the electrode in the pulp chamber and that in the saline solution drop covering the restoration was calculated. After subtraction of the dentin conductance (measurement of this conductance is described in the next paragraph), the final conductance value (which we call "marginal leakage" in this paper) was obtained. All measurements were performed in a thermo-controlled room at 37TC. Minimization of the influence of enamel conductance on the
6OHz,10 Vp-p
marginal leakage. -As shown in Fig. 3, marginal leakage conductance is distributed in parallel with that of the resin and the enamel, and in series with that of the dentin. In this electrical measuring procedure, the ideal conditions are that the current flows only through the microgap between the resin and the enamel, and through the dentin. The conductance of the resin was small enough to fulfill this ideal condition. After the final measurement (after 9000 cycles), the conductance of the restorative resin in each specimen was measured by the introduction of a saline solution drop on the resin surface without the margin being covered, as shown in Fig. 4B. This resin conductance value was 0.013 + 0.009 pLS (mean + S.D., n = 35), indicating no cohesive failure within the resin. The conductance of dentin could be measured accurately. Before the cavities were restored, their conductance values were measured by being filled with physiological saline (Fig. 4A). The dentin conductance value was determined to be 68 + 25 ALS (mean + S.D., n = 35). Each dentin conductance value was subtracted from the conductance between the pulp and the solution covering the restoration. The conductance of enamel might somewhat limit this electrical measurement method because enamel conductance is not as small as that of the resin composite. Therefore, so that current leakage through the enamel would be minimized, only the restoration and its margin were covered with a saline drop
ENAMEL
RE;ili02
DENTIN
PULP
Fig. 3-The distribution of conductance between the pulp and the resin restoration surface.
A
B - electrode
-*
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/ D Fig. 2-The experimental set-up. CRO
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Cathode-ray oscilloscope.
-
Fig. 4-Measurement of the conductance of the remaining dentin (A) and the resin restoration (B). E = enamel, D = dentin, electrode = silver electrode, saline solution = physiological saline solution.
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EFFECTS OF THERMAL STRESS ON MARGINAL LEAKAGE
Vol. 69 No. 10
that was as small as possible. So that a clear view of the drop's edges could be obtained, this solution was colored by 0.1% toluidine blue. Drops were made with a small syringe, and this procedure was controlled accurately under an operational microscope. The conductivity of the colored physiological saline solution was almost the same as that of the original uncolored one.
Results. Figs. 5-8 show the effect of thermal stress on the marginal leakage of specimens restored with each brand of resin composite. In each Fig., A shows the early period until 1000 cycles (nine days), and B shows the whole experimental period until 9000 cycles (47 days). Marginal leakage before application of thermal stress.Before the application of thermal stress (at 0 cycle), the marginal leakage of each resin composite (mean ± S.D.) was 1.6 + 0.4 p.S (n = 8) for Clearfil Posterior, 3.2 + 1.5 A.S (n = 8) for Clearfil Photo Posterior, 2.7 ± 0.8 p.S (n = 7) for P30, and 3.2 ± 1.5 A.S (n = 8) for P-50. The self-cured resin, Clearfil Posterior, showed less marginal leakage than the light-
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cured resins. There was large variation in the marginal leakage of different specimens restored with the same brand of resin, especially in light-cured resins. Effect of thermal stress on marginal leakage.-One cycle of thermal stress did not cause any distinguishable increase in marginal leakage. However, after 25 or 50 cycles, the marginal leakage in all specimens showed a significant increase, which was followed by a temporary decrease, probably due to resin expansion caused by absorption of water. After this temporary decrease, the marginal leakage of all specimens again increased gradually with the number of thermal cycles until the end of the experiment (9000 cycles). The lesser leakage that was found with self-cured Clearfil Posterior before thermal cycling disappeared during the experimental period, and finally, all four brands of resin showed a similar degree of marginal leakage. Specimens with a greater marginal leakage before the application of thermal stress showed a larger increase in marginal leakage with thermal stress, and vice versa. The marginal leakage values (mean + S.D.) for each resin at 7500 and 9000 cycles were 5.9 ± 1.4 puS and 6.5 ± 1.4 AxS (n = 8) for Clearfil Posterior, 6.3 ± 2.0 ,uS and 6.4 ± 1.8 AS (n = 8) for Clearfil Photo Posterior, 6.4 ± 1.6 A.S and 6.8 + 1.0 ALS (n = 7) for P-30, and 5.8 + 2.2 p.S and 5.7 ± 2.5 p.S (n = 8) for P-50. There was no significant difference among the four brands of resin composite.
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Fig. 6-The effect of thermal stress on the marginal leakage of restorations made with Clearfil Photo Posterior.
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MOMOI et al.
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J Dent Res October 1990
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Marginal leakage of restorations not subjected to thermal stress.-Fig. 9 shows the results for the specimen stored in a 370C physiological saline with no thermal stress. The significant increase in marginal leakage that was found in thermally stressed specimens was not observed throughout the experimental period.
Discussion. The gradual increase of marginal leakage with thernal stress.-An increase of marginal leakage, which was gradual rather than step-wise, occurred in all specimens (31 restorations) subjected to thermal stress (Figs. 5-8), but did not occur in the control specimen (four restorations) not subjected to thermal stress (Fig. 9). The increased leakage under thermal stress must be caused by differences in the thermal expansion of the tooth and the resin composite. In this experiment, restoration was performed at room temperature (22-240C), and then all specimens were stored in a 370C physiological saline solution for about one h before the first electrical measurements. In this situation, polymerization shrinkage should be almost completed at 370C, i.e., with no stress due to differences in the thermal expansion coefficient between the tooth and the resin at this temperature. When the temperature declines from 370C to 40C, the resin generates shrinkage stresses,
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(B) Fig. 8-The effect of thermal stress on the marginal leakage of restorations made with P-50.
while a temperature increase from 37TC to 60'C generates expansion stresses. First, we will discuss the former, i.e., the effect of decreasing the temperature from 37TC to 4TC. Braem et al. (1986, 1987) reported a Young's modulus for Clearfil Posterior and P-30 of about 20 GPa. On the other hand, the linear thermal expansion coefficient of the enamel is 11.4 x 10-6/1C (Souder and Paffenbarger, 1942), and that of the resin composite is deduced to be 2 to 3.5 times greater than that of the tooth (Kullmann and Potters, 1984). From the above values, the shrinkage stress generated by this temperature difference of 33TC can be calculated as about 8 19 MPa [20 x 109 Pa x 330C x (1 or 2.5) x 11.4 x 10-6/0C = 8 19 MPa]. The polymerization shrinkage stress that was not compensated for by the flow of resin should also be taken into consideration (Davidson and De Gee, 1984). Since the residual polymerization stress is influenced by the ratio of the bonded surface of the resin to unbonded surfaces of the resin (Feilzer et al., 1987), an accurate estimation of the residual shrinkage stress value is difficult. However, it had been reported that this residual shrinkage stress is about 5 MPa in many cases. Therefore, we calculated the value of the shrinkage stress at the bond between resin and tooth to be about 13 24 MPa (8 + 5 = 13, 19 + 5 = 24 MPa). The bond strength of resin to
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the same brand ofresin.-Marked variations of marginal leakage existed among specimens restored with the same brand of resin even before application of thermal stress, and this variation increased with the increase in thermal stress. Specimens with a large marginal leakage at the beginning had a larger leakage at the end, and vice versa. This result was not due to individual variations between the teeth, because no obvious correlation was found between the degree of marginal leakage and the individual teeth. The two teeth represented with the symbols X or 0 in Figs. 5-8 had rather larger marginal leakage values for all brands of resin composite, but in the other six teeth, no definite tendency was seen (Figs. 5-8). Therefore, may factor(s) relating have caused the variations between different restorations made with each brand of resin composite. Even though the teeth were restored under strict experimental conditions, the marginal leakage varied widely, suggesting the difficulty of the handling of resins under actual oral conditions, where restoration can easily be influenced by many undesirable factors. some
.j
0 2 5 11
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EFFECTS OF THERMAL STRESS ON MARGINAL LEAKAGE
Vol. 69 No. 10
16 20 24 28 32
47
Time after Filling (days) Fig. 9-Marginal leakage without thermal stress. To allow for eXomparison with the results of the thermal-stressed group (B in Figs. 5 -8), the time scale of the abscissa is not proportional.
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Acknowledgment. We thank Dr. D. Sullivan for help with the English.
enamel has been estimated as about 17 - 20 MPa (Zidan et al., 1980). Consequently, the shrinkage stress value is comparable with the bonding strength. This is in good agreement with our finding that a sudden increase of marginal leakage did not occur with only one thermal stress cycle, and that marginal leakage increased gradually with increasing numbers of thermal cycles (Figs. 5-8). This gradual increase in marginal leakage might be explained by the "peeling off' phenomenon. If the bond to the tooth is partly disrupted, the shrinkage stress might be concentrated at the margins of the bonded area. When this margin was subsequently peeled off during the temperature decline, the two stresses (thermal shrinkage stress and residual polymerization shrinkage stress) were released, and shrinkage of the resin occurred. The shrinkage caused by the residual polymerization stress will remain even when the specimen is returned to a temperature of 370C, i.e., the marginal leakage increases. These events may be repeated cycle by cycle with the recurrent application of thermal stress. With a temperature change from 37TC to 60'C, thermal expansion stresses put pressure on the cavity wall. Since the experimental cavities used in this study were small compared with the size of the bovine teeth, and since Young's modulus for enamel (83 GPa; Staines et al., 1981) is considerably larger than that for resin, it seems likely that the resin expands through the unrestricted free surface rather than by enlargement of the diameter of the cavity. Some configuration change of the resin might remain afterward (so-called "hysteresis"), i.e., the resin may be unable to resume its original diameter. Thus, hysteresis might also increase the marginal leakage.
Differences of marginal leakage among the four brands of resin composite.-In this study, we could not find significant differences among the four brands of resin composite in the final period of thermal stress (at 7500 to 9000 cycles). However, before application of thermal stress, the mean and standard deviation of the marginal leakage for self-cured resin was smaller than those for light-cured resins. This result might be explained by the slower polymerization process in self-cured resin (Davidson et at., 1984). The polymerization of light-cured resins occurs too fast for adequate compensation of their polymerization shrinkage by the resin flow, while polymerization of the self-cured resin occurred slowly enough to facilitate this flow. The marked variation in different specimens restored with
REFERENCES BEN-AMAR, A. (1989): Microleakage of Composite Resin Restorations: A Status Report for the American Journal of Dentistry, Am J Dent 2:175-180. BRAEM, M.; LAMBRECHTS, P.; VAN DOREN, V.; and VANHERLE, G. (1986): The Impact of Composite Structure on Its Elastic Response, J Dent Res 65:648-653. BRAEM, M.; LAMBRECHTS, P.; VANHERLE, G.; and DAVIDSON, C.L. (1987): Stiffness Increase During the Setting of Dental Composite Resins, J Dent Res 66:1713-1716. CRIM, G.A. and GARCIA-GODOY, F. (1987): Microleakage: The Effect of Storage and Cycling Duration, JProsthet Dent 57:574-576. DARBYSHIRE, P.A.; MESSER, L.B.; and DOUGLAS, W.H. (1988): Microleakage in Class II Composite Restorations Bonded to Dentin Using Thermal and Load Cycling, J Dent Res 67:585-587. DAVIDSON, C.L. and DE GEE, A.J. (1984): Relaxation of Polymerization Contraction Stress by Flow in Dental Composites, J Dent Res 63:146-148. DAVIDSON, C.L.; DE GEE, A.J.; and FEILZER, A. (1984): The Competition between the Composite-Dentin Bond Strength and the Polymerization Contraction Stress, J Dent Res 63:1396-1399. EAKLE, W.S. (1986): Effect of Thermal Cycling on Fracture Strength and Microleakage in Teeth Restored with a Bonded Composite Resin, Dent Mater 2:114-117. FEILZER, A.J.; DE GEE, A.J.; and DAVIDSON, C.L. (1987): Setting Stress in Composite Resin in Relation to Configuration of the Restoration, J Dent Res 66:1636-1639. KULLMANN, W. and PO3TERS, G. (1984): Vergleichende Untersuchungen zum Thermischen Expansionskoeffizienten an 50 Verschiedenen Kunststoff-Fiillungsmaterialien, Dtsch Zahndrztl Z 39:96-100. NAKANO, Y. (1985): A New Electrical Testing Method on Marginal Leakage of Composite Resin Restorations (in Japanese), Jpn J Conserv Dent 28:1183-1198. SCHERER, W.; KAIM, J.M.; LIPPMAN, N.; TAGLIANI, T.; and COOPER, H. (1989): Microleakage of Three Glass lonomer Cement Bases, Am J Dent 2:61-63. SOUDER, W. and PAFFENBARGER, G.C. (1942): Physical Properties of Dental Materials, U.S. Department of Commerce, National Bureau of Standards. Circular C433. U.S. Government Printing Office, Washington, D.C., pp. 12-13. STAINES, M.; ROBINSON, W.H.; and HOOD, J.A.A. (1981): Spherical Indentation of Tooth Enamel, J Mater Sci 16:2551-2556. ZIDAN, O.; ASMUSSEN, E.; and JORGENSEN, K.D. (1980): Correlation between Tensile and Bond Strength of Composite Resins, Scand J Dent Res 88:348-351.
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The effects of thermal stress on the marginal leakage of resin composite restorations in bovine teeth were investigated by a method that preserved the specimens. The changes in marginal leakage of specimens with increasing numbers of thermal cycles were measured by an electrical conductivity method. Four brands of posterior resin composites were used to fill cylindrical cavities (2.0 mm in diameter and 1.5 mm in depth) on the labial surfaces of bovine incisors, according to the manufacturer's instructions. Thermal cycling stress was applied to the specimens for up to about seven weeks (9000 cycles). During this time, the electrical conductance between the pulp and a drop of physiological saline solution covering the resin restoration was measured periodically by application of an electrical potential (60 Hz, 10 Vp p). Thermal stress increased the marginal leakage gradually, rather than step-wise. Even before application of any thermal stress, wide variations of marginal leakage were found among different specimens restored with the same brand of resin. Specimens with less initial leakage showed less increase in leakage, and vice versa, throughout the experimental period. J Dent Res 69(10):1659-1663, October, 1990
Introduction. In the mouth, teeth and restorations are unavoidably subjected to thermal stresses. There are several factors that affect the
marginal leakage of resin restorations (Ben-Amar, 1989). Among them, thermal stress is an important factor, especially in resin composite restorations, because of the high thermal expansion coefficient of resin, compared with that of the tooth (Souder and Paffenbarger, 1942; Kullmann and Potters, 1984). Destructive methods involving dye penetration (Crim and GarciaGodoy, 1987; Scherer et al., 1989) or silver nitrate staining (Darbyshire et al., 1988; Eakle, 1986) have commonly been used for evaluation of the marginal leakage of resin restorations. With these methods, specimens are destroyed at the time of evaluation, and the leakage evaluation can be performed only for certain sections. In addition to these defects, quantitative evaluation is very difficult. Therefore, no consideration has been given to the question of how the marginal leakage of resin composites occurs with thermal stress, i.e., whether the marginal leakage develops gradually or suddenly under thermal stress. Accordingly, in this study, we measured the change in marginal leakage with time by an electrical method (Nakano, 1985).
of electrodes into the tooth specimens, the pulp chambers were enlarged. Then, all teeth were stored in a 370C physiological saline solution for seven days. During this period, the solution was renewed, and the pulp chambers were cleaned several times. Four cylindrical cavities (2 mm in diameter, 1.5 mm in depth, with no bevel) were prepared on the labial surface of each tooth (Fig. 1) with a high-speed diamond bur (ISO #011, Shofu Dental Mfg. Co., Kyoto, Japan). An operational microscope was used to check for cracks around the cavity and the thickness of the enamel in the cavity wall. Nine teeth with no cracks and enamel 1.0 - 1.2 mm in thickness were selected for this experiment from about 50 teeth. Each cavity was filled with one of four brands of posterior resin composite: one selfcured resin, Clearfil Photo Posterior (Kuraray Co., Ltd., Kurashiki, Japan); and three light-cured resins-Clearfil Photo Posterior (Kuraray Co., Ltd.), P-30 (3M Co., St. Paul, MN), and P-50 (3M Co.). So that any influence of variations in individual tooth specimens could be avoided, each of the four cavities in a single tooth was filled with a different brand of resin, following the manufacturer's instructions, with etching and bonding. After the margins of the restorations were finished with a carbide finishing bur (#7902, Shofu Dental Mfg. Co., Kyoto, Japan), the absence of cracks or defects in the restored portion, particularly around the marginal enamel area, was confirmed by use of an operational microscope. This confirmation was also done at every electrical measurement of the marginal leakage. Consequently, 36 resin restorations in nine teeth (nine restorations with each of the four brands of resin) were produced for this experiment. We recognized an enamel crack adjacent to one filling with P-30 after 2000 thermal cycles. All results in this restoration were eliminated from this study. Thernal stress and electrical measurements.- Immediately after the restorations were finished, all specimens were stored in a 370C physiological saline solution. After one h of storage, the first electrical measurements were made before thermal
P-30
Clearfil Posterior
P-50
Clearfil Photo Posterior
Materials and methods. Specimens. -The roots of freshly extracted bovine incisors were removed, and their coronal parts were used. For insertion Received for publication February 2, 1990 Accepted for publication May 25, 1990
Fig. 1-The positions of the resin restorations on the extracted bovine teeth. Four cavities on the labial surface of each incisor were restored with four brands of posterior resin composite. The position of each one of the four brands of resin was changed tooth by tooth so that locational bias would be avoided.
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stress was applied. Successively,- eight specimens were thermocycled in temperature-controlled physiological saline baths using a thermal cycling machine (custom-made, Sunaka Co., Tokyo, Japan), for up to 47 days (9000 cycles). During this time, 21 measurements were performed electrically, as the number of thermal cycles was increased. One thermal cycling consisted of 370C (one min) -- 40C (two min) -* 370C (1 min) -- 60'C (2 min) -. The fluctuation of the temperature in each of the four baths was less than + 20C. To avoid any change in the conductivity of the physiological saline, due to evaporation of water, the solution level in the baths was kept constant by the automatic supply of distilled water. The conductivity of the saline in each of the baths was checked periodically during this experiment. The solution in each of the four baths was constantly stirred by a small propeller for maintenance of uniform thermal distribution within each bath. The ninth specimen was kept in the 370C physiological saline solution without thermal stress. During the same period, electrical measurements were also performed on this control specimen at the same time and in the same manner as for the thermally stressed teeth. Initially, measurements were performed after one, 25, 50, 75, and 100 thermal cycle(s). The set-up for the performance of the electrical measurements is shown in Fig. 2. After the tooth surface was wiped with a paper towel, the pulp chamber was refilled with physiological saline, and the restoration was covered with a drop of physiological saline containing toluidine blue. Then, silver electrodes were inserted into the pulp chamber and the drop of solution covering the restoration, and a sinusoidal voltage (60 Hz, 10 Vpp) was applied to the experimental circuit from a function generator (FS-2201, TOA Electronics Ltd., Tokyo, Japan). The voltage drop caused by the resistor (10, 15, or 20 Kfl, tolerance; ± 1%) in the circuit was measured with a cathode-ray oscilloscope (CRO, DM 1562A, Toshiba, Tokyo, Japan; input impedance, lMfl, 35 pF). From this voltage drop, the conductance between the electrode in the pulp chamber and that in the saline solution drop covering the restoration was calculated. After subtraction of the dentin conductance (measurement of this conductance is described in the next paragraph), the final conductance value (which we call "marginal leakage" in this paper) was obtained. All measurements were performed in a thermo-controlled room at 37TC. Minimization of the influence of enamel conductance on the
6OHz,10 Vp-p
marginal leakage. -As shown in Fig. 3, marginal leakage conductance is distributed in parallel with that of the resin and the enamel, and in series with that of the dentin. In this electrical measuring procedure, the ideal conditions are that the current flows only through the microgap between the resin and the enamel, and through the dentin. The conductance of the resin was small enough to fulfill this ideal condition. After the final measurement (after 9000 cycles), the conductance of the restorative resin in each specimen was measured by the introduction of a saline solution drop on the resin surface without the margin being covered, as shown in Fig. 4B. This resin conductance value was 0.013 + 0.009 pLS (mean + S.D., n = 35), indicating no cohesive failure within the resin. The conductance of dentin could be measured accurately. Before the cavities were restored, their conductance values were measured by being filled with physiological saline (Fig. 4A). The dentin conductance value was determined to be 68 + 25 ALS (mean + S.D., n = 35). Each dentin conductance value was subtracted from the conductance between the pulp and the solution covering the restoration. The conductance of enamel might somewhat limit this electrical measurement method because enamel conductance is not as small as that of the resin composite. Therefore, so that current leakage through the enamel would be minimized, only the restoration and its margin were covered with a saline drop
ENAMEL
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Fig. 3-The distribution of conductance between the pulp and the resin restoration surface.
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-
Fig. 4-Measurement of the conductance of the remaining dentin (A) and the resin restoration (B). E = enamel, D = dentin, electrode = silver electrode, saline solution = physiological saline solution.
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EFFECTS OF THERMAL STRESS ON MARGINAL LEAKAGE
Vol. 69 No. 10
that was as small as possible. So that a clear view of the drop's edges could be obtained, this solution was colored by 0.1% toluidine blue. Drops were made with a small syringe, and this procedure was controlled accurately under an operational microscope. The conductivity of the colored physiological saline solution was almost the same as that of the original uncolored one.
Results. Figs. 5-8 show the effect of thermal stress on the marginal leakage of specimens restored with each brand of resin composite. In each Fig., A shows the early period until 1000 cycles (nine days), and B shows the whole experimental period until 9000 cycles (47 days). Marginal leakage before application of thermal stress.Before the application of thermal stress (at 0 cycle), the marginal leakage of each resin composite (mean ± S.D.) was 1.6 + 0.4 p.S (n = 8) for Clearfil Posterior, 3.2 + 1.5 A.S (n = 8) for Clearfil Photo Posterior, 2.7 ± 0.8 p.S (n = 7) for P30, and 3.2 ± 1.5 A.S (n = 8) for P-50. The self-cured resin, Clearfil Posterior, showed less marginal leakage than the light-
1661
cured resins. There was large variation in the marginal leakage of different specimens restored with the same brand of resin, especially in light-cured resins. Effect of thermal stress on marginal leakage.-One cycle of thermal stress did not cause any distinguishable increase in marginal leakage. However, after 25 or 50 cycles, the marginal leakage in all specimens showed a significant increase, which was followed by a temporary decrease, probably due to resin expansion caused by absorption of water. After this temporary decrease, the marginal leakage of all specimens again increased gradually with the number of thermal cycles until the end of the experiment (9000 cycles). The lesser leakage that was found with self-cured Clearfil Posterior before thermal cycling disappeared during the experimental period, and finally, all four brands of resin showed a similar degree of marginal leakage. Specimens with a greater marginal leakage before the application of thermal stress showed a larger increase in marginal leakage with thermal stress, and vice versa. The marginal leakage values (mean + S.D.) for each resin at 7500 and 9000 cycles were 5.9 ± 1.4 puS and 6.5 ± 1.4 AxS (n = 8) for Clearfil Posterior, 6.3 ± 2.0 ,uS and 6.4 ± 1.8 AS (n = 8) for Clearfil Photo Posterior, 6.4 ± 1.6 A.S and 6.8 + 1.0 ALS (n = 7) for P-30, and 5.8 + 2.2 p.S and 5.7 ± 2.5 p.S (n = 8) for P-50. There was no significant difference among the four brands of resin composite.
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Fig. 6-The effect of thermal stress on the marginal leakage of restorations made with Clearfil Photo Posterior.
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MOMOI et al.
1662
J Dent Res October 1990
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Marginal leakage of restorations not subjected to thermal stress.-Fig. 9 shows the results for the specimen stored in a 370C physiological saline with no thermal stress. The significant increase in marginal leakage that was found in thermally stressed specimens was not observed throughout the experimental period.
Discussion. The gradual increase of marginal leakage with thernal stress.-An increase of marginal leakage, which was gradual rather than step-wise, occurred in all specimens (31 restorations) subjected to thermal stress (Figs. 5-8), but did not occur in the control specimen (four restorations) not subjected to thermal stress (Fig. 9). The increased leakage under thermal stress must be caused by differences in the thermal expansion of the tooth and the resin composite. In this experiment, restoration was performed at room temperature (22-240C), and then all specimens were stored in a 370C physiological saline solution for about one h before the first electrical measurements. In this situation, polymerization shrinkage should be almost completed at 370C, i.e., with no stress due to differences in the thermal expansion coefficient between the tooth and the resin at this temperature. When the temperature declines from 370C to 40C, the resin generates shrinkage stresses,
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(B) Fig. 8-The effect of thermal stress on the marginal leakage of restorations made with P-50.
while a temperature increase from 37TC to 60'C generates expansion stresses. First, we will discuss the former, i.e., the effect of decreasing the temperature from 37TC to 4TC. Braem et al. (1986, 1987) reported a Young's modulus for Clearfil Posterior and P-30 of about 20 GPa. On the other hand, the linear thermal expansion coefficient of the enamel is 11.4 x 10-6/1C (Souder and Paffenbarger, 1942), and that of the resin composite is deduced to be 2 to 3.5 times greater than that of the tooth (Kullmann and Potters, 1984). From the above values, the shrinkage stress generated by this temperature difference of 33TC can be calculated as about 8 19 MPa [20 x 109 Pa x 330C x (1 or 2.5) x 11.4 x 10-6/0C = 8 19 MPa]. The polymerization shrinkage stress that was not compensated for by the flow of resin should also be taken into consideration (Davidson and De Gee, 1984). Since the residual polymerization stress is influenced by the ratio of the bonded surface of the resin to unbonded surfaces of the resin (Feilzer et al., 1987), an accurate estimation of the residual shrinkage stress value is difficult. However, it had been reported that this residual shrinkage stress is about 5 MPa in many cases. Therefore, we calculated the value of the shrinkage stress at the bond between resin and tooth to be about 13 24 MPa (8 + 5 = 13, 19 + 5 = 24 MPa). The bond strength of resin to
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the same brand ofresin.-Marked variations of marginal leakage existed among specimens restored with the same brand of resin even before application of thermal stress, and this variation increased with the increase in thermal stress. Specimens with a large marginal leakage at the beginning had a larger leakage at the end, and vice versa. This result was not due to individual variations between the teeth, because no obvious correlation was found between the degree of marginal leakage and the individual teeth. The two teeth represented with the symbols X or 0 in Figs. 5-8 had rather larger marginal leakage values for all brands of resin composite, but in the other six teeth, no definite tendency was seen (Figs. 5-8). Therefore, may factor(s) relating have caused the variations between different restorations made with each brand of resin composite. Even though the teeth were restored under strict experimental conditions, the marginal leakage varied widely, suggesting the difficulty of the handling of resins under actual oral conditions, where restoration can easily be influenced by many undesirable factors. some
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EFFECTS OF THERMAL STRESS ON MARGINAL LEAKAGE
Vol. 69 No. 10
16 20 24 28 32
47
Time after Filling (days) Fig. 9-Marginal leakage without thermal stress. To allow for eXomparison with the results of the thermal-stressed group (B in Figs. 5 -8), the time scale of the abscissa is not proportional.
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Acknowledgment. We thank Dr. D. Sullivan for help with the English.
enamel has been estimated as about 17 - 20 MPa (Zidan et al., 1980). Consequently, the shrinkage stress value is comparable with the bonding strength. This is in good agreement with our finding that a sudden increase of marginal leakage did not occur with only one thermal stress cycle, and that marginal leakage increased gradually with increasing numbers of thermal cycles (Figs. 5-8). This gradual increase in marginal leakage might be explained by the "peeling off' phenomenon. If the bond to the tooth is partly disrupted, the shrinkage stress might be concentrated at the margins of the bonded area. When this margin was subsequently peeled off during the temperature decline, the two stresses (thermal shrinkage stress and residual polymerization shrinkage stress) were released, and shrinkage of the resin occurred. The shrinkage caused by the residual polymerization stress will remain even when the specimen is returned to a temperature of 370C, i.e., the marginal leakage increases. These events may be repeated cycle by cycle with the recurrent application of thermal stress. With a temperature change from 37TC to 60'C, thermal expansion stresses put pressure on the cavity wall. Since the experimental cavities used in this study were small compared with the size of the bovine teeth, and since Young's modulus for enamel (83 GPa; Staines et al., 1981) is considerably larger than that for resin, it seems likely that the resin expands through the unrestricted free surface rather than by enlargement of the diameter of the cavity. Some configuration change of the resin might remain afterward (so-called "hysteresis"), i.e., the resin may be unable to resume its original diameter. Thus, hysteresis might also increase the marginal leakage.
Differences of marginal leakage among the four brands of resin composite.-In this study, we could not find significant differences among the four brands of resin composite in the final period of thermal stress (at 7500 to 9000 cycles). However, before application of thermal stress, the mean and standard deviation of the marginal leakage for self-cured resin was smaller than those for light-cured resins. This result might be explained by the slower polymerization process in self-cured resin (Davidson et at., 1984). The polymerization of light-cured resins occurs too fast for adequate compensation of their polymerization shrinkage by the resin flow, while polymerization of the self-cured resin occurred slowly enough to facilitate this flow. The marked variation in different specimens restored with
REFERENCES BEN-AMAR, A. (1989): Microleakage of Composite Resin Restorations: A Status Report for the American Journal of Dentistry, Am J Dent 2:175-180. BRAEM, M.; LAMBRECHTS, P.; VAN DOREN, V.; and VANHERLE, G. (1986): The Impact of Composite Structure on Its Elastic Response, J Dent Res 65:648-653. BRAEM, M.; LAMBRECHTS, P.; VANHERLE, G.; and DAVIDSON, C.L. (1987): Stiffness Increase During the Setting of Dental Composite Resins, J Dent Res 66:1713-1716. CRIM, G.A. and GARCIA-GODOY, F. (1987): Microleakage: The Effect of Storage and Cycling Duration, JProsthet Dent 57:574-576. DARBYSHIRE, P.A.; MESSER, L.B.; and DOUGLAS, W.H. (1988): Microleakage in Class II Composite Restorations Bonded to Dentin Using Thermal and Load Cycling, J Dent Res 67:585-587. DAVIDSON, C.L. and DE GEE, A.J. (1984): Relaxation of Polymerization Contraction Stress by Flow in Dental Composites, J Dent Res 63:146-148. DAVIDSON, C.L.; DE GEE, A.J.; and FEILZER, A. (1984): The Competition between the Composite-Dentin Bond Strength and the Polymerization Contraction Stress, J Dent Res 63:1396-1399. EAKLE, W.S. (1986): Effect of Thermal Cycling on Fracture Strength and Microleakage in Teeth Restored with a Bonded Composite Resin, Dent Mater 2:114-117. FEILZER, A.J.; DE GEE, A.J.; and DAVIDSON, C.L. (1987): Setting Stress in Composite Resin in Relation to Configuration of the Restoration, J Dent Res 66:1636-1639. KULLMANN, W. and PO3TERS, G. (1984): Vergleichende Untersuchungen zum Thermischen Expansionskoeffizienten an 50 Verschiedenen Kunststoff-Fiillungsmaterialien, Dtsch Zahndrztl Z 39:96-100. NAKANO, Y. (1985): A New Electrical Testing Method on Marginal Leakage of Composite Resin Restorations (in Japanese), Jpn J Conserv Dent 28:1183-1198. SCHERER, W.; KAIM, J.M.; LIPPMAN, N.; TAGLIANI, T.; and COOPER, H. (1989): Microleakage of Three Glass lonomer Cement Bases, Am J Dent 2:61-63. SOUDER, W. and PAFFENBARGER, G.C. (1942): Physical Properties of Dental Materials, U.S. Department of Commerce, National Bureau of Standards. Circular C433. U.S. Government Printing Office, Washington, D.C., pp. 12-13. STAINES, M.; ROBINSON, W.H.; and HOOD, J.A.A. (1981): Spherical Indentation of Tooth Enamel, J Mater Sci 16:2551-2556. ZIDAN, O.; ASMUSSEN, E.; and JORGENSEN, K.D. (1980): Correlation between Tensile and Bond Strength of Composite Resins, Scand J Dent Res 88:348-351.
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