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Relationship between bond strength and microleakage measured in the same Class I restorations
Dent Mater 8:37-41, January, 1992
Relationship between bond strength and microleakage measured in the same Class I restorations C. PratP, M. Simpson 2, J. Mitchem 3, L. Tao 2, D.H. Pashley 2 Department of Conservative Dentistry, School of Dentistry, University of Bologna, Bologna, Italy 2Department of Oral Biology, School of Dentistry, Medical College of Georgia, Augusta, GA USA 3Department of Dental Materials, School of Dentistry, Oregon Health Sciences University, Portland, OR USA
Abstract. Microleakage measurements were made by use of a pressurized fluid method in Class I restorations prepared in extracted human teeth just prior to measuring the tensile bond strengths of the same restorations. The restorative materials included dentin bonding systems that are applied to smear layers as well as those which remove the smear layer. A light-cured glass-ionomer cement was also included. The results demonstrated that there was an inverse relationship between dentin bond strength and microleakage in some materials and that the bond strengths made to three-dimensional Class I cavities were much lower than those made to flat dentin surfaces. Measurement of microleakage by fluid filtration had no apparent effect on bond strength. Most dentin bonding studies have been done on flat dentin surfaces which minimize the development of interfacial stress due to polymerization contraction forces (Davidson, 1986; Feilzer et al., 1987). Recently, several laboratories have begun measuring dentin bond strength in three dimensional cavities prepared in dentin (Terkla et al., 1988, 1992; Wieczkowski et al., 1990; Stewart et al., 1990). These restorations can be influenced by both polymerization contraction and water sorption. Such bond testing may be more relevant to clinical practice than bonds made to flat surfaces. Microleakage develops between cavity walls and filling materials when these materials are poorly adapted to the cavity, when they pull off a wall during polymerization, when liners or smear layers dissolve, or when recurrent caries produces a gap between filling materials and tooth structure. Microleakage studies have largely been limited to Class 5 cavities prepared at the DEJ so that half the cavosurface margins are in enamel andhalfare in cementum/dentin (Gordon et al., 1986). Some authors have measured the width of marginal gaps in proximal Class I cavities (Munksgaard and Irie, 1987; Finger and Ohsawa, 1987). They assume that gap size is positively correlated to microleakage values (Irie et al., 1990). Several authors have then attempted to correlate bond strength values with marginal gap size, even though the former data were obtained by use of flat dentin surfaces and the latter were done with use of proximal Class I restorations (Munksgaard et aI., 1985; Komatsu and Finger, 1986). What is needed are experiments in which microleakage and bond strengths are determined on the same samples in a clinically relevant manner--that is, the tensile bond strength would be determined in cavities with three dimensions, following measurements of microleakage or marginal gap size. No such
experiments of this type have been reported in the literature. The purpose of these experiments was to measure the tensile bond strength of several dentin bonding systems in Class I cavities prepared in extracted human dentin and to compare those bond strengths with the degree ofmicroleakage exhibited by the same materials in the same restorations.
MATERIALS AND METHODS (I) Tooth Preparation .--Extracted human third molars were stored in isotonic saline containing 0.2% sodium azide (as an antimicrobial agent) at 4°C for less than one month. Crown segments were prepared by sectioning the roots from the crown at the CEJ using an Isomet saw (Buehler Ltd., Lake Bluff, IL). A second cut, parallel to the first, removed the occlusal enamel and left a flat dentin surface which had been in the middle of the coronal dentin. The pulpal soft tissue was removed with cotton forceps, with care taken not to create a smear layer on the pulpal side of the dentin. The minimum dentin thickness remaining between the pulpal floor of the cavity and the underlying pulp chamber was measured by means of a pincer-type caliper. The resulting crown segment was cemented, with cyanoacrylate (Zapit, DVA, Anaheim, CA 92807), to a 2 x 2 x 0.7 cm piece of Plexiglas. A 2 cm length of 18-gauge stainless steel tubing penetrated the Plexiglas and came into contact with the coronal part of the pulp chamber, which was filled with sterile phosphate-buffered saline (Dulbecco's, Grand Island Biologicals, Grand Island, NY 14072). The tubing was connected to a pressurized buffer reservoir which was used to measure the permeability of the dentin before and after restoration of the cavity (Pashley and Depew, 1986; Pashley et. al., 1988). (2) Cavity Preparation.--Class I cavities were prepared in the centers of the crown segments by use of an aluminum oxide stone (cat. #992, Dremel, Racine, WI) mounted in a precision drill press (Model 7010, Servo Products Co., Pasadena, CA). The stone was cooled with water and ran at a constant speed of 3500 rpm. The resulting cavity dimensions were 4 mm in diameter and 1.2 mm deep, with a total internal surface area of 0.271 cm2. The entire cavity and occlusal surface of the dentin were then treated with 0.5 mol/L EDTA for 2 min for removal of the smear layer and maximization of dentin permeability. The occlusal dentin outside the confines of the cavity was then treated for 2 rain with 3% monopotassium-monohydrogen oxalate (Protect, John O. Butler Co., Chicago, IL) so that all dentinal tubules outside the cavity would be occluded with calcium oxalate crystals (Pashley and Depew, 1986). Under
Dental Materials~January 1992 37
TABLE 1: EFFECTS OF BONDING PROCEDURES ON DENTIN PERMEABILITY/MICROLEAKAGE* Dentin Permeabilitv/Microleakaae as a Percent of Maximum Maximum
Smear Layer
Primed
Bonded
Debonded
Etched
Scotchbond/Silux
100
48.1 + 5.5(11)
22.4 _+ 4.2(11)1
42.7 + 5.9(11)
59.0 + 12.3
Vitrabond
100
34.2 + 5.5(11)
9.6 + 1.5(11)
16.7 + 2.5(9)
39.2 + 14.8
Tripton/Occlusin
100
54.2 + 9.4(11)
17.5 + 3.5(11)1
18.0 + 3.4(10)
37.7 + 7.0
Scotchbond 2/Silux
100
47.8 + 8.2(11)
8.7 _+ 2.4(11)
82.1 +53.0(11)
51.6 + 9.5
67.3 + 13.0 (11)1
19.0 + 4.7 Clearfil PB/PhotoPosterior 100 43.1 + 7.3(11) 80.7 + 16.3(11) I 16.3 ± 5.8(11)1 26.1 ± 5.3(11) Values are the x _+SEM of the maximum percent of the permeability of the specimens which was measured after use of 0.5 M EDTA (two min). Dentin permeability was re-measured after creation of smear layer, after treatment with primer, if required, after insertion of the material (bonded), and after breaking of the bond (debonded) groups (p < 0.05). After insertion of restorative materials into the cavity, fluid filtration was regarded as measuring microleakage rather than dentin permeability. Those values not connected by a vertical line in the same plane are significantly different at p < 0.05. Numbers in parentheses are the numbers of samples studied.
these conditions, all of the fluid permeation occurred within the dentin in the cavity preparation.
(3) Measurement of Microleakage/Dentin Permeability.-When the PBS in the pulp chamber was pressurized to 10 psi, the fluid filtered across only the EDTA-opened tubules within the empty cavity. This fluid flow was expressed as a hydraulic conductance (Lp) and was assigned a value of 100%, which represented the maximum permeability of the empty cavity. A smear layer was then created on that dentin within the cavity using the same slow-speed stone that was used to prepare the original cavity. The hydraulic conductance of the smear-layer-covered dentin was re-measured and expressed as a percent of the maximal (i.e., 100%) value. If the restorative system required treatment of the smear layer with a conditioner, then the dentin was so treated and the permeability of the dentin (i.e., hydraulic conductance or Lp) re-measured. The permeability of the dentin (Lp) was measured after debonding and again after challenging the de-bonded dentin surface with 37% phosphoric acid for 30 s to determine if the dentin surface was sensitive to acid treatment. (4) Bonding Procedures.--All bonding was done in the absence of a physiological pulpal pressure. There were two types of bonding done. Conventional dentin bonding was done on the center of flat occlusal dentin. Experimental dentin bonding was done in shallow Class I cavities prepared in the center of the flat occlusal dentin. All specimens were measured in the tensile mode 30 min after placement of the restorative materials. Four dentin bonding systems and one light-cured glassionomer cement (GIC) were tested (Table 1). They were all used according to their manufacturer's instructions. Vitrabond (3M Dental Products, St. Paul, MN) was mixed as directed, inserted into the cavity as a single increment, and then photo-cured for 60 s by means of an Optilux 50 (Demetron Research Corporation, CT 06810). No smear layer treatment was used. Tripton (Coe/ICI, Macclesfield, England) primer agent was applied directly to the smear-layer-covered dentin, spread gently with air, dried for five s, then covered with Tripton Universal Bonding Agent and light-cured for 20 s. The treated surface was then covered with Occlusin (ICI) composite material in a single step and light-cured for 40 s. Since the cavities were only 1.2 mm deep, they were all filled with a single increment of composite. Dual-cure Scotchbond (3M Dental Products, St. Paul, MN)
38 Prati et aL/Bond strength and microleakage in Class I cavities
was applied to the smear layer without any prior conditioning. It was photo-cured for 20 s and then covered with universal shade Silux (3M) as a single increment which was light-cured for 40 s. When the Scotchbond 2 system was used, the smear layer was treated with Scotchprep primer for 30 s, with constant agitation with a small sponge. This surface was then gently dried for 10 s so that a dry, reflective surface would be obtained. Scotchbond 2 adhesive was then applied and photocured for 20 s. Finally, Silux resin composite was applied as a single increment and photo-cured for 40 s. When the Clearfil Photo Bond system (Kuraray, Kyoto, Japan) was used, the smear layer was removed with 37% phosphoric acid gel. After 30 s of acid treatment, the cavity was rinsed with water for 10 s and dried with an air syringe for 10 s. Clearfil Photo Bond was mixed according to the manufacturer's instructions, applied to the cavity with a small brush, and photo-cured for 20 s. The bonding agent was then covered with Clearfll Photo Posterior resin composite (Kuraray) and photo-cured for 40 s. (5) Bond Testing.--After the microleakage of the restored cavities was measured, an orthodontic button (GAC International, Inc., Cat. #KOOO, Central Islip, NY) [approximately 3 mm in diameter, backed by a wire mesh] was bonded to the material by means of light-cured Clearfll Photo Posterior (Kuraray, Kyoto, Japan) composite. This permitted tensile forces to be applied to the restoration at 0.5 mm/min by use of an Instron universal testing machine. The specimens were tested 30 min after bonding. Some specimens subjected to bond tests were never subjected to fluid under pressure and served as a control group to those that were subjected to microleakage measurements. For comparative purposes, additional teeth were prepared with flat surfaces which are used in most conventional dentin bonding studies. Nylon cylindrical matrices with an internal diameter of 3 mm and a depth of 3 mm were used to bond the material to these flat surfaces. The final surface preparation was done with the same aluminum oxide stone as was used in the Class I cavities, so that the smear layers would be similar. Composite was added in two increments. After the first 1.5 mm of composite was inserted and cured, a straight pin dipped in Clearfil New Bond was placed head down in the composite, and the second 1.5-ram-thick increment was placed and cured. An adjustable chuck hung from the cross-head on a cable, like a plumb bob. The pin was placed in the chuck, which was then tightened firmly. The tooth specimen was then placed
TABLE2: BOND STRENGTH (MPa) TO HUMAN DENTIN Materials
3D Cavity a Plus Microleakage Test
3D Cavity, No Microleakage Test
Fiat Surface, No Microleakage Test
RDTb (m m)
0.63 _+ 0.13(0/6)
1.14 +
0.17 (0/7)
~.20 +
0.03(11)
1.92 _+ 0.32(1/11)
1.76 +_ 0.45(0/5)
1.52 +
0.39 (0/8)
1.30 +
0.03 (11)
Tripton
2.09 +
0.24 (0/11)
1.64 +
0.37 (0/5)
3.70 _+
1.23(0/6)
1.50 _+ 0.06 (11)
Scotchbond 2
3.03 +
0.36 (5/11)
3.76 +
0.6 (2/5)
6.85 +_
1.20 (0/8)
1.30 _+ 0.03 (11)
Scotchbond DC
0.78 +
Vitrabond
0.12(0/11 c)
1.20 + 0.07(11) Clearfil PB 3.90 + 0.32(1/11) 1.85 + 0.37(0/5) I 7.24 + 0.83 (0/7) a Bond strength in tensile mode~ + SEM, MPa). b RDT = remaining dentin thickness in mm. Numbers in parentheses indicate numbers of specimens tested. Vertical and horizontal lines connect groups that are not statistically different at p > 0.05. Those not connected by vertical lines in the same plane are statistically significantly different at p < 0.05. c The second numberisthe total number of samples tested. 0/11 means there were no partial cohesive failures out of 11samples and that 11/11 ofthe samples exhibited an adhesive failure. The first number in parentheses indicates the number of samples exhibiting partial cohesive failure.
beneath a restaining metal strap containing a hole large enough to permit the sample to be de-bonded but small enough to hold the specimen in place, creating a simple, self-aligning device. When the orthodontic buttons were used, the device of Powis et al. (1982) replaced the adjustable chuck. After the bond tests, both sides of the failed bond were inspected with a stereomicroscope (20x) for evaluation of whether the mode of failure was adhesive, cohesive, or both. (6) Statistical Analyses.--The force (in fkg) required to break the bonds was divided by total dentin surface areas to obtain the bond strength, which was expressed in MPa. The means and standard deviations of each group were compared for statistically significant differences, by one-way ANOVA. Duncan's multiple-range test was used to rank the materials and to perform multiple comparisons between and among the various treatments. Paired comparisons were performed by the paired t test. Statistical significance was defined as p < 0.05. Relationships between bond strengths and dentin permeability were tested by regression analysis. RESULTS The mechanical creation of smear layers in the cavities reduced dentin permeability from 100% (the EDTA-treated maximum value) to between 34.2 and 54.2% of maximum. There were no statistically significant differences between the groups in terms of their smear layer permeabilities (Table 1). In the two systems that use acidic primers to remove the smear layer, the permeability increased significantly from 47.8 to 67.3% in the Scotchbond 2 group and from 43.1 to 80.7% in the Clearfil PB group. When the cavities were restored (bonded values, Table 1), all of the materials produced a statistically significant reduction in fluid flow, with Vitrabond and Scotchbond 2 sealing the dentin better than the other materials (p < 0.05, Table 1). After the tensile strengths of the bonds were measured (de-bonding values in Table 1), the fluid flow through the empty cavity dentin exhibited little change in the Tripton, Vitrabond, and Clearfil PB groups, but increased in the Scotchbond DC and Scotchbond 2 groups, although due to the large variance, these increases were not statistically significant. When the debonded surfaces were challenged with 37% phosphoric acid, there were statistically significant increases in the permeability of the dentin in the Vitrabond and Tripton groups. The tensile bond strengths of the various groups are shown in Table 2. In those samples in which fluid was forced around the restoration to measure microleakage prior to bonding testing, the bond strengths ranged from a low value of 0.78 MPa
for Scotchbond DC to 3.90 MPa for Clearfil Photobond. When no fluid was forced around the restorations prior to bond testing, the values were approximately the same with the exception of the Clearfil Photo Bond group, which showed a lower mean value. Control specimens in which these materials were bonded to flat dentin surfaces exhibited higher bond strengths (p < 0.05), except for the Vitrabond and Tripton groups. The minimal remaining dentin thickness (RDT, Table 2) in the various groups ranged from 1.2 to 1.5 mm and was not statistically significantly different in any of the groups. When the bond strengths of the restored cavities were plotted against the apparent permeability (i.e., microleakage) of the restored cavities, only Scotchbond DC and Scotchbond 2 revealed negative correlations (Fig.). Regression analysis predicted that Scotchbond DC would show zero microleakage at a tensile bond strength of 1.1 MPa. Similar analysis of the Scotchbond 2 data gave a value of 4.6 MPa. DISCUSSION The results ofthis study demonstrate the feasibility ofmeasuring both microleakage and bond strength of dentin bonding systems in the same Class I restorations. The fluid-under-pressure system had been used previously to measure marginal leakage (Terkla et al., 1987; Derkson et al., 1986; Pashley and Depew, 1986; Pashley et al., 1988), but it had not been correlated with bond strengths. In Table 1, the permeability of the dentin prior to insertion of the materials was only between 34 and 54% of maximum, due to the presence of a smear layer. In the Scotchbond 2 and Clearfil Photo Bond groups, the smear layer was partially removed by primers, resulting in increased dentin permeability. However, after the materials were inserted, the largest reductions in apparent permeability were seen in those systems which removed the smear layers, Scotchbond 2 and Clearfil Photo Bond (54 and 65% reductions, respectively). The other three restorative systems placed on smear layers were less able to seal dentin. After insertion of a restorative material, the apparent permeability of the dentin was re-measured after 10 min. If the material sealed dentin perfectly, the fluid flow should have fallen to zero (Derkson et al., 1986; Pashley and Depew, 1986). If the material did not seal the dentin at all, the permeability would have remained unchanged from its prerestored value. Thus, changes in the permeability of the cavity following insertion of restorative materials provided a quantitative, non-destructive measure ofmicroleakage around any microgaps that might have existed between the material and
Dental Materials~January 1992 39
5 ~
O,~O Scotchbond 2 Scotchbond DC 4-
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m A
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~ O
a.
Scotchbond 2 y=e(1.517-0.065x) r =-0.68 p<0.01 N=11
3J¢ .
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c
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P
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Scotchbond DC Y=e(°128-°°21x) r =-0.48
k
2"0 f0
nd 1m
0 I
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20
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30
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50
Microleakage as Percent of Maximum Fig. Therewere inverserelationshipsbetweenthe bond strengthsof ScotchbondDC and Scotchbond2 and microleakagemeasuredas fluid filtration.
the walls of the cavity. After the restoration was de-bonded, the permeability of the Scotchbond DC/Silux-restored dentin returned to a value (i.e., 43%) which was not statistically different from its pre-bond (i.e., smear layer) value (Table 1). Both Vitrabond- and Tripton-treated dentin had de-bonded permeabilities that remained low, indicating that these materials left a residue on the smear layer that reduced its permeability. Scotchbond 2/Silux-bonded dentin exhibited a dentin permeability (but with a large variance) higher than the pre-bonded value. Presumably, this was due to removal of resin tags from the tubules when the bond failed. Clearfil Photo Bond-treated dentin revealed a low permeability after debonding, suggesting that the tubules remained plugged with resin even after the bond was broken. The de-bonded dentin surfaces were challenged with phosphoric acid for 30 s to determine if any of the original smear layer was accessible to the acid (Pashley et al., 1990). Such treatment increased the permeability of Scotchbond DC, Vitrabond, and Tripton groups (compare de-bonded values in Table 2 with values after phosphoric acid), although it was statistically significant only in the latter two groups. Acid challenge of Scotchbond 2/Silux- and Clearfil Photo Bondtreated dentin had no statistically significant effect on permeability. This was to be expected, since the original smear layer had been largely removed by primer treatment prior to the bonding procedure. Our results indicate that the bond strengths of these materials in cavities are lower than those obtained on flat surfaces. Terkla et al. (1988) were the first to measure bond strengths in Class I cavities. They obtained very low tensile bond strengths (approximately 0.3 MPa) using Scotchbond DC/P30. Stewart et al. (1990) obtained Scotchbond DC and Scotchbond 2/Silux bond strengths that were similar to ours and much lower than
40 Prati et aL/Bond strength and microleakage in Class I cavities
previously-reported bonds made to flat surfaces. When tensile stresses were applied to Class I cavities, the walls of the restoration were under shear stress and the pulpal floor was under tensile stress. Thus, the applied stresses were complex. Further, because the cavity contained opposing walls, there was opportunity for the development of stresses due to polymerization contraction. Samples which were bonded to flat dentin surfaces produced tensile bond strengths that were about twice as great as those made in three-dimensional cavities, presumably because there were no opposing dentin walls for stress development. We were concerned that forcing fluid under pressure around bonded resins may have weakened the bonds and contributed to the low measured bond strengths. However, control experiments were done in which bond strengths were measured in samples which were never exposed to fluid under pressure (third column, Table 2). There were no statistically significant differences between those treatments, indicating that the fluid under pressure had no effect on the measured bond strengths (Table 2). This confirmed a previous report by Derkson et al. (1987), that fluid filtration had no effect on dentin bond strengths. The bonding agents may have pooled at the line angles in the Class I cavities and may have been thin on the vertical cavity walls which may have "overthinned" the resin (Causton and Sefton, 1989). Thus, the thicknesses of the adhesive resins may have been greater on the flat dentin surfaces than in the cavities. However, the same conditions must exist in clinical practice when bonding agents are placed in Class I cavities in patients. The dentin used in these experiments was middle-to-deep dentin. Low bond strengths in deep dentin have been reported with Scotchbond DC (Causton, 1984; Mitchem and Gronas, 1986), Scotchbond 2 (McGuckin et al., 1991), and Clearfil Photo Bond (Prati et al., 1991). No influence of dentin depth on the bond strength of Vitrabond was found by Prati et al. (1991). No information is available on the effects of dentin depth on Tripton bond strengths. The significant negative correlations between bond strength and microleakage obtained with Scotchbond 2 (Fig.) indicate that high bond strengths are associated with low microleakage and vice versa. Clearfil Photo Bond gave a similar relationship, although it was not statistically significant (not shown). That type of relationship has been previously demonstrated (Munksgaard et al., 1985; Finger and Ohsawa, 1987), although not in the same samples. Thus, the previously published inverse relationship between bond strength and gap sizes or microleakage obtained with use of two different model systems is probably correct. Our method of measuring microleakage was based on hydrodynamics rather than on dye penetration or gap size. Had we measured these variables, it is likely that we would have seen significant microleakage. Our hydrodynamic method may simulate the clinical condition of post-operative sensitivity to masticatory, thermal, or osmotic stimuli. With the regression equation in the Fig.--that related the bond strength of bonding agents to "filtration microleakage"--zero microleakage (i.e., the y axis intercept) was predicted to occur at bond strengths of 1.1 MPa for Scotchbond DC and 4.6 MPa for Scotchbond 2. However, since the relationship was not statistically significant for Scotchbond DC, the predicted y axis value is also not statistically significant. The 4.6-MPa value is far lower than those predicted by other methods (17-20 MPa). This is not surprising, since it was obtained with a completely different method. Diffusion of dyes simulates the diffusion of lowmolecular-weight bacterial products that may occur through
microchannels around restorative materials. Even if these agents permeate through dentin to the pulp, and trigger an inflammatory reaction in the pulp, they may not cause pain, because they do not cause intratubular fluid movement. Fluid shifts across dentin are thought to activate mechanoreceptor nerves hydrodynamically, to cause sharp, well-localized pain, which is apparently unrelated to pulpal inflammation. Such fluid shifts may occur in response to biting forces or osmotic stimulation (i.e., sugar) of fluids in open channels around restorative materials. Since our method of measuring microleakage involved fluid filtration, our results could be interpreted as follows: Dentin sensitivity due to hydrodynamic stimuli should not occur in cavities restored with Scotchbond 2/ Silux if the bond strengths exceed 4.6 MPa (Fig.). However, even though these restorations might not permit enough fluid shifts to induce pain, they may still permit the diffusion of dyes or bacterial products around their margins. In summary, the methods used in this study demonstrated that microleakage can be quantitated non-destructively by use of fluid under pressure in Class I cavities prior to measurement of the bond strengths of adhesive restorations. The bond strengths of the materials tested were much less than those obtained on flat dentin surfaces. While the low bond strengths in three-dimensional cavities were probably due to partial debonding ofthe materials from some of the dentin surfaces, more research will be needed to rule out the possible contributions of other variables.
ACKNOWLEDGMENT The authors wish to thank Shirley Johnston for her excellent secretarial support in the preparation of this manuscript. This work was supported, in part, by grant DE06427 from the NIDR and by the Medical College of Georgia Dental Research Center. Received January 9, 1991/AcceptedAugust 23, 1991 Address correspondence and reprint requests to: D.H. Pashley Department of Oral Biology,School of Dentistry Medical College of Georgia, Augusta, GA 30912-1129
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Feilzer AJ, DeGee AJ, Davidson CL (1987). Setting stress in composite resin in relation to configuration of the restoration. J Dent Res 66:1636-1639. Finger WJ, Ohsawa M (1987). Effect of bonding agents on gap formation in dentin cavities. Oper Dent 12:100-104. Gordon M, Plasschaert AJM, Saiku JM, Pelzner RB (1986). Microleakage of posterior composite resin materials and an experimental urethane restorative material, tested in vitro above and below the cementoenameljunction. Quint Int 17:11-15. Irie M, Tanaka J, Nakai H, Hirota K, Tomioka K (1990). The marginal gap and bond strength of glass ionomers for base/ liner (abstract). J Dent Res (Spec Iss) 69:311. Komatsu M, Finger W (1986). Dentin-bonding agents: correlation of early bond strength with margin gaps. Dent Mater 2:257-262. McGuckin RS, Tao L, Thompson WO, Pashley DH (1991). Shear bond strength of Scotchbond, in vivo. Dent Mater 7:50-53. Mitchem JC, Gronas DG (1986). Effects of time after extraction and depth of dentin on resin dentin adhesives. J A m Dent Assoc 113:285-287. Munksgaard EC, Irie M (1987). Dentin-polymer bond established by gluma and tested by thermal stress. Scand J Dent Res 95:185-190. Munksgaard EC., Irie M, Asmussen E (1985). Dentin-polymer bond promoted by gluma and various resins. J Dent Res 64:1409-1411. Pashley DH, Depew DD (1986). Effects of the smear layer, copalite and oxalate on microleakage. Oper Dent 11:95102. Pashley EL, Tao L, Pashley DH (1988). The sealing properties of temporary filling materials. JProsthet Dent 60: 292-297. Pashley EL, Galloway SE, Pashley DH (1990). Protective effects of cavity liners on dentin. Oper Dent 15:10-17. Powis DR, Foller~s T, Merson SA, Wilson AD (1982). Iraproved adhesion of a glass ionomer cement to dentin and enamel. J Dent Res 61:1416-1422. Prati C, Pashley DH, Montanari G (1991). Hydrostatic intrapulpal pressure and bond strength of bonding system. Dent Mater 7:54-58. Stewart BL, Harcourt JK, Tyas MJ (1990). Comparison of bond strength to dentine in cavities restored before and after extraction (abstract). J Dent Res (Spec Iss) 69:945. Terkla LG, Brown AC, Hainisch AP, Mitchem JC (1987). Testing sealing properties of restorative materials against moist dentin. J Dent Res 66:1758-1764. Terkla LG, Mitchem JC, Hainisch AP, Mahler DB (1988). Influence of enamel on in vitro testing of dentin bonding agent (abstract). J Dent Res (Spec Iss) 67:364. Terkla LG, Mitchem JC, Mahler DB (1992). Evaluation of dentin bonding agents in clinically-simulated occlusal composite resin restorations. Am J Dent (in press). Wieczkowski G, Yu XY, Joynt RB, Davis EL (1990). Dentin bonding strength in a three-dimensional cavity preparation (abstract). J Dent Res (Spec Iss) 69:116.
Dental Materials~January 1992 41
Relationship between bond strength and microleakage measured in the same Class I restorations C. PratP, M. Simpson 2, J. Mitchem 3, L. Tao 2, D.H. Pashley 2 Department of Conservative Dentistry, School of Dentistry, University of Bologna, Bologna, Italy 2Department of Oral Biology, School of Dentistry, Medical College of Georgia, Augusta, GA USA 3Department of Dental Materials, School of Dentistry, Oregon Health Sciences University, Portland, OR USA
Abstract. Microleakage measurements were made by use of a pressurized fluid method in Class I restorations prepared in extracted human teeth just prior to measuring the tensile bond strengths of the same restorations. The restorative materials included dentin bonding systems that are applied to smear layers as well as those which remove the smear layer. A light-cured glass-ionomer cement was also included. The results demonstrated that there was an inverse relationship between dentin bond strength and microleakage in some materials and that the bond strengths made to three-dimensional Class I cavities were much lower than those made to flat dentin surfaces. Measurement of microleakage by fluid filtration had no apparent effect on bond strength. Most dentin bonding studies have been done on flat dentin surfaces which minimize the development of interfacial stress due to polymerization contraction forces (Davidson, 1986; Feilzer et al., 1987). Recently, several laboratories have begun measuring dentin bond strength in three dimensional cavities prepared in dentin (Terkla et al., 1988, 1992; Wieczkowski et al., 1990; Stewart et al., 1990). These restorations can be influenced by both polymerization contraction and water sorption. Such bond testing may be more relevant to clinical practice than bonds made to flat surfaces. Microleakage develops between cavity walls and filling materials when these materials are poorly adapted to the cavity, when they pull off a wall during polymerization, when liners or smear layers dissolve, or when recurrent caries produces a gap between filling materials and tooth structure. Microleakage studies have largely been limited to Class 5 cavities prepared at the DEJ so that half the cavosurface margins are in enamel andhalfare in cementum/dentin (Gordon et al., 1986). Some authors have measured the width of marginal gaps in proximal Class I cavities (Munksgaard and Irie, 1987; Finger and Ohsawa, 1987). They assume that gap size is positively correlated to microleakage values (Irie et al., 1990). Several authors have then attempted to correlate bond strength values with marginal gap size, even though the former data were obtained by use of flat dentin surfaces and the latter were done with use of proximal Class I restorations (Munksgaard et aI., 1985; Komatsu and Finger, 1986). What is needed are experiments in which microleakage and bond strengths are determined on the same samples in a clinically relevant manner--that is, the tensile bond strength would be determined in cavities with three dimensions, following measurements of microleakage or marginal gap size. No such
experiments of this type have been reported in the literature. The purpose of these experiments was to measure the tensile bond strength of several dentin bonding systems in Class I cavities prepared in extracted human dentin and to compare those bond strengths with the degree ofmicroleakage exhibited by the same materials in the same restorations.
MATERIALS AND METHODS (I) Tooth Preparation .--Extracted human third molars were stored in isotonic saline containing 0.2% sodium azide (as an antimicrobial agent) at 4°C for less than one month. Crown segments were prepared by sectioning the roots from the crown at the CEJ using an Isomet saw (Buehler Ltd., Lake Bluff, IL). A second cut, parallel to the first, removed the occlusal enamel and left a flat dentin surface which had been in the middle of the coronal dentin. The pulpal soft tissue was removed with cotton forceps, with care taken not to create a smear layer on the pulpal side of the dentin. The minimum dentin thickness remaining between the pulpal floor of the cavity and the underlying pulp chamber was measured by means of a pincer-type caliper. The resulting crown segment was cemented, with cyanoacrylate (Zapit, DVA, Anaheim, CA 92807), to a 2 x 2 x 0.7 cm piece of Plexiglas. A 2 cm length of 18-gauge stainless steel tubing penetrated the Plexiglas and came into contact with the coronal part of the pulp chamber, which was filled with sterile phosphate-buffered saline (Dulbecco's, Grand Island Biologicals, Grand Island, NY 14072). The tubing was connected to a pressurized buffer reservoir which was used to measure the permeability of the dentin before and after restoration of the cavity (Pashley and Depew, 1986; Pashley et. al., 1988). (2) Cavity Preparation.--Class I cavities were prepared in the centers of the crown segments by use of an aluminum oxide stone (cat. #992, Dremel, Racine, WI) mounted in a precision drill press (Model 7010, Servo Products Co., Pasadena, CA). The stone was cooled with water and ran at a constant speed of 3500 rpm. The resulting cavity dimensions were 4 mm in diameter and 1.2 mm deep, with a total internal surface area of 0.271 cm2. The entire cavity and occlusal surface of the dentin were then treated with 0.5 mol/L EDTA for 2 min for removal of the smear layer and maximization of dentin permeability. The occlusal dentin outside the confines of the cavity was then treated for 2 rain with 3% monopotassium-monohydrogen oxalate (Protect, John O. Butler Co., Chicago, IL) so that all dentinal tubules outside the cavity would be occluded with calcium oxalate crystals (Pashley and Depew, 1986). Under
Dental Materials~January 1992 37
TABLE 1: EFFECTS OF BONDING PROCEDURES ON DENTIN PERMEABILITY/MICROLEAKAGE* Dentin Permeabilitv/Microleakaae as a Percent of Maximum Maximum
Smear Layer
Primed
Bonded
Debonded
Etched
Scotchbond/Silux
100
48.1 + 5.5(11)
22.4 _+ 4.2(11)1
42.7 + 5.9(11)
59.0 + 12.3
Vitrabond
100
34.2 + 5.5(11)
9.6 + 1.5(11)
16.7 + 2.5(9)
39.2 + 14.8
Tripton/Occlusin
100
54.2 + 9.4(11)
17.5 + 3.5(11)1
18.0 + 3.4(10)
37.7 + 7.0
Scotchbond 2/Silux
100
47.8 + 8.2(11)
8.7 _+ 2.4(11)
82.1 +53.0(11)
51.6 + 9.5
67.3 + 13.0 (11)1
19.0 + 4.7 Clearfil PB/PhotoPosterior 100 43.1 + 7.3(11) 80.7 + 16.3(11) I 16.3 ± 5.8(11)1 26.1 ± 5.3(11) Values are the x _+SEM of the maximum percent of the permeability of the specimens which was measured after use of 0.5 M EDTA (two min). Dentin permeability was re-measured after creation of smear layer, after treatment with primer, if required, after insertion of the material (bonded), and after breaking of the bond (debonded) groups (p < 0.05). After insertion of restorative materials into the cavity, fluid filtration was regarded as measuring microleakage rather than dentin permeability. Those values not connected by a vertical line in the same plane are significantly different at p < 0.05. Numbers in parentheses are the numbers of samples studied.
these conditions, all of the fluid permeation occurred within the dentin in the cavity preparation.
(3) Measurement of Microleakage/Dentin Permeability.-When the PBS in the pulp chamber was pressurized to 10 psi, the fluid filtered across only the EDTA-opened tubules within the empty cavity. This fluid flow was expressed as a hydraulic conductance (Lp) and was assigned a value of 100%, which represented the maximum permeability of the empty cavity. A smear layer was then created on that dentin within the cavity using the same slow-speed stone that was used to prepare the original cavity. The hydraulic conductance of the smear-layer-covered dentin was re-measured and expressed as a percent of the maximal (i.e., 100%) value. If the restorative system required treatment of the smear layer with a conditioner, then the dentin was so treated and the permeability of the dentin (i.e., hydraulic conductance or Lp) re-measured. The permeability of the dentin (Lp) was measured after debonding and again after challenging the de-bonded dentin surface with 37% phosphoric acid for 30 s to determine if the dentin surface was sensitive to acid treatment. (4) Bonding Procedures.--All bonding was done in the absence of a physiological pulpal pressure. There were two types of bonding done. Conventional dentin bonding was done on the center of flat occlusal dentin. Experimental dentin bonding was done in shallow Class I cavities prepared in the center of the flat occlusal dentin. All specimens were measured in the tensile mode 30 min after placement of the restorative materials. Four dentin bonding systems and one light-cured glassionomer cement (GIC) were tested (Table 1). They were all used according to their manufacturer's instructions. Vitrabond (3M Dental Products, St. Paul, MN) was mixed as directed, inserted into the cavity as a single increment, and then photo-cured for 60 s by means of an Optilux 50 (Demetron Research Corporation, CT 06810). No smear layer treatment was used. Tripton (Coe/ICI, Macclesfield, England) primer agent was applied directly to the smear-layer-covered dentin, spread gently with air, dried for five s, then covered with Tripton Universal Bonding Agent and light-cured for 20 s. The treated surface was then covered with Occlusin (ICI) composite material in a single step and light-cured for 40 s. Since the cavities were only 1.2 mm deep, they were all filled with a single increment of composite. Dual-cure Scotchbond (3M Dental Products, St. Paul, MN)
38 Prati et aL/Bond strength and microleakage in Class I cavities
was applied to the smear layer without any prior conditioning. It was photo-cured for 20 s and then covered with universal shade Silux (3M) as a single increment which was light-cured for 40 s. When the Scotchbond 2 system was used, the smear layer was treated with Scotchprep primer for 30 s, with constant agitation with a small sponge. This surface was then gently dried for 10 s so that a dry, reflective surface would be obtained. Scotchbond 2 adhesive was then applied and photocured for 20 s. Finally, Silux resin composite was applied as a single increment and photo-cured for 40 s. When the Clearfil Photo Bond system (Kuraray, Kyoto, Japan) was used, the smear layer was removed with 37% phosphoric acid gel. After 30 s of acid treatment, the cavity was rinsed with water for 10 s and dried with an air syringe for 10 s. Clearfil Photo Bond was mixed according to the manufacturer's instructions, applied to the cavity with a small brush, and photo-cured for 20 s. The bonding agent was then covered with Clearfll Photo Posterior resin composite (Kuraray) and photo-cured for 40 s. (5) Bond Testing.--After the microleakage of the restored cavities was measured, an orthodontic button (GAC International, Inc., Cat. #KOOO, Central Islip, NY) [approximately 3 mm in diameter, backed by a wire mesh] was bonded to the material by means of light-cured Clearfll Photo Posterior (Kuraray, Kyoto, Japan) composite. This permitted tensile forces to be applied to the restoration at 0.5 mm/min by use of an Instron universal testing machine. The specimens were tested 30 min after bonding. Some specimens subjected to bond tests were never subjected to fluid under pressure and served as a control group to those that were subjected to microleakage measurements. For comparative purposes, additional teeth were prepared with flat surfaces which are used in most conventional dentin bonding studies. Nylon cylindrical matrices with an internal diameter of 3 mm and a depth of 3 mm were used to bond the material to these flat surfaces. The final surface preparation was done with the same aluminum oxide stone as was used in the Class I cavities, so that the smear layers would be similar. Composite was added in two increments. After the first 1.5 mm of composite was inserted and cured, a straight pin dipped in Clearfil New Bond was placed head down in the composite, and the second 1.5-ram-thick increment was placed and cured. An adjustable chuck hung from the cross-head on a cable, like a plumb bob. The pin was placed in the chuck, which was then tightened firmly. The tooth specimen was then placed
TABLE2: BOND STRENGTH (MPa) TO HUMAN DENTIN Materials
3D Cavity a Plus Microleakage Test
3D Cavity, No Microleakage Test
Fiat Surface, No Microleakage Test
RDTb (m m)
0.63 _+ 0.13(0/6)
1.14 +
0.17 (0/7)
~.20 +
0.03(11)
1.92 _+ 0.32(1/11)
1.76 +_ 0.45(0/5)
1.52 +
0.39 (0/8)
1.30 +
0.03 (11)
Tripton
2.09 +
0.24 (0/11)
1.64 +
0.37 (0/5)
3.70 _+
1.23(0/6)
1.50 _+ 0.06 (11)
Scotchbond 2
3.03 +
0.36 (5/11)
3.76 +
0.6 (2/5)
6.85 +_
1.20 (0/8)
1.30 _+ 0.03 (11)
Scotchbond DC
0.78 +
Vitrabond
0.12(0/11 c)
1.20 + 0.07(11) Clearfil PB 3.90 + 0.32(1/11) 1.85 + 0.37(0/5) I 7.24 + 0.83 (0/7) a Bond strength in tensile mode~ + SEM, MPa). b RDT = remaining dentin thickness in mm. Numbers in parentheses indicate numbers of specimens tested. Vertical and horizontal lines connect groups that are not statistically different at p > 0.05. Those not connected by vertical lines in the same plane are statistically significantly different at p < 0.05. c The second numberisthe total number of samples tested. 0/11 means there were no partial cohesive failures out of 11samples and that 11/11 ofthe samples exhibited an adhesive failure. The first number in parentheses indicates the number of samples exhibiting partial cohesive failure.
beneath a restaining metal strap containing a hole large enough to permit the sample to be de-bonded but small enough to hold the specimen in place, creating a simple, self-aligning device. When the orthodontic buttons were used, the device of Powis et al. (1982) replaced the adjustable chuck. After the bond tests, both sides of the failed bond were inspected with a stereomicroscope (20x) for evaluation of whether the mode of failure was adhesive, cohesive, or both. (6) Statistical Analyses.--The force (in fkg) required to break the bonds was divided by total dentin surface areas to obtain the bond strength, which was expressed in MPa. The means and standard deviations of each group were compared for statistically significant differences, by one-way ANOVA. Duncan's multiple-range test was used to rank the materials and to perform multiple comparisons between and among the various treatments. Paired comparisons were performed by the paired t test. Statistical significance was defined as p < 0.05. Relationships between bond strengths and dentin permeability were tested by regression analysis. RESULTS The mechanical creation of smear layers in the cavities reduced dentin permeability from 100% (the EDTA-treated maximum value) to between 34.2 and 54.2% of maximum. There were no statistically significant differences between the groups in terms of their smear layer permeabilities (Table 1). In the two systems that use acidic primers to remove the smear layer, the permeability increased significantly from 47.8 to 67.3% in the Scotchbond 2 group and from 43.1 to 80.7% in the Clearfil PB group. When the cavities were restored (bonded values, Table 1), all of the materials produced a statistically significant reduction in fluid flow, with Vitrabond and Scotchbond 2 sealing the dentin better than the other materials (p < 0.05, Table 1). After the tensile strengths of the bonds were measured (de-bonding values in Table 1), the fluid flow through the empty cavity dentin exhibited little change in the Tripton, Vitrabond, and Clearfil PB groups, but increased in the Scotchbond DC and Scotchbond 2 groups, although due to the large variance, these increases were not statistically significant. When the debonded surfaces were challenged with 37% phosphoric acid, there were statistically significant increases in the permeability of the dentin in the Vitrabond and Tripton groups. The tensile bond strengths of the various groups are shown in Table 2. In those samples in which fluid was forced around the restoration to measure microleakage prior to bonding testing, the bond strengths ranged from a low value of 0.78 MPa
for Scotchbond DC to 3.90 MPa for Clearfil Photobond. When no fluid was forced around the restorations prior to bond testing, the values were approximately the same with the exception of the Clearfil Photo Bond group, which showed a lower mean value. Control specimens in which these materials were bonded to flat dentin surfaces exhibited higher bond strengths (p < 0.05), except for the Vitrabond and Tripton groups. The minimal remaining dentin thickness (RDT, Table 2) in the various groups ranged from 1.2 to 1.5 mm and was not statistically significantly different in any of the groups. When the bond strengths of the restored cavities were plotted against the apparent permeability (i.e., microleakage) of the restored cavities, only Scotchbond DC and Scotchbond 2 revealed negative correlations (Fig.). Regression analysis predicted that Scotchbond DC would show zero microleakage at a tensile bond strength of 1.1 MPa. Similar analysis of the Scotchbond 2 data gave a value of 4.6 MPa. DISCUSSION The results ofthis study demonstrate the feasibility ofmeasuring both microleakage and bond strength of dentin bonding systems in the same Class I restorations. The fluid-under-pressure system had been used previously to measure marginal leakage (Terkla et al., 1987; Derkson et al., 1986; Pashley and Depew, 1986; Pashley et al., 1988), but it had not been correlated with bond strengths. In Table 1, the permeability of the dentin prior to insertion of the materials was only between 34 and 54% of maximum, due to the presence of a smear layer. In the Scotchbond 2 and Clearfil Photo Bond groups, the smear layer was partially removed by primers, resulting in increased dentin permeability. However, after the materials were inserted, the largest reductions in apparent permeability were seen in those systems which removed the smear layers, Scotchbond 2 and Clearfil Photo Bond (54 and 65% reductions, respectively). The other three restorative systems placed on smear layers were less able to seal dentin. After insertion of a restorative material, the apparent permeability of the dentin was re-measured after 10 min. If the material sealed dentin perfectly, the fluid flow should have fallen to zero (Derkson et al., 1986; Pashley and Depew, 1986). If the material did not seal the dentin at all, the permeability would have remained unchanged from its prerestored value. Thus, changes in the permeability of the cavity following insertion of restorative materials provided a quantitative, non-destructive measure ofmicroleakage around any microgaps that might have existed between the material and
Dental Materials~January 1992 39
5 ~
O,~O Scotchbond 2 Scotchbond DC 4-
0
m A
O
~ O
a.
Scotchbond 2 y=e(1.517-0.065x) r =-0.68 p<0.01 N=11
3J¢ .
O
c
•
P
O
Scotchbond DC Y=e(°128-°°21x) r =-0.48
k
2"0 f0
nd 1m
0 I
0
I
I
I
I
I
10
I
I
I
I
I
20
I
I
I
I
I
30
I
I
|
I
I
I
I
I
40
I
I
50
Microleakage as Percent of Maximum Fig. Therewere inverserelationshipsbetweenthe bond strengthsof ScotchbondDC and Scotchbond2 and microleakagemeasuredas fluid filtration.
the walls of the cavity. After the restoration was de-bonded, the permeability of the Scotchbond DC/Silux-restored dentin returned to a value (i.e., 43%) which was not statistically different from its pre-bond (i.e., smear layer) value (Table 1). Both Vitrabond- and Tripton-treated dentin had de-bonded permeabilities that remained low, indicating that these materials left a residue on the smear layer that reduced its permeability. Scotchbond 2/Silux-bonded dentin exhibited a dentin permeability (but with a large variance) higher than the pre-bonded value. Presumably, this was due to removal of resin tags from the tubules when the bond failed. Clearfil Photo Bond-treated dentin revealed a low permeability after debonding, suggesting that the tubules remained plugged with resin even after the bond was broken. The de-bonded dentin surfaces were challenged with phosphoric acid for 30 s to determine if any of the original smear layer was accessible to the acid (Pashley et al., 1990). Such treatment increased the permeability of Scotchbond DC, Vitrabond, and Tripton groups (compare de-bonded values in Table 2 with values after phosphoric acid), although it was statistically significant only in the latter two groups. Acid challenge of Scotchbond 2/Silux- and Clearfil Photo Bondtreated dentin had no statistically significant effect on permeability. This was to be expected, since the original smear layer had been largely removed by primer treatment prior to the bonding procedure. Our results indicate that the bond strengths of these materials in cavities are lower than those obtained on flat surfaces. Terkla et al. (1988) were the first to measure bond strengths in Class I cavities. They obtained very low tensile bond strengths (approximately 0.3 MPa) using Scotchbond DC/P30. Stewart et al. (1990) obtained Scotchbond DC and Scotchbond 2/Silux bond strengths that were similar to ours and much lower than
40 Prati et aL/Bond strength and microleakage in Class I cavities
previously-reported bonds made to flat surfaces. When tensile stresses were applied to Class I cavities, the walls of the restoration were under shear stress and the pulpal floor was under tensile stress. Thus, the applied stresses were complex. Further, because the cavity contained opposing walls, there was opportunity for the development of stresses due to polymerization contraction. Samples which were bonded to flat dentin surfaces produced tensile bond strengths that were about twice as great as those made in three-dimensional cavities, presumably because there were no opposing dentin walls for stress development. We were concerned that forcing fluid under pressure around bonded resins may have weakened the bonds and contributed to the low measured bond strengths. However, control experiments were done in which bond strengths were measured in samples which were never exposed to fluid under pressure (third column, Table 2). There were no statistically significant differences between those treatments, indicating that the fluid under pressure had no effect on the measured bond strengths (Table 2). This confirmed a previous report by Derkson et al. (1987), that fluid filtration had no effect on dentin bond strengths. The bonding agents may have pooled at the line angles in the Class I cavities and may have been thin on the vertical cavity walls which may have "overthinned" the resin (Causton and Sefton, 1989). Thus, the thicknesses of the adhesive resins may have been greater on the flat dentin surfaces than in the cavities. However, the same conditions must exist in clinical practice when bonding agents are placed in Class I cavities in patients. The dentin used in these experiments was middle-to-deep dentin. Low bond strengths in deep dentin have been reported with Scotchbond DC (Causton, 1984; Mitchem and Gronas, 1986), Scotchbond 2 (McGuckin et al., 1991), and Clearfil Photo Bond (Prati et al., 1991). No influence of dentin depth on the bond strength of Vitrabond was found by Prati et al. (1991). No information is available on the effects of dentin depth on Tripton bond strengths. The significant negative correlations between bond strength and microleakage obtained with Scotchbond 2 (Fig.) indicate that high bond strengths are associated with low microleakage and vice versa. Clearfil Photo Bond gave a similar relationship, although it was not statistically significant (not shown). That type of relationship has been previously demonstrated (Munksgaard et al., 1985; Finger and Ohsawa, 1987), although not in the same samples. Thus, the previously published inverse relationship between bond strength and gap sizes or microleakage obtained with use of two different model systems is probably correct. Our method of measuring microleakage was based on hydrodynamics rather than on dye penetration or gap size. Had we measured these variables, it is likely that we would have seen significant microleakage. Our hydrodynamic method may simulate the clinical condition of post-operative sensitivity to masticatory, thermal, or osmotic stimuli. With the regression equation in the Fig.--that related the bond strength of bonding agents to "filtration microleakage"--zero microleakage (i.e., the y axis intercept) was predicted to occur at bond strengths of 1.1 MPa for Scotchbond DC and 4.6 MPa for Scotchbond 2. However, since the relationship was not statistically significant for Scotchbond DC, the predicted y axis value is also not statistically significant. The 4.6-MPa value is far lower than those predicted by other methods (17-20 MPa). This is not surprising, since it was obtained with a completely different method. Diffusion of dyes simulates the diffusion of lowmolecular-weight bacterial products that may occur through
microchannels around restorative materials. Even if these agents permeate through dentin to the pulp, and trigger an inflammatory reaction in the pulp, they may not cause pain, because they do not cause intratubular fluid movement. Fluid shifts across dentin are thought to activate mechanoreceptor nerves hydrodynamically, to cause sharp, well-localized pain, which is apparently unrelated to pulpal inflammation. Such fluid shifts may occur in response to biting forces or osmotic stimulation (i.e., sugar) of fluids in open channels around restorative materials. Since our method of measuring microleakage involved fluid filtration, our results could be interpreted as follows: Dentin sensitivity due to hydrodynamic stimuli should not occur in cavities restored with Scotchbond 2/ Silux if the bond strengths exceed 4.6 MPa (Fig.). However, even though these restorations might not permit enough fluid shifts to induce pain, they may still permit the diffusion of dyes or bacterial products around their margins. In summary, the methods used in this study demonstrated that microleakage can be quantitated non-destructively by use of fluid under pressure in Class I cavities prior to measurement of the bond strengths of adhesive restorations. The bond strengths of the materials tested were much less than those obtained on flat dentin surfaces. While the low bond strengths in three-dimensional cavities were probably due to partial debonding ofthe materials from some of the dentin surfaces, more research will be needed to rule out the possible contributions of other variables.
ACKNOWLEDGMENT The authors wish to thank Shirley Johnston for her excellent secretarial support in the preparation of this manuscript. This work was supported, in part, by grant DE06427 from the NIDR and by the Medical College of Georgia Dental Research Center. Received January 9, 1991/AcceptedAugust 23, 1991 Address correspondence and reprint requests to: D.H. Pashley Department of Oral Biology,School of Dentistry Medical College of Georgia, Augusta, GA 30912-1129
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Dental Materials~January 1992 41