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The effect of two configuration factors, time, and thermal cycling on resin to dentin bond strengths
Biomaterials 24 (2003) 1013–1021
The effect of two configuration factors, time, and thermal cycling on resin to dentin bond strengths Richard B. Pricea,*, Tore De! randb, Pantelis Andreouc, Darcy Murphyd b
a Department of Dental Clinical Sciences, Dalhousie University, Halifax, Nova Scotia, Canada, B3H 3J5 Department of Oral Technology and Dental Materials Science, Malmo. University, Carl Gustafs vag . 34, Malmo, . Sweden, SE-214 21 c Community Health and Epidemiology, Dalhousie University, Halifax, Nova Scotia, Canada, B3H 3J5 d Dalhousie University, Halifax, Nova Scotia, Canada, B3H 3J5
Received 28 February 2002; accepted 4 September 2002
Abstract Most in vitro testing of bonding systems is performed using specimens made in a mold with a low configuration ðCÞ factor (ratio of bonded/unbonded surfaces) whereas clinically the C-factor is usually much greater. This study compared the effect of thermal cycling on the measured shear bond strength of 3M Single Bond dental adhesive bonded to dentin using molds with two different Cfactors. The hypothesis was that neither C-factor nor thermal cycling would affect measured bond strengths. Resin composite was bonded to human dentin in cylindrical molds with an internal diameter of 3:2 mm and either 1 mm or 2:5 mm deep. The 1 mm deep molds had a C-factor of 2.2 and the 2:5 mm deep molds had a C-factor of 4.1. Specimens were debonded either 10 min after they had been bonded to dentin, or after they had been stored for 7 days in water at 37711C; or after thermal cycling 5000 times for 7 days. Two-way ANOVA showed that overall both the C-factor and the storage condition had a significant effect on bond strength ðpo0:001Þ: There was a significant interaction ðpo0:001Þ between the C-factor and how the specimens had been stored. The GLM/ LSMEANS procedure with Sidak’s adjustment for multiple comparisons showed that overall the specimens made in the mold with a high C-factor (4.1) had a lower bond strength than those that had been made in the mold with a lower (2.2) C-factor ðpo0:001Þ: Thermal cycling had a negative effect on the bond strength only for specimens made in molds with a C-factor of 4.1 ðpo0:001Þ: r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Composite resins; Dental bonding/methods; Mechanical stress; Dental cavity preparation/classification
1. Introduction The shrinkage that occurs as resin composites polymerize has been cited to be a major reason for bonding failures in adhesive restorations [1]. This shrinkage may lead to microleakage around the restoration and subsequent secondary caries, which has been reported to be the most common reason for replacing resin composite restorations [2,3]. New dentin bonding systems (DBS) are often marketed without undergoing long term clinical trials and the dental profession has had to rely upon the results of in vitro bonding studies when choosing a new product [4–6]. After reviewing 50 investigations on bonding resin to dentin, al-Salehi and Burke [7] showed that there was little standardization in testing bond *Corresponding author. Tel.: +1-902-494-1226; fax: +1-902-4941662. E-mail address: [email protected] (R.B. Price).
strengths and the test conditions often did not reproduce the intraoral environment. Different research centers using the same materials, often produce different results which makes it difficult to predict clinical performance from in vitro tests [5,6,8]. Although bond strength testing can differentiate between those materials that have little adhesion to dentin and those that bond well, so far it has been impossible to correlate high in vitro bond strength data obtained for products [9,10] with good clinical performance [6,11] of the same products. This may be because previous in vitro studies: 1. Failed to reproduce the shape of typical preparations found in vivo. 2. Failed to measure the bond strength at an appropriate time. 3. Failed to reproduce the way and the amount of light energy that the restoration would receive in vivo. 4. Failed to reproduce intra oral conditions in the in vitro test.
0142-9612/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 6 1 2 ( 0 2 ) 0 0 4 4 1 - 6
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As the resin composite polymerizes and shrinks, the bond between the DBS and the tooth should be strong enough to prevent the resin from being pulled away from the tooth. To prevent debonding and microleakage at the resin–dentin interface and to resist the stresses produced during polymerization as the resin shrinks, it has been reported that bond strengths of 17–24 MPa are required [12–14]. However, even with bonding systems which have recorded 24 h in vitro shear bond strengths in excess of 17 MPa [10,15–18], microleakage [19–22], and gaps [4,23,24] still occur between the DBS and the tooth. In an in vivo study of six DBS which have produced 24 h in vitro bond strengths > 17 MPa; the polymerization shrinkage overcame the adhesive capability of all the dentin bonding systems and resulted in detachment of the resin from the tooth [23]. This suggests that the 24-h bond strengths recorded in vitro in a low stress environment are not able to predict whether adhesive failure will occur between the DBS and the tooth when resin composite is used as a restorative material in vivo. Previous studies to measure resin–dentin bond strengths have used metal molds [17], Teflon molds, or gelatin capsules filled with previously polymerized composite [9,10,12,15,16,18,24–27]. However, the design of the test mold can affect the bond strength and physical properties of the cured resin composite [28–31]. The design of the test mold has also been shown to have an effect on the stress generated during resin polymerization [32]. Feilzer et al. [33] reported that the ratio of the bonded to unbonded surfaces within the preparation, the configuration ðCÞ factor, can be calculated and used to predict those restorations which are most likely to exhibit bond failures between the resin and the tooth. Mean C-factors have been reported to be 4.03 for Class I, 1.85 for Class II, and 1.10 for cervical abrasion restorations [34]. According to the results of Feilzer et al. [33] restorations with a C-factor less than 1 are more likely to survive the polymerization contraction stresses and remain bonded to the tooth. This is supported by Yoshikawa et al. [35] who reported more microleakage as the C-factor increased from 2.3 to 3.0 and an in vivo study where the resin to dentin bond of 6 different DBS failed in all cases where the C-factor was 5 [23]. In bonding studies where the resin composite was bonded to the tooth at the bottom of the mold only, not to the sides [9,10,12,15–18,24–26], the ratio of the bonded to the unbonded surfaces was low. This resulted in a Cfactor less than 1 and imparted little stress on the bond to the tooth as the resin polymerized [33]. While this situation is similar to a Class VI restoration or a shallow cervical abrasion restoration, the C-factor is much greater in a Class I, II, or V restoration [34] where the resin composite is bonded to the sides as well as to the floor of the preparation (Fig. 1). Bouillaguet et al. reported a 20% reduction in micro-tensile bond
strengths in a Class II restoration with a C-factor of 1.4 compared to the bond strengths obtained on a flat dentin surface [30]. Hasegawa et al. reported that the high bond strengths recorded in low stress situations did not always prevent contraction gaps in a Class I restoration with a C-factor of 3 [24]. Therefore, if the resin composite can be bonded to the walls of the mold, as occurs in a tooth [24,27,30,31,36,37], this will place more stress on the bond between the resin and the tooth [32,33,37] and might provide a more clinically relevant test of bond strength [24,30]. There may then be a correlation between the measured bond strengths and the presence of contraction gaps. When Teflon or gelatin capsules are used as molds in bonding studies, then the DBS and composite will receive more light energy from the sides than in the in vivo situation. This will enhance bond strength and physical properties of the composite [28,29]. Similar amounts of light energy have been shown to penetrate through resin composite and tooth structure [38,39]. Therefore, if the mold is made out of resin composite instead of metal [27], then not only can the resin composite can be bonded to the walls of the mold, but a similar amount of light should reach the sides and bottom of the specimen as would occur in a tooth [38,39]. In many previous reports [9,10,25], the entire dentin surface was treated with the DBS and polymerized before the mold was placed on the dentin and the resin composite bonded to the dentin. This technique increases the bonded surface area (Fig. 2) and may artificially increase bond strength values [26]. To avoid this, the DBS on the dentin surface should be light activated only within the area enclosed by the mold and to which the resin composite is subsequently bonded. Many DBS are able to resist the effects of polymerization shrinkage and maintain the bond between dentin and the resin when the specimens are not subjected to additional stresses [10,15–18,21]. However, when Class II restorations were subjected to additional stress from functional loading, there was a significant increase in the microleakage of all eight DBS tested [21]. Thermal cycling also stresses the bond between resin and the tooth and, depending on the DBS, may affect bond strength [9,25]. However, only 19% of dentin bonding studies thermal cycle the specimens before testing [7]. This may be because the value of in vitro thermal cycling of specimens has been questioned [40]. Hasegawa et al. [15] reported that subjecting specimens to 500 thermal cycles had no effect on shear bond strength. Their testing protocol used a low C-factor split Teflon mold and the specimens were not thermal cycled until 4 weeks after bonding. The effects of thermal cycling may also depend on the bonding system being tested [9,25], but these conclusions may be due to the testing procedure used. Using a Teflon mold with a low C-factor,
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Fig. 1. Examples of how the C-factor changes with different preparation designs (modified after Feilzer et al. [33]).
Fig. 2. Schematic showing how the bonding surface area is increased if the DBS is not confined and is allowed to cover the tooth surface beyond the composite specimen (modified after Van Noort et al. [26]).
Miyazaki et al. [25] investigated the effect of thermal cycling up to 30,000 times on bond strength. They reported that there was a significant decrease in bond strength to bovine dentin for Single Bond (3M Dental, St. Paul, USA) after 3000 thermal cycles, but the bond strengths of two other DBS were unaffected after 30,000 thermal cycles. Different results may have been reported from these studies if a mold with a higher C-factor had been used. In 2001, Wilson [41] reiterated that laboratory studies need to be developed that can predict clinical success and, in an editorial in 2002, Christensen [42] commented that impressive in vitro resin–dentin bonds are transient when subjected to temperature changes in the mouth. Further investigation of the effects of using molds with clinically relevant C-factors on in vitro bond strength is required and the present study may explain some of the shortcomings of previous in vitro bonding studies. This study determined how similar cylindrical mold designs with C-factors of 2.2 and 4.1 affected the shear bond strength of a DBS to dentin and the effect of thermal cycling on the measured bond strengths. The hypothesis was that for specimens cured within the manufacturer’s tolerances, shear bond strengths would
not be affected as the C-factor increased, and that thermal cycling would not affect measured bond strengths.
2. Materials and methods Intact, non-carious, human third molar teeth were collected and stored in a solution of 0.5% chloramine-Thydrate (Aldrich Chemical Co., Milwaukee, USA) at 41C for up to 4 weeks [43]. The teeth were then washed thoroughly in running water and stored in water at 41C for at least a further 24 h; but no more than 1 week. Acrylic resin (Tray Resin, Caulk/Dentsply, Milford, USA) was used to mount the teeth so that each tooth had its buccal surface facing upwards and half of the tooth was above the resin. The buccal surface [43] of each tooth was ground flat to approximately halfway between the dentin–enamel junction and the pulp, using in turn 120, 240, and 400-grit silicon carbide (SiC) papers on a water cooled abrasive wheel. This depth was chosen to represent the depth of a typical cavity preparation. Since the surface finish produced by 600 grade SiC paper is similar to the finish produced by a
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carbide bur [44], one hour before the teeth were used, the dentin surface was lightly abraded with 600-grit SiC paper [10,15,16,25,26] on a water-cooled abrasive wheel. The teeth were then placed in a water bath at 37711C: Cylindrical resin composite molds were made to compare the effect of C-factor on bond strength. A flowable resin composite (3M Filtek Flow, Shade A2) was flowed into circular molds that were either 1 or 2:5 mm deep. The composite was irradiated for 60 s using an Optilux 500 (Demetron/Kerr, Danbury, USA) quartz tungsten halogen curing light equipped with a 13/ 8 Turbo Plus (Demetron/Kerr) light guide and then removed from the mold. The internal surface of the resin composite cylinder was roughened using 50 mm aluminium oxide grit blasting for 10 s to produce a matte finish. This procedure has been shown to produce a strong bond between new resin composite and previously cured composite [45]. The composite cylinders were checked to verify that they all had an external diameter of 6:6 mm; an internal diameter of 3:2 mm; and were either 1 or 2:5 mm deep. Using the following formula, the C-factor of the 1 mm deep cylinders was calculated to be 2.2 and the C-factor of the 2:5 mm deep cylinders was calculated to be 4.1 C¼
bonded area of composite ; unbonded area of composite
circumference of cylindrical mold depth þ area of one end bonded to dentin C¼ ; area of one unbonded end C¼
2pr h þ pr2 ; pr2
where h is the depth of composite mold (mm) and r the internal radius of composite mold (mm). Sixty resin composite cylinders were made 1 mm deep and 60 cylinders were made 2:5 mm deep. Polyester Hi Solvent Resistance Transparent tape (3M) was placed on the bottom of each cylinder to prevent the composite cylinder from bonding to the tooth (Fig. 3). The resin composite cylinder was clamped on to the dentin surface and the dentin enclosed by the cylinder was etched using phosphoric acid etch (3M) for 15 s and then washed thoroughly with distilled water at 37711C: Excess water was removed using Kimwipes EX-L (Kimberly-Clark Corp., Roswell, USA) tissue [16] leaving a moist dentin surface [18]. One drop of DBS (3M Single Bond) was placed on dentin surface inside the cylinder and then lightly brushed around for 10 s: The DBS was also brushed on to the inside of each resin composite cylinder to improve bonding between the old and new resin composite [45]. A second drop of DBS was then placed on the dentin surface, brushed around and an air syringe positioned approximately 6–8 in away was used to evaporate solvent from the DBS. The dentin
Fig. 3. Schematic showing how resin composite was used to make cylindrical molds with different C-factors.
and the DBS inside the cylinder was irradiated for 10 s using an Optilux 500 equipped with a 13/8 Turbo Plus light guide. Z100 resin composite (3M Dental, Shade A1) was placed into the mold, covered with a Mylar strip, and cured in one increment from the top of the mold for 40 s using the Optilux 500 light. The 2:5 mm increment thickness was within the manufacturer’s recommended maximum thickness of Z100 (3M) shade A1 composite, which can be adequately light cured in 40 s [46]. This increment was also considered something which might be done clinically in vivo. Due to the experimental design, the tip of the light guide was 2 mm from the top of the resin composite. The output from the curing light was checked at 2 mm using a Cure Rite radiometer (Dentsply/Caulk, Milford, USA) after every 20 specimens had been made to ensure that the power density was above 1000 mW=cm2 : Consequently, each specimen received at least 40 J=cm2 of energy from the Optilux 500 curing light. To account for variations in teeth and the effects of different dentin thicknesses on bond strength [31], the 120 teeth were randomly coded and assigned to one of three test groups for each C-factor (cylinder depth). A pilot project showed that it took about 7 min from starting to irradiate the DBS until the finished specimen was ready for debonding. To allow for operator variability, the early bond strengths were tested 10 min after irradiating the DBS. This time interval met the recommendation of al-Salehi and Burke that bond strengths should be measured 5–10 min after irradiating the restoration [7]. After bonding, all specimens were placed in a water bath at 37711C: One group ðn ¼ 20Þ for each cylinder depth was stored in water at 37711C and debonded 10 min from the time the DBS was irradiated. Another group was stored in water at 37711C and debonded 7 days from the time the DBS was irradiated. The final group was thermal cycled 5000 times from 51C to 551C with a 30 s dwell time and a 30 s intermediate time at a room temperature of 231C: After the thermal cycling, which took 7 days to complete, the
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specimens were placed into a 37711C water bath for at least 30 min before debonding. Although the ISO/TR 11405 document [43] on testing of adhesion to tooth structure recommends thermal cycling 500 times from 51C to 551C and back, 5000 thermal cycles were chosen to simulate 6 months in a patient’s mouth [40]. The 5– 551C temperatures with a 30 s dwell time in each bath were used in accordance with the ISO/TR 11405 document [43] that specifies a dwell time of at least 20 s: There are several methods to test the bond strength of a DBS (e.g., tensile, micro tensile or shear bond tests) [9,47,48]. Realizing its limitations, the shear bond strength test was chosen for this study because it has been reported by Øilo and Austrheim that tensile and shear test methods are equally representative when testing in vitro resin–dentin bond strengths [9]. To reduce the tensile, compressive and bending forces that can be introduced in the shear bond test, the load was applied using an Instron 1000 (Instron, Canton, USA) with a chisel edged blade that had a 0:5 mm blunt edge [43] positioned as close as possible to the tooth without actually touching it (Fig. 4). The load was applied at a crosshead speed of 2:8 mm which was within the 0.5– 5 mm=min range used by other researchers [7,10,15,16,18,25,31]. The shear bond strength data were compared using a general linear model analysis using po0:01 as the level of significance. The SAS GLM/LSMEANS procedure with Sidak’s adjustment for multiple comparisons [49] was used to determine the effects of time, thermal cycling, and C-factor on measured bond strength.
3. Results The mean bond strengths7standard deviations for the specimens are shown in Fig. 5. A power analysis showed that the power of the general linear model test was greater than 0.95 when comparing the effects of the three different storage conditions and 0.99 when comparing the effect of the Cfactor on shear bond strength. Two-way ANOVA showed that overall both the C-factor and the storage condition had a significant effect on bond strength ðpo0:001Þ: There was a significant interaction ðpo0:001Þ between the two factors (specimen C-factor and storage condition). The SAS GLM/LSMEANS procedure with Sidak’s adjustment for multiple comparisons showed that overall the specimens made in the mold with a high C-factor (4.1) had a lower bond strength than those that had been made in the mold with a lower (2.2) C-factor ðpo0:001Þ: Pairwise comparisons (Table 1) show that neither time ðp ¼ 0:0616Þ nor thermal cycling ðp ¼ 0:1037Þ had a significant effect on bond strength for specimens made in the molds with a C-factor of 2.2. Time also had no
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Fig. 4. Schematic showing how the resin to dentin bond was subjected to a shearing force.
significant effect on bond strength for specimens made in the molds with a C-factor of 4.1 ðp ¼ 0:1054Þ: However, thermal cycling had a significant negative effect on bond strength for the specimens with the greater C-factor ðpo0:0001Þ: At 7 days, the bond strength of the high C-factor specimens which had been thermally cycled was 47.2% less than that of the specimens that had been stored for 7 days in water at 371C: The hypothesis that shear bond strengths would not be affected as the C-factor increased was rejected since overall the specimens made in the mold with a high (4.1) C-factor had a lower bond strength than those which had been made in the mold with a lower (2.2) Cfactor ðpo0:001Þ: The hypothesis that thermal cycling would not affect bond strengths was accepted for the specimens made in the mold with a lower (2.2) C-factor ðp ¼ 1Þ; but it was rejected for the specimens made in the mold with a higher (4.1) C-factor ðpo0:0001Þ:
4. Discussion Overall, the specimens made in the mold with a high C-factor (4.1) had a lower bond strength than those which had been made in the mold with a lower (2.2) Cfactor ðpo0:001Þ: This agrees a previous study by Haller et al. who used a silanated brass mold to develop a Cfactor of 4 [27]. The result that thermal cycling did not have a significant effect ðp ¼ 1:0Þ on bond strength for specimens made in the molds with the lower C-factor supports the results reported by Hasegawa et al. who used a split Teflon mold with a low C-factor [15]. Thermal cycling stresses the resin–dentin bond [9,25].
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Fig. 5. Effect of time and thermal cycling on the mean shear bond strength of specimens with low and high configuration factors.
Table 1 Pairwise comparisons of the GLM/LSMEANS showing the effect of time and thermal cycling on the shear bond strength of specimens with configuration factors of 2.2 and 4.1 Specimen C-factor and treatment
P-Values Bond strength C-factor 2.2 C-factor 2.2 C-factor 2.2 C-factor 4.1 C-factor 4.1 C-factor 4.1 (MPa) 10 min 7 days 7 days 10 min 7 days 7 days + thermal cycling + thermal cycling
C-factor 2.2, 10 min C-factor 2.2, 7 days C-factor 2.2, 7 days + thermal cycling C-factor 4.1, 10 min C-factor 4.1, 7 days C-factor 4.1, 7 days + thermal cycling
14.1 17.1 16.9
— 0.0616 0.1037
0.0616 — 1.0000
0.1037 1.0000 —
0.0016 o0.0001 o0.0001
0.9636 0.0008 0.0015
o0.0001 o0.0001 o0.0001
9.8 12.7 6.7
0.0016 0.9636 o0.0001
o0.0001 0.0008 o0.0001
o0.0001 0.0015 o0.0001
— 0.1054 0.0565
0.1054 — o0.0001
0.0565 o0.0001 —
This added stress only adversely affected the bond between the resin and the tooth in the specimens made in a mold with the higher C-factor of 4.1 ðpo0:0001Þ: Overall, the bond strengths of the specimens were significantly lower at 10 min than after 7 days ðpo0:001Þ: The lower early bond strengths which increased with time support a previous study by Hasegawa et al. [15]. Although the bond strengths were less at 10 min than at 7 days (Fig. 5), pairwise comparisons (Table 1) showed that time did not have a significant effect on bond strength (p ¼ 0:0616 for the molds with a C-factor of 2.2, and p ¼ 0:1054 for the molds with a C-factor of 4.1). The early 10 min bond strengths to dentin achieved by a DBS are very important to the clinician. A DBS should provide adequate early resin–dentin bond strength to resist the polymerization shrinkage of the composite resin, the potential trauma of polishing the restoration, and the masticatory forces when the occlusion is checked. The mean 10 min shear bond strengths to dentin of 14:1 MPa in the 2.2 C-factor mold and 9:8 MPa in the mold with a C-factor of 4.1 were less than the desired 17–24 MPa [12–14]. This may explain why despite impressive 24 h bond strengths achieved in studies using molds with a C-factoro1; microleakage
[19–21], and gaps [4,23,24] still occur between the DBS and the tooth. The 2:5 mm deep mold with a C-factor of 4.1 represented a Class I preparation [34] and was the deepest mold that could be irradiated in one increment according to the manufacturer’s instructions [46]. The low C-factor mold was only 1 mm deep and was the shallowest mold that could be made with this technique. Z100 was chosen as the representative composite for several reasons: (a) Z100 is a commonly used resin composite [50]. (b) Compared to other resin composites, Z100 requires relatively little energy to polymerize [51]. (c) Measured bond strengths have usually been greater when resin composites with a high Youngs modulus (e.g., Z100) were used in bond strength studies using low C-factor molds [24,52]. (d) Uno et al. reported that increasing the C-factor from 2.5 to 4.0 had no effect on gap formation around compomers, but had a significant effect around a resin composite material [36]. This was thought to be due to the relatively low mechanical properties of the compomers that allowed some flow of the compomer and relaxation of the
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contraction stresses. This did not occur in the resin composite. Consequently, as the C-factor increased, the gap formation also increased. Z100 is a relatively stiff composite with a high modulus that polymerizes rapidly [53–56] and should generate the most stress at the resin–dentin interface [4,27,35,57]. The pulpal pressure was not simulated in this experiment because in vitro investigations have failed to show that it is necessary to simulate pulpal pressure [58]. Pameijer and Louw have also reported that it was not necessary to simulate the pulpal pressure because there was no difference between bonding to vital and non-vital teeth in vivo [59]. Despite the known effect of total energy density on resin polymerization, there is little standardization of the amount of energy used in bonding studies. The power density from a quartz tungsten halogen curing light can be measured by hand held dental radiometers. The Cure Rite radiometer provides a digital read out from 0 to 1999 mW=cm2 in 1 mW=cm2 increments in place of the analog meter found on some radiometers. Leonard et al. [60] reported that the power density values recorded by the Cure Rite radiometer were within 2% of those from a laboratory grade radiometer when using a 7:5 mm diameter light guide tip. The Turbo light guide used in this experiment was 8 mm in diameter and consequently the Cure Rite readings were taken as being accurate and were used to calculate the total energy received by the resin composite. Table 1 shows that thermal cycling did not have a significant effect ðp ¼ 1:0Þ on bond strength for specimens made in the molds with a lower C-factor, but thermal cycling had a very significant negative effect ðpo0:0001Þ on the bond strength of the specimens made with a C-factor of 4.1. This may have been because there was insufficient degree-of-conversion in the resin composite next to the DBS in the deeper mold. Therefore, this bottom layer furthest from the light would be more susceptible to breaking. The manufacturer of the Optilux curing lights (Demetron/Kerr) has recommended [61] that a power density greater than 300 mW=cm2 should be sufficient to polymerize composites up to 3 mm thick using the manufacturer’s recommended curing time. This is usually 40 s; which means that the restoration should receive at least 12 J=cm2 : Yap and Seneviratne have reported that 12 J=cm2 was enough energy to adequately polymerize a 2 mm increment of Z100 composite [62]. However, Unterbrink and Muessner reported that Z100 could be adequately polymerized to a depth of 4:5 mm when the specimen received 12 J=cm2 of energy [54]. Also, Bayne et al. reported a 6:15 mm depth of cure for Z100 when exposed to 24 J=cm2 of energy [56]. Since the specimens were given at least 40 J=cm2 of energy, this was considered more than adequate to polymerize the full
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depth of resin composite in both the 1 and 2:5 mm deep molds. Therefore, the significantly lower bond strengths of the specimens made in the molds with a high C-factor (4.1) compared to those that had been made in the molds with a lower (2.2) C-factor (Table 1) are more likely to be due to differences in the C-factor induced stresses than inadequate polymerization of the resin composite. Although most dentin bonding studies do not thermal cycle the specimens [7], the results of the present study (Table 1) showed that thermal cycling had a very significant negative effect on bond strength when molds with the higher C-factor representing a Class I preparation were used ðpo0:0001Þ: Using different composites and under different conditions, Yoshikawa et al. also reported that the micro-tensile 24 h bond strengths were 21–35% lower when the C-factor was increased from 1 to 3 [31]. Fig. 5 shows that after thermal cycling, the shear bond strength of the specimens made in the mold with C-factor of 4.1 was 60.2% lower than the bond strength measured using the mold with a C-factor of 2.2. Functional loading of Class II restorations has also been shown to significantly increase the microleakage at the dentin margin of eight different adhesive systems [21]. These reports, together with the results of the present study indicate that when the resin to dentin bond is subjected to clinically representative stresses, the bond may fail and microleakage occur. This may explain why gaps and microleakage occur around restorations in vivo [23] and why secondary caries has been reported to be the most common reason for replacing resin composite restorations [2,3]. The lower bond strengths measured from the specimens with the higher C-factor may explain why it has not been possible to predict that a DBS with high in vitro bond strengths will have good clinical performance [5,6,11]. The mold design used in this study is relatively simple to produce and allows in vitro bond strength tests and interfacial gap studies to be designed with different Cfactors. Since thermal cycling caused a significant 47.2% reduction (from 12.7 to 6:7 MPa) in bond strength in the specimens made in the mold with the higher C-factor of 4.1, it is recommended that bond strength studies should thermal cycle specimens before debonding. In vitro studies of bond strengths should also report the C-factor of the mold used in the study.
5. Conclusions For the one dentin bonding system and one resin composite tested, it was concluded that: 1. When the C-factor of the mold was increased from 2.2 to 4.1, overall the bond strength for Single Bond adhesive decreased ðpo0:001Þ:
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2. Thermal cycling 5000 times from 51C to 551C had a negative effect on the bond strength only for specimens made in molds with a C-factor of 4.1 ðpo0:001Þ:
Acknowledgements The authors would like to thank 3M ESPE for purchasing the thermal cycling machine for Dalhousie University. The resin composites and the curing light were kindly donated to Dalhousie University by the manufacturers.
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R.B. Price et al. / Biomaterials 24 (2003) 1013–1021 [40] Gale MS, Darvell BW. Thermal cycling procedures for laboratory testing of dental restorations. J Dent 1999;27:89–99. [41] Wilson NHF. Direct adhesive materials:current perceptions and evidence-future solutions. J Dent 2001;29:307–16. [42] Christensen GJ. Has tooth structure been replaced. J Am Dent Assoc 2002;133:103–5. [43] ISO Technical Report 11405. Dental materials—Guidance on testing of adhesion to tooth structure. Geneva: International Organization for Standardization, 1994. [44] McInnes PM, Wendt Jr. SL, Retief DH, Weinberg R. Effect of dentin surface roughness on shear bond strength. Dent Mater 1990;6:204–7. [45] Lucena-Martin C, Gonzalez-Lopez S, Navajas-Rodriguez de Mondelo JM. The effect of various surface treatments and bonding agents on the repaired strength of heat-treated composites. J Prosthet Dent 2001;86:481–8. [46] Z100 Product Instructions. 3M Dental, St. Paul, MN, 1998. [47] Pashley DH, Carvalho RM, Sano H, Nakajima M, Yoshiyama M, Shono Y, Fernandes CA, Tay F. The microtensile bond test: a review. J Adhes Dent 1999;1:299–309. [48] Sano H, Shono T, Sonoda H, Takatsu T, Ciucchi B, Carvalho R, Pashley DH. Relationship between surface area for adhesion and tensile bond strength—evaluation of a micro-tensile bond test. Dent Mater 1994;10:236–40. [49] Sidak Z. Rectangular confidence regions for the means of multivariate normal distributions. J Am Stat Assoc 1967;62:626–33. . IA, Wilson NH. Teaching Class I and Class II direct [50] Mjor composite restorations: results of a survey of dental schools. J Am Dent Assoc 1998;129:1415–21. [51] Abate PF, Zahra VN, Macchi RL. Effect of photopolymerization variables on composite hardness. J Prosthet Dent 2001;86:632–5.
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. [52] Mondragon E, Soderholm KJ. Shear strength of dentin and dentin bonded composites. J Adhes Dent 2001;3:227–36. [53] Price RB, Rizkalla AS, Hall GC. Effect of stepped light exposure on the volumetric polymerization shrinkage and bulk modulus of dental composites and an unfilled resin. Am J Dent 2000;13: 176–80. [54] Unterbrink GL, Muessner R. Influence of light intensity on two restorative systems. J Dent 1995;23:183–9. [55] Gladys S, Van Meerbeek B, Braem M, Lambrechts P, Vanherle G. Comparative physico-mechanical characterization of new hybrid restorative materials with conventional glass-ionomer and resin composite restorative materials. J Dent Res 1997;76:883–94. [56] Bayne SC, Thompson JY, Swift Jr EJ, Stamatiades P, Wilkerson M. A characterization of first-generation flowable composites. J Am Dent Assoc 1998;129:567–77. [57] Obici AC, Sinhoreti MA, de Goes MF, Consani S, Sobrinho LC. Effect of the photo-activation method on polymerization shrinkage of restorative composites. Oper Dent 2002; 27:192–8. [58] Moll K, Haller B. Effect of intrinsic and extrinsic moisture on bond strength to dentine. J Oral Rehabil 2000;27:150–65. [59] Pameijer CH, Louw NP. Significance of pulpal pressure during clinical bonding procedures. Am J Dent 1997;10:214–8. [60] Leonard DL, Charlton DG, Hilton TJ. Effect of curing-tip diameter on the accuracy of dental radiometers. Oper Dent 1999;24:31–7. [61] Demetron Curing Radiometer Operating Instructions. Demetron Research Corp. Danbury, CT, 1990. [62] Yap AU, Seneviratne C. Influence of light energy density on effectiveness of composite cure. Oper Dent 2001;26:460–6.
The effect of two configuration factors, time, and thermal cycling on resin to dentin bond strengths Richard B. Pricea,*, Tore De! randb, Pantelis Andreouc, Darcy Murphyd b
a Department of Dental Clinical Sciences, Dalhousie University, Halifax, Nova Scotia, Canada, B3H 3J5 Department of Oral Technology and Dental Materials Science, Malmo. University, Carl Gustafs vag . 34, Malmo, . Sweden, SE-214 21 c Community Health and Epidemiology, Dalhousie University, Halifax, Nova Scotia, Canada, B3H 3J5 d Dalhousie University, Halifax, Nova Scotia, Canada, B3H 3J5
Received 28 February 2002; accepted 4 September 2002
Abstract Most in vitro testing of bonding systems is performed using specimens made in a mold with a low configuration ðCÞ factor (ratio of bonded/unbonded surfaces) whereas clinically the C-factor is usually much greater. This study compared the effect of thermal cycling on the measured shear bond strength of 3M Single Bond dental adhesive bonded to dentin using molds with two different Cfactors. The hypothesis was that neither C-factor nor thermal cycling would affect measured bond strengths. Resin composite was bonded to human dentin in cylindrical molds with an internal diameter of 3:2 mm and either 1 mm or 2:5 mm deep. The 1 mm deep molds had a C-factor of 2.2 and the 2:5 mm deep molds had a C-factor of 4.1. Specimens were debonded either 10 min after they had been bonded to dentin, or after they had been stored for 7 days in water at 37711C; or after thermal cycling 5000 times for 7 days. Two-way ANOVA showed that overall both the C-factor and the storage condition had a significant effect on bond strength ðpo0:001Þ: There was a significant interaction ðpo0:001Þ between the C-factor and how the specimens had been stored. The GLM/ LSMEANS procedure with Sidak’s adjustment for multiple comparisons showed that overall the specimens made in the mold with a high C-factor (4.1) had a lower bond strength than those that had been made in the mold with a lower (2.2) C-factor ðpo0:001Þ: Thermal cycling had a negative effect on the bond strength only for specimens made in molds with a C-factor of 4.1 ðpo0:001Þ: r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Composite resins; Dental bonding/methods; Mechanical stress; Dental cavity preparation/classification
1. Introduction The shrinkage that occurs as resin composites polymerize has been cited to be a major reason for bonding failures in adhesive restorations [1]. This shrinkage may lead to microleakage around the restoration and subsequent secondary caries, which has been reported to be the most common reason for replacing resin composite restorations [2,3]. New dentin bonding systems (DBS) are often marketed without undergoing long term clinical trials and the dental profession has had to rely upon the results of in vitro bonding studies when choosing a new product [4–6]. After reviewing 50 investigations on bonding resin to dentin, al-Salehi and Burke [7] showed that there was little standardization in testing bond *Corresponding author. Tel.: +1-902-494-1226; fax: +1-902-4941662. E-mail address: [email protected] (R.B. Price).
strengths and the test conditions often did not reproduce the intraoral environment. Different research centers using the same materials, often produce different results which makes it difficult to predict clinical performance from in vitro tests [5,6,8]. Although bond strength testing can differentiate between those materials that have little adhesion to dentin and those that bond well, so far it has been impossible to correlate high in vitro bond strength data obtained for products [9,10] with good clinical performance [6,11] of the same products. This may be because previous in vitro studies: 1. Failed to reproduce the shape of typical preparations found in vivo. 2. Failed to measure the bond strength at an appropriate time. 3. Failed to reproduce the way and the amount of light energy that the restoration would receive in vivo. 4. Failed to reproduce intra oral conditions in the in vitro test.
0142-9612/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 6 1 2 ( 0 2 ) 0 0 4 4 1 - 6
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As the resin composite polymerizes and shrinks, the bond between the DBS and the tooth should be strong enough to prevent the resin from being pulled away from the tooth. To prevent debonding and microleakage at the resin–dentin interface and to resist the stresses produced during polymerization as the resin shrinks, it has been reported that bond strengths of 17–24 MPa are required [12–14]. However, even with bonding systems which have recorded 24 h in vitro shear bond strengths in excess of 17 MPa [10,15–18], microleakage [19–22], and gaps [4,23,24] still occur between the DBS and the tooth. In an in vivo study of six DBS which have produced 24 h in vitro bond strengths > 17 MPa; the polymerization shrinkage overcame the adhesive capability of all the dentin bonding systems and resulted in detachment of the resin from the tooth [23]. This suggests that the 24-h bond strengths recorded in vitro in a low stress environment are not able to predict whether adhesive failure will occur between the DBS and the tooth when resin composite is used as a restorative material in vivo. Previous studies to measure resin–dentin bond strengths have used metal molds [17], Teflon molds, or gelatin capsules filled with previously polymerized composite [9,10,12,15,16,18,24–27]. However, the design of the test mold can affect the bond strength and physical properties of the cured resin composite [28–31]. The design of the test mold has also been shown to have an effect on the stress generated during resin polymerization [32]. Feilzer et al. [33] reported that the ratio of the bonded to unbonded surfaces within the preparation, the configuration ðCÞ factor, can be calculated and used to predict those restorations which are most likely to exhibit bond failures between the resin and the tooth. Mean C-factors have been reported to be 4.03 for Class I, 1.85 for Class II, and 1.10 for cervical abrasion restorations [34]. According to the results of Feilzer et al. [33] restorations with a C-factor less than 1 are more likely to survive the polymerization contraction stresses and remain bonded to the tooth. This is supported by Yoshikawa et al. [35] who reported more microleakage as the C-factor increased from 2.3 to 3.0 and an in vivo study where the resin to dentin bond of 6 different DBS failed in all cases where the C-factor was 5 [23]. In bonding studies where the resin composite was bonded to the tooth at the bottom of the mold only, not to the sides [9,10,12,15–18,24–26], the ratio of the bonded to the unbonded surfaces was low. This resulted in a Cfactor less than 1 and imparted little stress on the bond to the tooth as the resin polymerized [33]. While this situation is similar to a Class VI restoration or a shallow cervical abrasion restoration, the C-factor is much greater in a Class I, II, or V restoration [34] where the resin composite is bonded to the sides as well as to the floor of the preparation (Fig. 1). Bouillaguet et al. reported a 20% reduction in micro-tensile bond
strengths in a Class II restoration with a C-factor of 1.4 compared to the bond strengths obtained on a flat dentin surface [30]. Hasegawa et al. reported that the high bond strengths recorded in low stress situations did not always prevent contraction gaps in a Class I restoration with a C-factor of 3 [24]. Therefore, if the resin composite can be bonded to the walls of the mold, as occurs in a tooth [24,27,30,31,36,37], this will place more stress on the bond between the resin and the tooth [32,33,37] and might provide a more clinically relevant test of bond strength [24,30]. There may then be a correlation between the measured bond strengths and the presence of contraction gaps. When Teflon or gelatin capsules are used as molds in bonding studies, then the DBS and composite will receive more light energy from the sides than in the in vivo situation. This will enhance bond strength and physical properties of the composite [28,29]. Similar amounts of light energy have been shown to penetrate through resin composite and tooth structure [38,39]. Therefore, if the mold is made out of resin composite instead of metal [27], then not only can the resin composite can be bonded to the walls of the mold, but a similar amount of light should reach the sides and bottom of the specimen as would occur in a tooth [38,39]. In many previous reports [9,10,25], the entire dentin surface was treated with the DBS and polymerized before the mold was placed on the dentin and the resin composite bonded to the dentin. This technique increases the bonded surface area (Fig. 2) and may artificially increase bond strength values [26]. To avoid this, the DBS on the dentin surface should be light activated only within the area enclosed by the mold and to which the resin composite is subsequently bonded. Many DBS are able to resist the effects of polymerization shrinkage and maintain the bond between dentin and the resin when the specimens are not subjected to additional stresses [10,15–18,21]. However, when Class II restorations were subjected to additional stress from functional loading, there was a significant increase in the microleakage of all eight DBS tested [21]. Thermal cycling also stresses the bond between resin and the tooth and, depending on the DBS, may affect bond strength [9,25]. However, only 19% of dentin bonding studies thermal cycle the specimens before testing [7]. This may be because the value of in vitro thermal cycling of specimens has been questioned [40]. Hasegawa et al. [15] reported that subjecting specimens to 500 thermal cycles had no effect on shear bond strength. Their testing protocol used a low C-factor split Teflon mold and the specimens were not thermal cycled until 4 weeks after bonding. The effects of thermal cycling may also depend on the bonding system being tested [9,25], but these conclusions may be due to the testing procedure used. Using a Teflon mold with a low C-factor,
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Fig. 1. Examples of how the C-factor changes with different preparation designs (modified after Feilzer et al. [33]).
Fig. 2. Schematic showing how the bonding surface area is increased if the DBS is not confined and is allowed to cover the tooth surface beyond the composite specimen (modified after Van Noort et al. [26]).
Miyazaki et al. [25] investigated the effect of thermal cycling up to 30,000 times on bond strength. They reported that there was a significant decrease in bond strength to bovine dentin for Single Bond (3M Dental, St. Paul, USA) after 3000 thermal cycles, but the bond strengths of two other DBS were unaffected after 30,000 thermal cycles. Different results may have been reported from these studies if a mold with a higher C-factor had been used. In 2001, Wilson [41] reiterated that laboratory studies need to be developed that can predict clinical success and, in an editorial in 2002, Christensen [42] commented that impressive in vitro resin–dentin bonds are transient when subjected to temperature changes in the mouth. Further investigation of the effects of using molds with clinically relevant C-factors on in vitro bond strength is required and the present study may explain some of the shortcomings of previous in vitro bonding studies. This study determined how similar cylindrical mold designs with C-factors of 2.2 and 4.1 affected the shear bond strength of a DBS to dentin and the effect of thermal cycling on the measured bond strengths. The hypothesis was that for specimens cured within the manufacturer’s tolerances, shear bond strengths would
not be affected as the C-factor increased, and that thermal cycling would not affect measured bond strengths.
2. Materials and methods Intact, non-carious, human third molar teeth were collected and stored in a solution of 0.5% chloramine-Thydrate (Aldrich Chemical Co., Milwaukee, USA) at 41C for up to 4 weeks [43]. The teeth were then washed thoroughly in running water and stored in water at 41C for at least a further 24 h; but no more than 1 week. Acrylic resin (Tray Resin, Caulk/Dentsply, Milford, USA) was used to mount the teeth so that each tooth had its buccal surface facing upwards and half of the tooth was above the resin. The buccal surface [43] of each tooth was ground flat to approximately halfway between the dentin–enamel junction and the pulp, using in turn 120, 240, and 400-grit silicon carbide (SiC) papers on a water cooled abrasive wheel. This depth was chosen to represent the depth of a typical cavity preparation. Since the surface finish produced by 600 grade SiC paper is similar to the finish produced by a
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carbide bur [44], one hour before the teeth were used, the dentin surface was lightly abraded with 600-grit SiC paper [10,15,16,25,26] on a water-cooled abrasive wheel. The teeth were then placed in a water bath at 37711C: Cylindrical resin composite molds were made to compare the effect of C-factor on bond strength. A flowable resin composite (3M Filtek Flow, Shade A2) was flowed into circular molds that were either 1 or 2:5 mm deep. The composite was irradiated for 60 s using an Optilux 500 (Demetron/Kerr, Danbury, USA) quartz tungsten halogen curing light equipped with a 13/ 8 Turbo Plus (Demetron/Kerr) light guide and then removed from the mold. The internal surface of the resin composite cylinder was roughened using 50 mm aluminium oxide grit blasting for 10 s to produce a matte finish. This procedure has been shown to produce a strong bond between new resin composite and previously cured composite [45]. The composite cylinders were checked to verify that they all had an external diameter of 6:6 mm; an internal diameter of 3:2 mm; and were either 1 or 2:5 mm deep. Using the following formula, the C-factor of the 1 mm deep cylinders was calculated to be 2.2 and the C-factor of the 2:5 mm deep cylinders was calculated to be 4.1 C¼
bonded area of composite ; unbonded area of composite
circumference of cylindrical mold depth þ area of one end bonded to dentin C¼ ; area of one unbonded end C¼
2pr h þ pr2 ; pr2
where h is the depth of composite mold (mm) and r the internal radius of composite mold (mm). Sixty resin composite cylinders were made 1 mm deep and 60 cylinders were made 2:5 mm deep. Polyester Hi Solvent Resistance Transparent tape (3M) was placed on the bottom of each cylinder to prevent the composite cylinder from bonding to the tooth (Fig. 3). The resin composite cylinder was clamped on to the dentin surface and the dentin enclosed by the cylinder was etched using phosphoric acid etch (3M) for 15 s and then washed thoroughly with distilled water at 37711C: Excess water was removed using Kimwipes EX-L (Kimberly-Clark Corp., Roswell, USA) tissue [16] leaving a moist dentin surface [18]. One drop of DBS (3M Single Bond) was placed on dentin surface inside the cylinder and then lightly brushed around for 10 s: The DBS was also brushed on to the inside of each resin composite cylinder to improve bonding between the old and new resin composite [45]. A second drop of DBS was then placed on the dentin surface, brushed around and an air syringe positioned approximately 6–8 in away was used to evaporate solvent from the DBS. The dentin
Fig. 3. Schematic showing how resin composite was used to make cylindrical molds with different C-factors.
and the DBS inside the cylinder was irradiated for 10 s using an Optilux 500 equipped with a 13/8 Turbo Plus light guide. Z100 resin composite (3M Dental, Shade A1) was placed into the mold, covered with a Mylar strip, and cured in one increment from the top of the mold for 40 s using the Optilux 500 light. The 2:5 mm increment thickness was within the manufacturer’s recommended maximum thickness of Z100 (3M) shade A1 composite, which can be adequately light cured in 40 s [46]. This increment was also considered something which might be done clinically in vivo. Due to the experimental design, the tip of the light guide was 2 mm from the top of the resin composite. The output from the curing light was checked at 2 mm using a Cure Rite radiometer (Dentsply/Caulk, Milford, USA) after every 20 specimens had been made to ensure that the power density was above 1000 mW=cm2 : Consequently, each specimen received at least 40 J=cm2 of energy from the Optilux 500 curing light. To account for variations in teeth and the effects of different dentin thicknesses on bond strength [31], the 120 teeth were randomly coded and assigned to one of three test groups for each C-factor (cylinder depth). A pilot project showed that it took about 7 min from starting to irradiate the DBS until the finished specimen was ready for debonding. To allow for operator variability, the early bond strengths were tested 10 min after irradiating the DBS. This time interval met the recommendation of al-Salehi and Burke that bond strengths should be measured 5–10 min after irradiating the restoration [7]. After bonding, all specimens were placed in a water bath at 37711C: One group ðn ¼ 20Þ for each cylinder depth was stored in water at 37711C and debonded 10 min from the time the DBS was irradiated. Another group was stored in water at 37711C and debonded 7 days from the time the DBS was irradiated. The final group was thermal cycled 5000 times from 51C to 551C with a 30 s dwell time and a 30 s intermediate time at a room temperature of 231C: After the thermal cycling, which took 7 days to complete, the
R.B. Price et al. / Biomaterials 24 (2003) 1013–1021
specimens were placed into a 37711C water bath for at least 30 min before debonding. Although the ISO/TR 11405 document [43] on testing of adhesion to tooth structure recommends thermal cycling 500 times from 51C to 551C and back, 5000 thermal cycles were chosen to simulate 6 months in a patient’s mouth [40]. The 5– 551C temperatures with a 30 s dwell time in each bath were used in accordance with the ISO/TR 11405 document [43] that specifies a dwell time of at least 20 s: There are several methods to test the bond strength of a DBS (e.g., tensile, micro tensile or shear bond tests) [9,47,48]. Realizing its limitations, the shear bond strength test was chosen for this study because it has been reported by Øilo and Austrheim that tensile and shear test methods are equally representative when testing in vitro resin–dentin bond strengths [9]. To reduce the tensile, compressive and bending forces that can be introduced in the shear bond test, the load was applied using an Instron 1000 (Instron, Canton, USA) with a chisel edged blade that had a 0:5 mm blunt edge [43] positioned as close as possible to the tooth without actually touching it (Fig. 4). The load was applied at a crosshead speed of 2:8 mm which was within the 0.5– 5 mm=min range used by other researchers [7,10,15,16,18,25,31]. The shear bond strength data were compared using a general linear model analysis using po0:01 as the level of significance. The SAS GLM/LSMEANS procedure with Sidak’s adjustment for multiple comparisons [49] was used to determine the effects of time, thermal cycling, and C-factor on measured bond strength.
3. Results The mean bond strengths7standard deviations for the specimens are shown in Fig. 5. A power analysis showed that the power of the general linear model test was greater than 0.95 when comparing the effects of the three different storage conditions and 0.99 when comparing the effect of the Cfactor on shear bond strength. Two-way ANOVA showed that overall both the C-factor and the storage condition had a significant effect on bond strength ðpo0:001Þ: There was a significant interaction ðpo0:001Þ between the two factors (specimen C-factor and storage condition). The SAS GLM/LSMEANS procedure with Sidak’s adjustment for multiple comparisons showed that overall the specimens made in the mold with a high C-factor (4.1) had a lower bond strength than those that had been made in the mold with a lower (2.2) C-factor ðpo0:001Þ: Pairwise comparisons (Table 1) show that neither time ðp ¼ 0:0616Þ nor thermal cycling ðp ¼ 0:1037Þ had a significant effect on bond strength for specimens made in the molds with a C-factor of 2.2. Time also had no
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Fig. 4. Schematic showing how the resin to dentin bond was subjected to a shearing force.
significant effect on bond strength for specimens made in the molds with a C-factor of 4.1 ðp ¼ 0:1054Þ: However, thermal cycling had a significant negative effect on bond strength for the specimens with the greater C-factor ðpo0:0001Þ: At 7 days, the bond strength of the high C-factor specimens which had been thermally cycled was 47.2% less than that of the specimens that had been stored for 7 days in water at 371C: The hypothesis that shear bond strengths would not be affected as the C-factor increased was rejected since overall the specimens made in the mold with a high (4.1) C-factor had a lower bond strength than those which had been made in the mold with a lower (2.2) Cfactor ðpo0:001Þ: The hypothesis that thermal cycling would not affect bond strengths was accepted for the specimens made in the mold with a lower (2.2) C-factor ðp ¼ 1Þ; but it was rejected for the specimens made in the mold with a higher (4.1) C-factor ðpo0:0001Þ:
4. Discussion Overall, the specimens made in the mold with a high C-factor (4.1) had a lower bond strength than those which had been made in the mold with a lower (2.2) Cfactor ðpo0:001Þ: This agrees a previous study by Haller et al. who used a silanated brass mold to develop a Cfactor of 4 [27]. The result that thermal cycling did not have a significant effect ðp ¼ 1:0Þ on bond strength for specimens made in the molds with the lower C-factor supports the results reported by Hasegawa et al. who used a split Teflon mold with a low C-factor [15]. Thermal cycling stresses the resin–dentin bond [9,25].
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Fig. 5. Effect of time and thermal cycling on the mean shear bond strength of specimens with low and high configuration factors.
Table 1 Pairwise comparisons of the GLM/LSMEANS showing the effect of time and thermal cycling on the shear bond strength of specimens with configuration factors of 2.2 and 4.1 Specimen C-factor and treatment
P-Values Bond strength C-factor 2.2 C-factor 2.2 C-factor 2.2 C-factor 4.1 C-factor 4.1 C-factor 4.1 (MPa) 10 min 7 days 7 days 10 min 7 days 7 days + thermal cycling + thermal cycling
C-factor 2.2, 10 min C-factor 2.2, 7 days C-factor 2.2, 7 days + thermal cycling C-factor 4.1, 10 min C-factor 4.1, 7 days C-factor 4.1, 7 days + thermal cycling
14.1 17.1 16.9
— 0.0616 0.1037
0.0616 — 1.0000
0.1037 1.0000 —
0.0016 o0.0001 o0.0001
0.9636 0.0008 0.0015
o0.0001 o0.0001 o0.0001
9.8 12.7 6.7
0.0016 0.9636 o0.0001
o0.0001 0.0008 o0.0001
o0.0001 0.0015 o0.0001
— 0.1054 0.0565
0.1054 — o0.0001
0.0565 o0.0001 —
This added stress only adversely affected the bond between the resin and the tooth in the specimens made in a mold with the higher C-factor of 4.1 ðpo0:0001Þ: Overall, the bond strengths of the specimens were significantly lower at 10 min than after 7 days ðpo0:001Þ: The lower early bond strengths which increased with time support a previous study by Hasegawa et al. [15]. Although the bond strengths were less at 10 min than at 7 days (Fig. 5), pairwise comparisons (Table 1) showed that time did not have a significant effect on bond strength (p ¼ 0:0616 for the molds with a C-factor of 2.2, and p ¼ 0:1054 for the molds with a C-factor of 4.1). The early 10 min bond strengths to dentin achieved by a DBS are very important to the clinician. A DBS should provide adequate early resin–dentin bond strength to resist the polymerization shrinkage of the composite resin, the potential trauma of polishing the restoration, and the masticatory forces when the occlusion is checked. The mean 10 min shear bond strengths to dentin of 14:1 MPa in the 2.2 C-factor mold and 9:8 MPa in the mold with a C-factor of 4.1 were less than the desired 17–24 MPa [12–14]. This may explain why despite impressive 24 h bond strengths achieved in studies using molds with a C-factoro1; microleakage
[19–21], and gaps [4,23,24] still occur between the DBS and the tooth. The 2:5 mm deep mold with a C-factor of 4.1 represented a Class I preparation [34] and was the deepest mold that could be irradiated in one increment according to the manufacturer’s instructions [46]. The low C-factor mold was only 1 mm deep and was the shallowest mold that could be made with this technique. Z100 was chosen as the representative composite for several reasons: (a) Z100 is a commonly used resin composite [50]. (b) Compared to other resin composites, Z100 requires relatively little energy to polymerize [51]. (c) Measured bond strengths have usually been greater when resin composites with a high Youngs modulus (e.g., Z100) were used in bond strength studies using low C-factor molds [24,52]. (d) Uno et al. reported that increasing the C-factor from 2.5 to 4.0 had no effect on gap formation around compomers, but had a significant effect around a resin composite material [36]. This was thought to be due to the relatively low mechanical properties of the compomers that allowed some flow of the compomer and relaxation of the
R.B. Price et al. / Biomaterials 24 (2003) 1013–1021
contraction stresses. This did not occur in the resin composite. Consequently, as the C-factor increased, the gap formation also increased. Z100 is a relatively stiff composite with a high modulus that polymerizes rapidly [53–56] and should generate the most stress at the resin–dentin interface [4,27,35,57]. The pulpal pressure was not simulated in this experiment because in vitro investigations have failed to show that it is necessary to simulate pulpal pressure [58]. Pameijer and Louw have also reported that it was not necessary to simulate the pulpal pressure because there was no difference between bonding to vital and non-vital teeth in vivo [59]. Despite the known effect of total energy density on resin polymerization, there is little standardization of the amount of energy used in bonding studies. The power density from a quartz tungsten halogen curing light can be measured by hand held dental radiometers. The Cure Rite radiometer provides a digital read out from 0 to 1999 mW=cm2 in 1 mW=cm2 increments in place of the analog meter found on some radiometers. Leonard et al. [60] reported that the power density values recorded by the Cure Rite radiometer were within 2% of those from a laboratory grade radiometer when using a 7:5 mm diameter light guide tip. The Turbo light guide used in this experiment was 8 mm in diameter and consequently the Cure Rite readings were taken as being accurate and were used to calculate the total energy received by the resin composite. Table 1 shows that thermal cycling did not have a significant effect ðp ¼ 1:0Þ on bond strength for specimens made in the molds with a lower C-factor, but thermal cycling had a very significant negative effect ðpo0:0001Þ on the bond strength of the specimens made with a C-factor of 4.1. This may have been because there was insufficient degree-of-conversion in the resin composite next to the DBS in the deeper mold. Therefore, this bottom layer furthest from the light would be more susceptible to breaking. The manufacturer of the Optilux curing lights (Demetron/Kerr) has recommended [61] that a power density greater than 300 mW=cm2 should be sufficient to polymerize composites up to 3 mm thick using the manufacturer’s recommended curing time. This is usually 40 s; which means that the restoration should receive at least 12 J=cm2 : Yap and Seneviratne have reported that 12 J=cm2 was enough energy to adequately polymerize a 2 mm increment of Z100 composite [62]. However, Unterbrink and Muessner reported that Z100 could be adequately polymerized to a depth of 4:5 mm when the specimen received 12 J=cm2 of energy [54]. Also, Bayne et al. reported a 6:15 mm depth of cure for Z100 when exposed to 24 J=cm2 of energy [56]. Since the specimens were given at least 40 J=cm2 of energy, this was considered more than adequate to polymerize the full
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depth of resin composite in both the 1 and 2:5 mm deep molds. Therefore, the significantly lower bond strengths of the specimens made in the molds with a high C-factor (4.1) compared to those that had been made in the molds with a lower (2.2) C-factor (Table 1) are more likely to be due to differences in the C-factor induced stresses than inadequate polymerization of the resin composite. Although most dentin bonding studies do not thermal cycle the specimens [7], the results of the present study (Table 1) showed that thermal cycling had a very significant negative effect on bond strength when molds with the higher C-factor representing a Class I preparation were used ðpo0:0001Þ: Using different composites and under different conditions, Yoshikawa et al. also reported that the micro-tensile 24 h bond strengths were 21–35% lower when the C-factor was increased from 1 to 3 [31]. Fig. 5 shows that after thermal cycling, the shear bond strength of the specimens made in the mold with C-factor of 4.1 was 60.2% lower than the bond strength measured using the mold with a C-factor of 2.2. Functional loading of Class II restorations has also been shown to significantly increase the microleakage at the dentin margin of eight different adhesive systems [21]. These reports, together with the results of the present study indicate that when the resin to dentin bond is subjected to clinically representative stresses, the bond may fail and microleakage occur. This may explain why gaps and microleakage occur around restorations in vivo [23] and why secondary caries has been reported to be the most common reason for replacing resin composite restorations [2,3]. The lower bond strengths measured from the specimens with the higher C-factor may explain why it has not been possible to predict that a DBS with high in vitro bond strengths will have good clinical performance [5,6,11]. The mold design used in this study is relatively simple to produce and allows in vitro bond strength tests and interfacial gap studies to be designed with different Cfactors. Since thermal cycling caused a significant 47.2% reduction (from 12.7 to 6:7 MPa) in bond strength in the specimens made in the mold with the higher C-factor of 4.1, it is recommended that bond strength studies should thermal cycle specimens before debonding. In vitro studies of bond strengths should also report the C-factor of the mold used in the study.
5. Conclusions For the one dentin bonding system and one resin composite tested, it was concluded that: 1. When the C-factor of the mold was increased from 2.2 to 4.1, overall the bond strength for Single Bond adhesive decreased ðpo0:001Þ:
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2. Thermal cycling 5000 times from 51C to 551C had a negative effect on the bond strength only for specimens made in molds with a C-factor of 4.1 ðpo0:001Þ:
Acknowledgements The authors would like to thank 3M ESPE for purchasing the thermal cycling machine for Dalhousie University. The resin composites and the curing light were kindly donated to Dalhousie University by the manufacturers.
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