Polymerization shrinkage-strain and microleakage in dentin-bordered cavities of chemically and light-cured restorative materials

dental materials Dental Materials 18 (2002) 521±528

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Polymerization shrinkage-strain and microleakage in dentin-bordered cavities of chemically and light-cured restorative materials M. Rosin a,*, A.D. Urban b, C. GaÈrtner c, O. Bernhardt a, C. Splieth a, G. Meyer a a

Department of Operative Dentistry, Periodontology and Pediatric Dentistry, University of Greifswald, Rotgerberstr. 8, 17489 Greifswald, Germany b Private Practice, Cologne, Germany c Department of Prosthodontics and Dental Materials, University of Greifswald, Rotgerberstr. 8, 17489 Greifswald, Germany Received 27 July 2000; revised 3 April 2001; accepted 19 June 2001

Abstract Objectives: The aim of this study was to evaluate in vitro the relationship between polymerization shrinkage and microleakage in dentinbordered restorations. Methods: Four light-cured restorative materials in combination with their respective dental bonding agents (DBA) were investigated: Tetric Ceram/Syntac classic (Vivadent), Solitaire/Gluma Solid bond (Heraeus Kulzer), De®nite/Etch & Prime 3.0 (Degussa), Solitaire 2/ Gluma Solid bond (Heraeus Kulzer). The chemically cured resin Degu®ll sc microhybrid (Degussa) in combination with ART Bond (ColteÁne) was also included. Polymerization shrinkage of the restorative materials was measured using three different methods (dilatometer, linometer, buoyancy method) and analyzed with ANOVA. For the determination of microleakage, caries-free human molars were embedded in acrylic resin and subsequently abraded with a wet abrasion machine to produce four level dentin surfaces. One hundred sixty cavities (3 mm diameter/1.5 mm deep) were randomly assigned to four groups of equal size. The groups were restored without (group 1 and 2) and with DBA (group 3 and 4), and either not subjected (group 1 and 3) or subjected (group 2 and 4) to 2000 cycles from 5±558C. Each group was further divided into ®ve material subgroups of eight cavities each. Microleakage was determined using a dye penetration test assessed at depths of 200, 400 and 600 mm into the ®llings. Data were analyzed with the Kruskal±Wallis and the Mann±Whitney test. Results: All three methods of measuring polymerization shrinkage (PS) generated the same, statistically secured ranking for the four lightcured restorative materials: PS De®nite , PS Tetric Ceram , PS Solitaire 2 , PS Solitaire. In the microleakage study, only a few statistically signi®cant differences were observed. Etch & Prime 3.0/De®nite in group 3 and Solid Bond/Solitaire 2 in group 4 tended to exhibit the least microleakage. Correlation coef®cients between aggregated shrinkage and microleakage data were 0.3 for group 3 and 20.2 for group 4. Signi®cance: The results do not suggest any correlation between polymerization shrinkage and microleakage in dentin of direct adhesive restorations. q 2002 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. Keywords: ORMOCER; Composite resin; Dental adhesive system; Polymerization shrinkage; Microleakage; Resin±dentin interface

1. Introduction In recent years, resin composites have become widely used as a restorative material in the posterior area [1]. Reasons for this include decreasing acceptance of the traditional material amalgam by patients and their increasing demands for esthetic restorations in molars and premolars [2]. Although the mechanical properties of modern composite resins have been improved, polymerization shrinkage still remains a clinically signi®cant problem [3]. The competition between the contraction stress in polymerizing resin composites and the bonds of adhesive resins to the wall * Corresponding author. Tel.: 149-3834-867167; fax: 149-3834867171. E-mail address: [email protected] (M. Rosin).

of restorations is one of the main causes of marginal failure and subsequent microleakage observed in resin restorations [4]. Microleakage, leading to marginal staining, secondary caries, and pulp pathology [5,6], has been reported as the predominant reason for replacement of composite resin restorations [7±9]. Recently, a restorative material (De®nite w, Degussa Dental) with a new type of matrix, a so-called ORMOCER w matrix, was introduced onto the market. An ORMOCER w (acronym for organically modi®ed ceramics) is an inorganic/ organic copolymer system based on multifunctional urethane- and thioether (meth)-acrylate alkoxysilanes [10,11]. The monomer precursors of the copolymer are synthesized by an NCO addition in which an isocyanatesubstituted silane is added to an OH-substituted (meth)acrylate [11]. The inorganic, condensible molecule segment

0109-5641/02/$-see front matter q 2002 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. PII: S 0109-564 1(01)00078-1

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Fig. 1. Schematic diagram of the linometer.

serves to construct an inorganic Si±O±Si network, which is synthesized via hydrolysis and polycondensation in the production process. The intraoral curing of an ORMOCER w-based restorative material is achieved by radical-induced polymerization (e.g. visible light or redoxinduced) of the attached (meth)-acrylate groups. One of the advantages of an ORMOCER w matrix is that by forming relatively large matrix molecules in the production process, the volume contraction incident to organic polymerization can be lessened [11]. In fact, the reported polymerization shrinkage of De®nite w is less than that of conventional composites [12±14]; 2.29 (0.49) Vol% for De®nite w and 3.03 (0.2) Vol% and 3.62 (0.19) Vol % for Tetric w Ceram (Vivadent, Schaan, Liechtenstein) and Solitaire w (Heraeus Kulzer, Wehrheim, Germany), respectively [12,13]. Reduced polymerization shrinkage would be particularly advantageous in the restoration of Classes I and II, where the unfavorable C-factor leads to a high tensile-stress build up [15]. In a clinical situation, dentinal margins in deep proximal boxes will be most susceptible to marginal gap formation [16,17]. In an in vitro study, the marginal integrity of restorations placed with two experimental ORMOCER ws in combination with various dental bonding agents was compared to the marginal integrity of two established composite resin systems [18]. One combination with ORMOCER w1 performed best in the enamel-bound segments of the cavities, and another combination with ORMOCER w2 revealed superior marginal integrity in the cervical dentin-bound cavity sections [18]. The aim of this study was to evaluate in vitro the relationship between polymerization shrinkage and microleakage in dentin-bordered restorations made of an ORMOCER wbased material and of chemically and light-cured resin composites.

2. Materials and methods 2.1. Measurements of polymerization shrinkage 2.1.1. Linometer Linear shrinkage was measured with a linometer built by the University of ZuÈrich (Fig. 1) using the method described by Park et al. [19]. Five samples per material were tested. Approximately 90.0 mm 3 of sample material were shaped into a ball and placed on the linometer's sample carrier, an aluminum disk. A sandblasted glass slide was laid on the sample and screwed down tight, thereby compressing the sample to a thickness of 1.67 mm. Photopolymerization was conducted through the glass slide for 40 s using a Translux EC polymerization unit (Heraeus Kulzer, Wehrheim, Germany); the use of a special aluminum shield maintained a distance of 1 mm between curing tip and slide, thus ensuring that no pressure was exerted upon the latter. Polymerization shrinkage lifted the aluminum disk toward the immobile glass slide, and an infrared micrometer measured the distance moved every 0.5 s for 300 s. Linear shrinkage was displayed in micrometer for the entire measurement period to generate a continuous shrinkage/time curve. 2.1.2. Buoyancy method (Archimedes` principle) First, the density of the unpolymerized (unset) material was determined as follows: a piece of nylon thread about 10 cm long (B ˆ 0.1 mm) (Migros, Switzerland) was weighed, and a loop was tied at one end. Approximately 1.5±2 g of material were shaped into a ball and hung by the thread from a metal frame on a microbalance (Mettler AE 200, NaÈnikon, Switzerland) to determine the exact weight. Subsequently, the sample on the string was submersed in a water-®lled beaker which sat on a bridge

M. Rosin et al. / Dental Materials 18 (2002) 521±528

two densities [Eq. (2)]:   densityunset £ 100% DV ˆ 1 2 1 2 densityset

523

…2†

Recently, a more easy way to determine polymerization shrinkage with the bouancy method using a simpler equation was published [20].

Fig. 2. Specimen for the microleakage study with four levelled surfaces and cavity preparations in exposed dentin.

over the balance's pan, thus not touching it. The weight of the sample with the nylon thread was determined in water. In addition, the temperature of the water was checked to be constant with a mercury thermometer after each measurement (DT ˆ 0.18C). The changes in weight on immersion in water were used to calculate the density of the material when unset and when set [Eq. (1)]. The calculation of the density of the set composite was analogous to that of the unset composite. ! munset1thread dry 2 mthread Densityunset material ˆ mpaste dry 2mpaste water £ densitywater

…1†

where munset1thread dry is the weight of unset material on thread in air; mthread is the weight of nylon thread; munset dry is the weight of unset material in air (minus thread); munset water is the weight of unset material in water; densitywater is the density of water at temperature measured. Six disk-shaped specimens of diameter 20 mm and height 1.5 mm with a de®ned volume (471 mm 3) were made from each material. Embedded in each disk was a nylon thread by which it could be hung. In order to produce homogeneous disks, the moulds ®lled to over¯owing with material are placed between two polyester strips (BuÈtz Folie, Taunusstein, Germany) supported by two ¯at metal plates. The upper plate was pressed toward the base in a press (Pugliese, Cuneo, Italy). The samples were then light cured for 3 min in a light oven (Spektramat, Ivoclar, Liechtenstein) from both sides through the polyester strips. After removal from the mould and trimming off any ¯ash, the weight of the samples was determined in air and then water, as described above. Subsequently, the samples were stored in an incubator (Ehret, Renggli, Switzerland) at 378C. Twenty-four hours later, the dry weight, the weight in water, and the water temperature were recorded according to the procedures mentioned above. Finally, polymerization shrinkage (DV) was calculated from the quotient of the

2.1.3. Dilatometer Volumetric shrinkage was measured with a dilatometer according to the method described by Reed et al. [21]. Three measurements per material were conducted. First, the samples were weighed (about 0.1 g) with a microbalance (AE 200, Mettler, NaÈnikon, Switzerland), and these data along with the already determined density of the composite paste were entered into the computer`s software program. The samples were placed on a glass plate which was clamped to the underside of the dilatometer's socket or funnel. This socket was then ®lled with mercury up to a given mark. The plunger, which transmits displacement to a linear variable displacement transducer (LVDT), was placed so that it ¯oated on the mercury. Following an adjustment phase of ca 7 min, the device automatically began registering measurements. The sample was light cured for 40 s through the glass plate using the dilatometer's built-in polymerization lamp (Heliolux DLX, Vivadent, Schaan, Liechtenstein). Polymerization shrinkage was calculated by the integrated software. Data were recorded continuously as a volume-shrinkage/time curve for the entire period of measurement (60 min). 2.2. Microleakage study Extracted caries-free human third molars were thoroughly cleaned of debris (curettes, pumice, water) and stored in 1% chloramine solution in a refrigerator until ready for use. A maximum storage time of 6 weeks was allowed. The teeth were embedded in a clear, self-curing acrylic resin (Technovit, Heraeus Kulzer, Wehrheim, Germany). Subsequently, the embedded teeth were abraded (360-grit SiC abrasive paper, Wirtz Buehler, DuÈsseldorf, Germany) with a wet abrasion machine (TF 250, Jean Wirtz, DuÈsseldorf, Germany), so that four level surfaces were produced. Upon each of those surfaces, the attempt was made to expose an area of dentin that was at least 4 mm in diameter. On suitable dentin surfaces, cavity preparations were made with a CAD/CAM milling machine (Isert-Electronic, Eiterfeld, Germany) using cylindershaped 30-mm grade diamond burs 1.4 mm in diameter (No. 806 314 15 65 14, Komet, Lemgo, Germany) at 5000 rpm with constant water cooling (Fig. 2). The bur was replaced after every tenth preparation. The preparations had a diameter of 3 mm and a depth of 1.5 mm. The margins were not treated further, so they had a 908 cavo-surface angle. As a rule, four cavities were prepared in each specimen; that is, one cavity on each side (Fig. 2). Cavities with

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Table 1 Overview of materials studied (R, restorative material; DBA, dental bonding agent) Material

Type

Manufacturer

Batch number

Tetric ceram Syntac classic Solitaire Gluma Solid Bond Solitaire 2 Gluma Solid Bond De®nite Etch & Prime 3.0 Degu®ll sc microhybrid ART Bond

R DBA R DBA R DBA R DBA R DBA

Vivadent a Vivadent Heraeus b Heraeus Heraeus Heraeus Degussa c Degussa Degussa ColteÁne d

926 797 927 163 33 19 00 27/21c 210 4302 5555 311 GG 417

a b c d

Vivadent, Schaan, Liechtenstein. Heraeus Kulzer, Dormagen, Germany. Degussa Dental, Hanau, Germany. ColteÁne, AltstaÈtten, Switzerland.

pulpal exposure (checked with 5 £ magni®cation loupes) were not used. Between abrasion of the teeth and ®lling of the cavities, the specimens were stored in saline. One hundred sixty cavity preparations were assigned to four groups of equal size. Each group was further divided into ®ve material subgroups of eight cavities each. The assignment of materials (Table 1) to the cavities was determined by a random number table. Groups 1 and 2 were restored with the restorative materials without using dental bonding agents (DBA). The respective materials were applied in one increment. Cavities were slightly over®lled, ®rmly pressed with a polyester strip (Hawe Neos, Bioggio, Switzerland) and photopolymerized for 40 s (Translux, Heraeus Kulzer, Hanau, Germany). In the Degu®ll sc microhybrid group, polymerization was allowed to take place for 10 min. Excess material was removed and surfaces were polished using 360-grit and subsequently 1200-grit SiC abrasive paper on the wet abrasion machine. In groups 3 and 4, the restorative materials were used in combination with the corresponding dental bonding systems. The dental bonding systems were applied according to the manufacturers` instructions and light cured for the recommmended time using the Translux light-curing unit. The ®lling and ®nishing procedure was essentially the same as in groups 1 and 2. The time elapsed between restoration and evaluation of microleakage was 7 days. To avoid swelling of the restorative materials due to water sorption, the specimens were not stored in water; all groups were instead kept in an incubator at 378C. To avoid loss of moisture during storage or water sorption during thermocycling, all specimens were individually sealed in water-tight, vacuum-sealed plastic bags. On day 5, groups 2 and 4 were removed from the incubator and subjected to 2000 thermocycles between 5 and 558. Dwell time in each temperature was 30 s, with a 15 s transfer time between baths. Following storage and thermocycling, the specimens were coated with nail polish except for 1 mm

around the restoration, making sure that the margin between embedding material and dentin surface was sealed. The specimens were then immersed in a 2% aqueous methyleneblue solution for 90 min and rinsed with running water for 1 min. The milling machine with a 30-mm grade diamond instrument (No. 806 314 15 65 14, Komet, Lemgo, Germany) was used to prepare (constant water cooling) 5 mm diameter cavities into the restorations and surrounding dentin exposing the interface at 200, 400 and then 600 mm depths. The margins of the 3 mm diameter restorations in the respective planes were fully visible and photographs of the three planes were taken with an optical microscope (BX 60, Olympus, Japan) at 3.5 £ magni®cation. The slides were digitized with a ®lm scanner (Canoscan 2700F, Canon, Japan), and dye penetration was assessed using a standard software package (Adobe photoshop). Stained margins were measured in arc-degrees of a full circle (Fig. 3). 2.3. Statistical analysis The data from the three methods of measuring polymerization shrinkage were analyzed with ANOVA. The Tukey HSD adjustment for multiple comparisons (signi®cance level a ˆ 0.05) was applied for the linometer and the buoyancy method. For the dilatometer data, Tamhane's T3 was used as a post-hoc test for pairwise comparisons, because unequal variances were present. The microleakage study was analyzed using nonparametric tests because the data were not normally distributed. A global test (Kruskal±Wallis test) was performed ®rst. The Mann±Whitney test was used for pairwise comparisons between materials within the four groups. Here, the level of signi®cance was adjusted to a *(k) ˆ 1 2 (1 2 a ) 1/k (where k ˆ the number of performed tests) by application of the error-rates method [22], which is a Bonferroni-type correction. The Mann±Whitney test was also performed to determine differences with respect to `treatment' between groups 1 and 2 and groups 3 and 4. Spearman's correlation coef®cients were calculated for aggregated shrinkage data and aggregated microleakage data from groups 3 and 4.

3. Results 3.1. Polymerization shrinkage measurements The results (means and standard deviations) of the three different methods of measuring polymerization shrinkage for the ®ve restorative materials are given in Table 2. Fig. 4 shows the dynamics of polymerization shrinkage of the various materials obtained from the linometer measurements. The results (p-values) of the statistical analysis of the shrinkage data obtained with the three different methods are summarized in Table 3.

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data for groups 3 and 4 (restorative materials with dental bonding agents). There was a tendency for Etch & Prime 3.0/De®nite to exhibit the least microleakage in group 3 (Table 5). When the specimens were subjected to thermocycling (group 4), there was a tendency for Solid Bond/ Solitaire 2 to perform best (Table 5). In the pairwise comparisons for the effect of thermocycling, signi®cant (a ˆ 0.05) differences were found for Solid Bond/Solitaire 2 at 400 mm and for Solid Bond/Solitaire and ART Bond/ Degu®ll sc microhybrid at 600 mm (Table 5). 3.3. Correlation analysis Fig. 3. Example from group 3 in the microleakage study. The photograph shows the 200 mm plane of a specimen with 54.7 arc degrees microleakage along the margin of the restoration.

Spearman's coef®cients between the aggregated polymerization shrinkage data from the three methods and the aggregated microleakage data from the three planes were 0.3 for group 3 and 20.2 for group 4.

3.2. Microleakage study The use of the restorative materials without the corresponding dental bonding agents (groups 1 and 2) resulted in microleakage along the entire cavity margins on the three planes investigated in nearly all specimes (Table 4). No differences between the materials or the treatments were observed (Table 4). Table 5 presents the microleakage

4. Discussion The formation of marginal gaps in direct resin restorations is mainly dependent on the polymerization shrinkage stresses of the restorative material, the quality and strength of the adhesive bond, and the con®guration of the cavity [23]. The aim of the present study was to assess the

Fig. 4. Dynamics of polymerization shrinkage of the materials investigated (data from the linometer measurements). The polymerization of Degu®ll sc microhybrid was not completed at the end of the measurements. Table 2 Mean polymerization shrinkage (standard deviations in parentheses) of the ®ve restorative materials measured with three different methods Material

Linometer n ˆ 5 (lin%)

Buoyancy method n ˆ 6 (vol%)

Dilatometer n ˆ 3 (vol%)

De®nite Tetric ceram Degu®ll sc mh a Solitaire 2 Solitaire

1.53 (0.15) 1.94 (0.04) ± 2.31 (0.03) 2.78 (0.18)

2.69 (0.11) 3.09 (0.13) 3.38 (0.23) 3.44 (0.17) 4.05 (0.16)

2.09 (0.08) 2.40 (0.03) 3.29 (0.88) 3.12 (0.07) 3.77 (0.06)

a

Values for the linometer not given because the material was not fully cured at the end of measurements.

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Table 3 Polymerization shrinkage measurements with the linometer, buoyancy method, and dilatometer. Results (P-values) of ANOVA for comparisons between pairs of materials Linometer a

De®nite Tetric Degu®ll Solitaire 2 Solitaire a b c

Buoyancy method b

De®nite

Tetric

± 0.000 ± 0.000 0.000

± ± 0.000 0.000

Degu®ll

± ± ±

Solitaire 2

De®nite

Tetric

± 0.000

± 0.002 0.000 0.000 0.000

± 0.035 0.009 0.000

Dilatometer c Degu®ll

± 0.977 0.000

Solitaire 2

De®nite

Tetric

Degu®ll

Solitaire 2

± 0.000

± 0.036 0.469 0.000 0.000

± 0.659 0.003 0.000

± 1.000 0.941

± 0.002

ANOVA with Tukey HSD adjustment for multiple comparisons (signi®cance level a ˆ 0.05), R 2 ˆ 0.963. ANOVA with Tukey HSD adjustment for multiple comparisons (signi®cance level a ˆ 0.05), R 2 ˆ 0.899. ANOVA with Tamhane T3 adjustment for multiple comparisons (signi®cance level a ˆ 0.05), R 2 ˆ 0.777.

the materials was generated. The chemically curing composite, Degu®ll sc microhybrid, could not be measured with the linometer and produced the highest standard deviations in the dilatometer and buoyancy tests (Table 2), most probably due to an uncontrolled incorporation of air bubbles during mixing. It was thus somewhat dif®cult to relate the polymerization shrinkage values of Degu®ll sc microhybrid to those found for the four light-cured materials. Most laboratory studies of microleakage are based on a categorical scoring system in a single plane, or very few planes, perpendicular to the surface of the restoration [24]. However, examining so few arbitrarily chosen planes in a three-dimesional cavity presents dif®culties in making quantitative assessments. The method we developed involves horizontal planes in which the entire circumference of the cavity margin is evaluated. The third dimension is included by taking measurements at increasing distances from the ®lling's surface. A bulk-®lled Class I situation was simulated, since this cavity design has been shown to produce the highest curing stress based on a most unfavorable C-factor [15]. Although not clinically relevant, a direct relation between polymerization shrinkage and microleakage can only be studied if the restorative material is not bonded to the cavity walls. Therefore, in a ®rst set of experiments, cavities were restored without the use of dental bonding agents. However,

in¯uence of polymerization shrinkage on marginal gap formation by measuring polymerization shrinkage of the restorative materials and determining their microleakage in dentin-bordered cavities. It was, however, not practicable to measure both polymerization shrinkage and microleakage with the same specimens, forcing the use of less powerful unmatched statistical testing. Therefore, correlation coef®cients could only be calculated for aggregated shrinkage data and aggregated microleakage data. We therefore hoped to acquire additional information on the basis of a clear ranking of the materials in the polymerization shrinkage and in the microleakage measurements. In the present study, an obvious ranking for the restorative materials with respect to the polymerization shrinkage (PS) was obtained: PS De®nite , PS Tetric Ceram , PS Solitaire 2 , PS Solitaire. All three methods of measuring polymerization shrinkage generated the same hierarchy for the four light-cured restorative materials (Table 2). Furthermore, all differences observed between the four light-cured materials within the various methods were statistically signi®cant (Table 3). Our data for De®nite, Tetric Ceram, and Solitaire are comparable to other data yielded by the buoyancy method (De®nite 2.29, Tetric Ceram 3.03, Solitaire 3.62) [12,13]. Linometer measurements by Watts [14] obtained higher absolute values (De®nite 1.65, Tetric Ceram 2.79, Solitaire 3.3), although the same ranking within

Table 4 Use of the restorative materials without dental bonding agents. Median and interquartile range of dyed margin lengths at three depths from the surface of the restorations (in arc degrees 1/3608) Group

Treatment

Material

n

200 mm

400 mm

600 mm

1

Without thermocycling

2

With thermocycling

Tetric Ceram Solitaire De®nite Solitaire 2 Degu®ll sc mh Tetric Ceram Solitaire De®nite Solitaire 2 Degu®ll sc mh

7 6 7 6 6 6 8 7 6 7

360 (0) 360 (0) 360 (0) 360 (0) 360 (0) 360 (0) 360 (0) 360 (0) 360 (0) 360 (0)

360 (43) 360 (9.8) 360 (0) 360 (19) 360 (40.8) 360 (0) 360 (0) 360 (0) 360 (0) 360 (0)

360 (63) 360 (11.5) 360 (38) 360 (61) 360 (116) 360 (22.8) 360 (0) 360 (0) 360 (36.8) 360 (0)

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527

Table 5 Use of the restorative materials with the corresponding dentinal bonding agents. Median and interquartile range of dyed margin lengths at three depths from the surface of the restorations (in arc degrees 1/3608) [groups connected by a line are signi®cantly different from each other (a * ˆ 0.05); for groups marked with the same symbol there are signi®cant differences between treatments (a ˆ 0.05)] Group

Treatment

3

Without thermocycling

4

With thermocycling

Material Syntac/Tetric Ceram Solid Bond/Solitaire Etch 00 3.0/De®nite Solid Bond/Solitaire 2 ART Bond/Degu®ll sc mh Syntac/Tetric Ceram Solid Bond/Solitaire Etch 00 3.0/De®nite Solid Bond/Solitaire 2 ART Bond/Degu®ll sc mh

the extent of microleakage observed under these conditions exceeded the anticipated magnitude by far (Table 4). Nearly all specimens, even in the group without thermocycling, exhibited microleakage along the entire cavity margin (Table 4). Consequently, no differences between the materials were observed. In a second set of experiments, the restorative materials were bonded with dental adhesives. To avoid uncontrolled incompatibilities [18] and to more closely simulate the clinical application of the materials, we decided to use the corresponding dental bonding agent from the manufacturer of the given restorative material, rather than employ one dental bonding agent for all materials. De®nite exhibited the least microleakage in the group without thermocycling, while in the group with thermocycling, presumably more clinically relevant, Solitaire 2 tended to perform best. No further differences between the materials were observed. In most cases, the dental bonding agent was able to mask the differences in shrinkage stresses with respect to the detrimental effect on marginal intergrity. The relatively equal performance of the materials in the microleakage study despite signi®cant differences in polymerization shrinkage was also re¯ected in the low correlation coef®cients between the aggregated polymerization shrinkage data from the three methods and the aggregated microleakage data from the three planes (0.3 for group 3 and 20.2 for group 4). Interestingly, there were only a few differences between thermocycled and nonthermocycled specimens of the same material (Table 5). The only slight effect of thermocycling in this study is presumably related to the fact that water had no direct access to the specimens. However, contact with water had to be avoided since the resulting hygroscopic expansion would have reduced polymerization shrinkage [25,26] and would have caused stress relaxation [27] to an unknown degree. Light-cured resin composites undergo a more immediate and rapid polymerization than chemically cured resin composites [28]. In our study, the different shrinkage dynamics of the chemically-cured Degu®ll sc microhybrid and the light-cured materials were quite

n 6 7 7 6 6 8 6 7 6 7

200 mm 223 (54.2) 210 (85) 203 (49) 178 (64) 178 (72.2) 239 (47) 210 (37.8) 204 (23) 137 (120.2) 168 (81)

400 mm

600 mm 33

3 7 7 5

174 (41.5) 87 (81) 5 7 7 7 7 60 (130) 7 115 (51.2)* 5 103 (40) 187 (72) 179 (99.2) 167 (76) 75 (40.8)* 124 (67)

3 106 (66.2) §5 47 (109) 3 2 (8) 60 (86.8) 5 65 (18.5) # 170 (128.2) 162 (109) § 87 (156) 41 (74.2) 120 (95) #

graphically demonstrated with the plotted data from our linometer measurements (Fig. 4). Because of the rapid polymerization, light-cured composites do not exist as long in the gel phase as do chemically cured resin composites, and thus permit less resin ¯ow [28]. Theoretically, the lower the capacity to ¯ow, the greater the contraction stress will be. Consequently, it has been demonstrated that light-cured resin composites generate higher polymerization shrinkage stresses than the analogous chemically cured composites [28]. Therefore, in our study, less microleakage could have been expected for Degu®ll sc microhybrid as compared to light-cured materials with equal or greater polymerization shrinkage, namely Solitaire and Solitaire 2. However, no differences between Degu®ll sc microhybrid and Solitaire or Solitaire 2 were observed (Tables 4 and 5). Interestingly, in group 3 at the 400 mm plane, Degu®ll sc microhybrid exhibited less microleakage than a lightcured material with less polymerization shrinkage, Tetric Ceram (Table 5). This again points to the dental bonding agents as the predominant factor in¯uencing marginal integrity in our study design. Degu®ll sc microhybrid was used with a dental bonding agent from another manufacturer (ART Bond) because Etch & Prime 3.0 was the only available dental bonding agent from the same manufacturer (Degussa). However, Etch & Prime 3.0 is, according to the manufacturer, incompatible with chemically cured resins, presumably because of the low pH or the water content of this self-conditioning adhesive. It is clear that under the experimental conditions of this study, the exact in¯uence of the dental bonding agents cannot be estimated. Since the distinct hierarchy among the investigated restorative materials regarding polymerization shrinkage is not re¯ected by the microleakage data, it may be possible that the dental bonding agents applied were able to redirect curing strains. Therefore, within the limitations of this study, it can be concluded that the polymerization shrinkage data of a material cannot predict the marginal integrity of direct restorations placed with that material.

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