- Email: [email protected]
Polymerization contraction stress in light-cured packable composite resins
dental materials Dental Materials 17 (2001) 253±259
www.elsevier.com/locate/dental
Polymerization contraction stress in light-cured packable composite resins H.Y. Chen*, J. Manhart, R. Hickel, K.-H. Kunzelmann Department of Restorative Dentistry, Dental School of the Ludwig-Maximilians University, Goethe Street 70, 80336 Munich, Germany Received 30 November 1999; revised 5 June 2000; accepted 28 June 2000
Abstract Objective: Determination of the polymerization contraction stress of packable composites (ALERT, Sure®l, Solitaire, Solitaire 2) and a packable ORMOCER material (De®nite) in comparison with a conventional hybrid composite (Tetric Ceram). Methods: Contraction force generated by the test materials (10 replications each) was measured by polymerizing the composites ®lled in a plastic tray between two aluminum attachments mounted in a Stress±Strain-Analyzer testing machine (specimen size: 4 £ 4 £ 2 mm, Cfactor 0.33). Contraction force was recorded for 300 s under a standard exposure condition (40 s, 800 mW/cm 2). Maximum contraction stress (MPa), force rate (N/s), relative force rate (%/s) of each material were statistically analyzed by ANOVA a 0:05 and post-hoc Tukey's test. Results: Maximum contraction stresses of the packable materials were 4.60 ^ 0.32 MPa (ALERT), 4.16 ^ 0.18 MPa (De®nite), 3.36 ^ 0.08 MPa (Solitaire 2), 3.33 ^ 0.23 MPa (Solitaire) and 3.13 ^ 0.18 MPa (Sure®l), which were signi®cantly higher than that of Tetric Ceram (2.51 ^ 0.14 MPa). Tetric Ceram exhibited the signi®cantly lowest force rate. Force/time curves were S-shaped. Solitaire especially showed a longer pre-gelation phase before contraction force was recorded. Signi®cance: High contraction stress and rapid contraction force development can lead to failure of bond to tooth structure. This study suggested that, packable composite resins are less capable of reducing the contraction stress during the early setting stage, thus not superior in maintaining the bond with cavity walls to conventional hybrid composite Tetric Ceram. q 2001 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. Keywords: Polymerization contraction stress; Stress development; Packable composite; ORMOCER; Hybrid composite
1. Introduction Polymerization shrinkage is one of the most critical properties of esthetic resin-based restorative materials. The contraction stress build-up that occurs during polymerization, adversely affects the maintenance of the bonded interface between composite resins and dental hard tissues, can potentially cause debonding and lead to clinical failure of the restorations [1]. Postoperative pain, fracture of the tooth and opening of the margins of restorations, which may result in microleakage of ¯uids, staining of marginal gaps and recurrent caries, are the most addressed clinical problems caused by polymerization shrinkage [2]. The amount of curing contraction stress build-up generated by light-curing bonded composite resins in the cavity is thus thought to be an important factor in determining the longevity of the restorations. * Corresponding author. Tel.: 149-89-5160-7619; fax: 149-89-51605344. E-mail address: [email protected] (H.Y. Chen).
Composite resins shrink during polymerization, mainly because the monomeric units of the polymer are located within covalent bond distance of each other, which is closer to one another than they were in the original monomer state [3,4]. In general, a majority of the shrinkage can be resolved in the early plastic state (before the polymerization gel point) by ¯ow, or minimizing contraction stresses by allowing the composite volume to change shape. Following gel formation, the polymerization process is accompanied by a rapid increase in elastic modulus. Contraction stress build-up occurs since subsequent shrinkage is obstructed and the material is rigid enough to resist suf®cient plastic ¯ow to compensate for the original volume [5]. Restoring an adhesive cavity, the composite material is bonded to the walls of the rigid tooth structure and thus is restrained from changing shape, except at the free surface, and further internal stresses will result. The contraction stress of composites has been generally measured by means of a tensilometer [6] or with strain gages [7,8]. The rigid contraction stress build-up was found to be of
0109-5641/01/$20.00 + 0.00 q 2001 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. PII: S 0109-564 1(00)00079-8
254
H.Y. Chen et al. / Dental Materials 17 (2001) 253±259
Table 1 Investigated materials Categorization
Brand name
Shade Batch number Filler weight (%) a
Average ®ller size (mm) b
Manufacturer
Packable composites
Solitaire Solitaire 2 Sure®l ALERT
A20 A3 A2 A2
25 VP011298JU 980315 8409.71
65 75 b 82 c 84 d
0.7±20 0.7±20 b 0.8 c 0.7 (glass ®bers: 60±80) e
Heraeus Kulzer, Wehrheim, Germany Heraeus Kulzer, Wehrheim, Germany Dentsply DeTrey, Konstanz, Germany Jeneric/Pentron, Wallingford, CT, USA
ORMOCER
De®nite
A2
060698
77 f
1±1.5 f
Degussa AG, Hanau, Germany
905431
g
Hybrid composite a b c d e f g
Tetric Ceram
A2
79
1.0
g
Vivadent, Schaan, Liechtenstein
Heraeus Kulzer, BRO 06./98. Heraeus Kulzer, personal communication Sure®l, technical manual, Dentsply, 98-08-12. ALERT, brochure130M-1/98, Jeneric/Pentron. Ref. [29]. De®nite, technical manual, Degussa. Tetric Ceram, scienti®c documentation 1997, Vivadent.
crucial importance clinically [2]. The magnitude of contraction stress was determined not only to depend on the cavity con®guration factor (C-factor), which describes the ratio of the bonded to unbonded surface area of the restorations [6,9], but also on the nature of the shrinkage material, notably the viscous-elastic properties [5,10]. At a given con®guration factor, less rigid materials and slower polymerization reaction may provide better ability to undergo plastic ¯ow during the early phases of polymerization, which causes reduction in polymerization contraction stress and less damage at the adhesive interface [5,11±18]. Packable composites are a new class of highly ®lled composite resins with a ®ller distribution that gives them a different consistency compared with hybrid composites. They are mainly characterized by less stickiness or stiffer viscosity than conventional composites, and are therefore claimed for stress-bearing posterior restorations as an alternative to amalgam based on an application technique that somewhat resembles amalgam placement [19]. ALERT (Jeneric/Pentron, Wallingford, CT, USA), Sure®l (Dentsply, Konstanz, Germany), Solitaire and Solitaire 2 (Heraeus Kulzer, Wehrheim, Germany) are among the commercially available packable composites, sometimes called ªcondensableº composites. Another packable resin material De®nite (Degussa AG, Hanau, Germany) is based on the ORMOCER technology. Instead of traditional monomer systems containing Bis-GMA, UDMA and TEGDMA, multifunctional urethane- and thioether(meth)acrylate alkoxysilanes as sol±gel precursors have been developed for synthesis of inorganic±organic copolymer ormocer composites as dental restorative materials. The alkoxysilyl groups of the silane allow the formation of an inorganic Si±O±Si-network by hydrolysis and polycondensation reactions, and the
(meth)acrylate groups are available for photochemically induced organic polymerization. After incorporation of ®ller particles, the ormocer composites can be manipulated by the dentist like a hybrid composite. ORMOCERs are characterized by this novel inorganic± organic copolymer in the formulation, which allow the modi®cation of mechanical parameters in a wide range [20±22]. Packable composites may allow a more convenient placement in posterior sites and may offer some technique advantages over conventional composites. However, their mechanical properties differ signi®cantly and are not consistently better than those of conventional hybrid composites [20]. The shrinkage properties of packable materials have been rarely reported. The purpose of this study was to investigate the polymerization contraction stress and the rate of contraction force development of packable composites (ALERT, Sure®l, Solitaire and Solitaire 2), an ORMOCER (De®nite), compared with a conventional hybrid composite (Tetric Ceram, Vivadent Schaan, Liechtenstein).
2. Materials and methods Four commercially available packable composites, an ORMOCER and a hybrid composite were tested in this study (Table 1). Wall-to-wall contraction was measured with a Stress± Strain-Analyzer (SSA T80, Engineering Consultancy, Peter Dullin, Jr., Munich, Germany) (Fig. 1). The experimental setup consisted of two opposing, identical, parallel aluminum stub attachments with a de®ned distance of 4 mm, one of which was connected to a load sensor (2000 N) and the other opposing to a Piezo-actuator
H.Y. Chen et al. / Dental Materials 17 (2001) 253±259
255
Fig. 1. Schematic illustration of the Stress±Strain-Analyzer (SSA T80) testing machine.
(Physik Instrumente PI GmbH and Co. Waldbronn, Germany) (Fig. 1). The Piezo-actuator served only as the holder for the aluminum attachments in the present setup. However, the piezo-actuator can ful®ll the compensation function of the test system in the setup by activating a corresponding measurement program. A plastic tray was ®xed on the testing machine, exactly holding the two attachments, building up a simulated cavity between the attachments (sample size: 4 mm (L) £ 4 mm (W) £ 2 mm (H)). The surfaces of the tray were covered with a thin PTFE ®lm applied to the unbonded surface area. The resin paste of the tested materials was inserted into the tray in a clinical manner in one increment. In order to be bonded to the attachments, the surfaces of the two attachments were pretreated with the ROCATEC-method (ESPE, Seefeld, Germany), followed by further application of a silane coupling agent (ESPE-Sil, ESPE, Seefeld, Germany) and left for 5 min in air before packing of the resin pastes into the tray. The con®guration factor C, being the ratio of bonded surface (the two attachments) to unbonded (free) surfaces, was 0.33 in this test setup, measured according to the method described by Feilzer et al. [6]. In addition, a compliance of the whole test setup including the compliance generated by the load cell and the aluminum attachments was determined to be 0.064 mm/N [23]. After squeezing out small amounts of excess paste, the composite resin was covered by a matrix to minimize air inhibition and polymerized for 40 s using a light-curing unit (Elipar Highlight, standard mode, ESPE). The intensity of the curing light was 800 mW/cm 2, veri®ed before the polymerization using a radiometer (Curing Radiometer Model 100, Demetron Research Corp., Danbury, USA). The contraction force
(N) generated by polymerizing the composite was continuously measured and recorded for 300 s after photo-initiation. Each experiment was conducted at room temperature (23±248C) and repeated 10 times for each material. A further conversion of recorded contraction force values (N) into contraction stress (MPa) was conducted. The mean values and standard deviations of 10 replications per material of the maximum contraction stress after 300 s were computed. By dividing the contraction force generated after 40 s light-exposure duration (N40 s) by the curing time (40 s), the force rate (N/s), as an indirect measure for the rate of polymerization shrinkage force development during the time period of lightcuring, was calculated and reported as mean values and standard deviations n 10 [24]. In addition, the relative force rate (%/s) was calculated by dividing the force rate after 40 s by the maximum force (recorded after 300 s). The relative force rate is an expression to present the development of contraction force in the period of light activation in relation to the maximum recorded force. Statistical analysis (SPSS 8.0, SPSS Inc., Chicago, IL) of contraction stress among the materials was carried out using one-way ANOVA. Post-hoc Tukey HSD multiple comparison tests were used to identify statistically homogeneous subsets a 0:05:
3. Results The mean values and standard deviations of the maximum contraction stresses after 300 s, force rates and relative force rates are listed in Table 2.
256
H.Y. Chen et al. / Dental Materials 17 (2001) 253±259
Fig. 2. Plot of contraction force vs. time for the tested materials within: (a) the whole observation period of 300 s; and (b) the ®rst 60 s of the observation period.
One-way ANOVA exhibited signi®cant differences p , 0:05 among the materials for the maximum contraction stresses as well as for the shrinkage force rates, and relative force rates. Pairwise multiple comparisons with Tukey's HSD test p , 0:05 revealed for ALERT and De®nite a signi®cantly higher maximum contraction stress (4.60 ^ 0.32 MPa, 4.16 ^ 0.18 MPa), force rate (0.68 ^ 0.04 N/s, 0.60 ^ 0.04 N/s) and relative force rate (1.85 ^ 0.12%/s, 1.81 ^ 0.15%/s), respectively, than for all other materials. No statistically significant differences in the maximum contraction stress of Sure®l, Solitaire and Solitaire 2 were found. The hybrid composite Tetric Ceram exhibited the signi®cantly lowest contraction stress and force rate compared with
all other tested materials. Solitaire 2 and Tetric Ceram revealed the lowest relative force rate (Table 2). Fig. 2a shows the contraction force vs. time curves for each tested material within the whole observation period of 300 s. Fig. 2b depicts the ®rst 60 s of the observation period in greater detail. All curves were generally S-shaped. ALERT and De®nite showed a similar trend towards faster polymerization kinetics and contraction force build-up. The contraction forces of ALERT and De®nite, immediately after the start of the light-exposure, showed a rapid increase in force amount. Sure®l and Solitaire 2 exhibited curves similar in appearance. The curve of Tetric Ceram was similar to those of Solitaire 2 and Sure®l during the early stage
Table 2 Maximum contraction stresses after 300 s, rates of contraction force development during light-curing (force rate) and force rates in percentage of maximum contraction forces (relative force rates) are detailed in mean values and standard deviations (in parentheses). Superscript letters indicate statistically homogeneous subsets (Tukey HSD test, a 0:05
Maximum contraction stress (MPa) Force rate (N/s) Relative force rate (%/s)
ALERT
Sure®l
Solitaire
Solitaire 2
De®nite
Tetric Ceram
4.60 (0.32) d 0.68 (0.04) e 1.85 (0.12) c
3.13 (0.18) b 0.39 (0.02) b 1.56 (0.06) a,b
3.33 (0.23) b 0.43 (0.04) c 1.64 (0.22) b
3.36 (0.08) b 0.39 (0.02) b 1.45 (0.06) a
4.16 (0.18) c 0.60 (0.04) d 1.81 (0.15) c
2.51 (0.14) a 0.30 (0.01) a 1.48 (0.10) a,b
H.Y. Chen et al. / Dental Materials 17 (2001) 253±259
of light curing, but tended towards a lower setting force level. Solitaire showed a distinct delay before the forces were recorded. 4. Discussion Along with the polymerization process, shrinkage stress build-up occurs after the material acquires stiffness [7,8]. In an adhesive restoration, further internal stresses will result due to restrained contraction by the bonded surfaces. The con®guration factor of the cavity is thus an important factor in the determination of the contraction stress in composite resins [6,9]. In this study, composite resins were inserted into a simulated cavity, sized 4 mm (L) £ 4 mm (W) £ 2 mm (H), surrounded by surfaces of the two aluminum attachments and the plastic tray. The thickness (H) of the composite material was 2 mm, which aimed to simulate a clinical increment. The surfaces of the aluminum attachments were silanized before each test to achieve an ef®cient bond. The Cfactor was de®ned at a low value of 0.33 for the present study. Feilzer emphasizes the importance of considering the effect of the compliance of the test setup and the compliance of the load-cell during stress measurement [6]. In this experimental setup with a very low C-factor of 0.33 (linear length of the specimen between two aluminum attachments was 4 mm), total compliance of the system, including load sensor and aluminum attachments, was determined to be 0.064 mm/N [23]. The Stress±Strain-Analyzer (SSA T80, Engineering Consultancy, Peter Dullin, Jr., Munich, Germany) has the option of measure stresses with and without compliance compensation by means of the Piezo-actuator using different measurement programs. The present data were recorded without compliance compensation because in a pilot study no differences could be observed in the mean stress values between the two modes of testing, which can be attributed to the stiff test setup (load cell 2000 N) with the same C-factor. Due to the increased noise level when using compliance compensation we decided to use the non-compensation mode for this experiment. Suf®cient contraction force generated by polymerizing the composite resins was measured and the contraction kinetics was represented by the contraction force vs. time curve (Fig. 2a and b). It is important, however, to consider compliance compensation if the C-factor is high or several test molds with different con®guration factors are compared. Resin systems shrink during polymerization mainly because the formation of a macromolecular network from discrete monomer species involves conversion of intermolecular distances of 0.3±0.4 nm into primary, covalent bonds with lengths of about 0.15 nm [4]. The polymerization of the resin matrix produces a gelation
257
in which the restorative material is transformed from a viscous-plastic phase with ¯ow into a rigid-elastic phase. Shrinkage that occurs in a cavity before the gel point is reached, while the monomer±polymer is still ¯owable, can be partially compensated by movement of molecules of the resin composite from the free surfaces of the restoration. This mode of compensation is not possible after gelation and, consequently, large stresses are built up in the composite. The amount of contraction stress has been determined to be dependent on the extent of the reaction, the stiffness of the composite and its ability to ¯ow previously [5,7,8,12,13]. Less rigid materials were observed to be better capable of reducing the contraction stresses than rigid materials [5,10]. In the present study, the results of the contraction stress determined were generally in agreement with the previous ®ndings. ALERT exhibited, signi®cantly, the highest maximum contraction stress of all tested materials. Similar results have been reported recently in a study on polymerization shrinkage force of condensable composites [24]. The high contraction stress generated by ALERT is likely to be related to the high ®ller load (Table 1) and the high elastic modulus [20] of this material. ALERT is reinforced by largesized glass-®bers (60 mm (L)) which highly in¯uence its physical and mechanical properties [20]. Great setting stress could be generated accompanying the rapid gain of elastic modulus following gel formation. Compared with ALERT, Sure®l exhibited a signi®cantly lower contraction stress corresponding to its lower elastic modulus [20]. However, the highly ®lled small-sized interlocking ®ller particles (0.8 mm average) may, to some extent, obstruct the composite to change shape during polymerization, resulting in an overall higher stress build-up than the hybrid composite Tetric Ceram. On the other hand, the resin matrix also has an important in¯uence on the properties of the composite materials, besides the ®ller system. Monomer structures of the resin matrix are important factors that in¯uence polymerization contraction of the composite resins [4]. The ORMOCER De®nite showed a high contraction stress in the study although it has relatively lower elastic modulus [20]. De®nite owns a resin system with inorganic±organic copolymers. This more rigid matrix of De®nite with high molecular weight molecules may allow less contraction stress compensation by ¯ow resulting in a high amount of residual rigid contraction stress. The rate of force development was represented as force rate and relative force rate (Table 2). The two representation modes of force development exhibited similar trends with ALERT and De®nite, revealing the most rapid force build-up. Solitaire 2 exhibited the lowest relative force rate that differed from the rank of its force rate in the study. The contraction force kinetics observed in this study were in agreement with
258
H.Y. Chen et al. / Dental Materials 17 (2001) 253±259
the previously described ®ndings on either the measurement of free shrinkage or the observation of contraction stress [6,11,12,25±27]. The force/time curves generally were S-shaped (Fig. 2a and b) within the light-exposure duration. Because of the effects of temperature changes due to the start and stop of the light-curing device, a depression at the start (indicating a slight expansion of test material because of temperature increase) and a great force increase at the end of light-curing (indicating a further contraction of the test material because of temperature decrease) were observed in the curves (Fig. 2b). ALERT and De®nite are assumed to be more rigid with the greatest restriction for the materials to change shape. The curves of these two materials showed only a short pre-gelation phase before the force was recorded, followed by a steep force growth and great residual contraction stress at the end. Solitaire especially showed a distinct delay before the contraction force was recorded (longer pre-gelation phase). Solitaire contains a ®ller system with porous SiO2 ®ller particles (32 wt.%), besides traditional Ba±Al±B±F±Si-glass ®llers (32.7 wt.%), which are claimed to integrate parts of the polymer matrix into the porosities of the ®ller particles. It is hypothesized that the porous ®ller particles in Solitaire, like voids and porosities in a material [11], may induce a delay in setting with an extended ¯ow phase so that a soft-start-like contraction kinetics was observed. However, it should be realized that high porosity may have a negative effect on the mechanical properties [28]. Signi®cantly lower values for physical and mechanical parameters were reported for Solitaire compared with other packable composite materials [20]. Solitaire 2 contains a distinctly reduced amount of porous ®ller particles (17 wt.%), besides traditional Ba±Al±B±F±Si-glass ®llers (58 wt.%), and is claimed by the manufacturer for improved mechanical properties. The packable resin-based materials ALERT, De®nite, Sure®l, Solitaire and Solitaire 2 exhibited signi®cantly higher maximum contraction stress and higher rate of contraction force development than the conventional hybrid composite Tetric Ceram. Tetric Ceram can be better in reducing the contraction stresses during the early setting stage than the packable composite resins. The moderate contraction force development of Tetric Ceram may bene®t the bond with the cavity walls.
References [1] Lutz F, Krejci I, Barbakow F. Quality and durability of marginal adaptation in bonded composite restorations. Dent Mater 1991;7:107±13. [2] Bausch JR, de Lange K, Davidson CL, Peters A, de Gee AJ. Clinical signi®cance of polymerization shrinkage of composite resins. J Prosthet Dent 1982;48:59±67.
[3] Venhoven BA, de Gee AJ, Davidson CL. Polymerization contraction and conversion of light-curing BisGMA-based methacrylate resins. Biomaterials 1993;14:871±5. [4] Peutzfeld A. Resin composites in dentistry: the monomer systems. Eur J Oral Sci 1997;105:97±116. [5] Davidson CL, Feilzer AJ. Polymerization shrinkage and polymerization shrinkage stress in polymer-based restoratives. J Dent 1997;25:435±40. [6] Feilzer AJ, de Gee AJ, Davidson CL. Setting stress in composite resin in relation to con®guration of the restoration. J Dent Res 1987;66:1636±9. [7] Sakaguchi RL, Sasik CT, Bunczak MA, Douglas WH. Strain gauge method for measuring polymerization contraction of composite restoratives. J Dent 1991;19:312±6. [8] Sakaguchi RL, Peters MC, Nelson SR, Douglas WH, Poort HW. Effects of polymerization contraction in composite restorations. J Dent 1992;20:178±82. [9] Choi KK, Condon JR, Ferracane JL. The effect of adhesive thickness on polymerization contraction stress of composite. J Dent Res 2000;79:812±7. [10] Dauvillier BS, Feilzer AJ, de Gee AJ, Davidson CL. Visco-elastic parameters of dental restorative materials during setting. J Dent Res 2000;79:818±23. [11] Feilzer AJ, de Gee AJ, Davidson CL. Setting stresses in composites for two different curing modes. Dent Mater 1993;9:2±5. [12] Feilzer AJ, de Gee AJ, Davidson CL. Quantitative determination of stress reduction by ¯ow in composite restorations. Dent Mater 1990;6:167±71. [13] Davidson CL, de Gee AJ. Relaxation of polymerization contraction stresses by ¯ow in dental composites. J Dent Res 1984;63:146±8. [14] Mehl A, Hickel R, Kunzelmann KH. Physical properties and gap formation of light-cured composites with and without softstart-polymerization. J Dent 1997;25:321±30. [15] Feilzer AJ, Dooren LH, de Gee AJ, Davidson CL. In¯uence of light intensity on polymerization shrinkage and integrity of restorationcavity interface. Eur J Oral Sci 1995;103:322±6. [16] Friedl KH, Schmalz G, Hiller K, MaÈrkl A. Marginal adaptation of Class V restorations with and without softstart-polymerization. Oper Dent 1999;25:26±32. [17] Goracci G, Mori G, de'Martinis LC. Curing light intensity and marginal leakage of resin composite restorations. Quintessence Int 1996;27:355±62. [18] Uno S, Asmussen E. Marginal adaptation of a restorative resin polymerized at reduced rate. Scand J Dent Res 1991;99:440±4. [19] Leinfelder KF, Bayne SC, Swift Jr. EJ. Packable composites: overview and technical considerations. J Esthet Dent 1999;11:234±49. [20] Manhart J, Kunzelmann KH, Chen HY, Hickel R. Mechanical properties and wear behavior of light-cured packable composite resins. Dent Mater 2000;16:33±40. [21] Wolter H, Storch W, Ott H. Dental ®lling materials (posterior composites) based on inorganic/organic copolymers (ORMOCERs). MACRO AKRON 1994;35:503. [22] Wolter H, Storch W, Ott H. New inorganic/organic copolymers (ORMOCERs) for dental applications. Mater Res Soc Symp Proc 1994;346:143±9. [23] Dullin P. Development of a measuring system for the determination of the polymerization behavior of dental composite restorative materials. (Entwicklung eines Mess-Systems zur Untersuchung des Polymerisationsverhaltens von zahnmedizinischen KompositfuÈllungswerkstoffen.) Thesis in Engineering Technology (Feinwerk und Mikrotechnik) University of Munich, 1998. p. 32±3 [24] Pearson JD, Bouschlicher MR, Boyer DB. Polymerization shrinkage forces of condensable composites. J Dent Res 1999;78:483 Abstract No. 3017. [25] Watts DC, Cash AJ. Determination of polymerization shrinkage in
H.Y. Chen et al. / Dental Materials 17 (2001) 253±259 visible-light-cured materials: methods development. Dent Mater 1991;7:281±7. [26] Labella R, Lambrechts P, Van Meerbeek B, Vanherle G. Polymerization shrinkage and elasticity of ¯owable composites and ®lled adhesives. Dent Mater 1999;15:128±37. [27] Bouschlicher MR, Rueggeberg FA, Boyer DB. Effect of stepped light
259
intensity on polymerization force and conversion in a photoactivated composite. J Esthet Dent 2000;12:23±32. [28] de Gee AJ. Some aspects of vacuum mixing of composite resins and its effect of porosity. Quintessence Int 1979;7:69±74. [29] Farah JW, Powers JM, editors. Condensable composites. Dent Advisor 1998;15:1±4.
www.elsevier.com/locate/dental
Polymerization contraction stress in light-cured packable composite resins H.Y. Chen*, J. Manhart, R. Hickel, K.-H. Kunzelmann Department of Restorative Dentistry, Dental School of the Ludwig-Maximilians University, Goethe Street 70, 80336 Munich, Germany Received 30 November 1999; revised 5 June 2000; accepted 28 June 2000
Abstract Objective: Determination of the polymerization contraction stress of packable composites (ALERT, Sure®l, Solitaire, Solitaire 2) and a packable ORMOCER material (De®nite) in comparison with a conventional hybrid composite (Tetric Ceram). Methods: Contraction force generated by the test materials (10 replications each) was measured by polymerizing the composites ®lled in a plastic tray between two aluminum attachments mounted in a Stress±Strain-Analyzer testing machine (specimen size: 4 £ 4 £ 2 mm, Cfactor 0.33). Contraction force was recorded for 300 s under a standard exposure condition (40 s, 800 mW/cm 2). Maximum contraction stress (MPa), force rate (N/s), relative force rate (%/s) of each material were statistically analyzed by ANOVA a 0:05 and post-hoc Tukey's test. Results: Maximum contraction stresses of the packable materials were 4.60 ^ 0.32 MPa (ALERT), 4.16 ^ 0.18 MPa (De®nite), 3.36 ^ 0.08 MPa (Solitaire 2), 3.33 ^ 0.23 MPa (Solitaire) and 3.13 ^ 0.18 MPa (Sure®l), which were signi®cantly higher than that of Tetric Ceram (2.51 ^ 0.14 MPa). Tetric Ceram exhibited the signi®cantly lowest force rate. Force/time curves were S-shaped. Solitaire especially showed a longer pre-gelation phase before contraction force was recorded. Signi®cance: High contraction stress and rapid contraction force development can lead to failure of bond to tooth structure. This study suggested that, packable composite resins are less capable of reducing the contraction stress during the early setting stage, thus not superior in maintaining the bond with cavity walls to conventional hybrid composite Tetric Ceram. q 2001 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. Keywords: Polymerization contraction stress; Stress development; Packable composite; ORMOCER; Hybrid composite
1. Introduction Polymerization shrinkage is one of the most critical properties of esthetic resin-based restorative materials. The contraction stress build-up that occurs during polymerization, adversely affects the maintenance of the bonded interface between composite resins and dental hard tissues, can potentially cause debonding and lead to clinical failure of the restorations [1]. Postoperative pain, fracture of the tooth and opening of the margins of restorations, which may result in microleakage of ¯uids, staining of marginal gaps and recurrent caries, are the most addressed clinical problems caused by polymerization shrinkage [2]. The amount of curing contraction stress build-up generated by light-curing bonded composite resins in the cavity is thus thought to be an important factor in determining the longevity of the restorations. * Corresponding author. Tel.: 149-89-5160-7619; fax: 149-89-51605344. E-mail address: [email protected] (H.Y. Chen).
Composite resins shrink during polymerization, mainly because the monomeric units of the polymer are located within covalent bond distance of each other, which is closer to one another than they were in the original monomer state [3,4]. In general, a majority of the shrinkage can be resolved in the early plastic state (before the polymerization gel point) by ¯ow, or minimizing contraction stresses by allowing the composite volume to change shape. Following gel formation, the polymerization process is accompanied by a rapid increase in elastic modulus. Contraction stress build-up occurs since subsequent shrinkage is obstructed and the material is rigid enough to resist suf®cient plastic ¯ow to compensate for the original volume [5]. Restoring an adhesive cavity, the composite material is bonded to the walls of the rigid tooth structure and thus is restrained from changing shape, except at the free surface, and further internal stresses will result. The contraction stress of composites has been generally measured by means of a tensilometer [6] or with strain gages [7,8]. The rigid contraction stress build-up was found to be of
0109-5641/01/$20.00 + 0.00 q 2001 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. PII: S 0109-564 1(00)00079-8
254
H.Y. Chen et al. / Dental Materials 17 (2001) 253±259
Table 1 Investigated materials Categorization
Brand name
Shade Batch number Filler weight (%) a
Average ®ller size (mm) b
Manufacturer
Packable composites
Solitaire Solitaire 2 Sure®l ALERT
A20 A3 A2 A2
25 VP011298JU 980315 8409.71
65 75 b 82 c 84 d
0.7±20 0.7±20 b 0.8 c 0.7 (glass ®bers: 60±80) e
Heraeus Kulzer, Wehrheim, Germany Heraeus Kulzer, Wehrheim, Germany Dentsply DeTrey, Konstanz, Germany Jeneric/Pentron, Wallingford, CT, USA
ORMOCER
De®nite
A2
060698
77 f
1±1.5 f
Degussa AG, Hanau, Germany
905431
g
Hybrid composite a b c d e f g
Tetric Ceram
A2
79
1.0
g
Vivadent, Schaan, Liechtenstein
Heraeus Kulzer, BRO 06./98. Heraeus Kulzer, personal communication Sure®l, technical manual, Dentsply, 98-08-12. ALERT, brochure130M-1/98, Jeneric/Pentron. Ref. [29]. De®nite, technical manual, Degussa. Tetric Ceram, scienti®c documentation 1997, Vivadent.
crucial importance clinically [2]. The magnitude of contraction stress was determined not only to depend on the cavity con®guration factor (C-factor), which describes the ratio of the bonded to unbonded surface area of the restorations [6,9], but also on the nature of the shrinkage material, notably the viscous-elastic properties [5,10]. At a given con®guration factor, less rigid materials and slower polymerization reaction may provide better ability to undergo plastic ¯ow during the early phases of polymerization, which causes reduction in polymerization contraction stress and less damage at the adhesive interface [5,11±18]. Packable composites are a new class of highly ®lled composite resins with a ®ller distribution that gives them a different consistency compared with hybrid composites. They are mainly characterized by less stickiness or stiffer viscosity than conventional composites, and are therefore claimed for stress-bearing posterior restorations as an alternative to amalgam based on an application technique that somewhat resembles amalgam placement [19]. ALERT (Jeneric/Pentron, Wallingford, CT, USA), Sure®l (Dentsply, Konstanz, Germany), Solitaire and Solitaire 2 (Heraeus Kulzer, Wehrheim, Germany) are among the commercially available packable composites, sometimes called ªcondensableº composites. Another packable resin material De®nite (Degussa AG, Hanau, Germany) is based on the ORMOCER technology. Instead of traditional monomer systems containing Bis-GMA, UDMA and TEGDMA, multifunctional urethane- and thioether(meth)acrylate alkoxysilanes as sol±gel precursors have been developed for synthesis of inorganic±organic copolymer ormocer composites as dental restorative materials. The alkoxysilyl groups of the silane allow the formation of an inorganic Si±O±Si-network by hydrolysis and polycondensation reactions, and the
(meth)acrylate groups are available for photochemically induced organic polymerization. After incorporation of ®ller particles, the ormocer composites can be manipulated by the dentist like a hybrid composite. ORMOCERs are characterized by this novel inorganic± organic copolymer in the formulation, which allow the modi®cation of mechanical parameters in a wide range [20±22]. Packable composites may allow a more convenient placement in posterior sites and may offer some technique advantages over conventional composites. However, their mechanical properties differ signi®cantly and are not consistently better than those of conventional hybrid composites [20]. The shrinkage properties of packable materials have been rarely reported. The purpose of this study was to investigate the polymerization contraction stress and the rate of contraction force development of packable composites (ALERT, Sure®l, Solitaire and Solitaire 2), an ORMOCER (De®nite), compared with a conventional hybrid composite (Tetric Ceram, Vivadent Schaan, Liechtenstein).
2. Materials and methods Four commercially available packable composites, an ORMOCER and a hybrid composite were tested in this study (Table 1). Wall-to-wall contraction was measured with a Stress± Strain-Analyzer (SSA T80, Engineering Consultancy, Peter Dullin, Jr., Munich, Germany) (Fig. 1). The experimental setup consisted of two opposing, identical, parallel aluminum stub attachments with a de®ned distance of 4 mm, one of which was connected to a load sensor (2000 N) and the other opposing to a Piezo-actuator
H.Y. Chen et al. / Dental Materials 17 (2001) 253±259
255
Fig. 1. Schematic illustration of the Stress±Strain-Analyzer (SSA T80) testing machine.
(Physik Instrumente PI GmbH and Co. Waldbronn, Germany) (Fig. 1). The Piezo-actuator served only as the holder for the aluminum attachments in the present setup. However, the piezo-actuator can ful®ll the compensation function of the test system in the setup by activating a corresponding measurement program. A plastic tray was ®xed on the testing machine, exactly holding the two attachments, building up a simulated cavity between the attachments (sample size: 4 mm (L) £ 4 mm (W) £ 2 mm (H)). The surfaces of the tray were covered with a thin PTFE ®lm applied to the unbonded surface area. The resin paste of the tested materials was inserted into the tray in a clinical manner in one increment. In order to be bonded to the attachments, the surfaces of the two attachments were pretreated with the ROCATEC-method (ESPE, Seefeld, Germany), followed by further application of a silane coupling agent (ESPE-Sil, ESPE, Seefeld, Germany) and left for 5 min in air before packing of the resin pastes into the tray. The con®guration factor C, being the ratio of bonded surface (the two attachments) to unbonded (free) surfaces, was 0.33 in this test setup, measured according to the method described by Feilzer et al. [6]. In addition, a compliance of the whole test setup including the compliance generated by the load cell and the aluminum attachments was determined to be 0.064 mm/N [23]. After squeezing out small amounts of excess paste, the composite resin was covered by a matrix to minimize air inhibition and polymerized for 40 s using a light-curing unit (Elipar Highlight, standard mode, ESPE). The intensity of the curing light was 800 mW/cm 2, veri®ed before the polymerization using a radiometer (Curing Radiometer Model 100, Demetron Research Corp., Danbury, USA). The contraction force
(N) generated by polymerizing the composite was continuously measured and recorded for 300 s after photo-initiation. Each experiment was conducted at room temperature (23±248C) and repeated 10 times for each material. A further conversion of recorded contraction force values (N) into contraction stress (MPa) was conducted. The mean values and standard deviations of 10 replications per material of the maximum contraction stress after 300 s were computed. By dividing the contraction force generated after 40 s light-exposure duration (N40 s) by the curing time (40 s), the force rate (N/s), as an indirect measure for the rate of polymerization shrinkage force development during the time period of lightcuring, was calculated and reported as mean values and standard deviations n 10 [24]. In addition, the relative force rate (%/s) was calculated by dividing the force rate after 40 s by the maximum force (recorded after 300 s). The relative force rate is an expression to present the development of contraction force in the period of light activation in relation to the maximum recorded force. Statistical analysis (SPSS 8.0, SPSS Inc., Chicago, IL) of contraction stress among the materials was carried out using one-way ANOVA. Post-hoc Tukey HSD multiple comparison tests were used to identify statistically homogeneous subsets a 0:05:
3. Results The mean values and standard deviations of the maximum contraction stresses after 300 s, force rates and relative force rates are listed in Table 2.
256
H.Y. Chen et al. / Dental Materials 17 (2001) 253±259
Fig. 2. Plot of contraction force vs. time for the tested materials within: (a) the whole observation period of 300 s; and (b) the ®rst 60 s of the observation period.
One-way ANOVA exhibited signi®cant differences p , 0:05 among the materials for the maximum contraction stresses as well as for the shrinkage force rates, and relative force rates. Pairwise multiple comparisons with Tukey's HSD test p , 0:05 revealed for ALERT and De®nite a signi®cantly higher maximum contraction stress (4.60 ^ 0.32 MPa, 4.16 ^ 0.18 MPa), force rate (0.68 ^ 0.04 N/s, 0.60 ^ 0.04 N/s) and relative force rate (1.85 ^ 0.12%/s, 1.81 ^ 0.15%/s), respectively, than for all other materials. No statistically significant differences in the maximum contraction stress of Sure®l, Solitaire and Solitaire 2 were found. The hybrid composite Tetric Ceram exhibited the signi®cantly lowest contraction stress and force rate compared with
all other tested materials. Solitaire 2 and Tetric Ceram revealed the lowest relative force rate (Table 2). Fig. 2a shows the contraction force vs. time curves for each tested material within the whole observation period of 300 s. Fig. 2b depicts the ®rst 60 s of the observation period in greater detail. All curves were generally S-shaped. ALERT and De®nite showed a similar trend towards faster polymerization kinetics and contraction force build-up. The contraction forces of ALERT and De®nite, immediately after the start of the light-exposure, showed a rapid increase in force amount. Sure®l and Solitaire 2 exhibited curves similar in appearance. The curve of Tetric Ceram was similar to those of Solitaire 2 and Sure®l during the early stage
Table 2 Maximum contraction stresses after 300 s, rates of contraction force development during light-curing (force rate) and force rates in percentage of maximum contraction forces (relative force rates) are detailed in mean values and standard deviations (in parentheses). Superscript letters indicate statistically homogeneous subsets (Tukey HSD test, a 0:05
Maximum contraction stress (MPa) Force rate (N/s) Relative force rate (%/s)
ALERT
Sure®l
Solitaire
Solitaire 2
De®nite
Tetric Ceram
4.60 (0.32) d 0.68 (0.04) e 1.85 (0.12) c
3.13 (0.18) b 0.39 (0.02) b 1.56 (0.06) a,b
3.33 (0.23) b 0.43 (0.04) c 1.64 (0.22) b
3.36 (0.08) b 0.39 (0.02) b 1.45 (0.06) a
4.16 (0.18) c 0.60 (0.04) d 1.81 (0.15) c
2.51 (0.14) a 0.30 (0.01) a 1.48 (0.10) a,b
H.Y. Chen et al. / Dental Materials 17 (2001) 253±259
of light curing, but tended towards a lower setting force level. Solitaire showed a distinct delay before the forces were recorded. 4. Discussion Along with the polymerization process, shrinkage stress build-up occurs after the material acquires stiffness [7,8]. In an adhesive restoration, further internal stresses will result due to restrained contraction by the bonded surfaces. The con®guration factor of the cavity is thus an important factor in the determination of the contraction stress in composite resins [6,9]. In this study, composite resins were inserted into a simulated cavity, sized 4 mm (L) £ 4 mm (W) £ 2 mm (H), surrounded by surfaces of the two aluminum attachments and the plastic tray. The thickness (H) of the composite material was 2 mm, which aimed to simulate a clinical increment. The surfaces of the aluminum attachments were silanized before each test to achieve an ef®cient bond. The Cfactor was de®ned at a low value of 0.33 for the present study. Feilzer emphasizes the importance of considering the effect of the compliance of the test setup and the compliance of the load-cell during stress measurement [6]. In this experimental setup with a very low C-factor of 0.33 (linear length of the specimen between two aluminum attachments was 4 mm), total compliance of the system, including load sensor and aluminum attachments, was determined to be 0.064 mm/N [23]. The Stress±Strain-Analyzer (SSA T80, Engineering Consultancy, Peter Dullin, Jr., Munich, Germany) has the option of measure stresses with and without compliance compensation by means of the Piezo-actuator using different measurement programs. The present data were recorded without compliance compensation because in a pilot study no differences could be observed in the mean stress values between the two modes of testing, which can be attributed to the stiff test setup (load cell 2000 N) with the same C-factor. Due to the increased noise level when using compliance compensation we decided to use the non-compensation mode for this experiment. Suf®cient contraction force generated by polymerizing the composite resins was measured and the contraction kinetics was represented by the contraction force vs. time curve (Fig. 2a and b). It is important, however, to consider compliance compensation if the C-factor is high or several test molds with different con®guration factors are compared. Resin systems shrink during polymerization mainly because the formation of a macromolecular network from discrete monomer species involves conversion of intermolecular distances of 0.3±0.4 nm into primary, covalent bonds with lengths of about 0.15 nm [4]. The polymerization of the resin matrix produces a gelation
257
in which the restorative material is transformed from a viscous-plastic phase with ¯ow into a rigid-elastic phase. Shrinkage that occurs in a cavity before the gel point is reached, while the monomer±polymer is still ¯owable, can be partially compensated by movement of molecules of the resin composite from the free surfaces of the restoration. This mode of compensation is not possible after gelation and, consequently, large stresses are built up in the composite. The amount of contraction stress has been determined to be dependent on the extent of the reaction, the stiffness of the composite and its ability to ¯ow previously [5,7,8,12,13]. Less rigid materials were observed to be better capable of reducing the contraction stresses than rigid materials [5,10]. In the present study, the results of the contraction stress determined were generally in agreement with the previous ®ndings. ALERT exhibited, signi®cantly, the highest maximum contraction stress of all tested materials. Similar results have been reported recently in a study on polymerization shrinkage force of condensable composites [24]. The high contraction stress generated by ALERT is likely to be related to the high ®ller load (Table 1) and the high elastic modulus [20] of this material. ALERT is reinforced by largesized glass-®bers (60 mm (L)) which highly in¯uence its physical and mechanical properties [20]. Great setting stress could be generated accompanying the rapid gain of elastic modulus following gel formation. Compared with ALERT, Sure®l exhibited a signi®cantly lower contraction stress corresponding to its lower elastic modulus [20]. However, the highly ®lled small-sized interlocking ®ller particles (0.8 mm average) may, to some extent, obstruct the composite to change shape during polymerization, resulting in an overall higher stress build-up than the hybrid composite Tetric Ceram. On the other hand, the resin matrix also has an important in¯uence on the properties of the composite materials, besides the ®ller system. Monomer structures of the resin matrix are important factors that in¯uence polymerization contraction of the composite resins [4]. The ORMOCER De®nite showed a high contraction stress in the study although it has relatively lower elastic modulus [20]. De®nite owns a resin system with inorganic±organic copolymers. This more rigid matrix of De®nite with high molecular weight molecules may allow less contraction stress compensation by ¯ow resulting in a high amount of residual rigid contraction stress. The rate of force development was represented as force rate and relative force rate (Table 2). The two representation modes of force development exhibited similar trends with ALERT and De®nite, revealing the most rapid force build-up. Solitaire 2 exhibited the lowest relative force rate that differed from the rank of its force rate in the study. The contraction force kinetics observed in this study were in agreement with
258
H.Y. Chen et al. / Dental Materials 17 (2001) 253±259
the previously described ®ndings on either the measurement of free shrinkage or the observation of contraction stress [6,11,12,25±27]. The force/time curves generally were S-shaped (Fig. 2a and b) within the light-exposure duration. Because of the effects of temperature changes due to the start and stop of the light-curing device, a depression at the start (indicating a slight expansion of test material because of temperature increase) and a great force increase at the end of light-curing (indicating a further contraction of the test material because of temperature decrease) were observed in the curves (Fig. 2b). ALERT and De®nite are assumed to be more rigid with the greatest restriction for the materials to change shape. The curves of these two materials showed only a short pre-gelation phase before the force was recorded, followed by a steep force growth and great residual contraction stress at the end. Solitaire especially showed a distinct delay before the contraction force was recorded (longer pre-gelation phase). Solitaire contains a ®ller system with porous SiO2 ®ller particles (32 wt.%), besides traditional Ba±Al±B±F±Si-glass ®llers (32.7 wt.%), which are claimed to integrate parts of the polymer matrix into the porosities of the ®ller particles. It is hypothesized that the porous ®ller particles in Solitaire, like voids and porosities in a material [11], may induce a delay in setting with an extended ¯ow phase so that a soft-start-like contraction kinetics was observed. However, it should be realized that high porosity may have a negative effect on the mechanical properties [28]. Signi®cantly lower values for physical and mechanical parameters were reported for Solitaire compared with other packable composite materials [20]. Solitaire 2 contains a distinctly reduced amount of porous ®ller particles (17 wt.%), besides traditional Ba±Al±B±F±Si-glass ®llers (58 wt.%), and is claimed by the manufacturer for improved mechanical properties. The packable resin-based materials ALERT, De®nite, Sure®l, Solitaire and Solitaire 2 exhibited signi®cantly higher maximum contraction stress and higher rate of contraction force development than the conventional hybrid composite Tetric Ceram. Tetric Ceram can be better in reducing the contraction stresses during the early setting stage than the packable composite resins. The moderate contraction force development of Tetric Ceram may bene®t the bond with the cavity walls.
References [1] Lutz F, Krejci I, Barbakow F. Quality and durability of marginal adaptation in bonded composite restorations. Dent Mater 1991;7:107±13. [2] Bausch JR, de Lange K, Davidson CL, Peters A, de Gee AJ. Clinical signi®cance of polymerization shrinkage of composite resins. J Prosthet Dent 1982;48:59±67.
[3] Venhoven BA, de Gee AJ, Davidson CL. Polymerization contraction and conversion of light-curing BisGMA-based methacrylate resins. Biomaterials 1993;14:871±5. [4] Peutzfeld A. Resin composites in dentistry: the monomer systems. Eur J Oral Sci 1997;105:97±116. [5] Davidson CL, Feilzer AJ. Polymerization shrinkage and polymerization shrinkage stress in polymer-based restoratives. J Dent 1997;25:435±40. [6] Feilzer AJ, de Gee AJ, Davidson CL. Setting stress in composite resin in relation to con®guration of the restoration. J Dent Res 1987;66:1636±9. [7] Sakaguchi RL, Sasik CT, Bunczak MA, Douglas WH. Strain gauge method for measuring polymerization contraction of composite restoratives. J Dent 1991;19:312±6. [8] Sakaguchi RL, Peters MC, Nelson SR, Douglas WH, Poort HW. Effects of polymerization contraction in composite restorations. J Dent 1992;20:178±82. [9] Choi KK, Condon JR, Ferracane JL. The effect of adhesive thickness on polymerization contraction stress of composite. J Dent Res 2000;79:812±7. [10] Dauvillier BS, Feilzer AJ, de Gee AJ, Davidson CL. Visco-elastic parameters of dental restorative materials during setting. J Dent Res 2000;79:818±23. [11] Feilzer AJ, de Gee AJ, Davidson CL. Setting stresses in composites for two different curing modes. Dent Mater 1993;9:2±5. [12] Feilzer AJ, de Gee AJ, Davidson CL. Quantitative determination of stress reduction by ¯ow in composite restorations. Dent Mater 1990;6:167±71. [13] Davidson CL, de Gee AJ. Relaxation of polymerization contraction stresses by ¯ow in dental composites. J Dent Res 1984;63:146±8. [14] Mehl A, Hickel R, Kunzelmann KH. Physical properties and gap formation of light-cured composites with and without softstart-polymerization. J Dent 1997;25:321±30. [15] Feilzer AJ, Dooren LH, de Gee AJ, Davidson CL. In¯uence of light intensity on polymerization shrinkage and integrity of restorationcavity interface. Eur J Oral Sci 1995;103:322±6. [16] Friedl KH, Schmalz G, Hiller K, MaÈrkl A. Marginal adaptation of Class V restorations with and without softstart-polymerization. Oper Dent 1999;25:26±32. [17] Goracci G, Mori G, de'Martinis LC. Curing light intensity and marginal leakage of resin composite restorations. Quintessence Int 1996;27:355±62. [18] Uno S, Asmussen E. Marginal adaptation of a restorative resin polymerized at reduced rate. Scand J Dent Res 1991;99:440±4. [19] Leinfelder KF, Bayne SC, Swift Jr. EJ. Packable composites: overview and technical considerations. J Esthet Dent 1999;11:234±49. [20] Manhart J, Kunzelmann KH, Chen HY, Hickel R. Mechanical properties and wear behavior of light-cured packable composite resins. Dent Mater 2000;16:33±40. [21] Wolter H, Storch W, Ott H. Dental ®lling materials (posterior composites) based on inorganic/organic copolymers (ORMOCERs). MACRO AKRON 1994;35:503. [22] Wolter H, Storch W, Ott H. New inorganic/organic copolymers (ORMOCERs) for dental applications. Mater Res Soc Symp Proc 1994;346:143±9. [23] Dullin P. Development of a measuring system for the determination of the polymerization behavior of dental composite restorative materials. (Entwicklung eines Mess-Systems zur Untersuchung des Polymerisationsverhaltens von zahnmedizinischen KompositfuÈllungswerkstoffen.) Thesis in Engineering Technology (Feinwerk und Mikrotechnik) University of Munich, 1998. p. 32±3 [24] Pearson JD, Bouschlicher MR, Boyer DB. Polymerization shrinkage forces of condensable composites. J Dent Res 1999;78:483 Abstract No. 3017. [25] Watts DC, Cash AJ. Determination of polymerization shrinkage in
H.Y. Chen et al. / Dental Materials 17 (2001) 253±259 visible-light-cured materials: methods development. Dent Mater 1991;7:281±7. [26] Labella R, Lambrechts P, Van Meerbeek B, Vanherle G. Polymerization shrinkage and elasticity of ¯owable composites and ®lled adhesives. Dent Mater 1999;15:128±37. [27] Bouschlicher MR, Rueggeberg FA, Boyer DB. Effect of stepped light
259
intensity on polymerization force and conversion in a photoactivated composite. J Esthet Dent 2000;12:23±32. [28] de Gee AJ. Some aspects of vacuum mixing of composite resins and its effect of porosity. Quintessence Int 1979;7:69±74. [29] Farah JW, Powers JM, editors. Condensable composites. Dent Advisor 1998;15:1±4.