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Polymerization shrinkage and hygroscopic expansion of contemporary posterior resin-based filling materials—A comparative study
journal of dentistry 35 (2007) 806–813
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Polymerization shrinkage and hygroscopic expansion of contemporary posterior resin-based filling materials—A comparative study Stefan Ru¨ttermann, So¨ren Kru¨ger 1, Wolfgang H.-M. Raab, Ralf Janda * Heinrich-Heine-University, Medical Faculty, Centre of Dentistry, Department of Operative and Preventive Dentistry and Endodontics, Moorenstr. 5, Geb. 18.13, D-40225 Du¨sseldorf, Germany
article info
abstract
Article history:
Objectives: To investigate the polymerization shrinkage and hygroscopic expansion of
Received 9 May 2007
contemporary posterior resin-based filling materials.
Received in revised form
Methods: The densities of SureFil (SU), CeramXMono (CM), Clearfil AP-X (CF), Solitaire 2 (SO),
7 July 2007
TetricEvoCeram (TE), and Filtek P60 (FT) were measured using the Archimedes’ principle
Accepted 29 July 2007
prior to and 15 min after curing for 20, 40 and 60 s and after 1 h, 24 h, 7 d, and 30 d storage at 37 8C in water. Volumetric changes (DV) in percent after polymerization and after each storage period in water were calculated from the changes of densities. Water sorption and
Keywords:
solubility were determined after 30 d for all specimens and their curing times. Two-way
Dentistry
ANOVA was calculated for shrinkage and repeated measures ANOVA was calculated for
Resin-based filling material
hygroscopic expansion ( p < 0.05).
Volumetric shrinkage
Results: DV depended on filler load but not on curing time (SU 2.0%, CM 2.6%,
Water sorption
CF 2.1%, SO 3.3%, TE 1.7%, FT 1.8%). Hygroscopic expansion depended on
Physical properties
water sorption and solubility. Except for SU, all materials showed DV +1% after water storage. Conclusion: Polymerization shrinkage depended on the type of resin-based filling material but not on curing time. Shrinkage was not compensated by hygroscopic expansion. # 2007 Elsevier Ltd. All rights reserved.
1.
Introduction
Although resin-based filling materials have improved greatly within the last decade with respect to their mechanical and esthetical properties, these materials still shrink significantly when polymerized.1–6 This property is still a challenging problem since in vitro studies show that the resulting shrinkage stress might be the main factor for adhesive failures, marginal disintegration and therefore, for recurrent caries.5–7 However, Sarret8 pointed out that the clinical
relevance of laboratory tests must be considered from the perspective of solving the remaining clinical challenges of current materials and of screening new materials. This review categorized the challenges as those related to the restorative materials, those related to the dentist, and those related to the patient and concluded that there is a general lack of data that correlates clinical performance with laboratory materials testing. Sarret even stated that there is a lack of evidence that indicates polymerization shrinkage is the primary cause of secondary caries. However, it is still commonly agreed that
* Corresponding author. Tel.: +49 6723 6020 750; fax: +49 211 81 19244. E-mail address: [email protected] (R. Janda). 1 Results are part of the thesis of S. Kru¨ger. 0300-5712/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jdent.2007.07.014
journal of dentistry 35 (2007) 806–813
resin-based filling materials, which are also increasingly used for posterior fillings that have significantly larger volumes than anterior ones, must be applied with the very time-consuming incremental technique to avoid marginal disintegration.7,9,10 Therefore, the extent of polymerization shrinkage gains even more importance. Furthermore, because of problems in adhering increments of the newly developed class of high viscosity posterior composite resins (packables), bulk-fill technique was recommended. However, the literature reported that despite the high filler content, the layering technique results in significantly less polymerization shrinkage.11–13 Reducing polymerization shrinkage significantly is still important to improve the quality of resin-based filling materials. Polymerization shrinkage depends on filler content,14,15 filler type16,17 and the composition of the resin matrix.6,7,18,19 In addition to the incremental technique there are several other approaches for minimizing shrinkage, such as different methods of light activation,9,20–23 to increase filler content significantly4,6,24–27,15 or the use of new, less shrinking monomers.19,28–30 The literature also reported that lower power densities were able to reduce the maximum polymerization rate and delay the formation of a rigid network. Conversion before the formation of a rigid network was also enhanced by using a lower power density. Considering that too premature gelation can lead to residual stress during shrinkage, the results indicated that the use of a lower power density can be effective in terms of delaying the onset of the formation of a rigid network, thus providing conditions for macromolecules to flow and relieve stress during shrinkage.31 Although the volumetric shrinkage values in cured composites were not affected by low light intensity, the contraction strain and polymerization’s exotherm were decreased.23 It was also shown that the degree of conversion and volumetric shrinkage showed a non-linear relationship to energy density. The degree of conversion showed a pronounced influence on stress. Increased inhibitor concentration reduced curing rate and contraction stress in composites, without compromising the final degree of conversion.32 Water sorption was also expected to compensate shrinkage to some extent by hygroscopic expansion, resulting in relaxation of internal stresses.33–36 But in general, water sorption is not desired since it causes an outward movement of residual monomers and ions as a consequence of solubility.37 Furthermore, absorbed water might weaken the material38 and excessive water uptake could even result in cuspal flexure or microcracks of the restored tooth.36 Furthermore, shrinkage occurs within seconds but water sorption takes days and did not even totally compensate shrinkage even after weeks.35 Thus, the positive influence of water sorption on marginal quality is very hypothetic. One goal of the present investigation was to investigate the volumetric polymerization shrinkages of contemporary posterior resin-based filling materials. Furthermore, the effects of curing time on shrinkage as well as of water sorption on hygroscopic expansion were examined. The working hypothesis was that (a) polymerization shrinkage depends on material, (b) curing time and shrinkage correlate and (c) hygroscopic expansion totally compensates shrinkage.
2.
807
Materials and methods
Six contemporary light curing resin-based posterior filling materials were chosen (Table 1) to investigate volumetric change as a function of curing time and water sorption. The tungsten halogen light Hilux Ultra Plus (Benlioglu Dental Inc. Ankara, Turkey) was used with the 11 mm diameter light guide and the constant polymerization mode (full light power from the start). Each time after a series of ten specimens was cured, the output of the curing device was checked with the Curing Light Meter (Benlioglu Dental Inc.). Irradiances between 750 and 850 mW cm2 (mean 800 67 mW cm2) were measured and no significant decrease of the output could be observed. Specimen preparation and tests were done at 22–23 8C and a relative humidity of 50% (room temperature: RT). The volumetric change was determined by measuring the density according to the Archimedes’ principle. This was done with the commercial Density Determination Kit of the analytical balance Mettler Toledo XS (Mettler Toledo GmbH, Greifensee, Switzerland). The kit provided a special holder for the specimens, a special container to keep the water, a computer, and appropriate software. The specimens were weighed in air and in water and the density was directly calculated in g cm3 by the software of the Mettler Toledo XS balance according to the equation: D¼
A ðD0 DL Þ þ DL AB
D is density of sample, A weight of sample in air, B weight of sample in water, D0 density of water at the exactly measured temperature in 8C according to the density table of distilled water, and DL is the air density (0.0012 g cm3). An internal balance correction factor (0.99985) of the Mettler Toledo XS balance software took air buoyancy of the adjustment weight into account.
2.1.
Determination of the polymerization shrinkage
From each test material ten uncured sphere-shaped specimens, each of approximately 0.1 g weight (CeramXMono = 0.1034 0.0042 g; Clearfil AP-X = 0.1283 0.0056 g; Filtek P60 = 0.1032 0.0145; Solitaire 2 = 0.1092 0.0056; SureFil = 0.1154 0.0066; TetricEvoCeram = 0.1180 0.0070), were carefully formed by hand in such a way that trapped air bubbles were avoided. Since the uncured materials were rather sticky, a thin polyester film (thickness 0.05) was fixed on the special holder of the balance and its mass was measured in air and in water. Next, the respective material sample was carefully placed on the polyester film and the mass of the whole assembly was measured again in air and in water. Slight deformations of the material during the test were of no importance since they do not influence the density. Furthermore, the weighing process was very fast (approximately 30–45 s), so that there was not very much time for the material to flow. The mass of each material was calculated by subtracting the mass of the polyester film from the mass of the whole assembly. Now the density of the uncured material (Dunpol) was computed. To measure the densities of the cured test materials, three groups of each test material were prepared, each comprised of 10 cylindrical specimens, 180 specimens in total. The specimens were prepared in a polyoxymethylene mould (dimension:
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journal of dentistry 35 (2007) 806–813
Table 1 – Test materials Material
Lot. no./Shade/ Literature
Filler load, approximately
Formulation
Manufacturer
mass% vol.% CeramXMono Microhybride composite with partial silicium-organically modified resin matrix (Nano-particle Hybride)
0510000198 M5 = A354–56
76
57
Resin matrix: methacrylate modified polysiloxane, dimethacrylate resin Inorganic filler: Ba–Al–borosilicate glass, pyrogenic SiO2 Photoinitiator: camphorquinone Synergist: ethyl-4-diemthylamino benzoate UV stabilizer Stabilizer: butylated hydroxy toluene
DeTrey Dentsply GmbH, Constance, Germany
Clearfil AP-X Microhybride (Hybride)
01122B A357,58
85.5
70
Resin matrix: Bis-GMA, Tegdma Inorganic filler: Ba-glass, silica, pyrogenic SiO2 Photoinitiator: camphorquinone Synergist: NI
Kuarary Co. Inc., Kurashiki, Japan
Filtek P60 Microhybride (Hybride)
6CK A359–62
84.0
62.34
Resin matrix: Bisphenol A Polyethylen Glycol Diether Dimethacrylate, UDMA, Bis-GMA, Bis-EMA, Tegdma Inorganic filler: zirconia, silica Photoinitiator: camphorquinone Synergist: NI
3 M Espe GmbH, Seefeld, Germany
Solitaire 2 Microhybride (Packable)
010257 A326,60,63,64
75
47.76
Resin matrix: Bis-GMA; HPMA, ETMA, PENTA Inorganic filler: Ba-Al-F-glass, Ba-glass, porous SiO2 Photoinitiator: camphorquinone Synergist: NI
Heraeus-Kulzer GmbH Hanau, Germany
SureFil High viscosity microhybride (Packable)
050802 C = A326,64–66
82
66
Resin matrix: Urethane modified Bis-GMA, Tegdma Inorganic filler: Ba-glass Photoinitiator: camphorquinone Synergist: NI
Dentsply/Caulk Inc., Milford, CT, USA
TetricEvoCeram Microhybride (Hybride)
H32690 A367,68
82.5
66
Resin matrix: UDMA, Bis-GMA, ethoxylated Bis-EMA Inorganic filler: Ba-glass, Al2O3, YbF3, pyrogenic SiO2, filler load: 48.5 mass-% Pre-polymer: filled with pyrogenic SiO2, filler load: 34.0 mass-% Photoinitiator: camphorquinone, diphenyl (2,4,6-trimethybenzoyl)-phosphine oxide Synergist: tertiary amine
Ivoclar Vivadent AG, Schaan, Liechtenstein
Bis-GMA: bisphenol-A-dimethacrylate, Bis-EMA: bisphenol-A-glycol dimethacrylate, UDMA: urethane dimethacrylate, Tegdma: triethylenglycol dimetacrylate, HPMA: 3-hydroxpropyl methacrylate, ETMA: ethyltriglycol methacrylate, PENTA: pentaerythrytol tetraacrylate, NI: no information.
10 0.1 mm diameter and 1 0.1 mm thickness) and their upper and lower sides were covered with a polyester film (thickness 0.05 mm) to avoid an inhibition layer. Group 1 was polymerized for 20 s, group 2 for 40 s and group 3 for 60 s from one side and in one step only by placing the light guide directly on the specimen’s surface. The masses m15 min and the densities D15 min of each group’s specimens were measured after 15 min of dry and dark storage at RT the first time. The volume change DV in % after polymerization was calculated from the densities according to the equation
DV ½% ¼
1 D15 min
1 Dunpol
!
1 100 Dunpol
2.2. Determination of hygroscopic expansion, water sorption and solubility Next, the volumes V15 min of the 15 min dry and dark stored specimens were determined according to ISO 404939 by measuring the specimen’s diameter from two perpendicular planes and the thickness from five measurements, one at the center and four at equally spaced points on the specimen’s circumference, using a mechanical caliper (Special Caliper, MIB Messzeuge GmbH, Spangenberg, Germany) and an accuracy of 0.02 mm. The determination with the caliper was done to avoid any moisture contact and to obtain the volume of the actually dry specimens. Further mass and density measurements were done after 1 h, 24 h, 7 d, and 30 d
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Table 2a – Volumetric shrinkages (DV, %) (standard deviations) of the test materials in % after polymerization and 15 min dark storage at 37 8C SureFil
CeramXMono
Clearfil AP-X
Solitaire 2
TetricEvo Ceram
Filtek P60
66.00
57.00
70.00
47.76
66.00
62.34
Curing [s]
DV [%]
DV [%]
DV [%]
DV [%]
DV [%]
DV [%]
20 40 60
1.9 (0.2) b 2.0 (0.3) b 1.9 (0.3) c
2.6 (0.6) abcd 2.6 (0.5) abc 2.4 (0.4) a
2.0 (0.1) c 2.1 (0.2) c 2.2 (0.3) b
3.1 (0.4) abcd 3.3 (0.4) abcd 3.7 (0.3) abcd
1.6 (0.3) d 1.7 (0.3) d 1.9 (0.3) d
Filler load [vol. (%)] Storage 15 min, dry, dark
1.8 (0.2) a 1.8 (0.3) a 1.8 (0.3) a
All materials shrank significantly after curing ( p < 0.05). Test material (filler load, vol.%).
a
storage in water at 37 1 8C in the dark and the respective volumes were calculated for each storage period according to V = m/D. The volumetric change after each storage period was calculated by subtracting the respective volumes of the waterstored specimens from the volumes of the 15 min dry-stored specimens. The results were expressed in percent. Prior to each measurement, the specimens were tempered to RT in a water bath for 10 min. Before weighing the specimens in the air, they were blot-dried with a cellulose pad. To determine water sorption and solubility, the specimens’ masses m30d were measured after 30 d storage in water. Next they were stored again at 37 1 8C in a vacuum desiccator and weighed every 24 h until constant mass was attained (deviation after every cycle < 0.1 mg) to obtain mredried. Water sorption after 30 d storage was calculated according to ISO 4049 with the formula Wsp = (m30d mredried)/V and solubility with the formula: Wsl = (m15 min mredried)/V: m15 min: specimen’s mass prior to water storage in mg, m30d: specimen’s mass after water storage at 37 8C for 30 days in mg, mredried: specimen’s mass after water storage at 37 8C for 30 days and drying in mg, V: specimen’s volume in mm3.
2.3.
Statistical analysis
Statistical analysis was conducted with SPSS software 12.0 (SPSS Software, Munich, Germany). Means and standard deviations were calculated. Normal distribution was tested by the Kolmogorov–Smirnov Test. Two-way ANOVA was calculated to observe the influence of curing time and material on polymerization shrinkage followed by a Bonferroni’s post hoc test to identify significant differences between the individual groups. Repeated measures ANOVA was used to reveal effects of material or curing time on hygroscopic expansion between the 15 min dry storage and the 1 h, 24 h 7 d, and 30 d storage at 37 8C in water followed by a Bonferroni’s
post hoc test to identify differences between the individual groups. Two-way ANOVA was used to find differences of water sorption or solubility between the materials or curing times and multiple comparisons were done again by a Bonferroni’s post hoc test. To identify correlations between material and shrinkage, hygroscopic expansion and water sorption after 30 d as well as hygroscopic expansion and solubility after 30 d an analysis according to Pearson was calculated. Statistical significance for all tests was considered as p < 0.05.
3.
Results
Results are shown in Tables 2–5. Statistical analysis revealed significant volumetric shrinkage for all materials after curing. Solitaire 2 shrank the most. SureFil, TetricEvoceram and Filtek P60 showed similar DV (Table 2a). Curing time did not significantly influence polymerization shrinkage. Significant differences of shrinkage were found between the test materials (Table 2b). Repeated measures ANOVA revealed significant hygroscopic expansion during water storage. The post hoc test did not show significant changes of volume between 15 min dry storage, 1 and 24 h wet storage at 37 8C but after 7 d water storage at 37 8C, all materials significantly absorbed water and expanded approximately 1% by volume except for SureFil (Tables 3a–3c). The materials did not continue to expand significantly between the 24 h and 7 d wet storage (Tables 3a–3c). ANOVA revealed significant influence of curing time on hygroscopic expansion. The 60 s cured samples of Clearfil AP-X, Solitaire 2 and Filtek P60 had significantly higher hygroscopic expansion than the 20 and 40 s cured ones (Tables 3a–3c). All materials significantly absorbed water. Filtek P60 had the highest and SureFil the lowest water sorption (Tables 4a
Table 2b – Significances between the materials related to the respective curing time ( p < 0.05) CeramXMono SureFil CeramXMono Clearfil AP-X Solitaire 2 TetricEvoCeram Filtek P60 ns: no significance.
20, 40 s 20, 40 s 20, 40, 60 s 20 s 20, 40, 60 s
Clearfil AP-X ns 20, 40 s 20, 40, 60 s ns ns
Solitaire 2
TetricEvoCeram
20, 40, 60 s 20, 40, 60 s 20, 40, 60 s
ns 20 s ns 20, 40, 60 s
20, 40, 60 s 20, 40, 60 s
ns
Filtek P60 ns 20, 40, 60 s ns 20, 40, 60 s ns
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journal of dentistry 35 (2007) 806–813
Table 3a – Changes (standard deviations) of volume (DV, %) of the test materials after various storage conditions in relation to the 15 min dry and dark storage Storage
Curing [s]
SureFil
CeramXMono
Clearfil AP-X
Solitaire 2
TetricEvo Ceram
Filtek P60
1 h, 37 8C, water, dark
20 40 60
0.2 (0.4) 0.1 (0.1) 0.0 (0.2)
0.2 (0.7) 0.2 (0.6) 0.1 (0.2)
0.0 (0.2) 0.0 (0.2) 0.1 (0.4)
0.2 (0.4) 0.1 (0.7) 0.2 (0.3)
0.1 (0.4) 0.4 (0.2) 0.2 (0.4)
0.1 (0.2) 0.1 (0.3) 0.0 (0.2)
24 h, 37 8C, water, dark
20 40 60
0.1 (0.4) 0.2 (0.2) 0.2 (0.3)
0.2 (0.6) 0.5 (0.5) 0.2 (0.2)
0.1 (0.3) 0.4 (0.1) 0.5 (0.2)
0.5 (0.4) 0.2 (0.4) 0.1 (0.3)
0.2 (0.5) 0.3 (0.3) 0.4 (0.2)
0.1 (0.2) 0.3 (0.3) 0.2 (0.2)
7 d, 37 8C, water, dark
20 40 60
0.0 (0.3) 0.0 (0.3) 0.2 (0.4)
0.8 (0.7) 0.7 (0.6) 0.6 (0.3)
0.3 (0.2) 0.7 (0.2) 0.9 (0.2)
0.3 (1.1) 0.4 (0.3) 0.7 (0.3)
0.9 (0.4) 1.1 (0.2) 1.1 (0.4)
0.7 (0.1) 0.9 (0.2) 0.9 (0.2)
30 d, 37 8C, water, dark
20 40 60
0.3 (0.3) 0.4 (0.3) 0.3 (0.3)
0.7 (0.7) 1.1 (0.6) 1.0 (0.3)
0.7 (0.1) 0.7 (0.1) 1.1 (0.3)
0.5 (0.4) 0.5 (0.2) 0.9 (0.3)
1.0 (0.3) 1.3 (0.2) 1.2 (0.4)
0.4 (0.2) 0.5 (0.2) 0.7 (0.2)
The bold values represent significantly different changes to the 15 min values ( p < 0.05).
Table 3b – Significances between the materials related to the respective curing time after 30 d storage in water at 37 8C ( p < 0.05) CeramXMono
Clearfil AP-X
Solitaire 2
TetricEvoCeram
40 s
60 s
60 s
20, 40, 60 s
SureFil
and 4b). SureFil, Clearfil AP-X, Solitaire 2, and 20 s cured samples of TetricEvoCeram significantly released soluble substances. No solubility was found for CeramXMono and Filtek P60 (Tables 4a and 4b). Water sorption and solubility did not depend on curing time. The correlation between filler content and volumetric shrinkage was significantly negative (Table 5). The correlation between water sorption and hygroscopic expansion was significantly positive but negative between solubility and hygroscopic expansion (Table 5).
4.
Discussion
Polymerization shrinkage and hygroscopic expansion of six contemporary posterior resin-based filling materials were examined and the correlations between shrinkage and filler content, water sorption and hygroscopic expansion as well as solubility and hygroscopic expansion were considered. One goal of the present investigation was to investigate polymerization shrinkage of contemporary posterior resin-based
Table 3c – Significant differences between the curing times related to the respective material after 30 d storage in water at 37 8C ( p < 0.05) Time [s] 60 Time [s]
20
40
Clearfil AP-X Solitaire 2 Filtek P60 Clearfil AP-X Solitaire 2 Filtek P60
Filtek P60 60 s
filling materials. It was speculated that the shrinkage of the test materials was reduced in comparison with materials in the years from 1980 to 1990.40–42 It was also examined if there was a correlation between shrinkage and curing time and if shrinkage might be compensated by hygroscopic expansion. The question of the influence of filler volume and type as well as of the resin matrix on volumetric behavior was also of interest. Specimens with a diameter of 10 0.1 mm and a thickness of 1 0.1 mm were irradiated from one side and in one step. Volumetric shrinkage and hygroscopic expansion were determined by measuring density according to the Archimedes’ principle, which is a well-established test method.40,43,44 The specimens were stored for 30 d in water at 37 8C to ensure that water sorption and hygroscopic expansion were complete.45,46 Laser micrometers were used to measure dimensional changes of resin-based filling materials47,48 and the obtained results are comparable with the ones of the present study (hygroscopic expansion of SureFil after 30 d water storage: Martin et al.48: 0.5%, present study: 0.4%). Equivalent values were also obtained with the gas pycnometer1 (gas pycnometer: Filtek P60 (20 s cured) DV = 1.84, SureFil (40 s cured) DV = 1.98/Archimedes: Filtek P60 (20 s cured) DV = 1.8 (0.2), SureFil (40 s cured) DV = 2.0 (0.3)). All test materials showed significant polymerization shrinkage that did not correlate with curing time. It is surprising that no influence of curing time was found in this experiment. An explanation might be that the high light density of 800 67 mW cm2 of the curing device in combination with the very thin specimens (1 mm thickness) resulted in the maximum possible density of the materials even after 20 s irradiation time was obtained after 20 s curing time. As expected, Solitaire 2 and CeramXMono, which have the lowest filler content, shrank most but no significant differences of shrinkage were found between the other test materials that were more heavily loaded (Table 2a and 2b). Although the resin
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Table 4a – Water sorption and solubility (standard deviation) of the test materials in mg mmS3 after 30 d water storage at 37 8C Curing [s]
SureFil
CeramXMono
Clearfil AP-X
Solitaire 2
TetricEvo Ceram
Filtek P60
Water sorption [mg mm3] 20 13.4 (1.8) 40 13.8 (0.9) 60 13.5 (1.9)
16.6 (1.9) 13.6 (2.2) 15.2 (3.4)
20.6 (1.5) 19.8 (1.4) 22.5 (2.5)
20.8 (1.5) 20.4 (2.3) 19.8 (1.7)
21.7 (3.7) 20.0 (1.2) 18.9 (3.2)
24.9 (3.3) 23.0 (1.6) 25.0 (1.3)
Solubility [mg mm3] 20 6.1 (0.3) 40 3.3 (1.9) 60 4.1 (1.3)
1.1 (2.9) 2.2 (2.5) 1.3 (1.8)
6.2 (2.2) 6.5 (1.0) 6.8 (1.5)
6.2 (1.4) 4.3 (1.6) 4.3 (1.8)
4.0 (2.1) 1.7 (1.3) 1.0 (1.0)
0.4 (1.8) 0.0 (2.3) 1.7 (1.5)
Significant values for water sorption and solubility are bold. Water sorption and solubility did not depend on curing time ( p < 0.05).
Table 4b – Significances between the materials related to the respective curing time ( p < 0.05) Water sorption SureFil SureFil CeramXMono Clearfil AP-X Solitaire 2 TetricEvoCeram Filtek P60
CeramXMono ns
20 s 40 s ns ns 20, 40 s
20, 40, 60 s 20 s ns ns
Clearfil AP-X
Solitaire 2
TetricEvoCeram
20, 40, 60 s 20, 40, 60 s
20, 40, 60 s 20, 40, 60 s ns
20, 40, 60 s 20, 40, 60 s ns ns
ns 40, 60 s 20, 40, 60 s
ns 20, 40 s Solubility
Filtek P60 20, 40, 60 s 20, 40, 60 s 20, 40 s 60 s 60 s
20 s
ns: no significance.
matrices of the test materials were different, a strong negative correlation (0.7668, p = 0.0000) was found between filler content and shrinkage. These results confirmed literature reporting the same correlation between filler content and polymerization shrinkage.6,24,49 Charton et al.2 showed that highly loaded resin-based filling materials have lower contraction stress during polymerization. They measured a maximum shrinkage stress 400 s after termination of polymerization of 1.29 (0.08) MPa for Solitaire 2 and of 0.92 (0.05) MPa for Clearfil AP-X. These contraction stress values and our results of polymerization shrinkage for the same materials after being cured for the same time (Solitaire 2: DV = 3.7 (0.3), Clearfil AP-X: DV = 2.2 (0.3), curing time: 60 s) complement one another since both properties are strongly correlating.5 The present study confirmed the results of Aw and Nicholls, who24 reported that Filtek P60 had significantly less shrinkage than Solitaire 2 which not only has less filler but also contains low molecular weight monomers like HPMA and ETMA (Table 1). They found a moderate association between filler volume and shrinkage and concluded, that filler size and resin chemistry are other factors that may also effect shrinkage. No significant improvement was achieved in comparison with resin-based filling materials measured from 1980 to 1990 with polymerization shrinkages between 1% and 3%.40–42 After 7 d storage in water at 37 8C, all materials except SureFil showed significant hygroscopic expansion (0.7% to
1%) that did not significantly further increase until the 30 d storage. Water sorption and hygroscopic expansion positively correlated (Table 5), coinciding with the literature.36,50 Although significant, it needs to be considered that this correlation is very low, meaning that not only water sorption but also other factors might influence hygroscopic expansion. It was also determined that hygroscopic expansion decreased with increasing solubility (Table 5), meaning that dissolved ingredients were exchanged by water molecules. No correlation between filler content and hygroscopic expansion was found, confirming that hydrophobicity or hydrophilicity, respectively, of the organic matrix51 and filler size, type and treatment must be considered as well. Ito et al.52 reported the dominating influence of the hydrophilicity on water sorption and Toledano et al.51 also showed that water sorption and solubility values were mainly influenced by the generic type of material and that variations occurring between materials of the same type may result from differences in resin matrix compositions. Clearfil AP-X, Solitaire 2 and Filtek P60 showed significantly increasing hygroscopic expansion with increasing curing times, possibly due to a more hydrophilic matrix or a higher degree of cross-linking. The absorbed water might have expanded the space between the polymer chains by reducing the intermolecular interactions, in particular when the water molecules are able to become attached to the hydrophilic parts of the macromolecules. This could be
Table 5 – Correlations of materials overall according to Pearson ( p < 0.05) Filler load/shrinkage 0.7668 ( p = 0.0000)
Water sorption/hygroscopic expansion, 30 d 0.1675 ( p = 0.0246)
Solubility/hygroscopic expansion, 30 d 0.2279 ( p = 0.0032)
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journal of dentistry 35 (2007) 806–813
supported by the fact that water sorption and solubility did not correlate with the curing time, indicating that the degree of conversion was probably complete. The results show that the polymerization shrinkage was not compensated by the hygroscopic expansion even after 30 d storage in water, as was expected by some authors.36,53
5.
Conclusion
Polymerization shrinkage and hygroscopic expansion of the tested contemporary resin-based filling materials were still in the same range when compared with materials investigated between 1980 and 1990. Thus, no progress was achieved regarding the investigated critical material properties and consequently, the bulk-fill technique still cannot be recommended. Therefore, part (a) of the working hypothesis was accepted but part (b) and part (c) were rejected.
references
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Polymerization shrinkage and hygroscopic expansion of contemporary posterior resin-based filling materials—A comparative study Stefan Ru¨ttermann, So¨ren Kru¨ger 1, Wolfgang H.-M. Raab, Ralf Janda * Heinrich-Heine-University, Medical Faculty, Centre of Dentistry, Department of Operative and Preventive Dentistry and Endodontics, Moorenstr. 5, Geb. 18.13, D-40225 Du¨sseldorf, Germany
article info
abstract
Article history:
Objectives: To investigate the polymerization shrinkage and hygroscopic expansion of
Received 9 May 2007
contemporary posterior resin-based filling materials.
Received in revised form
Methods: The densities of SureFil (SU), CeramXMono (CM), Clearfil AP-X (CF), Solitaire 2 (SO),
7 July 2007
TetricEvoCeram (TE), and Filtek P60 (FT) were measured using the Archimedes’ principle
Accepted 29 July 2007
prior to and 15 min after curing for 20, 40 and 60 s and after 1 h, 24 h, 7 d, and 30 d storage at 37 8C in water. Volumetric changes (DV) in percent after polymerization and after each storage period in water were calculated from the changes of densities. Water sorption and
Keywords:
solubility were determined after 30 d for all specimens and their curing times. Two-way
Dentistry
ANOVA was calculated for shrinkage and repeated measures ANOVA was calculated for
Resin-based filling material
hygroscopic expansion ( p < 0.05).
Volumetric shrinkage
Results: DV depended on filler load but not on curing time (SU 2.0%, CM 2.6%,
Water sorption
CF 2.1%, SO 3.3%, TE 1.7%, FT 1.8%). Hygroscopic expansion depended on
Physical properties
water sorption and solubility. Except for SU, all materials showed DV +1% after water storage. Conclusion: Polymerization shrinkage depended on the type of resin-based filling material but not on curing time. Shrinkage was not compensated by hygroscopic expansion. # 2007 Elsevier Ltd. All rights reserved.
1.
Introduction
Although resin-based filling materials have improved greatly within the last decade with respect to their mechanical and esthetical properties, these materials still shrink significantly when polymerized.1–6 This property is still a challenging problem since in vitro studies show that the resulting shrinkage stress might be the main factor for adhesive failures, marginal disintegration and therefore, for recurrent caries.5–7 However, Sarret8 pointed out that the clinical
relevance of laboratory tests must be considered from the perspective of solving the remaining clinical challenges of current materials and of screening new materials. This review categorized the challenges as those related to the restorative materials, those related to the dentist, and those related to the patient and concluded that there is a general lack of data that correlates clinical performance with laboratory materials testing. Sarret even stated that there is a lack of evidence that indicates polymerization shrinkage is the primary cause of secondary caries. However, it is still commonly agreed that
* Corresponding author. Tel.: +49 6723 6020 750; fax: +49 211 81 19244. E-mail address: [email protected] (R. Janda). 1 Results are part of the thesis of S. Kru¨ger. 0300-5712/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jdent.2007.07.014
journal of dentistry 35 (2007) 806–813
resin-based filling materials, which are also increasingly used for posterior fillings that have significantly larger volumes than anterior ones, must be applied with the very time-consuming incremental technique to avoid marginal disintegration.7,9,10 Therefore, the extent of polymerization shrinkage gains even more importance. Furthermore, because of problems in adhering increments of the newly developed class of high viscosity posterior composite resins (packables), bulk-fill technique was recommended. However, the literature reported that despite the high filler content, the layering technique results in significantly less polymerization shrinkage.11–13 Reducing polymerization shrinkage significantly is still important to improve the quality of resin-based filling materials. Polymerization shrinkage depends on filler content,14,15 filler type16,17 and the composition of the resin matrix.6,7,18,19 In addition to the incremental technique there are several other approaches for minimizing shrinkage, such as different methods of light activation,9,20–23 to increase filler content significantly4,6,24–27,15 or the use of new, less shrinking monomers.19,28–30 The literature also reported that lower power densities were able to reduce the maximum polymerization rate and delay the formation of a rigid network. Conversion before the formation of a rigid network was also enhanced by using a lower power density. Considering that too premature gelation can lead to residual stress during shrinkage, the results indicated that the use of a lower power density can be effective in terms of delaying the onset of the formation of a rigid network, thus providing conditions for macromolecules to flow and relieve stress during shrinkage.31 Although the volumetric shrinkage values in cured composites were not affected by low light intensity, the contraction strain and polymerization’s exotherm were decreased.23 It was also shown that the degree of conversion and volumetric shrinkage showed a non-linear relationship to energy density. The degree of conversion showed a pronounced influence on stress. Increased inhibitor concentration reduced curing rate and contraction stress in composites, without compromising the final degree of conversion.32 Water sorption was also expected to compensate shrinkage to some extent by hygroscopic expansion, resulting in relaxation of internal stresses.33–36 But in general, water sorption is not desired since it causes an outward movement of residual monomers and ions as a consequence of solubility.37 Furthermore, absorbed water might weaken the material38 and excessive water uptake could even result in cuspal flexure or microcracks of the restored tooth.36 Furthermore, shrinkage occurs within seconds but water sorption takes days and did not even totally compensate shrinkage even after weeks.35 Thus, the positive influence of water sorption on marginal quality is very hypothetic. One goal of the present investigation was to investigate the volumetric polymerization shrinkages of contemporary posterior resin-based filling materials. Furthermore, the effects of curing time on shrinkage as well as of water sorption on hygroscopic expansion were examined. The working hypothesis was that (a) polymerization shrinkage depends on material, (b) curing time and shrinkage correlate and (c) hygroscopic expansion totally compensates shrinkage.
2.
807
Materials and methods
Six contemporary light curing resin-based posterior filling materials were chosen (Table 1) to investigate volumetric change as a function of curing time and water sorption. The tungsten halogen light Hilux Ultra Plus (Benlioglu Dental Inc. Ankara, Turkey) was used with the 11 mm diameter light guide and the constant polymerization mode (full light power from the start). Each time after a series of ten specimens was cured, the output of the curing device was checked with the Curing Light Meter (Benlioglu Dental Inc.). Irradiances between 750 and 850 mW cm2 (mean 800 67 mW cm2) were measured and no significant decrease of the output could be observed. Specimen preparation and tests were done at 22–23 8C and a relative humidity of 50% (room temperature: RT). The volumetric change was determined by measuring the density according to the Archimedes’ principle. This was done with the commercial Density Determination Kit of the analytical balance Mettler Toledo XS (Mettler Toledo GmbH, Greifensee, Switzerland). The kit provided a special holder for the specimens, a special container to keep the water, a computer, and appropriate software. The specimens were weighed in air and in water and the density was directly calculated in g cm3 by the software of the Mettler Toledo XS balance according to the equation: D¼
A ðD0 DL Þ þ DL AB
D is density of sample, A weight of sample in air, B weight of sample in water, D0 density of water at the exactly measured temperature in 8C according to the density table of distilled water, and DL is the air density (0.0012 g cm3). An internal balance correction factor (0.99985) of the Mettler Toledo XS balance software took air buoyancy of the adjustment weight into account.
2.1.
Determination of the polymerization shrinkage
From each test material ten uncured sphere-shaped specimens, each of approximately 0.1 g weight (CeramXMono = 0.1034 0.0042 g; Clearfil AP-X = 0.1283 0.0056 g; Filtek P60 = 0.1032 0.0145; Solitaire 2 = 0.1092 0.0056; SureFil = 0.1154 0.0066; TetricEvoCeram = 0.1180 0.0070), were carefully formed by hand in such a way that trapped air bubbles were avoided. Since the uncured materials were rather sticky, a thin polyester film (thickness 0.05) was fixed on the special holder of the balance and its mass was measured in air and in water. Next, the respective material sample was carefully placed on the polyester film and the mass of the whole assembly was measured again in air and in water. Slight deformations of the material during the test were of no importance since they do not influence the density. Furthermore, the weighing process was very fast (approximately 30–45 s), so that there was not very much time for the material to flow. The mass of each material was calculated by subtracting the mass of the polyester film from the mass of the whole assembly. Now the density of the uncured material (Dunpol) was computed. To measure the densities of the cured test materials, three groups of each test material were prepared, each comprised of 10 cylindrical specimens, 180 specimens in total. The specimens were prepared in a polyoxymethylene mould (dimension:
808
journal of dentistry 35 (2007) 806–813
Table 1 – Test materials Material
Lot. no./Shade/ Literature
Filler load, approximately
Formulation
Manufacturer
mass% vol.% CeramXMono Microhybride composite with partial silicium-organically modified resin matrix (Nano-particle Hybride)
0510000198 M5 = A354–56
76
57
Resin matrix: methacrylate modified polysiloxane, dimethacrylate resin Inorganic filler: Ba–Al–borosilicate glass, pyrogenic SiO2 Photoinitiator: camphorquinone Synergist: ethyl-4-diemthylamino benzoate UV stabilizer Stabilizer: butylated hydroxy toluene
DeTrey Dentsply GmbH, Constance, Germany
Clearfil AP-X Microhybride (Hybride)
01122B A357,58
85.5
70
Resin matrix: Bis-GMA, Tegdma Inorganic filler: Ba-glass, silica, pyrogenic SiO2 Photoinitiator: camphorquinone Synergist: NI
Kuarary Co. Inc., Kurashiki, Japan
Filtek P60 Microhybride (Hybride)
6CK A359–62
84.0
62.34
Resin matrix: Bisphenol A Polyethylen Glycol Diether Dimethacrylate, UDMA, Bis-GMA, Bis-EMA, Tegdma Inorganic filler: zirconia, silica Photoinitiator: camphorquinone Synergist: NI
3 M Espe GmbH, Seefeld, Germany
Solitaire 2 Microhybride (Packable)
010257 A326,60,63,64
75
47.76
Resin matrix: Bis-GMA; HPMA, ETMA, PENTA Inorganic filler: Ba-Al-F-glass, Ba-glass, porous SiO2 Photoinitiator: camphorquinone Synergist: NI
Heraeus-Kulzer GmbH Hanau, Germany
SureFil High viscosity microhybride (Packable)
050802 C = A326,64–66
82
66
Resin matrix: Urethane modified Bis-GMA, Tegdma Inorganic filler: Ba-glass Photoinitiator: camphorquinone Synergist: NI
Dentsply/Caulk Inc., Milford, CT, USA
TetricEvoCeram Microhybride (Hybride)
H32690 A367,68
82.5
66
Resin matrix: UDMA, Bis-GMA, ethoxylated Bis-EMA Inorganic filler: Ba-glass, Al2O3, YbF3, pyrogenic SiO2, filler load: 48.5 mass-% Pre-polymer: filled with pyrogenic SiO2, filler load: 34.0 mass-% Photoinitiator: camphorquinone, diphenyl (2,4,6-trimethybenzoyl)-phosphine oxide Synergist: tertiary amine
Ivoclar Vivadent AG, Schaan, Liechtenstein
Bis-GMA: bisphenol-A-dimethacrylate, Bis-EMA: bisphenol-A-glycol dimethacrylate, UDMA: urethane dimethacrylate, Tegdma: triethylenglycol dimetacrylate, HPMA: 3-hydroxpropyl methacrylate, ETMA: ethyltriglycol methacrylate, PENTA: pentaerythrytol tetraacrylate, NI: no information.
10 0.1 mm diameter and 1 0.1 mm thickness) and their upper and lower sides were covered with a polyester film (thickness 0.05 mm) to avoid an inhibition layer. Group 1 was polymerized for 20 s, group 2 for 40 s and group 3 for 60 s from one side and in one step only by placing the light guide directly on the specimen’s surface. The masses m15 min and the densities D15 min of each group’s specimens were measured after 15 min of dry and dark storage at RT the first time. The volume change DV in % after polymerization was calculated from the densities according to the equation
DV ½% ¼
1 D15 min
1 Dunpol
!
1 100 Dunpol
2.2. Determination of hygroscopic expansion, water sorption and solubility Next, the volumes V15 min of the 15 min dry and dark stored specimens were determined according to ISO 404939 by measuring the specimen’s diameter from two perpendicular planes and the thickness from five measurements, one at the center and four at equally spaced points on the specimen’s circumference, using a mechanical caliper (Special Caliper, MIB Messzeuge GmbH, Spangenberg, Germany) and an accuracy of 0.02 mm. The determination with the caliper was done to avoid any moisture contact and to obtain the volume of the actually dry specimens. Further mass and density measurements were done after 1 h, 24 h, 7 d, and 30 d
809
journal of dentistry 35 (2007) 806–813
Table 2a – Volumetric shrinkages (DV, %) (standard deviations) of the test materials in % after polymerization and 15 min dark storage at 37 8C SureFil
CeramXMono
Clearfil AP-X
Solitaire 2
TetricEvo Ceram
Filtek P60
66.00
57.00
70.00
47.76
66.00
62.34
Curing [s]
DV [%]
DV [%]
DV [%]
DV [%]
DV [%]
DV [%]
20 40 60
1.9 (0.2) b 2.0 (0.3) b 1.9 (0.3) c
2.6 (0.6) abcd 2.6 (0.5) abc 2.4 (0.4) a
2.0 (0.1) c 2.1 (0.2) c 2.2 (0.3) b
3.1 (0.4) abcd 3.3 (0.4) abcd 3.7 (0.3) abcd
1.6 (0.3) d 1.7 (0.3) d 1.9 (0.3) d
Filler load [vol. (%)] Storage 15 min, dry, dark
1.8 (0.2) a 1.8 (0.3) a 1.8 (0.3) a
All materials shrank significantly after curing ( p < 0.05). Test material (filler load, vol.%).
a
storage in water at 37 1 8C in the dark and the respective volumes were calculated for each storage period according to V = m/D. The volumetric change after each storage period was calculated by subtracting the respective volumes of the waterstored specimens from the volumes of the 15 min dry-stored specimens. The results were expressed in percent. Prior to each measurement, the specimens were tempered to RT in a water bath for 10 min. Before weighing the specimens in the air, they were blot-dried with a cellulose pad. To determine water sorption and solubility, the specimens’ masses m30d were measured after 30 d storage in water. Next they were stored again at 37 1 8C in a vacuum desiccator and weighed every 24 h until constant mass was attained (deviation after every cycle < 0.1 mg) to obtain mredried. Water sorption after 30 d storage was calculated according to ISO 4049 with the formula Wsp = (m30d mredried)/V and solubility with the formula: Wsl = (m15 min mredried)/V: m15 min: specimen’s mass prior to water storage in mg, m30d: specimen’s mass after water storage at 37 8C for 30 days in mg, mredried: specimen’s mass after water storage at 37 8C for 30 days and drying in mg, V: specimen’s volume in mm3.
2.3.
Statistical analysis
Statistical analysis was conducted with SPSS software 12.0 (SPSS Software, Munich, Germany). Means and standard deviations were calculated. Normal distribution was tested by the Kolmogorov–Smirnov Test. Two-way ANOVA was calculated to observe the influence of curing time and material on polymerization shrinkage followed by a Bonferroni’s post hoc test to identify significant differences between the individual groups. Repeated measures ANOVA was used to reveal effects of material or curing time on hygroscopic expansion between the 15 min dry storage and the 1 h, 24 h 7 d, and 30 d storage at 37 8C in water followed by a Bonferroni’s
post hoc test to identify differences between the individual groups. Two-way ANOVA was used to find differences of water sorption or solubility between the materials or curing times and multiple comparisons were done again by a Bonferroni’s post hoc test. To identify correlations between material and shrinkage, hygroscopic expansion and water sorption after 30 d as well as hygroscopic expansion and solubility after 30 d an analysis according to Pearson was calculated. Statistical significance for all tests was considered as p < 0.05.
3.
Results
Results are shown in Tables 2–5. Statistical analysis revealed significant volumetric shrinkage for all materials after curing. Solitaire 2 shrank the most. SureFil, TetricEvoceram and Filtek P60 showed similar DV (Table 2a). Curing time did not significantly influence polymerization shrinkage. Significant differences of shrinkage were found between the test materials (Table 2b). Repeated measures ANOVA revealed significant hygroscopic expansion during water storage. The post hoc test did not show significant changes of volume between 15 min dry storage, 1 and 24 h wet storage at 37 8C but after 7 d water storage at 37 8C, all materials significantly absorbed water and expanded approximately 1% by volume except for SureFil (Tables 3a–3c). The materials did not continue to expand significantly between the 24 h and 7 d wet storage (Tables 3a–3c). ANOVA revealed significant influence of curing time on hygroscopic expansion. The 60 s cured samples of Clearfil AP-X, Solitaire 2 and Filtek P60 had significantly higher hygroscopic expansion than the 20 and 40 s cured ones (Tables 3a–3c). All materials significantly absorbed water. Filtek P60 had the highest and SureFil the lowest water sorption (Tables 4a
Table 2b – Significances between the materials related to the respective curing time ( p < 0.05) CeramXMono SureFil CeramXMono Clearfil AP-X Solitaire 2 TetricEvoCeram Filtek P60 ns: no significance.
20, 40 s 20, 40 s 20, 40, 60 s 20 s 20, 40, 60 s
Clearfil AP-X ns 20, 40 s 20, 40, 60 s ns ns
Solitaire 2
TetricEvoCeram
20, 40, 60 s 20, 40, 60 s 20, 40, 60 s
ns 20 s ns 20, 40, 60 s
20, 40, 60 s 20, 40, 60 s
ns
Filtek P60 ns 20, 40, 60 s ns 20, 40, 60 s ns
810
journal of dentistry 35 (2007) 806–813
Table 3a – Changes (standard deviations) of volume (DV, %) of the test materials after various storage conditions in relation to the 15 min dry and dark storage Storage
Curing [s]
SureFil
CeramXMono
Clearfil AP-X
Solitaire 2
TetricEvo Ceram
Filtek P60
1 h, 37 8C, water, dark
20 40 60
0.2 (0.4) 0.1 (0.1) 0.0 (0.2)
0.2 (0.7) 0.2 (0.6) 0.1 (0.2)
0.0 (0.2) 0.0 (0.2) 0.1 (0.4)
0.2 (0.4) 0.1 (0.7) 0.2 (0.3)
0.1 (0.4) 0.4 (0.2) 0.2 (0.4)
0.1 (0.2) 0.1 (0.3) 0.0 (0.2)
24 h, 37 8C, water, dark
20 40 60
0.1 (0.4) 0.2 (0.2) 0.2 (0.3)
0.2 (0.6) 0.5 (0.5) 0.2 (0.2)
0.1 (0.3) 0.4 (0.1) 0.5 (0.2)
0.5 (0.4) 0.2 (0.4) 0.1 (0.3)
0.2 (0.5) 0.3 (0.3) 0.4 (0.2)
0.1 (0.2) 0.3 (0.3) 0.2 (0.2)
7 d, 37 8C, water, dark
20 40 60
0.0 (0.3) 0.0 (0.3) 0.2 (0.4)
0.8 (0.7) 0.7 (0.6) 0.6 (0.3)
0.3 (0.2) 0.7 (0.2) 0.9 (0.2)
0.3 (1.1) 0.4 (0.3) 0.7 (0.3)
0.9 (0.4) 1.1 (0.2) 1.1 (0.4)
0.7 (0.1) 0.9 (0.2) 0.9 (0.2)
30 d, 37 8C, water, dark
20 40 60
0.3 (0.3) 0.4 (0.3) 0.3 (0.3)
0.7 (0.7) 1.1 (0.6) 1.0 (0.3)
0.7 (0.1) 0.7 (0.1) 1.1 (0.3)
0.5 (0.4) 0.5 (0.2) 0.9 (0.3)
1.0 (0.3) 1.3 (0.2) 1.2 (0.4)
0.4 (0.2) 0.5 (0.2) 0.7 (0.2)
The bold values represent significantly different changes to the 15 min values ( p < 0.05).
Table 3b – Significances between the materials related to the respective curing time after 30 d storage in water at 37 8C ( p < 0.05) CeramXMono
Clearfil AP-X
Solitaire 2
TetricEvoCeram
40 s
60 s
60 s
20, 40, 60 s
SureFil
and 4b). SureFil, Clearfil AP-X, Solitaire 2, and 20 s cured samples of TetricEvoCeram significantly released soluble substances. No solubility was found for CeramXMono and Filtek P60 (Tables 4a and 4b). Water sorption and solubility did not depend on curing time. The correlation between filler content and volumetric shrinkage was significantly negative (Table 5). The correlation between water sorption and hygroscopic expansion was significantly positive but negative between solubility and hygroscopic expansion (Table 5).
4.
Discussion
Polymerization shrinkage and hygroscopic expansion of six contemporary posterior resin-based filling materials were examined and the correlations between shrinkage and filler content, water sorption and hygroscopic expansion as well as solubility and hygroscopic expansion were considered. One goal of the present investigation was to investigate polymerization shrinkage of contemporary posterior resin-based
Table 3c – Significant differences between the curing times related to the respective material after 30 d storage in water at 37 8C ( p < 0.05) Time [s] 60 Time [s]
20
40
Clearfil AP-X Solitaire 2 Filtek P60 Clearfil AP-X Solitaire 2 Filtek P60
Filtek P60 60 s
filling materials. It was speculated that the shrinkage of the test materials was reduced in comparison with materials in the years from 1980 to 1990.40–42 It was also examined if there was a correlation between shrinkage and curing time and if shrinkage might be compensated by hygroscopic expansion. The question of the influence of filler volume and type as well as of the resin matrix on volumetric behavior was also of interest. Specimens with a diameter of 10 0.1 mm and a thickness of 1 0.1 mm were irradiated from one side and in one step. Volumetric shrinkage and hygroscopic expansion were determined by measuring density according to the Archimedes’ principle, which is a well-established test method.40,43,44 The specimens were stored for 30 d in water at 37 8C to ensure that water sorption and hygroscopic expansion were complete.45,46 Laser micrometers were used to measure dimensional changes of resin-based filling materials47,48 and the obtained results are comparable with the ones of the present study (hygroscopic expansion of SureFil after 30 d water storage: Martin et al.48: 0.5%, present study: 0.4%). Equivalent values were also obtained with the gas pycnometer1 (gas pycnometer: Filtek P60 (20 s cured) DV = 1.84, SureFil (40 s cured) DV = 1.98/Archimedes: Filtek P60 (20 s cured) DV = 1.8 (0.2), SureFil (40 s cured) DV = 2.0 (0.3)). All test materials showed significant polymerization shrinkage that did not correlate with curing time. It is surprising that no influence of curing time was found in this experiment. An explanation might be that the high light density of 800 67 mW cm2 of the curing device in combination with the very thin specimens (1 mm thickness) resulted in the maximum possible density of the materials even after 20 s irradiation time was obtained after 20 s curing time. As expected, Solitaire 2 and CeramXMono, which have the lowest filler content, shrank most but no significant differences of shrinkage were found between the other test materials that were more heavily loaded (Table 2a and 2b). Although the resin
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journal of dentistry 35 (2007) 806–813
Table 4a – Water sorption and solubility (standard deviation) of the test materials in mg mmS3 after 30 d water storage at 37 8C Curing [s]
SureFil
CeramXMono
Clearfil AP-X
Solitaire 2
TetricEvo Ceram
Filtek P60
Water sorption [mg mm3] 20 13.4 (1.8) 40 13.8 (0.9) 60 13.5 (1.9)
16.6 (1.9) 13.6 (2.2) 15.2 (3.4)
20.6 (1.5) 19.8 (1.4) 22.5 (2.5)
20.8 (1.5) 20.4 (2.3) 19.8 (1.7)
21.7 (3.7) 20.0 (1.2) 18.9 (3.2)
24.9 (3.3) 23.0 (1.6) 25.0 (1.3)
Solubility [mg mm3] 20 6.1 (0.3) 40 3.3 (1.9) 60 4.1 (1.3)
1.1 (2.9) 2.2 (2.5) 1.3 (1.8)
6.2 (2.2) 6.5 (1.0) 6.8 (1.5)
6.2 (1.4) 4.3 (1.6) 4.3 (1.8)
4.0 (2.1) 1.7 (1.3) 1.0 (1.0)
0.4 (1.8) 0.0 (2.3) 1.7 (1.5)
Significant values for water sorption and solubility are bold. Water sorption and solubility did not depend on curing time ( p < 0.05).
Table 4b – Significances between the materials related to the respective curing time ( p < 0.05) Water sorption SureFil SureFil CeramXMono Clearfil AP-X Solitaire 2 TetricEvoCeram Filtek P60
CeramXMono ns
20 s 40 s ns ns 20, 40 s
20, 40, 60 s 20 s ns ns
Clearfil AP-X
Solitaire 2
TetricEvoCeram
20, 40, 60 s 20, 40, 60 s
20, 40, 60 s 20, 40, 60 s ns
20, 40, 60 s 20, 40, 60 s ns ns
ns 40, 60 s 20, 40, 60 s
ns 20, 40 s Solubility
Filtek P60 20, 40, 60 s 20, 40, 60 s 20, 40 s 60 s 60 s
20 s
ns: no significance.
matrices of the test materials were different, a strong negative correlation (0.7668, p = 0.0000) was found between filler content and shrinkage. These results confirmed literature reporting the same correlation between filler content and polymerization shrinkage.6,24,49 Charton et al.2 showed that highly loaded resin-based filling materials have lower contraction stress during polymerization. They measured a maximum shrinkage stress 400 s after termination of polymerization of 1.29 (0.08) MPa for Solitaire 2 and of 0.92 (0.05) MPa for Clearfil AP-X. These contraction stress values and our results of polymerization shrinkage for the same materials after being cured for the same time (Solitaire 2: DV = 3.7 (0.3), Clearfil AP-X: DV = 2.2 (0.3), curing time: 60 s) complement one another since both properties are strongly correlating.5 The present study confirmed the results of Aw and Nicholls, who24 reported that Filtek P60 had significantly less shrinkage than Solitaire 2 which not only has less filler but also contains low molecular weight monomers like HPMA and ETMA (Table 1). They found a moderate association between filler volume and shrinkage and concluded, that filler size and resin chemistry are other factors that may also effect shrinkage. No significant improvement was achieved in comparison with resin-based filling materials measured from 1980 to 1990 with polymerization shrinkages between 1% and 3%.40–42 After 7 d storage in water at 37 8C, all materials except SureFil showed significant hygroscopic expansion (0.7% to
1%) that did not significantly further increase until the 30 d storage. Water sorption and hygroscopic expansion positively correlated (Table 5), coinciding with the literature.36,50 Although significant, it needs to be considered that this correlation is very low, meaning that not only water sorption but also other factors might influence hygroscopic expansion. It was also determined that hygroscopic expansion decreased with increasing solubility (Table 5), meaning that dissolved ingredients were exchanged by water molecules. No correlation between filler content and hygroscopic expansion was found, confirming that hydrophobicity or hydrophilicity, respectively, of the organic matrix51 and filler size, type and treatment must be considered as well. Ito et al.52 reported the dominating influence of the hydrophilicity on water sorption and Toledano et al.51 also showed that water sorption and solubility values were mainly influenced by the generic type of material and that variations occurring between materials of the same type may result from differences in resin matrix compositions. Clearfil AP-X, Solitaire 2 and Filtek P60 showed significantly increasing hygroscopic expansion with increasing curing times, possibly due to a more hydrophilic matrix or a higher degree of cross-linking. The absorbed water might have expanded the space between the polymer chains by reducing the intermolecular interactions, in particular when the water molecules are able to become attached to the hydrophilic parts of the macromolecules. This could be
Table 5 – Correlations of materials overall according to Pearson ( p < 0.05) Filler load/shrinkage 0.7668 ( p = 0.0000)
Water sorption/hygroscopic expansion, 30 d 0.1675 ( p = 0.0246)
Solubility/hygroscopic expansion, 30 d 0.2279 ( p = 0.0032)
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supported by the fact that water sorption and solubility did not correlate with the curing time, indicating that the degree of conversion was probably complete. The results show that the polymerization shrinkage was not compensated by the hygroscopic expansion even after 30 d storage in water, as was expected by some authors.36,53
5.
Conclusion
Polymerization shrinkage and hygroscopic expansion of the tested contemporary resin-based filling materials were still in the same range when compared with materials investigated between 1980 and 1990. Thus, no progress was achieved regarding the investigated critical material properties and consequently, the bulk-fill technique still cannot be recommended. Therefore, part (a) of the working hypothesis was accepted but part (b) and part (c) were rejected.
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