Journal of Investigative and Clinical Dentistry (2014), 5, 1–5
ORIGINAL ARTICLE Dental Materials Science
Effect of configuration factor on gap formation in hybrid composite resin, low-shrinkage composite resin and resin-modified glass ionomer Parvin M. Boroujeni, Sayyed M. Mousavinasab & Elham Hasanli Department of Restorative Dentistry, Faculty of Dentistry, Islamic Azad University Khorasgan Branch, Isfahan, Iran
Keywords configuration factor, gap formation, glass ionomer, resin composite. Correspondence Elham Hasanli, Department of Restorative Dentistry, Faculty of Dentistry, Islamic Azad University Khorasgan Branch, Isfahan, Iran. Tel: +98-917-3033708 Fax: +98-311-5354053 Email: [email protected] Received 25 April 2013; revised 2 August 2013; accepted 3 November 2013. doi: 10.1111/jicd.12082
Abstract Aim: Polymerization shrinkage is one of the important factors in creation of gap between dental structure and composite resin restorations. The aim of this study was to evaluate the effect of configuration factor (C-factor) on gap formation in a hybrid composite resin, a low shrinkage composite resin and a resin modified glass ionomer restorative material. Methods: Cylindrical dentin cavities with 5.0 mm diameter and three different depths (1.0, 2.0 and 3.0 mm) were prepared on the occlusal surface of 99 human molars and the cavities assigned into three groups (each of 33). Each group contained three subgroups depend on the different depths and then cavities restored using resin modified glass ionomer (Fuji II LC Improved) and two type composite resins (Filtek P90 and Filtek Z250). Then the restorations were cut into two sections in a mesiodistal direction in the middle of restorations. Gaps were measured on mesial, distal and pulpal floor of the cavities, using a stereomicroscope. Results: Data analyses using Kruskal–Wallist and Mann–Whitney tests. Increasing C-factor from 1.8 to 3.4 had no effect on the gap formation in two type composite resins, but Fuji II LC Improved showed significant effect of increasing C-factor on gap formation. Taken together, when C-factor increased from 1.8 up to 3.4 had no significant effect on gap formation in two tested resin composites. Although, Filtek P90 restorations showed smaller gap formation in cavities walls compared to Filtek Z250 restorations. Conclusions: High C-factor values generated the largest gap formation. Siloranebased composite was more efficient for cavity sealing than methacrylate-based composites and resin modified glass ionomer.
Introduction The mechanical characteristics of light-curing resin composites are responsible for their utilization in both anterior and posterior restorations. Nevertheless, one of the problems that could intervene with their clinical implementation is the shrinkage stress made during their polymerization reaction.1 The gap between methacrylate monomers present in polymeric matrices (by Van der Waals attraction forces) are 0.3–0.4 nm. Rupture of the double carbon bonds of methacrylate monomers would lead to reduction of the gap maintained between methacª 2014 Wiley Publishing Asia Pty Ltd
rylate monomers in polymer matrices and establish a 0.15 nm gap (long covalent bonds).2 As a result, the material undergoes a reduction in volume, which can be construed as densification.3 The polymerization shrinkage could lead to gap formation, fluid infiltration and bacterial presence at the tooth–composite interface and to postoperative sensitivity.4,5 Variables, such as resin monomer, type and concentration of filler particles, and photoinitiators, affect this process.6 The cavity shape is a significant criteria in conserving the composite–dentin bond.1 Feilzer and others established the configurationfactor concept (C-factor: bonded to unbounded surfaces 1
P. M. Boroujeni et al.
Configuration factor effect on gap formation
ratio), and it was considered that in most of the clinically relevant cavity configurations, stress-release flow is not sufficient to preserve adhesion to dentin by dentin-bonding agents.7 Considering the importance of the multifactorial prospect of the polymerization shrinkage process, the current study investigated the influence of the C-factor on restoration interface sealing in a hybrid composite resin, a low-shrinkage composite resin and a resin-modified glass ionomer. Therefore, the null hypothesis in this study was increasing the C-factor has an equal effect on gap formation in the three restorative materials and also an equal effect on gap formation in different walls. Materials and methods Two commercially marketed resin composites, chosen in accordance with their different types of matrix, were tested: a microfilled hybrid resin composite (Z250; 3M ESPE, St Paul, MN, USA), a low-shrinkage siloran-based resin composite (P90; 3M ESPE), and a resin-modified glass ionomer (Fuji II LC Improved GC, Tokyo, Japan). All the specimens in the current study were light cured with a light-emitting diode (LED) unit (Dentamerical; Litex695, Taipei, Taiwan) for 20 sec. Light intensity was calibrated at 1000 mw/cm2 with a radiometer (DemetronInc, Danbury, CT, USA) and after curing of each specimen the light intensity was counted by that radiometer. Selection and preparation of teeth and restorative procedure Ninety-nine caries-free human molars were collected during 2 weeks and stored in 0.5% chloramines solution and randomly divided into three groups of 33 each. All the occlusal surfaces were wet ground in a polishing machine with 150- and 600-grit SiC papers (Sof-Lex discs; 3M ESPE) until flat dentin surfaces were acquired. The roots were fixed in polyester resin inside PVC cylinders (1.5 mm in diameter) with the flat dentin planes parallel to the top surfaces. The cylinders were stabilized in a special samplealigning device, and cylindrical class I cavities 5.0 mm in diameter having three different depths (1.0, 2.0 and 3.0 mm) were produced in all flat dentin surfaces with a diamond bur (#4054; KG Sorensen, SP, Brasilia, Brazil) in a high-speed handpiece. Each bur was used for preparing five cavities: the cavity depths were controlled by using a digital caliper (MPI/E-101; Mitutoyo, Tokyo, Japan). The C-factor was attaining by using the C-factor = ((2prh) + pr2)/pr2 formula. Where r is the cavity radius and h is the cavity depth. Therefore, the C-factor for the three different prepared designed depths were 1.8, 2.6, and 3.4 respectively. Thirty-three cavities (11 from C-factor =1.8, 11 from C-factor = 2.6 and 11 from C-factor = 3.4) were restored 2
in bulk with Filtek P90. The second 33 cavities were restored in bulk with Filtek Z250 and the last 33 cavities were restored in bulk with Fuji II LC Improved, in accordance with the manufacturer’s instructions. The materials to be cured were covered with a polyester strip and lightcured with a LED unit (Dentamerical) for 20 sec and light intensity was calibrated at 1000 mw/cm2. Nine experimental groups, in accordance with materials and C-factor, were produced (n = 11). After storing the samples in distilled water (37°C/24 h) the restorations were cut into two sections in a mesiodistal direction in the middle of restorations using a diamond disk (Vafaei Industrial, Veluna Park Industrial Devices, Tehran, Iran). During cutting with the diamond disk a water-coolant stream was applied. Gap measurement and statistical analysis The cross-cut surfaces of the restorations were observed using a high-resolution stereomicroscope (Olympus/ SZX9, Tokyo, Japan). This method has an advantage in avoiding the necessity of desiccating the specimens, which may lead to separation of the bonding due to the contraction of the tooth substrate. Gap widths were measured on mesial, distal, and pulpal floor of the cavities at 9600 magnification (Figure 1). Data analysis was carried out using Kruskal–Wallistand and Mann–Whitney tests (P < 0.05). Results The results are shown in Figures 2 and 3. Increasing the C-factor from 1.8 to 3.4 had no effect on gap formation in the two types of resin composites tested, but in Fuji II LC Improved there was a significant effect of C-factor
Figure 1. Representative images of the cross-cut surface (Fuji II LC Improved, cavity floor). The top-right image is at 9600 magnification.
ª 2014 Wiley Publishing Asia Pty Ltd
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Configuration factor effect on gap formation
Discussion
Figure 2. Mean gap width for different C-factor values and restorative materials. h CF 1.8, CF 2.6, & CF 3.4.
Figure 3. Mean gap width and location of cavities. h Mesal wall, Distal wall, & Floor.
changes on gap formation (P < 0.05). Statistical analysis of C-factor changes on gap formation for Filtek P90 and Filtek Z250 were 0.534 and 0.148 respectively. Smallest gap size was measured for the cavities restored with Filtek P90 (6 lm), whereas the largest gap formation was found in cavities restored with Fuji II LC Improved in C-factor 3.4 (197.2 lm), followed by C-factor 2.6 and C-factor 1.8. In Fuji II LC Improved for each of the three C-factors, there was no significant difference between mean gap width in mesial and distal wall; and mean gap width in pulpal floor was more than mesial and distal wall (P < 0.05). For Filtek Z250, there was no significant difference between mean gap width in mesial and distal wall; and only in C-factor 3.4 was mean gap width in pulpal floor more than mesial and distal wall (P < 0.05). In Filtek P90 there was no significant difference between mean gap width in mesial, distal wall and pulpal floor. ª 2014 Wiley Publishing Asia Pty Ltd
For evaluation of the interface, various techniques have been applied. In this study, the cross-cut surface of the restorations was observed using a high-resolution stereomicroscope. This method has an advantage in avoiding the necessity of desiccating the specimens, which may lead to separation of the bonding due to the contraction of the tooth substrate. On the other hand, cutting process has been accomplished under a flow of water. The results of the present study showed that there was no difference in gap sized formed between a low-shrinkage composite and hybrid resin composite with increasing C-factor, but in resin modified glass ionomer there was shown significant effect of C-factor changes on gap formation (P < 0.05). Despite innovative advances and excellent applications in methacrylate-based restorative materials, polymerization shrinkage is still considered as their main drawback.8 Shrinkage stress produced during resin composite polymerization can be responsible for maintaining the interface tooth-restorative material and for the consequential failure of the restoration.1 Light-curing starts the conversion of monomer molecules to a polymer network, a stage that causes resin composite shrinkage because of closer packing of the molecules and conversion of the resin composite from a viscous-plastic state to a rigid-plastic state.9 First, shrinkage stresses in a cavity are offset by viscous flow of the resin composite, but a short time after light-curing begins, viscous flow is decreased and the resin composite begins to transmit stresses to the cavity walls.7,9 Filler content, type of organic matrix and flexural modulus have a direct effect on shrinkage stresses and marginal adaptation in cavities restored with light-curing resin composites.10 Several approaches have been proposed to decrease polymerization shrinkage and the influence of contraction stress on composite resins; such as incremental placement techniques, applying a low-modulus intermediate layer, modification of the current resin composites.11 Some marketed low-shrinkage restorative composites are BisGMA-based and use increased filler levels or do not utilize low-molecular weight dimethacrylates as strategies to decrease polymerization shrinkage. Other materials incorporate conventional dimethacrylates with new high-molecular-weight monomers, for instance, tricyclodecane-urethane dimethacrylate or dimmer dicarbamate dimethacrylate.12 A novel category of resin matrix, so-called silorane, was developed based on ring-opening monomers for a low-shrinkage resin composite.11,13,14 The silorane molecule renders a siloxane core with four oxirane rings attached that open as soon as polymerization occurs and bond to other monomers.12 A siloranebased composite resin seems to be one of as the features
3
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Configuration factor effect on gap formation
of low-shrinkage composites. Mechanical features of the silorane-based composite resin were shown to be comparable to clinically successful methacrylate-based composite materials,8 and this resin composite showed better characteristics than the methacrylate-based composites in setting contraction and marginal adaptation.15 The microleakage of experimental silorane-based composite resin was less than commercial methacrylate-based composites in mesio occluso distal cavities.16 In order to decrease microleakage problems, silorane-based materials might be a better replacement for methacrylate-based composites.13 In a clinical study, silorane-based composite showed good durability, but not considerably better than the methacrylate-based composites in class II cavities.17 Filtek P90 is a material consisting of a new monomer technology that uses a combination of a siloxane backbone along with oxirane molecules and a cationic ring-opening polymerization process resulting in a polysilorane polymer. Filtek Z250 is a successful methacrylate-based composite. According to the composition of the materials applied in this study, a difference in gap width among materials was expected. There are differences in filler content and this may justify the different behavior with regard to shrinkage stress and so the gap formation values that were observed. The polymerization reaction includes three phases: pregel, gel and post-gel. In the pre-gel phase, a viscous behavior is shown by the resin composite, and shrinkage stresses produced during the polymerization reaction can be released by the material flow.1,18,19 Polymer chains are diffused in a linear mode and have mobility that allows tensions initiated by polymerization shrinkage to be dissipated by flowing.20 As the reaction is promoted, the post-gel phase begins the first cross-links between chains, making flow laborious and simultaneously promoting the increase of mechanical properties and flexural modulus, which involve inducing tensions in the restoration.20 Influence of the confinement conditions constraining the resin composite (usually expressed as the bonded to unbounded ratio, known as the C-factor), especially plays a critical role in gap formation.7 In the present study, cylindrical cavities with different C-factors were prepared by varying the depth but keeping diameter the same. The results showed that high C-factor values (3.4) indicated large gap formation (Figure 2). This can be explained by the stress relieving flow not being sufficient in this case to maintain adhesion to dentin by dentin bonding agents. On the other hand, lower C-factor values (1.8 and 2.6) allowed more resin composite relaxation.21 Large gap formation was commonly observed at floor of cavities in all tested groups, and this could be related to high shrinkage stress in these areas22 (Figure 3). In microleakage studies, cavity depth was found to have a stronger effect than diameter.22 In agreeing with these results, the present research always 4
used the same diameter and different depths to create experimental groups with different C-factor values. In the current study, smallest gap sizes were found in cavities restored with Filtek P90. On this point this study agrees with studies by El-Sahn,23 Motaz24 and Bagis.13 Filler content has controversial influence on contraction pattern. An increase in volume content results in a decrease in volumetric shrinkage since the resin volume is minimized. On the other hand the high filler volume leads to stiffer materials with high elastic modulus.15 Resin-modified glass ionomer restorations caused the largest gaps to be found in restorations. Flow ability and not possible to pack resin-modified glass ionomer in cavities caused to found largest gaps in tooth-cavity interfaces restored with resin modified glass ionomre. Low elasticity and low viscosity lead to poor marginal adaptation and reduced contraction stresses during polymerization of resin-modified glass ionomer. In research by Dacic25 have shown more marginal adaptation in composite-resin restorations than resin-modified glass ionomer. However, Suprabha26 found more microleakage in resin-modified glass ionomer restorations than composite resin restorations. Conditioning before filling cavities with resin-modified glass ionomer might prevent creation of an ion exchange layer, so causing larger gaps. Results showed that increasing the C-factor from 1.8 to 3.4 had no effect on gap formation in two types of composite resin restorations, but in Motaz’s study increasingthe C-factor from 1 to 5 had a significant effect on gap formation in composite restorations.24 It may be that different methods for measuring the C-factor are the cause of this difference in results because in that research instead of changing cavity height, they prevented bond surfaces increasing the C-factor. Watts et al.27 found that increasing C-factor from 1.88 to 3.75 had no effect on gap contraction stresses in composite resin restorations. In summary, based on the findings attained in this in vitro study, which simulated clinical restorative procedures in a tooth cavity, gap formation is a multifactorial phenomenon that depends on several factors, such as restorative materials and C-factor.3,7,9,10 Moreover, it is also important to study other factors, such as the incremental technique and use of liner materials in order to promote restoration sealing. Conclusions Considering the limitations of this in vitro study, it was possible to conclude that the high C-factor values generated the largest gap formation. Furthermore, Silorane-based composite was more efficient for cavity sealing than methacrylate-based composites and resin-modified glass ionomer. The outcome suggests that further research should be conducted in order to provide better sealing of cavities. ª 2014 Wiley Publishing Asia Pty Ltd
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Configuration factor effect on gap formation
References 1 Davidson CL, de Gee AJ, Feilzer A. The competition between the composite-dentin bond strength and the polymerization contraction stress. J Dent Res 1984; 63: 1396–9. 2 Peutzfeldt A. Resin composites in dentistry: Monomer systems. Eur J Oral Sci 1997; 105: 97–116. 3 Braga RR, Ballester RY, Ferracane JL. Factors involved in the development of polymerization shrinkage stress in resin-composites: a systematic review. Dent Mater 2005; 21: 962–70. 4 Eick JD, Welch FH. Polymerization shrinkage of posterior composite resins and its possible influence on postoperative sensitivity. Quintessence Int 1986; 17: 103–11. 5 Krejci I, Lutz F. Marginal adaptation of Class V restorations using different restorative techniques. J Dent 1991; 19: 24–32. 6 Condon JR, Ferracane JL. Reduction of composite contraction stress through non-bonded microfiller particles. Dent Mater 1998; 14: 256–60. 7 Feilzer AJ, de Gee AJ, Davidson CL. Setting stress in composite resin in relation to configuration of the restoration. J Dent Res 1987; 66: 1636–9. 8 Ilie N, Hickel R. Macro-, micro, and nano mechanical investigations on silorane and methacrylate based composite. Dent Mater 2009; 25: 810–9. 9 Peutzfeldt A, Asmussen E. Determinants of in vitro gap formation of resin composites. J Dent 2004; 32: 109–15. 10 Silikas N, Eliades G, Watts DC. Light intensity effects on resin-composite degree of conversion and shrinkage strain. Dent Mater 2000; 16: 292–6. 11 Navarra CO, Cadenaro M, Armstrong SR et al. Degree of conversion of Filtek Silorane Adhesive System and
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Clearfil SE Bond within the hybrid and adhesive layer: an in situ Raman analysis. Dent Mater 2009; 25: 1178– 85. Boaro LC, Goncalves F, Guimaraes TC, Ferracane JL, Versluis A, Braga RR. Polymerization stress, shrinkage and elastic modulus of current lowshrinkage restorative composites. Dent Mater 2010; 26: 1144–50. Bagis YH, Baltacioglu IH, Kahyaogullari S. Comparing microleakage and the layering methods of siloranebased resin composite in wide Class II MOD cavities. Oper Dent 2009; 34: 578–85. Duarte SJR, Phark JH, Varjao FM, Sadan A. Nanoleakage ultramorphological characteristics, and microtensile bond strengths of a new low-shrinkage composite to dentin after artificial aging. Dent Mater 2009; 25: 589–600. Papadogiannis D, Kakaboura A, Palaghias G, Eliades G. Setting characteristics and cavity adaptation of lowshrinkage resin composites. Dent Mater 2009; 25: 1509–16. Sabatini C, Blunck U, Denehy G, Munoz C. Effect of pre-heated composites and flowable liners on class II gingival margin gap formation. Oper Dent 2010; 35: 663–71. Soares CG, Carracho HG, Braun AP, Borges GA, Hirakata LM, Spohr AM. Evaluation of bond strength and internal adaptation between the dental cavity and adhesives applied in one and two layers. Oper Dent 2010; 35: 69–76. Bausch JR, de Lange K, Davidson CL, Peters A, de Gee AJ. Clinical significance of polymerization shrinkage of composite resins. J Prosthetic Dent 1982; 48: 59–67. Emami N, Soderholm KJ, Berglund LA. Effect of light power density vari-
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ations on bulk curing properties of dental composites. J Dent 2003; 31: 189–96. Ferracane JL. Developing a more complete understanding of stresses produced in dental composites during polymerization. Dent Mater 2005; 21: 36–42. Davidson CL, de Gee AJ. Relaxation of polymerization contraction stresses by flow in dental composites. J Dent Res 1984; 63: 146–8. Braga RR, Boaro LC, Kuroe T, Azevedo CL, Singer JM. Influence of cavity dimensions and their derivatives (volume and “C” factor) on shrinkage stress development and microleakage of composite restorations. Dent Mat 2006; 22: 818–23. El-Sahn NA, El-Kassas DW, El-Damanhoury HM, Fahmy OM, Gomaa H, Platt JA. Effect of C-factor on microtensile bond strengths of low-shrinkage composites. Oper Dent 2011; 36: 281–92. Ghulman Motaz A. Effect of cavity configuration (C Factor) on the marginal adaptation of low-shrinking composite: a comparative ex vivo study. Int J Dent 2011; 23: 8. Dacic S, Dacic DS, Radicevic G et al. Marginal GAP and alteration of enamel around adhesive restorations of teeth (in vitro SEM investigation). Sci J Fac Med Nis 2011; 28: 109–18. Suprabha BS, Sudha P, Vidya M. A comparative evaluation of sealing ability of restorative materials used for coronal sealing after root canal therapy. Indian Soc Pedod Prev Dent J 2001; 19: 137–42. Watts DC, Satterthwaite JD. Axial shrinkage-stress depends upon both C-factor and composite mass. Dent Mater 2008; 24: 1–8.
5
ORIGINAL ARTICLE Dental Materials Science
Effect of configuration factor on gap formation in hybrid composite resin, low-shrinkage composite resin and resin-modified glass ionomer Parvin M. Boroujeni, Sayyed M. Mousavinasab & Elham Hasanli Department of Restorative Dentistry, Faculty of Dentistry, Islamic Azad University Khorasgan Branch, Isfahan, Iran
Keywords configuration factor, gap formation, glass ionomer, resin composite. Correspondence Elham Hasanli, Department of Restorative Dentistry, Faculty of Dentistry, Islamic Azad University Khorasgan Branch, Isfahan, Iran. Tel: +98-917-3033708 Fax: +98-311-5354053 Email: [email protected] Received 25 April 2013; revised 2 August 2013; accepted 3 November 2013. doi: 10.1111/jicd.12082
Abstract Aim: Polymerization shrinkage is one of the important factors in creation of gap between dental structure and composite resin restorations. The aim of this study was to evaluate the effect of configuration factor (C-factor) on gap formation in a hybrid composite resin, a low shrinkage composite resin and a resin modified glass ionomer restorative material. Methods: Cylindrical dentin cavities with 5.0 mm diameter and three different depths (1.0, 2.0 and 3.0 mm) were prepared on the occlusal surface of 99 human molars and the cavities assigned into three groups (each of 33). Each group contained three subgroups depend on the different depths and then cavities restored using resin modified glass ionomer (Fuji II LC Improved) and two type composite resins (Filtek P90 and Filtek Z250). Then the restorations were cut into two sections in a mesiodistal direction in the middle of restorations. Gaps were measured on mesial, distal and pulpal floor of the cavities, using a stereomicroscope. Results: Data analyses using Kruskal–Wallist and Mann–Whitney tests. Increasing C-factor from 1.8 to 3.4 had no effect on the gap formation in two type composite resins, but Fuji II LC Improved showed significant effect of increasing C-factor on gap formation. Taken together, when C-factor increased from 1.8 up to 3.4 had no significant effect on gap formation in two tested resin composites. Although, Filtek P90 restorations showed smaller gap formation in cavities walls compared to Filtek Z250 restorations. Conclusions: High C-factor values generated the largest gap formation. Siloranebased composite was more efficient for cavity sealing than methacrylate-based composites and resin modified glass ionomer.
Introduction The mechanical characteristics of light-curing resin composites are responsible for their utilization in both anterior and posterior restorations. Nevertheless, one of the problems that could intervene with their clinical implementation is the shrinkage stress made during their polymerization reaction.1 The gap between methacrylate monomers present in polymeric matrices (by Van der Waals attraction forces) are 0.3–0.4 nm. Rupture of the double carbon bonds of methacrylate monomers would lead to reduction of the gap maintained between methacª 2014 Wiley Publishing Asia Pty Ltd
rylate monomers in polymer matrices and establish a 0.15 nm gap (long covalent bonds).2 As a result, the material undergoes a reduction in volume, which can be construed as densification.3 The polymerization shrinkage could lead to gap formation, fluid infiltration and bacterial presence at the tooth–composite interface and to postoperative sensitivity.4,5 Variables, such as resin monomer, type and concentration of filler particles, and photoinitiators, affect this process.6 The cavity shape is a significant criteria in conserving the composite–dentin bond.1 Feilzer and others established the configurationfactor concept (C-factor: bonded to unbounded surfaces 1
P. M. Boroujeni et al.
Configuration factor effect on gap formation
ratio), and it was considered that in most of the clinically relevant cavity configurations, stress-release flow is not sufficient to preserve adhesion to dentin by dentin-bonding agents.7 Considering the importance of the multifactorial prospect of the polymerization shrinkage process, the current study investigated the influence of the C-factor on restoration interface sealing in a hybrid composite resin, a low-shrinkage composite resin and a resin-modified glass ionomer. Therefore, the null hypothesis in this study was increasing the C-factor has an equal effect on gap formation in the three restorative materials and also an equal effect on gap formation in different walls. Materials and methods Two commercially marketed resin composites, chosen in accordance with their different types of matrix, were tested: a microfilled hybrid resin composite (Z250; 3M ESPE, St Paul, MN, USA), a low-shrinkage siloran-based resin composite (P90; 3M ESPE), and a resin-modified glass ionomer (Fuji II LC Improved GC, Tokyo, Japan). All the specimens in the current study were light cured with a light-emitting diode (LED) unit (Dentamerical; Litex695, Taipei, Taiwan) for 20 sec. Light intensity was calibrated at 1000 mw/cm2 with a radiometer (DemetronInc, Danbury, CT, USA) and after curing of each specimen the light intensity was counted by that radiometer. Selection and preparation of teeth and restorative procedure Ninety-nine caries-free human molars were collected during 2 weeks and stored in 0.5% chloramines solution and randomly divided into three groups of 33 each. All the occlusal surfaces were wet ground in a polishing machine with 150- and 600-grit SiC papers (Sof-Lex discs; 3M ESPE) until flat dentin surfaces were acquired. The roots were fixed in polyester resin inside PVC cylinders (1.5 mm in diameter) with the flat dentin planes parallel to the top surfaces. The cylinders were stabilized in a special samplealigning device, and cylindrical class I cavities 5.0 mm in diameter having three different depths (1.0, 2.0 and 3.0 mm) were produced in all flat dentin surfaces with a diamond bur (#4054; KG Sorensen, SP, Brasilia, Brazil) in a high-speed handpiece. Each bur was used for preparing five cavities: the cavity depths were controlled by using a digital caliper (MPI/E-101; Mitutoyo, Tokyo, Japan). The C-factor was attaining by using the C-factor = ((2prh) + pr2)/pr2 formula. Where r is the cavity radius and h is the cavity depth. Therefore, the C-factor for the three different prepared designed depths were 1.8, 2.6, and 3.4 respectively. Thirty-three cavities (11 from C-factor =1.8, 11 from C-factor = 2.6 and 11 from C-factor = 3.4) were restored 2
in bulk with Filtek P90. The second 33 cavities were restored in bulk with Filtek Z250 and the last 33 cavities were restored in bulk with Fuji II LC Improved, in accordance with the manufacturer’s instructions. The materials to be cured were covered with a polyester strip and lightcured with a LED unit (Dentamerical) for 20 sec and light intensity was calibrated at 1000 mw/cm2. Nine experimental groups, in accordance with materials and C-factor, were produced (n = 11). After storing the samples in distilled water (37°C/24 h) the restorations were cut into two sections in a mesiodistal direction in the middle of restorations using a diamond disk (Vafaei Industrial, Veluna Park Industrial Devices, Tehran, Iran). During cutting with the diamond disk a water-coolant stream was applied. Gap measurement and statistical analysis The cross-cut surfaces of the restorations were observed using a high-resolution stereomicroscope (Olympus/ SZX9, Tokyo, Japan). This method has an advantage in avoiding the necessity of desiccating the specimens, which may lead to separation of the bonding due to the contraction of the tooth substrate. Gap widths were measured on mesial, distal, and pulpal floor of the cavities at 9600 magnification (Figure 1). Data analysis was carried out using Kruskal–Wallistand and Mann–Whitney tests (P < 0.05). Results The results are shown in Figures 2 and 3. Increasing the C-factor from 1.8 to 3.4 had no effect on gap formation in the two types of resin composites tested, but in Fuji II LC Improved there was a significant effect of C-factor
Figure 1. Representative images of the cross-cut surface (Fuji II LC Improved, cavity floor). The top-right image is at 9600 magnification.
ª 2014 Wiley Publishing Asia Pty Ltd
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Configuration factor effect on gap formation
Discussion
Figure 2. Mean gap width for different C-factor values and restorative materials. h CF 1.8, CF 2.6, & CF 3.4.
Figure 3. Mean gap width and location of cavities. h Mesal wall, Distal wall, & Floor.
changes on gap formation (P < 0.05). Statistical analysis of C-factor changes on gap formation for Filtek P90 and Filtek Z250 were 0.534 and 0.148 respectively. Smallest gap size was measured for the cavities restored with Filtek P90 (6 lm), whereas the largest gap formation was found in cavities restored with Fuji II LC Improved in C-factor 3.4 (197.2 lm), followed by C-factor 2.6 and C-factor 1.8. In Fuji II LC Improved for each of the three C-factors, there was no significant difference between mean gap width in mesial and distal wall; and mean gap width in pulpal floor was more than mesial and distal wall (P < 0.05). For Filtek Z250, there was no significant difference between mean gap width in mesial and distal wall; and only in C-factor 3.4 was mean gap width in pulpal floor more than mesial and distal wall (P < 0.05). In Filtek P90 there was no significant difference between mean gap width in mesial, distal wall and pulpal floor. ª 2014 Wiley Publishing Asia Pty Ltd
For evaluation of the interface, various techniques have been applied. In this study, the cross-cut surface of the restorations was observed using a high-resolution stereomicroscope. This method has an advantage in avoiding the necessity of desiccating the specimens, which may lead to separation of the bonding due to the contraction of the tooth substrate. On the other hand, cutting process has been accomplished under a flow of water. The results of the present study showed that there was no difference in gap sized formed between a low-shrinkage composite and hybrid resin composite with increasing C-factor, but in resin modified glass ionomer there was shown significant effect of C-factor changes on gap formation (P < 0.05). Despite innovative advances and excellent applications in methacrylate-based restorative materials, polymerization shrinkage is still considered as their main drawback.8 Shrinkage stress produced during resin composite polymerization can be responsible for maintaining the interface tooth-restorative material and for the consequential failure of the restoration.1 Light-curing starts the conversion of monomer molecules to a polymer network, a stage that causes resin composite shrinkage because of closer packing of the molecules and conversion of the resin composite from a viscous-plastic state to a rigid-plastic state.9 First, shrinkage stresses in a cavity are offset by viscous flow of the resin composite, but a short time after light-curing begins, viscous flow is decreased and the resin composite begins to transmit stresses to the cavity walls.7,9 Filler content, type of organic matrix and flexural modulus have a direct effect on shrinkage stresses and marginal adaptation in cavities restored with light-curing resin composites.10 Several approaches have been proposed to decrease polymerization shrinkage and the influence of contraction stress on composite resins; such as incremental placement techniques, applying a low-modulus intermediate layer, modification of the current resin composites.11 Some marketed low-shrinkage restorative composites are BisGMA-based and use increased filler levels or do not utilize low-molecular weight dimethacrylates as strategies to decrease polymerization shrinkage. Other materials incorporate conventional dimethacrylates with new high-molecular-weight monomers, for instance, tricyclodecane-urethane dimethacrylate or dimmer dicarbamate dimethacrylate.12 A novel category of resin matrix, so-called silorane, was developed based on ring-opening monomers for a low-shrinkage resin composite.11,13,14 The silorane molecule renders a siloxane core with four oxirane rings attached that open as soon as polymerization occurs and bond to other monomers.12 A siloranebased composite resin seems to be one of as the features
3
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Configuration factor effect on gap formation
of low-shrinkage composites. Mechanical features of the silorane-based composite resin were shown to be comparable to clinically successful methacrylate-based composite materials,8 and this resin composite showed better characteristics than the methacrylate-based composites in setting contraction and marginal adaptation.15 The microleakage of experimental silorane-based composite resin was less than commercial methacrylate-based composites in mesio occluso distal cavities.16 In order to decrease microleakage problems, silorane-based materials might be a better replacement for methacrylate-based composites.13 In a clinical study, silorane-based composite showed good durability, but not considerably better than the methacrylate-based composites in class II cavities.17 Filtek P90 is a material consisting of a new monomer technology that uses a combination of a siloxane backbone along with oxirane molecules and a cationic ring-opening polymerization process resulting in a polysilorane polymer. Filtek Z250 is a successful methacrylate-based composite. According to the composition of the materials applied in this study, a difference in gap width among materials was expected. There are differences in filler content and this may justify the different behavior with regard to shrinkage stress and so the gap formation values that were observed. The polymerization reaction includes three phases: pregel, gel and post-gel. In the pre-gel phase, a viscous behavior is shown by the resin composite, and shrinkage stresses produced during the polymerization reaction can be released by the material flow.1,18,19 Polymer chains are diffused in a linear mode and have mobility that allows tensions initiated by polymerization shrinkage to be dissipated by flowing.20 As the reaction is promoted, the post-gel phase begins the first cross-links between chains, making flow laborious and simultaneously promoting the increase of mechanical properties and flexural modulus, which involve inducing tensions in the restoration.20 Influence of the confinement conditions constraining the resin composite (usually expressed as the bonded to unbounded ratio, known as the C-factor), especially plays a critical role in gap formation.7 In the present study, cylindrical cavities with different C-factors were prepared by varying the depth but keeping diameter the same. The results showed that high C-factor values (3.4) indicated large gap formation (Figure 2). This can be explained by the stress relieving flow not being sufficient in this case to maintain adhesion to dentin by dentin bonding agents. On the other hand, lower C-factor values (1.8 and 2.6) allowed more resin composite relaxation.21 Large gap formation was commonly observed at floor of cavities in all tested groups, and this could be related to high shrinkage stress in these areas22 (Figure 3). In microleakage studies, cavity depth was found to have a stronger effect than diameter.22 In agreeing with these results, the present research always 4
used the same diameter and different depths to create experimental groups with different C-factor values. In the current study, smallest gap sizes were found in cavities restored with Filtek P90. On this point this study agrees with studies by El-Sahn,23 Motaz24 and Bagis.13 Filler content has controversial influence on contraction pattern. An increase in volume content results in a decrease in volumetric shrinkage since the resin volume is minimized. On the other hand the high filler volume leads to stiffer materials with high elastic modulus.15 Resin-modified glass ionomer restorations caused the largest gaps to be found in restorations. Flow ability and not possible to pack resin-modified glass ionomer in cavities caused to found largest gaps in tooth-cavity interfaces restored with resin modified glass ionomre. Low elasticity and low viscosity lead to poor marginal adaptation and reduced contraction stresses during polymerization of resin-modified glass ionomer. In research by Dacic25 have shown more marginal adaptation in composite-resin restorations than resin-modified glass ionomer. However, Suprabha26 found more microleakage in resin-modified glass ionomer restorations than composite resin restorations. Conditioning before filling cavities with resin-modified glass ionomer might prevent creation of an ion exchange layer, so causing larger gaps. Results showed that increasing the C-factor from 1.8 to 3.4 had no effect on gap formation in two types of composite resin restorations, but in Motaz’s study increasingthe C-factor from 1 to 5 had a significant effect on gap formation in composite restorations.24 It may be that different methods for measuring the C-factor are the cause of this difference in results because in that research instead of changing cavity height, they prevented bond surfaces increasing the C-factor. Watts et al.27 found that increasing C-factor from 1.88 to 3.75 had no effect on gap contraction stresses in composite resin restorations. In summary, based on the findings attained in this in vitro study, which simulated clinical restorative procedures in a tooth cavity, gap formation is a multifactorial phenomenon that depends on several factors, such as restorative materials and C-factor.3,7,9,10 Moreover, it is also important to study other factors, such as the incremental technique and use of liner materials in order to promote restoration sealing. Conclusions Considering the limitations of this in vitro study, it was possible to conclude that the high C-factor values generated the largest gap formation. Furthermore, Silorane-based composite was more efficient for cavity sealing than methacrylate-based composites and resin-modified glass ionomer. The outcome suggests that further research should be conducted in order to provide better sealing of cavities. ª 2014 Wiley Publishing Asia Pty Ltd
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Configuration factor effect on gap formation
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