Temperature-dependent polymerization shrinkage stress kinetics of resin-composites

d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 654–660

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Temperature-dependent polymerization shrinkage stress kinetics of resin-composites D.C. Watts a,b,∗ , A. Alnazzawi a,c a b c

School of Dentistry and Photon Science Institute, University of Manchester, UK Institute of Materials Science and Technology, Friedrich-Schiller-University, Jena, Germany School of Dentistry, Taibah University, Medina, Saudi Arabia

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objectives. To determine temperature dependence of shrinkage stress kinetics for a set of

Received 24 February 2014

resin composites formulated with dimethacrylate monomer matrices.

Accepted 6 March 2014

Methods. Six representative resin composites with a range of resin matrices were selected. Two of them were considered as low shrinking resin composites: Kalore and Venus Diamond. The shrinkage stress kinetics at 23 ◦ C and 37 ◦ C were measured continuously using

Keywords:

a Bioman instrument for 60 min. Stress levels between materials were compared at two

Resin composite

intervals: 2 min and 60 min. Specimen temperatures were controlled by a newly designed

Polymerization

heating device. Stress measurements were monitored for 1 h, after irradiation for 40 s at

Shrinkage

550 mW/cm2 (energy density = 22 J/cm2 ). Three specimens (n = 3) were used at each temper-

Stress

ature per material.

Temperature

Results. Shrinkage stress at 23 ◦ C ranged from 2.93 MPa to 4.71 MPa and from 3.57 MPa to 5.42 MPa for 2 min and 60 min after photo-activation, respectively. The lowest stressrates were recorded for Kalore and Venus Diamond (0.34 MPa s−1 ), whereas the highest was recorded for Filtek Supreme XTE (0.63 MPa s−1 ). At 37 ◦ C, shrinkage stress ranged from 3.27 MPa to 5.35 MPa and from 3.36 MPa to 5.49 MPa for 2 min and 60 min after photoactivation, respectively. Kalore had the lowest stress-rate (0.44 MPa s−1 ), whereas Filtek Supreme XTE had the highest (0.85 MPa s−1 ). Materials exhibited a higher stress at 37 ◦ C than 23 ◦ C except for Kalore and Venus Diamond. Positive correlations were found between shrinkage stress and stress-rate at 23 ◦ C and 37 ◦ C (r = 0.70 and 0.92, respectively). Significance. Resin-composites polymerized at elevated temperature (37 ◦ C) completed stress build up more rapidly than specimens held at 23 ◦ C. Two composites exhibited atypical reduced stress magnitudes at the higher temperature. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

∗

Corresponding author at: University of Manchester School of Dentistry, Oxford Road, Manchester M13 9PL, UK. Tel.: +44 1612756749. E-mail address: [email protected] (D.C. Watts).

http://dx.doi.org/10.1016/j.dental.2014.03.004 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 654–660

1.

Introduction

Since resin composites were developed and introduced commercially in the late 1950s to restore the appearance and function of biological tooth tissues they have been progressively re-formulated to improve their mechanical and physical properties [1,2,9]. However, shrinkage and subsequent shrinkage stress that occur during polymerization are still disadvantages [3]. This may lead to deleterious clinical complications, such as post-operative pain, marginal discoloration, recurrent caries, cusp deflection and enamel micro-cracks [1,4]. Shrinkage stress is the result of confining the shrinking material by bonding to cavity walls. During the early stage of the polymerization reaction, resin composites may flow and relax any developing stress. However, there is a very short period before the material becomes stiff and unable to deform readily. Then stress starts to increase [1]. Thus, material properties, geometry, and boundary conditions are contributory factors that interact with each other to determine the resulting stress. The degree of conversion, polymerization rate, shrinkage strain, elastic modulus, pre-gel flow, and post-gel shrinkage are among material factors that play a role in stress development [2,5–8,10]. Previous studies have proposed different ways to reduce shrinkage and its accompanying stress, via incremental placement techniques, use of stress absorbing liners, soft-start light curing, and redesigning of materials [1,3]. Several low shrinking resin composites have been introduced, including Kalore and Venus Diamond. Kalore is based on a novel monomer (DX-511), which is a modified UDMA and has a high molecular mass in comparison to Bis-GMA (895 g/mole vs. 512 g/mole) [9,11]. Venus Diamond is based on TCD-DI-HEA monomer which is described as a low shrinkage monomer with low viscosity [3,9]. Previous studies showed the extent to which ambient temperature affects key properties of resin composites, including degree of conversion [12,13], rate of polymerization [13], shrinkage strain and its rate [14,15], and also the elastic and viscous moduli [16,17]. Increasing temperature decreases resin composite viscosity, increases free volume and improves molecular mobility. Increasing degree of conversion is normally accompanied by increased shrinkage. Apart from effects of increasing crosslinking, higher temperature facilitates polymer chain segmental movement that is manifest in greater compliance and lower elastic modulus [10,17]. Many studies have taken place that explores the polymerization stress phenomenon. However, the majority were conducted at ambient room temperature. The objective of this study was to investigate the effect of increasing specimen temperature from room temperature (23 ◦ C) to body temperature (37 ◦ C) on shrinkage stress kinetics for a representative range of resin composites. The null hypotheses were (1) temperature has no effect on shrinkage stress, (2) there is no difference in shrinkage stress at 2 min and 60 min, and (3) there is no relation between shrinkage stress and stress rate.

2.

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Materials and methods

Six commercial photo-activated resin-composites were selected on the basis of their matrix monomer compositions and filler loading (Table 1). The Bioman instrument [18–20] was used in this study to measure shrinkage stress kinetics (Fig. 1) at 23 ◦ C and 37 ◦ C. This instrument incorporates a stiff cantilever load cell with a compliant end attached rigidly to a 2 cm thick stainless steel base-plate. The compliant end has an integral clamp holding a circular steel rod (10 mm in diameter, 22 mm long) in a vertical and perpendicular orientation to the load cell axis. Resin composite specimens were placed between two surfaces: the lower surface of the rod and a 3 mm thick glass plate surface. These two surfaces were lightly sandblasted to promote bonding of the composite specimens. The glass plate was held firmly in position by a metallic clamp and a hollow cylindrical bolt that allowed for passage of a straight light-curing optic to contact the lower surface of the glass plate. The specimen gap between the rod and glass plate was adjusted with the aid of a feeler gauge so that following placement of the specimen paste it would be 0.8 mm. Resin composite paste of standardized mass (0.12–0.15 g) was introduced into this space to form specimen disks of 10 mm diameter and 0.8 mm thickness (configuration factor = 6.25). After a period for temperature equilibration, the specimen was then photo-activated from below, by transillumination of the glass plate. During polymerization, the stress created within the resin composite, between the opposing steel and glass surfaces, caused displacement of the compliant end of cantilever recording a signal via its strain-gauge load cell. This signal was amplified by a calibrated strain indicator (Model 3800, Vishay, Measurements Group, Raleigh, NC, USA) and was connected to a personal computer via data-logging hardware and software (Picotech, Cambridge, UK). The stress value in MPa was then obtained from the load, recorded a 1 s intervals, divided by the specimen disk area. A newly designed heating device incorporating a heating element and connected to a power supply (Farnell E20-28, Farnell Ltd., Wetherby, UK) was used to provide a heat source for stress measurements at 37 ◦ C (Fig. 2). The power supply was set at 5.00 Volts to raise the heating element to 75 ◦ C which was shown via thermocouple methods to set the resin composite specimen temperature to 37 ◦ C. The resin composite specimen temperature reached 37 ◦ C then stabilized for the next 60 min (Fig. 3). A light cure unit, with a modified straight curing tip and delivering a calibrated irradiance of 550 mW/cm2 , (MARC RC, BlueLight Analytics, Inc., Halifax, Canada) was used to irradiate the specimen from beneath the glass plate for 40 s (energy density = 22 J/cm2 ). Polymerization shrinkage stress was monitored for 60 min. Statistical software (SPSS ver. 18, IBM Inc., USA) was used, and data were analysed by one-way analysis of variance (ANOVA). Prior to post hoc tests, data were analysed for equal variances using the homogeneity test (p < 0.05). For data of shrinkage stress at 23 ◦ C and 37 ◦ C, equal variances can be assumed, thus Multiple pair-wise comparisons using a Tukey’s post hoc test were conducted to establish homogenous

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Table 1 – Investigated resin composites. Code

Resin composites

Filler loading wt.%a

vol.%

GRO GCK

Grandio G C Kalore

87 82

71.4 69

VDD FXE

Venus Diamond Filtek Supreme XTE

81.2 78.5

64 63.3

GDP

Gradia Direct Posterior

77

65

GDA

Gradia Direct Anterior

73

64

a

Resin matrix

Lot no.

Manufacturer

Bis-GMA, TEGDMA DX-511, UDMA and dimethacrylate co monomers TCD-DI-HEA UDMA BIS-GMA, BIS-EMA (6), TEGDMA, PEGDMA and UDMA UDMA and dimethacrylate co-monomers UDMA and dimethacrylate co-monomers

581793 0903171

Voco, Cuxhaven Germany GC Europe

010035 N147105

Heraeus Kulzer, Germany 3 M ESPE Germany

0905201

GC Europe

0901134

GC Europe

Manufacturer data.

Fig. 1 – Bioman instrument.

subsets at p = 0.05. Differences between groups for temperature and time effects were assessed using a paired t-test.

3.

Results

Shrinkage stress data at 23 ◦ C and 37 ◦ C for all materials are plotted as time-dependent curves in Figs. 4 and 5, respectively. Table 2 summarizes mean values of shrinkage stress (MPa) for 2 min and 60 min, and the stress rates at 23 ◦ C and

37 ◦ C. Shrinkage-stress-rate (MPa s−1 ) kinetic plots, obtained by numerical differentiation, are shown in Figs. 6–8. At 23 ◦ C and 2 min after photo-activation, the lowest shrinkage stress was recorded by Kalore (2.93 MPa) – significantly less than the other materials (p ≤ 0.026). The highest stress was found for Filtek Supreme XTE (4.71 MPa) – significantly greater than other materials (p < 0.001). At 23 ◦ C and 60 min after photo-activation, Kalore again demonstrated the lowest shrinkage stress (3.75 MPa) that was significantly different (p < 0.001) from other materials except Grandio (p = 0.361).

Fig. 2 – Heating device above the glass plate, shown in the retracted position.

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Table 2 – Shrinkage stress (MPa) and stress rate (MPas−1 ) at 23 ◦ C and 37 ◦ C. At 23 ◦ C

Materials 2 min GRO GCK VDD FXE GDP GDA

At 37 ◦ C

60 min a,A*

3.29 (0.10) 2.93 (0.15)b,A 3.68 (0.15)c,A 4.71 (0.08)d,A* 3.69 (0.16)c,A* 4.03 (0.04)e,A*

Rate a,B

3.80 (0.12) 3.57 (0.16)a,B 4.54 (0.17)b,B+ 5.42 (0.09)c,B 4.39 (0.17)b,B 4.76 (0.11)b,B

2 min

0.44 (0.042) 0.34 (0.035) 0.34 (0.012) 0.63 (0.004) 0.42 (0.006) 0.40 (0.036)

60 min a,A*

3.79 (0.09) 3.27 (0.04)b,A 3.62 (0.09)a,b,A 5.35 (0.11)c,A* 4.21 (0.21)d,A* 4.58 (0.18)d,A*

Rate a,A

3.91 (0.06) 3.36 (0.05)b,A 3.83 (0.08)a,A+ 5.49 (0.05)c,A 4.43 (0.35)d,A 4.76 (0.19)d,A

0.60 (0.069) 0.44 (0.033) 0.49 (0.019) 0.85 (0.027) 0.79 (0.044) 0.78 (0.063)

Within each column different superscript small letter indicates significant differences between materials (p < 0.05). Within each temperature different superscript capital letter in the same row indicates significant differences in the same material (p < 0.05). Within same time and different temperature, asterisk (*) and plus (+) superscript in the same row indicates significant differences in the same material (p < 0.05).

Again, Filtek Supreme XTE had the significantly greatest (p < 0.001) shrinkage stress (5.42 MPa). At 37 ◦ C and 2 min after photo-activation, Kalore again had the lowest shrinkage stress among all examined materials (3.27 MPa) and that was significantly different (p ≤ 0.012)

Fig. 3 – Graph of temperature vs. time of heating element and resin composite specimen.

Fig. 4 – Time-dependence shrinkage stress at 23 ◦ C, up to 2 min and 60 min for six resin composites, irradiated for 40 s at 550 mW/cm2 .

from other materials except for Venus Diamond (p = 0.065). Again the significantly highest (p < 0.001) shrinkage stress was seen with Filtek Supreme XTE (5.49 MPa). These trends were maintained after 60 min, with Kalore (3.27 MPa) having the significantly lowest (p ≤ 0.05) shrinkage stress and Filtek Supreme XTE having the significantly highest (p ≤ 0.002) shrinkage stress. Thus all materials recorded numerically greater mean shrinkage stresses at 60 min compared to 2 min for both temperatures. However, although these differences were statistically significant at 23 ◦ C (p ≤ 0.007), they were not statistically significant at 37 ◦ C (p ≥ 0.112). At 2 min after photo-activation, all materials demonstrated a higher mean shrinkage stress at 37 ◦ C than at 23 ◦ C with the exception of Venus Diamond. This difference was statistically significant for all materials (p ≤ 0.028), except for Venus Diamond (p = 0.687), and also for Kalore (p = 0.068). After 60 min all materials demonstrated rather similar mean shrinkage stress magnitudes at both temperatures, except for Kalore and Venus Diamond that had an atypical reduced mean shrinkage stress at 37 ◦ C compared to 23 ◦ C. However, only for Venus Diamond was that difference statistically significant (p = 0.006). Turning to consider shrinkage stress-rates, (Figs. 6–8) all the plots show initial auto-acceleration followed by a peakrate leading to auto-deceleration. Hence we now compare the detailed peak stress-rates. The lowest peak-rates at 23 ◦ C were recorded for Kalore and Venus Diamond (0.34 MPa s−1 ), while the highest was recorded for Filtek Supreme XTE (0.63 MPa s−1 ) (Fig. 6). At 37 ◦ C, Kalore again demonstrated the lowest peakrate (0.44 MPa s−1 ), and Filtek Supreme XTE demonstrated the highest peak-rate (0.85 MPa s−1 ) (Fig. 7). All materials showed higher peak stress rates at 37 ◦ C compared to 23 ◦ C (Fig. 8). Peak shrinkage stress-rates are plotted as a function of maximum shrinkage-stress (Fig. 9) at the two temperatures. Linear regressions gave positive correlations for both 23 ◦ C and 37 ◦ C data (r = 0.70 and 0.92, respectively). The maximum shrinkage stresses between materials were compared as a function of filler-loadings (Figs. 10 and 11). Similar plots were obtained at both key time-points (2 and 60 min) and for both temperatures, with quadratic regression curves indicating stress-minima for filler loadings of about 68% (v/v). The correlation coefficients are indicated in the figures.

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Fig. 5 – Time-dependence shrinkage stress at 37 ◦ C, up to 2 min and 60 min for six resin composites, irradiated for 40 s at 550 mW/cm2 .

4.

Discussion

Shrinkage stress is a local physical condition resulting from the interaction of material properties, geometry, and boundary conditions [32]. Although numerous methods have been proposed to measure shrinkage, the principle underlying these methods is similar [1,6]. However, results for the same material may differ greatly between methods due to differences in testing configuration and instrument compliance. The Bioman instrument used in the current study uses a beam of fixed compliance and proved to be convenient, and gave reproducible results [19,20]. The heating device was carefully designed to provide heat locally. Therefore, specimen temperature could be reliably increased without causing any change in the temperature of load cell and its temperature-sensitive strain gage. In the current study, final (1 h) shrinkage stress magnitudes ranged from 3.54 to 5.42 MPa and from 3.27 to 5.49 MPa (at 23 ◦ C and 37 ◦ C, respectively). Even at body temperature (37 ◦ C) – which can exceed some oral temperatures – the measured

Fig. 6 – Time-dependent shrinkage stress-rate at 23 ◦ C.

Fig. 7 – Time-dependent shrinkage stress-rate at 37 ◦ C.

stress levels for examined materials were less than the bond strength of good dental adhesives – typically in the range of 15–30 MPa for fully cured resin composites when bonded to enamel [21–23], and 13–23 MPa when bonded to dentin [23,24]. However, detailed comparisons od bond-strengths are beyond the scope of this discussion. Kalore demonstrated the lowest shrinkage stress among all examined materials at both temperatures and both times. Additionally, it had a lower peak stress-rate than other materials at both temperatures. This behavior could result from the high molecular mass of the base monomer within Kalore; DX-511 (895 g/mole vs. 512 g/mole for Bis-GMA), which allows Kalore to have fewer reactive sites. Thus, for equal mass, Kalore may undergoes a lower number of reactions during polymerization compared to other dimethacrylate resin composites [25]. The monomer has a long rigid core and two flexible side arms [26]. Previous studies showed that Kalore has a lower shrinkage strain and low modulus in comparison to conventional resin composites [8,27]. Therefore, the lower stress recorded by Kalore may be explained by its lower shrinkage, polymerization rate, modulus and possibly degree of conversion compared to other materials.

Fig. 8 – Maximum rate of shrinkage stress at 23 ◦ C and 37 ◦ C. The maximum rate occurs about 3–5 s after start of irradiation.

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Fig. 9 – Association map of maximum shrinkage stress (at 60 min) and maximum stress rate (at 3–5 s) for a series of resin composites at 23 ◦ C and 37 ◦ C.

Filtek Supreme XTE showed the highest shrinkage stress and peak stress-rate among all examined materials at both temperatures and both times. This higher stress may be explained by the greater polymerization rate and faster attainment of stiffness that may reduce the capacity for flow and stress relief. Takahashi et al. [28] investigated the effects of shrinkage strain, shrinkage stress, and Young’s modulus of resin composites on marginal adaptation. They concluded that resin composites with low shrinkage strain, low shrinkage stress, and low stiffness resulted in superior cavity adaptation. Shrinkage stress in resin composites is produced by the interplay of several factors: degree of conversion, shrinkage strain, reaction rate and elastic modulus [1]. Polymerization of resin composites at higher temperature compared to room temperature may be accompanied by an increase in degree of conversion, shrinkage strain, and polymerization rate [12,13]. Although most of the examined materials showed a higher stress after 2 min at 37 ◦ C compared to that at 23 ◦ C,

Fig. 10 – Early shrinkage-stress at 2 min versus filler loading (vol.%) at 23 ◦ C and 37 ◦ C for six resin composites.

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Fig. 11 – Maximum shrinkage-stress at 60 min versus filler loading (vol.%) at 23 ◦ C and 37 ◦ C for six resin composites.

Venus Diamond recorded a lower magnitude. Additionally, there were non-significant differences for Kalore. However, after 60 min and at 37 ◦ C, Kalore and particularly Venus Diamond showed the ability to compensate for increasing stress and eventually recorded lower final stresses than those at 23 ◦ C. This is consistent with behavior noted by Yamamoto et al. [29] and Yamasaki et al. [30]. This may be explained by the effect of increasing temperature on the elastic and viscous moduli [16,17]. Thus, the first hypothesis was partially rejected. Photo-curing of resin composites is characterized by a fast reaction rate which reduces the time available for material flow [1]. Thus, stress starts to increase instantly after photoactivation and continues to increase with time, despite the phenomenon of auto-deceleration following the peak rate [18]. In the current study, all materials showed higher stress at one hour after photo-activation, than that at 2 min, for both temperatures. At 23 ◦ C, examined materials showed increases in maximum stress of more than 15%, from 2–60 min, whereas less than 6% was observed at 37 ◦ C. However, the increases in stress at 37 ◦ C were not statistically significant. A higher temperature leads to expansion of resin composites and increasing free volume, which may facilitate polymer chain movement and result in greater compliance of the resin matrix. Previous studies [10,16,17] confirmed that the elastic and viscous moduli are dependent on measurement temperature and concluded that increasing temperature leads to a decrease in elastic modulus, and an increase in viscous modulus. Therefore, the second hypothesis was partially rejected. In the present study, the analysis of shrinkage stress-rate showed positive correlations with shrinkage stress at both temperatures. Previous studies reported that the polymerization rate is reflected in the shrinkage strain-rate, and the rate of increase in elastic modulus. As a result, the shrinkage stress rate is directly affected by the polymerization rate [1,31]. Thus, the greater the stress-rate the higher final shrinkage stress (Fig. 9) and, therefore, the third hypothesis was rejected.

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Conclusions

Within the limitations of this study, and the temperature range considered, it can be concluded that: (1) The main consequence of increasing the measurement temperature was an increase in the shrinkage-stress rate rather than a major change in the final shrinkage stress magnitude. (2) Some materials exhibited a definite decrease in final stress at higher temperature, though this was atypical. (3) Final stress magnitudes were minimized for materials with filler loading of circa 68% (v/v). (4) Even at oral temperatures (37 ◦ C) the measured stress for examined materials was less than the bond strength of many dental adhesives to enamel and dentin, although this finding is system-dependent and so should be interpreted cautiously.

references

[1] Watts DC, Schneider LFJ, Marghalani HY. Bond-disruptive stresses generated by resin composite polymerization in dental cavities. J Adhes Sci Technol 2009;23:1023–42. [2] Cramer NB, Stansbury JW, Bowman CN. Recent advances and developments in composite dental restorative materials. J Dent Res 2011;90:402–16. [3] Marchesi G, Breschi L, Antoniolli F, Di Lenarda R, Ferracane J, Cadenaro M. Contraction stress of low-shrinkage composite materials assessed with different testing systems. Dent Mater 2010;26:947–53. [4] Tantbirojn D, Pfeifer C, Braga R, Versluis A. Do low-shrink composites reduce polymerization shrinkage effects? J Dent Res 2011;90:596–601. [5] 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. [6] Ferracane J. Developing a more complete understanding of stresses produced in dental composites during polymerization. Dent Mater 2005;21:36–42. [7] Cadenaro M, Codan B, Navarra CO, Marchesi G, Turco G, Di Lenarda R, et al. Contraction stress, elastic modulus, and degree of conversion of three flowable composites. Eur J Oral Sci 2011;119:241–5. [8] Van Ende A, Mine A, De Munck J, Poitevin A, Van Meerbeek B. Bonding of low-shrinking composites in high C-factor cavities. J Dent 2012;40:295–303. [9] Ferracane JL. Resin composite—state of the art. Dent Mater 2011;27:29–38. [10] Stansbury JW. Dimethacrylate network formation and polymer property evolution as determined by the selection of monomers and curing conditions. Dent Mater 2012;28:13–22. [11] Terry DA, Leinfelder KF, Blatz MB. Achieving aesthetic and restorative excellence using an advanced biomaterial: part I. Intl J Cont Dent 2010;1:68–81. [12] Daronch M, Rueggeberg FA, De Goes MF. Monomer conversion of pre-heated composite. J Dent Res 2005;84:663–7.

[13] Daronch M, Rueggeberg FA, De Goes MF, Giudici R. Polymerization kinetics of pre-heated composite. J Dent Res 2006;85:38–43. [14] Baroudi K, Saleh AM, Silikas N, Watts DC. Shrinkage behaviour of flowable resin-composites related to conversion and filler-fraction. J Dent 2007;35:651–5. [15] Atai M, Watts DC. A new kinetic model for the photopolymerization shrinkage-strain of dental composites and resin-monomers. Dent Mater 2006;22:785–91. [16] Mesquita RV, Axmann D, Geis-Gerstorfer J. Dynamic visco-elastic properties of dental composite resins. Dent Mater 2006;22:258–67. [17] Mesquita RV, Geis-Gerstorfer J. Influence of temperature on the visco-elastic properties of direct and indirect dental composite resins. Dent Mater 2008;24:623–32. [18] Watts DC, Marouf AS, Al-Hindi AM. Photo-polymerization shrinkage-stress kinetics in resin-composites: methods development. Dent Mater 2003;19:1–11. [19] Watts DC, Satterthwaite JD. Axial shrinkage-stress depends upon both C-factor and composite mass. Dent Mater 2008;24:1–8. [20] Palin WM, Hadis MA, Leprince JG, Leloup G, Boland L, Fleming GJ, et al. Reduced polymerisation stress of MAPO-containing resin composites with increased curing speed, degree of conversion and mechanical properties. Dent Mater 2014;30. Available online 12 March 2014 (in press). [21] Perdigao J, Thompson J, Toledano M, Osorio R. An ultra-morphological characterization of collagen-depleted etched dentin. Am J Dent 1999;12:250–5. [22] Fritz UB, Finger WJ. Bonding efficiency of single-bottle enamel/dentin adhesives. Am J Dent 1999;12:277–82. [23] De Munck J, Vargas M, Van Landuyt K, Hikita K, Lambrechts P, Van Meerbeek B. Bonding of an auto-adhesive luting material to enamel and dentin. Dent Mater 2004;20: 963–71. [24] Scherrer SS, Cesar PF, Swain MV. Direct comparison of the bond strength results of the different test methods: a critical literature review. Dent Mater 2010;26:e78–93. [25] Naoum SJ, Ellakwa A, Morgan L, White K, Martin FE, Lee IB. Polymerization profile analysis of resin composite dental restorative materials in real time. J Dent 2012;40:64–70. [26] Ilie N, Hickel R. Resin composite restorative materials. Aust Dent J 2011;56:59–66. [27] Wei Y-J, Silikas N, Zhang Z-T, Watts DC. Hygroscopic dimensional changes of self-adhering and new resin-matrix composites during water sorption/desorption cycles. Dent Mater 2011;27:259–66. [28] Takahashi H, Finger WJ, Wegner K, Utterodt A, Komatsu M, Wöstmann B, et al. Factors influencing marginal cavity adaptation of nanofiller containing resin composite restorations. Dent Mater 2010;26:1166–75. [29] Yamamoto T, Kubota Y, Momoi Y, Ferracane JL. Polymerization stresses in low-shrinkage dental resin composites measured by crack analysis. Dent Mater 2012;28:e143–9. [30] Yamasaki LC, De Vito Moraes AG, Barros M, Lewis S, Francci C, Stansbury JW, et al. Polymerization development of low-shrink resin composites: reaction kinetics, polymerization stress and quality of network. Dent Mater 2013;29:e169–79. [31] Pfeifer CS, Ferracane JL, Sakaguchi RL, Braga RR. Factors affecting photopolymerization stress in dental composites. J Dent Res 2008;87:1043–7. [32] Fok ASL. Shrinkage stress development in dental composites - an analytical treatment. Dent Mater 2013;29:1108–15.