Shrinkage stress in light-cured composite resins: Influence of material and photoactivation mode

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Shrinkage stress in light-cured composite resins: Influence of material and photoactivation mode Christophe Charton a,∗ , Pierre Colon b , Fernand Pla a a b

Laboratoire des Sciences du G´enie Chimique, UPR 6811, INPL, ENSIC, 1, rue Grandville, BP 451 F-54.001 NANCY Cedex, France Laboratoire de Biomat´eriaux Dentaires, UFR d’Odontologie de Paris VII, 5, rue Garanci`ere, 75.006 Paris, France

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objectives: The aim of this study was to record the effect of composite type and photoac-

Received 23 September 2005

tivation mode on the stresses resulting from polymerization of five established composite

Received in revised form

resins: packable (Solitaire, Solitaire 2), micro-hybrid (Aelitefil, Z100) and hybrid (Clearfil AP-

14 March 2006

X).

Accepted 22 July 2006

Methods: A mechanical testing machine was used to record the polymerization contraction stress (MPa) of cylindrical composite specimens (d = 5 mm; h = 2 mm; C-factor = 1.25) at 0.1 s intervals over a period of 400 s. The samples were photopolymerized using

Keywords:

a halogen light curing device under two types of light exposure: group 1, Standard

Composite resin

(800 mW·cm−2 × 60 s); group 2, Exponential (logarithmic increase from 150 to 800 mW·cm−2

Polymerization

over 15 s + 800 mW·cm−2 × 45 s). The stress rate (SR: slopeMPa>0–60 s ) and the maximum shrink-

Shrinkage stress

age stress (MSS: MPa400 s ) of each material (five replications) were statistically analysed by

Viscous flow

one-way ANOVA/Scheffe’s test and Pearson’s correlation procedure (˛ = 0.05). Finally, Stu-

Light-curing

dent’s t-test (two matched series) enabled the assessment of the effect of the irradiation method on the results. Results: For group 1, in decreasing order, the MSS was 1.51 ± 0.07 MPa (Solitaire) statistically equivalent to 1.45 ± 0.06 MPa (Aelitefil), 1.29 ± 0.08 MPa (Solitaire 2), and 1.04 ± 0.03 MPa (Z100) statistically equivalent to 0.92 ± 0.05 MPa (Clearfil AP-X). Z100 showed the highest SR (0.045 ± 6 × 10−3 ) and Solitaire, the lowest (0.017 ± 2 × 10−3 ). There was no correlation between SR and MSS (r < −0.33, p < 0.05). For group 2, the MSS and SR values were distributed in a similar way to those from group 1. There was a negative correlation between SR and MSS (r < −0.43 and p < 0.01). The exponential ramp successfully reduced the MSS (−3.9%) and SR (−11%) values. Significance: There is no relationship between composite resin type, stress rate and shrinkage stress levels. The slower stress rate development, resulting from ramped light intensity, helped slightly to reduce the maximum polymerization stress. © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

The marginal integrity of composite resins restorations depends on many factors. It includes the hydric absorption, which occurs after the polymerization process and the dif-

∗

ference between the thermal expansion coefficient of the tooth and the restorative material, but especially the polymerization shrinkage related to the dimethylacrylate-based matrix of these products [1–4]. In the course of their polymerization, composite resins shrink due to their passage

Corresponding author at: 10, rue de Laxou, 54000 Nancy, France. Tel.: +33 383 287 508. E-mail address: [email protected] (C. Charton). 0109-5641/$ – see front matter © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2006.06.034

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from a liquid to solid state (sol–gel transition). Shrinkage is the result of the reduction in the intermolecular distance between monomeric units, necessary to maintain thermodynamic equilibrium during the liquid–solid transformation. In the liquid, the monomers are subjected to weak interactions (van der Waals forces and hydrogenic linkages) at distances of 0.3–0.4 nm. In the reticulated polymer the links are covalent at 0.154 nm, whereas the double vinyl links, which react are only lengthened by 0.019 nm [5]. Therefore, the shrinkage depends on the concentration volume of reactive groups, as well as the type, the flexibility and the ability of the polymeric network to reach a spatial configuration that minimizes the system’s free energy. Polymerization leads to gelation, characterized by an increase in viscosity [6]. The gel point corresponds to the passage of a monomer-oligomer solution from a viscous-plastic phase into a rigid visco-elastic phase [7] with infinite viscosity and an elastic modulus of nil [8]. Shrinkage strain is an interesting fundamental value. But in a clinical situation, this value changes due to the adhesive process, and shrinkage stresses are created instead. If the composite is efficiently bonded into a cavity during polymerization, stress occurs when the crosslinking density prevents the accommodation of shrinkage strain by viscoelastic flow of the polymer, except on the free surface area [9,10]. The shrinkage stresses should be considered as the principal mechanism responsible for problems concerning the interface [4,11,12]. If contraction forces exceed the bonding strength at the interface, the resulting interfacial gap can lead to staining, marginal leakage [13–17], post-operative sensitivity [18,19] and recurrent caries [18,20,21]. If the interface is preserved, the contraction forces can be transferred to neighboring dental structures causing cuspal deflection [22–24] or fractures in the enamel [13,25]. Classically, stress development and relaxation are related to local conditions such as the C-factor corresponding to the bonded/unbonded surface area ratio [26], the compliance of the substrate corresponding to the surrounding tooth structure [13,22,27] and the thickness of the adhesive layer. Also they depend to the composite itself (filler content [28,29], ratio and type of comonomers [30], concentration and system of photoinitiators [31,32]) and finally, to the type of technique used (incremental technique [33–35], parameters of curing [36–39]). The volumetric shrinkage of currently used composites ranges from 2 to 6% [40]. Several investigations have shown the influence of increasing composite filler content on reducing volumetric shrinkage [41–44]. However, high filler levels lead to increased elastic modulus and, as a consequence, do not result in less polymerization shrinkage stress [29,45,46]. A previous study verified a positive linear correlation between filler content and contraction stress in commercial composite materials, suggesting that the stiffness of the material would be the main factor responsible for stress development [28]. The Hooke’s law analogy should not be used, because many previous studies have proven that it is rather simplistic and does very little in terms of explaining stress development, due to the composite’s viscoelastic behavior [47]. The kinetic of polymerization has also been related to stress development during the curing process. A slower reaction may provide better ability to undergo plastic flow dur-

ing the early phases of polymerization, which causes reduction in polymerization stress and, therefore, less damage at the adhesive interface [9,48]. In several studies the curing rate was controlled by varying the light intensity delivered to the samples. Whereas an increase of irradiance is desirable for achieving a high degree of conversion (DC) and improve mechanical properties in the shortest possible exposure time, lower irradiance over longer exposure times is required to maintain the integrity of the tooth-restoration interface [37,49] while preserving an identical DC [50]. This finding probably results from an extended period of viscous flow in the pre-gel phase within the setting resin. However, the increased exposure duration required to obtain a radiant exposure (units = mJ·cm−2 or mW·cm−2 s) resulting in an equivalent DC may not be acceptable to clinicians. When initially a low irradiance is increased to a high value, either stepped (Elipar* Highlight [39,51–53]), either exponentially (Elipar* Trilight [54]), the resulting “soft-start” light energy application phase may result in an improved marginal adaptation and a DC equivalent to that of a single, continuous low-intensity photoactivation. The aim of this study was to verify the influence of the composite material and photoactivation method (continuous versus ramped) on shrinkage stress development.

2.

Materials and methods

Two packable, two micro-hybrid and one hybrid composite were tested in this study (Table 1). As the light absorption coefficient can affect the DC, all composites were selected in A2 shade.

2.1.

Shrinkage stress test

Polymethylacrylate (PMMA) sticks (diameter: d = 5 ± 0.05 mm) from Altuglas (CDF Chimie, Carling, France) were cut to obtain rods with a cylindrical section and a surface at a 90◦ angle to the longitudinal axis. In order to optimise adhesion, mechanical retention and the surface energy of the flat surfaces of the rod ends were increased through two combined treatments: improvement of roughness by finishing with silicon carbide polishing discs (Isomet, Buehler, Evanston, Illinois, USA) with 180 ␮m grain size and etching with a semi-gel of 35% orthophosphoric acid (Ultra-Etch, Bisco, Itasca, Illinois 60143, USA) for 30 s. The acid was then rinsed under air/water spray and the rods allowed to air-dry for 60 s. The DC of PMMA is approximately 70% [55], which leaves 30% of unreacted carbon–carbon double bonds (C C) available. Thus, a chemical adhesion can occur by the formation of carbon–carbon single bonds (C–C) between the double methylacrylate linkages counterpart of the PMMA and those of the various monomers of the bonding resin. This may help the bonding between rod and composite resin. The bonding resin Scotchbond 1 (3MESPE, Saint Paul, Minnesota 55101, USA) was then applied to the etched rod ends using a fine brush. The Elipar* Trilight curing device (3M-ESPE Dental AG, Seefeld, Germany) was used in standard 80 s mode (800 mW·cm−2 for 80 s) to polymerize the adhesive, with its light tip placed parallel at 2 mm from the rods’ flat surfaces.

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Table 1 – Chemical composition and mechanical/physical properties of the materials tested Brand name

Aelitefil

Clearfil AP-X

Manufacturer

Kuraray Co., Osaka 530-8611; Japan

Type Shade Batch number Filler weight (%) Filler volume (%) Filler type

Bisco dental Products, Itasca, Illinois 60143; USA Micro-hybrid A2 H-621A2 74a 65a –

Filler size (␮m) Monomer composition

Shrinkage (vol.%) Flexural modulus (GPa)

Z100

Solitaire

Solitaire2

3M-ESPE, St Paul, MN 55101; USA

Heraeus kulzer Inc., Wehrheim; Germany

Heraeus kulzer Inc., Wehrheim; Germany

Micro-hybrid A2 3021A2 84.5c 71c Zirconium/silica glassc

0.04–0.8a Bis-GMA and UEDMAa

Hybrid A2 00593A 86b 70b Silanated barium glass and silanated colloidal silicab 0.1–15b Bis-GMA (7%) and TEGDMA (6%)b

Packable A2 030038 65d 90d Porous SiO2 glass (32%) and Ba–Al–F–Si glass (33%)d 0.7–25d HPMA, ETMA, bis-GA and tetrafunctional monomersd

2.1a 10.3a

1.9b 16.6b

2.4c 13c

Packable A2 010249 75e 57e Porous SiO2 glass (18%) and Ba–Al–F–Si glass (57%)e 0.7–25e TEGDMA, UEDMA, bis-GA and tetrafunctional monomerse 3.15e 8e

0.01–3.3c Bis-GMA (7.5%) and TEGDMA (7.5%)c

4.5d 6d

Note: other values of absolute flexural modulus were obtained for Solitaire (4.4 GPa66 ; 5.1 GPa67 ), Solitaire2 (13.5 GPa89 ) and Z100 (21 GPa89 ). a b c d e

Aelitefil, technical manual, Bisco, 2003-10-11. AP-X, scientific documentation 2003, Kuraray Co. Z100, technical information 2003, 3M-ESPE. Ref. [71]. Solitaire 2: features and properties, Heraeus kulzer, BRO 12./2003.

The lower rod was held in a screw jaw connected to the electro-mechanical mobile crossbar of a mechanical testing machine Instron 1185 (Instron Corp., Canton, Massachusetts, USA), which was micro-processor controlled by a testing software set for a tensile test configuration (Test Corp., Munich, Germany). The upper rod was held by another screw jaw connected to a fixed 1 kN load cell Instron 2511-317 (Instron Corp., Canton, Massachusetts, USA). The rods were aligned precisely in a vertical position. When the upper and lower rods were put into close contact with each other – controlled by a minimal load recording – the load cell/software couple was zeroed. This reference measurement enabled accurate positioning of the rod ends at the desired distance (h) during testing. The rods were positioned at a distance from each other, which enabled access for inserting the composite specimens. The samples were handled under the overhead fluorescent light, as the amount of premature composite curing, which can be attributed to ambient fluorescent lighting has been shown to be negligible [56]. An increment of composite resin was placed on the PMMA lower rod. Then the electromechanical mobile crossbar of the machine was automatically reset to place the rod ends at a distance of 2 mm, with accuracy of 1 ␮m. As in the methodology of Feilzer et al. [26], the displacement control system was programmed so that the crossbar maintained the initial distance. The configuration factor (C-factor), being the ratio between the bonded and unbonded (free) surface areas, was 1.25 in this test set-up, calculated according to the method described by Feilzer et al. [26] (h = 2 mm, d = 5 mm, C-factor = d/2h = 1.25). The samples were polymerized with a quartz-tungstenhalogen-based dental light-curing unit (Elipar* Trilight, 3M-

ESPE Dental AG, Seefeld, Germany) using two different curing modes. Group 1 with a conventional high single-irradiance curing mode (Standard: 800 mW·cm−2 ) with 60 s exposure. Group 2 was a ramped-irradiance curing mode (Exponential: 150 mW·cm−2 with a logarithmic increase to 800 mW·cm−2 in 15 s + 800 mW·cm−2 × 45 s). The tip of the light unit was placed perpendicularly to the long axis and centred on the middle of the specimen. The distance between the light tip and the composite surface was 1 mm (Fig. 1). The irradiance of the curing light checked with a radiometer (Curing Radiometer Model 100, Demetron Research Corp., USA) was constant during the experiment. The contraction force generated by composite polymerization was continuously recorded at 0.1 s intervals over a period of 400 s from the initial exposure. The force value (N) was converted to stress (MPa) by dividing it by the cross-section area of the PMMA rod. The shrinkage stress/time curves were recorded for all trials and each experiment group (five specimens per material). The experiments were conducted at room temperature (21–22 ◦ C). The maximum shrinkage stress (MSS) was evaluated at the end of the test (T400 s ). In addition, the stress rate (SR) was calculated as corresponding to the slope obtained between the onset of stress (MPa > 0) and the end of the irradiation (T60 s ).

2.2.

Statistical analysis

The mean values and standard deviations were computed for MSS and SR. These values were compared using the one-way ANOVA/Scheffe’s test. Pearson’s correlation procedure was used to evaluate the effects of SR on MSS. Finally, Student’s ttest (two matched series) enabled the assessment of the effect

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Fig. 1 – Schematic illustration of the Instron 1185 test system and the light guide of the curing system device.

of the irradiation method on the results. All statistical testing was performed at a preset alpha of 0.05.

the stress rate, resulting in a plateau at the end of which the maximum shrinkage stress was recorded.

3.1.

3.

Group 1: standard exposure mode

Results

The mean values and standard deviations of the stress rate (slope on the MPa > 0 and T60 s period) and maximum shrinkage stress after 400 s are listed in Table 2. This data was taken from the stress/time curves (Fig. 2). In these trials, the load cell recorded the traction force needed to strain the specimen in order to keep its initial height. There was a short time lag between the beginning of light curing and the first stress recorded, then all curves were S-shaped. The first inflexion point in the curve was followed by a rapid linear increase in stress during which the maximum stress rate was recorded, followed by a second inflexion point beginning the moment the lamp was switched off (T60 s ), then a gradual decrease in

The one-way ANOVA/Scheffe’s test revealed that the Z100 stress/time curve showed faster polymerization kinetics because of its highest stress rate (SR) – or the steepest slope – and it was statistically higher (0.045 ± 6.10−3 ) than all other materials tested. Solitaire had the significantly lowest SR value (0.017 ± 2.10−3 ) while there was no difference between Clearfil AP-X, Solitaire 2 and Aelitefil. Fig. 3 depicts the first 70 s of the observation period. It gives the profile of each slope in detail. Solitaire showed a distinct delay before the stresses were recorded. Solitaire and Aelitefil had a significantly higher maximum shrinkage stress (MSS: 1.51 ± 0.07 MPa, 1.45 ± 0.06 MPa, respectively), than any of the other materials. Clearfil AP-X had the lowest MSS (0.92 ± 0.05 MPa) without a significant dif-

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Table 2 – Details of the stress rate (SR) of the TMPa > 0–60 s period and maximum shrinkage stress (MSS) after 400 s are given in mean values and standard deviations (in brackets) for group 1 (standard exposure mode) and group 2 (exponential exposure mode) Stress rate (slopeMPa>0–60 s ) Group 1 standard mode Aelitefil 0.037 (2 × 10−3 ) b Cleafil AP-X 0.032 (4 × 10−3 ) b Solitaire 0.017 (2 × 10−3 ) a Solitaire2 0.032 (4 × 10−3 ) b Z100 0.045 (6 × 10−3 ) c Group 2 exponentiel mode Aelitefil 0.033 (2 × 10−3 ) b Clearfil AP-X 0.031 (2 × 10−3 ) b Solitaire 0.017 (2 × 10−3 ) a Solitaire2 0.027 (3 × 10−3 ) b Z100 0.04 (3 × 10−3 ) c

matched series: the exponential mode curves both had a lower SR and MSS than their counterparts in standard mode. The exponential ramp successfully reduced the values studied by showing higher efficiency of the SR (−11%) than the MSS (−3.9%).

Maximum shrinkage stress (MPa400 s )

4. 1.45 (0.06) c 0.92 (0.05) a 1.51 (0.07) c 1.29 (0.08) b 1.04 (0.03) a 1.4 (0.09) c 0.9 (0.03) a 1.49 (0.07) c 1.24 (0.05) b 0.97 (0.05) a

The various letters indicate statistically homogeneous subsets (one-way ANOVA/Scheffe’s test, ˛ = 0.05).

ference compared to Z100 (1.04 ± 0.03 MPa). Solitaire MSS was 164% of the Clearfil AP-X value. The simple Pearson’s correlation calculation enables one to conclude that there is no correlation between the SR and the MSS (r < −0.33, p < 0.05).

3.2.

915

Group 2: exponential exposure mode

The SR and MSS statistical results follow the same order as those of group 1. The simple Pearson’s correlation calculation shows that a negative SR-MSS correlation even exists with this mode (r < −0.43 and p < 0.01): the materials with the highest reticulation rate had the lowest final stresses. To assess the overall influence of the exponential mode on the results, a Student’s t-test was used to compare two

Discussion

The shrinkage before the gel point is not regarded as having clinical relevance [57], because it can be partially compensated for by the movement of molecules, notably from the free surface of the material. After gelation the material is rigid enough, so that plastic flow becomes impossible. Consequently, stress polymerization occurs inside the composite resin. In an adhesive restoration, further internal stresses will result, due to restrained contraction by the bonded surfaces. Therefore, it is crucial to determine the shrinkage stresses of established composite resins to understand their clinical behavior [4]. Nevertheless, it is necessary to remember that a recent study showed that triaxial stress activity exists in our typical uniaxial contraction stress setting, and that not all stress is being measured [58]. Authors emphasize the importance of considering the effect of the compliance of the test set-up and the compliance of the load-cell during stress measurement [26,58]. Effectively, it is a crucial element because it can result in relatively large recording errors [4,59,60]. In the current experimental set-up, it was not necessary to determine the total compliance of the device because the test design had a feedback system. The parameters of the microprocessor, which piloted the mechanical test machine, were set so that the electro-mechanical cross-beam maintained the original sample length (h) throughout the duration of the test. Thus the load-cell measured the actual forces developed by the sample, with a constant C-factor.

Fig. 2 – Graphs showing shrinkage stress vs. time for the materials tested in: (a) group 1 (standard exposure mode); (b) group 2 (exponential exposure mode). -·-·-· the dotted line shows the end of irradiation (T60 s ).

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Fig. 3 – Graphs showing shrinkage stress vs. time of the materials tested for the first 70 s of the observation period in: (a) group 1 (standard exposure mode); (b) group 2 (exponential exposure mode).

The configuration factor of the cavity is an important element in the determination of the contraction stress in composite resins [26,61]. In this study, the C-factor was 1.25. This value corresponds to the configuration factor, which one encounters in the majority of clinical situations with C-factor values of approximately 1–2, as in Class II and Class III restorations [26]. Finally, a pilot study showed that the tensile bond strength between the composite and the PMMA rod was 6 MPa, therefore, much higher than the maximum stress recorded in the contraction stress test (1.7 MPa). Indeed, no interfacial failure occurred during the stress tests. All manufacturers of composites recommend exposure times of 20 s for their materials but the authors’ intention was rather to maximise the degree of conversion. Braga and Ferracane [62] reported that a minimum energy density of about 11.040 mJ·cm−2 was required to stabilise the DC and volume shrinkage values of an experimental composite. Another study carried out on commercial composites [63] related a DC stabilised with an energy density about 27.000 mJ·cm−2 (30 s irradiation time, using an irradiance of 900 mW·cm−2 ). In order to obtain the higher DC for all tested materials in this study, an energy density about 48.000 mJ·cm−2 (60 s exposure time, using an irradiance of 800 mW·cm−2 ) was chosen. Regarding the curves (Fig. 2), an S-shape was noticed after about 25 s light-exposure time. During this phase thermal expansion stress interfers with shrinkage stress. Authors explain this phenomenon by the elevation of the temperature related to both the light-curing device and the reticulation exothermy [45,46,49,64,65]. One notes that this thermal dilatation stress, which thwarts the shrinkage stress, has been able to slow down and even temporarily stop (Clearfil AP-X and Z100) the development of shrinkage stress (Fig. 3). It is therefore crucial that this thermal dilatation stress should be taken into account because the composites with the highest thermal dilatation coefficients could confuse the results which are recorded at moments too close to this phenomenon. In order

to be free from the influence of thermal dilatation stress, the authors carried out a complementary study, which enabled them to note that, for their product volume, the time for a return to a state of thermodynamic equilibrium was 400 s. The MSS are recorded after 400 s in order to get rid of the influence of this thermal expansion. The shrinkage stress values recorded during polymerization depend on the conversion, the monomers (size, molecular weight and functionality) and the filler rate (weight and volume). The one-way ANOVA/Scheffe’s test showed that whatever the irradiation mode, the largest stresses were observed with Solitaire. Significantly lower values for physical and mechanical parameters were reported for this product compared to other packable, microfilled and micro-hybrid composite materials [66–68]. This fact may be attributed to the low filler content for this product, and because Solitaire contains high porous SiO2 filler particles (32 wt.%) which may have a negative effect on the mechanical properties [69]. Its high shrinkage stress (1.51 ± 0.07 MPa; group 1) may be attributed to a higher resin-monomer volume fraction (35 wt.%), and because this composite is based on multifunctional-acrylate monomer chemistry (multifunctional methacrylic acid ester instead of acrylic monomer) [70]. This specific matrix presents a higher DC than those containing conventional mixtures of dimethacrylates, which might explain the high stress values [62]. This higher DC theory has two main origins. (1) The acrylate monomers have a low molecular weight. Therefore, the number of double C C links per weight unit is higher, therefore more bond formation is possible [71,72]. (2) The acrylates are less sterically hindered because they lack the methyl group of the methacrylates. For this reason, acrylates have higher mobility during polymerization, which increases the DC. Thus it is found that small increases in the degree of conversion produced substantial increases in stress [62]. The very high volume filler rate (90%) of Solitaire provides a large surface area to volume ratio, and therefore extensive possibil-

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ities for surface interactions with polymerizing monomers. In the same way as an “internal” C-factor, it may prevent the matrix from changing shape during polymerization, resulting in an overall higher stress build-up [47]. Effectively, it has been suggested that covalent bonds and physical molecular interactions occurring between the silane molecules and the polymerizing resin during the reaction, also constrain the ‘free ´ shrinking’ of the polymer. Soderholm et al. used the term of ‘hoop’ stresses around the fillers [73]. Aelitefil showed shrinkage stress levels (1.45 ± 0.06 MPa; group 1), which were not significantly statistically different to those of Solitaire. In the current results, Aelitefil constitutes the exception because with a filler rate of 74 wt.% and elastic modulus of 10.3 GPa, intermediary compared to other products, it produces high shrinkage stress. Its rigidity is due both to the filler rate and also to its organic matrix, composed of two rigid monomers with a voluminous skeleton: bis-GMA and UEDMA. These large monomers with a high molecular weight present a more limited number of double links per weight unit than the small monomers [74]. This constitutes an advantage with low volume shrinkage (2.1%) [75] but also a disadvantage because they are tetrafunctional – which increases the density of the structures formed by reducing molecular mobility – and present limited flexibility. Aelitefil engenders high shrinkage stress because of its limited viscoelastic relaxation capacity [76]. As Solitaire, Solitaire2 constitutes a matrix with multifunctional methacrylic acid ester, but with a large quantity of acrylate monomers which have been substituted by a long chain of monomers (UEDMA and TEGDMA). Therefore, the Solitaire2 monomer mixture includes fewer double bonds per mol than Solitaire, which improves its shrinkage and at the same time, its shrinkage stress. Finally its higher filler rate (75 wt.%) gives Solitaire2 better behavior in terms of shrinkage stress (1.29 ± 0.08 MPa; group 1). This observation has not been validated by Chen et al. [45], who recorded statistically identical values between Solitaire and Solitaire2. The higher light irradiance in the current study, which increased the DC, could explain this difference in results. The study carried out by Ernst et al. [77] confirms the results of the current study. In their work Solitaire2 shows stress levels significantly higher than those of Clearfil AP-X. In standard and exponential modes, the lowest maximum stress values were recorded for Z100 (1.04 ± 0.03 MPa; group 1) and Clearfil AP-X (0.92 ± 0.05 MPa; group 1) with only a numeric difference. These two composites have very similar structure and matrix chemical composition. Their bis-GMA/TEGDMA ratio optimises their DC values [30], their physical properties [78] and consequently their flexural modulus (Table 1), which is not incompatible with low stress levels. Their high weight filler rate would explain their low volume shrinkage [42] and shrinkage stress [79]. Finally, due to its original system for recording shrinkage stresses (Bioman shrinkage stress Instrument), Watts et al. [80] also reported low shrinkage stress values for Clearfil AP-X compared to other hybrid and condensable composites. The literature generally relates that the less rigid materials are more capable of reducing shrinkage stresses than rigid materials. This assertion has been verified by numerous studies concerning the marginal integrity of the mate-

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rials [81–84]. Works concerning the in vitro analysis of the shrinkage stresses have confirmed this theory [28,46,49,51,53]. However the materials tested were of different types (photoinitiated composite versus chemically-initiated composite) [36] or the assembly did not include a feed-back system capable of maintaining the distance (h) throughout the trial [45,49,59,60,65]. The data from the current study were not able to establish a correlation between shrinkage stress and the rigidity of the material. In fact, whatever the irradiation method, the results showed that the lowest maximum shrinkage stress values were recorded for Clearfil AP-X and Z100 with a numeric difference only, the latter being the most rigid composites of the products tested. On the other hand, the most flexible composite Solitaire, showed the highest maximum stress levels. This observation confirms the results from numerous other authors [48,63,65,85–88]. The assertion of the authors in the current study was also confirmed by the work of Ernst [77] which relates that the composite producing the highest shrinkage stress was an intermediate value of rigidity (Solitaire2: 4.4 ± 0.1 MPa; flexural modulus: 13.5 GPa [89]), whereas one of the composites studied producing low shrinkage stress had a high rigidity (Filtek P60: 3.3 ± 0.1 MPa; flexural modulus: 19.7 GPa [89]). Some of the results of Chen [45] confirmed the authors’ remarks because in Chen’s study on packable composites, Surefil a product known for its high rigidity (flexural modulus: 18.7 GPa [89]), produced shrinkage stress (3.13 ± 0.18 MPa) which showed no significantly statistical difference to those of Solitaire (3.33 ± 0.23 MPa) and Solitaire2 (3.36 ± 0.08 MPa; flexural modulus: 13.5 GPa [89]); composites known for being more flexible. Two highly filled composites (Clearfil AP-X and Z100) presenting lower polymerization shrinkage and higher elastic modulus (Table 1) led to lower contraction stress. A high filler rate could reduce shrinkage stress because it implicates a reduction of matrix responsible for the contraction phenomenon [41]. Watts et al. [79] verified that increasing the filler content in an experimental composite led to lower polymerization shrinkage, higher elastic modulus and lower contraction stress. On the other hand, studies carried out on commercial composites found a strong positive correlation between filler level and contraction stress [28,46,53], which would indicate the strong influence of the resin matrix composition and especially its DC [62,63], in determining the development of the contraction stress behavior of the composites. These findings also confirm that Hooke’s law should not be directly applied to composite resin polymerization because this is a dynamic process where the contraction and the elastic modulus both increase with time. This phenomenon cannot be predicted by simply considering the value of elastic modulus [47]. Polymerization rate depends on the temperature, mobility and reactivity of the monomers, the concentration of photoinitiator, reducing agent and inhibitor [32]. Some authors calculate the polymerization rate in a simplified arithmetical way by dividing the contraction stress generated at the end of light-exposure by the curing time [45,46]. More than the parameters of kinetic polymerization, this type of calculation characterises the extent of the reaction [62], i.e. the ratio matrix quantity-DC, offering an anticipated approach to the maximum shrinkage stress levels recorded at the end of the trials. Like many authors [62–64,90], those in the current study

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opted for the calculation of the slope situated between the onset of stress (MPa > 0) and the end of irradiation (T60 s ), characterising the stress rate (SR). In fact, it is just after the onset of stress that the higher contraction rate, due to a higher concentration of light-activated radicals, induces a further increase in the stress at the interfacial bond. The authors’ approach does not take the pre-gel phase into account but it does not depend on the DC or the quantity of resin. The one-way ANOVA/Scheffe’s test showed that, whatever the irradiation mode, Solitaire has the lowest SR of all the materials, which confirms the results of other studies [45,46,71,90]. It has been suggested that the reason for this slower polymerization rate may be the shielding from the 400 to 500 nm light of the photoinitiator molecules within the porous filler. At the same time, this product also has a delay in early stress-kinetics (Fig. 3) which Watts and Al-Hindi named the “intrinsic soft-start” phenomenon [71]. It is hypothesized that the porous SiO2 filler particles (32 wt.%) in Solitaire may induce this delay in setting with an extended flow phase. Z100 presented the shortest polymerization rate in groups 1 and 2. This could be explained by the fact that its matrix contains a large quantity of TEGDMA (50 wt.%), tetrafunctional monomer with a low molecular weight, which increase the concentration of the vinyl group (C CH2 ), and at the same time, the reticulation rate [90,91]. Pearson’s correlation procedure has demonstrated that whatever the irradiation mode, SR has no effect on MSS. This result revealed that continuous irradiation for 60 s achieved a clinically relevant conversion; a hypothetical slowness of polymerization did not provide a significant benefit towards the reduction of overall stress. When the cross-linked network density and elastic modulus increased drastically during the post-gel stage, the polymer network’s ability to relieve the stress through chain viscoelastic relaxation became significantly restricted. This assertion agrees with the literature because some studies confirmed that to achieve a large capability to relieve stress, a much longer post-cure relaxation time is required between the gel and the vitrification stages [36,51,53,92,93]. The viscous flow in the pre-gel phase constitutes one of the principal mechanisms at the origin of the relaxation and therefore the reduction of shrinkage stress [92]. To obtain optimal results, one should induce a slowing down of the reticulation rate before the gel point by reducing the formation of free radicals. The most tried and tested solution is to reduce the conversion rate by diminishing the initiation rate through the curing light irradiance. Soft-start or pulse delay curing techniques generate the delay of the gel-point setting [53,64,94–97], which allows an increase in the flow time of the material before mobility is restricted by vitrification. The current results confirm those found in the literature because the exponential ramp successfully reduced the SR (−11%) and in consequence, the MSS values (−3.9%). Most of the basic studies have shown that a reduced light intensity or slower curing technique enables one to reduce shrinkage stresses [53,64] and better marginal adaptation [25,37,39,50,94,98]. However, the efficiency of “soft-start” light intensity on gap formation remains disputed today [99]. According to the current results, these photopolymerization techniques often show a limited effect on polymerization shrinkage [86] and consequently, on polymerization stress. They can also show variable efficiency

according to the materials [95]. These varying research work results could be explained by the differences in energy densities applied to the samples. In fact the higher the radiant exposure, the better the material is cured and the more the curing technique efficiency is limited [44]. This observation may lead to the misinterpretation of positive results in investigations on polymerization stress or marginal integrity [100]. Moreover, since the majority of shrinkage stress is developed during and after vitrification when almost no stress relaxation can be observed [93], the benefit of using lower initial light intensity to relieve the shrinkage stress prior to the vitrification stage is limited (MSS: −3.9%).

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