d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1280–1289
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The effect of ultra-fast photopolymerisation of experimental composites on shrinkage stress, network formation and pulpal temperature rise Luc D. Randolph a,b,c,∗ , William M. Palin d , David C. Watts e , Mathieu Genet a,c , Jacques Devaux b,c , Gaetane Leloup a,b,c,f , Julian G. Leprince a,b,c,f a
Louvain Drug Research Institute, Université catholique de Louvain, Brussels, Belgium Institute of Condensed Matter and Nanosciences, Bio- and Soft- Matter, Université catholique de Louvain, Louvain-la-Neuve, Belgium c CRIBIO (Center for Research and Engineering on Biomaterials), Brussels, Belgium d Biomaterials Unit, University of Birmingham, College of Medical and Dental Sciences, School of Dentistry, St Chad’s Queensway, Birmingham, B4 6NN, UK e School of Dentistry and Photon Science Institute, University of Manchester, UK f School of Dentistry and Stomatology, Université catholique de Louvain, Brussels, Belgium b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. to complement our previous work by testing the null hypotheses that with short
Received 8 August 2014
curing times and high DC, TPO-based resin composites would exhibit (1) higher polymer-
Received in revised form
ization stresses and consequently display (2) higher temperature rise and (3) higher flexural
3 September 2014
modulus, flexural strength and hardness, compared to a conventional CQ-based experimen-
Accepted 3 September 2014
tal composite. Methods. Two experimental resin composites using either Lucirin-TPO or camphorquinone/DMAEMA as photoinitiators were prepared. Light curing was carried out
Keywords:
using spectral outputs adapted to the absorption properties of each initiator. Different irra-
MAPO
diation protocols were selected (0.5, 1, 3, 9 s at 500, 1000 and 2000 mW/cm2 for Lucirin-TPO
Experimental
based composites and 20 or 40 s at 1000 mW/cm2 for Lucirin-TPO and camphorquinone-
Dental composite
based composites). Degree of conversion (DC) was measured in real time by means of FT-NIR
Irradiation parameters
spectroscopy. Pulpal temperature rise (T) was studied in a tooth model. Polymerization
Irradiance
stress was monitored using the Bioman instrument. For cured specimens, flexural modulus
Polymerization kinetics
and flexural strength were determined using a three point bending platform and Vickers
Polymerization stress
hardness was determined with a microhardness indentor on samples prior to and after
Tooth model
24 h incubation in 75/25 ethanol/H2 O. Premolars were restored with both materials and
Temperature rise
microleakage at the teeth/composite interfaces following restoration was assessed.
Microleakage
Results. Lucirin-TPO-based composites irradiated at radiant exposures of 3 J/cm2 and more exhibited significantly higher DCs, associated with increased flexural moduli and hardness compared to CQ-based composites. For an ultra-short irradiation time of 1 s at 1000 mW/cm2 , TPO-composites displayed similar polymerization stresses compared to CQ-controls with
∗ Corresponding author at: Université catholique de Louvain Louvain Drug Research Institute Avenue E. Mounier 73, B-1200 Brussels Belgium. E-mail address: [email protected] (L.D. Randolph). http://dx.doi.org/10.1016/j.dental.2014.09.001 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 ) 1280–1289
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yet a 25% increase for flexural modulus and 40% increase for hardness measured after EtOH/H2 O sorption. Higher stress rates were however observed in all curing protocols compared to CQ-composites. Microleakage was similar between TPO and CQ-composites irradiated at 1000 mW/cm2 for 3 and 20 s respectively, while a significant increase was observed for TPO-composites irradiated for 1 s. T measured through a 0.6 mm thick dentin layer were all below 5.5 ◦ C; TPO-composites exhibited similar or lower values compared to controls. Significance. The use of Lucirin-TPO in resin composites along with appropriate curing conditions may allow for a major reduction of irradiation time while improving mechanical properties. The amount of stress observed during polymerization in TPO-based composites can be similar to those using CQ and the cohesion at the restoration-tooth interface was not affected by short curing times. Contrary to other studies, we found that the temperatures increases measured during polymerization were all well below the 5.5 ◦ C threshold for the pulp. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
According to clinical studies, the major reasons for failure of resin composite restorations are secondary caries, restoration fracture and need for endodontic treatment [1–4]. From a biomimetic standpoint, the materials used should display the same characteristics as the replaced tissues. For dentin, hardness lies in the range of 50–60 HV [5], elastic modulus is around 20–25 GPa [6]. Current commercial resin composites exhibit flexural moduli and strength in the range 2.4–14.7 GPa and 45–145 MPa, respectively and Vickers hardness falls in the range of 18.6–77.7 HV/20 [7]. The wide variation of results may be explained by the different formulations but also the quality of conversion as higher degrees of conversion (DC) usually lead to more rigid [8] and harder structures [9]. Besides mechanical improvement, the materials should present a reliable interface with the dental tissues (generating minimal shrinkage stress on cure) and be safe for the pulp tissue (low monomer elution, low temperature rise in the pulp chamber, etc.). At the same time, resin composites should meet the patients and practitioners requirements, notably the reduction of chairside procedures times, for example by shortening curing time. Despite the clear clinical interest of reducing curing time by improving polymerization kinetics, higher rates could result in greater stresses at the dentin-composite interface, which may in turn induce cusp deflection and/or gaps [10]. However the development of polymerization shrinkage stresses is a complex phenomenon, which was shown to be affected by multiple factors including not only polymerization rate, but also degree of conversion, volumetric shrinkage and elastic modulus [11]. For example, polymerization stress was shown to evolve non-linearly with conversion [12] while volumetric changes are the combination of both contraction due to the polymerization and contraction/expansion related to thermal effects [13]. Finally, an additional possible concern of faster polymerization with higher conversion and higher irradiances is the heat generated during polymerization, which could damage the underlying pulp [14,15]. Besides, reaction temperature rise (T) was also positively correlated to polymerization shrinkage [16].
In a recent paper by our group, using a model experimental BisGMA/TegDMA composite formulation, we demonstrated the possibility of improving the degree of conversion (DC) and significantly reducing monomer elution despite a significant reduction of curing time (1 s for a 2 mm composite layer at 1000 mW/cm2 ) [17]. This was achieved first by replacing the common ketone (camphorquinone)/amine photoinitiation system (CQ) with a Norrish Type I monoacylphosphine oxide photoinitiator, namely Lucirin-TPO (TPO), and also by choosing appropriate irradiation parameters (wavelength range, irradiance and irradiation time). Several other studies have already demonstrated that the curing light source is critical in realizing the potential of TPO and that switching from CQ to TPO, and other photoinitiators with higher molar absorptivity is advantageous for materials development [18–22]. Further, the clinicians’ desire for reduced curing time (3 s, or less) has seen an influx to the dental market of high irradiance (>2000 mW/cm2 ) LED light curing units that may not provide sufficient cure of some composite material types [23,24]. If such short curing times are desired, there is a requirement for the development of materials chemistry, rather than simply increasing the power of the curing light source. Overall, both intrinsic and extrinsic parameters should be taken into account in order to optimize the polymerization efficacy and resulting material properties [25]. Despite these clear improvements, it remains unclear whether the dramatic increase in reaction speed seen in TPO-based composites [19,20], associated with improved conversion [17,19,20] and mechanical properties [20], will be detrimental in terms of shrinkage stress at short curing times. Since the generation of polymerization stress throughout cure is not an intrinsic material property, its comprehensive evaluation requires the investigation of other central parameters, i.e. degree of conversion, temperature rise and elastic modulus. Hence, the aim of the present study was to complement our previous work [17] by testing the null hypotheses that with short curing times and high DC, TPO-based resin composites would exhibit (1) higher polymerization stresses and consequently display (2) higher temperature rise and (3) higher flexural modulus, flexural strength and hardness, compared to
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a conventional CQ-based experimental composite cured for 20 or 40 s at 1000 mW/cm2 .
2.
Materials and methods
2.1.
Materials
Two experimental composites materials were prepared, based on a 70/30 wt% Bis-GMA/TegDMA resin (Sigma–Aldrich), using two different photoinitiators systems in equimolar concentration (1.34 × 10−5 mol cm−3 ): either camphorquinone and dimethylaminoethyl methacrylate (altogether CQ, from Sigma Aldrich) or Lucirin-TPO (TPO, from BASF). Initiators accounted for 0.2/0.8 wt% and 0.42 wt% for CQ and TPO, respectively. Fillers were silanated barium glass (G018186/K6, d50 = 3 ± 1 m, Schott AG, Landshut Germany) and methacrylsilane treated fumed silica (12 nm, Evonik Industries, Germany) introduced in amounts of 65/10 wt% respectively. The fillers were dispersed using a dual asymmetric centrifuge (Speed mixer, FlackTek, USA).
2.2.
Flexural bending test
Rectangular specimens for the flexural bending test were polymerized in Teflon moulds of dimensions 25 × 2 × 2 mm (n = 5 following ISO4049). Light-curing was carried out with an AURA light device (Lumencor, USA) providing narrow spectral outputs. Irradiation could be applied independently at two wavelength ranges: 395–415 nm and 455–485 nm for TPO and CQ-based composites, respectively (Fig. 1). Light irradiance calibrations were carried out as described previously [17]. Irradiation was applied in 5 successive and non-overlapping cycles starting at the center of the sample, then extended towards the ends. During each irradiation, Mylar films were used to cover the top and bottom surfaces of the samples and opaque masks with a 5 mm diameter window (the dimension
of the light guide) were used to cover the adjacent sample areas and thereby reduce overlapping irradiation areas due to light diffusion. The bars were stored for 24 h in the dark at room temperature before being tested. The CQ-based composites (controls) were irradiated for 20 and 40 s at 1000 mW/cm2 . TPO-composites were irradiated at three different irradiances for: 2000 mW/cm2 for 9, 3, 1 or 0.5 s, 1000 mW/cm2 for 40 s 20, 9, 3, 1 or 0.5 s and 500 mW/cm2 for 9, 3, 1 or 0.5 s. Flexural testing was conducted using a universal testing machine (Lloyd LRX Plus, Lloyd Instruments), with a 20 mm span between supports and 0.75 mm/min crosshead speed. Flexural modulus was calculated according to Eq. (1) and flexural strength was calculated according to Eq. (2), where F is the load (N), l is the distance between supports (mm), w and t are the width and thickness of the sample (mm) and d is the deflection due to the load F (mm). E=
Fl3 4wt3 d
rupt =
2.3.
(2)
Vickers hardness
5 mm diameter, 2 mm thickness disc-shaped specimens were prepared in split Teflon moulds and were irradiated following the same protocol as described in the previous section (n = 3). Following irradiation, the Mylar films were removed, samples were extracted from the moulds and were stored for 24 h in the dark at room temperature before being tested. A first series of hardness measurements was then carried out. Subsequently, samples were incubated for another 24 h in 75/25 vol% ethanol/H2 O; period after which hardness was again measured. In both cases, measurements were carried out on the upper surface (two per surface) with a microhardness tester (Durimet,Leitz, Wetzlar, Germany) using a 100 g load (F = 0.1 kgf) and 30 s indentation period. Hardness calculation was done using mean diagonal values of indentations (d) and Eq. (3). HV =
2.4.
Fig. 1 – Emission spectra of the AURA light engine two independent outputs (Lumencor) compared to the absorption spectra of Camphorquinone (CQ) and Lucirin-TPO (TPO).
3Fl 2wt2
(1)
1.854F d2
(3)
Polymerization stress
The stress generated during the polymerization reaction was measured using a single cantilever device, the Bioman instrument, as previously described [26]. In the present study the specimen layer was restricted to 0.9 ± 0.05 mm to limit the flowing of the material. The light source was located below the glass slide at a distance of 7.5 mm from the bottom surface of the sample and calibrations were repeated in order to obtain actual irradiances of 500, 1000 and 2000 mW/cm2 . The force (stress) generated by the material was measured via the strain-gage signal of the cantilever load cell and using Eq. (4) (n = 3). p =
kV S
(4)
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1280–1289
where V is the voltage difference read at the sensor during the measurement, k is a constant such as k V is equivalent to Newtons and S is the surface of the sensor (mm2 ). Maximum polymerization stress was determined as the value reached after 4 min of acquisition and rates were calculated as the first derivative against time.
2.5.
Temperature analysis
The temperature variations associated with polymerization and irradiation were monitored using an experimental setup. A composite specimen was placed in a Teflon mold (2 mm thick, 5 mm inner diameter), itself resting on a molar tooth which was machined to leave a 0.6 mm thick dentin roof, accommodate a temperature sensor in the pulp chamber and present a flat occlusal surface (Fig. 2(a)). The tooth was partially immersed in a temperature-controlled water bath (37 ± 1 ◦ C) as shown in Fig. 2 b). Irradiation was carried out as in other analyses, with the light guide in contact with the Mylar strip covering the composite specimen. Temperature was recorded with a 4-wire platinum sensor and data acquisition module (HSRTD-3-100-A-1 M and PT-104A respectively, Omega, UK) every 0.1 s with an accuracy of 0.15 ◦ C (n = 3). The maximum temperature changes are reported (T).
2.6.
Degree of polymerization
Real-time DC measurement was performed using Fourier Transform Near Infrared Spectroscopy (FT-NIRS) as previously described (n = 3) [19]. Briefly, resin composites were placed in white Teflon disk-shaped moulds (12 × 2 mm) with lower surfaces in contact with a glass microscope slides (1.2 mm thick). Degrees of conversion were determined in real-time by monitoring the changes in peak height associated with the reaction of vinyl CH2 . DCs reported in Table 1 represent mean values at 110 s of monitoring.
2.7.
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Microleakage
15 wisdom teeth extracted for therapeutic reasons were distributed into 3 groups (n = 5), CQ 20 s, TPO 1 s and TPO 3 s, 1000 mW/cm2 . They were stored in 0.5% chloramine solution at 6 ◦ C until cavity preparation. After been cleaned (soft tissue and calculus were removed), two class V cavities (2 mm depth, 4 mm width and 2 mm height) were prepared per tooth on the buccal and lingual surfaces under continuous water irrigation. The cavities extended 1 mm on either side of the cementenamel junction, with a bevel on the enamel side. The gold standard total-etch adhesive Optibond FL (Kerr) was selected and applied following the manufacturer’s instructions. Briefly, the cavity was conditioned with 37.5% phosphoric acid (Gell Etchant, Kerr) during 30 s for enamel and 15 s for dentin. The primer was applied, followed by the bonding, and light irradiation was carried out using the AURA light engine (emitting at 455–485 nm) during 20 s at 1000 mW/cm2 to ensure protocol consistency. Cavities were then restored in a single layer using the TPO or CQ-based resin composites and using the appropriate lamp outputs as described in previous sections. Restored teeth were then polished using Sof-Lex Finishing discs (3M-ESPE) and root apices were sealed with a resinmodified glass-ionomer (Fuji II, GC) to prevent the dye from penetrating through the root canals. The teeth were then thermocycled for 100 cycles between two water-baths kept at temperatures of 1.4 ± 0.5 ◦ C and 60 ± 1 ◦ C. A cycle consisted of 10 s immersion in one bath, 30 s transfer-time and another 10 s immersion in the other bath. Immediately following thermocycling the teeth were placed in a 0.5% basic fushin solution for 24 h and kept at 37 ◦ C ± 1 ◦ C. Finally, the teeth were embedded in epoxy resin and cut to obtain three 0.5 mm-thick sections per tooth. Given the inherent structural inhomogeneity of dentin, each section was considered as an independent system, yielding a total of 30 sections per group to be scored (3 sections per cavity, 2 cavities per tooth and 5 teeth per group). Microleakage was recorded under 10x magnification at the enamel and dentin interface by a blinded evaluator: a score of ‘0 was assigned in the absence of dye penetration, while ‘1 or ‘2 were assigned for dye penetration along the enamel or gingival floor up to or exceeding half the restoration depth, respectively. A score of ‘3 was given when the dye stained the axial wall.
2.8.
Statistical analysis
Statistical analyses were performed using the JMP statistical software (SAS Institute Inc.). All data sets were analyzed using one-way analysis of variance (ANOVA) and with Tukey tests (˛ = 0.05) except for the microleakage results. In this case, paired group comparison were carried out using the nonparametric Wilcoxon test (˛ = 0.05).
3.
Fig. 2 – Tooth preparation (a), general setup for pulpal temperature rise analysis (b) and evolution of temperature for TPO and CQ composites irradiated at 1000 mW/cm2 (c).
Results
Polymerization occurred faster in TPO-composites compared to the controls as seen in Fig. 3. Final DCs were significantly higher (p < 0.05) in TPO-composites compared to CQ controls for irradiation times equal to or greater than 3 s (Table 1).
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Table 1 – Mean values for final DC, maximum polymerization stress, stress rate and Tmax of all TPO and CQ-composites. Standard deviation in parentheses. Same superscript connects statistically similar values (p < 0.05). Irradiation time (s)/Irradiance (mW/cm2 ) TPO 0.5/500 1/500 3/500 9/500 0.5/1000 1/1000 3/1000 9/1000 20/1000 40/1000 0.5/2000 1/2000 3/2000 9/2000 CQ 20/1000 40/1000
Radiant energy (J/cm2 )
Final DC (%)
Final p (MPa)
Stress rate (MPa/s)
T (◦ C)
0.25 0.5 1.5 4.5 0.5 1 3 9 20 40 1 2 6 18
42 (2)H 56 (1)F,G 65 (1)B,C 68 (1)A,B 54 (2)G 63 (2)D,E 70 (1)A 69 (1)A 70 (1)A 69 (1)A 64 (1)C,D 68 (2)A,B 70 (1)A 70 (1)A
2.4 (0.2)F 2.9 (0.2)E,F 3.4 (0.2)D,E 4.4 (0.3)A,B 3.2 (0.2)D,E 3.7 (0.2)B,C,D 4.3 (0.2)A,B,C 4.6 (0.1)A 4.6 (0.3)A 4.9 (0.1)A 3.6 (0.2)C,D 3.5 (0.4)D,E 4.6 (0.3)A 4.9 (0.3)A
0.9 (0.1)E 1.3 (0.1)D,E 1.2 (0.1)D,E 1.3 (0.1)D,E 1.6 (0.3)C,D 1.9 (0.2)A,B,C 1.9 (0.1)A,B,C 1.7 (0.1)B,C,D 2.0 (0.3)A,B,C 2.0 (0.1)A,B,C 2.3 (0.4)A,B 2.3 (0.2)A,B 2.1 (0.2)A,B,C 2.5 (0.2)A
0.43 (0.05)I 0.54 (0.05)H,I 0.75 (0.03)E,F,G 0.92 (0.01)D,E 0.59 (0.03)G,H,I 0.73 (0.06)F,G 0.83 (0.03)E,F 1.14 (0.09)C 1.52 (0.04)B 1.88 (0.06)A 0.68 (0.03)F,G,H 0.81 (0.07)E,F 1.02 (0.13)C,D 1.45 (0.05)B
20 40
57 (1)E,F 60 (1)E,F
3.5 (0.2)D,E 3.5 (0.3)D,E
0.2 (0.1)F 0.2 (0.1)F
1.21 (0.08)C 2.05 (0.07)A
Under optimal conditions, TPO-composites display an average 10% conversion increase. Polymerization stresses increased in a logarithmic pattern with time, maximum stress rate being reached in less than 3 s for TPO-composites and between 5 and 10 s for CQ-controls (Fig. 4). T increased with time, reaching a maximum between 20 and 40 s, depending on the curing parameters (Fig. 2(c)). In the case of CQ-controls, a sharp drop of T curves could be observed, while T decrease was more progressive for all TPO-composites. For 20 and 40 s curing modes, Tmax was reached when the curing light was switched off (as indicated by red arrows on Fig. 2 c)). However, Tmax was reached 10–20 s after the light was switched off for TPO-materials cured during 9 s or less. When plotting maximum stress and Tmax against radiant energy, different trends were observed (Fig. 5 c)). In TPO-composites, the maximum polymerization stress reached a plateau at approximately 5 J/cm2 while Tmax continued to increase linearly above that level of radiant exposure. With similar irradiation parameters (20 or 40 s at 1000 mW/cm2 ), TPO-composites and controls exhibited similar Tmax as controls, but the latter showed lower maximum stress (Table 1, Fig. 5). No improvements were observed when curing time was increased from 20 to 40 s for CQ controls: they displayed similar DCmax and maximum polymerization stress. TPO-composites displayed higher polymerization stresses compared to the CQ controls when irradiated for 3 s or more for irradiances greater than 1000 mW/cm2 . Maximum stress rates were all significantly higher for TPO-composites, by at least a factor 5 (Fig. 3 and Table 1). Flexural modulus was in most cases significantly higher in TPO-composites compared with controls, except for three curing parameters (0.5 and 1 s at 500 mW/cm2 and 0.5 s at 1000 mW/cm2 (Fig. 6(a)). Flexural strength values were similar between TPO and controls except for the 500–0.5 protocol (Fig. 6 b)). Upper surface hardness increased both with irradiation time and irradiance in TPO-composites. Hardness values were significantly higher than in controls for irradiation times greater than 1s at 1000 and 2000 mW/cm2 (Fig. 6(c). After incubation in the ethanol solution, hardness dropped by 39 to 47%
in controls while the drop in TPO-composites ranged between 21 and 36%. Hardness values of TPO-composites after storage in ethanol solution were all similar to or higher than controls. No microleakage was observed at the enamel/composite interface for all samples. For the dentin/composite interface, wilcoxon’s test on paired groups indicated a significantly higher microleakage for the TPO 1 s group compared to CQ 20 s (p = 0.0274) and TPO 3 s (p = 0.0102). TPO 3 s and CQ 20 s did not show any significant difference (p = 0.6894). Fig. 7 shows the fraction of scores attributed to the different groups.
4.
Discussion
The three null hypotheses, namely that TPO-based composites cured at short curing times and displaying high DC would result in (1) higher polymerization stresses, (2) higher temperature rises, and (3) higher flexural modulus, flexural strength and hardness compared to a conventional CQ-based composite, were all rejected. First, TPO-based materials generated higher polymerization stresses than those generated by the controls, but only for some curing protocols. Second, temperature increases were in most cases lower than those measured for the CQ controls. Third, flexural modulus and hardness were significantly higher in TPO-composites than in CQ-controls for most but not all curing protocols, while flexural strength of TPO-composites and CQ-controls were similar (except for one condition). The curing kinetics measured with FT-NIRS were in accordance with previous literature comparing CQ and TPO materials [19], where TPO-based composites reached their maximum DC in less than 5 s when using high irradiances (1000 mW/cm2 or above). In the present work, 95% of DC was reached in about 2–3 s (Fig. 3). In accordance with our previous study on the same materials/curing conditions [17], TPO-composites did not benefit from an increase in irradiance and/or irradiation time above a given threshold. More precisely this corresponded to irradiation protocols of 3 s irradiation time at 1000 and 2000 mW/cm2 (3–6 J/cm2 ) and 9 s
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1280–1289
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Fig. 4 – Evolution of polymerization stress for TPO-composites (black) and CQ-composites (grey) irradiated at 1000 mW/cm2 (a). Evolution of stress rates for the same materials (b).
Fig. 3 – Evolution of DCs over time in TPO-composites irradiated at 2000 mW/cm2 (a), TPO and CQ-composites irradiated at 1000 mW/cm2 (b) and TPO-composites irradiated at 500 mW/cm2 (c). Red vertical arrows indicate irradiation times and are reproduced with the same length on (a), (b) and (c).
at 500 mW/cm2 (4.5 J/cm2 ) (Table 1). These findings confirmed what was previously reported using electron paramagnetic resonance [17], wherein plotting radical concentration against radiant energy revealed a plateau following 3 J/cm2 , and therefore, 3 s of irradiation at 1000 mW/cm2 yielded optimal curing with sub-optimal states below that threshold. In the present work, the existence of a radiant exposure threshold was confirmed for polymerization stress, with a plateau value reached around 3–5 J/cm2 (Fig. 5(b)). In comparison, the build-up of polymerization stress in CQbased controls was much slower than TPO-composites, with no difference between 20 or 40 J/cm2 . This is in accordance with the DC data (Fig. 3), where DC of controls reached 95% of the final DC value after 20 s of irradiation. This slower stress
build-up could be expected, since the polymerization reaction of CQ-based composites has been reported to depend more on irradiation time rather than on irradiance [8,27,28]. Specifically regarding the direct comparison of CQ vs TPO materials, it was shown that the latter cure better when irradiated for 45 s at 400 mW/cm2 than at 6 s at 3000 mW/cm2 [19] (both 18 J/cm2 ). Altogether these results tend to indicate that while CQ-based RBC require long irradiation times, TPO-composites have the ability to cure in a fraction of that time, hence requiring a much lower energy to reach full cure [17]. Polymerization stress is a corollary of chain lengthening, entanglement and cross-linking during curing [10]. The evolution of stress in resin composites occurs non-linearly with conversion [12,29]. In dimethacrylate resins or resin composite systems, polymerization proceeds following a diffusion-controlled mechanism [30] and in final stages, crosslinking induces a major change in viscosity. Beyond this point, any increase in conversion results in further contraction in a highly rigid material and hence increased stress and the present results for TPO-composites follow such a pattern (Table 1). As noted previously, at radiant energies of 3 J/cm2 and above, the stress reaches a plateau with final values of about 4–5 MPa (Fig. 5 b)). Since maximum DC is reached at a similar threshold, the increase, then saturation of maximum stress with increasing radiant energy
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Fig. 5 – Average final DC versus maximum polymerization stress (a), maximum polymerization stress and Tmax versus radiant energy (b) and (c) respectively for all TPO (blue) and CQ-composites (white).
is logically linked to the evolution in the final stages of polymerization. Contrary to stress curves, Tmax curves continued increasing in a linear fashion above the 3 J/cm2 radiant energy threshold, and up to 40 J/cm2 (Fig. 5(c)). To discriminate between T due to the reaction exotherm and T from the curing light, it could be approximated that the temperature rise purely related to the polymerization reaction corresponded to T in specimens with saturated DC and short curing time (e.g. 3 s at 1000 mW/cm2 ). For these parameters, the light was indeed switched off long before T curves reached their maximum (Fig. 2(c)). The amount of heat purely due to the chemical reaction was then about 0.8 ◦ C for TPOcomposites (through 0.6 mm dentin, and with the specific
experimental setup of this work). Any additional temperature increase should then be attributed to excess energy provided by the LCU and not used by the composite for additional propagation [13,16,19]. Following this reasoning, the increase of T at short curing times is concomitant to the increase of stress and conversion until the threshold. Above the latter however, longer irradiation times induced composite over-heating without any noticeable change in conversion measured within the timeframe of the experiment. Similarly, for CQ-composites, the increase from 20 to 40 s did not result in DC increase, but an almost two-fold T increase was observed. As mentioned in previous work, curing parameters should be optimized to reach optimal curing but at the same time avoid unnecessary composite – and pulp – overheating [31]. It has been previously described that the onset of vitrification is delayed in TPO-composites [19,20] compared to CQ-based materials, possibly as a result of transfer reactions due to the TPO system [32]. In methacrylate networks, such mechanism is known to promote higher conversion degrees and reduced stress [33]. This trend was confirmed in the present study, since similar stresses were indeed observed in CQ- and TPO-based materials, despite the higher DCs measured in the latter. Vitrification is the stage where the polymer matrix fully rigidifies. It may be defined as the point where kinetically, the exotherm of the reaction itself is no longer sufficient to provide mobility to propagating radicals in an increasingly viscous medium. In this situation any additional heat provided by the LCU to the system could be expected to postpone that vitrification by providing additional mobility. In the present study, we used an open system where the temperature measurement had high clinical relevance but lacked the adiabatic quality of systems such as differential scanning calorimetry. However, given the precision of our measurement, it still made sense comparing temperature increases in relation with matrix formation. The additional heat provided by the LCU was clearly not enough to significantly increase conversion, possibly indicating that T increase was insufficient to increase Tg in TPO-composites [34]. Here Tg is defined as the temperature above which a cross-linked matrix fully relaxes [35]. In order for the heating from the LCU to have a major impact on DC, it should preferably occur during polymerization, i.e. when T of both LCU and reaction are cumulated. This would be difficult to observe in TPO-composites since the polymer matrix was already highly crosslinked after 3 s as indicated by FTNIRS (Fig. 2) and previous results [19,20]. In CQ-composites where polymerization is inherently slower, this is more likely to occur, but again it did not appear to be the case from DC values, which are at best similar, and in general lower than TPO-composites. Hence, these observations tended to show that the levels of temperature rise in the materials were insufficient to significantly move Tg , but also relatively safe for the pulp tissues when placed on a 0.6 mm thick wet dentin layer (
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
The effect of ultra-fast photopolymerisation of experimental composites on shrinkage stress, network formation and pulpal temperature rise Luc D. Randolph a,b,c,∗ , William M. Palin d , David C. Watts e , Mathieu Genet a,c , Jacques Devaux b,c , Gaetane Leloup a,b,c,f , Julian G. Leprince a,b,c,f a
Louvain Drug Research Institute, Université catholique de Louvain, Brussels, Belgium Institute of Condensed Matter and Nanosciences, Bio- and Soft- Matter, Université catholique de Louvain, Louvain-la-Neuve, Belgium c CRIBIO (Center for Research and Engineering on Biomaterials), Brussels, Belgium d Biomaterials Unit, University of Birmingham, College of Medical and Dental Sciences, School of Dentistry, St Chad’s Queensway, Birmingham, B4 6NN, UK e School of Dentistry and Photon Science Institute, University of Manchester, UK f School of Dentistry and Stomatology, Université catholique de Louvain, Brussels, Belgium b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. to complement our previous work by testing the null hypotheses that with short
Received 8 August 2014
curing times and high DC, TPO-based resin composites would exhibit (1) higher polymer-
Received in revised form
ization stresses and consequently display (2) higher temperature rise and (3) higher flexural
3 September 2014
modulus, flexural strength and hardness, compared to a conventional CQ-based experimen-
Accepted 3 September 2014
tal composite. Methods. Two experimental resin composites using either Lucirin-TPO or camphorquinone/DMAEMA as photoinitiators were prepared. Light curing was carried out
Keywords:
using spectral outputs adapted to the absorption properties of each initiator. Different irra-
MAPO
diation protocols were selected (0.5, 1, 3, 9 s at 500, 1000 and 2000 mW/cm2 for Lucirin-TPO
Experimental
based composites and 20 or 40 s at 1000 mW/cm2 for Lucirin-TPO and camphorquinone-
Dental composite
based composites). Degree of conversion (DC) was measured in real time by means of FT-NIR
Irradiation parameters
spectroscopy. Pulpal temperature rise (T) was studied in a tooth model. Polymerization
Irradiance
stress was monitored using the Bioman instrument. For cured specimens, flexural modulus
Polymerization kinetics
and flexural strength were determined using a three point bending platform and Vickers
Polymerization stress
hardness was determined with a microhardness indentor on samples prior to and after
Tooth model
24 h incubation in 75/25 ethanol/H2 O. Premolars were restored with both materials and
Temperature rise
microleakage at the teeth/composite interfaces following restoration was assessed.
Microleakage
Results. Lucirin-TPO-based composites irradiated at radiant exposures of 3 J/cm2 and more exhibited significantly higher DCs, associated with increased flexural moduli and hardness compared to CQ-based composites. For an ultra-short irradiation time of 1 s at 1000 mW/cm2 , TPO-composites displayed similar polymerization stresses compared to CQ-controls with
∗ Corresponding author at: Université catholique de Louvain Louvain Drug Research Institute Avenue E. Mounier 73, B-1200 Brussels Belgium. E-mail address: [email protected] (L.D. Randolph). http://dx.doi.org/10.1016/j.dental.2014.09.001 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 ) 1280–1289
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yet a 25% increase for flexural modulus and 40% increase for hardness measured after EtOH/H2 O sorption. Higher stress rates were however observed in all curing protocols compared to CQ-composites. Microleakage was similar between TPO and CQ-composites irradiated at 1000 mW/cm2 for 3 and 20 s respectively, while a significant increase was observed for TPO-composites irradiated for 1 s. T measured through a 0.6 mm thick dentin layer were all below 5.5 ◦ C; TPO-composites exhibited similar or lower values compared to controls. Significance. The use of Lucirin-TPO in resin composites along with appropriate curing conditions may allow for a major reduction of irradiation time while improving mechanical properties. The amount of stress observed during polymerization in TPO-based composites can be similar to those using CQ and the cohesion at the restoration-tooth interface was not affected by short curing times. Contrary to other studies, we found that the temperatures increases measured during polymerization were all well below the 5.5 ◦ C threshold for the pulp. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
According to clinical studies, the major reasons for failure of resin composite restorations are secondary caries, restoration fracture and need for endodontic treatment [1–4]. From a biomimetic standpoint, the materials used should display the same characteristics as the replaced tissues. For dentin, hardness lies in the range of 50–60 HV [5], elastic modulus is around 20–25 GPa [6]. Current commercial resin composites exhibit flexural moduli and strength in the range 2.4–14.7 GPa and 45–145 MPa, respectively and Vickers hardness falls in the range of 18.6–77.7 HV/20 [7]. The wide variation of results may be explained by the different formulations but also the quality of conversion as higher degrees of conversion (DC) usually lead to more rigid [8] and harder structures [9]. Besides mechanical improvement, the materials should present a reliable interface with the dental tissues (generating minimal shrinkage stress on cure) and be safe for the pulp tissue (low monomer elution, low temperature rise in the pulp chamber, etc.). At the same time, resin composites should meet the patients and practitioners requirements, notably the reduction of chairside procedures times, for example by shortening curing time. Despite the clear clinical interest of reducing curing time by improving polymerization kinetics, higher rates could result in greater stresses at the dentin-composite interface, which may in turn induce cusp deflection and/or gaps [10]. However the development of polymerization shrinkage stresses is a complex phenomenon, which was shown to be affected by multiple factors including not only polymerization rate, but also degree of conversion, volumetric shrinkage and elastic modulus [11]. For example, polymerization stress was shown to evolve non-linearly with conversion [12] while volumetric changes are the combination of both contraction due to the polymerization and contraction/expansion related to thermal effects [13]. Finally, an additional possible concern of faster polymerization with higher conversion and higher irradiances is the heat generated during polymerization, which could damage the underlying pulp [14,15]. Besides, reaction temperature rise (T) was also positively correlated to polymerization shrinkage [16].
In a recent paper by our group, using a model experimental BisGMA/TegDMA composite formulation, we demonstrated the possibility of improving the degree of conversion (DC) and significantly reducing monomer elution despite a significant reduction of curing time (1 s for a 2 mm composite layer at 1000 mW/cm2 ) [17]. This was achieved first by replacing the common ketone (camphorquinone)/amine photoinitiation system (CQ) with a Norrish Type I monoacylphosphine oxide photoinitiator, namely Lucirin-TPO (TPO), and also by choosing appropriate irradiation parameters (wavelength range, irradiance and irradiation time). Several other studies have already demonstrated that the curing light source is critical in realizing the potential of TPO and that switching from CQ to TPO, and other photoinitiators with higher molar absorptivity is advantageous for materials development [18–22]. Further, the clinicians’ desire for reduced curing time (3 s, or less) has seen an influx to the dental market of high irradiance (>2000 mW/cm2 ) LED light curing units that may not provide sufficient cure of some composite material types [23,24]. If such short curing times are desired, there is a requirement for the development of materials chemistry, rather than simply increasing the power of the curing light source. Overall, both intrinsic and extrinsic parameters should be taken into account in order to optimize the polymerization efficacy and resulting material properties [25]. Despite these clear improvements, it remains unclear whether the dramatic increase in reaction speed seen in TPO-based composites [19,20], associated with improved conversion [17,19,20] and mechanical properties [20], will be detrimental in terms of shrinkage stress at short curing times. Since the generation of polymerization stress throughout cure is not an intrinsic material property, its comprehensive evaluation requires the investigation of other central parameters, i.e. degree of conversion, temperature rise and elastic modulus. Hence, the aim of the present study was to complement our previous work [17] by testing the null hypotheses that with short curing times and high DC, TPO-based resin composites would exhibit (1) higher polymerization stresses and consequently display (2) higher temperature rise and (3) higher flexural modulus, flexural strength and hardness, compared to
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a conventional CQ-based experimental composite cured for 20 or 40 s at 1000 mW/cm2 .
2.
Materials and methods
2.1.
Materials
Two experimental composites materials were prepared, based on a 70/30 wt% Bis-GMA/TegDMA resin (Sigma–Aldrich), using two different photoinitiators systems in equimolar concentration (1.34 × 10−5 mol cm−3 ): either camphorquinone and dimethylaminoethyl methacrylate (altogether CQ, from Sigma Aldrich) or Lucirin-TPO (TPO, from BASF). Initiators accounted for 0.2/0.8 wt% and 0.42 wt% for CQ and TPO, respectively. Fillers were silanated barium glass (G018186/K6, d50 = 3 ± 1 m, Schott AG, Landshut Germany) and methacrylsilane treated fumed silica (12 nm, Evonik Industries, Germany) introduced in amounts of 65/10 wt% respectively. The fillers were dispersed using a dual asymmetric centrifuge (Speed mixer, FlackTek, USA).
2.2.
Flexural bending test
Rectangular specimens for the flexural bending test were polymerized in Teflon moulds of dimensions 25 × 2 × 2 mm (n = 5 following ISO4049). Light-curing was carried out with an AURA light device (Lumencor, USA) providing narrow spectral outputs. Irradiation could be applied independently at two wavelength ranges: 395–415 nm and 455–485 nm for TPO and CQ-based composites, respectively (Fig. 1). Light irradiance calibrations were carried out as described previously [17]. Irradiation was applied in 5 successive and non-overlapping cycles starting at the center of the sample, then extended towards the ends. During each irradiation, Mylar films were used to cover the top and bottom surfaces of the samples and opaque masks with a 5 mm diameter window (the dimension
of the light guide) were used to cover the adjacent sample areas and thereby reduce overlapping irradiation areas due to light diffusion. The bars were stored for 24 h in the dark at room temperature before being tested. The CQ-based composites (controls) were irradiated for 20 and 40 s at 1000 mW/cm2 . TPO-composites were irradiated at three different irradiances for: 2000 mW/cm2 for 9, 3, 1 or 0.5 s, 1000 mW/cm2 for 40 s 20, 9, 3, 1 or 0.5 s and 500 mW/cm2 for 9, 3, 1 or 0.5 s. Flexural testing was conducted using a universal testing machine (Lloyd LRX Plus, Lloyd Instruments), with a 20 mm span between supports and 0.75 mm/min crosshead speed. Flexural modulus was calculated according to Eq. (1) and flexural strength was calculated according to Eq. (2), where F is the load (N), l is the distance between supports (mm), w and t are the width and thickness of the sample (mm) and d is the deflection due to the load F (mm). E=
Fl3 4wt3 d
rupt =
2.3.
(2)
Vickers hardness
5 mm diameter, 2 mm thickness disc-shaped specimens were prepared in split Teflon moulds and were irradiated following the same protocol as described in the previous section (n = 3). Following irradiation, the Mylar films were removed, samples were extracted from the moulds and were stored for 24 h in the dark at room temperature before being tested. A first series of hardness measurements was then carried out. Subsequently, samples were incubated for another 24 h in 75/25 vol% ethanol/H2 O; period after which hardness was again measured. In both cases, measurements were carried out on the upper surface (two per surface) with a microhardness tester (Durimet,Leitz, Wetzlar, Germany) using a 100 g load (F = 0.1 kgf) and 30 s indentation period. Hardness calculation was done using mean diagonal values of indentations (d) and Eq. (3). HV =
2.4.
Fig. 1 – Emission spectra of the AURA light engine two independent outputs (Lumencor) compared to the absorption spectra of Camphorquinone (CQ) and Lucirin-TPO (TPO).
3Fl 2wt2
(1)
1.854F d2
(3)
Polymerization stress
The stress generated during the polymerization reaction was measured using a single cantilever device, the Bioman instrument, as previously described [26]. In the present study the specimen layer was restricted to 0.9 ± 0.05 mm to limit the flowing of the material. The light source was located below the glass slide at a distance of 7.5 mm from the bottom surface of the sample and calibrations were repeated in order to obtain actual irradiances of 500, 1000 and 2000 mW/cm2 . The force (stress) generated by the material was measured via the strain-gage signal of the cantilever load cell and using Eq. (4) (n = 3). p =
kV S
(4)
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1280–1289
where V is the voltage difference read at the sensor during the measurement, k is a constant such as k V is equivalent to Newtons and S is the surface of the sensor (mm2 ). Maximum polymerization stress was determined as the value reached after 4 min of acquisition and rates were calculated as the first derivative against time.
2.5.
Temperature analysis
The temperature variations associated with polymerization and irradiation were monitored using an experimental setup. A composite specimen was placed in a Teflon mold (2 mm thick, 5 mm inner diameter), itself resting on a molar tooth which was machined to leave a 0.6 mm thick dentin roof, accommodate a temperature sensor in the pulp chamber and present a flat occlusal surface (Fig. 2(a)). The tooth was partially immersed in a temperature-controlled water bath (37 ± 1 ◦ C) as shown in Fig. 2 b). Irradiation was carried out as in other analyses, with the light guide in contact with the Mylar strip covering the composite specimen. Temperature was recorded with a 4-wire platinum sensor and data acquisition module (HSRTD-3-100-A-1 M and PT-104A respectively, Omega, UK) every 0.1 s with an accuracy of 0.15 ◦ C (n = 3). The maximum temperature changes are reported (T).
2.6.
Degree of polymerization
Real-time DC measurement was performed using Fourier Transform Near Infrared Spectroscopy (FT-NIRS) as previously described (n = 3) [19]. Briefly, resin composites were placed in white Teflon disk-shaped moulds (12 × 2 mm) with lower surfaces in contact with a glass microscope slides (1.2 mm thick). Degrees of conversion were determined in real-time by monitoring the changes in peak height associated with the reaction of vinyl CH2 . DCs reported in Table 1 represent mean values at 110 s of monitoring.
2.7.
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Microleakage
15 wisdom teeth extracted for therapeutic reasons were distributed into 3 groups (n = 5), CQ 20 s, TPO 1 s and TPO 3 s, 1000 mW/cm2 . They were stored in 0.5% chloramine solution at 6 ◦ C until cavity preparation. After been cleaned (soft tissue and calculus were removed), two class V cavities (2 mm depth, 4 mm width and 2 mm height) were prepared per tooth on the buccal and lingual surfaces under continuous water irrigation. The cavities extended 1 mm on either side of the cementenamel junction, with a bevel on the enamel side. The gold standard total-etch adhesive Optibond FL (Kerr) was selected and applied following the manufacturer’s instructions. Briefly, the cavity was conditioned with 37.5% phosphoric acid (Gell Etchant, Kerr) during 30 s for enamel and 15 s for dentin. The primer was applied, followed by the bonding, and light irradiation was carried out using the AURA light engine (emitting at 455–485 nm) during 20 s at 1000 mW/cm2 to ensure protocol consistency. Cavities were then restored in a single layer using the TPO or CQ-based resin composites and using the appropriate lamp outputs as described in previous sections. Restored teeth were then polished using Sof-Lex Finishing discs (3M-ESPE) and root apices were sealed with a resinmodified glass-ionomer (Fuji II, GC) to prevent the dye from penetrating through the root canals. The teeth were then thermocycled for 100 cycles between two water-baths kept at temperatures of 1.4 ± 0.5 ◦ C and 60 ± 1 ◦ C. A cycle consisted of 10 s immersion in one bath, 30 s transfer-time and another 10 s immersion in the other bath. Immediately following thermocycling the teeth were placed in a 0.5% basic fushin solution for 24 h and kept at 37 ◦ C ± 1 ◦ C. Finally, the teeth were embedded in epoxy resin and cut to obtain three 0.5 mm-thick sections per tooth. Given the inherent structural inhomogeneity of dentin, each section was considered as an independent system, yielding a total of 30 sections per group to be scored (3 sections per cavity, 2 cavities per tooth and 5 teeth per group). Microleakage was recorded under 10x magnification at the enamel and dentin interface by a blinded evaluator: a score of ‘0 was assigned in the absence of dye penetration, while ‘1 or ‘2 were assigned for dye penetration along the enamel or gingival floor up to or exceeding half the restoration depth, respectively. A score of ‘3 was given when the dye stained the axial wall.
2.8.
Statistical analysis
Statistical analyses were performed using the JMP statistical software (SAS Institute Inc.). All data sets were analyzed using one-way analysis of variance (ANOVA) and with Tukey tests (˛ = 0.05) except for the microleakage results. In this case, paired group comparison were carried out using the nonparametric Wilcoxon test (˛ = 0.05).
3.
Fig. 2 – Tooth preparation (a), general setup for pulpal temperature rise analysis (b) and evolution of temperature for TPO and CQ composites irradiated at 1000 mW/cm2 (c).
Results
Polymerization occurred faster in TPO-composites compared to the controls as seen in Fig. 3. Final DCs were significantly higher (p < 0.05) in TPO-composites compared to CQ controls for irradiation times equal to or greater than 3 s (Table 1).
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Table 1 – Mean values for final DC, maximum polymerization stress, stress rate and Tmax of all TPO and CQ-composites. Standard deviation in parentheses. Same superscript connects statistically similar values (p < 0.05). Irradiation time (s)/Irradiance (mW/cm2 ) TPO 0.5/500 1/500 3/500 9/500 0.5/1000 1/1000 3/1000 9/1000 20/1000 40/1000 0.5/2000 1/2000 3/2000 9/2000 CQ 20/1000 40/1000
Radiant energy (J/cm2 )
Final DC (%)
Final p (MPa)
Stress rate (MPa/s)
T (◦ C)
0.25 0.5 1.5 4.5 0.5 1 3 9 20 40 1 2 6 18
42 (2)H 56 (1)F,G 65 (1)B,C 68 (1)A,B 54 (2)G 63 (2)D,E 70 (1)A 69 (1)A 70 (1)A 69 (1)A 64 (1)C,D 68 (2)A,B 70 (1)A 70 (1)A
2.4 (0.2)F 2.9 (0.2)E,F 3.4 (0.2)D,E 4.4 (0.3)A,B 3.2 (0.2)D,E 3.7 (0.2)B,C,D 4.3 (0.2)A,B,C 4.6 (0.1)A 4.6 (0.3)A 4.9 (0.1)A 3.6 (0.2)C,D 3.5 (0.4)D,E 4.6 (0.3)A 4.9 (0.3)A
0.9 (0.1)E 1.3 (0.1)D,E 1.2 (0.1)D,E 1.3 (0.1)D,E 1.6 (0.3)C,D 1.9 (0.2)A,B,C 1.9 (0.1)A,B,C 1.7 (0.1)B,C,D 2.0 (0.3)A,B,C 2.0 (0.1)A,B,C 2.3 (0.4)A,B 2.3 (0.2)A,B 2.1 (0.2)A,B,C 2.5 (0.2)A
0.43 (0.05)I 0.54 (0.05)H,I 0.75 (0.03)E,F,G 0.92 (0.01)D,E 0.59 (0.03)G,H,I 0.73 (0.06)F,G 0.83 (0.03)E,F 1.14 (0.09)C 1.52 (0.04)B 1.88 (0.06)A 0.68 (0.03)F,G,H 0.81 (0.07)E,F 1.02 (0.13)C,D 1.45 (0.05)B
20 40
57 (1)E,F 60 (1)E,F
3.5 (0.2)D,E 3.5 (0.3)D,E
0.2 (0.1)F 0.2 (0.1)F
1.21 (0.08)C 2.05 (0.07)A
Under optimal conditions, TPO-composites display an average 10% conversion increase. Polymerization stresses increased in a logarithmic pattern with time, maximum stress rate being reached in less than 3 s for TPO-composites and between 5 and 10 s for CQ-controls (Fig. 4). T increased with time, reaching a maximum between 20 and 40 s, depending on the curing parameters (Fig. 2(c)). In the case of CQ-controls, a sharp drop of T curves could be observed, while T decrease was more progressive for all TPO-composites. For 20 and 40 s curing modes, Tmax was reached when the curing light was switched off (as indicated by red arrows on Fig. 2 c)). However, Tmax was reached 10–20 s after the light was switched off for TPO-materials cured during 9 s or less. When plotting maximum stress and Tmax against radiant energy, different trends were observed (Fig. 5 c)). In TPO-composites, the maximum polymerization stress reached a plateau at approximately 5 J/cm2 while Tmax continued to increase linearly above that level of radiant exposure. With similar irradiation parameters (20 or 40 s at 1000 mW/cm2 ), TPO-composites and controls exhibited similar Tmax as controls, but the latter showed lower maximum stress (Table 1, Fig. 5). No improvements were observed when curing time was increased from 20 to 40 s for CQ controls: they displayed similar DCmax and maximum polymerization stress. TPO-composites displayed higher polymerization stresses compared to the CQ controls when irradiated for 3 s or more for irradiances greater than 1000 mW/cm2 . Maximum stress rates were all significantly higher for TPO-composites, by at least a factor 5 (Fig. 3 and Table 1). Flexural modulus was in most cases significantly higher in TPO-composites compared with controls, except for three curing parameters (0.5 and 1 s at 500 mW/cm2 and 0.5 s at 1000 mW/cm2 (Fig. 6(a)). Flexural strength values were similar between TPO and controls except for the 500–0.5 protocol (Fig. 6 b)). Upper surface hardness increased both with irradiation time and irradiance in TPO-composites. Hardness values were significantly higher than in controls for irradiation times greater than 1s at 1000 and 2000 mW/cm2 (Fig. 6(c). After incubation in the ethanol solution, hardness dropped by 39 to 47%
in controls while the drop in TPO-composites ranged between 21 and 36%. Hardness values of TPO-composites after storage in ethanol solution were all similar to or higher than controls. No microleakage was observed at the enamel/composite interface for all samples. For the dentin/composite interface, wilcoxon’s test on paired groups indicated a significantly higher microleakage for the TPO 1 s group compared to CQ 20 s (p = 0.0274) and TPO 3 s (p = 0.0102). TPO 3 s and CQ 20 s did not show any significant difference (p = 0.6894). Fig. 7 shows the fraction of scores attributed to the different groups.
4.
Discussion
The three null hypotheses, namely that TPO-based composites cured at short curing times and displaying high DC would result in (1) higher polymerization stresses, (2) higher temperature rises, and (3) higher flexural modulus, flexural strength and hardness compared to a conventional CQ-based composite, were all rejected. First, TPO-based materials generated higher polymerization stresses than those generated by the controls, but only for some curing protocols. Second, temperature increases were in most cases lower than those measured for the CQ controls. Third, flexural modulus and hardness were significantly higher in TPO-composites than in CQ-controls for most but not all curing protocols, while flexural strength of TPO-composites and CQ-controls were similar (except for one condition). The curing kinetics measured with FT-NIRS were in accordance with previous literature comparing CQ and TPO materials [19], where TPO-based composites reached their maximum DC in less than 5 s when using high irradiances (1000 mW/cm2 or above). In the present work, 95% of DC was reached in about 2–3 s (Fig. 3). In accordance with our previous study on the same materials/curing conditions [17], TPO-composites did not benefit from an increase in irradiance and/or irradiation time above a given threshold. More precisely this corresponded to irradiation protocols of 3 s irradiation time at 1000 and 2000 mW/cm2 (3–6 J/cm2 ) and 9 s
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1280–1289
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Fig. 4 – Evolution of polymerization stress for TPO-composites (black) and CQ-composites (grey) irradiated at 1000 mW/cm2 (a). Evolution of stress rates for the same materials (b).
Fig. 3 – Evolution of DCs over time in TPO-composites irradiated at 2000 mW/cm2 (a), TPO and CQ-composites irradiated at 1000 mW/cm2 (b) and TPO-composites irradiated at 500 mW/cm2 (c). Red vertical arrows indicate irradiation times and are reproduced with the same length on (a), (b) and (c).
at 500 mW/cm2 (4.5 J/cm2 ) (Table 1). These findings confirmed what was previously reported using electron paramagnetic resonance [17], wherein plotting radical concentration against radiant energy revealed a plateau following 3 J/cm2 , and therefore, 3 s of irradiation at 1000 mW/cm2 yielded optimal curing with sub-optimal states below that threshold. In the present work, the existence of a radiant exposure threshold was confirmed for polymerization stress, with a plateau value reached around 3–5 J/cm2 (Fig. 5(b)). In comparison, the build-up of polymerization stress in CQbased controls was much slower than TPO-composites, with no difference between 20 or 40 J/cm2 . This is in accordance with the DC data (Fig. 3), where DC of controls reached 95% of the final DC value after 20 s of irradiation. This slower stress
build-up could be expected, since the polymerization reaction of CQ-based composites has been reported to depend more on irradiation time rather than on irradiance [8,27,28]. Specifically regarding the direct comparison of CQ vs TPO materials, it was shown that the latter cure better when irradiated for 45 s at 400 mW/cm2 than at 6 s at 3000 mW/cm2 [19] (both 18 J/cm2 ). Altogether these results tend to indicate that while CQ-based RBC require long irradiation times, TPO-composites have the ability to cure in a fraction of that time, hence requiring a much lower energy to reach full cure [17]. Polymerization stress is a corollary of chain lengthening, entanglement and cross-linking during curing [10]. The evolution of stress in resin composites occurs non-linearly with conversion [12,29]. In dimethacrylate resins or resin composite systems, polymerization proceeds following a diffusion-controlled mechanism [30] and in final stages, crosslinking induces a major change in viscosity. Beyond this point, any increase in conversion results in further contraction in a highly rigid material and hence increased stress and the present results for TPO-composites follow such a pattern (Table 1). As noted previously, at radiant energies of 3 J/cm2 and above, the stress reaches a plateau with final values of about 4–5 MPa (Fig. 5 b)). Since maximum DC is reached at a similar threshold, the increase, then saturation of maximum stress with increasing radiant energy
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Fig. 5 – Average final DC versus maximum polymerization stress (a), maximum polymerization stress and Tmax versus radiant energy (b) and (c) respectively for all TPO (blue) and CQ-composites (white).
is logically linked to the evolution in the final stages of polymerization. Contrary to stress curves, Tmax curves continued increasing in a linear fashion above the 3 J/cm2 radiant energy threshold, and up to 40 J/cm2 (Fig. 5(c)). To discriminate between T due to the reaction exotherm and T from the curing light, it could be approximated that the temperature rise purely related to the polymerization reaction corresponded to T in specimens with saturated DC and short curing time (e.g. 3 s at 1000 mW/cm2 ). For these parameters, the light was indeed switched off long before T curves reached their maximum (Fig. 2(c)). The amount of heat purely due to the chemical reaction was then about 0.8 ◦ C for TPOcomposites (through 0.6 mm dentin, and with the specific
experimental setup of this work). Any additional temperature increase should then be attributed to excess energy provided by the LCU and not used by the composite for additional propagation [13,16,19]. Following this reasoning, the increase of T at short curing times is concomitant to the increase of stress and conversion until the threshold. Above the latter however, longer irradiation times induced composite over-heating without any noticeable change in conversion measured within the timeframe of the experiment. Similarly, for CQ-composites, the increase from 20 to 40 s did not result in DC increase, but an almost two-fold T increase was observed. As mentioned in previous work, curing parameters should be optimized to reach optimal curing but at the same time avoid unnecessary composite – and pulp – overheating [31]. It has been previously described that the onset of vitrification is delayed in TPO-composites [19,20] compared to CQ-based materials, possibly as a result of transfer reactions due to the TPO system [32]. In methacrylate networks, such mechanism is known to promote higher conversion degrees and reduced stress [33]. This trend was confirmed in the present study, since similar stresses were indeed observed in CQ- and TPO-based materials, despite the higher DCs measured in the latter. Vitrification is the stage where the polymer matrix fully rigidifies. It may be defined as the point where kinetically, the exotherm of the reaction itself is no longer sufficient to provide mobility to propagating radicals in an increasingly viscous medium. In this situation any additional heat provided by the LCU to the system could be expected to postpone that vitrification by providing additional mobility. In the present study, we used an open system where the temperature measurement had high clinical relevance but lacked the adiabatic quality of systems such as differential scanning calorimetry. However, given the precision of our measurement, it still made sense comparing temperature increases in relation with matrix formation. The additional heat provided by the LCU was clearly not enough to significantly increase conversion, possibly indicating that T increase was insufficient to increase Tg in TPO-composites [34]. Here Tg is defined as the temperature above which a cross-linked matrix fully relaxes [35]. In order for the heating from the LCU to have a major impact on DC, it should preferably occur during polymerization, i.e. when T of both LCU and reaction are cumulated. This would be difficult to observe in TPO-composites since the polymer matrix was already highly crosslinked after 3 s as indicated by FTNIRS (Fig. 2) and previous results [19,20]. In CQ-composites where polymerization is inherently slower, this is more likely to occur, but again it did not appear to be the case from DC values, which are at best similar, and in general lower than TPO-composites. Hence, these observations tended to show that the levels of temperature rise in the materials were insufficient to significantly move Tg , but also relatively safe for the pulp tissues when placed on a 0.6 mm thick wet dentin layer (
