DENTAL-2532; No. of Pages 5
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Influence of temperature on volumetric shrinkage and contraction stress of dental composites Leontine A. Jongsma ∗ , Cees J. Kleverlaan Department of Dental Materials Sciences, Academic Centre for Dentistry Amsterdam (ACTA), University of Amsterdam and VU University Amsterdam, The Netherlands
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. To test the influence of temperature on contraction stress and volumetric shrink-
Received 16 July 2014
age of Clearfil AP-X, Venus Diamond, Premise and Filtek Z250.
Accepted 24 March 2015
Methods. Volumetric shrinkage measurements were carried out using mercury dilatometry,
Available online xxx
while a constraint tensilometer set-up was used for the measurement of contraction stress. Measurements were carried out with a composite temperature of 23, 30, 37, and 44 ◦ C.
Keywords:
Results. Volumetric shrinkage increases with higher temperature. Premise and Venus Dia-
Resin composite
mond show lower volumetric shrinkage than Clearfil AP-X and Filtek Z250. Clearfil AP-X
Contraction stress
shows the highest contraction stress which slightly increases with higher temperatures.
Volumetric shrinkage
The other composites only show an increase in contraction stress between 23 and 30 ◦ C.
Temperature
Significance. Heating of dental composites results in a higher volumetric shrinkage. However,
Composite pre-heating
the contraction stress does not change significantly due to increased temperature above 30 ◦ C. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Heating of dental composites prior to placement is often carried out in clinical practice to improve the handling of these materials. Especially when materials are relatively viscous, the preheating will result in a less viscous material with better flow properties [1] which makes placement easier, especially if narrow crevices have to be filled. With the increased flow and adaptation, microleakage may also be reduced [2,3]. Polymerization of a dental resin composite is important in order to obtain material properties that will enhance good clinical performance [4]. On the other hand, polymerization leads to shrinkage strain and contraction stress which might
affect the integrity of the bond to dentin [5,6]. Temperature is one of the many factors which can influence the polymerization efficiency of a dental resin composite [7]. The difference between room temperature and the temperature in the oral cavity may lead to a different polymerization rate, degree of cure [8], and therefore different shrinkage properties, due to improved monomer mobility at higher temperatures, allowing more of the polymerization reaction to occur before reaching of the gel-point [9]. It has been reported that the mechanical properties of dental resin composites are considerably higher after curing at temperatures of up to 60 ◦ C [10]. It is well established that preheating will lead to higher monomer conversion [8,9,11–14]. The rapid polymerization at higher temperatures leads to a
∗ Corresponding author at: ACTA, Department of Dental Materials Sciences, Gustav Mahlerlaan 3004, 1081 LA Amsterdam, The Netherlands. Tel.: +31 205980236. E-mail address: [email protected] (L.A. Jongsma).
http://dx.doi.org/10.1016/j.dental.2015.03.009 0109-5641/© 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Jongsma LA, Kleverlaan CJ. Influence of temperature on volumetric shrinkage and contraction stress of dental composites. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.03.009
DENTAL-2532; No. of Pages 5
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Table 1 – Materials used in this study. Product
Manufacturer
Color
Batch number
Clearfil AP-X
Kuraray
A2
1102 A
Premise
Kerr
Enamel A2
3543967
Venus Diamond
Heraeus Kulzer
A2
010038/010036
Filtek Z250
3M ESPE
A2
N256375
Composition Bis-GMA, TEGDMA, 70 vol.% fillers (silanated barium glass fillers, silanated silica fillers, silanated colloidal silica), dl-camphorquinone, catalysts, accelerators, pigments, others. Ethoxylated bisphenol-A-dimethacrylate, TEGDMA, 70 vol.% fillers of barium glass (size 0.4 m) and silica fillers (size 0.02 m), light cure initiators and stabilizers. TCD-di-HEA, UDMA, 63.5–65.1 vol.% fillers of barium aluminum fluoride glass and highly discrete nanoparticles (size 5 nm to 20 m), rheology modifier, initiator system, stabilizers, pigments. Bis-EMA, UDMA, Bis-GMA, TEGDMA, 60 vol.% silane treated ceramic filler (size 0.01–3.5 m), benzotriazol, EDMAB.
Kuraray Medical Inc., Okoyama, Japan; KerrHawe SA, Boggio, Switzerland; Hereaus Kulzer GmbH, Hanau, Germany; 3M ESPE, Seefeld, Germany. Bis-GMA: bisphenol A diglycidylmethacrylate; TEGDMA: triethylene glycol dimethacrylate; TCD-di-HEA: tricyclodecane-urethane dimethacrylate; UDMA: diurethane dimethacrylate; Bis-EMA: bisphenol A polyethylene glycol diether dimethacrylate; EDMAB: ethyl 4-dimethyl aminobenzoate.
higher degree of cure, but may also lead to elevated stress formation and faster reaching of the gel-point [9]. Although a high degree of cure is very desirable because the higher final double bond conversion will lead to better material properties [4], contraction stress may form a threat to the integrity of the bond to dentin [5,6]. It is known that curing at a higher temperature leads to higher shrinkage of dental composites [15,16] and resin cements [17], although according to some authors this effect is little [18] and not clinically significant [19]. Lohbauer et al. showed a significant effect of preheating on shrinkage after 5 min, but this result was not statistically significant after 24 h [20]. There is, however, very little known about the effect of temperature on contraction stress. Few publications have been published concerning the relationship between contraction stress and temperature. It has been demonstrated that residual stress is increased with increasing temperature [21]. Furthermore, recently published research shows an increase of “shrinkage stress rate” with increasing temperature [22]. This study showed a moderate increase in contraction stress at higher temperatures for most composites, although some materials showed decreased values. More research is needed to fully understand the effects of temperature on contraction stress. The purpose of this study was therefore to test the influence of temperature on contraction stress and volumetric shrinkage of several commercially available dental resin composites.
2.
Materials and methods
Four commercially available resin composites were tested: Clearfil AP-X A2, Premise Enamel A2, Venus Diamond A2, and Filtek Z250 A2 (Table 1). Volumetric shrinkage measurements were performed with the use of mercury dilatometry as reported by de Gee et al.
[23] at 23 ± 0.1 ◦ C and at 30, 37 and 44 ± 0.1 ◦ C. The standard procedure and operation of the dilatometer consisted of: (i) a layer of high vacuum grease (Dow Corning Corporation, Midland, MI, USA) was applied on the flat surface of the glass stopper for separation. (ii) An amount of approximately 300 mg of a resin composite paste was applied on the greased surface of the stopper and flattened to a thickness of approximately 1.5 mm. (iii) After the stopper was inserted into the dilatometer the specimens were light activated with an Elipar Trilight (3M-ESPE, Seefeld, G) for 40 s in standard mode (ca 750 mW/cm2 ), ensuring complete curing. Recording was started at the moment that the light source was switched on. (iv) After the specimens were removed from the dilatometer, the grease was washed off with ether and the density measured by means of pycnometry with a Mettler AT261 DeltaRange. From each material three specimens (n = 3) were measured during a period of 30 min. Non-compliant contraction stress measurements were performed at 23, 30, 37, and 44 ± 0.1 ◦ C with an Instron 6022 Tensilometer (Instron Ltd., Wycombe, UK) as reported previously [24]. The glass plate (4 mm thick) was sandblasted with Al2 O3 (50 m) and remaining Al2 O3 was removed with compressed air. The surface was treated with Ceramic Primer (3M, St. Paul, MN, USA) and SE Bond (Kuraray Medical Inc., Okoyama, Japan) according to manufacturer’s instructions. The bolt head (3.2 mm diameter) was wet-ground on 600 grit SiC sandpaper and then sandblasted with Al2 O3 (50 m) and treated with SE Bond and light cured for 20 s. The composite was applied between the glass plate and bolt head, and the cross-head was moved to set the distance between the glass plate and bolt head to 0.8 mm. This configuration resulted in C-value of 2 (C = d/2h = 3.2/1.6) [25]. The specimens were light cured through the glass plate with an Elipar Trilight (ESPE, Seefeld, Germany) for 40 s in standard mode, ensuring complete curing. From the start of light curing the contraction stress development was measured during 30 min. The stress at 30 min was used for the statistical analysis. The axial contraction of the specimens
Please cite this article in press as: Jongsma LA, Kleverlaan CJ. Influence of temperature on volumetric shrinkage and contraction stress of dental composites. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.03.009
ARTICLE IN PRESS
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Clearfil AP-X Shrinkage (vol.%) 2.3 (0.1)a 23 ◦ C 30 ◦ C 2.7 (0.1)1 37 ◦ C 2.8 (0.1)1,2,a,b 44 ◦ C 3.0 (0.1)2,a Overall
a
2.7 (0.3)
Premise
Venus Diamond
Filtek Z250
2.0 (0.1)b 2.4 (0.1)1,a 2.6 (0.1)1,2,a 2.7 (0.1)2
2.0 (0.1)b 2.4 (0.1)a 2.7 (0.2)a,b 3.0 (0.1)a
2.4 (0.1)1,a 2.5 (0.2)1,a 2.9 (0.2)b 3.3 (0.1)
b
2.4 (0.3)
b
2.5 (0.4)
3.5
Volumetric Shrinkage (%)
Table 2 – Shrinkage (vol.%) and contraction stress (MPa) of the resin composites 30 min after start of cure at different temperatures with their standard deviations in parentheses.
3.0
2.5
Clearfil AP-X Premise Venus Diamond Filtek Z250
2.0
a
2.8 (0.4)
20 Contraction stress (MPa) 17.7 (0.5)1 9.1 (0.5)a,b 23 ◦ C 30 ◦ C 19.2 (0.1)1 13.4 (1.4)1,a,b 37 ◦ C 20.3 (0.4)1,2 14.4 (0.7)1,a,b 44 ◦ C 23.4 (0.6)2 14.2 (0.2)1,a Overall
20.1 (2.2)
12.8 (2.4)
7.1 (1.4)a 10.5 (0.8)1,a 12.0 (0.7)1,a 10.7 (0.4)1
10.8 (4.4)b 15.7 (0.9)1,b 15.9 (1.7)1,b 15.6 (0.7)1,a
10.1 (2.1)
14.5 (3.1)
1,2
No statistically significant difference between different temperature groups within the same composite resin. a,b No statistically significant difference between composite resins within the same temperature group.
Results
3.1.
Influence of temperature on volumetric shrinkage
The results of the volumetric shrinkage 30 min after start of cure of the different composites at different temperatures are summarized in Table 2 and graphically depicted in Fig. 1. Two-way analysis of variance showed a significant effect on composite (F = 31.6; P < 0.001) and temperature (F = 149.5; P < 0.001). There was also a significant interaction between composite and temperature (F = 5.1; P < 0.001). When looking at the mean overall results of the composites without considering the used temperature, Premise and Venus Diamond show a statistically significant lower volumetric shrinkage compared to the other two composites, who do not differ from each other. Considering the differences between temperatures within the individual cements, it can be seen that overall, there is a statistically significant increase in volumetric shrinkage with higher temperature. However, this is not spread equally over the tested composites. With the composites Clearfil AP-X and Premise there is a large difference in volumetric shrinkage between 23 and 30 ◦ C. With a temperature higher than 30 ◦ C, however, the effect of temperature on shrinkage weakens, with no more statistically significant
35
40
45
Fig. 1 – Mean volumetric shrinkage (vol.%) at 30 min as a function of temperature.
differences between temperature groups. On the other hand, Venus Diamond and Filtek Supreme show a more gradual increase in shrinkage with increasing temperature (see Fig. 1).
Influence of temperature on contraction stress
The results of the contraction stress of the different composites at different temperatures are summarized in Table 2 and graphically depicted in Fig. 2. Two-way analysis of variance showed a significant effect on composite (F = 113.6; P < 0.001) and temperature (F = 31.0; P < 0.001). There was, however, no significant interaction between composite and temperature (F = 1.6; P = 0.162). When looking at the mean contraction stress of the composites without taking temperature into account, there were statistically significant differences between all tested composites, with Clearfil AP-X showing the highest and Venus Diamond showing the lowest stress. Regarding the differences between temperatures within the different composites, it can be seen that the highest increase in contraction stress occurs between 23 and 30 ◦ C, as there are no further significant differences between temperature groups. There is a general trend for an increase in contraction stress between 23 and 30 ◦ C, after which the contraction stress curve flattens. 25
Contraction Stress (MPa)
3.
30
Temperature (ºC)
3.2. was continuously counteracted by a feedback displacement of the cross-head to keep the height of the specimen constant. This simulated a restoration in a fully rigid situation where the cavity walls do not yield to the contraction forces. The mean contraction stress of each of the resin composites was determined from three measurements (n = 3). One- and two-way analysis of variance (ANOVA) and Tukey’s post hoc tests were performed (SigmaStat version 3.0, SPSS Version 15.0 Inc., Chicago, USA) to analyze the differences between groups. The significance level ˛ was set at 0.05.
25
20 15 10
Clearfil AP-X Premise Venus Diamond Filtek Z250
5 0 20
25
30
35
40
45
Temperature (ºC) Fig. 2 – Mean contraction stress (MPa) at 30 min as a function of temperature.
Please cite this article in press as: Jongsma LA, Kleverlaan CJ. Influence of temperature on volumetric shrinkage and contraction stress of dental composites. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.03.009
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An exception is Clearfil AP-X, which shows a more gradual increase in contraction stress, with statistically significant differences in contraction stress only between 23 and 44 ◦ C, and between 30 and 44 ◦ C.
4.
Discussion
The visco-elastic behavior and the reaction kinetics of composites are strongly dependent on the temperature. If the temperature increases, the polymerization reaction will take place at a higher speed, resulting in a higher degree of cure and better cross-linking. This also results in an increased volumetric shrinkage, as found in this study. One might expect a higher contraction stress as a result of this increased volumetric shrinkage. The higher temperature, however, also results in changed visco-elastic behavior of the composite material, e.g. better flow capacity, preventing the build-up of high contraction stresses with increasing temperature. The results show that for the investigated resin composites, increased temperature resulted in better flow capacity, preventing high contraction stresses. This resulted in a high degree of cure [26], a relatively low increase in contraction stress and therefore most probably improved mechanical properties. To the author’s knowledge, only one previous study [22] has been published regarding the influence of temperature on contraction stress. In this study, it was concluded that the contraction stress rate was increased at a higher temperature, even though the contraction stress itself only increased moderately. The measured contraction stress values in this study were significantly lower than the values measured in our study, because Watts et al. used a compliant test setup. This makes direct comparison difficult. The relatively low increase in contraction stress (compared to the higher increase of the volumetric shrinkage) at higher temperatures is in concordance with this study, however. Residual stress has been published, and was found to increase at higher temperatures of 40–60 ◦ C compared to room temperature. This was explained by the faster polymerization, allowing less time for viscous flow and earlier reaching of the gel-point [21]. In this study, it was found that the highest increase in contraction stress takes place between room temperature and 30–37 ◦ C. There was a statistically significant difference between contraction stress at room temperature and at body temperature for all tested composites. Taking into consideration that composites will probably reach a temperature of between 30 and 37 ◦ C quickly after application in a cavity [27], and probably before light-curing is started, it is questionable what the clinical relevance of in vitro contraction stress measurements are, which are usually carried out at room temperature. A temperature higher than body temperature did not cause a further increase in contraction stress for any of the tested composites, however. Pre-heating of dental resin composites leads to a higher degree of conversion [1]. Over-heating of the composite material, however, may not be beneficial since intrapulpal temperature increases of more than 6.5 ◦ C may lead to significant pulp injury [28], such as pain and possible necrosis of the pulp. Lohbauer et al. measured the rise in pulpal temperature at 2 mm away from the composite. A composite temperature
of 68 ◦ C lead to an increased pulpal temperature of only 1.2 ◦ C. The subsequent temperature rise by combined effect of the exothermic polymerization reaction and the curing light however, lead to an increase in pulpal temperature of 4.2 ◦ C [20]. The influence of the curing light may therefore be of more significance than the pre-heating of the composite. This is supported by the finding that the tooth material serves as a heat sink, quickly lowering the composite temperature [27]. In many in vitro studies composites are pre-heated to high temperatures which are not clinically relevant. Pre-heating of the composite within clinically relevant temperatures may not lead to a significant increase in monomer conversion and therefore mechanical properties, which can be explained by the rapid drop in composite temperature during handling [2]. Moreover, the possible advantageous effect of composite pre-heating in terms of advanced degree of cure is reported to be less pronounced after 24 h when compared to immediate measurements, because of the delayed polymerization which allows the polymerization of the composite applied at room temperature to catch up with the polymerization of the pre-heated composite [2,20]. However, it enhances composite adaptation to cavity walls [2]. Although pre-heating leads to better flow properties, in vitro results, however, do not show differences in microleakage between composites at room temperature and at 60 ◦ C [1]. Furthermore, it is clear from this study that the differences in contraction stress and volumetric shrinkage between the tested composites at body temperature are much less evident than at room temperature, which questions these material properties as direct predictors for clinical success. It might be recommended that most of the in vitro studies, which are generally carried out at room temperature, should be carried out at “mouth” temperature (32–37 ◦ C).
5.
Conclusions
Within the limitations of this study, it can be concluded that pre-heating of dental composites results in an increased volumetric shrinkage. The contraction stress between 23 ◦ C and higher temperatures is increased at a statistically significant level. Above 30 ◦ C, however, there are no further increases in contraction stress with higher temperatures, except for Clearfil AP-X, which shows a more gradual increase in contraction stress with increasing temperature.
Acknowledgement We would like to thank Ms. J. Rezende for carrying out the contraction stress and volumetric shrinkage experiments.
references
[1] Deb S, Di Silvio L, Mackler HE, Millar BJ. Pre-warming of dental composites. Dent Mater 2011;27:e51–9. [2] Froes-Salgado NR, Silva LM, Kawano Y, Francci C, Reis A, Loguercio AD. Composite pre-heating: effects on marginal adaptation, degree of conversion and mechanical properties. Dent Mater 2010;26:908–14.
Please cite this article in press as: Jongsma LA, Kleverlaan CJ. Influence of temperature on volumetric shrinkage and contraction stress of dental composites. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.03.009
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[3] Wagner WC, Aksu MN, Neme AM, Linger JB, Pink FE, Walker S. Effect of pre-heating resin composite on restoration microleakage. Oper Dent 2008;33:72–8. [4] Lovell LG, Lu H, Elliott JE, Stansbury JW, Bowman CN. The effect of cure rate on the mechanical properties of dental resins. Dent Mater 2001;17:504–11. [5] Davidson CL, de Gee AJ, Feilzer A. The competition between the composite–dentin bond strength and the polymerization contraction stress. J Dent Res 1984;63:1396–9. [6] Braga RR, Ballester RY, Ferracane JL. Factors involved in the development of polymerization shrinkage stress in resin-composites: a systematic review. Dent Mater 2005;21:962–70. [7] Leprince JG, Palin WM, Hadis MA, Devaux J, Leloup G. Progress in dimethacrylate-based dental composite technology and curing efficiency. Dent Mater 2013;29:139–56. [8] Price RB, Whalen JM, Price TB, Felix CM, Fahey J. The effect of specimen temperature on the polymerization of a resin-composite. Dent Mater 2011;27:983–9. [9] Daronch M, Rueggeberg FA, De Goes MF, Giudici R. Polymerization kinetics of pre-heated composite. J Dent Res 2006;85:38–43. [10] Bausch JR, de Lange C, Davidson CL. The influence of temperature on some physical properties of dental composites. J Oral Rehabil 1981;8:309–17. [11] Cook WD. Thermal aspects of the kinetics of dimethacrylate photopolymerization. Polymer 1992;33:2152–61. [12] Lovell LG, Newman SM, Bowman CN. The effects of light intensity, temperature, and comonomer composition on the polymerization behavior of dimethacrylate dental resins. J Dent Res 1999;78:1469–76. [13] Daronch M, Rueggeberg FA, De Goes MF. Monomer conversion of pre-heated composite. J Dent Res 2005;84:663–7. [14] Trujillo M, Newman SA, Stansbury JW. Use of near-IR to monitor the influence of external heating on dental composite photopolymerization. Dent Mater 2004;20: 766–77. [15] El-Korashy DI. Post-gel shrinkage strain and degree of conversion of preheated resin composite cured using different regimens. Oper Dent 2010;35:172–9.
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[16] Elhejazi AA. The effects of temperature and light intensity on the polymerization shrinkage of light-cured composite filling materials. J Contemp Dent Pract 2006;7:12–21. [17] Kitzmuller K, Graf A, Watts D, Schedle A. Setting kinetics and shrinkage of self-adhesive resin cements depend on cure-mode and temperature. Dent Mater 2011;27:544–51. [18] Osternack F, Caldas D, Almeida J, Souza E, Mazur R. Effects of preheating and precooling on the hardness and shrinkage of a composite resin cured with QTH and LED. Oper Dent 2012. [19] Walter R, Swift Jr EJ, Sheikh H, Ferracane JL. Effects of temperature on composite resin shrinkage. Quintessence Int 2009;40:843–7. [20] Lohbauer U, Zinelis S, Rahiotis C, Petschelt A, Eliades G. The effect of resin composite pre-heating on monomer conversion and polymerization shrinkage. Dent Mater 2009;25:514–9. [21] Prasanna N, Pallavi Reddy Y, Kavitha S, Lakshmi Narayanan L. Degree of conversion and residual stress of preheated and room-temperature composites. Indian J Dent Res 2007;18:173–6. [22] Watts DC, Alnazzawi A. Temperature-dependent polymerization shrinkage stress kinetics of resin-composites. Dent Mater 2014;30:654–60. [23] de Gee AJ, Davidson CL, Smith A. A modified dilatometer for continuous recording of volumetric polymerization shrinkage of composite restorative materials. J Dent 1981;9:36–42. [24] Kleverlaan CJ, Feilzer AJ. Polymerization shrinkage and contraction stress of dental resin composites. Dent Mater 2005;21:1150–7. [25] Feilzer AJ, De Gee AJ, Davidson CL. Setting stress in composite resin in relation to configuration of the restoration. J Dent Res 1987;66:1636–9. [26] Rueggeberg F, Tamareselvy K. Resin cure determination by polymerization shrinkage. Dent Mater 1995;11:265–8. [27] Rueggeberg FA, Daronch M, Browning WD, MF DEG. In vivo temperature measurement: tooth preparation and restoration with preheated resin composite. J Esthet Restor Dent 2010;22:314–22. [28] Zach L, Cohen G. Pulp response to externally applied heat. Oral Surg Oral Med Oral Pathol 1965;19:515–30.
Please cite this article in press as: Jongsma LA, Kleverlaan CJ. Influence of temperature on volumetric shrinkage and contraction stress of dental composites. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.03.009
ARTICLE IN PRESS d e n t a l m a t e r i a l s x x x ( 2 0 1 5 ) xxx–xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Influence of temperature on volumetric shrinkage and contraction stress of dental composites Leontine A. Jongsma ∗ , Cees J. Kleverlaan Department of Dental Materials Sciences, Academic Centre for Dentistry Amsterdam (ACTA), University of Amsterdam and VU University Amsterdam, The Netherlands
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. To test the influence of temperature on contraction stress and volumetric shrink-
Received 16 July 2014
age of Clearfil AP-X, Venus Diamond, Premise and Filtek Z250.
Accepted 24 March 2015
Methods. Volumetric shrinkage measurements were carried out using mercury dilatometry,
Available online xxx
while a constraint tensilometer set-up was used for the measurement of contraction stress. Measurements were carried out with a composite temperature of 23, 30, 37, and 44 ◦ C.
Keywords:
Results. Volumetric shrinkage increases with higher temperature. Premise and Venus Dia-
Resin composite
mond show lower volumetric shrinkage than Clearfil AP-X and Filtek Z250. Clearfil AP-X
Contraction stress
shows the highest contraction stress which slightly increases with higher temperatures.
Volumetric shrinkage
The other composites only show an increase in contraction stress between 23 and 30 ◦ C.
Temperature
Significance. Heating of dental composites results in a higher volumetric shrinkage. However,
Composite pre-heating
the contraction stress does not change significantly due to increased temperature above 30 ◦ C. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Heating of dental composites prior to placement is often carried out in clinical practice to improve the handling of these materials. Especially when materials are relatively viscous, the preheating will result in a less viscous material with better flow properties [1] which makes placement easier, especially if narrow crevices have to be filled. With the increased flow and adaptation, microleakage may also be reduced [2,3]. Polymerization of a dental resin composite is important in order to obtain material properties that will enhance good clinical performance [4]. On the other hand, polymerization leads to shrinkage strain and contraction stress which might
affect the integrity of the bond to dentin [5,6]. Temperature is one of the many factors which can influence the polymerization efficiency of a dental resin composite [7]. The difference between room temperature and the temperature in the oral cavity may lead to a different polymerization rate, degree of cure [8], and therefore different shrinkage properties, due to improved monomer mobility at higher temperatures, allowing more of the polymerization reaction to occur before reaching of the gel-point [9]. It has been reported that the mechanical properties of dental resin composites are considerably higher after curing at temperatures of up to 60 ◦ C [10]. It is well established that preheating will lead to higher monomer conversion [8,9,11–14]. The rapid polymerization at higher temperatures leads to a
∗ Corresponding author at: ACTA, Department of Dental Materials Sciences, Gustav Mahlerlaan 3004, 1081 LA Amsterdam, The Netherlands. Tel.: +31 205980236. E-mail address: [email protected] (L.A. Jongsma).
http://dx.doi.org/10.1016/j.dental.2015.03.009 0109-5641/© 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Jongsma LA, Kleverlaan CJ. Influence of temperature on volumetric shrinkage and contraction stress of dental composites. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.03.009
DENTAL-2532; No. of Pages 5
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ARTICLE IN PRESS d e n t a l m a t e r i a l s x x x ( 2 0 1 5 ) xxx–xxx
Table 1 – Materials used in this study. Product
Manufacturer
Color
Batch number
Clearfil AP-X
Kuraray
A2
1102 A
Premise
Kerr
Enamel A2
3543967
Venus Diamond
Heraeus Kulzer
A2
010038/010036
Filtek Z250
3M ESPE
A2
N256375
Composition Bis-GMA, TEGDMA, 70 vol.% fillers (silanated barium glass fillers, silanated silica fillers, silanated colloidal silica), dl-camphorquinone, catalysts, accelerators, pigments, others. Ethoxylated bisphenol-A-dimethacrylate, TEGDMA, 70 vol.% fillers of barium glass (size 0.4 m) and silica fillers (size 0.02 m), light cure initiators and stabilizers. TCD-di-HEA, UDMA, 63.5–65.1 vol.% fillers of barium aluminum fluoride glass and highly discrete nanoparticles (size 5 nm to 20 m), rheology modifier, initiator system, stabilizers, pigments. Bis-EMA, UDMA, Bis-GMA, TEGDMA, 60 vol.% silane treated ceramic filler (size 0.01–3.5 m), benzotriazol, EDMAB.
Kuraray Medical Inc., Okoyama, Japan; KerrHawe SA, Boggio, Switzerland; Hereaus Kulzer GmbH, Hanau, Germany; 3M ESPE, Seefeld, Germany. Bis-GMA: bisphenol A diglycidylmethacrylate; TEGDMA: triethylene glycol dimethacrylate; TCD-di-HEA: tricyclodecane-urethane dimethacrylate; UDMA: diurethane dimethacrylate; Bis-EMA: bisphenol A polyethylene glycol diether dimethacrylate; EDMAB: ethyl 4-dimethyl aminobenzoate.
higher degree of cure, but may also lead to elevated stress formation and faster reaching of the gel-point [9]. Although a high degree of cure is very desirable because the higher final double bond conversion will lead to better material properties [4], contraction stress may form a threat to the integrity of the bond to dentin [5,6]. It is known that curing at a higher temperature leads to higher shrinkage of dental composites [15,16] and resin cements [17], although according to some authors this effect is little [18] and not clinically significant [19]. Lohbauer et al. showed a significant effect of preheating on shrinkage after 5 min, but this result was not statistically significant after 24 h [20]. There is, however, very little known about the effect of temperature on contraction stress. Few publications have been published concerning the relationship between contraction stress and temperature. It has been demonstrated that residual stress is increased with increasing temperature [21]. Furthermore, recently published research shows an increase of “shrinkage stress rate” with increasing temperature [22]. This study showed a moderate increase in contraction stress at higher temperatures for most composites, although some materials showed decreased values. More research is needed to fully understand the effects of temperature on contraction stress. The purpose of this study was therefore to test the influence of temperature on contraction stress and volumetric shrinkage of several commercially available dental resin composites.
2.
Materials and methods
Four commercially available resin composites were tested: Clearfil AP-X A2, Premise Enamel A2, Venus Diamond A2, and Filtek Z250 A2 (Table 1). Volumetric shrinkage measurements were performed with the use of mercury dilatometry as reported by de Gee et al.
[23] at 23 ± 0.1 ◦ C and at 30, 37 and 44 ± 0.1 ◦ C. The standard procedure and operation of the dilatometer consisted of: (i) a layer of high vacuum grease (Dow Corning Corporation, Midland, MI, USA) was applied on the flat surface of the glass stopper for separation. (ii) An amount of approximately 300 mg of a resin composite paste was applied on the greased surface of the stopper and flattened to a thickness of approximately 1.5 mm. (iii) After the stopper was inserted into the dilatometer the specimens were light activated with an Elipar Trilight (3M-ESPE, Seefeld, G) for 40 s in standard mode (ca 750 mW/cm2 ), ensuring complete curing. Recording was started at the moment that the light source was switched on. (iv) After the specimens were removed from the dilatometer, the grease was washed off with ether and the density measured by means of pycnometry with a Mettler AT261 DeltaRange. From each material three specimens (n = 3) were measured during a period of 30 min. Non-compliant contraction stress measurements were performed at 23, 30, 37, and 44 ± 0.1 ◦ C with an Instron 6022 Tensilometer (Instron Ltd., Wycombe, UK) as reported previously [24]. The glass plate (4 mm thick) was sandblasted with Al2 O3 (50 m) and remaining Al2 O3 was removed with compressed air. The surface was treated with Ceramic Primer (3M, St. Paul, MN, USA) and SE Bond (Kuraray Medical Inc., Okoyama, Japan) according to manufacturer’s instructions. The bolt head (3.2 mm diameter) was wet-ground on 600 grit SiC sandpaper and then sandblasted with Al2 O3 (50 m) and treated with SE Bond and light cured for 20 s. The composite was applied between the glass plate and bolt head, and the cross-head was moved to set the distance between the glass plate and bolt head to 0.8 mm. This configuration resulted in C-value of 2 (C = d/2h = 3.2/1.6) [25]. The specimens were light cured through the glass plate with an Elipar Trilight (ESPE, Seefeld, Germany) for 40 s in standard mode, ensuring complete curing. From the start of light curing the contraction stress development was measured during 30 min. The stress at 30 min was used for the statistical analysis. The axial contraction of the specimens
Please cite this article in press as: Jongsma LA, Kleverlaan CJ. Influence of temperature on volumetric shrinkage and contraction stress of dental composites. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.03.009
ARTICLE IN PRESS
DENTAL-2532; No. of Pages 5
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d e n t a l m a t e r i a l s x x x ( 2 0 1 5 ) xxx–xxx
Clearfil AP-X Shrinkage (vol.%) 2.3 (0.1)a 23 ◦ C 30 ◦ C 2.7 (0.1)1 37 ◦ C 2.8 (0.1)1,2,a,b 44 ◦ C 3.0 (0.1)2,a Overall
a
2.7 (0.3)
Premise
Venus Diamond
Filtek Z250
2.0 (0.1)b 2.4 (0.1)1,a 2.6 (0.1)1,2,a 2.7 (0.1)2
2.0 (0.1)b 2.4 (0.1)a 2.7 (0.2)a,b 3.0 (0.1)a
2.4 (0.1)1,a 2.5 (0.2)1,a 2.9 (0.2)b 3.3 (0.1)
b
2.4 (0.3)
b
2.5 (0.4)
3.5
Volumetric Shrinkage (%)
Table 2 – Shrinkage (vol.%) and contraction stress (MPa) of the resin composites 30 min after start of cure at different temperatures with their standard deviations in parentheses.
3.0
2.5
Clearfil AP-X Premise Venus Diamond Filtek Z250
2.0
a
2.8 (0.4)
20 Contraction stress (MPa) 17.7 (0.5)1 9.1 (0.5)a,b 23 ◦ C 30 ◦ C 19.2 (0.1)1 13.4 (1.4)1,a,b 37 ◦ C 20.3 (0.4)1,2 14.4 (0.7)1,a,b 44 ◦ C 23.4 (0.6)2 14.2 (0.2)1,a Overall
20.1 (2.2)
12.8 (2.4)
7.1 (1.4)a 10.5 (0.8)1,a 12.0 (0.7)1,a 10.7 (0.4)1
10.8 (4.4)b 15.7 (0.9)1,b 15.9 (1.7)1,b 15.6 (0.7)1,a
10.1 (2.1)
14.5 (3.1)
1,2
No statistically significant difference between different temperature groups within the same composite resin. a,b No statistically significant difference between composite resins within the same temperature group.
Results
3.1.
Influence of temperature on volumetric shrinkage
The results of the volumetric shrinkage 30 min after start of cure of the different composites at different temperatures are summarized in Table 2 and graphically depicted in Fig. 1. Two-way analysis of variance showed a significant effect on composite (F = 31.6; P < 0.001) and temperature (F = 149.5; P < 0.001). There was also a significant interaction between composite and temperature (F = 5.1; P < 0.001). When looking at the mean overall results of the composites without considering the used temperature, Premise and Venus Diamond show a statistically significant lower volumetric shrinkage compared to the other two composites, who do not differ from each other. Considering the differences between temperatures within the individual cements, it can be seen that overall, there is a statistically significant increase in volumetric shrinkage with higher temperature. However, this is not spread equally over the tested composites. With the composites Clearfil AP-X and Premise there is a large difference in volumetric shrinkage between 23 and 30 ◦ C. With a temperature higher than 30 ◦ C, however, the effect of temperature on shrinkage weakens, with no more statistically significant
35
40
45
Fig. 1 – Mean volumetric shrinkage (vol.%) at 30 min as a function of temperature.
differences between temperature groups. On the other hand, Venus Diamond and Filtek Supreme show a more gradual increase in shrinkage with increasing temperature (see Fig. 1).
Influence of temperature on contraction stress
The results of the contraction stress of the different composites at different temperatures are summarized in Table 2 and graphically depicted in Fig. 2. Two-way analysis of variance showed a significant effect on composite (F = 113.6; P < 0.001) and temperature (F = 31.0; P < 0.001). There was, however, no significant interaction between composite and temperature (F = 1.6; P = 0.162). When looking at the mean contraction stress of the composites without taking temperature into account, there were statistically significant differences between all tested composites, with Clearfil AP-X showing the highest and Venus Diamond showing the lowest stress. Regarding the differences between temperatures within the different composites, it can be seen that the highest increase in contraction stress occurs between 23 and 30 ◦ C, as there are no further significant differences between temperature groups. There is a general trend for an increase in contraction stress between 23 and 30 ◦ C, after which the contraction stress curve flattens. 25
Contraction Stress (MPa)
3.
30
Temperature (ºC)
3.2. was continuously counteracted by a feedback displacement of the cross-head to keep the height of the specimen constant. This simulated a restoration in a fully rigid situation where the cavity walls do not yield to the contraction forces. The mean contraction stress of each of the resin composites was determined from three measurements (n = 3). One- and two-way analysis of variance (ANOVA) and Tukey’s post hoc tests were performed (SigmaStat version 3.0, SPSS Version 15.0 Inc., Chicago, USA) to analyze the differences between groups. The significance level ˛ was set at 0.05.
25
20 15 10
Clearfil AP-X Premise Venus Diamond Filtek Z250
5 0 20
25
30
35
40
45
Temperature (ºC) Fig. 2 – Mean contraction stress (MPa) at 30 min as a function of temperature.
Please cite this article in press as: Jongsma LA, Kleverlaan CJ. Influence of temperature on volumetric shrinkage and contraction stress of dental composites. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.03.009
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An exception is Clearfil AP-X, which shows a more gradual increase in contraction stress, with statistically significant differences in contraction stress only between 23 and 44 ◦ C, and between 30 and 44 ◦ C.
4.
Discussion
The visco-elastic behavior and the reaction kinetics of composites are strongly dependent on the temperature. If the temperature increases, the polymerization reaction will take place at a higher speed, resulting in a higher degree of cure and better cross-linking. This also results in an increased volumetric shrinkage, as found in this study. One might expect a higher contraction stress as a result of this increased volumetric shrinkage. The higher temperature, however, also results in changed visco-elastic behavior of the composite material, e.g. better flow capacity, preventing the build-up of high contraction stresses with increasing temperature. The results show that for the investigated resin composites, increased temperature resulted in better flow capacity, preventing high contraction stresses. This resulted in a high degree of cure [26], a relatively low increase in contraction stress and therefore most probably improved mechanical properties. To the author’s knowledge, only one previous study [22] has been published regarding the influence of temperature on contraction stress. In this study, it was concluded that the contraction stress rate was increased at a higher temperature, even though the contraction stress itself only increased moderately. The measured contraction stress values in this study were significantly lower than the values measured in our study, because Watts et al. used a compliant test setup. This makes direct comparison difficult. The relatively low increase in contraction stress (compared to the higher increase of the volumetric shrinkage) at higher temperatures is in concordance with this study, however. Residual stress has been published, and was found to increase at higher temperatures of 40–60 ◦ C compared to room temperature. This was explained by the faster polymerization, allowing less time for viscous flow and earlier reaching of the gel-point [21]. In this study, it was found that the highest increase in contraction stress takes place between room temperature and 30–37 ◦ C. There was a statistically significant difference between contraction stress at room temperature and at body temperature for all tested composites. Taking into consideration that composites will probably reach a temperature of between 30 and 37 ◦ C quickly after application in a cavity [27], and probably before light-curing is started, it is questionable what the clinical relevance of in vitro contraction stress measurements are, which are usually carried out at room temperature. A temperature higher than body temperature did not cause a further increase in contraction stress for any of the tested composites, however. Pre-heating of dental resin composites leads to a higher degree of conversion [1]. Over-heating of the composite material, however, may not be beneficial since intrapulpal temperature increases of more than 6.5 ◦ C may lead to significant pulp injury [28], such as pain and possible necrosis of the pulp. Lohbauer et al. measured the rise in pulpal temperature at 2 mm away from the composite. A composite temperature
of 68 ◦ C lead to an increased pulpal temperature of only 1.2 ◦ C. The subsequent temperature rise by combined effect of the exothermic polymerization reaction and the curing light however, lead to an increase in pulpal temperature of 4.2 ◦ C [20]. The influence of the curing light may therefore be of more significance than the pre-heating of the composite. This is supported by the finding that the tooth material serves as a heat sink, quickly lowering the composite temperature [27]. In many in vitro studies composites are pre-heated to high temperatures which are not clinically relevant. Pre-heating of the composite within clinically relevant temperatures may not lead to a significant increase in monomer conversion and therefore mechanical properties, which can be explained by the rapid drop in composite temperature during handling [2]. Moreover, the possible advantageous effect of composite pre-heating in terms of advanced degree of cure is reported to be less pronounced after 24 h when compared to immediate measurements, because of the delayed polymerization which allows the polymerization of the composite applied at room temperature to catch up with the polymerization of the pre-heated composite [2,20]. However, it enhances composite adaptation to cavity walls [2]. Although pre-heating leads to better flow properties, in vitro results, however, do not show differences in microleakage between composites at room temperature and at 60 ◦ C [1]. Furthermore, it is clear from this study that the differences in contraction stress and volumetric shrinkage between the tested composites at body temperature are much less evident than at room temperature, which questions these material properties as direct predictors for clinical success. It might be recommended that most of the in vitro studies, which are generally carried out at room temperature, should be carried out at “mouth” temperature (32–37 ◦ C).
5.
Conclusions
Within the limitations of this study, it can be concluded that pre-heating of dental composites results in an increased volumetric shrinkage. The contraction stress between 23 ◦ C and higher temperatures is increased at a statistically significant level. Above 30 ◦ C, however, there are no further increases in contraction stress with higher temperatures, except for Clearfil AP-X, which shows a more gradual increase in contraction stress with increasing temperature.
Acknowledgement We would like to thank Ms. J. Rezende for carrying out the contraction stress and volumetric shrinkage experiments.
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