doi:10.1111/iej.12309
Polymerization kinetics of resin cements after light activation through fibre posts: an in vitro study
A. Khabeer1, J. Whitworth2 & S. Rolland2 1
Department of Restorative Dental Sciences, College of Dentistry, University of Dammam, Dammam, Saudi Arabia; and 2School of Dental Sciences, Newcastle University, Newcastle-upon-Tyne, UK
Abstract Khabeer A, Whitworth J, Rolland S. Polymerization kinetics of resin cements after light activation through fibre posts: an in vitro study. International Endodontic Journal, 48, 261–267, 2015.
Aim To measure the polymerization of light-cured (Variolink Veneer, VLV) and dual-cured (Variolink II, VLII) resin luting cements after light activation through different lengths of fibre post ex vivo. Methodology Degree of conversion after prolonged direct light activation (PLA) [12 min after LED light activation for 3 min] was determined using ATRFTIR. Models were then produced to allow samples of VLV and VLII (n = 5 each group) to be light-activated through 3, 6 and 9 mm lengths of fibre post (Fiber Lux). Degree of conversion was assessed by ATR-FTIR and expressed as a percentage of the degree of conversion achieved after PLA. Data were analysed using ANOVA and Tukey’s test (P < 0.05). Results Mean [SD] degree of conversion for VLV and VLII after PLA was 57.97% [1.51] and 54.71%
Introduction Bonded fibre posts are increasingly employed in the restoration of root filled teeth as many laboratory investigations have suggested a modulus of elasticity similar to dentine (Ferrari et al. 2000) and improved
Correspondence: Sarah Rolland, School of Dental Sciences, Newcastle University, Framlington Place, Newcastle upon Tyne NE2 4BW, UK (Tel.: +44 (0) 191 222 8242; Fax: +44 (0) 191 222 6137; e-mail: [email protected]).
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
[3.77], respectively. Light activation of VLV through a 3 mm post produced 81.62% of the PLA value, compared with 71.03% for the 6 mm and 46.04% for the 9 mm post. Conversion after activation through 9 mm posts was significantly less than through 3 mm and 6 mm posts (P < 0.05). For VLII, activation through a 3 mm post produced 66.51% of the PLA value, compared with 54.38% for the 6 mm and 41.56% for the 9 mm post. A significant decrease in degree of conversion was noted for VLII as post length increased (P < 0.05). Conclusions The degree of conversion for VLV reduced after light activation through 9 mm posts when compared to 3 mm and 6 mm posts, whilst the degree of conversion for VLII decreased with every increase in post length. Keywords: degree of conversion, dual cured, fibre post, FTIR, light cured, light transmission. Received 4 March 2013; accepted 26 April 2014
bond strengths (Naumann et al. 2008) compared with conventional posts and cements. The detailed dynamics of adhesive behaviour within post channels is, however, under-researched, and concerns have been expressed about the quality of curing as a function of compromised light penetration (Cekic-Nagas & Ergun 2011). It has been reported that the degree of conversion of composite resin has an inverse relationship with the length of post channel and explanations may include the increasing distance between the activating light source and the material (Le Bell et al. 2003). The use of a translucent fibre post may be
International Endodontic Journal, 48, 261–267, 2015
261
Curing through fibre posts Khabeer et al.
Coltolux light meter (ColteneWhaledent) before each test.
expected to allow deeper penetration of light and improved conversion (Shadman et al. 2012), but inadequate conversion rates have been reported in middle and apical root thirds (Faria-e-Silva et al. 2008, Kim et al. 2009). Incomplete curing of adhesive cements may therefore contribute to poor physical properties of the cement and ultimately poor bonding and clinical performance (Roberts et al. 2004). Dual-cured resin cements have been recommended, but light activation may still be needed and the details of curing behaviour are unclear (Attar et al. 2003). This study aimed to evaluate the degree of conversion of a light-cured and a dual-cured resin luting cement after light activation through various lengths of fibre post. The hypothesis tested was that the degree of conversion of the dual-cured material would be consistent after light activation through different post lengths, whilst the degree of conversion of the light-cured material would be reduced after activation through longer posts.
Determination of degree of conversion after prolonged, direct curing Degree of conversion after prolonged direct light activation (PLA) was monitored for both test materials (n = 5) using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR; Perkin Elmer Spectrum 100, PerkinElmer, Waltham, MA, USA) with a diamond sensing element in ATR mode at 4 cm1 resolution in the wavelength range of 750–2000 cm1. Materials were dispensed directly onto the FTIR sensing element and a glass coverslip placed over as a spacer device to produce specimens 1 mm thick. Two readings were taken of uncured material, before light activation for 3 min followed by 12 min in dark conditions before ATR-FTIR scanning (15 min from the start of light activation). The degree of conversion (DoC) was calculated by comparing the ratio of aliphatic carbon–carbon double bonds (C=C) at 1640 cm1 and C=O at 1710 cm1 in cured and uncured materials using the following formula: 1 Rpolymerised DoC ¼ 100 Runpolymerised
Materials and methods A light-curing [Variolink Veneer (VLV); IvoclarVivadent, Schaan, Liechtenstein] and a dual-curing (Variolink II (VLII); IvoclarVivadent) resin luting cement were included (Table 1). Materials were stored at 2 °C before use. Experiments were performed at room temperature and materials manipulated according to the manufacturer’s instructions. All posts were parallel sided glass fibre (Translucent Fiber Lux posts, ColteneWhaledent, Mahwah, NY, USA) with a diameter of 1.50 mm. The curing light was a LED light [Flashlite 1401; Discus Dental, Inc., Culver City, CA, USA], whose average output intensity was measured as 980 mW cm2 with a
where R ¼
peak height 1640 cm1 peak height 1710 cm1
Degree of conversion after light activation through fibre posts Posts were trimmed perpendicular to length with a water-cooled diamond saw to give post lengths of 3 mm, 6 mm and 9 mm measured from their coronal
Table 1 Materials used in this study Material
Code
Variolink Veneer (Light-cured luting composite) Variolink II (Dual-cured luting composite)
VLV
IvoclarVivadent
Medium Value 0
P47768
VLII
IvoclarVivadent
Base: Transparent Catalyst: Transparent/ Low Viscosity –
Base: P52019 Catalyst: P71536
ParaPostâ Fiber Lux
–
Manufacturer
neWhaledent Colte
Shade
LOT
–
Composition UDMA, TEGDMA, Silicon dioxide, Ytterbiumtrifluoride, initiators, stabilizers and pigments. Bis-GMA,UDMA, TEGDMA, Silica, Barium glass, Ytterbiumtrifluoride, spheroid mixed oxide, initiators, stabilizers and pigments.
60% Fibreglass; 40% Resin
Bis-GMA, Bisphenol A-glycidyldimethacrylate; UDMA, Urethane dimethacrylate; TEGDMA, Triethylene glycol dimethacrylate.
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© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
Khabeer et al. Curing through fibre posts
Translucent fibre post Impression material 1 mm thick luting composite 6 mm
FTIR crystal
3 mm
Figure 1 Diagrammatic representation of model used in the study. The horizontal lines represent the sectioning of the mould and post at 6 mm and 3 mm.
heads. Models were created that replicated post channels (Fig. 1) prepared in modified Eppendorf tubes with a 1.5 mm diameter 9 1 mm thick disc placed in the centre of the open end to provide space for the test material. The fibre post was secured in place and Virtualâ hydrophilic vinylpolysiloxane impression material (IvoclarVivadent) poured around the disc and post to the desired length. Three models were prepared, for posts of 3 mm, 6 mm and 9 mm lengths. VLV was syringed directly into the apical end of the mould, whilst the base and catalyst of VLII were mixed according to the manufacturer’s instructions. All procedures were performed in dimmed light to reduce the influence of ambient light on the curing. Two FTIR readings were taken of uncured material, after which the material was light-activated for 10 s through the post followed by a further FTIR reading, taking 10 s. This cycle of curing and FTIR measurements was repeated 10 times for each material/post combinations, giving a total light curing of 100 s. The data collected after curing through the posts were then manipulated using the formula: DoC %of prolonged Cure ¼ 100 Mean DoC after 15min Data were manipulated and analysed using MS Excel and SPSS version 16.0 (SPSS Inc., Chicago, IL, USA) using independent sample t-test, ANOVA and Tukey’s multiple range test (P < 0.05).
DoC for the two materials was not significantly different (t-test; P > 0.05). Table 2 shows percentage of the PLA value for VLV and VLII after light activation through 3 mm, 6 mm and 9 mm posts. For all the groups, there was a trend of reduced conversion as the length of the post increased. For VLV, no significant difference was found between the conversion through 3 mm and 6 mm posts (P > 0.05), whilst conversion through 9 mm posts was significantly less than through 3 mm and 6 mm posts (P < 0.05). For VLII, a significant decrease in degree of conversion was noted as post length increased (P < 0.05). The degree of conversion was not significantly different between VLV and VLII when materials were light-activated through each length of posts (t-test; P > 0.05). Figures 2 and 3 compare the percentage of the degree of conversion achieved by activating test materials through 3 mm, 6 mm and 9 mm posts.
Discussion This laboratory study evaluated the degree of conversion of light-cured VLV and dual-cured VLII after irradiating through 3 mm, 6 mm and 9 mm lengths of light transmitting fibre post. Several techniques, such as differential thermal analysis (Imazato et al. 1995), differential scanning calorimetry (Antonucci & Toth 1983), Raman spectroscopy (Shin et al. 1993), electron paramagnetic resonance (Sustercic et al. 1997) and nuclear magnetic resonance (Morgan et al. 2000), have been deployed to determine the degree of conversion of resin materials. In this study, ATR-FTIR was used to determine the degree of conversion as it is a nondestructive technique, simple to use, with high optical output and the ability to monitor the setting reaction in real time as it progresses (Oh et al. Table 2 Mean [SD]% of prolonged, direct curing value for VLV and VLII after light activation for 40 s through different lengths of post. (n = 5 samples) Variolink Veneer
Depth
Mean (%)
Control 3 mm 6 mm 9 mm
91.76a 81.62ab 71.03bc 46.04
Results Degree of conversion after prolonged direct light activation The mean (SD) degree of conversion for VLV and VLII after prolonged direct light activation (PLA) was 57.97% (1.51) and 54.71% (3.77), respectively. The
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
Variolink II
Std. deviation
Mean (%)
Std. deviation
2.39 5.34 6.09 12.79
82.98 66.51 54.38 41.56
3.08 2.11 6.32 8.62
Superscript letters indicate groups with no significant difference (ANOVA, Tukey’s multiple range test, P < 0.05).
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60 55 Degree of conversion (%)
50 45 40 35 30 25 20 15
VLV 3 mm
10
VLV 6 mm
5 0
VLV 9 mm 0
10
20
30
40 50 60 70 Light activation time (s)
80
90
100
110
Figure 2 Degree of conversion (%) of VLV after light activation through 3 mm, 6 mm and 9 mm fibre posts. (n = 5 samples). 60 55 Degree of conversion (%)
50 45 40 35 30 25 20 15
VLII 3 mm
10
VLII 6 mm
5
VLII 9 mm
0 0
10
20
30
40
50
60
70
80
90
100
110
Light activation time (s)
Figure 3 Degree of conversion (%) of VLII after light activation through 3 mm, 6 mm and 9 mm fibre posts. (n = 5 samples).
2006). ATR-FTIR utilizes two basic mechanisms: total internal reflection and its attenuation. Internal reflection takes place when the infrared light is passed through a crystal with a high reflective index, leading to the formation of evanescence wave that extends beyond the crystal. This extended evanescence comes in contact with the sample leading to the absorption of energy by the sample. This results in the attenuation of the evanescent wave, and the energy from this attenuated wave is passed to the detector and a spectrum produced (Katon 1996). The degree of conversion was calculated by comparing the ratio of the aliphatic C=C peak at 1640 cm1 and the C=O peak at 1710 cm1. Previ-
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ous studies have used the aromatic C=C peak at around 1610 cm1 (Spencer et al. 2000, Faria-e-Silva et al. 2008, Kim et al. 2009), but this peak was absent for VLV, which is likely to be due to the absence of Bis-GMA, therefore ruling out the use of this peak for investigation. In this study, two scans of uncured samples were taken 20 s apart before light activation which showed no noticeable degree of conversion even though the chemical and light-activated elements of dual-curing cements work independently (Braga et al. 2002). A similar finding has also been reported by Tezvergil-Mutluay et al. (2007) who noted that VLII achieved 0.05). These lower values may be attributed to the inability of VLII to undergo chemical reaction in the early phases of polymerization. As shown in Figures 2 and 3, there was no plateauing for both the materials during the test periods indicating further potential for polymerization. Nonetheless, for both VLV and VLII, there was a general trend for a lower degree of conversion as the length of the post increased, which is likely to be due to a reduction in light intensity with increasing post length (Goracci et al. 2008), resulting in incomplete polymerization in the deeper zones of post channel (Kim et al. 2009). Moreover, the degree of conversion of resin luting cements is also dependant on the translucency and composition of the fibre post as suggested by Kim et al. (2009) who reported higher degrees of conversion for VLII when light activated through translucent quartz or glass fibre posts, compared with an opaque silicon-zirconium glass fibre post. The Fiber Lux post used in this study is glass fibre, and light transmission through this post is significantly greater than through an opaque glass fibre post from the same manufacturer, but light transmission drops by approximately 95% between 1 mm and 9 mm length (Ho et al. 2011). Other factors that may contribute to the polymerization of resin cements in post channels include the diameter of the fibre post and depth of cure has been shown to be directly proportional to the diameter of the fibre post (Lui 1994). However, in clinical situations, the selection of the post diameter is dependent on the type and anatomy of the root canal and the mechanical needs of the restoration. In the present study, translucent Fiber Lux posts with a diameter of 1.5 mm were used to facilitate handling and the production of wider diameter of test samples allowing better positioning of the samples on the FTIR crystal. These results may therefore not translate automati-
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cally to posts of similar design but with different diameter. One of the purposes of determining the degree of conversion can be related to the retention of the post in the root canal as a higher degree of conversion is related to higher bond strength (Giachetti et al. 2004). However, retention of the post also depends on the bond between the post and cement as well as between the cement and root dentine. In addition, incomplete conversion may result in increased amounts of residual monomers, which might leak into the periodontal tissues resulting in adverse effects (Schmid-Schwap et al. 2009). Therefore, attempts should be made to select a translucent fibre post with the highest light transmitting capability, in conjunction with a light-curing unit that allows maximal conversion of the particular luting cement. Further research needs to be carried out to fully understand the kinetics of polymerization in the deeper zones of post channels.
Conclusion The hypothesis that the degree of conversion of the dual-cured material (VLII) would be consistent through different length posts after light activation was rejected. Degree of cure of VLII reduced significantly as the post length was increased. The hypothesis that the degree of conversion of the light-activated material (VLV) would reduce with increasing post length was partially supported – the degree of cure observed following curing through 9 mm post length was significantly less than through 3 and 6 mm post length.
Acknowledgements Submitted in partial fulfilment for degree of MClinDent in Restorative Dentistry, Newcastle University, UK. Mr. Andrew Yates is thanked for his technical support.
References Antonucci JM, Toth EE (1983) Extent of polymerization of dental resins by differential scanning calorimetry. Journal of Dental Research 62, 121–5. Attar N, Tam LE, McComb D (2003) Mechanical and physical properties of contemporary dental luting agents. Journal of Prosthetic Dentistry 89, 127–34. Braga RR, Cesar PF, Gonzaga CC (2002) Mechanical properties of resin cements with different activation modes. Journal of Oral Rehabilitation 29, 257–62.
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
Khabeer et al. Curing through fibre posts
Cekic-Nagas I, Ergun G (2011) Effect of different light curing methods on mechanical and physical properties of resincements polymerized through ceramic discs. Journal of Applied Oral Sciences 19, 403–12. Emami N, Soderholm KJ (2005) Influence of light-curing procedures and photo-initiator/co-initiator composition on the degree of conversion of light-curing resins. Journal of Materials Science. Materials in Medicine 16, 47–52. Faria e Silva AL, Arias VG, Soares LE, Martin AA, Martins LR (2007) Influence of fiber-post translucency on the degree of conversion of a dual-cured resin cement. Journal of Endodontics 33, 303–5. Faria-e-Silva AL, Casselli DS, Lima GS, Ogliari FA, Piva E, Martins LR (2008) Kinetics of conversion of two dualcured adhesive systems. Journal of Endodontics 34, 1115–8. Ferracane JL, Berge HX, Condon JR (1998) In vitro aging of dental composites in water–effect of degree of conversion, filler volume, and filler/matrix coupling. Journal of Biomedical Materials Research 42, 465–72. Ferrari M, Vichi A, Garcia-Godoy F (2000) Clinical evaluation of fiber-reinforced epoxy resin posts and cast post and cores. American Journal of Dentistry 13, 15B–8B. Giachetti L, Scaminaci Russo D, Bertini F, Giuliani V (2004) Translucent fiber post cementation using a light-curing adhesive/composite system: SEM analysis and pull-out test. Journal of Dentistry 32, 629–34. Goracci C, Corciolani G, Vichi A, Ferrari M (2008) Lighttransmitting ability of marketed fiber posts. Journal of Dental Research 87, 1122–6. Grandini S, Goracci C, Monticelli F, Borracchini A, Ferrari M (2005) SEM evaluation of the cement layer thickness after luting two different posts. The Journal of Adhesive Dentistry 7, 235–40. Ho YC, Lai YL, Chou IC, Yang SF, Lee SY (2011) Effects of light attenuation by fibre posts on polymerization of a dual-cured resin cement and microleakage of post-restored teeth. Journal of Dentistry 39, 309–15. Imazato S, Tarumi H, Kobayashi K, Hiraguri H, Oda K, Tsuchitani Y (1995) Relationship between the degree of conversion and internal discoloration of light-activated composite. Dental Materials Journal 14, 23–30. Katon JE (1996) Infrared microspectroscopy. A review of fundamentals and applications. Micron 27, 303–14. Kim YK, Kim SK, Kim KH, Kwon TY (2009) Degree of conversion of dual-cured resin cement light-cured through three fibre posts within human root canals: an ex vivo study. International Endodontic Journal 42, 667–74. Le Bell AM, Tanner J, Lassila LV, Kangasniemi I, Vallittu PK (2003) Depth of light-initiated polymerization of glass fiber-reinforced composite in a simulated root canal. The International Journal of Prosthodontics 16, 403–8. Lui JL (1994) Depth of composite polymerization within simulated root canals using light-transmitting posts. Operative Dentistry 19, 165–8.
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Morgan DR, Kalachandra S, Shobha HK, Gunduz N, Stejskal EO (2000) Analysis of a dimethacrylate copolymer (bisGMA and TEGDMA) network by DSC and 13C solution and solid-state NMR spectroscopy. Biomaterials 21, 1897– 903. Naumann M, Sterzenbach G, Rosentritt M, Beuer F, Frankenberger R (2008) Is adhesive cementation of endodontic posts necessary? Journal of Endodontics 34, 1006–10. Oh SJ, Lee SC, Park SY (2006) Photopolymerization and photobleaching of n-butyl acrylate/fumed silica composites monitored by real time FTIR-ATR spectroscopy. Vibrational Spectroscopy 42, 273–7. Rahiotis C, Patsouri K, Silikas N, Kakaboura A (2010) Curing efficiency of high-intensity light-emitting diode (LED) devices. Journal of Oral Science 52, 187–95. Roberts HW, Leonard DL, Vandewalle KS, Cohen ME, Charlton DG (2004) The effect of a translucent post on resin composite depth of cure. Dental Materials 20, 617– 22. dos Santos Alves Morgan LF, Peixoto RT, de Castro Albuquerque R, Santos Correa MF, de Abreu Poletto LT, Pinotti MB (2008) Light transmission through a translucent fiber post. Journal of Endodontics 34, 299–302. Schmid-Schwap M, Franz A, K€ onig F et al. (2009) Cytotoxicity of four categories of dental cements. Dental Materials 25, 360–8. Shadman N, Atai M, Ghavam M, Kermanshah H, Ebrahimi SF (2012) Parameters affecting degree of conversion of dual-cure resin cements in the root canal: FTIR analysis. Journal of the Canadian Dental Association 78, c53. Shin WS, Li XF, Schwartz B, Wunder SL, Baran GR (1993) Determination of the degree of cure of dental resins using Raman and Ft-Raman spectroscopy. Dental Materials 9, 317–24. Spencer P, Wang Y, Walker MP, Wieliczka DM, Swafford JR (2000) Interfacial chemistry of the dentin/adhesive bond. Journal of Dental Research 79, 1458–63. Stricker EJ, Gohring TN (2006) Influence of different posts and cores on marginal adaptation, fracture resistance, and fracture mode of composite resin crowns on human mandibular premolars. An in vitro study. Journal of Dentistry 34, 326–35. Sustercic D, Cevc P, Funduk N, Pintar MM(1997) Determination of curing time in visible-light-cured composite resins of different thickness by electron paramagnetic resonance. Journal of materials science. Materials in medicine 8, 507–10. Teixeira EC, Teixeira FB, Piasick JR, Thompson JY (2006) An in vitro assessment of prefabricated fiber post systems. Journal of the American Dental Association 137, 1006–12. Tezvergil-Mutluay A, Lassila LV, Vallittu PK (2007) Degree of conversion of dual-cure luting resins light-polymerized through various materials. Acta Odontologica Scandinavica 65, 201–5.
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Polymerization kinetics of resin cements after light activation through fibre posts: an in vitro study
A. Khabeer1, J. Whitworth2 & S. Rolland2 1
Department of Restorative Dental Sciences, College of Dentistry, University of Dammam, Dammam, Saudi Arabia; and 2School of Dental Sciences, Newcastle University, Newcastle-upon-Tyne, UK
Abstract Khabeer A, Whitworth J, Rolland S. Polymerization kinetics of resin cements after light activation through fibre posts: an in vitro study. International Endodontic Journal, 48, 261–267, 2015.
Aim To measure the polymerization of light-cured (Variolink Veneer, VLV) and dual-cured (Variolink II, VLII) resin luting cements after light activation through different lengths of fibre post ex vivo. Methodology Degree of conversion after prolonged direct light activation (PLA) [12 min after LED light activation for 3 min] was determined using ATRFTIR. Models were then produced to allow samples of VLV and VLII (n = 5 each group) to be light-activated through 3, 6 and 9 mm lengths of fibre post (Fiber Lux). Degree of conversion was assessed by ATR-FTIR and expressed as a percentage of the degree of conversion achieved after PLA. Data were analysed using ANOVA and Tukey’s test (P < 0.05). Results Mean [SD] degree of conversion for VLV and VLII after PLA was 57.97% [1.51] and 54.71%
Introduction Bonded fibre posts are increasingly employed in the restoration of root filled teeth as many laboratory investigations have suggested a modulus of elasticity similar to dentine (Ferrari et al. 2000) and improved
Correspondence: Sarah Rolland, School of Dental Sciences, Newcastle University, Framlington Place, Newcastle upon Tyne NE2 4BW, UK (Tel.: +44 (0) 191 222 8242; Fax: +44 (0) 191 222 6137; e-mail: [email protected]).
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
[3.77], respectively. Light activation of VLV through a 3 mm post produced 81.62% of the PLA value, compared with 71.03% for the 6 mm and 46.04% for the 9 mm post. Conversion after activation through 9 mm posts was significantly less than through 3 mm and 6 mm posts (P < 0.05). For VLII, activation through a 3 mm post produced 66.51% of the PLA value, compared with 54.38% for the 6 mm and 41.56% for the 9 mm post. A significant decrease in degree of conversion was noted for VLII as post length increased (P < 0.05). Conclusions The degree of conversion for VLV reduced after light activation through 9 mm posts when compared to 3 mm and 6 mm posts, whilst the degree of conversion for VLII decreased with every increase in post length. Keywords: degree of conversion, dual cured, fibre post, FTIR, light cured, light transmission. Received 4 March 2013; accepted 26 April 2014
bond strengths (Naumann et al. 2008) compared with conventional posts and cements. The detailed dynamics of adhesive behaviour within post channels is, however, under-researched, and concerns have been expressed about the quality of curing as a function of compromised light penetration (Cekic-Nagas & Ergun 2011). It has been reported that the degree of conversion of composite resin has an inverse relationship with the length of post channel and explanations may include the increasing distance between the activating light source and the material (Le Bell et al. 2003). The use of a translucent fibre post may be
International Endodontic Journal, 48, 261–267, 2015
261
Curing through fibre posts Khabeer et al.
Coltolux light meter (ColteneWhaledent) before each test.
expected to allow deeper penetration of light and improved conversion (Shadman et al. 2012), but inadequate conversion rates have been reported in middle and apical root thirds (Faria-e-Silva et al. 2008, Kim et al. 2009). Incomplete curing of adhesive cements may therefore contribute to poor physical properties of the cement and ultimately poor bonding and clinical performance (Roberts et al. 2004). Dual-cured resin cements have been recommended, but light activation may still be needed and the details of curing behaviour are unclear (Attar et al. 2003). This study aimed to evaluate the degree of conversion of a light-cured and a dual-cured resin luting cement after light activation through various lengths of fibre post. The hypothesis tested was that the degree of conversion of the dual-cured material would be consistent after light activation through different post lengths, whilst the degree of conversion of the light-cured material would be reduced after activation through longer posts.
Determination of degree of conversion after prolonged, direct curing Degree of conversion after prolonged direct light activation (PLA) was monitored for both test materials (n = 5) using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR; Perkin Elmer Spectrum 100, PerkinElmer, Waltham, MA, USA) with a diamond sensing element in ATR mode at 4 cm1 resolution in the wavelength range of 750–2000 cm1. Materials were dispensed directly onto the FTIR sensing element and a glass coverslip placed over as a spacer device to produce specimens 1 mm thick. Two readings were taken of uncured material, before light activation for 3 min followed by 12 min in dark conditions before ATR-FTIR scanning (15 min from the start of light activation). The degree of conversion (DoC) was calculated by comparing the ratio of aliphatic carbon–carbon double bonds (C=C) at 1640 cm1 and C=O at 1710 cm1 in cured and uncured materials using the following formula: 1 Rpolymerised DoC ¼ 100 Runpolymerised
Materials and methods A light-curing [Variolink Veneer (VLV); IvoclarVivadent, Schaan, Liechtenstein] and a dual-curing (Variolink II (VLII); IvoclarVivadent) resin luting cement were included (Table 1). Materials were stored at 2 °C before use. Experiments were performed at room temperature and materials manipulated according to the manufacturer’s instructions. All posts were parallel sided glass fibre (Translucent Fiber Lux posts, ColteneWhaledent, Mahwah, NY, USA) with a diameter of 1.50 mm. The curing light was a LED light [Flashlite 1401; Discus Dental, Inc., Culver City, CA, USA], whose average output intensity was measured as 980 mW cm2 with a
where R ¼
peak height 1640 cm1 peak height 1710 cm1
Degree of conversion after light activation through fibre posts Posts were trimmed perpendicular to length with a water-cooled diamond saw to give post lengths of 3 mm, 6 mm and 9 mm measured from their coronal
Table 1 Materials used in this study Material
Code
Variolink Veneer (Light-cured luting composite) Variolink II (Dual-cured luting composite)
VLV
IvoclarVivadent
Medium Value 0
P47768
VLII
IvoclarVivadent
Base: Transparent Catalyst: Transparent/ Low Viscosity –
Base: P52019 Catalyst: P71536
ParaPostâ Fiber Lux
–
Manufacturer
neWhaledent Colte
Shade
LOT
–
Composition UDMA, TEGDMA, Silicon dioxide, Ytterbiumtrifluoride, initiators, stabilizers and pigments. Bis-GMA,UDMA, TEGDMA, Silica, Barium glass, Ytterbiumtrifluoride, spheroid mixed oxide, initiators, stabilizers and pigments.
60% Fibreglass; 40% Resin
Bis-GMA, Bisphenol A-glycidyldimethacrylate; UDMA, Urethane dimethacrylate; TEGDMA, Triethylene glycol dimethacrylate.
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© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
Khabeer et al. Curing through fibre posts
Translucent fibre post Impression material 1 mm thick luting composite 6 mm
FTIR crystal
3 mm
Figure 1 Diagrammatic representation of model used in the study. The horizontal lines represent the sectioning of the mould and post at 6 mm and 3 mm.
heads. Models were created that replicated post channels (Fig. 1) prepared in modified Eppendorf tubes with a 1.5 mm diameter 9 1 mm thick disc placed in the centre of the open end to provide space for the test material. The fibre post was secured in place and Virtualâ hydrophilic vinylpolysiloxane impression material (IvoclarVivadent) poured around the disc and post to the desired length. Three models were prepared, for posts of 3 mm, 6 mm and 9 mm lengths. VLV was syringed directly into the apical end of the mould, whilst the base and catalyst of VLII were mixed according to the manufacturer’s instructions. All procedures were performed in dimmed light to reduce the influence of ambient light on the curing. Two FTIR readings were taken of uncured material, after which the material was light-activated for 10 s through the post followed by a further FTIR reading, taking 10 s. This cycle of curing and FTIR measurements was repeated 10 times for each material/post combinations, giving a total light curing of 100 s. The data collected after curing through the posts were then manipulated using the formula: DoC %of prolonged Cure ¼ 100 Mean DoC after 15min Data were manipulated and analysed using MS Excel and SPSS version 16.0 (SPSS Inc., Chicago, IL, USA) using independent sample t-test, ANOVA and Tukey’s multiple range test (P < 0.05).
DoC for the two materials was not significantly different (t-test; P > 0.05). Table 2 shows percentage of the PLA value for VLV and VLII after light activation through 3 mm, 6 mm and 9 mm posts. For all the groups, there was a trend of reduced conversion as the length of the post increased. For VLV, no significant difference was found between the conversion through 3 mm and 6 mm posts (P > 0.05), whilst conversion through 9 mm posts was significantly less than through 3 mm and 6 mm posts (P < 0.05). For VLII, a significant decrease in degree of conversion was noted as post length increased (P < 0.05). The degree of conversion was not significantly different between VLV and VLII when materials were light-activated through each length of posts (t-test; P > 0.05). Figures 2 and 3 compare the percentage of the degree of conversion achieved by activating test materials through 3 mm, 6 mm and 9 mm posts.
Discussion This laboratory study evaluated the degree of conversion of light-cured VLV and dual-cured VLII after irradiating through 3 mm, 6 mm and 9 mm lengths of light transmitting fibre post. Several techniques, such as differential thermal analysis (Imazato et al. 1995), differential scanning calorimetry (Antonucci & Toth 1983), Raman spectroscopy (Shin et al. 1993), electron paramagnetic resonance (Sustercic et al. 1997) and nuclear magnetic resonance (Morgan et al. 2000), have been deployed to determine the degree of conversion of resin materials. In this study, ATR-FTIR was used to determine the degree of conversion as it is a nondestructive technique, simple to use, with high optical output and the ability to monitor the setting reaction in real time as it progresses (Oh et al. Table 2 Mean [SD]% of prolonged, direct curing value for VLV and VLII after light activation for 40 s through different lengths of post. (n = 5 samples) Variolink Veneer
Depth
Mean (%)
Control 3 mm 6 mm 9 mm
91.76a 81.62ab 71.03bc 46.04
Results Degree of conversion after prolonged direct light activation The mean (SD) degree of conversion for VLV and VLII after prolonged direct light activation (PLA) was 57.97% (1.51) and 54.71% (3.77), respectively. The
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
Variolink II
Std. deviation
Mean (%)
Std. deviation
2.39 5.34 6.09 12.79
82.98 66.51 54.38 41.56
3.08 2.11 6.32 8.62
Superscript letters indicate groups with no significant difference (ANOVA, Tukey’s multiple range test, P < 0.05).
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60 55 Degree of conversion (%)
50 45 40 35 30 25 20 15
VLV 3 mm
10
VLV 6 mm
5 0
VLV 9 mm 0
10
20
30
40 50 60 70 Light activation time (s)
80
90
100
110
Figure 2 Degree of conversion (%) of VLV after light activation through 3 mm, 6 mm and 9 mm fibre posts. (n = 5 samples). 60 55 Degree of conversion (%)
50 45 40 35 30 25 20 15
VLII 3 mm
10
VLII 6 mm
5
VLII 9 mm
0 0
10
20
30
40
50
60
70
80
90
100
110
Light activation time (s)
Figure 3 Degree of conversion (%) of VLII after light activation through 3 mm, 6 mm and 9 mm fibre posts. (n = 5 samples).
2006). ATR-FTIR utilizes two basic mechanisms: total internal reflection and its attenuation. Internal reflection takes place when the infrared light is passed through a crystal with a high reflective index, leading to the formation of evanescence wave that extends beyond the crystal. This extended evanescence comes in contact with the sample leading to the absorption of energy by the sample. This results in the attenuation of the evanescent wave, and the energy from this attenuated wave is passed to the detector and a spectrum produced (Katon 1996). The degree of conversion was calculated by comparing the ratio of the aliphatic C=C peak at 1640 cm1 and the C=O peak at 1710 cm1. Previ-
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ous studies have used the aromatic C=C peak at around 1610 cm1 (Spencer et al. 2000, Faria-e-Silva et al. 2008, Kim et al. 2009), but this peak was absent for VLV, which is likely to be due to the absence of Bis-GMA, therefore ruling out the use of this peak for investigation. In this study, two scans of uncured samples were taken 20 s apart before light activation which showed no noticeable degree of conversion even though the chemical and light-activated elements of dual-curing cements work independently (Braga et al. 2002). A similar finding has also been reported by Tezvergil-Mutluay et al. (2007) who noted that VLII achieved 0.05). These lower values may be attributed to the inability of VLII to undergo chemical reaction in the early phases of polymerization. As shown in Figures 2 and 3, there was no plateauing for both the materials during the test periods indicating further potential for polymerization. Nonetheless, for both VLV and VLII, there was a general trend for a lower degree of conversion as the length of the post increased, which is likely to be due to a reduction in light intensity with increasing post length (Goracci et al. 2008), resulting in incomplete polymerization in the deeper zones of post channel (Kim et al. 2009). Moreover, the degree of conversion of resin luting cements is also dependant on the translucency and composition of the fibre post as suggested by Kim et al. (2009) who reported higher degrees of conversion for VLII when light activated through translucent quartz or glass fibre posts, compared with an opaque silicon-zirconium glass fibre post. The Fiber Lux post used in this study is glass fibre, and light transmission through this post is significantly greater than through an opaque glass fibre post from the same manufacturer, but light transmission drops by approximately 95% between 1 mm and 9 mm length (Ho et al. 2011). Other factors that may contribute to the polymerization of resin cements in post channels include the diameter of the fibre post and depth of cure has been shown to be directly proportional to the diameter of the fibre post (Lui 1994). However, in clinical situations, the selection of the post diameter is dependent on the type and anatomy of the root canal and the mechanical needs of the restoration. In the present study, translucent Fiber Lux posts with a diameter of 1.5 mm were used to facilitate handling and the production of wider diameter of test samples allowing better positioning of the samples on the FTIR crystal. These results may therefore not translate automati-
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cally to posts of similar design but with different diameter. One of the purposes of determining the degree of conversion can be related to the retention of the post in the root canal as a higher degree of conversion is related to higher bond strength (Giachetti et al. 2004). However, retention of the post also depends on the bond between the post and cement as well as between the cement and root dentine. In addition, incomplete conversion may result in increased amounts of residual monomers, which might leak into the periodontal tissues resulting in adverse effects (Schmid-Schwap et al. 2009). Therefore, attempts should be made to select a translucent fibre post with the highest light transmitting capability, in conjunction with a light-curing unit that allows maximal conversion of the particular luting cement. Further research needs to be carried out to fully understand the kinetics of polymerization in the deeper zones of post channels.
Conclusion The hypothesis that the degree of conversion of the dual-cured material (VLII) would be consistent through different length posts after light activation was rejected. Degree of cure of VLII reduced significantly as the post length was increased. The hypothesis that the degree of conversion of the light-activated material (VLV) would reduce with increasing post length was partially supported – the degree of cure observed following curing through 9 mm post length was significantly less than through 3 and 6 mm post length.
Acknowledgements Submitted in partial fulfilment for degree of MClinDent in Restorative Dentistry, Newcastle University, UK. Mr. Andrew Yates is thanked for his technical support.
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