not always indicate the type of photoinitiators in their ... concentration of photoinitiator in the material, the number ... formed free radicals attack the C=C bonds.
PHOTOINITIATORS IN DENTISTRY: A REVIEW ARIO SANTINI BDS, DDS, PHD, FDS RCPS(GLASG), DIPFMED, DGDP, FFGDP(UK), FADM, DIRECTOR OF BIOMATERIALS RESEARCH, EDINBURGH DENTAL INSTITUTE, THE UNIVERSITY OF EDINBURGH, UK AND ALSO IRANZIHUATL TORRES GALLEGOS DDS, MSC, PHD, PHYSICS INSTITUTE AND FACULTY OF DENTISTRY, SAN LUIS POTOSI UNIVERSITY, MEXICO AND CHRISTOPHER M. FELIX BSC, CHIEF SCIENCE OFFICE, BLUELIGHT ANALYTICS INC., CANADA
ABSTRACT Polymerization of Resin Based Composites (RBCs) initiated by a light curing unit activating photoinitiators. Different RBCs require different light energy levels for proper curing. Manufacturers are now producing RBCs with more than one initiator and not all of these will be properly polymerised with blue LED lights. An added problem is that manufacturers do not always indicate the type of photoinitiators in their materials. This review discusses the importance of matching the spectral output of LCUs to the absorption spectra of RBCs and the consequences of spectral mismatch. Resin based composites (RBCs) were first introduced in the 1960s1 and with development of effective and reliable dentine bonding systems2, have been used routinely as a filling material for both anterior and posterior teeth. The early RBCs were either chemically cured two component materials or photo-initiated materials that used UV initiators in the beginning and then transitioned to visible light initiators such as camphorquinine which was introduced in 1978.3 The first report of a light curing material was of an ultraviolet (UV) cured fissure sealant.4 However, due to the limited penetration depth of the UV light and the potential health hazards, this system was quickly abandoned. The advancement of science yielded light curing materials which contributed to a significant clinical progress over the UV and chemically cured RBCs.4 Additional advancements to direct RBC restoration materials included luting agents for ceramic restorations, pit and fissure sealants and resin modified glass ionomers.
The amount of activated photo initiator depends on the concentration of photoinitiator in the material, the number of photons to which the material is exposed and the energy of the photons (wavelength), the latter depending on the curing light.6 The most common photoinitiator in dental materials today is camphorquinone, which has a peak activity around 470 nanometres.6 The factors affecting polymerization include filler type, size and loading, the thickness and shade of the restorative material, the effectiveness of light transmission (eg. light guide tips being free from debris and scratches), exposure time, distance of the light source from the restorative material and light intensity.7 It is important to note that the photoinitiator activation occurs at specific wavelengths, in other words, the optimum efficiency is obtained when the peak absorptivity of the photoinitiator corresponds with the spectral emission from the LCU. Commercially available curing units have different light intensities and light sources, with energy levels in QTH, LED and other LCUs ranging from 300 to more than 2000 mW/cm.
Polymerization in an RBC is initiated by a light curing unit (LCU); this technology is based on the use of photoreactive systems that absorb light irradiation from the LCUs at appropriate wavelength. Then the photoinitiators contained in the RBCs, absorb the incoming photons from the LCU and the monomers in the molecular structure become excited and in that active state, there is a change from monomers into a polymer network6. The success of this technology hinges on matching the spectral emission of the LCU with the requirements of the photoinitiator system to convert the monomers into a polymer network.
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1
Figure 1: Absorption spectrum of camphorquinone Figure 2: Absorption spectrum of Lucirin TPO Figure 3: Absorption spectrum of phenylpropanedione (PPD)
2
CQ/tertiary amine photoinitiator system Since its invention by Dart and Nemcek in 19728 the most common photoinitiator system in RBCs is the camphorquinone/ tertiary amine (CQ/TA). When CQ absorbs the light (wavelength of maximum absorbance: 468nm6), it excites and interacts with the TAs, then forms a photoexcited complex. In that state, CQ abstracts a hydrogen atom from the TAs, producing free radicals both on CQ and TAs. The formed free radicals attack the C=C bonds of monomers, resulting in the formation of new radicals with a much longer chain than before (propagating radicals). The same process continues through the chain reaction until the reaction process terminates.9 The peak sensitivity of CQ is near 470 nm in the blue wavelength range (Figure 1). Even though CQ/TA systems have good acceptance, they present some disadvantages; used in very small amounts (ppm), the yellow-colored CQ influences the composite colour10*. Another major problem is that the α-diketone group, derived from CQ, has peak absorption in the visible range, resulting in fast photopolymerization under ambient light (fluorescent lamps and dental lamps) and giving a short therapeutic operation time.11 To solve the problems of CQ/TA, other initiators are now incorporated in RBCs, for example, phenyl-propanedione (PPD) and diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide (TPO)12.
Lucirin TPO and PPD photoinitiator system TPO is an acyl phosphine oxide. It has been shown that TPO results in higher degree of conversion (DC) than
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Figure 4: Light output form two curing lights: LED (B) conventional QTH (C) superimposed on the absorption spectrum of CQ (A)
CQ/TA when is cured with halogen16, or polywave17 LCUs; in addition, TPO exhibits greater colour stability compared to CQ/TA. That is the reason TPO may be especially useful in extra white shades of RBCs, often required in bleached teeth since it eliminates the unwanted yellow effect of CQ/TA18. Its absorption spectrum extends from 380nm to about 425nm (Figure 2). Phenyl Propanedione (PPD) is also a photoinitiator, its absorption spectrum extends from below 350 nm to approximately 490 nm. (Figure 3).
Spectral mismatching Different RBCs require different colours of light for proper curing and this depends on the photoinitiators present in the materials. The manufacturers are producing materials with different initiators and not all of these materials could be properly polymerized with blue LED lights. An additional problem is that manufacturers are not indicating the type of photoinitiator incorporated in their materials.
The RBCs that only contain CQ/TA system, only require the blue spectral range (420-540 nm); however, if the RBCs contain CQ plus TPO and/or PPD, then light in both the blue (420-540 nm) and violet (360-420 nm) ranges are required. Not all curing lights deliver the required colour(s) of light and using a spectrally mismatched combination of LCU and RBC is possible. QTH LCUs have a broad spectral range corresponding with the broad spectral absorption of CQ. (Figure 4) Moreover, its spectral range extends into the violet range and so it can activate photoinitiators such as TPO if they were incorporated in the RBCs. The emission spectrum of commercially available single
6 Figure 5: Single peaks LCUs cover the wavelength range of circa 420 to 515 nm Figure 6: Dual peak LCUs cover the wavelength range of circa 380 to 520 nm
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peak LCUs (420-490 nm) is limited to be effective in activation CQ/TA19, 20 but not so useful when TPO type photoinitiators are present19, 21, 22. In an attempt to overcome the problem of the emission-absorption mismatch of TPO and PPDcontaining materials, dual peak LED LCUs (Figure 5) have been introduced. They have a primary emission peak at around 460 nm to cover the absorption spectrum of the CQ/TA and an additional peak at around 400 nm to match TPO and PPD. This should be taken into account when curing bleached shades of RBCs, even if the manufacturers do not indicate the presence of TPO in their materials.
Relative photosensitivity Not all of the photoinitiators have the same sensitivity. Figure 7 illustrates the peak absorption of TPO and CQ/tertiary amines systems. TPO is about five times more sensitive than CQ. When the spectral output of both single and dual peak LCUs are superimposed (Figure 8) on the absorption spectra of CQ and TPO, the following points become obvious: • Single peak LCUs deliver light energy mainly in the blue spectral range • Dual peak deliver light energy mainly in the blue and violet spectral range • Though violet light is delivered, the peak delivery is still mismatching the peak absorption of TPO and lies in the wave range where absorption of photons by TPO is low. This is similar for PPD Most major dental supply manufacturers market their own RBCs and LCUs. As they determine the composition of the RBCs and the light intensities of the LCUs and recommend irradiation times for both adhesives and RBCs, it may be assumed that the wavelength or intensity of the curing light matches that of the RBC. This may not be the case if RBCs and LCUs from different manufacturers are used by clinicians. The quality of light produced by a dental light-curing unit (LCU) has a direct influence on the polymerization of restorative materials and is highly dependent upon the intensity or strength of irradiation, the peak wavelength emission, the interactions between these functions, and their compatibility with the individual restorative material.
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Figure 7: Relative photosensitivity of CQ and TPO
The darker and more opaque materials generally produce a shallower depth of cure or need longer curing times because less light transmits through the increment of RBC reaching the initiators at the bottom. It is often not possible to polymerize thick increments reliably unless the material is highly translucent or contains only a limited amount of light-refracting fillers. The conventional initiator systems employed in tooth-coloured materials with enamel-like translucency quickly reach their limits when they are faced with the demand for a quick and reliable cure in increments that are thicker than the usual 2mm. This is due, in part, to the fact that though violet light is delivered, the peak delivery is still mismatching the peak absorption of TPO and lies in the wave range where absorption of photons by TPO is low.
REFERENCES 1
2
3
4
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Bowen RL, Rodriguez MS. Tensile strength and modulus of elasticity of tooth structure and several restorative materials. J Am Dent Assoc 1962; 64: 378-387. Leinfelder KF. A conservative approach to placing posterior composite resin restorations. J Am Dent Assoc 1996; 127: 743-748. Bassiouny MA, Grant AA. A visible light-cured composite restorative. Clinical open assessment. Br Dent J 1978; 145: 327-330. Buonocore M. Adhesive sealing of pits and fissures for caries prevention, with use of ultraviolet light. J Am Dent Assoc 1970; 80: 324-330. Solomon CS, Osman YI. Evaluating the efficacy of curing lights. Sadj 1999; 54: 357-362. Abate PF, Zahra VN, Macchi RL. Effect of photopolymerization
variables on composite hardness. J Prosthet Dent 2001; 86: 632-635. 7 Aguirar F, Lazzari C, Lima D, Ambrosano G, Lovadino J. Effect of light curing tip distance and resin shade on microhardness of a hybrid resin composite. Braz Oral Res 2005 19: 302-306. 8 Dart EC, Nemcek J. Photopolymerizable composition. Great Britain Patent Specification No. 1408265 (1971). Japanese Patent No. Toku-Kou-Sho 54-10986 (1979) (Imperial Chemical Industry Co. Ltd.). 9 Lee D-S, Jeong T-S, Kim S, Kim HI, Kwon YH. Effect of dual-peak LED unit on the polymerization of coinitiatorcontaining composite resins. Dental Materials Journal 2012;31:656–61. 10 Janda R, Roulet JF, Kaminsky M, Steffin G, Latta M. Color stability of resin matrix restorative materials as a function of the method of light activation. European Journal of Oral
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Figure 8: Spectral output of single and dual-peak LCUs superimposed on the absorption spectra of CQ and TPO translucency. Figure 9 shows the peak absorption of Ivocerin which corresponds with the peak emission of light in the violet range of dual peaked LCUs.
Figure 9: Spectra of single and dualpeaked LCUs superimposed on the absorption spectra of CQ, TPO, PPD and Ivocerin
Summary The polymerization of RBCs is directly influenced by the quality of light produced by the dentist’s LCU, including the strength of irradiation, peak wavelength emission and the interaction between that and the constituents of the individual RBC. Emissionabsorption matching of the LCU light output and photoinitiator light absorption is critical to adequate polymerization.
Ivocerin is a new initiator that features a high absorption coefficient and is therefore highly effective even if used in only small quantities. It is a germanium-based initiator and complements the current range of standard photoinitiators and allows for increased quantum efficiency and is therefore far more effective than CQ/AT or TPO.
Dentists should be aware not only of the tip irradiance, as stated by manufacturer, of their LCU but also of the spectral range. Manufacturers should also make a clear statement of the contained photoinitiators in their RBCs.
This photoinitiator is said to produce highly reactive polymerization and only very small amounts are required. Thus photoinitiator colour (compare CQ/TA system) is not a problem and allows it to be used in tooth-coloured materials with enamel like
Science 2004;112:280-5 11 Ikemura K, Ichizawa K, Jogetsu Y, Endo T. Synthesis of a novel camphorquinone derivative having acylphosphine oxide group, characterization by UV-VIS spectroscopy and evaluation of photo polymerization performance. Dental Materials Journal 2010;29:122-31. 12 Schneider LF, Pfeifer CS, Consani S, Prahl SA, Ferracane JL. Influence of photoinitiator type on the rate of polymerization, degree of conversion, hardness and yellowing of dental resin composites. Dental Materials 2008;24:1169-77. 13 Emami N, Söderholm KJ. Influence of light-curing procedures and photo initiator/co-initiator composition on the degree of conversion of lightcuring resins. Journal of Material Science: Materials in Medicine 2005;16:47-52. 14 Lygre H, Hl PJ, Solheim E, Moe G.
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16
17
18
Organic leachables from polymerbased dental fi lling materials. European Journal of Oral Science 1999;107:378-83. Park YJ, Chae KH, Rawls HR. Development of a new photoinitiation system for dental light-cure composite resins. Dental Materials 1999;15:120-7. Leprince JG, Hadis M, Shortall AC, Ferracane JL, Devaux J, Leloup G, et al. Photoinitiator type and applicability of exposure reciprocity law in filled and unfilled photoactive resins. Dental Materials, 2011;27:157-64 Miletic V, Santini A. Micro-Raman spectroscopic analysis of the degree of conversion of composite resins containing different initiators cured by polywave or monowave LED units. Journal of Dentistry 2012;40:106-13 Arikawa H, Takahashi H, Kanie T, Ban S. Effect of various visible light
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20
21
22
photoinitiators on the polymerization and color of light-activated resins. Dental Materials Journal 2009;28:454-60 Santini A. Current status of visible light activation units and the curing of light activated resin-based composite materials. Dental Update 2010;37:214-6, 218-20, 223-7. Ilie N, Hickel R. Can CQ be completely replaced by alternative initiators in dental adhesives? Dental Materials Journal 2008;27:221-8. Arikawa H, Takahashi H, Kanie T, Ban S. Effect of various visible light photoinitiators on the polymerization and color of light-activated resins. Dental Materials Journal 2009;28:454-60. Ikemura K, Ichizawa K, Yoshida M, Ito S, Endo T. UV–vis spectra and photoinitiation behaviors of acylphosphine oxide and bisacylphosphine oxide derivatives in
unfilled, light-cured dental resins. Dental Materials Journal 2008;27:765–74. 23 Miletic V, Santini A. Micro-Raman spectroscopic analysis of the degree of conversion of composite resins containing different initiators cured by polywave or monowave LED units. Journal of Dentistry 2012;40:106-13. 24 Miletic V, Santini A. Optimizing the concentration of 2,4,6trimethylbenzoyldiphenylphosphine oxide initiator in composite resins in relation to monomer conversion. Dental Materials Journal 2012;31:717–23. 25 Santini A, Miletic V, Swift MD, Bradley M. Degree of conversion and microhardness of TPO-containing resin-based composites cured by polywave and monowave LED units. Journal of Dentistry 2012;40:577– 84.
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