ADVANCES IN DENTAL MATERIALS GARRY JP FLEMING1
Resin-based composites (RBCs)
ABSTRACT The dental market is replete with new restorative materials marketed on the basis of novel technological advances in materials chemistry, bonding capability or reduced operator time and/or technique sensitivity. This paper aims to consider advances in current materials, with an emphasis on their role in supporting contemporary clinical practice.
Introduction I believe it is fair to say that dental materials development lies far behind most disciplines in the health sciences, with dental amalgam remaining the most commonly used material for restoring posterior natural dentition in UK dental practice.1 Just ask yourself: what is new in dental materials in the last 20 years? The embracing of zirconia restorations is phenomenal but the first conference on zirconia, The Science and Technology of Zirconia, was held in 1981,2 almost 20 years before zirconia’s adoption by dentistry. In 1983, at the second conference on zirconia, The Science and Technology of Zirconia II, it was reported that ‘the atmosphere was one of pregnant anticipation.’2 Our experience of the embracement of zirconia for dental restorations 10 years on is that performance is plagued by problems with chipping fractures, which are commonplace.3
1a
Historically, chemically cured RBCs composed of bisphenol-A and glycidyl methacrylate (BisGMA) monomeric resin with a quartz or aluminosilicate glass filler particulate were reported in 1958.4 Investigations by Bowen identified the necessity for the addition of a comonomer (triethylene glycol dimethacrylate: TEGDMA), which resulted in a decrease in the monomer viscosity and facilitated the incorporation of filler particles; this was patented in 1962,5 heralding the beginning of aesthetic restorative dentistry. A visiblelight irradiated RBC patented in 19786 transformed aesthetic restorative dentistry when employed using ‘total etch’ adhesives developed in the 1980s.7 However, since the patenting of chemically cured RBCs,5 it should be noted that the majority of RBC monomeric formulations available today contain BisGMA/TEGDMA and/or other dimethacrylates (Figure 1). Technical advances in RBC technologies, such as resins of higher molecular weight, hydrogenated dimer acids and novel oxirane and silorane chemistries have been commercialised.8 Advances in RBC filler technologies have resulted in Bowen’s original quartz or aluminosilicate glass fillers4 being
1b
1
Garry JP Fleming
Senior Lecturer and Head of Materials Science Unit, School of Dental Sciences, Dublin Dental University Hospital, Trinity College Dublin, Ireland
54
P R I M A R Y D E N TA L J O U R N A L
superseded by numerous ceramic filler particles of various compositions, densities, volume fractions and mean particle sizes, with a general trend in the reduction in average filler particle size9 towards the nano-filler scale. Dentistry has embraced the nanotechnology to such an extent that almost all restorative materials currently available are marketed based on revolutionary nanotechnology. However, in modernday RBCs, despite the claimed technical advances in technologies over Bowen’s patented formulation,5 the major disadvantages currently include: 1) an insufficient depth of cure which requires an incremental RBC restoration technique,10 and 2) the generation of polymerisation shrinkage stress11 on polymerisation notwithstanding difficulties in 3) light irradiation techniques.12 These disadvantages will be the focus of the current manuscript. Insufficient depth of cure When restoring very large class II restorations with conventional RBC materials, either triangular-shaped or horizontal increments of maximum 2mm thickness are routinely required. To counteract for the insufficient depth of cure of modern-day RBCs, manufacturers have launched bulk-fill flowable RBCs for use beneath conventional RBC materials, and bulk-fill RBCs which do not necessitate the use of a conventional RBC material. Bulk-fill RBCs One of the first bulk-fill RBCs developed was a packable posterior bulk-fill (x-tra
1c
1d
V O L 3 N O 2 M AY 2 0 1 4
fil; VOCO GmbH, Cuxhaven, Germany). The first article to commentate on its use concluded: ‘the manufacturer’s claims that the recently marketed x-tra fil could be cured to a depth of 4mm appear to be vindicated and the performance in terms of flexure strength, water uptake and biocompatibility are comparable with conventional RBCs.’13 It was probably not until well after the launch of SDR (Smart Dentine Replacement, Dentsply; circa 2012) that bulk-fill RBCs became popular. When SDR was launched as a new material (for which the manufacturers provided limited independent data, as all manufacturers do) the claims of a ‘polymerisation modulator’ and ‘self-levelling consistency’ for cavity adaption14 were audacious. Studies on the clinical performance of bulk-fill flowable RBC base materials are limited to investigations on marginal quality,15 cuspal deflection,16 cuspal deflection in conjunction with microleakage17 and adhesion to cavitybottom dentine.18 It should be noted that these studies15-18 collectively advocated the clinical use of bulk-fill flowable RBC bases. In terms of mechanical properties of bulk-fill flowable RBC bases, Czasch and Ilie19 used Fourier transform infrared (FTIR) spectroscopy to determine the degree of conversion (DC) of the bulk-fill flowable RBC base materials with a lightemitting diode (LED) light curing unit (LCU). We used a similar approach for bulk-fill flowable RBC base materials investigated at irradiation depths up to
8mm (in 1mm increments) using a quartz tungsten halogen (QTH) LCU.20 From these DC studies,19-20 I believe that the bold claims that bulk-fill flowable RBC bases have a reported depth of cure in excess of 4mm can be confirmed – if adequately irradiated (see MARC-PS below). Stress generation The irradiation of a photosensitised (light-curable) RBC containing camphorquinone (or other photosensitiser) results primarily in the formation of an excited-state photosensitiser molecule and may result in the formation of a free radical which can initiate a polymerisation chain reaction.21 The free radical polymerisation reaction is accompanied by a closer packing of molecules whereby methacrylate RBCs shrink in volume by 2-4% upon light irradiation.11 The generation of stress that may compromise the adhesive margin of the restoration is multi-factorial, dependent upon the onset of gelation of the resin matrix,12 associated shrinkage, the elastic modulus (stiffness) of the material, polymerisation rate, and the ratio of bonded to non-bonded surface area.22-23 Higher molecular weight resins and hydrogenated dimer acids, which have been advocated in place of TEGDMA, reduce the overall generation of stress during light irradiation by decreasing the concentration of carbon-to-carbon double bonds.8 However, while the elastic modulus of RBC formulations has improved steadily with increasing filler volume fraction9 and the volumetric
Figure 1: Common resins in methacrylate RBCs a Bisphenol A glycol dimethacrylate b Triethylene glycol dimethacrylate c Urethane dimethacrylate d Bisphenol A ethoxylated dimethacrylate
55
ADVANCES IN DENTAL MATERIALS
range of available shades and limited depth of cure. However, it would be a calamity if dental manufacturers shied away from revolutionary technological advances in monomeric resin formulation in favour of ‘safer’ methacrylate formulations.
2a
2b
Figure 2: Two novel oxirane-based resins used in EXL 596 a 3',4'-Epoxycyclohexanamethyl3,4-epoxycyclohexane carboxylate b Bisphenol F diglycidyl ether
shrinkage has been reduced over time by the elimination of TEGDMA,11 there are still doubts as to the clinical relevance of a reduction in volumetric shrinkage of an RBC material from 4% to 2% in terms of the real-world practicalities of dentistry. In this respect, the development of Filtek Silorane (3M ESPE, St. Paul, MN) was truly revolutionary in that the monomeric constituents were novel (nondimethacrylate) chemistries.24 I had the opportunity to work on the predecessor materials to Filtek Silorane in the early 2000s and although by 2004 our mechanical testing protocols had shown that the oxirane RBC material EXL 596 (Figure 2) was precluded from use, the silorane RBC (Figure 3) did show significant promise.125-26 In 2008, Filtek Silorane was launched and in my opinion this was the first significant technological advancement for dentistry since 1994. For the first time since Bowens’ original RBC patented formulation, methacrylates were not the monomeric resin constituents of a commercial RBC. Unfortunately the material has not gained traction in the dental community, facing challenges such as being tacky to handle, a poor
Light irradiation Light irradiation of RBCs has also advanced, with high-powered LED LCUs superseding their QTH equivalents. The spectral output of gallium nitride blue LEDs falls within the absorption spectrum (400-500nm) of the camphorquinone photoinitiator present in most lightactivated RBCs; hence, no filters are required in LED LCUs.12 Additionally, QTH LCUs have been reported to suffer from intensity output degradation over time, such that one third of all QTH LCUs examined in dental practices had an inadequate intensity to polymerise an RBC sufficiently. A word of caution is required regarding the use of LED LCUs. There is an assumption amongst dental LCU manufacturers that there is reciprocity between LCU output intensity (I) and time (t), whereby the radiant energy (E) the RBC will receive is E = I × t.21 Therefore, the employment of high-intensity LED LCUs enables a corresponding reduction in irradiation time whilst achieving the same mechanical properties. Whilst QTH LCUs have a nominal output of about one-quarter (500 mW/cm2) that of third-generation LED LCUs (2000 mW/cm2), manufacturers have suggested that the recommended
Figure 3: The novel Filtek Silorane chemistry
56
P R I M A R Y D E N TA L J O U R N A L
Figure 4: The Managing Accurate Resin Curing patient simulator
irradiation time can be halved when the higher-output LED LCU is used.21 This clearly shows some recognition that reciprocity does not apply, in that if it did, only a quarter of the irradiation time should be necessary. The reciprocity question has not prevented what is now essentially the conventional wisdom: that irradiation times can be significantly reduced with high-intensity LCUs.12 In short, I would advocate not reducing the irradiation time and to essentially ‘cook’ the RBC. This is the only chance you will get to do so, and remember that not all the light goes where you want it to go (see MARC-PS below). Managing Accurate Resin Curing (MARC) When it comes to advances in dentistry, it is my opinion that the recently developed Managing Accurate Resin Curing patient simulator (MARC-PS; BlueLight Analytics, Halifax, Canada) is the ultimate step forward (Figure 4). MARC-PS has a laboratory-grade spectrophotometer inside a mannequin head and typodont where a light detector is located at the base of a Class I preparation in the maxillary left second molar 2mm from the cavosurface margin
V O L 3 N O 2 M AY 2 0 1 4
and 4mm from the cusp tip.27,28 In general, RBC manufacturers tend not to state how much energy is required to polymerise an RBC, since the energy required is a function of the depth of the cavity, shade, opacity and monomeric resin constituent.44-47 However, let us say that you have an LCU with output intensity I = 500mW/cm2 and the manufacturer recommends t = 20s irradiation time. Ideally, the radiant energy the RBC will receive is E = 10J/cm2 (E=I × t). What MARC-PS achieves is that the actual amount of radiant energy delivered to the typodont is quantified accurately using the laboratory-grade spectrophotometer. Most dental students irradiate RBCs while holding the orange shield paddle in the opposite hand to the LCU. The 20s irradiation is generally spent talking to the dental nurse, and rarely do the students look where the light guide actually is. This is the ‘Harry Potter’ lightcuring technique – put the wand (light guide) in the patient’s mouth, turn it on and magically the restoration cures! In essence, the top of the restoration is cured but you may well have only cured the surface, leaving the underneath soft
and uncured (not unlike a crème brûlée). In tests in a dental clinical training setting, 75% and 84% of students delivered less than the recommended radiant energy.27-28 However, after receiving instruction using MARC-PS, this was reduced to 0% and 5%.27-28 This is not a gimmick, and I would encourage anybody going to international meetings or trade shows both to check out MARC-PS and to read the Dental Update editorial about it.29
57
ADVANCES IN DENTAL MATERIALS
Resin dentine bonding systems The traditional adhesive bonding classification system of different adhesive generations is complex at best, and the system proposed by Van Meerbeek et al 7,30-31 is at least simplified. Resin dentine bonding systems were classified as ‘traditional,’ including etch-and-rinse adhesives where the first step involves using a separate etch with phosphoric acid gel to allow for the removal of the smear layer and to expose the collagen fibres in the dentine, thereby increasing the surface available for bonding.7,30-31 The priming second step is then followed by the application of the adhesive resin for the third step.7 Simplified two-step etch-and-rinse adhesives combining the primer and adhesive resin were classified as ‘simplified’ resin dentine bonding systems. Conventional wisdom7 suggests that the traditional and simplified etch-and-rinse adhesives are the optimum in resin dentine bonding. Self-etch adhesives, which eliminate the preliminary phosphoric acid etching gel step, boasting reduced technique sensitivity and operator time, were termed ‘all-in-one’ resin dentine bonding systems. Although undoubtedly userfriendly, the effectiveness of ‘all-in-one’ resin dentine bonding systems has been
REFERENCES 4 1
2
3
58
Lucarotti PS, Holder RL, Burke FJT. Outcome of direct restorations placed within the general dental services in England and Wales (Part 1): variation by type of restoration and re-intervention. Journal of Dentistry 2005; 33: 805-815. Claussen N, Ruhle M, Heuer AH (eds.) Advances in Ceramics Vol. 12, Science and Technology of Zirconia II, The American Ceramic Society, Colombus, Ohio, 1983. Anusavice KJ. Standardizing failure, success, and survival decisions in clinical studies of ceramic and metal-ceramic fixed dental prostheses. Dental Materials
5
6
7
2012; 28: 102-111. Bowen RL. Synthesis of a silica-resin direct filling material: Progress report. Journal of Dental Research 1958; 37: 90-91. Bowen RL. Dental filling materials comprising of vinyl-silane treated fused silica and binder consisting of the reaction product of bisphenol and glycidyl methacrylate. 1962. US Patent 3,066,112. Dart EC, Cantwell JB, Traynor JR, Taworzyn JF, Nemeck J. Method of repairing teeth using a composition which is curable by visible light. 1978. US Patent 4,089,763. Van Meerbeek B, Peumans M, Poitevin A, Mine A, Van Ende A, Never A, De Munck J. Relationship between bond-strength tests and
questioned, since bond strength testing revealed (albeit product-dependently7) that simplification of the application procedure reduced bond effectiveness.30-31 A new family of bonding agents known as ‘universal’ or ‘multi-mode’ adhesives have been recently introduced into the dental market.32-33 These are one-step self-etch adhesives that can be employed with or without a separate step of etching with acid. Therefore, ‘universal’ or ‘multi-mode’ adhesives offer versatility to the dentist during RBC restoration placement by including monomeric constituents capable of producing chemical adhesion to sound natural dentition.32-33 It is exceptionally difficult to assess adhesive performance in the laboratory. Bond strength testing is controversial, and in the peer-reviewed dental literature, articles reporting the bond strength of a certain material to dentine are frequently at odds with other published studies.34 From 1994 to 2009, in excess of 200 peer-reviewed bond strength papers were published annually in the dental literature.35 We have been employing a cuspal deflection and microleakage protocol to determine adhesive performance of different bonding systems.17,25 In summary,
clinical outcomes. Dental Materials 2010; 26: e100-e121. 8 Leprince JG, Palin WM, Hadis MA, Devaux J, Leloup G. Progress in dimethacrylate-based dental composite technology and curing efficiency. Dental Materials 2013; 29: 139-156. 9 Mitra SB, Wu D, Holmes BN. An application of nanotechnology in advanced dental materials. Journal of American Dental Association 2003; 134: 1382-1390. 10 Sakaguchi RL, Douglas WH, Peters MC. Curing light performance and polymerization of composite restorative materials. Journal of Dentistry 1992; 20: 183-188. 11 Davidson CL, Feilzer AJ. Polymerisation shrinkage and
12
13
14 15
polymerisation shrinkage stress in polymer-based restoratives. Journal of Dentistry 1997; 25: 435-440. Pelissier B, Jacquot B, Palin WM, Shortall AC. Three generations of LED lights and clinical implications for optimizing their use. 1: from past to present. Dental Update 2011; 38: 660-662, 664-666, 668-670. Fleming GJP, Awan M, Cooper PR, Sloan AJ. The potential of a resincomposite to be cured to a 4mm depth. Dental Materials 2008; 24: 522-529. Product specification for SDR (Dentsply Caulk, Milford, DE, USA). Roggendorf MJ, Kramer N, Appelt A, Naumann M, Frankenberger R. Marginal quality of flowable 4-mm
P R I M A R Y D E N TA L J O U R N A L
we believe etch-and-rinse adhesives to be the optimum; however, the performance of the self-etch and ‘universal’ adhesives was very much dependent on their pH.36-37 This result was based on Yoshida et al’s adhesiondecalcification (AD) concept,36-37 which suggests a trend towards ‘mild self-etch’ adhesives (pH~2.0) with a dentine interaction depth of 1-2µm.7 Mild selfetch adhesives performed best in mean total cuspal deflection and cervical microleakage score,36-37 while ‘ultra-mild self-etch’ solutions (pH>2.5 with a dentine interaction depth of
Resin-based composites (RBCs)
ABSTRACT The dental market is replete with new restorative materials marketed on the basis of novel technological advances in materials chemistry, bonding capability or reduced operator time and/or technique sensitivity. This paper aims to consider advances in current materials, with an emphasis on their role in supporting contemporary clinical practice.
Introduction I believe it is fair to say that dental materials development lies far behind most disciplines in the health sciences, with dental amalgam remaining the most commonly used material for restoring posterior natural dentition in UK dental practice.1 Just ask yourself: what is new in dental materials in the last 20 years? The embracing of zirconia restorations is phenomenal but the first conference on zirconia, The Science and Technology of Zirconia, was held in 1981,2 almost 20 years before zirconia’s adoption by dentistry. In 1983, at the second conference on zirconia, The Science and Technology of Zirconia II, it was reported that ‘the atmosphere was one of pregnant anticipation.’2 Our experience of the embracement of zirconia for dental restorations 10 years on is that performance is plagued by problems with chipping fractures, which are commonplace.3
1a
Historically, chemically cured RBCs composed of bisphenol-A and glycidyl methacrylate (BisGMA) monomeric resin with a quartz or aluminosilicate glass filler particulate were reported in 1958.4 Investigations by Bowen identified the necessity for the addition of a comonomer (triethylene glycol dimethacrylate: TEGDMA), which resulted in a decrease in the monomer viscosity and facilitated the incorporation of filler particles; this was patented in 1962,5 heralding the beginning of aesthetic restorative dentistry. A visiblelight irradiated RBC patented in 19786 transformed aesthetic restorative dentistry when employed using ‘total etch’ adhesives developed in the 1980s.7 However, since the patenting of chemically cured RBCs,5 it should be noted that the majority of RBC monomeric formulations available today contain BisGMA/TEGDMA and/or other dimethacrylates (Figure 1). Technical advances in RBC technologies, such as resins of higher molecular weight, hydrogenated dimer acids and novel oxirane and silorane chemistries have been commercialised.8 Advances in RBC filler technologies have resulted in Bowen’s original quartz or aluminosilicate glass fillers4 being
1b
1
Garry JP Fleming
Senior Lecturer and Head of Materials Science Unit, School of Dental Sciences, Dublin Dental University Hospital, Trinity College Dublin, Ireland
54
P R I M A R Y D E N TA L J O U R N A L
superseded by numerous ceramic filler particles of various compositions, densities, volume fractions and mean particle sizes, with a general trend in the reduction in average filler particle size9 towards the nano-filler scale. Dentistry has embraced the nanotechnology to such an extent that almost all restorative materials currently available are marketed based on revolutionary nanotechnology. However, in modernday RBCs, despite the claimed technical advances in technologies over Bowen’s patented formulation,5 the major disadvantages currently include: 1) an insufficient depth of cure which requires an incremental RBC restoration technique,10 and 2) the generation of polymerisation shrinkage stress11 on polymerisation notwithstanding difficulties in 3) light irradiation techniques.12 These disadvantages will be the focus of the current manuscript. Insufficient depth of cure When restoring very large class II restorations with conventional RBC materials, either triangular-shaped or horizontal increments of maximum 2mm thickness are routinely required. To counteract for the insufficient depth of cure of modern-day RBCs, manufacturers have launched bulk-fill flowable RBCs for use beneath conventional RBC materials, and bulk-fill RBCs which do not necessitate the use of a conventional RBC material. Bulk-fill RBCs One of the first bulk-fill RBCs developed was a packable posterior bulk-fill (x-tra
1c
1d
V O L 3 N O 2 M AY 2 0 1 4
fil; VOCO GmbH, Cuxhaven, Germany). The first article to commentate on its use concluded: ‘the manufacturer’s claims that the recently marketed x-tra fil could be cured to a depth of 4mm appear to be vindicated and the performance in terms of flexure strength, water uptake and biocompatibility are comparable with conventional RBCs.’13 It was probably not until well after the launch of SDR (Smart Dentine Replacement, Dentsply; circa 2012) that bulk-fill RBCs became popular. When SDR was launched as a new material (for which the manufacturers provided limited independent data, as all manufacturers do) the claims of a ‘polymerisation modulator’ and ‘self-levelling consistency’ for cavity adaption14 were audacious. Studies on the clinical performance of bulk-fill flowable RBC base materials are limited to investigations on marginal quality,15 cuspal deflection,16 cuspal deflection in conjunction with microleakage17 and adhesion to cavitybottom dentine.18 It should be noted that these studies15-18 collectively advocated the clinical use of bulk-fill flowable RBC bases. In terms of mechanical properties of bulk-fill flowable RBC bases, Czasch and Ilie19 used Fourier transform infrared (FTIR) spectroscopy to determine the degree of conversion (DC) of the bulk-fill flowable RBC base materials with a lightemitting diode (LED) light curing unit (LCU). We used a similar approach for bulk-fill flowable RBC base materials investigated at irradiation depths up to
8mm (in 1mm increments) using a quartz tungsten halogen (QTH) LCU.20 From these DC studies,19-20 I believe that the bold claims that bulk-fill flowable RBC bases have a reported depth of cure in excess of 4mm can be confirmed – if adequately irradiated (see MARC-PS below). Stress generation The irradiation of a photosensitised (light-curable) RBC containing camphorquinone (or other photosensitiser) results primarily in the formation of an excited-state photosensitiser molecule and may result in the formation of a free radical which can initiate a polymerisation chain reaction.21 The free radical polymerisation reaction is accompanied by a closer packing of molecules whereby methacrylate RBCs shrink in volume by 2-4% upon light irradiation.11 The generation of stress that may compromise the adhesive margin of the restoration is multi-factorial, dependent upon the onset of gelation of the resin matrix,12 associated shrinkage, the elastic modulus (stiffness) of the material, polymerisation rate, and the ratio of bonded to non-bonded surface area.22-23 Higher molecular weight resins and hydrogenated dimer acids, which have been advocated in place of TEGDMA, reduce the overall generation of stress during light irradiation by decreasing the concentration of carbon-to-carbon double bonds.8 However, while the elastic modulus of RBC formulations has improved steadily with increasing filler volume fraction9 and the volumetric
Figure 1: Common resins in methacrylate RBCs a Bisphenol A glycol dimethacrylate b Triethylene glycol dimethacrylate c Urethane dimethacrylate d Bisphenol A ethoxylated dimethacrylate
55
ADVANCES IN DENTAL MATERIALS
range of available shades and limited depth of cure. However, it would be a calamity if dental manufacturers shied away from revolutionary technological advances in monomeric resin formulation in favour of ‘safer’ methacrylate formulations.
2a
2b
Figure 2: Two novel oxirane-based resins used in EXL 596 a 3',4'-Epoxycyclohexanamethyl3,4-epoxycyclohexane carboxylate b Bisphenol F diglycidyl ether
shrinkage has been reduced over time by the elimination of TEGDMA,11 there are still doubts as to the clinical relevance of a reduction in volumetric shrinkage of an RBC material from 4% to 2% in terms of the real-world practicalities of dentistry. In this respect, the development of Filtek Silorane (3M ESPE, St. Paul, MN) was truly revolutionary in that the monomeric constituents were novel (nondimethacrylate) chemistries.24 I had the opportunity to work on the predecessor materials to Filtek Silorane in the early 2000s and although by 2004 our mechanical testing protocols had shown that the oxirane RBC material EXL 596 (Figure 2) was precluded from use, the silorane RBC (Figure 3) did show significant promise.125-26 In 2008, Filtek Silorane was launched and in my opinion this was the first significant technological advancement for dentistry since 1994. For the first time since Bowens’ original RBC patented formulation, methacrylates were not the monomeric resin constituents of a commercial RBC. Unfortunately the material has not gained traction in the dental community, facing challenges such as being tacky to handle, a poor
Light irradiation Light irradiation of RBCs has also advanced, with high-powered LED LCUs superseding their QTH equivalents. The spectral output of gallium nitride blue LEDs falls within the absorption spectrum (400-500nm) of the camphorquinone photoinitiator present in most lightactivated RBCs; hence, no filters are required in LED LCUs.12 Additionally, QTH LCUs have been reported to suffer from intensity output degradation over time, such that one third of all QTH LCUs examined in dental practices had an inadequate intensity to polymerise an RBC sufficiently. A word of caution is required regarding the use of LED LCUs. There is an assumption amongst dental LCU manufacturers that there is reciprocity between LCU output intensity (I) and time (t), whereby the radiant energy (E) the RBC will receive is E = I × t.21 Therefore, the employment of high-intensity LED LCUs enables a corresponding reduction in irradiation time whilst achieving the same mechanical properties. Whilst QTH LCUs have a nominal output of about one-quarter (500 mW/cm2) that of third-generation LED LCUs (2000 mW/cm2), manufacturers have suggested that the recommended
Figure 3: The novel Filtek Silorane chemistry
56
P R I M A R Y D E N TA L J O U R N A L
Figure 4: The Managing Accurate Resin Curing patient simulator
irradiation time can be halved when the higher-output LED LCU is used.21 This clearly shows some recognition that reciprocity does not apply, in that if it did, only a quarter of the irradiation time should be necessary. The reciprocity question has not prevented what is now essentially the conventional wisdom: that irradiation times can be significantly reduced with high-intensity LCUs.12 In short, I would advocate not reducing the irradiation time and to essentially ‘cook’ the RBC. This is the only chance you will get to do so, and remember that not all the light goes where you want it to go (see MARC-PS below). Managing Accurate Resin Curing (MARC) When it comes to advances in dentistry, it is my opinion that the recently developed Managing Accurate Resin Curing patient simulator (MARC-PS; BlueLight Analytics, Halifax, Canada) is the ultimate step forward (Figure 4). MARC-PS has a laboratory-grade spectrophotometer inside a mannequin head and typodont where a light detector is located at the base of a Class I preparation in the maxillary left second molar 2mm from the cavosurface margin
V O L 3 N O 2 M AY 2 0 1 4
and 4mm from the cusp tip.27,28 In general, RBC manufacturers tend not to state how much energy is required to polymerise an RBC, since the energy required is a function of the depth of the cavity, shade, opacity and monomeric resin constituent.44-47 However, let us say that you have an LCU with output intensity I = 500mW/cm2 and the manufacturer recommends t = 20s irradiation time. Ideally, the radiant energy the RBC will receive is E = 10J/cm2 (E=I × t). What MARC-PS achieves is that the actual amount of radiant energy delivered to the typodont is quantified accurately using the laboratory-grade spectrophotometer. Most dental students irradiate RBCs while holding the orange shield paddle in the opposite hand to the LCU. The 20s irradiation is generally spent talking to the dental nurse, and rarely do the students look where the light guide actually is. This is the ‘Harry Potter’ lightcuring technique – put the wand (light guide) in the patient’s mouth, turn it on and magically the restoration cures! In essence, the top of the restoration is cured but you may well have only cured the surface, leaving the underneath soft
and uncured (not unlike a crème brûlée). In tests in a dental clinical training setting, 75% and 84% of students delivered less than the recommended radiant energy.27-28 However, after receiving instruction using MARC-PS, this was reduced to 0% and 5%.27-28 This is not a gimmick, and I would encourage anybody going to international meetings or trade shows both to check out MARC-PS and to read the Dental Update editorial about it.29
57
ADVANCES IN DENTAL MATERIALS
Resin dentine bonding systems The traditional adhesive bonding classification system of different adhesive generations is complex at best, and the system proposed by Van Meerbeek et al 7,30-31 is at least simplified. Resin dentine bonding systems were classified as ‘traditional,’ including etch-and-rinse adhesives where the first step involves using a separate etch with phosphoric acid gel to allow for the removal of the smear layer and to expose the collagen fibres in the dentine, thereby increasing the surface available for bonding.7,30-31 The priming second step is then followed by the application of the adhesive resin for the third step.7 Simplified two-step etch-and-rinse adhesives combining the primer and adhesive resin were classified as ‘simplified’ resin dentine bonding systems. Conventional wisdom7 suggests that the traditional and simplified etch-and-rinse adhesives are the optimum in resin dentine bonding. Self-etch adhesives, which eliminate the preliminary phosphoric acid etching gel step, boasting reduced technique sensitivity and operator time, were termed ‘all-in-one’ resin dentine bonding systems. Although undoubtedly userfriendly, the effectiveness of ‘all-in-one’ resin dentine bonding systems has been
REFERENCES 4 1
2
3
58
Lucarotti PS, Holder RL, Burke FJT. Outcome of direct restorations placed within the general dental services in England and Wales (Part 1): variation by type of restoration and re-intervention. Journal of Dentistry 2005; 33: 805-815. Claussen N, Ruhle M, Heuer AH (eds.) Advances in Ceramics Vol. 12, Science and Technology of Zirconia II, The American Ceramic Society, Colombus, Ohio, 1983. Anusavice KJ. Standardizing failure, success, and survival decisions in clinical studies of ceramic and metal-ceramic fixed dental prostheses. Dental Materials
5
6
7
2012; 28: 102-111. Bowen RL. Synthesis of a silica-resin direct filling material: Progress report. Journal of Dental Research 1958; 37: 90-91. Bowen RL. Dental filling materials comprising of vinyl-silane treated fused silica and binder consisting of the reaction product of bisphenol and glycidyl methacrylate. 1962. US Patent 3,066,112. Dart EC, Cantwell JB, Traynor JR, Taworzyn JF, Nemeck J. Method of repairing teeth using a composition which is curable by visible light. 1978. US Patent 4,089,763. Van Meerbeek B, Peumans M, Poitevin A, Mine A, Van Ende A, Never A, De Munck J. Relationship between bond-strength tests and
questioned, since bond strength testing revealed (albeit product-dependently7) that simplification of the application procedure reduced bond effectiveness.30-31 A new family of bonding agents known as ‘universal’ or ‘multi-mode’ adhesives have been recently introduced into the dental market.32-33 These are one-step self-etch adhesives that can be employed with or without a separate step of etching with acid. Therefore, ‘universal’ or ‘multi-mode’ adhesives offer versatility to the dentist during RBC restoration placement by including monomeric constituents capable of producing chemical adhesion to sound natural dentition.32-33 It is exceptionally difficult to assess adhesive performance in the laboratory. Bond strength testing is controversial, and in the peer-reviewed dental literature, articles reporting the bond strength of a certain material to dentine are frequently at odds with other published studies.34 From 1994 to 2009, in excess of 200 peer-reviewed bond strength papers were published annually in the dental literature.35 We have been employing a cuspal deflection and microleakage protocol to determine adhesive performance of different bonding systems.17,25 In summary,
clinical outcomes. Dental Materials 2010; 26: e100-e121. 8 Leprince JG, Palin WM, Hadis MA, Devaux J, Leloup G. Progress in dimethacrylate-based dental composite technology and curing efficiency. Dental Materials 2013; 29: 139-156. 9 Mitra SB, Wu D, Holmes BN. An application of nanotechnology in advanced dental materials. Journal of American Dental Association 2003; 134: 1382-1390. 10 Sakaguchi RL, Douglas WH, Peters MC. Curing light performance and polymerization of composite restorative materials. Journal of Dentistry 1992; 20: 183-188. 11 Davidson CL, Feilzer AJ. Polymerisation shrinkage and
12
13
14 15
polymerisation shrinkage stress in polymer-based restoratives. Journal of Dentistry 1997; 25: 435-440. Pelissier B, Jacquot B, Palin WM, Shortall AC. Three generations of LED lights and clinical implications for optimizing their use. 1: from past to present. Dental Update 2011; 38: 660-662, 664-666, 668-670. Fleming GJP, Awan M, Cooper PR, Sloan AJ. The potential of a resincomposite to be cured to a 4mm depth. Dental Materials 2008; 24: 522-529. Product specification for SDR (Dentsply Caulk, Milford, DE, USA). Roggendorf MJ, Kramer N, Appelt A, Naumann M, Frankenberger R. Marginal quality of flowable 4-mm
P R I M A R Y D E N TA L J O U R N A L
we believe etch-and-rinse adhesives to be the optimum; however, the performance of the self-etch and ‘universal’ adhesives was very much dependent on their pH.36-37 This result was based on Yoshida et al’s adhesiondecalcification (AD) concept,36-37 which suggests a trend towards ‘mild self-etch’ adhesives (pH~2.0) with a dentine interaction depth of 1-2µm.7 Mild selfetch adhesives performed best in mean total cuspal deflection and cervical microleakage score,36-37 while ‘ultra-mild self-etch’ solutions (pH>2.5 with a dentine interaction depth of
