General Information on How to Use Dental LED Curing Light to Cure Resin-based Composites
Following are some information on using ATOMO LED curing light to cure resin-based composites.
Visible light cured resin-based composites are the predominant restorative materials for both anterior and posterior restorations. In 2000, 94% of U.S. dentists used visible-light curing units.1 Light-cured composites allow the dentist to actively initiate the polymerization step being a significant advantage compared to autocured composites.2 Furthermore, a meticulous layering technique was employed to reduce polymerization shrinkage to be applicable even in larger stressbearing cavities in re-dentistry.3 This enables the dentist to generate esthetic and durable restorations such as pit and fissure sealants, direct and indirect resin composite restorations, and luting of ceramic restorations. Even resin-modified glass ionomers rely on photopolymerization.4 The mode of curing has regularly changed during the last 30 years. By 2007, the era of light emitting diodes has been definitively established (Fig. 1). Today, four main types of polymerization sources are available: Halogen bulbs, plasma arc lamps, argon ion lasers, and light emitting diodes. Furthermore, different curing protocols were designed to improve photopolymerization, as there is soft-start, step-curing, or oscillating irradiation.5 Based on this background, the present review focused on questions arising from this change in technology: 1. Have clinical recommendations changed in terms of shorter polymerization intervals? 2. Are there differences in curing depth? 3. Are there differences in polymerization kinetics and shrinkage performance? Fig. 1. Achievements in the area of lighting with different technologies. 4. Which are the actual recommendations for curing through tooth-colored restorations? 5. Is heat generation a clinically relevant problem?
Polymerization of resin composites
Important for any polymerization is the resin matrix of composites, mainly di- or tri-esters of methacrylic acid. Those have proven ability to survive under intraoral conditions, since only methacrylates are found to be linked to different organic parts such as aliphatic chains, polyethers and, and aromatic ring structures. The most common molecule is the so-called Bowenmonomer BisGMA (2,2-bis [4-(2-hydroxy-3-methacrylyloxyFig. 1. Achievements in the area of lighting with different technologies. 2 136 Kramer et al Fig. 2. Energy profiles of QTH and LED lamps. propoxy)phenyl]propane). The main advantage is its considerably reduced polymerization shrinkage compared to pure methacrylates and high crosslinking ability. Another common monomer is the aromatic UDMA (urethane dimethacrylate). UDMA is characterized by its contribution to color stability, hydrophobicity, high viscosity, and good diametral tensile strength.6-8 Radicalic polymerization is either initiated by redox systems in autopolymerizing resins being always delivered in two components or by visible light at a wavelength of 468 nm. Classic initiator systems are di-benzoylperoxide for self-curing systems and camphorquinone for photopolymerization with blue light, or in former times butyl hydroxytoluol or lucirine for ultraviolet light curing (< 390 nm).6 In addition to the resin matrix, camphorquinone still serves as photoinitiator in almost all commercially available composite materials. Its absorption range was found to be between 370-500 nm with a peak at 468 nm.9 This light spectra is responsible for effective light curing; however, sufficient intensity of the light source is a fundamental requirement to achieve acceptable material properties for intraoral use even in stress-bearing cavities and to prevent discoloration and premature degradation.10 The depth of cure is dependent on different co-factors such as filler particle size and distribution, color and optical translucency of the composite, and refractive index ratio of the single components being used.11-15 Therefore, a minimal intensity at the most efficient wavelength is needed over a defined irradiation period. At a given depth, curing will not occur without the inhibitor being consumed by the generated radicals in this particular region. Initiators divide C=C double bonds thus leading to crosslinking and build-up of a three-dimensional methacrylic network. The phase of growing chains is determined when monomer molecules are consumed or when two radicals react. In order to obtain good incorporation of fillers and to reduce setting stress, smaller and highly mobile co-monomers are added to the matrix, such as TEGDMA (triethyleneglycol dimethacrylate) or Bis-DMA (bisphenol dimethacrylate). These co-monomers inhibit quick setting after the polymerization is initiated. A high amount of these dimethacrylates guarantees high conversion rates; however, polymerization shrinkage and American Journal of Dentistry, Vol. 21, No. 3, June, 2008 hygroscopic expansion are increased.16 Polymerization of resin-based composites leads to a highly crosslinked structure, but steric hindrance causes residual unsaturation by pendant methacrylate groups. The degree of conversion is defined as the percentage of reacted C=C double bonds. It affects several important parameters such as flexural strength, fatigue, solubility, discoloration, and biocompatibility.17-19 It has been reported that the same degree of conversion is produced by a fixed amount of energy density, leading to the recommendation of an energy density of 21-24 J/cm2 for proper polymerization of a 2 mm portion of resinbased composites.20-22 Energy densities (J/cm2 or mWs/cm2 ), i.e. the product of light intensity (mW/cm2 ) and irradiation time(s), have been suggested to account for variations in irradiation intensity, time and mode. The same degree of conversion is produced by a fixed amount of energy (energy density: J/cm2 ), independent of variations in light irradiance.20.
Quartz tungsten halogen (QTH) QTH lamps have been the standard curing units for several years, despite a remarkably low efficiency compared to heat generation.5 Since QTH lamps emit a rather wide range of wavelengths, band-pass filters are required to limit the wavelength between 370 and 550 nm in order to fit the peak absorption of camphorquinone.23 QTH lamps have a limited lifespan of 100 hours with consecutive degradation of bulb, reflector, and filter caused by high operating temperatures and considerable quantity of heat being produced during operating cycles.24 This implicates a reduction of curing efficiency over time by aging of the components. Many QTH lamps used in dental offices operate beneath the minimum power output specified by the manufacturers.25 This may even deteriorate over time due to insufficient maintenance of the light sources and especially the light tips. With QTH lamps, 5% of the total energy is visible light, 12% heat, and 80% light emitted in the infrared spectrum (Fig. 2).26,27 Plasma arc curing (PAC) Plasma arc curing lamps emit at higher intensities28 and were primarily designed to save irradiation time as an economic factor. PAC lamps emit light from glowing plasma being composed of a gaseous mixture of ionized molecules such as xenon molecules and electrons. PAC units are characterized by high intensities in a narrow range of wavelengths around 470 nm. Due to the described high energy output of plasma arc systems, the manufacturers of these lamps repeatedly claimed that 3 seconds of PAC irradiation would achieve similar material properties compared to 40 seconds curing with QTH lamps. However, this claim has been fully rejected.27,29-32 Today, recommendations for PAC lights are based on 3 x 3 seconds.33 Argon-ion lasers (AL) Argon lasers emit blue-green light of activated argon ions in selected wavelengths (between 450 and 500 nm) and are therefore suitable for light-curing of resin-based composites.34 Argon-ion lasers operating with 250 ± 50 mW/cm2 for 10 seconds achieve improved curing of light-activated restorative Fig. 2. Energy profiles of QTH and LED lamps. 3 American Journal of Dentistry, Vol. 21, No. 3, June, 2008 Fig. 3. Characteristic spectra of QTH and LED lamps. materials in a shorter period of time resulting in equal or even superior physical properties as compared to the conventional QTH systems on the market.35 On the other hand, heat generation during polymerization combined with considerably high initial shrinkage stresses have been reported to be problematic.36,37 Compared to QTH, argon-ion lasers obtain higher conversion rates34 and polymerization depths.38 In general, the literature in the field reflects a strong divergence of opinions covering many aspects of the efficiency of laser curing compared to conventional light curing.38 Light emitting diodes (LED) To solve the previously described problems being connected with conventional QTH technology, solid-state LEDs were introduced to the market.39 Whereas halogen bulbs operate with a hot filament, LEDs use junctions of doped semiconductors (p-n junctions) for the generation of light. In gallium nitride LEDs under forward biased conditions, electrons and holes recombine at the LEDs p-n junction leading to the generation of blue light. A small polymer lens in front of the p-n junction partially collimates the light.40 The spectral emittance of gallium nitride blue LEDs cover the absorption spectrum of camphorquinone so that no filters are required in LED light curing units.41 Recent reports revealed that blue LED lamps offer the highest photo polymerization efficiency.23 LEDs are less energy-consuming compared to QTHs and do not require external cooling in the majority of products on the market. Moreover, LED lamps have a lifetime of several thousands of hours without a significant intensity loss. LEDs emit approximately 15% visible light and 85% heat (Fig. 2). In the direction of the curing tip, LEDs are mainly not emitting heat; however, 85% heat is produced in a backward direction.26 LEDs were subjected to dramatic changes in technology over the last 10 years (Fig. 3). The development of recent generations of high power LEDs is comparable to advances in high tech computer technology. Not so long ago, the power density of early LED generations was very low which forced the manufacturers to build complicated arrangements of 10 to 15 diodes into one lamp (Fig. 4). This was the reason why the first generation of LED curing units could not compete with conventional QTH units.27,42 The initial expectations towards a possible reduction of curing times by use of first generation LEDs could not be confirmed.43 Today, LED technology has considerably changed towards Light curing in the LED era 137 Fig. 4. Diagram of old-styled conventional LED lamps. Fig. 5. Diagram of recent high-power LEDs. high power LEDs being capable of delivering a rather high output with one single diode inside the curing unit (Fig. 5).1 On the other hand, heat generation became a clinical concern for gingival and pulpal tissues using power LEDs. This is caused by the so-called photodynamic effect. Facing maximum light intensities of up to 2,000 mW/cm2 , the problem of heat generation should be seriously taken into account. Power LEDs definitely rely on external cooling. Curing time For the incremental technique for layering of resin composites, the maximum thickness of each individual comFig. 3. Characteristic spectra of QTH and LED lamps. Fig. 4. Diagram of old-styled conventional LED lamps. Fig. 5. Diagram of recent high-power LEDs. 4 138 Kramer et al posite layer was advocated to be < 2 mm with a required curing time of 40 seconds for each layer.44-46 In order to achieve a maximum conversion rate, some authors recommended curing at lower intensities (< 500 mW/cm2 ) within extended polymerization intervals.47 More recently, this paradigm was questioned more often when facing high output curing lights. Koran & Kürschner10 evaluated the variables hardness, adhesion, shrinkage, viscosity and degree of polymerization at different light intensities and different polymerization times with QTH. At energy densities > 17,000 mW/cm2 , no further improvement of mechanical properties was achievable. This leads to the conclusion that with latest generation LED units providing output levels consistently between 1,500-2,000 mW/cm2 , polymerization time can be reduced to 20 seconds.48 Experiences with plasma arc curing demonstrated that 3 x 3 seconds light curing with constant high energy densities are sufficient for appropriate polymerization of hybrid resin composites.33 Nevertheless, in the majority of surveys dealing with curing light intensity and curing time in private dental offices, curing units often lack maintenance and thus provide weak performance, combined with curing times often being limited to 20 seconds.2,49 Therefore, compensation of these practically relevant problems by higher energy output may be the most important point in recent photopolymerization technology. Derived from these observations, and based on the surveys published, most of the resin composite restorations in dental offices may not be sufficiently cured with all consequent disadvantages such as higher abrasion and less biocompatibility. This situation could be changed in favor of more durable and biocompatible resin composite restorations in the future. Curing depth Caughman et al11 postulated clinical guidelines for photocuring in 1995, indicating secure polymerization of resin composites for layers < 2 mm at 280 mW/cm2 . For QTH light units and 3 mm layers, even at 800 mW/cm2 and 80 seconds exposure time (energy density of 64,000 mW/cm2 ) no adequate polymerization was achieved.50 These results question the recommended bulk curing of packable resin composites.51 A recent study52 demonstrated a linear relationship between light intensity of both QTH and LED lamps and curing depth. Interestingly, even prolonged curing times did not guarantee higher curing depths.52,53 If the light tip is placed at a distance of more than 6 mm from the resin composite surface, polymerization depth is affected.53 At a distance of 12 mm from the light tip, no appropriate curing of resin composites was achieved, being independent of the type of light (QTH vs. PAC) and the curing mode (soft start vs. standard).54 Light-curing through ceramic restorations is still a considerable problem. For ceramic inlays, dual-cured and solely light-cured resin composites are described.55 The polymerization depth of solely light-curing resin composites depends on the thickness of the ceramic itself, and from the shade and material of the inlay (ceramic or composite).56 Safe polymerization beneath ceramic inlays is possible up to approximately 3 mm distance from the polymerization tip.57,58 However, with darker inlay shades, curing of luting composites American Journal of Dentistry, Vol. 21, No. 3, June, 2008 is reduced already with ceramic thicknesses of more than 2 mm.59 The same is true for LED units of the first generation exhibiting weak curing potential through a ceramic layer thickness exceeding 2 mm.13 Compared with light-curing resin composites, dual-cured materials exhibit improved curing through ceramic discs.60 QTH units have been reported to be more efficient compared to PAC units.60 Degree of conversion The degree of conversion of a methacrylic resin composite is defined as the percentage of reacted C=C bonds. This ratio substantially affects many properties including mechanical properties, solubility, dimensional stability, color change and biocompatibility of the resin composite.21,61,62 Thus, the degree of conversion plays an important role in determining the ultimate success of a light activated direct restoration.32,63 Degree of conversion is commonly measured by Fourier transform infrared reflectance spectroscopy (FTIR). This method has been reported to produce highly reliable results.64 Calculation is based on the measurement of the net peak absorbance area of the C=C bonds and the aromatic C-C bonds as reference. The net absorbance peak area ratio of cured to uncured material provides the percentage of converted double bonds. Based on that method, a variety of correlations could be proved. It has generally been observed that the higher the conversion in resin composites the higher the polymerization shrinkage will be.61 However, by applying increased light intensities, composites restrain this stress relief much more by not allowing enough flow to reduce internal stress.65 Applied moderate light intensities, in contrast, activate a reduced concentration of initiator molecules to form the network more slowly, thus allowing the material to flow during the early stages of curing.10,61 Within a narrow range, the same degree of conversion is produced by a fixed energy density, independent from variations of light irradiance and exposure time.20 Polymerization at extremely high light intensities was found not to result in adequate curing, due to inferior measured flexural moduli and inferior depths of polymerization.32 Recent work20 has shown a close correlation between energy density and degree of conversion. Also, increased energy densities lead to superior physical and mechanical properties,62 such as fracture strength and the surface degree of conversion, but worse bulk properties like reduced flexural fatigue limits and in-depth degree of conversion. Bulk properties were found to be improved by applying moderate light intensities.66 An important effect on the final mechanical behavior is derived from the applied energy densities and thus from the induction of internal stresses.10 It is conceivable that the different irradiation protocols and thus the build-up of internal stresses will lead to different polymer structures, even though the degree of conversion is the same. A reduced intensity polymerization is probably associated with relatively few centers of polymer growth which may result in a relatively low crosslinked structure.67 High light intensity in the initial phase of the irradiation period will, in contrast, initiate a multitude of growth centers, resulting in a highly crosslinked polymer. Even with a high degree of conversion, a resin composite based on a 5 American Journal of Dentistry, Vol. 21, No. 3, June, 2008 polymer with few crosslinks may be more sensitive to crack initiation or visco-elastic degradation.67 Furthermore, crosslinking differences might be derived from the use of different monomers. A significantly increased degree of conversion was found for UDMA compared to BisGMA monomers.21 For clinically successful restorative dentistry, a minimum degree of conversion has not yet been precisely established. Nevertheless, a negative correlation of in vivo abrasive wear depth with dual conversion has been established for dual conversion values in the range between 55 to 65%.61 Polymerization of adhesives In order to achieve durable bonds between tooth dental hard tissues and directly applied resin composites, a separate polymerization of the adhesive is routinely performed. This is proven to be beneficial especially for dentin aspects.68,69 However, differences relating to the outcome of polymerization of adhesives with different curing units are scarcely reported, with some advantages for QTH lights.70 Regarding the duration of the separate light-curing step, manufacturers normally recommend a 10-second period. An unpublished study from our laboratory confirmed this; for Heliobond,a part of the Syntaca adhesive system, no separate curing resulted in significantly more gaps in adhesive Class V restorations after thermomechanical loading. A completely different situation is found when adhesives placed under ceramic or composite inlays are polymerized. Hikita et al71 named critical factors related to adhesive luting of indirect tooth-colored adhesives. When no separate light-curing of a solely light-curing adhesive is carried out prior to cementation, adhesive performance may be poor. A previous study72 also demonstrated the separate light-curing to be beneficial prior to the application of luting resins; however, light-curing of the bonding resin prior to the insertion of the luting composite produced unacceptable large diameters of the luting spaces.72,7§ One possible solution of this polymerization problem beneath tooth-colored inlays may be the introduction of truly dual-polymerizing adhesive/composite combinations.74 However, after 4 years of clinical service no difference was found compared to a not separately cured light-curing adhesive for bonding of a dual-cured resin composite. Recent results75 indicated that the separate light-curing of the adhesive prior to adhesive cementation may be an overestimated phenomenon. Due to the fact that the dentist desperately tries to thoroughly air-thin the adhesive to avoid pooling, it may not polymerize any more due to oxygen inhibition.75 The most promising way to effectively seal the dentin may therefore be an adhesive lining or build-up, which is referred to as immediate dentin sealing or resin coating technique. Both techniques guarantee contamination-free reliable curing of the adhesive and therefore better sealing as represented by high dentin bond strengths.75-78 Polymerization kinetics, strain, stress Major shortcomings of resin-based composites are inferior conversion and its intrinsic polymerization shrinkage. However, from the clinical point of view, these properties are alLight curing in the LED era 139 ways in conflict with each other. Increased conversion enhances resistance to wear and flexural fatigue, while an increased polymerization shrinkage and thus a higher stress level in bonded resin-based composite restorations is expected.19 During polymerization, dental resin composites transform from plastic viscous through a rubbery visco-elastic into an elastic glassy stage. Initially, the composite remains in its viscous stage and is then able to flow prior to reaching the glassy stage. After passing the gel-point, steric hindrance becomes prominent and with that elastic properties are measurable.79 The elastic modulus increases with growing conversion reaching its final level at the glassy stage. Therefore the degree of conversion has a substantial effect on finally obtained mechanical properties and wear resistance being independent of the cure method.80 In order to investigate changes in modulus and viscoelastic properties of resin-based composites, dynamic mechanical thermal analysis is routinely used. Different polymeric transitions can be identified under changing testing conditions.19 Curing protocols Different light curing protocols are available such as soft start, step curing, or oscillating irradiation. These special curing modes have been considered to increase the degree of conversion for better material properties, and to decrease internal stress to achieve better marginal quality in bonded resin composite restorations.42,65 The introduction of step curing may be interpreted as the first attempt to reduce initial shrinking stress by delaying the gel phase.81,82 During the pre-gel phase, the resin composite flows, and the stresses with the structures are relieved.79 After gelation, flow ceases and cannot compensate for shrinkage stresses.83 Using step curing, the polymerization process is started for 10 seconds on a low level of intensity (100 mW/cm2 ). Consecutively, the light unit automatically increases the power output to 700 mW/cm2 . First results were promising and indicated less polymerization stress compared to conventional QTH curing,81 however, after 10 years, no unanimous proof for improved marginal adaptation could be found.84 Soft start polymerization means also starting at a lower level (100 mW/cm2 ); however, the increase to the final power density (800 mW/cm2 ) takes an exponential curve. This special curing protocol is offered with different QTH and LED models. Nevertheless, the effectiveness of soft start curing is not unanimously clarified. On one hand, a certain reduction of polymerization stresses was shown.31,85 On the other hand, a true beneficial effect on marginal quality of resin composite restorations was not proven.86-88 Only in Class V cavities, some positive effects have been reported.89 The pulse-activated polymerization uses short impulses of high intensity (e.g. e-lightb : 10 pulses for 2 seconds each 750 mW/cm2 ). To date, no enhanced polymerization kinetics have been found.85 The so-called pulse-delay technique was repeatedly investigated in vitro. With this technique, the restoration is initially irradiated with short pulses of light energy (prepoly-merization at low light, e.g. 3 seconds or 20 seconds with 100 mW/cm2 ). After a short waiting period of 3 minutes, the final polymerization is carried out for 30 seconds at high intensi- 6 140 Kramer et al ties.90 Also, a significant advantage could not be found.91 Finally this method is controversial due to the low degree of polymerization at the cavity floor.91 Heat generation as a biological concern The most significant temperature rise during the application of direct resin composite restorations is found during lightcuring.92 The heat generated by photopolymerization can theoretically damage pulpal and gingival tissues.93 Dentin is reported to behave as a good isolating substrate, however, in deep cavities a thin remaining dentin thickness may be problematic.94 With less than 1 mm dentin thickness remaining, a critical temperature rise of 5.6°C inside the pulp chamber has to be taken into account.95 When LEDs have been introduced to the market, lower heat generation was expected involving less risk of tissue damage.96 However, even in modern power LEDs, up to 93% of the total energy amount is still heat.26 Facing the fact that recent generations of high power LEDs reach output intensities of up to 2,000 mW/cm2 , the problem in deep cavities and those close to the gingiva may be even more serious compared to QTH curing units.96 Therefore, some manufacturers already included special curing modes for adhesives and first layers for cavity floor dentin in deep areas being close to the pulp. Inside the resin composite, temperature was found to rise up to 10°C, therefore in deep cavities LEDs and QTHs are recommended with lower energy.97,98 Darker shades also promote heat generation within the resin composite.99 Polymerization from the clinical view Dealing with photopolymerization of resin-based composites, it must be taken into account that clinical circumstances may often differ considerably from laboratory conditions. There is a difference between resin-based composites light cured in the laboratory under perfect conditions with 100% access to cavity and materials cured in the oral cavity. Also in the oral cavity, a shallow Class V in an upper first incisor is easier to irradiate than a deep Class II in a second molar. Therefore, clinical recommendations always have to be adjusted to the individual clinical situation. Due to the fact that in the majority of cases the dental assistant is guiding the photopolymerization steps, a certain amount of knowledge is fundamental for clinical success. How many assistants know that the distance from the light tip may be one of the most crucial factors in adhesive dentistry? Nevertheless, the recent achievements make the older guidelines questionable. In 1995, an exposure time of 60 seconds at 280 mW/cm2 was strictly recommended.11 Today, with the latest generation of LED units, curing time of 2 mm thick increments of resin composite can be reduced to 20 seconds to obtain durable results. Curing depth is fundamentally dependent on the distance of the resin composite to the light source, but only decisive when exceeding 6 mm. The polymerization kinetics can be modified for better marginal adaptation by soft start polymerization, however, in the majority of cavities this may not be the case. It is still not proven whether modified polymerization protocols improve clinical long-term success. Adhesives should be lightAmerican Journal of Dentistry, Vol. 21, No. 3, June, 2008 cured separately for at least 10 seconds when the resin composite is directly applied. This also affects heat generation inside the pulp chamber, so finally heat generation with highpower photopolymerization units should not be underestimated as a biological problem for both gingival and pulpal tissues.
ATOMO Dental produces the best-in-class LED curing light. ATOMO LED curing light has the following features:
- ATOMO Dental LED Curing Light equips with top quality high power LED (manufactured by CREE), super luminance, pure blue light and excellent solidification effect.
- Device type: cordless style, battery replaceable.
- 3 operation modes: Full Power, Progressive, Pulse.
- Optional working time: 3s, 5s, 10s, 15s, 20s, 25s, 30s, 35s, 40s.
- Super capacity Li-ion battery, a full charge can be used for more than 500 times continuously under 10s working time mode.
- Low standby power consumption with 70 days standby time
- Stable output of light intensity, so that the solidification effect is not affected by the consumption of remaining power.
- Curing composite resins to depth of 4 mm.
- Light guide is autoclaveable.
- Metal shell handpiece.
Comparison of Depth of Cure, Hardness and Heat Generation of LED and High Intensity QTH Light Sources
To compare curing performance of a second generation LED curing light with a high power tungsten quartz halogen (QTH).
A hybrid composite resin (Filtek Z 250, 3M, USA) was used as test material and cured using a second generation LED light (Translux Power Blue™, Heraus Kulzer ,Germany) or a very high power QTH light unit (EMS, Switzerland). A two split aluminum mold was used to prepare ten samples with LED light source cured for forty seconds and ten samples prepared using high power QTH light unit, cured for four or six seconds recommended exposure time. Hardness, depth of cure (DOC) and thermal rise during exposure time by these light sources were measured. The data submitted to analysis of variance (ANOVA), Tukey’s and student’s t tests at 5% significance level.
Significant differences were found in hardness, DOC of samples cured by above mentioned light sources and also in thermal rises during exposure time. The curing performance of the tested QTH was not as well as the LED light. TPB light source produced the maximum hardness (81.25, 73.29, 65.49,55.83 and 24.53 for 0 mm, 1 mm, 2 mm, 3 mm and 4 mm intervals) and DOC (2.64 mm) values with forty seconds irradiation time and the high power (QTH) the least hardness (73.27, 61.51 and 31.59 for 0 mm, 1 mm and 2 mm, respectively) and DOC (2 mm) values with four seconds irradiation time. Thermal rises during 4 s and 6 s curing time using high power QTH and tested LED were 1.88°C, 3°C and 1.87°C, respectively.
The used high power LED light produced greater hardness and depth of cure during forty seconds exposure time compared to high power QTH light with four or six seconds curing time. Thermal rise during 6 s curing time with QTH was greater compared to thermal changes occurred during 40 s curing time with tested LED light source. There was no difference seen in thermal changes caused by LED light with 40 s and QTH light with 4 s exposure time.
Light emitting diode (LED) light curing units are becoming increasingly popular in dental practice. Most of the first-generation LED light units were unable to cure composite resin in the manner of quartz-tungsten-halogen (QTH) light sources. Since the spectral output of the LEDs is concentrated in the blue wavelength range, high power LED curing lights are capable of polymerizing some resins as well as, or better than, some QTH lights. LED curing units are characterized by a relatively narrow emission spectrum and lower heat generation than QTH curing units.
There have been reports that exposure causes less temperature rise with a conventional curing light than with an LED curing light and that higher pulp chamber temperature changes are induced by high output curing units than by conventional curing units. Likewise, previous findings that had indicated less temperature rise with LED units than with QTH units have been debated in another study.
The degree of conversion of composite resins is influenced by the spectral distribution and intensity of the curing light as well as the shade, opacity, and chemical composition of the resin-based composite. While both LED lights and quartz-tungsten-halogen lights are believed capable of curing resin-based composites, some differences are observed in the performance of the cured resin. Moreover, both the composite material and its curing time have a significant association with the resulting degree of polymerization.
It has been shown that LED light units, like conventional halogen light sources, are capable of curing the camphorquinone-based composites to an acceptable degree of polymerization and such resin composites show similar strain behavior whether an LED or halogen light curing unit is used to polymerize the resin composite.
The ability to reduce exposure time by using high power LED or QTH lights may improve clinical time management. A very high power QTH light curing unit has been introduced into the market, claiming to cure resin composite with a thickness of more than 2 mm within a short exposure time. The aim of this study was to assess that high power light unit in terms of the curing performance and the temperature rise during irradiation as compared with those for a second-generation LED light source.
MATERIALS AND METHODS
A hybrid composite resin with A3 shade (Filtek Z250, 3M ESPE, St Paul, MN, USA) was used in this study as the test material. The curing light sources used in this study were a second-generation LED light unit with 860 mW/cm2 intensity (Translux Power Blue, Heraeus Kulzer, Germany) and a very high power QTH light unit with 2890 mW/cm2 (Swiss Master Light, EMS, Switzerland). The light intensity of the units was checked using the builtin digital radiometer of an Optilux 501 light curing unit.
A two-split aluminum mold with a semicircular column-shaped hole (4 mm in diameter and 8 mm deep) was used to prepare samples for measuring the depth of cure and the hardness. The mold was placed on a sheet of Mylar strip and then the resin was compressed to produce a flat surface before it was covered by a clear polyester strip (Matrix Tape Refill, 3M) and finally was photopolymerized. For the control group, a Translux Power Blue (TPB) was used in continuous light photoactivation mode for 40 s. Ten samples were prepared with the TPB light source. For the test group, a QTH light unit (EMS) was used in fast cure mode for either 4 s or 6 s. Ten samples were prepared with the EMS light source for each exposure time (4 s or 6 s). The cure depth of the resin was determined using a standard technique (ISO 4049:2000). Immediately after irradiation, the uncured material was scraped away with a spatula. The height of the cylinder of set resin was measured with a digital micrometer (Digital Cal, Switzerland) to a precision of ±0.01 mm, and the result was divided by two. Vickers hardness values were determined at 1.0 mm intervals along the depth of the cured samples, on a flat surface that was parallel to the direction of the light source, using a universal indenter (Leitz Wetzlar, Germany).
A digital thermometer (Temp Alert, Dual Thermo, China) was used to measure the temperature rise during exposure for a cured disc of resin composite (Filtek Z250, 3M, USA) with A3 shade, 2 mm thickness, and 13 mm diameter. A circular mold was prepared from an elastomeric base material in order to support the thermocouple under the cured resin disc. For each curing light and exposure time, groups of ten measurements were performed at 30 min intervals with the room temperature controlled.
The data were submitted to analysis of variance (ANOVA) as well as Tukey’s and Student’s t tests at the 5% significance level.
Analysis of the data revealed significant (P<.05) differences among the samples cured by the light sources in terms of Vickers hardness, depth of cure, and thermal changes during irradiation.
The hardness values of the 0 mm, 1 mm, and 2 mm levels all indicated significant differences between the samples cured by the TPB light unit and those cured by the EMS light unit (regardless of the 4 s or 6 s exposure time). Note that hardness values of the 3 mm and 4 mm levels were measurable only in those samples cured by the TPB light unit. Indeed, the maximum Vickers hardness values were produced by the TPB light unit, which gave 81.25, 73.29, 65.94, 55.83, and 24.53 for the 0 mm to 4 mm levels, respectively. The minimum hardness values were produced by the EMS light unit with a 4 s exposure time, which gave 73.27, 61.51, and 31.59 for the 0 mm to 2 mm levels, respectively. Between the samples irradiated by the EMS light unit for 4 s and 6 s, there was no significant difference in the hardness value at the 0 mm or 1 mm level, but there was at the 2 mm level. In samples cured with the TPB light unit, the 1 mm and 2 mm levels attained hardness values greater than or equal to 80% of the corresponding surface hardness values. In samples cured with the EMS light unit for 4 s, only the 1 mm level attained 80% of the corresponding surface hardness values. In samples cured with the EMS unit for 6 s, the 2 mm level attained hardness values that were closer to 80% of the corresponding surface hardness values than in samples cured for only 4 s.
Depth of cure
The depth of cure (DOC) reached its maximum of 2.64 mm in samples cured by the LED light unit and its minimum of 2.00 mm in those cured by the QTH light with a 4 s exposure time. There was thus a significant (P<.05) difference in DOC between samples cured with the LED light unit and those cured with the QTH light unit. However, this significant difference was also found between samples cured with the EMS light unit for 4 s or 6 s of exposure time, since the DOC was 2.00 mm or 2.15 mm, respectively.
There was only an insignificant (P>.05) difference between the mean temperature rise generated by a 40 s irradiation with LED light (1.87 °C, SD=0.34) and that generated by a 4 s irradiation with QTH light (1.88°C, SD=0.19). However, for a 6 s irradiation with QTH light the analogous difference (3°C, SD=0.27) was considerable. Note that the difference in temperature rise between 4 s and 6 s of irradiation with the QTH light unit was also significant.
In this study, the curing performance of a high power halogen light unit (EMS) was matched against that of a second-generation LED light unit (TPB) with the aim of determining whether the halogen light unit with its short recommended exposure time is capable of curing composites as completely as the LED light unit with its 40 s irradiation time.
In general, larger hardness values are indicators of more extensive polymerization. The depth of cure for light activated dental resin composites has thus often been evaluated indirectly by measuring the hardness of the material at specific depths. It has been suggested that the depth of cure be defined as the level above which the hardness value of the cured resin composite is greater than or equal to 90% (or recently 80%) of the surface hardness value.
The resin composite used in this study contains camphorquinone as the photoinitiator, and generally such resin composites can be more efficiently cured using LED light units.From 78% to 95% of the light emitted by a blue LED unit is within the wavelength range 450–500 nm, as opposed to 56% for a conventional halogen unit. Therefore, only light within this range can best activate campherquinon.However, some studies have indicated that a longer curing time is needed to reach a similar depth of cure and to create optimal performance for resin composite materials when using an LED light unit rather than a conventional tungsten halogen light unit.
For a given irradiation time, it can be anticipated that the microhardness of the composite decreases as the thickness of resin cured increases. In this study the LED light gave better performance than the high power halogen light with respect to hardness and depth of cure. In samples cured with the LED (TPB) light unit, the 1 mm and 2 mm levels attained hardness values greater than or equal to 80% of the corresponding surface hardness values. However, in the samples fast cured with the QTH (EMS) light unit, only the 1 mm levels attained 80% of the surface hardness values. The wavelength range of the LED light used in this study was 440–480 nm, which explains why greater degrees of hardness and conversion were achieved by this LED light with a 40 s irradiation than by the QTH light with either a 4 s or 6 s irradiation.
The halogen curing light produced significantly lower depth of cure (DOC) and hardness values than the LED curing light. Although the QTH light unit delivered a much greater power density, suggesting a higher degree of polymerization, the LED light benefitted from a much longer exposure time. The QTH light unit was only partly able to compensate for this difference, increasing the DOC but not the hardness of the resin composite. This result may be due to the reduced photoactivation time used by the EMS unit, representing a lower amount of energy and a shorter period of time for light to penetrate deeper into the material, since part of the light necessary for polymerization is absorbed and scattered by the resin composite that has already been polymerized.
Another related factor may be that, although much emitted light can satisfy the camphorquinone (CQ) absorption curve and initiate a polymerization reaction, the highest probability of light absorption corresponds with the peak at 465 nm. While the output of the halogen curing light has a broad spectrum, a great portion lies outside the CQ absorption curve, so 80% of the energy from the halogen lamp is outside the useful curing range.
A photoactivated resin-based composite can be fully polymerized at reduced light intensity while the final conversion value remains high. Curing the composite with a high intensity light and short exposure time, such as when using the EMS unit, shortens the pregelation phase and prevents a slow and ordered chain growth.
According to manufacturer information, Filtek Z250 contains filler particles that range in size from 0.01 to 3.5 μm. Most activation light sources that are commercially available have a peak in the range 450–500 nm. Research has shown that light scattering in the resin composite is maximal when the filler particle size is half the wavelength of the activating light, resulting in a lower transmission coefficient and smaller depth of cure. The transmission coefficient is influenced by the wavelength of the light, the refractive indices of the resin and fillers, and the nature and amount of the filler particles.In comparison with the LED light, the EMS light has a much greater light intensity, which actually increases light scattering and light attenuation, resulting in less camphorquinone activation and resin conversion. This may also partly explain the decreased DOC in the samples cured with the EMS light in this study.
The light intensity and exposure time are known to be the most important factors in temperature change.Moreover, the temperature rise is known to increases with the power density of the LED or QTH unit, but yielding a greater rise for a given power density when using a QTH unit, as corroborated by the findings of this study.
Another study indicated that photocuring blue light sources increase the temperature in tooth tissue during in situ polymerization of resin composite and that a higher power density QTH light source (Swiss Master Light) caused a greater increase in tooth temperature than a high power LED light. The temperature rise was greater with increased exposure time, as found in this study as well.
In this study, increasing the period of irradiation with the QTH light from 4 s to 6 s only affected the DOC of the resin composite and the hardness of its 2 mm level. This can be partly attributed to a temperature rise by from 1.88°C to 3°C while curing the resin composite.
The thermal variations that occur during the photoactivation of composite resins are related both to the exothermic polymerization of the materials and to the heat output from the dental curing light units. A cured composite, such as the cured composite disc in this study, is capable of reducing the ability of irradiated light to increase temperature. This has been shown by a study in which the temperature rise caused by irradiation was less via a previously cured composite than via an initially unpolymerized composite.
According to the results of this study, composite increments less than 2 mm in thickness should be cured using a high power QTH light source for a short period of time, especially in deep cavities to ensure proper curing of the composite. Applying thicker increments and increasing the exposure time to compensate will increase temperature rise and endanger pulp chamber vitality.
Hardness values produced were greater with the LED light than with the QTH light.
Depth of cure obtained was higher with the LED light.
Depth of cure produced by the QTH light was higher for a recommended 6 s irradiation than for a shorter 4 s exposure.
Thermal changes were greater using the QTH light for 6 s than using it for 4 s or the LED light for 40 s.
Thermal changes were the same whether using the LED light for 40 s or the QTH light for 4 s.
ATOMO LED curing light has the highest light intensity (2000±200 mW/cm²), and can cure resin-based composites up to 4mm thickness within 10 seconds. This is a new record in all dental curing light.
Please contact ATOMO Dental for more detailed information:
1241 Quarry Lane #125
Pleasanton, CA 94566
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