Comparative efficiency of plasma and halogen light sources on composite micro-hardness in different curing conditions

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1 Article Comparative efficiency of plasma and halogen light sources on composite micro-hardness in different curing conditions DIETSCHI, Didier, MARRET, N, KREJCI, Ivo Abstract Recent developments have led to the introduction of high power curing lights, which are claimed to greatly reduce the total curing time. This study evaluated the effectiveness of a plasma-curing device (Apollo 95 E) and a halogen device (Heliolux DLX), in different curing conditions. Reference DIETSCHI, Didier, MARRET, N, KREJCI, Ivo. Comparative efficiency of plasma and halogen light sources on composite micro-hardness in different curing conditions. Dental Materials, 2003, vol. 19, no. 6, p PMID : DOI : /s (02) Available at: Disclaimer: layout of this document may differ from the published version.

2 Dental Materials 19 (2003) Comparative efficiency of plasma and halogen light sources on composite micro-hardness in different curing conditions D. Dietschi*, N. Marret, I. Krejci Division de Cariologie, d Endodontie et de Pédodontie, Section de Médecine Dentaire, Faculté de Médecine, Université de Genève, 19 Rue Barthélémy Menn, 1205 Geneva, Switzerland Received 11 May 2000; revised 30 June 2001; accepted 4 December 2001 Abstract Objectives. Recent developments have led to the introduction of high power curing lights, which are claimed to greatly reduce the total curing time. This study evaluated the effectiveness of a plasma-curing device (Apollo 95 E) and a halogen device (Heliolux DLX), in different curing conditions. Method. Vicker s micro-hardness values were performed on 1 and 2 mm thick composite discs cured in a natural tooth mold by direct irradiation or indirect irradiation through composite material (2 or 4 mm) and dental tissues (1 mm enamel or 2 mm enamel-dentin). Measures were, respectively, performed after a 1, 3, 6 s (SC, step curing mode) or 18 s (3 SC) exposure to the plasma light, and a 5, 10, 20 or 40 s exposure to the halogen light. Results. With the PAC light used, a 3 s irradiation in the direct curing condition was necessary to reach hardness values similar to those obtained after a 40 s exposure to the halogen light. Using the indirect curing condition, hardness values reached after an 18 s exposure (3 SC mode) with the plasma light were either equivalent or inferior to those obtained with 40 s halogen irradiation. Significance. Direct polymerization with the plasma light used requires longer exposure times than those initially proposed by the manufacturer. The effectiveness of plasma generated light was lowered by composite or natural tissues, and therefore requires an important increase in the irradiation time when applied to indirect polymerization. The practical advantage of this polymerization method is less than expected, when compared to traditional halogen curing. q 2003 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved Keywords: Composite resin; Polymerization; Curing; Plasma light; Vicker s micro-hardness 1. Introduction Light activated composite resins are now the most widely used restorative and luting materials. The main advantage of this activation mode over the chemical one is the control the operator has over the working time. However, a definite amount of energy, which is defined as power multiplied by curing time, is necessary to obtain the optimal conversion rate of the composite resin. This amount largely depends on the composite shade, opacity, thickness, initiator, composition, increment size and restoration s configuration [1 5]. If any of these variables are not adequately addressed, the composite resin material may remain less than optimally * Corresponding author. Tel.: þ /165; fax: þ address: ddietschi@medecine.unige.ch (D. Dietschi). converted, which in turn, may result in an inadequate clinical performance. As regards halogen curing lamps, manufacturers are constantly increasing their power. This proved especially useful for curing composites through a restoration or dental tissues. Nevertheless, in clinical conditions, the time needed for proper light curing of luting composites or for the multiincremental build-up of large restorations with halogen lamps is quite extensive [2,3]. High power curing devices such as argon lasers and plasma arc lights were therefore developed, which should theoretically decrease curing time. According to the available data, it seems that Apollo 95 E, a widely used PAC light unit, offers a curing efficiency comparable to halogen devices, as determined by measurements of conversion and contraction rates, polymerization stress, flexural strength, elasticity modulus, Knoop or Vicker s micro-hardness and evaluation of marginal leakage /03/$ - see front matter q 2003 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved PII: S (02)

3 494 D. Dietschi et al. / Dental Materials 19 (2003) Table 1 Experiment variables and group distribution for Z100 direct curing (grouping a) (n ¼ 5) Groups Sample thickness (mm) or marginal adaptation. However, most of the existing data were produced in vitro, in clinically non-relevant experimental set-ups and conditions and have not yet been published. The aim of the present study was then to compare the curing effectiveness of one plasma and one halogen curing units, based on composite hardness measurements of samples polymerized in cavities made on natural teeth. The hypothesis that curing times with the Apollo 95E plasma light should be adapted to the composite reactivity, increment thickness as well as the type and thickness of materials interposed between the light source and composite surface was evaluated. 2. Materials and method 2.1. Mold fabrication Curing device PAC QTH Exposure duration (s) Z A1-1 1 þ 1 Z A3-1 1 þ 3 Z ASC-1 1 þ 6 (SC) Z H þ 40 Z ASC-2 2 þ 6 (SC) Z A3-2 2 þ 3 Z H5-2 2 þ 5 Z H þ 10 Z H þ 20 Z H þ 40 Group coding refers to experimental variables: Z ¼ Z100; A ¼ PAC curing or H ¼ QTH curing; third and fourth digits ¼ exposure time or curing mode; last digit ¼ sample thickness. Natural tooth molds were fabricated in order to simulate light reflection and absorption found in clinical conditions. For that purpose, the crowns of intact human third molars (n ¼ 6; two teeth per mold type) were flattened using Table 2 Experiment variables and group distribution for Tetric Ceram direct curing and 2 mm sample thickness (grouping b) (n ¼ 5) Groups Curing device Exposure duration (s) Apollo Heliolux T ASC-2 þ 6 (SC) T A3-2 þ 3 T A1-2 þ 1 T H40-2 þ 4 Group coding refers to experimental variables: T ¼ Tetric Ceram; A ¼ PAC curing or H ¼ QTH curing; third and fourth digits ¼ exposure time or curing mode; last digit ¼ sample thickness. a dental cast trimmer (Model Trimmer, Produits Dentaires; Vevey, Switzerland) in order to provide a flat surface of about 10 mm in diameter. Subsequently, the teeth were embedded in self-curing resin (Technovit 4071, Kulzer; Bad Wehrheim, Germany), leaving the outermost 3 mm uncovered. The root side was flattened parallel to the coronal surface and the teeth were mounted on a surveyor table and standardized cylindrical cavities were prepared under water spray with a 5 mm diameter diamond bur (No. 847, Komet- Brasseler; Lemgo, Germany), mounted on an electrically driven hand piece. Cavity depths were 0.5, 1.0, and 2.0 mm (^0.1 mm), respectively. In order to provide separable molds, the teeth were cut perpendicularly to the tooth long axis by using a slow rotating diamond coating saw, under water irrigation (Isomet, Buehlers, Uzwil, Switzerland). An elastic band placed in a peripheral channel allowed for perfect repositioning and stabilization of the mold during sample fabrication. To prevent bacteria or fungus growth in the storage medium, molds were kept in an isotonic and disinfectant saline solution containing 0.5% thymol, at 4 8C, before being used Sample fabrication Composite discs of varying thickness (0.5, 1 or 2 mm) were produced from two fine hybrid composites (Z100, 3M; Minneapolis, MN; batch no ; Tetric Ceram; Vivadent, Schaan, FL; batch no. A01088). These specific composite brands were selected because of their different reactivity and proper activation by QTH and PAC light sources [6,7]. For that purpose, the natural tooth molds were filled with the restorative material and covered by a 40 mm transparent polyester foil (Universal Strip, Odus Dental; Dietiken, CH). The material was light cured, either directly through the polyester foil (direct curing mode) or through interposed materials; the distance between the curing tip and polyester foil or interposed material was kept constant at 0.5 mm to simulate the optimal clinical situation. The interposed material was a 2 or 4 mm thick composite disc (A2 shade, Z100; 3M), to simulate irradiation through a restoration, or 1 mm thick enamel and 2 mm thick dentinenamel discs, to simulate transcuspal irradiation. The composite samples were polymerized with either a quartz tungsten halogen light (QTH) (Heliolux DLX, Vivadent) or xenon plasma PAC light (PAC) (Apollo 95 E, DMD; Westlake Village, CA). The relative light intensity of the QTH and PAC curing units was, respectively, 650 and.1000 mw/cm 2 (as measured with a radiometer: Demetron-Kerr; Orange, CA) [15]. Different exposure times were applied: 1, 3, 6 s (SC, step curing mode) or 18 s (3 SC) for the PAC light, and 5, 10, 20 or 40 s for the QTH light. Five samples were fabricated for each group. The 29 selected combinations of the aforementioned test variables corresponding to the experimental groups are given in Tables 1 3.

4 D. Dietschi et al. / Dental Materials 19 (2003) Table 3 Experiment variables and group distribution for indirect curing (groupings c and d) (n ¼ 5) Groups Composite Curing device Exposure duration (s) Interposed material PAC QTH 2 mm CP 4 mm CP 1 mm E 2 mm E/D Z CP4 A3 Z100 þ 3 þ Z CP4 ASC Z100 þ 6 (SC) þ Z CP4 A3SC Z100 þ 18 (3 SC) þ Z CP4 H40 Z100 þ 40 þ Z CP2 A3 Z100 þ 3 þ Z CP2 ASC Z100 þ 6 (SC) þ Z CP2 H40 Z100 þ 40 þ Z E ASC Z100 þ 6 (SC) þ Z E A3SC Z100 þ 18 (3 SC) þ Z E H40 Z100 þ 40 þ Z ED A3SC Z100 þ 18 (3 SC) þ Z ED H40 Z100 þ 40 þ T ED ASC Tetric þ 6 (SC) þ T ED A3SC Tetric þ 18 (3 SC) þ T ED H40 Tetric þ 40 þ Group coding refers to experimental variables: Z ¼ Z100 and T ¼ Tetric Ceram; A ¼ PAC curing or H ¼ QTH curing; last or two last digits ¼ exposure time or curing mode; CP ¼ composite resin; E ¼ enamel; ED ¼ enamel-dentin. The samples were then removed from the mold and the top surface was marked with a permanent pen. Samples were kept in saline, at 37 8C, for 14 days, to maximize postpolymerization [8,9]. Consequently, the five samples of each group were embedded, face to face, in a slow selfcuring resin, producing a maximal exothermic temperature of 21 8C during curing (Epo-Thin, Buehlers). Samples were kept in position during resin setting by special clips (Sample-Klip, Buehlers). After 18 h, the resin cylinders including the samples were mounted in a semi-automated polishing machine and first ground to expose samples in their largest diameter (Roto Force, Merck, Dietikon, Switzerland). Thereafter, samples were polished to a specular gloss. The polishing sequence was as follows: (1) Wet grinding with Grit 80 and 120 SiC papers for the time required to reach the specimen s largest diameter, with a progressive force (5 30 N). (2) Wet pre-polishing with Grit 220 SiC paper; 300 rpm for 5 min with a progressive force (5 30 N). (3) Wet polishing with Grit 1200 SiC paper, 300 rpm for 5 min, with a progressive force (5 30 N). (4) Specular polishing: sprays of diamond powder (6 and 3 mm grains) on a MD-PAN w (Struers; Rødovre, Denmark) tissue, with lubricants (DP lubricant green and OP-U suspension, Struers), at 150 rpm, with a progressive force (5 30 N). After each polishing step, the sample holders were dismounted and the embedded sample surfaces were thoroughly cleaned with warm water. Surface quality was also assessed between polishing steps, under a binocular microscope Micro-hardness measurements Micro-hardness measurements were used as an indirect evaluation of degree of conversion [10]. After polishing, samples were stabilized parallel to base of the hardness measurements device (Shimazu HMV-2000, Gnehm; Horgen) by pressing the sample over a thin layer of a plasticizing material (Plasticine; Buehlers). Vicker s hardness measurements were obtained using a 300 g load for all Z100 specimens, while those of groups ZA1/1, ZEDA3SC, ZEDH40 and Tetric samples, were measured using a 100 g load. This change in the load applied to the Vicker s diamond was necessary to maintain the indentation size within the dimensions of the device-measuring window. The appropriate load was applied for 30 s and the indentation size was recorded 10 s later. For samples of 1 and 2 mm thickness, Vicker s hardness profiles were established with four measurements per sample at 10 mm underneath the sample top surface, four measurements at the center of the sample and four measurements at 10 mm above the sample bottom. For groups simulating the indirect curing mode, with a 0.5 mm uniform composite thickness, five measurements were taken, 10 mm above the sample bottom Statistics Mean hardness values were calculated for each group and at the three locations (top, middle and bottom), when applicable for statistical comparisons. One factor ANOVA and Scheffé F-tests were applied to explore differences between groups, at a 5% confidence level.

5 496 D. Dietschi et al. / Dental Materials 19 (2003) Table 4 Mean Vicker s hardness values (^SD) Groups 3. Results The results of the Vicker s hardness measurements are presented in Table 4 and Figs The statistical analysis of the data is presented in Tables 4 and Direct curing Mean (^SD) Top Middle Bottom Z A (5.8) 85.4 (4.34) Z H (4.6) a, A (6.1) a, A (3.1) a, B Z A (2.5) a (3.4) a (2.5) a Z ASC (3.1) a, A, B (2.6) a, A (5.0) a, B Z H (2.6) a, A 96.8 (6.4) a, A, 89.6 (4.7) a, B B Z H (2.1) a, b, (1.9) a, A 98.2 (13.0) a, A A Z H (6.0) b, c, A, B (6) b, A (5.1) a, c, B Z ASC (1.8) c, d, A, B (1.2) b, c, A (2.0) b, c, B Z A (4.0) d, A (6.7) c, A (5.1) b, A Z H (8.0) d, A (3) c, A (6.1) b, c, A T A (3.3) A 45.6 (2.3) 26.4 (5.0) A T A (1.7) a, A 55.6 (2.9) a, A 52.0 (2.7) a, A T ASC (3.9) a, A 58.4 (5.4) a, A 54.0 (3.7) a, A T H (2.9) a, A 54 (4.6) a, A 57.0 (2.4) a, A Z CP4 A (7.1) a Z CP4 ASC 62.2 (2.8) a Z CP4 A3SC 103 (10.2) b Z CP4 H (6.8) b Z CP2 A (3.0) Z CP2 ASC (3.34) a Z CP2 H (4.0) a Z E ASC 112 (3.0) a Z E A3SC 103 (5.9) a, b Z E H (2.1) b T ED ASC 48.2 (3.6) a, A 45.2 (8.2) a, A 31.0 (3.7) T ED A3SC 52 (2.4) a, A 51.6 (3.4) a, A 44.4 (3.6) a T ED H (3.6) a, A 54.8 (2.2) a, A 50.0 (5.1) a, A Z ED A3SC 76.8 (12.5) a, A 80.0 (5.9) A 47.6 (2.4) Z ED H (10.9) a, A (9.4) A 84.2 (7.8) A For comparison between products (columns), means with same lower case letter are not statistically different at p ¼ 0.05 using the Scheffé F-test. For comparison between measurement locations (rows), means with same capital letter are not statistically different at p ¼ 0.05 using the Scheffé F-test. One millimeter samples cured for 1 s with the PAC device showed hardness values lower than the 3 s and SC (6 s) curing modes or 40 s curing with the QTH device, for each measurement site (top, middle and bottom) (Fig. 1 and Table 5 Z100, indirect polymerization; comparison between the different interposed materials (ANOVA) Comparison p Statistical differences (at p, 0.05) ZA3 t ¼ CP2 vs. CP4 ZASC F ¼ CP4 vs. CP2, E ZA3SC t ¼ ZH40 F ¼ CP4 vs. E Group coding refers to experimental variables: Z ¼ Z100 and T ¼ Tetric Ceram; A ¼ PAC curing or H ¼ QTH curing; last or two last digits ¼ exposure time or curing mode; CP ¼ composite resin; E ¼ enamel. Table 4). For the direct curing groups, the hardness values taken in the sample middle were usually slightly higher than on the top (Table 4). The hardness values decreased with decreasing exposure duration for the 2 mm Z100 samples polymerized with QTH light (Fig. 2 and Table 4). Exposure of samples to the QTH light for 5 or 10 s produced significantly lower hardness values than for 20 s (middle) and 40 s, or 3 s and SC with PAC light. A 3 s exposure using the PAC light gave higher hardness values than a 20 s curing with QTH light, at the middle and bottom surfaces. For the 2 mm Tetric groups, the results obtained after a 3 s or SC exposure to the PAC light and 40 s exposure to the QTH light were not statistically different, but there was a significant decrease in hardness values for a 1 s exposure to the PAC light (Fig. 3 and Table 4). In the direct curing mode and with each of the two materials evaluated, 3 s or SC modes with the PAC light proved equivalent to a 40 s exposure to the QTH light Curing through composite and tooth substance When curing through composite and tooth substance, (interposition of 2 and 4 mm of composite or 1 mm of enamel), the hardness values increased while extending the exposure of samples to the PAC light (from 3 s, SC to 3 SC modes) (Figs. 4 and 5 and Table 4). The curing efficiency of QTH light proved significantly superior to the 3 s mode with PAC light, for each interposed material (Fig. 4 and Table 4). The SC mode of the PAC light was less efficient than 3 SC or 40 s with the QTH light, when 4 mm of composite was interposed (Fig. 4 and Table 4). The interposition of 4 mm of composite significantly reduced the composite hardness, in comparison with 2 mm of composite (Z100 3 s and SC after PAC curing) or 1 mm of enamel (Z100 Apollo SC and Heliolux 40 s) (Fig. 4 and Tables 4 and 5). With interposition of 2 mm enamel-dentin, the hardness values of Z100 samples exposed to the QTH light were superior to those obtained with the PAC light, including the 3 SC mode (Fig. 5 and Table 4). Tetric samples polymerized with the PAC light in the SC mode presented a hardness inferior to those polymerized in the 3 SC mode or exposed 40 s to the QTH light (Fig. 5 and Table 4). For all

6 D. Dietschi et al. / Dental Materials 19 (2003) Fig. 1. Vicker s hardness measurements for Z100, 1 mm thick samples (direct cure). samples polymerized with the PAC light through 2 mm enamel-dentin, hardness values at the bottom surface were significantly inferior to those at the middle and top locations (Fig. 5 and Table 4). 4. Discussion Plasma arc lights are increasingly used by dentists because they supposedly allow for a reduction of curing time, due to their elevated light output, which is considered a significant advantage over conventional halogen light curing units. Actually, it is well known that the degree of conversion of a given light-curing composite is mainly influenced by the light intensity of the curing device, total exposure duration, and distance between the exit window and resin surface [3,5]. Early advertising of the Apollo 95 E PAC unit advocated a 1 s curing time, which justifies the enormous and immediate interest taken in this device. Though this claim has now been revised, the question of minimal exposure times remains burning, not only in general, but also for specific restorative procedures making use of the plasma lamps Sample fabrication In this study, three different clinical conditions were simulated: the direct polymerization when curing through a polyester strip only or the polymerization through a composite workpiece or through tooth substance, thus simulating complex restorative procedures such as the three-sited light curing technique [11], the trans-wall polymerization technique [12] or the luting of an indirect restoration. Because the reflectance properties of the mold Fig. 2. Vicker s hardness measurements for Z100, 2 mm thick samples (direct cure).

7 498 D. Dietschi et al. / Dental Materials 19 (2003) Fig. 3. Vicker s hardness measurements for Tetric Ceram, 2 mm thick samples (direct cure). material may influence the depth of cure [13], natural extracted human teeth were used as the mould material, in order to mimic the clinical situation as closely as possible. The QTH curing unit and a 40 s exposure duration served as a reference [2,3,11]. Besides that, shorter exposure durations were also evaluated to get better insight into the influence of exposure durations on hardness values. Consequently, the variable tested in the present investigation was exposure duration. This factor may be considered the most important clinical variable because it is the only parameter that is directly influenced by the dentist s selection. Both restorative composites tested here are widely used but exhibit rather dissimilar polymerization behavior. Tetric is less sensitive to light than is Z100 [7]. As mentioned in the specific sections, Tetric was not submitted to some experimental conditions, especially those which produced insufficient hardness values with Z100, the more reactive brand. The choice of these two materials was also based on the fact that the plasma lamp can adequately polymerize both of them [6] Micro-hardness measurements The degree of resin cure is one of the critical parameters, which may influence physical properties of composite materials [14] and thus the clinical behavior of light curing restorations. Surface hardness is a good predictor for resin Fig. 4. Vicker s hardness measurements for Z100, 0.5 mm thick samples, cured through 2 and 4 mm of composite, and 1 mm of enamel.

8 D. Dietschi et al. / Dental Materials 19 (2003) Fig. 5. Vicker s hardness measurements for Z100 and Tetric Ceram, 2 mm thick samples, cured through 2 mm of enamel-dentin. conversion [9]. It is especially sensitive to small changes of the polymer cross-linkage in areas of high conversion and is quite simple to use [18]. In addition, it allows for measurements at specific locations within the sample; for this study, evaluations were made at the top, middle and at the bottom of the specimens. Though hardness values may not be used for a direct comparison among materials, they are a valuable tool for relative measurements within the same material and its simplicity facilitates the evaluation of a large number of specimens [15], making it suitable for comparing different curing techniques Direct irradiation For a given experimental condition and curing procedure, the hardness values of Z100 were about twice the values of Tetric Ceram. This observation is in agreement with the literature [7] and can be explained by the different composition of the two restorative materials. A 40 s irradiation of samples with a high intensity QTH unit served as the positive control to judge on the efficacy of other curing modes. A 1 s exposure to the PAC light was therefore generally insufficient, even for 1 mm thick samples and the reactive composite Z100. No measurement was made in the middle of these samples because of the very low hardness values already obtained at the top surface. On the other hand, a 3 s curing with plasma lamp of 1 and 2 mm thick samples was not significantly different from the 40 s reference curing with the halogen lamp. In view of these results, it was considered useless to test 1 mm samples of Tetric Ceram. When trying to determine the minimal curing time on 2 mm thick samples, for both materials, it appeared that only the 3 s or SC curing modes with the PAC light systematically equaled 40 s QTH curing. For Z100, a 5 s curing with the QTH unit produced hardness values surprisingly close to the control group (Fig. 2). With such highly reactive composite resins, the advantage of the plasma lamp is probably less decisive. When looking at the PAC lamp results, there was almost no decrease in hardness in the depth of the samples, whereas with the QTH lamp, a certain decrease in hardness was observed with Z100, although not being present with Tetric Ceram. Therefore, differences among top, middle and bottom values in all directly cured groups were not statistically significant. However, the hardness profiles revealed a trend towards the highest hardness in the middle of the samples in all directly cured samples. The lower hardness at the top of the samples can be explained by the fact that composite cured against a smooth surface (sample covered with a polyester foil) exhibits an increased resin content [16,17] and/or by compromised polymerization due to oxygen inhibition from the ambient air [18] which can be present even if the composite surface is covered by a mylar strip [19]. The lower hardness at the bottom of the samples may be a result of the attenuation due to the scattering of the curing light by the composite resin [20]. The deviation of the results was generally small, with both curing devices, which indicates a good reproducibility and reliability of the curing protocol Indirect polymerization Trends observed using indirect curing differs markedly from those related to direct exposure. The least critical situation was recorded if 0.5 mm of Z100 was cured through 1 mm enamel. In this set-up, a 40 s QTH light curing resulted in hardness values similar to the ones of direct curing. However, exposure to the PAC light for one SC cycle delivered hardness values that were about 10% less than in the direct curing group with 2 mm thick samples. Even three SC curing cycles were not able to produce the same level of hardness as after 40 s curing with the QTH

9 500 D. Dietschi et al. / Dental Materials 19 (2003) lamp. Obviously, even a thin layer of enamel, which has a high level of transparency [21], greatly reduces the efficacy of the PAC lamp. Logically, no measurements were performed with the PAC light, for the 3 s indirect curing mode, when 1 mm of enamel was interposed, since very low hardness values were obtained with the SC curing mode under the same conditions. This effect was even more pronounced with interposition of Z100 composite material. After a 3 s cure with the PAC light, a 50% hardness reduction was observed between 2 mm samples directly cured and 0.5 mm samples, cured beneath 2 mm of composite. However, with a 40 s exposure to QTH light, very little difference was observed. Apparently, the light density attenuation effect of interposed restorative materials [22] is maximal within the narrow emission spectrum of the PAC light. By extending exposure in the SC mode, this effect was largely compensated for. In this case, only about 10% decrease in hardness was observed in the indirect group with the interposition of 2 mm of composite, when compared to 2 mm thick composite samples directly cured. However, if the thickness of the cured composite above the 0.5 mm uncured test samples was extended to 4 mm, thus representing thicker inlays and onlays [23], even three SC cycles with the PAC lamp were not sufficient to obtain the same hardness as after 40 s exposure to the QTH lamp. Three SC cycles correspond to about a 20 s exposure. When curing 2 mm thick composite samples through 2 mm thick enamel/ dentin-plates, thus simulating the situation in complex direct restorative procedures, the hardness reduction at the bottom of samples was highly pronounced with the PAC light. This is in agreement with the literature, where dentin exhibited a high attenuation effect of visible light [24]. However, the present results emphasize that, with indirect duration, the time needed for proper polymerization with the PAC light has to be considerably extended, even with a highly reactive composite material, such as Z Conclusions High power density is considered necessary for adequate composite resin cure within a short time frame. The increased power density generated by the PAC light was able to significantly reduce the exposure duration for direct polymerization of 1 and 2 mm thick composite samples in comparison with QTH light. This reduction can lead to a significantly reduced chair time, especially when incremental techniques are applied. However, using indirect polymerization, a distinct prolongation of exposure duration was required for the PAC light. An undifferentiated recommendation of exposure durations for the PAC lamp is therefore not appropriate. Rather, meticulous guidelines with respect to exposures must be established for each single clinical indication and specific brand to ensure properly cured restorations. References [1] Nomoto R, Uchida K, Hirasaw AT. Effect of light intensity on polymerization of light-cured composite resins. Dent Mater J 1994; 13: [2] Rueggberg FA, Caughman WF, Curtis JW, Davis HC. A predictive model for the polymerization of photo-activated resin composites. Int J Prosthod 1994;7: [3] Rueggeberg FA, Cauchman WF, Curtis JW. Effect of light intensity and exposure duration on cure of resin composites. Oper Dent 1994; 19: [4] Boushlicher MR, Vargas MA, Boyer DB. Effect of composite type, light intensity, configuration factor and laser polymerization on polymerization contraction forces. Am J Dent 1997;10: [5] Hansen EK, Asmussen E. Visible-light curing units: correlation between depth of cure and distance between exit window and resin surface. 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