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Degree of Conversion and Mechanical Properties of a BisGMA:TEGDMA Composite as a Function of the Applied Radiant Exposure Fernanda C. Calheiros,1 Ma´rcia Daronch,1,2 Frederick A. Rueggeberg,2 Roberto R. Braga1 1

Department of Biomaterials and Oral Biochemistry, School of Dentistry, University of Saõ Paulo, Saõ Paulo, Brazil

2

Department of Oral Rehabilitation, School of Dentistry, Medical College of Georgia, Augusta, Georgia

Received 6 February 2007; revised 15 May 2007; accepted 17 May 2007 Published online 16 July 2007 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.b.30897

Abstract: Objective: Verify the influence of radiant exposure (H) on composite degree of conversion (DC) and mechanical properties. Methods: Composite was photoactivated with 3, 6, 12, 24, or 48 J/cm2. Properties were measured after 48-h dry storage at room temperature. DC was determined on the flat surfaces of 6 mm 3 2 mm disk-shaped specimens using FTIR. Flexural strength (FS) and modulus (FM) were accessed by three-point bending. Knoop microhardness number (KHN) was measured on fragments of FS specimens. Data were analyzed by one-way ANOVA/Tukey test, Student’s t-test, and regression analysis. Results: DC/top between 6 and 12 J/cm2 and between 24 and 48 J/cm2 were not statistically different. No differences between DC/top and bottom were detected. DC/bottom, FM, and KHN/top showed significant differences among all H levels. FS did not vary between 12 and 24 J/cm2 and between 24 and 48 J/cm2. KHN/bottom at 3 and 6 J/cm2 was similar. KHN between top and bottom was different up to 12 J/cm2. Regression analyses having H as independent variable showed a plateau region above 24 J/cm2. KHN increased exponentially (top) or linearly (bottom) with DC. FS and FM increased almost linearly with DC/bottom up to 55% conversion. Conclusions: DC and mechanical properties increased with radiant exposure. Variables leveled off at high H levels. ' 2007 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 84B: 503–509, 2008

Keywords:

radiant exposure; composite; mechanical properties; degree of conversion

INTRODUCTION A positive correlation between degree of conversion (DC) and mechanical behavior of resin composites has been demonstrated in several studies.1–4 Increased DC also improves material’s biocompatibility by reducing the amount of residual monomers leached to the oral environment.5,6 As a direct relationship between DC and radiant exposure (i.e., energy dose or H, unit: J/cm2) has been well established,7,8 clinicians are compelled to employ high H levels during photoactivation by increasing irradiance (unit: mW/cm2) and/or exposure.9–11 However, an often overlooked aspect is the fact that the relationship between DC and H is not linear, and studies show a tendency for DC to stabilize above 10 J/cm2.7,9,12 As a consequence, it is likely that mechanical properties would not significantly improve above a certain H value. Correspondence to: R. R. Braga (e-mail: [email protected]) Contract grant sponsor: FAPESP (The State of Saõ Paulo Research Foundation); Contract grant numbers: 03/13002-0, 04/05975-0. ' 2007 Wiley Periodicals, Inc.

Moreover, the use of increased H values is associated with the development of high polymerization stress levels,8,13,14 which has been demonstrated to be directly related with the occurrence of microleakage of composite restorations in vitro.15,16 Dental literature displays an intense controversy in terms of defining H values necessary for optimal mechanical properties. While some studies suggested that at the surface of a restoration H levels as low as 4–6 J/cm2 would be enough,17,18 others advised that doses below 12 J/cm2 could jeopardize composite’s compressive and diametral tensile strengths.19 Higher values, between 21 and 24 J/cm2, have been indicated to ensure homogeneous polymerization of 2-mm thick increments.20,21 An ideal H value would provide optimized mechanical properties, together with a minimum polymerization stress development. Two recent studies by the same research group verified for a commercial composite that DC and mechanical properties increased significantly with radiant exposure; however, polymerization contraction also increased, which poses a risk to the integrity of the bonded interface.22,23 In fact, a previous study evaluating an experimental composite 503

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containing 1 BisGMA:1 TEGDMA and 75% inorganic filler (in weight) indicated that successive twofold increases in H, from 3 J/cm2 until 24 J/cm2 resulted in significant increases in polymerization stress among the four H levels evaluated.24 Specific to the composite investigated,24 polymerization stress development did not reach a plateau even at a relatively high H level. Thus, it is clinically important to determine whether lower H values could be employed without compromising the material’s mechanical performance. Therefore, the purpose of this study was to evaluate the influence of different H values on mechanical properties (flexural strength (FS), flexural elastic modulus (FM), and Knoop microhardness), DC, and homogeneity of cure of an experimental composite with organic matrix containing equal amounts (in weight) of BisGMA and TEGDMA. The hypothesis of the study was that an energy dose threshold would exist, above which increases in H would not promote significant increases in DC or mechanical properties. MATERIALS AND METHODS An experimental composite was prepared containing 1:1 (in weight) of bisphenol A glycidyl dimethacrylate (BisGMA; 2,2-bis[4-(2-hydroxy-3-methylacryloxypropoxy)-phenyl]propane) and triethylene glycol dimethacrylate (TEGDMA; Esstech, Essington, PA), camphorquinone (photoinitiator, Sigma Aldrich Brasil), 2-dimethylaminoethyl methacrylate (co-initiator, 98%, Sigma Aldrich), and crystalline butyl hydroxytoluene (inhibitor, Sigma Aldrich) in 0.5 wt % each. Fumed silica (0.04 mm, Aerosil DT 4 and VP DT 5, Degussa Brasil) and barium glass particles (2 mm, Vigodent, Rio de Janeiro, Brasil) were added in amounts corresponding, respectively, to 5 and 70% of the total mass of composite. A quartz-tungsten-halogen light unit was used for specimen preparation (VIP Junior, Bisco, Schaumburg, IL). An output irradiance of 500 mW/cm2 was employed under different exposures (6, 12, 24, 48, and 96 s), resulting in H values of 3, 6, 12, 24, and 48 J/cm2. The variability in irradiance among five successive exposures was determined with a laboratory-grade power meter (DAS 2100, Labsphere, N. Sutton, NH), and was found to be less than 2%. During specimen preparation, irradiance was periodically checked with a dental radiometer (Curing Radiometer, model100 P/N 10503, Demetron/Kerr, Danbury, CT). All specimens were stored dry for 48 h at room temperature (228C) prior to analysis or mechanical testing.

type D, Du Pont, Wilmington, DE) pressed by a microscope slide. The glass slide was removed prior to photoactivation, and the tip of the light guide was positioned contacting the mylar strip in a normal angle with the surface of the specimen. The presence of the mylar strip interposed between the light guide and the composite was found to cause a reduction in irradiance of 50 mW/cm2. After photoactivation, specimens were stored as previously described. Infrared spectra of top and bottom surfaces were obtained by coaddition of 16 scans at a resolution of 2 cm1 between 1680 and 1550 cm1. Monomer conversion was calculated based on the changes in the ratios of aliphatic (1640 cm1) to aromatic (1610 cm1) carbon double bonds absorption peaks in the uncured and cured states.25–27 Three replications for each test condition were made. Flexural Strength and Flexural Modulus

FS and FM were measured by three-point bending using a universal testing machine (Instron 5565, Canton, MA) at a crosshead speed of 0.5 mm/min. Ten specimens of each experimental condition were prepared using a stainless steel mold (10 mm 3 2 mm 3 2 mm). Photoactivation and storage were similar to the procedure described for DC. Load was applied on the irradiated surface of the specimen, with 6-mm span between the supports. FS was calculated using the formula: Fs ¼

3LD 2wh2

ð1Þ

where Fs is the FS (in MPa), L is the failure load (in Newtons), D is the distance between the supports (in mm), w is the width, and h is the height of the specimen (both in mm). FM was calculated using data obtained from the initial linear portion of the load 3 displacement curve, according to the formula: Ef ¼

LD3 3 103 4wh3 d

ð2Þ

where Ef is the FM (in GPa), L is the load (in N), D is the distance between the supports (in mm), w is width of the specimen and h is the height of the specimen (both in mm), and d is the displacement of the crosshead (in mm) at load L. Knoop Microhardness

Degree of Conversion

Composite DC was determined by Fourier transform infrared spectroscopy (Digilab FTS-40, Bio-Rad, Cambridge, MA), using an attenuated total reflectance unit (MKII Golden Gate, SPECAC, Smyrna, GA, EUA). Brass rings (6 mm diameter, 2 mm height) were filled with composite and covered with a mylar strip (0.08-mm thick, Mylar

After the three-point bending test, three fragments of each experimental condition were randomly selected for KHN determination. Top (irradiated) and bottom (2 mm depth) surfaces of the specimens were analyzed. On each surface, five indentations (25 g, 10 s dwell time) were performed using a microhardness tester (HMV-2T, Shimadzu Corporation, Kyoto, Japan). Journal of Biomedical Materials Research Part B: Applied Biomaterials DOI 10.1002/jbmb

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DEGREE OF CONVERSION AND PROPERTIES RELATED TO RADIANT EXPOSURE

Statistical Analyses

For all tests, data were analyzed using one-way ANOVA. Tukey test was used for multiple comparisons, with global significance level set at 5%. For DC and KHN, Student’s ttests were performed to compare results obtained from top and bottom surfaces at each level of H. Homogeneity of cure was estimated based on Student’s test results for KHN, as previous studies revealed its higher sensitivity compared to DC.14,28 Finally, regression analyses were performed using either H or DC as independent variables, and KHN, FS, or FM as dependent variables.

RESULTS Averages and standard deviations are shown in Table I. For DC, statistically significant differences among groups were detected for both top and bottom surfaces (p < 0.001 for both). DC/top increased significantly between 3 and 6 J/cm2 and between 12 and 24 J/cm2. On the opposite surface, statistically significant increases in DC were observed among all H levels. Student’s t-tests did not reveal any differences in DC between top and bottom surfaces (p > 0.05). FS increased significantly between each consecutive level of H up to 12 J/cm2 (p > 0.001). The use of 24 J/ cm2 resulted in statistically similar strength to values obtained with both 12 and 48 J/cm2. For FM and KHN/top, statistically significant differences were found among all the levels of H (p < 0.001). For KHN/bottom, only the averages obtained with 3 and 6 J/cm2 were statistically similar (p < 0.001). Student’s t-tests comparing KHN on top and bottom surfaces revealed statistically significant differences for 3 J/cm2 (p < 0.001), 6 J/cm2 (p < 0.05), and 12 J/cm2 (p < 0.001). Figure 1 shows the regression analyses with H as independent variable. In all cases, a strong fit was obtained through exponential functions (R2 values between 0.997 and 0.999). Figure 2 shows the regression analyses between the mechanical properties and DC. On the irradiated surface, an exponential relationship was found between DC and KHN, while at the bottom, the correlation was clearly linear. FS

and FM showed an almost linear increase with DC/bottom up to 55% conversion, reaching a plateau above that. DISCUSSION The hypothesis of the present study was only partially confirmed. In spite of the evident tendency to level off above 24 J/cm2, statistically significant increases in DC/bottom, FM, and KHN (top and bottom) still were observed between 24 and 48 J/cm2. Only FS and DC/top did not increase significantly above 24 J/cm2. These results disagree with previous findings by the same group of researchers where two commercial composites did not show statistically significant increases in DC or FM between 6 and 36 J/cm2, while FS reached a maximum at 12 J/cm2.14 There are two possible explanations for this discrepancy. The first one refers to differences in formulation between commercial materials and the experimental composite tested here. The efficiency of photoactivation can be improved by the choice of and balance between camphorquinone, the tertiary amine, and inhibitor,29 in a way that high DC levels would be reached at a relatively low H. Another explanation concerns the storage of specimens prior to testing. While in the present investigation specimens were stored for 48 h at room temperature (228C), in the previous investigation, storage was for 24 h at 378C. Composite samples with initially lower DC benefit more from storage at high temperatures,30 presenting a more efficient ‘‘dark cure.’’ As a result, it is possible for them to reach DC and mechanical properties similar to those displayed by samples photoactivated under higher radiant exposures.12,30,31 In the present study, storage at room temperature was chosen in order to minimize the influence of temperature in the dark cure period. Notwithstanding, it is licit to suppose that at oral temperature conditions, the effect of H on DC and physical–chemical properties would be less evident than those reported here. The relationship between DC and H evidenced a tendency of DC stabilization at high radiant exposures, which was previously reported.8–10 Under constant irradiance, free radical concentration also increases logarithmically with exposure, becoming nonsignificant above a certain limit,

TABLE I. Average and Standard Deviation of Degree of Conversion (Top and Bottom), Flexural Strength, Flexural Modulus, and Knoop Microhardness (Top and Bottom) of an Experimental Composite Photoactivated with Different Radiant Exposures

Radiant Exposure (J/cm2) 3 6 12 24 48

Degree of Conversion (%) Top 33.1 43.5 48.2 57.0 58.0

(3.7) (2.5) (3.0) (0.7) (1.5)

Flexural Strength (MPa)

Bottom C B B A A

31.3 37.8 44.7 55.5 61.3

(2.2) (2.1) (1.9) (0.8) (2.5)

E D C B A

31.2 (5.9) D 65.8 (14.9) C 116.6 (15.8) B 132.3 (20.8) AB 142.9 (15.8) A

Flexural Modulus (GPa) 0.2 0.8 2.2 2.9 3.1

(0.04) E (0.1) D (0.1) C (0.2) B (0.1) A

Knoop Microhardness Top

Bottom

2.9 (0.1) E* 6.4 (1.1) D* 14.5 (0.6) C* 23.2 (0.9) B 26.1 (0.3) A

1.0 (0.0) D 2.0 (0.4) D 9.1 (0.9) C 19.9 (2.3) B 25.5 (1.5) A

In the same column, similar letters indicate lack of statistical difference ( p > 0.05). Asterisk indicates statistically significant difference between top and bottom (Student’s t-test, p > 0.05).

Journal of Biomedical Materials Research Part B: Applied Biomaterials DOI 10.1002/jbmb

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Figure 1. Regression analyses using radiant exposure as independent variable. Error bars represent 61 SD.

according to findings of a study using electron spin resonance spectrometry.32 Therefore, it is possible that free-radical generation at high H levels did not differ substantially. A second hypothesis, more commonly mentioned in the literature, refers to the decreasing free-radical mobility during polymer reticulation.32 Because free-radical polymerization of dimethacrylates is diffusion-controlled, the high concentration of free radicals generated at increased H levels would lead to a rapid decrease in the mobility of the reaction medium due to formation of more microgel regions, hindering significant increases in DC beyond a certain threshold. Though several studies agree upon the influence of H on mechanical properties, there are many aspects involved in the experimental design, such as composite formulation, photoactivation protocol, specimen storage, and testing procedure that preclude a direct comparison among them. For instance, while some authors reported significant improvements in KHN, flexural and compressive strength between 6 and 39 J/cm2,18,19 others did not find differences in FS and FM between 6 and 36 J/cm2.14 In this investigation, composite behavior varied according to the response evaluated. Differences among groups were less frequent for FS,

in comparison to FM and KHN. Moreover, in contrast to studies that found a positive linear correlation between H and fatigue strength, FS and FM in a commercial composite,23,33 the present results revealed that mechanical properties increased with H according to exponential functions, in tandem with the behavior displayed by DC. Though some authors have not observed a correlation between DC and properties like diametral tensile strength and microhardness,34 others demonstrated that increases in DC corresponded to increases in fracture toughness, wear resistance, FS, FM, and KHN.1,2 The present investigation disclosed a few interesting aspects regarding the relationship between DC and mechanical properties. While KHN, for instance, presented a linear correlation with DC when both were determined on the bottom of the 2-mm specimen (in agreement with previous studies28,35), on the top surface it increased exponentially with DC. Because no statistically significant differences were observed between DC measured on the top and on the bottom of the specimens at each H level, the accentuated increase in KHN observed on the top suggests the existence of a more densely crosslinked polymer structure. Differences in crosslinking density may result from variations in conversion rate in different depths Journal of Biomedical Materials Research Part B: Applied Biomaterials DOI 10.1002/jbmb


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Figure 2. Regression analyses using degree of conversion as independent variable. Flexural strength and modulus were plotted against DC/bottom only.

of the sample.36 As light travels through the composite, attenuation may reduce its irradiance in 70–74% per millimeter,37 due to reflection, absorption, and dispersion phenomena taking place at the surface and as a function of depth.38–40 Therefore, it is possible that a more linear polymer was formed at the bottom of the specimen due to a slower polymerization caused by the fewer number of free radicals generated, in comparison to the irradiated surface. Besides, prolonged exposure times employed in higher H levels may have promoted a more intense temperature raise on the top of the specimen,41–43 allowing more mobility to the reactive species, which may increase crosslinking.30,44–46 Mechanical properties evaluated by flexural testing are influenced by characteristics of the specimens in areas where it is subjected predominantly to tensile stresses.47 In the present study, the 2-mm thick flexural bar was tested in a way that the nonirradiated surface would be the one subjected to more severe tensile stress. For this reason, regressions involving FM and FS used DC/bottom as the independent variable. The results agree with the theory that after polymer network formation reaches a certain threshold, mechanical strength becomes less dependent of DC.3 It must be kept in mind that regression Journal of Biomedical Materials Research Part B: Applied Biomaterials DOI 10.1002/jbmb

analyses used DC measurements performed in specimens with different geometry than those used for flexural and microhardness testing. Therefore, it can be speculated that differences in internal temperature excursion during photocuring may cause conversion of the larger disk-shaped specimens to be somewhat higher than that of the bars used in the flexural test. As observed in this investigation, the use of a radiant exposure of 48 J/cm2 resulted in the highest values found for DC at 2-mm depth, as well as for FM and KHN. However, results for DC/top, FS, and homogeneity of cure (estimated by the comparison of KHN at the top and bottom of the specimen) indicated that the use of radiant exposures higher than 24 J/cm2 is questionable. In fact, it has been shown that composites with very high DC (obtained by photoactivation followed by heat treatment) displayed a reduction in FS due to increased friability.48 Also, as mentioned in the introduction, high energy doses may cause DC to reach levels where polymerization stress increases exponentially, leaving the restoration’s bonded interface at risk.8,13,14,16 Therefore, further studies are necessary to access the real benefit of the use of high radiant exposures in view of physical–chemical and, ultimately, clinical performance of direct restorative composites.

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CONCLUSION DC and mechanical properties increased with radiant exposure. For the experimental composite used in this study, FM, Knoop microhardness (top and bottom), and DC on the nonirradiated surface presented statistically significant increases between 24 and 48 J/cm2. On the other hand, DC on the irradiated surface and FS did not benefit from a radiant exposure above 24 J/cm2. Likewise, a minimum of 24 J/cm2 was necessary in order to obtain homogeneity of cure in 2-mm thick specimens. Authors thank Esschem for donating the monomers, and Degussa and Vigodent for donating the inorganic fillers used in composite preparation.

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