STOMATOLOGY EDU JOURNAL 2017, Volume 4, Issue 3 SEJ_3-2017_Online - Page 19

further progress of the polymerization process, the organic matrix becomes a gelatinous state, and the reaction rate decreases (glass effect). 15 The variation of DC during irradiation of the FRC was well described by the superposition of two exponential functions. The first exponential function, characterizing the gel phase, is described by the parameters “a” and “b”, while the second exponential function is characterizing the glass phase and is described by the parameters “c” and “d”. The experimental data reveal that parameter “a” decreases with increased specimen thickness, but only in increments thicker than 2 mm, while parameter “b” decreases with increased specimen thickness already starting from thin layers. Both effects demonstrate that the decrease of the C-C double bonds in the gel phase is slower in thicker layers compared to the top, due to the exponential decrease (Lambert’s Law) of light transmission with the composite’s thickness. 16 It has been previously shown that merely 24% ÷ 44% of the incident blue light and 9 ÷ 14% of the incident violet light of a LCU is transmitted through 2 mm commercially available bulk-fill resin composite increments. These values are yet consistently reduced in 4 mm increments (9 ÷ 24% and 3 ÷ 9%, respectively) 7 and may further change to the detriment of curing quality when LCUs with low irradiances are used or, as usual in clinical practice, when the LCU cannot be applied perpendicularly and directly on the restoration surface. While the effect of light attenuation was not directly reflected in DC, as evidenced by the statistically similar DC values measured in 2 and 4 mm increments, the polymerization kinetic describes a slower polymerization (lower parameter a and b) as well as a lower maximal rate of polymerization (Rate max = 20.0÷22.1 and 13.5÷15.7, respectively). The lower amount of photons reaching 4-mm layers compared to 2-mm will activate less efficiently the photo-initiator, inducing less nuclei of polymerization and therefore longer polymer chains and a lower cross-linking. This can have as a result a lower modulus of elasticity, although the amount of C-C double bond conversion (DC) is similar. This effect might be in accordance with data presented by Omran et al. by evaluating the bond strength of various resin composites to dentin, revealing that EverX Posterior can be safely applied in bulks of 4-mm increments, similarly to other analysed bulk-fill composites, but its performance was better in 2-mm thick increments. 17 In addition to that, parameters “c” and “d” describing the glass phase reveal that “c” is not altered up to 4-mm thick increments, while “d” has a very low value and is of less relevance. The low variation of parameters “c” and “d” demonstrates that the reaction kinetic is less thickness-dependent in the glass phase compared to the gel phase. Since the analyzed material is indicated to be used in large posterior cavities in optional larger incremental thickness compared to conventional resin-composites layered in 2 mm increments, a direct comparison with the material category of high-viscosity bulk-fill resin-composites seems pertinent. This comparison is possible owing to identical specimen geometry, test type and test parameters as well as specimen storage conditions. Accordingly, EverX Posterior may be ranged for the flexural strength (128.30 ± 8.38) MPa in the middle of the above specified material category, characterised by values varying among 99.9±10.7 MPa (Admira Fusion x-tra) and 142.8±12.9 MPa (SonicFill). Similar considerations apply to the Vickers hardness (92.00 ± 15.86 N/ mm²) when compared to the high-viscosity bulk- fill resin-composite category, delimited by the values 77.1±5.6 N/mm² (Admira Fusion x-tra) and 133.5±32.0 N/mm² (X-tra Fil). The modulus of elasticity measured in either macro and micro scale was rather interrelated to the upper values of the material category (8.38± 0.87) GPa vs 4.5±0.8 (Tetric EvoCeram Bulk Fill) to 9.5±0.6 (X-tra Fil) and for Y HU (17.82±1.82) GPa vs 13.4±0.8 GPa (Tetric EvoCeram Bulk Fill) to 22.2±1.7 GPa (X-tra Fil). 9 Polymerisation kinetics in a fibre reinforced resin-based composite 5. Conclusions In the present study and considering the tested FRC, the polymerization kinetic allowed identifying a slower start of the polymerization process depending on material’s depth. This was associated with a lower maximal rate of C-C double bond conversion, although DC values recorded in 2 and 4 mm depth were statistically similar. There is, however, evidence that the quality of curing at 4-mm depth is lower compared to the top of the material. The null hypothesis must therefore be rejected. The analyzed FRC revealed mechanical properties situated within the range of the high viscosity bulk- fill resin-composites category, with a high modulus of elasticity (upper range of the mentioned material categories. Acknowledgement The author kindly acknowledges the company GC for supporting ca. 20% of the costs involved in the presented study. References 1. 2. Shouha P, Swain M, Ellakwa A. The effect of fiber aspect ratio and volume loading on the flexural properties of flowable dental composite. Dent Mater. 2014;30(11):1234-1244. doi: 10.1016/j.dental.2014.08.363. [Full text links] [PubMed] Vallittu PK. High-aspect ratio fillers: fiber-reinforced composites and their anisotropic properties. Dent Mater. 2015;31(1):1-7. doi: 10.1016/j.dental.2014.07.009. [Full text links] [PubMed] Stomatology Edu Journal 3. 4. van Dijken JW, Sunnegårdh-Grönberg K. Fiber-reinforced packable resin composites in Class II cavities. J Dent. 2006;34(10):763-769. doi: 10.1016/j.jdent.2006.02.003. [Full text links] [PubMed] Ellakwa A, Shortall A, Marquis P. Influence of fibre position on the flexural properties and strain energy of a fibre- reinforced composite. J Oral Rehabil. 2003;30(7):679-682. doi: 10.1046/j.1365-2842.2003.01121.x [Full text links] 169