Influence of Multiple Peak Light-emitting-diode Curing Unit Beam Homogenization Tips on Microhardness of Resin Composites

Abstract

SUMMARY This study evaluated the effect of light curing unit (LCU) guide type (regular or homogenizing) on top and bottom microhardness of conventional and bulk-fill resin-based composites (RBCs). A polywave light-emitting-diode (LED) LCU (Bluephase Style, Ivoclar Vivadent AG) was used with two different light guides: a regular tip (RT, 935 mW/cm2 emittance) and a homogenizer tip (HT, 851 mW/cm2 emittance). Two conventional RBCs (Herculite Ultra [HER], Kerr Corp; Tetric EvoCeram [TEC], Ivoclar Vivadent AG) and two bulk-fill RBCs (SonicFill [SOF], Kerr Corp; Tetric EvoCeram Bulk Fill [TBF], Ivoclar Vivadent AG) were tested. Disc-shaped samples (10 mm Ø), 2-mm thick for conventional composites and 4-mm thick for bulk-fill composites were prepared. Samples were light cured according to manufacturer-recommended times. Knoop microhardness values (KHN) were obtained on the top and bottom surfaces of each specimen at locations correlated with the output of the three LED chips emitting blue (456 nm) or violet light (409 nm). Beam profile analysis using both light guides was also performed. Microhardness of each composite was analyzed using three-way analysis of variance and Tukey honestly significant difference post hoc test (α=0.05). Beam profile images showed better light distribution across the surface of the HT light guide. Use of the HT decreased KHN of HER at the locations of the blue LED chips at bottom of the sample but had no effect on the top surface. For TEC, use of HT increased KHN of all three LED locations at the top surface. Use of the HT increased KHN of SOF at locations corresponding to one of the blue and the violet LED chips at the bottom surface. For TBF, HT increased KHN at all top surface locations. All RBCs showed higher mean KHN at the top compared with the bottom surfaces. In general, all composites presented a higher KHN at the blue LED areas regardless of the surface or the tip used. Results suggest that the homogenizer light guide resulted in significantly increased microhardness at the top, in composite resins containing alternative photoinitiators; however, that effect was not observed at the bottom surfaces.

Flexural strength and microhardness of bulk‐fill restorative materials

“…5,6 To overcome those problems, manufacturers developed bulk-fill materials with flowable or highviscosity consistencies. 7 These materials present reduced polymerization shrinkage stress compared to conventional RBCs, 3,5,8,9 and increased depths of cure [10][11][12][13] and are indicated for use in a single increment of 4 mm thick or more, without diminishing their physical properties or the quality of restored teeth. 14 Clinically, a reduction in operative time has been proposed as a positive factor for selecting bulk-fill products; 10,15 however, for light cured bulk-fill RBCs, studies report insufficient polymerization at high depths, that might compromise the clinical success of the restorations as a result of increased cytotoxicity, 16 susceptibility to marginal defects, [16][17][18] and reduced hardness.…”

The light-curing unit: An essential piece of dental equipment

“…To better activate these alternative photoinitiators, some LED curing lights (Figure 2c,d) now include additional LED emitters that produce additional light in the violet range of wavelengths 2,20,21,32,36 . Figure 2 (c,d) illustrates that the number and location of these violet and blue light emission peaks can vary between manufacturers, as does the relative contribution of each wavelength peak to the total radiant power output from the LCU.…”

“…However, unless the LCU is carefully designed, the addition of several different wavelength LED emitters in the LCU can negatively affect the total amount of blue light present and the overall uniformity of the emitted light beam. This will then change the spectral irradiance received across the resin surface, which may then produce an uneven polymerisation within the RBC 2,21,32,36,49,50 .…”

“…irradiance from the same LCU is viewed using a beam profiler camera through narrow bandpass filters that only allow either violet (400 AE 5 nm) or blue (460 AE 5 nm) wavelengths of light through, the spectral uniformity of the light emitted from the LCU in the violet and blue regions becomes quantifiable 33,36,49 . Thus, dentists should look for a good optical design in the LCU that can homogenise the light so that both the irradiance and the emission spectrum are uniformly distributed across the light tip, that is there are no 'hot spots'.…”

Acompanhamento de três anos de restaurações do tipo bulk-fill flow em dentes posteriores: ensaio clínico controlado randomizado duplo-cego

“…Some recent randomized clinical trials showed that restorations with paste-like bulk-fill composite resins also presented satisfactory results over time 35,36 . In terms of clinical procedures, caution with the light-curing unit is extremely important, since evidences showed that stable irradiance, homogeneity of light beam and wavelength components may influence the degree of conversion, depth of cure and wear of bulk-fill composite resins depending on the type and brand of each bulk-fill material [37][38][39][40] . Indeed, a disadvantage of restoring posterior teeth with bulk-fill composite resin could be that the susceptibility to variance in the irradiance of the light-curing units was shown to be material dependent 21 .…”