Influence of Light Curing Energy on Dentin Bond Strength

Purpose: Treatment based on the minimal intervention (MI) concept is now widely performed, and the use of restoration treatment using composite resin (CR) has increased. In the molar regions, CR restoration is widely applied to ClassⅡ cavities. However, in these cavities, a septum, such as a sectional matrix, is attached corresponding to the anatomical morphology of the tooth, simultaneously with the formation of a cavity with a narrow width minimizing tooth grinding for MI, through which light for polymerization is unlikely to reach the gingival side wall in the deepest region of the cavity. In this study, using halogen-type and light-emitting diode (LED) curing units, we measured the tensile bond strength at various irradiation distances and light intensities to investigate the influence of the irradiation energy of the curing units on the bond strength of CR restorations. Methods: A frozen extracted bovine tooth was thawed under running water. The labial side of the crown was cut using a model trimmer to flatten the dentin surface, and then polished with waterproof #600 abrasive paper. A metal mold with a 3-mm inner diameter and 2-mm height was fixed to the adherend surface with double-sided adhesive tape. The irradiators used in this study consisted of a halogen-type curing unit, Curing Light XL3000 (“XL” ; 3M ESPE) and LED-type curing units, Elipar S10 (“S10” ; 3M ESPE) and PENCURE 2000 (“PC” ; J. Morita MFG.). Light irradiation using PC was used in the normal mode (PC-N) and high-power mode (PC-H). Results: When light was irradiated at 60° from the lingual direction to the occlusal surface, the light intensity decreased significantly compared with that at an irradiation angle of 90°. When the irradiation distance was increased from 2 to 7, 12, and 22 mm, the light intensity decreased, and the bond strength also decreased. When the light intensity of the curing units was increased from 100 to 200, 400, and 600 mW/cm2, the bond strength improved. When the light intensity of the curing units was set at 100, 200, 400, and 600 mW/cm2 and the irradiation time was prolonged so as to obtain a uniform amount of light energy, the adhesive strength at the light intensity of 100 and 200 mW/cm2 decreased significantly compared with that of 600 mW/cm2 even though the irradiation time was increased. Conclusion: When the irradiation distance was increased from 2 to 7, 12, and 22 mm, the adhesive strength also decreased, and when the light intensity of the irradia