Effect of the irradiance distribution from light curing units on the local micro-hardness of the surface of dental resins

• Each light curing unit has an individual irradiance distribution.
• The surface hardness map corresponds directly to the irradiance distribution.
• Longer exposure times lift the hardness pattern to higher values.
• Longer exposure times do not lead to more homogeneity of the hardness distribution.

ObjectiveAn inhomogeneous irradiance distribution from a light-curing unit (LCU) can locally cause inhomogeneous curing with locally inadequately cured and/or over-cured areas causing e.g. monomer elution or internal shrinkage stresses, and thus reduce the lifetime of dental resin based composite (RBC) restorations. The aim of the study is to determine both the irradiance distribution of two light curing units (LCUs) and its influence on the local mechanical properties of a RBC.MethodsSpecimens of Arabesk TOP OA2 were irradiated for 5, 20, and 80 s using a Bluephase® 20i LCU in the Low mode (666 mW/cm2), in the Turbo mode (2222 mW/cm2) and a Celalux® 2 (1264 mW/cm2). The degree of conversion (DC) was determined with an ATR-FTIR. The Knoop micro-hardness (average of five specimens) was measured on the specimen surface after 24 h of dark and dry storage at room temperature.ResultsThe irradiance distribution affected the hardness distribution across the surface of the specimens. The hardness distribution corresponded well to the inhomogeneous irradiance distributions of the LCU. The highest reaction rates occurred after approximately 2 s light exposure. A DC of 40% was reached after 3.6 or 5.7 s, depending on the LCU. The inhomogeneous hardness distribution was still evident after 80 s of light exposure.SignificanceThe irradiance distribution from a LCU is reflected in the hardness distribution across the surface. Irradiance level of the LCU and light exposure time do not affect the pattern of the hardness distribution – only the hardness level. In areas of low irradiation this may result in inadequate resin polymerization, poor physical properties, and hence premature failure of the restorations as they are usually much smaller than the investigated specimens. It has to be stressed that inhomogeneous does not necessarily mean poor if in all areas of the restoration enough light intensity is introduced to achieve a high degree of cure.