Altering of optical and mechanical properties in high-translucent CAD-CAM resin composites during aging

Objectives: This study was aimed to monitor the alteration of the optical and mechanical properties of high-translucent CAD-CAM resin composites (HTRCs) during aging. Corrections for the measured transmitted irradiance are proposed.

Methods: Individual sets (n = 6) of plane-parallel test specimens (0.5-, 2- and 4-mm) of seven HTRCs were prepared. The optical properties (absorbance, transmittance, reflectance, linear absorption coefficient, and transmitted irradiance) were assessed by regular spectrometric testing methodologies and were corrected to account for light reflection. Several edge chipping resistance parameters (edge force at different edge distances, edge chip resistance), as well as mechanical parameters obtained from depth-sensing indentation (Vickers hardness, indentation modulus, creep, elastic and plastic indentation work), were considered. Aging involved storage for two weeks in artificial saliva at 37 °C, followed by thermal aging (10,000 thermocycles, 5 °C-55 °C) and storage in alcohol/water solution, employing non-aged specimens as a reference. Filler size and shape were analyzed by scanning electron microscopy.

Results: Reflectance varied from 11% to 27% and was altered during aging. Linear absorption coefficients of 0.281 mm^-1 to 0.369 mm^-1 were obtained for non-aged specimens, with very low changes during aging. A difference between the true and measured irradiance was identified to decrease exponentially with the specimen's thickness. The impact of HTRC was stronger than the impact of aging.

Conclusions: Reflectance determined by spectrometric analysis was identified as a possible criterion to estimate surface degradation during aging. Corrections for thin and translucent specimens are needed when determining transmitted irradiance through restorations. Depth-sensing indentation was evidenced as the most discriminatory testing methodology for aging, but the results do not correlate with the edge chipping resistance parameters.