Investigation of the influence of Gamma aging on colour stability of dental composites

In dentistry, dental composites (composite resins) are materials used in restorations such as the treatment of tooth deformities and tooth deficiencies. The colour of the composite material to be used should have the most matching appearance with the natural tooth colour of each patient. Therefore the material with the most appropriate shade is preferred in dentistry. Colour changes that may occur in composite materials used over time due to environmental factors and various reasons are studied with some certain accelerated aging techniques. Upon reviewing studies addressing aesthetic concerns, no literature was found regarding the influence of natural radiation, which we are inevitably exposed to, on the colour change of dental materials. The main purpose of this study is to explore how natural gamma radiation affects the colour stability of composite resins commonly used in dentistry. For this purpose, dental composites in shades A2 and A3 from three different manufacturers were selected for examination (Spectra ST HV (Dentsply Sirona, Germany), Clearfil Majesty Esthetic (Kuraray, Japan) and FiltekTM Z250 (3M ESPE, USA)). 120 measurement specimens, in the form of 2mm high and 10mm diameter disks, were prepared from the selected dental composite samples. Thus, 12 measurement groups (N=10) were produced from 6 different materials, which are subject to two different accelerated artificial aging processes (TS, thermal cycle and G, gamma aging processes). VITA Easyshade V (VITA Zahnfabrik, Germany) digital spectrophotometer was used for colour measurements before and after the aging process. Each measurement performed with the spectrophotometer was carried out on a white background in a colour measurement cabinet that eliminated environmental conditions and provided an artificial midday light source (D65: daylight 6500 °K). For each specimen, measurements were taken from 5 points close to the center, the arithmetic means of 5 measurements were recorded as L*, a* and b* colour coordinate data in the CIE (Commission International De L'eclairage) colour system. In the TS aging process, for 1 year of artificial aging, the specimens were subjected to a 1000-cycle thermal cycle in the Dentester device (the hot and cold water chambers of the device were adjusted to temperatures of 5 and 55 °C, respectively, to imitate intra-oral temperature changes). In the G aging process, the specimens were exposed to a 1 mSv dose of gamma radiation corresponding to 1-year aging. Accelerated gamma aging processes of 1, 2 and 5 years were applied to composite samples at Ankara University Nuclear Sciences Institute, 137Cs Calibration Laboratory and colour measurements were performed with a digital spectrophotometer after each process. The change in colour stability of dental composites depending on time and type of aging process was investigated by calculating colour difference (〖∆E〗_ab^*) values. Before starting the aging process, L0*, a0* and b0* colour coordinate values for the samples belonging to 12 groups were measured and recorded as control group (CG) data. After 1, 2 and 5 years of aging, the data of L*, a* and b* colour coordinates obtained from colour measurements and CG data were compared statistically. Since all data groups were dependent, data groups with normal distribution were subjected to the Dependent Groups t-Test, and groups without normal distribution were subjected to the Wilcoxon Signed Ranks Test and analysed with the SPSS 29.0 program. In pairwise comparisons based on CG data, a statistically significant difference was found in 94 of 108 comparisons (p<0.05). In general, it can be said that as a result of both aging processes, the colour coordinates of dental composite samples differ depending on the years. Using the obtained L*, a* and b* colour coordinate values, 〖∆E〗_ab^* colour difference values were calculated for 1, 2 and 5 years of TS and G aging processes. Whether the resulting 〖∆E〗_ab^* groups were normally distributed was assessed using the Shapiro-Wilk normality test. To be able to compare the data in 〖∆E〗_1^*, 〖∆E〗_2^* and 〖∆E〗_5^* groups for TS and G aging processes, the calculated data were statistically analysed in the SPSS 29.0 program using the Dependent Groups t-Test for groups with normal distribution and the Wilcoxon Signed Rank Test for groups with non-normal distribution. For both aging processes, 〖∆E〗_ab^* data groups of 1 and 2 years aging were found to display a significant difference (p<0.05) apart from the Clearfil Majesty Esthetic specimens with A2 shade, FiltekTM Z250 specimens with A2 and A3 shades and Spectra ST HV specimens with A2 shade in TS process, and the specimens of the FiltekTM Z250 composite with A3 shade in G process. For aging processes of 2 and 5 years, significant differences were observed among the 〖∆E〗_ab^* data groups (p<0.05), except for the specimens of Spectra ST HV with A2 shade in TS process. When the 1 and 5-year 〖∆E〗_ab^* data were examined for both aging processes, it was determined that the colour difference values in all related groups exhibited statistically significant differences (p <0.05). Within the limits of this study, it was revealed that the aging processes of TS and G significantly affect the colour stability of dental composites. In the study, it was found that accelerated G aging was approximately 2 times more effective on colour change in the short term (between 1 and 2-year measurements) and about 1.5 times more effective in the long term (between 1 and 5-year measurements) when compared to artificial TS aging. It has been concluded that the gamma aging process, which has not been taken into consideration until now, should be included as one of the accelerated artificial aging methods in colour measurement studies on dental materials.