The effect of specimen preparation on degree of conversion and hardness of dental resin composites

Purpose: Material characterization of dental resin-based composites in research laboratories involves cutting and slicing procedures that may impact the reliability and reproducibility of data. The purpose of this research is to study the effect of wet- and dry-slicing and polishing procedures on the degree of conversion (DC) and surface microhardness (VHN) of resin-based composite (RBC) materials. Materials and Methods: 3M Filtek\(^{TM}\) One bulk-fill restorative material was prepared in white nylon mold 13 mm inner & 24 mm external diameter and 1 - 5 mm thickness increment of 1 mm, were light-activated using the 3M ESPE Elipar\(^{TM}\) Deepcure-S curing unit (2.468 W/cm\(^2\); 20 s; 49.4 J/cm\(^2\); radiant exposure). A total of 72 specimens were prepared and divided into two main groups (Figure 2.1). Lower surfaces of specimens were characterized using surface microhardness measurements (Vickers Hardness; Duramin Struers, ver. 0.04; 9.807 N, 15 s), and by two methods of attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR): 1) centrally located single point measurements over a 3mm diameter; and 2) multiple mapped measurements at 500µm increments over 13mm diameter. Polymer surface was further characterized by surface reflection measurements and scanning electron microscopy (SEM). Data generated were analyzed using one-way ANOVA and post-hoc Turkey comparison (p=0.05). Results/Discussion: The dry-sliced & polished specimens (DSPS) and wet-sliced & polished specimens (WSPS) revealed: for single point DC, DSPS was 40 ± 13 % higher than WSPS for 6 h specimens and DSPS was 41 ± 14 % higher than WSPS for 24 h specimens; for mapped DC, DSPS was 54 ± 6 % higher than WSPS for 6 h specimens and DSPS was 25 ± 11 % higher than WSPS for 24 h specimens; for hardness values, DSPS was 19 ± 3 % higher than WSPS for 6 h specimens and DSPS was 21 ± 5 % higher than WSPS for 24 h specimens. The WSPS and non-sliced & non-polished specimens (NSNP) revealed: for single point DC, WSPS was 33 ± 11 % lower than NSNP for 6 h specimens and WSPS was 50 ± 17 % lower than NSPS for 24 h specimens; for mapped DC, WSPS was 50 ± 17 % lower than NSPS for 6 h specimens and WSPS was 26 ± 9 % lower than 24 h specimens; for hardness values, WSPS was 5 ± 2 % lower than NSPS for 6 h specimens and no significant difference between both 24 h specimen groups. The DSPS and NSNP revealed: for single point DC, DSPS increased by 19 ± 2 % than NSNP for 6 h specimens and DSPS increased by 5 ± 2 % than NSNP for 24 h specimens; for hardness values, DSPS increased by 13 ± 4 % than NSNP for 6 h specimens and DSPS increased by 27 ± 5 % than NSNP for 24 h specimens, and for mapped DC, no significant difference between the groups (p>0.05) for both (6 h & 24 h). The reasons for the above results may be due to heat generated due to friction which resulted to increase diffusion and cross-linking. Wu, et al 2016 & Peng, et al 2019, explained that the interfacial friction heat generation instantaneous increase of temperature to the flow temperature of the composite matrix, which confirmed the observed bonded surface of SEM images of dry sliced specimens and washing away of loosed particles on the surface of the specimens by ionized water, during slicing and polishing processes. Conclusions: Experimental procedures during specimen preparations have a significant effect on study outcomes. This research demonstrated the effect of wet/dry slicing and polishing where dry-sliced specimens exhibited significantly higher DC and hardness compared to wet-sliced & non-sliced specimens. Researchers should note the limitations of their study design and implement appropriate control measures to minimize experimental artifacts. Better designed, controlled experiments that simulate clinical conditions will minimize these experimental artifacts and will improve reliability and reproducibility to improve consistency between both studies and practice, which may lead, ultimately, to a better understanding of RBC cure characteristics, and performance