Head and neck cancer patients receiving radiotherapy are more susceptible to have decay in the restoration margins; therefore, the ideal material to restore their carious teeth should be resistant to erosion as well as mechanical strength and hamper carious lesions’ formation[17]. When dental materials are exposed to ionizing radiation, it causes structural changes by interacting with their surface, which will occur clearly relying on the material’s chemical components. Among these components is the silane coupling agent which is mostly organic silicide (X3SiY) where X may be alkoxy, chlorine or acetoxy groups and Y may be epoxy, vinyl or amino groups. Through hydrolysis, X groups transform to the alkoxy group and create covalent or hydrogen bonding together with the alkoxy group exist on the inorganic filler particles’ surface while Y is reactive groups that attach with organic monomers and therefore enhance adhesion of filler-matrix interface[18]. According to De Amorim et al.[19] silanized flouroaluminosilicate fillers and methacrylate monomers produced a more stable bond between fillers and monomers in resin modified glass ionomer, thus post radiation morphology was conserved.
Concerning resin composite, SEM images showed that a slight disintegration of materials’ surfaces occurred after exposure to ionizing radiation, which enhanced the emergence of subsurface filler particles that were dispersed below the resin-rich surface layer[20]. It was also reported that ionizing radiation depleted the bond between resin matrix and larger fillers. So, larger particles would have been separated from the polymerized resin matrix, revealing smaller particles that were entrapped into the polymerized resin matrix. At the instant that ionizing radiation can cause a degradation of the resin matrix, it can enhance polymerized chains’ linking after photoactivation through molecular excitation and continual polymerization of the non-polymerized surface layer[21]. Polymerized chains can form crosslinks through hydrogen bonds between OH or NH groups and ether or carbonyl groups, as well as among themselves, especially for hydroxyl-hydroxyl groups of monomers[22]. In such a manner, resin composites possess a higher percentage of dimethacrylate monomers that present these groups, such as Bis-GMA, UDMA, and ethoxylated bisphenol A dimethacrylate (Bis-EMA)[19]. Clinically, dental restorations are exposed to radiotherapy in the presence of saliva so in our study, artificial saliva was used as it is the most reliable storage medium.
In the present study, the effect of radiotherapy at a therapeutic dose of 60 Gy on surface roughness and color change of Filtek Z350 nanocomposite and giomer restorative materials was investigated. The surface roughness of dental materials is habitually affected by mechanical and chemical factors, and an increase in roughness is unenviable because it promotes the adhesion of bacteria and the aggregation of chromogenic substances, which leads to an elevated risk of secondary caries and discoloration of the restoration, respectively[19].
Results of the current study documented that the tested null hypothesis, which stated that ionizing radiation, would not alter the surface properties of the tested materials regarding surface roughness and color change was partially rejected. There was no significant difference in surface roughness between both materials at 24 hrs. storage period for both control and irradiated groups as well as irradiated 6 m group. This was in partial agreement with Ibrahim et al.[7] who found non-statistically significant difference in surface roughness between giomer and Filtek Z350 restorative materials when polished according to the manufacturer’s instructions. They stated that a smoother surface finish could be provided by adhesion of smaller size filler particles to resin matrix[23]. In contrast, giomer has a smaller filler particle size than other esthetic restorative materials, which may enhance to its relatively smooth surface.
However, there was significant higher values in non-irradiated giomer group stored for 6 months in comparison to nanocomposite. Both investigated materials contain Bis-GMA and TEGDMA oligomers in their matrices. These types of polymers are known of their high hydrophilic nature that might be related to strong hydrogen bonds formed between their hydroxyl groups and water molecules, which leads to growing tendency for water sorption. In comparison to other oligomers, TEGDMA in composition is more heterogeneous and has higher flexibility. As heterogeneous network increases, the micropores created between polymer clusters become larger and the quantity of absorbed water increases. TEGDMA chains become more prone to swell and fit in with higher amounts of water which explains the ability of both materials to absorb water; however, the presence of urethane dimethacrylate (UDMA) oligomer in the composition of Filtek Z-350 nanocomposite which is more hydrophobic than other mentioned oligomers made it more resisting to water sorption[24].
Nevertheless, the amount of absorbed water may be affected by other factors rather than resin matrix composition. Giomers incorporate surface pre-reacted glass polyacid zones that become a constituent of the giomer filler structure. These zones have the ability to create an osmotic pressure that probably increases water sorption. That is why Beautifil II may be affected by water sorption than Filtek Z-350. Resin matrix solubility goes together with its capability for water sorption, because unreacted components can only be leached-out when water penetrates the material[25]. In addition, Beautifil II higher solubility could be revealed by the glass fluoridation method, the pre-reacted glass-ionomer technology appealed in giomer manufacturing is accountable for formation of a stable phase of GIC in the material’s matrix. The large amount of fluoride release in giomers is due to the more extensive acid-base reaction and hydrogel layer of glass fillers, which consequently increases the amount of solubility. Furthermore, the density of cross-links and degree of conversion within each material might be the reason for the difference in solubility between both resin-based materials. So storage of giomer was found to affect its surface roughness unlike Filtek Z-350 nanocomposite, which might be assigned again to water sorption and dissolution of resin matrix[26].
In this study, surface roughness significantly increased after 6 months storage in a slightly alkaline artificial saliva of pH = 7.2 for all tested groups. This was in accordance with Ibrahim et al.[7] who stated that there was a significant difference in the surface roughness of Filtek Z350XT and Beautifil-Bulk Restorative pre and post thermocycling. It is well known that water can cause deterioration of physical properties of composites through porosity and inter-molecular space by entering polymer chains, so hydrolysis of the polymer chains’ bond takes place. Reduction in the physical properties of the composites is attributable to the separation of the polymer chains by molecules, which do not form principal chemical bond chains. Such process can be controlled by external factors like presence of a catalyst, which is a substance that can hasten the reaction rate at a certain temperature without changing or being used by the reaction itself. The catalyst can also reduce the energy required for the reaction progress[27]. Similarly, the effect of pH of the saliva on the composite surface roughness acts only as a catalyst, so pH does not react with the saliva or composite, but affects reaction rates. The acidity is affected not only from outside, but also from inside through carboxylic acid which can hasten polymer degradation by lowering the pH. The components of TEGDMA or Bis-GMA are degraded causing the breaking of polymer chains into monomer ones, thus debilitate the materials’ physical power[28].
Results also revealed a non-statistically significant difference in roughness between control and gamma irradiated groups except for giomer 6 m groups in both tested materials. This agreed with Turjanski et al.[29] who stated that there was no statistically significant difference between control and irradiated groups. Their results showed that the surface roughness did not change statistically for most materials after the 35-days period, regardless of exposure to radiotherapy. The effect of radiotherapy on the surface roughness of restorative materials is vague, as the available literature provides mixed results. A scanning electron microscopy study by de Amorim et al.[19] demonstrated that radiation could reveal filler particles in subsurface layers of composites and even detached the coating resin layer of glass ionomers, causing surface roughening which hinges on the exposed particles’ size. The potential of radiation to effect on surface roughness seems to depend on the material, as in previous study, an elevation in roughness was noticed only for a resin-modified glass ionomer, while no significant difference was observed for resin composites and conventional glass ionomers[30]. The stability of the surface roughness of resin composites subjected to radiotherapy is supported by a former study that reported an unchanged roughness of a microfilled and packable composite[31]. Antagonistic results were reported by Lima et al.[32] in 2019 who reported a significant increase in surface roughness in composites, glass ionomers, and resin-modified glass ionomers after radiotherapy. In 2020, Ugurlu et al.[33] investigated roughness by atomic force microscopy, and radiation had no effect on a giomer and a conventional glass ionomer; however, a significant increase in surface roughness was observed for ceramic and zirconia-reinforced glass ionomers, which was interpreted by high absorption of radiation by the reinforcing fillers.
Regarding color change, there was statistical and clinically significant differences between control and irradiated groups for both materials in both storage periods. At both intervals, Giomer had significantly higher color change than composite (p < 0.05). For both materials, color change measured after 6 months was significantly higher than 24 hours value (p < 0.05). This was in accordance with Turjanski et al.[29] who stated that Tetric EvoCeram showed higher discoloration in the irradiated group than the control group. In the comparisons among the materials, the highest values were observed for the glass-ionomer-based materials, compared to lower values found for the resin-based materials. Although the dental restorations discoloration never leads to disastrous failure, recent restorative materials are expected to keep a stable aesthetic appearance throughout the employment life of the restoration. Using a visible light spectrophotometer to measure the color change in the L* a* b* color space and calculate the delta E* value as the summed color change is an appropriate method to establish whether the discoloration is detectable to the human eye. The effect of radiotherapy on the color change of dental restorative materials has been rarely investigated; a single former study reported a significant effect for a composite, a resin-modified glass ionomer, and a compomer[34]. The authors of that study hypothesized that the discoloration may have resulted from the formation and entrapment of free radicals or decomposition products of phenolic stabilizers. In spite of the lack of more detailed studies on the color change induced by radiation of methacrylate polymers used in dental composites, discoloration can be anticipated because polymeric materials are known to change color on exposure to radiation[35].
Regarding the color change results between 24 hrs. and 6 months, for control and irradiated specimens, there was no significant difference between both materials (p > 0.05). For both materials, specimens in the control groups had significantly higher discoloration than irradiated specimens (p < 0.05). This was in partial agreement with Turjanski et al.[29] who stated that all the materials tested showed some discoloration with aging. For resin composites, this is due to the post-reaction gradually changing the refractive index of the polymer, while a similar color change also takes place in glass ionomers during lengthy maturation[36]. In their study, the glass ionomer materials were close to or slightly exceeded the perceptibility threshold, designating that their maturation-induced discoloration would be visually detectable. Resin composites had comparatively lower delta E* values, indicating better color stability. Based on this and other studies, we are still distant from the common sense for the best clinical approach, restorative materials, and strategies for patients receiving radiotherapy. Therefore, in vivo studies are needed in order to properly investigate the direct effects of radiation on various esthetic restorative materials.