Oral cancer represents one of the most frequently encountered malignancies in dental practice, predominantly affecting anatomical sites in the head and neck region. Its management is guided by factors such as tumour location, staging, histopathological differentiation, and overall patient health status (1). Radiotherapy, either alone or in combination with surgery, is a cornerstone treatment modality. It operates through high-energy ionizing radiation, which targets tumor cells while minimizing collateral damage to surrounding healthy tissues (2).

However, ionizing radiation can interact not only with biological tissues but also with restorative dental materials, potentially altering their physicochemical properties (3). This is of particular concern in patients undergoing cervicofacial radiation, where restorative interventions often become more complex. These patients may experience compromised adhesion of restorative materials, leading to restoration failure and increased risk of secondary caries (4, 5). Prior studies have highlighted reduced bond strength and changes in microhardness of enamel and dentin post-irradiation (6), yet limited evidence exists comparing how modern restorative materials—such as bulk-fill composites, resin-modified glass ionomers (RMGIs), and alkasite-based materials—respond to therapeutic doses of radiation (7).

In restorative dentistry, material longevity and biocompatibility are paramount. A key mechanical property that influences the durability of dental restorations is surface microhardness, which reflects a material's resistance to plastic deformation and wear (8). While traditional amalgam restorations have demonstrated long-term clinical success, their use has declined due to aesthetic limitations and concerns about mercury content. Consequently, contemporary restorative practice increasingly favors tooth-colored materials such as resin composites and glass ionomer cements (9).

Among these, newer classes of materials—such as bulk-fill composites, nano-hybrid composites, and alkasite-based restoratives—have been introduced to enhance aesthetics, simplify clinical protocols, and improve mechanical resilience. Bulk-fill composites, for example, allow for incremental layering and exhibit lower polymerization stress, while maintaining adequate depth of cure (10). Charisma Topaz, a nano-hybrid composite, offers enhanced strength and wear resistance owing to its TCD-matrix structure (11). Cention N, an alkasite-based material, features bioactive fillers that release calcium, fluoride, and hydroxide ions, promoting remineralization under acidic conditions (12).

Despite these advancements, limited data exist on how ionizing radiation—common in cancer therapy—affects the surface properties and structural integrity of such materials. Preliminary studies suggest radiation may degrade the resin matrix, disrupt filler-matrix bonding, and reduce surface hardness, thus potentially shortening clinical lifespan (7, 13).

While conventional composites have been widely used in daily practice, new formulations such as nano-hybrid composites and alkasites offer improvements in filler technology, polymer matrix resilience, and ion release. However, their durability under radiation exposure has not been sufficiently validated. A clearer understanding of material degradation, especially at the microstructural level, is needed to guide clinicians in material selection for patients undergoing head and neck radiation therapy (8).

This study aims to evaluate the effect of therapeutic ionizing radiation on the microhardness and micromorphology of three widely used restorative materials: 3M™ Filtek™ Bulk Fill (bulk-fill composite), Charisma Topaz One (nano-hybrid composite), and Cention N (alkasite). Scanning electron microscopy (SEM) and Vickers microhardness testing were employed to assess surface-level changes following two clinically relevant radiation protocols.

We hypothesized that exposure to therapeutic radiation would lead to a significant decrease in surface microhardness and visible morphological degradation of all tested restorative materials, with differences depending on their formulation and filler content.

This study evaluated the effects of ionizing radiation on the surface microhardness and micromorphology of three contemporary restorative materials. Across all groups, exposure to both standard and hypofractionated radiation protocols led to a measurable reduction in Vickers hardness values and notable changes in surface morphology. These findings confirm that restorative materials can undergo physical degradation under clinically relevant radiotherapy conditions (7). The observed reduction in microhardness post-irradiation may be attributed to chain scission within the resin matrix, oxidative degradation of polymer networks, and debonding at the filler–matrix interface due to differential thermal expansion. Water uptake in irradiated specimens could further plasticize the matrix, contributing to the reduction in hardness (6, 14, 17).

Beyond natural tooth structures, ionizing radiation has also been reported to affect the mechanical and morphological properties of restorative materials. Prior studies have noted degradation in composite resin integrity and changes in their surface topography following therapeutic radiation exposure (7, 18). Our results are consistent with earlier reports showing that ionizing radiation can reduce the mechanical strength of resin-based materials and alter surface texture. Much of this research has been conducted using animal models or outdated materials. The present study sought to address this gap by evaluating the impact of clinically relevant radiation doses on three widely used modern restorative materials: 3M™ Filtek™ Bulk Fill, Charisma Topaz One, and Cention N.

The findings from this study clearly reject the null hypothesis, as exposure to ionizing radiation produced measurable changes in both surface microhardness and micromorphology of the tested materials. Filtek™ Bulk Fill demonstrated the least reduction in hardness values post-irradiation. This may be attributed to its higher filler loading, deeper polymerization capability, and the resilience of its Bis-GMA/UDMA-based resin matrix (10).

While both radiation protocols resulted in a reduction in hardness values, the difference between Protocol A (70 Gy in 35 fractions) and Protocol B (45 Gy in 5 fractions) was not statistically significant (p > 0.05). This suggests that material degradation may depend more on total radiation dose than dose fractionation. Nevertheless, subtle microstructural changes observed under SEM imaging support the hypothesis that the radiation-induced damage is material-specific and likely influenced by compositional factors, such as filler size, resin matrix type, and polymerization efficiency.

Charisma Topaz One and Cention N showed more pronounced degradation, with the latter exhibiting surface voids and disrupted filler-matrix integrity on SEM. This is least in Filtek™ Bulk Fill. The detachment of larger filler particles in bulk-fill resins can be explained by the breakdown of the filler–resin interface, likely exacerbated by polymer matrix embrittlement under radiation (19, 20). Interestingly, the smaller and more uniformly distributed fillers in Charisma Topaz appeared to resist matrix detachment better, suggesting that nano-hybrid composites may offer improved resilience in irradiated environments.

The number of exposed filler particles may be correlated with the Vickers hardness values observed, as more exposed fillers often indicate breakdown of the surrounding matrix and thus reduced resistance to indentation (21). Despite this, all three materials demonstrated acceptable levels of mechanical performance post-irradiation, reinforcing their potential use in restorative procedures for oncology patients.

A notable strength of this study is the use of artificial saliva during the irradiation process, which better simulates the oral environment compared to the distilled water used in earlier studies (13). This approach likely improved the external validity of the findings.

However, certain limitations should be acknowledged. This study is not without limitations. As an ex vivo model, it lacks several oral environment variables, including Salivary enzyme interactions, Temperature fluctuations and thermal cycling and Mechanical fatigue due to mastication. Also, only two radiation protocols were tested, which may not capture the full variability encountered clinically in vivo, dose distribution can vary based on tumor site, tissue heterogeneity, and beam angulation.

Additionally, as SEM analysis in this study was qualitative, future research should incorporate quantitative profilometry or atomic force microscopy for surface roughness assessment. Nanoindentation testing and polymer crosslink density analysis could further elucidate micromechanical changes. Studies comparing fractionated and single-dose regimens are warranted to determine the clinical relevance of laboratory findings.

This ex vivo study demonstrated that exposure to therapeutic doses of ionizing radiation significantly affects the microhardness and surface morphology of restorative dental materials. All three tested materials—Filtek™ Bulk Fill, Charisma Topaz One, and Cention N—underwent measurable degradation, with the bulk-fill composite exhibiting greater resistance to these changes.

The findings suggest that material composition, particularly filler type and resin matrix characteristics, influences the extent of radiation-induced alterations. While these results contribute to our understanding of restorative material behavior in irradiated environments, further in vivo studies are needed to confirm long-term clinical relevance and performance under real-world oral conditions.