Effect of Different Mouthwashes on the Hardness and Color Stability of CAD/CAM Materials: An In Vitro Study

Abstract

Objective: To evaluate the effect of different mouthwashes on the microhardness and color stability of two CAD/CAM restorative materials. Methods: A total of 60 rectangular samples (2 mm × 7 mm × 12 mm) were prepared by sectioning two CAD/CAM materials (NICE and Lava Ultimate) and divided into six groups according to material type and immersion solution: distilled water (DW, control), hydrogen peroxide (HP), and povidone-iodine (PVP-I). Microhardness and color parameters (L*, a*, b*) were measured at baseline and after 30 days of immersion, and the ΔE₀₀ color difference was calculated. Data were analyzed using t-tests, two-way and one-way ANOVA, and Tukey’s post hoc test. Results: After 30 days of immersion, both materials showed a significant decrease in microhardness following treatment with HP and PVP-I (NICE from ~823 to ~720 HV and ~709 HV; LAVA from ~197 to ~142 HV and ~113 HV, respectively). Regarding color, ΔE₀₀ values exceeded the clinically acceptable threshold (ΔE₀₀ > 1.8), with no significant differences between the two materials (p > 0.05). Within each material, ΔE₀₀ was significantly higher in both mouthwash groups compared to the control (p < 0.001), but no significant difference was observed between the two antimicrobial solutions (p > 0.05). Conclusion: Within the limitations of this study, the microhardness and color stability of both CAD/CAM restorations may be susceptible to degradation after prolonged exposure to HP and PVP-I mouthwashes.

1. Introduction

The choice of restorative material is of vital importance for clinical success and patient satisfaction. Such materials must not only possess high mechanical strength and wear resistance but also maintain stable color and favorable esthetic performance over time [1]. Since the introduction of computer-aided design and computer-aided manufacturing (CAD/CAM) technology into dentistry in 1980 [2], this method for producing restorations has gradually become a widely used option for dental restorations due to its high strength, excellent esthetic properties, and ease of processing—particularly when combined with intraoral scanning technology, which significantly increases manufacturing efficiency and improves the patient’s treatment experience [3].

The main types of restorative materials used in CAD/CAM technology are resin blocks and glass–ceramic materials [4,5]. Resin blocks are pre-polymerized at high temperature and pressure [6], providing better mechanical and polymerization properties than conventional composites. Glass–ceramic materials must undergo special heat treatment to achieve crystallization, forming a homogeneous crystalline network structure within the material [5]. All materials are subject to various internal and external factors that can cause changes in their mechanical and esthetic properties [7].

Various systemic diseases require the use of antimicrobial mouth rinses or antiseptic solutions as part of oral infection prevention and control. COVID-19 is an infectious disease caused by the SARS-CoV-2 virus, which spreads primarily through direct contact or droplet transmission from infected individuals [8]. Because of this, the use of mouthwashes was recommended to reduce the risk of contagion, although to date there is no scientific evidence of its effectiveness. In daily clinical practice, many professionals use pre-procedural mouth rinses containing formulations such as chlorhexidine (CHX), hydrogen peroxide (HP), or povidone-iodine (PVP-I), among others [9,10]. While the effect of chlorhexidine is already well studied, the other two mouthwashes have not yet been sufficiently investigated and are therefore the subject of this experimental study. These acidic or oxidizing substances can have certain effects on the properties of restorative materials [11]—including hardness or roughness—by accelerating degradation of the surface and internal structure of the material [4,12], flexural strength [13], or color stability [14]. Some of these changes may weaken the bond strength between the material and dental tissue, thereby reducing the restoration’s longevity [15].

Given their widespread use in clinical practice, it is important to investigate the effects of different substances on the properties of these CAD/CAM materials. The general aim of this study is to evaluate the effect of immersion in two antiseptic solutions for 30 days on the color and microhardness of CAD/CAM resin and ceramic blocks.

The null hypothesis proposed in this study is that the two antiseptics will have no effect on the hardness and color stability of either CAD/CAM material.

2. Materials and Methods

2.1. Sample Size Calculation

The study design followed a 2 × 3 factorial model including two CAD/CAM materials (Lava Ultimate and NICE) and three solutions (distilled water, hydrogen peroxide, and povidone-iodine), for a total of six experimental groups. To estimate the required sample size, the significance level was set at α = 0.05 with a statistical power of 0.80. The effect size was set at f = 0.25. A sample size of nine specimens per group was obtained to detect differences between groups in terms of color or hardness. Considering possible interactions and result stability, ten specimens per group were used.

2.2. Materials Used and Specimen Preparation

Two CAD/CAM materials were selected. NICE blocks are manufactured from glass–ceramic materials. LAVA blocks are resin blocks pre-polymerized at high temperature and pressure [6], which enhances the mechanical and polymerization properties of the resin compared to conventional composites. The composition of both types of blocks is detailed in

Table 1. Data regarding the tested materials and mouthwashes.

Material/Abbreviation/pH	Manufacturer	Composition
N!ce® (NICE)	Straumann (Basel, Switzerland)	SiO2 64–70%, Li2O 10.5–12.5%, Al2O3 10.5–11.5%, Na2O 1–3%, K2O 0–3%, P2O5 3–8%, ZrO2 0–0.5%, CaO 1–2%
Lava Ultimate (LAVA)	3M ESPE (St. Paul, MN, USA)	20% polymeric matrix (Bis-GMA, UDMA, Bis-EMA, and TEGDMA), 80% nanoceramic fillers (20 nm silica, 4–11 μm zirconia, and infiltrated nanoclusters of both).
Hydrogen peroxide (HP), (pH = 3)	Peroxfarma (Barcelona, Spain)	3% hydrogen peroxide and purified water
Povidone-iodine (PVP-I), (pH = 3.67)	NORMON (Madrid, Spain)	7% w/v povidone-iodine, disodium phosphate, and purified water
Distilled water (Control), (pH = 7)	Bosque verde (Cheste, Valencia, Spain)	Distilled water

. Basic mecanical properties of the CAD/CAM materials are detailes in

Table 2. Basic mechanical properties of the CAD/CAM materials evaluated in this study [16,17].

Material	Young’s Modulus (GPa)	Tensile Strength (MPa)	Compressive Strength (MPa)	Flexural Strength (MPa)
NICE	60–70	90–110	900–1200	350 ± 50
Lava Ultimate	12–14	100–120	350–400	204

.

Samples measuring 2 × 7 × 12 mm were sectioned from the CAD/CAM blocks using a precision saw (Struers, Copenhagen, Denmark) equipped with a water-cooling system, yielding a total of 60 specimens (n = 30 NICE and n = 30 LAVA). Each specimen was polished using silicon carbide abrasive papers (Buehler, Lake Bluff, IL, USA) of varying grit sizes (P600 to P1200) to obtain a homogeneous surface. The specimens of each material were randomly divided into three subgroups (n = 10) according to the immersion solution, as shown in Figure 1. CAD/CAM blocks used in the study are show in Figure 2.

2.3. Immersion in Different Solutions

Three solutions (HP, PVP-I and DW-control-) were selected as immersion media. A total period of 30 days was established, during which each sample was immersed twice a day, for one minute each time, in 25 mL of the corresponding solution [10]. To ensure the stability and pH of the immersion solution, these were renewed daily. All samples were kept at 37 °C during the experiment. The characteristics of the solutions are summarized in Table 1.

2.4. Vickers Hardness Test

The Vickers surface hardness of each sample was measured using a microhardness tester (HMV-G20S, Shimadzu Corp., Kyoto, Japan) equipped with a Vickers diamond indenter. A load of 9.807 N was applied for 10 s. Measurements were taken for each sample under baseline conditions and 30 days after daily immersion in the different solutions under study. Measurements were taken at three points in the central area of the sample, using the average value of the three measurements as the final value for that sample for analysis.

2.4.1. Color Measurement

Chromatic coordinates in the CIELab color space—lightness (L*), red/green axis (a*), and yellow/blue axis (b*)—were recorded for all specimens at baseline and after 30 days of immersion, using a Vita EasyShade spectrophotometer (Vita, Bad Säckingen, Germany) under standardized lighting conditions. Prior to each measurement, the spectrophotometer was calibrated according to the manufacturer’s instructions. For each specimen, three repeated measurements were taken from the central area, and the average value was recorded. Color change (ΔE₀₀) was calculated using the CIEDE2000 formula.

$$\mathit{\Delta }{E}_{00}=\sqrt{{\left({\displaystyle \frac{\mathit{\Delta }L}{k_L·S_L}}\right)}^2+{\left({\displaystyle \frac{\mathit{\Delta }{C}^{\prime }}{k_C·S_C}}\right)}^2+{\left({\displaystyle \frac{\mathit{\Delta }{H}^{\prime }}{k_H·S_H}}\right)}^2+\left(R_T\left({\displaystyle \frac{\mathit{\Delta }{C}^{\prime }}{k_C·S_C}}\right)\left({\displaystyle \frac{\mathit{\Delta }{H}^{\prime }}{k_H·S_H}}\right)\right)}$$

The perception threshold was set at ΔE₀₀ = 0.8 [6,11], and the threshold of acceptability was set at ΔE₀₀ = 1.8 [5,10].

2.4.2. Statistical Analysis

Statistical analysis was performed using IBM SPSS Statistics 29 (New York, NY, USA), and the normality of all data was verified using the Shapiro–Wilk test. The comparison of material hardness was analyzed using independent samples t-tests, while paired samples t-tests were used to evaluate changes in the hardness of the same material before and after treatment with different solutions. The effects of material type, immersion solution, and their interactions on color change (ΔE₀₀) were analyzed using two-way analysis of variance (ANOVA) and multiple post hoc comparisons using Tukey’s HSD tests. In addition, paired sample t-tests were used to analyze changes in chromaticity parameters (L*, a*, b*) before and after treatment for each material, in order to determine the specific coordinates of color change. The significance level for all statistical analyses was set at p < 0.05.

3. Results

3.1. Microhardness Analysis

The baseline hardness values for the NICE group were significantly higher than those for the LAVA group, both initially and after 30 days of immersion in the different solutions (p < 0.001). The two materials were statistically different at all experimental time points and under all treatment conditions (Figure 3).

Regarding NICE, a significant decrease in hardness was observed in all treatment groups compared with the baseline value (822.89 ± 35.61 VHN), with the HP (pH: 3) group showing a value of 720.00 ± 43.46 VHN (p < 0.001), the PVP-I (pH: 3,67) group 709.22 ± 45.15 VHN (p < 0.001), and the control group 791.67 ± 11.19 VHN (p < 0.05). The HP (720.00 ± 43.46 VHN) and PVP-I (709.22 ± 45.15 VHN) groups also showed a significant reduction compared with the control group (791.67 ± 11.19 VHN) (p < 0.001), with no significant differences between the two antiseptics (p > 0.05), as shown in Figure 3. Conversely, LAVA exhibited significant differences in all treatment groups compared with baseline values (197.22 ± 9.04 VHN), with hardness values of 174.56 ± 13.65 VHN, 141.88 ± 20.88 VHN, and 112.72 ± 12.99 VHN after immersion in distilled water, HP, and PVP-I, respectively (p < 0.001).

3.2. Color Analysis

Paired-samples t-tests were performed to analyze the effect of the three solutions on the color parameters of the two CAD/CAM materials (NICE and LAVA) before and after treatment, for L*, a*, and b*.

Table 3. Mean difference (± SD) of chromatic coordinates between baseline and 30 days immersion in the study solutions.

Material	Solution	ΔL (Mean± SD)	Δa (Mean ± SD)	Δb (Mean ± SD)
NICE	Distilled water	0.438 ± 1.104	−0.275 ± 0.266	1.300 ± 0.540
	Hydrogen peroxide	−1.975 ± 1.308	−0.138 ± 0.213	2.363 ± 0.560
	Povidone-iodine	−0.950 ± 2.100	−0.550 ± 0.245	1.700 ± 1.155
LAVA	Distilled water	−1.000 ± 0.484	−0.375 ± 0.167	0.575 ± 0.871
	Hydrogen peroxide	−0.963 ± 1.687	−0.963 ± 0.407	1.225 ± 2.204
	Povidone-odine	−1.000 ± 0.703	−1.025 ± 0.255	2.525 ± 0.341

Bold values indicate significant differences between pre-and post-treatment values for each material (p < 0.05).

presents the values of the increments for each parameter, calculated as the difference between the post-treatment value and the pre-treatment value. Lightness decreased in all treatment groups and materials, except in the NICE group after immersion in distilled water. The a* parameter (red–green axis) shifted toward green, while the b* parameter (yellow–blue axis) shifted toward yellow.

These variations allowed calculation of the overall perceived color change using the ΔE₀₀ (CIEDE2000) formula. CAD/CAM materials exhibited different degrees of color change after 30 days of immersion in the three solutions, with ΔE₀₀ values above the perceptibility threshold (ΔE₀₀ = 0.8) in all experimental groups. For both NICE and LAVA, immersion in HP and PVP-I produced ΔE₀₀ values exceeding the clinical acceptability threshold (ΔE₀₀ = 1.8) (Figure 4).

Significant differences were found between the control group (DW) and both HP and PVP-I groups (p < 0.001), but not between the two antiseptic solutions (p > 0.05). In the NICE group, HP produced the highest ΔE₀₀ (2.10 ± 0.34), followed by PVP-I (1.90 ± 0.32), both significantly different from the control (1.04 ± 0.40) (p < 0.05), with no difference between the two antiseptics (p > 0.05). In the LAVA group, the greatest color change was also observed in the HP group (2.15 ± 0.30), followed by PVP-I (1.88 ± 0.29), both significantly different from the control (0.97 ± 0.27; p < 0.001), but not from each other (p > 0.05).

4. Discussion

The null hypothesis proposed in this study was rejected, as both the microhardness and the color of the two types of CAD/CAM blocks changed after immersion in the antiseptic solutions tested.

The better the mechanical properties of a restorative material, the lower the likelihood of defects that could accumulate pigmentation and biofilm [16]. The results obtained in the present study are consistent with previously published studies indicating that CAD/CAM glass ceramics have higher microhardness than CAD/CAM resin composites [17].

Regarding the effect of different solutions on the hardness of CAD/CAM blocks, a previous study evaluated water absorption and solubility of eight materials (including the two used in this study) after eight months of storage in distilled water and artificial saliva. All materials exhibited significant changes in water absorption and solubility in distilled water. Although that study did not directly measure hardness, it observed structural changes in the polymeric matrix that could lead to reduced properties, indirectly supporting the idea that CAD/CAM material hardness decreases in a distilled water environment [18], consistent with our findings.

pH is considered an important factor affecting material stability. The pH values of the solutions used in this study were 3 for HP and 3.67 for PVP-I, both markedly acidic. The literature describes that exposure to acidic media negatively affects the polymerization network of resin composites, leading to structural degradation and reduced mechanical properties [19]. For CAD/CAM ceramic materials subjected to acid immersion, a significant reduction in microhardness has been observed in citric acid solutions, with the mechanism related to the dissolution of oxide ions from the glass-phase structure in acidic environments, altering the mechanical stability of the ceramic matrix [20]. Hydrogen peroxide (H₂O₂) undergoes spontaneous or catalyzed decomposition, releasing reactive oxygen species according to: 2H₂O₂ → 2H₂O + O₂. This process generates hydroxyl radicals (•OH) and other reactive species that can attack the polymeric network of resin-based materials [21], cleaving C=C bonds in the methacrylate matrix and leading to chain scission and softening [22]. In the case of povidone-iodine, the release of molecular iodine (I₂) and triiodide (I₃⁻) ions can cause oxidative interactions with organic moieties at the material’s surface: PVP–I ⇌ PVP + I2 + I3−. These iodine species may contribute both to degradation of the resinous phase and to extrinsic staining, as iodine complexes can adsorb and penetrate into superficial defects. For glass–ceramic materials such as NICE, immersion in acidic solutions promotes partial leaching of alkali ions from the silicate network, weakening Si–O–Si bonds and resulting in superficial softening and altered optical properties [23].

In the present study, a reduction in microhardness was observed for both materials after immersion in all tested solutions. For LAVA, immersion in PVP-I reduced hardness more significantly than HP, whereas for NICE blocks, both solutions had a similar impact on microhardness. This phenomenon may be closely related to the structural properties of NICE, whose main component, SiO₂, has a dense structure dominated by inorganic glassy phases, conferring good chemical resistance so that external oxidants primarily affect only the superficial layer [24]. Some authors report that low concentrations of HP have limited effect on the microhardness of ceramic materials [25], whereas others, using energy-dispersive X-ray spectroscopy (EDX), report that HP concentrations around 6% can decrease the SiO₂ content on the surface of feldspathic porcelains, which is associated with reduced hardness [26].

The chemical mechanisms of PVP-I and HP differ in their effect on restorative materials. The former produces sustained oxidation through the release of iodide ions [27], while the latter triggers rapid oxidation Via free radical generation [28]. It is possible that the depth of action of both solutions on NICE is limited by its dense structure, resulting in a similar degree of superficial structural change and mechanical property degradation after 30 days of immersion. Notably, there is a lack of information on the effect of PVP-I on the mechanical properties of glass–ceramic CAD/CAM materials, so the present results provide preliminary insight. Moreover, given that both solutions are acidic, their low pH may cause interference with the oxide-bonding structure in glass materials, acting as a common factor influencing hardness degradation.

For LAVA blocks, the PVP-I solution caused greater hardness loss than HP. This suggests that the effect of PVP-I on resin-based materials may be more severe. Both HP and PVP-I have been reported to penetrate the polymeric network, affecting hardness, roughness, and color change in resin composites [29]. Literature also suggests that peroxides cause separation of resin composite polymer chains and weakening of their double bonds, leading to surface and subsurface softening, which impacts the durability and clinical success of restorations [30]. Although evidence on the effect of PVP-I on resin-based CAD/CAM materials is still limited, some authors suggest it may produce a greater negative effect on material properties than HP [31], consistent with our findings.

Regarding color change, both antiseptic solutions significantly worsened the color of the two types of CAD/CAM blocks compared with the control group. Both materials contain glassy phases, and it has been reported that glass-phase matrices are more susceptible to ionic solubilization and surface structure breakdown in acidic environments, which, in addition to altering mechanical properties, also affects the optical properties of materials [32].

HP induces redox reactions and free radical release, leading to matrix structure alteration, increased surface roughness, and potential color change [33]. Although the exact mechanism of PVP-I’s action is not fully understood, it has been suggested that pigment particles from its components may adsorb onto the material surface, affecting its color stability. The potential for color change may also be related to the higher content of colorant components and the relatively high viscosity of the solution itself [34]. These factors are consistent with the present findings.

CIEDE2000 analysis showed ΔE₀₀ values above the clinical acceptability threshold (ΔE₀₀ = 1.8) [35] after immersion in both antiseptic solutions, indicating that the color change would be considered clinically unacceptable, particularly in esthetic areas.

The results of the study indicate that when treating patients with restorations of this type, we must carefully assess whether to use these solutions. HP solutions are preferable if the restorations are CAD/CAM resin composites. In addition to the chlorhexidine and povidone-iodine solutions evaluated in this study, the literature describes other alternatives such as rinses based on essential oils, cetylpyridinium chloride or octenidine, which have shown significant antimicrobial activity. Future research could explore in greater detail the impact of these formulations on the stability and optical properties of restorative materials [36].

The oral environment is dynamic, with factors such as salivary enzymes, dental plaque, diet, temperature changes, and mechanical abrasion from toothbrushing or mastication influencing the behavior of CAD/CAM materials [37]. Therefore, although the results of this study are relevant under experimental conditions, further studies that approximate real intraoral conditions are needed. Designing future studies with longer observation periods, exposing materials to conditions simulating clinical use, and conducting long-term clinical trials are crucial to verify the clinical relevance of these In Vitro observations. Furthermore, regarding surface roughness, it is important to note that a specific analysis of this parameter was not included in this study. Although CAD/CAM blocks exhibit a highly homogeneous microstructure, future research could complement the findings of this study by using specific surface roughness characterization techniques (profilometry or AFM) to further investigate this relationship [38,39].

5. Conclusions

Both HP and PVP-I reduced the surface microhardness of CAD/CAM blocks, regardless of their manufacturing material. Both materials exhibited discoloration upon immersion in the evaluated antiseptics, with ΔE₀₀ values above the 50:50 clinical acceptability threshold. These findings indicate that prolonged exposure to oxidizing mouthwashes may compromise the properties of CAD/CAM restorations, regardless of their composition. From a clinical perspective, these results highlight the need for careful consideration when prescribing long-term antiseptic rinses for patients with extensive CAD/CAM restorations in esthetic areas, as both functional durability and visual appearance may be affected. Future studies should expand the analysis by incorporating surface roughness measurements, high-resolution imaging techniques, and longer observation periods under conditions simulating the oral environment.