Effect of 32% Hydrogen Peroxide Bleaching on Surface Properties and Staining Susceptibility of CAD/CAM Hybrid and Feldspathic Ceramics Under Simultaneous Thermocycling–Staining Challenge

Abstract

Objectives

Resistance to post-bleaching staining and preservation of surface properties are critical to long-term durability of restorations. This study evaluated the effect of 32% hydrogen peroxide (H₂O₂) in-office bleaching gel on surface roughness (Ra), microhardness, and staining susceptibility of three hybrid computer-aided design/computer-aided manufacturing (CAD/CAM) materials compared with a feldspathic ceramic, subjected to a combined thermocycling–staining protocol.

Materials and Methods

Slice specimens (n = 14/material) were prepared from hybrid ceramic materials: Vita Enamic (VE); Lava Ultimate (LU); Shofu Block HC (SB); and from a feldspathic ceramic material, VitaBlocs Mark II (VB), which served as the control. Seven slices per material underwent CIEDE2000 color difference analysis (ΔE₀₀) after simultaneous staining and thermocycling (1,000 cycles, 5–55°C), with or without a prior bleaching procedure using 32% H₂O₂ gel for three 15-minute cycles. The remaining slices were evaluated for surface roughness (Ra) and Vickers microhardness (VH) before and after bleaching with 32% H₂O₂.

Statistical Analysis

Data were analyzed using two-way ANOVA and the Tukey test (α = 0.05).

Results

The results showed that bleaching produced no significant changes in ΔE₀₀, Ra, or Vickers hardness number (VHN) for VE, LU, or SB (P > 0.05). VB alone showed a significant reduction in hardness (454 → 323 HV; p < 0.001). Post-staining ΔE₀₀ values remained below the 1.8 clinical acceptability threshold for LU and SB but slightly exceeded it for the non-bleached VE group (≤2.2) and were high for VB (>8). Regardless of bleaching, VB exhibited the greatest color change, whereas SB had the lowest hardness. Ra values for SB and VB were above the critical threshold for bacterial adhesion (0.02 µm).

Conclusions

Although a single session of 32% H₂O₂ in-office bleaching significantly decreases the hardness of unglazed feldspathic ceramic, it does not compromise the surface integrity or color stability of hybrid CAD/CAM materials. Therefore, the tested hybrid materials can be safely subjected to 32% H₂O₂-based in-office bleaching without requiring surface pretreatment or postoperative replacement due to material degradation. However, restorative materials should be selected based on a comprehensive assessment of esthetic and mechanical performance to ensure long-term clinical success.

Keywords

CAD/CAM - hybrid ceramic - microhardness - color stability - staining susceptibility - surface roughness - in-office bleaching - hydrogen peroxide

Introduction

Over the past two decades, the growing demand for esthetic dentistry has made eliminating both extrinsic and intrinsic tooth discoloration a routine clinical request as patients increasingly desire a brighter, whiter smile.[1] Among the conservative options available—micro-abrasion, macro-abrasion, veneering, and bleaching—chairside bleaching is widely regarded as the least invasive means of achieving predictable color improvement in sound enamel.[2] Contemporary bleaching agents employ hydrogen peroxide (H₂O₂) or its precursor, carbamide peroxide, which liberates H₂O₂ on decomposition.[3] These formulations may be activated chemically or by photo-activation to accelerate free-radical release.[4]

Hybrid computer-aided design/computer-aided manufacturing (CAD/CAM) ceramics were introduced to couple the dentin-like elasticity of resin composites with the favorable optical, mechanical, and biological attributes of feldspathic porcelain. This elastic–ceramic synergy allows the material to absorb functional stress without permanent deformation or catastrophic fracture.[5] Clinical observations further indicate that hybrids are wear-resistant, kinder to opposing dentition, and require shorter milling times than conventional ceramics.[6] [7] Despite excellent chipping resistance and straightforward intra-oral maintenance, their relatively low flexural strength (150–240 MPa) currently limits indications to single-unit restorations—veneers, inlays, onlays, and crowns (especially implant-supported crowns where no periodontal ligament shock absorption exists).[7]

Bleaching agents may inadvertently get in contact with the pre-existing restorations during the routine in-office bleaching session.[8] Moreover, the literature shows that bleaching agents demonstrate effective outcomes in the color recovery of stained restorations.[9] [10] Multiple investigations have explored how peroxide-based bleaching influences restorative materials, yet outcomes remain inconsistent. Some authors report increased surface roughness and up to a 15% reduction in hardness for feldspathic porcelain after exposure to 10 to 16% carbamide peroxide.[11] Significant color shifts have likewise been documented in feldspathic ceramics following 10% H₂O₂ or carbamide peroxide bleaching.[12] Conversely, other studies found negligible changes in certain esthetic materials under similar condition.[13] [14] Mechanistically, free radical penetration and subsequent alkali ion leaching may induce glass-phase dissolution within ceramics, elevating surface roughness beyond the 0.2 µm plaque-retention threshold and thereby predisposing to secondary caries, periodontal inflammation, and compromised esthetics.[15] [16] Variations across earlier studies likely stem from differences in material composition, baseline properties, bleaching agent concentration, and application protocols. Notably, limited evidence exists regarding peroxide effects on the latest generation of hybrid ceramics, whose unique polymer–ceramic networks may respond differently to oxidative challenges.[17] Clarifying these interactions is essential to guide pre-bleaching precautions and post-bleaching maintenance. Furthermore, prior research has primarily focused on the impact of low-concentration at-home bleaching products on the surface and optical properties of various restorative materials, with less attention paid to the influence of high-concentration in-office bleaching products and the susceptibility of restorative materials to staining after bleaching. In the limited studies that have evaluated post-bleaching staining resistance, either hot or cold coffee or red wine has typically been used, and investigations have been primarily confined to bleaching protocols involving 10 to 16% carbamide peroxide or 40% non-light-activated H₂O₂.[18] [19] [20] [21] [22] [23] The potential influence of other commonly consumed staining agents, such as tea and citrus juices, as well as the role of thermocycling, has not been adequately explored. Repeated thermal fluctuations are known to induce microcrack formation and interfacial gaps opening within restorative materials, thereby facilitating deeper penetration of staining solutions and accelerating discoloration.[24] [25] [26]

To the best of our knowledge, the effect of light-activated 32% H₂O₂ in-office bleaching on the staining susceptibility of CAD/CAM restorative materials has not yet been investigated. Furthermore, no previous study has assessed this effect under simultaneous thermocycling and staining conditions using both hot and cold beverages, which more closely replicate the thermal and chemical challenges encountered in the oral environment. Consequently, the present study addresses this important gap by introducing a clinically relevant experimental model to evaluate the combined effects of bleaching, temperature variation, and staining on CAD/CAM hybrid and feldspathic ceramics.

Accordingly, this in vitro study investigated the influence of an in-office 32% H₂O₂ regimen on surface roughness, microhardness, and stain susceptibility of three commercially distinct hybrid ceramics—Vita Enamic (VE) (VITA Zahnfabrik), Lava Ultimate (LU) (3M ESPE), and Shofu Block HC (SB) (Shofu)—compared with VitaBlocs Mark II feldspathic ceramic (VB) (VITA Zahnfabrik) subjected to a combined thermocycling–staining protocol. The null hypotheses tested in this study were as follows:

• (1) In-office bleaching with 32% H₂O₂ does not significantly increase the susceptibility of hybrid ceramics to staining;

• (2) In-office bleaching does not cause detrimental changes in the surface roughness or microhardness of hybrid ceramics; and

• (3) The effects of in-office bleaching on surface and optical properties do not differ significantly between hybrid ceramics and feldspathic ceramic materials.

Materials and Methods

Materials

Three hybrid CAD/CAM blocks were examined—one polymer-infiltrated ceramic (VE; VITA Zahnfabrik, Bad Säckingen, Germany), and two resin nano-ceramics (LU; 3M ESPE, St. Paul, MN, USA and SB; Shofu Inc., Kyoto, Japan). A feldspathic ceramic (VB; VITA Zahnfabrik, Bad Säckingen, Germany) served as the control. Nominal compositions are summarized in [Table 1].

Table 1 Materials used in the study and their composition

Material	Type	Content[a]	Manufacturer
Vita Enamic	Polymer infiltrated ceramic	SiO2, Al2O3, Na2O, K2O, B2O3, ZrO2, CaO, UDMA, TEGDMA	VITA Zahnfabrik
Lava Ultimate	Resin nano ceramic (RNC)	Resin: Bis-GMA, UDMA, Bis-EMA, TEGDMA Fillers: SiO2, ZrO2, Si/ZrO2 cluster	3M ESPE
Shofu Block HC	Zirconium silicate nanoparticles reinforced pre- polymerized resin matrix	Filler composition: 61%, incl. zirconium silicate, silicon dioxide, micro-fumed silica Resin matrix: UDMA, TEGDMA	Shofu
VitaBlocs Mark II	Leucite-reinforced glass ceramic	Mixture of feldspathic crystalline particles embedded in a glassy matrix	VITA Zahnfabrik

Abbreviations: Bis-EMA, ethoxylated bisphenol A glycol dimethacrylate; BisGMA, bisphenol A glycol dimethacrylate; TEGDMA, triethylene glycol dimethacrylate; UDMA, urethane dimethacrylate.

^a Manufacturer's data.

Sample Size Calculation

Sample size was estimated using G*Power (version 3.1), based on the means and standard deviations reported by Ghaemi et al,[27] which yielded a large effect size (Cohen's f = 0.4). A priori power analysis indicated that a minimum of six specimens per group would be required to detect significant differences at a significance level of α = 0.05 and power of 80%. To align with the methodology of previous studies evaluating the effect of bleaching on color stability of restorative materials, seven specimens per group were included in the present study.

Specimen Preparation

Each block was embedded in acrylic resin and sectioned with a low-speed precision saw (IsoMet 1000, Buehler, Lake Bluff, IL, USA) to obtain 14 slabs per material (thickness = 2 mm). Surfaces were standardized under water-cooling with 240-grit silicon-carbide papers (CarbiMet, Buehler Ltd., USA) for 60 seconds on a rotary polisher (MetaServ 250 Grinder-Polisher, Buehler Ltd., USA). Final gloss was achieved with a two-step ceramic polishing kit (Komet 4313B.204, Lemgo, Germany) to simulate the clinical surface condition post-cementation and occlusal adjustment.

In-office Bleaching Protocol

A 32% H₂O₂ gel (Fläsh, WHITEsmile, Birkenau, Germany) was applied to the polished top surfaces, after baseline measurements, for three consecutive 15-minute cycles and photo-activated with the manufacturer's lamp (Flash Whitening Lamp XG, Birkenau, Germany), operating at a wavelength of 460 nm and an irradiance of 190 mW/cm². To ensure consistency, photo-activation was conducted at a standardized distance by positioning the lamp's light guide head directly on the benchtop surface, maintaining uniform proximity to all specimens. After each bleaching cycle, the gel was carefully aspirated, and the specimens were thoroughly rinsed with sterile saline prior to the application of fresh gel. All bleaching procedures were performed under controlled environmental conditions, with the room temperature maintained at 24°C.

Surface Roughness (Ra) and Microhardness (VHN)

Seven slabs from each group were randomly selected. Baseline roughness (Ra₀) was measured with a contact profilometer (Surftest 211, Mitutoyo, Tokyo, Japan; 2 µm diamond stylus; cut-off 0.25 mm; evaluation length 1.25 mm). Vickers microhardness (VHN₀) was determined with a micro-hardness tester (FM-800, Future-Tech, Kanagawa, Japan; 500 g load, 15 seconds dwell). Three readings per specimen were averaged. Vickers hardness (VHN) was calculated automatically (VHN = 1.854 F/D²). Three readings per specimen were averaged. After re-polishing, bleaching was performed according to the protocol described earlier, then measurements were repeated (Ra_W, VHN_W). Each surface property was evaluated by one trained operator, who performed all corresponding measurements.

Color Stability

The remaining seven slabs per material were used for color analysis. Color measurements were recorded at baseline for both the top and bottom surfaces, after staining for the top bleached surface (BS) and the bottom surface (S). A spectrophotometer (VITA EasyShade V, Vita Zahnfabrik, Bad Säckingen, Germany) was used over a white background. After calibration, the tip was placed perpendicular to each surface, and three readings were averaged per sample. Color analysis was performed by a single trained operator.

Staining and Aging

Specimens underwent 1,000 thermocycles (5 °C tea–orange juice mixture ↔55 °C coffee; dwell time: 30 s, transfer time: 15 s), following Ren et al.[28] Solutions were placed in stainless-steel baths of a programmable thermocycler (SD Mechatronik, Westerham, Germany). Post-cycling, Specimens were brushed lightly with commercial toothpaste (Close-Up White Now; 10 seconds), rinsed (de-ionized water, 30 seconds), and air-dried. Color was re-measured on the bleached (bleached-stained, BS) and unbleached (stained, S) surfaces of each specimen.

Color change was quantified using the CIEDE2000 (ΔE₀₀) color-difference formula, in accordance with the standard developed by the Commission Internationale de l'Éclairage (CIE).[29] The ΔE₀₀ value was calculated using the following equation:

Where ΔL', ΔC', and ΔH' represent differences in lightness, chroma, and hue, respectively. RT accounts for interactions between chroma and hue in the blue region. SL, SC, and SH are weighting functions. KL, KC, and KH are correction factors. A ΔE₀₀ value of 1.8 was considered the clinical acceptability threshold.[30]

Statistical Analysis

Data (Ra, VHN, ΔE₀₀) were analyzed with two-way ANOVA (factors = material, bleaching) followed by Tukey post-hoc tests (α = 0.05) using IBM SPSS Statistics v29.0.1 (IBM Corp., Armonk, NY, USA). Results are reported as mean ± standard deviation. Pearson correlation analysis was performed to explore potential associations between surface roughness, microhardness, and color change (ΔE₀₀BS) following bleaching and staining procedures.

Results

The two-way ANOVA revealed that the staining susceptibility (ΔE₀₀) of the tested materials was not significantly influenced by 32% H₂O₂ in-office bleaching (p = 0.883); therefore, the first null hypothesis was accepted. Similarly, in-office bleaching had no significant effect on surface roughness (Ra) (p = 0.603). In contrast, Vickers hardness number (VHN) was significantly affected by bleaching (p < 0.001), leading to rejection of the second null hypothesis.

The interaction between in-office bleaching and material type showed no significant effect on Ra or ΔE₀₀ (p = 0.981 and p = 0.963, respectively). However, material type significantly influenced ΔE₀₀, Ra, and VHN (p < 0.001). Moreover, VHN was significantly affected by the interaction between material type and bleaching (p < 0.001), supporting rejection of the third null hypothesis.

Post hoc comparisons revealed that 32% H₂O₂ had no statistically significant effect on the VHN of the tested CAD/CAM hybrid materials (VE, LU, and SB) (P > 0.05), as shown in [Table 2] and [Fig. 1]. In contrast, VB exhibited a significant reduction in VHN following bleaching (p < 0.001), although it retained the highest hardness values among all materials. SB consistently demonstrated the lowest VHN values, with significantly lower post-bleaching hardness compared with LUB, VEB, and VBB (p < 0.05).

Table 2 Mean (±standard deviation) of Vickers microhardness (VHN), surface roughness (Ra, μm), and color change (ΔE₀₀) of the tested materials before and after bleaching and staining (P-values represent the differences between the means of the bleached and unbleached subgroups within each material)

Material	VHN0 (before bleaching)	VHNB (after bleaching)	Ra0 (μm) (before bleaching)	RaB (μm) (after bleaching)	ΔE00S (after staining)	ΔE00BS (after bleaching and staining)
Vita Enamic (VE)	164.43 ± 9.92ᵃ	164.86 ± 13.30ᵃ	0.103 ± 0.023ᵃ	0.106 ± 0.017ᵃ	2.16 ± 0.58ᵃ	1.83 ± 0.30ᵃ
P-value	1.000		1.000		1.000
Lava Ultimate (LU)	90.53 ± 2.82ᵇ	99.44 ± 6.69ᵇ	0.099 ± 0.027ᵃ	0.108 ± 0.029ᵃ	1.43 ± 0.57ᵃ	1.10 ± 0.44ᵃ
P-value	0.938		1.000		1.000
Shofu Block HC (SB)	71.08 ± 4.50ᵇ	66.64 ± 4.68ᶜ	0.276 ± 0.080ᵇ	0.266 ± 0.094ᵇ	1.04 ± 0.32ᵃ	0.93 ± 0.59ᵃ
P-value	0.999		1.000		1.000
VitaBlocs Mark II (VB)	454.53 ± 25.63ᶜ	322.56 ± 17.12ᵈ	0.224 ± 0.062ᵇ	0.232 ± 0.053ᵇ	8.30 ± 1.73ᵇ	8.30 ± 2.02ᵇ
P-value	<0.001		1.000		1.000

Note: Within each column, values with the same superscript lowercase letter are not statistically significantly different (P > 0.05).

Fig. 1 Vickers microhardness (VHN) values of four computer-aided design/computer-aided manufacturing (CAD/CAM) materials before and after in-office bleaching with 32% H₂O₂. Asterisks indicate statistically significant differences between baseline and post-bleaching means within the same material (*p < 0.001). Error bars represent ± the standard deviation. Shofu, Shofu Block HC; VB, VitaBlocs Mark II.

Moreover, as displayed in [Table 2] and [Figs. 2] and [3], post hoc comparisons showed that 32% H₂O₂ in-office bleaching had no statistically significant effect on ΔE₀₀ or Ra for VE, LU, SB, or VB (p = 1.000). After staining, VB exhibited the highest ΔE₀₀ values in the unbleached (ΔE₀₀S) and bleached–stained (ΔE₀₀BS) groups, exceeding the clinical acceptability threshold and being significantly higher than those of VE, LU, and SB (p < 0.05). No significant differences in ΔE₀₀ were observed among VE, LU, and SB (P > 0.05). Bleached hybrid material groups generally demonstrated lower ΔE₀₀ values than their unbleached counterparts, following aging and staining; however, these differences were not statistically significant. Statistical analysis also revealed that VE and LU exhibited significantly lower Ra values than SB and VB (p < 0.05), while no significant difference in Ra was observed between SB and VB.

Fig. 2 Surface roughness (Ra, μm) of four computer-aided design/computer-aided manufacturing (CAD/CAM) materials before and after in-office bleaching with 32% H₂O₂. No statistically significant differences were observed between baseline and post-bleaching means for any material (p > 0.05). Error bars represent ± the standard deviation. Shofu, Shofu Block HC; VB, VitaBlocs Mark II.

Fig. 3 Color difference (ΔE₀₀) values for four computer-aided design/computer-aided manufacturing (CAD/CAM) materials after staining only (S) and bleaching followed by staining (BS). Asterisks indicate statistically significant differences between feldspathic ceramic and all other groups (*p < 0.001). Error bars represent ± the standard deviation. Shofu, Shofu Block HC; VB, VitaBlocs Mark II.

Correlation analysis revealed a strong, statistically significant positive relationship between microhardness and the extent of stain susceptibility following bleaching and staining (ΔE₀₀BS) (r = 0.967, p = 0.033). Conversely, the correlation between surface roughness and color change was weak and not statistically significant (r = 0.365, p = 0.635).

Discussion

Polymer-infiltrated ceramic network (PICN) and resin-nanoceramic blocks were conceived to unite the optical fidelity of porcelains with the stress-absorbing resilience of resin, thereby meeting the growing demand for restorations that preserve esthetics and surface integrity over time. Our results confirm that this design goal is largely achieved: a single 32% H₂O₂ bleaching session did not alter surface roughness (Ra) or color change (ΔE₀₀) in VE, LU, or SB, whereas the feldspathic control, VB, showed the greatest color change after staining and high surface roughness regardless of the bleaching treatment. The absence of a bleaching effect on Ra and ΔE₀₀ is consistent with earlier work on feldspathic porcelain and composites, suggesting that the highly cross-linked, pre-polymerized matrices of CAD/CAM hybrids confer resistance to peroxide challenge.[12] [31] [32]

Although profilometry is inherently two-dimensional,[33] the stable Ra values recorded here still carry clinical weight. Both VE and LU remained far below the 0.2 µm plaque-retention threshold, while SB and unglazed VB hovered just above it—differences attributable to filler size, porosity, and the loss of glaze during finishing.[34] [35] Importantly, bleaching neither exacerbated these rougher profiles nor generated new surface defects, thereby maintaining the wear compatibility of the restorations.

VE and SB showed no significant change in VHN after bleaching, and LU exhibited a modest increase, echoing the peroxide-related hardening reported by Rodrigues et al.[11] By contrast, VB lost nearly 30% of its Vickers hardness, an outcome attributed to peroxide-mediated silicate oxide (SiO₂) leaching from the glass matrix.[36] [37] [38] Despite this reduction, VitaBlocs remained the hardest group, implying that fracture resistance may still be adequate if occlusal thickness is sufficient. Therefore, the hardness behavior was material-dependent.

LU and SB remained below the 1.8 ΔE₀₀ acceptability limit after thermocycling and staining; VE sat marginally above it, whereas VB exceeded it markedly, mirroring the stain susceptibility of feldspathic ceramics reported by Ghinea et al.[39] The observation that bleaching slightly reduced ΔE₀₀ in all hybrid groups supports the stain-lightening effect of peroxide on resin-containing ceramics.[40] [41]

These patterns align with the known chemistry of peroxide interaction. Roughening and hardness loss in composites are linked to polymer-matrix erosion,[17] [42] driven by highly diffusive free radicals.[43] [44] The prefabricated blocks tested here incorporate a densely cross-linked methacrylate network that resists oxidative cleavage,[45] explaining the minimal changes observed. Feldspathic porcelains, on the other hand, lack a protective polymer phase and present larger, more open silicate domains vulnerable to ion loss and pigment adsorption.[46]

In the present study, feldspathic ceramics were included as a control material and were polished using the same standardized protocol applied to the hybrid ceramics to ensure surface comparability. This approach is supported by previous studies demonstrating that polished feldspathic ceramics may exhibit surface roughness values comparable to, or even lower than, those of glazed surfaces.[47] [48] In addition, according to the manufacturer, VitaBlocs Mark II is designed for time-efficient fabrication and polishing without the need for an additional glazing firing step.[49] Furthermore, several studies investigating staining susceptibility and surface properties of CAD/CAM ceramics have employed polished VitaBlocs Mark II specimens without glazing.[50] [51]

The strong positive correlation between microhardness after bleaching and the extent of stain susceptibility following bleaching suggests that harder materials may exhibit more perceptible color shifts under the tested bleaching and staining conditions. Conversely, the weak correlation between surface roughness and stain susceptibility indicates that roughness changes may not strongly influence color alterations in the tested materials. These findings underscore the potential relevance of material hardness in predicting esthetic stability post bleaching.

Clinical Relevance

Within the study parameters, chairside 32% H₂O₂ bleaching can be safely offered to patients restored with the tested hybrid ceramics, as no additional polishing or glaze application appears necessary. In contrast, feldspathic restorations may benefit from pre-bleaching surface glazing and/or post-bleaching reglazing to offset potential reduction in hardness and color alteration.

Findings are confined to one bleaching concentration, a single thermocycling regimen, and a fixed staining protocol. The lack of formal reliability testing (e.g., intra-observer reliability analysis) is acknowledged as a limitation of the present study. Although literature-derived, profilometer-based roughness omits three-dimensional topography. Moreover, the ΔE₀₀ acceptability threshold may not mirror every clinical scenario.[30] [52] Coupling profilometry with SEM, testing multiple peroxide concentrations and application times, and conducting in vivo or in situ trials will clarify long-term clinical performance. Investigations into the chemical interactions that increased LU's hardness could also guide the formulation of peroxide gels tailored for restorative safety.

Conclusion

Within the limitations of this in vitro study, the application of a 32% H₂O₂-based in-office bleaching agent did not deteriorate the surface roughness, microhardness, or color stability of the tested CAD/CAM hybrid materials. Unglazed VitaBlocs Mark II exhibited a significant reduction in microhardness following bleaching; however, it retained the highest VHN among the tested materials. Despite its superior hardness, the extent of color change in VB after aging and staining was beyond the threshold of clinical acceptability.

Conflict of Interest

None declared.

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Publication History

Article published online:
12 February 2026