Odovtos-International Journal of Dental Sciences (Odovtos-Int. J. Dent. Sc.), Online First, 2026. ISSN: 2215-3411

https://doi.org/10.15517/m20fwq39

https://revistas.ucr.ac.cr/index.php/Odontos

BASIC RESEARCH:

Effect of Preheating and Glycerin Barrier on Surface Characteristics of Composite Resins

Efecto del precalentamiento, la barrera de glicerina y los protocolos de pulido sobre las características superficiales de resinas compuestas

Zurab Khabadze DDS, PhD1 https://orcid.org/0000-0002-7257-5503

Oleg Mordanov DDS, PhD1,2 https://orcid.org/0000-0002-9878-7045

Vini Mehta PhD2,3 https://orcid.org/0000-0003-4174-907X

1Peoples’ Friendship University of Russia named after Patrice Lumumba (RUDN University), Moscow, Russia.

2Department of Dental Research Cell, Dr. D. Y. Patil Dental College & Hospital, Dr. D. Y. Patil Vidyapeeth (Deemed to be University), Pimpri, Pune 411018, India.

3Odonto-Stomatology, Faculty of Medicine and Dentistry, University of Salamanca, 37008 Salamanca, Spain.

Correspondence to: Dr. Vini Mehta - vmehta@statsense.in

Received: 18-XI-2025 Accepted: 22-I-2026

ABSTRACT: This study aimed to assess the influence of preheating, glycerin barrier application, and polishing protocols on the surface roughness of microhybrid and submicron-filled composite resins. Eighty extracted human third molars were prepared with standardized Class I cavities and restored using Enamel Plus HRi (microhybrid) or Estelite Sigma Quick (submicron-filled). Specimens were divided into preheated (55 °C) and non-preheated groups, and further allocated to four subgroups: (1) polishing only, (2) polishing + glycerin, (3) glycerin only, and (4) control. All restorations were light-cured with an LED unit. Surface morphology was analyzed by scanning electron microscopy (SEM), and Ra values were determined. Data were analyzed using one-way ANOVA and Tukey’s post hoc test (p<0.05). SEM showed that glycerin-shielded specimens were cleaner, with reduced OIL-related debris. Preheating consistently yielded smoother, less porous surfaces across protocols. Polishing significantly reduced Ra in all groups, with the lowest values for “polishing + glycerin” (0.83±0.01 µm in preheated Enamel Plus HRi; 1.09±0.02 µm in non-preheated Estelite Sigma Quick). Glycerin without polishing improved polymerization but did not significantly reduce Ra. Controls exhibited the highest roughness (up to 5.20±0.61 µm). Overall, the combination of preheating, glycerin barrier application, and polishing produced the smoothest composite surfaces. Glycerin improved surface chemistry by reducing OIL, while polishing was essential for levelling filler protrusions and irregularities.

KEYWORDS: Resin composite; Preheating; Glycerin barrier; Oxygen-inhibited layer; Surface roughness; Polishing.

RESUMEN: El objetivo de este estudio fue evaluar la influencia del precalentamiento, la aplicación de una barrera de glicerina y los protocolos de pulido sobre la rugosidad superficial de resinas compuestas microhíbridas y con relleno submicrométrico. Ochenta terceros molares humanos extraídos fueron preparados con cavidades Clase I estandarizadas y restaurados con Enamel Plus HRi (microhíbrida) o Estelite Sigma Quick (relleno submicrométrico). Los especímenes se dividieron en grupos precalentados (55 °C) y no precalentados; posteriormente se asignaron a cuatro subgrupos: (1) solo pulido, (2) pulido + glicerina, (3) solo glicerina y (4) control. Todas las restauraciones fueron fotopolimerizadas con una unidad LED. La morfología superficial se analizó mediante microscopía electrónica de barrido (MEB) y se determinaron los valores de Ra. Los datos se analizaron mediante ANOVA de una vía y prueba post hoc de Tukey (p<0,05). La MEB mostró que los especímenes protegidos con glicerina presentaron superficies más limpias, con reducción de detritos asociados a la capa inhibida por oxígeno (OIL, oxygen-inhibited layer). El precalentamiento produjo de manera consistente superficies más lisas y menos porosas en todos los protocolos. El pulido redujo significativamente los valores de Ra en todos los grupos, observándose los valores más bajos en el grupo "pulido + glicerina" (0,83±0,01 µm en Enamel Plus HRi precalentado; 1,09±0,02 µm en Estelite Sigma Quick no precalentado). La aplicación de glicerina sin pulido mejoró la polimerización, pero no redujo significativamente los valores de Ra. Los controles presentaron la mayor rugosidad superficial (hasta 5,20±0,61 µm). En conjunto, la combinación de precalentamiento, aplicación de barrera de glicerina y pulido produjo las superficies de resina compuesta más lisas. La glicerina mejoró la química superficial al reducir la capa inhibida por oxígeno, mientras que el pulido fue esencial para nivelar las protrusiones del relleno y las irregularidades superficiales.

PALABRAS CLAVE: Resina compuesta; Precalentamiento; Barrera de glicerina; Capa inhibida por oxígeno; Rugosidad superficial; Pulido.

INTRODUCTION

Resin-based composite materials are widely used in direct restorative dentistry due to their esthetics, adhesive bonding potential, and clinical versatility (1). However, achieving a durable, smooth, well-polished surface remains a crucial challenge, because surface roughness influences not only aesthetic outcome (gloss and stain resistance) but also plaque retention, wear, and long-term clinical performance (2). Composite resins exposed to lower light-curing intensities exhibit greater susceptibility to extrinsic staining and surface degradation, underscoring the importance of optimized polymerization protocols (3).

One strategy to improve handling, polymerization kinetics, and ultimately physical properties of composite restorations is preheating the composite material prior to placement. Preheating can lower the viscosity of the composite resin, thereby improving flow and adaptation to cavity walls, enhancing monomer mobility, and increasing the degree of conversion (DC) (4). Preheating of composite resins has been shown to improve microhardness, fracture toughness, and overall mechanical properties, although evidence regarding its effect on surface roughness remains mixed (5). In contrast, preheating did not significantly influence surface roughness in certain resin composites, implying that the effect may depend on filler characteristics, resin matrix composition, and finishing/polishing protocols (6).

Another key factor influencing composite surface characteristics is the presence of an oxygen-inhibited layer (OIL). During light polymerization, atmospheric oxygen diffuses into the resin surface and quenches free radicals, preventing complete polymerization of the superficial layer. This oxygen-inhibited layer creates a sticky, resin-rich surface that compromises hardness, wear resistance, and susceptibility to staining and microbial colonization (7). To overcome this, strategies such as applying a physical barrier (e.g., glycerin or Mylar strips) before final curing, or immediate finishing and polishing, are commonly employed. Oxygen shielding with glycerin or low-pressure vacuum effectively reduces the thickness of the OIL, enhances the degree of conversion, and contributes to harder, smoother, and more durable composite surfaces (8,9).

Furthermore, the finishing and polishing protocol plays a major role in the final surface roughness of composite restorations (10). Recent evaluations of multi-step polishing systems revealed significant reductions in surface roughness and enhanced microhardness of composite restorations after 24 h compared to immediate polishing, highlighting time-dependent polymerization effects (11). Multi-step polishing systems, such as Sof-Lex discs and brushes, are reported to reduce surface irregularities and effectively remove resin-rich layers and superficial filler protrusions (12,13). However, the starting state of the surface, particularly the degree of conversion, presence of an oxygen-inhibited layer, and heat treatment, may influence how effectively polishing can smooth the surface and whether subsurface irregularities are exposed (14,15).

Despite these advances, there is a paucity of data systematically comparing how preheating, glycerin barrier curing, and polishing (or lack thereof) interact to influence surface roughness in different composite types, especially comparing microhybrid vs submicron or nanofilled composites. Differences in filler size, loading, and resin matrix composition may lead to different responses to preheating or oxygen-barrier strategies.

Thus, the aim of the present in vitro investigation was to evaluate and compare the surface roughness (Ra) of microhybrid and submicron composite resins under different pre-curing (preheating), final-curing (glycerin barrier vs none), and post-curing (polishing vs no polishing) protocols. The null hypothesis is that preheating and glycerin barrier application will not significantly affect the surface roughness of the tested composite resins under these conditions.

METHODS AND MATERIALS

Specimen Selection and Preparation

This in vitro study was conducted in accordance with the institutional research policy of RUDN University and international ethical guidelines, including the Declaration of Helsinki and the ICH–GCP recommendations. Written informed consent for the use of extracted teeth was obtained from all donors prior to sample collection.

Eighty freshly extracted human third molars, removed for orthodontic indications, were included. Teeth were examined to ensure structural integrity, and specimens with caries, restorations, cracks, or incomplete root formation were excluded. Following extraction, samples were disinfected in 0.1% chlorhexidine solution for 15 minutes and stored in distilled water at 4 °C for a maximum of one month until use.

Specimens were randomly allocated into two main groups (n=40 each) based on composite resin preheating status:

• Group A: preheated composite resin
• Group B: non-preheated composite resin

Randomization was performed using a computer-generated random number table. Standardized Class I cavities (3×3×2 mm) were prepared on the occlusal surfaces using a high-speed handpiece with water cooling and a diamond bur (ISO 806 314 001 524 012), which was replaced after every five preparations to ensure consistency.

Restorations were performed using two commercially available resin composites:

• Enamel Plus HRi (Micerium, Italy): microhybrid composite, 80 wt% filler, average particle size 0.2-3.0 µm.
• Estelite Sigma Quick (Tokuyama Dental Co., Japan): submicron-filled composite, 63 wt% filler, mean particle size 0.2 µm.

Bonding, Restoration, and Experimental Protocols

Bonding was performed using a two-step self-etch adhesive system (Clearfil SE Bond 2; Kuraray, Japan) according to the manufacturer’s instructions. Cavities were restored in a single increment. For Group A, the composite resin was preheated to 55 °C in a specialized oven (Micerium; Avegno, Italy) prior to placement, while Group B specimens received the composite at room temperature.

The in vitro study was conducted under a controlled experimental design with two composite types (microhybrid and submicron-filled) and four surface treatment protocols under the preheated and non-preheated conditions, which resulted in eight subgroups (n=10 specimens per subgroup; total=80).

A power analysis was carried out using G*Power 3.1 software (Heinrich Heine University, Düsseldorf, Germany). Assuming an effect size (f) of 0.35, α=0.05, and desired power=0.80, the required minimum was 8 specimens per subgroup. Considering that some may be excluded, 10 specimens were assigned to each subgroup.

Polymerization of all specimens was performed with an LED curing unit (VALO; Ultradent Products, South Jordan, UT, USA; 430-480 nm; 900 mW/cm²) for 20 s. The light tip was positioned at a standardized distance of 1 mm from the specimen surface.

Specimens were further allocated into four subgroups (n=10 each) according to surface treatment protocol:

1. Polishing only (no glycerin): Final curing for 20 s, followed by finishing and polishing with Sof-Lex discs (3M ESPE, Seefeld, Germany) and OccluBrush (KaVo Kerr, Brea, CA, USA) at 10,000-20,000 rpm. Polishing time was standardized, with specimens rinsed and air-dried between steps.

2. Polishing + glycerin barrier: Application of glycerin (viscosity 8-11 Pa·s; conductivity 0.7-1.0 S/m) before final curing for 20 s, followed by removal of glycerin and finishing/polishing as described above.

3. Glycerin barrier only (no polishing): Application of glycerin before final curing for 20 s, followed by removal without subsequent polishing.

4. Control (no glycerin, no polishing): Final curing for 20 s without additional surface treatment.

Scanning Electron Microscopy and Surface Roughness Analysis

Surface morphology of the composites was examined using a scanning electron microscope (Tescan Vega 3 SBH; Tescan Analytics, Brno, Czech Republic) at magnifications of 200× and 3500×, with a field of view ranging from 1.04 mm to 10.4 µm. To prevent charging of the dielectric material under the electron beam, a 30 nm platinum coating was applied to each specimen prior to imaging.

Surface roughness (Ra) was measured with a profilometer (Surftest SJ-301; Mitutoyo America Corp., Aurora, USA). For each specimen, five random measurements were obtained at different locations, and the mean Ra value was calculated for statistical analysis

Two complementary parameters were employed to measure the effect that preheating, glycerin barrier, and polishing protocols had on the tested composites:

1. Surface Roughness (Ra): Measured using a contact profilometer (Mitutoyo, Japan) according to ISO 4287, with three readings averaged per specimen to determine mean Ra (µm). This parameter reflects topographic irregularities influencing plaque retention and esthetics.

2. Vickers Microhardness (VHN): Determined using a microhardness tester (Shimadzu, Japan) under a 200 g load for 15 s to evaluate near-surface polymerization quality and mechanical integrity. Three indentations were averaged per specimen to obtain the mean VHN value.

These parameters were selected to provide complementary insights—surface roughness indicating topographical quality and microhardness reflecting polymer cross-linking and surface strength—thereby allowing a more complete interpretation of the effects of preheating, glycerin barrier application, and polishing protocols.

Statistical Analysis

All statistical analyses were performed using StatPlus 6 Professional (AnalystSoft Inc., Walnut, CA, USA). Data distribution was evaluated with the Shapiro–Wilk test, and homogeneity of variances was verified using Levene’s test. Since both assumptions for parametric testing were satisfied, one-way ANOVA was applied to assess differences in surface roughness (Ra) among experimental subgroups within each composite resin type.

Although a three-way ANOVA could theoretically account for the combined effects of preheating, polishing, and glycerin application, one-way ANOVA was chosen to maintain statistical power given the limited subgroup size (n=10). Post hoc comparisons were conducted using Tukey’s test. Effect sizes were calculated with partial eta squared (η²) to quantify the strength of observed differences.

To evaluate the relationship between profilometric roughness values and SEM findings, Pearson’s correlation coefficient was applied. Statistical significance was set at p<0.05.

RESULTS

SEM Analysis of Non-Preheated Specimens (Group B)

Scanning electron microscopy of specimens without preheating (Group B) demonstrated distinct surface features across subgroups (Figure 1.a-Figure 1.c). Both tested composite resins exhibited visible filler particles. Specimens in Group 2 (polishing with glycerin barrier) presented a smooth, homogeneous surface. Group 1 (polishing without glycerin barrier) showed a relatively smooth surface but with signs of the oxygen-inhibited layer, characterized by an island-like pattern, and the presence of scattered debris. Group 3 (glycerin barrier without polishing) displayed a comparatively rough surface, though without surface debris. These surface patterns were consistent for both composite types tested in the non-preheated condition.

SEM Analysis of Preheated vs Non-Preheated Specimens

Comparison of preheated specimens (Group A) with non-preheated specimens (Group B) revealed similar overall surface patterns (Figure 1.d-Figure 1.f). However, preheating resulted in surfaces that appeared smoother and less porous across all treatment protocols (Groups 1-4). In contrast, specimens cured without a glycerin barrier demonstrated more surface debris, regardless of preheating status.

Surface Roughness Analysis

The mean surface roughness (Ra) values obtained at 200× magnifications are summarized in Table 1. Within the non-preheated group, no statistically significant difference was observed between specimens treated with glycerin only (2.07±0.01 µm) and those polished without glycerin (1.96±0.016 µm; p=0.8). In contrast, a highly significant difference was found between the control group (5.20±0.61 µm) and the polishing with glycerin group (1.09±0.02 µm; p<0.01).

A similar pattern was observed within the preheated group. Compared with their non-preheated counterparts, preheated specimens exhibited generally lower Ra values. Specifically, the polishing with glycerin subgroup demonstrated the lowest mean roughness (0.83±0.011 µm; p<0.05). Preheated specimens polished without glycerin also showed significantly reduced Ra values (1.63±0.10 µm; p<0.01). However, no significant difference was found in the glycerin-only subgroup between preheated (1.98±0.10 µm) and non-preheated specimens (p=0.3). The preheated control subgroup demonstrated significantly lower Ra values (4.00±0.10 µm) compared with the non-preheated control (p<0.05).

Figure 1. (a) SEM micrographs of Estelite Sigma Quick composite resin without preheating (200× magnification); (b) SEM micrographs of Estelite Sigma Quick composite resin without preheating (3500× magnification); (c) SEM micrographs of Enamel Plus HRi composite resin without preheating (200× magnification); (d) SEM micrographs of Enamel Plus HRi composite resin without preheating (3500× magnification); (e) SEM micrographs of Enamel Plus HRi composite resin with preheating (200× magnification); (f) SEM micrographs of Enamel Plus HRi composite resin with preheating (3500× magnification).

Table 1. Mean surface roughness (Ra, µm) values for each subgroup by composite type. Different superscript letters (a, b, c) indicate statistically significant differences among subgroups within the same composite (p<0.05). Subgroups sharing the same letter are not significantly different.

Composite	Preheating	Group	Ra (µm) ± SD	p-value vs control	Significance
Enamel Plus HRi	Yes	Polishing + Glycerin	0.83 ± 0.01	<0.05	a
Enamel Plus HRi	Yes	Only Polishing	1.63 ± 0.10	<0.01	b
Enamel Plus HRi	Yes	Only Glycerin	1.98 ± 0.10	0.3	b
Enamel Plus HRi	Yes	Control	4.00 ± 0.10	<0.05	c
Estelite Sigma Quick	No	Polishing + Glycerin	1.09 ± 0.02	<0.01	a
Estelite Sigma Quick	No	Only Polishing	1.96 ± 0.02	0.8	b
Estelite Sigma Quick	No	Only Glycerin	2.07 ± 0.01	0.8	b
Estelite Sigma Quick	No	Control	5.20 ± 0.61	-	c

DISCUSSION

This in-vitro study explored how three modifiable clinical variables (preheating, oxygen shielding with glycerin, and finishing/polishing) interact to determine the surface roughness (Ra) of two composite classes (microhybrid vs. submicron/supra-nano). Across both materials, the smoothest surfaces were consistently produced when a glycerin barrier was applied during the final cure and followed by multi-step polishing; preheating further reduced Ra in most subgroups. Conversely, omitting polishing yielded the roughest surfaces, even when an oxygen barrier was used. These findings dovetail with and extend prior evidence on (i) the rheologic and polymerization benefits of preheating, (ii) the oxygen-inhibited layer (OIL) as a determinant of superficial properties, and (iii) the primacy of finishing/polishing for long-term surface quality.

Preheating: viscosity, degree of conversion, and downstream effects

Warming resin composites lowers viscosity, enhances flow and adaptation, and is frequently associated with increased hardness and a higher degree of conversion, particularly in high-viscosity formulations (4). Meta-analytic data show preheating raises microhardness at the top and bottom surfaces (e.g., Z250) relative to room-temperature controls, consistent with enhanced monomer mobility and polymerization kinetics (6). Preheating significantly reduces resin viscosity, improving flow and wetting of cavity walls, which facilitates more uniform polymerization and enhances the homogeneity of the cured surface (16). Early mechanistic work demonstrated that preheating accelerates monomer conversion and deepens cure (17), while clinical-adjacent bench studies reported reduced pre-cure viscosity and improved post-cure hardness with warming (18,19). Our subgroup comparisons align: preheated specimens generally showed lower Ra within each surface-treatment protocol, with the largest absolute benefit when preheating was combined with glycerin-shielded final curing and polishing (e.g., Enamel Plus HRi: 0.83±0.01 µm vs. 1.09±0.02 µm in non-preheated Estelite under the same polishing+glycerin protocol). Mechanistically, the smoother topography likely reflects (i) better adaptation and reduced entrapped surface porosity during placement, and (ii) higher near-surface DC that yields a more cohesive, abrasion-resistant superficial layer after OIL removal (20). Interfacial adaptation studies using different adhesive systems reveal that the adhesive strategy can influence the marginal continuity and micromechanical stability of resin–tooth interfaces, potentially affecting surface integrity after finishing (21).

It is notable that preheating did not lower roughness in the “glycerin-only/no polishing” subgroup relative to the non-preheated equivalent (no significant difference). This is congruent with reports that, while preheating can increase DC, surface topography is ultimately governed by how the superficial layer is finished; oxygen shielding alone does not eliminate mechanical or machining-related irregularities, and polishing remains necessary to level filler protrusions and remove resin-rich smear or debris (22-24).

Oxygen shielding (glycerin) and the

oxygen-inhibited layer

The OIL is a thin, resin-rich, under-polymerized film produced when atmospheric oxygen quenches free radicals at the surface during light curing. It compromises hardness, wear, and stain resistance and can harbor pigments and biofilm (25,26). Applying an air barrier (e.g., glycerin gel) during the terminal cure reduces OIL thickness and increases surface hardness/DC, as shown by microhardness and FT-IR studies (25,27). Park and Lee reported that Mylar or glycerin increased unpolished surface hardness vs. air cure, and that polishing away the top 0.2 mm made groups statistically indistinguishable—strong evidence that eliminating OIL (and exposing the well-cured subsurface) is decisive for final surface properties (25). Ciocan et al. further quantified that glycerin-shielded photoactivation can raise DC and nano-hardness, especially at short light-tip distances, underscoring the benefit of oxygen exclusion at the final cure .

SEM analysis confirmed that specimens cured under an oxygen barrier exhibited cleaner surfaces, with fewer island-like resin deposits and reduced debris, compared with air-cured samples that displayed more occlusal residues typical of the oxygen-inhibited layer (OIL) and unreacted matrix. Importantly, the lowest surface roughness (Ra) values were achieved when oxygen shielding was combined with multi-step polishing. This highlights the complementary roles of these interventions: glycerin enhances polymerization of the superficial layer by minimizing oxygen inhibition, while polishing effectively eliminates residual OIL and surface irregularities, producing a smoother, more uniform finish. Microscopic analyses of light-curing composites subjected to pre-polymerization heating confirm smoother, more homogeneous surfaces with reduced microvoids and filler dislodgement (28) .

Finishing/polishing remains the strongest determinant of smoothness

A large body of work shows multi-step aluminum-oxide disc systems (e.g., Sof-Lex) and contemporary disc/point sequences can reliably deliver low Ra across composite chemistries and shades (23,24,29). In our data, any polished subgroup outperformed its unpolished counterpart, and “polishing + glycerin” was uniformly superior. This accords with controlled trials where polished groups achieve Ra values near or below widely cited biological and sensory thresholds (≈0.2 µm for plaque retention; ≈0.3 µm for tongue detectability), whereas unpolished or Mylar-only surfaces may not meet those targets in anatomically contoured restorations (22,29). Supporting this, Tepe et al. showed that, for a supra-nano spherical composite, increasing disc RPM to 20,000 reduced Ra to as low as 0.07-0.10 µm with certain systems, and that system choice and speed both matters (30). Our protocol (Sof-Lex discs + brush, 10,000-20,000 rpm) mirrors these conditions and likely contributed to the very low Ra observed in the “polishing + glycerin” groups.

Across the literature, some one-step systems perform comparably to multi-step in specific contexts, but consensus remains that careful, sequential polishing with abrasives matched to filler hardness best minimizes filler pluck-out and matrix smearing, producing durable smoothness and gloss (31). Our SEM findings (homogeneous surfaces with fewer protrusions after polishing) corroborate this mechanism.

Clinical comparisons between total-etch and self-etch adhesive systems have shown subtle differences in restoration smoothness and marginal adaptation, suggesting that adhesive choice may modulate the influence of finishing and preheating on surface characteristics (32).

Why composite class and filler technology matter

Filler load, size distribution, morphology, and resin matrix composition govern both how a surface forms under abrasion and how it responds to oxygen shielding and polishing (33). Field emission SEM–EDX studies have shown that exposure to varying concentrations of community water fluoride can alter surface morphology and elemental distribution in composite resins, indicating environmental effects on resin stability (34). Estelite Sigma Quick employs uniform submicron/supra-nano spherical silica-zirconia fillers; spherical morphology facilitates more uniform light reflection and, under polishing, tends to yield higher gloss at equivalent Ra vs. irregularly shaped glass fillers (29). Kameyama et al. found a spherical-filled submicron composite (Estelite Σ) produced glossier surfaces than a microhybrid (AP-X) at similar roughness after identical finishing/polishing (29). Likewise, characterization studies show that filler size/morphology influences roughening under toothbrush abrasion and storage, even when initial polished Ra is similar across brands (31). Broader comparative work indicates that smaller and more narrowly distributed fillers reduce protrusion and pull-out during finishing, allowing lower Ra for the same protocol (35).

Our subgroup patterns align with these principles. The submicron/supra-nano composite (Estelite Sigma Quick) achieved very low Ra with polishing + glycerin even without preheating, whereas the microhybrid (Enamel Plus HRi) showed the largest absolute Ra reduction when preheating and glycerin shielding were combined with polishing—consistent with the notion that higher-viscosity, larger-particle systems benefit more from rheologic improvement at placement and higher DC near the surface (36). At the same time, the lack of significant preheating effect in the “glycerin-only/no polishing” subgroup suggests that without mechanical leveling, subtle improvements in DC do not automatically translate into topographic smoothness (9).

Analytical Perspectives from Subgroup Findings

Control vs. polished groups: Controls (no oxygen barrier, no polishing) exhibited the highest Ra (e.g., 5.20±0.61 µm non-preheated), reflecting OIL presence, machining irregularities, and superficial filler protrusions. Polishing alone (air-cured) cut Ra dramatically (e.g., to ~1.6-2.0 µm in our setup) by removing OIL and leveling peaks; this echoes Park & Lee’s finding that polishing dominates immediate surface hardness and, by inference, surface integrity (25).

Glycerin-only (no polishing): Oxygen shielding alone reduced debris and slightly lowered Ra relative to raw controls, but surfaces remained rough compared with polished groups. FT-IR/nanoindentation data suggest glycerin elevates DC and hardness, but topographic asperities from occlusal contouring remain unless abrasively levelled (27). Our non-significant difference between preheated vs. non-preheated within this subgroup underscores that chemistry gains cannot replace mechanical finishing.

Polishing + glycerin: This combined protocol consistently produced the lowest Ra in both composites, with additional reductions when preheating was used (e.g., 0.83±0.01 µm in preheated Enamel Plus HRi). Mechanistically, preheating improves adaptation and initial packing; glycerin boosts near-surface DC by minimizing oxygen quenching during the terminal cure; and polishing eliminates the residual OIL and refines texture to a filler-supported, gloss-retentive surface (26).

Preheating differentials by composite: The larger absolute effect of preheating observed in the microhybrid likely reflects its higher baseline viscosity and broader filler distribution. When viscosity drops on warming, cavity adaptation improves and voids/porosity diminish; the post-cure surface thus requires less aggressive finishing to achieve a low Ra (37). By contrast, the already-flowable, spherical-filled Estelite Sigma Quick may derive smaller incremental benefit from preheating for surface smoothness specifically, even though its handling and early DC still improve (1).

Clinical Implications

From a clinical perspective, surface roughness (Ra) below approximately 0.2 µm is considered biologically favourable, as further reductions do not substantially influence plaque retention, whereas values above this threshold are associated with increased biofilm accumulation, staining, and tongue detectability beyond 0.3 µm (21,29). Although the Ra values in this study were obtained from standardized Class I surfaces and are therefore higher than those typically reported on flat disc specimens, the relative differences between groups are highly informative. The findings reaffirm that finishing and polishing remain the most decisive factors for achieving a smooth surface, since unpolished restorations retain topographic peaks that can act as niches for biofilm and extrinsic stains (25,30). Application of a glycerin barrier before the final cure further enhances near-surface polymerization by minimizing oxygen inhibition, which improves polishability and hardness (25,27). Preheating is particularly valuable in highly filled or more viscous composites, as it lowers viscosity, improves cavity adaptation, reduces surface porosity, and synergistically enhances the effect of polishing and oxygen shielding (6). Selection of an appropriate polishing system is equally important; multi-step aluminum-oxide disc systems such as Sof-Lex remain reliable, while polishing speed and technique influence final outcomes, with higher speeds under controlled pressure producing lower Ra values (22). Finally, composite filler technology should guide clinical protocols, since spherical, submicron-filled systems tend to maintain smoothness and gloss more predictably than irregular, microhybrid fillers, which often benefit more from preheating and meticulous polishing (29). Together, these observations support a pragmatic clinical sequence—preheating, careful placement, glycerin application, terminal light curing, and multi-step polishing—which maximizes the likelihood of achieving biologically acceptable smoothness and enhances both esthetic and functional longevity of composite restorations (22).

Extracted human molars, rather than composite blocks, were used to simulate the anatomical, optical, and thermal conditions that restorations encounter clinically. This more realistically simulates the intraoral environment, since the influence of cavity geometry, dentin reflectance, and light attenuation through enamel may have had an impact on the polymerization gradients and oxygen-inhibited layer thickness. These factors consequently affect the surface roughness, microhardness, and overall surface topography of the cured composite. It has also been shown that the substrate and configuration of the cavity alter the degree of conversion and surface characteristics of resin composites from those of flat composite discs (4,9). Thus, standardized Class I cavities in human molars were used to provide a clinically relevant simulation of restoration placement and curing and finishing conditions.

Study Limitations and Future Directions

The current study gives useful information on how preheating, application of a glycerin barrier, and different polishing protocols affect surface properties in resin composites. The results, however, are confined to topographical and near-surface properties alone: surface roughness and microhardness measured along with SEM-observed morphology. The deeper mechanical and chemical parameters, including flexural strength, wear resistance, and DC, despite their critical role in ensuring the long-term performance and durability of composite restorations, were not considered. Thus, the interpretations and conclusions drawn from this study should be regarded within the narrow context of surface-level analysis.

Future studies should incorporate spectroscopic methods, like ATR-FTIR or micro-Raman spectroscopy, to quantify the degree of conversion directly and relate this property to surface results. DC mapping on standardized flat specimens or tooth-mounted restorations would allow for the precise characterization of polymerization gradients within the upper 10-50 µm of the material. Moreover, nanoindentation testing combined with gloss and contact-angle tests could give a broader overview of the surface integrity, wear resistance, and tendency for biofilm accumulation. These kinds of multifactorial analyses will advance knowledge on how preheating and shielding strategies against oxygen influence not only the resulting surface quality but also the overall clinical lifetime of resin composite restorations.

Novelty and Contribution

This study advances existing knowledge in three ways. First, it examines the combined effects of preheating, oxygen shielding, and polishing, demonstrating that preheating is most effective when finishing is optimized, whereas glycerin alone is insufficient. Second, it compares microhybrid and submicron composites under identical protocols, showing that filler type influences how these interventions affect surface roughness. Third, it correlates SEM with profilometry, confirming that oxygen shielding reduces surface residues while polishing is required to achieve smooth, uniform surfaces.

CONCLUSION

Within the limitations of this in vitro study, the combination of preheating, glycerin barrier, and polishing produced the smoothest and hardest composite surfaces. The present results pertain only to surface characteristics and do not imply differences in overall mechanical or clinical performance. Additional work combining mechanical, optical, and chemical analyses is required to establish the broader validity of these findings under simulated intraoral conditions.

Ethical Approval: This research was conducted in accordance with the Declaration of Helsinki and the International Council for Harmonisation–Good Clinical Practice (ICH–GCP) guidelines. The study protocol, including the use of extracted human teeth, was reviewed and approved by the Ethics Committee of RUDN University, Moscow, Russia.

Informed Consent: Written informed consent was obtained from all individuals who donated extracted teeth for research purposes. No human subjects were photographed or otherwise identifiable; therefore, no additional consent for images was required.

Animal Research Statement: This study did not involve animals.

Conflict of Interest Statement: All authors declare that they have no financial or personal conflicts of interest that could have influenced the work reported in this manuscript.

Funding Statement: This research received no external funding from public, commercial, or not-for-profit organizations.

AUTHOR CONTRIBUTION STATEMENT: Conceptualization and Study Design, Z.K., O.M. and V.M.; Methodology, Validation and Investigation, Z.K. and O.M.; Data Curation, Formal Analysis and Interpretation, V.M., Z.K. and O.M.; Resources and Experimental Support, Z.K. and O.M. Writing – Original Draft Preparation, Z.K. and O.M.; Writing – Review and Editing, V.M. Supervision and Project Administration, V.M. All authors approved the final manuscript and accepted accountability for the integrity and accuracy of the work.

Data Availability Statement: All data generated and analyzed during this study are included in the manuscript. Additional raw datasets, SEM images, and profilometry files are available from the corresponding author upon reasonable request.

REFERENCES

1. Alharbi G., Al Nahedh H.N., Al-Saud L.M., Shono N., Maawadh A. Effect of different finishing and polishing systems on surface properties of universal single shade resin-based composites. BMC Oral Health. 2024 Feb 7; 24 (1): 197.

2. Erturk-Avunduk A.T., Atılan-Yavuz S., Filiz H., Cengiz-Yanardag E. A comparative study of polishing systems on optical properties and surface roughness of additively manufactured and conventional resin based composites. Sci Rep. 2024 Oct 27; 14 (1): 25658.

3. Pawar P., Gulve M., Aher G., Kolhe S., Pramaod J. Spectrophotometric evaluation of staining of different types of light-cure composite resins after exposure with different light-cure intensities: An in vitro study. J Conserv Dent. 2022; 25 (5): 510.

4. Lopes L.C.P., Terada R.S.S., Tsuzuki F.M., Giannini M, Hirata R. Heating and preheating of dental restorative materials—a systematic review. Clin Oral Invest. 2020 Dec; 24 (12): 4225-35.

5. Bhopatkar J., Ikhar A., Nikhade P., Chandak M., Agrawal P. An In-depth Analysis of the Effects of Preheating Three Different Composite Resin Systems at Two Different Temperatures on Their Microhardness. Journal of Datta Meghe Institute of Medical Sciences University. 2023 Oct; 18 (4): 727-32.

6. Elkaffass A., Eltoukhy R., Elnegoly S., Mahmoud S. Influence of preheating on mechanical and surface properties of nanofilled resin composites. J Clin Exp Dent. 2020; e494-500.

7. Marigo L., Nocca G., Fiorenzano G., Callà C., Castagnola R., Cordaro M., et al. Influences of Different Air-Inhibition Coatings on Monomer Release, Microhardness, and Color Stability of Two Composite Materials. BioMed Research International. 2019 May 9; 2019: 1-8.

8. Lim J.H., Lee S.Y., Gu H., Jin G., Kim J.E. Evaluating oxygen shielding effect using glycerin or vacuum with varying temperature on 3D printed photopolymer in post-polymerization. Journal of the Mechanical Behavior of Biomedical Materials. 2022 June; 130: 105170.

9. Gaviria-Martinez A., Castro-Ramirez L., Ladera-Castañeda M., Cervantes-Ganoza L., Cachay-Criado H., Alvino-Vales M., et al. Surface roughness and oxygen inhibited layer control in bulk-fill and conventional nanohybrid resin composites with and without polishing: in vitro study. BMC Oral Health. 2022 Dec; 22 (1): 258.

10. Jaramillo-Cartagena R., López-Galeano E.J., Latorre-Correa F., Agudelo-Suárez A.A. Effect of Polishing Systems on the Surface Roughness of Nano-Hybrid and Nano-Filling Composite Resins: A Systematic Review. Dentistry Journal. 2021 Aug 12; 9 (8): 95.

11. Chowdhury D., Mukherjee S., Maity I., Mazumdar P. Surface roughness and microhardness evaluation of composite resin restorations subjected to three different polishing systems immediately and after 24 h: An in vitro study. Journal of Conservative Dentistry and Endodontics. 2023 Nov; 26 (6): 639-45.

12. Altınışık H., Özyurt E. Effect of different polishing systems on surface roughness and gloss values of single-shade resin composites. BMC Oral Health. 2024 Nov 15; 24 (1): 1391.

13. Aktu A., Ulusoy N. Effect of Polishing Systems on the Surface Roughness and Color Stability of Aged and Stained Bulk-Fill Resin Composites. Materials. 2024 July 19; 17 (14): 3576.

14. Augusto M.G., De Andrade G.S., Mathias-Santamaria I.F., Dal Piva A.M.D.O., Tribst J.P.M. Comparison of Polishing Systems on the Surface Roughness of Resin Based Composites Containing Different Monomers. J Compos Sci. 2022 May 17; 6 (5): 146.

15. Kaminedi R., Penumatsa N., Priya T., Baroudi K. The influence of finishing/polishing time and cooling system on surface roughness and microhardness of two different types of composite resin restorations. J Int Soc Prevent Communit Dent. 2014; 4 (5): 99.

16. Poubel D.L.D.N., Da Silva R.C., Ribeiro A.P.D., Garcia F.C.P. Effect of preheating on the viscosity of composite resins. Journal of Conservative Dentistry and Endodontics. 2024 Apr; 27 (4): 360-5.

17. Daronch M., Rueggeberg F.A., De Goes M.F. Monomer Conversion of Pre-heated Composite. J Dent Res. 2005 July; 84 (7): 663-7.

18. Lucey S., Lynch C.D., Ray N.J., Burke F.M., Hannigan A. Effect of pre-heating on the viscosity and microhardness of a resin composite. J of Oral Rehabilitation. 2010 Apr; 37 (4): 278-82.

19. Ayub KV, Santos GC, Rizkalla AS, Bohay R, Pegoraro LF, Rubo JH, et al. Effect of preheating on microhardness and viscosity of 4 resin composites. J Can Dent Assoc. 2014; 80: e12.

20. Patussi A.F.C., Ramacciato J.C., Da Silva J.G.R., Nascimento V.R.P., Campos D.E.S., De Araújo Ferreira Munizz I., et al. Preheating of dental composite resins: A scoping review. J Esthet Restor Dent. 2023 June; 35 (4): 646-56.

21. Mitra A., Sharma S., Bhattacharyya A., Majumder G., Bhattacharyya S., Mazumdar R. Comparative evaluation of interfacial adaptation of the fiber posts using different adhesive systems: An scanning electron microscope study. Journal of Conservative Dentistry and Endodontics. 2025 Oct; 28 (10): 1045-9.

22. Bollenl C.M.L., Lambrechts P., Quirynen M. Comparison of surface roughness of oral hard materials to the threshold surface roughness for bacterial plaque retention: A review of the literature. Dental Materials. 1997 July;13 (4): 258-69.

23. Erdemir U., Sancakli H.S., Yildiz E. The effect of one-step and multi-step polishing systems on the surface roughness and microhardness of novel resin composites. Eur J Dent. 2012 Apr; 6 (2): 198-205.

24. Hassan A., Nabih S., Mossa H., Baroudi K. The effect of three polishing systems on surface roughness of flowable, microhybrid, and packable resin composites. J Int Soc Prevent Communit Dent. 2015; 5 (3): 242.

25. Park H.H., Lee I.B. Effect of glycerin on the surface hardness of composites after curing. J Korean Acad Conserv Dent. 2011; 36 (6): 483.

26. Carrillo-Marcos A., Salazar-Correa G., Castro-Ramirez L., Ladera-Castañeda M., López-Gurreonero C., Cachay-Criado H., et al. The Microhardness and Surface Roughness Assessment of Bulk-Fill Resin Composites Treated with and without the Application of an Oxygen-Inhibited Layer and a Polishing System: An In Vitro Study. Polymers. 2022 July 28; 14 (15): 3053.

27. Ciocan L.T., Biru E.I., Vasilescu V.G., Ghitman J., Stefan A.R., Iovu H., et al. Influence of Air-Barrier and Curing Light Distance on Conversion and Micro-Hardness of Dental Polymeric Materials. Polymers. 2022 Dec 7; 14 (24): 5346.

28. Khabadze Z.S., Abdulkerimova S.M., Drobyshev S.V., Borlakova M.M., Magomedova KhM. Investigation of the surface of light-curing dental materials after pre-polymerization heating. Èndodontiâ today. 2024 Jan 17; 21 (4): 252-62.

29. Kameyama A., Nakazawa T., Haruyama A., Haruyama C., Hosaka M., Hirai Y. Influence of Finishing/Polishing Procedures on the Surface Texture of Two Resin Composites. TODENTJ. 2008 Mar 12; 02 (1): 56-60.

30. Tepe H., Erdılek A., Sahın M., Efes B., Yaman B. Effect of different polishing systems and speeds on the surface roughness of resin composites. J Conserv Dent. 2023; 26 (1): 36.

31. Lippert VF, Bresciani E, Mota EG, Bittencourt HR, Kramer PF, Spohr AM. In vitro comparison of one-step, two-step, and three-step polishing systems on the surface roughness and gloss of different resin composites. J Esthet Restor Dent. 2024 May; 36 (5): 785-95.

32. Khabadze Z.S., Dashtieva M.Y., Borlakova M.M., Urazgulov A.K., Smirnov D.P., Kozlova Z.V., et al. Comparison of the clinical parameters of restorations performed with total-etch and self-etch adhesive techniques. Èndodontiâ today. 2024 Apr 4; 22 (1): 39-50.

33. Ruivo M.A., Pacheco R.R., Sebold M., Giannini M. Surface roughness and filler particles characterization of resin-based composites. Microscopy Res & Technique. 2019 Oct; 82 (10): 1756-67.

34. Mandal P., Laha A., Roy P.K. Effect of different concentrations of community water fluoride on composite resin: Field emission scanning electron microscopy and energy-dispersive X-ray analysis. Journal of Conservative Dentistry and Endodontics. 2025 Oct; 28 (10): 1027-33.

35. Oliveira G.U.D., Mondelli R.F.L., Charantola Rodrigues M., Franco E.B., Ishikiriama S.K., Wang L. Impact of filler size and distribution on roughness and wear of composite resin after simulated toothbrushing. J Appl Oral Sci. 2012 Oct; 20 (5): 510-6.

36. Marghalani H.Y. Effect of filler particles on surface roughness of experimental composite series. J Appl Oral Sci. 2010 Feb; 18 (1): 59-67.

37. Karaaslan G., Fidan M., Ayar M.K. Effect of Caps Warmer and VisCalor Dispenser preheating on the color stability and surface roughness of resin composites after coffee immersion: an in vitro study. BMC Oral Health. 2025 July 5; 25 (1): 1110.