Abstract
Background
Preheating resin composites can reduce viscosity and improve handling; however, resin syringes or capsules used in multiple clinical applications may undergo repeated heating and cooling. The purpose of this study was to examine the effect of repeated preheating on the polymerization shrinkage kinetics and microhardness of resin composites.
Methods
Three capsule-type nanohybrid resin composites, Filtek Z350 XT (FZ), Estelite Sigma Quick (ESQ), and EsCom250 (EC), were tested under three conditions: non-heated, one-cycle preheating, and five-cycle preheating. Under the one-cycle preheating condition, the specimens were preheated once before curing. Under the five-cycle preheating condition, the specimens underwent five preheating cycles before curing, with cooling to room temperature between cycles. Preheating was performed using a composite warmer set to 55 °C. The polymerization shrinkage strain, maximum shrinkage rate, and time to maximum shrinkage rate were measured using a real-time displacement-based system. The Vickers microhardness was measured on the upper and lower specimen surfaces after 24 h of water storage. Data were analyzed using two-way analysis of variance (ANOVA) with material type and preheating conditions as fixed factors, including their interactions. Post hoc comparisons were performed for each material when appropriate.
Results
Two-way ANOVA showed significant material × preheating-condition interactions for polymerization shrinkage strain and maximum shrinkage rate, but not for time to maximum shrinkage rate or upper- or lower-surface Vickers microhardness. Within-material comparisons showed that the preheating conditions significantly affected the shrinkage strain in the FZ and EC, the maximum shrinkage rate in all three materials, and the time to maximum shrinkage rate in the ESQ and EC. Five-cycle preheating reduced the maximum shrinkage rate in all materials and delayed the time to reach the maximum shrinkage rate in ESQ and EC. Preheating did not reduce the Vickers microhardness of the upper or lower surfaces of any of the materials.
Conclusions
Within the limitations of this in vitro study, one-cycle preheating produced limited, material-dependent changes in polymerization shrinkage kinetics and did not reduce Vickers microhardness. Five repeated preheating cycles altered the shrinkage kinetics in a material-dependent manner but did not reduce the Vickers microhardness.
Supplementary Information
The online version contains supplementary material available at https://doi.org/10.1186/s12903-026-09107-7.
Keywords: Resin composite, Repeated preheating, Polymerization shrinkage, Vickers microhardness
Background
Resin composites are widely used in restorative dentistry because of their esthetic properties, adhesive placement, and acceptable clinical performance, and their indications have expanded with developments in material formulation and direct restorative techniques [1, 2]. Because resin composite placement is technique sensitive, clinical procedures that improve handling, adaptation, and polymerization behaviors may be important for predictable restorations.
Preheating resin composites is one such clinical technique. Since the concept was introduced more than four decades ago [3], contemporary evidence has indicated that increased temperatures can temporarily reduce viscosity and improve dispensing, flow, handling, and cavity adaptation [4–7]. Reduced viscosity may also facilitate adaptation to the cavity walls and reduce the incorporation of air voids during placement, whereas higher temperatures may increase radical and monomer mobility during polymerization and improve degree of conversion or some mechanical properties. However, these reported benefits are not uniform, and their effects on polymerization kinetics, shrinkage behavior, shrinkage stress, and mechanical properties are material- and protocol-dependent.
At the restoration level, these rheological changes may have clinically relevant consequences beyond the ease of manipulation. Conventional pre-warmed composites have been reported to exhibit better adaptation to cavity walls and between increments, fewer large voids, and higher strength than room-temperature composites. However, this advantage is time-sensitive; after removal from a warmer environment, restorative composites lose heat rapidly, and the effective working temperature depends on the material formulation, heating protocol, and handling delay [7, 8]. These factors support the evaluation of preheating under conditions resembling repeated chairside use.
Most laboratory studies evaluated resin composites after a single preheating procedure. In clinical practice, resin syringes or capsules used in multiple applications may undergo repeated heating and cooling before use. However, direct evidence of repeated heating and cooling remains limited, and previous studies have mainly addressed mechanical properties or clinical performance rather than real-time polymerization shrinkage kinetics [9–12]. Recent studies also support the potential benefits of cervical adaptation while emphasizing material- and device-dependent behavior and the need to consider thermal safety [13–15]. Thus, the effects of repeated preheating on shrinkage kinetics and microhardness of conventional nanohybrid resin composites remain unclear.
Therefore, this study evaluated the polymerization shrinkage strain, maximum shrinkage rate, time to reach the maximum shrinkage rate, and Vickers microhardness of three resin composites under non-heated, one-cycle preheating, and five-cycle preheating conditions. The null hypotheses were that for each tested resin composite, one-cycle and five-cycle preheating would not affect the polymerization shrinkage strain, maximum shrinkage rate, time to maximum shrinkage rate, or upper- and lower-surface Vickers microhardness compared with the non-heated condition.
Methods
This is an in vitro study of dental materials. Three capsule-type nanohybrid resin composites were evaluated: Filtek Z350 XT (FZ; 3 M ESPE, St Paul, MN, USA); Estelite Sigma Quick (ESQ; Tokuyama, Tokyo, Japan); and EsCom250 (EC; Spident, Incheon, Republic of Korea). The details of the materials are listed in Table 1.
Table 1.
Composite materials tested
| Product | Shade | Type | Manufacturer | Resin matrix | Filler content (wt%/vol%) |
Lot no. |
|---|---|---|---|---|---|---|
|
Filtek Z350 XT (FZ) |
A2B | Nanohybrid | 3 M ESPE, St Paul, MN, USA | Bis-GMA, UDMA, Bis-EMA |
20 nm silica, 4–11 nm zirconia, zirconia/silica clusters (78.5/63.3) |
NF41769 |
|
Estelite Sigma Quick (ESQ) |
A2 | Nanohybrid | Tokuyama, Tokyo, Japan | Bis-GMA, TEGDMA |
200 nm spherical silica-zirconia filler (82.0/71.0) |
E8777 |
|
EsCom250 (EC) |
A2 | Nanohybrid | Spident, Incheon, Republic of Korea | Bis-GMA, UDMA, TEGDMA |
16–1200 nm zirconia filler, Ba glass, fumed silica (77.1/65.0) |
52A21050 |
Bis-EMA ethoxylated bisphenol-A dimethacrylate, Bis-GMA bisphenol-A glycidyl methacrylate, TEGDMA triethylene glycol dimethacrylate, UDMA urethane dimethacrylate
A temperature of 55 °C was selected based on previous reports suggesting that excessive heating is unnecessary and that temperatures above approximately 54 °C may not further increase conversion in a clinically useful manner [16, 17]. Each material was assigned to one of three conditions: non-heated (room temperature, 25 °C), one-cycle preheating, and five-cycle preheating. Under the one-cycle preheating condition, the specimens were preheated once before curing. Under the five-cycle preheating condition, the specimens were preheated five times before curing and cooled to room temperature between cycles. One preheating cycle was defined as 30 min of heating in a composite warming device (Calset Unit 21; AdDent, Danbury, CT, USA) set to 55 °C followed by 30 min of cooling at room temperature. The five-cycle protocol was selected as the standardized laboratory model of repeated thermal exposure, which is consistent with previous studies evaluating repeated preheating of resin composites [9, 10]. The 30-min heating and 30-min cooling intervals were used to create complete heating-and-cooling cycles under controlled conditions, allowing the material to cool to room temperature before reheating.
To characterize temperature loss during the handling interval before light curing, the temperature changes in the preheated resin composites were measured at room temperature. Each preheated resin composite was dispensed onto a glass slide, and temperature was recorded every second for 2 min using a thermometer (Testo 915i; Testo, Titisee-Neustadt, Germany). The relative decrease in temperature was calculated using the highest recorded post-placement temperature as the reference value.
A schematic of the real-time displacement-based measurement setup is shown in Fig. 1. Polymerization shrinkage strain, maximum shrinkage rate, and the time to reach the maximum shrinkage rate were measured for each material and preheating condition (n = 15 per subgroup) using a real-time displacement-based system (µ-BioMechanics and software; IB Systems, Republic of Korea). A standardized amount (16 mm3) of the composite was shaped into a disk approximately 0.5 mm thick and 5–6 mm in diameter between glass surfaces. After 10 s of baseline recording, the specimens were light-cured for 20 s using a 900 mW/cm2 LED curing unit (P-LED Controller, IB Systems). The polymerization shrinkage was recorded for 300 s, and the shrinkage strain was calculated from the shrinkage displacement and specimen thickness. The polymerization rate curves were derived from the shrinkage-strain curves, and the maximum shrinkage rate and time required to reach the maximum shrinkage rate were determined.
Fig. 1.
Schematic diagram of the real-time displacement-based instrument used for polymerization shrinkage measurement
For the Vickers microhardness testing, disk-shaped specimens were prepared in an e-PTFE mold with a diameter of 5 mm and a depth of 2 mm for each material and preheating condition (n = 15 per subgroup). The specimens were covered with a celluloid strip to obtain flat surfaces and light-cured for 20 s using a 900 mW/cm2 LED curing unit (S-lite; Monitex Industrial, Taiwan). The specimens were stored in distilled water for 24 h. The Vickers microhardness was measured on the upper and lower surfaces using a microhardness tester (MMT-X; Matsuzawa, Japan) under a load of 100 gf for 10 s. Three randomly selected indentations were made on each surface, and the mean value was used for analysis.
For analysis, the sample size was set at 15 specimens per material and preheating condition, based on previous comparable in vitro resin-composite studies evaluating preheating-related changes in polymerization shrinkage, polymerization kinetics, and hardness or mechanical properties [18–20], and to provide sufficient replication for within-material comparisons among preheating conditions.
Statistical analyses were performed using the SPSS Statistics (version 25; IBM Corp., Armonk, NY, USA) and R software (version 4.5.2; R Foundation for Statistical Computing, Vienna, Austria). Specimen-level raw data were analyzed using two-way analysis of variance (ANOVA), with material type and preheating conditions as fixed factors. The material × preheating condition interaction was examined to determine whether the effect of preheating differed among materials. When a significant interaction was detected, post hoc comparisons were performed within each material to compare the preheating conditions, with adjustments for multiple comparisons. Between-material post hoc comparisons were not used to support product-level superiority because the primary objective was to evaluate the effect of repeated preheating on each resin composite. Residual normality was assessed using the Shapiro-Wilk test, and homogeneity of variance was assessed using Levene’s test centered at the median. Effect sizes were reported as partial η2. The significance level was set at p < 0.05.
Results
When resin composites preheated at 55 °C were dispensed for testing, the highest post-placement temperatures were approximately 48–50 °C, indicating immediate temperature loss during transfer from the warmer to the testing position. The temperature decreased rapidly during the early handling period. By approximately 40–60 s after placement, approximately 80% of the initial temperature elevation above room temperature had been lost. Thereafter, the temperature gradually decreased to room temperature (Fig. 2).
Fig. 2.
Temperature-time curves after removal of preheated resin composites from the warming device. FZ, Filtek Z350 XT; ESQ, Estelite Sigma Quick; EC, EsCom250
Two-way ANOVA results, including partial η2 values, are summarized in Table 2. Significant material × preheating-condition interactions were observed for the polymerization shrinkage strain and maximum shrinkage rate, indicating that the effect of preheating was material-dependent. Therefore, these outcomes were interpreted primarily through within-material comparisons. For the time to maximum shrinkage rate and upper- and lower-surface Vickers microhardness, the material × preheating-condition interactions were not significant. The preheating conditions had significant effects on the maximum shrinkage rate and time to maximum shrinkage rate, but not on the polymerization shrinkage strain or upper- or lower-surface Vickers microhardness. Assumption checks showed deviations from residual normality for all outcomes (Shapiro-Wilk p ≤ 0.025) and heterogeneity of variance across the material × preheating-condition subgroups (Levene’s test, all p < 0.001). Because the design was balanced, with equal subgroup sizes in all cells, the ANOVA results were interpreted together with partial η2 values and descriptive within-material comparisons.
Table 2.
Two-way ANOVA results for polymerization shrinkage kinetics and Vickers microhardness
| Outcome | Effect | df | F value | p value | Partial η2 |
|---|---|---|---|---|---|
| Polymerization shrinkage strain (%) | Material | 2, 126 | 82.158 | < 0.001 | 0.566 |
| Preheating condition | 2, 126 | 2.624 | 0.076 | 0.040 | |
| Material × Preheating condition | 4, 126 | 9.959 | < 0.001 | 0.240 | |
| Maximum shrinkage rate (%/s) | Material | 2, 126 | 286.845 | < 0.001 | 0.820 |
| Preheating condition | 2, 126 | 53.701 | < 0.001 | 0.460 | |
| Material × Preheating condition | 4, 126 | 20.038 | < 0.001 | 0.389 | |
| Time to maximum shrinkage rate (s) | Material | 2, 126 | 18.338 | < 0.001 | 0.225 |
| Preheating condition | 2, 126 | 7.608 | < 0.001 | 0.108 | |
| Material × Preheating condition | 4, 126 | 0.599 | 0.664 | 0.019 | |
| Upper-surface Vickers microhardness (VHN) | Material | 2, 126 | 890.044 | < 0.001 | 0.934 |
| Preheating condition | 2, 126 | 0.174 | 0.840 | 0.003 | |
| Material × Preheating condition | 4, 126 | 1.788 | 0.135 | 0.054 | |
| Lower-surface Vickers microhardness (VHN) | Material | 2, 126 | 369.250 | < 0.001 | 0.854 |
| Preheating condition | 2, 126 | 2.953 | 0.056 | 0.045 | |
| Material × Preheating condition | 4, 126 | 0.730 | 0.573 | 0.023 |
Two-way ANOVA was performed using the material type and preheating conditions as fixed factors (n = 15 per material × preheating-condition subgroup)
The df values are the degrees of freedom of the numerator and denominator. Effect size is reported as partial η2
The polymerization shrinkage strain-time curves are shown in Fig. 3, and the polymerization shrinkage strain values are shown in Table 3 and Fig. 4A. For Fig. 3, representative individual time-series curves were selected for visualization within each material; for each material-preheating condition subgroup, the selected curve was the specimen whose final polymerization shrinkage strain was closest to the corresponding subgroup mean among the 15 measurements. In FZ, within-material comparisons showed a significant difference among preheating conditions (p = 0.004), and the one-cycle preheating condition showed a lower shrinkage strain (2.04 ± 0.08%) than the non-heated (2.15 ± 0.13%) and five-cycle preheating (2.15 ± 0.07%). In ESQ, preheating condition did not significantly affect shrinkage strain (p = 0.121), with values ranging from 1.86 ± 0.07% to 1.93 ± 0.11%. In EC, a significant difference was observed among preheating conditions (p < 0.001), and the five-cycle preheating showed a lower shrinkage strain (2.08 ± 0.18%) than the one-cycle preheating (2.34 ± 0.14%).
Fig. 3.
Polymerization shrinkage strain-time curves by material. Curves represent individual specimens selected for each material-preheating condition subgroup. FZ, Filtek Z350 XT; ESQ, Estelite Sigma Quick; EC, EsCom250
Table 3.
Polymerization shrinkage kinetic parameters by material and preheating condition
| Polymerization shrinkage strain (%) | Maximum shrinkage rate (%/s) | Time to maximum shrinkage rate (s) | |||||||
|---|---|---|---|---|---|---|---|---|---|
| Material | Non-heated | 1 cycle | 5 cycles | Non-heated | 1 cycle | 5 cycles | Non-heated | 1 cycle | 5 cycles |
| FZ |
2.15 ± 0.13a (2.07–2.22) |
2.04 ± 0.08b (2.00-2.08) |
2.15 ± 0.07a (2.11–2.20) |
0.33 ± 0.02a (0.32–0.34) |
0.32 ± 0.02ab (0.31–0.33) |
0.31 ± 0.02b (0.30–0.32) |
1.56 ± 0.22a (1.44–1.68) |
1.57 ± 0.18a (1.47–1.67) |
1.65 ± 0.24a (1.52–1.78) |
| ESQ |
1.93 ± 0.11a (1.87–1.99) |
1.86 ± 0.07a (1.83–1.90) |
1.88 ± 0.07a (1.84–1.92) |
0.38 ± 0.02a (0.37–0.39) |
0.37 ± 0.02a (0.36–0.38) |
0.34 ± 0.02b (0.33–0.36) |
1.48 ± 0.11ab (1.42–1.54) |
1.40 ± 0.12a (1.33–1.47) |
1.54 ± 0.19b (1.43–1.65) |
| EC |
2.21 ± 0.17ab (2.11–2.30) |
2.34 ± 0.14a (2.26–2.42) |
2.08 ± 0.18b (1.98–2.18) |
0.49 ± 0.05a (0.46–0.52) |
0.50 ± 0.03a (0.48–0.52) |
0.38 ± 0.04b (0.36–0.40) |
1.34 ± 0.15a (1.26–1.43) |
1.30 ± 0.13a (1.23–1.38) |
1.49 ± 0.12b (1.42–1.56) |
Values are mean ± SD with 95% confidence intervals shown in parentheses (n = 15 per subgroup)
Different lowercase letters indicate statistically significant differences among preheating conditions within each material and parameter (p < 0.05)
Fig. 4.
Polymerization shrinkage kinetic parameters according to material and preheating condition. A Polymerization shrinkage strain. B Maximum shrinkage rate. C Time to reach the maximum shrinkage rate. Bars represent mean values, and error bars represent standard deviations (n = 15 per subgroup). Asterisks indicate significant pairwise differences among the preheating conditions within the same material (*p < 0.05, **p < 0.01, ***p < 0.001). FZ, Filtek Z350 XT; ESQ, Estelite Sigma Quick; EC, EsCom250
The maximum shrinkage rate and time to reach the maximum shrinkage rate are shown in Table 3 and Fig. 4B and C. Within-material comparisons showed that the preheating conditions significantly affected the maximum shrinkage rate in the FZ (p = 0.033), ESQ (p < 0.001), and EC (p < 0.001). Five-cycle preheating produced a lower maximum shrinkage rate than the non-heated condition in FZ (0.31 ± 0.02 vs. 0.33 ± 0.02%/s) and lower values than both non-heated and one-cycle preheating in ESQ (0.35 ± 0.02 vs. 0.38 ± 0.02 and 0.37 ± 0.02%/s) and EC (0.38 ± 0.04 vs. 0.49 ± 0.05 and 0.50 ± 0.03%/s). Regarding the time to reach the maximum shrinkage rate, the preheating condition had no significant effect on the FZ (p = 0.494), whereas significant differences were observed in the ESQ (p = 0.044) and EC (p = 0.001). The five-cycle preheating showed later peak timing than the one-cycle preheating in ESQ (1.54 ± 0.19 vs. 1.40 ± 0.12 s) and later peak timing than the non-heated and one-cycle preheating in EC (1.49 ± 0.12 vs. 1.34 ± 0.15 and 1.30 ± 0.13 s).
The Vickers microhardness values of the upper and lower surfaces are presented in Table 4 and Fig. 5. The preheating conditions did not significantly affect the upper- or lower-surface microhardness of any material. Within-material comparisons showed no significant effect of preheating conditions on upper-surface microhardness (FZ, p = 0.139; ESQ, p = 0.500; EC, p = 0.730) or lower-surface microhardness (FZ, p = 0.189; ESQ, p = 0.256; EC, p = 0.435). Upper-surface microhardness values were 87.22 ± 4.72 VHN for one-cycle FZ, 57.31 ± 2.04 VHN for five-cycle ESQ, and 57.21 ± 4.81 VHN for five-cycle EC; and lower-surface values were 72.14 ± 4.56, 50.29 ± 1.71, and 48.20 ± 4.08 VHN, respectively.
Table 4.
Vickers microhardness by preheating condition and specimen surface
| Upper-surface | Lower-surface | |||||
|---|---|---|---|---|---|---|
| Material | Non-heated | 1 cycle | 5 cycles | Non-heated | 1 cycle | 5 cycles |
| FZ |
85.59 ± 4.52 (83.08–88.09) |
87.22 ± 4.72 (84.61–89.84) |
83.85 ± 4.37 (81.43–86.27) |
72.14 ± 4.56 (69.62–74.66) |
68.56 ± 6.13 (65.16–71.95) |
68.83 ± 6.69 (65.12–72.54) |
| ESQ |
57.06 ± 3.34 (55.21–58.91) |
56.20 ± 2.49 (54.82–57.58) |
57.31 ± 2.04 (56.18–58.44) |
49.92 ± 3.29 (48.10-51.74) |
48.65 ± 3.15 (46.90–50.40) |
50.29 ± 1.71 (49.34–51.23) |
| EC |
56.92 ± 3.52 (54.97–58.87) |
56.16 ± 2.53 (54.76–57.56) |
57.21 ± 4.81 (54.54–59.87) |
48.20 ± 4.08 (45.94–50.46) |
46.43 ± 3.67 (44.39–48.46) |
47.37 ± 3.37 (45.50-49.24) |
Values are mean ± SD with 95% confidence intervals shown in parentheses, VHN (n = 15 per subgroup)
No significant pairwise differences were observed among preheating conditions within each material and surface
Fig. 5.
Upper- and lower-surface Vickers microhardness by material and preheating condition. Bars represent mean values, and error bars represent standard deviations (n = 15 per subgroup). No significant pairwise differences were observed among the preheating conditions within the same material and surface. FZ, Filtek Z350 XT; ESQ, Estelite Sigma Quick; EC, EsCom250
The original temperature-time data, polymerization shrinkage strain-time data, and the 15 specimen-level quantitative outcome data are provided as Supplementary Data 1, 2, and 3, respectively.
Discussion
This study tested whether repeated preheating changes the polymerization shrinkage kinetics and Vickers microhardness of three nanohybrid resin composites. A single preheating cycle caused only limited changes in shrinkage kinetics and did not reduce microhardness. Five preheating cycles changed the shrinkage kinetics, whereas the microhardness remained unchanged after 24 h. These findings indicate that the null hypothesis was only partially supported. Preheating affected some shrinkage-kinetic outcomes in a material-dependent manner, particularly after five preheating cycles, whereas the Vickers microhardness after 24 h was not reduced by either one- or five-cycle preheating under the tested conditions.
The analysis was designed to compare the preheating conditions within each material; it did not test for differences among the FZ, ESQ, and EC. Descriptively, ESQ exhibited a lower shrinkage strain, whereas EC exhibited a higher maximum shrinkage rate and an earlier rate peak. The FZ showed higher Vickers microhardness values, which may reflect differences in the resin matrix composition, filler type, and filler loading. These observations should not be interpreted as statistically significant between-product differences.
The significant material × preheating-condition interactions for the polymerization shrinkage strain and maximum shrinkage rate indicated that the effect of repeated preheating was not uniform across the tested composites. This material-specific response is consistent with previous experimental reports showing that prewarming or increased composite temperature can alter the flow, shrinkage, contraction stress, shrinkage force, and monomer conversion differently among resin-composite formulations [21–23]. These differences may reflect variations in the resin matrix composition, filler loading, filler morphology, and filler-matrix coupling, which can influence viscosity, heat retention, polymerization kinetics, and shrinkage development. In the present study, five-cycle preheating was associated with lower shrinkage-related outcomes in some materials; however, the findings do not establish a general preheating effect that applies uniformly to all resin composites.
Single-cycle preheating had little effect on shrinkage kinetics in this study, whereas earlier works reported increases in polymerization shrinkage or changes in polymerization behavior after composite preheating [17, 18, 24, 25]. This discrepancy may be partly related to the heat loss during specimen preparation and transfer. Temperature measurements after preheating showed rapid cooling after removal from the warming device, and clinically relevant handling intervals can reduce the temperature available for light activation [7, 26, 27]. In the present study, most of the temperature elevation above room temperature was lost within approximately 40–60 s after placement, a period that may overlap with the specimen preparation and transfer before light activation.
The lower shrinkage strain and maximum shrinkage rate observed after five preheating cycles require further investigation. One possible explanation is that repeated heating and cooling may affect the resin matrix and filler-matrix interface because the resin matrix and filler phases have different coefficients of thermal expansion [28]. Such thermal cycling can alter material flow or polymerization kinetics before light activation. Because this study did not measure degree of conversion, filler-matrix integrity, monomer elution, or shrinkage stress, the proposed mechanism and its clinical relevance require further verification. In particular, the shrinkage strain and rate describe the dimensional change and polymerization kinetics, whereas the shrinkage stress is more directly related to the bonded restoration behavior, interfacial stress, and marginal integrity. Future studies should combine repeated preheating protocols with direct conversion analysis, shrinkage-stress measurements under clinically relevant bonded configurations, and evaluation of filler-matrix integrity.
The microhardness results agree with reports in which preheating did not reduce the hardness or mechanical properties measured after 24 h [20, 29–31]. A systematic review also found that preheating can improve or maintain the surface hardness, depending on the material and protocol [32]. Because hardness was measured after 24 h of water storage, post-irradiation polymerization may have reduced the early differences among the groups. Microhardness is commonly used as an indirect indicator of degree of conversion; however, it is not interchangeable with direct spectroscopic measurements [33, 34]. Therefore, the absence of a reduction in the Vickers microhardness should be interpreted as evidence that the final surface hardness was maintained under the present conditions and not as direct evidence that the degree of conversion was unchanged. Therefore, the similar values among the preheating-cycle groups indicate that repeated preheating for up to five cycles did not significantly reduce the final surface hardness of these materials under the present curing conditions. The differences in absolute hardness were more likely to reflect the resin matrix composition, filler type, and filler loading than the number of preheating cycles.
Evidence related to repeated preheating under clinically relevant conditions remains limited but is relevant to the present question. Randomized clinical trials have suggested that repeated preheating of bulk-fill resin composites for up to ten cycles does not worsen postoperative hypersensitivity or 24-month clinical performance [11, 12]. Micro-CT evidence also suggests that preheating can improve the cervical adaptation of paste-like bulk-fill composites in Class II cavities [13]. These findings suggest that repeated preheating may be relevant in selected clinical protocols, although composite preheating should not be considered as routine in all restorative practices.
The present five-cycle protocol should be interpreted as a controlled in vitro model, rather than a direct reproduction of every clinical workflow. In clinical practice, resin composites may return to a warmer state during prolonged or multiple restorative procedures; however, the number and duration of warming episodes can vary according to the warming device, material packaging, amount of material used, and operator workflow. The 30-min heating and cooling periods were used to standardize repeated thermal exposure and ensure cooling to room temperature between cycles, not to imply that this exact timing occurs routinely in clinical practice. Moreover, the available clinical and adaptation studies have not directly established the effects of repeated preheating on the materials and outcomes tested. The present study used capsule-type nanohybrid composites, a controlled 55 °C protocol, and shrinkage kinetics and 24-h Vickers microhardness as outcomes; therefore, the clinical implications should be interpreted cautiously.
Thermal safety remains protocol-dependent when warmed composites are used clinically. Pulp temperature changes depend on the warming device, restorative site, remaining dentin thickness, and the cooling strategy [14, 15]. The present study did not evaluate pulpal temperature because it was designed to test material behavior outside a tooth model. Future studies should examine syringe-type composites, pulpal temperature changes, and longer or more variable heating protocols under clinically relevant handling conditions.
Conclusions
Within the limitations of this in vitro study, one-cycle preheating at 55 °C produced limited, material-dependent changes in polymerization shrinkage kinetics and did not reduce upper- or lower-surface Vickers microhardness after 24 h in the tested resin composites. Five-cycle preheating altered some shrinkage-kinetic parameters in a material-dependent manner but did not reduce the 24-h Vickers microhardness under the tested conditions. These findings should be interpreted based on test materials, preheating protocols, and in vitro outcomes. Further studies using direct degree-of-conversion analysis and polymerization shrinkage-stress measurements under clinically relevant bonded configurations are required to determine the clinical relevance of the observed shrinkage-kinetic changes.
Supplementary Information
Acknowledgements
Not applicable.
Abbreviations
- FZ
Filtek Z350 XT
- ESQ
Estelite Sigma Quick
- EC
EsCom250
- ANOVA
Analysis of variance
- SD
Standard deviation
- LVDT
Linear variable differential transformer
- LED
Light-emitting diode
- e-PTFE
Expanded polytetrafluoroethylene
- SPSS
Statistical Package for the Social Sciences
- VHN
Vickers hardness number
- micro-CT
Micro-computed tomography
Authors’ contributions
Daeyeong Kim: Methodology, Investigation, Formal analysis, Writing-original draft. Je Seon Song: Methodology, Formal analysis, Writing-review and editing. Dohyun Kim: Conceptualization, Methodology, Visualization, Writing-original draft, Writing-review and editing. Yooseok Shin: Conceptualization, Methodology, Resources, Supervision, Writing-original draft, Writing-review and editing.
Funding
This study was supported by the Yonsei University College of Dentistry Fund (grant number: 6-2025-0016).
Data availability
The original experimental data supporting the findings of this study are provided in Supplementary Data 1, 2, and 3. Additional information is available from the corresponding author on request.
Declarations
Ethics approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Competing interests
The authors declare no competing interests.
Footnotes
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Contributor Information
Dohyun Kim, Email: dohyun0kim@yuhs.ac.
Yooseok Shin, Email: densys@yuhs.ac.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Data Availability Statement
The original experimental data supporting the findings of this study are provided in Supplementary Data 1, 2, and 3. Additional information is available from the corresponding author on request.