Degree of Conversion of Nanoceramic and Microhybrid Composites Activated by Different Polymerization Modes at Different Intervals: An In Vitro Comparative Study

INTRODUCTION

Recently, resin composite reclamation has been sought after in esthetic dentistry. In this manner, there have been endeavors to continually improve the clinical, mechanical, and esthetic properties of this material. The few elements impacting the properties of resin composites are polymeric matrix composition, filler molecule type, and content, and the degree of conversion (DC) after the polymerization response, which can impact the dissolvability and sorption behavior of resin composites.¹ The degree of curing composite resins, spectral distribution, and intensity of the curing light used, as well as the shade or color, stability, opacity, and chemical composition of the resin-based composite itself, are the prime considerations when choosing these materials for restorations. In addition, both the composite resin components and their curing time have a significant association with the resulting level of polymerization. Also, both the composite segments and its relieving time have a critical relationship with the subsequent degree of polymerization.² The appraisal of DC% polymerization progress is viewed as a helpful instrument in describing and understanding the polymerization kinetics utilizing diverse composite formulations and restoration techniques.³

Light initiation that uses a high introductory irradiance gives a higher DC. However, this initiation mode may incite a high shrinkage worry in the material, thus increasing marginal gap formation at the cavity wall–resin composite interface. Soft-start light-activation modes have been introduced in an attempt to overcome this drawback in composite polymerization.^4,5

The physical and mechanical properties of the dental composite are influenced by the degree of change. The lower level of transformation leads to unwanted results with deficient properties, diminished bond qualities, expanded breakdown at the edges, diminished biocompatibility, more prominent staining, and corruption, poor wear obstruction, and color stability, less holding grip, and minor micro leakage.

Notwithstanding numerous reports about the impacts of light-emitting diode (LED) on the composite resin, most examinations have concentrated distinctly on 24-hour investigations. However, the mechanical properties of the resin composite immediately after light activation deserve some concern as the restorative material is submitted to all stress-related to finishing procedures and occlusal adjustment.⁶ For this reason, it is crucial to determine the DC values of resin composite for a longer duration. Faced with these findings, the purpose of this study was to investigate the influence of LED light source on the DC between new nanoceramic and microhybrid resin composites exposed to light at two different polymerization modes and measured at different intervals. There is a severe paucity of data related to DC of different composite resins, with different polymerization modes and at different times, hence the hypothesis associated with the present study is that nanoceramic resins are better cured as compared with microhybrid resins.

MATERIALS AND METHODS

The study was conducted after obtaining approval from the Institutional Ethical Committee. This study evaluated the DC% of two composite resins, namely Ceram.X universal nanoceramic composite and Spectrum TPH microhybrid composite. The composites are listed in Table 1. Both the composite resins were of A2 shade. The composite samples were light-cured using four Bluephase G2 LED curing units (batch no. 215502, wavelength range 385–515 nm, power density 1,200 mW/cm² ± 10% with tip diameter ~0.61 cm² and circular tip design). The light output of the curing unit was assessed regularly after every five exposures using a light radiometer device (Bluephase Meter; Ivoclar Vivadent, Schaan Liechtenstein, Germany).

Fourier-transform infrared spectrophotometer (Thermo Electron Scientific Instruments Corporation, Waltham, Massachusetts, USA) was used to assess DC%. The specifications for the same are mentioned in Table 2.

A total of 80 cylindrical samples of composites (5 mm diameter × 2 mm thickness) were fabricated using a Teflon mold and randomly divided into eight subgroups.

A Mylar strip (Frasaco, Tettnang, Germany) was placed on top of a microscopic glass slide on which the mold was placed. The mold was filled completely with Ceram.X or Spectrum TPH composite material, and the top surface of the mold was covered with one more Mylar strip followed by microscopic glass slide (1 mm thickness). The glass slide was slightly compressed to extrude the excess material out of the mold. The light cure unit (Bluephase G2) was used to polymerize the composite samples in soft start mode at 1,200 mW/cm² light intensity by positioning the light guide tip to be in contact with the glass slide on the top surface of the specimen and to standardize the distance from the light cure tip. The Mylar strips gave a smooth finish for the sample by avoiding any incidence of scattering of the light beam. To standardize the sample thickness, a digital caliper (Ultra Cal Mark III; Fowler Sylvac, Switzerland) was used. After polymerizing, the samples were removed from the mold, and placed in a lightproof dark container and stored in an incubator (Cultura; Ivoclar Vivadent) for 2 days and 7 days at 37°C, 90% ± 10% relative humidity before DC% testing.

Flowchart 1 shows various group divisions of the study.

After cleaning and the background is corrected, the composite samples (polymerized) were placed onto the small crystal area. Once the composite samples have been placed on the crystal area, the pressure arm was positioned over the crystal/sample area. Force was applied to the sample, pushing it onto the diamond surface. The data were obtained by Spectrum™ FT-IR software, “Preview Mode,” which allows the quality of the spectrum to be monitored in real-time while fine-tuning the exerted force. The pressure was applied until the strongest spectral bands were obtained with an intensity extending beyond 70% T.

The composite samples (unpolymerized) (n = 4) of each composite resin were placed in the crystal area and then pressed in a similar way, and then a spectrum was obtained with the same parameters as for the polymerized specimens.

The number of double carbon bonds that are converted into single bonds provides the DC% of the composite resin and helps to measure the DC. The FTIR spectra of both uncured and cured samples were analyzed using an accessory of the diffuse reflectance. The measurements were recorded in the absorbance operating under the following conditions: 32 scans, a 4 cm⁻¹ resolution, and a 400–4,000 cm⁻¹ wavelength. Degree of conversion was determined by estimating the changes in peak height ratio of the absorbance intensities of aliphatic Cm̂C peak at 1,637 cm⁻¹ and that of an internal standard peak of aromatic Cm̂C at 1,608 cm⁻¹ during polymerization, in relation to the uncured material.

Degree of conversion % for each specimen was calculated using the following equation:⁷

The data obtained were computed and statistically analyzed using IBM SPSS Statistics version 20 (Statistical Package for the Social Sciences; SPSS Inc., Chicago, IL, USA). Two-way and three-way ANOVA were used in testing statistical significance for the effect of the material, the curing times, and duration. The significance level was set at p ≤ 0.05.

RESULTS

The present study compared the DC% between nanoceramic composite Ceram X and microhybrid composite Spectrum TPH cured for 20 seconds and 40 seconds and DC% measured at 2 days and 7 days after photoactivation.

The comparison of Ceram X measured at 2 days and 7 days cured for 20 seconds and 40 seconds, respectively, showed a statistically significant difference (p = 0.000). The mean was highest at 7 days interval in both curing times of 20 seconds (50.53) and 40 seconds (55.44). The mean difference between 2 days and 7 days interval at 20 seconds and 40 seconds curing time was 6.86 and 7.01, respectively. Moreover, this mean-difference was statistically significant (p < 0.05) (Table 3).

The mean difference at 2 days between 20 seconds and 40 seconds was statistically significant in Ceram.X (p = 0.001) but was statistically not significant in Spectrum TPH (p = 0.168). The mean difference at 7 days between 20 seconds and 40 seconds was statistically significant in Ceram.X (p = 0.002) but was statistically not significant in Spectrum TPH (p = 0.116) (Table 4).

There was no statistically significant difference between Ceram.X and Spectrum TPH cured for 20 seconds and measured at 2 days (p = 0.168), but it was statistically significant when cured for 20 seconds and measured at 7 days (p < 0.000).

The mean difference (5.991) between Ceram.X and Spectrum TPH was statistically significant (p < 0.05) (Table 5).

Three-way ANOVA shows a statistically significant difference in DC (p < 0.05). However, the interaction effect was statistically significant only between composite resin (A) and curing time (B) and composite resin (A) and duration (C) (p < 0.05) (Table 6).

DISCUSSION

The present examination assessed the DC% of two composite resins to be specific, Ceram X and Spectrum TPH, cured with an LED light unit with polymerization times of 20 seconds and 40 seconds at 2 days and 7 days. Both the composite resins were of A2 conceal to limit any impact of shading on polymerization of the composites.^8,9

In the present examination, DC was assessed by FTIR as it empowers checking the curing procedure during polymerization and furthermore gives an immediate estimation of unreacted methacrylate groups, and it has been acknowledged as a dependable method for distinguishing the Cm̂C bond extending vibrations before and after polymerization of composite resins.^1,10

The aftereffects of the present investigation showed that DC estimations of Ceram X acquired are similar to the study led by Prasanna et al.,¹¹ who revealed a DC% of 50–52% when contrasted with 43–55% in our examination independent of time presentation. However, Jafarzadeh et al.¹² found values for Ceram X extending from 36% to 65% when cured with quartz–tungsten–halogen (QTH) light for 20 seconds and demonstrated a higher DC running from 65 to 78% when restored with LED, paying little heed to the profundity of the fix of the example. This result conflicted with the results of the present investigation. In our examination, DC% of Spectrum TPH cured for 40 seconds detailed DC% of 41–47%, which is similar to the consequences of the study by Rahiotis et al.,⁴ which revealed DC% of 46.2%. On the contrary, Ozturk et al.¹³ obtained the values of 59.8% in contrast with 41–47% in our examination independent of the introduction time and time interval. This could be the result of the different spectral distributions and intensity of the curing light used.

A few specialists are of the view that the nano filled composite pitch demonstrates a lower DC% contrasted with microhybrid composites.^14,15 The equivalence is clarified by the way that composite resins with nanoparticles demonstrated an altogether lower light transmittance in contrast with small-scale mixture composites in which the filler range is in micrometers. The light going through the resin composite is dispersed by the little filler particles, and light transmittance is decreased. In nano-filled composites, the light power is constricted as a result of the light-dispersing impact, in this way diminishing the DC. On the contrary, in the half and half composite, the light transmittance is higher, prompting high DC values.^16,17

The distinction in the DC estimations of the composites in the present examination could be identified with the synthetic structure of the materials. Ceram X contains granulated round submicron glass fillers, which could have permitted even more light transmittance in contrast with Spectrum TPH, and consequently, the higher DC esteems. Notwithstanding the fillers, the monomer substance of the composites could be the contributing element for the distinctions in DC esteems between the materials.

It has been accounted that the proportion of filler present in the resins impacts DC. The higher filler rate can prompt insufficient restoration of the composites.¹⁸ The two composites utilized in our examination had fillers running between 72% and 76% by weight. So the result of the present investigation conflicted with that by Yoon et al.,¹⁸ which is that filler rate could impact the DC. The investigation upheld the perspective of Price et al.,¹⁹ who revealed that the connection between filler rate and light transmission was not even.

The composites in this investigation were cured utilizing an LED unit either for 20 seconds or for 40 seconds at 1200 mW/cm² light power with soft start mode. The primary explanation behind utilizing this new age of LED unit was the way that higher thickness irradiance would improve the curing adequacy of composite resins.²⁰ Additionally, they appear to be exceptionally effective in contrast with QTH restoring units and dispose of the requirement for cooling fans. Warming of the illuminated composite materials by LED lights is likewise minimal.¹³ Furthermore, during polymerization; the composite is first submitted to a low irradiance pursued by higher irradiance, in this way permitting shrinkage stress, by polymer chain relaxation. It has likewise been demonstrated that soft-start polymerization mode does not diminish the DC of dental composites.¹⁶ The DC% of the composite materials is proportional to light exposure time,²¹ and accordingly, the level of restoration is expanded if the light introduction time of the materials is increased.²² The present study answered both, as the composite materials showed higher DC rates when presented to 40 seconds curing time when contrasted with 20 seconds curing time.

The higher DC% in the samples exposed to increased curing time could be related to the greater light energy used in the material. These discoveries were in concurrence with the discoveries of another investigation that showed that when light vitality was diminished, the DC decreased considerably.²³ Previous examinations have revealed that the noteworthy polymerization was accomplished inside 24 hours of exposure to light restoration and very little change in postfix DC esteems, which was in logical inconsistency with the consequence of our investigation. Ceram.X composite demonstrated a huge change in DC esteems from 2 days to 7 days. The expansion in postfix DC esteems could be clarified by the way that a few segments of the radicals created during composite polymerization are caught inside the heterogeneous system and can endure for increased periods. As time progresses, the caught radicals can experience pendant gatherings, bringing about extra conversion.²⁴

CONCLUSION

Within the limitations of the present investigation, the result is that the nanoceramic composite Ceram.X exhibited high DC in contrast with that of microhybrid Spectrum TPH with respect to diverse polymerization modes. DC was significantly altered by the time interval and mode of polymerization. Degree of conversion % test is a dependable test to evaluate the biocompatibility of composite and physical properties by estimation of its leftover unresponded monomer.

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