Comparative evaluation of plasticization of composite resin by preheating, sonic, and ultrasonic activation on its adaptation to the tooth surface - A FESEM study

Abstract

Introduction:

By reducing the viscosity of composite resin, its adaptability to the tooth surface can be improved.

Aim:

This study aims to comparatively evaluate the efficacy of plasticization of composite resin by preheating, sonic, and ultrasonic activation on the marginal adaptation to the tooth surface.

Materials and Methods:

Standardized box-shaped Class I cavities (3 mm × 3 mm × 2 mm) were prepared on 32 extracted mandibular molars. Following etching and bonding, samples were divided into four groups depending on the technique used to plasticize the composite resin (n = 8 each): Group 1 – hand instrument (no plasticization done), Group 2 – preheated composite resin, Group 3 – sonically driven plasticization, and Group 4 – ultrasonically driven plasticization. Further, composite resins were compacted using a hand instrument and light-cured for 20 s. Samples were then stored in an incubator at 37°C and 100% humidity for 24 h to allow complete polymerization. Models were split in buccolingual direction to expose the bonding interface. Marginal adaptation was analyzed using FESEM (Field Emission Scanning Electron Microscope) at ×5000–×20,000.

Statistical Analysis Used:

The data were statistically analyzed using one-way ANOVA and post hoc test.

Results:

Post hoc pairwise comparison of marginal adaptation revealed statistically significant differences between Group 1 – hand instrument (no plasticization) with all other groups adapted using the plasticization technique. Ultrasonic plasticization demonstrated the best marginal adaptation, followed by preheated composite, then sonic plasticization, while the hand instrument placement technique showed the least favorable results.

Conclusion:

Plasticization of composite resin enhances the marginal adaptation to the tooth surface and produces gap-free margins.

INTRODUCTION

Over the years, composite resins have evolved from macro-filled to micro-filled, hybrid, and nanohybrid formulations. Advances in material science have contributed to improvements in the mechanical performance and clinical reliability of composite resin restorations.[1] Attempts to improve the mechanical properties in the form of high filler content have led to increased viscosity of composite resins.[2] This increase in viscosity adversely affects the rheological behavior of the material, which may subsequently influence marginal adaptation.[3]

Achieving an optimal marginal seal remains essential for the long-term predictability of adhesive restorations.[4] Marginal discrepancies may occur due to the high viscosity of heavily filled composites and stresses generated during polymerization shrinkage.[5] The primary factors influencing shrinkage stress and gap formation in composite resin restorations include the degree of polymerization shrinkage, the elastic modulus, and the flow characteristics of the material.[6] Composite resins undergo significant volumetric shrinkage during polymerization. As monomers cross-link to form covalent bonds within the polymer network, the intermolecular distance decreases, resulting in volumetric contraction.[7]

To improve marginal adaptation, the viscosity of composite resin can be reduced using plasticization techniques such as preheating, sonic activation, and ultrasonic-driven plasticization. These techniques enhance material flow without reducing filler content or compromising mechanical properties.[8] The viscosity of composite resin materials and the insertion technique can influence film thickness and the overall quality of the restoration.[9]

Preheating composite resins improves their rheological properties, reduces film thickness, and enhances monomer conversion during polymerization. These effects contribute to improved marginal adaptation, reduced microleakage, and increased hardness.[10] However, this technique has not been widely adopted in clinical practice due to concerns such as monomer evaporation at elevated temperatures and rapid heat loss during transfer from the heating device to the cavity.[11] For instance, composites heated to 60°C may lose approximately 50% of their temperature within 2 min and up to 90% within 5 min, thereby reducing the clinical benefits of preheating.[12] Consequently, alternative techniques for reducing viscosity have been explored.

Sonic activation temporarily reduces composite viscosity through a thixotropic response, facilitating improved flow along cavity walls.[13] Once sonic energy ceases, viscosity increases, thereby improving packing ability.[14] Despite its advantages, the requirement for specialised equipment has limited its routine clinical use.

Ultrasonic plasticization represents another approach, wherein ultrasonic tips modify the consistency of high-viscosity composite resins. This technique utilizes ultrasonic mechanical energy to generate internal heat within the material, thereby reducing viscosity and enabling effective adaptation and compaction against cavity walls. Ultrasonic vibrations induce a thixotropic effect, reduce film thickness, improve wettability, and enhance marginal adaptation.[15]

Various techniques are available for evaluating interfacial gaps and marginal adaptation, including dye penetration, microcomputed tomography, and scanning electron microscopy (SEM).[8,16] These methods allow visualisation of the tooth–restoration interface. Field Emission Scanning Electron Microscopy (FESEM), however, offers superior resolution with magnifications ranging from approximately ×10 to over ×1,000,000, enabling precise evaluation of interfacial gaps.[17]

In the present study, FESEM was employed to assess the marginal adaptation of composite resin restorations and evaluate the tooth–restoration interface.

MATERIALS AND METHODS

Thirty-two freshly extracted human mandibular molars, extracted for periodontal reasons, were selected for the study. Teeth with caries, restorations, fractures, previous endodontic treatment, or visible cracks were excluded.[8] All specimens were cleaned of soft-tissue remnants using an ultrasonic scaler (Waldent MaxPiezo 3+) and disinfected in 0.1% thymol for 24 h.

Standardized Class I cavities (3 mm buccolingual × 3 mm mesiodistal × 2 mm depth) were prepared using a #245 carbide bur followed by an SF-13 diamond bur under continuous water irrigation. Enamel and dentin were etched with 37% phosphoric acid gel for 15 s, rinsed thoroughly, and gently air-dried for 5 s. A universal adhesive (Single Bond Universal, 3M ESPE, USA) was applied with agitation for 10 s, gently air-thinned for 5 s, and light-cured for 20 s using an LED curing light (1200 mW/cm²).

The specimens were randomly allocated into four groups (n = 8) and restored using a nanohybrid composite resin (3M ESPE Filtek Z250 XT, USA):

• Group 1: Hand instrument (no plasticization): Composite resin was placed in a single increment using a Teflon-coated instrument (GDC) and cured for 20 s

• Group 2: Preheated composite: Composite resin was preheated to 50°C for 5 min using a composite warmer (Endoking, India), placed in a single increment, adapted with a hand instrument, and light-cured

• Group 3: Sonic plasticization: Composite resin was placed conventionally, followed by sonic activation for 2 min using a sonic tip (Glin Sonic Endo Activator, China), moved circumferentially along the cavity walls before hand adaptation and light curing

• Group 4: Ultrasonic plasticization: Following composite placement, ultrasonic plasticization was performed using an ultrasonic tip (Waldent MaxPiezo 3+) at a power setting of 5–6 for 2 min. The composite was then adapted using a hand instrument and light-cured for 20 s.

All specimens were stored at 37°C and 100% humidity for 24 h to ensure complete polymerization. Each tooth was sectioned buccolingually to expose the tooth–restoration interface. Root portions were discarded, and both crown halves were retained for evaluation.

Chromium sputtering of the specimens was performed using the Quorum Q150T S Plus unit. The samples were then analyzed using field emission scanning electron microscopy (FESEM) (Thermo Scientific Apreo 2S) to evaluate the tooth–restoration interface and assess the adaptation of the composite restorative material to the tooth margins at magnifications ranging from ×5000 to ×20,000. FESEM images were acquired for analysis. The entire interface was evaluated, and marginal gap measurements (μm) were obtained using XT Microscope Server software. The collected data were subjected to statistical analysis.

Statistical analysis was performed using one-way ANOVA for intergroup comparison, followed by Post hoc tests for pairwise analysis using SPSS v23.0 (Statistical analysis was performed using SPSS software (Version 23.0; IBM Corp., Armonk, NY, USA)). Significance level was set at P < 0.05.

RESULTS

Marginal gap values varied among the experimental groups. Group 1 (hand instrument) demonstrated the highest mean marginal gap (2.000 ± 0.8124 μm). Group 2 (preheated composite) showed reduced gap values (0.900 ± 0.6969 μm), followed by Group 3 (sonic plasticization) (1.050 ± 0.6547 μm). Group 4 (ultrasonic plasticization) exhibited the lowest marginal gap values (0.700 ± 0.5757 μm) [Figures 1–4].

Figure 1.

The marginal gap achieved by Group 1: hand instrument (no plasticization) with average value – 0.6 μm

Figure 4.

No marginal gap achieved by Group 4: ultrasonically driven plasticization

Figure 2.

The marginal gap achieved by Group 2: preheated composite with average value – 0.3 μm

Figure 3.

The marginal gap achieved by Group 3: sonically driven plasticization with average value – 0.4 μm

One-way ANOVA revealed statistically significant differences among the groups (P = 0.004). Post hoc analysis demonstrated a significant difference between Group 1 and Groups 2, 3, and 4, whereas no statistically significant differences were observed among Groups 2, 3, and 4 [Graph 1].

Graph 1.

Intergroup comparison of marginal adaptation (ANOVA)

Overall, ultrasonic plasticization resulted in the most favorable marginal adaptation, while hand instrumentation showed the least favorable outcome.

DISCUSSION

Marginal integrity is a key determinant of restoration longevity and is influenced by both material properties and placement techniques.[18] In the present study, marginal adaptation was quantitatively assessed, allowing precise measurement of interfacial gaps and offering an advantage over qualitative microleakage methods.[9] Marginal failure is often associated with the limited flow of highly filled composite resins, which may hinder complete adaptation in confined cavity areas.[8]

Ultrasonic plasticization demonstrated superior marginal adaptation, likely due to high-frequency mechanical vibrations that enhance thixotropic behavior, redistribute filler particles, and reduce film thickness by approximately 21%–49%.[19] The findings of the present study are consistent with those reported by Sajjan et al. who observed improved marginal adaptation with ultrasonically plasticized composite resins.[8]

Preheated composite resin also exhibited improved marginal adaptation. Increased temperature enhances monomer mobility and reduces intermolecular forces, thereby improving flow and reducing film thickness.[8] Montaser M. (2021) similarly observed better marginal adaptation with preheated composite compared with other insertion methods.[9]

Temperatures exceeding 60°C may result in monomer evaporation, resin degradation, and increased polymerization shrinkage, potentially compromising bond strength and restoration durability. Therefore, controlled preheating below this threshold is recommended.[9]

Sonic activation temporarily reduces composite viscosity by inducing oscillatory energy, resulting in acoustic streaming and improved monomer mobility.[9] In addition, sonic vibrations facilitate filler redistribution within the resin matrix, thereby reducing void formation.[20]

Hand instruments exhibited the least favorable marginal adaptation, likely due to reliance solely on manual compaction. Unlike plasticization techniques, it does not provide mechanical mechanisms to reduce viscosity, enhance flow, or minimize polymerization stress, all of which are critical for optimal marginal adaptation.[6]

Clinically, techniques that improve marginal adaptation by reducing composite viscosity allow better adaptation to cavity walls. This improved seal minimizes microleakage, reduces postoperative sensitivity, and contributes to improved clinical outcomes.

Despite the valuable findings, this study has certain limitations. As an in vitro investigation, it does not fully replicate the complex oral environment. In addition, only one composite resin formulation was evaluated, which may limit the generalisability of the results. Temperature control during preheating under clinical conditions also remains a challenge.

CONCLUSION

Within the limitations of this ex vivo study, all three plasticization techniques demonstrated improved marginal adaptation compared with conventional hand placement. Ultrasonic plasticization showed the greatest effectiveness, followed by preheating and sonic activation, whereas hand instrumentation resulted in the least favorable marginal adaptation. Ultrasonic plasticization may therefore be considered a reliable and effective technique for improving marginal integrity in routine composite resin restorations.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.