Due to the biocompatibility issues on mercury-containing products, restorative dental practice fosters an increased use of reliable materials other than amalgam.¹ Dental resin-based composites have evolved enormously in the recent past, offering esthetic and functional advantages with minimal preparation techniques.² Despite significant improvements, volumetric polymerization shrinkage during curing of resin composites ranges between 1% and 6%, and thus formation of interfacial gaps was the crucial issue with composite restorations.¹,³ A recent systematic review and meta-analysis reported the annual failure rate of 1%–3% for posterior composite restorations with bulk fracture and secondary caries being the main reasons for those failures.² Several studies stated that the bonded interface between the tooth and the restorative composite is a weak link by the formation of microgaps, allowing microleakage and bacterial invasion leading to secondary caries formation.³,⁴

With the improvements in technology, contemporary nanohybrid composites exhibited enhanced physical and mechanical properties with reduced shrinkage stress during polymerization compared to microhybrids.⁵,⁶ However, a continuous, stable, 3-dimensional collagen-polymer network between adhesive and dentin substrate is hard to achieve. The disparity between the demineralized dentin depth and adhesive monomer infiltration activates the hydrolytic degradation of hybrid layer and endogenous collagenolytic degradation.⁷,⁸ In addition, composite resin restorative materials accumulate more dental plaque biofilm, manifesting potential impact on recurrent caries formation.⁹ To counteract these issues, several hybrid-type restoratives have been launched, and the manufacturers promoted them as “bioactive restoratives.”

The “hybrid” restorative materials by incorporating bioactive agents such as bioglass, hydroxyapatite, calcium aluminate, or phosphate in the polymer matrix can preserve the interfacial adhesive integrity by promoting remineralization.¹⁰ In addition, resin-modified glass-ionomer-based bioactive restoratives exhibited minimum polymerization shrinkage, i.e., <1.7%, and thus can reduce microgap formation and leakage at dentin restorative interface.¹¹

The Activa bioactive (Pulpdent, Watertown, USA) is the first marketed “smart dental restorative” material, claimed to stimulate remineralization by releasing fluoride, calcium, sodium, and silicon ions. It is a dual-cure, hydrophilic resin-modified glass-ionomer containing bioglass and a patented polybutadiene diurethane dimethacrylate (Embrace) resin.¹² Earlier in vitro studies demonstrated enhanced wear and fracture resistance with high flexural strength for Activa bioactive, and they advocated its use for restoring occlusal stress-bearing areas.¹³,¹⁴ Nevertheless, a recent randomized in vivo assessment of Class-II restorations with Activa reported a high failure rate and unacceptable clinical performance.¹⁵,¹⁶

Hydroxyapatite modified glass-ionomer restorative cement, the Micron bioactive (Prevest DenPro, Jammu, India), was recently introduced into the market. This bioactive restorative has shown excellent adhesion to the tooth, prolonged fluoride release, and exhibited high compressive strength with the mineralizing potential property.¹⁷ The patented “alkasite” filler is responsible for the increased release of hydroxyl ions from the micron material, which regulates the pH during scathing attacks, thereby preventing demineralization.¹²

The Predicta Bioactive (Parkell, NY, USA), a bulk-fill dual-cure resin composite, has high compressive, tensile, and flexural strengths and is suitable for direct and indirect restorations. According to manufacturer claim, Predicta with its bioactive property provides a strong bond with the tooth by sealing the interfacial microgaps and protecting against microleakage.¹⁸

Internal adaptation refers to restorative material's ability to adapt to the dental substrate internally. Poor interfacial adaptation of a restorative material may result in plaque accumulation, microleakage, marginal discoloration, secondary caries, fracture, and restoration loss.⁴,¹⁶,¹⁹ Hence, a longitudinal assessment of internal adhesive defects is required to gauge the newly introduced restorative material's clinical applications. For evaluating internal adaptation of the restorations, nondestructive micro-computed tomography (micro-computed tomography [CT]) imaging is a more authentic technique. This technique allows 3-dimensional evaluation of the entire prepared tooth effectively, irrespective of the shape or dimensions of a specimen. In addition, repeated evaluations on the same specimen can be done quantitatively at baseline and after the aging process.²⁰

Considering the potential advantages of bioactive materials, this study investigated the internal adaptation of Activa, Micron, and Predicta-bulk bioactive restoratives compared to a nanohybrid composite in Class II cavities. The null hypothesis tested was (i) the internal adaptation of Class II restoration is not dependent on the type of restorative or different tooth-restorative interfacial locations. (ii) Aging with thermo-mechanical cyclic loading application did not have a detrimental influence on the integrity of tooth-restorative interface.

In this in vitro micro-CT assessment study, sixty noncarious, fully formed mandibular molar teeth with approximal mesiodistal and buccolingual dimensions (±1 mm) were collected for the study following the protocols approved by the Dr. NTR University of Health Sciences (D188601021).Teeth were stored in saline till the experimental period that was not more than 3 months after extraction. Sample size estimation was done depending on the data acquired from the pilot study using G* power 3.1 software (version 2.0) Aichach, Bavaria Germany. Based on power analysis with a Type I error of 5% and confidence interval of 95%, the minimal sample size calculated was 12 teeth per group. Thus, 20 specimens for each group were considered to assess the internal adaptation of restorative materials.

Restorative procedure

Standardized Class II mesio and disto-occlusal (MO and DO) cavities with 3.5 mm depth and 3 mm wide occlusally, 5 mm deep proximally from cervical to occlusal marginal ridge area with a gingival seat width of 1.5 mm were prepared in all 60 teeth samples. Cavities were prepared using # Ex 41 straight fissure diamond abrasive (Mani, Tochigi, Japan) and #245 carbide burs (SS White, New Jersey, USA) and discarded each bur after every five tooth preparations.

The materials used, their composition, and application methods are presented in Table 1. After cleaning with distilled water, all the prepared cavities were randomly assigned into three groups (n = 20 each). The mesio-occlusal cavities of all 60 teeth were etched with N-etch (Ivoclar, Liechtenstein, Europe) and after Tetric N-Bond adhesive application, restored with Tetric N-Ceram nanohybrid composite (Ivoclar, Schaan, Europe) using a horizontal incremental layering technique. Each increment was allowed to cure for about 20 s using Bluephase C8 light-emitting diode unit (Ivoclar, Schaan, USA) having an irradiance of 800 mW/cm².{Table 1}

Disto-occlusal cavities were restored with one of the three tested bioactive restorative materials (n = 20 each). Activa material was available as a two-paste system that can be dispensed through the spiral nozzle by co-extrusion. After conditioning the disto-occlusal cavities of 20 teeth with 37% phosphoric acid for about 15 s, the Tetric N-Bond adhesive was light cured for 10 s. The flowable Activa was dispensed first into the proximal cavity with slow pressure allowing the material to flow ahead of the nozzle tip. The cavities were filled with two increments and each increment was allowed to self-cure for about 30 s followed by light curing for 20 s, based on manufacturer's instructions.

In the Micron group, disto-occlusal cavities (n = 20) were conditioned with 10% polyacrylic acid for 20 s and thoroughly cleansed with water. Two scoops of Micron powder along with two drops of liquid were dispensed and mixed vigorously for about 30 s. The material was deposited proximally, then in the occlusal cavity, and was condensed by Teflon coated plastic filling instruments-1102-T2 and 1208T2 (Addley, Germany) to circumvent void formation. The restoration was contoured and allowed to self-cure for 2 min by following the manufacturer's instructions.

In disto-occlusal cavities (n = 20) of Predicta bioactive group samples, after etching and Tetric N-bond application, the material was expressed into the cavity with slow pressure, filling the deepest portion to occlusal surface. To reduce the shrinkage stresses, the material was allowed to self-cure for 2 min, followed by light curing for 20 s. Using flame-shaped TR-25EF (Mani, Tochigi, Japan), diamond abrasives, marginal overhangs, and gross irregularities of the restorations were removed. Sof-Lex medium to fine-grit flexible disks (3M ESPE, MN, USA) and polishing cups (Shofu, San Marcos, USA) were used at slow speed to finish and polish the restorations.

All the restored teeth samples were stored in artificial saliva (Jiana Lifescience, Mumbai, India) to mimic intraoral conditions during the whole experimental period till the post TMC evaluations. The baseline inter-facial defects were digitized using a micro-CT scanner (X-Radia Versa 500, Zeiss, Germany) within 2 weeks after the completion of restorations.

Thermo-mechanical cyclic loading

After recording the baseline values of interfacial microgaps, teeth were loaded mechanically and thermically cycled. Thermocycling was done at 5°C to 55°C for 10,000 cycles with 5 s transfer time and 30 s dwell time in the cold and hot cycle.²¹ The restorations were simultaneously subjected to 100,000 mechanical load cycles of 50 Newtons using a round piston of 5 mm diameter at 1 Hz frequency.²²

Interfacial adaptation evaluation with micro-computed tomography

To analyze the internal adaptation of the materials, teeth were mounted in acrylic blocks and placed in the micro-CT scanner. The parameters were set as follows; a rotation of 360° vertically, a 0.5° rotational step, 100 kV voltage, and an exposure time of 20 min with 100 mA° beam current using a 0.5 mm aluminum filter. Three-dimensional images were generated and reconstructed using Scout and Scan TM control system 10.7.3679.13921 (Zeiss, Germany) software Figure 1. The images were assessed for microgaps, and data was analyzed using AVIZO software (FEI, Thermo scientific, Germany). All the values obtained in micrometers were converted into percentages by dividing the observed microgaps length by the total length of that particular wall/floor.²⁰Figure 1

Micro-computed tomography images of Predicta group: (a) Axial wall before TMC, (b) Axial wall after TMC, (c) Pulpal wall before TMC, (d) Pulpal wall after TMC, (e) Buccal, lingual, and gingival walls before TMC, (f) Buccal, lingual, and gingival walls after TMC.

The outcome obtained indicates lower the percentage of microgaps; the better will be the adaptation of material to the cavity walls. One-way ANOVA results revealed significantly different mean microgap percentages between the groups at baseline and after artificial aging (P ≤ 0.001) Figure 2 and Table 2. The mean interfacial gap percentage was significantly high for Micron bioactive and minimum for Predicta bioactive material. Paired t-test results revealed a significant increase in the interfacial mean gap percentages among the groups after thermomechanical cyclic loading (P = 0.005), and the increase in microgaps was not different among the groups (P = 0.4256).Figure 2

Box-plot diagram showing comparison of interfacial gaps percentages among four groups before and after thermomechanical cyclic loading.

Microgap formation due to localized failure of a bond at the tooth-restorative interface is one of the most critical aspects, while it may affect the strength and longevity of a restoration.²³ The bioactive restoratives containing bioglass may facilitate the dissemination of fluoride, calcium, and phosphate ions into dental hard tissues, motivating remineralization by forming a hydroxyapatite layer along the tooth-restorative interface.²⁴ Deposition of minerals around denuded collagen prevents enzymatic and hydrolytic degradation of hybrid layer by minimizing matrix metalloproteinase (MMPs) and cathepsin activity and thus improves the dynamic tissue behavior besides restoring dentin mineral content.²⁵

At the time of remineralization, glass particle dissolution depends on the type of aqueous medium present. To simulate the intraoral conditions, restored teeth samples in the study were immersed in artificial saliva having ionic constituents similar to plasma throughout the experimental period.²⁶,²⁷ Intraorally, restorations get exposed to occlusal masticatory and parafunctional forces along with intermittent temperature changes. These thermal changes bring about contraction and expansion of restorative materials, permit water and oral fluids to penetrate through resin matrix, and accelerate hydrolysis at the interface leading to adhesive bond failure.²⁸ Among the different techniques available, thermomechanical load cycling is the most effective method to mimic artificial aging.²¹ Hence, the teeth were subjected to 10,000 thermal cycles and one lakh mechanical load cycles with an intermittent occlusal load of 50 Newtons in the study to simulate intraoral functioning for 1-year period.²²

Different teeth may have dissimilar mineral content properties and are subjected to varying types of age-related changes. To avoid bias and achieve better standardization, restorations were executed on the same tooth for test and control materials in the study. Though large Class I occlusal preparations might have high C-factor by generating maximum polymerization shrinkage stresses, microleakage and secondary caries occurrence were noted most commonly at the cervical margins of proximal restorations.²⁹ In consequence, occluso-proximal restorations were done in the study to assess the internal adaptation of the tested materials.

It was observed that interfacial microgap formation significantly differed among the tested materials. Moreover, thermomechanical cyclic loading significantly increased the percentage of microgaps interfacially for all the tested materials. These results suggest that both the first and second null hypotheses were rejected. Predicta bulk bioactive material has shown superior interfacial adaptation with minimal microgap formation. With dual-cure ability, Predicta restorative can attain high compressive, tensile, and flexural strengths with bulk placement.³⁰ Besides, it releases calcium, phosphate, and fluoride ions to stimulate apatite mineral formation and might block the microgaps formed.³¹ In addition, these bioactive restoratives have shown minimal shrinkage during polymerization facilitating better interfacial adaptation of the Predicta restorative. With its remineralizing ability, Predicta bioactive can reduce postoperative sensitivity, prevent secondary caries occurrence, seals the margins of a restoration better against microleakage, and improves the durability of the restoration.¹⁸

Activa bioactive has shown less percentage of interfacial gaps compared to Tetric N-Ceram and Micron bioactive restorative materials. Activa is a hybrid resin material having physical properties similar to composite resins and biological properties simulating that of glass ionomers.³⁰ However, some earlier studies reported less amount of fluoride release and more amount of microleakage with Activa bioactive.¹²,²⁷ A recent in vitro study recommended combining mechanical grinding of the substrate and chemical adhesive application to obtain optimal bonding outcomes with Activa restorative.³² The better adaptability of Activa to the tooth surface in the present study can be attributed to the Tetric N-bond adhesive ability to provide a strong bond. It was manifested earlier that in contrast to G-Bond, 5^th generation bonding agents can provide better adaptability with less microleakage for Activa restorative.³¹

Despite the manufacturer's claim that Micron bioactive has excellent adhesion to tooth and antibacterial property, it produced more percentage of interfacial microgaps among the tested bioactive restoratives in this study. Insufficient smear layer removal and lack of adhesive agent application might have caused defective adhesion of Micron to the prepared walls. The Tetric N Ceram composite resin disclosed a significantly higher percentage of microgap formation than Predicta and Activa bioactive restoratives in the study. Lack of chemical interaction with the tooth substrate and passive bonding of adhesive components of the Tetric N-Ceram might have caused the formation of microgaps maximum.

Bond degradation with increased interfacial gap percentages was noticed in all the tested teeth samples after thermomechanical cyclic loading. It was hypothesized that the bioactive materials could promote ion diffusion through the bonded interface and thereby increase the matrix to mineral ratio and minimize the nanoleakage.¹⁰,¹²,¹⁴ The released fluoride ions inhibit pro- and active MMP-2 and MMP-9 activity and prevent bond degradation by diffusing phosphate and calcium ions through hybrid layers. These ions may precipitate and crystallize with the formation of Ca-PO4/MMP complexes inhibiting the MMP's activity.³³ During functional, occlusal activity, the generated stresses get transmitted through rigid, brittle filler particles to the flexible resin matrix. High interfacial stresses may be generated due to the larger interfacial area between the matrix and filler particles during thermal changes and mechanical stresses intraorally.³⁴ In spite of their proven ion-releasing ability, artificial aging has negatively influenced the interfacial adaptation of bioactive restoratives in the study.

In agreement with the outcome of previous studies, the interfacial gaps observed were maximum on the pulpal and axial walls compared to gingival, buccal, and lingual walls.²⁰,³⁵ Dentinal walls of pulpal and axial walls are closer to the pulp. These walls, due to increased volume of fluid-filled dentinal tubules and elevated dentin permeability, impart suboptimal conditions for bonding. Compared to buccal and lingual walls, gingival interfacial gap percentages were more for all the tested materials. In the proximal cavity, gingival-occlusal height was longer than the buccolingual width and thus created more polymerization shrinkage stresses at the gingival seat area. Furthermore, insufficiency of enamel for bonding at gingival seat area makes this region less favorable for bonding.³⁶

The study results indicate the use of bioactive restoratives as they exhibited less interfacial microgap formation compared to nanohybrid composites. Though in vitro testing of restorative materials is done for preclinical investigations, the outcome wouldn't necessarily translate the clinical presentation. Moreover, none of these new bioactive restoratives revealed good evidence regarding their long-term clinical performance, though very few in vivo studies were conducted till now.

The following conclusions can be drawn from the study results:

Predicta and Activa bioactive restoratives presented less percentage of interfacial microgaps compared to nanohybrid composite

Interfacial gap percentages on pulpal and axial walls were maximum and minimal on buccal, lingual walls

Thermal and mechanical cyclic loading has a negative impact on the internal adaptation of bioactive restorative materials.

Financial support and sponsorship

Nil.

Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or nonfinancial in this article.

1 Ferracane JL Buonocore Lecture.Placing dental composites – A stressful experienceOper Dent 247 57 2 Ástvaldsdóttir Á Dagerhamn J van Dijken JW Naimi-Akbar A Sandborgh-Englund G Tranæus S Longevity of posterior resin composite restorations in adults – A systematic reviewJ Dent 934 54 3 Maske TT Hollanders AC Kuper NK Bronkhorst EM Cenci MS Huysmans MC A threshold gap size for in situ secondary caries lesion developmentJ Dent 36 40 4 Spencer P Ye Q Park J Topp EM Misra A Marangos O Adhesive/Dentin interface: The weak link in the composite restorationAnn Biomed Eng 1989 2003 5 Chen MH Update on dental nanocompositesJ Dent Res 549 60 6 Frankenberger R Reinelt C Krämer N Nanohybrid vs.fine hybrid composite in extended class II cavities: 8-year resultsClin Oral Investig 125 37 7 Tsujimoto A Barkmeier WW Takamizawa T Latta MA Miyazaki M Influence of thermal stress on simulated localized and generalized wear of nanofilled resin compositesOper Dent 380 90 8 Breschi L Mazzoni A Ruggeri A Cadenaro M Di Lenarda R De Stefano Dorigo E Dental adhesion review: Aging and stability of the bonded interfaceDent Mater 90 101 9 Svanberg M Mjör IA Orstavik D Mutans streptococci in plaque from margins of amalgam, composite, and glass-ionomer restorationsJ Dent Res 861 4 10 Jones JR Review of bioactive glass: From Hench to hybridsActa Biomater 4457 86 11 Alrahlah A Diametral tensile strength, flexural strength, and surface microhardness of bioactive bulk fill restorativeJ Contemp Dent Pract 13 9 12 Porenczuk A Jankiewicz B Naurecka M Bartosewicz B Sierakowski B Gozdowski D A comparison of the remineralizing potential of dental restorative materials by analyzing their fluoride release profilesAdv Clin Exp Med 815 23 13 Pameijer CH Garcia-Godoy F Morrow BR Jefferies SR Flexural strength and flexural fatigue properties of resin-modified glass ionomersJ Clin Dent 23 7 14 Francois P Fouquet V Attal JP Dursun E Commercially available fluoride-releasing restorative materials: A review and a proposal for classificationMaterials (Basel) 2313 15 van Dijken JW Pallesen U Benetti A A randomized controlled evaluation of posterior resin restorations of an altered resin modified glass-ionomer cement with claimed bioactivityDent Mater 335 43 16 Bhadra D Shah NC Rao AS Dedania MS Bajpai N A 1-year comparative evaluation of clinical performance of nanohybrid composite with Activa™ bioactive composite in class II carious lesion: A randomized control studyJ Conserv Dent 92 6 17 Khere CH Hiremath H Sandesh N Misar P Gorie N Evaluation of antibacterial activity of three different glass ionomer cements on streptococcus mutans: An in-vitro antimicrobial studyMed Pharm Rep 288 93 18 Available from: https://pdf medicalexpo[Last accessed on Jan ]. medicalexpocom/pdf/parkell-inc/predicta/73596-224730html [Last accessed on 2021 Jan 21] 19 Haak R Näke T Park KJ Ziebolz D Krause F Schneider H Internal and marginal adaptation of high-viscosity bulk-fill composites in class II cavities placed with different adhesive strategiesOdontology 374 82 20 Han SH Park SH Comparison of internal adaptation in class II bulk-fill composite restorations using micro-CTOper Dent 203 14 21 Han SH Park SH Micro-CT evaluation of internal adaptation in resin fillings with different dentin adhesivesRestor Dent Endod 24 31 22 Armstrong S Breschi L Özcan M Pfefferkorn F Ferrari M Van Meerbeek B Academy of Dental Materials guidance on in vitro testing of dental composite bonding effectiveness to dentin/enamel using micro-tensile bond strength (μTBS) approachDent Mater 133 43 23 Hannig M Friedrichs C Comparative in vivo and in vitro investigation of interfacial bond variabilityOper Dent 3 11 24 de Morais RC Silveira RE Chinelatti M Geraldeli S de Carvalho Panzeri Pires-de-Souza F Bond strength of adhesive systems to sound and demineralized dentin treated with bioactive glass ceramic suspensionClin Oral Investig 1923 31 25 Khoroushi M Mousavinasab SM Keshani F Hashemi S Effect of resin-modified glass ionomer containing bioactive glass on the flexural strength and morphology of demineralized dentinOper Dent E1 10 26 Roulet JF Hussein H Abdulhameed NF Shen C In vitro wear of two bioactive composites and a glass ionomer cementDeutsch Int 24 30 27 Owens BM Phebus JG Johnson WW Evaluation of the marginal integrity of a bioactive restorative materialGen Dent 32 6 28 Breschi L Maravic T Cunha SR Comba A Cadenaro M Tjäderhane L Dentin bonding systems: From dentin collagen structure to bond preservation and clinical applicationsDent Mater 78 96 29 Korkmaz Y Ozel E Attar N Effect of flowable composite lining on microleakage and internal voids in class II composite restorationsJ Adhes Dent 189 94 30 Zhang K Zhang N Weir MD Reynolds MA Bai Y Xu HH Bioactive dental composites and bonding agents having remineralizing and antibacterial characteristicsDent Clin North Am 669 87 31 Kaushik M Yadav M Marginal microleakage properties of activa bioactive restorative and nanohybrid composite resin using two different adhesives in non carious cervical lesions – An in vitro studyJ West Afr Coll Surg 1 14 32 Alsahhaf A Bond strength of resin composite of bioactive restorative materials using different surface treatmentsBiosci Biotech Res Commun 95 100 33 Sauro S Makeeva I Faus-Matoses V Foschi F Giovarruscio M Maciel Pires P Effects of ions-releasing restorative materials on the dentine bonding longevity of modern universal adhesives after load-cycle and prolonged artificial saliva agingMaterials (Basel) 722 34 Blumer L Schmidli F Weiger R Fischer J A systematic approach to standardize artificial aging of resin composite cementsDent Mater 855 63 35 Sampaio CS Garcés GA Kolakarnprasert N Atria PJ Giannini M Hirata R External marginal gap evaluation of different resin-filling techniques for class II restorations – A micro-CT and SEM analysisOper Dent E167 75 36 Schiltz-Taing M Wang Y Suh B Brown D Chen L Effect of tubular orientation on the dentin bond strength of acidic self-etch adhesivesOper Dent 86 91 37