Tridimensional Evaluation of the Interfacial Gap in Deep Cervical Margin Restorations: A Micro-CT Study

Resin composites are the most widely used materials in direct posterior restorations, and consequently, interest in the longevity and reliability of these materials has grown over time.¹  Although widely used, composites still present various problems. During the polymerization phase, these materials are subject to shrinkage stress, which can lead to debonding, causing an interfacial gap.^2,3  The resulting poor marginal seal is associated with postoperative sensitivity, secondary caries, periodontal problems, and infiltration of bacteria, liquids, and molecules, leading to marginal discoloration and failure of the restoration itself.^4,5 

The clinical prognosis for adhesive restorations with margins made entirely of enamel is excellent,^6,7  but the same cannot be stated for deep cavities; with cervical margins extending beyond the cemento-enamel junction (CEJ), it is difficult to obtain an effective and durable marginal seal.^8–10  It has been reported that restorations with cervical margins in dentin and cementum are more susceptible to marginal staining, postoperative sensitivity, and the formation of secondary caries.^4,11 

Different techniques and materials have been tested to improve the sealing of deep cervical margins, such as the open-sandwich technique,^12,13  the use of ceramic inlays,^14–16  and margin elevation with composites.¹⁶ 

Several studies have shown how the use of flowable composites, interposed between the cavity floor of the interproximal box and the restorative material, can reduce the interface stress related to volumetric contraction while curing.^17,18  These materials have a low elasticity modulus, which allows increased elastic deformation and therefore greater absorption of contraction stress caused by polymerization; this minimizes the interfacial gap, especially in the cervical area.^8,19  Another property of these materials is improved wetting, which facilitates adaptation, ensuring a more intimate contact with the cavity walls.²⁰  However, flowable resins have inferior mechanical properties compared with conventional composites.²¹ 

Some studies have reported a better marginal fit with use of flowable composite layering with reduced thickness.^22,23  However, Malmström and others showed that neither the thickness nor the presence of flowable composite as an initial cervical increment significantly influenced the marginal microleakage,²⁴  probably due to lower mechanical properties and a reduced resistance to deformation compared with conventional resin composites.²⁵ 

Bulk-fill flowable composites have been introduced to minimize internal polymerization stresses via a longer pre-gel phase. A study by Moorty and others showed that minor contraction stress exerted by bulk-fill flowable composites translates into reduced cuspal deflection compared with traditional composites placed with an oblique layering technique.²⁶  However, a laboratory study by Furness and others showed that bulk-fill materials, both flowable and nonflowable, resulted in a similar gap-free marginal interface compared with conventional composites.²⁷ 

Interfacial gap formation has been evaluated in the literature by means of different destructive tests,^28–32  hindering a more detailed analysis of the interface before and after polymerization. Microcomputed Tomography (micro-CT) enables creation of a three-dimensional (3D) map of the tooth–restoration interface and detection of the deepest marginal leakage,^33,34  allowing a detailed assessment not only of the entity but also of the topography of the interfacial gap.

To the best of our knowledge, no studies have performed a tridimensional interfacial gap analysis when the margin elevation technique with different resin composite materials and techniques for deep class II cavities is necessary. Thus, the aim of the present laboratory study was to evaluate the volumetric interfacial gap of composite restorations in class II cavities, with enamel and dentin cervical margins, before and after cyclic fatigue.

The null hypotheses were that the tridimensional interfacial gap during cervical margin elevation technique is not influenced by (1) the material used or (2) cyclic fatigue.

Forty-eight intact human maxillary premolars, extracted for periodontal reasons within the last three months, were selected and stored in distilled water after disinfection with an ultrasonic device. The selected teeth had no carious lesions, demineralization, cracks, or signs of wear. Two class II cavities, one mesial and one distal, of similar shape and size were created on each specimen by the same operator. The cavities were 4 mm in the buccal–lingual direction and 3 mm in the mesio-distal direction; the mesial cavity had an enamel cervical margin 1 mm above the cementoenamel junction (CEJ), whereas the distal cavity had a dentin cervical margin 1 mm below the CEJ. A circumferential steel matrix was applied (Automatrix, Dents-ply, Sirona, Germany) and tightened until a perfect fit with the cervical margin was achieved. Then, all specimens were subjected to the same adhesive procedure: selective enamel etching for 40 seconds with 35% phosphoric acid (K-etchant, Kuraray Noritake Dental, Mie, Japan), rinsing for 30 seconds, and air-drying. A two-step self-etch adhesive system (Clearfil SE Bond2, Kuraray Noritake Dental) was then applied following the manufacturer’s instructions and lightly air-dried before light-curing for 40 seconds with a light-emitting diode (LED) lamp (Cefalux2, VOCO, Cuxhaven, Germany). Specimens were then divided into six groups (n=8 each) using the following restoration techniques.

• Group 1 (G1): A 1-mm-thick horizontal layer of flowable resin (Grandioso Heavy Flow, VOCO) was applied over the cervical margin. The restoration was then finalized with 2-mm-thick oblique layers of nanofilled composite (Grandioso, VOCO).

• Group 2 (G2): A 1-mm-thick horizontal layer of ormocer flowable resin (Admira Fusion Flow, VOCO) was applied over the cervical margin. The restoration was then finalized with 2-mm-thick oblique layers of nanofilled ormocer (Admira Fusion, VOCO).

• Group 3 (G3): The same technique used in G1 was applied but with 2 mm of flowable composite (Grandioso Heavy Flow, VOCO).

• Group 4 (G4): The same technique used in G2 was applied but with 2 mm of flowable ormocer (Admira Fusion Flow, VOCO).

• Group 5 (G5): A nanohybrid composite (Filtek Supreme XTE, 3M ESPE, St Paul, MN, USA) was used, applying 2-mm-thick oblique layers.

• Group 6 (G6): A bulk restoration was performed using a bulk nanofilled composite (Filtek Bulk-Fill Posterior, 3M ESPE).

A summary concerning the used materials is given in Table 1.

In all specimens, each composite layer was light-cured with an LED lamp (Cefalux2, VOCO) at 1400 mW/cm² for 20 seconds. Finishing and polishing procedures were then performed with abrasive disks (SofLex, 3M ESPE) and silicon points (Enhance, Dentsply).

The marginal adaptation of each restoration was evaluated using a micro-CT scanner (SkyScan 1172, Bruker, Billerica, MA, USA). High-resolution scans were performed on each specimen using the following parameters: voltage = 100 kV; current = 100 μA; aluminum and copper (Al+Cu) filter; pixel size = 10 μm; averaging = 5; rotation step = 0.1°; and total scan duration = five hours.

A CS-4.4 chewing simulator (SD Mechatronik, Feldkirchen-Westerham, Germany) was used for fatigue-cycling mechanical aging of the specimens. The resilience of the human periodontium was simulated by coating the roots of the teeth with a 1-mm polyether layer (Impregum, 3M ESPE). A 6-mm-diameter steatite sphere was used with the following settings: occlusal load = 50 N; frequency = 1 Hz; downward speed = 16 mm/s; and sliding movement = 2 mm over the buccal triangular crest. All restored specimens had a standardized anatomy and were similarly positioned to center the sphere exactly on the central fossa of the tooth. The test was performed for 1,000,000 cycles in distilled water.

To reveal interfacial gap progression between the restorations and the tooth after cyclic fatigue, specimens were subjected to a second scan, with the same baseline parameters used to ensure consistency of the grayscale values. NRecon (Bruker, Kontich, Belgium) was used to reconstruct samples and obtain DICOM files (Digital Imaging and Communications in Medicine; .dcm) with the same Hounsfield unit (Hu) parameters and the following software corrections: beam hardening = 30%, smoothing = 3, smoothing kernel = 2 gaussian, and ring artifact correction = 7. A novel tridimensional method was used to analyze the interfacial gap. Using Mimics software (ver. 20.0, Materialise, Leuven, Belgium), thresholding of voids surrounding the restoration was performed automatically to include all voids surrounding the restoration in a 200-μm range. The Hu values representative for gap voids (−1024/990) were selected by an expert operator on the first sample and therefore applied to all samples. Using dynamic region growing and region growing functions, only internal and marginal gaps were included in the present analysis (Figures 1 and 2).

Volumetric calculation of the resulting mask was performed by the software, and volume data (expressed in cubic millimeters) were collected for both the dentin (Figure 3) and enamel interfaces (Figure 4).

To evaluate the effects of materials and techniques, a two-way analysis of variance (ANOVA) and post hoc Tukey tests were performed. The significance level was set to 95% (α=0.05). All statistical analyses were performed using the Stata software package (Stata-Corp, College Station, TX, USA).

Interfacial volumetric gaps (±SD; expressed in cubic millimeters) are shown in Tables 2 (enamel cervical margins) and 3 (dentin cervical margins). The results of the ANOVA showed that restoration technique (p=0.001) and chewing simulation (p=0.00001) significantly influenced the interfacial gap on dentin but not on enamel. The post hoc test showed that, for deep dentin margins, flowable resins, either 1 or 2 mm, were better able to seal the interface before the chewing simulation but were more prone to interfacial degradation than nano-hybrid and bulk-fill composites. After cyclic fatigue, only the dentin margins closed with 2 mm of flowable composites showed greater interfacial gap than the other groups. On enamel margins, no differences were found between the restoration techniques tested.

Based on the results of the present study, the first null hypothesis was partially rejected because the use of flowable resins yielded a significantly better marginal seal on deep cervical margin elevation at baseline. The sealing ability of flowable resins also showed a significant reduction after artificial aging; therefore, the second null hypothesis was rejected.

Several laboratory studies have tested the performance of adhesive systems by evaluating marginal gap formation around restorations of extracted teeth.³⁵  This method assumes that if the forces generated by polymerization shrinkage or thermo-mechanical strain exceed the bond strength, an observable gap will form at the margin of the restoration. Although there is no clear correlation between laboratory gap formation and interfacial failures observed clinically, it is reasonable to assume that this marginal gap formation is clinically relevant.³⁶  Many studies have found that all current adhesives appear incapable of completely sealing the restoration margins and thus preventing microleakage,^37,38  especially for cavities with deep cervical margins.

Many techniques have been used to assess micro-leakage, and the results vary considerably.²⁸  Traditional laboratory methods to detect microleakage between a restoration and composite use organic dyes, such as basic fuchsin, methylene blue, and rhodamine, in conjunction with microscopy techniques²⁹  or transmission electron microscopy.³⁰  The disadvantage of these analyses include invasiveness, semiquantitative results, and limited ability to represent tridimensional geometry.³¹  Using scanning electron microscopy (SEM) to determine the presence of internal cracks or voids requires sample sectioning, which eliminates the possibility of evaluating the effects of artificial aging on samples after a baseline analysis. Epoxy replicas can also be used and evaluated with SEM, but this allows analysis only of the external margin. Moreover, such results can be affected by the accuracy of the impression, and a weak-to-moderate correlation with clinical findings has been reported.³²  More recently, optical coherence tomography (OCT) has been used to evaluate interfacial adaptation and microleakage in composite restorations. OCT is a time-domain low-coherence interferometric technique that provides high-resolution cross-sectional (two-dimensional) or volumetric (tridimensional) images without x-ray exposure by quantifying the reflection of infrared light from dental structures.³⁹  The resolution of OCT is approximately 5–15 μm, which is more than radiographic or current clinical CT images.⁴⁰  OCT initially showed some limits in detecting gaps in deep cavities due to the light transmission ability through dental tissues and materials, but new techniques and equipment are overcoming this problem.⁴¹  However, a recently published review by Sahyoun and others stated that image scaling, deformable registration, and fusion methods still must be implemented to superimpose OCT data onto another 3D surface.⁴²  In the present study, micro-CT was used without any radio-opaque tracer to evaluate interfacial gaps. This approach has the advantages of being nondestructive, quantitative, and tridimensional. Specifically, it allows tridimensional visualization of the spatial distribution of the interfacial leakage along the cavity walls and floor, which cannot be obtained easily using traditional techniques that require sectioning of the specimen. These features make this method markedly more comprehensive. By contrast, traditional methods for microleakage studies can provide only limited, or even unrepresentative, information unless multiple sections of the sample are analyzed.³⁴  The volumetric evaluation of interfacial gaps between tooth hard tissues and restorations allows not only standardized, tridimensional measurement of the gap progression after cyclic fatigue, but also qualitative visualization of where the gap occurs. Stress propagation along adhesive restoration interfaces could be related to several factors and visualized by superimposition of baseline and after-chewing scans to determine the weakest point of the restoration–tooth interface.

When considering interface analysis between resin composite and tooth tissue, the adhesive system could represent a variable in marginal gap formation, particularly when margins are positioned apical to the CEJ. Bonding to dentin is different compared with enamel due to morphologic, histologic, and compositional differences; dentin contains a substantial proportion of water and organic materials, which impairs the bonding mechanism.⁴³  To improve the evaluation of restorative materials conducted in the present study, the same adhesive protocol was applied to all samples, as described in the previous section. Previous studies have shown an absence of resin tags in the cementum area,^44,45  which reinforces the notion that cervical margins have the weakest bonding strength among all areas of class II restorations. A two-step self-etch adhesive procedure with pre-etching of the peripheral enamel was performed on each sample in this study. As demonstrated in the literature, a mild etching effect causes a reduction in bond strength to enamel compared with that achieved using phosphoric acid–selective enamel etching.⁴⁶  This allows a considerable increase in the depth of resin penetration (longer resin tags), resulting in a better adhesion performance along enamel margins.⁴⁷  The results of the present study showed that the restoration techniques did not significantly influence the interfacial gap values of enamel margins at baseline or after chewing simulation. This can be explained by the greater adhesive reliability achievable on enamel substrate.⁴⁸ 

Consistent with previous studies, class II cervical margins in dentin, which usually yield to the deep margin elevation technique, showed a significantly greater marginal gap than those in enamel because dentin is a highly hydrophilic tissue that is only partially dehydratable and, therefore, more difficult to infiltrate by hydrophobic adhesives.²⁹  Water may persist within the adhesive layer on solvent evaporation, permeate the adhesive interface from the outer environment, or diffuse from the wet underlying dentin substrate. The amount of water uptake within the interface increases with time as bond strength decreases. In nonaqueous media, long-term preservation of dentin bond strength seems to be strongly linked to interface sealing.⁴⁹ 

The results of the present study also showed that marginal gaps along the dentin margins, treated with a deep margin elevation technique, were smaller when a flowable composite was used as the first horizontal layer, independent of its layer thickness and type. This finding is consistent with the results of two previous studies. Fabianelli and others reported that the open-sandwich technique was associated with significantly less dye penetration than the closed-sandwich technique,¹²  whereas Korkmaz and others reported that the closed-sandwich technique required greater operator skill and achieved worse marginal adaptation.⁵⁰  Sadeghi and Lynch showed how better marginal adaptation could be achieved using an intermediate layer of flowable composites or compomers, especially when the cavity extended below the CEJ.⁵¹  Moreover, it has been widely reported that stresses generated during placement of a composite restoration can significantly influence the immediate marginal leakage, especially when dentin margins are present.⁵²  With a low elastic modulus and better wettability, flowable composites can create an intermediate flexible layer between the adhesive system and the composite resin, reducing contraction stress and improving the restoration seal.⁵³ 

Several authors reported significant effects of flow on the cavity floor, reducing microleakage in class II restorations. A previous study evaluated microleakage with and without flowable liners and concluded that flowable composites reduced, but did not eliminate, microleakage at the gingival cavosurface margins apical to the CEJ.⁵¹  The use of flow materials could reduce C-factor effects, leading to a reduction in polymerization stress and associated problems when applied in a 1-mm-thick layer. Lowering the C-factor may reduce the internal stresses within the composite restoration. However, the benefit of the gingival liner for reducing polymerization contraction stress is still somewhat controversial: some studies have reported that the use of flow did not reduce microleakage in class II restorations with margins below the CEJ,^7,54  whereas other studies exhibited discordant results regarding microleakage.^7,9,55,56  However, the methods used to evaluate marginal gaps were not precise or standardized, leading to greater variability in results due to differences in sample preparation, sectioning, and data collection procedures. Contrasting results were also presented by Kim and Park, who used micro-CT to evaluate the internal adaptation of composites.¹⁶  They showed that bulk-fill and layered resin composites had similar marginal sealing quality over dentin. However, differences in restorative materials, flowable liners, adhesive systems, and above all, testing procedures may explain variations in results.

The present laboratory study showed how flowable composites exhibited greater interfacial deterioration than nanohybrid composites, with a significantly increased gap volume after artificial aging procedures, especially when applied in thicknesses of 2 mm. Furthermore, the 3D analysis of the interfacial gap progression after cyclic fatigue allowed visualization of the microleakage increasing more at the level of the angle between the axial pulp wall and gingival floor. This could be related to the flowable resin’s mechanical properties: Bayne and others evaluated the filler percent, wear, compressive strength, diametral tensile strength, indented biaxial flexure strength, and toughness of eight flowable and two hybrid composites.²⁵  Mechanical properties were approximately 60%–90% of conventional composites, resulting in a conclusion that flowable materials should be used with caution in high stress-bearing areas. More recently, Baroudi and others found that the edge-fracture resistance of flowable composites was lower toward the margins than toward the center of a restoration,⁵⁷  explaining how, in the present study, the area between the axial pulpal wall and gingival floor, near the cervical margin, showed the greatest increase in the interfacial gap. Even if the flowable composite’s elastic modulus permits a stress-absorbing action, its higher amount of monomer could attenuate its mechanical resistance in long-term simulations, particularly in the area were internal stress consequent to functional loads concentrates. This would explain how flowable resins, if applied in 2-mm layers, showed greater interfacial deterioration. Pongprueksa and others also reported that conventional composites released significantly fewer monomers than flowable or bulk-fill composites, and a higher total monomer elution was recorded for both flowable composites, irrespective of the application method.⁵⁸  It is therefore assumable that higher quantities of unpolymerized monomers could lead to overlapping deformations. By consequence, fatigue microfailures would be more likely to appear in early stages of the simulation compared with a rigid material, with a more regular molecular structure that can dissipate forces.

All composite materials performed significantly better on enamel than on dentin. All flowable materials, regardless of the first horizontal layer thickness, were able to create a significantly better marginal seal than nanohybrid composites at baseline. However, nanohybrid and bulk-fill composites may be able to better maintain a marginal seal over time, because their use was not associated with any significant alteration of the marginal seal after mechanical treatment. Our results suggest that longitudinal clinical trials are necessary for precise clinical indications on the ideal approach to restoring cavities with deep cervical margins.