Interfacial nanoleakage and internal cement thickness of three esthetic crown systems

Abstract

Objectives

To evaluate interfacial nanoleakage expression of the combination of different cementation procedures and different crown systems.

Methods

Forty-five human premolars prepared to receive single crowns were randomly divided into three groups ( n = 15) based on the materials to be used for crown fabrication and cementation: group 1: Zirc (Ivoclar-Vivadent) cemented with Multilink Automix; group 2: Ivoclar disilicate IPS Empress 2 luted with Excite DSC in combination with Variolink II; group 3: AAdva Zirconia (GC) cemented with G-Cem Automix. The specimens were then assessed for interfacial nanoleakage expression and the amount of silver deposits along the interface was quantified. The thickness of the cement was assessed at 5 different levels: cervical margins, midway between the cervical margin and the occlusal wall along the axial walls and at the occlusal wall. The nanoleakage scores and the cement thickness were analyzed with Kruskall–Wallis non-parametric Analysis of Variance and Dunn’s Multiple-range post hoc test.

Results

Group 2 showed significantly less nanoleakage expression than group 1 ( p < 0.05). The groups can be ranked in the following order 3 < 1 < 2 with regard to the cement thickness.

Conclusions

There was no correlation among combination of different cementation procedures and different crown systems and interfacial nanoleakage. Also the cement thickness and the degree of nanoleakage cannot be related. The amount of cement found at the cervical margins of all groups showed discrepancies within a clinical acceptable range.

Introduction

In recent years patients are demanding a higher esthetic quality in their dental work, so metal-free restorations are now commonly being used in Prosthodontics .

There are different techniques to fabricate all-ceramic crowns : one involves the heat-press method, which reminds us of the technique used for metal ceramic crowns , and it produces a high strength core primarily consisting of luting disilicate glass ; one features a computer aided design and manufacturing CAD/CAM system. This second technique focuses on precise and consistent manufacturing of Zirconia ceramics (ZrO 2 ) .

All of these different materials and techniques involved in the manufacturing of crowns have significant effects on the strength of the final restoration, as well as on the marginal fit .

Zirconia ceramic has up to twice the bending strength and fracture toughness of the conventional ceramics . Based on these excellent physical properties, zirconia has been advocated as a framework material for all-ceramic fixed partial dentures (FPDs) .

Zirconia crowns have shown a good marginal adaptation, giving the clinician an esthetic alternative to metal ceramic crowns. The widely studied marginal fit, which ranges between 50 and 100 microns for conventional crowns, is an important and well documented issue for all prosthetic devices .

Lithium disilicate, as it has been previously stated, is a material which features great esthetics and which has a reliability higher or comparable to veneered zirconia and metal ceramic systems .

Poor marginal adaptation of restorations increases plaque retention and changes the distribution of the microflora, which can subsequently induce the onset of periodontal disease . Moreover, microleakage from the oral cavity can cause endodontic inflammation and as a consequence pulp necrosis .

As Pioch states the term “nanoleakage” was introduced to describe a specific type of leakage within the dentin margin of the restoration . Consequently the seal ability and resistance to the varying stresses of luting agents used to cement the crown systems are extremely important . Several materials can be used to cement different types of crowns. Nowadays the use of resin luting agents is recommended for the cementation of all-ceramic systems, and it strengthens the crown . Unfortunately these resin luting agent systems require a multistep application technique, which can be a complex procedure, and it is reported to be highly sensitive to the operator capacity . Due to this setback, self-adhesive cements are now commonly used for the cementation of all-ceramic crowns, in the spirit to simplify the cementation procedure .

Many tests have been performed with these materials, observing external and internal adaptation of esthetic crowns, but few studies have ever used nanoleakage analysis to test infiltration after the cementation of full crowns .

Therefore the purpose of the present study was to evaluate the seal provided by the combination of different cementation procedures and different all-ceramic crown systems (zirconia and disilicate-based ceramics) as suggested by the manufacturer; by examining the nanoleakage pattern along the cement dentin interface.

The tested null-hypothesis was that there were no statistically significant differences between different crown systems and different cements in terms of interfacial nanoleakage expression and framework adaptation.

Materials and methods

Tooth preparation

Forty-five human premolars, extracted for periodontal reasons were used in this study. The teeth were stored in 0.5% Chloramine T solution at 4 °C immediately after extraction to prevent bacterial growth. An expert operator created standardized preparations to receive single crowns. The occlusal surface was reduced by approximately 1.5 mm and the axial walls to 12°, reducing the tooth circumferentially of about 1.5 mm. The cervical preparation margins, 1-mm circumferential rounded chamfer, were placed in dentin and followed the course of the CEJ using a circular chamfer milling bur (Intensiv SA, Montagnola, Switzerland, #020847) under water cooling. The preparations were then polished with fine and extra-fine diamond burs. Smooth and round preparation surfaces are recommended to minimize internal stress, as force will concentrate on sharp edges and line angles. To standardize the preparations the measurements of the removed dental tissue were undertaken in two steps. Firstly, standardized milling burs with calibrated diameter were used, after which this measurement was checked using a digital caliper with a precision of 0.01 mm.

Impressions were taken of each prepared tooth using a polyether material (Impregum, 3M ESPE, Seefeld, Germany, #354210). Master casts were then prepared with type IV extra-hard stone following the manufacturer’s mixing recommendations.

The specimens were then randomly divided into 3 groups ( n = 15) according to the type of crown they would subsequently receive.

The amount of internal relief and resulting tightness of fit was controlled with the cement space thickness setting of the design software. Virtual spacer was set at 30 μm.

For each type of crown the cementation procedure recommended by the manufacturer was to be followed. Therefore, the following experimental groups were defined ( Table 1 ):

Table 1
Manufacturers and composition of adhesive materials used.
Groups Cement Adhesive used Manufacturer
Group 1 Multilink Automix Self-cured composite None Ivoclar-Vivadent, Schaan, Liechtenstein
Group 2 Variolink II Dual-cured composite Excite DSC Ivoclar-Vivadent, Schaan, Liechtenstein
Group 3 G-Cem Dual-cured self-adhesive resin cement None GC, Tokyo, Japan

Group 1. Zirc (Ivoclar-Vivadent, Schaan, Liechtenstein) cemented with the manufacturer’s recommended luting agent Multilink Automix (Ivoclar-Vivadent);

Group 2. Lithium disilicate glass ceramic core IPS Empress 2 (Ivoclar-Vivadent, Schaan, Liechtenstein) luted with Excite DSC (Ivoclar-Vivadent in combination with Variolink II (Ivoclar-Vivadent));

Group 3. AAdva Zirconia (GC, Tokyo, Japan) cemented with G-Cem (GC).

Cementation procedure

Group 1: Multilink Automix

The two Multilink Primer A/B were mixed in a 1:1 mixing ratio and then applied to all preparation surfaces using a microbrush with a scrubbing action for 15 s. A reaction time of 30 s is recommended on the enamel and 15 s on the dentin. Multilink Primer excess was dispersed with a strong stream of air until the mobile liquid film was no longer visible. No light-curing was applied.

Multilink Automix was then applied to the inner surface of the restoration. The excess material was removed immediately with a microbrush.

Luting procedures were performed under a constant pressure of 1 kg (0.098 MPa) until polymerization of the cement was complete. The material was left to auto-cure for the first 5 min and then an additional polymerization was performed on the palatal and buccal side for 40 s.

Group 2: Excite DSC and Variolink II

Phosphoric acid gel was applied to the prepared enamel and then flowed onto the prepared dentin. The etchant was left to react for 15–30 s on the enamel and for 10–15 s on the dentin. The etchant gel was then thoroughly removed with a vigorous water spray for at least 5 s. Excess moisture was removed leaving the dentin surface with a glossy wet appearance. Excite DSC was applied to the enamel and dentin with a scrubbing action for 10 s. Variolink II was then applied to the inner surface of the restoration. The crown was placed in situ with slight pressure and the excesses were removed with a microbrush. The restoration was left to set under constant pressure for 20 s and then light-curing was performed for 40 s for segment.

Group 3: G-Cem

The internal surface of the restoration was coated with G-Cem and seated immediately under constant pressure. The surfaces were light-cured for 2–4 s each and then the excess were removed using a microbrush.

Maintaining constant pressure of 1 kg (0.098 MPa) the material was light-cured on the palatal and buccal side for 40 s. The material was the left to set for another 4 min.

All specimens were stored in a laboratory oven at 37 °C and 100% relative humidity for 24 h and then prepared for interfacial nanoleakage analysis.

Nanoleakage analysis

Specimens for interfacial nanoleakage analysis were prepared according to procedures described in previous studies . Teeth were covered with nail varnish up to 1 mm from the margins of the crowns, then specimens were immersed in a 50 wt% ammonical AgNO 3 solution and finally in a photo-developing solution.

The specimens were then totally embedded in epoxy resin (Epoxy embedding medium kit, #45359, St. Louis, MO) and cut longitudinally (oral-buccaly) to a thickness of 1 mm. From each specimen 5–6 sections were made. A custom designed diamond blade was used to cut zirconia at low speed under water cooling (Low speed Isomet Saw 1000, Buehler, Milano, Italy).

Two central slab were chosen from each specimen for a total of 30 slabs per group. The section per group was then randomly divided into 2 groups: one used for nanoleakage ( n = 15) and one for Sem analysis ( n = 15 per group). The sections ( n = 90) were then ground down to a thickness of approximately 40 μm using wet carbide papers mounted on a specially designed grinding machine (Micromet, Remet, Bologna, Italy). The slices were stained with acid fuchsin and observed with a transmitted light microscope (Nikon Eclipse, Nikon). Images of all interfaces were obtained at 100× magnification, and the amount of silver deposits along the interface between cement and dentin was quantified by two observers in accordance with the following score method proposed by Saboia et al. :

  • 0 = no nanoleakage

  • 1 = <25% of adhesive interface (cement–dentin) showing nanoleakage

  • 2 = 25 ≤ 50% of adhesive interface showing nanoleakage

  • 3 = 50 ≤ 75% of adhesive interface showing nanoleakage

  • 4 = >75% of adhesive interface showing nanoleakage

Two investigators, who had been blinded with regard to the restorative materials and crown system tested in each group, after being calibrated, independently scored the specimens. Intra-examiner reliability was assessed by the Kappa test ( K = 0.88).

SEM analysis

After nanoleakage evaluation, 15 sections per group were randomly selected and processed for scanning electron microscope analysis (SEM) (JSM 6060 LV, JEOL, Tokyo, Japan). On each section the cement thickness was measured in microns at the cervical margins ( Fig. 1 , points a and e), along the axial walls at midway between the cervical margin and the occlusal wall ( Fig. 1 , points b and d), and at the occlusal wall ( Fig. 1 , point c). The points a and e, and b and d, were considered as one only point as the results showed no statistical difference. The value given to a and e, and b and d was the average between those points.

Fig. 1
Points at which the SEM readings were taken. Point a refers to the cervical margins, point b along the axial walls at midway between the cervical margin and the occlusal wall and point c the occlusal wall.

Statistical analysis

Nanoleakage scores were compared among the groups using the Kruskall–Wallis Analysis of Variance followed by the Dunn’s Multiple Range test for post hoc comparisons.

As the pooled data set of cement thicknesses measured in the 3 groups at different levels did not respect the assumptions for Two-Way Analysis of Variance (ANOVA), two separate One-Way ANOVA’s were run in order to compare cement thicknesses at different levels within the same group and between different groups at the same level. The One-Way ANOVA was followed by the Tukey test for post hoc comparisons as needed.

In all the analyses the level of significance was set at p < 0.05.

Materials and methods

Tooth preparation

Forty-five human premolars, extracted for periodontal reasons were used in this study. The teeth were stored in 0.5% Chloramine T solution at 4 °C immediately after extraction to prevent bacterial growth. An expert operator created standardized preparations to receive single crowns. The occlusal surface was reduced by approximately 1.5 mm and the axial walls to 12°, reducing the tooth circumferentially of about 1.5 mm. The cervical preparation margins, 1-mm circumferential rounded chamfer, were placed in dentin and followed the course of the CEJ using a circular chamfer milling bur (Intensiv SA, Montagnola, Switzerland, #020847) under water cooling. The preparations were then polished with fine and extra-fine diamond burs. Smooth and round preparation surfaces are recommended to minimize internal stress, as force will concentrate on sharp edges and line angles. To standardize the preparations the measurements of the removed dental tissue were undertaken in two steps. Firstly, standardized milling burs with calibrated diameter were used, after which this measurement was checked using a digital caliper with a precision of 0.01 mm.

Impressions were taken of each prepared tooth using a polyether material (Impregum, 3M ESPE, Seefeld, Germany, #354210). Master casts were then prepared with type IV extra-hard stone following the manufacturer’s mixing recommendations.

The specimens were then randomly divided into 3 groups ( n = 15) according to the type of crown they would subsequently receive.

The amount of internal relief and resulting tightness of fit was controlled with the cement space thickness setting of the design software. Virtual spacer was set at 30 μm.

For each type of crown the cementation procedure recommended by the manufacturer was to be followed. Therefore, the following experimental groups were defined ( Table 1 ):

Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Interfacial nanoleakage and internal cement thickness of three esthetic crown systems
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