Sample Preparation

The procedures followed are summarized in Figure 1. Extracted human molars (n=84) stored in 0.5% chloramine solution from the time of extraction were hand-scaled, cleaned with slurry of pumice, mounted in acrylic resin (Palavit G, Kulzer, Wehrheim, Germany) and stored in physiological saline solution until used. Diamond burs (Cerinlay Set, Intensiv, Viganello Lugano, Switzerland) in a high-speed hand-piece with sufficient water cooling were used to prepare the PCC-cavities (width about 5.0 mm/depth about 4.0 mm/angle about 5°) with coverage of functional cusps (about 1.5 mm/angle of 5°) and butt joint preparations (about 1.0–11.5 mm/angle 5°) as previously reported.11 Non-functional cusps were not covered. Proximal margins were placed 1 mm below the cemento/enamel junction, within cementum/dentin. Rounded line angles were prepared (Figure 2a). After preparation, the teeth were randomly assigned to three groups: 1) Conventional luting technique (n=36), 2) resin-coating luting technique (n=36) or 3) recently introduced self-adhesive universal resin cement (n=12). Within these three groups, the following materials were tested (Table 1): 1) EVC: Excite/Variolink II (Vivadent, Schaan, Liechtenstein), PAC: ED-Primer/Panavia-F 2.0 (Kuraray Medical Co, Tokyo, Japan), SYC: Syntac Classic/Variolink II (Vivadent), 2) EVR: Excite/Tetric Flow (Vivadent) resin-coat prior to luting with Excite/Variolink II, PAR: Clearfil-SE-Bond/Protect-Liner F (Kuraray Medical Co) resin-coat prior to luting with Panavia-F 2.0, SYR: Syntac Classic/Tetric Flow resin-coat prior to luting with Excite/Variolink II, 3) REX: RelyX Unicem (3M ESPE, Seefeld, Germany).

View Figure

• Download as Powerpoint
• Download Figure

View Figure

• Download as Powerpoint
• Download Figure

Table 1 Overview. Steps of Several Luting Procedures to the Point of Examination (Groups 1–3)

View Fullscreen

Table 2 Application Methods of the Resin-coating Technique (Group 2: EVR, PAR and SYR)

View Fullscreen

A CAD/CAM method (Cerec, Sirona, Bensheim, Germany) was used for the fabrication of PCC. The PCCs were fabricated from Vita Mark II ceramic blocs (Vita Zahnfabrik, Bad Säckingen, Germany) using the Cerec 3 system and corresponding software in the function mode (Sirona, software: Cerec 3, 1.0 [600]). After adjustment to the cavities and conditioning of the ceramic with hydrofluoric acid and silane coating, the PCCs were inserted by using one of the seven different luting techniques and material systems. The luting procedures are summarized in Table 1.

The Group 2 specimens (EVR, PAR, SYR) were resin-coated with the respective adhesive system and flowable resin following the manufacturer's instructions (Figure 2b and 3; Table 2). The exact procedures are described separately in Table 2.

View Figure

• Download as Powerpoint
• Download Figure

After insertion of the restorations, excess luting material was removed prior to curing. Light curing was performed using the Elipar II halogen light-curing unit (3M ESPE, Seefeld, Germany; 800 mW/cm²) for 40 seconds from each aspect (mesial-distal-occlusal-oral). Following the insertion procedures, finishing was performed with finishing diamonds (Komet/Brasseler, Lemgo, Germany) and the restorations were polished with flexible discs (Sof-Lex, 3M Dental Products Division, St Paul MN, USA). The samples were stored in physiological saline solution at 37°C for 24 hours. Subsequently, the samples were exposed to thermocycling (TC: 5000 cycles at 8°–55°C, 30 seconds/cycle) and mechanical loading (ML: 500000 cycles at 72.5 N, 1.6 Hz) simultaneously.

Silver Staining

After TCML, microleakage was determined at proximal locations for ceramic/composite-, resin-coating/composite-(where practical) and dentin/composite-interfaces separately by means of dye penetration (Figure 4). The specimens were covered with nail varnish, except for areas within 1 mm from restoration margins, and placed in 50% silver nitrate aqueous solution (S-6506: Sigma-Aldrich Chemie GmbH, Steinheim, Germany) for two hours in the dark, followed by six hours exposure to a photo-developing solution/fluorescent light (Rodinal B & W Film Developer, AGFA, Leverkusen, Germany). For multiple sectioning, the specimens were mounted onto stubs using acrylic resin and consecutively sectioned longitudinally in the mesio-distal direction into as many sections as possible (6–8) using a rotating diamond saw (blade thickness 300 μm) (Innenlochsäge Leitz 1600, Leitz, Germany) and water cooling. Section thickness was about 300 μm, with each section presenting two sites for the evaluation of silver staining. Digital images of the sections were recorded for both sites. Microleakage, indicated by silver staining along the ceramic/composite, resin-coating/composite (where practicable) and dentin/composite interfaces, was evaluated on the multiple sections with an image analyzing system (Optimas 6.1, Stemmer, München, Germany). The extent of microleakage along the interface was expressed as the percentage of penetration into the depth of the cavity towards the pulpal wall along the cervical cavity margin (100%) (Figure 4). The maximum measurement per tooth, noted separately for each interface, was selected as the characteristic descriptive value, representative of the respective tooth. The median of n=12 maximum values was selected as the characteristic descriptive value representative of each sample. Additionally, all silver staining values (medians of maxima) were pooled for each luting technique and material separately, irrespective of all other parameters.

View Figure

• Download as Powerpoint
• Download Figure

Statistical Analysis

A nonparametric statistical analysis was used to analyze the data, because of the lack of a normal distribution. Medians and 25%-75% percentiles for each of the different criteria were determined separately for all interfaces. Statistical analysis was performed using the Mann-Whitney-U-test for pairwise comparison among the groups. The level of significance was set to α =0.05. For evaluation of the influence of the luting material/method in general, the level of significance was adjusted to α *(k)=1-(1-α)^1/k by application of the Error-Rates Method (k=number of paired tests performed).

Method

In this investigation, a TCML device was used. It allowed for the simultaneous application of static load and thermal stressing in order to simulate clinical conditions within the limitations of the protocol used.12 In an in vitro study on marginal adaptation of adhesive ceramic inlays, Krejci and others showed that the initial marginal integrity to enamel and dentin deteriorated after thermocycling and mechanical loading, reporting that 120,000 chewing (loading) cycles approximated six months of clinical use.13 The settings in the current study approximated 2–2.5 years of clinical use.

Furthermore, silver staining was chosen to determine the degree of marginal deterioration. When silver staining is used, enhanced contrast may be achieved when compared to other dye penetration methods.14–15 The use of silver nitrate in combination with a photo developing solution as a chemical tracer is only distinct from the use of dyes in that the tracers rely on the reaction between both chemicals used.1416 Silver stains resist disruption by the coolant when sectioning the tooth, although they are passively deposited.17–18 In general, the interpretation of silver staining results is subject to controversy. The clinical relevance of microleak-age data is critically discussed in the literature. However, the relative improvement of marginal adaptation exhibited by resin-coating and the new self-adhesive resin cement RelyX Unicem in vitro is anticipated to also be reflected in vivo.

Treatment of Prepared Specimens

For the restoration of prepared specimens, machinable feldspathic ceramic blocks and fabrication of PCCs with the Cerec 3 system were chosen.11 No temporary restorations were applied in order to avoid possible additional influence of the temporary cement on the adhesive bond between the resin-coat or dentin and luting agent in this investigation.5

Materials

Within the conventional luting technique group, the resin luting materials Variolink II and Panavia F2.0 have been chosen for the evaluation, as they are very commonly applied in adhesive dentistry and their material properties are well documented in the literature.310 For Variolink II, a two-step and three-step adhesive system were used for resin-coating in order to study the influence of the adhesive system. Panavia F 2.0 was used with the corresponding self-etching adhesive (ED-Primer). Within the resin-coating luting group, the addition of a low-viscosity composite, Tetric Flow or Protect-Liner F and the corresponding adhesive systems, Excite, Clearfil SE-Bond or Syntac Classic, were used in order to stay within the product line of each manufacturer. Furthermore, the recently introduced self-adhesive universal resin cement RelyX Unicem was included in this evaluation, because of the promising material properties referred to in the recent literature and because no information was available regarding its suitability for luting PCCs.8–9

Differences Between Interfaces

In general, the silver staining values were significantly higher at the dentin/composite interface than at the ceramic/composite interface (Table 3). These findings are in line with the data of previous studies by Jayassoriya and others and Paul and others3–4 regarding the conventional and resin-coating luting groups, as well as Behr and others and De Munck and others regarding the recently introduced resin-based luting material.8–9 It should be noted that the data of these investigations only refer to inlay restorations. Furthermore, little or no microleakage could be detected at the interface resin-coat/luting material, indicating that bonding to the resin-coat did not present a limitation as might have been expected (Figure 5; Table 3). This finding is in accordance with data reported by Jayassoriya and others, who also found the least microleakage at the resin-coat/luting material interface.3

Improvement by Resin-coating

With respect to the silver staining values at the dentin/composite interface, a significant improvement in marginal adaptation within the resin-coating group was generally observed when compared to the conventional luting group. The results of the current investigation are in accordance with the data reported in the literature for inlay restorations, where resin-coating has been shown to be a means to reduce marginal leakage at the dentin/composite interface.3–419 For the conventional luting technique, Feilzer and others have reported the development of high polymerization stress in thin bonded layers due to the unfavorable configuration factor (bonded to unbonded surface).20 The lining of the cavity with a flowable resin-composite layer following preparation allows for free shrinkage of the thin layer due to the favorable ratio of bonded to unbonded surfaces, thus producing a primary stable bond to the tooth structure. This bond claims to withstand polymerization shrinkage stresses that develop when the ceramic restoration is finally luted to the resin-coated surface, with the resin-coat acting as an additional stress absorber.26 This may explain the significant differences in the occurrence of marginal imperfections represented by silver staining at the dentin margin between Groups 1 (conventional luting) and 2 (resin-coating). In addition, the regional variability of the dentin,21–22 the depth of the cavity,23 the burs selected for cavity preparation24 and the distance from the light-curing unit25–26 may additionally affect the differences observed.

Differences Within the Resin-coating Group

The dual-cured resin cement Panavia F2.0 showed the best overall values within the resin-coating group. Only the ceramic/composite interface microleakage values of the SYR combination showed a tendency to be superior to the data of Panavia (Figure 5; Table 3). With respect to the resin cement Variolink II, luting with the three-step total-etch DBA Syntac Classic revealed significantly less microleakage at each interface than bonding with the two-step total etch DBA Excite (Figure 5).

However, it was not possible to completely eliminate marginal deterioration as indicated by silver staining. Therefore, further improvement of the techniques and materials is necessary to achieve gap-free marginal sealing of a restoration. Furthermore, the use of the resin-coating technique implies an additional step during the operative procedures. This makes the resin-coating technique more technique-sensitive and time-consuming. Additionally, a further interface is generated using a low-viscosity composite that shows comparatively high polymerization shrinkage, which may enhance gap formation.27 The polymerization of the luting composite to the cured flowable composite of the resin-coat is also critically discussed in the literature with respect to marginal deterioration.2027

Improvement by the Recently Introduced Self-adhesive Universal Resin Cement

The best overall microleakage data in this investigation were found with the self-adhesive luting material RelyX Unicem. These results are in line with in vitro investigations by Abo-Hamar and others, who investigated bond strength, and Behr and others, who evaluated the marginal adaptation of RelyX Unicem at the dentin/composite interface of all-ceramic crowns.810

In the current study, due to technical reasons, only the influence of the materials at the dentinal margin was studied. A study by De Munck and others showed that marginal adaptation of RelyX Unicem was significantly lower at the enamel interface than at the dentin/composite interface. Therefore, these authors advise selective acid-etching of the enamel prior to the application of RelyX Unicem.9 Further clinical studies are needed to show whether the improvements observed in the current investigation can be confirmed clinically and are similar to clinical findings determined with conventional luting procedures.28–29

Panavia Versus RelyX Unicem

It should be noted that the silver staining values at the dentin/composite interface for the Panavia resin-coating group are in the same order of magnitude as the values for RelyX Unicem. However, based upon the results of the current study, the use of RelyX Unicem for luting all-ceramic restorations can be preferably recommended for reducing microleakage and gap formation at the dentin/restoration interface, considering that RelyX Unicem involves fewer steps, therefore it is less time-consuming and technique sensitive.