2.1

Specimen preparation

A standardized MO cavity was prepared on teeth using a round end paralel diamond bur (883H.146.016 F G – Brasseler USA Dental, Savannah, GA) with water coolant so that the buccopalatal width of the occlusal isthmus was one third, and the proximal box width was two thirds of the buccopalatal width of the crown. The gingival floor was prepared 1 mm above the cemento-enamel junction (CEJ). All internal angles were rounded and the cavosurface margins were at 90°. After finalizing the MO cavity preparation, access cavity preparation was perfromed with a round-end diamond bur (850−014 M SSWhite, Lakewood, NJ, USA) with water cooling and root canal treatment was made in the prepared teeth. The working length was created with the direct method by subtracting 1 mm from the real root length defined by introducing a number 10 K-file (Maillefer-Dentsply, Ballaigues, Switzerland) until it was visible through the apical foramen. The root canals were prepared using rotary ProTaper Universal files (Dentsply, Maillefer, Ballaigues, Switzerland). The ProTaper sequence (S1, S2, F1, F2) was used for the preparation at the working length. Irrigation was done after every instrument with 2 ml of 2.5% NaOCl solution and the canal space was saturated with irrigant during the instrumentation phase. After root canal cleaning and shaping, the roots were dried using 96% alcohol and paper points. Root canal obturation was made by matched single-cone obturation with a master cone (F2 gutta-percha, Maillefer-Dentsply, Ballaigues, Switzerland) and sealer (AH plus; Dentsply De Trey GmbH, Konstanz, Germany). The guttapercha was cut back to the level of the orifice and the access cavity was temporarily filled with Fuji Triage Pink (GC Europe, Leuven, Belgium). Fuji Triage Pink was applied to the apical part of the root in order to prevent leakage through the apex. The teeth were stored wet in an incubator (mco-18aic, Sanyo, Japan) for one week (at 37 °C, 100% relative humidity). After this the temporary material was removed and the MO cavity, including the access cavity was refreshened with a diamond bur.

In Group A1-A3 the root canal was no longer invaded by any preparation and teeth were restored with MO filling without any radicular reinforcement. In Group B1-B2 a very shallow post space preparation was carried out by a 1.2 GC Fiber Post drill to a depth of 3 mm apical from the root canal orifice. In Group C1-C4 post space preparation was carried out by a 1.2 GC Fiber Post drill to a depth of 6 mm apical from the root canal orifice. After cutting back the gutta-percha, the root canal was washed with chlorhexidine and dried with paper points.

All specimen had the same adhesive treatment. Tofflemire (1101C 0.035, Hawe-Neos, Italy) matrix band was applied prior to the adhesive treatment of the cavity and the root canal, the enamel was selectively acid-etched with 37% phosphoric acid for 15 s and washed with water. The coronal cavity and the root canal were rinsed with 2 ml of water and dried with paper points and air. A dual-cure one-step self-etch adhesive system (G-Premio Bond and DCA, GC Europe, Leuven, Belgium) was used for bonding procedure according to the manufacturer’s instructions using a microbrush-X disposable applicator (Pentron Clinical Technologies, LLC, USA). Extra adhesive was eliminated by suction drying (Evacuation Tip – Starryshine, Anaheim, CA, USA) within 0.5 cm from the occlusal cavity (without contact). The excess adhesive resin at the bottom of the canal was eliminated using a paper point. The adhesive was light-cured for 60 s using an Optilux 501 quartz-tungsten-halogen light-curing unit (Kerr Corp., Orange, CA, USA). The light-curing tip was always located in close contact (1−2 mm) with the tooth surface. The average power density of the light source, measured with a digital radiometer (Jetlite light tester; J. Morita USA Inc. Irvine, CA, USA) before the bonding procedure, was 840 ± 26.8 mW/cm ² . After light-curing the adhesive, the missing interproximal walls were build-up with conventional composite (G-aenial Posterior PJ-E, GC Europe, Leuven, Belgium) using the centripetal technique, thus transforming the MO cavity into a class I. cavity. This interproximal wall was light cured for 40 s.

Nine different techniques were used to restore the specimens in Group A1-C4. ( Fig. 1 ):

Shematic figure representing the test groups (Group A1-C4). Gr. A1: Packable SFRC core; Gr. A2: Flowable SFRC core; Gr. A3: Conventional composite core; Gr. B1: Packable SFRC directly layered as post (3 mm) and core; Gr. B2: Flowable SFRC directly layered as post (3 mm) and core; Gr. C1: Packable SFRC directly layered as post (6 mm) and core; Gr. C2: Flowable SFRC directly layered as post (6 mm) and core; Gr. C3: Individually-made unidirectional FRC post luted by flowable SFRC; Gr. C4: Conventional unidirectional FRC post luted by flowable SFRC.

Group A1: The cavities were restored with packable SFRC (everX Posterior, GC Europe, Leuven, Belgium) applied in a bulk-fill technique. The material was placed in single increment according to the anatomy of the dentine, leaving 1.5-2 mm occlusally for the final composite layers as prescribed by the manufacturer. The SFRC increment was light cured from the occlusal surface for 40 s. The last occlusal layer was conventional composite material (G-aenial Posterior PJ-E) covering the SFRC.

Group A2: The cavities were restored with flowable SFRC (everX Flow, GC Europe, Leuven, Belgium) applied in a bulk-fill technique. The material was placed in single increment according to the anatomy of the dentine, leaving 1.5-2 mm occlusally for the final composite layers as prescribed by the manufacturer. The SFRC increment was light cured from the occlusal surface for 40 s. The last occlusal layer was conventional composite material covering the SFRC.

Group A3: The cavities were restored with conventional composite material (G-aenial Posterior) applied with an oblique incremental technique. The material was placed in consecutive 2 mm thick increments. Each increment was light cured from the occlusal surface for 40 s.

Group B1: The cavities including the 3 mm deep post space were restored with packable SFRC applied in a horizontal layering technique. The material was placed in 2 increments (approx. 4 mm thick each) according to the anatomy of the dentine. The light curing of the layers and covering with a final occlusal layer of composite material was performed as in Group A1.

Group B2: The cavities including the 3 mm deep post space were restored with flowable SFRC the same way as described in Group B1.

Group C1: The cavities and the 6 mm deep post space were reconstructed with the Bioblock technique described by Fráter et al., [ ] building a direct layered post and core from packable SFRC. An approx. 4 mm thick increment of using a microbrush-X disposable applicator (Pentron Clinical Technologies, LLC, USA). A light transmitting FRC post (1.2 mm GC Fiber post, GC Europe, Leuven, Belgium) was inserted into the post space to aid the transmission of the light to the apically positioned layers. The ‘light transmitting’ post was withdrawn with 0.5–1 mm from the surface of the uncured SFRC layer not to have direct contact with it. This apical layer was light cured through the fiber post for 80 s. The rest of the cavity was restored as described in Group A1.

Group C2: The cavities and the 6 mm deep post space were restored with the Bioblock technique with the use of flowable SFRC. The coronal portion of the cavity was restored as described in Group A2

Group C3: The teeth received an individually-made unidirectional FRC post (everStick Post, GC Europe, Leuven, Belgium). Before the adhesive treatment, the posts of 1.2 mm diameter was tried in and cut to a length 2 mm below the level of the occlusal cavity margins with a sterile scrissors. Luting of the posts and the core build-up was performed with flowable SFRC. Flowable SFRC was applied in an approx. 4 mm thick layer into the post space. After insertion of the post, light curing was performed for 60 s. The coronal portion of the cavity was restored as described in Group A2.

Group C4: The teeth received a conventional unidirectional FRC post (GC Fiber post, GC Europe, Leuven, Belgium). Before the adhesive treatment, the conventional translucent FRC posts of 1.2 mm diameter was tried in and cut to a length 2 mm below the level of the occlusal cavity margins with a water-cooled diamond disc (Isomet 2000; Buehler Ltd., Lake Bluff, IL, USA) and cleaned with alcohol after try in. The posts received silanization of the surface (Ceramic Primer, GC Europe, Leuven, Belgium) following the manufacturer’s recommendation. After silanization, the post surface was bonded with the same bonding agent used for the cavity. Luting of the posts and the core build-up was performed with flowable SFRC. Flowable SFRC was applied in an approx. 4 mm thick layer into the post space. After insertion of the post, light curing was performed for 60 s. The coronal portion of the cavity was restored as described in Group A2.

Finally, for all restored specimens, glycerine gel (DeOx Gel, Ultradent Products Inc., Orange, CA, USA) was applied and final polymerization from each side for 40 s was performed. The restorations were finished with a fine granular diamond burr (FG 7406-018, Jet Diamonds, USA and FG 249-F012, Horico, Germany) and aluminum oxide polishers (OneGloss PS Midi, Shofu Dental GmbH, Ratingen, Germany).

2.2

Mechanical loading of the specimen

The restored specimens were stored in distilled water at 37 °C for a week. To simulate the periodontal ligament, the root surface of each tooth was coated with a layer of liquid latex separating material (Rubber-Sep, Kerr, Orange, CA) prior to embedding. Specimens were embedded in methacrylate resin (Technovit 4004, Heraeus-Kulzer) at 2 mm from the cementoenamel junction (CEJ) to simulate the bone level. For mechanical testing, the restored specimens were submitted to an accelerated fatigue-testing protocol [ ] by a hydraulic testing machine (Instron ElektroPlus E3000, Norwood, MA, USA) at an angle of 135 degrees to the long axis of each tooth. Testing was carried out in two parts. During the first part of testing (simulation of normal forces) cyclic isometric loading was applied on the triangular ridge of the buccal cusp of the tooth using a round-shaped metallic tip (with a diameter of 5 mm). The palatal cusp was slightly reduced to aid the propoer positioning of the testing tip. A cyclic load was applied at a frequency of 5 Hz, starting with gradually increasing static loading till 100 N in 5 s, followed by cyclic loading in 100 N steps, up to 500 N, 5000 cycles per step. The specimens were loaded until fracture occurred or 25,000 cycles were reached. The total number of survived cycles were recorded for each specimen for the survival analyses.

The specimens that survived 25,000 cycles were then loaded with extremely high forces simulating parafunction (clenching or bruxism). During this part of the testing, cyclic isometric loading was continued. Cyclic load was applied at a frequency of 5 Hz, continuing with gradually increasing static loading up to 600 N in 5 s, followed by cyclic loading in 100 N steps, up to 1000 N, 5000 cycles per step. The specimens were loaded until fracture occurred or 30,000 cycles were reached (within this second phase). The total number of survived cycles were recorded for each specimen for the survival analyses.

The failed specimens were visually examined under an optical microscope for the type, location and direction of failure, with two-examiner agreement. According to Scotti and co-workers, a distinction was made between repairable and irrepairable fractures, where a repairable fracture is above the CEJ, meaning that in case of fracture, the tooth can be restored, while an irrepairable fracture extends below the CEJ and the tooth is likely to be extracted [ ].

2.3

Microgap detremination test

Four groups (C1-4), each consisting of 5 endodontically treated and restored teeth, were investigated in the microgap detremination test. The teeth (n = 20) were restored in the same way as mentioned earlier. Teeth were sectioned mid-sagitally in the mesio-distal plane using a ceramic cutting disc operating at a speed of 100 rpm (Struers, Glasgow, Scotland) under water cooling. In each group, one of the sectioned restoration that contains the post was further grind and polish using #4000-grit silicon carbide papers at 300 rpm under water cooling using an automatic grinding machine (Rotopol-1; Struers, Copenhagen, Denmark). Then, sectioned teeth were painted with permanent marker, and polish gently for few seconds. The dye penetration along post/core margins of each section was evaluated independently using a stereo-microscope (Heerbrugg M3Z, Heerbrugg, Switzerland) at a magnification of 6.5x and the extent of dye penetration was recorded in mm as a percentage of the total margin length [ ].

2.4

Microhardness test

Microhardness of luting composite inside the canal was measured using a Struers Duramin hardness microscope (Struers, Copenhagen, Denmark) with a 40 objective lens and a load of 1.96 N applied for 10 s. Each sectioned restoration was subjected to 5 indentations on the top (coronal part) and the bottom (apical part) of the canal for indication of polymerization [ , ]. The diagonal length impressions were measured and Vickers values were converted into microhardness values by the machine. Microhardness was obtained using the following equation:

where H is Vickers hardness in kg/mm ² , P is the load in grams and d is the length of the diagonals in μm.

2.5

Statistical analysis

Statistical analyses were performed in SPSS 21.0 (IBM, USA). Groups were defined according to the method of restoration (or the lack thereof for the control group). The number of survived cycles was analyzed descriptively for each group and with the Kaplan-Meier method across the groups (with the Breslow test for the pairwise analyses). The frequency of restorable and non-restorabe fractures was calculated for each group.