The study conformed to the tenets of the Declaration of Helsinki, and it was approved by the Regional Human Biomedical Research Ethics Committee at the University of Szeged, Hungary. All restorative materials used in this study are made by the same manufacturer and used according to the manufacturer’s instructions.
One hundred and eight maxillary premolar teeth, extracted for periodontal or orthodontic reasons, were selected for this investigation. The newly extracted premolars were directly inserted in sodium hypochlorite (5.25%) for 5 min and then kept in saline (0.9%) for a maximum of 12 weeks at room temperature before use. After extraction, with the aid of hand scalers, the root surface was cleaned from the covered soft tissue. The teeth selection criteria regarding their integrity and radicular dimensions were the same as in our previous method . Regarding coronal dimensions, 90% of the teeth ranged between 9 and 10 mm bucco-palatally, measured at the widest bucco-palatal dimension. The average mesio-distal dimension was between 7 and 7.5 for 90% of the samples. Ten percent maximum deviation was allowed in the remaining 10% of the samples. The height of the crowns was between 7.5 and 8 mm in 90% of the samples, and + /0.5 mm deviation was allowed in the rest of the samples.
The teeth were randomly distributed across 9 study groups of 12 specimens each (A1 = control, A2–3, B1–3, C1–3). Specimens with average dimensions (within the above-mentioned 90%) were randomly distributed across the 9 groups, and specimens with non-average dimensions (the remaining 10%) were evenly distributed across the same 9 groups. MOD cavity preparation and root canal treatment were performed by the same trained operator. A standardized MOD cavity was prepared using a round-end parallel diamond (881.31.014 FG, Brasseler USA Dental, Savannah, GA) with water cooling. So the cavity was prepared so that the buccopalatal width of the occlusal isthmus was one-third of the intercuspal width, and the proximal box width was half of the buccopalatal width of the crown. The gingival floor was located 1 mm above the cemento-enamel junction (CEJ). All internal angles were rounded and the cavosurface margins were at 90°. After finalizing the MOD cavity preparation, access cavity preparation was carried out with a round-end diamond bur (850–014 M SSWhite, Lakewood, NJ, USA) with water cooling, and root canal treatment was performed in the prepared teeth.
The working length was defined utilizing the direct method, that is, by subtracting 1 mm from the real root length, which was determined 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 (Maillefer-Dentsply). The ProTaper sequence (S1, S2, F1, F2) was used for the preparation of the working length. Irrigation was done after every instrument with 2 ml of 2.5% NaOCl solution, and the canal space was filled 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 done by matched single-cone obturation with a master cone (F2 gutta-percha, Maillefer-Dentsply) and sealer (AH plus; Dentsply De Trey GmbH, Konstanz, Germany). The gutta-percha 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 to prevent leakage through the apex. The teeth were stored wet in an incubator (mco-18aic, Sanyo, Japan) for 1 week (at 37 °C, 100% relative humidity). After this, the temporary material was removed, and the MOD cavity including the access cavity was refreshened with a diamond bur.
In Groups A2-3, B2-3, and C2-3, 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, as proposed in one of our previous studies . After cutting back the gutta-percha, the root canal was washed with chlorhexidine and dried with paper points.
Finally, in all groups marked “B” and “C,” all cusps were reduced by 2 mm of their original height.
All specimens received the same adhesive treatment. A Tofflemire (1101C 0.035, KerrHawe, Bioggio, Switzerland) matrix band was applied before the adhesive treatment of the cavity and the root canal, and 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) was used for bonding, according to the manufacturer’s instructions, using a microbrush-X disposable applicator (Pentron Clinical Technologies, LLC, USA). Excess adhesive was eliminated by suction drying (Evacuation Tip–Starryshine, Anaheim, CA, USA) applied approximately 0.5 cm from the occlusal cavity (without contact). 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 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/cm2.
In the control group and group B1, the missing interproximal walls were built up with conventional composite (G-aenial Universal Injectable, shade A3, GC Europe), while in groups A2, A3, B2, and B3, the missing interproximal walls were built up with flowable SFRC (everX Flow, dentine shade, GC Europe) using the centripetal technique, thus transforming the MOD cavity into a class I cavity. This interproximal wall was light-cured for 40 s.
The teeth/groups were restored according to different restorative approaches (see Fig. 1 and Table 1).
Group A1 = control group
The cavities were restored with conventional PFC composite material (G-aenial Universal Injectable) 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.
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 flowable SFRC. The thickness of the increments was approximately 4 mm, and a microbrush-X disposable applicator (Pentron Clinical Technologies, LLC, USA) was used. A light-transmitting FRC post (1.2 mm GC Fiber post, GC Europe) was inserted into the post space to aid the transmission of the light to the apically positioned layers. The “light transmitting” post was withdrawn 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 with two 4-mm-thick layers of flowable SFRC (everX Flow, bulk shade, GC Europe). The material was placed to a level according to the anatomy of the dentine, leaving 2 mm occlusally for the final PFC composite. These SFRC increments were light-cured from the occlusal surface for 40 s. The last occlusal layer was conventional composite material (G-aenial Universal Injectable) covering the SFRC as described in Group A1.
The teeth received a custom-made unidirectional FRC post (everSTICK POST, GC Europe). Before the adhesive treatment, the posts of 1.2-mm diameter were tried in and cut to a length of 2 mm below the level of the occlusal cavity margins with sterile scissors. Luting of the posts and the core build-up was performed with flowable SFRC as described by Fráter et al. . Flowable SFRC was applied in approx. 4-mm-thick layers 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.
The cavities were restored with conventional PFC composite material (G-aenial Universal Injectable) as described in Group A1 to the level of the occlusal reduction. The previously reduced cusps were built back by conventional PFC composite with the aid of a silicon index.
The cavities and the 6-mm-deep post space were reconstructed with flowable SFRC as in Group A2 to the level of reduction. The previously reduced cusps were built back by conventional PFC composite with the aid of a silicon index.
The cavities and the 6-mm-deep post space were reconstructed with a custom-made FRC post (everSTICK POST) as in Group A3 to the level of reduction. The previously reduced cusps were built back with the conventional composite material with the aid of a silicon index.
The cavities were restored with conventional composite material as described in Group A1 (G-aenial Universal Injectable) in the form of a core build-up, leaving 2-mm space for the overlay on the occlusal and also on the interproximal surfaces.
The cavities and the 6-mm-deep post space were reconstructed with flowable SFRC as in Group A2 to the level of reduction in the form of a core build-up, leaving 2-mm space for the overlay on the occlusal and also on the interproximal surfaces.
The cavities and the 6-mm-deep post space were reconstructed with a custom-made FRC post (everSTICK POST) as in Group A3 in the form of a core build-up, leaving 2-mm space for the overlay on the occlusal and also on the interproximal surfaces.
Groups C1, C2, and C3 then received indirect CAD/CAM overlays according to the following steps:
After refining the cavity margins, a polyether impression (Permadyne, 3 M ESPE) was taken of each prepared specimen, using a simultaneous mixing technique according to the manufacturer’s instructions. The impressions were poured with type IV dental stone (FUJIROCK, GC Europe) and 2-mm-thick CAD/CAM composite resin overlays (CERASMART 270, GC Europe) were prepared by the same technician for each prepared specimen.
Luting was performed as follows: the intaglio surface of the composite overlays was treated with hydrofluoric acid for 20 s. After rinsing and drying, the overlays were silanized (G-Multi PRIMER, GC Europe) and dried. As for the teeth, the enamel margins were etched with 37% phosphoric acid for 30 s, rinsed with water, and air-dried. The surface of the composite core was roughened with a diamond bur, and a bonding agent was applied on the core (G-Premio Bond). Then the same bonding agent was applied on the intaglio of the restoration and left undisturbed for 10 s. Using an air syringe, the surface of the restoration was dried for 5 s with maximum air pressure. The overlays were luted with dual-cure resin cement (G-CEM LinkForce, GC Europe). The luting agent was applied onto the intaglio surface of the overlays, and the overlays were applied on the teeth under finger pressure until complete adaptation. After removing the excess material, glycerine gel (DeOx Gel, Ultradent Products Inc., Orange, CA, USA) was applied, and photopolymerization from each side for 40 s with Optilux 501 was performed.
Embedding and the simulation of the periodontal ligaments were carried out as described in our previous studies [31, 35]. During the mechanical testing, the specimens were submitted to an accelerated fatigue-testing protocol [29, 31] by a hydraulic testing machine (Instron ElektroPlus E3000, Norwood, MA,USA) at an angle of 135 degrees to the long axis of each tooth. 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). To aid the proper positioning of the testing tip, the palatal cusp was slightly reduced. The 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 1000 N, 5000 cycles per step. The specimens were loaded until fracture occurred or 50,000 cycles were reached. The total number of survived cycles was recorded for each specimen for the survival analyses.
After recording failure load, each specimen was visually examined for the type and location of the failure, as well as the direction of failure. According to Scotti and co-workers, a distinction was made between restorable and non-restorable fractures under an optical microscope (Carl Zeiss Omni Pico, Oberkochen, Germany) with a two-examiner agreement. A restorable fracture is above the CEJ, meaning that in case of fracture, the tooth can be restored, while a non-restorable fracture extends below the CEJ, and the tooth is likely to be extracted .
Statistical analysis was performed in SPSS 23.0 (IBM Corp., Somers, NY, USA). Kaplan-Meyer survival analysis was conducted, followed by pairwise post hoc comparisons (Breslow). The frequency of restorable and non-restorable fractures was calculated for each group.