2.1

Resin cement and bonding agent fabrication

The 2,2 bis[4-(2-hydroxy-3-methacryloxypropoxy)phenyl]-propane (Bis-GMA), triethyleneglycol dimethacrylate (TEGDMA), camphorquinone (CQ), ethyl 4-dimethylaminobenzoate (EDMAB), benzoyl peroxide (BPO), ethanol (≥99.5%), 2MP, and 2-hydroxyethyl methacrylate (HEMA) were purchased from Sigma–Aldrich (St. Louis, MO, USA) and used without further purification ( Table 1 ). The IBOA, EHA, 1,6-hexanediol diacrylate (HDDA), and tripropylene glycol diacrylate (TPGDA) were purchased from Double Bond Chemical (Taipei, Taiwan) and used without further purification ( Table 1 ). The resin matrix comprised bis-GMA and TEGDMA at a 1:1 molar ratio. The photoactive and chemically active systems comprised 0.25% CQ, 0.5% EDMAB, and 0.5% BPO. The IE cement was prepared by adding 10 wt.% of the resin matrix using the following composition: 25% IBOA, 25% EHA, 25% HDDA, and 25% TPGDA. Three concentrations of 2MP (30, 35, and 40 wt.%) were prepared to fabricate the bonding agents as follows:

• (1)

  2MP-30%: 35% resin matrix, 17.5% HEMA, 30% 2MP, and 17.5% ethanol;

• (2)

  2MP-35%: 33% resin matrix, 16% HEMA, 35% 2MP, and 16% ethanol; and

• (3)

  2MP-40%: 30% resin matrix, 15% HEMA, 40% 2MP, and 15% ethanol.

2.2

Microtensile bond test for measurement of coronal dentin-resin cement adhesion

Extracted permanent molars from people aged 16–40 years were used after obtaining informed consent from the donors. Permission to collect human teeth was obtained from the Ethical Committee of National Taiwan University Hospital (Case No. 201105080RC). Crowns with caries, restorations, or fractures were discarded. Any remaining soft tissue was removed from the tooth surfaces with a dental scaler (Sonicflex 2000, KaVo Co, Biberbach, Germany) under running water. All teeth were stored in distilled water containing 0.2% thymol at 4 °C to inhibit microbial growth, and the storage medium was replaced every week to minimize deterioration.

Forty-two molars were randomly divided into 7 groups with 6 molars in each group. While fully hydrated, each molar was cut using a low-speed diamond-wafering blade (Isomet, Buehler Ltd, Lake Bluff, IL, USA) immediately below the occlusal pit and fissure and perpendicular to the long axis of the tooth. The dentin surfaces were subsequently wet-polished using 600 grit silica paper to create a uniform flat surface, followed by sonic vibration in distilled water for 30 s to remove any superficial debris from the cutting and polishing procedures. Kerr Gel Etchant (37.5% phosphoric acid) was applied to etch the surfaces of the teeth from Groups 1–3 for 15 s before the surfaces were thoroughly rinsed and air-dried. The fabricated adhesive systems (2MP-30%, 2MP-35%, and 2MP-40%) were subsequently applied for 15 s, and absorbent paper points were used to remove the excess adhesive. A light-curing machine (SmartLite, Dentsply, New York, PA, USA) was used to light cure the surfaces for 10 s at an intensity of ≥800 mW/cm ² . Energy output was measured with a power meter (Cure Rite, Dentsply Caulk, Milford, DE, USA). Subsequently, the dentin surface was encircled with a plastic ring (7 mm in diameter) and filled with an adequate amount of the IE cement and then light cured for 40 s. The teeth specimens were placed in 100% humidity at 37 °C for 24 h or 5 months. Following the same procedures used for Groups 1 to 3, teeth in Groups 4 to 7 were treated with 4 different resin cements (NX3, Variolink II, RelyX Unicem, and Panavia F 2.0) ( Table 2 ). NX3 and Variolink II are etch-and-rinse types of resin cements, and the corresponding bonding agents are Optibond Solo Plus (Kerr, CA, USA) and Excite DSC (Ivoclar Vivadent, Schaan, Liechtenstein), respectively. RelyX Unicem is a self-adhesive type of resin cement that does not use a bonding agent. Panavia F 2.0 is a self-etching type of resin cement and uses ED primer (Kuraray Medical Inc., Tokyo, Japan) as the bonding agent.

After treatment, the 6 molars in each group were further sectioned into 30 beams (1.0 mm × 1.0 mm) using a low-speed saw (Isomet, Buehler Ltd., Lake Bluff, IL, USA) under water-cooling. The Microtensile testing (μTBS) for the beams was then performed using the non-trimming technique. The beams were fixed using a cyanoacrylate adhesive (Zapit, DVA, Anaheim, CA, USA) and then subjected to tensile forces in a microtensile testing machine (Microtensile Tester, Bisco, Inc., Schaumburg, IL, USA) at a crosshead speed of 1 mm/min. The cross-sectional area at the site of fracture was measured to the nearest 0.01 mm using a digital caliper (Model CD-6BS; Mitutoyo, Tokyo, Japan) to calculate the tensile bond strength (MPa).

The 30 beams in each group were further divided into two subgroups, with 15 beams in each subgroup. One subgroup was tested with 24 h of storage time and the other with 5 months of storage. Accordingly, the total sample size of each subgroup was n = 15.

2.3

Push-out bond strength test for measurement of root canal dentin-resin cement adhesion

The palatal roots of the maxillary molars or distal roots of mandibular molars were used to perform a push-out bond strength test. The distal roots of mandibular molars that had 2 root canals were excluded from this test. The apical portion of each root (approximately 3 mm) was cut away to obtain a 9 mm root specimen. Each canal was prepared until the apical opening could be passed with an ISO size 80, 0.02 taper file. The prepared root specimen was vertically restrained using an apparatus comprising 2 aligned cylindrical steel dies secured with 3 screws . Under copious distilled water-cooling, a multi-drilling machine (LT-848; Dengyng Instruments Co Ltd., Taipei, Taiwan) was used to drill a 1.8 mm diameter hole along the center of each root specimen. The drilled canals were at least 1 mm away from the edge of the specimen.

A custom-made alignment device was employed to mount each prepared root vertically in a custom-made aluminum cylinder (3 cm diameter, 2 cm height). The aligning device contained a base plate with 3 orientation screws and 1 central guiding pin. Each prepared root was first positioned in the cylinder using the central guiding pin. After a thin layer of petroleum jelly was applied to the inner wall of the cylinder, the root was embedded by pouring a self-curing acrylic resin (Tempron; GC Corp, Tokyo, Japan) into the space between the fringe of the root and the cylinder wall. The cylinder was removed after the acrylic resin had set and a resin block with a mounted root segment was obtained. All specimens were immersed in an ultrasonic cleaner (Delta; Mandarin Scientific Co Ltd., Taipei, Taiwan) filled with 2.5% NaOCl for 1 min, followed by 17% EDTA for 2 min to remove the smear layer, and finally by distilled water for 2 min.

Forty-two resin blocks were randomly divided into 7 groups with 6 blocks in each group. Treatment of the canals in the root segments was identical to the discussed coronal dentin treatment for the microtensile tests. Subsequently, the top and bottom surfaces of the root segments were light-cured for 40 s each with a light intensity of ≥800 mW/cm ² . The root segment was then placed in 100% humidity at 37 °C for either 24 h or 5 months. Each block was then serially sectioned to create 1-mm root slices for subsequent push-out bond strength tests using a high-speed diamond-wafering blade (Isomet 2000 Precision High-Speed Saw; Buehler Ltd). The thickness was verified using an electronic vernier scale (CD-10CX; Mitutoyo Co Ltd., Tokyo, Japan). The top and bottom root slices were discarded, ultimately producing 5 root slices from each block; namely, 30 slices for each group. Each group was then divided into two subgroups, with one testing with 24 h of storage time and the other with 5 months of storage, resulting in a total sample size of n = 15 for each subgroup. The push-out bond strengths of the above mentioned 7 resin cements to root dentin were then measured.

The push-out apparatus comprised 2 cylindrical steel dies aligned with 2 dowels and secured with 3 screws. A 1.75 mm diameter hole and a 1.85 mm diameter hole were positioned at the center of the upper and lower dies, respectively. A 1.7 mm diameter cylindrical carbon steel rod was used as a plunger. The push-out apparatus was mounted on an Instron universal testing machine (Merlin series, Mini-55; Instron Corp., Canton, MA, USA), and a constant crosshead speed of 0.5 mm/min was set to push the filling cement from the root slice. The push-out bond strength was calculated using the following equation: push-out bond strength = force/π × diameter × thickness.

2.4

Fracture toughness test for measurement of resin cement-root canal dentin adhesion

Extracted permanent teeth with single roots longer than 25 mm were used to perform the fracture toughness test. A total of 210 specimens were prepared from root dentin (1.5 mm × 3 mm × 25 mm) and randomly divided into 7 groups with 30 specimens in each group. The 30 specimens in each group were further divided equally into two subgroups; 15 specimens were tested with 24 h of storage and the remaining 15 specimens with 5 months of storage. The total sample size was n = 15 for each subgroup. Resin cements were also prepared using a flat steel mold with a slot in dimension of 3 mm × 3 mm × 25 mm. The remaining surface treatments are identical to those of the push-out bond strength test. The interfacial fracture toughness between the resin cement and dentin was measured using an asymmetric double cantilever beam (ADCB) method . A razor blade of known thickness D , driven by a servomotor at a constant speed (5 × 10 ⁻⁶ m/s), was inserted into the dentin/resin cement interface, as shown in Fig. 1 . A crack was initiated ahead of the razor edge. Steady state crack propagation was observed after several minutes.

Diagram of the asymmetric double cantilever beam apparatus for measuring the fracture toughness of the dentin/resin cement interface. a represents the crack length and D is the thickness of the razor blade. h ₁ and h ₂ are the thickness of the dentin and resin cement, respectively.

Based on Kanninen’s calculation for a small crack in a bi-material with finite elasticity, the fracture toughness of the interface can be measured from the following equation :

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