A novel resin cement was developed.
The phosphate groups of 2MP formed hydrogen bonds with the collagen.
The hydrogen bonds prominently decreased in number in the specimens that were stored for 5 months.
The detachment of fiber posts from root canals is primarily caused by the loss of adhesion between dentin and cement; therefore, the purpose of this study was to formulate a novel resin cement that improves the bond strength of fiber posts to the dentin–cement interface.
Three concentrations (30, 35, and 40 wt.%) of bis[2-(methacryloyloxy)-ethyl] phosphate (2MP) were prepared as dentin bonding agent components. Isobornyl acrylate (IBOA) and ethylhexylacrylate (EHA) were used as key components to fabricate the resin cement (named IE cement). The adhesive strengths of IE cement to coronal and root canal dentin were tested after placement of specimens in a water bath at 100% humidity and 37 °C for either 24 h or 5 months. The microtensile bond test, the push-out bond test, and the fracture toughness test were performed. Four commercially available resin cements (Nexus ® third generation (NX3), Variolink II, RelyX Unicem, and Panavia F 2.0) were used for comparisons. X-ray photoelectron spectroscopy (XPS) was used to analyze the interaction of collagen extracted from human dentin and 2MP as well as the fracture surfaces of the specimens submitted to the microtensile bond test.
The 35% concentration of 2MP, in combination with IBOA and EHA, was the most effective for improving the IE cement’s bond strength to dentin. The XPS results revealed that the phosphate groups of 2MP formed hydrogen bonds with the collagen and that such bonds prominently decreased in number in the specimens that were stored for 5 months.
The combination of 2MP, IBOA, and EHA can effectively increase the adhesive strength of IE cement to dentin via hydrogen bond formation.
Endodontically treated teeth with insufficient coronal structure generally require radicular posts for crown restoration . Traditionally, cast metal posts and cores covered with porcelain fused to metal crowns have been the choice for anterior restoration. However, fiber posts have gradually been accepted as a replacement for cast metal posts (Young’s modulus, E = approximately 150–200 GPa) because the esthetic outcomes for anterior teeth are more pleasing and because the elastic moduli of these posts ( E = approximately 40 GPa) are approximately that of dentin ( E = approximately 18 GPa), thus providing even stress distribution along the tooth . Fiber posts commonly consist of a high proportion of reinforcing fibers embedded in highly cross-linked epoxy resins . The use of adhesive resin cements in fiber post luting has been recommended to improve retention, reduce microleakage, and enhance resistance to tooth fracture . Based on the hybridization of the demineralized surface and formation of resin tags, the bonding mechanism of resin cements to root canal dentin is micromechanical . However, the adhesion of fiber post to root canal dentin can be improved. Specifically, the adhesion between the resin cement and dentin is considered weaker than that between the resin cement and the fiber post .
Various methods have been tested to enhance the bond strength of fiber posts. These methods include a light-cured or self-cured adhesive system that adheres to light-transmitting translucent posts, treatment of the post surface with silane solution (Monobond S) or with various chemicals (hydrogen peroxide, sodium ethoxide, potassium permanganate, and hydrofluoric acid), sandblasting, and silicate/silane coating . However, to our knowledge, no consistent or effective method has been recognized.
Bis[2-(methacryloyloxy)-ethyl] phosphate (2MP) comprises a pendant phosphate group and 2 methacrylate groups, and it has been incorporated into oligo(polyethylene glycol) fumarate hydrogel to improve the attachment and differentiation of human fetal osteoblast cells . It has also been evaluated as a model self-etching adhesive . Isobornyl acrylate (IBOA) and ethylhexylacrylate (EHA) have been used as a pressure-sensitive adhesion system . IBOA-based copolymers have been applied to coatings , microfluidic devices for controlling microfluidic flow , and drug delivery . Although 2MP has been used as an ingredient in self-etching adhesives, its combined use with IBOA and EHA as a resin cement to increase the bond strength of fiber posts to human dentin requires further investigation.
Because fiber post retention to root canals could be improved and the weakest adhesion occurs between the resin cement and the dentin, the development of novel adhesive systems that increase the bond strength at the cement–dentin interface is crucial to improve the clinical success rate of fiber posts . The purpose of this study was to formulate a novel type of resin cement (named IE cement). The adhesive strength of the IE cements to coronal and root canal dentin was tested after placement of specimens in a water bath at 100% humidity and 37 °C for either 24 h or 5 months using the microtensile bond test, push-out bond test, and fracture toughness test. The adhesive strength of the new formulation was compared to that of Nexus ® third generation (NX3), Variolink II, RelyX Unicem, and Panavia F 2.0 resin cements, commercial materials that have been on the market for several years. To investigate the interaction between collagen and 2MP, collagen was extracted from human dentin and coated on a silicon wafer. 2MP was then applied on collagen and the samples were analyzed using X-ray photoelectron spectroscopy (XPS) to detect chemical bond formation. The fractured surfaces of the specimens derived from the microtensile test were also examined using XPS for comparison.
Materials and methods
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:
2MP-30%: 35% resin matrix, 17.5% HEMA, 30% 2MP, and 17.5% ethanol;
2MP-35%: 33% resin matrix, 16% HEMA, 35% 2MP, and 16% ethanol; and
2MP-40%: 30% resin matrix, 15% HEMA, 40% 2MP, and 15% ethanol.
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.
|NX3 (Optibond Solo Plus)||2-step etch-and-rinse||Kerr, Orange, CA, USA||monomers: methacrylic acid; mineral fillers|
|Variolink II (Excite DSC)||2-step etch-and-rinse||Ivoclar Vivadent, Schaan, Liechtenstein||Bis-GMA, urethane dimethacrylate, triethylene glycol dimethacrylate, ytterbium trifluoride, barium glass, silica|
|RelyX Unicem||Self-adhesive||3M ESPE, St. Paul, MN, USA||TEGDMA, substituted dimethacrylate, methacrylated phosphoric acid esters, calcium hydroxide, sodium persulfate, sodium p-toluene sulfinate, glass powder, silica|
|Panavia F 2.0 (ED primer)||Self-etch||Kuraray Medical Inc., Tokyo, Japan||Paste A: MDP, Bis-GMA, filler, benzoyl peroxide, photoinitiator|
|Paste B: Bis-GMA, filler, sodium fluoride, amine|
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.
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.
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.
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 :
G c = 3 D 2 8 a 4 ( E 1 E 2 h 1 3 h 2 3 ) ( C 1 2 E 2 h 2 3 + C 2 2 E 1 h 1 3 ) ( C 1 3 E 2 h 2 3 + C 2 3 E 1 h 1 3 ) 2