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Journal of the Formosan Medical Association (2016) xx, 1e8
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.jfma-online.com
ORIGINAL ARTICLE
A mesoporous biomaterial for biomimetic crystallization in dentinal tubules without impairing the bonding of a self-etch resin to dentin Yu-Chih Chiang a,b, Yin-Lin Wang a,b, Po-Yen Lin c, Yen-Yi Chen a,b, Chih-Yu Chien d, Hong-Ping Lin e, Chun-Pin Lin a,b,* a Graduate Institute of Clinical Dentistry, School of Dentistry, National Taiwan University, Taipei, Taiwan b National Taiwan University Hospital, College of Medicine, National Taiwan University, Taipei, Taiwan c Department of Dentistry, School of Dentistry, National Yang-Ming University, Taipei, Taiwan d Graduate Institute of Oral Biology, School of Dentistry, National Taiwan University, Taipei, Taiwan e Department of Chemistry, National Cheng Kung University, Tainan, Taiwan
Received 24 December 2015; received in revised form 7 January 2016; accepted 9 January 2016
KEYWORDS biomimetic; crystal growth; dentinal tubule; mesoporous materials; self-etch adhesive
Abstract Background/Purpose: CaCO3@mesoporous silica reacted with phosphoric acid (denoted as CCMS-HP) enables the growth of calcium phosphate crystals in dentinal tubules. This study tested whether CCMS-HP could be used to form a biomimetic barrier on the exposed dentin for prevention of dentin sensitivity without impairing the bonding of Single Bond Universal (SBU) self-etch adhesive to the dentin. Methods: Twenty-four dentin disks were prepared and divided into three groups: (1) SBU group (n Z 8), in which SBU self-etch adhesive was bonded to the dentin disk directly; (2) CCMS-HP group (n Z 8), in which CCMS-HP was applied onto the dentin surface; and (3) CCMS-HP/SBU group (n Z 8), in which the dentin surface was first treated with CCMS-HP and then boned by SBU. The permeation depth of crystals into the dentinal tubules was examined and measured with a scanning electron microscope. The shear bonding strength of SBU and CCMS-HP/SBU to dentin was also measured. Results: The mean crystal permeation depth was 35.8 6.9 mm for the CCMS-HP/SBU group and 33.6 12.2 mm for the CCMS-HP group; no significant difference was found between the two groups. Moreover, the mean shear bonding strength was 22.7 6.7 MPa for the CCMS-HP/SBU group and 23.3 7.0 MPa for the SBU group. There was also no significant
Conflicts of interest: All named authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest, or nonfinancial interest in the subject matter or materials discussed in this manuscript. * Corresponding author. Department of Dentistry, National Taiwan University Hospital, Number 1, Chang-Te Street, Taipei 10048, Taiwan. E-mail address: [email protected] (C.-P. Lin). http://dx.doi.org/10.1016/j.jfma.2016.01.005 0929-6646/Copyright ª 2016, Formosan Medical Association. Published by Elsevier Taiwan LLC. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: Chiang Y-C, et al., A mesoporous biomaterial for biomimetic crystallization in dentinal tubules without impairing the bonding of a self-etch resin to dentin, Journal of the Formosan Medical Association (2016), http://dx.doi.org/10.1016/ j.jfma.2016.01.005
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Y.-C. Chiang et al. difference between the two groups. Conclusion: CCMS-HP can be used to form a biomimetic barrier for prevention of dentin sensitivity because it neither impedes the bonding of SBU to dentin nor impairs the shear bonding strength between the SBU and dentin. Copyright ª 2016, Formosan Medical Association. Published by Elsevier Taiwan LLC. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/).
Introduction Dentin, a complex structure that contains a number of dentinal tubules, is the major component of human teeth and protects the pulp tissue. These tiny tubules exhibit external openings in dentinoenamel junction and internal connection to the innermost layer of the tooth pulp cavity. A healthy or intact tooth is covered by crown enamel and root cementum to protect the transparent nature of dentinal tubules. Dentin exposure would cause short and sharp pain if the fluid flows within the exposed dentinal tubules and the deformation of nerve endings at the Raschkow plexus in response to outside stimuli. The cavity preparation during caries management also exposes the dentinal tubules prior to filling the cavity. Some of the fluidfilled dentinal tubules are exposed during the process of tooth preparation, which allows fluid movement from the pulp toward the cut dentin surface. The lumen of the dentinal tubules varies in size from the dentinoenamel junction (0.5e0.9 mm) to the pulpal wall (2e3 mm).1 When caries lesion is deep, the cavity is prepared close to the dental pulp, where the amount and movement of fluid within dentinal tubules move more and faster based on PoiseuilleeHagen law.2 After the cavity is filled, the patient may feel sensitive to thermal change, chewing action, or air flow, or even feel spontaneous sensitivity. This phenomenon is known as postoperative sensitivity.3 Al-Omari et al3 and Gordan et al4 reported that postoperative sensitivity after amalgam restoration ranged from 27% (lesions are located in the middle third of dentin) to 58% (lesions are located in the inner third of dentin). Microleakage around restorations is believed to cause postoperative sensitivity. The demand for esthetic adhesive restorations has revolutionized restorative dental procedures and encouraged the use of tooth-colored dental composite resin. However, the dimethacrylate-based dental composites are always accompanied by 1e6% volumetric shrinkage when they are light cured.5,6 Many efforts have been conducted to minimize the polymerization shrinkage to reduce the postoperative sensitivity and marginal leakage. Dentin bonding agents (adhesive) have been known to be effective in reducing microleakage, but the regional differences of dentin, cavity configuration, and adhesive systems/quality may result in unpredictable shrinkage behavior.6e8 Thus, the clinical assessment still indicates high frequency of postoperative sensitivity using light-cured dental composite as filling materials, especially in Class II mesialeocclusaledistal cavity restorations (26%).9,10 Various surface treatments of prepared cavity in tooth aimed at overcoming the postoperative sensitivity problem
have been reported. Calcium hydroxide has been traditionally used as a cavity liner or capping material, although many practitioners recognize its potential for stimulating reparative dentin formation. However, the insufficient mechanical properties and inability to bond to restorations would compromise its performance. Thus, it is now widely believed that occluding patent dentinal tubules is a crucial strategy for preventing bacterial invasion, reducing dentinal tubule permeability, and subsequently preventing the progression of caries lesion.11e13 Glass-ionomer cement and resinmodified glass ionomer may be the promising capping materials to occlude the exposed dentin, but they do not have effective bonding to the covered dental composite resin. Therefore, it is preferable to develop biomaterials for forming calcium phosphate (CaeP) or hydroxyapatite (HA)like crystalline precipitates as biomimetic barriers that can provide lasting dentinal tubule occlusion.14e16 However, it is difficult to bring materials inside the tubules within a short time and at enough long depths because of the tiny dimensions of hydrophilic dentinal tubules. For this reason, we have used mesoporous silica as a carrier for CaCO3 nanoparticles [CaCO3@mesoporous silica (CCMS)].12 The mesoporous silica is templated using ecofriendly gelatin. When the CCMS is mixed with an appropriate amount of phosphoric acid (denoted as CCMS-HP; molar ratio of Ca/P Z 1:1), the paste releases significant amounts of calcium ions and hydrogen phosphate (HPO2 4 ) species, which react with dentin surface. The supersaturated environment on exposed 2 dentin surface enables Ca2þ and PO3 4 =HPO4 ions to permeate the dentinal tubules and form dicalcium phosphate dehydrate (DCPD), tricalcium phosphate, or hydroxyapatite-like (HAp) crystals at a depth of w40 mm. The CCMS-HP paste effectively occludes patent dentinal tubules within 10 minutes through the formation of a biomimetic CaeP protective hybrid barrier. In vitro biocompatibility tests (WST-1 and lactate dehydrogenase) and alkaline phosphatase assays show high cell viability and mineralization ability in a transwell dentin disk model treated with CCMS-HP. Biocompatibility tests (WST-1 and lactate dehydrogenase) show high cell viability in a transwell dentin disk model treated with CCMS-HP (p < 0.05). The alkaline phosphatase assays also indicate CCMS-HP has the potential mineralization ability of pulpedentin complex. In this study, we propose a novel strategy for treating the exposed dentin surface of tooth cavity with CCMS-HP to form a biomimetic hybrid crystal layer on the exposed dentin surface prior to bonding to the dental adhesive system in order to prevent the postoperative sensitivity. In this study, we tested whether CCMS-HP could be used to form a biomimetic barrier on the exposed dentin for
Please cite this article in press as: Chiang Y-C, et al., A mesoporous biomaterial for biomimetic crystallization in dentinal tubules without impairing the bonding of a self-etch resin to dentin, Journal of the Formosan Medical Association (2016), http://dx.doi.org/10.1016/ j.jfma.2016.01.005
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A mesoporous biomaterial for biomimetic crystallization prevention of dentin sensitivity without impairing the bonding of Single Bond Universal (SBU; 3M Deutschland, Neuss, Germany) self-etch adhesive to the dentin surface.
Methods Preparation of nano-CaCO3@mesoporous silica reacted with phosphoric acid The synthesis procedure of nano-CaCO3@mesoporous silica reacted with phosphoric acid (CCMS-HP) has been previously reported by Chiang et al.11 The brief procedure is described as follows. The gelatin (1.0 g) was added into distilled water (25.0 g) as a templet and stirred at 40 C for 15 minutes. An acidified silicate solution with an approximate pH of 5.0 was prepared (2.0M, 40 C) and reacted with gelatin solution to form a silicateegelatin gel solution. The gel solution was hydrothermally treated at 100 C for 24 hours, filtered, dried, and calcined at 600 C to gather the mesoporous silica. Calcined mesoporous silica (0.5 g) was mixed with an aqueous solution of CaCO3 (0.84 g) and oxalic acid (0.09 g) as the CaCO3 precursor. The mixture was dried at 100 C to obtain powder and then calcined at 200e400 C in air to form the CCMS. CCMS-HP paste was prepared by mixing with 30% phosphoric acid at a molar ratio of Ca/P Z 1:1 prior to application.
Specimen preparation A total of 24 intact human molars were collected and stored in distilled water containing 0.2% thymol at 4 C. All human teeth used in this study were obtained from 18- to 40-yearold patient after appropriate informed written consent was provided. A precision diamond saw (ISOMET 2000; BUEHLER Ltd., Lake Bluff, IL, USA) was used to expose and flatten dentin surface perpendicular to long axis of the tooth. Dentin
Figure 1
3 disks with 2-mm thickness were obtained through apical cuts. All dentin disks were pretreated with 17% EDTA and divided into three groups with each group having eight dentin disks for the evaluation of occlusion in dentinal tubules. The remaining part of teeth with exposed dentin surfaces were divided into two groups with each group having eight specimens for shear bonding strength test. The flow chart of experimental procedure is summarized in Figure 1.
Occlusive treatment in dentinal tubules Twenty-four dentin disks were divided into three groups: (1) SBU group (n Z 8), in which SBU self-etch adhesive was bonded to the dentin disk directly; (2) CCMS-HP group (n Z 8), in which CCMS-HP paste was applied onto the dentin surface by means of a microbrush (Microbrush International, Grafton, WI, USA) for 10 minutes. The paste was then wiped out from the dentin surfaces, which were then rinsed with distilled water; and (3) CCMS-HP/SBU group (n Z 8), in which the dentin surface was first treated with CCMS-HP and then boned by SBU self-etch adhesive. Dentin disks were split to examine the depth of material penetration within the dentinal tubules with a scanning electron microscope (SEM; S-2400, Hitachi, Tokyo, Japan). The mean crystal permeation depths of the three groups were compared using one-way analysis of variance and post hoc Tukey’s test with SPSS software version 17 (SPSS Inc., Chicago, IL, USA). The statistical significance was set at a Z 0.05.
Shear bonding strength test The shear bonding strength between the SBU self-etch resin and the dentin was tested in two experimental groups: (1) SBU group (n Z 8), in which SBU self-etch resin was boned to exposed dentin surface directly; (2) CCMS-HP/SBU group
Schematic diagram of the experimental groups.
Please cite this article in press as: Chiang Y-C, et al., A mesoporous biomaterial for biomimetic crystallization in dentinal tubules without impairing the bonding of a self-etch resin to dentin, Journal of the Formosan Medical Association (2016), http://dx.doi.org/10.1016/ j.jfma.2016.01.005
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Y.-C. Chiang et al. Table 1
Effects of capping materials on permeation depth in dentinal tubules.
Capping materials (n Z 8)
SBU
Occlusion pattern Permeation depth, mm (mean SD)a
Resin tag 5.1 1.7
a
CCMS-HP
CCMS-HP/SBU
Biomimetic crystal growth 33.6 12.2 b
Resin tag & biomimetic crystal growth 35.8 6.9 b
CCMS-HP Z CaCO3@mesoporous silica reacted with phosphoric acid; SBU Z Single Bond Universal; SD Z standard deviation. a Values with the same superscript letter indicate that there were no significant differences (p > 0.05).
(n Z 8), in which the dentin surface was firstly treated with CCMS-HP and then bonded by SBU self-etch resin. All bonded specimens were stored in distilled water at 37 C for 24 hours. The shear bonding strength tests were carried out using a universal testing machine (Instron 5566, Norwood, MA, USA) at a crosshead speed of 1 mm/min (load cell 500N). The differences were analyzed with Student t test with statistical significance set at a Z 0.05. The fractography of debonded specimens were examined by SEM micrographs.
Results Occlusive assessment in dentinal tubules Table 1 shows the mean crystal permeation depths of the three experimental groups. We found that the mean
crystal permeation depth was 35.8 6.9 mm for the CCMSHP/SBU group and 33.6 12.2 mm for the CCMS-HP group; no significant difference was found between the two groups. The permeation depth of resin tags formed by SBU was about 5.1 1.7 mm. Figure 2 shows that dentinal tubules were well sealed in the CCMS-HP/SBU group. The compact adhesive joint formed by composite resin, selfetch adhesive, and dentin was noted (Figure 2A). Significant integration of the formed occlusion resin plugs and crystals into the dentinal tubules was observed at the higher magnification (Figure 2B). Crystalline particles were found surrounding the penetrated resin tags (Figure 2B). Some crystals formed by the treatment of CCMS-HP underneath the SBU adhesive layer were noted (Figure 2C). At a penetration depth of 60 mm, the deeper part of dentinal tubules, some long crystals were found in dentinal tubules as well (Figure 2D).
Figure 2 SEM micrographs of the longitudinal section of dentin treated with CCMS-HP/SBU. (A) A hybrid bonded layer of SBU with a thickness of 12e15 mm on the dentin surface. Adhesive resin tags penetrated into dentinal tubules are noted. Composite resin (asterisk) is observed on SBU layer. (B) At higher magnification, formed crystalline particles surrounding resin tags (arrow) that filled the gap between resin tags and tubule wall are observed. (C) Crystallization (pointer) is found between the dentin and SBU layer. Tight biomimetic crystals formed within dentinal tubules are noted (asterisks). (D) Some crystals could be found in the deeper part of dentinal tubules up to a depth of 60 mm. CCMS-HP Z CaCO3@mesoporous silica reacted with phosphoric acid; SBU Z Single Bond Universal; SEM Z scanning electron microscope. Please cite this article in press as: Chiang Y-C, et al., A mesoporous biomaterial for biomimetic crystallization in dentinal tubules without impairing the bonding of a self-etch resin to dentin, Journal of the Formosan Medical Association (2016), http://dx.doi.org/10.1016/ j.jfma.2016.01.005
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A mesoporous biomaterial for biomimetic crystallization
Shear bonding strength test The data of the mean shear bonding strength between the SBU self-etch adhesive and the dentin are summarized in Figure 3. The mean shear bonding strength was 22.7 6.7 MPa for the CCMS-HP/SBU group and 23.3 7.0 MPa for the SBU group. There was no significant difference between the two groups. Figure 4 shows the fractography analysis of the dentin surface after the shear bonding strength test. A shallow dentin structure could be cut out; however, crystal plugs remained in the dentinal tubules. The fractography of composite resin side is shown in Figure 5. Resin tags combined with crystal plugs were pulled out from the dentin surface.
Discussion Postoperative sensitivity after light-initiated composite resin filling is still one of the most troublesome and common complaints expressed by patients.17,18 Postoperative sensitivity has been associated with the polymerization shrinkage of dental composite resin.19,20 The resulting cusp deformation or marginal gap may cause fluid movement in dentinal tubules. Thus, the sealing/occlusion of dentinal tubules is crucial for reducing the postoperative sensitivity. Dentin adhesives can be one of the solutions to seal the dentinal tubules. However, its hydrophobic monomer may decrease the effectiveness because hydrophilic fluid fills in
5 the dentinal tubules. The outward hydrostatic pressure of the pulp in a human tooth (14.1 cm H2O) makes it difficult to seal the dentinal tubules. In addition, lamina limitans is also a challenge to dentin adhesives because of its proteinlike membrane.21 Thus, if biomimetic crystals (calcium phosphates) can precipitate sufficiently deep in the dentinal tubule to reduce the dentin permeability, fluid movement in dentinal tubules can be limited and the postoperative pain can be prevented or relieved after the restorative procedure (Figure 6). Ishikawa et al22 demonstrated that an acidic solution containing Ca2þ and HPO2 4 may seal the dentinal tubules to a depth of 4e10 mm. However, this shallow occlusion does not guarantee a positive clinical outcome. Our previous studies have reported that DP-bioglass mixed with phosphoric acid can occlude dentinal tubules with crystallization to a depth of 60 mm.23,24 However, the long reaction time (3 days) and lower occlusion percentage limit the use of DP-bioglass and compromise the desired clinical outcome because the caries treatment cannot be completed in one visit. In crystallography, supersaturation of solutions plays a vital role in crystal precipitation and may overcome the challenges associated with the permeation of ions into the tiny dentinal tubules. Chiang et al11 demonstrated that nano CaO@mesoporous silica reacted with 30% phosphoric acid can create highly supersaturated Ca2þ and HPO2 ion-containing paste on dentin surfaces and 4 form a CaHPO4$2H2O crystal precipitation in the dentinal tubules to a depth of 100 mm. This mesoporous silica biomaterial also exhibits a significant reduction in dentin permeability (15% of the fluid conductance).11 In this study, we found that nano CCMS mixed with phosphoric acid could accelerate ion permeation and crystal precipitation in dentinal tubules to a depth of w40 mm by the 3 resulting high supersaturated Ca2þ, HPO2 4 , and PO4 ions 12 environment on the dentin surface. The crystals that grew in dentinal tubules could be DCPD, or tricalcium phosphate, or HAp. The subsequent self-etch adhesive procedure performed after CCMS-HP dentin treatment did not reduce the crystal permeation in dentinal tubules (in the CCMS-HP/SBU group, a mean crystal permeation depth of 35.8 6.9 mm was noted) compared to the CCMS-HP treatment without the subsequent dentin adhesive bonding (in the CCMS-HP group, a mean crystal permeation depth of 33.6 12.2 mm was found). It indicates that the covered self-etch adhesive can confirm or protect the precipitation reaction in dentinal tubules. The self-etch adhesive (SBU; 3M Deutschland) used in this study is an ethanol-based solution with a pH of 2.6, which can be categorized as a mild acid adhesive. The acidic monomer solution will neither increase the pH value on the dentin surface nor impair the Ca2þ and HPO2 ion 4 permeation in dentinal tubules to form calcium phosphates crystals. The chemical reactions are suggested as follows: Ca2þ þ H2PO4 / CaHPO4 þ Hþ
Figure 3 Shear bonding strength test indicates nonsignificant difference between the CCMS-HP/SBU and SBU groups (a Z 0.05). Bar Z standard deviation (n Z 8). CCMS-HP Z CaCO3@mesoporous silica reacted with phosphoric acid; SBU Z Single Bond Universal.
Ca2þ þ HPO2 / CaHPO 4 (DCPD) HPO2 þ Hþ 4 H2PO 4
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Figure 4 Fractography analysis of a represented specimen of debonded dentin surface. (A) Pointer indicates some dentin structure was cut and crystals remain in dentinal tubules. (B) At higher magnification, prominent fractured crystals from dentinal tubules are noted.
Thus, as soon as DCPD precipitates, the hydrogen ion byproduct from the self-etch adhesive creates a mildly acidic 2 þ environment in which H2PO 4 and HPO4 can react with H ions to form a buffer solution. Subsequently, the increase in pH by a reaction with the dentin surface is favorable for the growth of crystals with a higher Ca/P ratio. The main components of the SBU dental self-etch adhesive are the 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) monomer, dimethacrylate resins, 2hydroxyethyl methacrylate (HEMA), and polyalkenoic acid copolymer. HEMA is a popular monomer frequently used in dental adhesives in order to increase their wetting ability and hydrophilicity.25 However, HEMA released from dental adhesives negatively influences the dental pulp tissue by diffusing through the remaining dentin structure of the tooth cavity.26,27 The crystals grown in dentinal tubules under the dental adhesive layer acted as a protective biomimetic barrier to prevent the diffusion of cytotoxic monomer into the dentinal tubules (Figures 2 and 4). This biomimetic barrier in dentin also demonstrates a biocompatibility and a potential ability for dentinepulp remineralization.12 SBU dental self-etch adhesive is one of the most recent novelties in adhesive dentistry and was introduced as “universal” or “multimode” adhesives. These kinds of material are simplified adhesives, usually containing all
bonding components in a single bottle. Bonding to dentin using traditional etch and rinse adhesive system may be challenging because an ideal moisture condition should be maintained to avoid the collapse of the collagen matrix and to allow proper infiltration of the adhesive into the demineralized substrate.28 Thus, the purpose of choosing the self-etch adhesive in our study was to eliminate the additional etching step. In the present study, CCMS-HP paste could produce an acidic environment on the dentin surface, which in turn allowed the acidic self-etch adhesive working well on the dentin surface. Polyalkenoic acid copolymer in the dental adhesive could contribute to a higher bonding strength to caries-affected dentin.29 In addition to the contribution of a micromechanical bond to dentin (Figure 2), the SBU self-etch adhesive system is believed to incorporate a chemical interaction with the calcium in dentin because of the 10-MDP functional monomer, which has been regarded as a promising monomer for chemical bonding to the HA of dentin.30,31 In summary, the proposed novel strategy for treating the exposed dentin surface of tooth cavity with CCMS-HP prior to bonding to a dental adhesive system has a great potential to prevent postoperative sensitivity by forming a biomimetic calcium phosphate crystal barrier in the dentinal tubules. Based on the present findings, the formation of biomimetic crystal barrier in dentinal tubules was not
Figure 5 Fractography analysis of a represented specimen of debonded composite rod surface. (A) Resin tags/crystal plugs are pulled out from the dentin surface. (B) At higher magnification, integrated crystal layer and resin tags are noted.
Please cite this article in press as: Chiang Y-C, et al., A mesoporous biomaterial for biomimetic crystallization in dentinal tubules without impairing the bonding of a self-etch resin to dentin, Journal of the Formosan Medical Association (2016), http://dx.doi.org/10.1016/ j.jfma.2016.01.005
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A mesoporous biomaterial for biomimetic crystallization
Figure 6 Schematic diagram of biomimetic crystals formation and resin tags in dentinal tubules. Gaps between resin tags and tubule walls could be filled by calcium phosphate (CaP) crystals. SEA Z self-etch adhesive, HL Z hybrid layer.
impeded by the subsequent bonding procedure using SBU self-etch adhesive. Moreover, the mesoporpous silica biomaterial (CCMS-HP)-formed biomimetic barrier did not impair the bonding of SBU self-etch adhesive to dentin. Therefore, we conclude that CCMS-HP can be used to form a biomimetic barrier for prevention of dentin sensitivity because it neither impedes the bonding of SBU to dentin nor impairs the shear bonding strength between the SBU and dentin.
Acknowledgments This study was partly supported by the funding from the National Science Council Taiwan (103-2314-B-002-111-MY2 and NSC101-2314-B-002-098-MY2) and the National Taiwan University Hospital (100-N1766). The authors also wish to thank the staff of the Core Labs, Department of Medical Research, National Taiwan University Hospital, Taipei, Taiwan for technical support.
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7 7. Pashley EL, Tao L, Matthews WG, Pashley DH. Bond strengths to superficial, intermediate and deep dentin in-vivo with 4 dentin bonding systems. Dent Mater 1993;9:19e22. 8. Zhang L, Wang DY, Fan J, Li F, Chen YJ, Chen JH. Stability of bonds made to superficial vs. deep dentin, before and after thermocycling. Dent Mater 2014;30:1245e51. 9. Mahmoud SH, Ali AK, Hegazi HA. A three-year prospective randomized study of silorane- and methacrylate-based composite restorative systems in class II restorations. J Adhes Dent 2014;16:285e92. 10. Briso AL, Mestrener SR, Delicio G, Sundfeld RH, BedranRusso AK, de Alexandre RS, et al. Clinical assessment of postoperative sensitivity in posterior composite restorations. Oper Dent 2007;32:421e6. 11. Chiang YC, Chen HJ, Liu HC, Kang SH, Lee BS, Lin FH, et al. A novel mesoporous biomaterial for treating dentin hypersensitivity. J Dent Res 2010;89:236e40. 12. Chiang YC, Lin HP, Chang HH, Cheng YW, Tang HY, Yen WC, et al. A mesoporous silica biomaterial for dental biomimetic crystallization. ACS Nano 2014;8:12502e13. 13. Love RM, Jenkinson HF. Invasion of dentinal tubules by oral bacteria. Crit Rev Oral Biol Med 2002;13:171e83. 14. Tanase CE, Sartoris A, Popa MI, Verestiuc L, Unger RE, Kirkpatrick CJ. In vitro evaluation of biomimetic chitosanecalcium phosphate scaffolds with potential application in bone tissue engineering. Biomed Mater 2013;8:025002. 15. LeGeros RZ. Calcium phosphates in oral biology and medicine. Monogr Oral Sci 1991. Basel, Karger. 16. He C, Xiao G, Jin X, Sun C, Ma PX. Electrodeposition on nanofibrous polymer scaffolds: rapid mineralization, tunable calcium phosphate composition and topography. Adv Funct Mater 2010;20:3568e76. 17. Reis A, Loguercio AD, Schroeder M, Luque-Martinez I, Masterson D, Maia LC. Does the adhesive strategy influence the post-operative sensitivity in adult patients with posterior resin composite restorations? A systematic review and meta-analysis. Dent Mater 2015;31:1052e67. 18. van Dijken JWV, Pallesen U. A randomized controlled three year evaluation of “bulk-filled” posterior resin restorations based onstress decreasing resin technology. Dent Mater 2014; 30:E245e51. 19. Segura A, Donly KJ. In-vitro posterior composite polymerization recovery following hygroscopic expansion. J Oral Rehabil 1993;20:495e9. 20. Fok ASL. Shrinkage stress development in dental compositesdan analytical treatment. Dent Mater 2013;29:1108e15. 21. Thomas HF. The lamina limitans of human dentinal tubules. J Dent Res 1984;63:1064e6. 22. Ishikawa K, Eanes ED, Tung MS. The effect of supersaturation on apatite crystal formation in aqueous solutions at physiologic pH and temperature. J Dent Res 1994;73:1462e9. 23. Lee BS, Tsai HY, Tsai YL, Lan WH, Lin CP. In vitro study of DPbioglass paste for treatment of dentin hypersensitivity. Dent Mater J 2005;24:562e9. 24. Lee BS, Kang SH, Wang YL, Lin FH, Lin CP. In vitro study of dentinal tubule occlusion with solegel DP-bioglass for treatment of dentin hypersensitivity. Dent Mater J 2007;26: 52e61. 25. Moretto SG, Russo EM, Carvalho RC, De Munck J, Van Landuyt K, Peumans M, et al. 3-Year clinical effectiveness of one-step adhesives in non-carious cervical lesions. J Dent 2013; 41:675e82. 26. Chang HH, Guo MK, Kasten FH, Chang MC, Huang GF, Wang YL, et al. Stimulation of glutathione depletion, ROS production and cell cycle arrest of dental pulp cells and gingival epithelial cells by HEMA. Biomaterials 2005;26:745e53. 27. Lee BS, Jan YD, Huang GS, Huang CH, Chou HY, Wang JS, et al. Effect of dentin bonding agent diffusing through dentin slices
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8 on the reactive oxygen species production and apoptosis of pulpal cells. J Formos Med Assoc 2015;114:339e46. 28. Pashley DH, Tay FR, Breschi L, Tjaderhane L, Carvalho RM, Carrilho M, et al. State of the art etch-and-rinse adhesives. Dent Mater 2011;27:1e16. 29. Nakajima M, Sano H, Zheng L, Tagami J, Pashley DH. Effect of moist vs. dry bonding to normal vs. caries-affected dentin with Scotchbond Multi-Purpose Plus. J Dent Res 1999;78:1298e303.
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Journal of the Formosan Medical Association (2016) xx, 1e8
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ORIGINAL ARTICLE
A mesoporous biomaterial for biomimetic crystallization in dentinal tubules without impairing the bonding of a self-etch resin to dentin Yu-Chih Chiang a,b, Yin-Lin Wang a,b, Po-Yen Lin c, Yen-Yi Chen a,b, Chih-Yu Chien d, Hong-Ping Lin e, Chun-Pin Lin a,b,* a Graduate Institute of Clinical Dentistry, School of Dentistry, National Taiwan University, Taipei, Taiwan b National Taiwan University Hospital, College of Medicine, National Taiwan University, Taipei, Taiwan c Department of Dentistry, School of Dentistry, National Yang-Ming University, Taipei, Taiwan d Graduate Institute of Oral Biology, School of Dentistry, National Taiwan University, Taipei, Taiwan e Department of Chemistry, National Cheng Kung University, Tainan, Taiwan
Received 24 December 2015; received in revised form 7 January 2016; accepted 9 January 2016
KEYWORDS biomimetic; crystal growth; dentinal tubule; mesoporous materials; self-etch adhesive
Abstract Background/Purpose: CaCO3@mesoporous silica reacted with phosphoric acid (denoted as CCMS-HP) enables the growth of calcium phosphate crystals in dentinal tubules. This study tested whether CCMS-HP could be used to form a biomimetic barrier on the exposed dentin for prevention of dentin sensitivity without impairing the bonding of Single Bond Universal (SBU) self-etch adhesive to the dentin. Methods: Twenty-four dentin disks were prepared and divided into three groups: (1) SBU group (n Z 8), in which SBU self-etch adhesive was bonded to the dentin disk directly; (2) CCMS-HP group (n Z 8), in which CCMS-HP was applied onto the dentin surface; and (3) CCMS-HP/SBU group (n Z 8), in which the dentin surface was first treated with CCMS-HP and then boned by SBU. The permeation depth of crystals into the dentinal tubules was examined and measured with a scanning electron microscope. The shear bonding strength of SBU and CCMS-HP/SBU to dentin was also measured. Results: The mean crystal permeation depth was 35.8 6.9 mm for the CCMS-HP/SBU group and 33.6 12.2 mm for the CCMS-HP group; no significant difference was found between the two groups. Moreover, the mean shear bonding strength was 22.7 6.7 MPa for the CCMS-HP/SBU group and 23.3 7.0 MPa for the SBU group. There was also no significant
Conflicts of interest: All named authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest, or nonfinancial interest in the subject matter or materials discussed in this manuscript. * Corresponding author. Department of Dentistry, National Taiwan University Hospital, Number 1, Chang-Te Street, Taipei 10048, Taiwan. E-mail address: [email protected] (C.-P. Lin). http://dx.doi.org/10.1016/j.jfma.2016.01.005 0929-6646/Copyright ª 2016, Formosan Medical Association. Published by Elsevier Taiwan LLC. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: Chiang Y-C, et al., A mesoporous biomaterial for biomimetic crystallization in dentinal tubules without impairing the bonding of a self-etch resin to dentin, Journal of the Formosan Medical Association (2016), http://dx.doi.org/10.1016/ j.jfma.2016.01.005
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Y.-C. Chiang et al. difference between the two groups. Conclusion: CCMS-HP can be used to form a biomimetic barrier for prevention of dentin sensitivity because it neither impedes the bonding of SBU to dentin nor impairs the shear bonding strength between the SBU and dentin. Copyright ª 2016, Formosan Medical Association. Published by Elsevier Taiwan LLC. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/).
Introduction Dentin, a complex structure that contains a number of dentinal tubules, is the major component of human teeth and protects the pulp tissue. These tiny tubules exhibit external openings in dentinoenamel junction and internal connection to the innermost layer of the tooth pulp cavity. A healthy or intact tooth is covered by crown enamel and root cementum to protect the transparent nature of dentinal tubules. Dentin exposure would cause short and sharp pain if the fluid flows within the exposed dentinal tubules and the deformation of nerve endings at the Raschkow plexus in response to outside stimuli. The cavity preparation during caries management also exposes the dentinal tubules prior to filling the cavity. Some of the fluidfilled dentinal tubules are exposed during the process of tooth preparation, which allows fluid movement from the pulp toward the cut dentin surface. The lumen of the dentinal tubules varies in size from the dentinoenamel junction (0.5e0.9 mm) to the pulpal wall (2e3 mm).1 When caries lesion is deep, the cavity is prepared close to the dental pulp, where the amount and movement of fluid within dentinal tubules move more and faster based on PoiseuilleeHagen law.2 After the cavity is filled, the patient may feel sensitive to thermal change, chewing action, or air flow, or even feel spontaneous sensitivity. This phenomenon is known as postoperative sensitivity.3 Al-Omari et al3 and Gordan et al4 reported that postoperative sensitivity after amalgam restoration ranged from 27% (lesions are located in the middle third of dentin) to 58% (lesions are located in the inner third of dentin). Microleakage around restorations is believed to cause postoperative sensitivity. The demand for esthetic adhesive restorations has revolutionized restorative dental procedures and encouraged the use of tooth-colored dental composite resin. However, the dimethacrylate-based dental composites are always accompanied by 1e6% volumetric shrinkage when they are light cured.5,6 Many efforts have been conducted to minimize the polymerization shrinkage to reduce the postoperative sensitivity and marginal leakage. Dentin bonding agents (adhesive) have been known to be effective in reducing microleakage, but the regional differences of dentin, cavity configuration, and adhesive systems/quality may result in unpredictable shrinkage behavior.6e8 Thus, the clinical assessment still indicates high frequency of postoperative sensitivity using light-cured dental composite as filling materials, especially in Class II mesialeocclusaledistal cavity restorations (26%).9,10 Various surface treatments of prepared cavity in tooth aimed at overcoming the postoperative sensitivity problem
have been reported. Calcium hydroxide has been traditionally used as a cavity liner or capping material, although many practitioners recognize its potential for stimulating reparative dentin formation. However, the insufficient mechanical properties and inability to bond to restorations would compromise its performance. Thus, it is now widely believed that occluding patent dentinal tubules is a crucial strategy for preventing bacterial invasion, reducing dentinal tubule permeability, and subsequently preventing the progression of caries lesion.11e13 Glass-ionomer cement and resinmodified glass ionomer may be the promising capping materials to occlude the exposed dentin, but they do not have effective bonding to the covered dental composite resin. Therefore, it is preferable to develop biomaterials for forming calcium phosphate (CaeP) or hydroxyapatite (HA)like crystalline precipitates as biomimetic barriers that can provide lasting dentinal tubule occlusion.14e16 However, it is difficult to bring materials inside the tubules within a short time and at enough long depths because of the tiny dimensions of hydrophilic dentinal tubules. For this reason, we have used mesoporous silica as a carrier for CaCO3 nanoparticles [CaCO3@mesoporous silica (CCMS)].12 The mesoporous silica is templated using ecofriendly gelatin. When the CCMS is mixed with an appropriate amount of phosphoric acid (denoted as CCMS-HP; molar ratio of Ca/P Z 1:1), the paste releases significant amounts of calcium ions and hydrogen phosphate (HPO2 4 ) species, which react with dentin surface. The supersaturated environment on exposed 2 dentin surface enables Ca2þ and PO3 4 =HPO4 ions to permeate the dentinal tubules and form dicalcium phosphate dehydrate (DCPD), tricalcium phosphate, or hydroxyapatite-like (HAp) crystals at a depth of w40 mm. The CCMS-HP paste effectively occludes patent dentinal tubules within 10 minutes through the formation of a biomimetic CaeP protective hybrid barrier. In vitro biocompatibility tests (WST-1 and lactate dehydrogenase) and alkaline phosphatase assays show high cell viability and mineralization ability in a transwell dentin disk model treated with CCMS-HP. Biocompatibility tests (WST-1 and lactate dehydrogenase) show high cell viability in a transwell dentin disk model treated with CCMS-HP (p < 0.05). The alkaline phosphatase assays also indicate CCMS-HP has the potential mineralization ability of pulpedentin complex. In this study, we propose a novel strategy for treating the exposed dentin surface of tooth cavity with CCMS-HP to form a biomimetic hybrid crystal layer on the exposed dentin surface prior to bonding to the dental adhesive system in order to prevent the postoperative sensitivity. In this study, we tested whether CCMS-HP could be used to form a biomimetic barrier on the exposed dentin for
Please cite this article in press as: Chiang Y-C, et al., A mesoporous biomaterial for biomimetic crystallization in dentinal tubules without impairing the bonding of a self-etch resin to dentin, Journal of the Formosan Medical Association (2016), http://dx.doi.org/10.1016/ j.jfma.2016.01.005
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A mesoporous biomaterial for biomimetic crystallization prevention of dentin sensitivity without impairing the bonding of Single Bond Universal (SBU; 3M Deutschland, Neuss, Germany) self-etch adhesive to the dentin surface.
Methods Preparation of nano-CaCO3@mesoporous silica reacted with phosphoric acid The synthesis procedure of nano-CaCO3@mesoporous silica reacted with phosphoric acid (CCMS-HP) has been previously reported by Chiang et al.11 The brief procedure is described as follows. The gelatin (1.0 g) was added into distilled water (25.0 g) as a templet and stirred at 40 C for 15 minutes. An acidified silicate solution with an approximate pH of 5.0 was prepared (2.0M, 40 C) and reacted with gelatin solution to form a silicateegelatin gel solution. The gel solution was hydrothermally treated at 100 C for 24 hours, filtered, dried, and calcined at 600 C to gather the mesoporous silica. Calcined mesoporous silica (0.5 g) was mixed with an aqueous solution of CaCO3 (0.84 g) and oxalic acid (0.09 g) as the CaCO3 precursor. The mixture was dried at 100 C to obtain powder and then calcined at 200e400 C in air to form the CCMS. CCMS-HP paste was prepared by mixing with 30% phosphoric acid at a molar ratio of Ca/P Z 1:1 prior to application.
Specimen preparation A total of 24 intact human molars were collected and stored in distilled water containing 0.2% thymol at 4 C. All human teeth used in this study were obtained from 18- to 40-yearold patient after appropriate informed written consent was provided. A precision diamond saw (ISOMET 2000; BUEHLER Ltd., Lake Bluff, IL, USA) was used to expose and flatten dentin surface perpendicular to long axis of the tooth. Dentin
Figure 1
3 disks with 2-mm thickness were obtained through apical cuts. All dentin disks were pretreated with 17% EDTA and divided into three groups with each group having eight dentin disks for the evaluation of occlusion in dentinal tubules. The remaining part of teeth with exposed dentin surfaces were divided into two groups with each group having eight specimens for shear bonding strength test. The flow chart of experimental procedure is summarized in Figure 1.
Occlusive treatment in dentinal tubules Twenty-four dentin disks were divided into three groups: (1) SBU group (n Z 8), in which SBU self-etch adhesive was bonded to the dentin disk directly; (2) CCMS-HP group (n Z 8), in which CCMS-HP paste was applied onto the dentin surface by means of a microbrush (Microbrush International, Grafton, WI, USA) for 10 minutes. The paste was then wiped out from the dentin surfaces, which were then rinsed with distilled water; and (3) CCMS-HP/SBU group (n Z 8), in which the dentin surface was first treated with CCMS-HP and then boned by SBU self-etch adhesive. Dentin disks were split to examine the depth of material penetration within the dentinal tubules with a scanning electron microscope (SEM; S-2400, Hitachi, Tokyo, Japan). The mean crystal permeation depths of the three groups were compared using one-way analysis of variance and post hoc Tukey’s test with SPSS software version 17 (SPSS Inc., Chicago, IL, USA). The statistical significance was set at a Z 0.05.
Shear bonding strength test The shear bonding strength between the SBU self-etch resin and the dentin was tested in two experimental groups: (1) SBU group (n Z 8), in which SBU self-etch resin was boned to exposed dentin surface directly; (2) CCMS-HP/SBU group
Schematic diagram of the experimental groups.
Please cite this article in press as: Chiang Y-C, et al., A mesoporous biomaterial for biomimetic crystallization in dentinal tubules without impairing the bonding of a self-etch resin to dentin, Journal of the Formosan Medical Association (2016), http://dx.doi.org/10.1016/ j.jfma.2016.01.005
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Y.-C. Chiang et al. Table 1
Effects of capping materials on permeation depth in dentinal tubules.
Capping materials (n Z 8)
SBU
Occlusion pattern Permeation depth, mm (mean SD)a
Resin tag 5.1 1.7
a
CCMS-HP
CCMS-HP/SBU
Biomimetic crystal growth 33.6 12.2 b
Resin tag & biomimetic crystal growth 35.8 6.9 b
CCMS-HP Z CaCO3@mesoporous silica reacted with phosphoric acid; SBU Z Single Bond Universal; SD Z standard deviation. a Values with the same superscript letter indicate that there were no significant differences (p > 0.05).
(n Z 8), in which the dentin surface was firstly treated with CCMS-HP and then bonded by SBU self-etch resin. All bonded specimens were stored in distilled water at 37 C for 24 hours. The shear bonding strength tests were carried out using a universal testing machine (Instron 5566, Norwood, MA, USA) at a crosshead speed of 1 mm/min (load cell 500N). The differences were analyzed with Student t test with statistical significance set at a Z 0.05. The fractography of debonded specimens were examined by SEM micrographs.
Results Occlusive assessment in dentinal tubules Table 1 shows the mean crystal permeation depths of the three experimental groups. We found that the mean
crystal permeation depth was 35.8 6.9 mm for the CCMSHP/SBU group and 33.6 12.2 mm for the CCMS-HP group; no significant difference was found between the two groups. The permeation depth of resin tags formed by SBU was about 5.1 1.7 mm. Figure 2 shows that dentinal tubules were well sealed in the CCMS-HP/SBU group. The compact adhesive joint formed by composite resin, selfetch adhesive, and dentin was noted (Figure 2A). Significant integration of the formed occlusion resin plugs and crystals into the dentinal tubules was observed at the higher magnification (Figure 2B). Crystalline particles were found surrounding the penetrated resin tags (Figure 2B). Some crystals formed by the treatment of CCMS-HP underneath the SBU adhesive layer were noted (Figure 2C). At a penetration depth of 60 mm, the deeper part of dentinal tubules, some long crystals were found in dentinal tubules as well (Figure 2D).
Figure 2 SEM micrographs of the longitudinal section of dentin treated with CCMS-HP/SBU. (A) A hybrid bonded layer of SBU with a thickness of 12e15 mm on the dentin surface. Adhesive resin tags penetrated into dentinal tubules are noted. Composite resin (asterisk) is observed on SBU layer. (B) At higher magnification, formed crystalline particles surrounding resin tags (arrow) that filled the gap between resin tags and tubule wall are observed. (C) Crystallization (pointer) is found between the dentin and SBU layer. Tight biomimetic crystals formed within dentinal tubules are noted (asterisks). (D) Some crystals could be found in the deeper part of dentinal tubules up to a depth of 60 mm. CCMS-HP Z CaCO3@mesoporous silica reacted with phosphoric acid; SBU Z Single Bond Universal; SEM Z scanning electron microscope. Please cite this article in press as: Chiang Y-C, et al., A mesoporous biomaterial for biomimetic crystallization in dentinal tubules without impairing the bonding of a self-etch resin to dentin, Journal of the Formosan Medical Association (2016), http://dx.doi.org/10.1016/ j.jfma.2016.01.005
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A mesoporous biomaterial for biomimetic crystallization
Shear bonding strength test The data of the mean shear bonding strength between the SBU self-etch adhesive and the dentin are summarized in Figure 3. The mean shear bonding strength was 22.7 6.7 MPa for the CCMS-HP/SBU group and 23.3 7.0 MPa for the SBU group. There was no significant difference between the two groups. Figure 4 shows the fractography analysis of the dentin surface after the shear bonding strength test. A shallow dentin structure could be cut out; however, crystal plugs remained in the dentinal tubules. The fractography of composite resin side is shown in Figure 5. Resin tags combined with crystal plugs were pulled out from the dentin surface.
Discussion Postoperative sensitivity after light-initiated composite resin filling is still one of the most troublesome and common complaints expressed by patients.17,18 Postoperative sensitivity has been associated with the polymerization shrinkage of dental composite resin.19,20 The resulting cusp deformation or marginal gap may cause fluid movement in dentinal tubules. Thus, the sealing/occlusion of dentinal tubules is crucial for reducing the postoperative sensitivity. Dentin adhesives can be one of the solutions to seal the dentinal tubules. However, its hydrophobic monomer may decrease the effectiveness because hydrophilic fluid fills in
5 the dentinal tubules. The outward hydrostatic pressure of the pulp in a human tooth (14.1 cm H2O) makes it difficult to seal the dentinal tubules. In addition, lamina limitans is also a challenge to dentin adhesives because of its proteinlike membrane.21 Thus, if biomimetic crystals (calcium phosphates) can precipitate sufficiently deep in the dentinal tubule to reduce the dentin permeability, fluid movement in dentinal tubules can be limited and the postoperative pain can be prevented or relieved after the restorative procedure (Figure 6). Ishikawa et al22 demonstrated that an acidic solution containing Ca2þ and HPO2 4 may seal the dentinal tubules to a depth of 4e10 mm. However, this shallow occlusion does not guarantee a positive clinical outcome. Our previous studies have reported that DP-bioglass mixed with phosphoric acid can occlude dentinal tubules with crystallization to a depth of 60 mm.23,24 However, the long reaction time (3 days) and lower occlusion percentage limit the use of DP-bioglass and compromise the desired clinical outcome because the caries treatment cannot be completed in one visit. In crystallography, supersaturation of solutions plays a vital role in crystal precipitation and may overcome the challenges associated with the permeation of ions into the tiny dentinal tubules. Chiang et al11 demonstrated that nano CaO@mesoporous silica reacted with 30% phosphoric acid can create highly supersaturated Ca2þ and HPO2 ion-containing paste on dentin surfaces and 4 form a CaHPO4$2H2O crystal precipitation in the dentinal tubules to a depth of 100 mm. This mesoporous silica biomaterial also exhibits a significant reduction in dentin permeability (15% of the fluid conductance).11 In this study, we found that nano CCMS mixed with phosphoric acid could accelerate ion permeation and crystal precipitation in dentinal tubules to a depth of w40 mm by the 3 resulting high supersaturated Ca2þ, HPO2 4 , and PO4 ions 12 environment on the dentin surface. The crystals that grew in dentinal tubules could be DCPD, or tricalcium phosphate, or HAp. The subsequent self-etch adhesive procedure performed after CCMS-HP dentin treatment did not reduce the crystal permeation in dentinal tubules (in the CCMS-HP/SBU group, a mean crystal permeation depth of 35.8 6.9 mm was noted) compared to the CCMS-HP treatment without the subsequent dentin adhesive bonding (in the CCMS-HP group, a mean crystal permeation depth of 33.6 12.2 mm was found). It indicates that the covered self-etch adhesive can confirm or protect the precipitation reaction in dentinal tubules. The self-etch adhesive (SBU; 3M Deutschland) used in this study is an ethanol-based solution with a pH of 2.6, which can be categorized as a mild acid adhesive. The acidic monomer solution will neither increase the pH value on the dentin surface nor impair the Ca2þ and HPO2 ion 4 permeation in dentinal tubules to form calcium phosphates crystals. The chemical reactions are suggested as follows: Ca2þ þ H2PO4 / CaHPO4 þ Hþ
Figure 3 Shear bonding strength test indicates nonsignificant difference between the CCMS-HP/SBU and SBU groups (a Z 0.05). Bar Z standard deviation (n Z 8). CCMS-HP Z CaCO3@mesoporous silica reacted with phosphoric acid; SBU Z Single Bond Universal.
Ca2þ þ HPO2 / CaHPO 4 (DCPD) HPO2 þ Hþ 4 H2PO 4
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Figure 4 Fractography analysis of a represented specimen of debonded dentin surface. (A) Pointer indicates some dentin structure was cut and crystals remain in dentinal tubules. (B) At higher magnification, prominent fractured crystals from dentinal tubules are noted.
Thus, as soon as DCPD precipitates, the hydrogen ion byproduct from the self-etch adhesive creates a mildly acidic 2 þ environment in which H2PO 4 and HPO4 can react with H ions to form a buffer solution. Subsequently, the increase in pH by a reaction with the dentin surface is favorable for the growth of crystals with a higher Ca/P ratio. The main components of the SBU dental self-etch adhesive are the 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) monomer, dimethacrylate resins, 2hydroxyethyl methacrylate (HEMA), and polyalkenoic acid copolymer. HEMA is a popular monomer frequently used in dental adhesives in order to increase their wetting ability and hydrophilicity.25 However, HEMA released from dental adhesives negatively influences the dental pulp tissue by diffusing through the remaining dentin structure of the tooth cavity.26,27 The crystals grown in dentinal tubules under the dental adhesive layer acted as a protective biomimetic barrier to prevent the diffusion of cytotoxic monomer into the dentinal tubules (Figures 2 and 4). This biomimetic barrier in dentin also demonstrates a biocompatibility and a potential ability for dentinepulp remineralization.12 SBU dental self-etch adhesive is one of the most recent novelties in adhesive dentistry and was introduced as “universal” or “multimode” adhesives. These kinds of material are simplified adhesives, usually containing all
bonding components in a single bottle. Bonding to dentin using traditional etch and rinse adhesive system may be challenging because an ideal moisture condition should be maintained to avoid the collapse of the collagen matrix and to allow proper infiltration of the adhesive into the demineralized substrate.28 Thus, the purpose of choosing the self-etch adhesive in our study was to eliminate the additional etching step. In the present study, CCMS-HP paste could produce an acidic environment on the dentin surface, which in turn allowed the acidic self-etch adhesive working well on the dentin surface. Polyalkenoic acid copolymer in the dental adhesive could contribute to a higher bonding strength to caries-affected dentin.29 In addition to the contribution of a micromechanical bond to dentin (Figure 2), the SBU self-etch adhesive system is believed to incorporate a chemical interaction with the calcium in dentin because of the 10-MDP functional monomer, which has been regarded as a promising monomer for chemical bonding to the HA of dentin.30,31 In summary, the proposed novel strategy for treating the exposed dentin surface of tooth cavity with CCMS-HP prior to bonding to a dental adhesive system has a great potential to prevent postoperative sensitivity by forming a biomimetic calcium phosphate crystal barrier in the dentinal tubules. Based on the present findings, the formation of biomimetic crystal barrier in dentinal tubules was not
Figure 5 Fractography analysis of a represented specimen of debonded composite rod surface. (A) Resin tags/crystal plugs are pulled out from the dentin surface. (B) At higher magnification, integrated crystal layer and resin tags are noted.
Please cite this article in press as: Chiang Y-C, et al., A mesoporous biomaterial for biomimetic crystallization in dentinal tubules without impairing the bonding of a self-etch resin to dentin, Journal of the Formosan Medical Association (2016), http://dx.doi.org/10.1016/ j.jfma.2016.01.005
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A mesoporous biomaterial for biomimetic crystallization
Figure 6 Schematic diagram of biomimetic crystals formation and resin tags in dentinal tubules. Gaps between resin tags and tubule walls could be filled by calcium phosphate (CaP) crystals. SEA Z self-etch adhesive, HL Z hybrid layer.
impeded by the subsequent bonding procedure using SBU self-etch adhesive. Moreover, the mesoporpous silica biomaterial (CCMS-HP)-formed biomimetic barrier did not impair the bonding of SBU self-etch adhesive to dentin. Therefore, we conclude that CCMS-HP can be used to form a biomimetic barrier for prevention of dentin sensitivity because it neither impedes the bonding of SBU to dentin nor impairs the shear bonding strength between the SBU and dentin.
Acknowledgments This study was partly supported by the funding from the National Science Council Taiwan (103-2314-B-002-111-MY2 and NSC101-2314-B-002-098-MY2) and the National Taiwan University Hospital (100-N1766). The authors also wish to thank the staff of the Core Labs, Department of Medical Research, National Taiwan University Hospital, Taipei, Taiwan for technical support.
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