|Year : 2014 | Volume
| Issue : 2 | Page : 41-46
Modification of dentin surface to enamel-like structure: A potential strategy for improving dentin bonding durability, desensitizing and self-repairing
Hongye Yang1, Yake Wang2, Siying Liu1, Jinmei Lei1, Cui Huang2
1 The State Key Laboratory Breeding Base of Basic Science of Stomatology (Hubei-MOST) and Key Laboratory for Oral Biomedical Ministry of Education, School and Hospital of Stomatology, Wuhan University, Wuhan, Hubei Province, China
2 The State Key Laboratory Breeding Base of Basic Science of Stomatology (Hubei-MOST) and Key Laboratory for Oral Biomedical Ministry of Education; Department of Prosthodontics, School and Hospital of Stomatology, Wuhan University, Wuhan, Hubei Province, China
|Date of Web Publication||2-Jun-2014|
The State Key Laboratory Breeding Base of Basic Science of Stomatology (Hubei-MOST) and Key Laboratory for Oral Biomedical Ministry of Education, School and Hospital of Stomatology, Wuhan University, Wuhan, Hubei Province
Source of Support: The research that led to the proposed hypothesis was
supported by the National Nature Science Foundation of China (No. 81371191), Conflict of Interest: None
Introduction: Current theories of dentin bonding are based on the concept of "hybrid layer". However, the histological complexity of dentin, as well as the vulnerability of the hybrid layer, goes against the long-term effect of dentin bonding. At the same time, post-operative sensitivity is more likely to occur after traditional adhesive restoration. The Hypothesis: Compared to dentin bonding, enamel bonding exhibits a more optimal immediate and long-term performance, owing to its higher degree of mineralization, well-arranged enamel crystals and the porous structure after etching. Moreover, "enamel hypersensitivity" is never going to happen due to the lack of tubules existing in dentin. In light of this phenomenon, we brought up the concept and the proposal method to form an "enamel-like" dentin, simulating enamel structure to achieve satisfying durability of dentin bonding and obtain good performance for preventing post-operative sensitivity. With the application of mesoporous silicon bi-directionally binding to hydroxyapatite of dentin itself and hydroxyapatite nanorods synthetized in vitro, we may be able to form an enamel-like "functional layer" via ion-regulating self-assembly. Evaluation of Hypothesis: This paper explains how to achieve dentin enamel-like modification by chemical methods, especially, details the strategies and possible mechanisms of the hypothesis. Validation of the hypothesis is more likely to eliminate the adverse effect of dentinal fluid, improve long-term performance of dentin bonding, offer strategies for desensitizing treatment and self-repairing carious-affected dentin, and furthermore, provide the possibility to introduce new theories of dentin bonding.
Keywords: Dentin modification, dentin bonding, desensitizing, enamel-like structure, self-repairing
|How to cite this article:|
Yang H, Wang Y, Liu S, Lei J, Huang C. Modification of dentin surface to enamel-like structure: A potential strategy for improving dentin bonding durability, desensitizing and self-repairing. Dent Hypotheses 2014;5:41-6
|How to cite this URL:|
Yang H, Wang Y, Liu S, Lei J, Huang C. Modification of dentin surface to enamel-like structure: A potential strategy for improving dentin bonding durability, desensitizing and self-repairing. Dent Hypotheses [serial online] 2014 [cited 2019 Jun 20];5:41-6. Available from: http://www.dentalhypotheses.com/text.asp?2014/5/2/41/133421
| Introduction|| |
With the continuous improvement of living standards, esthetic tooth-colored restoration is gaining more popularity and wider acceptance in daily clinical practice. As the foundation of esthetic restoration, dentin bonding has undergone revolutionary changes following the pioneer approach of Buonocore in 1955.  Contemporary dentin adhesive systems show favorable immediate and short-term bonding effectiveness. However, the durability and stability of resin-dentin bonds remain questionable.  Bonding durability is responsible for the service life of adhesive restoration, because degradation can weaken retention and marginal adaptation, and eventually result in loss of bonded restorations overtime.  The National Institute of Dental and Craniofacial Research (NIDCR) 2009-2013 strategic plan reported that the mean replacement of resin restoration is only 5.7 years.  Therefore, how to improve the durability of resin-dentin bonding has become an urgent question and tough challenge in the field of dentistry.
Based upon the current adhesion strategy, resin-dentin bonding is a unique form of tissue engineering, in which exposed collagen fibers are used as the scaffold for resin infiltration to form a micro-mechanical interlocking, the so-called hybrid layer or resin-dentin inter-diffusion zone.  Actually, the hybrid layer is a flawed structure and always remains the weakest area of adhesive restorations. Recent researches are mainly concentrated on finding more practical and durable means to preserve the hybrid layer integrity.  These methods include the introduction of ethanol wet-bonding technology, the application of matrix metalloproteinases (MMPs) inhibitors, increasing hybrid layer collagen cross-linking, and biomimetic remineralization.  However, no consensus is obtained until now because of the limitations of strategies and technologies themselves, and still they are very technique-sensitive processes. New methods to improve durability of dentin bonding are still needed.
At the same time, post-operative sensitivity is frequently encountered with traditional adhesive restoration.  Although, most dentinal tubules can be sealed initially by permeation of self-etching or etch-and-rinse dentin adhesives, exudation of tubular fluid may cause entrapment of water bubbles along the adhesive-composite interface when the volatile solvent evaporates quickly before polymerization of the adhesive.  These bubbles may induce rapid fluid movement during mastication and then result in a transient stabbing, that is, post-operative sensitivity.  Therefore, researchers are committed to finding more effective strategies not only to enhance bonding durability of adhesive restoration but to prevent the occurrence of post-operative sensitivity.
| The Hypothesis|| |
The adhesion is a complex physical and chemical process, and the durability of adhesion to tooth tissue not only depends on the performance of adhesive agent used, but closely relates to the structural characteristics and mechanical properties of bonding substrate.  Fundamentally, the vulnerability and easy degradability of hybrid layers are associated with the morphology and physiology of dentin structure, such as the high organic content (collagen), tubular structure, the outward flow of tubule fluid and structural topographic variations. The dynamic tissue inevitably affects the performance of bonding durability.  Compared with dentin bonding, the bond to enamel is more reliable and stable regardless of immediate or long-term effectiveness.  This is mainly attributed to the superior structure of enamel. Firstly, selective dissolution of hydroxyapatite (HAP) crystals through etching creates lots of etch pits like porous honeycomb, followed by in situ polymerization of adhesive resin, which is readily absorbed by capillary attraction, therefore, enveloping individually exposed HAP crystals and forming high-performance mechanical locking.  Secondly, the degradation of resin-enamel interface rarely happens due to the stable structure of enamel such as the high mineral content (95%), regular arranged structure of enamel crystals and no fragile collagen.  Besides, "enamel hypersensitivity" is never going to happen due to the lack of tubules existing in dentin.
Inspired by this, our hypothesis is that if dentin surface is inorganically modified to enamel-like structure, it would be very likely to obtain reliable bonding durability and eliminate post-operative sensitivity.
| Strategies and Possible Mechanisms|| |
How to achieve enamel-like modification of dentin surface? Compared with the microstructure of enamel, there are at least three key issues needed to be resolved. Firstly, collagen fibers of dentin should be removed. Secondly, dentinal tubules should be compactly occluded. Finally, regularly arranged HAPs crystals should be formed on dentin surface. The possible mechanisms of dentin enamel-like modification are showed in [Figure 1].
|Figure 1: The possible mechanisms of dentin enamel-like modification with the application of deproteinization, MSNs and HAP nanorods|
Click here to view
Deproteinization of dentin
The presence of collagen fiber networks of dentin is the basis of the hybrid layer.  However, various studies have reported that collagen is not necessary for effective adhesion. ,,, The instability of collagen makes the adhesive protocol highly technique-sensitive.  Its low surface free energy could reduce adhesive monomer infiltration through the nanospaces of the collagen network,  leaving exposed collagen at the dentin-adhesive interface.  These denudes collagen are also full of water, which acts as a functional medium for the hydrolysis of resin matrices by esterases.  Furthermore, the collagen fibers within hybrid layer cannot resist the degradation of host-derived MMPs and cysteine cathepsins. , In this connection, some researches have pointed out the removal of collagen fibrils from the demineralized dentin as a possible way to minimize the hybridization technique sensitivity and optimize bonding. ,
In the first step of this project, we are aiming to achieve dentin deproteinization with the application of sodium hypochlorite (NaOCl). NaOCL is a well-known, non-specific proteolytic agent that is widely used in various dental procedures. It is capable of removing organic material (collagen) and altering the dentin composition to HAP-rich structure similar to enamel. It can also kill bacteria remaining in dentin to some extent due to its anti-bacterial property, thereby preventing the occurrence of secondary caries. 
After complete remove of collagen fibers, the deproteinized dentin surface is characterized with tubules with wide openings, many minor orifices in the tubular dentin, numerous microporous inorganic structure, and increased permeability. , Some researchers showed that deproteinization alone have increased immediate bond strength. ,,, It reminded us that the inorganic modified dentin surface can still apply to dentin bonding. However, the long-term bonding effectiveness is insufficient due to no elimination of the dentinal tubules liquid.
| Mesoporous Silicon for Blocking Dentinal Tubules|| |
As we know, tubule fluid flowing in the dentinal tubules compromises long-term bonding strength when dentin suffers from chemical, thermal, evaporative, tactile or osmotic stimuli.  Therefore, the second step, we plan to reduce hydraulic conductance of outward fluid flow from dentin. According to Brannstrom's "Hydrodynamic Theory,"  which is the most universally accepted mechanism, the most effective way to eliminate the impact of liquid is to block dentinal tubules.  But tubule occlusion may reduce the subsequent formation of adhesive resin tags. Do they affect bonding strength ultimately? Studies showed that full resin penetration through the hybrid layer presented a more important role on bond strength than that of resin tags formed within dentinal tubules. , This has also been confirmed by us in earlier experiment.  We plugged dentinal tubules using a calcium-containing desensitizer before adhesive restoration. It was found that the bonding strength between dentin and composite resin still presented a satisfactory result.  This phenomenon may be attributed to the foundation of a functional layer as a result of the uniform inorganic modification with mineral deposition, forming a substrate conducive to dentin bonding. Therefore, it is feasible to achieve a win-win objective for either durable bonding or post-operative sensitivity prevention through the application of tubule occlusion on deproteinized dentin surface. But, what would be the ideal biomaterial to produce a close connection with HAP (the main component of deproteinized dentin), fully enter into tubules and plug them tightly?
Recently, mesoporous silica nanoparticles (MSNs) have attracted widespread interest for their potential biomedical applications  due to their high surface area, high surface energy, high porous volume and well-ordered hexagonal pore structure, excellent thermal and mechanical stabilities, strong penetration and adsorption performance.  MSNs offer many advantages as a separation media for carrying cargo such as proteins, enzymes, drugs, genes and nanoparticles.  Lin et al., proved that the MSNs can penetrate extensively into dentinal tubules and closely integrated with the ingredients within the tubular wall.  Ishikawa et al., suggested that higher calcium ion concentrations and lower pH value promoted the formation of precipitates and helped them penetrate more deeply into the dentinal tubules.  Therefore, in the second step, the MSNs loaded nanoparticles become our choice.
The MSNs (MCM-41), we want to apply, are synthesized using a simple multi-cycle method in a more eco-friendly and economical way based on Ng EP et al.,'s new report.  The same amount of MSNs separately located calcium oxide (CaO) and phosphoric acid (H 3 PO 4 ) are mixed well with distilled water, forming a paste that is smeared evenly on deproteinized dentin surface for three times. After clotted in about 10 minutes, the remaining patch on surface are rinsed gently with water spray for 30 s, leaving a thin white coating on deproteinized dentin surface. During the coating procedure, the Ca 2+ and HPO 4 2− ions are easily and rapidly released due to high surface-area-to-volume ratio of the MSNs, resulting in the formation of CaHPO 4·2H2 O precipitates that plug the tubules tightly, acting as a barrier to prevent invasion of tubule fluid on bonding surface.  At the same time, the nucleation of CaP, when the pH value rises, also contributes to the adhesion between MSNs and underlying deproteinized dentin.  Last but not least, the MSNs will firmly bound to the HAP of deproteinized dentin by chemical reaction of generating silicon-stabilized tricalcium phosphate (Si-TCP), leading to the formation of a silicon-rich layer. 
| Self-Assembly of Hydroxyapatite Nanorods|| |
After the MSNs plugging dentinal tubules, the influence of tubule fluid can be eliminated and the inorganic content of dentin increases.  Theoretically speaking, we may stand a chance of witnessing a durable bonding strength and perfect desensitizing effects through above measures. However, the MSNs alone cannot form regularly arranged enamel-like structures. We want to continue to form an enamel-like layer, which has a similar structure and function with that of human enamel.
Fully developed enamel mainly consists of bundles of nanorod-like HAP crystals, which can form regularly arranged structure parallel to each other through multi-stage assembling.  All those gorgeous mechanical, physical, chemical and biological properties of enamel are attributed to this unique hierarchical structure.  There are so many strategies to develop well-aligned HAP crystals such as electrochemical deposition, magnetic crystallization, soaking organic film and so on, in which self-assembly of HAP has received much more attention due to its lower cost and higher efficiency.  Yamagishi K reported on "Nature" that fluoride HAP coating can be used for enamel reconstruction.  Meanwhile, MSNs have been found to allow fast deposition of HAP layer with strong bonds to the surface. , Inspired by this, we would like to use HAP nanorods coating for reconstruction of enamel-like layer. The HAP nanorods (approximately 10 nm in cross section and 50-100 nm in length along the c-axis) are synthesized in vitro according to Wang X's methods.  Then they are coated on the silicon-rich layer of conditioned dentin surface after the application of MSNs. The HAP nanorods will firmly bound to silicon-rich layer via a chemical reaction that generates Si-TCP.  Meanwhile, HAP self-assembly occurs under the regulation and control of MSNs, and regularly arranged enamel-like structures are formed gradually. ,
Up to this point, we would have modified dentin surface to enamel-like structure with the application of deproteinization, MSNs and HAP nanorods in turn.
| Evaluation of the Hypothesis|| |
This paper is the first attempt to operate enamel-like modification for dentin surface to obtain reliable long-term bonding strength, to prevent post-operative sensitivity, and to self-repair carious dentin. Moreover, this paper explains how to achieve dentin enamel-like modification by chemical methods, especially, details the strategies and possible mechanisms.
The realization of the hypothesis is more likely to obtain reliable bonding durability, because there is a strong resemblance in structure and function between enamel-like layer and real enamel.  Dentists can directly operate adhesive restoration on enamel-like dentin, which is a more favorable substrate for long-term bonding, just like enamel. Due to tubule occlusion by MSNs, most tubule fluids are separated from oral environment, and the nerve ending of dental pulp may not easily response to external stimuli in daily life. Thus, it is possible that dentin enamel-like modification can prevent the occurrence of post-operative sensitivity to a large extent. Even sometimes, this modification could replace the application of adhesive restoration if there is just a minor dental defect such as superficial dentin caries. The regularly arranged enamel-like structure can tightly attach and fill the defects, and self-repair the appearance and function of the damaged tooth.
Although patients' mouths are the ultimate testing environment for the hypothesis, however, it is hard to make appropriate standardization due to factors such as inter-operators variability, subjects' differences, patient compliance and recall failure. Therefore, in primary stage, some easy, rapid and realistic in vitro models will be established to help predict the behavior and effectiveness of dentin enamel-like modification during clinical life.
Regarding to bonding durability, we propose to test our hypothesis via the use of several artificial aging models, such as long-term water storage, thermocycling, NaOCl storage, pH cycling and mechanical loading. In particular, after completing adhesive restoration on enamel-modified dentin surface, specimens will suffer a range of artificial aging treatments mentioned above, and then some classic assessment index, such as degree of conversion, microtensile bond strengths (MTBS), fracture evaluation and nanoleakage expression, will be evaluated objectively. By doing this, it is possible to predict, as close as possible to clinical conditions, the influence of enamel-like modification on dentin-bonding durability.
In order to test the desensitizing validity of our hypothesis, dentin slices, approximately 0.8 mm in thickness, will be prepared. After operating enamel-like modification, scanning electron microscopy (SEM), atomic force microscopy (AFM), and confocal laser scanning microscopy (CLSM) will be used to assess tubule occlusion. Hydraulic conductance will be used to evaluate the validity of enamel-like modification in arresting dentin fluid movement.  The robustness of the occlusion under the simulated pulpal pressure, and the acid resistance of the occlusion will also be assessed. 
For testing the effectiveness of self-repairing, dentin substrate will suffer pH cycling for 14 days to resemble caries-affected dentin, and then a 250-300 μm thick artificial carious lesion model will be established, with which a stepwise demineralization occurs from the surface to the base of the lesion.  Then, we will manage enamel-like modification on prepared dentin substrate. After that, assessment will be conducted to test its wear resistance, longevity in saliva, and biocompatibility.
To sum up, we brought up the concept and the proposal method to form an "enamel-like" dentin with the application of MSNs bi-directionally binding to HAP of dentin itself and HAP nanorods synthetized in vitro. This structure will provide sufficient and stable bonding strength, offer more possibilities in improving long-term stability of dentin bonding, and further allow us to put forward a new theory embracing dentin bonding, desensitizing and self-repairing systems.
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