LITERATURE REVIEW

CHARACTERIZATION OF ENDODONTICALLY TREATED DENTIN¹

Lidis Marina Torres Reyes²; Carolina Torres Rodríguez³

¹ This article is one of the requirements to qualify for the Master's Degree in Dentistry from Universidad Nacional de Colombia

² Candidate to the Master's Degree in Dentistry, Universidad Nacional de Colombia, Bogotá, Colombia

³ Ph.D. Associate Professor, Department of Oral Health, School of Dentistry, Universidad Nacional de Colombia, Bogotá, Colombia. Email: ctorresr@unal.edu.co

SUBMITTED: APRIL 9/2011-APPROVED: SEPTEMBER 24/2013

Torres LM, Torres C. Characterization of endodontically treated dentin: a review. Rev Fac Odontol Univ Antioq 2014; 25 (2):.

ABSTRACT

INTRODUCTION: restoring endodontically treated teeth involves treating dentin with disinfectants and acidic substances that improve adhesion but generate structural irreversible changes. The objective of this article was to describe and analyze the changes in structure and mechanical properties produced by 5.25% sodium hypochlorite, 17% ethylenediaminetetraacetic acid (EDTA), and 2% chlorhexidine on endodontically treated dentin, as described in the literature. METHODS: the following databases were consulted: Scient Direct, Pubmed, Scielo, and EbscoHost, using these keywords: "Human Dentin" and "Root Canal" and "Human Dentin and Change", "Dentin Treated" and "Treated dentin and Chlorhexidine". We selected 67 articles that met these criteria: thoroughly describe dentin changes when it is irrigated with 5.25% sodium hypochlorite and 17% ethylenediaminetetraacetic acid (EDTA), and 2% chlorhexidine at different treatment stages. RESULTS AND CONCLUSIONS: : we found out that treated dentin suffers changes in terms of dentinal architecture loss, ionic content and organic matrix, and reduction in microhardness, nanohardness, and compressive and tensile strength. Endodontically treated dentin is a chemically altered substrate with affected properties and poor adhesion durability.

Key words: dentin, sodium hypochlorite, EDTA, tooth, root canal treatment, chlorhexidine.

INTRODUCTION

The objective of endodontic treatment is to remove vital or necrotic gelatinous mass pulp remains that accumulate in the pulpal chamber and root canals, in order to decontaminate it. For this purpose, debridement is performed with manual and mechanical instruments inside the canal, followed by irrigation.^{1, 2} Tissue debridement with manual and rotary instruments produces dentinal smear, a layer about 2-2.5 Μm thick that accumulates on the surface of dentinal tubules and inside them and is composed of bacteria, toxins, hydroxyapatite crystals, saliva, and blood, making root canal irrigation necessary.^{2, 3}

Root canal irrigation seeks to stimulate debridement, lubrication, disinfection, tissue dissolution, collagen removal, and dentinal dehydration, by using active chemicals. According to Teixeira, De Vasconcelos, Cecchin and Jaju, the chemical agent most widely used for this purpose is sodium hypochlorite at different concentrations, used either isolated or alternated with 17% ethylenediaminetetraacetic acid, EDTA.^4-7

Sodium hypochlorite is a salt formed by the bonding of chemical compounds, hypochlorous acid (HOCl), and sodium hydroxide (NaOH). It is hypertonic and alkaline, its pH is higher than 11, and is antibacterial, acts as a solvent of organic matrix, oxidizes and hydrolyzes protein, and removes intracellular fluids as well as magnesium and carbonate ions.8 In addition, it destroys fungi, spores, and viruses. It has low toxicity when used at low concentrations. Its concentration varies between 1 and 5.25%.^{2 ,8}

Although 5.25% sodium hypochlorite is used as a single irrigant in most root canals, it does not completely remove the inorganic portion of dentinal smear, and in narrow canals it does not appropriately moisten canal walls.2 Therefore, some studies have proposed to alternate hypochlorite with chelating agents that expand the root canal lumen facilitating access. The most widely used is ethylenediaminetetraacetic acid, EDTA, though this protocol is controversial.^{9, 10}

In its chemical structure, ethylenediaminetetraacetic acid, EDTA, has six potential sites for binding metal ions. It forms stable complexes with calcium ions and demineralizes superficial root canal walls by simplifying preparation in narrow canals.¹¹ However, it is not provided with disinfectant capacities, does not remove the organic component of dentinal smear and it is believed that when combined with sodium hypochlorite it inactivates chlorine, removing its proteolytic capacity.¹²

Another substance commonly used to irrigate canals is chlorhexidine gluconate, a molecule of symmetrical structure with two fractions of ionizable guanidine.13 It is considered to be an effective broad-spectrum antimicrobial agent against gram-positive and gram-negative bacteria, and fungi. It has a special property: substantivity, which enables long-lasting association with a substrate, prolonging its antimicrobial effect.^{13, 14}

The antimicrobial activity of chlorhexidine is pH dependent and its bactericidal effect occurs because the positively charged molecule penetrates bacterial walls, interacting with their phosphate groups (negatively charged) and altering their osmotic balance.^{15, 16} In the dentin, chlorhexidine adheres to the dentinal tubules and disinfects them, preventing bacterial adhesion.^{17, 18} When dentinal tissue is mineralized and chlorhexidine has a neutral pH, the positively charged molecule is attracted to the negative charges of the hydroxyapatite phosphate groups. Conversely, when the dentine is demineralized, the molecule reduces its bonding to phosphate groups.¹⁹ According to Basrani and Haapasalo, chlorhexidine digluconate has the same effectiveness as conventional medicines and irrigants.^{20, 21} As a disadvantage, it is unable to dissolve tissue and dentinal smear, and that is why Haapasalo suggested to alternate this solution with sodium hypochlorite to ensure sealing gutta percha on root canal walls.²¹

Despite the benefits of the aforementioned irrigants, Öztürk, Santos, Campos and Parodi reported that such irrigants can alter dentin composition as well as interaction with adhesive materials.^22-25

The dental restoration phase comes after endodontic treatment and is aimed at restoring tooth function and aesthetics. At this stage, it is important to avoid bacterial infiltration in the canal, so it is recommended to irrigate again with sodium hypochlorite and EDTA to remove dentinal smear and to improve adherence,²⁰ according to the restoration protocol of choice.^{3, 11}

METHODS

This was a topic review by searching the following databases: Scient Direct, Pubmed, EbscoHost, and Scielo, using these keywords: Human Dentin AND Fiber post, Root Canal and Dentin and Change, Root canal irrigants review, Mechanical Properties and Dentin Treated dentin and Clorhexidine. The search was limited to studies that treated dentin with sodium hypochlorite, EDTA, and chlorhexidine. For data analysis, the articles were classified according to type of irrigant and time of application.

RESULTS

The search yielded 264 articles, of which 67 were selected and analyzed. The collected information allowed the researchers to describe healthy dentin and its changes when it is endodontically treated and subjected to the action of sodium hypochlorite, EDTA, and chlorhexidine.

Dentin

Dentin is the structural axis of the tooth and is its mineralized tissue with the most volume. In 1996, Pashley²⁶ described dentin as a porous biological compound formed by a matrix of collagen filled with hydroxyapatite crystals. Dentin has several phases: an organic phase corresponding to 20%, of which 90% is collagen type I, and the remaining 10% are non-collagen proteins.^{27, 28}

Its organic phase is composed of type I collagen, which is a fibrous insoluble protein formed from tropocollagen molecules, which in turn are composed of three coiled polypeptide chains linked by hydrogen bonds that make them compact providing the tissue with strength.^{29, 30} Each polypeptide chain has a specific sequence of repetitive amino acids, like this: glycine, proline and hydroxyproline. Since glycine has hydrogen in its side chain, it behaves as a basic/acid amino acid, i.e., it is amphoteric, providing the collagen molecule with special features.²⁹

The other phase of dentin is inorganic, representing 70% of its tissue and made up of hydroxylapatite—also called biological apatite—, which belongs to the family of ion-substituted calcium orthophosphates, arranged on crystals smaller than enamel.³¹ Apatite is a complex ion network of calcium phosphate crystals, hydroxyls and fluorides of variable composition. This makes dentin apatite crystals less stable and more reactive.³¹ The rest of dentinal tissue, about 10-12%, is water.

Histologically, dentin consists of two structures: the dentinal tubules and the intertubular matrix. Dentinal tubules are cylindrical structures located along the dentin and covered by highly mineralized peritubular dentin that provides them with rigidity. Inside the dentinal tubules are the odontoblast cytoplasmic extensions or Thomes fibrils.³²

The number of dentinal tubules varies depending on the tooth's area. The deep dentin, closer to the pulp, has about 25.300-32.300 tubules per mm² and surface dentin has about 13.700-16.500 per mm². While in the radicular dentin the number of tubules reaches 24.000 per mm² near the pulp and 12.000 per mm² far from the pulp.³³

Dentinal tubules are surrounded by intertubular matrix, which separates dentinal tubules from their neighbors. It is comprised of collagen fibers that form a mesh, where crystals of hydroxyapatite with lower degree of mineralization accumulate. Intertubular matrix differs in hardness depending on where it is located; near the pulp it presents low Knoop hardness values: 64, 75-65, 05, and away from the pulp it has higher values: 72, 53-73, 75.34

Is should be noted that there are differences between coronal and root dentin: the latter has fewer dentinal tubules of reduced area and more intertubular dentin than the former. Another difference is that root dentin collagen has a larger diameter and is differently oriented. These structural variations can cause significant differences in mechanical properties.^{33, 35, 36}

Since dentin is composed of organic substance, mineral substance and water, it is considered a heterogeneous composite with special features such as viscoelasticity and anisotropy, which have a time-dependent behavior and allow different load distribution along the three axes.^{37, 38}

The structure of dentin has been traditionally studied by different methods, ranging from microscope polarized light or contrast of phases, immunohistochemistry, fluorescence, and scanning electron microscopy, which were used to study coronal dentin. This is why there were few known details about the differences between root and coronal dentin.³⁰

Currently, it is possible to analyze the structure and composition of biological tissues and their properties at nanoscales. For example, through atomic force microscopy one can calculate the nanohardness and the modulus of elasticity of specific sites, such as coronal or root inter- and peritubular dentin.^{12, 30}

In fact, Inoue et al reported that the nanohardness and the modulus of elasticity of mineralized dentin vary depending on its location. According to their studies, intertubular coronal dentin hardness was 0.81 ± 0m5 Gpa and root dentine 0m55 ± 0m02 Gpa. They also found out that the coronal dentin's modulus of elasticity equals 26.60 ± 2.19 Gpa, while the root dentin was 20.89 ± 1.10 Gpa.³⁹

Similarly, according to Kishen, Palamara and Kinney, the modulus of elasticity values present differences. These could be attributed to measurement errors (due to the small size of samples which makes them difficult to handle), differences in each experiment's measurement area, and the type of treatment the sample received according to the instrument used (dry or wet samples).^40-42

Thus, Kinney et al reported a maximum modulus of elasticity value of 28.3 Gpa, obtained by simulating an isotropic model on dry dentin, by measuring in a direction perpendicular to the dentinal tubule, while in wet dentin they reported a value of 24.4 Gpa. This means that the elastic modulus increased 4 Gpa when the dentin sample was dehydrated.⁴²

In contrast, Ziskind et al observed a gradual reduction in modulus of elasticity in intertubular dentin, with values of 17 Gpa, while in peritubular dentin the values reach 40-42 Gpa.⁴³

In synthesis, the variation of mechanical properties in dentin, based on dentinal tubules characteristics, is a product of the higher or lower mineral content in them and the orientation of collagen fibers.⁴²

Effect of 5.25% sodium hypochlorite, 17% EDTA, and chlorhexidine gluconate on endodontically treated teeth

The success of endodontic therapy depends on appropriate debridement of the pulp tissue and the complete elimination of bacteria and their toxins from the root canal system. This is why it requires mechanical instrumentation with manual and rotary instruments followed by irrigation with chemicals that cleanse the canal and prevent its reinfection.⁴⁴

The consulted literature showed that endodontically treated teeth suffer four main changes: water loss, dentinal tissue loss, microstructural-composition changes, and changes in mechanical properties (microhardness, nanohardness, modulus of elasticity and tensile strength).

It has been reported that after canal treatment, teeth lose 10% of water.1 The other reported change is dental tissue loss associated with: 1) the presence of caries, extensive restorations, or trauma. 2) the root canal access made in order to access the root canal, breaking the tooth sealing and causing deflection, and 3) chemical-mechanical preparation of the canal system by means of files and manual or mechanical rotary instruments.⁴⁵

In terms of changes to the microstructure and composition of dentin, the time of application and the irrigant concentration required to cause adverse effects were taken into account.

Sim, Grigoratos and Toledano reported that treating dentin with 5% sodium hypochlorite for two minutes produces dissolution of collagen and collagen-mineral bond as well as changes in apatite crystallinity, resulting in a surface rich in apatite crystals similar to that of dental enamel. Therefore, the substrate becomes more brittle, decreases its physical properties and produces a very weak bond.^46-48

Also, Toledano reported that 5% sodium hypochlorite not only causes removal of dentin collagen matrix, but also the loss of part of the mineral phase of it, making it weaker than a non-treated dentin.⁴⁸

In terms of change in mechanical properties, specifically microhardness and nanohardness, Fuentes et al, in a study that compared the Knoop microhardness of dentin treated with sodium hypochlorite and mineralized dentin, obtained a microhardness reduction of 60.3 to 30, respectively.34 In the same way, Toledano reported that the Knoop hardness of dentin treated with hypochlorite is 50% lower that the mineralized dentin.⁴⁸

Similarly, Sayin et al showed that the only fact of treating dentine with sodium hypochlorite and EDTA reduces surface microhardness. These authors also found out a relationship between the percentage of change in radicular dentin microhardness and the measuring area. That is to say, coronal and medium dentin treated with hypochlorite and EDTA have the highest changes percentages ranging from 7.72% to 29.4%, whereas apical dentin suffered minor changes compared to the control group (dentin irrigated with distilled water).⁴⁹

The abovementioned results were confirmed by Pascon and Patil, who reported that treatment with sodium hypochlorite at different concentrations (2.5%, 3.5% and 5.25%) alters the mechanical properties of root dentin, such as microhardness, roughness, elastic modulus, flexural resistance, and fatigue. In addition, root dentin experiences volumetric shrinkage and changes in apatite crystallinity, which in turn produces changes in hardness.^{50, 51}

Cheron et al also compared the nanohardness of vital dentin and endodontically treated dentin, obtaining a decrease in the endodontically treated dentin from 0.84 ± 0.25 to 0.84 ± 0.18 respectively.⁵²

In terms of changes in the elastic modulus of the treated dentin, Sim and Grigoratos, by using a machine of trials and applying flexural loads, reported that treating dentine with 5% sodium hypochlorite reduces the elastic module in mineralized dentin from 5,2 x 10¹⁰ Nm^-2 to 3,2 x 10¹⁰ Nm^-2).^{46, 47} Other authors, such as Marshall and Cheron, evaluated the elastic modulus of the intertubular dentin of endodontically treated teeth using atomic force microscopy and found that the elastic modulus increases with respect to the mineralized dentin from 17.8 ± 2.9 Gpa to 18.9 ± 2.9 Gpa.^{33, 52}

The dentinal changes caused by irrigation with chlorhexidine have not been thoroughly studied and are controversial; these changes are basically related to root dentin microhardness and to the effects on root canal adhesion.^{23, 53}

Microhardness analysis is related to the composition of dentinal surface. According to Oliveira, irrigating root dentin with 2% chlorhexidine for 15 minutes significantly reduces the Vickers microhardness of root dentin. This changes from 30.73 to 20.89, depending on the depth to which the measurement is taken.⁵³ In the same way, Saghiri reported that 5 minutes of treatment with chlorhexidine reduces dentin microhardness and causes erosion.⁵⁴ On the contrary, Ari reported that treatment with chlorhexidine does not affect dentin microhardness, although the concentration used for his study was 0.2%.55

Regarding the effect of chlorhexidine on adherence, the experiments use chlorhexidine in two forms, solution and gel, at a concentration that ranges from 0.2 to 1 and 2% with short treatment times, establishing differences depending on the adhesive systems used.^56-58

Consequently, the changes in microstructure, composition, and properties of endodontically treated dentin at different scales could explain why dental fracture is so common in endodontically treated teeth.¹

DISCUSSION

The results of this review show that the substances used to treat dentin produce irreversible changes in it, depending on irrigant solution type and exposure time.

The solutions under analysis were 5.25% sodium hypochlorite, 17% EDTA, and 2% chlorhexidine gluconate. According to Beltz et al, hypochlorite dissolves 90% of the organic components of dentin, and 17% EDTA dissolves 70% or more of its inorganic components,⁵⁹ while chlorhexidine adheres to dentinal hydroxyapatite but is unable to dissolve organic tissue.^{19, 60} This means that they are effective as disinfectants, chelating agents and irrigants, but they modify dentin composition. As for the time needed, Beltz reported that using 10 ml of 17% EDTA for 1 min or 20 ml for 3 minutes is enough for it to be effective.⁵⁹ Siqueira reported that irrigating the root canal with 5% sodium hypochlorite for 40 min is effective, bearing in mind that its deleterious effect on dentin depends on concentration and exposure time.^{61, 62}

In this sense, in their in vitro study Vianna et al reported that 1 ml of 2% chlorhexidine solution for one minute or less is effective to eliminate facultative (Enterococus Faecalis) and anaerobic (Stafilococus Aureus and Candida albicans) bacteria, agreeing with Gomes B in 2003. But in practical terms, when irrigating with chlorhexidine solution, Vianna et al suggest to rigorously instrument the canal to ensure pulp tissue removal.^{15, 63}

Two of the identified dentinal changes are water loss and structural integrity loss, highlighted by Faria et al as the main causes of tooth fracture.1 This means that eliminating pulp chamber sealing during endodontic treatment produces the first disadvantage of treated versus untreated tooth. One must make sure then that the tooth really needs this treatment.

Consequently, the tooth suffers substantial changes in terms of the analyzed mechanical properties: microhardness, modulus of elasticity, compressive and tensile strength, due to collagen degradation (peptides breakdown and chlorination of its terminal group) and to the loss of the mineral phase.³⁶

Saleh et al reported that irrigating the root canal with 5% sodium hypochlorite and 17% EDTA for 60 seconds significantly reduces the dentin's Knoop microhardness. The authors also found a reduction of Knoop microhardness in superficial dentin.⁶⁴

Regarding the change in modulus of elasticity of the treated dentin, according to Mountouris et all the results are controversial, since they reported that the modulus of elasticity in the coronal dentin did not suffer changes, unlike the modulus of elasticity in root dentin, which decreased depending on the type of irrigant and the origin of samples.⁶⁵ These differences can be explained because the coronal dentin has higher content of calcium and a higher Ca/P ratio, and its properties are dependent of the tubules density rather than the intertubular dentin, as Kinney stated.⁴² Other sources of change include the measurement instrument, the scale used and sample preparation (whether it is wet or dry),⁶⁶ so that the various studies cannot be comparable.

The differences between peritubular and intertubular dentin concerning the presence or absence of mineral and the three-dimensional orientation of collagen type I fibers, with evident spaces and overlaps, provide the tissue with the ability of absorbing loads and respond to them in a heterogeneous manner. This explains the close relationship between the structure of a tissue and its mechanical properties.67 We can then explain the variability at different scales of the in vitro studies consulted.

Concerning chlorhexidine, it should be noted that it is a worthy alternative as root canal irrigant and intracanal medication, thanks to its antimicrobial activity, substantivity and low toxicity, but it is unable to dissolve organic matter,¹⁶ i.e., it wouldn't be indicated in presence of pulp tissue remains.

However, in order to take advantage of the benefits of irrigant solutions, it has been suggested to combine them—although in some cases this causes negative effects on them, as described by Haapasalo et al—. These authors found out that combining chlorhexidine with sodium hypochlorite produces a precipitate of orange-brown color containing metal ions and para-chloroaniline, a substance with mutagenic potential.²¹

Few studies were found concerning the changes in mechanical properties of root dentin when it is irrigated with chlorhexidine. Oliveira et al noted that chlorhexidine directly affects the dentinal structure components, resulting in reduction of their microhardness.⁵³

The present review allows the authors to inform clinicians of the risks of treating dentin with 5% sodium hypochlorite, 17% EDTA, and 2% chlorhexidine to disinfect the root canal, remove dentinal smear, and improve adhesion, as it leaves the tooth with uncertain prognosis and listed among those that will possibly suffer fractures.

Given the changes described in the literature, it is recommended to use irrigants at a low concentration (1% sodium hypochlorite) and to establish minimum requirements as well as application times during treatments, in order to ensure endodontic treatment success and to reduce the adverse effects on dentin.

Also, it is suggested to use 17% EDTA exclusively for sclerotic canals treatment, and to re-consider its use in restorative protocols, since it implies treating the dentin two times, modifying its mechanical properties and reducing the tooth's biomechanical response. In the case of 2% chlorhexidine, it is proposed as a second choice after sodium hypochlorite, as it is unable to dissolve necrotic tissue. Given the scarce literature about the effects of this substance on the mechanical properties of root dentin, further studies are recommended.

Finally, although the focus of this review was 2% chlorhexidine as irrigant, it is important to note that this substance has been included in restorative protocols in order to improve dentinal adhesion, so it is necessary to perform further studies on adhesive systems, substance concentration, substance formulation, and time of application.

CONCLUSIONS

This topic review suggests that the analyzed dentinal treatments promote the substrate's irreversible deterioration due to a loss in dentinal architecture as well as loss of ions, calcium, phosphate, magnesium, carbonates, water, and collagen denaturation, which in turn affects the adhesion processes favoring failures in the final restoration.

REFERENCES