Effects of Irrigating Solutions on Dentin: Comparison
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Irrigating solutions play an important role in the eradication of intracanal microbes and debris dissolution during endodontic treatment. Different combinations of solutions and protocols have been advocated, with sodium hypochlorite (NaOCl), ethylenediamine tetra acetic acid (EDTA), and chlorhexidine (CHX) remaining the most widely used ones by many clinicians. Although these solutions provide efficient inorganic dissolution and antimicrobial capacity, their use has also been reported to cause undesired effects on root dentin composition and mechanical and biomechanical properties, such as microhardness, surface roughness, bond strength, and matrix metalloproteinase (MMP) activity.

  • endodontics
  • irrigating solutions
  • matrix metalloproteinases

1. Introduction

To achieve an effective chemomechanical preparation of the root canal system space, several irrigant solutions have been adopted in root canal treatment, which include, but are not limited to, the following: ethylenediamine tetra acetic acid (EDTA); citric acid (CA); sodium hypochlorite (NaOCl); chlorhexidine (CHX); iodine potassium iodide; hydrogen peroxide; local anesthetic; and saline and/or water [38][1].
Irrespective of the various irrigating solutions available, a systematic review revealed that there is no difference between these several solutions, and the deficit in well-conducted clinical studies should be taken into consideration when considering a “no difference” result as opposed to taking it for granted [39][2].
Although irrigating solutions may differ in their actions and chemical properties [40][3], from the range of characteristics an ideal irrigant should possess, only a few of them, when used alone, offer some spectrum of the ideal properties [39][2]. Some of these include germicidal, fungicidal, nontoxic, nonirritating, stability in solution, noninterference with tissue repair, prolonged antimicrobial effect, and relatively inexpensive [33,41][4][5].

2. Effect of Sodium Hypochlorite

NaOCl is a broad-spectrum antimicrobial solution used in endodontics to eliminate biofilms of species and microorganisms, such as enterococcus, actinomyces, and candida species [42][6]. Consequently, it is widely recognized for its effective antibacterial activity, and at minimal concentrations, it destroys bacteria rapidly [3][7]. Furthermore, it possesses tissue dissolution ability and endotoxin deactivation, and it is nonallergenic [40][3]. It is also inexpensive, has a long shelf-life, and is easily available [43,44,45][8][9][10]. Regardless of the many advantages of NaOCl, its relative toxicity, inability to remove the smear layer, and unpleasant taste have been condemned [46,47][11][12].
It has been recorded that a concentration of NaOCl as high as 10% has been used by dentists [48][13]. Although an increase in the solution concentration enhances its tissue dissolution effect [49][14], it could also adversely affect dentin properties [50][15]. Coupled with an increase in the concentration of hypochlorite solution, other factors that may enhance the tissue dissolution effect of the irrigant include an increase in pH, prolonged exposure time, increase in temperature, and ultrasonic agitation [40,51,52][3][16][17].
As a nonspecific oxidizing agent, the adverse effect of NaOCl is also concentration dependent as with its antimicrobial and tissue dissolving effects [53][18]. For this reason, it has been utilized for hard tissue deproteination [54][19]. As an oxidizing agent, its ability to fragment peptide chains and to chlorinate protein terminal groups to produce N-chloramines [55,56][20][21] is responsible for its deleterious effects on the dentin surface [25][22]. Reports have also demonstrated that sodium hypochlorite changes the mechanical properties of dentin by the degradation of the dentin’s organic constituents, and this is because the organic constituent of dentin is 22% of its weight and the irrigant can easily deplete it if dentine is demineralized [25,57][22][23].
It was noted from a study carried out on bovine dentin that alterations of the chemical and physical properties occur within the timespan of endodontic treatment [58][24]. Based on the study of the effect of NaOCl on human root dentin by Marending et al. [25][22], it was demonstrated that a hypochlorite solution dissolves the organic constituent of dentin but leaves the inorganic constituents unaltered. However, because of the varying methodologies and experimental parameters (i.e., lack of standardization), studies have revealed contradicting results concerning the effect of sodium hypochlorite on the organic constituent of dentine [25][22].
Compared to a physiological saline solution, the mechanical properties of dentin, such as flexural strength and the modulus of elasticity, have been reported to become remarkably reduced after exposure to a greater than 3% weight per volume of NaOCl for 2 h [8,59][25][26]. Additionally, a 2.5% NaOCl solution decreases the flexural strength of dentin after 24 min exposure [26][27], while with a 5.25% NaOCl solution, both the elastic modulus and flexural strength of dentin decrease after a 2 h exposure [8][25]. Even a lower 0.5% hypochlorite solution has a comparatively lesser effect on the flexural strength and modulus of the elasticity of dentin when compared to the 6.0% concentration, which is used by several clinicians in the United States of America [8][25]. The changes in the flexural strength and elastic modulus caused by a high concentration of NaOCl may lead to a decrease in the properties by 50% [25][22]. On the other hand, a contradicting study by Machnick et al. [60][28] observed that the flexural strength and elastic modulus were not affected by the sodium hypochlorite concentration.
Corresponding to the study by Lee et al. [61][29], who revealed that cracks were on the dentin surfaces after exposure to 5% NaOCl, Marending et al. 2007 [25][22] also noted crack lines in the dentin specimens after exposure to a 5% hypochlorite solution for 2 h. These results support the hypothesis that the microhardness of root dentin is dependent on the concentration of the hypochlorite used [58][24]. The microhardness of dentin was depleted after immersion in 1.0% sodium hypochlorite for 15 min [62][30] and 5.0% hypochlorite solution for 60 s [30][31], which also corroborates the findings that there is a greater effect on the microhardness when an increased concentration is used.
According to Marending et al. [25][22], the permeability of dentin is also altered by exposure to NaOCl. The study revealed that dentin permeability was significantly increased after exposure of dentin bars to a 1.0% hypochlorite solution but most especially to a 5.0% concentration. This experiment was carried out by subjecting the dentin specimens treated with NaOCl in a basic fuchsin dye. Changes of the sealing ability and adhesion of the resin-based cement to the dentinal collagen are also consequences of the effect of NaOCl on the dentin collagen [63,64,65][32][33][34]. This effect may produce equal or superior bonding results for some dentin-bonding systems [66,67,68,69,70][35][36][37][38][39].
Ari et al. [71][40] also revealed that 2.5% to 5.25% of NaOCl caused a remarkable increase in the surface roughness, while other data recorded no effect on roughness [14][41]. A study using the in silico approach observed an increase in stress and strain concentrations with the use of a hypochlorite solution [60,72][28][42]. There was a 15.9% increase in tensile strain and a 33.5% increase in compressive strain after a high hypochlorite solution was used as an irrigant [8][25].
Reports also show that alterations of the dentin matrix are a result of all NaOCl concentrations [71,73][40][43]. Zhang et al. [23][44] reported that a 5.25% hypochlorite solution caused greater dentinal erosion compared to a 1.3% concentration, and the mechanism by which NaOCl removed the dentin organic phase was by infiltration into the apatite-encapsulated collagen matrix. This was possible due to the low molecular weight of the irrigant [23][44]. Another study on the tissue dissolution and modifications in the dentin composition by different NaOCl concentrations observed that the uninterrupted degeneration of the dentin surface collagen was evident by the time-dependent effect in the increase in the amide II/phosphate ratio [74][45]. Preceding studies in line with this outcome also proved that the removal of the dentin organic phase was time-dependent [23][44]. Furthermore, Tartari et al. [74][45] also confirmed that the degeneration of dentin collagen by a hypochlorite solution was concentration dependent. In this study, both the concentration and time-dependent effects of NaOCl were evident by the decrease in the carbonate/phosphate ratio after 30 s immersion in all concentrations. Apart from the degenerating effects on collagen, another study demonstrated the effect of NAOC on chondroitin sulfate and more specifically on type I collagen, while others demonstrated the loss of immunoreactivity of both glycosaminoglycans and type I collagen after treatment with sodium hypochlorite [75][46].
The consequences of damage to the collagen matrix of dentine included a less resilient and a more fragile substrate [25[22][47],76], which may encourage the generation of fatigue cracks when cyclic forces are applied and ultimately cause a decrease in the resistance to crown and root fractures [23,77][44][48]. The deproteination ability of the hypochlorite solution led to an unequal effect thus producing an unbound hydroxyapatite and collagen-sparse dentine subsurface rich in apatite [76,78][47][49]. The deproteination effect also caused additional dentin changes evident by the “moth-eaten” appearance seen on a scanning electron microscope (SEM) [66,79][35][50]. The microscopy analysis revealed undamaged intertubular dentin surface as effects of NaOCl on the dental tissue [80][51].
In as much as there have been insufficient studies on the direct quantitative effects of low concentrations (0.5–2.25%) and short exposure periods (1–10 min) of NaOCl on dentin deproteination, 0.5% sodium hypochlorite causes a lesser effect on dentin deproteination than 1.0% and 2.25% concentrations [81][52]. Deproteination by 5.0% NaOCl for 10 min decreased the bond strength between the fiber posts and dentin surfaces [82][53].
To maintain an aseptic environment in the root canal while trying to limit dentin deproteination, it is advised that an ideal NaOCl concentration at a suitable exposure time is used [76][47]. Hence the use of hypochlorite concentrations, such as 1.0% and 2.5%, which have been found to promote organic tissue dissolution as well as limited destruction to the dentin structure, have been advised [74][45]. Although preceding studies revealed that 0.5% NaOCl is effective only for the removal of bacteria on the dentin surface layer, other studies recommended the use of a low 0.5% hypochlorite solution concentration as a routine irrigating solution but at a longer exposure period because it achieved excellent antimicrobial activity and had an insignificant effect on dentin deproteination [14][41].

3. Effect of Decalcifying Agents

The various levels of actions of decalcifying agents on mineral dentin depend on the concentration, immersion time, and decalcifying capability [74][45]. Irrespective of the concentration and time, EDTA and other chelating agents can cause harmful effects. Exposure to 17% EDTA for 3 min can dissolve the inorganic smear layer of dentin, and because EDTA has been found to remove the smear layer [14][41], other chelating agents, such as etidronic acid (HEDP), tetrasodium ethylenediamine tetraacetic acid (EDTANa4), and peracetic acid (PAA), have also been used as substitutes of EDTA [74][45] to remove the smear layer. It has been noted that in 5 min, HEDP and EDTANa4 completed this action [28[54][55],83], while in 1 min, PAA could remove the inorganic components of the smear layer [84][56].
The study by Tartari and Bachmann [74][45] revealed that changes in the components of dentin were evident by the ratios of amide III/phosphate (PO43− v3) and carbonate (CO32− v2)/phosphate (PO43− v3) bands. At different rates, increments in the amide II/phosphate ratio indicated dentin demineralization and dentin surfaces sparse in apatite but rich in collagen, while a decrease in the phosphate and carbonate apatite bands by the different decalcifying agents at different rates was also observed. At 9% and 18%, HEDP did not change the amide III/phosphate ratio; however, at 5% and 10%, EDTANa4 minimally reduced the phosphate group compared to the other decalcifying agents. At a 2% concentration, PAA eliminated the phosphate group and exposed the collagen matrix, which ultimately caused a significant increase in the amide III/phosphate ratio. The results by Tartari and Bachmann [74][45] reveal that HEDP and alkaline EDTANa4 have little effect on dentin demineralization, while EDTAHNa3 and PAA, at concentrations greater than 0.5%, may have a greater demineralizing ability and should be prudently used [21][57].
The process by which EDTA removes calcium ions (Ca2+) from mineral tissues is shown to destroy the dentin matrix [17][58], and it has been noted that calcium was depleted from the dentin surface to a depth of almost 150μm after exposure to 17% EDTA for 2 h [85][59]. Although EDTA destroys the dentin matrix [17][58], its solo use as an irrigant could inhibit dentin dissolution because of the accumulation of an organic matrix on the canal surface [75][46].
EDTA has been found to be a major cause of dentinal erosion [86][60], and because the erosion of the canal wall is a result of prolonged used of the solution due to its ability to demineralize root dentin [87[61][62],88], efforts have been made to weaken the constant chelating effect of EDTA for longer periods [89,90][63][64]. The use of a lower EDTA concentration (1%) was recommended by Şen et al. [90][64] to prevent extreme root canal dentin erosion but also to provide sufficient smear layer removal. Apart from EDTA’s effect on the dentinal wall, 19% citric acid has also been reported to enlarge the superficial part of dentinal tubules [91,92][65][66].
Although the bond strength between endodontic sealers, such as AH plus (Dentsply DeTrey, Konstanz, Germany), and dentin is enhanced due to the chemical bond between the material and the amino groups of dentin that is exposed by the demineralizing effect of strong decalcifying agents (EDTAHNa3 & PAA) [65][34], the generation of a weak bond and an increase in interfacial degradation could occur due to insignificant penetration of the material into the demineralized dentin caused by significant decalcification in the walls of the root canal [28,93,94,95,96][54][67][68][69][70].
Additionally, deleterious effects on dentin’s surface roughness provoked by EDTA exposure has been reported [8,24][25][71]. Ari et al. in 2004 [71][40] also reported a substantial increase in the surface roughness on root canal dentine by 17% EDTA. Other dentin properties, such as micro and nanohardness, have also been reported to be changed using chelating agents [14][41], and this corroborates the studies by Saleh et al. in 1999 [30][31] and Cruz-Filho et al. in 2001 [97][72] that demonstrated a decrease in dentin’s microhardness by EDTA.
Moreover, the teeth fracture resistance was found to be markedly reduced by 17% EDTA when compared to 5% EDTA after a 10 min exposure, even though after a 1 min immersion, it was reported that 5% EDTA had a greater effect on the reduction of teeth fracture resistance when compared to 17% EDTA [98][73]. Additionally, demonstrations by Uzunoglu et al. [98][73] revealed that when compared to roots rinsed with distilled water only, 17% or 5% EDTA produced a higher fracture resistance of the roots after 1 min exposure, which was explained by the ability to remove the smear layer, and this has been shown to improve the bond strength of resin-based sealers to dentin [98][73].
Following dentin exposure to 17% EDTA for 2 h, the flexural strength and the modulus of elasticity were reduced by one third and by half, respectively [99][74]. Because mechanical properties, especially strength, are less likely to be affected after the use of EDTA for shorter periods, it is recommended to reduce the influence of high-concentration EDTA (15% or 17%) for it to be used during shorter periods (approximately 2 min) [98,100,101,102,103,104,105][73][75][76][77][78][79][80].

4. Effect of Chlorhexidine

Chlorhexidine (CHX), a cationic biguanide with a broad spectrum, is the most used antiseptic product [3][7]. It is used as a disinfectant, preservative, and antiseptic in the medical, pharmaceutical, and dental fields [106,107][81][82]. The bis-biguanide is a strong basic salt, and in its original form (chlorhexidine acetate and hydrochloride), it is most stable, but due to its insolubility in water [40[3][83],108], chlorhexidine digluconate became its replacement [109][84].
Based on the effect of CHX on the mechanical properties of dentin, only one study proved there was no effect [105][80]. The association of chlorhexidine molecules with the surface of dentin was responsible for the activity of CHX [110][85]. The removal of the smear layer by this interaction also increased chlorhexidine activity [111,112][86][87].
Ari et al. [71][40] observed that the microhardness of dentin was markedly decreased by all irrigating solutions (NaOCl, H2O2, EDTA, etc.) except CHX. They noted that the surface roughness of dentin was not affected by 0.2% CHX gluconate, and their results also reveal that this solution was the only one, amongst the other irrigating solutions, that did not affect the constituents of dentin. Due to its nontoxic effect, no negative effect on microhardness, and roughness of dentin, it was suggested that 0.2% CHX gluconate was a suitable irrigant to be used in endodontics [71][40]. Further, chlorhexidine did not influence the fracture resistance of endodontically treated teeth after irrigation, while insufficient smear layer removal using chelating agents may inhibited its beneficial effects [98][73]. It has also been noted that the demineralization of dentin and exposure of dentinal tubules by 17% EDTA reinforced the effect of chlorhexidine [113][88].
Compared to other irrigating solutions, CHX showed the greatest bond strength values [114][89]; by reducing the wetting angle and raising the surface energy, it improved the penetration and bond strength of AH Plus (Dentsply, Petrópolis, Rio de Janeiro, Brazil) and wettability of dentin [115][90]. Worthy of note is the fact that CHX can prevent the destruction of collagen by host-derived proteases MMPs [95,116][69][91].

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