Natural Extracts as Biological Smear Layer Removing Agents

INTRODUCTION Smear layer was defined as an amorphous layer consisting of inorganic dentin debris and organic tissues, including vital and necrotic pulp tissues, bacterial cells, and blood cells.[1] The existence of smear layer may block the canal irregularities and prohibit the penetration of root canal disinfectants to eliminate the hidden microorganisms.[2] Moreover, it can compromise the sealing ability and adaptation of root canal obturating materials.[3] Different chemotherapeutic agents were used for smear layer removal, including sodium hypochlorite (NaOCl), ethylenediaminetetraacetic acid (EDTA), alternative use of EDTA associated with NaOCl, synthetic acids (citric and acetic acids),[3] and organic acids (maleic acid and glycolic acid (GA)).[45] NaOCl is the gold standard tissue-dissolving agent. It failed to completely remove the smear layer and only dissolved its organic component.[36] Although all chemical chelators like EDTA and malic acid are effective for smear layer removal, they may induce certain toxicity to periapical tissues and render tooth weakening by removing the calcium ions and reducing the microhardness of radicular dentin.[17] Recently, some authors focussed on the use of natural extracts as biological smear layer removing agents to reduce the risk factors on tissue reaction and mechanical properties of radicular dentin.[7] The objectives of this review were to identify whether natural irrigants are better than synthetic conventional irrigants for smear layer removal and to analyze their influence on the mechanical and chemical radicular dentin properties. The research question was addressed in terms of PICOS format (problem, interventions, comparators, outcomes, and study designs) as follows: P—smear layer removal and mechanical and chemical radicular dentin properties, I—natural irrigants (fruit and plant extracts), C—synthetic conventional irrigants, O—efficacy, and S—in-vitro studies. MATERIALS AND METHODS LITERATURE SEARCH STRATEGY The articles’ selection was based on an electronic search using MEDLINE via PubMed (http://www.ncbi.nlm.nih.gov/pubmed), Scopus (http://www.scopus.com), Web of Science (https://www.webofknowledge.com), Google Scholar (http://scholar.google.com), and Saudi Dental Library (https://kau.deepknowledge.io/KAU). Date limit was set from 2000 to 2020. Manual search was done through peer-reviewed journals, including “Journal of Endodontics” and “International Endodontic Journal” from 1990 to 2020. The initial search was performed using keywords “Natural extracts” and “Endodontic irrigants.” The last search was carried out on June 2, 2020, as filter was performed using the following keywords: “Fruit and plant extracts” or “Apple vinegar” or “Grape extract” or “Glycolic acid” or “Phytic acid” and “Smear layer removal.” STUDIES SELECTION The inclusion criteria were in-vitro studies that compared the impact of natural (fruit or plant extracts) irrigants/chelators with synthetic conventional irrigants/chelators on smear layer removal and their effect on the mechanical and chemical properties of radicular dentin, including microhardness, roughness, strength, calcium ions concentration/release, and erosion. Full-text articles either published or accepted for publication in the English language were included. The exclusion criteria comprised articles that evaluated other properties of fruit or plant extracts (e.g., antimicrobial activity and cytotoxicity) or articles that tested other natural irrigants/chelators for smear layer removal than the targeted solutions. SCREENING Literature search results were imported to EndNote library (X7 version, Thomson Reuters, New York, NY, USA). After de-duplication, all researchers independently screened the titles and abstracts of the articles for the relevance and existence of eligibility criteria. Discrepancies in the screening of titles/abstracts and articles in full text were resolved by discussion. DATA CHARTING A charting table was developed to record the key information of the included studies based on the review question. The data were retrieved by one reviewer (S.T.A.Z.) and reviewed by others (H.A.B. and A.A.M.S.). Data of charting table included the following: the tested irrigating solutions (intervention and comparators); solutions’ concentration; final rinse if present; application time; properties evaluated; and the main findings. RESULTS The flow chart was designed based on the guidelines of PRISMA extension for scoping reviews,[8] as shown in Figure 1. A total of 3469 citations have been collected from electronic databases after duplicates’ elimination. About 3146 were excluded based on title and abstract, whereas 323 articles in full text were retrieved and then tested for eligibility. After full-text reading, a total of 36 studies met the inclusion criteria.Figure 1: Flow chart of the study according to PRISMA extension for scoping reviewsAmong the selected articles, several types of fruit and plant extracts were used, including apple vinegar (10 studies),[9101112131415161718] apple cider vinegar (4 studies),[19202122] pomegranate and grape vinegars (1 study),[20] grape seed extract (GSE) (6 studies),[232425262728]Citrus aurantifolia (2 studies),[2930] white vinegar (1 study),[31] GA (3 studies),[432,33] and phytic acid (10 studies)[34353637383940414243] [Table 1].Table 1: Articles included on natural extracts ordered chronologically for each solutionSMEAR LAYER REMOVAL From 21 studies, 13 studies[49,1112,1415,1721,2233,3638,42] indicated that the natural extracts had better or similar effectiveness, compared with synthetic conventional agents such as EDTA or NaOCl, while the other 8 studies[1618,23293031,3435] showed better effectiveness of EDTA. Fruit extract Apple vinegar-apple cider vinegar determined better smear layer removal in six studies[1112,1415,1722] and no significant difference in two studies,[921] compared with chemical agents including EDTA, NaOCl, acetic acid, citric acid, or malic acid. However, they determined lower effect than EDTA,[1618] chitosan,[22] and SmearClear.[16] When compared with EDTA, one study showed that different concentrations of GSE (13%, 6.5%, and 3.25%) were effective to clean the apical third[23] and two studies showed the effectiveness of C. aurantifolia (CAu) to partially clean the coronal and middle thirds.[2930] However, both solutions were not as good as 17% EDTA. The effect of CAu increased when mixed with Sapindus mukorossi (2:1 concentration), particularly with ultrasonic agitation.[30] Plant extract White vinegar showed less chelating action[31]; however, GA was equally effective at different concentrations (5%, 10%, and 17%)[33] or different pH values (1.2 and 5)[4] to 17% EDTA or 10% citric. When compared with EDTA, 0.5–1% phytic acid (IP6) determined better effect in one study,[36] equal effect in two studies,[3842] and less effect in two studies.[3435] It had similar effect to Q-mix.[35] Two studies showed ineffectiveness of IP6 to clean the apical region.[3842] EFFECT OF NATURAL IRRIGANTS ON MECHANICAL AND CHEMICAL DENTIN PROPERTIES From a total of 24 studies, 15 studies demonstrated no alteration in strength or microhardness,[2425,2832] less detrimental effect on calcium ions (Ca2+) loss, erosion or microhardness,[1314,1718,2634,3739,42] or even positive effect on roughness or flexural strength,[2733] compared with synthetic conventional solutions such as NaOCl, EDTA, or citric acid. Roughness, microhardness, and dentin strength Apple vinegar determined lower surface roughness than 15–17% EDTA[10] or 10% citric acid,[13] whereas pomegranate, apple cider, and grape vinegars presented higher roughness values, which were nearly similar to each other, than that obtained by NaOCl and CHX.[20] All these used solutions did not alter the dentin microhardness, when used for 15 min, whereas they reduced the microhardness after 30 min of treatment.[20] GSE maintained the dentin microhardness[28] and strength[2425,27] with no significant difference with the untreated group. However, when preceded with 5.25% NaOCl, it induced reduction in microhardness lower than that obtained by 17% EDTA.[26] Regarding GA, 17% GA reduced micro-hardness and increased the roughness of the dentin surface in two studies,[433] whereas one study showed no significant alternation in flexural dentin strength induced by 5–17% GA[32] versus 17% EDTA or 10% citric acid. With regard to IP6, 0.5–1% applied for 3 or 5 min had less detrimental effect on dentin microhardness than 17% EDTA[3739] or similar to 0.2% chitosan.[39] Calcium ions concentration/mineral content Apple vinegar recorded Ca2+ concentration similar to that of 5% acetic acid and smaller than 17% EDTA in two studies,[1418] but greater than 0.2% chitosan and 15% EDTA, respectively, in one study.[22] When compared with EDTA, apple vinegar rendered the highest calcium dentin content in one study,[17] whereas 1% IP6 produced more calcium loss[41] particularly when associated with NaOCl.[40] In one study, there were no chemical changes after using different concentrations of GA (5%, 10%, and 17%), 17% EDTA, or citric acid.[33] However, other two studies showed reduction in apatite/collagen ratio with the lowest significant value obtained with increasing GA concentration (17%).[432] Erosion When compared with 17% EDTA, GSE[26] as well as 1% IP6 at any application time (1, 3, or 5 min)[3442] induced the least significant dentin erosion. Only one study showed that the highest significant erosion was induced in the middle region by 0.5–1% IP6, with no significant difference at the cervical region.[36] DISCUSSION Based on the current literature review, most studies found that apple vinegar, GSE, CAu, 5–10% GA, and 0.5–1% IP6 effectively removed the smear layer better or similar to synthetic agents. Natural irrigants had low risk factors on radicular dentin microhardness, roughness, strength, Ca2+ release, and erosion, especially when used for a short period of time. Most of the included studies reported the efficiency of apple vinegar than EDTA,[1417,22] 1–5% NaOCl,[1112] 2.5% NaOCl associated with 17% EDTA as final flush,[15] and 10% sodium citrate,[18] or even no significant difference compared with chemical chelators.[921] However, the chelating action was reduced with diluted apple vinegar.[15] Although GSE proved its efficiency with different concentrations (3.25%, 6.5%, and 13%) and CAu exhibited partially clean coronal and middle parts,[29] their effects were not as good as 17% EDTA.[23] The smear layer removing ability of vinegars is multifactorial.[44] The different effects of multiple types of vinegars may be attributed to their different compositions and pH values. Apple vinegar and apple cider vinegar are two types of vinegars extracted from apple fruit. Both contain acetic (main component), malic, citric, formic, lactic, and succinic acids with lesser alcohol and more acetic acid in apple vinegar.[45] The chelating effect of apple vinegar could be affected by its acidic pH (2.71).[46] However, GSE contains weak acid (about 74–78% proanthocyanidin (PA)).[23] C. aurantifolia is a genus citrus fruit, resembles orange leaves, and contains 6–8% citric acid and 2% potassium citrate.[2930] Due to its high surface tension, it failed to penetrate into the apical region. S. mukorossi is an Indian fruit containing saponin. The mixture of CAu and S. mukorossi improved the chelating effect of citric acid by the surfactant action of saponin that lowered the surface tension of CAu and promoted better penetration.[2930] However, white vinegar yields from food fermentation, such as sugar, beets, and potatoes, mainly consist of 5% acetic acid with pH of 2.4 and showed no-to-moderate smear layer removal.[31] GA[433] and IP6[3638] are natural plant extracts, recently used as smear layer removing agent. Compared with 17% EDTA, 5–17% GA exhibited similar effect.[433] GA is a hydroxyacetic acid derived from sugar cane and sweet vegetables. Its chelating efficiency may be attributed to its acidic pH (1.2 and 5) and surface tension.[4] With regard to IP6, variable results were obtained, ranging from better effect,[36] similar effect,[3842] or lower effect[3435] compared with 17% EDTA. IP6 is a natural plant compound extracted from cereals, legumes, oil seeds, nuts, and rice bran. It has six reactive phosphate groups with unique binding affinity to certain dietary minerals such as calcium.[47] Its chelating effectiveness may be attributed to its negatively charged phosphate groups that have strong chelating capacity to divalent cations such as Ca+2.[48] The concentration of the solution also has impact role, as 1% IP6 was more effective than 0.5%.[36] The conflicting results of IP6 could be pH-dependent factor. Furthermore, the action of chelator claims to be a time-dependent process, in which IP6 applied for 3 min exhibited more open dentinal tubules with less debris, compared with 1 min application time.[34] However, no statistical significant difference was reported between apple vinegar applied for 1 or 3 min and 17% EDTA applied for 3 min in their smear layer removing ability.[17] Regarding the chelating effect at different root canal levels, it has been reported that either natural extract chelators including apple vinegar, GSE, CAu, and IP6 or chemical chelators like EDTA were ineffective to clean the apical third.[1323,2934,3538] This might be due to the presence of sclerotic dentin in the apical region,[49] which impairs the irrigant’s flow. The removal of smear layer has a significant influence on chemo-mechanical dentin properties. Although both chemical and natural chelators exhibited reduction in dentin microhardness, it seems that the natural chelators have less impact on reducing radicular dentin microhardness when compared with synthetic ones, especially when associated with NaOCl.[2650] It has been reported that GSE did not alter the dentin microhardness[28] or strength[2425,27] with no significant difference versus the untreated dentin or even produced higher flexural strength compared with NaOCl.[27] However, when the irrigating process was preceded with 5.25% NaOCl, GSE-induced reduction in microhardness value is significantly lower than that obtained by 17% EDTA.[26] This action could be attributed to the great PAs content in GSE. Moreover, PAs have been found to strengthen the mechanical dentin properties as they prevent the biodegradation and breakdown of collagen fibers and limit its enzymatic degradation and matrix metalloprotease enzyme.[5152] In contrast, apple vinegar[13] and IP6[3739] produced lesser reduction in dentin microhardness, but GA provided greater reduction[33] than EDTA and citric acid.[32] On the contrary, one study showed the greatest microhardness reduction and the highest surface roughness produced by 1% IP6 than 17% EDTA and 10% citric acid, respectively.[43] The dentin mechanical properties seem to be influenced by several factors. Concerning the pH factor, it has been reported that more acidic pH of IP6 (1.2)[43] can induce aggressive dentin effect compared with IP6 with less acidic pH (3.2)[39] or EDTA with neutral pH.[43] Moreover, the use of IP6 for prolonged contact time (5 min)[37] produced more percentage reduction in microhardness compared with shorter contact time (3 min).[39] In addition, 30 min application of apple cider, pomegranate, and grape vinegars significantly reduced the dentin microhardness compared with 15 min contact time.[20] Regarding the chelator’s concentration, it has been reported that the higher a solution’s concentration, the greater the demineralizing action.[53] About 17% GA[33] and 1% IP6[37] produced significant microhardness reduction compared with their lower concentrations (5–10%) and (0.5%), respectively. The solution’s surface tension accounts for its mechanical effect on dentin. The solution with low surface tension can easily penetrate into the dentin surface, allow efficient smear layer removal with greater dentin softening associated with more denaturation of collagen fibril, extract dentin mineral, increase Ca2+ concentration in the surrounding solution, and allow changes in the apatite/collagen ratio.[433] With increase in GA concentration (17%), its surface tension was decreased, which allows its better penetration to dentinal tubules and reduces dentin microhardness.[32] The higher surface roughness is a benefit value to improve the adhesion of obturating material to dentin by creating micromechanical bonds. The high concentration and acidic pH of the chelator have an influence on the surface roughness. This fact was confirmed by Dal Bello et al.,[33] in which the surface roughness increased by 17% versus 5–10% GA. Moreover, the more acidic pH of IP6 (1.2) significantly increased the surface roughness compared with EDTA with neutral pH.[43] The changes in microhardness and surface roughness are also affected by Ca2+ removal from the dentin surface by the irrigant, which is affected by attractive and repulsive forces between the metal ions. It was suggested that due to the ionization of carboxylic group of EDTA molecule, hydrogen atoms are released in the surrounding environment and compete with dentin calcium ions.[13] In contrast, acetic acid of apple vinegar is a weak acid with low concentration of H+ ions that may be responsible for inefficient calcium removal.[14] These results are confirmed by three studies, in which the apple vinegar recorded low amount of chelated Ca2+ in solution[1418] or the highest dentin calcium content[17] compared with EDTA. On the contrary, IP6 caused more calcium loss from radicular dentin compared with EDTA[41] without much altering dentin microhardness.[3739] It seems that the preceding use of NaOCl could improve IP6 performance. The use of NaOCl dissolves the organic portion of smear layer, allows acids to dissolve the inorganic portion, penetrates into dentinal tubules, and decalcifies them.[54] The smear layer removal has an impact on disintegration of intertubular and peritubular dentin that promotes canal wall erosion, which, in turn, may jeopardize the sealing ability of the obturating material and allow greater bacterial penetration with further tooth weakening.[42] The erosive ability of the chelator could be related to pH and application time. IP6, with weak acidic pH (3), considered a weak chelating agent with no-to-moderate erosive ability when applied for 1 and 3 min, respectively.[34] However, 1% IP6 showed moderate-to-severe erosion when applied for 1 min in association with 5% NaOCl.[36] The use of NaOCl after chelator may be the cause of further dentin erosion.[55] GSE produced mild erosion when compared with moderate–severe erosion induced with 17% EDTA.[2636] The mild erosive effect of GSE could be related to the PAs constituent that inhibits the degradation of intertubular and peritubular dentin. It was suggested that the excessive dentin erosion induced progressive opening of dentinal tubules. CONCLUSION Under the circumstances of this review, most of the included studies revealed that apple or apple cider vinegars, GSE, CAu mixed with S. mukorossi, 5–10% GA, and 0.5–1% IP6 effectively removed the smear layer better or similar to synthetic conventional agents. Natural irrigants/chelators had limited risk factors on radicular dentin microhardness, roughness, strength, calcium ions release, and erosion, particularly when used for a short time and with low concentration. All root canal irrigants, either natural (interventions) or chemical (comparators), failed to completely clean the apical root canal region. LIMITATIONS The first is despite the fact that the major databases were used for the literature search, papers that were not listed in these sources may have been neglected. The second limitation concern is the lack of critical appraisal of included study validity, with all evidence treated as equally valid. STRENGTH The current literature review is valuable to map the efficacy of natural irrigants versus synthetic ones for smear layer removal in endodontics. FINANCIAL SUPPORT AND SPONSORSHIP Autosupport by authors themselves. CONFLICTS OF INTEREST There was no conflict of interest. AUTHORS’ CONTRIBUTIONS The authors equally contributed to the current study. ETHICAL POLICY AND INSTITUTIONAL REVIEW BOARD STATEMENT Not applicable. PATIENT DECLARATION OF CONSENT Not applicable. DATA AVAILABILITY STATEMENT Data used in the current article are available for all readers. ACKNOWLEDGEMENTS None.