INTRODUCTION
Calcium hydroxide has been successfully used in endodontic therapy since its introduction in 1920 by B. W. Hermann.[1] It is a white, odorless powder with a pH of 12.5–12.8.[1] The alkaline nature of calcium hydroxide accounts for its mechanism of action, attributing to its ability to stimulate hard tissue formation and induce antimicrobial properties.[2] The ionic dissociation of calcium hydroxide releases hydroxyl ions that account for its antimicrobial action. The hydroxyl ions further cause lipid peroxidation that induces the destruction of phospholipids and structural changes in the cellular membrane of the bacterial cell.[3] These peroxides act as free radicals and result in extensive membrane damage through the initiation of autocatalytic chain reactions. The high pH of calcium hydroxide, causes alkalization of the narrow range of pH of bacterial cellular enzymes that bring about denaturation of the protein structure of these enzymes.[34] Moreover, the hydroxyl ions are also efficient to react with bacterial DNA and result in splitting of the strands inhibiting DNA replication and disarranging the cellular activity.
In endodontics, calcium hydroxide is widely employed as an intracanal dressing, for apexification and apexogenesis procedures, wherein it induces dentin bridge formation, for direct and indirect pulp capping, as well as pulp regeneration procedures.[2]
Today, calcium hydroxide has been replaced for various endodontic procedures by mineral trioxide aggregate, due to a high success rate, less pulpal inflammation and more predictable formation of a hard dentin bridge of the latter.[5] However, Kawashima et al. suggested the use of calcium hydroxide as a short-term intracanal medicament which is still a preferred approach in endodontics and dental traumatology.[6]
It has been proposed that the link between hydroxyapatite and collagenous fibrils determine the strength of dentin.[7] The soaring pH of calcium hydroxide may cause the dentinal structure to collapse owing to the destruction of carboxylate and phosphate groups.[89] This destruction and dissolution within the dentinal structure reduces the hardness of the treated tissue. Various studies that have been conducted draw attention to the fact that root canal medicaments may affect the physical and chemical properties of radicular dentin.[24101112]
Past literature also reveals that long-term exposure to calcium hydroxide may weaken the dentin, thereby increasing the susceptibility of the tooth to fracture.[213] The difference in findings of the previously conducted studies indicates an obvious controversy regarding the negative reverberation of calcium hydroxide on the mechanical properties of dentin that may impact the fracture resistance of teeth, or how long calcium hydroxide can be exposed to radicular dentin without compromising its mechanical properties.[2]
The last systematic review was published in 2013, in which all studies that had an impact on the mechanical properties were included.[2] This review included the literature on animal as well as human teeth.[14] A review was conducted on the reliability of bovine teeth as a substitute for human teeth in dental research.[14] The findings indicated that there exists inconsistent data in regard to whether bovine teeth can be used as a substitute for human teeth in dental research.[14] This outcome pinpoints to the fact that there is a need to narrow down literature and focus exclusively on studies conducted on human dentition so as to replicate clinical scenarios and obtain a scientifically acceptable end result. Subsequently, new studies have been conducted which immensely contribute to the literature and need to be amalgamated to provide an updated overview.
Therefore, the aim of this review is to provide an update on the effect of calcium hydroxide on the fracture strength and microhardness of dentin in human teeth on the basis of recent available literature.
MATERIALS AND METHODS
Research question
What will be the effect of calcium hydroxide on fracture resistance and microhardness of dentin in human teeth?
Protocol and registration
The protocol of the review is registered with Open Science Framework (OSF) with DOI No. 10.17605/OSF.IO/ZU8V4 and registered from osf.io/7quh2.
Literature search process
PubMed and Scopus were searched for articles till March 12th, 2022. Using the search keywords, Calcium Hydroxide, dentin, dentine, intracanal medicament, metapex, medicament, tooth, fracture, hardness, microhardness, fracture strength, and root dentin, 556 and 627 articles were found for PubMed and Scopus databases, respectively. The review was conducted according to the preferred reporting items for systematic reviews and meta-analyses checklist.[15]
Selection criteria
Only those studies were selected in a systematic review that have one control group, are based on human dentition only, have taken any time of duration of exposure taken into consideration, have at least five samples per group, have at least one experimental group with root/radicular dentin filled or exposed to calcium hydroxide and fracture resistance checked before obturation. The minimum number of required samples for a statistically accurate result is five to six, therefore, this is the rationale behind having to include studies with at least five samples per group.[16] Only the articles written in English were considered to be included in the systematic review. Studies having no control group, conducted on bovine and/or ovine teeth, not having a calcium hydroxide group, aim of the study not matched as per the aim of the review, fracture resistance checked after obturation and case reports, letters, short communications, case series, reviews were excluded.
Study selection
According to the defined inclusion/exclusion criteria, articles were screened based on the title, abstracts and full-text analysis. Articles were also selected from the citation list in order to ensure to identify studies that might have been missed by the database search.
Quality assessment
The methodological quality of the included studies was evaluated using an adaptation of the parameters used in a previous systematic review performed with in vitro studies.[1718] The data parameters of the included studies will be evaluated using the following parameters: (a) teeth randomization, (b) standardization of tooth type, (c) teeth free of caries/restoration/cracks, (d) standardization of duration of exposure, (e) standardization of type of testing machine used, (f) single operator protocol execution, and (g) statistical analysis.
Data extraction
Data extraction from the included studies was performed, and data were tabulated under the headings as author and year, type of teeth used, standardization of teeth, presence of any restorations, cracks, fractures, type of mechanical test used, duration of exposure, and pertinent outcome of the study [Table 1].
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Table 1: Data extraction under the headings as author and year, type of teeth used, standardization of teeth, presence of any restorations, cracks, fractures, type of mechanical test used, duration of exposure, and pertinent outcome of the study
RESULTS
A total of 1183 articles were retrieved from PubMed and Scopus databases, including articles that were hand searched [Figure 1]. After exclusion of duplicates, the title/abstract evaluation of the remaining articles was done that yielded 39 articles. Of these 39, 29 studies were excluded and the remaining ten studies were included in this review. Table 2 shows the number of studies and their reason for exclusion.
Figure 1: Preferred Reporting Items for Systematic Reviews and Meta-Analyses literature search flow diagram
Table 2: Number of studies and their reason for exclusion
The methodological quality of included studies was assessed under the following sub-headings: teeth randomization, standardization of tooth type, teeth free of caries/restoration/cracks/fractures, standardization of duration of exposure, standardization of type of testing machine used, single operator protocol execution, and statistical analysis. From the ten included studies, five studies were estimated to have a low risk of bias, and five were judged as to have a moderate risk of bias [Table 3].
Table 3: Risk of bias assessment for the included studies in this review
Due to the heterogenicity in the types of groups, methods, and techniques, the pertinent outcomes could not be standardized, and therefore, conducting a meta-analysis was not possible. In this review, ten studies that evaluated fracture resistance and microhardness of dentin in human teeth, majority studies indicated that prolonged exposure of calcium hydroxide to dentin had a negative impact, thereby affecting the strength of the tooth. The studies included in the systematic review indicate that exposure of more than 30 days (1 month) of calcium hydroxide to dentin increases the fracture susceptibility of teeth. In four of the six experiments that evaluated the microhardness of dentin on exposure to calcium hydroxide, an inverse relationship was observed between the duration of exposure to calcium hydroxide and the microhardness of dentin.
DISCUSSION
The aim of this review was to identify and systematically analyze studies that assessed the effect of calcium hydroxide as an intracanal medicament in endodontics on the fracture resistance and microhardness of dentin in human teeth. The intracanal medication with calcium hydroxide was evaluated at different time intervals that is 1 day (24 h), 3 days, 7 days (1 week), 2 weeks, 1 month (30 days or 4 weeks), 3 months, 6 months (180 days), and 12 months (180 days). In this review, only those studies were undertaken that were conducted on mature permanent human teeth as the previous study speculated that incompletely formed or immature roots and resorption defects could make the root prone to fracture due to thin dentin walls.[227]
Based on the past literature so far, it was reported that calcium hydroxide as intracanal dressing causes changes in the surface of root canals and softens the tooth tissue.[479111218] A previous systematic review revealed that modulus of elasticity was the least affected property on exposure to calcium hydroxide; therefore, it was not taken into consideration in this review.[2] This review, in total, reports four studies that evaluated fracture resistance and five that evaluated microhardness and one study that considered both parameters. As per this review, it was seen that in majority studies increased exposure of calcium hydroxide to dentin decreased fracture resistance and microhardness, thereby affecting the strength of the tooth on the whole.
This decrease is often elucidated by the alkaline nature of calcium hydroxide that might result in neutralization, dissolution, and degradation of proteins in dentin that are involved in linking collagen fibers to hydroxyapatite crystals. Moreover, Kawamoto et al. hypothesized that the high pH of calcium hydroxide can be one of the paramount factors that cause denaturation and destruction of inorganic content of root dentin and the collagen network making the tooth susceptible to fracture.[28] One of the rational explanations behind the relatively long-term exposure required to bring about a change in mechanical properties is that the collagen fibrils are amalgamated with hydroxyapatite crystals and these are not readily available to calcium hydroxide.[28]
However, further studies are required to understand the mechanism behind the changes in mechanical properties in a manner that closely replicates clinical scenarios.
Limitations
However, one of the important facts to consider in interpretation of the results from the included in vitro studies is that these do not truly represent clinical situations. This is due to the fact that the amount, type, and manner of force application as performed in in-vitro studies do not replicate the physiological forces that the teeth undergo in a real-life situation. In an in vitro set up, the forces are applied to dentin bars, cylinders, or dentin discs thereby results from these studies may or may not be accurate in recreating a true clinical scenario. Another instance is that teeth function differently within the oral cavity in harmony with specialized periodontium and in vitro studies fail to reproduce such an environment.
Another limitation of the included studies lies in the substantial heterogenicity of the study designs reflected upon by the varied sample size, tooth type, and anatomy and direction, and load of applied force. It is also important to note that in these in vitro setups, microhardness was tested by immersing the dentin discs in testing solutions which is not the case clinically.
CONCLUSION
The majority of studies included in this review indicated that long-term calcium hydroxide (more than 1 month) negatively affected the fracture resistance and microhardness of dentin in human teeth. Therefore, calcium hydroxide as an intracanal medicament should be used cautiously in order to ensure the long-term strength of the tooth.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
Acknowledgment
The authors would like to thank Dr. Vivek Aggarwal (Professor, Department of Conservative Dentistry and Endodontics, Faculty of Dentistry, Jamia Millia Islamia, New Delhi, India) for his guidance and advice, which helped in doing the systematic review.
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