Intracanal medicaments have been used for disinfection purposes between the sessions of endodontic procedures. The time it takes for the medicament to be effective varies depending on the procedure and potency of the material. ¹ , ²

Root fracture is associated with several known factors that weaken tooth structure, including decays, access cavity and canal preparation, and presumably, intracanal medicaments. ³ , ⁴ , ⁵ Increased root fragility related to intracanal medicaments may be due to demineralization effect of these materials which in long-term intervals can have a negative effect on the radicular structure. ⁶

Calcium hydroxide (CH) is a widely accepted well studied endodontic material with high pH and wide antimicrobial activity that is being used as intracanal dressing. ⁷ The disadvantages include ineffectiveness against Enterococcus faecalis and Candida albicans and also its denaturating effect on dentinal structures which leads to decreased fracture resistance. ⁶ , ⁸ , ⁹ This adverse effect might be more significant because of need for prolonged use of this material to achieve effectiveness, especially in the treatment of traumatized teeth. ⁶

Chlorhexidine (CHX) is an effective antimicrobial agent, notably, against CH resistant microflora. ⁹ Therefore, the combination of CHX and CH has been suggested to avoid persistent endodontic infections. Furthermore, CHX can bind to dentin for a prolonged antibacterial effect. ¹⁰

Double antibiotic paste (DAP) is a mixture of metronidazole and ciprofloxacin and has been used in endodontic regeneration with favorable results. ¹¹ Nevertheless, there are concerns about its negative effect on the mechanical properties of radicular dentin which have been attributed to the strong demineralizing effect of this low pH mixture. ¹²

Simvastatin is a 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor used basically for cardiovascular disease with documented safety and low price. ¹³ Interesting properties of this drug have led to innovative use of it in endodontics. It has been suggested that angiogenesis potential and anti-inflammatory and bone regenerating properties of simvastatin might be of use in endodontic regeneration. ¹⁴ Nevertheless, experimental studies on simvastatin are limited, and biomechanical aspects of its use have not been studied thoroughly yet.

Root fracture is one of the most undesirable complications after endodontic treatment that seriously affects the prognosis. ¹⁵ Thus, the aim of this study was to assess the effect of CH, CH combined with CHX, DAP, and simvastatin as intracanal medicaments on fracture resistance of the human root.

This was an in vitro study. After receiving local ethics committee approval, 120 single-rooted mandibular premolars, extracted for periodontal and orthodontic reasons were obtained and stored in chloramine T 0.5% until the study time. All specimens were examined with magnifying loop and plain radiography for the exclusion of any cracked, calcified, previously treated or undeveloped teeth. After scaling and root planning (Cavitron, Dentisply, Ltd, Weybridge, UK), all teeth were decoronated below the CEJ with diamond disk (Sp 1600 Microtome, Leica, Na Block, Germany) under water coolant to leave 13 mm of length. Working length was determined with K-file #15 (Dentisply Mailefer, Balbigue, Switzerland) instrumentation was done using the crown-down technique with rotary system (Dentisply Mailefer, Balbigue, Switzerland) up to main apical file F3 and #35. 2.5% Sodium hypochlorite (MORVABON, Tehran, Iran) was used for irrigation. Finally, 5.25% sodium hypochlorite and 17% ethylenediaminetetraacetic acid (MORVABON, Tehran, Iran) were applied for smear layer removal, and canals were rinsed and dried with paper points.

All roots were randomly assigned to five study groups (24 roots in each group); Group 1: CH (MORVABON, Tehran, Iran), Group 2: Combined use of CH and CHX 2% (MORVABON, Tehran, Iran), Group 3: Simvastatin (Pursina, Tehran, Iran); simvastatin 1% gel was prepared according to method used by Dianat et al., ¹⁶ Group 4: DAP (Metronidazole (Abidi, Tehran, Iran) and Ciprofloxacin (Farabi, Isfahan, Iran); two 250 mg Metronidazole tablet and one 500 mg Ciprofloxacin tablet were grinded and equal portions were mixed with normal saline to form a paste. Group 5: Control group without addition of any substances.

All the root canals in the experimental and control groups were sealed with Cavite dressing (Cavisol, Golchay, Tehran, Iran). All specimens in each group were randomly assigned into two equal subgroups depending on the incubation time. Subgroup A specimens were incubated for 1 week and Subgroup B for 1 month in 37° and 100% moist in incubator. At the end of the incubation period, canals were rinsed with 5.25% sodium hypochlorite and normal saline and were obturated with gutta-percha and AH26 sealer (Dentisply, Dentrey, kostanz, Germany) with lateral compaction technique and were again incubated for 24 h with previous settings. In the control group, all specimens were obturated with the same technique after canal preparation.

To simulate the PDL and create a 0.2-mm gap external surface of each root was covered with a thin layer of melted wax and dried. Calipers (Yates-Motloid, Chicago, Illinois, USA) were used to assess the thickness of the wax layer in two different root levels. Then roots were embedded in self-curing acrylic cylinders (AcroPars, Tehran, Iran). After polymerization was completed, roots were removed and the wax layer was removed with warm water. Simulated acrylic sockets were filled with poly vinyl siloxane (Impregum Soft, 3M ESPE, Seefeld, Germany) and roots were reinserted in sockets immediately. Finally, samples were tested for fracture resistance with the universal testing machine (Hounsfield Test Equipment, Model: H5K-S, Surrey, England).

The statistical analysis of the data was performed using the SPSS software version 17.0 (SPSS Inc., Chicago, IL, USA) where P = 0.05 was considered statistically significant. The Kolmogorov–Smirnov test was used to assess the normality of the data.

Descriptive findings are presented in Table 1. According to the results, the highest fracture resistance after 1 week was seen in CH and CHX group (643.53 ± 445.13) and CH group (613.62 ± 345.13), respectively, while the least fracture resistance value was measured in the control group (185.55 ± 231.12). Due to nonnormal distribution of data in two groups in the 1-week interval, Mann–Whitney U-test was used for the comparative analysis. According to the results, the difference between CH and CHX group and CH group with the control group was statistically significant (P = 0.03 and 0.02, respectively), but there was no significant difference between these two groups or other experimental groups with the control group Table 2.{Table 1}{Table 2}

Based on the values measured after 1 month, the highest fracture resistance was related to Simvastatin group with an average of 657.78 ± 449.01 and the least amount was recorded in DAP group (308.08 ± 238.52). Normality of data and homogeneity of variances was confirmed in the 1-month interval. Thus, according to the one-way ANOVA test results, fracture resistance amount was not significantly different between the study groups (P = 0.13).

According to Wilcoxon test results, there was no statistically significant difference between the fracture resistance of each study group in 1 week as compared to fracture resistance in 1 month Table 3.{Table 3}

This study was conducted to investigate the effect of different intracanal medicaments on the fracture resistance of radicular dentin. According to our findings, CH and CH combined with CHX, DAP, and simvastatin do not have a significant negative effect on root fracture resistance when used as intracanal medicament for <1 month.

Although CH has been widely used for endodontic treatments, its antimicrobial inefficiency in persistent endodontic infections is considerable. ¹⁷ Addition of CHX as a potent antimicrobial agent has long been studied to overcome this problem, but the results have been controversial. ¹⁸

An unexpected finding of our study was the significant increase in fracture resistance of samples treated with CH (with or without CHX) after 1 week of incubation. This observed effect might be associated with denaturating effect of CH which might result in better penetration of obturation materials into radicular dentin and subsequent improvement in fracture resistance. However, the lack of this observation on the samples incubated for 1 month might reflect the negative influence of progressive denaturation on fracture resistance. Further specifically designed studies with larger sample sizes are required to evaluate these findings.

Regarding the current study's findings, it can be suggested that addition of CHX to CH is mechanically harmless for dentinal structure in less than a month and might be rational for a better antimicrobial coverage. This finding is consistent with Prabhakar et al. study results. ¹⁹ However, conducting randomized clinical studies is crucial to support this claim.

Simvastatin is known to have antimicrobial activity against bacteria and reduces the formation of Staphylococcus aureus biofilms. ²⁰ , ²¹

DAP and simvastatin had no significant effect on fracture resistance. To our knowledge, this is the first study to assess the biomechanical effects of these materials on radicular dentin. Anti-inflammatory effects of simvastatin through the reduction of interleukin 6 and 8 have been established. Therefore, the local application of simvastatin as an intracanal medicament or as a combination with previously studied materials might lead to improved results without jeopardizing fracture resistance of radicular dentin. ²²

One of the advantages of this study compared to previous similar studies is that human premolars were used instead of bovine specimens. ³ , ²³ The simulation of PDL and its shock absorbent properties are another advantage of this study. These properties have been overlooked in many similar previous studies. In 2013, Yassen et al. ²⁴ assessed the effect of intracanal medicaments on root fracture using 5 mm cervical root cylinders as specimens and reported a significant time-dependent decrease on fracture resistance with DAP and CH application; however, a closer look at the mechanism of this negative effect highlights the importance of PDL simulation. Mechanical changes have been related to collagen degradation of radicular dentin due to the high pH of CH as first described by Andreasen et al. ³ in 2002. Decreased collagen component leads to increased brittleness which can be adjusted by PDL shock absorber effect. Contrary to Yassen et al. findings, when PDL simulation was applied by Zarei et al. ²⁵ in 2012, no significant reduction in fracture resistance was seen in 1 month. These findings are in agreement with the present study, although different testing machines were used.

In this study, second follow-up was carried out after 1 month because the duration of application of CH in most of the endodontic treatments is in this range. ²⁶ However, in some regenerative treatment cases, CH has been used for longer periods of time, which might be harmful for dentinal structures. ²⁷

According to Olcay et al., ²⁸ weakening of coronal structure after endodontic treatment is a crucial factor in root fracture susceptibility and is more important than intracanal medicament type. Hence, in this study, roots were decoronated to eliminate crown weakness as a confounding factor.

In conclusion, under the limitations of this study, CH, CH + CHX, DAP, and simvastatin do not have a negative effect on root fracture resistance at 1-month interval when used as intracanal medicaments. Thus, the studied medicaments can be used in regenerative treatments without concerns about negative mechanical effects on root resistance and future studies can focus on other aspects of them.

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Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or non-financial in this article.