Short-term Effect of Four Root Filling Materials on the Flexural Strength of Human Root Dentin

Background: This study aimed to assess the effects of calcium hydroxide, Biodentine, calcium-enriched mixture (CEM) cement, and mineral trioxide aggregate (MTA) on root dentin flexural strength after a 30-day exposure period. Methods: This in vitro experimental study evaluated 25 freshly extracted sound human incisors with no caries or restorations. The apical 5 mm and the coronal two-thirds of the crowns were cut such that all samples had 10 mm length. Dentin samples (n=20 in each group) were then exposed to 2 mm thickness of calcium hydroxide, Biodentine, CEM cement, MTA, or saline (control) in petri dishes for 30 days. Finally, dentin samples were subjected to a three-point bending test after the intervention, and the flexural strength data were analyzed using one-way ANOVA, Tukey’s test, and t test. Results: Thirty-day exposure to all four biomaterials decreased the flexural strength of root dentin (P < 0.05). The four groups were significantly different in terms of the flexural strength of root dentin (P = 0.001). The flexural strength of root dentin was significantly lower following exposure to calcium hydroxide (P = 0.003), Biodentine (P = 0.011), CEM cement (P = 0.001), and MTA (P = 0.007) compared to saline. The reduction in strength following exposure to calcium hydroxide was higher than that in Biodentine, CEM cement, and MTA groups (P < 0.05) while the latter three were not significantly different in this respect (P > 0.05). Conclusions: In general, all four tested biomaterials decrease the dentin strength although this reduction is more prominent by calcium hydroxide. *Correspondence to Hadiseh Abbaspourrokni, Email: drhadisehabbaspourrokni@ yahoo.com Background Root canal treatment is a commonly performed dental procedure for preserving teeth with necrotic or infected pulp tissue. Tooth fracture is a common clinical complication in endodontically treated teeth, which often necessitates tooth extraction (1). Irrigating solutions, intracanal medicaments, and root filling materials affect the properties of dentin, making it more susceptible to fracture (2). Considering the extensive use of new products in the root canal treatment of teeth, some concerns still exist regarding their adverse effects on dental structures, especially dentin. Materials with minimal or no adverse effects are obviously preferred and have gained more popularity. Calcium hydroxide is the most commonly used intracanal medicament and pulp capping agent due to its high pH, optimal antibacterial properties, and the induction of hard tissue formation (3,4). It is applied into the root canal for short or long periods of time, Open Access Scan to access more free content Short-term Effect of Four Root Filling Materials on the Flexural Strength of Human Root Dentin Received December 8, 2020 Accepted March 17, 2021 ePublished March 30, 2021


Background
Root canal treatment is a commonly performed dental procedure for preserving teeth with necrotic or infected pulp tissue. Tooth fracture is a common clinical complication in endodontically treated teeth, which often necessitates tooth extraction (1). Irrigating solutions, intracanal medicaments, and root filling materials affect the properties of dentin, making it more susceptible to fracture (2).
Considering the extensive use of new products in the root canal treatment of teeth, some concerns still exist regarding their adverse effects on dental structures, especially dentin. Materials with minimal or no adverse effects are obviously preferred and have gained more popularity.
Calcium hydroxide is the most commonly used intracanal medicament and pulp capping agent due to its high pH, optimal antibacterial properties, and the induction of hard tissue formation (3,4). It is applied into the root canal for short or long periods of time, barrier in a process called apexification. However, it has been reported that long-term use of calcium hydroxide may weaken the root structure and lead to fracture of immature teeth (8).
Limited studies have focused on changes in the fracture strength of dentin in response to exposure to endodontic materials. For instance, Rosenberg et al (9) evaluated the effect of calcium hydroxide on the microtensile strength of the dentin of maxillary incisors and showed that it weakened the dentin structure by 23%-43.9%.
Mineral trioxide aggregate (MTA) is a silicate-based cement introduced in 1993 as a root-end filling material (10). Since then, it has been commonly applied as apical plug and for root perforation repair, pulp capping, pulpotomy, treatment of root resorption, and root filling (10,11). It is composed of several oxides combined with some other hydrophilic components, which are crystalized in the presence of moisture. The main components of MTA include tricalcium silicate, tricalcium aluminate, tricalcium oxide, and silicate oxide (12). The hydration of powder results in the formation of a colloidal gel, which is solidified within a few hours. Evidence indicates that MTA provides a better seal compared to amalgam, intermediate resin material, and super ethoxy benzoic acid, and better adapts to the prepared root end dentinal wall (13). The pH of the freshly mixed MTA is alkaline similar to that of calcium hydroxide (14). A limited body of research has addressed the effect of MTA on root dentin strength. For example, White et al (15) reported a 33% reduction in bovine dentin strength following the application of MTA for 5 weeks. Based on the findings of another study, immature teeth filled with MTA were stronger than those filled with calcium hydroxide (16).
Calcium-enriched mixture (CEM) cement has clinical applications similar to those of MTA while with a different chemical composition (17). It has a high alkaline pH level and releases calcium hydroxide (18). It can also produce hydroxyapatite crystals using endogenous and exogenous ions. It has biocompatibility similar to that of MTA whereas superior working time and handling properties. However, it causes no staining and has a shorter setting time in comparison with MTA (18). It has a film thickness, flow, and sealing ability comparable to those of MTA (17) but with lower cytotoxicity (19,20). In addition, its antibacterial activity is comparable to that of hydroxide apatite while higher than that of MTA (21). It is also used as a root-end filling material (22) and for pulp capping, pulpotomy, internal root resorption repair, and furcal perforation repair (23,24). Moazami et al (25) showed that long-term use of CEM cement caused a reduction in dentin strength although this reduction was compensated in the long term.
Biodentine is another silicate-based cement introduced in 2009 for dentin replacement. It can be used for root perforations, pulp capping, apexification, root resorption, and root-end filling in endodontic surgery (26), and is supplied in the form of a powder and liquid. The powder is composed of tricalcium silicate (as its main constituent), calcium carbonate (filler), zirconium oxide (opaquer), dicalcium silicate, calcium oxide, and iron oxide. Its liquid form is composed of an aqueous solution of water-soluble polymers, along with calcium chloride (for decreasing the setting time). The physical properties of Biodentine (e.g., its flexural strength, modulus of elasticity, and Vickers hardness number) are higher than those of MTA. Biodentine is denser than MTA and has less porosities. It is also alkaline. Rajasekharan et al (27) confirmed that Biodentine is neither cytotoxic nor genotoxic. In general, Biodentine is believed to have superior physical and biological properties compared to other tricalcium cements such as MTA.
MTA and calcium hydroxide are frequently used in immature traumatized teeth for inducing apical closure. The intertubular and peritubular dentin in such teeth have not developed well. Andreasen et al (28) reported that the proteolytic activity of calcium hydroxide can weaken the tooth structure by 50% and lead to its fracture. MTA also has similar proteolytic activity.
Given that no previous study has compared the effects of Biodentine, calcium hydroxide, CEM cement, and MTA on the flexural strength of dentin, this study sought to evaluate and compare the effects of these materials on dentin flexural strength.

Materials and Methods
This in vitro, experimental study investigated 25 freshly extracted sound mature human incisors with no caries or restoration.
The teeth were stored in saline for hydration prior to the experiment. The sample size was calculated to be 20 samples in each group assuming alpha =0.05, 80% power, standard deviations of 24 and 39 for calcium hydroxide and MTA, and 10% error according to a study by Moazami et al (25).
The apical 5 mm and the coronal two-thirds of the crowns were cut using a high-speed diamond bur (D&Z, Wiesbaden, Germany) under water coolant such that all samples had a length of 10 mm. Next, a 2.5 mm drill (D&Z, Wiesbaden, Germany) was used parallel to the root canal longitudinal axis to widen the canal. Moreover, a medium-size trephine bur (Meisinger, Düsseldorf, Germany) was used to standardize the external diameter of the samples. Dentin cylinders with 2.5 and 5.5 mm internal and external diameters and 10 mm length were obtained as such. These cylinders were then longitudinally divided into four segments with a diamond disc (JOTA, Hirshensprungstrasse, Switzerland). Next, the samples were randomly divided into five groups (n=20). Samples in each group were placed in petri dishes, and the dentin surface was subjected to root filling materials as follows: • Group 1: Petri dish containing 2 mm thickness of creamy calcium hydroxide (Merck, Darmstadt, pairwise comparisons of the flexural strength of the groups. The results demonstrated that dentin exposed to calcium hydroxide had significantly lower flexural strength in comparison with dentin exposed to Biodentine, CEM cement, and MTA (P < 0.05). Dentin flexural strength was nearly the same in Biodentine, CEM cement, and MTA groups (P > 0.05).

Discussion
This study assessed the effects of calcium hydroxide, Biodentine, CEM cement, and MTA on dentin flexural strength after a 30-day exposure period. The results revealed that 30-day exposure to all the aforementioned biomaterials decreased the flexural strength of root dentin. Importantly, the reduction in strength following exposure to calcium hydroxide was greater than that in Biodentine, CEM cement, and MTA groups while the latter three were not significantly different in this regard. Some previous studies confirmed the weakening effect of calcium hydroxide on dentin (19,30), which is in agreement with our findings. For instance, Moazami et al (25) reported that calcium hydroxide, MTA, and CEM cement decrease dentin strength. However, dentin samples in the CEM cement group regained their lost strength after one week. Thus, CEM cement was found to be the most suitable biomaterial in terms of preserving dentin strength. The reduction in dentin strength in calcium hydroxide, MTA, and CEM cement groups was attributed to the destruction of dentin protein structures as the result of the alkalinity of these compounds. Similarly, White et al (15) evaluated the effect of calcium hydroxide, sodium hypochlorite, Germany); • Group 2: Petri dish containing 2 mm thickness of CEM cement (BioniqueDent, Tehran, Iran); • Group 3: Petri dish containing 2 mm thickness of MTA (Dentsply, Tulsa, OK, USA); • Group 4: Petri dish containing 2 mm thickness of Biodentine (Septodont, Lancaster PA, France); • Group 5: Four petri dishes containing saline as the control group. All materials were prepared according to the manufacturer's instructions. The samples remained in petri dishes for 30 days (29). Distilled water was added to petri dishes every 3-4 days in order to hydrate the samples. All dishes were stored at 37°C and 100% humidity. After the completion of the storage period, each sample was rinsed with water. Dentin cylinders were then subjected to a three-point bending test using an Instron universal testing machine (Fantam, Mashhad, Iran). Each dentin cylinder was subjected to load application to its center point. The load was applied at a crosshead speed of 1 mm/s. The load at fracture was recorded in Newton and converted to megapascal (MPa).
The obtained data were analyzed using SPSS (SPSS Inc., IL, the USA) version 20. The effect of the type of root filling material on the flexural strength of dentin was evaluated using one-way ANOVA and Kruskal-Wallis tests for normally and non-normally distributed data, respectively. In the case of the presence of a significant difference, Tukey's test or independent t test was applied for pairwise comparisons, and P < 0.05 was considered statistically significant. Table 1 presents the flexural strength of root dentin following exposure to root filling materials. According to the t-test, the flexural strength of root dentin following exposure to calcium hydroxide (P = 0.003), Biodentine (P = 0.011), CEM cement (P = 0.001), and MTA (P = 0.007) was significantly lower than that of saline.

Results
According to one-way ANOVA test results, the four groups significantly differed in terms of the flexural strength of root dentin (P = 0.001). Table 2 provides the  and MTA on dentin strength and hardness and reported a reduction in dentin strength after 5 weeks of exposure to the materials. They concluded that such a decline was owing to the alkaline effect of materials and the subsequent destruction of dentin proteins. In another study, Sahebi et al (29) explained that the reduction in dentin strength following exposure to MTA and CEM cement can be due to the gradual release of calcium hydroxide and the subsequent destruction of dentin proteins because of its alkaline pH. Similar to our study, the reduction in strength caused by MTA and CEM cement in their study was less than that caused by calcium hydroxide, which can be due to the lower amount of released calcium hydroxide from MTA and CEM cement compared to the use of pure calcium hydroxide.
In some studies on the strength and fracture resistance of dentin following exposure to biomaterials, the researchers filled the entire root canal with the respective root filling material and then applied force while some others, similar to the present study, applied force on dentin cylinders, which can yield different results. Some studies claim that the filling of the root canal with MTA increases its fracture resistance (32,33). Such controversial results can be due to differences in methodology. Our methodology was similar to that of Moazami et al (25) and Sahebi et al (29), both reporting a decrease in dentin strength following exposure to biomaterials, which is in line with our findings. They further (25) explained that filling the weak root canals with materials with a modulus of elasticity similar to that of dentin yields a higher strength compared to empty root canals. However, similar to our study, the samples were rinsed prior to load application in order to eliminate all the filling material residues, and the load was applied to dentin cylinders rather than a filled root canal (25).
According to our findings, Biodentine caused a reduction in dentin strength although the magnitude of this decline was less than that of the other three materials and had no statistically significant difference with MTA and CEM cement groups. This reduction can be due to the alkalinity of Biodentine and the destruction of dentin proteins. Sawyer et al (34) evaluated the effect of Biodentine and MTA on the flexural strength of dentin and concluded that they both decrease the flexural strength of dentin, which is in accordance with our study findings.
Considering the current findings and those of similar previous studies regarding the effect of calcium hydroxide, Biodentine, CEM cement, and MTA on root dentin strength, it seems that long-term use of calcium hydroxide and the filling of the entire root canal with calcium hydroxide paste increase the risk of root fracture compared to the application of other three root filling materials. On the other hand, the entire canal length is not filled with calcium silicate-based cements in vital pulp therapy or apical plug placement; instead, they are applied with 3-4 mm height, which decreases the risk of root fracture in their application. Nonetheless, further studies with longer exposure time and longer follow-ups are required to better elucidate this issue. Eventually, this study had an in vitro design, which limits the generalizability of results to the clinical setting. Thus, future clinical studies are needed to cast a final judgment in this respect.

Conclusions
In general, it appears that all four tested biomaterials decreased the dentin strength although this reduction was more prominent by calcium hydroxide.