To evaluate and compare the compressive strength of root dentin exposed to calcium hydroxide, mixed with various vehicles for a period of 30 days - An in vitro study.

Background: Long-term use of intracanal medicaments such as calcium hydroxide (CH) reduces the fracture resistance of dentin. Aim: This study aimed to evaluate and compare the compressive strength of root dentin exposed to CH, mixed with various vehicles after 30 days of placement. Settings and Design: Sixty single-rooted extracted human permanent premolars were collected, and complete instrumentation was done. Samples were divided into six groups. Materials and
Methods: In Group I, no medicament was placed. In Group II, Group III, and Group IV, the root canals were filled with CH powder mixed with normal saline, distilled water, and local anesthetic solution. In Group V and Group VI, the root canals were filled with Metapaste and RC-Cal. Access cavities were sealed using IRM and were stored for 30 days. Teeth sectioned to obtain a 5 mm thick dentin cylinder subjected to compressive loading to evaluate the fracture resistance using a universal testing machine. Statistical Analysis: Results were analyzed using one-way ANOVA and multiple comparisons using Bonferroni test.
Results: Highest mean compressive strength was recorded in Group I followed by Group II, Group IV, Group VI, Group III, and Group V. Conclusions: CH when used as a root canal dressing material will reduce the compressive strength of the teeth irrespective of the form of CH and vehicle. Copyright:

INTRODUCTION

Bacteria cause all pulpal and periapical pathologies, disinfecting the pulp area is a critical step in root canal treatment. Due to the polymicrobial nature of endodontic infection, intracanal medication is required.[1]

Due to its greater activity and lower cytotoxicity to the periradicular tissues, calcium hydroxide (CH) remains the preferred chemical.[1] Hermann (1920) was the first to develop it, and its application in endodontics is well established. However, long-term exposure of root canal dentin to CH weakens dentin, resulting in endodontically treated teeth deteriorating (Andreasen et al.(2002) and Rosenberg et al.(2007).[23] The most common treatment for infected mature teeth with apical periodontitis is to insert CH in the root canal for 1–4 weeks. As a result, it is critical to understand the short-term effects of CH on root dentin strength in human mature teeth.[34]

Various studies were conducted on the effects of CH on the strength of root dentin at different time intervals.[2345] Not many studies have evaluated the short-term effects of vehicles used to mix CH on the strength of root dentin. Hence, the present study was undertaken with an objective to evaluate the compressive strength of root dentin exposed to CH mixed with various vehicles and to compare the difference in compressive strength between various vehicles after 30 days of placement.

The tested null hypothesis was that there would be no difference in the fracture resistance of radicular dentin when CH is mixed with various vehicles, which were used for a duration of 30 days.

MATERIALS AND METHODS

Sample size derivation and sample collection

The sample size was calculated using G Power 3.1 software.[25] At a level of significance set at 5%, power of the study 80%, and for an expected effect size of 0.506 (obtained from the pilot study), it was calculated that 10 samples per group were required.

Sixty single-rooted human mandibular premolars extracted for orthodontic reasons (18–35 years) with straight root and single root canal were selected for the study. Teeth with root cracks, caries, restorations, and previous endodontic treatments were excluded. After extraction, the soft tissues, dental calculus, and stains were immediately removed from teeth and stored in normal saline at 4°C for further use.

Sample preparation

Preoperative radiographs of teeth were taken to assess the root canal anatomy. An endodontic access cavity was prepared using an ISO size 014 round bur. Working length was determined by inserting a #10 K-file (Mani, Japan) into the canal until it just exits the apical foramen and then subtracting 1 mm from the obtained length. Biomechanical preparation was carried out with ProTaper (Dentsply, Maillefer, Switzerland) rotary file system up to the size F3 according to the manufacturer's instructions. During preparation, the canals were irrigated with sodium hypochlorite (NaOCl). The root canals were dried using absorbent paper points. The teeth were then randomly divided into 6 groups.

In Group I, the root canals were not filled with CH. The root canals of the teeth in the remaining groups were filled with CH paste using a Lentulo spiral (Dentsply, Maillefer, Switzerland) in a slow-speed handpiece and spun into the canal space up to the cementoenamel junction. The paste was prepared by mixing CH powder with normal saline in Group II, distilled water in Group III, and local anesthetic solution in Group IV. The root canals of the teeth were filled with Metapaste (Meta, Biomed, Korea), a proprietary Ca (OH)2 product in Group V, and with RC-Cal (Prime Dental Products. Pvt. Ltd) in Group VI.

Radiographs were taken to ensure complete filling of the canals with Ca (OH)2 paste. All the teeth were sealed coronally with a cotton pellet and Intermediate Restorative Material (IRM) (Dentsply, USA) and apically by IRM. Prepared teeth were then stored in normal saline at 37°C that was replenished weekly for 30 days at room temperature. After 30 days, the crowns of the teeth were sectioned at the cementoenamel junction by a diamond abrasive disk (Horico, Germany). The apical sections of all the roots were sectioned with the diamond abrasive disk leaving behind 5 mm of root portion. The obtained 5-mm thick dentin cylinder was polished with 600 grit silicon carbide paper to produce smooth, flat ends to prevent any interference with the jaws of universal testing machine. The dentin cylinders were subsequently preserved in normal saline to prevent dehydration.

Sample testing

Each dentin cylinder was then tested for the compressive force required to fracture it using a universal testing machine at a crosshead speed of 2 mm/min. The samples were placed between the jaws of testing machine [Figure 1]. Force was applied in a vertical direction with the point of application being centered in the root canal until the dentin cylinder fractured [Figure 2]. The force required to fracture the specimens was noted and the values obtained were subjected to statistical analysis carried out using one-way ANOVA and multiple comparisons using Bonferroni test.

Figure 1

Dentin cylinder placed between the jaws of the Universal Testing Machine

Figure 2

Fractured dentin cylinder

RESULTS

Descriptive statistical analysis was carried out in the present study. Results on continuous measurements were presented on mean ± standard deviation (minimum–maximum) [Table 1] and results on categorical measurements were presented in number (%) [Table 2].

Table 1

Representation of mean compressive strength and standard deviation

Group	Mean±SD	SEM	95% CI for mean		Minimum-maximum
			Lower bound	Upper bound
Group I	246.50±12.68	4.01	237.43	255.57	226-269
Group II	201.70±14.94	4.72	191.01	212.39	176-220
Group III	197.40±11.23	3.55	189.37	205.43	180-213
Group IV	200.80±15.16	4.79	189.96	211.64	172-219
Group V	196.60±22.00	6.96	180.87	212.33	163-227
Group VI	199.30±21.57	6.82	183.87	214.73	167-229

SD: Standard deviation, SEM: Standard error of mean, CI: Confidence interval

Table 2

Representation of analysis of variance

Source	df	SS	Mean SS	F	P
Between groups	5	18863.750	3772.750	13.406	<0.001*
Within groups	54	15197.100	281.428	-	-
Total	59	34060.850	-	-	-

*P<0.001 significant difference. df: Degrees of freedom, SS: Sum of squares, Mean SS: Mean sum of squares, F: F statistics

Table 1 represents the mean fracture resistance to fracture. The highest mean compressive strength was recorded in Group I (no medicament was placed), followed by Group II (CH + normal saline), Group IV (CH + local anesthetic solution), Group VI (RC-Cal), Group III (CH + distilled water), and Group V (Metapaste).

Graph I represented the mean fracture resistance of all the groups after 30 days of intracanal medicament placement. When compared with the control group, all the experimental groups showed a reduction in the mean compressive strength to fracture. However, no statistically significant difference was found when comparing in between the experimental groups.

Graph 1

Mean compressive strength among the groups

The difference in mean compressive strength among the groups was found to be statistically significant (P < 0.001). A statistically significant difference was found between Groups I and II (P < 0.001), Group I and III (P < 0.001), Group I and IV (P < 0.001), Group I and V (P < 0.001) as well as Group I and VI (P < 0.001). No significant difference was found between the other pair of groups (P > 0.05).

DISCUSSION

During root canal therapy, CH is the most widely utilized intracanal medication between appointments.[1]

The fundamental benefit of CH is that it causes a high pH in the surrounding tissues by releasing hydroxyl ions, which diffuse from the substance into the surrounding tissues through the apical foramen or dentinal tubules. These hydroxyl ions include highly oxidizing free radicals that kill bacteria by causing cytoplasmic membrane damage, protein denaturation, and DNA damage.[1] It has always been difficult to treat teeth with immature root development and extensive periapical lesions with endodontic treatment. Heithersay and Frank were the first to introduce apexification with the use of CH. Since then, there are many studies establishing the use of long-term CH dressing in immature teeth with open apex, inflammatory resorptions, and perforations.[4]

Sahebi et al. in 2010 found that 60% of all endodontically treated teeth with immature root development suffered cervical fractures because of minor impacts.[4] In 1992, Cvek presented a similar finding. Such studies raised the possibility that endodontic therapy had damaged the tooth structure, which was substantiated by histological evidence of circumpulpal dentin alterations in replanted teeth following long-term CH treatment.[46]

Dentin's flexural strength is determined by a close relationship between its two primary constituents, hydroxyapatite crystals and the collagenous network. Acid proteins and proteoglycans with phosphate and carboxylate groups make up part of the organic matrix. Between the collagen network and the hydroxyapatite crystals, these chemicals may act as bonding agents. Due to its alkaline nature, CH may neutralize, dissolve, or denature some of the acidic components that act as bonding agents, weakening the dentin.[234]

When CH is exposed to pulp tissue for just 1 week, it has a dissolving effect. The pulp tissue is denaturated and hydrolyzed to achieve this.[6] These phenomena could have something to do with the pH alterations in dentin that occur following CH therapy. CH could cause severe dentin to change if this is the case. This may decrease the organic support of the dentin structure, affecting the dentin's mechanical qualities.[2]

According to the findings of our investigation, there was a statistically significant difference between the CH groups and the control group.

All the experimental groups that received various kinds of CH as an intracanal medicament had a statistically significant higher mean compressive strength than the control group. As a result, the null hypothesis was rejected.

The mean compressive strength of the teeth which were not filled with CH (control group) was 246.5 kg, whereas the mean compressive force required to fracture the teeth filled with CH mixed with various vehicles was 201.7, 197.4, 200.8, 196.6, and 199.3 kg in Groups II, III, IV, V, and VI, respectively. The values indicate that there is a significant reduction in the mean compressive forces required to fracture the teeth that are treated with CH [Table 2].

Between the experimental groups in which CH powder mixed with various vehicles such as normal saline, distilled water, local anesthetic solution, and the paste forms of CH pastes such as Metapaste (Meta Biomed) and RC Cal (Prime dental Corp), there was a difference in the mean compressive forces required to fracture the sample, but it is not statistically significant.

There was no statistically significant difference in the mean compressive forces necessary to fracture the dentin cylinder between the experimental groups, which included CH combined with various vehicles. The experimental groups’ values were ordered in decreasing order as follows: Group II, IV, VI, III, and V.

Although there is no statistically significant difference between the groups, there was a subtle difference in the paste forms of CH used in Group V (Metapaste) and Group VI (RC-Cal). This could be due to the vehicles used in the pastes. Metapaste has polypropylene glycol, but RC-Cal has an aqueous solution. The dissociation of Ca+ ions or the pH of the paste could possibly explain the reason for the difference in the mean fracture resistance of teeth to fracture in the respective groups.

When CH powder and distilled water were used, dentin strength was reduced less than when CH and saline were used together. This could be due to CH ions in distilled water dissociating slightly. This is in line with the findings of research by Pacios MG, Hasheminia SM, Kawamoto R.[789]

The findings revealed that CH, in any form, when administered as an intracanal medicament, reduces the strength of the root dentin. As a result, the kind of CH has no influence on the compressive forces needed to fracture a tooth.

There have been numerous studies showing the weakening of the structure of root dentin by CH. The results of our study are in consistent with the report of Andreasen et al. who demonstrated the weakening of tooth root dentin after exposing to CH in 1 week.[46] Rosenberg et al. in measuring the effect of CH on the micro tensile fracture strength of teeth reported weakening of root dentin by as much as 43.9% from 7 days to 84 days.[5] This study confirmed the work of White et al. who reported the reduction of the strength of bovine root dentin after exposure to CH and mineral trioxide aggregate as much as 32% and 33%.[3]

The present study was consistent with another study by Kawamoto et al. which demonstrated that exposure of bovine dentin to CH can make it prone to fracture.[9] Sahebi et al. evaluated the effect of short-term effect of CH application on the strength dentin against the compressive forces. They concluded that 30 days application of CH reduced the compressive strength of root dentin by almost 15%.[4]

However, the results of our study were contradictory to the study conducted by Doyon et al. They conducted a study to determine if exposure to intracanal CH alters the fracture resistance of human root dentin using single-rooted human permanent teeth for a period of 30 days to 180 days. They concluded that after 180 days, the roots of the teeth exposed to CH showed a significant decrease in peak load at fracture than those of the control group, whereas the roots of the teeth which were stored for 30 days could not show any statistically significant reduction in the peak load to fracture.[10]

Similar findings are reported in a study by Kahler SL in which three commercially available CH pastes and compared with formulations prepared with CH powder mixed with sterile saline in ovine teeth. It suggests that the reduction in the fracture strength of lamb's teeth was not associated with CH. Rather it was related to the stage of root development in the in vitro studies.[11]

Limitations

Only radiographic evaluation was carried out to check the filling of the canals with CH, which is not a standardized method

Other limitations of the model are that in vitro studies under experimental conditions do not reflect the conditions of teeth in vivo that have a functioning periodontium

Apart from evaluation of compressive strength, stereomicroscopic or scanning electron microscope evaluation

Also, in an acute dental trauma, teeth are more likely to be affected by a high impact force rather than slow load-to-fracture testing used in the in vitro experiments.

CONCLUSIONS

The following conclusions were drawn from the present study:

The teeth that were not filled with CH showed the highest strength

CH when used as a root canal dressing material will reduce the compressive strength of the teeth, irrespective of the form of CH and vehicle

As a temporary root canal dressing material, CH can be used with various vehicles. The form of CH and vehicle does not have any influence on the compressive strength

Hence, if a decision is made to use CH, it should be of shorter duration.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.