| Abstract|| |
Aim: The purpose of this study was to investigate temperature-dependent rheological properties of three endodontic sealers MTA Fillapex (Angelus, Brazil), AH Plus (Dentsply, Germany), and EndoREZ (Ultradent, USA).
Materials and Methods: Five samples of each group of endodontic sealers (n = 30) were freshly mixed and placed on the plate of a rheometer (MCR 301, AntonPaar, Physica) and examined at 25° C and 37° C temperature, respectively. Rheological properties of the sealers were calculated according to the loss modulus (G"), storage modulus (G'), loss factor (Tan δ), and complex viscosity (ç*) using dynamic oscillatory shear tests.
Results: Statistical analysis (Wilcoxon signed-rank test) demonstrated that MTA Fillapex exhibited higher loss modulus (G" > G') and a crossover region. AH Plus and EndoREZ had a higher storage modulus (G' > G") at both temperatures. Loss factor (Tan δ) of MTA Fillapex was the highest compared to AH Plus, followed by EndoREZ. With a temperature change from 25°C to 37°C, MTA Fillapex exhibited a decrease while AH Plus exhibited an increase and, EndoREZ exhibited the least change, in complex viscosity (ç* ).
Conclusions: EndoREZ exhibited better rheological properties compared to the other two test sealers.
Keywords: Endodontic sealers; rheology; viscoelasticity
|How to cite this article:|
Rai RU, Singbal KP, Parekh V. The effect of temperature on rheological properties of endodontic sealers. J Conserv Dent 2016;19:116-9
|How to cite this URL:|
Rai RU, Singbal KP, Parekh V. The effect of temperature on rheological properties of endodontic sealers. J Conserv Dent [serial online] 2016 [cited 2019 Nov 13];19:116-9. Available from: http://www.jcd.org.in/text.asp?2016/19/2/116/178683
| Introduction|| |
Endodontic sealers form an integral part of the obturation systems to create and maintain a three-dimensional seal that extends to the apical constriction and includes canal irregularities, isthmus, fins, ramifications, and micro voids between the master and accessory cones.  They help to prevent leakage, reducing the possibility of residual bacteria from the canal invading the periapical tissues and thus help to resolve the periapical pathosis. 
According to Grossman, an ideal root canal sealer should provide the following: An excellent seal when set, dimensional stability, a slow setting time to ensure sufficient working time, insolubility to tissue fluids, bacteriostaticity, nonstaining have adequate adhesion with canal wall, radiopacity, and biocompatibility. 
The properties of root canal sealers have an impact on the quality of the final root canal filling. The flow characteristics of endodontic sealers may determine how effectively they obturate the accessory canals and core filling material. Whereas adequate flowability allows for the filling of irregularities, high flow may result in apical extrusion, leading to injury of the periapical tissues because of the cytotoxicity of the sealers. 
Several methods have been proposed to evaluate the flow of endodontic sealers. The International Organization for Standardization (ISO) 6876-2002/12 and the American Dental Association (ADA) specification no. 57 for endodontic sealers use the measurement of the diameter of material flowing between two glass plates under a specified weight for a specified time. These tests are preliminary tests and do not require measurement of viscosity which is a quantitative parameter for the evaluation of rheological properties of endodontic sealers and may help to achieve an ideal flow pattern. ,
Rheological studies are useful to investigate the time-dependent viscoelasticity of the materials that are related to the working and setting times. Their measurement could give a more accurate indication of when the material is changing from liquid-like behavior to solid-like behavior and how this affects their handling and sealing properties. Various types of rheometers such as capillary extrusion rheometers, custom-made capillary rheometer, rotating spindle geometry rheometer, and control stress rheometers with cone-plate geometry used to study the flow properties of sealers have been investigated. 
The rheological properties of endodontic sealers are affected by various factors like temperature, humidity, pressure, etc. of the material. The flow of endodontic materials can, in addition, be affected by the geometry of the delivery system, the geometry of the root canal, and the physical conditions within the canal. Significant effects on the rheology of endodontic materials will be found as well from their chemical composition, the molecular weight of macromolecules, their molecular weight distribution, and the molecular architecture.
Rheological properties include loss modulus (G"), storage modulus (G'), loss factor (Tan δ), and complex viscosity (ç*). G' is a measure of stored energy and represents the elastic component of the material. G" represents the viscosity of the materials and is a measure of the energy lost as heat. Tan δ is the ratio of the viscous part to the elastic part and ç* reflects the response of the microstructure of the materials to the shearing motion of extremely low strength.  The effects of temperature, time, shear rate, and powder-liquid ratio, humidity, particle size distribution, molecular weight on rheological properties can be compared by these parameters. ,
There are few studies to distinguish rheological characterization of endodontic sealers at different temperatures. The purpose of this in vitro study, therefore, is to compare the rheological properties of three root canal sealers through dynamic oscillatory shear tests and to investigate the effect of changes in temperature on their rheological behavior.
| Materials and methods|| |
Rheological properties of three endodontic sealers, MTA Fillapex (Angelus, Brazil - #26239), AH Plus (Dentsply, Germany - #1307000833), and EndoREZ (Ultradent, USA - #B875J) were measured using a high-performance strain-controlled Compact Modular Rheometer - (MCR 301-PHYSICA) (Anton Paar, Germany), with cone and plate geometry (cone angle Ό 0.1 radian and cone diameter Ό 25 mm), and Peltier plate-temperature control (±0.1° C). 
The endodontic sealers (n = 30) were freshly mixed according to the manufacturers' instructions, weighed precisely 1 mg with a digital balance (Toledo, Germany) and were placed on the plate of the rheometer. The relative humidity impact was minimized by ensuring that there was an extra sealer within the outer rim of the cone. ,
Rheological properties of the sealers including G", G', Tan δ and ç* were studied using dynamic oscillatory shear test. Amplitude sweep test was performed to obtain an optimum strain rate with the frequency fixed at 10s−1 . After establishing the linear region, measurement was taken as a function of frequency (from 0.01 to 100 s−1 ) using RHEOPLUS/32 software. The sealers were tested at 25°C (n = 15) and 37°C (n = 15).
The results of all measurements were analyzed statistically (P < 0.005) using Wilcoxon signed-rank test.
| Results|| |
Storage modulus (G') and loss modulus (G")
MTA Fillapex [Graph 1 [Additional file 1]] exhibited G" more than G' and a crossover region, with a frequency of 0.01s−1 at 25° C and a much higher frequency of 0.05 s−1 at 37° C. AH Plus [Graph 2 [Additional file 2]] and EndoREZ [Graph 3 [Additional file 3]], exhibited a higher G' than G", indicating elastic behavior.
Loss factor (Tan δ)
The loss factor of MTA Fillapex has been highest among all sealers [Graph 4 [Additional file 4]]. AH Plus exhibited an increase in Tan (δ) with increase in temperature. EndoREZ exhibited similar values at both the temperatures.
Complex viscosity (η*)
EndoREZ exhibited very high ç*, indicating highly cross-linked microstructure and resistance to flow. Additionally, it exhibited the least change at both temperatures [Graph 5 [Additional file 5]]. At higher temperatures, the ç* for AH Plus was more than MTA Fillapex. The ç* of all materials reduced with an increase in frequency.
| Discussion|| |
Rheology is defined as the science of the deformation and the flow of matter.  It is of importance in endodontics, as it relates to the flow of sealers within the root canal system and along the root canal wall. It will have an effect on whether endodontic sealers extend to the apical foramen without excessive extrusion, whether they flow into lateral canals and enter into the dentinal tubules, their working and setting time, and their behavior with the change in pressure or temperature. ,
Two temperatures were selected; 25°C (room temperature) and 37°C (mouth temperature). ISO standard for flow of endodontic sealers does not specify a temperature for the procedure, but it is an important factor in determining and comparing flow in materials. 
MTA Fillapex which is calcium salicylate-based resin cement revealed G" greater than G' that attributes to gel-like viscoelastic behavior. It exhibited a crossover region in which the G" crossed the G' at both temperature conditions. This indicated, setting down behavior of MTA Fillapex at lower frequencies, and a change in molecular structure, a prospective structural breakdown beyond crossover frequency, which may be due to complexation type of auto catalytic reaction.
The G' of AH Plus was higher than its G" over the entire experimental frequency range at both temperatures. The results indicated a dominant elastic solid-like behavior of this sealer. These findings may be due to epoxy-amine addition reaction that resulted to form a physical network in the structure of this sealer compared to the other sealers. These results were in accordance to the study done by Khedmat et al. 
EndoREZ exhibited higher G' than G'' over the entire frequency range at both temperatures. At the same time, in comparison to the other two sealers, it exhibited the least change in storage and G" at different temperature conditions. EndoREZ has a single methacrylate-based polymerization reaction that may explain its dominant viscous behavior.
It has been shown that the debonding of polymer adhesive systems is related to the values of Tan δ.  The Tan δ of MTA Fillapex was consistently higher at all frequency at both temperatures. The Tan δ of EndoREZ was least and compared to AH Plus; no significant difference was observed at both temperatures. This indicated that MTA Fillapex for an optimum formulation is less adhesive than EndoREZ and AH Plus at both temperatures.
As this is the first study to define loss factor (Tan δ) of endodontic sealers, there is no previous data available for comparison. Further study on the effect of entanglement molecular weight on the values of Tan δ may help to determine an optimum formulation for endodontic sealers.
The ç* of MTA Fillapex has been consistently lower compared to EndoREZ and AH Plus over the entire frequency range at both temperature conditions. This indicated the highest level of flowability.  EndoREZ, on the other hand, had exhibited highest values of ç* indicating its resistance to flow.
Interestingly, EndoREZ had exhibited the least deviation in η*, when subjected to a temperature change from 25°C to 37°C, indicating the highest degree of microstructural stability of the composition of sealer material, compared to the other two sealers in the study. AH Plus at the same time had exhibited an increase in the ç* with an increase in temperature from 25°C to 37°C.  MTA Fillapex exhibited a lowering of ç* with an increase in temperatures from 25°C to 37°C.
It is necessary to distinguish between the rheometric characterization of materials, with suggested clinical implications, and hypothesis-based studies of rheological behavior in clinical situations. More rheometric characterization studies should be published to contribute to the literature on dental rheology and to inform further rheological study of endodontic materials in the clinical situation.
| Conclusion|| |
Within the limitation of this experimental study it can be concluded that:
- The study in Rheology allows a "quantitative measurement" of viscoelastic behaviour of any material which can be used as a "finger print" or "benchmark" to study its properties.
- MTA Fillapex sealer was more sensitive to temperature changes. MTA Fillapex had a crossover region from liquid to solid like behaviour indicative of a prospective structural breakdown and less adhesive over exposed variable temperature and stress conditions.
- At both temperature conditions and over the entire frequency range, EndoREZ and AH Plus exhibited dominant solid viscous phase with stable molecular structure and least structural changes.
- EndoREZ exhibited better rheological properties compared to the other two test sealers.
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Conflicts of interest
There are no conflicts of interest.
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Roshni U Rai
Flat No. B 504, Aashirwad 1, Poonam Sagar Complex, Mira Road East, Thane, Mumbai - 401 107, Maharashtra
Source of Support: None, Conflict of Interest: None