Temperature evolution of preheated irrigant injected into a root canal ex vivo

Abstract

Objectives

The aim of this study is to test the influence of the temperature of the surrounding medium, flow rate, duration of irrigation, and apical patency on the evolution of the temperature of irrigants injected in a root canal.

Materials and methods

Thermocouples were inserted into an incisor at different positions to monitor irrigant temperature during and after injection at 21, 45, or 60 °C. The tooth was immersed in a water bath at 21 and 37 °C.

Results

Preheated syringes were used for up to 2.5 min before being cooled down from 60 to below 45 °C. The irrigant temperature was higher apically than at coronal levels (P ≤ 0.028). The duration of irrigation had no influence on the average temperatures during delivery (P ≥ 0.337), but the apical patency lowered the intracanal temperature (P = 0.004). The highest temperature measured on the outside of the tooth was 39 °C.

Conclusions

Preheating the irrigant at 60 °C resulted in temperatures higher than 45 °C throughout the root canal, during irrigant delivery. After completion, the temperature dropped rapidly.

Clinical relevance

These results contribute to a better understanding of the optimum irrigant delivery time at given temperature, the cooling rate of irrigant in the syringe, and the influence of heated irrigant temperature in the periodontium, which should guide the preheated syringe turnover.

Appendix. Numerical model of temperature evolution inside a tooth during irrigant injection

Numerical simulations of heat conduction and heat dissipation can provide detailed information on the temperature changes over time, for various wall thicknesses and root canal dimensions, as was shown previously [1–3]. Such simulations have never been employed to study root canal irrigation with preheated irrigants. Here, we introduce a numerical model that simulates the thermodynamics of root canal irrigation.

Details on the numerical model

A one-dimensional time-dependent axisymmetric cylindrical model was created to calculate the temperature evolution at a single location in the tooth model (Fig. 5). Unless indicated otherwise, the dimensions at the apical location of the tooth described in the main article were used; however, a circular cross section was simulated.

Fig. 5

Sketch of the one-dimensional numerical model (thick black line) with the simulation parameters and boundary conditions indicated. Red dots depict the locations at which the temperature was evaluated

The center domain represented the irrigant in the root canal lumen. This domain was surrounded by a second domain that represented the wall of the root canal. The third domain represented the surrounding medium. In each of these domains, the heat conduction equation [4] was solved, using a finite element solver (COMSOL v4.2, COMSOL AB, Stockholm, Sweden):

$$ \frac{1}{r}\frac{\partial }{\partial r}\left( r\frac{\partial T}{\partial r}\right)=\frac{1}{D}\frac{\partial T}{\partial t} $$

(4)

with T the temperature as a function of time t at a certain point r from the center of the root canal. D represents the thermal diffusivity for each material (Table 1).

Table 1 Thermal properties of irrigants, root material, and surrounding medium as used in the numerical model

The numerical grid consisted of approximately 5000 elements with a maximum size of 5 μm; a higher resolution was implemented near interfaces. Grid independency was verified. The thermal properties used in the numerical model are listed in Table 1 and are assumed constant within the temperature ranges considered here.

Axial symmetry was imposed at the centre of the root canal (r = 0 mm). At a distance of r = 25 mm from the centre of the root canal, the surrounding medium was assumed to remain at a fixed temperature T _medium of 37 °C. The irrigant was assumed to have an initial temperature T _irrigant of 60 °C; the tooth wall was initially at the same temperature as the surrounding medium.

Validation was performed using the geometry of the tooth used in the experiments, at the apical position. In order to obtain simulation results for the sensor on the outer surface at the middle position, an irrigant temperature of 45 °C was assumed and the wall thickness at the middle position was used in the simulation.

The wall thickness d was varied in the range 0.25 to 1.50 mm in steps of 0.25 mm [5–7] in order to study its influence on the temperature of the inner and outer wall. The radius of the root canal was fixed at 0.256 mm, corresponding to the apical position of the tooth used in the experiments.

In order to compare in vivo irrigation (roots embedded in periodontal ligament (PDL) and bone at 37 °C) to the ex vivo experiments, simulations were performed with air, water, or PDL and bone as surrounding medium. The PDL was assumed to have a thickness of 0.2 mm [8] and embedded in alveolar bone of 10 mm [9]. The results were compared to the “baseline case”: the tooth in a water bath at 37 °C, irrigated with distilled water at 60 °C.

The cementum layer covering the root was not considered separately, because it is relatively thin and has a thermal diffusivity very similar to dentin [10].

Numerical results for the temperature evolution

The numerical model of the tooth resulted in temperature evolutions that looked very similar to those found experimentally at the apical position (Fig. 6). The temperature decrease showed a slope similar to those observed experimentally; however, there is up to 2 °C difference. Also, at the outer middle sensor position, the agreement between measurement and simulation was good, although in the experiment, the temperature decreased faster.

Fig. 6

Comparison of the measurement (solid lines) inside the real root canal and the simulation (dashed lines), at the apical (red) and middle outside (cyan) sensor positions. The blue dashed line represents the temperature of the irrigant inside the root canal instead of inside the syringe. The external temperature is 37 °C, and the irrigant temperature is 60 °C. Standard deviation is left out here for clarity but can be found in Fig. 3

These differences can be attributed to the absence of convection in the numerical model. Furthermore, the tooth geometry may be different than simulated; e.g., the tooth is oval shaped instead of circular, which can result in more dissipation of heat as the surface-to-volume ratio is different. Additionally, in the experiment, the outside thermocouple is glued to the outer surface and acts as a cooling fin.

Simulation of the tooth embedded in PDL and bone rather than water (Fig. 7a) leads to a slower dissipation of heat in the tissue, which resulted in a higher temperature (+1.26 °C) of the tooth’s outer surface and a slower decrease in temperature inside the tooth. For a smaller wall thickness, e.g., near the apex, the temperature at the outside of the tooth was predicted to reach values up to 52 °C, suggesting that tissue damage may occur [11]. With a larger wall thickness, there is more dentin to be heated up, resulting in a lower temperature of the periodontal tissue.

Fig. 7

Simulation results for the temperature of the irrigant and at the apical and middle outside sensor location. In a, the surrounding material is varied between water and human tissue (both initially at 37 °C) and in b between water and air (both initially at 21 °C). In c, the wall thickness is varied from 0.25 to 1.5 mm (with 0.85 mm the “experimental” case)

With air surrounding the tooth, the heat dissipation to the environment is predicted to be greatly reduced (Fig. 7b), which caused buildup of heat inside the tooth. The temperature at the inner surface of the tooth increased up to the irrigant temperature. The temperature at the outer surface of the tooth increased up to 57.23 °C.

It should be noted that the thermal properties of the dentin of the tooth used in this study are not exactly known, as these properties vary between different sections of the tooth and between teeth, depending, e.g., on their age [3, 12].

Numerical simulations for the different wall thicknesses (Fig. 7c) showed that the wall thickness had no relevant effect on the temperature inside the root canal, leading to a maximum of 0.1 °C temperature differences compared to the standard case. The temperature at the outer surface of the tooth decreased with increasing wall thickness; it was 7.07 °C higher for a wall thickness of 0.25 mm and 3.29 °C lower at 1.50 mm compared to the standard case.

The numerical model can provide quick insight into a variety of control parameters, such as the wall thickness and surrounding material. The numerical model used in this study can also be used to investigate the possibility of using a heat source inside the root canal instead of preheating the irrigant. For example, an ultrasonically oscillating file is known to be able to heat up the liquid [13]; however, it requires a dynamic model to estimate the acoustic power required to heat the irrigant.

The one-dimensional model should be further expanded into a full three-dimensional model of a tooth during irrigation, since there is heat transport into all three directions that the present one-dimensional model does not take into account. Such a model should also include more complex root canal geometries as well as convection of the injected irrigant [14].

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