Numerical Investigation of the Effect of Hot and Cold Fluids on Dental Implants Made of Different Materials

1. Introduction

Despite all the advances in oral and dental health, many people lose teeth, often due to tooth decay, periodontal disease or injury. For many years, bridges and dentures were the only treatment options available for people with missing teeth. However, today a brand new, more reliable, more aesthetic and more permanent method is available. Dental implants are replacement tooth roots. Implants provide a strong foundation for fixed (permanent) or removable artificial teeth made to match natural teeth. In other words, a dental implant is an artificial tooth "root" that is artificially placed in the mouth to support dental prostheses such as dentures or bridges. They are screws made of tissue-friendly materials and inserted into the jawbone that act as a root for the missing tooth [1].

A dental implant consists of three main parts; a dental implant body, an abutment and a crown [2]. A dental implant is surgically inserted in the jawbone in place of the absent tooth's root. The abutment is attached to the implant body by an abutment fixation screw and extends through gums into the mouth to support the attached new artificial teeth.

The earliest stone and ivory implants are reported to have been seen in China and Egypt. Gold and Ivory dental implants were also reported in the 16^th and 17^th centuries [3]. Metal implants made of gold, lead, iridium, tantalum, stainless steel and cobalt alloys also began to be used in the early 20^th century. Between these two eras, various polymers have been used as dental implants, including ultra-high molecular weight polyurethane, polyamide, polytetrafluoroethylene, and polyurethane.

Figure 1. Structure of the dental implant system [2].

Figure 1. Structure of the dental implant system [2].

Today, with the extensive research work and developments in the field of biomaterials available for dental implants, newer materials such as zirconia, roxsolid, surface-modified titanium implants have emerged. These materials not only meet to functional requirements, but are also aesthetically pleasing [4,5]. The metals used initially were stainless steels, which were gradually replaced by cobalt-chromium alloys. Although titanium has been in use since the late forties, it has only gained widespread attention relatively recently. Titanium and its alloys are used more and more compared to cobalt-chromium alloys and their application area is expanding [6].

A dental implant is a structure made of alloplastic materials that are implanted into or through the oral tissues or bone under the mucosa and periosteum to provide retention and support for a fixed or removable dental prosthesis [7]. Metals are an ideal type of material for implants due to their excellent mechanical properties. Apart from the pure titanium and titanium alloys commonly used today, metal implant materials also include stainless steel, gold alloys, cobalt, chromium, chromium alloys etc. [8].

The fact that dental implants are produced from materials with different physical properties shows that the lifetime of the implants and their resistance to breakage and cracking will also change. For this reason, the results of implants with different materials have been repeatedly examined in the literature, both in clinical settings and in various numerical and experimental studies conducted in the engineering world. Many of these studies have been on the mechanical effects of implants. In these studies, most of which are based on numerical analysis, as in present study, commercial software base on Finite Element Analysis Method (FEM) were used [9,10,11] In some recent studies additionally to the mechanical behavior, the thermal behavior of the dental implants have also been handled under microscope. Especially the effects of drinking hot beverages or food on teeth and the heat transfer during the drilling of the dental surgery are searched [12,13].

Since extremely high or cold temperatures may cause irreversible damages on the tissues and organs, it is important to know the effect of dental implant materials to different temperatures. Under the conditions of extremely hot drinking some oral-burn syndrome may become which is also associated with dental implant loss or damage [14]. For a normal human being regarding the age or gender a temperature between 50 °C and 60 °C is reported to be the dangerous temperature range for both oral tissues and dental implants [14]. However the later studies showed that the temperature produced intra-orally during hot water consumption may reach 67 °C and even 77 °C [15]. It was also reported that metal implants with high conductivity could conduct extreme temperatures to the osseo integration interface. Additionally, high temperatures might be a risk factor during the healing process following implant insertion [14].

The heat generation that occurs during the surgical mounting of an implant in the mouth is another thermal effect research that attracts the researchers. In the literature, there have been some recent studies on the subject, where the authors attempted to model and investigate the effects of thermal loads on the bone implant interface [16,17,18,19].

As it is seen, although the effect of hot food or beverages or heat-emitting procedures on dental implants has been investigated, studies investigating the effect of cold environment have not been found in the literature. Furthermore, unlike the studies in the literature examining thermal events related to dental implants, in this study it is assumed that the temperature change occurs not by conduction, but by the contact of the fluid, that is, by convection. Therefore, the problem is solved as a Computational Fluid Dynamics (CFD) problem.

In present study presence of cold (5°C) liquid, hot (60°C) liquid and cold air under freezing point (-10°C) in human mouth are tested numerically. The most preferred implant materials such as titanium and zirconium, as well as gold and cobalt are considered to be the materials of implant bodies. Porcelain and zirconium materials are chosen for the crown materials attached to the implant bodies with abutment screw. The effects of those different thermal conditions on the dental implants and natural tooth are determined by means of ANSYS-CFX commercial software.

2. Materials and Methods

2.1. Materials

This study numerically examines the thermal effect of hot water (60 °C), cold water (5°C) and cold air (-10°C) affecting four implanted teeth and a natural tooth, totally five teeth in the lower jaw bone. The purpose of using water at 5 °C is to examine the temperature change on the teeth in case of drinking cold water, juices, beverages, etc. Similarly, the purpose of using water at 60 °C is to observe the temperature effects of tea and coffee-like fluids on natural tooth and dentures. Finally, air was used at -10 °C, representing breathing air in cold weather. Table 1 shows the thermophysical properties of air and water at the temperatures in question.

As previously told, four dental implants and one natural tooth have been assumed to be on the jawbone. Figure 2 is illustrated to represent the arrangement of the teeth. The material of implant bodies are selected as zirconium, titanium, cobalt and gold. By the way, the crown materials are assumed to be the porcelain and zirconium.

Table 2 shows the four cases (coded as C1, C2, C3 and C4) that are simulated. For the implant bodies the material was introduced so that the first two implants (Implant 1 and Implant 2) and the last two implants (Implant 3 and Implant 4) are the same material. The thermophysical properties of materials of implant body and crown are listed in Table 3.

2.2. Numerical Method

Numerical analysis in the context of dental implants refers to the application of mathematical and computational methods to simulate, model, and analyze various aspects of the implant process. This can include studying the biomechanics of implants, stress distribution, osseointegration, thermal effects, and other factors that influence the success and longevity of dental implants. Present analysis helps in designing implants that can withstand the thermal loads exerted during normal functioning. For this aim, ANSYS-CFX the well-known commercial software is used to analyze the effect of cold and hot fluids on the dental implant teeth. The solutions are obtained by determining 1000 iterations. Convergence values for the solution are entered as 10^-6. The governing equations, including the continuity, momentum and energy equations for 3-D incompressible laminar flow are presented as:

Continuity:

$$\overrightarrow{\nabla }.\overrightarrow{\nabla }=0$$

(1)

Momentum (Navier-Stokes) equations:

$$\left(\overrightarrow{V}.\overrightarrow{\nabla }\right)\overrightarrow{V}=-{\displaystyle \frac{1}{\rho }}\overrightarrow{\nabla }{P}^{\text{'}}+v{\nabla }^2\overrightarrow{V}$$

(2)

General heat transfer equations:

$$\rho C_p{\displaystyle \frac{\partial T}{\partial t}}+k\nabla .q=g\left(T,t,x\right)$$

(3)

where ρ, C_p and k represent density, specific heat and thermal conductivity, respectively.

2.3. Physical Model

In order to save time and number of elements, the modeling process is considered to be the single jaw. Four implant bodies are placed side by side on the jawbone. Four crowns, each separately, were placed on the upper parts of the implant bodies. The purpose of modeling crowns and implant bodies separately is to allow the observation of the temperature effects of different implant body and crown materials.

Since the inlet velocity of water and air is not high enough to be turbulent the flow regime is considered to be laminar with Re = 1000. Solutions are obtained for steady-state conditions. The body core temperature, and the temperatures at walls (mouth and jaw) are assumed to be 37.5 °C. Figure 3a shows the jawbone with dental implants and natural tooth. Figure 3b represents whole solution domain. Finally Figure 3c is the domain with boundary conditions.

Meshing step is the most important step of this study. In order to achieve correct results, the number of elements in the mesh structure must be formed appropriately. The 'Mesh Numbers' obtained for each element in the model (jawbone, crown, natural tooth, implant body) are given in Table 4.

In order to determine the accuracy of the mesh number determined for this study, three mesh structures that include 145950, 169049 and 96153 nodes in all areas were studied. The crown-implant body interface mean temperature value is extracted from each model. It is sufficient that results are converging to each other, less than 10%. Various mesh structures of jawbone, natural tooth, implant bodies and crowns are given in Figure 4.

3. Results

Now the attention will be turned to the results of the numerical simulations. For representing the temperature values three lines are considered as shown in Figure 5, namely Line 1, Line 2 and Line 3. Each line has 100 data on it. Line 1 is selected especially to show the temperatures on the joint section of crown and implant body. Line 2 shows the data on crown and Line 3 represents the data on implant body.

Variation of Surface Temperatures

The interface temperatures of the prominent models C1, C2, C3 and C4 in this study are given in Figure 6, Figure 7, Figure 8 and Figure 9 respectively. In Figure 6, the surface temperature distribution is shown for the case, C1. While the surface temperature of the natural tooth is equal to the contact fluid temperature during water contact at 5 °C and 60 °C, the surface temperatures of all four dentures are 8.91 °C and 57.63 °C. In the case of air contact at -10 °C, the surface temperature of the four dentures is equal to 34.78 °C, while the surface temperature of the natural tooth is calculated as 2.53 °C. The surface temperatures of the crowns are close to the liquid temperature in hot and cold water contacts, but the surface temperatures of the crowns are 34.74 °C during air contact at -10 °C, that is, the surface temperatures do not change with air contact as much as it does at liquid contact. This can be explained by the density and specific heat capacity of water and air.

It is clearly seen in Figure 7 that the general temperature tendency for C2 seems to be similar for C1. The surface temperatures of the crowns during the contact of water at 60 °C and 5 °C are 57.63 °C and 8.93 °C, respectively. The crown surface temperature is 34.75 °C during air contact at -10 °C.

When C3 model given in Figure 8 is examined, the surface temperatures of the crowns in case of water contact at 60 °C and 5 °C are 58.24 °C and 7.13 °C, respectively. However, in case of air contact at -10 °C, surface temperature of the crown is 18.75 °C. This result is lower than the ones of model C2.

In Figure 9, the surface temperatures of the crowns at 60 °C and 5 °C temperatures in liquid contact situations are approximately similar to the previous models. However, with air contact at -10 °C, the surface temperature of the crown is 34.76 °C. This value is similar to the values obtained for C1 and C2, and it is considerably higher than the 18.75°C value of the C3 model under the same conditions. From this point of view, it can be concluded that the C3 model has more disadvantageous than other three models in cold weather conditions.

There is no significant changing in the average temperatures of natural tooth and other four implant teeth in all cases. However, the highest average temperature of 44.998 °C and 45.096 °C is observed at the interface of the C1 and the lower part of the implant body, where zirconium and titanium are used. In the models C2, C3 and C4, approximately similar results are obtained for the average temperatures of the upper part of the teeth and implants, the interface and the lower parts of the implant body and tooth root. In C1, C2, C3 and C4 models, the surface temperatures of the crown vary between approximately 59.4°C - 59.83°C. At the interfaces, mean temperatures are obtained as approximately 45 °C for the four implant cases. Considering the implant body temperature of C3, the temperature is obtained as 44.044 °C on Implant 1 and Implant 2 and 44.079 °C on Implant 3 and Implant 4. Thus, it is seen that the implant bodies of the C3 are less affected by temperature than the other three models.

While checking the average temperatures for models subjected to water at 5°C and 60 °C previously, for models exposed to airflow at -10 °C to model breathing in cold weather, the upper part crown showed similar temperature values in the use of zirconium (C1, C3, C4), with 34.614 °C. When the porcelain is used as the crown, the average temperature was 16.381 °C. As can be seen, the temperature is decreased by almost half. When the upper part is a natural tooth, different effects are seen in each case.

Figure 10 is plotted to show the comparison of all cases on the same line when water temperature is considered to be 5°C. It is seen that the temperature distributions almost overlap in C1, C2 and C4, where zirconium is used as a crown, and the surface temperatures of the teeth decrease to around 8-9 °C. The temperature of porcelain prosthesis expressed in C3 model decrease to 7.16 °C. From this, it can be concluded that using the C3 model is not recommended considering the tooth sensitivity to the thermal effects.

Figure 11 is plotted to show the comparison of all cases on Line 1, which crosses all teeth. Here the temperatures of C1 model is equal to the temperatures of C2 model. In C4 model, there is a decrease, but this decrease is not as high as in C1 model. When the temperature distribution in C3 is examined, it is seen that the decrease is very small. It is undesirable that the temperature difference is high at the interface (Line 1), which is the junction point of the implant body and the crown. Sudden temperature differences cause thermal stresses and tooth sensitivity.

Figure 12 shows the temperature distributions along Line 3 having water in the mouth at 5 °C. The temperatures of C2 are close equal to the temperatures of C4, and the temperatures of C1 are close to temperatures of C2. Hence only two cases are plotted. In C1 model, temperature on the implant screws decreases to 35.5 °C, while in C3 model temperature decreases to 36.8 °C. Therefore, the C3 model with less temperature decrease is preferable.

Temperatures with contour plots for all tested conditions of the C1 case is given in Figure 13. The figures on the left show the general temperature distribution over the surfaces. The figures on the right are the temperature distributions at the interfaces of the implant body and the crown. In all three cases, it is seen that the temperature effects occur more than the implant bodies, especially at the interfaces and the lower parts of the crowns.

In Figure 14, the temperature changes of the tooth root and implant body in the jawbone are given when the material pairs in C3 and C4 come into contact with the air at -10 °C. It is seen that the greatest effect on tooth root and implant body temperature is in C4. Especially between Implant 2 and Implant 3, the temperature decreased by 0.271% compared to C1.

In Figure 15, the temperature changes of the tooth root and implant body in the jawbone are seen when the material pairs in C3 and C4 come into contact with water at 60 °C. When C3 model is used, the temperatures on the implant body is nearly 37.02 °C. It is observed that the highest temperature in this region reaches to 37.8 °C when C4 is used. An approximately 1.35% increase is observed in C4 model comparatively to C3 model.

In a numerical study of Wang et al. [20] when 60°C temperature is subjected to the teeth then it may cause an increase in temperature of up to 47 °C along the surface of the dental implant. Feuerstein et al [14] measured some temperatures in a clinical study under the hot beverages thermal conditions, and found that they are all lower than the calculated ones from Wang et al [20]. In fact, both values were higher than the temperature value of transient changes in osteoblasts [21]. The 60 °C water exposed into the mouth gives agreeable values to Wang’s calculated values.

4. Conclusions

In this study, numerical solutions are obtained by studying 4 different models with different combinations for 3 different fluid conditions. The following conclusions can be listed for the models named C1, C2, C3 and C4.

• Temperatures on the upper surface of the tooth, which is initially considered to be at body temperature, is least affected, and that the temperatures at the interface, which is the junction of the crown and implant body, and temperature at the lower point of the implant are close to the temperatures of the natural tooth in the same region.
• In case of drinking water at 5 °C temperature, it is concluded that the porcelain used as crown material in C3 is not as suitable as the zirconium used as prosthetic material as in C1 and C4.
• During liquid contact at 60 °C, the zirconium material used as the crown material in the C4 model is found to be more successful than the porcelain, which is the crown material in the C3 model, since it is less affected by the temperature.
• The model in which the temperature of the crown decreases the most in case of contact with air at -10 °C is the model expressed in C3. The model with the least decrease in surface temperature is the C4 model, in which zirconium is used as the crown.
• In the case of drinking water at 5 °C, the greatest decrease in interface temperatures is seen in the crown and implant body expressed in C1 model. When the temperature distribution in C3 is examined, it is seen that the decrease is very small, and this result makes C3 more successful.
• In the case of drinking water at 60 °C, it is seen that there is no increase in the interface temperatures of the teeth designed in the C3 model, but it reaches a maximum of 38.5 °C in the C4 model. This result shows that the C3 model is more suitable at the interface.
• All of the models examined in air contact at -10 °C showed similar behavior in terms of temperature at the interfaces.
• Since there is a negligible (0.8 °C) temperature drop in the implant bodies of the model called C3 in contact with water at 5 °C, the C3 model is found to be successful under these conditions.
• In case of drinking water at 60 °C, the temperature of the implant screws of the C3 model increases up to a maximum of 36.93 °C. This situation can be interpreted as keeping the temperature constant and can be expressed as the success of the C3 model.
• Among the models examined in air contact at -10 °C, the temperature drop in the implant screw of C3 is at most 36.8 °C. In this case, it is concluded that the C3 model is more successful than the other models

It's important to note that while numerical analysis provides valuable insights, it should be complemented by experimental studies and clinical observations to ensure the accuracy and applicability of the findings in real-world scenarios.