Biophysics -101 I5G Chapter 3: Heat Transfer in Teeth PDF

# Chapter 3: Heat Transfer in Teeth ## Introduction - Human teeth consist of hard components (enamel, dentine, cementum) and soft pulp tissue and sensory fibres. - Human teeth are sensory tissue with pulp tissue, a soft connective tissue that contains nerve fibres and nerve endings that extend int...

# Chapter 3: Heat Transfer in Teeth ## Introduction - Human teeth consist of hard components (enamel, dentine, cementum) and soft pulp tissue and sensory fibres. - Human teeth are sensory tissue with pulp tissue, a soft connective tissue that contains nerve fibres and nerve endings that extend into the dentinal tubules. - These pulpal nerve terminals are crucial in sensing thermal stimuli. - Although heat transfer in human teeth is a common occurrence in both daily life and clinical dentistry, there is a lack of knowledge regarding the actual amount of heat transfer that takes place during dental procedures. - This is important as trauma must be limited to a stressed pulp, where the accumulation of thermal, microbial, chemical, and mechanical factors can compromise its vitality. ## Heat generation - There are various stages during dental treatment that generate heat affecting the intrapulpal temperature: - Cutting of the tooth structure by high-speed dental handpieces (HSDH). - Exothermic reactions during the polymerization of light or self-cured restorative materials. - During the polishing step. - However, little is known about the effect of various factors which can increase the intrapulpal temperature. - Measuring the intrapulpal temperature in human patients would be unethical and infeasible. - Previous studies have adopted in vitro simulation models to conduct research on changes in intrapulpal temperature. ## Comprehensive Understanding of Heat Generation - This section aims to provide a comprehensive understanding of the heat generation during dental treatments affecting intrapulpal temperatures. - To address the issue, the following will be discussed: 1. The human tooth structure and the mechanism of heat transfer of enamel and dentine. 2. Factors affecting intrapulpal temperature during tooth preparation (cutting), crown fabrication, light curing and polishing. 3. In vitro and in vivo methodologies used to study the intrapulpal temperature, along with opportunities and challenges. ## Vulnerability to Heat Exposure - Regardless, (the pulp is still vulnerable to impairment, particularly to heat exposure during tooth preparation and extensive restorative procedures. - Pulp insults are mainly the result of heat changes, desiccation, exposure to chemicals, and bacterial infection. - The normal intrapulpal baseline temperature appears to range between 34 and 35°C, with increases in intrapulpal temperature exceeding 42 to 42.5°C sufficient to cause irreversible damage. - Since an increase in intrapulpal temperature does not necessarily produce an increase in pulpal blood flow, the pulp is highly susceptible to damage. - Consequently, for the pulp which may already be dealing with the effect of thermal changes from tooth preparation, any previous inflammatory changes and limited perfusion may lead to the potential loss of pulpal vitality. ## Irreversible Biological Effects - In vitro studies show that irreversible biological effects result when intrapulpal temperature increases by more than 5.5°C (that is, the intrapulpal temperature exceeds 42.4°C). - It was found that 15% of the experimental teeth developed irreversible pulpitis or necrosis when this temperature was reached. ## Mechanism of Thermal Insult to a Human Tooth - When heat is transferred to the pulp, it can cause various histopathological changes which may lead to irreversible injury. - The mechanism of injury includes protoplasm coagulation, expansion of the liquid in the dentinal tubules, increased outwards flow from the tubules, vascular injuries, and tissue necrosis. - Because of the variance in thermosphysical properties and microstructure between the layers in human teeth, heat transfer may also result in thermal stresses that lead to cracking within the different layers. - An intrapulpal temperature rise above 43°C activates nerve fibres, leading to a reactive increase of blood circulation which assists in the dissipation of any heat advancing towards the dental pulp. ## Role of Periodontal Tissues in Thermal Control - Additionally, the surrounding periodontal tissues could also play a significant role in promoting heat convection, thus limiting the intrapulpal temperature rise. - Perfused blood serves as a heat sink under heating and as a source of heat when subjected to cooling. - However, the overall influence of pulpal blood flow on heat transfer is thought to be minimal due to its relatively low blood volume. ## Tooth Heat Transfer - The relatively low values for thermal conductivity (TC) and diffusivity of enamel and dentine help protect the deeper tissues from thermal insults. - The characteristic arrangements of the inner structures of teeth have a significant influence on heat excursion in teeth. - More attention is given to dentine since it is often the layer in direct contact with provisional materials and the layer likely to be involved in the heat transfer that takes place from the surface of the tooth preparation to the pulp chamber. - Enamel and dentine are hard components with a high percentage of mineral content, but their thermoplastic properties are different. - TC indicates the ability of a material to conduct heat, and the TD is the measure of the speed with which a temperature change will proceed through an object. - The TC and TD of enamel are larger than dentine, respectively. - The TC and TD of both enamel and dentine are relatively low compared to those of the pulp; therefore, these two layers are effectively thermal insulators and protect the pulp from harmful thermal irritation. ## Thermal Effects on Bones and Teeth - When the bone experiences higher temperatures or prolonged exposure to heat, it first darkens and becomes charred. - This is followed by the calcination of the bone, which renders it white since the organic content has been lost. - Teeth are generally assumed to go through these similar changes, especially since they do discolor in a manner similar to bones. - The makeup of the dental hard tissues is different from bones, and the reactions to the effects of heating should also be different. - Enamel has a lower organic content than bones. - Enamel has a different fundamental organization (rods rather than osteons), which may yield a unique pattern of heat-related alteration. - Some studies of burned teeth show that the progression in dental color change as a result of thermal exposure was similar to bones. - The teeth first turn black or brown. - These colors then give way to blues and grays which are eventually replaced with white. ## Modern Dental Practices - Modern dental practices have incorporated lasers as a tool in addition to conventional treating procedures which generate heat. ## Categories of Heat Transfer in Teeth - The discussion of heat transfer in teeth will be divided into five categories: 1. The tooth morphology and properties. 2. The normal thermal response and the extent of possible damage. 3. The effects of restorative processes or materials. 4. The preventive applications in dentistry, including the use of lasers. - Almost all of the heat transfer processes in teeth take place in the conductive mode.