# Temperature Changes in Composite Materials during Photopolymerization
## 1 Introduction  
Composite materials are widely used in every dental practice. As the result of many years of technological development and improvements, as well as changes in their composition, composite materials are very durable, while being esthetically pleasing and popular among dentits. At the same time, numerous manufacturers have introduced increasingly advanced units featuring modifying polymerization programs and power adjustments, so as to ensure the best mechanical performance of composite materials. However, many dentits fail to use polymerization units in the correct manner, by focusing mainly on the anatomical representation and the esthetics of fillings. In the survey carried out by [PERSON] et al., almost one third of dentits failed to use proper eye protection against blue light while up to 78.3% of respondents were unaware of the irradiance values of the polymerization lamp they used [1]. Many respondents did not check the quality of the light produced by the polymerization unit. Other studies have shown that preclinical dental students and dentits in their internship years use polymerization lamps in an incorrect manner, not delivering the required amount of energy to the composite layer. Following a briefing on the use of polymerization tips, the number of study subjects who failed to deliver the minimum required energy to the filling was significantly reduced [2]. Many dentists do not analyze polymerization unit power and modes before purchasing, study instructions of dental composite polymerization protocols, or obey polymerization procedures. This is an essential element, which can affect the final treatment result. As materials science advanced, more and more dental practitioners abandoned the use of base layers, even in deep caries; combined with the increased lamp powers and changes in composite material compositions, this might lead to increased temperatures being achieved during polymerization, and leading to pulp damage. The critical value for pulp damage (temperature increased by 5.5 \({}^{\circ}\)C) was first reported by [PERSON] and [PERSON] in 1965 on the basis of a study conducted in five adult rhesus macaque monkeys. The authors demonstrated necrosis of 15% of the tissue as the pulp temperature was increased by 5.5 \({}^{\circ}\)C [3]. However, not all studies confirmed this value as critical for pulp damage. [PERSON] et al. observed no histological changes within the pulp as its temperature increased by 5.5 \({}^{\circ}\)C [4]. In the study by [PERSON] et al., a 60-s exposure led to the highest increase in pulp temperatures, exceeding 5.5 \({}^{\circ}\)C for certain teeth. However, the authors noted that the critical temperature of pulp damage leading to potential necrosis had been determined by testing the teeth of monkeys rather than humans [5]. There are various methods of protecting the pulp against thermal injury: cooling with water during preparation or using liners, such as calcium hydroxide, mineral trioxide aggregate (MTA), or glass-ionomer cements (GIC) [6; 7; 8]. Polymerization lamp choice is essential to avoid pulp overheating. On the other hand, the lamps emitting lower levels of energy can disturb the process of polymerization, thus disabling the acquisition of optimal mechanical parameters, including the tensile strength and the flexural strength. Since the tensile strength of composites is much lower than their compressive strength, and since tensile strength is typically much more affected by internal flaws, this property is likely the most appropriate test of strength. However, it is usually substituted by measurements of flexural strength as a potentially simpler testing method, well-related to tensile failure. Flexure testing is the standard means for the strength testing of dental composites (ISO 4049), and has been shown to correlate with material wear in some studies [9]. Manufacturers keep upgrading their composite materials using different resins or fillers. [PERSON] et al.