This study aimed to evaluate the action of cold and heat on the surface roughness and Knoop microhardness of dental restorative materials present in the teeth of victims of freezing or carbonization. The authors started from the null hypothesis that there would be no difference in the properties studied in each material, irrespective of low or high temperatures and periods to which they were subjected. The results demonstrated that the hypothesis tested could be partially accepted, because the heat promoted significant changes (p < .05) in the surface roughness of GIC and amalgam, and in the microhardness of GIC; however, the action of the cold caused no significant change (p > .05) in the properties of any of the restorative materials studied.
For the exposure to the cold, the temperatures of 2.5 °C, − 20 °C, and − 80 °C were selected, due to the possibility of precisely maintaining these temperatures using refrigerators. The 7- and 30-day periods correspond to the short and the longer time intervals that bodies can be found under conditions of extreme cold.
The analyses of the changes in the surface roughness and microhardness, in different periods, could contribute to estimating the period between death and the necroscopic exam of the victim’s mouth. The elucidation of the time of death may be essential for establishing the medical and legal causa mortis (Huntington et al. 2007) and also to present evidence of whether the corpse suffered antemortem or post-mortem lesions or displacements, thus contributing to the experts’ investigations.
Estimating the time of death by the analysis of the cooling and rigidity of the corpse and the emergence of hypostatic stains is not possible in cases of late occurrence of the corpses (Levy et al. 2010). However, the presence of dental remnants may confirm the analysis of the roughness and hardness of the restorative material as a practical and accessible chronological thanatology technique and not only as an auxiliary identification method.
For the exposure to the heat, a pilot study indicated to establish, as analysis parameters, the temperatures of 100 °C, 200 °C, and 300 °C, because up to 300 °C the three materials selected could still have the changes in surface roughness and microhardness analyzed. Above this temperature, there was catastrophic damage to the structure of the materials.
For the temperature to reach 300 °C inside the mouth, it would be necessary for the fires to provide ambient temperatures around 800 °C to 1000 °C for extended periods, such as 60 min or longer (Marella et al. 2012). For this study, the period of 15 min was selected for the exposure to the heat, following a previously described methodology (Patidar et al. 2010; Pol et al. 2015), in which the authors concluded that the changes that occurred in the materials after 60 min were no different from those that occurred after they were exposed to the heat for 15 min.
Bovine teeth were selected as the substrate for performing the cavity preparations and the restorative procedures because they are more easily obtainable than human teeth. Moreover, when in non-carious conditions, they do not generate any harm to the tests proposed, since the aim of this study was to evaluate the changes that occurred in the materials and not specifically in the dental structures.
Bovine teeth make it easier to standardize the samples, with a lower risk of infection, besides the bioethical issues (Wang et al. 2012; Zhang et al. 2013). They present similarities to the human dental tissues, particularly concerning the orientation of the enamel prisms, an equivalent percentage by weight of calcium, and protein matrix composed of the same amino acids (Wang et al. 2012). As regards dentin, there are some controversies relative to the similarity between the teeth of both species. However, there is a consensus in the literature that in shallow cavities, with a depth of 2 mm, bovine dentin has been shown to be feasible for adhesion, providing adequate bond strength when compared with human dentin (Zhang et al. 2013).
The analysis of the surface roughness in the CR restorations possibly found in a post-mortem examination would not confirm whether the victim suffered the action of the heat, in cases in which the report on the external condition of the corpse did not reveal this situation. This is because the heat did not cause sufficient changes (p > .05) in its surface roughness. In GIC restorations, high surface roughness suggests exposure of the body to the heat. The dryness of GIC due to evaporation and consequent loss of water (syneresis), exposing its filler particles and leading to the appearance of small cracks, indicates that the temperature of exposure was higher than 300 °C.
The surface roughness of CR, on an average, was always lower than that of GIC, corroborating the results of Liberman and Geiger 1994, de Araújo et al. 1998, according to whom GICs are rougher because of the composition of the material itself. Relative to particle size, CRs have smaller inorganic particles, while conventional GICs have larger particles (Gladys et al. 1997). This leads to more significant irregularities in the surface of GIC.
Teeth restored with amalgam, in which the material shows high values of surface roughness, suggest exposure of the victim’s mouth to temperatures over 300 °C. At this temperature, the formation of bubbles occurs resulting from evaporation of the mercury present in its composition (Patidar et al. 2010), which generates increases in its surface roughness.
In the cold tests, the low temperatures, up to − 80 °C, did not structurally modify any of the materials to the point of changing their roughness. Thus, analyzing the surface roughness by itself, it is not possible to affirm whether or not the materials suffer the action of cold, or to which temperature they were subjected, up to − 80 °C, even if they were exposed for prolonged periods of up to 30 days.
However, discrimination could be made between CR and GIC using the surface roughness analysis, because they follow different patterns, irrespective of the action of the cold or heat. As the exposure temperature increased up to 300 °C, the heat only made the distinction more evident due to the changes that occurred in GIC.
Analysis of the microhardness of a restorative material is commonly related to its mechanical strength and degree of conversion. The advantage of the study of the Knoop microhardness is the possibility of finding a correlation between the hardness of different materials (Ferracane 1985), since amalgam, differently from CR and GIC, is considered a friable material. It does not only deform when subjected to stress but it also fractures (Willems et al. 1993).
When exposed to the heat, the CR did not undergo significant changes (p > .05) in its Knoop microhardness. So, it is not possible to define whether the CR was exposed to the heat, or to which temperature it was subjected, up to 300 °C, analyzing this property by itself; these results are similar to those found by Basting et al. 2002. However, the heat could lead to the degradation of CR due to combustion and volatilization of the organic components with consequent loss of mass (Dionysopoulos and Watts 1989).
The loss, in mass, of the organic matrix and the resultant increase in the inorganic portion due to this loss may lead to increase in microhardness (Dionysopoulos and Watts 1989). The heat would also be an additional polymerization factor, which would increase the degree of conversion of the CR, providing the matrix with a more homogeneous and resistant structure, improving its mechanical properties (Loza-Herrero et al. 1998), including microhardness.
In GIC, a significant increase (p < .05) in its microhardness was verified, and this was more intense the higher the exposure temperature, up to 300 °C. At 100 °C, the microhardness increased by approximately five times, compared with the mean value before the action of the heat. At 200 °C, the elevation was even more significant, attaining an increase of around 20 times the value of the initial mean. When the materials were exposed to 300 °C, the rate of increase in microhardness was maintained at a very high level and was a little higher compared with the increase at 200 °C.
It is possible to distinguish the tooth-colored materials based on the elevation of the microhardness of GIC by itself, at 200 °C and 300 °C. The action of the heat leads to a loss of water and reduction in the matrix of GIC, which causes a proportional increase in the percentage of filler particles. Studies (Okada et al. 2001; Cattani-Lorente 1994) have observed an increase in some mechanical properties, such as the hardness, of the GIC due to drying of this material.
Some authors (Okada et al. 2001; Cattani-Lorente 1994; Mojon et al. 1996) have affirmed that an increase in the microhardness values of the GICs usually occurs with the passage of time. This increase is probably related to the acid-base reaction that occurs in a more slowly and continuously way, in which protons attack and degrade the structure of the aluminosilicate glass, releasing calcium, strontium and aluminum ions that react with the carboxylic groups (Xie et al. 2000).
However, in the tests conducted at low temperatures in this study, even after 30 days of exposure, no significant change (p > .05) in the microhardness occurred in any of the materials studied, including GIC. Perhaps 30 days period has been too short for these changes to occur, or the action of the cold may have retarded the acid-base reaction of GIC. In spite of this, the discrimination between the tooth-colored materials can be made by the analysis of this property by itself, because the hardness pattern of the CR is quite distinguished from that one of the GIC, a fact that may be related to the type and quantity of inorganic particles by volume in its composition.
Concerning the amalgam, as there was no significant change (p > .05) in its microhardness due to the heat, it is not possible to determine whether amalgam was exposed to the heat or to which temperature, up to 300 °C, by the analysis of this property by itself. Researchers (Willems et al. 1993; Patsurakos and Moberg 1990) have reported that the microhardness of the amalgam is a time-dependent property, probably due to the lower content of mercury, that evaporates over the course of time, and also to the more significant quantity of crystallization reactions, that leads to more γ phases than γ1 and γ2 phases; these latter two present lower hardness values than the phase γ (Patsurakos and Moberg 1998).
However, in the tests conducted at low temperatures in this study, even after 30 days of exposure, no relevant change occurred in the microhardness of the amalgam. Perhaps the 30 days period has been insufficient for the occurrence of these changes. However, as this study is related to forensic purposes, such period could be considered extensive concerning the permanence or the find of a human body or bones in environmental conditions of extreme cold.
In the microhardness analysis, high standard deviation was verified in the readouts of the three materials evaluated, both in the heat and cold tests. For the tooth-colored materials, this may have occurred because there is the possibility of the penetrator of the microhardness tester to hit the harder filler particles or even the matrix, at the moment in which it presses against the material, leading to a significant difference in the microhardness values for the same restoration. In the heat tests, another limiting factor relative to the GIC and adverse for the microhardness evaluation was the appearance of microcracks in the restoration that occurred after heating to 300 °C. When putting pressure on the restoration surface, the penetrator may make the mark precisely on one of these cracks, or in their proximities, and cause small movement of a portion of the restoration, which would undoubtedly influence the microhardness evaluation.
Concerning the amalgam, the same could occur with the crystals of phase γ1 and phase γ2, being the first more resistant than the latter. Thus, the penetrator tip can hit one phase or the other, leading to a significant standard deviation.
