Friday, October 28, 2011

OBSERVATIONS ON DENTAL PROSTHESES AND RESTORATIONS SUBJECTED TO HIGH TEMPERATURES

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To improve the human identification process possibilities by the aid of forensic odontology. Particularly we considered to carry out an experimental study to learn more about the changes that dental remains, restorative materials and prosthetic devices undergo when exposed to very high temperatures, defining their behaviour and morphology with the aim to edit a reference table.


In large scale disasters associated with fire the damage caused by heat can make medico-legal identification of human remains difficult and as a result teeth, restorations and prostheses which are resistant to even very high temperatures can be resorted to.


This in vitro study present the behaviour and morphology of sound and filled teeth and dental prostheses exposed to a range of high temperatures.


Various specimens were used for the study (1) one healthy, unrestored tooth, () one molar with one previously class I restoration in amalgam and one added class V restoration in amalgam (specifically carried out for the research), () one premolar with a class V restoration of the vestibular surface in composite, and one with a class V restoration of the lingual surface with compomer, (4) one fixed prosthetic crown or bridge of aesthetic resinous material (polycarbonate, acrylic based products or composite materials), (5) one fixed prosthetic crown or bridge of metal alloy covered with aesthetic resinous material (acrylic based products or composite materials), (6) one removable partial prosthesis made of base metal casting alloys and denture base acrylic resins, and (7) one fixed prosthetic crown or bridge in metal-feldspatic ceramic dental systems.


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The tests of exposure to heat were carried out in a oven, and the specimens were heated to one of the six pre-established temperatures 00, 400, 600, 800, 1000 and 1100°C at a rate of increase of 0°C/minute. As soon as each target heat had been reached the samples were removed from the oven and allowed to cool to room temperature.


Two complete sets of samples (groups 1 to 7) were each subjected to each temperature and samples were examined macroscopically and then observed by stereomicroscopy and SEM.


The results table was edited reporting the macroscopic and microscopic findings for each specimen related to the different temperature levels.


Our experiments showed that dental tissues, prosthetic devices and restorative materials undergo a range of changes which correlate well with the various temperatures to which they were exposed. These changes are a consequence of the nature of the materials and their physicochemical characteristics, but individual components can remain recognisable and identifiable even at very high temperatures. For example, at 1100 °C it was possible to recover and identify residues of amalgam restorations while the prostheses in metal-porcelain contained residues of cement (which obviously had not been directly exposed to heat since it was protected by both the porcelain and the alloy). At the same temperature the teeth were well recognisable and not completely destroyed thanks to their mineralised structure. Our experiments did not take into account possible factors present in real-life circumstances, i.e. the protection afforded by soft and hard tissues surrounding the dental components and/or devices, nor any other externally worn items. For example, the root of a tooth should be even more resistant to thermal insults since it is sheltered within the bone. These in vivo circumstances prevent direct exposure to fire which otherwise always cause violent evaporation of organic components with consequent explosion of the crown. This phenomenon occurred in our experiments starting from a temperature of 800 °C. Incineration of soft tissues and any other organic material can produce a metallic-coloured layer covering the teeth which can modify the real colour change. It is therefore very important to carry out SEM and stereomicroscopic analyses in order to identify the real presence of restorative materials, particularly when only fragments of the teeth remain available for analysis, for example the occasional finding of an endodontic filling material in a premolar repaired with composite and compomer after complete shattering of the crown was observed by the stereomicroscope. Furthermore, amalgam releases mercury vapour which reacts easily in the presence of gold or alloys, further causing colour changes to the tooth; this occurred in our experiments starting at a temperature of 400 °C. Only at a temperature of 00 °C the teeth did not show signs of fractures, whereas as the temperature rose cracks, fissures and fragmentation of both the crown and the root occurred, although in two cases, at 600 and 800 °C, the teeth fractured when handled. This highlights two important points first, that calcined teeth, being completely dehydrated, are very delicate, and secondly, that fractures may precede the fire because in real-life situations trauma is often associated with the high temperatures caused by major fires. In our experiments, once the desired temperature had been reached, the specimens were removed from the oven and left to cool, thus all the materials were exposed to a single, brief, thermal insult. In reality various factors can further modify recovered remains the duration of the exposure to fire, the way in which the fire develops, the rate of increase of temperature, and substances used to extinguish the fire.


From our experiments we conclude that (1) some prosthetic devices and restorative materials seem resistant to temperatures higher than those theoretically predicted; () when a restorative material is lost because of detachment or change of state, its prior presence can be detected by both SEM and stereomicroscopy of the surface morphology of the remaining cavity; () it seems possible to reach a reasonably reliable estimation of the temperature of exposure from an analysis of the teeth and restorative materials.





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