Does thermal injury affect teeth during dentistry?
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2017-01-16 |
| Journal | Equine Veterinary Education |
| Authors | R. J. Pascoe |
| Institutions | Bell Equine Veterinary Clinic |
| Citations | 1 |
Abstract
Section titled “Abstract”The use of motorised dental equipment has become commonplace in equine dentistry. The risk of thermal trauma from their use will be evaluated using current evidence. The search terms ‘equine dental thermal’, ‘equine teeth thermal’, ‘equine teeth heat’ and ‘equine dental heat’ were used to search for relevant literature in the Medline database via the unbound Medline app for iOS devices and using Google Scholar. The literature searches performed were screened for relevance to the question ‘Does thermal injury affect teeth during dentistry?’ The searches for ‘equine dental thermal’ provided in vitro theoretical research from Allen et al. (2004), Wilson and Walsh (2005), and O’Leary et al. (2013). A search for ‘equine dental heat’ resulted in further in vitro research from Haeussler et al. (2014). Finally, a research paper by Haeussler et al. (2013) was revealed by the search ‘equine teeth heat’ on Google Scholar. No clinical case studies could be found, however, the previous searches revealed related papers that detailed or discuss concerns related to thermal trauma (Dixon et al. 2008; White and Dixon 2010; Marshall et al. 2012; Earley and Rawlinson 2013). Further secondary searches of references contained in the previous located papers found that Zach and Cohen (1965) was cited in all. Secondary sources used were Dixon and du Toit (2011). Zach and Cohen (1965) studied the effects of externally applied heat to teeth in Macaque monkeys. This paper found that temperature rises of <2.2°C did not produce histological evidence of damage to the pulpal tissues. Once temperature rises of >5.5°C were seen these resulted in pulpal necrosis through protein denaturisation in 15% of teeth. The 5.5°C increase is suggested in Allen et al. (2004), Wilson and Walsh (2005), O’Leary et al. (2013), and Haeussler et al. (2013, 2014) to be a threshold where pulp damage becomes likely and therefore should be avoided. Although it is reasonable to assume that pulpal tissues in mammalian species may respond in similar fashion to thermal insult, it is unclear whether anatomical differences in hypsodont teeth when compared with brachydont teeth influence the outcome of such insults in a positive or negative manner. Haeussler et al. (2014) suggested that thermal insults in the hypsodont tooth may be of greater significance, as the outer layer of the pulp is comprised of a single layer of odontoblasts whose responsibility is the production of secondary and tertiary (reparative) dentine in response to occlusal stimuli. Damage or disruption might lead to a decreased ability to produce dentine, thereby resulting in pulp exposure from the continued wear. All the in vitro studies showed that increases in the pulpal temperature could be produced through the use of motorised instruments. Allen et al. (2004) measured temperature increases with thermocouples placed 15 and 25 mm from the occlusal surface. They compared the application of a rotary disc instrument with a carbide blade to the occlusal surface of mandibular teeth for periods of 1 min uncooled, 2 min uncooled and 2 min with water-cooling. For the thermocouple placed 15 mm from the surface, the mean peak increase of 6.6°C was seen after 1 min of grinding with the peak temperature increase being seen sometime after the cessation of work. In the group treated for 2 min, this site recorded mean increases of 24.3°C. Where continuous water-cooling was used, the temperature increase was −0.1°C (with a range of −1.2 to +2.4°C). The length of time used in this particular study might be considered unrealistic in the clinical scenario but larger overgrowths being corrected may require reduction of prolonged periods even with breaks between, and the continued rise in temperature after instrument use is of concern when reduction is performed in this manner. Wilson and Walsh (2005) placed thermocouples within the pulp chamber as close as possible to the occlusal extent of the pulp horns of maxillary cheek teeth and coupled to the dentine using a heat-sinking compound. In this study, they compared two instruments, a rotary disk instrument with a tungsten chip and an axial tungsten carbide burr. These instruments were used for periods of 15 and 20 s on teeth taken from two horses at post-mortem. In the younger horse, higher temperature increases were seen and, in those teeth rasped for 20 s, rises of 4.5°C were seen, close to the threshold for pulp damage. Of greatest concern in this study was the length of time it took for teeth to cool back to normal temperature, which was between 5 min for the younger horse and 10 min for the more mature horse. Intermittent water-cooling reduced temperature increases and continuous water-cooling prevented any increase in the temperatures recorded. O’Leary et al. (2013) compared three diamond coated disk instruments when used on maxillary cheek teeth. The instruments were used in intervals of 30, 45, 60 and 90 s and results were compared with use of intermittent water-cooling, continuous water-cooling and against secondary dentine depth. The increased duration of use was found to increase the likelihood of critical temperatures being reached by 7.3, 8.9 and 24.3 times, respectively. Water-cooling following reduction was found to be protective as was increased secondary dentine depth, and continuous water-cooling was shown to eliminate temperature increases. One particular instrument with a larger diameter diamond disk was found to increase the likelihood of reaching critical temperatures by 8-fold, although this instrument was found to remove dental material twice as fast as the other instruments used in the study. In teeth that reached critical temperatures there was a mean of 5 mm of dental material removed, whereas in teeth where critical temperatures were not reached, the mean was 3 mm. It was suggested that maximum amounts of material to be removed were 3-4 mm, although it was acknowledged that there was considerable variation in these results within each group. Of concern was that 20% of teeth that were ground for 30 s exceeded the critical temperature threshold in the absence of water-cooling. Haeussler et al. (2013) conducted a study looking at the thermal conductivity of equine cheek teeth. Measurements confirmed that the distance of the pulp horn from the occlusal surface was a significant factor for heat conduction within the tooth. Position of the pulp within the tooth was not significant in influencing the time span for heating of the pulp. When considering the variability of the depth of secondary dentine found by White and Dixon (2010) and Marshall et al. (2012) the risk of thermal trauma will be hard to predict without 3D diagnostic imaging to ascertain the occlusal surface to pulp distance, something that is impractical in the clinical situation. Haeussler et al. (2014) looked at both rotational speed and head position as factors in temperature increases in maxillary and mandibular teeth. In this study the thermocouples were placed a uniform 5 mm from the occlusal surface to reduce variability in results created by individual anatomy. Grinding directly over pulp horns resulted in a mean time to increase by 5.5°C of 143 s and an average cooling period of 356 s. A key finding was an increase in pulp temperature of 1°C continued after the cessation of grinding, implying that the heat transmission continues with the dental tissues acting like a heat sink and repeated grinding episodes could result in incremental increases in temperature. This study also tried to replicate the use of instruments in routine rasping of buccal and lingual points. In contrast to other studies, they measured the time to reach critical temperatures. Doubling rotational speed resulted in reduction in time to reach critical temperatures by 52% in maxillary and 78% in mandibular teeth suggesting differing responses to heating effects of motorised instruments, possibly reflecting their relative mass. Doubling the rotation speed halved the time to remove dental material. This suggests that, particularly for mandibular teeth, lower rotational speeds should be used. Allen et al. (2004), Wilson and Walsh (2005), and Haeussler et al. (2013) all highlight the possibility that pulpal blood flow and the oral environment (periodontium, gingiva, saliva) could influence the risk associated with thermal trauma. However, secondary papers highlighted by Haeussler et al. (2013) (Pohto and Scheinin 1958; Raab 1991) suggest that the contribution to heat removal in teeth in brachydont species is minimal. This is logical since the enamel and dentine do not contain vasculature that might assist with cooling (Dixon and du Toit 2011). There were no papers documenting direct cases where thermal trauma had resulted in clinical problems, although concerns arising from the problem were mentioned in a number of papers (Dixon et al. 2008; White and Dixon 2010; Marshall et al. 2012), in relation to diastema widening procedures and studies on the depth of secondary dentine. Earley and Rawlinson (2013) discussed the ramifications of thermal trauma in relation to treatment of incisor and canine teeth; however, they relied on evidence drawn from O’Leary et al. (2013) and, given the differences seen between maxillary and mandibular teeth reported by Haeussler et al. (2014), it is difficult to support the implementation of this when applied to small teeth with different anatomy. There were no other studies discussing thermal trauma in relation to incisors or canines. As O’Leary et al. (2013), Wilson and Walsh (2005), and Haeussler et al. (2013) all showed increased protection from secondary dentine depth, the variation seen in secondary dentine depth documented by White and Dixon (2010) and Marshall et al. (2012) increases the concern that thermal insults to the pulp may arise. The lack of clinical evidence is difficult to interpret, and it is suggested by Haeussler et al. (2013) that odontoblasts respond producing tertiary dentine when exposed to noxious stimuli. This may allow teeth to manage thermal insults in many cases or simply delay the onset of clinical signs such that links between the two are difficult to prove definitively. The research clearly indicates that there is a likelihood of uncooled motorised instruments causing significant heating of dental tissues during their use. The consequences of this heating remain unestablished in the clinical situation, although it is clear that temperature increases to levels likely to cause direct thermal trauma to the pulp in other species are possible with inappropriate use. Finally, using water-cooled instruments can eliminate temperature increases. No conflicts of interests have been declared.
Tech Support
Section titled “Tech Support”Original Source
Section titled “Original Source”References
Section titled “References”- 2013 - A reliable measuring method for heat transfer in equine cheek teeth
- 1991 - Vitalmikroskopische Untersuchungen zur Mikrozirkulation der Zahnpulpa