Pulpar temperature changes during mechanical reduction of equine cheek teeth: Comparison of different motorised dental instruments, duration of treatments and use of water cooling

Authors


Correspondence email: lears01@gmail.com;

Summary

Reasons for performing study

Although equine motorised dental instruments are widely used, there is limited information on their thermal effect on teeth. The recently described variation in subocclusal secondary dentine depth overlying individual pulp horns may affect heat transmission to the underlying pulps.

Objectives

This study compared the effect of 3 different equine motorised dental instruments on the pulpar temperature of equine cheek teeth with and without the use of water cooling. It also evaluated the effect of subocclusal secondary dentine thickness on pulpar temperature changes.

Methods

A thermocouple probe was inserted into the pulp horns of 188 transversely sectioned maxillary cheek teeth with its tip lying subocclusally. Pulpar temperature changes were recorded during and following the continuous use of 3 different equine motorised dental instruments (A, B and C) for sequential time periods, with and without the use of water cooling.

Results

Using motorised dental instrument B compared with either A or C increased the likelihood that the critical temperature was reached in pulps by 8.6 times. Compared with rasping for 30 s, rasping for 45, 60 and 90 s increased the likelihood that the critical temperature would be reached in pulps by 7.3, 8.9 and 24.7 times, respectively. Thicker subocclusal secondary dentine (odds ratio [OR] = 0.75/mm) and water cooling (OR = 0.14) were both protective against the likelihood of the pulp reaching the critical temperature.

Conclusions

Prolonged rasping with motorised dental instruments increased the likelihood that a pulp would be heated above the critical temperature. Increased dentinal thickness and water cooling had protective roles in reducing pulpar heating.

Potential relevance

Motorised dental instruments have the potential to seriously damage equine pulp if used inappropriately. Higher speed motorised dental instruments should be used for less time and teeth should be water cooled during or immediately after instrument use to reduce the risk of thermal pulpar damage.

Introduction

Motorised dental instruments are widely used in equine dentistry. Whilst it is widely accepted that there is a risk of pulpar injury or even necrosis from excessive thermal insult to teeth when using motorised dental instruments, there is limited information on the effect of motorised dental instrument use on inducing temperature changes within the underlying pulp [1, 2].

During dental rasping, energy not used in the cutting process is mainly transformed into heat. The amount of heat transmitted to the tooth depends on the rasping time, cooling technique (if used), type of burr, physical characteristics of the grinding surface and speed and torque of the instrument used [3]. These properties can vary widely between commercially available equine motorised dental instruments. Additionally, most motorised dental instruments do not have water cooling systems. The current recommendation for use of motorised dental instruments that do not have cooling systems is to intermittently apply water using a dental syringe when there is a pause in rasping or when it is finished (Operator manual, Powerfloat)a, which may be less effective than continuous water cooling during motorised dental instrument use. There have been limited comparisons of the degree of pulpar heating produced by different commercially available motorised dental instruments but one study found a significant difference between heat production of an air driven die-grinder with a tungsten-carbide burr and an electrical driven tungsten chip disc [2].

The level of thermal change that can be tolerated by vital dental pulp has been defined by in vivo animal studies [4]. In Macau monkeys, reversible histological changes were noted when pulpar temperature increased by 3.3°C; an increase of >5.5°C caused pulpar necrosis in 15% of teeth and increase of >16.7°C resulted in pulpar necrosis in all teeth [4]. Accordingly, a 5.5°C temperature increase is widely accepted as the ‘critical temperature threshold’ value when assessing thermal pulpar injuries during heat generating dental procedures.

A previous study that evaluated pulpar heating in equine maxillary cheek teeth measured temperature changes at a predetermined depth below the occlusal surface of the tooth [1]. However, recent anatomical studies have shown a wide individual variation in depth of subocclusal secondary dentine in equine cheek teeth [5-8]. This subocclusal secondary dentine variation may potentially affect the results of studies investigating the heating effect of motorised dental instruments.

Aims

This study compared the effects of 3 different equine dental motorised dental instruments on the temperature of the occlusal aspect of pulp horns when used for a variety of time periods, with and without the use of water cooling, and also evaluated the effect of subocclusal secondary dentine thickness on this temperature increase.

The hypotheses for this study were:

  1. Pulpar temperature increase is associated with the individual motorised dental instrument used and the length of time it is used for.
  2. Pulpar temperature increase is inversely related to the depth of subocclusal secondary dentine overlying the pulp horns.
  3. The application of water applied in a continuous or intermittent fashion is protective against pulpar temperature increase.

Materials and methods

One hundred and eighty-eight grossly normal maxillary cheek teeth were extracted from 37 equine cadaver heads classified as small, medium or large. Ages of the skulls were estimated by incisor morphology [9-11] in 26 skulls (contributing 127 teeth) and from passport documentation in 11 skulls (contributing 61 teeth). The cheek teeth were transversely sectioned 40 mm beneath their occlusal surface with a 99-TS23OM water cooled tile saw with a 20.3 cm diameter, 0.81 mm thick continuous rim, diamond tipped bladeb. The teeth were secured in a clamp and were orientated to mimic their in vivo position, with their occlusal aspect positioned ventrally. A type K miniature (0.25 mm diameter) thermocouple probec with an operating range of -100–800°C linked to a hand held type K thermocouple digital indicatorc (range -50°C–999°C, accuracy ± 0.3% of reading) was inserted into either pulp horns 3, 4 or 6 [12] in a retrograde fashion until the tip of the thermocouple touched the occlusal aspect of the pulp horn (Fig 1).

Figure 1.

Experimental set-up: transversely sectioned cheek tooth with thermocouple placed into a pulp horn (passed to occlusal aspect of pulp horn) and attached to a thermocouple digital indicator. The tooth is held in a clamp with rubber padding to minimise conduction heat loss. The motorised dental instrument is applied at the occlusal aspect of the tooth.

A pilot study (n = 118 pulp horns) was performed to ascertain if the subocclusal secondary dentine depth could be estimated by measuring the external length of the sectioned tooth and subtracting from this the length of the thermocouple that could be inserted into the pulp horn (Fig 2). The teeth in the pilot study were then sectioned and subocclusal secondary dentine depths were measured using a digital caliper (Draper Expert 46610 0–150 mm dual reading digital Vernier Caliper)d. For 116 teeth, the amount of dental material removed by the motorised dental instrument was also calculated at the end of the experiment by measuring the height reduction of the treated tooth.

Figure 2.

Diagram showing pilot study measurements of estimated subocclusal secondary dentine (SO2D). A: external length of the sectioned tooth. B: length of thermocouple probe that can be inserted into the pulp horn (PH). Estimated depth of suboccusal secondary dentine (C) = A–B.

A minimum of 10 teeth were used for each of the 17 experimental protocols with 13 teeth used for 2 groups (A30, B60), 12 teeth used for 2 groups (A90, C60), 11 teeth used for 8 groups (A30W, A60W, A90W, B30, B30W, B45W, C30, C90) and 10 teeth used for the other 5 groups (A45, B60, C45, A45W, B45) (A, B, C correspond to motorised dental instrument used, see below for details, 30–90 correspond to time length in seconds instrument applied for, W corresponds to application of water at cessation of rasping).

Three commercially available equine dental motorised dental instruments were used. Instrument A (maximum 4000 rpm)a had a 26 mm diameter circular diamond coated disk burr, Instrument B (maximum 9000 rpm)e had a 30 mm diameter circular diamond coated disk burr and Instrument C (maximum 8300 rpm)f had a 25 mm diameter circular diamond coated disk burr. Motorised dental instruments were used by the same operator for all experiments, at maximal speed for periods of 30, 45, 60 and 90 s of continuous rasping. When pulpar temperatures were found to consistently rise above critical temperature for an experimental group, subsequent longer time periods of experimentation were not conducted for that particular motorised dental instrument.

Instruments A and B did not have an inbuilt water cooling system and ‘water cooled’ experiments for these motorised dental instruments consisted of the application of 60 ml of room temperature water immediately after the cessation of the rasping period using a syringe. Instrument C had an inbuilt water cooling mechanism that gave a continuous flow of water (approximately 50 ml water/min) over the tooth during rasping, Instrument C was therefore not evaluated without the use of water cooling. The pressure of application of the rasping disc could not be standardised but one operator completed all experiments and attempted to provide a standard amount of pressure.

All experiments were conducted at room temperature and temperature changes recorded from the digital thermocouple immediately before and then every 15 s during and after cessation of rasping for the first 5 min and then every 30 s until the temperature had returned to baseline or had stabilised for a minimum period of 5 min.

Data analysis

For the pilot study, comparison of estimated and actual subocclusal secondary dentine depth was evaluated using a Spearman's rank correlation test. Pulp horn number was evaluated for variability in measurements of subocclusal secondary dentine using a Mann–Whitney test. Statistical significance was set at (P<0.05).

Variables included in univariable and multivariable statistical analyses were: horse number, age, head size (small, medium or large), Triadan number of the tooth, pulp horn, number, subocclusal secondary dentine depth, environmental temperature, water application (Yes/No), water temperature, water applied continuously or with a dental syringe at the end of rasping, Instrument (A, B or C), time spent rasping and temperature changes at the occlusal aspect of the pulp before, during and after cessation of rasping. The following values were also calculated and included in the univariable and multivariable models: maximum increase in pulpar temperature (Tmax), mean Tmax for each experimental group, whether or not the pulpar temperature increased above the critical temperature (+ 5.5°C from baseline), time spent above the critical temperature threshold, the total amount of tooth reduction (mm) for each experiment.

Three outcomes were investigated using multivariable analysis:

  1. Maximum pulpar temperature achieved.
  2. Time spent above critical temperature.
  3. Did pulpar temperature rise above the critical temperature threshold of 5.5°C (Yes/No).

Multivariable linear regression models were used for the first 2 outcomes and a multivariable logistic regression model for the third outcome. Horse number was included as a random effect in all 3 models as some horses provided more than one tooth for testing. The association between instrument type and each outcome was investigated while accounting for other potential explanatory variables. Variables were retained in the final models if they significantly improved the fit of the model (likelihood ratio statistic <0.05). Continuous variables were manipulated and categorised to identify the best fitting form of each variable. Categorical variables were recategorised, where necessary, to produce the final model with the best fit.

Results

For the pilot study, good correlation was found between the actual and estimated subocclusal secondary dentine depth (Spearman's ρ = 0.98, P<0.01). The estimated subocclusal secondary dentine depth was used for all subsequent analyses.

There was a statistical difference found in subocclusal secondary dentine depth between the 3 different pulp horns evaluated: PH3 vs. PH4 (P = 0.012), PH4 vs. PH6 (P = 0.040), PH3 vs. PH6 (P = 0.006). Pulp horn 3 had a median subocclusal secondary dentine depth value of 12 mm, pulp horn 4 of 11 mm and pulp horn 6 of 8.5 mm. There was a significant but weak negative correlation between subocclusal secondary dentine depth and age (Spearman's ρ = -0.3225, P<0.01). The mean subocclusal secondary dentine depth for all (n = 188) teeth was 11.9 mm (range 3–25 mm). There was a significant difference in subocclusal secondary dentine depth for the following experimental groups that underwent rasping for the same time period; Instrument A for 30 s vs. Instrument C for 30 s (P<0.01), Instrument B for 30 s vs. Instrument C for 30 s (P<0.001), Instrument B for 45 s vs. Instrument B for 45 s with water (P<0.05), Instrument A for 60 s vs. Instrument B for 60 s (P<0.05) (Bonferroni multiple comparison test; P<0.05) (Fig 3).

Figure 3.

Box and whiskers plot showing range of subocclusal secondary dentine (SO2D) depth for teeth in each experimental protocol. A, B and C = motorised dental instruments; 30, 45, 60, 90 = time spent rasping in s; W = experiments performed with water cooling.

For the 116 teeth in which the amount of tooth removed was calculated, the mean amount of dental material removed/second is shown in Table 1. Instrument B removed significantly more dental material than Instruments A (P = 0.001) and C (P = 0.01). There was no difference in the rate of dental material removal between Instruments A and C (P>0.05).

Table 1. Mean rate of crown height reduction using 3 different equine motorised dental instruments
MDINo. teethMean mm of reduction/ss.d.
  1. aSignificant difference between A vs. B (P<0.001) and B vs. C (P<0.01).
  2. MDI = motorised dental instrument.
A400.06 mm/s0.009
Ba320.12 mm/s0.04
C440.07 mm/s0.005

Graphs showing the mean temperature change achieved for each group of teeth undergoing rasping for 30 and 45 s are shown in Figs 4 and 5, respectively. It should be noted that the subocclusal secondary dentine depths for these groups of teeth are not necessarily comparable and will have affected the raw data in terms of temperature change. Variability in subocclusal secondary dentine between experimental groups is shown in Figure 3.

Figure 4.

Mean pulpar temperature changes with motorised dental instruments (MDIs) A, B and C, rasping for 30 s (30) ± water application (W). Note that these figures have not been corrected to account for variable subocclusal secondary dentine between groups.

Figure 5.

Mean pulpar temperature changes with motorised dental instruments (MDIs) A, B and C, rasping for 45 s (45) ± water application (W). Note that these figures have not been corrected to account for variable subocclusal secondary dentine between groups.

Multivariable analysis

Results of the final multivariable linear regression model for outcomes 1 and 2 (including horse as a random effect) are shown in Table 2. Results of the final multivariable logistic regression model for outcome 3 (including horse as a random effect) are shown in Table 3.

Table 2. Multilevel multivariable linear regression models describing the association between all statistically significant explanatory variable and outcomes 1 (Tmax), and 2 (time spent above critical temperature)
Statistically significant explanatory variablesOutcome variable
1. Tmax (°C)2. Time spent above critical temperature (s)
Relative increase in Tmax (°C)P value95% CIRelative increase in time spent > critical temperature (s)P value95% CI
  1. CI = confidence interval; MDI = motorised dental instrument; SO2D = subocclusal secondary dentine.
MDI8.1<0.0015.3−10.895.6<0.00157.5–133.6
B (compared with A or C)
Time spent grinding (s)      
≥450.4<0.0010.2–0.6
(compared with 30)      
6058.50.00122.7–94.2
90143.5<0.00189.8–197.0
(compared with ≤45)      
Increase in SO2D depth (per mm)-0.7<0.001-1.0–-0.4-5.80.007-9.9–-1.6
Water used?      
Yes (compared with No)-111.3<0.001-157.0–-65.7
Table 3. Multilevel multivariable logistic regression model describing the association between several explanatory variables and the likelihood that pulpar temperature rose above the critical temperature
Explanatory variableOdds ratioP value95% confidence interval
  1. MDI = motorised dental instrument.
MDI
A or C1  
B8.60.0012.48–29.8
Time spent grinding (s)
301  
457.250.0012.22–23.7
608.880.0012.49–31.7
9024.7<0.0014.98–122.6
Increase in SO2D depth (per mm)0.75<0.0010.66–0.86
Water used?
No1  
Yes0.140.0010.05–0.44

Discussion

The results of this study proved our 3 hypotheses: Firstly, that pulpar temperature increase (in terms of all 3 outcome variables) is associated with the individual motorised dental instrument used and duration of its use. Secondly, that pulpar temperature increase (in terms of all 3 outcome variables) is inversely related to the depth of subocclusal secondary dentine overlying the pulp horns. Finally, that for Instruments A and B, the application of water during and after the cessation of rasping is protective against pulpar temperature increase (in terms of whether the pulp horn went over the critical temperature threshold and the length of time spent above this threshold).

This study is the first to account for individual differences in subocclusal secondary dentine depth between teeth and thus measure the effect of subocclusal secondary dentine depth on pulpar heating. A very fine thermocouple was used that was placed as close as possible to the occlusal aspect of pulp horns 3, 4, or 6. The (palatally situated) pulp horns 3 and 4 were selected as these were found to be the widest and are reported to extend further occlusally than the buccally situated pulp horns [13] and were therefore easiest to fit the thermocouple probe into. We acknowledge that for routine removal of sharp enamel points, the buccal pulp horns (1 and 2) of maxillary teeth are more likely to be closest to the motorised dental instrument burr. However, removal of enamel points should be achievable in a much shorter time length than the rasping periods used in this study. Rasping periods of >30 s are more applicable for reduction of more significant focal dental overgrowths, where multiple pulp horns are likely to be susceptible to thermal damage. Recent studies have shown that subocclusal secondary dentine thickness in overgrown teeth can be as low and as variable as normal values [5-8]. Marshall et al. suggest that due to this variability, large overgrowths should be gradually reduced in stages and that rasping for prolonged periods on a single tooth should be avoided [8].

Positioning the thermocouple probe in a retrograde fashion into the pulp horn ensured absence of direct contact between the thermocouple tip and the motorised dental instrument or the water applied to the occlusal aspect of the tooth, thus giving an accurate value of pulpar temperature changes. In contrast to the experimental design used previously by Allen et al. [1], our results showed a lag phase in pulpar temperature changes during the early stages of rasping and following cessation of rasping, similar to that reported by Wilson and Walsh [2] that was inversely related to the depth of subocclusal secondary dentine.

Our experimental design was limited by our inability to calibrate the pressure applied to the cheek teeth occlusal surface by the instruments. In order to minimise this variability, the experimental protocols were conducted by one clinician with the same force as would routinely be used on clinical cases. It would appear from our results that the cutting efficiency of an individual motorised dental instrument has a significant effect on its heat production. The cutting efficiency of any motorised dental instrument may be related to its maximal rpm, differences in the shape, size, distribution and numbers of abrasive particles on the different diamond coated disc burrs. In order to minimise variability caused by wear of the discs, we used new diamond coated disc burrs for each of the 17 experimental groups and cleaned them between experiments.

Thermal pulpar changes in laboratory models have been shown to replicate in vivo human studies [14]. However, the same may not be true for equine teeth which are hypsodont and while the equine pulpo-dentinal interface has many similarities to human teeth [15], equine teeth have relatively large, vascular pulps and presumably high blood flow in vivo, especially when young [16]. Consequently, the cooling effect of circulating blood in equine pulp is likely to have temperature sparing effects [17] that was not accounted for in the current in vitro model. The critical temperature threshold in equine teeth has yet to be defined and the value used in this study was based on a primate study [4].

Use of Instrument B increased the risk of exceeding the critical temperature threshold by 8-fold compared with Instruments A and C, and also significantly increased the time that the pulp horns spent above this threshold temperature. However, Instrument B was also found to have a significantly faster rate of attrition, wearing away dental material at approximately twice the rate of the other 2 motorised dental instruments and therefore would only need to be used for a shorter period of time when reducing overgrown cheek teeth. Instrument B had a slightly larger burr head and higher operating speed (9000 rpm) than Instruments A (4000 rpm) and C (8300 rpm), which is likely to have contributed to its high rate of dental reduction and increased heat production. Although the operating speed of Instrument C was also high, it may be that its intrinsic water cooling system was protective against heat production, but Instrument C certainly did not reduce the tooth as fast as Instrument B. In human dentistry, dental burrs applied with increasing pressure, rpm and area in contact with the tooth increase heat production [18].

It has been suggested that all equine dental motorised dental instruments should be used with water cooling, at least in an intermittent fashion, to reduce pulpar heating [1, 2, 19]. The results of this study support this recommendation, with the application of water in this study reducing the likelihood of a pulp horn exceeding the critical temperature threshold by 7-fold. Research in human dentistry indicates that there is no benefit to intermittent dental grinding if there is a lack of water coolant [20]. Instrument C was used with a continuous flow of water during rasping but we did not find any significant pulpar temperature sparing advantage of continuous water cooling as compared with Instrument A, which had water applied at the end of the rasping period. The use of water application by either technique appeared to have significant temperature sparing effects on pulps, as has been previously reported [1, 2].

The current findings are similar to those of a previous study [1] where temperature increases above the critical threshold were much higher and prolonged when rasping times were increased. In this study, compared with rasping for 30 s, rasping for 45, 60 and 90 s increased the likelihood of pulp exceeding the critical temperature threshold by 7.3, 8.9 and 24.7 times, respectively. It would be misleading to indicate that motorised dental instruments rasping for 30 s is safe in all teeth as 11/56 (19.7%) teeth ground for 30 s with the 3 different instruments (Instrument A n = 1, Instrument B n = 7, Instrument C n = 3) ± water exceeded the critical temperature threshold. Some of these teeth had thin subocclusal secondary dentine (A30: 5 mm, B30: 9, 12, 13 mm, B30W: 11, 11, 11, 12 mm, C30: 5, 10, 13 mm; mean subocclusal secondary dentine of all teeth exceeding threshold = 11.2 mm), whereas the other 45 teeth ground for 30 s whose pulp did not rise above the critical temperature threshold had a mean subocclusal secondary dentine depth of 13.1 mm.

The rate and maximum temperature rise within pulp was found to be inversely related to the depth of overlying subocclusal secondary dentine. Recent studies have shown a very wide individual variation of subocclusal secondary dentine in equine cheek teeth [5-8], but some focally overgrown teeth have reduced subocclusal secondary dentine, assumed to be due to reduced occlusal stimulatory contact [8]. Therefore, the risk of causing both gross pulpar exposure and thermal pulpar necrosis is likely to be higher in overgrown teeth compared with the normal teeth used in this study.

One group of authors suggested that a horse's age may influence the development of thermal stress in dental pulp and reported that both heating and cooling times were longer in older teeth [2]. We found a significant but weak correlation between increasing age and thinner subocclusal secondary dentine, but this finding has not been backed up by other studies [6, 8]. Additionally, we found both heating and cooling times to be longer in teeth with increased subocclusal secondary dentine, suggesting that although a thicker layer of subocclusal secondary dentine may be protective against heating, it may also insulate the pulp once heating has occurred [6, 8].

It would be useful to be able to give guidelines for motorised dental instrument use; for instance, how long a given motorised dental instrument can be used for or how much tooth can be safely removed before the thermal heating effect becomes dangerous. However, our study confirmed that the high variability in subocclusal secondary dentine depth in horses' cheek teeth and the subsequent effects of subocclusal secondary dentine depth on thermal heating and the risk of pulpar exposure makes a true ‘safety’ guideline impossible to give without knowing the subocclusal secondary dentine of the individual tooth/pulp horn being rasped. In the live horse, this would require the use of computed tomography and is therefore not practical in most cases. It might also have been useful to have examined shorter time periods of rasping in this study. The mean amount of tooth removed from the 41 teeth that exceeded the critical temperature threshold was 5 mm whereas the mean amount of tooth removed from the 75 teeth that did not exceed the critical temperature threshold was 3 mm but the range of values were highly variable (1–13 and 1–8 mm, respectively). These findings would suggest that, on average, teeth should not be rasped in excess of 3–4 mm at any one time.

Further studies need to be conducted on comparing the rasping efficiency and heat production of different types of burrs and specific safety margins regarding the time spent rasping should be given for individual motorised dental instruments by their manufacturers [21, 22].

Conclusions

This is the first study to quantifiably measure the effect of subocclusal secondary dentine on the pulp horn temperature changes associated with the use of 3 different equine dental motorised dental instruments. Prolonged rasping of cheek teeth and the absence of water cooling significantly increases the likelihood of pulps reaching the critical temperature threshold and causing irreversible pulpar damage. Current recommendations that cheek teeth overgrowths should be reduced gradually by only a few mm on several occasions due to the proposed risk of pulp exposure are supported by this study. Unless the subocclusal secondary dentine depth of a tooth is known definitive guidelines for safe rasping times cannot be given. We found almost 20% of teeth that were ground for 30 s exceeded the critical temperature threshold.

Authors' declaration of interests

No competing interests have been declared.

Source of funding

Funding for this project was provided by the University of Edinburgh.

Acknowledgement

Our thanks to Craig Pennycook for his technical help.

Authorship

John Mark O'Leary: contributed to study design, data collection, study execution, data analysis and interpretation and preparation of the manuscript. Timothy Barnett: contributed to data collection, study execution and preparation of the manuscript. Tim Parkin: contributed to statistical analysis of data collection and interpretation. Padraic Dixon: contributed to study design and preparation of the manuscript. Safia Barakzai: contributed to study design, data analysis and interpretation and preparation of the manuscript.

Manufacturers' addresses

  1. aD&B Equine Enterprises Inc., Calgary, Alberta, Canada.

  2. bBuehler, Coventry, Warwickshire, UK.

  3. cTC Direct, Uxbridge, UK.

  4. dDraper, Evesham, Worcestershire, UK.

  5. eHorse Dental Equipment Inc., Chateaubourg, France.

  6. fEquine Dental Instruments Inc., Elmwood, Wisconsin, USA.

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