Temperature Measurements During Dental Cutting Using an Internally Cooled Diamond Dental Bur

Transcription

1 Journal of Materials Science and Engineering B 2 (10) (2012) D DAVID PUBLISHING Temperature Measurements During Dental Cutting Using an Internally Cooled Diamond Dental Bur Uri Ben-Hanan, Michael Regev and Herbert Judes 1. Mechanical Engineering Department, Ort Braude College of Engineering, Karmiel, Israel 2. Head, Mechanical Engineering Department, Ort Braude College of Engineering, Karmiel, Israel 3. School of Dental Medicine, Tel Aviv University, Tel Aviv, Israel Received: June 17, 2012 / Accepted: July 12, 2012 / Published: October 25, Abstract: The common used cooling method in the existing dental handpieces implies water jets or water spraying towards the bur center. This method has two main drawbacks. First, in many cases the coolant is not applied at the heat source area; Second, the produced mist disturbs the visibility of the grinding zone. The purpose of the current study was to evaluated two methods of cooling using a high speed turbine dental handpiece, namely, the standard air-water spray directed towards the bur versus applying the coolant via a hollow bur. The experiments were conducted using glass ceramic and extracted molars. The temperature was recorded by means of thermocouples during work while applying the two cooling methods. The first cooling method by using a standard handpiece and the second, via a hollow diamond bur through which water was driven by a specially constructed device added to the handpiece. The results showed a superior cooling ability using the hollow tube diamond bur. Subjectively the operators reported improved visibility of the field of operation using the internal cooling method. Statistical analysis yielded superior cooling capacity in the case of internal cooling. It was therefore concluded that. Internal cooling was found to be superior compared with the conventional cooling method. It is likely to require a smaller volume of water, however, this point requires further research. This study indicates that during cavity preparation the internal cooling method is advantageous with respect to the commonly used cooling methods. In addition, improved visibility can be achieved. Key words: Dental burs, pulp chamber temperature, tooth temperature rise, water cooling, cavity preparation. 1. Introduction The potential damaging effect of temperature increase on pulpal tissue during dental treatment has always been a concern. Cavity preparation can be regarded as a source of temperature increase and therefore special attention should be paid to this point when developing a new dental bur cooling system. An in vivo qualitative estimation of the temperature increase resulting in trauma to the dental pulp was published by Zach, et al. [1]. They showed that healthy pulps failed to recover from intrapulpal temperature increase of 11.1 C in about 60% of the cases while fifteen percent of the teeth heated to Corresponding author: Uri Ben-Hanan, Dr., research fields: control and mechatronic systems. ubenhana@braude.ac.il. 5.5 C failed to recover. A few studies dealing with in vitro temperature rise in the pulp chamber during cavity preparation have been published, the main results of part of them are summarized below. Öztürk, et al. [2] reported on an in vitro assessment of temperature changes in the pulp chamber during 4 different tooth preparation techniques and the effect of 3 different levels of water cooling. They placed a tip of a thermocouple in the center of the pulp chamber of 120 extracted human premolar teeth and tried different combinations of loads applied to the handpiece and air pressure together with water flow of 40 ml/min, 15 ml/min and 0 ml/min. For all preparation techniques without water cooling the temperature rise within the pulp chamber

2 552 Temperature Measurements During Dental Cutting Using an Internally Cooled Diamond Dental Bur varied between 7.1 C and 19.7 C. Ottl, et al. [3] studied the relationship between different grits of diamond burs on the temperature response within a pulp chamber of extracted human molars during tooth preparation with an air turbine handpiece. The temperature within the pulp increased by a maximum of 2.5 C for fine, 2.6 C for coarse, and 3.2 C for ultracoarse burs. Watson et al. [4] recorded temperature changes during cavity preparation They used for their study both diamond and tungsten carbide (WC) burs with two different types of handpieces. The contribution of the water spray was studied by them as well, comparing the temperature changes occurring during cavity preparation with and without a water spray. They found that in all cavities cut under water spray, irrespective of handpiece, speed or bur, there was a decrease of the mean temperature in the pulp chamber from the starting temperature 37 C. Laforgia, et al. [5] also recorded temperature changes in twelve extracted teeth during cavity preparation. They found that no temperature rise occurred as long as water spray was applied. Lauer, et al. [6] came to the conclusion that the temperature of the cooling water should not exceed 35 C. When water temperatures were between 42.1 C and 44.4 C prolonged periods of tooth preparation resulted in increases of approximately 3 C in the temperature of the pulp chamber. Conversely, the temperature of the pulp chamber was reduced with cooling water temperatures between 29.8 C and 33.7 C. Cavalcanti, et al. [7] studied the combined effect of load applied on the handpiece and the cooling water flow. Two cavity preparation techniques were applied: a low-load of 29 g to 85 g tooth preparation technique and a high load of 150 g to 246 g technique. Temperature increase for the high-load technique was 16.4 C without cooling, C with 30mL/min cooling water flow and 9.96 C with 45mL/min cooling water flow. Temperature increase for the low-load technique was 9.54 C without cooling, 1.56 C with 30 ml/min cooling water flow and 0.04 C decrease with 45 ml/min cooling water flow. Vanderlei, et al. [8] recorded a maximum pulpal temperature increase of 4.9 C during cavity preparation by using a high-speed handpiece and diamond burs. Consani, et al. [9] performed cutting tests using different types carbide and diamond burs and water at 19 C was used as coolant. The average temperature measured by Consani, et al. [9] during cutting was C, the maximum and minimum temperatures measured were C and C, respectively. Evans, et al. [10] studied the influence of the cutting method, namely dry or wet, and recorded pressure and temperature changes within the pulp chamber during cutting. They claimed that the temperature increase did not exceed 2 C at any stage during dry cutting while temperature increase during wet cutting was even lower. Galindo, et al. [11] evaluated the different variables involved in tooth cutting to characterize intrapulpal temperature generation, cutting efficiency, and bur durability when using conventional and channeled diamond burs. The baseline temperature of the dental pulp chamber was 34 C. Their results showed that the temperature recorded during cutting was always lower than the baseline temperature. It can be seen from the above references that when using the standard coolants, either a water stream or an air-water spray the temperature within the pulp chamber does not exceed the critical values reported by Zach, et al. [1], however, use of these coolants results in poor visibility of the processed area, especially when an air-water spray is applied. Cooling by air alone was claimed to be feasible by Schuchard, et al. [12], however, Lloyd, et al. stated that air alone is substantially inferior compared with water stream or air-water spray [13]. The current study focuses, therefore, on a totally different concept, an internal cooling method via hollow burs. This technique if successfully applied to diamond burs may, in the authors opinion, lead to an improvement in visibility as well as improved cooling

3 Temperature Measurements During Dental Cutting Using an Internally Cooled Diamond Dental Bur 553 efficiency during dental cutting. 2. Materials and Methods Cutting experiments were performed on both Macor machinable glass ceramic plates, the mechanical and thermal properties of which are known to be similar to those of human enamel, and on extracted human teeth. The temperature was recorded in-situ while applying external and internal cooling, as explained in the following paragraphs. 2.1 Glass Ceramic The experimental system for the glass ceramic machining includes a specimen holder designed to hold a mm MACOR glass ceramic plate and a leading screw mechanism driven by a DC motor with an incremental encoder for measuring the hand piece displacement and advance rate. The handpiece advance rate was controlled by using an NI motion control board, NI PCI 7344, supported by an NI LabVIEW software package with a PID controller. The burs used for this study were made from a 1.67 mm diameter stainless steel tube with a 1.21 mm inner diameter; they were produced by Strauss & Co., Ra anana, Israel. The external tube diameter was reduced to 1.6 mm and then coated with diamond particles. Each bur was long enough to go through the handpiece as can be seen from Fig. 1. The cooling water was sprayed towards the bur from two nozzles thus yielding an external cooling as can be seen in Fig. 1a. The internal cooling was enabled by introducing the water into the bur with the aid of an injection needle. A special adaptor was designed for water supply to a KaVo 625 handpiece and attached to the top of the handpiece with an hypodermic needle, mounted down its center. This assured a water supply straight to the center and deep enough into the bur in order to avoid any leakage. The cooling water flow for both external and internal cooling was the same. The rotation speed of the KaVo 625 handpiece was 360,000 20,000 rpm. Three K type thermocouples were used for the temperature measurements. The thermocouples were placed 2 mm from the edge of the glass ceramic plate and 0.5 mm away from the groove made by the bur during the grinding of the glass ceramic, one at each side and the third one below the bur as can be seen in Fig. 2. A special silicon grease was inserted to the holes where the thermocouples where inserted to assure good heat conductivity between the thermocouple and the glass ceramic workpiece. Data acquisition was performed by using a NI 9211 data acquisition module with sampling rate of 12 counts per second so that the readings of each thermocouple were recorded 4 times per second with sensitivity of less than 0.07 C. Twenty four cutting experiments were conducted on glass ceramic, two feed rates were chosen for this study, namely mm/s and mm/s while the cooling method was changed after each experiment from internal to external and vice versa. Alternating the cooling method enabled comparison of the cooling efficiencies at the same degree of bur wear out. It should be noted that the water flow was kept equal for each pair of experiments in order to keep their comparative value. The depth of the cut was set to 4.2 mm. A general view of the system is shown in Fig Extracted Human Molars Cutting experiments were performed on extracted lower molars as well. The pulp chamber of each tooth Fig. 1 Schematic drawing of the two cooling techniques (a) external, (b) internal.

4 554 Temperature Measurements During Dental Cutting Using an Internally Cooled Diamond Dental Bur Fig. 2 Schematic drawing the thermocouples position (a) side view, (b) front view. Fig. 3 The experimental system (a) general view, (b) handpiece with injection head and hollow bur. was cleaned and filled with special silicon grease, characterized by high thermal conductivity prior to the experiments. A K type thermocouple was then inserted into the pulp chamber with special silicon grease to ensure good thermal conductivity. The pulp chamber was eventually sealed with resin in order to hold the thermocouple in place and to avoid water from entering the pulp chamber. The experimental setup included a water reservoir with a heating element and a stirrer so that the water temperature could be kept constant. The heated water was pumped from the reservoir into a smaller bath where the tooth holder and the tooth were submerged. The water temperature was around 34 C, similar to the temperature prevailing inside the human mouth. The water supply adaptor was the same one used for the glass ceramic cutting experiments. Eleven cutting experiments were conducted on extracted teeth; the water flow in all cases was 50 ml/min. Two cuts were made in each tooth, in seven cutting experiments external cooling was applied for the first cut while internal cooling was applied for the second cut. In the

5 Temperature Measurements During Dental Cutting Using an Internally Cooled Diamond Dental Bur 555 remaining four cutting experiment the internal cooling was applied to the first cut while the external cooling was applied to the second one, this was done in order to eliminate any dependence on the order of the experiments. The temperature was recorded during the cutting experiment using an NI 9211 data acquisition module. 3. Results 3.1 Glass Ceramic Curves of the temperature recorded during experiments No. 9 and No. 10 are given in Fig. 4. Internal cooling was applied during Test 9 while external cooling was applied during Test 10. These two curves show the temperature recorded by the three thermocouples as a function of the distance travelled by the bur and may be referred to as the temperature variation as a function of time during the cutting test. The thermocouples were located, as can be seen from Fig. 4, two millimeters away from the edge of the plate along the cutting path. It can be seen from Fig. 4 that the maximum temperature measured was higher in the case of external cooling. The cutting experiment results are summarized in Tables 1 and 2, table 1 refers to feed rate of mm/s while Table 2 refers to mm/s. The temperature values given in these tables are the mean values of the temperatures recorded by the respective thermocouples during cutting. The recording process started when the bur was 0.25 mm away from the thermocouple and ended when it was 0.25 beyond the thermocouple. Referring to Fig. 1b, thermocouple 1 is located on the left side of the bur, thermocouple 2 is located right below the bur path and thermocouple 3 is located on the right side of the bur. 3.2 Extracted Molars An example of a graph of the temperature developed in the tooth during Test 4 as a function of time is given in Fig. 5, the solid curve refers the temperature measured in-situ while using external cooling and the broken one refers to the internal cooling. The mean temperatures measured within the pulp chamber during each cutting test are listed in Table 3. It can be seen that in all cases where internal cooling was applied the mean temperature was lower than the mean temperature measured where external cooling was applied, however, the tooth was heated up to a temperature about 34 C prior to the cutting experiment and therefore the temperature difference between the initial temperature and the maximum temperature was calculated and compared. (a) (b) Fig. 4 Temperature measured with the various thermocouples versus the distance traveled by the bur (a) while applying internal cooling (b) while applying external cooling

6 556 Temperature Measurements During Dental Cutting Using an Internally Cooled Diamond Dental Bur Table 1 Cutting test results, feed rate mm/s. Test No. TC1-mean TC2-mean TC3-mean TC1-SD ( C) TC2-SD ( C) TC3-SD ( C) Cooling moded Internal External Internal External Internal External Internal External Internal External TC, thermocouple; SD, standard deviation. Table 2 Cutting test results, feed rate mm/s. Test No. TC1-mean TC2-mean TC3-mean TC1-SD ( C) TC2-SD ( C) TC3-SD ( C) Cooling mode Internal External Internal External Internal External Internal External Internal External Internal External Internal External TC, thermocouple; SD, standard deviation. Fig. 5 Temperature during machining of an extracted tooth versus time with the two cooling methods.

7 Temperature Measurements During Dental Cutting Using an Internally Cooled Diamond Dental Bur 557 Table 3 Test No. Extracted teeth cutting test results. Mean temp ( C)- external cooling Temperature rise ( C)- external cooling Mean temp ( C)- internal cooling Temperature rise ( C)- internal cooling 4. Discussion 4.1 Glass Ceramic As stated earlier, alternating the cooling method (external or internal) after each cutting test enabled comparing the cooling efficiencies at the same degree of the bur wear. It can be seen from Table 2 that the temperatures recorded by all the three thermocouples were higher in the case of the external cooling for almost every pair of cutting tests. The only exception are Experiments 3 and 4 in which higher mean temperatures were recorded by Thermocouples 1 and 3 in the case of internal cooling, however, the differences are relatively small and in the case of thermocouple No. 1 are within one standard deviation. The statistical analysis presented herein is based on ANOVA with repeated measures test. The main and most important conclusion is that the capacity of the internal cooling was found to be higher, the mean temperatures measured by each of the thermocouples were found to be lower with p value lower than 0.01 in the case of feed rate of mm/s and p = in the case of feed rate of mm/s. Having calculated the mean values of all the readings of each one of the thermocouples while applying external cooling and comparing them to the respective mean values referring to internal cooling, other important conclusions were drawn as described in the following paragraph. The values calculated are summarized in Table 4. Looking at Table 4 one can see that in the case of feed rate of mm/s for both internal and external cooling the mean reading of thermocouple no. 1 is higher than that of No. 2 which is higher than that of No. 3 In the case of feed rate of mm/s with internal cooling the mean reading of thermocouple no. 1 is higher than that of No. 2 which is higher than that of No. 3 while with external cooling the highest mean value was obtained for thermocouple No. 2 and the lowest one for thermocouple No. 3. It should be mentioned that the differences between the mean readings of the three thermocouples are lower in the case of the internal cooling compared with those of the external cooling for both mm/s and mm/s. Applying ANOVA with repeated measures test on the above data it turns out that the location of the thermocouple influences the temperature measured with p = 0.01 for feed rate of mm/s and with p = 0.04 for feed rate of mm/s. As for the interaction between the location of the thermocouple and the cooling method, it shows that this interaction is statistically established as well, ANOVA with repeated measures test yielded p = for feed rate of mm/s and p = 0.01 for feed rate of mm/s. In summary, it may be concluded that besides being more efficient, the internal cooling yields a more uniform temperature than the conventional external cooling.

8 558 Temperature Measurements During Dental Cutting Using an Internally Cooled Diamond Dental Bur Table 4 Total mean value readings for each thermocouple. Thermocouple no. Mean Std. deviation ( C) Cooling mode Feed rate (mm/s) Internal Internal Internal External External External Internal Internal Internal External External External Extracted Molars The mean temperature change was found to be, 1.2 C where external cooling was applied with a standard deviation of 1.7 C while in the case of internal cooling, the mean temperature change was found to be -3.5 C with a standard deviation of 1.0 C. Paired t-test yielded that the significance value for these results is p < It may be claimed, therefore, that with both cooling methods the tooth is exposed to cooling rather than heating, however, for a given water flow the internal cooling yields a grade better cooling in addition to its major advantage, namely, better visibility during work as was reported both by one of the authors and by a dentistry student working with the internal cooling system. 5. Conclusions Two cooling methods were evaluated durin dental grinding, the standard air-water spray vs. applying the water via a hollow dental bur. Cutting tests were performed on both glass ceramic and extracted molars. The internal cooling was found to be more efficient, and yielded a more uniform temperature than the conventional external cooling when tested on glass ceramic. The internal cooling was found to be superior when applied on extracted molars, namely, yielded better cooling for a given water flow Apart from being being more efficient, the major advantage of internal cooling is the better visibility it enables. Acknowledgments The authors wish to thank Mr. E. Agazade, Mr. A. Hadar and Mr. I. Oz for their assistance with the cutting experiments; thanks are also due to Mrs. I. Gelernter for the statistical analysis. This research project was supported by MAGENTON grant No from the Israeli Ministry of Industry, Trade & Labor. References [1] L. Zach, G. Cohen, Pulp response to externally applied heat, Oral. Surg. Oral. Med. Oral. Pathol. 19 (1965) [2] B. Öztürk, A. Üşümez, A.N. Öztürk, F. Ozer, In vitro assessment of temperature change in the pulp chamber during cavity preparation, J. Prosthet. Dent. 91 (2004) [3] P. Ottl, H.C. Lauer, Temperature response in the pulpal chamber during ultrahigh-speed tooth preparation with diamond burs of different grit, J. Prosthet. Dent. 80 (1998) [4] T.F. Watson, D. Flanagan, D.G. Stone, High and low torque handpieces: Cutting dynamics, enamel cracking and tooth temperature, British Dent. J. 188 (2000) [5] P.D. Laforgia, C. Morea, A. Desiate, Temperature change in the pulp chamber during complete crown preparation, J. Prosthet. Dent. 65 (1991)

9 Temperature Measurements During Dental Cutting Using an Internally Cooled Diamond Dental Bur 559 [6] H.C. Lauer, E. Kraft, W. Rothlauf, T. Zwingers, Effects of the temperature of cooling water during high-speed and ultrahigh-speed tooth preparation, J. Prosthet. Dent. 63 (1990) [7] B.N. Calvalcanti, C. Otani, S.M. Rode, High speed cavity preparation technique with different water flows, J. Prosthet. Dent. 87 (2002) [8] A.D. Vanderlei, A.L.S Borges, B.N. Cavalcanti, S.M. Rode, Ultrasonic versus high-speed cavity preparation: analysis of increases in pulpal temperature and time to complete preparation, J. Prosthet. Dent 100 (2008) [9] S. Consani, L. Ruhnke, Temperature developed during the cutting of tooth tissue, Bull Tokyo Dent Coll 17 (1976) [10] D.J. Evans, P.R. Wilson, The effect of tooth preparation on pressure measured in the pulp chamber: A laboratory study, Int. J. Prosthodont 12 (1999) [11] D.F. Galindo, C. Ercoli, P.D. Funkenbusch, T.D. Greene, M.E. Moss, H.J. Lee, et al., Tooth preparation: A study on the effect of different variables and a comparison between conventional and channeled diamond burs, Journal of Prosthodnotics 13 (2004) [12] A. Schuchard, C.E. Watkins, Thermal and histologic response to high-speed and ultrahigh-speed cutting in tooth structure, J. Amer. Dent. Assn. 71 (1965) [13] B.A. Lloyd, J.A. Rich, W.S. Brown, Effect of cooling techniques on temperature control and cutting rate for high-speed dental drills, J. Dent. Res. 57 (1978)