Heat generated by grinding during removal of ceramic brackets Mark E. Vukovich, DDS, MCID (Orth),* David P. Wood, DDS, MCID (Orth),** and Tom D. Daley, DDS, MSc, FRCD(C)*** London, Ontario, Canada

When ceramic brackets fracture during treatment or at the time of debonding, it may become necessary to remove residual fragments by grinding with a handpiece. However, the grinding of ceramic surfaces may generate temperatures high enough to have detrimental effects on dental pulp. Intrapulpal temperature measurements were made on extracted teeth during bracket grinding with a small thermocouple probe fixed to the pulpal wall subjacent to the bracket position. These measurements were" then compared with established threshold temperatures that have been reported to cause pulpal pathosis. We removed 122 ceramic brackets (A-Company Starfire, GAC Allure, and Unitek Transcend) from eight extracted teeth by grinding with high-speed diamond burs or low-speed green stones, both with and without air or water coolant. It was determined that low-speed grinding without coolant resulted in a significant (p < 0.001) increase in pulp chamber temperature for all three types of brackets. Neither high-speed nor low-speed grinding during bracket removal caused a rise in pulp chamber temperature when combined with air or water coolant. (AM J ORTHODOENTOFACORTHOP1991 ;99:505-12.)

T h e use of tooth-colored or clear ceramic brackets has gained popularity in recent years. Superior esthetics and an ev~iluation of the shear bond strength of ceramic brackets strongly suggest that ceramic brackets should offer a practical alternative to metal brackets, t Ceramic brackets made of crystalline aluminum oxide are considered in this study. Two forms of this ceramic bracket material are currently in use: singlecrystal alumina Starfire ("A"-Company, San Diego, Calif.) brackets and polycrystalline alumina Allure (GAC International, Central Islip, N.Y.) and Transcend brackets (Unitek/3M, Monrovia, Calif.). A shallow scratch on the surface of the ceramic material will drastically reduce the fracture strength, whereas the same scratch on a metal surface will have little, if any, effect on its fracture tendencies under load. 2 Hence, care must be taken during treatment not to scratch bracket surfaces with instruments or to overstress tie wings during ligation because such abuse can result in bracket fracture. Fracture can also occur during debracketing.

In the event of bracket fracture, the remaining ceramic fragments must be removed from the tooth by shear forces with a sharp instrument or by grinding with rotary instruments. Grinding ceramic material from the labial surface of a tooth may generate heat, which could damage the dental pulp. In vivo examination by Pohto and Scheinin 3 of dental pulps exposed to heat revealed that an elevation in temperature from 37 ° C to 39 to 42 ° C resulted in an increase in circulation (vasodilation). Temperatures at and above 44 ° C caused aggregates of red blood cells to form. If temperatures of 46 ° to 50 ° C were maintained for 30 seconds, thrombosis and a standstill of circulation occurred. zach and Cohen 4 carded out an in vivo study that actually correlated the rise in temperature on the surface of the tooth with an internal rise in temperature. They found that an increase in pulpal temperature to 42.2 ° C caused pulpal necrosis in 15% of the teeth in their Macaca sample. A rise in temperature tO 47.7 ° C caused necrosis in 60% of the teeth. The objectives of this study are as follows:

*Private practice in orthodontics, Thunder Bay, Ontario. **Clinic Director, Graduate Orthodontics at the University of Western Ontario. ***Associate Professor, Department of Pathology at the University of Western Ontario. . 811116694

1. To determine whether pulp temperature rises during the removal of ceramic brackets by grinding 2. To compare pulp temperature elevation with the previously published critical values for pulp survival 3. To compare the temperature elevation caused by



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Am. J. Orthod. Dentofac. Orthop. June 1991




Fig. 1. Apparatus for recording changes in pulpal temperature that result from grinding a ceramic bracket off labial surface of tooth.

different methods of grinding and cooling (i.e., highspeed and diamond burs versus low-speed and green stones, both with and without air or water cooling) 4. To establish a standardized protocol for grinding ceramic bracket fragments from teeth without significant risk of thermal injury to the pulps MATERIALS AND METHOD

Many studies have dealt with evaluation of the amount of heat that reaches the pulp chamber and its effects on dental pulp tissue during operative or other procedures. 4'59 Thermocouple sensors planted in artificial pulp tissue were used in these studies to monitor the changes in pulp temperature. A similar method was used in this study (Fig. 1). Premolars that had been extracted from adolescent orthodontic patients were collected and stored in a 10% formalin solution immediately after extraction to prevent dehydration. The roots of these teeth were then embedded in epoxy resin to simulate the insulating effect of alveolar bone and periodontal soft tissues. ~° To simulate in vivo conditions within the teeth, a replacement for pulp tissue was used. Type Z9 heat sink compound (GC Electronics, Rockford, II1.) was placed around the thermocouple tip and in the pulp chamber to allow for natural dispersion of heat." Minute AWG No. 40 type J (iron-constantan) thermocouple sensors were fabricated and placed in the pulp

chambers of 20 extracted teeth ,in holes that had been drilled into each tooth from the lingual side. 4"8'9 The pulp chambers were filled with the conducting medium, and the thermocouples were inserted to contact the labial dentin. They were secured in place with restorative resin. The teeth were radiographed to allow evaluation of the positions of the thermocouples, tz Eight teeth proved satisfactory for use in the study, while the other 12 were rejected on the basis of poor placement of the thermocouples or internal damage to the lat~ial dentine. The thermocouple sensors were connected to a temperature-recording device (Omega Engineering model 383, Stamford, Conn.) and to a chart recorder (Fisher Scientific Recordall Series 5000, Nepan, Ontario), set at the 100 mV range and a chart speed of 2.5 or 5.0 cm/min, to record the changes in temperature during the grinding period (Fig. I). The accuracy of the apparatus was checked against a calibrated water bath and found to be correct to within 0.5 ° C. Standard high-speed (Midwest American, Dublin, Ireland) handpieces operating at 32 psi and Shorty lowspeed (Midwest American, Dublin, Ireland) contraangle handpieces operating at 40 psi, as recommended by the manufacturer, were used for the removal of the brackets. High-speed drilling was carried out at approximately 120gm, and low-speed drilling at approximately 220 gin. We observed during the experiments

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-~ 0.05) was found between the GAC and Unitek brackets at either time interval. The statistical temperature difference between the ACompany Starfire bracket and either the GAC Allure or the Unitek Transcend brackets was the same at both intervals of time and reached the p < 0.05 level of significance. This minor statistical difference was determined to be clinically insignificant for the following reasons: 1. After both 24 seconds and 60 seconds of grinding, all three types of brackets had exceeded the thresh-

old values for heat sufficient to initiate necrosis, regardless of any actual difference in temperature. 2. Aside from the initial lag in temperature increase, the heat curves for all th~:ee brackets were nearly parallel (Fig. 3). The difference between the A-Company brackets and the GAC and Unitek brackets is probably due to different physical properties of the single-crystal alumina (A-Company Starfire) and polycrystalline alumina (GAC Allure and Unitek Transcend) materials. The removal of a ceramic bracket from the surface of a vital lower incisor tooth by means of a low-speed bur without coolant resulted in numerous histologic pulp changes (Figs. 4 and 5). These changes were characteristic of pulp tissue that has been exposed to temperatures exceeding 42.2 ° C4'6 and implied that removal of brackets by this method may cause total necrosis in at least 15% of teeth. Nyborg and Brannstrom, ~4 however, reported a relative lack of (aspirated) nuclei in the dentinal tubules. This result is attributed to the direct heating and probable desiccation of dentin in their study. The enamel was intact and the dentin unexposed during the removal of the bracket in our experiments. OBSERVATIONS

Several observations from our experiments are worth mentioning. Occasionally, large fragments of ceramic material fractured off during grinding. Conceivably, these fragments could injure the operator or assistant or be swallowed by the patient. After grinding off several brackets, operators re-


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Fig. 5. This view shows microblistering that has led to disruption of odontoblastic area. Intra- and interodontoblastic vesicles (V) and "wheat sheafing" ON) are visible. Microblisters (M) are present in the cell-free zone of Weil. Karyorrhexis and pyknosis of the odontoblastic nuclei are seen. (Magnification x 320.)

ported eye irritation and itchy skin on the hands, which was attributed to the ceramic dust produced by grinding the brackets. In light of this observation, perhaps it would be wise to recommend that patients and operators avoid wearing contact lenses during bracket grinding. High-volume suction adjacent to the site of grinding is also recommended to reduce the amount of dust spread about the operatory. In any event, the need for gloves and eye protection during the removal of ceramic brackets in such cases is apparent. CONCLUSION It was found that high temperatures that significantly exceed known thresholds for pulpal damage are generated by low-speed grinding of ceramic brackets without coolant. Temperatures significantly lower than threshold values were reached during the removal of brackets by grinding with either high-speed or lowspeed handpieces, with water or air as coolant. These results lead us to the conclusion that removal of ceramic brackets from teeth with low-speed green stone burs and no coolant may cause permanent damage or necrosis of dental pulps. The potential for damage by this method is further illustrated by the histologic changes observed after the in vivo removal of a ceramic bracket from a lower incisor. Several common-sense clinical recommendations can be made. Water coolant provides the most signif-

icant cooling o f the grinding sites in both high-speed and low-speed bracket removal and is suggested in all cases. High-volume suction adjacent to the site of grinding is also recommended, to reduce the number of ceramic particles spread about the operatory area. Patients and operators should be advised to wear eye protection and to avoid wearing contact lenses during the removal of ceramic brackets. We thank the following persons and companies: Livio Rigutto and Dr. J. Wanklin, for their assistance in the experiment; Dr. C. R. Hill, of Saskatchewan, Canada, and ACompany, GAC, and Unitek, for the brackets used in our study. REFERENCES I. Gwinnett AJ. A comparison of shear bond strengths of metal and ceramic brackets. A~,l J ORTIIODDENTOFACORTnOP1988; 94:346-8. 2. Scott GE Jr. Fracture toughness and surface cracks--The key to understanding ceramic brackets. Angle Orthod 1988;58: 5-8. 3. PohtoM, Scheinin A. Microscopicobservationson livingdental pulp, Part 2. Aeta Odontol Scand 1957;24:303-14. 4. Zaeh L, Cohen G. Pulp response to externally applied heat. Oral Surg Oral Med Oral Pathol 1965;19:515-30. 5. Anderson D1, Van Praagh G. Preliminary investigation of the temperatures produced in burring. Br Dent J 1942;73:62-4. 6. Lisanti VF, Zander HA. Thermal injury to norn~al dog=teeth: in vivo measurements of pulp temperature increases and their effects on the pulp tissue. J Dent Res 1952;31:548-58.


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7. Harnett J, Smith W. The production of heat in the dental pulp by use of the air turbine. J Am Dent Assoe 1961;63: 210-4. 8. Sheridan JJ, Brawley G, Hastings J. Electrothermal debracketing. Part I. AM J ORTIIOD1986;89:21-7. 9. Sheridan JJ, Brawley G, Hastings J. Electrothermal debracketing. Part II. AM J OR'rrtoD 1986;89:141-5. 10. Jacobs H, Thompson R, Brown W. Heat transfer in teeth. J Dent Res 1973;52:248-52. 11. Cooley R, Barkmeier W. Temperature rise in the pulp chamber caused by twist drills. J Prosthet Dent 1980;44:426-9. 12. Grusser OJ, Kollmann W, Lerch J, Mijatovie E. Changes in pulp

Am. J. Orthod. Dentofac. Orthop. June 1991

temperature during thermostimulationof human teeth by a simple electrically controlled then'node. Pflugers Arch 1985;403:273-7. 13. Zach L, Cohen G. Thermogenesis in operative techniques: comparison of four methods. J Prosthet Dent 1962;12:977. 14. Nyborg H, Brannstrom M. Pulp reaction to heat. J Prosthet Dent 1968;19:605-12. Reprint requests to: Dr. M. E. Vukovich 100-44 S. AIgoma St. Thunder Bay, Ontario P7B 3A9 Canada

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Heat generated by grinding during removal of ceramic brackets.

When ceramic brackets fracture during treatment or at the time of debonding, it may become necessary to remove residual fragments by grinding with a h...
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