Method for Measuring Temperature Changes in the Tooth during Restorative Procedures ARTHUR W. APLIN, FRED M. SORENSON, and KENNETH R. CANTWELL Department of Operative Dentistry University of Oregon Dental School, Portland, Oregon

With the advent of handpieces operating in the 100,000 r.p.m.-and-over range, there has been much renewed interest in the possible effects of temperature change within teeth during cavity preparation. Many attempts to quantitate temperature rise in cavity preparation have been made and reported. Considerable variation in the reported data is not surprising in view of the many technical factors that might act to influence significantly the production and measurement of temperature. Several important aspects of this problem that have not been adequately described or standardized in the past are (1) fidelity of temperature reception and recording, (2) quantitative and qualitative aspects of force application of potential thermogenic sources, and (3) site and detector orientation within the tooth for thermal observation. It is the purpose of this paper to describe methods and instrumentation that allow control and evaluation of these important variables. MATERIALS AND METHODS

Temperature measurement.-Because our major interest in temperature production and detection in teeth has been pointed toward peak or maximum temperature changes realized, rather than toward total heat introduced into the system, thermocouples were utilized as the thermal detectors throughout these experiments. The thermocouple has been employed as a thermal detecting device by many investigators. The application of such a device in the accurate determination of the magnitude of transient temperature changes in tooth structure requires the consideration of various factors. Dansgaard and Jarbyl have discussed some of the possible sources of error in thermal measurements to which the thermocouple is liable, and the reader is referred to this or other works for more detailed discussion of basic theory of thermo-

couple operation. Temperature recording.-The temperature changes, as detected by the thermocouples, throughout all experiments, including that subsequently described, were recorded using an automatic recording potentiometer.* Manufacturer's specifications for this instrument give the lag time as 1 second to 99 per cent full scale and a combined 0.3 per cent error in full-scale response to an initiate signal. This investigation was supported by USPHS research grant D-914 (C3) from the National Institute of Dental Research, National Institutes of Health, Bethesda, Maryland. Received for publication September 18, 1962. * Leeds-Northrup Speedomax G, Leeds & Northrup Company, Philadelphia, Pennsylvania. 925

926 APLIN, SORENSON, AND CANTWELL

July-August 1963 Prior to each experiment a temperature recording range of 50° F. was established for J. dent. Res.

this instrument. Periodic calibration checks against a known voltage input signal from a manual potentiometer of the same manufacturer were made throughout each experimental run. To exclude posisble influence of transient temperature change within the laboratory during the experimental period, the external temperature-reference junction was imbedded within a brass cylinder of approximately 8 pounds weight and containing an accurate mercury thermometer, readable to 0.2° F. The thermocouples employed were prepared from insulated chromel-alumel wires by fusion in an oxygen-methane microflame, following which the bead of the thermocouple was contoured to conform to the dimensions of the original wires.* One of the sources of error in temperature recording as herein described rests with the use of a thermocouple of the considerable cross-sectional area found in 28 gauge wires and the resulting bead. Reduction in cross-sectional area has been suggested' as a means to restrict heat conduction from the site of observation, but in practice the ease of manipulation and insertion (from specimen to specimen) of a larger-gauge thermocouple (smaller cross-sectional area) into a hole slightly larger than a No. I round bur is difficult to accomplish with any tactile assurance of adequate contact within the area of interest. It has also been our experience that the larger-gauge thermocouples, when used in this manner, rapidly deteriorate in conformation on repeated insertion into teeth. Since the nature and limit of biological response to transient thermal stimulation of pulpal tissues are as yet undetermined, it would seem that the compromise of thermal detecting accuracy for ease of handling as here outlined is justified. Thermocouple seating reproducibility.-One factor in the fidelity of temperature detection by non-stationary (not fixed with cement, etc.) thermocouples is the uniformity of repeated manual seating of the thermocouple in a reposit of questionable or varying composition. Previous studies by others who have employed the manual positioning of the thermocouple within a bur hole in a tooth have given little consideration to this problem of reproducibility of placement. To determine the influence of this factor on the variation in temperature reception of a thermocouple seated in a bur hole in dentin, the following study was carried out. A matrix within the extracted tooth structure was constructed that contained recesses for 2 thermocouples and a heating element. The recess for the heating element, simulating a class V cavity preparation, was placed at the site of the buccal pit of a lower first molar to a depth of approximately 1 mm.,using a No. 8 carbide bur. The heating element (an oversized copper-tipped wood-burning tool) was then milled to approximate closely the form and size of the recess. The holes for the thermocouples were placed nearly at right angles to one another, such that one thermocouple would yield an accurate account of the heating element output, while the other (the test thermocouple), at approximately 2 mm. distance, would portray the change in thermal events at a site within the body of the tooth. Both the thermocouple recesses were oriented in the same horizontal plane within the matrix as illustrated in Figure 1. The monitoring thermocouple A was firmly cemented into position within the tooth matrix in immediate proximity to the heating element. The test thermocouple B, under all experimental conditions, was held in position within the bur hole by frictional contact only. * B & S gauge 28, Brown & Sharpe (supplied by Leeds & Northrup Company, Philadelphia, Pennsylvania).

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TEMPERA TURE CHANGES DURING RESTORATIVE PROCEDURES 927

The heating element was attached to a wheeled vehicle that could be drawn up and down an inclined track. By this method, constant pressure and reproducible seating of the heating element within the simulated cavity preparation were obtained. The system was situated in a thermally insulated enclosure during the interval of heat insertion (45 seconds). Because a single-channel recorder was used, a double-pole double-throw switch was utilized to allow the near-simultaneous recording of temperatures from each thermocouple within the matrix. Under a controlled regimen of heating and cooling of the matrix, temperatures at the monitoring thermocouple were observed to initiate from room temperature and level off at the end of the heating period at N To SPEEDOMAX RECORDER

CHROMEL ALUMEL

N PLE~~~~~~~L

3LASSWOOL

stabilizedd at 150O F above Room Temperature)\

VOLTAGE REGULATOR FIG. 1.-Cavity matrix for heater and thermocouple

48.10 ± 0.60 F. Correspondingly, the temperatures recorded by the test thermocouple varied from 120 to 160 F., subject to the conditions of the experiment. The difference between the temperatures recorded at these two sites reflects the heat transmitted through a constant volume, and therefore constant heat capacity, of tooth structure. The constancy of the observed gradients was a measure of the reproducibility of the test thermocouple response under varying conditions of insertion and environment. In anticipation of possible significant variations in detected temperature gradients that might arise if thermocouple contact with the walls of the prepared hole varied on removal and reinsertion, a series of tests in which a small drop of mercury was placed in the test thermocouple hole prior to thermocouple insertion was included in the experimental design. It was felt that this procedure might produce a more uniform contact between tooth and thermocouple on subsequent reinsertions. In Table 1 are presented the mean temperature differences obtained in the manner previously in-

928 APLIN, SORENSON, AND CANTWELL

J. dent. Res. July-August 1963

dicated. The test of the analysis of variance at the 5 per cent significance level led to the following conclusions: (1) Reproducible seating of this size thermocouple can be accomplished under the conditions stated, independent of the presence of mercury in the test reception site; i.e., (a) no significant differences were found in mean temperature gradients among samples with fixed insertion (left in place) and reinsertion conditions of thermocouple B, and (b) no significant interaction between contions of insertion and mercury environment of thermocouple test site B was observed. (2) The addition of a small droplet of mercury increased the validity of temperature measurement of this system about 10-20 per cent within the temperature range studied (i.e., mean temperature gradients of samples with mercury present at thermocouple test site B were significantly less than those without mercury at site B). The magnitude of this increased validity of temperature measurement of thermocouple B, following the introduction of mercury into hole B, reflected an average TABLE 1* MEAN TEMPERATURE GRADIENTS (TA- TB) 0 F. RECORDED BETWEEN SITES A AND B AFTER 45-SECOND HEATING CONDITIONS OF TEST SITE B CONDITIONS OF THEDRMOcouPLE B

1. Left in place ........ 2. Reinserted ......... *

Mercury Present

Mercury Absent

32.70 (±0.30)

35.20 (±0.48) 35.60 (±0.42)

32.20 (±0.36)

Std. deviations based on sample sizes of 22 observations.

fractional increase in the rise above room temperature at site B and is offered as an estimate of changes demonstrating the advantages of the use of mercury in this particular system. If these results are taken to indicate that considerable reliability in repeated measurements within the bur holes in dentin is obtainable, the care required to achieve this reproducibility cannot be overemphasized. In regard to the latter precaution, the ability of mercury to act as a thermal contact surface may tend to negate a portion of the care of thermocouple insertion required. A question might be raised as to the state of hydration of the tooth matrix during the course of experimental thermocouple insertion, which, of course, could result in differences in the thermal conductivity of the area of conduction. However, both the dry storage and heating from preliminary trials, plus the sequence of treatments "with" and "without" mercury present in the hole, would tend to minimize or eliminate hydration change as a significant variable in this study. Specimen preparation.-The question of where to measure temperature changes within tooth structure has been approached by others for several in vitro experimental conditions,2-7 and some evidence is available to indicate, as a practical beginning, that the site of thermal observation at or near 0.5 mm. from the thermogenic source would yield near-maximum thermal differences. In view of the functional relationship between maximum peak temperature and the thickness of insulation between heat detector and heat source, an attempt was made to seek some standardization of the

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T'EWPERA TURE CIHANGES DURING RESTORATJVE PROCEDURES 929

thickness of tooth structure separating the thermal detector and the course of heat production. Kramer8 has indicated, from preliminary studies of histopathologic reaction to rotary instrumentation, that a central distribution of thermally induced alteration of dentinstaining reactions exists, which would support the contention that a symmetric alignment of thermal detector and source might prove more receptive to maximum changes than an otherwise eccentric site of observation. In these experiments, therefore, attempts to utilize symmetric alignment of the thermal detectors and heat source were made. Following the selection of the site of observation of heat generation in a tooth, a No. round bur hole is placed from the lingual surface in such a manner as to allow

FiG. 2.-Ends of micrometer in position of measurement

the desired degree of manipulation of the external portion of the tooth (i.e., cutting, polishing, etc.) and finish with a reasonably constant thickness of material separating the site of thermogenesis and the thermal detector. The desired orientation of the thermocouple hole is accomplished in the following manner: (1) the tooth is firmly held in a clamp on the table surface of a small hobby drill press possessing a depth stop; (2) the depth stop is set by using an accurate. gauge block (±0.005 mm.), and the No. I round bur is allowed to cut to the depth stop, thus producing a hole ending at the specified distance (thickness of gauge block) from the specimen exterior; and (3) the distance is then measured (accuracy ±+ 0.02 mm.), using a 0-1-inch micrometer. The magnitude of reduction in this dimension by further manipulation, such as the cutting of a cavity preparation, can then be determined by subsequent measurement with the micrometer. Figure 2 shows a section of a specimen with ends of the micrometer in place. This procedure aids in the orientation of the heating source under study and allows the measurement of remaining dentin thickness as well as the determination of depth of cut of the cavity preparation. Following the placement of the No. -1 round bur hole, this hole is washed free of

930 A PLIN, SORENSON, A NVD CA lTWELL

July--A ugust 1963 debris and dried with a blast of air. The specimen is then mounted in a brass ring such that the vertical and horizontal orientation of the tip of the thermocouple hole remains constant in each mounted specimen, thus facilitating specimen placement and orientation in a force-measuring device. A small droplet of mercury is deposited in and seated to the full depth of the thermocouple hole with sharp tapping. The specimen is placed in the force-measuring unit, J. dent. Res.

FIG. 3. Radiographic check of thermocouple seating

and the thermocouple is inserted and fastened at the full depth of the hole. Placement of the mercury droplet and the thermocouple is checked radiographically to insure uniformity of placement of the thermal detecting materials throughout all procedures

(Fig. 3).

Force measurem-iient.-Several systems for determining the force applied to the tooth during cutting have been described in reports dealing with cavity preparation. None of these methods, it would seem, holds much promise of ascertaining either (1) the distinctive magnitude and quality of the forces encountered in the rendition of the clinical cavity preparation or (2) any delicate, manual, multidirectional action

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TEMPERATURE CHANGES DURING RESTORATIVE PROCEDURES 931

in or on the tooth. The accommodation of higher rotational-speed cutting instruments into accepted dental usage has not diminished the need for knowledge of these forces. It is for these reasons that an attempt was made to derive a system whereby these forces could be studied and measured. A diagrammatic presentation of the forcetransducing system that was devised and found to be very useful is illustrated in Figure 4. The operation of this device was as follows: (1) the light beam from source (H),

FIG. 4.-Force-loading device, showing thermocouple orientation. A = tooth mounted in plaster of Paris; B = thermocouple stabilized within tooth; C = 15/10,000-inch steel diaphragm; D = strain-gauge attached to diaphragm; E = strain-gauge leads to amplifier; F = leads to 12-volt d.c. supply; G = rubber compression damper; H = light source, I = collimating lens system; J photocell; K = iris; L = photocell leads to amplifier.

focused on the iris of a photocell (I), was deflected in proportion to the applied horizontal load to the tooth; (2) a vertical load applied to the tooth was transmitted (with minor frictional loss) to the strain-gauge transducer diaphragm at C; and (3) each of the resulting variations in electrical signal associated with these applied forces, after suitable manipulation, was fed to an individual channel of a galvanometric recorder. With such an arrangement, the horizontal resultants and the downward vertical components of loading of the tooth during operative procedures were recorded. Typical recordings achieved with this instrument are shown in Figures 5-8. Figures

932 APLIN, SORENSON, AND CANTWELL

J. dent. Res. July-August 1963

show the horizontal and vertical components of an intermittently and constantly applied cutting force. In order that the direction of preferential loading in the horizontal plane might be identified, three pie-shaped photocells (arranged to form a circle with each apex at the center) have been utilized in place of the single photocell shown in the diagram. Simultaneous recording of signals from each of these photocells allows the evaluation of direction, as well as the magnitude and character of the applied horizontal forces. Grams Force

Grams

Load

Vo Vertical Force

H/orizontol Force

-40

-50

Unload 32

0

Time in Seconds

32

Time in Seconds

0

Grams Force

Horizontal Force

Grams Force

Vertical Force 50

_> jJ~

28

~

Wood0

~~~~~~~

Time in Seconds

I1 °

28

Time in Seconds

0

Fo

FIG. 5.-Horizontal cutting force of intermittently applied load. FIG. 6.-Vertical cutting force of intermittently applied load. FIG. 7.-Horizontal cutting force of constantly applied load. FIG. 8.- Vertical cutting force of constantly applied load.

An examination of the force-loading transducer system diagramed in Figure 4 suggests the possibility of certain inertial aberrations of the applied load. However, by suitable damping at G, recordings typified in Figures 5-8 are achieved. These offer considerably more versatility and precision in determinations of transient loading to the tooth than are obtainable by other methods, such as spring scales or pivotal balance systems. As can be seen, these force tracings yield not only the magnitude (after suitable calibration) but the frequency and, to the degree that frequency, sequential position, and direction of application may describe, the "operator characteristics" of force application.

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TEMPERATURE CHANGES DURING RESTORATIVE PROCEDURES 933 SUMMARY

Methods of instrumentation and specimen handling that allow standardization of procedures and controlled evaluation of important variables in temperature production and detection within extracted or synthetic teeth have been developed and tested. Data presented in this report suggest that reproducible seating of a 28-gauge thermocouple in a clean dental bur hole within teeth can be accomplished if some care in placement is exercised. The addition of a small droplet of mercury into this repository prior to thermocouple placement offers minimal irregularities in the contact of thermocouple and dentin, as well as a measure of increased validity of temperature measurement. A method for orienting the thermal detector and the heating source in relation to the tooth specimen has been outlined. This procedure allows accurate measurement of the remaining dentin separating heat source and thermal detector without destroying the specimen. Instrumentation that makes possible the determination of magnitude, character, and frequency of horizontally and vertically applied cutting loads to the tooth concomitant with temperature detection and recording has been described. REFERENCES 1. DANSGAARD, W., and JARBY, S. Measurement of Non-stationary Temperatures in Small Bodies, Odont. T., 66:472-502, 1958. 2. HENSCHEL, C. J. Heat Impact of Revolving Instruments on Vital Dentin Tubules, J. dent. Res., 22:323-33, 1943. 3. ANDERSON, D. J., and VAN PRAAGH, G. Preliminary Investigation of the Temperature Produced in Burring, Brit. dent. J., 73:62-64, 1942. 4. PEYTON, F. A., and VAUGHN, R. C. Thermal Changes Developed during the Cutting of Tooth Tissue, Fortn. Rev., Chicago dent. Soc., 20:9-23, 1950. 5. WALSH, J. P., and SYMMONS, H. F. A Comparison of the Heat Production and Mechanical Efficiency of Diamond Instruments, Stones, and Burs at 3,000 and 60,000 r.p.m., N.Z. dent. J., 45: 28-32, 1949. 6. WOLCOTT, R. B., PAFFENBARGER, G. C., and SCHOONOVER, J. C. Direct Resinous Filling Materials: Temperature Rise during Polymerization, J. Amer. dent. Ass., 42:253-63, 1951. 7. HUDSON, D. C., and SWEENEY, W. T. Temperatures Developed in Rotating Dental Cutting Instruments, J. Amer. dent. Ass., 48:127-33, 1954. 8. KRAMER, I. H. R. Changes in Dentin during Cavity Preparation Using Turbine Handpieces, Brit.

dent. J., 109:59-64, 1960.