The thermomechanical properties of gutta-percha. Part IV. A thermal profile of the warm gutta-percha packing procedure Alvin Goodman, D.D.S., M,Sc. D., * Herbert Schilder, D.D .S., ** and Winthrop Aldrich, Ph.D.,*** Boston, Mass.

BOSTON UNIVERSITY SCHOOL OF GRADUATE DENTISTRY

A thermal profile of the warm gutta-percha technique was produced by the thermocouple instrumentation of natural teeth and the subsequent monitoring of intraradicular temperature changes during the packing procedure. Although variations in thermal patterns resulted from individual differences in timing and instrumentation, certain clinically accepted patterns of activity produced consistent, representative temperature ranges to which the gutta-percha was subjected. The maximum regional temperature determined for bulk gutta-percha in the body of the canal was 80” C., while the over-all peak temperature recorded in the apical region was 45” C. Thermal penetration of the gutta-percha was expectedly limited, with significant thermal effects rarely exhibited more than 4 to 6 mm. into the material.

The following article is the next in the Boston University series on gutta-percha. It represents Part IV of a report entitled “The Thermomechanical Properties of Gutta-Percha,” being presented in serial form in this Journal. Parts I, II, and III have appeared earlier in sequential order, Part I (Oral Surg. 37: 946-963, June, 1974) reported on the investigation of the compressibility of gutta-percha. It was a study to determine if gutta-percha, after being subjected to packing under pressure, could be expected to assist in a seal at the dentin-gutta-percha interface through the mechanism of molecular “spring-back.” Compaction via condensation was explained in detail. Part II (Oral Surg. 37: 954-961, June, 1974) gave a comprehensive review of the history and molecular chemistry of gutta-percha. in order to design experiments to investigate the thermal and mechanical properties of gutta-percha, it was necessary to correlate this information with the experimental phase of the project. important background information is contained therein. Part 111 (Oral Surg. 38: 109-l 14, July, 1974) reported the determination of those temperatures at which molecular changes occur in gutta-percha polymer chains during heating and cooling. These phase transition temperatures, familiar to the molecular chemistry and materials engineering communities, are directly related to and responsible for the volumetric behavior of gutta-percha. It is suggested that the reader familiarize himself with the previously published segments of this series in order that he understand each succeeding chapter.

From a thesis submitted by Dr. Goodman in partial fulfillment of the V olume changes of gutta-percha on heating and requirements for the degree of Master of Science at Boston Univer- cooling have long been reported in the dental litera- sity School of Graduate Dentistry. *Instructor, School of Graduate Dentistry. **Assistant Dean, Professor and Chairman, Department of End- odontics, School of Graduate Dentistry. ***Assistant Professor, College of Engineering, Boston University.

544

ture. I-’ Recently published material has suggested

possible changes for thermal-volumetric changes as re- lated to molecular transformations in the polymer.s. y

No new data on the quantification of volume changes

0030-4220/81/050544+08$00.80/0 0 1981 The C. V. Mosby CO.

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Thermomechanical properties of gutta-percha 545

Fig. 1. Longitudinally sectioned cuspid. Fig. 3. The :rmocouple channels prepared at 2 mm. throl %' lout the linear extent of the root.

intervals

Fig. 2. Preparations for thermocouples in dentinal wall. A, Prepared channel for thermocouple leads with internal bevel on pulpal wall. B, A thermocouple head in position.

in gutta-percha have been presented since 1918.3 If the endodontic potentialities of gutta-percha are to be exploited, newer methods of quantifying volumetric changes must be investigated and accurate studies must be performed at clinically meaningful temperatures. Preliminary studies were required, however, before such experiments could be realistically designed or the data inteliigently interpreted. The current article com- pletes the presentation of the preliminary data.

Once the crystalline phase transformations of the gutta-percha polymer were recognized,8 a calorimetric

Fig. 4. Thermocouple leads wedged in position on the external surface of the root.

analysis was performed to demonstrate these transfor- mations in the gutta-percha material used in endodon- tics today and to determine at what points in the thermal cycle they take place.s

It then became necessary to monitor the intraradic- ular temperature ranges to which the polymer is sub- jected during actual warm gutta-percha compaction procedureslO and to demonstrate variations in those ranges at one level of the root canal space as opposed to another.

546 Goodman. Schilder. and Aldrich Oral S ,urg. May. I 1981

Fig- wall

I1 r the gutta-percha were to attain a uniform tempera-

5. Thermocouple heads set in sequence in the root canal

ture throughout its mass, that temperature alone would be used in future volume studies. If the temperatures attained were to vary regionally, within the canal, further studies would be required at the maximal tem- perature attained in both the maximally and minimally elevated portions of the canal. It would also be impor- tant to determine whether the temperature ranges in the gutta-percha included the phase-transformation points. If subsequent temperature-volume analyses were to subject gutta-percha to temperature ranges not realisti- cally incurred, misleading results would be produced relative to potential dimensional alterations within the root canal space.

METHODS AND MATERIALS

Eight single-rooted human teeth were prepared for the warm gutta-percha packing procedure by cleaning and shaping the root canals with serial reaming and filing. The teeth were then sectioned longitudinally through the prepared root canal space with a Keller bone saw, producing two facing halves for each tooth (Fig. 1). In a single half of each specimen, channels were bored through the dentin from the internal pulpal wall to the external surface of the root. Unitek twist drills and appropriate reamers were used to create open- ings approximately 0.010 inch in diameter (Fig. 2, A) designed to accommodate two 0.005 inch termocouple leads threaded from the pulpal side of the root. Pre- pared internal bevels allowed the head of each ther- mocouple to be “counter sunk” into a position flush with the pulpal wall without permitting its being pulled through the channel itself (Fig. 2, B).

Fig. 6. Mounting the tooth and thermocouple assembly in acrylic. A, Aluminum mold and specimen poured with quick-cure acrylic. B, Separating aluminum mold. C, Speci- men encased in acrylic block.

By this procedure, preparations were made through- out the linear extent of the root at 2 mm. intervals for placement of thermocouple channels, beginning 2 mm. from the apex until the cervical line was approached. Six such channels were required for each tooth (Fig. 3). Omega precision fine-wire thermocouples were thread- ed into the preparations, wedged into place with small wooden pegs, and cemented with epoxy resin (Figs. 4 and 5). Chrome-constantan thermocouples were cho- sen because of their sensitivity and high millivolt re- sponse at any given temperature. The two halves of each specimen were then cemented back together with epoxy, locking the thermocouples within the walls of the reassembled teeth.

Each specimen, with its tooth and thermocouple as- sembly, was then encased in an acrylic block. This was accomplished with an aluminum mold designed to provide for the sequential separation of the six ther- mocouple stations with their positive and negative leads (Fig. 6, A, B, and C). After separation from the mold, the specimens were stored ,at body temperature in an incubator to await subsequent steps in the procedure. As each specimen, with its attached thermocouple assem- bly, was removed from the incubator, it was placed on a

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Thermomecbanicrrl properties of gmfl-percha 547

Fig. 7. Assembled workboard.

Fig. 8. Assembled workboard with millivolt recorder (right).

flask mantle controlled by a rheostat which acted as a heating pad. This maintained the acrylic block within a narrow range of + 1.5 C. of body temperature at all times during the experimental procedures (Fig. 7).

The positive and negative leads from each ther- mocouple station within the tooth were wired to the individual channels of a “Brush Datapoint” millivolt recorder (Fig. 8). This instrument is a singlepoint re- corder capable of scanning, at variable speeds up to l/16 second, eight separate channels. In effect, this allowed practically simultaneous recording of all six thermocouple stations. One of the two additional chan- nels on the recorder was used for calibration and moni- toring of room temperature. The remaining channel was disconnected. The thermocouple stations recording temperature changes were identified for data collection as follows:

Fig. 9. Radiograph of prepared cuspid with fitted gutta-p cone.

bercha

Fig. 10. Cuspid after monitoring the packing procedl ure .

Station I- 12 mm. from the apex Station 2-10 mm. from the apex Station 3-8 mm. from the apex Station 4-6 mm. from the apex Station 5-4 mm. from the apex Station 6-2 mm. from the apex Members of the endodontic faculty were asked

a master gutta-percha cone for a particular speci evaluate it radiographically (Fig. 9). and pack the while being monitored. Sealer was omitted to pr insulation of the thermocouple heads from the tern

to fit .men, : case event Ipera-

548 Goodman, Schilder, and Aldrich Oral Surg. May. 1981

Fig. 11. Example of thermal pattern demonstrating regional temperature changes occurring experimentally in gutta-percha with the insertion of a heat carrier (material at the level of the cervical line.)

ture of the gutta-percha. Each staff member was ad- vised to proceed as he usually would clinically, stop- ping to take additional radiographs at his discretion. Eight specimens were packed (Fig. 10) and monitored while the data recorded on the strip chart produced thermal profiles for each warm gutta-percha packing procedure.

RESULTS

The scale of 0” to 100“ C. on the strip chart was more than adequate for the temperature range encountered during the monitoring procedure. In only isolated in- stances did the recording needle move completely off the scale. This occurred when the heat carrier itself came inadvertently into direct contact with a ther- mocouple head.

A generally recognizable pattern, of which Fig. 11 is an example, was produced at each level of penetration of the heat carrier into the gutta-percha. Fig. 11 specifi- cally demonstrates the regional temperature changes that occur throughout the tooth when a hot heat carrier is inserted into the gutta-percha when the material is at the cervical line at an early stage in the packing procedure.

The regional temperatures of the gutta-percha in this example are as follows: Station l-80” C.; Station 2-62” C.; Station 3-56” C.; and Station 4-46” C. Station 5 recorded only a 0.5” C. increase above base temperature, while at Station 6, 2 mm. from the apex, there was no evident thermal effect. Every thermal re- cording obtained while the gutta-percha was at this specific level produced similar patterns of temperature distribution.

Fig. 12 represents an example of temperatures re- corded when heat was applied to the gut&perch3 at successively deeper levels of the root canal space. Fig. 12, A, for example, represents the temperature eleva- tions which occurred when the heat carrier was plunged into the gutta-percha after it was compacted to the level of Station 2. Fig. 12, B is the pattern recorded when the material was at Station 3, and Fig. 12, C represents the pattern produced when the material was reduced to the level of Station 4. As the operator approached deeper stations within the root canal with the heating and plugging procedure (Fig. 13), bypassed stations were disconnected, contributing significantly to the clarity of the recordings obtained.

Displacement to the left indicates elevation of tem- perature. The highest temperatures were recorded by the stations closest to the initial contact with the heat source, namely, the level of the compacted gutta- percha. Deeper stations always recorded less elevation in temperature, but this reduced temperature rise was not linear. As the level of gutta-percha is brought closer to the apical channels, a similar pattern of temperature elevation is repeated in the remaining gutta-percha mass. It was consistently observed, however, that even after the gutta-percha was reduced to levels deep within the prepared canals, there was still only slight elevation in the temperature of the remaining gutta-percha in the most apical region.

The individual working style of each staff member involved in the study was revealed in the thermal profile that he produced. Observation of the graphed results presented patterns unique to individual clini-

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Thermomechanical properties of gutta-percha 549

Fig. 12. Examples of temperature changes recorded experi- mentally with gutta-percha at successively lower levels of the root canal. A, Gutta-percha at Station 2 (10 mm. from the apex). B, Gutta-percha at Station 3 (8 mm. from the apex). C, Gutta-percha at Station 4 (6 mm. from the apex).

cians relative to the tempo with which the instrumenta- tion was accomplished. Delayed, nonrhythmic delivery of the heat source, for example, produced thermal pat- terns with definite peaks but without heat build-up, re- sulting in rapid retreat to essentially the original base temperature (Fig. 14). More rapid, rhythmic cycling of the heat carrier between flame and tooth, on the other hand, produced a somewhat different profile (Fig. 15). In such cases temperatures were lower in the beginning but became progressively higher. Heat build-up in the system produced a shift in the base line to the left, representing increases in base temperature for the entire mass.

MSCUSSION

The data obtained demonstrated that regional varia- tions do occur within the gutta-percha in different por- tions of the root canal. Most significant was the obser- vation that the temperature of the apical gutta-percha was, on only one occasion, elevated as high as 45” C. and rarely above 40” C.

The thermocouple monitoring of natural teeth sub-

Fig. 13. Radiograph of cuspid with gutta-percha compacted to the level of Station 3.

jetted to the warm gutta-percha technique supported the reputation of gutta-percha as a restricted thermal conductor, evidenced in this study by the difficulty in transmitting heat through the material to apical regions of tbe prepared root canal space. Significant thermal effects were rarely exhibited more than 4 to 6 mm. into the material and infrequently more than 2 to 3 mm. beyond the point of deepest penetration of the heat carrier. Although the original technique of vertical condensation of warm gutta-percha calls for 3 to 4 mm. penetrations into the gutta-percha mass with the heat carrier, plunges of 2 to 5 mm. were actually observed in this study. Significantly, temperature elevation beyond the point of deepest penetration became more difficult as the level of the remaining gutta-percha ap- proached the apical end of the canal.

An evaluation of more than 150 feet of strip-chart data revealed interesting variations in thermal profiles from one operator to another, relative to speed and rhythm of heat application. When individual operators varied the application of heat (by design or by acci- dent), minor changes were affected in the temperature ranges recorded at various levels of the root canal space. Clinically accepted guidelines for the warm gutta-percha technique, however, incorporated into all their patterns of activity made it possible to identify representative temperatures to which gutta-percha is ac- tually subjected within the teeth. There is no question that, in extremely localized areas immediately adjacent to the heat source, temperatures were above these val- ues. Since the experiment was designed to record re- gional temperatures of the bulk material at 2 mm. inter-

550 Goodtnatl, Schilder, and Aldrich Oral Surg. May. 1981

Fig. 14. Pattern of thermal activity recorded with delayed, nonrhythmic delivery of the heat source.

vals, these localized effects were masked. Although exceeded in some instances and not reached in others, a representative maximum regional temperature for gutta- percha in the body of the root canal was determined to be 80” C. This is well above the alpha to amorphous transformation temperatures for the various dental gutta-percha materials, as determined by differential scanning calorimetry.g This representative maximum of 80” C. found in the body of the gutta-percha mass could be applied usefully as the upper limit of the temperature range to be used in further planned thermal-volumetric determinations.‘” It is noteworthy that no significant morphologic changes occur in the polymer anywhere near this temperature.

Circumstances were different in the apical region. Thermal penetration to the apex was restricted. Rhyth- mic use of the heat carrier did produce an increase in base temperature of as much as 4” C. as a result of heat build-up in the entire system. Gutta-percha was dem- onstrated to be moldable apically at temperatures 2” to 4” C. above body temperature. Indeed, elevations of more than 2” C. above base temperature, producing a plugging temperature of 40” C., were infrequently ob- served at Station 6. This is significantly below the beta

Fig. 15. Pattern of thermal activity recorded with rapid, rhythmic delivery of the heat source.

to alpha transformation point and highly reassuring with respect to the potential effectiveness of vertically condensed gutta-percha in sealing the apical end of the root canal. In the temperature ranges below 46” C., all popular, commercially available gutta-percha cones but two undergo no molecular phase transformation with heating and cooling.

Considering the importance of sealing the apical portion of the root canal space, the maximum peak temperature ever recorded at Station 6 became increas- ingly important. Accordingly, although the routinely observed maximum was less than 42” C., future vol- umetric investigations were planned using the atypical but “peak” recorded apical temperature of 45” C. to subject the warm gutta-percha technique to the severest possible volumetric test.

SUMMARY

Clinically significant temperatures were sought for carrying out subsequent thermovolumetric studies for gutta-percha. A thermal profile of the warm gutta- percha packing procedure revealed minor variations in

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thermal patterns among individual operators as a result of their personal application of the technique. Certain clinically accepted patterns of activity made it possible to determine representative intraradicular temperature ranges for gutta-percha. A maximum representative regional temperature determined for bulk gutta-percha in the body of the canal was 80” C., while the peak temperature ever recorded for the material in the apical region was 45” C. Thermal penetration was limited. with significant thermal effects rarely exhibited more than 4 to 6 mm. into the material.

CONCLUSIONS

1. Clinically acceptable patterns of activity, com- mon to all knowledgeable operators, produced consis- tent, representative temperature ranges to which gutta- percha is subjected regionally within the roots of teeth.

2. The representative maximum regional tempera- ture to which bulk gutta-percha is subjected in the body of the canal during the warm gutta-percha packing pro- cedure is 80” C.

3. The maximal temperature range to which gutta- percha is elevated in the apical region is 40” to 42” C., although 45” C. was recorded as the peak temperature in one instance.

4. There are variations in the thermal profile for gutta-percha as used in the warm gutta-percha packing procedure because of individual differences in in- strumentation, timing, and personal contributions to the technique.

5. Thermal penetration of gutta-percha is limited during the packing procedure, with significant thermal effects rarely exhibited more than 4 to 6 mm. into the material from the point of entry of the heat source, or more than 2 to 3 mm. from the point of deepest pene- tration.

6. Sufficient data have been collected relative to

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8.

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Tl~ert~~omr~hanical properties qf grttta-perch 551

temperature changes experienced by gutta-percha dur- ing vertical condensation to warrant a detailed investi- gation of volumetric changes of gutta-percha at maxi- mum temperatures of 80” and 45” C.

REFERENCES I. Flagg. J. Foster: Gum-Percha; a Permanent Filling Material,

Trans. N. I’. Odontol. Sot., pp. 24-41, 1888. ” Webster. A. E.: The Place of Gutta-Percha in Dentistry, Domin-

ion Dent. J. 43: 103-104. 1931. Price, Weston A.: Report of Laboratory Investigations on the Physical Properties of Root Filling Materials and the Efficiency of Root Fillings for Blocking Infection From Sterile Tooth Struc- tures, J. Natl. Dent. Assoc. 5: 1259-1280, 1918. Massler, Maury, and Osaevsky, Abraham: Sealing Qualities of Various Filling Materials, J. Dent. Child. 21: 228-243, 1954. Parris, Leonard. and Kapsimalis, Peter: The Effects of Tempera- ture Changes on the Sealing Properties of Temporary Filling Materials. ORAI. SURG. 13: 982-989. 1960. Best, E. J.. Gurney, B. F.. and Sowle. T.: Utilization of Di- mensional Changes in Gutta-Percha Cones to Simplify Filling of the Root Canal. Dent. Digest 69: 14.20, 1963. Gurney, B. F., Best, E. I., and Ciervasio, G.: Physical Mea- surements of Gutta-Percha. ORAL SURG. 32: 260-270, 1971. Goodman, Alvin. Schilder. Herbert, and Aldrich, Winthrop: The Thermomechanical Properties of Gutta-Percha. Part II. The History and Molecular Chemistry of Gutta-Percha, ORAL SURG. 37: 954-961, 1974. Schilder, Herbert, Goodman. Alvin, and Aldrich, Winthrop: The Thermomechanical Properties of Gutta-Percha. Part III. The Determination of Phase Transition Temperatures for Gutta- Percha, ORAL SURG. 38: 109-I 14, 1974. Schilder, Herbert: Filling Root Canals in Three Dimensions, Dent. Clin. North Am.. pp. 723-744. November, 1967. Schilder, Herbert: Cleaning and Shaping the Root Canal, Dent. Clin. North Am.. pp. 269-296, April, 1974. Schilder, Herbert, Goodman. Alvin, and Aldrich. Winthrop: The Thermomechanical Properties of Gutta-Percha. Part V. The Determination of Volume Changes in Bulk Gutta-Percha as a Function of Temperature, J. Ended. (In press.)

Reprint reqmsts to: Dr. Herbert Schilder Boston University School of Graduate Dentistry 100 East Newton St. Boston, Mass. 02118