0099-2399/92/1806-0263/$03.00/0 Printed in U.S.A.
JOURNAL OF ENDODONTICS
VOL. 18, NO. 6, JUNE1992
Copyright © 1992 by The American Association of Endodontists
Thermomechanical Analysis of Dental Gutta-percha Joseph Marciano, DCD, DSO, Pierre Michailesco, DCD, DSO, Emmanuel Charpentier, DCD, Luiz C. Carrera, MSc, and Marc J. M. Abadie, PhD
Samples of three common commercial gutta-percha endodontic filling points (Hygienic, Mynol, and Maillefer Pink) and a sample of natural gutta-percha were submitted to thermomechanical analysis under pressures ranging from 0.01 to 0.2 N. Samples of Mynol and Maillefer filling points were thermally treated and then submitted to thermomechanical analysis in parallel with differential scanning calorimetric analysis. The results show that thermomechanical analysis gives results distinct from those obtained by classic dilatometry and that it is a technique well suited to the study of the thermoplastic properties of guttapercha. An analysis of the results shows that the amount of the inorganic component used in a commercially available endodontic point has a strong influence on its thermomechanical properties. Thermodynamical properties and the "thermal history" of the gutta-percha are also important. Both temperature and force should be controlled in order to assess the thermomechanical properties of endodontic filling points, while the latter have not yet been codified in clinical procedures.
whose temperatures can be increased at a constant rate. The tip of a probe is in contact with the top of the sample to be tested and exerts a calibrated constant force on it (pressure). The thermal expansion of the sample is transmitted to the probe, whose displacement can be measured on a gauge, allowing for measurement of the linear expansion coefficient. This procedure can be fully automated; the oven heat input is computer controlled to follow a programmed plan, sample thickness measurements are taken and recorded, and the linear thermal expansion coefficient is computed on-line. Automatization allows for the use of TMA in routine testing. Furthermore, TMA appliances can be twinned to differential scanning calorimetry (DSC) equipment. Hence, it is possible to correlate variations of the linear thermal expansion coefficient to phase transitions in the gutta-percha as measured by DSC.
MATERIALS AND METHODS Commercially available samples of natural gutta-percha and Mynol, Hygienic, and Maillefer Pink gutta-percha endodontic points were used. The TMA instrument was a Mettler TMA 40, coupled to a DSC instrument Mettler TA 4000. The sample frame and probe of this equipment are made of quartz glass, whose very
According to Grossmann (1), one of the most important criteria for success in endodontics is the tridimensional stability of the root canal filling material. Gutta-percha has been found by various authors, Weine (2), Nguyen (3), and Schilder (4), to have very desirable properties for endodontic obturation. Among other factors, its thermal dilatometric properties are important. These properties have been studied by Dean (5), Leeper and Schlessinger (6), and by Schilder et al. (7). All of these authors used classic dilatometric methodology. These methods are currently being challenged by automated methods such as thermomechanical analysis (TMA). The objective of this article was to study the dilatometric properties of dental gutta-percha points by means of TMA.
Oven
Calibration force
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PRINCIPLES OF THERMOMECHANICAL ANALYSIS
source
Figure 1 is a diagram of the principle of TMA equipment; a sample of the material to be studied is heated in an oven
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Sample thickness measuring gauge
FIG 1. Principle of thermomechanical analysis.
263
264
Marciano et al.
Journal of Endodontics
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small thermal expansion is negligible, thus simplifying the analysis of results. The gutta-percha samples were compressed between two parallel glass plates in order to obtain a sheet 0.6- _.+0.5-ram thick. A 5- × 5-mm square sample was cut from this sheet for analysis. For all manipulations, the TMA instrument oven was programmed to obtain a constant rise in temperature at a rate of 5°C/min, starting at 20"C. Since different probes were available, a preliminary experiment compared the performances of two probes: a pointed, sharp tip probe and an hemispherical (2 m m in diameter) probe. This comparison was made on samples of Mynol guttapercha points, using a pressure of 0.1 N. Samples of all materials were tested by TMA, using pressures of 0.01, 0.025, 0.05, 0.1, and 0.2 N. Since it is known that the "thermal history" of a guttapercha sample has an influence on its thermal properties, the TMA results obtained on fresh samples of gutta-percha ob-
tained from Mynol and Maillefer Pink points were compared with results obtained after thermal treatment of these points (left at 60°C for 30 min and cooled to 20°C before TMA). These comparisons were made with a pressure of 0.025 N. For this experiment, DSC scanning was done in parallel on a similar sample, using a temperature increase rate of 5°C/rain. RESULTS The results obtained from Mynol filling point samples with pressures of 0.01 and 0.I N are given in Figs. 2 and 3. These curves are typical of all of the curves obtained in this study. Two curves were computed for each experiment: the curve on the right gives the position of the probe tip as a function of the temperature. The curve on the left is the derivative of this position over time. Since temperature is a linear function of time, this derivative is also the derivative of the probe tip position along temperature.
Vol. 18, No. 6, June 1992
Analysis of Gutta-percha 220
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When there is no penetration of the probe into the sample, the position of the probe measures the thickness of the sample and its derivative along temperature is hence the linear thermal expansion coefficient. In all of the experiments an expansion of the samples was observed with the maximum occurring around 50°C, followed by penetration of the probe into the samples. The penetration was initially fast and then slowed. Hence, the necessity to study not only the thermal expansion of the samples, but also the characteristics of the penetration of the probe into the samples resulted in expression of penetration depth as a fraction of the original thickness of the sample. Amount of data obtained is too large to be reported; it can however be summarized graphically. Figure 4 plots the temperature at which penetration of the probe in samples of Mynol gutta-percha under a pressure of 0.1 N was observed, for both the sharp probe and the hemispherical probe.
Penetration (%) FIG 7. Temperature at which a given penetration is observed in Hygienic samples.
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266
Marciano et al.
Journal of Endodontics 160
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points. Abscissa, penetration (percentage of sample); ordinate, temperature at which this penetration is observed. Figures 5 to 8 plot the temperature at which probe penetration is observed under various pressures for each of the four materials under study• Figure 9 plots the maximal linear thermal expansion coefficient under various pressures for all of the materials under study. Figures l0 and 11 plot the temperature at which a given penetration is observed for fresh and thermically treated annealed Mynol gutta-percha samples and for fresh and thermically treated Maillefer gutta-percha samples, respectively• Figures 12 to 15 are the results of DSC scanning of fresh and thermically treated Mynol gutta-percha samples and fresh and thermically treated Maillefer gutta-percha samples• Thermodynamic transition temperatures can be determined from these curves. DISCUSSION The results obtained from testing Mynol gutta-percha points are typical of all of the results obtained and can be
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used as a model for discussion purposes. The linear thermal expansion coefficient is initially small, and then increases up to a maximum around 50°C. It is worth noting that this temperature is approximately the temperature at which a phase transition (~ --, a) occurs in gutta-percha. This phase transition is verifiable by DSC examination. The maximal speed of probe penetration is observed just after the maximal expansion occurs. This can be explained as follows. At the beginning of probe penetration, the contact
Analysis of Gutta-percha
Vol. 18, No. 6, June 1992
267
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medean forces become larger, slowing the penetration. A comparison of the curves obtained with probe pressures of 0.01 and 0.1 N is important. With the smaller probe pressure, the measured linear thermal expansion coefficient is larger (2 × 10-2/*C compared with 10-2/°C), the maximal speed of penetration is smaller (2.5 um/s compared with 6 #m/s) and even at 150"C, the sample is not fully penetrated. With a pressure of 0.1 N the sample is totally penetrated at 100*C. Hence, it can be concluded that probe pressure used for TMA is an important factor in the interpretation of results and penetration of the samples is an artifact inherent to the method used. This conclusion justifies the fact that probe pressures superior to 0.2 N were not used for this study. The importance of probe pressure in interpretation of TMA results is even more apparent in Fig. 5. With light probe pressures (0.01 to 0.05 N) the temperature needed to reach a given penetration is highly dependent on the pressure as is shown by the slopes of the regression lines which are steep. Conversely, with high probe pressure, the penetration is fast, almost instantaneous, and the temperature at which it occurs is almost constant.
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surface between the probe and the sample is almost a point. When the probe begins to penetrate the sample, this contact surface widens; hence, the forces of friction as well as Archi-
Observed (Fig. 4) was the fact that the sharp-tipped probe did not penetrate as well as did the spherical-tipped probe. A 10% penetration was observed at 40°C with the hemispherical probe and at 48°C with the sharp one. Both results were obtained with Mynol points under a probe pressure of 0.1 N. Since attention was called to the penetration characteristics
268
Marciano et al,
Journal of Endodontics
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FIG 17. Comparison of the materials under study with a study pressure of 0.025 N. Abscissa, penetration (percentage of sample); ordinate, temperature at which this penetration is observed.
FIG 19. Comparison of the materials under study with a study pressure of 0.1 N. Abscissa, penetration (percentage of sample); ordinate, temperature at which this penetration is observed.
of the probe tip, the hemispherical probe was chosen for this study.
percha samples. This could be related to the fact that these different materials have inorganic components in their composition. According to Marciano and Michailesco (8), Mynol gutta-percha points have a slightly higher inorganic component than do Hygienic gutta-percha points. According to Marciano (9), MaiUefer gutta-percha points have a higher inorganic component than do Mynol gutta-percha points. And, of course, natural gutta-percha has none at all. The resistance to penetration by probes of these materials are ranked according to their inorganic content. This relationship is illustrated by Figs. 16 to 20 where the temperature necessary to get a given penetration of the probe for all of the materials under study with a given pressure is plotted. The theologic properties of the different materials can be readily compared as follows: for small pressures (0.1 N), the relationship between material hardness and its inorganic content is sharp.
Effect of Pressure on the Penetration
In Figs. 5 to 8 the temperature at which various pressures were observed under different probe pressures was plotted for each of the materials in study. It was observed that the pressure-temperature relationship is almost linear for all of the materials. Among these materials natural gutta-percha exhibits little resistance to penetration even with pressures of 0.01 N. The samples were fully penetrated at temperatures around 80°C. Conversely, Maillefer gutta-percha point samples exhibited the best resilience, while Mynol and Hygienic gutta-percha point samples gave similar results, that is intermediate between natural gutta-percha and the Maillefer gutta-
Analysis of Gutta-percha
Vol. 18, No. 6, June 1992 100
for a pressure of 0.2 N, this relationship still exists but the difference between materials is rather small. These observations justify the decision to use only moderate probe pressures for this study. Characterization of the Thermorheologic Properties: The Penetration Coefficient The results obtained show that the rheologic properties of gutta-percha are highly dependent on temperature• A given probe penetration can be achieved either by a rise in temperature or by a rise in pressure. Hence, it is justifiable to speak of thermorheologic properties of gutta-percha. The thermorheologic properties of a given sample of gutta-percha as determined by this experiment can be summarized in the following way. Let F be given penetration, dP a given variation of pressure (except for natural gutta-percha, only consider variations of pressure between 0.025 and 0.2 N, i.e. dP = 0.175 N in what follows), and d T the difference between temperatures at which penetration F is observed with respective low and high pressures. The variation of penetrability exhibited by gutta-percha can be characterized by a penetration coefficient, Cp, which can be computed as: d(dT) F
Cp-
de
This penetration coefficient (whose dimensions are *C/N) is a measure of the sensitivity of a given sample to heating. For the materials under study, the coefficients can be reconstructed from Figs. 5 to 8 and are given in Table 1. These values are in accord with previously discussed results. Table 1 also gives values determined by Rootare et at. (10), which are close to the results of the study. It should be noted that, since natural gutta-percha is extremely soft, measurements made with pressures of 0.01 and 0.1 N for this material were used. Effect of Thermal Treatment Figures 10 and 11 show that thermal treatment before a TMA determination modifies the thermomechanical properties of the samples. Both the Maiilefer Pink and Mynol gutta-percha samples were penetrated further after thermal treatment, this softening being more marked for the Maiilefer than for the Mynol gutta-percha. Figures 12 to 15 show that the transition enthalpy of the Maillefer Pink gutta-percha samples is lowered by 26.4% by thermal cycling, while this lowering is only 14.1% for the Mynol gutta-percha percha sample. In a previous study (11), this result was interpreted as a reduction of crystallinity of the samples. Such a reduction could explain the fact that thermally treated samples are softer than are fresh samples. Variations of the Maximal Linear Thermal Expansion Coefficient The linear thermal expansion coefficient increases with temperature, up to the thermodynamic transition point. The
269
CHARGE - 0.2N
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A
MAILLEFER PINK
70
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i
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i
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70
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80
i
90
•
100
Penetration (%) FIG 20. Comparison of the materials under study with a study pressure of 0.2 N. Abscissa, penetration (percentage of sample); ordinate, temperature at which this penetration is observed. TABLE 1. Penetration coefficients (in °C/N) as observed in our study (Cp) and as measured by Rootare et al. (10) (PC) Material
Cp
PC
Maillefer Pink Hygienic Mynol Natural gutta-percha
1.32 1.20 0.85 0.22
-1.16 0.80 0.38
maximal linear expansion coefficient (Fig. 9) was measured just before the thermodynamic transition. The results obtained are highly paradoxical. Maillefer Pink gutta-percha samples, which have an important inorganic component, exhibit a much greater thermal expansion than does natural (not-composite) gutta-percha, while one should reasonably expect a smaller expansion of materials containing inorganic components• This finding could be explained by the fact that gutta-percha tends to flow or creep, even before its thermodynamic transition. Inorganic components which harden gutta-percha tend to lower this flow. Hence, the softer the material is, the higher the flow will be. What has been measured as an expansion coefficient might be in fact a measure of the superposition of two phenomena that is both a thermal expansion and a flow. The effect would be to lower the observed expansion. Further studies using TMA at low temperatures are needed to confirm or refute this hypothesis. Maximal Speed of Penetration Figure 21 plots the maximal speed of penetration of the probe in the sample, measured just after the thermodynamic transition, as a function of the pressure; in fact, the inverse of the force exerted on the probe. An important finding is that the maximal speed of penetration and the nature of the sample are almost independent; the inorganic content of the material has no importance here. This is confirmed by the fact that the relationship between pressure and the maximal speed of penetration is very close to a negative exponential. All of the
270
Marciano et al.
Journal of Endodontics
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strate the importance of the thermodynamic properties of the gutta-percha used in the points. As a consequence, the thermal history of these points is important to their clinical properties. Although great importance has been given to the codification and control of temperature in endodontic procedures, the control of pressure has not received such attention. The results of this study show that this parameter is important in clinical applications. Dr. Marciano and Dr. Michailesco are members of the Faculte de Chirurgie Dentaire de Montpellier, Montpellier, France. Dr. Charpenter is a member of the Departement de rlnformation Medicale, CHRU Caremau, NTmes, France. Mr. Carrera and Dr. Abadie are members of the Laboratoire d'Etude des Materiaux Polymeres/MaterJaux Avances Organiques (LEMP/MAO), Universite Montpellier II, Sciences et Techniques du-Languedoc, Montpellier, France. Address requests for reprints to Dr. Joseph M;-:ciano, Centre Medical Specialise des Collines d'Estanove, 65, Route de Laverune, F-34 070 Montpellier, France.
"O 2 > i
i
i
i
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)
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curves converge to a limit value of 6 um/s which could be attained for an infinite pressure. This is probably characteristic of the fluidity of gutta-percha. These curves also confirm that 0.2 N is the maximal admissible pressure for this kind of experiment. The most important conclusion of this study is that the nature and amount of the inorganic components of guttapercha used in endodontic points strongly influences their mechanical properties and thermal behavior. These components do not, however, allow a good control of their effective dilatometric properties, which depend also on the mechanical aspects of their use. The results suggest that better suited proportions of inorganic components should be researched. The existence of discrepancies among the thermomechanical behaviors of fresh and thermically treated samples demon-
References 1. Grossman LI. Endodontic practice. 10th ed. Philadelphia: Lea & Febiger, 1981. 2. Weine FS. Endodontic therapy. 3rd ed. St. Louis: CV Mosby, 1982. 3. Nguyen NT. Obturation of the root canal system. In: Cohen S, Burns SC, eds. Pathways of the pulp. 3rd ed. St. Louis: CV Mosby, 1984. 4. Schilder H. Filling root canals in three dimensions. Dent Clin North Am 1967;11:723-44. 5. Dean JL. Heat treatment and polymorphism of gutta-percha and balata. IRI Transactions 1932;8:25-37. 6. Leeper HM, Schlessinger W. Gutta II interconversion of alpha and beta forms. J Poly Sci 1953;11:307-23. 7. Schilder H, Goodman A, Aldrich W. The thermomechanical properties of gutta-percha. Part V: Volume changes in bulk gutta-percha as a function of temperature and its relationship to molecular phase transformation. Oral Surg Oral Med Oral Patho11985;59:285-96. 8. Marciano J, Michailesco PM. Dental gutta-percha: chemical composition, x-ray identification, enthalpic studies and clinical applications. J Endodon 1989;11:149-53. 9. Marciano J. Propri6t6s physicochimiques de la gutta-percha. Application en Endodontie [Thesis]. Montpellier, France: Universite de Montpellier, 1991. 10. Rootare HM, Powers JM, Smith RL. Thermal analysis of dental guttapercha. In: Porter RS, Johnson Jr, eds. Analytical calorimetry. Vol 4. New York: Plenum Press, 1977. 11. Marciano J, Michailesco P, Charpentier E, Breysse F, Carrera LC, Abadie MJM. Thermal phase transitions of dental gutta-percha: differential scanning calorimetry analysis. J Dent Res (in press).
JOURNAL OF ENDODONTICS
VOL. 18, NO. 6, JUNE1992
Copyright © 1992 by The American Association of Endodontists
Thermomechanical Analysis of Dental Gutta-percha Joseph Marciano, DCD, DSO, Pierre Michailesco, DCD, DSO, Emmanuel Charpentier, DCD, Luiz C. Carrera, MSc, and Marc J. M. Abadie, PhD
Samples of three common commercial gutta-percha endodontic filling points (Hygienic, Mynol, and Maillefer Pink) and a sample of natural gutta-percha were submitted to thermomechanical analysis under pressures ranging from 0.01 to 0.2 N. Samples of Mynol and Maillefer filling points were thermally treated and then submitted to thermomechanical analysis in parallel with differential scanning calorimetric analysis. The results show that thermomechanical analysis gives results distinct from those obtained by classic dilatometry and that it is a technique well suited to the study of the thermoplastic properties of guttapercha. An analysis of the results shows that the amount of the inorganic component used in a commercially available endodontic point has a strong influence on its thermomechanical properties. Thermodynamical properties and the "thermal history" of the gutta-percha are also important. Both temperature and force should be controlled in order to assess the thermomechanical properties of endodontic filling points, while the latter have not yet been codified in clinical procedures.
whose temperatures can be increased at a constant rate. The tip of a probe is in contact with the top of the sample to be tested and exerts a calibrated constant force on it (pressure). The thermal expansion of the sample is transmitted to the probe, whose displacement can be measured on a gauge, allowing for measurement of the linear expansion coefficient. This procedure can be fully automated; the oven heat input is computer controlled to follow a programmed plan, sample thickness measurements are taken and recorded, and the linear thermal expansion coefficient is computed on-line. Automatization allows for the use of TMA in routine testing. Furthermore, TMA appliances can be twinned to differential scanning calorimetry (DSC) equipment. Hence, it is possible to correlate variations of the linear thermal expansion coefficient to phase transitions in the gutta-percha as measured by DSC.
MATERIALS AND METHODS Commercially available samples of natural gutta-percha and Mynol, Hygienic, and Maillefer Pink gutta-percha endodontic points were used. The TMA instrument was a Mettler TMA 40, coupled to a DSC instrument Mettler TA 4000. The sample frame and probe of this equipment are made of quartz glass, whose very
According to Grossmann (1), one of the most important criteria for success in endodontics is the tridimensional stability of the root canal filling material. Gutta-percha has been found by various authors, Weine (2), Nguyen (3), and Schilder (4), to have very desirable properties for endodontic obturation. Among other factors, its thermal dilatometric properties are important. These properties have been studied by Dean (5), Leeper and Schlessinger (6), and by Schilder et al. (7). All of these authors used classic dilatometric methodology. These methods are currently being challenged by automated methods such as thermomechanical analysis (TMA). The objective of this article was to study the dilatometric properties of dental gutta-percha points by means of TMA.
Oven
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PRINCIPLES OF THERMOMECHANICAL ANALYSIS
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FIG 1. Principle of thermomechanical analysis.
263
264
Marciano et al.
Journal of Endodontics
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small thermal expansion is negligible, thus simplifying the analysis of results. The gutta-percha samples were compressed between two parallel glass plates in order to obtain a sheet 0.6- _.+0.5-ram thick. A 5- × 5-mm square sample was cut from this sheet for analysis. For all manipulations, the TMA instrument oven was programmed to obtain a constant rise in temperature at a rate of 5°C/min, starting at 20"C. Since different probes were available, a preliminary experiment compared the performances of two probes: a pointed, sharp tip probe and an hemispherical (2 m m in diameter) probe. This comparison was made on samples of Mynol guttapercha points, using a pressure of 0.1 N. Samples of all materials were tested by TMA, using pressures of 0.01, 0.025, 0.05, 0.1, and 0.2 N. Since it is known that the "thermal history" of a guttapercha sample has an influence on its thermal properties, the TMA results obtained on fresh samples of gutta-percha ob-
tained from Mynol and Maillefer Pink points were compared with results obtained after thermal treatment of these points (left at 60°C for 30 min and cooled to 20°C before TMA). These comparisons were made with a pressure of 0.025 N. For this experiment, DSC scanning was done in parallel on a similar sample, using a temperature increase rate of 5°C/rain. RESULTS The results obtained from Mynol filling point samples with pressures of 0.01 and 0.I N are given in Figs. 2 and 3. These curves are typical of all of the curves obtained in this study. Two curves were computed for each experiment: the curve on the right gives the position of the probe tip as a function of the temperature. The curve on the left is the derivative of this position over time. Since temperature is a linear function of time, this derivative is also the derivative of the probe tip position along temperature.
Vol. 18, No. 6, June 1992
Analysis of Gutta-percha 220
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When there is no penetration of the probe into the sample, the position of the probe measures the thickness of the sample and its derivative along temperature is hence the linear thermal expansion coefficient. In all of the experiments an expansion of the samples was observed with the maximum occurring around 50°C, followed by penetration of the probe into the samples. The penetration was initially fast and then slowed. Hence, the necessity to study not only the thermal expansion of the samples, but also the characteristics of the penetration of the probe into the samples resulted in expression of penetration depth as a fraction of the original thickness of the sample. Amount of data obtained is too large to be reported; it can however be summarized graphically. Figure 4 plots the temperature at which penetration of the probe in samples of Mynol gutta-percha under a pressure of 0.1 N was observed, for both the sharp probe and the hemispherical probe.
Penetration (%) FIG 7. Temperature at which a given penetration is observed in Hygienic samples.
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266
Marciano et al.
Journal of Endodontics 160
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points. Abscissa, penetration (percentage of sample); ordinate, temperature at which this penetration is observed. Figures 5 to 8 plot the temperature at which probe penetration is observed under various pressures for each of the four materials under study• Figure 9 plots the maximal linear thermal expansion coefficient under various pressures for all of the materials under study. Figures l0 and 11 plot the temperature at which a given penetration is observed for fresh and thermically treated annealed Mynol gutta-percha samples and for fresh and thermically treated Maillefer gutta-percha samples, respectively• Figures 12 to 15 are the results of DSC scanning of fresh and thermically treated Mynol gutta-percha samples and fresh and thermically treated Maillefer gutta-percha samples• Thermodynamic transition temperatures can be determined from these curves. DISCUSSION The results obtained from testing Mynol gutta-percha points are typical of all of the results obtained and can be
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used as a model for discussion purposes. The linear thermal expansion coefficient is initially small, and then increases up to a maximum around 50°C. It is worth noting that this temperature is approximately the temperature at which a phase transition (~ --, a) occurs in gutta-percha. This phase transition is verifiable by DSC examination. The maximal speed of probe penetration is observed just after the maximal expansion occurs. This can be explained as follows. At the beginning of probe penetration, the contact
Analysis of Gutta-percha
Vol. 18, No. 6, June 1992
267
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medean forces become larger, slowing the penetration. A comparison of the curves obtained with probe pressures of 0.01 and 0.1 N is important. With the smaller probe pressure, the measured linear thermal expansion coefficient is larger (2 × 10-2/*C compared with 10-2/°C), the maximal speed of penetration is smaller (2.5 um/s compared with 6 #m/s) and even at 150"C, the sample is not fully penetrated. With a pressure of 0.1 N the sample is totally penetrated at 100*C. Hence, it can be concluded that probe pressure used for TMA is an important factor in the interpretation of results and penetration of the samples is an artifact inherent to the method used. This conclusion justifies the fact that probe pressures superior to 0.2 N were not used for this study. The importance of probe pressure in interpretation of TMA results is even more apparent in Fig. 5. With light probe pressures (0.01 to 0.05 N) the temperature needed to reach a given penetration is highly dependent on the pressure as is shown by the slopes of the regression lines which are steep. Conversely, with high probe pressure, the penetration is fast, almost instantaneous, and the temperature at which it occurs is almost constant.
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surface between the probe and the sample is almost a point. When the probe begins to penetrate the sample, this contact surface widens; hence, the forces of friction as well as Archi-
Observed (Fig. 4) was the fact that the sharp-tipped probe did not penetrate as well as did the spherical-tipped probe. A 10% penetration was observed at 40°C with the hemispherical probe and at 48°C with the sharp one. Both results were obtained with Mynol points under a probe pressure of 0.1 N. Since attention was called to the penetration characteristics
268
Marciano et al,
Journal of Endodontics
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FIG 19. Comparison of the materials under study with a study pressure of 0.1 N. Abscissa, penetration (percentage of sample); ordinate, temperature at which this penetration is observed.
of the probe tip, the hemispherical probe was chosen for this study.
percha samples. This could be related to the fact that these different materials have inorganic components in their composition. According to Marciano and Michailesco (8), Mynol gutta-percha points have a slightly higher inorganic component than do Hygienic gutta-percha points. According to Marciano (9), MaiUefer gutta-percha points have a higher inorganic component than do Mynol gutta-percha points. And, of course, natural gutta-percha has none at all. The resistance to penetration by probes of these materials are ranked according to their inorganic content. This relationship is illustrated by Figs. 16 to 20 where the temperature necessary to get a given penetration of the probe for all of the materials under study with a given pressure is plotted. The theologic properties of the different materials can be readily compared as follows: for small pressures (0.1 N), the relationship between material hardness and its inorganic content is sharp.
Effect of Pressure on the Penetration
In Figs. 5 to 8 the temperature at which various pressures were observed under different probe pressures was plotted for each of the materials in study. It was observed that the pressure-temperature relationship is almost linear for all of the materials. Among these materials natural gutta-percha exhibits little resistance to penetration even with pressures of 0.01 N. The samples were fully penetrated at temperatures around 80°C. Conversely, Maillefer gutta-percha point samples exhibited the best resilience, while Mynol and Hygienic gutta-percha point samples gave similar results, that is intermediate between natural gutta-percha and the Maillefer gutta-
Analysis of Gutta-percha
Vol. 18, No. 6, June 1992 100
for a pressure of 0.2 N, this relationship still exists but the difference between materials is rather small. These observations justify the decision to use only moderate probe pressures for this study. Characterization of the Thermorheologic Properties: The Penetration Coefficient The results obtained show that the rheologic properties of gutta-percha are highly dependent on temperature• A given probe penetration can be achieved either by a rise in temperature or by a rise in pressure. Hence, it is justifiable to speak of thermorheologic properties of gutta-percha. The thermorheologic properties of a given sample of gutta-percha as determined by this experiment can be summarized in the following way. Let F be given penetration, dP a given variation of pressure (except for natural gutta-percha, only consider variations of pressure between 0.025 and 0.2 N, i.e. dP = 0.175 N in what follows), and d T the difference between temperatures at which penetration F is observed with respective low and high pressures. The variation of penetrability exhibited by gutta-percha can be characterized by a penetration coefficient, Cp, which can be computed as: d(dT) F
Cp-
de
This penetration coefficient (whose dimensions are *C/N) is a measure of the sensitivity of a given sample to heating. For the materials under study, the coefficients can be reconstructed from Figs. 5 to 8 and are given in Table 1. These values are in accord with previously discussed results. Table 1 also gives values determined by Rootare et at. (10), which are close to the results of the study. It should be noted that, since natural gutta-percha is extremely soft, measurements made with pressures of 0.01 and 0.1 N for this material were used. Effect of Thermal Treatment Figures 10 and 11 show that thermal treatment before a TMA determination modifies the thermomechanical properties of the samples. Both the Maiilefer Pink and Mynol gutta-percha samples were penetrated further after thermal treatment, this softening being more marked for the Maiilefer than for the Mynol gutta-percha. Figures 12 to 15 show that the transition enthalpy of the Maillefer Pink gutta-percha samples is lowered by 26.4% by thermal cycling, while this lowering is only 14.1% for the Mynol gutta-percha percha sample. In a previous study (11), this result was interpreted as a reduction of crystallinity of the samples. Such a reduction could explain the fact that thermally treated samples are softer than are fresh samples. Variations of the Maximal Linear Thermal Expansion Coefficient The linear thermal expansion coefficient increases with temperature, up to the thermodynamic transition point. The
269
CHARGE - 0.2N
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100
Penetration (%) FIG 20. Comparison of the materials under study with a study pressure of 0.2 N. Abscissa, penetration (percentage of sample); ordinate, temperature at which this penetration is observed. TABLE 1. Penetration coefficients (in °C/N) as observed in our study (Cp) and as measured by Rootare et al. (10) (PC) Material
Cp
PC
Maillefer Pink Hygienic Mynol Natural gutta-percha
1.32 1.20 0.85 0.22
-1.16 0.80 0.38
maximal linear expansion coefficient (Fig. 9) was measured just before the thermodynamic transition. The results obtained are highly paradoxical. Maillefer Pink gutta-percha samples, which have an important inorganic component, exhibit a much greater thermal expansion than does natural (not-composite) gutta-percha, while one should reasonably expect a smaller expansion of materials containing inorganic components• This finding could be explained by the fact that gutta-percha tends to flow or creep, even before its thermodynamic transition. Inorganic components which harden gutta-percha tend to lower this flow. Hence, the softer the material is, the higher the flow will be. What has been measured as an expansion coefficient might be in fact a measure of the superposition of two phenomena that is both a thermal expansion and a flow. The effect would be to lower the observed expansion. Further studies using TMA at low temperatures are needed to confirm or refute this hypothesis. Maximal Speed of Penetration Figure 21 plots the maximal speed of penetration of the probe in the sample, measured just after the thermodynamic transition, as a function of the pressure; in fact, the inverse of the force exerted on the probe. An important finding is that the maximal speed of penetration and the nature of the sample are almost independent; the inorganic content of the material has no importance here. This is confirmed by the fact that the relationship between pressure and the maximal speed of penetration is very close to a negative exponential. All of the
270
Marciano et al.
Journal of Endodontics
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strate the importance of the thermodynamic properties of the gutta-percha used in the points. As a consequence, the thermal history of these points is important to their clinical properties. Although great importance has been given to the codification and control of temperature in endodontic procedures, the control of pressure has not received such attention. The results of this study show that this parameter is important in clinical applications. Dr. Marciano and Dr. Michailesco are members of the Faculte de Chirurgie Dentaire de Montpellier, Montpellier, France. Dr. Charpenter is a member of the Departement de rlnformation Medicale, CHRU Caremau, NTmes, France. Mr. Carrera and Dr. Abadie are members of the Laboratoire d'Etude des Materiaux Polymeres/MaterJaux Avances Organiques (LEMP/MAO), Universite Montpellier II, Sciences et Techniques du-Languedoc, Montpellier, France. Address requests for reprints to Dr. Joseph M;-:ciano, Centre Medical Specialise des Collines d'Estanove, 65, Route de Laverune, F-34 070 Montpellier, France.
"O 2 > i
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curves converge to a limit value of 6 um/s which could be attained for an infinite pressure. This is probably characteristic of the fluidity of gutta-percha. These curves also confirm that 0.2 N is the maximal admissible pressure for this kind of experiment. The most important conclusion of this study is that the nature and amount of the inorganic components of guttapercha used in endodontic points strongly influences their mechanical properties and thermal behavior. These components do not, however, allow a good control of their effective dilatometric properties, which depend also on the mechanical aspects of their use. The results suggest that better suited proportions of inorganic components should be researched. The existence of discrepancies among the thermomechanical behaviors of fresh and thermically treated samples demon-
References 1. Grossman LI. Endodontic practice. 10th ed. Philadelphia: Lea & Febiger, 1981. 2. Weine FS. Endodontic therapy. 3rd ed. St. Louis: CV Mosby, 1982. 3. Nguyen NT. Obturation of the root canal system. In: Cohen S, Burns SC, eds. Pathways of the pulp. 3rd ed. St. Louis: CV Mosby, 1984. 4. Schilder H. Filling root canals in three dimensions. Dent Clin North Am 1967;11:723-44. 5. Dean JL. Heat treatment and polymorphism of gutta-percha and balata. IRI Transactions 1932;8:25-37. 6. Leeper HM, Schlessinger W. Gutta II interconversion of alpha and beta forms. J Poly Sci 1953;11:307-23. 7. Schilder H, Goodman A, Aldrich W. The thermomechanical properties of gutta-percha. Part V: Volume changes in bulk gutta-percha as a function of temperature and its relationship to molecular phase transformation. Oral Surg Oral Med Oral Patho11985;59:285-96. 8. Marciano J, Michailesco PM. Dental gutta-percha: chemical composition, x-ray identification, enthalpic studies and clinical applications. J Endodon 1989;11:149-53. 9. Marciano J. Propri6t6s physicochimiques de la gutta-percha. Application en Endodontie [Thesis]. Montpellier, France: Universite de Montpellier, 1991. 10. Rootare HM, Powers JM, Smith RL. Thermal analysis of dental guttapercha. In: Porter RS, Johnson Jr, eds. Analytical calorimetry. Vol 4. New York: Plenum Press, 1977. 11. Marciano J, Michailesco P, Charpentier E, Breysse F, Carrera LC, Abadie MJM. Thermal phase transitions of dental gutta-percha: differential scanning calorimetry analysis. J Dent Res (in press).
