Discussion - References.
Several parameters influence the heating of guttapercha during vertical compaction, particularly because canal systems are complex and narrow. The main parameter to be considered is the low heat conductive capability of dentine (1.36 _10_3 cal/cm2 8C; Craig & Peyton 1961); other factors influencing the heating of gutta-percha are the dentine thickness (Figdor et al.1983), the dimension of the residual roots (Fors et al. 1985) and the periradicular blood circulation (Hardie1986).
The various influences related to the heating of guttapercha can be classified into mechanical, physical, chemical and biological.
Mechanical factors are related to the fact that guttapercha must be plasticized to become adapted to the canal, however, it should not be too soft in order to allow control of the material. Two aspects influence the degree of softening: the necessity to reach and fill narrow and small spaces that require a soft gutta-percha, and the clinical necessity of precise positioning of the cone at the apex in particular when the patency of the orifice is maintained. Moreover, with very soft gutta-percha, the pluggers would penetrate the material rather than compacting it; this could generate empty spaces inside the mass of the gutta-percha during the removal of the instruments (Jurcak et al.1992).
Chemical-physical aspects are related to the fact that the gutta-percha heating process should remain within the range from 37 to 42-45 8C. Temperatures higher than 45 8C give volumetric changes during the cooling owing to phase changes in the structure of the material (Schilder et al. 1985). Heating gutta-percha over 100 8C gives irreversible modifications to the molecular structure (Marciano & Michailesco1989).
Biological problems relate to possible damage to periradicular tissues when the gutta-percha is heated inside the root and when the residual dentine walls are thin (Lee et al.1998).
Factors affecting heat distribution relate to the compaction technique (Silver et al. 1999) and the use of an endodontic cement, that acts as a heat insulator for the periodontal region. Indeed, it has been reported that an endodontic cement can decrease the temperatures by 1-2 8C on the root surface (Barkhordar et al.1990). Considering this finding and to avoid unpredictable influences in the temperatures of the gutta-percha, the endodontic cement was purposely omitted during the specimen preparation particularly because differences in the thickness of the cement layer within the specimens could have affected the results.
Problems may be encountered during the application of warm vertical compaction techniques in small canals asit can be difficult to obtain real thermal effects on the apical portion of the root-canal filling materials. It has been demonstrated (Goodman et al. 1981) that an increase of 4 8C of the apical gutta-percha (over the body temperature of 37 8C) is the ideal level to obtain the correct softening for excellent compaction and good control of the material in accordance with the Schilder’s technique (Schilder 1967). Moreover, it has been reported that using traditional heat-carriers (heated on flame) in wide canals (Marlin & Schilder 1973) the gutta-percha in the coronal third can reach an average temperature of approximately 50 8C, whilst getting closer to the apex the increase of temperature (DT) dramatically drops; at the apical third of the canal DT ison ly 2-4 8C, thus temperature rise over 42 8Care rare. Other similar data were recorded filling wide canals (Goodman et al. 1981, Marciano &Michailesco1989) and confirmed that minimal increases in temperature of gutta-percha are obtainable when approaching the apex even if the carrier is heated with a flame at very high temperature. In thin root canals, these difficulties in heating gutta-percha at the correct temperature increase dramatically.
The use of an electric heat carrier such as Touch’n Heat (models5 001, 5002 and 5004) produces different thermal phenomena (Analytic Technology Corp. 1993). In fact, the heating source is the tip insert itself that can be continuously heated, moreover very thin and flexible tips a re available allowing them to reach apical regions in curved and thin canals.
The System-BHeat Source (AnalyticTechnology Corp. 1997a), compared to the Touch’nHeat, presents more technological capabilities such as the display for the temperature control of the tip and the advantage of combining heating and compaction allowing the Buchanan’s‘ Continuous Wave’ obturating technique (Buchanan1996).
The suggested working temperature for the device using the smallest tip at a distance of 5-7 mm from the apex is185 8C (Buchanan1998). In the present study, different temperature settings were selected in order to evaluate the effective heating power of the device (Table 1) as previous studies have revealed that at the temperature reading on the unit’s liquid crystal display was in accurate and higher than achieved at the tip (Blum et al. 1997, Silver et al. 1999). These discrepancies have been confirmed in this study, thus a higher temperature (250 8C) than the suggested one (200 8C) was set for the down-pack. In fact by setting the tested System-B Heat Source at the temperature of 250 8C on the display, the highest temperature recorded was 160 8C, 2 mm from the tip (Table 1): this result was obtained by putting the thermocouple indirect contact with the instrument.
Several in vitro studies recorded different values of temperature of the gutta-percha during the warm vertical compaction technique (Blum et al. 1997, DuLac et al. 1999, Silver et al. 1999). The different results probably depend upon three factors: the dimension of canals, the periradicular temperature, and the use of a system of thermo-dispersion that can simulate the periradicular blood circulation by maintaining it at a constant temperature of 37 8C. Overall, in such narrow canals there is a small contact area between the tip of the instrument and the coronal part of the apical gutta-percha, reducing the transfer of heat.
The results of the present study revealed that the use of the System-B Heat Source in root canals immersed in a thermostatic bath of water at a constant temperature of 37 8C induces insignificant temperature increases on the apical gutta-percha (0.5 _0.5 8C for group 1 and 0.9 _1.1 8C for group 2) and in all specimens, this increase was lower than the 4 8C previously identified as optimal for the warm compaction. These findings were not related to insufficient penetration of the insertion tip inside canals as the mean distances from the apex were lower (3.26 mm in group 1 and 2.86 mm in group 2) than the suggested ones under normal clinical conditions( usually up to 5-7 mm from the apex). To obtain changes of 3-4 8C within the apical gutta-percha in narrow canals it would be necessary to use instruments at a higher temperature, which might negatively affect periradicular tissues, particularly at the CEJ where dangerous valuesc an be recorded (Hand et al. 1976,Weller & Koch 1995). An increase of 10 8C in periradicular tissues is considered to be dangerous (Eriksson & Albrektsson1983, Fors et al.1985, Hardie1986, Gutmann et al.1987a, Gutmann et al.1987b, Saunders 1990,Weller & Koch 1995), even if effective bone necrosis was found only after maintaining a constant temperature of 44- 47 8C for at least 1min (Eriksson & Albrektsson 1983). Previous studies revealed that traditional heat carriers and Touch’n Heat can create a10 8C increase on the outer surface of the root (Blum et al. 1997, Silver et al. 1999), whilst the System-B Heat Source usually cannot reach this level (Lee et al.1998, Romero et al.1998). The results of this study also confirmed that the DT at point C was not higher than 10 8C in any of the specimens (mean value of 4 8C forgroup1and 3.8 8C for group 2), thus confirming the safety of the clinical use of the System-BHeat Source. Moreover, under normal clinical conditions the endodontic cement, by increasing thermal insulation of the endodontic canal system, reduces the possibility of creating damage to periradicular tissues using the System-B Heat Source.
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