Discussion - References.
In the present study, extracted whole mandibular molar roots were used as the experimental model. Because root-canal morphology can affect the application and removal of calciumhydroxide and canal obturation, distribution of roots with each group was made on the basis of root and canal shape.
Because it is difficult to remove calcium hydroxide paste completely from the root canal (Guignes et al. 1991, Margelos et al.1997, Lambrianidis et al.1999), there is a need to determine whether the remaining calcium hydroxide paste has any beneficial or adverse effect on the filling. There have been several studies regarding the influence of remaining calciumhydroxide on the apical seal of root-canal fillings. Porkaew et al. (1990) and Holland et al. (1995) reported that apical leakage was less in teeth that received calcium hydroxide dressings than in non-medicated control teeth. Holland et al. (1995) explained the reduced leakage in calcium hydroxide medicated teeth on the basis that the residual calcium hydroxide was incorporated into the sealer during obturation, which caused a decrease in the permeability of the sealer itself or that calcium hydroxide was transported or mechanically forced into the dentinal tubules, blocking them off and decreasing dentinal permeability. They also suggested that the improved seal with plugs of calcium hydroxide was due to the fact that the plug provides a stop or matrix against which the gutta-percha and sealer may be condensed more effectively.
However, in the present study, calcium hydroxide medicated root canals showed significantly more apical leakage than ones without medication. Because calcium hydroxide cannot be removed completely from the canal, it is likely that the remaining calcium hydroxide may interfere with the sealability of fillings when a zinc oxide-eugenol sealer is used. A number of explanations may account for the difference between the present study and those of Holland et al. (1995). Firstly, it is not likely that a decrease in dentinal permeability will lead to a decrease in apical leakage. Leakage at the apex occurs between the root-canal wall and the sealer, between the sealer and the gutta-percha, or within the sealer itself. In the present apical-leakage study, all the specimens were coated with nail varnish on their root surface to exclude any possible leakage through the dentine surface to leave only apical pathways. Secondly, when calcium hydroxide was mixed with zinc oxide-eugenol sealer, a calcium hydroxide-eugenol compound was reported to be produced, which was more soluble, less sealing had a thicker film-thickness and higher water sorption value than the original sealer (Park et al. 1999). In another study (Park 1999), calcium hydroxide and zinc oxide-eugenol were shown to form calcium eugenolate, or that calcium bonds to eugenol by an ionic bond which can be broken when water is present. These reports imply that, when a zinc oxide-eugenol sealer is used, calcium eugenolate is formed which weakens the sealer in the long term.
Even though there is no reported technique that can remove calcium hydroxide completely from the canal (Lambrianidis et al. 1999), root-canal irrigation with NaOCl and EDTA solutions have given better results than NaOCl alone (Margelos et al. 1997, Calt & Serper 1999). However, in the present study, there was no significant difference in leakage between these two groups. A combined approach to remove calcium hydroxide from the canal with irrigation and an instrumentation demonstrated no difference in the apical seal between K-files one size larger than the MAF, and K-files of the same size as the MAF. Holland et al. (1995) also reported similar results with size 40-70 files.
Calcium hydroxide has been reported to have a decolourizing effect on methylene blue dye (Kontakiotis et al. 1997). Wu et al. (1998) also studied the decolourizing effect of six dental filling materials on1%methylene blue dye, and found that calciumhydroxide had decolourized the dye by 74%. The decolourizing effect of calcium hydroxide is related to itβs high alkalinity and this varies according to its form, that is, paste, cone or sealer (Esberard et al.1996,Wu et al.1998, Calt et al.1999, Economides et al. 1999). On the contrary, India ink was not shown to be decolourized by calcium hydroxide (Caliskan et al. 1998). This might be one of the possible explanations for the results of the present study using India ink being different from previous studies with methylene blue (Porkaew et al.1990, Holland et al.1995).
India ink particles are known to be much larger than methylene blue, but are still small enough to penetrate cracks within root-canal sealers (Yoshikawa et al. 1997). Ahlberg et al. (1995) compared linear leakage patterns of methylene blue and India ink, and showed that methylene blue penetrated more deeply and with more variance. This penetration-depth difference between methylene blue dye and India ink might be another possible explanation for differences between the present study using India ink and previous studies using methylene blue.
In the present study, the set sealer showed relatively uneven thickness, some cracks and voids in the calcium hydroxide-medicated canals. These appearances are in agreement with other studies (Margelos et al.1997, Park et al.1999) and can be explained by the report (Margelos et al. 1997) that a faster setting of sealer prevented full sealing of gutta-percha, and this may be related to the thicker appearance of the sealer in the present study. The present appearance of a thicker sealer is also in accordance with the finding of Calt & Serper (1999) that when calcium hydroxide was left on the root-canal surface, the sealer could not penetrate into dentinal tubules.
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