0099-2399/98/2404-0236503.00/0 JOURNAL OF ENDOOONTICS
Printed in U.S.A.
Copyright © 1998 by The American Association of Endodontists
VOL. 24, NO. 4, APRIL1998
pH Measurement of Root Canal Sealers Tsui-Hsien Huang, DDS, and Chia-Tze Kao, DDS, MMS
The purpose of this study was to compare the surface pH level of three different type sealers after mixing at various time intervals in vitro. The cements were mixed according to the manufacturer's instructions. They were incubated to set in 100% humidity at 37°C for I h, 24 h, 5 days, 8 days, 2 wk, 3 wk, 4 wk, 5 wk, and 7 wk. pH was calculated by a Twin pH meter. The pH levels of the three sealers were different at various time intervals (p < 0.0001). The resin-based cement had a acid pH level (pH < 7.0). The calcium hydroxide-based cement showed a higher alkalinity pH level (pH > 7.0). The zinc oxide-eugenol-based cement showed a similar pH level to the calcium hydroxide cement at the end of the measurement. We postulated that, in endodontic therapy when those healing is needed, the alkaline-based sealer is the choice.
M A T E R I A L S AND METHODS
Preparation of Samples and Test Solutions The following three root canal filling materials were used: Canals (ZOE-based cement; Taipei, Taiwan), pulp canal sealer (calcium hydroxide-based cement; Kerr, Detroit, MI), and AH26 (resin-based cement; De-Trey, Zurich, Switzerland). The cements were mixed according to the manufacturer's instructions. Multidishes (Nunc, Delta Denmark) were used to hold the cements to be tested. Three groups of 8 dishes contained the three same types of cement tested. Each dish was immersed in 0.5 ml of pH 7.0 buffer solution. The control group contained buffer solution only. They were incubated in 100% humidity at 37°C for 1 h, 24 h, 5 days, 8 days, 2 wk, 3 wk, 4 wk, 5 wk, and 7 wk.
pH Determination and Statistical Test The pH level of the sealers in each dish was determined with the Twin pH meter (Horiba, Japan) at each test interval. The measurement was made by wiping the sample with a piece of sampling sheet (Horiba, Japan) that was soaked in buffer solution and placed on a sensor. The sampling sheet is a special paper with strong retention characteristics. Before testing at each time, the pH meter was corrected by the manufacturer's instructions. The sheets were placed in the dish and incubated with the sealer from the beginning to the end of the test. The pH levels of the controls, in which no sealer had been placed, were determined at the same time intervals as those in the other groups. After the pH levels of the solutions in the control and test groups were determined at the various times, mean and standard deviation were calculated, the significance of differences was determined with one-way analysis of variance (ANOVA), followed by Student-Newman-Keuls multiple range comparisons (9).
There are three types of root canal sealers used in endodontic treatment. They are the zinc oxide-eugenol (ZOE)-based cements, the calcium hydroxide-based cements, and the resin-based cements. Studies have shown that the resorption process may be arrested by proper endodontic therapy (1, 2). A combination of sealing ability and biocompatibility of a root canal sealer is important for favorable prognosis in root canal treatment. The chemical tissue irritation effect is most important in considering sealer properties before selecting one (3). The pH level will affect experimental wound healing of human fibroblasts in vitro (4). It has been postulated that the high pH level of calcium hydroxide in root canal fillings promotes a state of alkalinity in adjacent tissues, a condition that favors repair (5). This influence on the pH level has been shown to be nonspecific and of low toxicity because of the low solubility of calcium hydroxide (6). Granchi et al. (7) reported that some root canal sealers could hamper the periapex healing processes by inhibiting cell proliferation through a selective action on different phases of the cell cycle. Denli and Eskitascioglu (8) demonstrated that the acidity of cements changed considerably, depending on time. The purpose of this study was to compare the surface pH level of three different type sealers at various time intervals in vitro.
RESULTS The mean pH levels at the nine times for all four samples are shown in Table 1. One-way ANOVA was performed to determine if there were any statistically significant differences between the pH determinations for the four samples at nine different times. Results show that for each of the nine intervals, A N O V A had an F probability of p > 0.0001; therefore, the Student-Newman-Keuls tests were performed each time.
Vol. 24, No. 4, April 1998
TABLE 1. Mean pH levels of the three root canal sealers at nine different times Time 1 h 24 h 5 days 8 days 2 wk 3 wk 4 wk 5 wk 7 wk
Canals 5.67 6.79 7.10 7.19 8.03 8.04 7.58 7.36 7.43
_+ 2.04 +_ 0.19 -+ 0.09 _+ 0.14 +- 0.13" + 0.13" -+ 0.15" -+ 0.15 _+ 0.21
Sealer Paste 7.75 7.08 7.10 7.15 7.73 7.63 7.26 7.40 7.60
+ 0.38* _+ 0.19" _+ 0.08 _+ 0.08 _+ 0.31 -+ 0.13 +_ 0.11 _+ 0.12 +_ 0.08*
AH26 5.69 5.85 5.86 5.93 6.11 6.53 6.19 6.31 6.38
_+ 0.30 _+ 0.08~ + 0.08¢ _+ 0.07¢ _+ 0.12¢ _+ 0.14¢ +_ 0.15~ _+ 0.08¢ _+ 0.05¢
Control 6.9 6.9 6.9 6.9 6.9 6.9 6.9 6.9 6.9
-+ 0 + 0 -+ 0 -+ 0 + 0 -+ 0 +- 0 -+ 0 +_ 0
ANOVA p (F value) 7.81" 126.141" 544.28T 398.391" 200.971" 274.021" 222.73t 211.751" 197.72
* Significantly higher than all other groups (p < 0.05). t Statistically significantly different at p < 0.0001. :~Significantly lower than all other groups (p < 0.05).
At 1 h, calcium hydroxide cement had higher pH levels (p < 0.0l). At 24 h, the calcium hydroxide group had higher pH levels (p < 0.001) than the other groups. At 5 and 8 days, the resin group had a lower pH level (p < 0.001) than the other groups. There was no difference between the ZOE group and calcium hydroxide group. At 2, 3 and 4 wk, the ZOE group had higher pH levels (p < 0.001) than the other groups. At the 5th wk, the resin-based cement had the lowest pH level (p < 0.001). At the 7th wk, calcium hydroxide had a higher pH levels (p < 0.001) than the other groups, and the resin group had the lowest pH level. DISCUSSION The optimal condition for hard tissue resorption to take place is an acid pH of the tissues; at an acid pH, the acid hydrolases are active and a demineralization of the mineral component of the tissue occurs (10). Some hydroxyapatites show a considerable increase of pH in aqueous solution. A soluble amorphous phase is responsiblefor the increase in pH (11 ). From this point of view, the increase of pH level is good for bone repair. Many studies have found that most cements show a low solubilityin distilled water, that reduction in pH can produce a marked increasedin solubility (12-14). In our study, to modify the true oral condition,we used pH 7.0 buffer solution as a solvent to detect the pH levels of the samples. By using the Twin pH detector, pH values can be detected easily. The zinc oxide cement pH levels increased as time increased, up to 3 wk, then decreased to the 7th wk. The reason may be that the acid cement degraded by leaching out ions is accelerated by low pH. Sensitivity to a particular pH depends on how firmly the metal ions are complexed in the cement. However, the setting reaction of eugenol-based cement is reversible, and their degradation involves a gradual loss of eugenol and a hydrolysis of zinc eugenolate chelate causing the cement to revert to zinc oxide (14, 15). Calcium hydroxide-based cement pH levels increased as time increased. In the bone repair process, the rise in pH would be unfavorable for osteoclastic acid hydrolase activity, because the optimal pH for these enzymes ranges from 5 to 5.5, and their activity might be inhibited (10, 16). An alkaline pH might activate alkaine phosphateses which, it has been suggested, play an important role in hard tissue formation (17). The pH of a hydroxide salt solution is a function of its solubility and its ionization constant (i.e. its propensity to dissociate into positive-charged cations and negative-charged hydroxyl groups).
Therefore, the cation moeity determines the ultimate alkalinity of the compound (6). Gordon et al. (6) showed that calcium hydroxide exerts a nonspecific pH effect that is not overly toxic by virture of its low solubility. In contrast to calcium hydroxide pH levels, the resin-based cement exhibited a lower pH level in all test groups. Many investigators have studied the elution of these unbound molecules into aqueous media. Results of these studies indicated that elution of leachable components from composites is rapid. The quantity of leachables has been correlated to the degree of cure of the polymer network. The composition and solubility characteristics of the extraction solvent influence the kinetics and mechanism of the elution process (18). Elution is generally thought to occur via diffusion of molecules through the resin matrix, and is therefore dependent on size and chemical characteristics of the leachable species. Drs. Huang and Kao are lecturers, Dental Department, Chung Shan Medical and Dental College, Taiwan, Republic of China. Address requests for reprints to Dr. Tsui-Hsien Huang, Dental Department, Chung Shah Medical College Hospital, 23, Section 1, Taichung Kang Road, Taichung, Taiwan, Republic of China.
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