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The possible role of pH changes during EDTA demineralization of teeth
The possible role of pH changes during EDTA demineralization of teeth Victor Calve Pkrez, Biochem BS, Maria Ester Medina Girdenas, DDS, and Ulises Sbnchez Planells, DDS, * Santiago, Chile FACULTY
OF DENTISTRY,
UNIVERSITY
OF CHILE
Demineralization of dentin is a dynamic process. The pH of EDTA solutions inside pulp cavities decreases as demineralization occurs. Increasing EDTA concentration from 0.1 mol/L to 0.5 mol/L causes a faster acid accumulation and a higher demineralization rate. The efficiency of EDTA solutions decreases as time goes on, probably because of the acid released from EDTAHNa, (the predominating species under neutral conditions). Not all the available EDTA reacts after a few hours. The so-called auto/imitation might be due to this acidification of the EDTA solution. (ORAL SURG ORAL MED ORAL PATHOL 1989;68:220-2)
T
he efficiency of teeth root demineralization by EDTA salts is one of the subjects currently being “reevaluted” by some endodontists.’ Among the major factors affecting root canal cleansing by EDTA solutions, acidity can be mentioned playing an important role in three possible ways. First, the chelating ability of EDTA increases as acidity decreases2 Second, solubility of teeth mineral (hydroxyapatite, HA) increases as pH decreases (on acid medium).3 Third, pH enhances “the penetrability of EDTA” into small spaces.4 A wide range of pH conditions has been investigated by different authors,“7 all of whom suggested very different pH optimum conditions. However, most commercial preparations average a pH of 7.3, possibly looking for a greater solubility of hydroxyapatite and an improved chelating efficiency of EDTA solutions. As mentioned earlier by Nikiforuk and Sreebny,’ when large volumes of EDTA were employed, no pH changes were observed. However, in root canals, we can only place as much as 0.05 ml of EDTA solutions, Under these conditions, the concentration of EDTA on the mineral surface must decrease as reaction advances. In addition, under neutral pH conditions, the sodium salt of EDTA (at pH 7.3,99% is EDTAHNa,) is supposed to exchange hydrogen
*Professor of Chemistry
220
and Head, Department
of Chemistry.
ion by calcium during chelation2 with a subsequent decrease in pH. Since pulpal dentin contains hydroxyapatite (HA, a calcium mineral), it is feasible that acid may be released during demineralization. The purpose of this research was to explore the dynamic of teeth demineralization by EDTA, the rate of acid evolution, and the EDTA consumption during demineralization by different EDTA concentrations. MATERIALS
AND METHODS
Extracted, intact human molars were cleaned and coated with dental wax. The crown was cut off with a diamond circular saw, leaving open the pulp cavity of the teeth. The roots were cleaned and organic material was mechanically extracted from the pulp cavity* Measurements
Five different EDTA concentrations were placed in the cavities, and pH (acidity) was measured by using a small tip glass electrode (Fig. 1, A). The pH electrode was soaked in EDTA out of the cavity before the experiment. The pH changes were recorded by means of a Corning-l 12 digital pH-meterrecorder device (Medfield, Massachusetts). After demineralization, EDTA solutions were poured out of the pulp cavities into small flasks and back-titrated in accordance with the diddiette meth-
Volume 68 Number 2
Changes in pH during EDTA demineralization
of teeth
221
6.6 - PH
4
t (i-k)
'2
'3
Fig. 1. A, Radiography of small tip (3 mm) glass electrode placed on pulp cavity. B, Typical pH evolution during demineralization of pulp cavities for different EDTA HNa, concentrations. Control without EDTA (C) contains 0.1 mol/L imidazole. HCL on the pulp cavity of 25” C. EDTA concentrations are indicated beSideeach tracing.
od.* EDTA concentration changes inside root canals and cavities were measured up to 3 hours after the treatment on 12 different teeth.
0.86
log EDTA
log PH l
.
,**
RESULTS
Recordings of pH changes during demineralization of teeth pulp cavities are shown in Fig. 1, B. The greater the EDTA concentration, the more the pH decreased. All tracings level off after a few hours. Neither EDTA itself nor a buffer inside the cavity changed the pH, as the control shows. Data obtained for pH changes in EDTA solutions were redrawn as log pH on Fig. 2, b (starred line). These results were predicted by the chelation theory.2 On the other hand, log (EDTA) decreasedas the reaction proceeded (Fig. 2, A, dotted line), which is in agreement with the results obtained by Seidberg and Schilder’ by weighing undissolved dentin after EDTA treatment. Fairly good correlation was obtained in both cases, as evaluated by Student’s t test (p < 0.005). The extent of EDTA consumption was followed to more than 80% completion by titration. After 3 hours, we observed no further reaction, just as Seidberg and Schilder reported previously. DISCUSSION
These findings suggest the following: The pH decreased because of EDTA’s proton
l
.
.
* -0.2
.
- -0.6 0.85
,-1.0 0.82
I 1 t (hr)
I 2
I
* -1.4
3
Fig. 2. Relationship between EDTA consumption and acid formation (pH) during demineralization of pulp tissues on 12 different teeth. (a) log (EDTA) . . . n = 12; r = 0.993; p < 0.005. (b) log pH ***n = 22; r = 0.990; p < 0.005.
exchange by Ca+2.As pH approachespkEDTA (6.1), the pH levels off as a result of the buffer effect of EDTA near pH 6.1. On the other hand, the acid released could react with hydroxyapatite, affecting the solubility of the dentin.3T9 l The coexistence of protonation (2) and complex formation (1) is the likely mechanism. Chelation prevails to a greater extent under neutral conditions.
222 Calve, Medina, and Shnchez Increasing the hydrogen ion reverses the complex formation (1). (1) EDTA HS3+ Ca+z = EDTA Cae2+ H+ (2) EDTA H-3 + H+ = EDTA H;* As the reaction proceeds, acid accumulates and protonation of EDTA prevails (2), thus decreasing the rate of demineralization (1). (The rate of acid demineralization of dentin is much slower than the demineralization rate of EDTA). This might be one of the mechanisms for “self-limitation.” On the other hand, as pH decreases,the solutions might not be so penetrating, as several authors have determined.4,‘o l The higher the EDTA concentration, the faster the pH decreases, thus indicating that the proton comes from EDTAHNa,. CONCLUSIONS
1. EDTA solutions inside teeth cavities change their pH during demineralization. 2. As pH decreases,dentin demineralization rate decreases, thus “limiting” the amount of dissolved dentin. 3. These changes might alter the physical properties of cleansing solutions, thus having an impact on their efficiency (i.e., penetrability). 4. These observations can be rationalized in terms of creating a constant pH medium to keep high demineralization rates up to 99% completion of the reaction. 5. In the past, the importance of reaction kinetics was underestimated. However, this chemical approach can explain several facts already described by endodontists and currently under “reevaluation.”
ORAL SURC ORAL MED ORAL PATHOL August 1989 This research received University of Chile.
a grant
(M-2728-8712)
from the
REFERENCES 1. Dow PR. EDTA-time of reevaluation. lnt Endod J 1984; 17~2-4. 2. Dwyer FP, Mellor DP. Chelating agents and metal chelates. New York: Academic Press, 1964:283-333. 3. Theuns HM, van Dijk JWE, Driessens FCM, Groeneveld A. Effect of time, degree of saturation, pH and acid concentration of buffer solutions on the rate of in vitro demineralization of human enamel. Arch Oral Biol 1985;30:37-42. 4. Ravnik C, Sand HF, March T. Enamel lesions produced in vitro by solutions of EDTA and EDTA-sodium salts. Acta Odontol Stand 1962;20:349-58. 5. Nikiforuk G, Sreebny L. Demineralization of hard tissues by organic chelating agents at neutral pH. J Dent Res 1953; 32:859-67. 6. Cury JA, Bragotto C, Valdrighi L. The demineralization efficiency of EDTA solutions on dentin. I. Influence of pH. ORAL SURC ORAL MED ORAL PATHOL 1981;52:446-8. 7. Seidberg BH, Schilder H. An evaluation of EDTA in endodontics. ORAL SURC ORAL MED ORAL PATHOL 1974;37:60920. 8. Vogel A. A textbook of quantitative inorganic analysis. 2nd ed. Chapter 111. London: Longmans, Green & Co, 1951: 535-6. 9. Brown WE. Physicochemical mechanism of dental caries. J Dent Res 1974;53(suppl 2):204-16. 10. Marwan A, Patonai FJ. The effect of decreasing surface tension on the flow of irrigating solutions in narrow root canals. ORAL SURG ORAL MED ORAL PATHOL 1982;53: 524-6.
Reprint requests to: Ulises Sanchez Planells Fact&ad Odontologia U. de Chile Avda. Sta. Maria 57 1. Santiago, Chile
OF DENTISTRY,
UNIVERSITY
OF CHILE
Demineralization of dentin is a dynamic process. The pH of EDTA solutions inside pulp cavities decreases as demineralization occurs. Increasing EDTA concentration from 0.1 mol/L to 0.5 mol/L causes a faster acid accumulation and a higher demineralization rate. The efficiency of EDTA solutions decreases as time goes on, probably because of the acid released from EDTAHNa, (the predominating species under neutral conditions). Not all the available EDTA reacts after a few hours. The so-called auto/imitation might be due to this acidification of the EDTA solution. (ORAL SURG ORAL MED ORAL PATHOL 1989;68:220-2)
T
he efficiency of teeth root demineralization by EDTA salts is one of the subjects currently being “reevaluted” by some endodontists.’ Among the major factors affecting root canal cleansing by EDTA solutions, acidity can be mentioned playing an important role in three possible ways. First, the chelating ability of EDTA increases as acidity decreases2 Second, solubility of teeth mineral (hydroxyapatite, HA) increases as pH decreases (on acid medium).3 Third, pH enhances “the penetrability of EDTA” into small spaces.4 A wide range of pH conditions has been investigated by different authors,“7 all of whom suggested very different pH optimum conditions. However, most commercial preparations average a pH of 7.3, possibly looking for a greater solubility of hydroxyapatite and an improved chelating efficiency of EDTA solutions. As mentioned earlier by Nikiforuk and Sreebny,’ when large volumes of EDTA were employed, no pH changes were observed. However, in root canals, we can only place as much as 0.05 ml of EDTA solutions, Under these conditions, the concentration of EDTA on the mineral surface must decrease as reaction advances. In addition, under neutral pH conditions, the sodium salt of EDTA (at pH 7.3,99% is EDTAHNa,) is supposed to exchange hydrogen
*Professor of Chemistry
220
and Head, Department
of Chemistry.
ion by calcium during chelation2 with a subsequent decrease in pH. Since pulpal dentin contains hydroxyapatite (HA, a calcium mineral), it is feasible that acid may be released during demineralization. The purpose of this research was to explore the dynamic of teeth demineralization by EDTA, the rate of acid evolution, and the EDTA consumption during demineralization by different EDTA concentrations. MATERIALS
AND METHODS
Extracted, intact human molars were cleaned and coated with dental wax. The crown was cut off with a diamond circular saw, leaving open the pulp cavity of the teeth. The roots were cleaned and organic material was mechanically extracted from the pulp cavity* Measurements
Five different EDTA concentrations were placed in the cavities, and pH (acidity) was measured by using a small tip glass electrode (Fig. 1, A). The pH electrode was soaked in EDTA out of the cavity before the experiment. The pH changes were recorded by means of a Corning-l 12 digital pH-meterrecorder device (Medfield, Massachusetts). After demineralization, EDTA solutions were poured out of the pulp cavities into small flasks and back-titrated in accordance with the diddiette meth-
Volume 68 Number 2
Changes in pH during EDTA demineralization
of teeth
221
6.6 - PH
4
t (i-k)
'2
'3
Fig. 1. A, Radiography of small tip (3 mm) glass electrode placed on pulp cavity. B, Typical pH evolution during demineralization of pulp cavities for different EDTA HNa, concentrations. Control without EDTA (C) contains 0.1 mol/L imidazole. HCL on the pulp cavity of 25” C. EDTA concentrations are indicated beSideeach tracing.
od.* EDTA concentration changes inside root canals and cavities were measured up to 3 hours after the treatment on 12 different teeth.
0.86
log EDTA
log PH l
.
,**
RESULTS
Recordings of pH changes during demineralization of teeth pulp cavities are shown in Fig. 1, B. The greater the EDTA concentration, the more the pH decreased. All tracings level off after a few hours. Neither EDTA itself nor a buffer inside the cavity changed the pH, as the control shows. Data obtained for pH changes in EDTA solutions were redrawn as log pH on Fig. 2, b (starred line). These results were predicted by the chelation theory.2 On the other hand, log (EDTA) decreasedas the reaction proceeded (Fig. 2, A, dotted line), which is in agreement with the results obtained by Seidberg and Schilder’ by weighing undissolved dentin after EDTA treatment. Fairly good correlation was obtained in both cases, as evaluated by Student’s t test (p < 0.005). The extent of EDTA consumption was followed to more than 80% completion by titration. After 3 hours, we observed no further reaction, just as Seidberg and Schilder reported previously. DISCUSSION
These findings suggest the following: The pH decreased because of EDTA’s proton
l
.
.
* -0.2
.
- -0.6 0.85
,-1.0 0.82
I 1 t (hr)
I 2
I
* -1.4
3
Fig. 2. Relationship between EDTA consumption and acid formation (pH) during demineralization of pulp tissues on 12 different teeth. (a) log (EDTA) . . . n = 12; r = 0.993; p < 0.005. (b) log pH ***n = 22; r = 0.990; p < 0.005.
exchange by Ca+2.As pH approachespkEDTA (6.1), the pH levels off as a result of the buffer effect of EDTA near pH 6.1. On the other hand, the acid released could react with hydroxyapatite, affecting the solubility of the dentin.3T9 l The coexistence of protonation (2) and complex formation (1) is the likely mechanism. Chelation prevails to a greater extent under neutral conditions.
222 Calve, Medina, and Shnchez Increasing the hydrogen ion reverses the complex formation (1). (1) EDTA HS3+ Ca+z = EDTA Cae2+ H+ (2) EDTA H-3 + H+ = EDTA H;* As the reaction proceeds, acid accumulates and protonation of EDTA prevails (2), thus decreasing the rate of demineralization (1). (The rate of acid demineralization of dentin is much slower than the demineralization rate of EDTA). This might be one of the mechanisms for “self-limitation.” On the other hand, as pH decreases,the solutions might not be so penetrating, as several authors have determined.4,‘o l The higher the EDTA concentration, the faster the pH decreases, thus indicating that the proton comes from EDTAHNa,. CONCLUSIONS
1. EDTA solutions inside teeth cavities change their pH during demineralization. 2. As pH decreases,dentin demineralization rate decreases, thus “limiting” the amount of dissolved dentin. 3. These changes might alter the physical properties of cleansing solutions, thus having an impact on their efficiency (i.e., penetrability). 4. These observations can be rationalized in terms of creating a constant pH medium to keep high demineralization rates up to 99% completion of the reaction. 5. In the past, the importance of reaction kinetics was underestimated. However, this chemical approach can explain several facts already described by endodontists and currently under “reevaluation.”
ORAL SURC ORAL MED ORAL PATHOL August 1989 This research received University of Chile.
a grant
(M-2728-8712)
from the
REFERENCES 1. Dow PR. EDTA-time of reevaluation. lnt Endod J 1984; 17~2-4. 2. Dwyer FP, Mellor DP. Chelating agents and metal chelates. New York: Academic Press, 1964:283-333. 3. Theuns HM, van Dijk JWE, Driessens FCM, Groeneveld A. Effect of time, degree of saturation, pH and acid concentration of buffer solutions on the rate of in vitro demineralization of human enamel. Arch Oral Biol 1985;30:37-42. 4. Ravnik C, Sand HF, March T. Enamel lesions produced in vitro by solutions of EDTA and EDTA-sodium salts. Acta Odontol Stand 1962;20:349-58. 5. Nikiforuk G, Sreebny L. Demineralization of hard tissues by organic chelating agents at neutral pH. J Dent Res 1953; 32:859-67. 6. Cury JA, Bragotto C, Valdrighi L. The demineralization efficiency of EDTA solutions on dentin. I. Influence of pH. ORAL SURC ORAL MED ORAL PATHOL 1981;52:446-8. 7. Seidberg BH, Schilder H. An evaluation of EDTA in endodontics. ORAL SURC ORAL MED ORAL PATHOL 1974;37:60920. 8. Vogel A. A textbook of quantitative inorganic analysis. 2nd ed. Chapter 111. London: Longmans, Green & Co, 1951: 535-6. 9. Brown WE. Physicochemical mechanism of dental caries. J Dent Res 1974;53(suppl 2):204-16. 10. Marwan A, Patonai FJ. The effect of decreasing surface tension on the flow of irrigating solutions in narrow root canals. ORAL SURG ORAL MED ORAL PATHOL 1982;53: 524-6.
Reprint requests to: Ulises Sanchez Planells Fact&ad Odontologia U. de Chile Avda. Sta. Maria 57 1. Santiago, Chile