Solvent Action of Sodium Hypochlorite on Bovine Pulp and Physico-Chemical Properties of Resulting Liquid
Júlio César Emboava SPANÓ1
Eduardo Luiz BARBIN1
Tanit Clementino SANTOS1
Luiz Fernando GUIMARÃES2
Jesus Djalma PÉCORA1
1Department of Endodontics, Faculty of Dentistry of Ribeirão Preto,
University of São Paulo, Ribeirão Preto, SP, Brazil
2Faculty of Pharmaceutical Sciences of Ribeirão Preto, University
of São Paulo, Ribeirão Preto, SP, Brazil
Correspondence: Prof. Dr. Jesus Djalma Pécora, Departamento de Odontologia Restauradora, Faculdade de Odontologia de Ribeirão Preto, Universidade de São Paulo, 14040-904, Ribeirão Preto, SP, Brasil. e-mail: pecora@forp.usp.br
Braz Dent J (2001) 12(3): 154-79 ISSN 0103-6440
INTRODUCTION | MATERIAL AND METHODS | RESULTS | DISCUSSION | RESUMO | REFERENCES
The solvent effect of 4 concentrations of sodium hypochlorite (0.5, 1.0,
2.5, and 5.0%) on bovine pulp tissue and the level of residual chlorine,
pH and surface tension before and after dissolution were studied in vitro.
A fragment of bovine pulp was submersed in sodium hypochlorite that circulated
in an apparatus with a peristaltic pump and Luer Lok syringe. The higher
the concentration of sodium hypochlorite the faster the dissolution of the
pulp tissue. All concentrations of sodium hypochlorite reduced the pH and
the surface tension and the higher concentrations of the solution had the
least consumption of chlorine during tissue dissolution. Thus this study
indicated that residual chlorine was directly proportional to concentration
in the process of pulp tissue dissolution and that there was residual chlorine
at all concentrations used.
Key words: sodium hypochlorite, bovine pulp, solvent action.
INTRODUCTION
Sodium hypochlorite has proven to be an excellent solution for the chemical-biomechanical
preparation of root canals. One of its most important characteristics, the
capability of tissue dissolution (1-5), has been studied by numerous researchers
(2-12). However, there is little reported in scientific literature on the
mechanism of action of sodium hypochlorite on organic material and the physico-chemical
properties of this solution after dissolution of organic tissue as well as
the effect of pH on tissue dissolution.
The objective of this investigation was to study, in vitro, the effect of
four concentrations of sodium hypochlorite on the rate of bovine pulp tissue
dissolution at environmental temperature and evaluate the level of residual
chlorine, pH and surface tension after dissolution.
MATERIAL AND METHODS
Four concentrations of sodium hypochlorite (0.5, 1.0, 2.5, and 5.0%) were
prepared from a 10% solution and stored in amber glass flasks at 9oC for
three days. The solutions were removed from the refrigerator to reach room
temperature before use.
Central incisors were removed from bovine mandibles and stored in cool saline
for 2 h. The crowns were fractured, and the pulp was removed manually from
the root canal surface with a #10 K file (Maillefer, Ballaigues, Switzerland),
wrapped in plastic and frozen for 2 days. The pulp was subsequently thawed
at room temperature and a 10-mm long fragment of the central portion was
removed by transverse cuts with a scalpel. Each fragment was weighed on a
precision scale (MLW, Bonn, Germany).
For the evaluation of dissolution, a urethane hose was connected to the needle
end of a Luer Lok glass syringe (B-D Yale, São Paulo, SP, Brazil)
and to the entrance of a peristaltic pump. The exit of the pump was connected
to another segment of urethane hose that was placed at the other end of the
syringe. A nylon net with the pulp fragment was place inside the syringe
and 15 ml of sodium hypochlorite circulated at a constant flow of 64 ml/min.
The time required for the dissolution of the pulp fragment was confirmed
by 16X magnification and recorded.
Evaluation of the residual chlorine was done by titration. pH was evaluated
with a Digimed meter and surface tension was analyzed with a tensiometer
(Fisher Scientific, Pittsburgh, PA, USA) before and after each test of pulp
tissue dissolution which was repeated 5 times for each solution at 65 ±
5% humidity and 22 ± 2oC.
The Kruskal-Wallis test and the regression/correlation test were used for
statistical analysis with p<0.01 for the hypothesis of equality (H0).
RESULTS
Table 1 shows the means of the dissolution of bovine pulp tissue, the level
of residual chlorine, the pH and the surface tension before and after dissolution.
The speed of pulp dissolution was obtained with the equation v = m/t, where
v is the velocity of dissolution measured in mg/s, m is the fragment mass
in mg, and t is the time of total dissolution of the pulp fragment in seconds.
There was a statistically significant difference at 1% among all multiple
comparisons (Kruskal-Wallis) and the regression/correlation test showed a
directly proportional relation between solution concentration and speed of
dissolution.
For the evaluation of residual chlorine, the concentration of chlorine before
dissolution was compared to that after dissolution by the equation Clr =
(Cla x 100)/Clb, where Clr is the residual chlorine, Cla is the chlorine
concentration after pulp tissue dissolution and Clb is the chlorine concentration
before dissolution. There was a statistically significant difference at 1%
for all multiple comparisons. The regression/correlation test showed a directly
proportional relation between sodium hypochlorite concentration and residual
chlorine.
The regression/correlation test showed an inversely proportional relation
between sodium hypochlorite concentration and the reduction in pH and a directly
proportional relation between the concentration and the reduction of surface
tension.
DISCUSSION
Sodium hypochlorite is found in aqueous solution dissociated into sodium
hydroxide and hypochloric acid and is represented by the following chemical
reaction: NaOCl+H2O Û NaOH+HOCl. When sodium hypochlorite comes in
contact with organic material, several chemical reactions take place, i.e.
fatty acids react with sodium hydroxide creating soap and glycerol (saponification
reaction), amino acids react with sodium hydroxide creating salt and water
(neutralization reaction) and also react with hypochlorous acid creating
chloramine and water. These reactions occur simultaneously and synergistically
leading to liquefaction of organic tissue.
In agreement with other researchers, the present experiment showed that the
greater the concentration of sodium hypochlorite, the more rapid was the
organic tissue dissolution (4,6-10).
All solutions were strongly alkaline and after pulp dissolution, the pH decreased.
This is due to the interaction of NaOH with organic material by the reactions
of saponification and neutralization. It was also noted that with a greater
initial concentration, there was less of a reduction of pH in the resulting
solution, which is due to more hydroxyl ions in solutions of greater concentration.
With a higher initial concentration there was a greater reduction of surface
tension of the final solution. This probably occurred due to the larger quantity
of sodium hydroxide that reacted with fatty acids producing soap, which reduces
surface tension.
The consumption of active chlorine was greater at lower concentrations of
sodium hypochlorite. This indicates that there is a greater interaction of
HOCl with organic material producing chloramines which is in agreement with
the surface tension results that showed a greater interaction of sodium hydroxide
with organic material at higher concentrations.
In an attempt to elucidate the mechanism of action of the sodium hypochlorite
solutions on organic material, two experiments were carried out using the
same method as used for the dissolution test. The velocity of tissue dissolution
of 4 hypochlorite solutions: 0.5% sodium hypochlorite with a pH of 11.6 and
with a pH of 9, and 0.36% sodium hypochlorite with a pH of 11.6 and with
a pH of 7.0. The reduction of pH was accomplished with the addition of boric
acid. The velocity of dissolution by 0.5% sodium hypochlorite with a pH of
9 was 0.007 mg/s, of 0.5% sodium hypochlorite with a pH of 11.6 was 0.014
mg/s, of 0.36% sodium hypochlorite with a pH of 7.0 was 0.003 mg/s, and of
0.36% sodium hypochlorite with a pH of 11.6 was 0.010 mg/s. The reduction
of pH and consequently the change of the dynamic chemical equilibrium with
the formation of hypochlorous acid decreased the velocity of tissue dissolution.
This is due to the neutralization reaction between boric acid and sodium
hydroxide that decreases the saponification and neutralization reactions.
In the second experiment, 15% aqueous NaOH was used to test pulp tissue dissolution.
After 6 h, NaOH did not totally dissolve the pulp tissue although there were
chromatic changes and an alteration in consistency. This indicates that pulp
tissue dissolution is due to the combined synergistic effect between sodium
hydroxide and hypochlorous acid, present in sodium hypochlorite. Each component
reacts with certain components of the pulp.
Applying the results of this investigation to clinical use, we conclude that
at all concentrations studied, there was dissolution of pulp tissue and residual
chlorine. Thus, the clinician could use a concentration in function of instrumentation
time of necrosed root canals. For long instrumentation times, the clinician
could use less concentrated solutions and in cases with more rapid instrumentation
techniques (rotary), more concentrated solutions may be chosen provided there
is no extrusion at the periapex.
RESUMO
Spanó JCE, Barbin EL, Santos TC, Guimarães LF, Pécora
JD. Ação solvente da solução de hipoclorito de
sódio sobre o tecido pulpar bovino e as propriedades físico-quimica
do líquido resultante. Braz Dent J 2001;12(3):154-157.
Estudou-se, in vitro, a ação solvente de quatro concentrações
da solução de hipoclorito de sódio (0,5, 1,0, 2,5 e
5,0%) sobre o tecido pulpar bovino, o teor de cloro residual, pH e a tensão
superficial antes e após a dissolução. Um fragmento
do tecido pulpar bovino era submerso na solução de hipoclorito
de sódio que circulava em um aparato composto de uma bomba peristáltica
e uma seringa Luer Lok e anotava-se o tempo até a total dissolução
deste fragmento. De acordo com a metodologia empregada e com os resultados
obtidos concluiu-se que: quanto maior a concentração da solução
de hipoclorito de sódio maior a velocidade de dissolução
do tecido pulpar, todas as soluções de hipoclorito de sódio
apresentaram redução do pH e da tensão superficial após
os testes de dissolução e as soluções de maior
concentração apresentaram menor consumo de cloro para a realização
da dissolução tecidual.
Unitermos: hipoclorito de sódio, polpa bovina, ação
solvente.
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Accepted January 10, 2000
