DR. SATHYA KUMAR Postgraduate Student
DEPARTMENT OF CONSERVATIVE DENTISTRY & ENDODONTICS
SRI RAMACHANDRA DENTAL COLLEGE AND HOSPITALS CHENNAI
CONTENTS Introduction Biochemical Actions Initial Mineralization Calcium Hydroxide Induced Mineralization The Dentine Bridge Direct Pulp Capping Direct Pulp Capping Pulpotomy Histology Replantation of the Teeth Perforative Types of Resorption Apexification Root Resorption Idiopathic: Repair of Iatrogenic Perforations Types of Vehicles Aqueous Vehicles Proprietary Brands Mechanisms of Antimicrobial Activity of Calcium Hydroxide Mechanisms of Antimicrobial Activity 1. Damage to the bacterial cytoplasmic membrane 2. Protein denaturation 3. Damage to the DNA Root Canal Dis-Infection
Influence of the Vehicle on the Antimicrobial Activity Physical Barrier INTRODUCTION Since the introduction of calcium hydroxide Ca (OH) 2 to dentistry by Hermann (1920, 1930), this medicament has been indicated to promote healing in many clinical situations. However, the initial reference to its use has been attributed to Nygren (1838) for the treatment of the 'fistula dentalis', while Codman (1851) was the first to attempt to preserve the involved dental pulp.
According to Cvek (1989) calcium hydroxide became more widely known in the 1930s through the pioneering work of Hermann (1936) and the introduction of this material in the United States (Teuscher & Zander 1938). The first reports dealing with successful pulpal healing using calcium hydroxide appeared in the literature between 1934 and 1941. Since then, and mainly after the Second World War, the clinical indications for its use were expanded and now this chemical is considered the best medicament to induce hard tissue deposition and promote healing of vital pulpal and periapical tissues (Garcia 1983).
Although the overall mechanisms of action of calcium hydroxide are not fully understood, many articles have been published describing its biological properties, which are achieved by the dissociation in the Ca and OH ions. The role of the high pH and the ionic activity in the healing process, diffusion through dentinal tubules, influence on apical Microleakage and some clinical topics, such as the importance of the interappointment restoration, are examples of how this material has been evaluated since its introduction.
Many substances have been added to the powder to improve properties such as the antibacterial action, radiopacity, flow and consistency.
BIOCHEMICAL ACTIONS Initial mineralization Before considering the specific effects of calcium hydroxide, it will be helpful to describe briefly the theories of mineralization within mesenchymal tissues. It is now widely accepted that an epitactic mechanism operates following the initial seeding of a collagenous tissue. Only certain types of collagen such as those found in dentine and bone, mineralize in this way. The process is probably the result of juxtaposition of charged groups on adjacent macromolecules which give rise to the
epitactic centers. These centers require a nucleation site from which hydroxyapatite crystal growth can proceed. Chondroitin sulphate as the seed whilst others, conversely considered it to be an inhibitor of mineralization. Other substances which have been postulated as initiators of mineralization include a Vitamin D dependent protein which is capable of binding calcium, phosphoproteins and phospholipids.
One safety factor may be the presence in the blood and tissue of substances such as pyrophosphate ions which pyrophosphates are metabolized
Pyrophosphates are a member of the alkaline phosphates group, which many explain why these enzymes are invariably present in mineralizing tissues.
Calcium hydroxide induced mineralization It seems that calcium hydroxide has the unique potential to induce mineralization, even in tissues, which have not been programmed to mineralize (Mitchell & Shankwalker 1958, Binnie 1967). However, Rasmussen & Mjor (1971) could not verify that calcium hydroxide induced mature bone formation under these circumstances, but found that when the material was placed in direct contact with host tissue it induced the formation of fibrous tissue, with the occasional formation of areas of
immature bone. These workers also found that when calcium hydroxide was separated from host tissue by Millipore filters, no significant reactions occurred, indicating that the material did not have any effect once it had diffused some distance. It is also likely that the calcium ions present in the applied calcium hydroxide do not become incorporated in the mineralized repair tissue, which derives its mineral content solely from the dental pulp, presumably via the blood supply. These observations indicate that calcium hydroxide is an initiator rather than a substrate for repair.
The high pH may also activate alkaline phosphate activity, which is postulated to play an important role in hard tissue formation. The optimum pH for alkaline phosphates activity is 10.2 a level of alkalinity, which is produced by many calcium hydroxide preparations.
The dentine bridge There appears to be some variation in the way in which a dentine bridge is formed depending on the pH of the material that is used to dress the tooth. I the case of a high pH material such as Pulpdent a necrotic zone is formed adjacent to the material and the dentine bridge then forms between this layer and underlying vital pulp. The necrotic tissue eventually degenerates and
Hydroxide. Nirschl et al reported that a high rate of success (94.4%) can be seen in cases done with calcium hydroxide. The mode of action of Ca(OH)2 on bacteria is not yet completely understood. Yet, it is able to practically avert the caries process.
Direct pulp capping Involves the application of a medicament to the exposed pulp in an attempt to pre-care vitality. Observe has described the histopathology of the pulp and concluded that the cells of the pulp are the same as those of loose comparative tissues and that these cells could differentiate and that healing could occur in the dental pulp.
In 1930, Hermann introduced calcium hydroxide as a successful pulp-capping agent.
In 1938, Tauscher and Zander introduced calcium hydroxide in the United States and historically confirmed complete dentinal bridging with healthy radicular pulp tissues leading to degeneration.
According to Mo Walter et al, the advantage of calcium hydroxide is when applied to exposed pulp, it does not exert a persistent stimulating
effect on reparative dentine, which would lead to eventual pulp obliteration.
When calcium hydroxide is applied directly to (Stanley and Landy) showed that pulp adjacent to each appeared to be chemically cauterized) pulp tissue, there is necrosing on the adjacent pulp tissue and an inflammation of the contiguous tissue. Certain formation occurs at the junction of the necrotic tissue and the contiguous vital inflamed tissue. Although calcium hydroxide works effectively, the exact mechanism is not yet understood. The compounds of similar alkalinity cause liquefaction when applied to pulp tissue.
Direct Pulp Capping A study was done by the University if Michigan to understand the cellular changes taking place in the pulp following direct capping with calcium hydroxide. The study showed an increase in the number of cells in the odontoblritiz and the cell free Fones over time, with an concurrent decrease in the number of cells in the center of the pulp, suggesting that this was where the cells were originated from. The study also showed that at least 2 DNA Replications had occurred between initial treatment and the final odontoblast-like cell differentiation. The study was done in Primater after injecting them with thymidine.
In some case internal resorption is seen following caprior with calcium hydroxide. In others complete dentin mineralization of the remaining pulp takes place so that the camels are occluded. However, due to the low incidence of those assurances, it does not seem to be justified to avoid calcium hydroxide.
It was postulated that calcium would diffuse 'rom' dwening into the pulp and participate in the formation of reactive dentin. Experiments with radioactive ions however have shown that calcium ions from the dressing do not enter into the formation of a new ideation and that the calcium for the dentin bridge comes from the blood stream. The pulpal response varies under different commercial form of calcium hydroxide.
Dycal or life: here the dentin forms directly under the dressing. The chemically altered tissue created by the calcium hydroxide is resorbed first and the bridge is then formed in contact with the capping material.
Pulp dent: With calcium hydroxide powder (pulp dent) the bridge forms at the junction of the chemically altered tissue and the vital inflamed pulpal tissue. The altered tissue degenerates and leaves a void
between the capping material and the bridge. So radiographically the bridge would be better appreciated with pulp dent than with Dycal or life.
The quality of the dentin bridge was found to be good. Reolit (hard setting CaOH2) has a neutral pH and has proved to be unsuccessful for pulp capping while hydreax (hard setting CaOH 2) has been giving conflicting reports.
PULPOTOMY The pulpotomy procedure involves removing pulp tissue that has inflammatory or degenerative channels leaving behind the remaining vital tissue, which is then covered with a pulp-capping agent to promote healing at the amount site or an agent to cause fixation of the underlying tissue.
In young permanent teeth, CaOH2 pulpotomy is the treatment or choice.
The use of hard setting materials is the best. For Teener
amputations, CaOH2 powder is carefully beard against pulp stem. Care must be taken not to pack the powder into the pulp tissue as this will cause greater inflammation and failure or in the case of success, there will be increase calcification of the remaining pulp tissues around the particles of calcium hydroxide. Doyle et al have compared calcium hydroxide and
for increased pulpotomies in primary teeth and found a better rate of success with form.
HISTOLOGY Superficial part of the pulp, just beneath the coach is necrotic, under this a layer of calcium proteinate was formed. Initially a 3 one of inflammation was seen under the necrotic layer which then transforms into a dentinal bridge. Systemic calcium is used. CaOH2 helps in maintaining alkalinity, which is necessary for optimum hard tissue formation. Pulpal merenchymal cells differentiate into odontoblasts, forming the dentinal bridge. CaOH2 is also used in deep pulpotomy.
In a study done on dogs, immature teeth in the Tokyo Dental College and published in 1990, a calcium hydroxide iodoform paste was found to be very effective for pulpotomies on immature permanent teeth. Calcium hydroxide paste are used is root canal sealers and calcium hydroxide plugs produce a periapical response that overall is indistinguishable from that produced by dentin plugs.
However another study shows that the calcification observed with dentin plugs is more complete than that observed with calcium hydroxide plugs. In an in-vivo study done in the University of Sanis Cataria, Brazil,
canine teeth roads were cleaned, shaped and filled with CaOH 2 sealer. Partial closure of the apices was seen and all over filled specimens showed chronic inflammatory reaction on the periapical region. Study done by Verdon and Holz shows that CaOH2 sealer shows almost complete resorption of over fillings in I st year and a slight improvement in the radiologic appearance of the periapical legions was also seen is Ist years.
In cases of root fracture, where the apical segment of the root is vital and the coronal section is non-vital and a space exists between the coronal and radicular segment, CaOH2 can be used to fill in the coronal segment and promote healing. After a healing period of about 6 months, calcium hydroxide can be replaced by Gutta Percha.
The indications are: 1. There is no evidence of a periapical radiolucency at the end of the apical section 2. The apical segment of the pulp seems vital as evidenced by bleeding on a paper point 3. A radial space is evident between or adjacent to the fractural segments.
4. There is evidence of internal or external resorption in the coronal segment. 5. A wide root-canal space makes a fitting of the root canal with guttapercha difficult, with a strong possibility of extrusion of the guttapercha or cement into the fracture site. 6. There is no communication between the fracture site and the oral cavity.
REPLANTATION OF THE TEETH Anderson first reported the use of calcium hydroxide in the treatment of inflammatory root resorption, which follows the reimplantation of teeth. The tooth is re- implant and splinted in position before the pulp is extirpated and the access is sealed using o temporary restoration.
Placement of calcium hydroxide should be delayed by 1 to 2 weeks following re implantation. Anderson has shown that immediate insertion of calcium hydroxide in re implantation teeth causes noticeably more replacement resorption that is seen in teeth with extenuated pulp or GuttaPercha fillings. He postulated that calcium hydroxide diffuses out through the apical foramen. Further injuring the periodontal ligament. He has also pointed out that inflammatory resorption is initiated at around 2 weeks
about the time that the periodontium is healing which is the ideal time for the placement of calcium hydroxide fillings. The mode of action of calcium hydroxide is uncertain. Tornstad. L et al have shown an increase in the Ph of dentin in teeth in which the canal has been filled with calcium hydroxide. They speculate that the rise in Ph stimulates repair while reducing osteoclasts in an acidic environment is necessary for osteoclastic activity.
Since CaOH2 is an absorbable material, it will eventually dissipate out of the canal in some teeth so these cases will have to be reviewed every 2 or 3 months and the CaOH 2 filling replaced if necessary. After a minimal period of 1 year, the CaOH 2 can be removed and gutta-percha placed.
PERFORATIVE TYPES OF RESORPTION CaOH2 in a suitable vehicle such as can promote healing and formation of in the same manner it can be used for the treatment of accidental perforations. The rationale is 1. CaOH2 has an antibacterial effect. 2. It has an alkaline pH, which causes osteoclastic activity and promotes repair, which was demonstrated by Tronsted et al.
APEXIFICATION Kaiser first reported the use of calcium hydroxide for apexification in 1964 and the technique was popularized by the work of Frank.
Calcium hydroxide alone or in combination with other materials has been the most clearly accepted material to promote apexification. This procedure is done in non-vital teeth the pulp tissue is extirpated the canals are cleaned and filled with a paste temporality to promote calcification at the apex. The calcium hydroxide powder has been mixed with camphorated chlrophenol (CMCP), metacresyl acetate, cresanol (a mixture of CMCP and Metacresylacetate), physiologic saline ringer's solution distilled water, LA solution. All have been reported to stimulate apexification. Tricalcium Phosphate and collagen-calcium phosphate gel has been reported to also produce apexification in both humans and animals.
ROOT RESORPTION Idiopathic: Calcium hydroxide is frequently used as a dressing for the treatment of both internal and external inflammatory root resorption in order to halt the progress and encourage re-mineralization. It is doubtful weather the materials has any real beneficial effect on internal resorption,
as this is now considered to be sustain by infection within the dentinal tubules coronal to the resorptive process.
Following the replacement of an avulsed tooth or transplantation of a tooth Once avulsed tooth has been splinted in position for 2 weeks the root canal should be thoroughly cleaned and dressed with calcium hydroxide for a period of 3-6 months, prior to the placement of a conventional root filling. Although it has been shown calcium and hydroxyl ions do not diffuse through the dentin, the calcium hydroxide may still penetrate through lateral canals. Calcium hydroxide treatment has no effect on replacement resorption (ankylosis) once it has become established. The principle of managing transplanted teeth once pulpal necrosis has been confirmed, are essentially the same as those that relate to replantation.
REPAIR OF IATROGENIC PERFORATIONS The calcium hydroxide sealer, Sealapex, was used by Beavers et al 1986 to treat root canal perforations. They observed bone healing and in growth of trabeculae into the perforation after 42 days, there was also reparative cementum formation and ankylosis.
TYPES OF VEHICLES Aqueous vehicles Water: some chemicals characteristics of such a paste were evaluated by different authors, including its pH (Conrado et al 1965, Leonardo et al 1992), ionic dissociation and its diffusion through dentin. The antibacterial effect was studied by martins et al while the solvent action was evaluated by Hasselgern et al 1988.
This paste has been evaluated for the tissue reaction when implanted in rat subcutaneous connective tissue for its ability to induce hard deposition \in apexification procedure in non- vital dogteeth and in replacement resorption in replanted rat teeth.
In human clinical studies this paste has been indicated for capping of vital pulp tissue after pulpotomy (Russo et al ) as a long term dressing in cases of non-vital teeth associated large periapical lesions and in apexification procedures.
Sterile water. In humans this paste has been indicated for direct pulp capping, pulpotomy and apexogenesis, Goldmen 1974 Sheehy and Roberts 1997, Apexification procedure. And as an apical plug before
Gutta Percha filling in non-vital teeth with an open apex and in cases of internal resorption with perforation of the dentinal wall.
Distilled water. It is important to highlight that Crabb (1965) was the first to use this paste in the treatment of large periapical lesions he said " perhaps the locally destructive action of calcium hydroxide with its high pH, acting as chemical cattery, might effect breakdown of the epithelium"
Clinically, it has been employed for the induction of hard tissue deposition in apexification procedure (Saad 1988, Yang et al 1990), in pulpotomy of deciduous or permanent teeth as a temporary dressing after vital pulp extirpation and in non-vital teeth with associated chronic periapical disease, in internal resorption, in perforation and to arrest external cervical resorption after bleaching of pulpless teeth (Santos 1996).
It has been suggested that iodoform or bismuth carbonate should be added to improve the radiopacity of the paste (Holland et al 1981, Rezende 1982).
An old suggestion proposed by Yacometti (1952) was to add penicillin to a calcium hydroxide-distilled water paste to be used as a pulp capping material.
4. Sterile distilled water: This paste was evaluated for human direct pulp capping in apexification procedures and in animal studies, for its intradentinal calcium diffusion.
5. Bidistilled water: According to Laurichesse (1980) it was Albou who first used bid stilled water as the vehicle of the paste in normal clinical cases. However, in cases of infected non-vital teeth, some drops of camphorated parachlorophenol were added to the paste.
6. Sterile bidistilled water: This vehicle was recommended by Breillat et al (1983 a.b) for human apexogenesis and specification procedures.
7. Saline or sterile saline: According to the United States Pharmacopea (1989) saline is prepared by dissolving 9 g of sodium chloride in water to make 1000ml. When this paste was implanted in vital tissues, the reactions were evaluated by Pissoitis & Spangberg (1980). In animal studies, the paste was evaluated in direct pulp capping and in the
apexification of immature non-vital dog and monkey teeth and to arrest inflammatory resorption in replanted dogteeth.
Clinically, it was evaluated in human non-vital immature teeth (Cvek 1972), in preparations (Bogaers 1997), in internal resorption at the site of an intra-alveolar root fracture in external inflammatory root resorption, in infected teeth with associated cutaneous sinus tract, in endodontic re-treatment after endodontic and surgical failures and as a dressing partial pulpectomy.
Recently, Yoshiba et al (1994) proposed a new formulation, adding a Tricalcium phosphate to the calcium hydroxide powder and saline for capping amputee pulps. Sazak et al (1996) have suggested adding Ledermix to a calcium hydroxide-saline paste to be used after pulpotomy with the purpose of reducing postoperative pain and inflammation
8. Anaesthetic solutions: It is interesting to note that most these solutions have an acid pH, but when mixed with the calcium hydroxide powder, the final paste has a high pH which is maintained over time. Further more, they promote a rapid ionic release.
As the final paste lacks radiopacity, some authors add barium sulphate (one part) to calcium hydroxide powder (four parts) (Dumsha & Gotmann 1985). Marasis (1996) believes this proportion is not necessary for a high radiopacity and usesa 1:8 ratio to increase the antibacterial property of the paste. Teplitsky (1986) suggested adding one drop of camphorated chlorophenol when used as a dressing in infected non-vital cases.
9. Ringer's solution: United States Pharmacopeia (1989) sodium chloride (8.6 g), Potassium chloride (0.3 g), calcium chloride (0.33 g) and water to 1000 ml.
Historically, it was Granath (1959) who was the first to describe the use of such a paste in cases of traumatic injuries, although some authors believe he was also the first to employ a calcium hydroxide paste in root-end induction procedures. This is not correct because the oldest reference in which a calcium hydroxide paste was used for root-end hard tissue deposition is Marmasse (1953).
10. Methylcellulose and carboxymethylcellulose: Maisto & Capurro (1964) introduced a paste composed of equal volumes of calcium
hydroxide powder and iodoform mixed with a 5% aqueous solution of Methylcellulose.
Laurichesse (1980) proposed the following modification of the original formula; calcium hydroxide and iodoform in a ratio 2/3:1/3, two drops of camphorated parachlorophenol and a 3% aqueous solution of Methylcellulose as the vehicle.
More recently, Giro et al (1993) proposed the use of carboxy methyl cellulose or according to the United States Pharmacopeia (1989), polycarboxymethylether of cellulose as the vehicle in the following formula: 0.5 g of calcium hydroxide to 0.5 ml of a 1.66% solution of carboxymethylcellulose.
11. Anionic detergent solution: It is well known that detergents decrease the surface tension between two surfaces and facilitate substance penetration. This is perhaps the reason why calcium hydroxide powder has been mixed with an aqueous detergent solution to increase the action of the v deeper into the tissues.
Unfortunately, only two studies have appeared in the literature dealing with these substances. Barbosa et al. (1994) tested the
antibacterial effect of a paste composed of calcium hydroxide and sodium lauryl diethyleneglycol ether sulphate and Peniche et al (1996) evaluated the pH of a paste containing calcium hydroxide and sodium lauryl sulphate.
PROPRIETARY BRANDS Calyxyl: This paste is the oldest manufactured calcium hydroxide paste and was introduced by Hermann (1920). Employed as a dressing with the purpose of maintaining vital pulp tissue and inducing healing by the formation of a calcified
MECHANISMS OF ANTIMICROBIAL ACTIVITY OF CALCIUM HYDROXIDE Mechanisms of antimicrobial activity Most of the endodontopathogens are unable to survive in the highly alkaline environment provided by calcium hydroxide (Heithersay 1975). Since the pH of calcium hydroxide is about 12.5, several bacterial species commonly found in infected root canals are eliminated after a short period when in direct contact with this substance (Bystrom et al 1985)
Antimicrobial activity of calcium hydroxide is related to the release of hydroxyl ions in an aqueous environment. Hydroxyl ions are highly
oxidant free radicals that show extreme reactivity, reacting with several biomolecules. This reactivity is high and indiscriminate, so this free radical rarely diffuses away from sites of generation. Their lethal effects on bacterial cells are probably due to the following mechanisms:
1. Damage to the bacterial cytoplasmic membrane The bacterial cytoplasmic membrane possesses important functions to the survival of the cell, such as (1) selective permeability and transport of solutes; (ii) electron transport and oxidative phosphorylation in aerobic species; (iii) excretion of hydrolyticexoenzymes; (iv) bearing enzymes and carrier molecules that function in the biosynthesis of DNA, cell wall polymers, and membrane lipids; and (v) bearing the receptors and other proteins of the chemotactic and other sensory transduction systems.
Hydroxyl ions induce lipid peroxidation, resulting in the destruction of phospholipid structural components of the cellular membrane. Hydroxyl ions remove hydrogen atoms from unsaturated fatty acids, generating a free lipidic radical. This free lipidic radical reacts with oxygen, resulting in the formation of lipidic peroxide radical, which removes another hydrogen atom from a second fatty acid, generating another lipidic peroxide. Thus peroxides themselves act as free radicals,
initiating an auto catalytic chain reaction and resulting in further loss of unsaturated fatty acids and extensive membrane damage.
2. Protein denaturation Cellular metabolism is highly dependent on enzymatic activities. Enzymes have optimum activity and stability in a narrow range of pH, which turns around neutrality. The alkalinization provided by calcium hydroxide induces the breakdown of ionic bonds that maintain the tertiary structure but the polypeptide chain is randomly unraveled in variable and irregular special conformation. These changes frequently result in the loss of biological activity of the enzyme and disruption of the cellular metabolism. Structural proteins may also be damaged by hydroxyl ions.
3. Damage to the DNA Ions react with the bacterial DNA and induce the splitting of the strands. Genes are then lost. DNA replication is inhibited and the cellular activity is disarranged. Free radicals may also induce lethal mutations.
It is difficult to establish in a chronological sense which is the main mechanism involved in the death of bacterial cells after exposure to a strong base.
It has been suggested that the calcium hydroxide to absorb carbon dioxide may contribute to its antibacterial activity. Hence, carbon dioxide supply to remaining bacteria in the root canal system may be maintained from the outside. In addition, bacteria located in ramifications have direct access to carbon dioxide from the periradicular tissues. There is little reason to consider that calcium hydroxide impedes the carbon dioxide supply to bacteria.
ROOT CANAL DIS-INFECTION Several studies have demonstrated that calcium hydroxide exerts lethal effects on bacterial cells. These effects were observed only when the substance was in direct contact with bacteria in solution. In such conditions, the concentration of hydroxyl ions is very high, reaching incompatible levels to bacterial survival. Clinically, this direct contact is not always possible.
Bases of alkaline metals, such as NaOH2 and KOH show high solubility and thereby may diffuse more than calcium hydroxide across the culture medium. Both bases have pronounced antibacterial activity. On the other hand, high solubility and diffusibility increases the cytotoxic effects of these substances, they are not indicated for use in endodontic practice.
Killing of bacteria by calcium hydroxide will depend on the availability of hydroxyl ions in solution, which is higher where the paste is applied. Calcium hydroxide exerts an bacterial effects in the root canal as they retain a very high pH.
Bacteria inside dentinal tubules may constitute an important reservoir from which root canal infection or reinfection occurs during and after endodontic treatment (Oguntebi 1994). Occasionally, these remaining bacteria may cause a persistent infection that jeopardizes the outcome of endodontic therapy. Therefore, treatment strategies that are directed towards the elimination of tubule infection are necessary and must include medicaments that penetrate dentinal tubules and kill bacteria.
To act effectively as an intracanal dressing, the hydroxyl ions must be able to diffuse through dentine and pulpal tissue remnants. Studies have revealed that hydroxyl ions derived from a calcium hydroxidemedication do diffuse through root dentine.
To be effective against bacteria located inside the dentinal tubules the hydroxyl ions from calcium hydroxide should diffuse into dentine at sufficient concentrations. It has been reported that dentine has buffering
ability because of the presence of proton donors, such as H2PO4, H2CO3, HCO3-, in the hydrated layer of hydroxyapatite, which furnish additional protons to keep the pH unchanged (Wang & Home 1998, Nerwich et al. 1993). Therefore, in order to have antibacterial effects within dentinal tubules, the ionic diffusion of calcium hydroxide should exceed the dentine buffer ability, reaching pH levels sufficient to destroy bacteria. After short-term use of calcium hydroxide, microorganisms are probably exposed to lethal levels of hydroxyl ions at the tubule orifice.
Another factor can also help to explain the inefficacy of calcium hydroxide in disinfecting dentinal tubules. The arrangement of the bacterial cells colonizing the root canal walls can reduce the antibacterial effects of v since the cells located at the periphery of colonies can protect those located more deeply inside the tubules.
A short dressing with calcium hydroxide appears to eliminate mainly bacterial cells in direct contact with this substance, such as bacteria located in the main root canal or in the circumpulpal dentine. These areas are also commonly affected by the chemomechanical procedures.
Bacteria may survive after intracanal medication for several reasons. First, bacterial strains present in the root canal infection may be intrinsically resistant to the medicament. Secondly, bacterial cells may be enclosed within anatomical variations inaccessible to the medicament. Thirdly, the medicament may be neutralized by tissue components and by bacterial cells or products, losing its antibacterial effects. Fourthly, medicaments may remain the root canal system for insufficient time to reach and kill bacterial cells. Finally, bacteria may alter their pattern of gene expression after changes in the environmental conditions. This alteration may allow them to survive in unfavourable environments.
INFLUENCE OF THE VEHICLE ON THE ANTIMICROBIAL ACTIVITY Most of the substances used as a vehicle for calcium hydroxide donâ€™t have significant antibacterial activities. They include distilled water, saline solution and glycerin. Other substances such as camphorated paramonochlorophenol (CMCP) and metacresylacerate are known to posses this property. Frank (1996) recommended mixing v with CMCP in apexification procedures. Some authors criticized this by considering it unnecessary to add antimicrobial agents to calcium hydroxide, especially those that have been shown to be tissue irritating (Cvek et al. 1976, Anthony et al 1982).
Calcium hydroxide CMCP glycerin paste effectively killed bacteria in the tubules after 1 h exposure, except foe E. faecalis that required 1 day of exposure. Studies using an agar diffusion test have revealed that the calcium hydroxide /CMCP paste had pronounced antibacterial activity against facultative and anaerobic bacteria which was superior to pastes containing calcium hydroxide in entire substances.
1. The small concentration of released paramonochlorophenol (MCP). Calcium
paramonochlorophenolate, which is a weak salt that progressively releases MCP and hydroxyl ions the surrounding medium (Anthony et al. 1982). It is well known that a substance may have either beneficial or deleterious effects, depending on its concentration. 2. The denaturing effect of calcium hydroxide on connective tissue, which may prevent the tissue penetration of MCP reducing its toxicity (Siqueira 1997). 3. The fact that the effect on peri radicular tissues is probably associated with the antimicrobial effect of the paste, which allows natural healing to occur without persistent infectious irritation.
PHYSICAL BARRIER In addition to eliminating remaining viable bacteria unaffected by the chemomechanical preparation of the root canal intracanal medicaments have been advocated for other reasons. They should also act as a physicochemical barrier, precluding the proliferation of residual microorganisms and preventing the reinfection of the root canal by bacteria from the oral cavity (Siqueira 1997). Canals filled with calcium hydroxide/saline solution and calcium hydroxide/CMCP/glycerin showed entire recontamination within an average of 14.7 and 16.5 days, respectively. Calcium hydroxide pastes were significantly more effective than CMCP in preventing root canal recontamination by bacteria from saliva.
The filling ability of calcium hydroxide pastes is probably more important in retarding root canal recontamination than the chemical effect. Because calcium hydroxide has low water solubility, it is dissolved in saliva, remaining in the canal for a long period, delaying the bacterial progression towards the apical foramen. Despite the vehicle used, calcium hydroxide seems to act as an effective physical barrier. Medicaments that act as a barrier can kill remaining microorganisms by withholding substrate for growth and by limiting space for multiplication.
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