Classification of filling materials for root canal.
I. Plastic non-hardening:
II. Plastic hardening materials.
III. Solid filling materials (pins):
Requirements for root canal filling materials.
1. The cement should be tacky when mixed so as to provide good adhesion, when set, between it and the canal wall.
2. It should make an hermetic seal.
3. It should be radiopaque so that it can be visualized in the X-ray.
4. The particles of powder should be very fine so that they can mix easily with the cement liquid.
5. It should not shrink upon setting.
6. It should not stain tooth structure.
7. It should be bacteriostatic, or at least should not encourage bacterial growth.
8. It should set slowly.
9. It should be insoluble in tissue fluids.
10. It should be tissue tolerant, i.e., nonirritating to periapical tissue.
11. It should be soluble in a common solvent if it be necessary to remove the root canal filling.
Plastic hardening filling materials cure after introduction into the root canal and almost never absorbed in it. They are sometimes called sealants, endosealants or isolation materials. For this purpose, can be used a number of plastic hardening materials, very different in their properties.
These include most of the materials that are now used. They are divided into several groups:
1. Zinc phosphate cements.
2. ements based on zinc oxide and eugenol.
3. Glass ionomer cements.
4. Materials based on synthetic resins (epoxy, acrylic, resorcinol-formalin resin).
5. Polymeric materials containing calcium hydroxide.
6. Materials based on calcium phosphate.
Clinically, the purpose of the hardening paste (sealant) is to provide a volumetrically stable filling which will adapt to the variable contours and incongruities of the shape of the root canal wall and that of the core material.
For this purpose, sealants that remain soft are not adequate; neither alone nor in combination with one or several cones used for root canal obturation as these pastes are soluble and is absorbed by viable tissue. This absorption may lead to re-infection of the root canal system.
As a rule, obturation of more than 90 % of the canal's volume is aimed for in any technique used for root canal obturation with a volumetrically stable core material. The hardening root canal obturation paste should not exceed 10 % of the total volume of obturation.
Root canal obturation pastes that remain soft are not suitable for the final sealing of a root canal, neither alone nor in combination with cones.
In the past, numerous sealants with very divergent chemical compositions were developed. The objective was always to develop a paste for use in root canal obturation that is dimensionally stable, insoluble, and at the same time biocompatible. As a rule, the pastes described below are used for final root canal obturation.
Pastes based on zinc oxide/eugenol
Zinc oxide eugenol pastes without added medical substances
These two-component preparations will set after mixing powder and liquid due to the formation of a friable, porous zinc eugenolate ([C10H11O2]2Zn). Excess free eugenol is always bound to eugenolate and zinc oxide, even if mixed according to the manufacturer's instructions, and may compromise the integrity and so weaken the zinc eugenolate complex.
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This eugenolate is soluble in tissue fluid. Therefore, these pastes are to be considered comparatively soluble.
Furthermore, pastes based on zinc oxide eugenol b>shrink by approx. 0.3 % to 1 % during their setting process.
After mixing the sealant's ingredients, they are supposed to have slightly cytotoxic effects, eugenol more so than zinc oxide. This effect is time dependent as the cytotoxicity progressively decreases the longer the material is in-situ.
Zinc oxide eugenol pastes with added medical substances
These preparations are sealants to which, in contrast with pure zinc oxide cements, various pharmacologically active substances have been added.
Root canal obturation pastes containing medicinal additives are materially little different to those made purely of zinc oxide eugenol.
But on account of the substances that have been added, mostly paraformaldehyde and/or an anti-inflammatory agent (corticosteroid), they assume, from a biological point of view, a special place with regard to their clinical use. As these substances have potential local as well as systemic side effects, their use needs to be carefully considered.
Sealants that contain paraformaldehyde (e.g. N2) release formaldehyde both during and after the curing process. They are considered neurotoxic, cytotoxic, genotoxic, and have a strong allergenic potential. The healing of existing periapical inflammation has been shown to be prolonged even when these sealants are used exclusively within the root canal itself. The use of root canal filling pastes with added paraformaldehyde is to be considered obsolete in modern endodontics.
Corticosteroids employ different mechanisms for local immunosuppression. Therefore, when sealants containing corticosteroids (such as Endomethasone N) are used, this will frequently lead to uncontrolled local proliferation of micro-organisms even when a disinfectant or antibiotic is added. In most cases, this results in worsening of the inflammation in periapical tissue.
Use of root canal obturation pastes with admixed paraformaldehyde and/or corticosteroids is to be considered obsolete and clearly contradicts the biological principles of modern endodontics.
Epoxy resin based pastes. As an older epoxy resin, the product AH 26 contains hexamethylenetetramine, which is the result of the reaction between formaldehyde and ammonia. For this reason, AH26 releases a small amount of formaldehyde for a short time immediately after it has been mixed, with a peak being reached two days after mixing. However, the release of formaldehyde cannot be measured at a later date even with the most sensitive of analysis procedures. On account of the formaldehyde that is released initially, AH26 does exhibit low-level cytotoxicity, which clearly diminishes after a few weeks and which there after can virtually no longer be demonstrated. When fully cured, all epoxy resins may be considered largely biologically inert.
AH Plus , which from a physical and material point of view can be considered to be the equivalent or even the improved follow-on product of AH26, is considered not to release any formaldehyde as the substance responsible for this, hexamethylenetetramine, is no longer present in AHPlus. On the basis of the data currently available, AH Plus can be considered, from the point of view of its biocompatibility, to be an improved product in comparison with AH 26.
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The material properties of epoxy-resin based root canal obturation pastes can, with regard to their viscosity, water solubility and radio-opacity, be rated as being good to very good. AH-Plus in particular is all but insoluble. Overall, these sealants demonstrate excellent sealing capacity and good volumetric stability, whereby AH-26 in contrast with most of the other sealants does not shrink but instead expands initially during the curing process by around 4 to 5 %.
In summary, from both a clinical and a material point of view, it can be said that epoxy-resin based root canal obturation pastes are worth recommendation.
