Methacrylate Resin Based Sealers: A Review

INTRODUCTION

Root canal sealers have several important functions which include

1. sealing off of voids and multiple foramina, patent accessory canals
2. create a bond between the core filling material and the root canal wall,
3. providing a lubricating effect during the placement of the filling core and trapping any remaining bacteria.

Due to the relative biological and technical importance of sealers, their chemical and physical properties have been the subject of considerable discussion since their initial development in the early twentieth century.¹

CLASSIFICATION OF ENDODONTIC SEALERS

Endodontic sealers can be classified widely as:

1. Zinc oxide eugenol based sealers (tubliseal, grossman’s formula, roth’s 801)
2. Calcium hydroxide based sealers (sealapex, apexit)
3. Glass ionomer based sealers
4. Resin based sealers (AH Plus, AH26, Diaket, Epiphany, Endorez, Hydron).

Need for Resin-based Sealers:

Resin based sealer have the desirable property of creating monobloc in the root canal. Several strategies have been used to create root canal monobloc. Coating gutta percha with poly butadiene-di-isocynate-methacrylate adhesive which has a hydrophilic portion which is compatible with methacrylate resin and hydrophobic portion compatible with polyisoprene substrate of gutta percha leading to strong union between gutta percha and resin adhesive leading to formation of the root canal monobloc to achieve total bond and hence a total seal of the canal space. Use of a polycaprolactone and dimethacrylate-containing resin blend to form a filled thermoplastic composite (Resilon) that can be used an alternative root filling material instead of gutta percha.²

There are two types of resin based sealers namely epoxy resin based and methacrylate resin based.

Methacrylate resin based sealers are classified according to the different generations. The first generation of methacrylate based sealers, Hydron appeared in the mid-1970’s when dentinal bonding were at their initial stage of development. The second generation of bondable sealers were non-etching and hydrophilic in nature and does not require the adjunctive use of a dentin adhesive. In order to simplify bonding procedures, new generations of self-etching (third generation) and self-adhesive (fourth generation) resin composites have been introduced to restorative dentistry during the last 5 years and are awaiting trials for clinical use.³ Methacrylate sealers create a solid, bonded, continuous material from one dentinal wall of the canal to the other forming a fluid proof seal without forming any gaps or voids.

Resins added to the different sealers in different generations are:

1. 1st generation –hydron
2. 2nd generation-endorez
3. 3rd generation-epiphany,fiberfil
4. 4th generation –metaseal

Chronological Classification To date

4 generations of methacrylate resin–based sealers have been introduced. The first generation Hydron (Hydron Technologies, Inc, Pompano Beach, FL) appeared in the mid-1970s when scientific foundations behind dentin bonding were at their infancy stage of development. The use of poly [2-hydroxyethyl methacrylate] (poly [HEMA]) as the major ingredient rendered the sealer very hydrophilic, E.g., Hydron.⁴

The second generation of bondable sealer is nonetching and hydrophilic in nature and does not require the adjunctive use of a dentin adhesive. It is designed to flow into accessory canals and dentinal tubules to facilitate resin tag formation for retention and seal after smear layer removal with NaOCl and ethylenediaminetetraacetic acid (EDTA). e.g.- EndoREZ.⁵

The third-generation self-etching sealers contain a self-etching primer and a dual-cured resin composite root canal sealer. The use of self-etching primers reintroduced the concept of incorporating smear layers created by hand/rotary instruments along the sealer-dentin interface. An acidic primer is applied to the dentin surface that penetrates through the smear layer and demineralizes the superficial dentin. FibreFill root canal sealant (Pentron Clinical Technologies, Wallingford, CT) is an example of a third generation methacrylate resin–based sealer. Another third-generation methacrylate resin–based sealer that incorporates the use of self-etching primers became commercially available with the introduction of Resilon.⁶

The fourth-generation methacrylate resin–based sealers (e.g., Meta- SEAL, Parkell Inc; RealSeal SE, SybronEndo) are functionally analogous to a similar class of recently introduced selfadhesive resin luting composites in that they have further eliminated the separate etching/ bonding step. Acidic resin monomers that are originally present in dentin adhesive primers are now incorporated into the resin-based sealer/composite to render them self-adhesive to dentin substrates. The combination of an etchant, a primer, and a sealer into an all-in-one self-etching, self-adhesive sealer is advantageous in that it reduces the application time as well as errors that might occur during each bonding step. MetaSEAL is the first commercially available fourth generation self-adhesive dual-cured sealer.⁶

ADVANTAGES

1. When used with resilon forms ‘Monoblock’ which further improves the seal.
2. Realseal has greater root fracture resistance compared to AH Plus.
3. Good radiopacity but less then AH Plus.
4. Slow polymerization of the dual-curable sealers would improve the chance for the relief of shrinkage stress via resin flow.
5. They showed that roots filled with Resilon/Epiphany exhibited significantly higher fracture load values than those filled with gutta-percha/AH-26 when the specimens were subjected to vertical loading forces.
6. EndoREZ was found to be well-tolerated by connective tissues and bone tissue.
7. Methacrylate resin–based sealers used with Resilon or gutta-percha were more effectively removed, with less remnant filling material than conventional sealer/gutta-percha combinations.
8. Smartpoint expands only laterally on absorbing water from the tooth, adopting the canal shape. Smart paste also expands on hydration to form a perfect seal.
9. FibreFill R.C.S. is reported to have good sealing and adhesive properties to radicular dentin.
10. For methacrylate resin-based sealers, thin films had higher bond strength than thick.
11. The chemical union between the polyisoprene component of the gutta-percha and the polybutadiene end of the EndoRez resin coating.⁷

DISADVANTAGES

1. Epiphany and metaseal is cytotoxic even after dilutions.
2. Resilon/Epiphany (RealSeal) filled canals also contained significantly more voids and gaps than those filled with gutta-percha and conventional sealers.
3. Lower push-out strengths than gutta percha/conventional nonbonding sealer combinations.
4. Greater C-factor causing more polymerisation shrinkage hence more gap formation and microleakage.
5. The chemical coupling between contemporary methacrylate resin–based sealers to root filling materials is generally weak or insufficiently optimized.
6. Creeping of incompletely polymerized resinous sealers, which results in failure along the sealer-dentin interface.
7. Presence of residual monomers in the root canals.
8. Epiphany in both freshly mixed and set conditions showed a severe to moderate cytotoxic effect, and its cytotoxicity actually increased with time, posing significant cytotoxic risks.
9. Epiphany is insoluble in the solvents commonly used in dentistry. Thus, removal of resin sealers from fins, accessory canals, or canal isthmi remains a challenge.
10. Solubility values for Epiphany and AH Plus were 3.41% but according to ADA it should be less than 3%.
11. Unreacted monomers, leachable monomers from the incompletely polymerized Smart paste sealer can leak through the apical foramen after water sorption and swelling and cause inadvertent harmful detrimental effects on the periodontal tissues.
12. Diffusion of water into resin matrices might result in the rapid deterioration of the physical/mechanical properties of a resin, compromising the durability of resin-dentin bonds by hydrolysis and microcrack formation.⁷

HYDRON:

It was introduced by Wichterle and Lim in the year 1960. It is a rapid setting air injectable root canal filling material which is hydrophobic and is used as a root canal sealer without the use of a core. It is a polymer of HEMA (hydroxyl ethyl methacrylate). It requires the use of a special syringe and needle. Hydron is a biocompatible material that confirms the shape of the root canal because of its plasticity.⁸ Hydron was designed to be injected into a root canal and to be polymerized in situ for en masse root filling. It was reported to be easy to use because of its injectability, nonirritating, highly adaptable to the canal walls, nonsupportive of bacterial growth, and able to be calcified in the event of inadvertent extrusion of the sealer into the periapical regions. However, the sealer came to a disastrous end and became obsolete in the 1980s because discrepancies between the manufacturer’s claims and laboratory/clinical findings on its physical/ clinical properties and biocompatibility became apparent soon after its commercialization. The sealer caused severe inflammatory reaction, absorption of the material, severe leakage, as well as water sorption and swelling.⁹

ENDOREZ:

It is hydrophilic, non-etching dual cure radiopaque sealer supplied in a two-spense mixing and delivery syringe. The matrix constitutes of urethane dimethacrylate resin, barium sulphate and zinc oxide. Hydrophilic nature of this sealer when used in wet environment of the root canal system can be very effective in penetrating dentinal tubules and forming long resin tags.¹⁰ EndoREZ (Ultradent Products Inc, South Jordan, UT) is a dual cured radiopaque hydrophilic methacrylate sealer that might be used in the wet environment of the root canal system and is very effective in penetrating dentinal tubules and adapting closely to the canal walls. Although EndoREZ is recommended for use with either a conventional gutta-percha cone or with specific EndoREZ points (resin-coated gutta-percha), low bond strength to the dentinal wall was reported with conventional uncoated gutta-percha. To facilitate rapid cure of EndoREZ, an accelerator that is compatible with EndoREZ has recently become available.⁹

EPIPHANY:

It is a dual curing, hydrophilic resin sealer, used with resilion core materials. It is dispensed with an automixing barrel for ease of use and accurate mixing. An adhesive primer containing sulfonic acid terminated functional monomer, HEMA and water and filler content is 70% by weight. Matrix is constituted by mixture of bis-GMA, UDMA and ethoxylatedbis-GMA. It has excellent sealing ability because of monobloc creation which adheres to dentinal walls. Drawbacks include water sorption leading to disruption of bonds. The self-etching primers are further reduced from a 2-bottle system to a single-bottle system. These self-etching primers/adhesives mostly contain 2-acrylamido-2-methyl-propanesulfonic acid (AMPS) as the functional acidic monomer. In the single-bottle type self-etching primer, the functional acidic monomers, solvents, water that is necessary for ionization of the acidic monomers, and self-cured catalysts are incorporated into ‘‘one-component’’ (i.e., incorporated inside a single bottle). This is similar to the so-called one-component type all-in-one adhesives that are currently available in restorative dentistry. By combining self-etching adhesives and methacrylate resin–based sealers with Resilon, the manufacturer introduced what they advertized as ‘‘a new era’’ in root canal obturation. An ethoxylated bisphenol-A-dimethacrylate (EBPADMA)– based resinous solvent (e.g., RealSeal Thinning Resin, SybronEndo) is also included in these systems to adjust the sealer viscosity. However, addition of the thinning solvent to the sealer without photoactivation did not increase adhesion to dentin.⁹

Fiberfill

FibreFill root canal sealant (Pentron Clinical Technologies, Wallingford, CT) is an example of a third generation methacrylate resin–based sealer that is designed for filling canals with fiber reinforced obturators that are attached to thermoplastic root filling material tip. The resin sealer is used in combination with a self-cured, self-etching primer system (Fibrefill Primer A and B). Bonding between adhesive systems and dentin depends on the penetration of monomers into the conditioned dentin surface to create micromechanical interlocking between the dentin collagen and resin, forming a hybrid layer. FibreFill R.C.S. is reported to have good sealing and adhesive properties to radicular dentin.⁹

MetaSEAL:

Metaseal is a fourth-generation self-adhesive dual cure sealer, available in powder-liquid form. It is a radiopaque, insoluble material that can be used either with Resilon or normal guttapercha. Powder contains zirconia oxide filler, silicon dioxide fillers, polymerisation initiators and liquid contains 4-meta monofunctional methacrylate monomers and photoinitiators. Formation of hybrid dentin is the major mechanism of bonding.⁹ Acidic resin monomers that are originally present in dentin adhesive primers are now incorporated into the resin-based sealer/composite to render them self-adhesive to dentin substrates. The combination of an etchant, a primer, and a sealer into an all-in-one self etching, self-adhesive sealer is advantageous in that it reduces the application time as well as errors that might occur during each bonding step. MetaSEAL is the first commercially available fourth generation self adhesive dual-cured sealer. The inclusion of an acidic resin monomer, 4- methacryloyloxyethyl trimellitate anhydride (4-META), makes the sealer self-etching, hydrophilic, and promotes monomer diffusion into the underlying intact dentin to produce a hybrid layer after polymerization. The sealer purportedly bonds to thermoplastic rootfilling materials as well as radicular dentin via the creation of hybrid layers in both substrates. MetaSEAL is also marketed as Hybrid Bond SEAL (Sun Medical Co Ltd, Shiga, Japan) in Japan and had been reported to produce similar or slightly inferior sealing properties as conventional nonbonding epoxy resin–based sealers.¹¹ The idea of incorporating 4-META as a resin monomer component for root canal sealers is not new. Endoresin-2 was similar to Hydron because it was designed to be used as an injectable type of root filling material rather than a root canal sealer. SuperBond RC Sealer (Sun Medical Co Ltd) is a liquid-and-powder type sealer. Its polymer powder consists of the same constituents as the commercialized Polymer (L-Type radiopaque) in Super-Bond C&B resin (Sun Medical C Ltd). Reports claim that it possesses reasonable sealing ability when compared with conventional root canal sealers.¹²

RealSeal SE

It is the simplified dual-cured version of RealSeal and uses a polymerizable methacrylate carboxylic acid anhydride (ie, 4-META) as the acidic resin monomer. It might be used with Resilon cones or pellets by using cold lateral or warm vertical techniques or with the more recently introduced RealSeal 1 (SybronEndo), a carrier-based Resilon obturator system.¹³

CONCLUSION

Resin-based root canal sealers show promising results as root canal sealers. However, discrepancies in the results of some studies reveal that these sealers do not fulfil all of the requirements demanded of an ideal root canal sealer. Further studies are required to clarify the clinical outcomes associated with the use of these sealers.

REFERENCES