Continuing Education Activity

Root canal therapy is performed to remove affected dental pulp tissue, clean and disinfect the root canal system, and ultimately seal the canal system using a variety of available materials. The filling of root canals, referred to as obturation, utilizes a variety of materials to ensure that any residual microorganisms are completely entombed, thus preventing future proliferation. Operators commonly use a combination of a root canal sealer and core filling material to accomplish this goal. Both root canal sealers and core materials are available in various compositions and therefore differ in their indications and uses. This activity reviews the materials used to fill root canals and highlights the key differences between materials as well as indications and contraindications to improve treatment outcomes for patients who undergo root canal therapy.

Objectives:

• Describe the importance of filling a root canal.
• Summarize the characteristics of an ideal root canal filling material.
• Explain the difference between a root canal sealer and a core filling material.
• Identify the most commonly utilized root canal filling materials.

Introduction

The filling of root canals, commonly referred to as obturation, is a critical step in successful root canal treatment (RCT). The goals of obturation are twofold: to create an apical seal and fill the root canal without incorporating voids within the filling material.[1][2] Successful obturation eliminates leakage of contaminants into the channel, such as saliva, bacteria, and periapical tissue fluid, and entraps any residual microorganisms within the canal space.[2]

The ability to adequately obturate a root canal is directly related to the preceding step of cleaning and shaping. Cleaning and shaping refer to the mechanical and chemical preparation of a root canal to remove organic and inorganic matter. A canal that has been poorly cleaned and shaped will result in equally poor obturation and is one of the strongest indicators of treatment failure.[2]

Root canals are 3D structures with complex and unique anatomy, and no two are alike. Achieving a 3D obturation with complete coronal, lateral, and apical seals is essential for long-term success.[1] Radiographs are still the main method used to visualize root canals during both the diagnostic and treatment stages. However, one of their major weaknesses is that they only provide a 2D representation of 3D canals.[3] As a result, it is impossible to assess the quality of the canal seal created during the obturation stage. The introduction of micro-CT imaging into the field of endodontics has dramatically enhanced the accuracy of diagnosis, cleaning and shaping, and ultimately obturation.[4]

There are countless materials and techniques for obturation. Most techniques employ the use of a sealer and a core filling material in order to ensure complete obturation. This activity will discuss available sealers and proceed to core obturation materials.

Classification

Root Canal Sealers

The role of the sealer is critical for establishing a fluid-tight seal of the root canal as it fills the vacant space between the prepared dentinal wall and the core filling material.[1][5] It also reaches lateral canals and tubules that are not accessible to gutta-percha cones and provides antimicrobial action.[5]

Most sealers are toxic before setting, so care should be taken to avoid the extrusion of sealers into the periapical tissues.[6] Sealers induce different levels of periapical inflammation; however, this effect is usually transient and does not interfere with tissue healing.[7] Sealers are partly reabsorbed after exposure to tissue fluids. The core material should occupy almost all of the canal, and sealers only applied in small quantities.

Several types of sealers are available, which vary in formulation and properties. They are classified into five main groups according to their composition: zinc oxide eugenol (ZOE) based, calcium hydroxide based, glass ionomer-based, resin-based, and bioceramic sealers.

The properties of an ideal sealer have been outlined in the literature. It is important to note that no material has been proven to eliminate leakage completely nor fulfill all the ideals.

Properties of an ideal sealer material:

• Ability to establish a fluid-tight seal
• Radiopaque (to be visible on radiographs)
• Antimicrobial
• Non-shrinking upon setting
• Non-staining
• Non-irritating to periapical tissues
• Insoluble in tissue fluid
• Biocompatible[1][8]

Core Obturation Materials

Core materials exist to fill the bulk of the root canal space and act as a plug to prevent any leakage or ingress of foreign material. They take up most of the area within the canal and rely on the sealer to fill any voids or irregular spaces between it and the canal walls. Although other materials have been fabricated in recent years, gutta-percha remains the most widely used and accepted core filling material.

Properties of an ideal core obturation material:

• Ease of manipulation
• Dimensional stability
• Excellent sealing ability (apically and laterally)
• Non-irritating to periapical tissues
• Radiopaque (to be visible on radiographs)
• Non-staining or discoloring
• Antimicrobial
• Biocompatible[1][5]

Properties

Root Canal Sealers

Zinc Oxide Eugenol Sealers

Zinc oxide eugenol (ZOE)-based sealers are the most used sealers in clinical endodontics. They present as a powder-liquid or a two-paste preparation.[5] Although ex vivo laboratory studies have demonstrated cytotoxic properties in ZOE sealers due to the presence of eugenol, they have proven to be clinically useful in both animal and human studies.[5][9]

Notable properties of ZOE sealers include absorption if they are extruded into the surrounding tissues and anti-inflammatory and antimicrobial effects.[10] However, upon setting, ZOE sealers become weak and porous, making them susceptible to decomposition if tissue fluids leak.[9][10] Alternatively, other sealers are available that do not include eugenol and therefore mitigate any of its potentially deleterious effects. These sealers are aptly referred to as non-eugenol sealers.

Calcium Hydroxide Sealers

Calcium hydroxide is a widely used material in clinical dentistry due to its therapeutic properties for dental pulp in procedures such as pulp capping and apexification.[11][12] The key therapeutic properties behind its widespread use are its ability to promote the healing of periapical tissues and its antimicrobial effect.[13] The exact process through which calcium hydroxide exerts its action is still unknown, but some mechanisms have been proposed. Calcium hydroxide is highly alkaline (pH of 12), which is believed to trigger reparative and remineralization activities. It may also denature specific intracanal proteins and arrest external root resorption, subsequently promoting healing.[9][13][12][9] However, the exact mechanism of action of calcium hydroxide remains under investigation.

Several calcium hydroxide-based sealers are available, which vary in terms of antibacterial effect, cytotoxicity, and solubility.[13][12] Calcium hydroxide-based sealers have lower cytotoxicity compared to other groups of sealers.[13] Their sealing capacity to provide an appropriate seal has proven similar to ZOE sealers.[14] Solubility could be a concern.[13]

Glass Ionomer Sealers

Clinicians advocating for the use of glass ionomer (GI) sealers as an obturation material do so due to their dentin-bonding properties.[15] Dentin bonding results from the reaction between the polyacrylic acid in the GI and the hydroxyapatite on the dentinal wall and allows for excellent adhesion between the obturation material and the prepared canal wall.[16] Several major advantages arise from the improved adherence, including superior apical sealing, increased resistance to root fracture, and resistance to solubility in tissue fluids.[16][17] Additional advantages include antimicrobial properties and sustained fluoride release. However, the dentinal walls are hard to reach with primer solutions in the middle and apical thirds, potentially limiting the adhesion of the sealer to the walls.[17][18] Furthermore, the dentin-bonded GI sealer creates a homogenous solid material that is difficult to remove in the case of endodontic retreatment.[18]

Resin Sealers

Resin sealers are non-eugenol sealers that provide excellent adhesion to the canal walls.[9] These sealers can be separated based on their constituent elements as epoxy resin-based or methacrylate resin-based sealers. Methacrylate resin-based sealers are further classified into four distinct generations, each with its own unique formulation. The generations vary according to properties such as primer type and non-etching vs. self-etching capabilities.[9][19] Resin-based sealers have several beneficial properties, including excellent adhesion and seal, as well as good tolerability from apical tissues.[9][19] However, it has also been shown to produce an initial inflammatory reaction with the potential for allergic and mutagenic responses.[20]

Bioceramic Sealers

Bioceramic sealers are biocompatible materials that use either calcium silicate or calcium phosphate as their main constituent.[21] Mineral trioxide aggregate (MTA) is a newer material that is currently used as a root canal sealer in addition to its many other uses within the field of endodontics. The formula of MTA is primarily tricalcium silicate with added bismuth oxide, a radiopaque powder for enhanced visualization on radiography.[21][22] 

MTA is activated with the introduction of water and produces a highly alkaline mixture with a pH of 12 and beneficial hydrates of calcium silicate. These hydrates make MTA an excellent endodontic material as they form on the surface of existing calcium silicate particles and continue to grow inwards, effectively creating a barrier.[21][22] One major disadvantage of bioceramic sealers is that the enhanced adhesion between the material and the dentin wall makes endodontic retreatment and post-preparation increasingly challenging.[21][22]

Core Obturation Materials

Silver Cones

Silver cones (also referred to as silver points) are a historic obturation material that is no longer used in modern endodontics. The original basis for their use stemmed from the belief that they had an oligodynamic property (heavy metals' capacity to exert a bactericidal effect) that led to the destruction of intracanal microbes.[5] This belief has been proven to be false, and silver points have been shown to have several major flaws as obturation materials.[23] One major disadvantage is that they corrode when exposed to saliva or tissue fluid, creating cytotoxic products, and do not completely fill most root canals resulting in leakage.[23][24]

Gutta-Percha

Gutta-percha (GP) is the most commonly used core obturation material in modern endodontics.[25] They are manufactured as cones comprised mostly of zinc oxide and gutta-percha with added plasticizers and radiopacifiers.[5] Gutta-percha is a naturally-occurring thermoplastic material that originates from specific trees in the Malaysian region.[25]

The popularity of gutta-percha as an obturation material stems from its variable chemical composition. GP exists in two crystalline phases, alpha, and beta, and can cycle between these phases when heated is applied or removed. The alpha phase of GP occurs when it is heated and produces a tacky material that can flow under pressure. Alternatively, the beta phase is when the GP is unheated, forming a solid mass of compactable material.[1][25]

Gutta-percha has many advantages, such as ease of manipulation, radiopacity, low toxicity, and plasticity. But, it does not bond to dentin and shrinks when cooled.[5][25]

Gutta-percha cones are available in standardized 0.02 taper as specified by the International Organization of Standardization (ISO) and non-standardized sizes.[5] This allows clinicians to mechanically prepare the canal space using various commercial rotary systems that also use standardized file sizes. When the canal is prepared to a given file size, a GP cone of the same size can be inserted, adapting to the canal.[1][5][25]

Bioceramic Materials

Bioceramics, in particular MTA, can also be utilized as a core filling material. In addition to the aforementioned chemical properties of MTA, it is commonly used as an endodontic material due to its many beneficial characteristics. These include a long set time, a low relative compressive strength, high fracture resistance, and both antibacterial and antifungal properties.[26][27] Furthermore, MTA materials have been consistently proven in the literature to be biocompatible due to their high pH and ability to form hydroxyapatite, an organic substance that promotes bony growth.[28] There are several notable uses of MTA in endodontics, including in the repair of perforations during endodontic treatment, internal root resorption, apexogenesis, and pulp capping procedures.[11][29]

Other Core Materials

There are various other core obturation materials commercially available; however, no statistical difference in terms of reduction in leakage has been noted between them and gutta-percha.[1][5] These materials include GI-impregnated gutta-percha cones and resin-based obturation systems.[30] Using a resin core in addition to a resin sealer is believed to allow the formation of a 'monoblock,' or bonded homogenous mass of material within the root canal.[31] However, this has not yet been achieved with current materials.[31]

Advantages and Disadvantages

Root Canal Sealers

Zinc Oxide Eugenol (ZOE) Sealers

Advantages:

• Absorption, if extruded past the apex
• Anti-inflammatory effects
• Antimicrobial effects[5][9]

Disadvantages:

• Transition into a weak, porous state upon setting
• Shrinkage upon setting[5][9]

Calcium Hydroxide Sealers

Advantages

• Antimicrobial properties
• Milder cytotoxicity
• Promotes tissue healing[13]

Disadvantages

• Solubility

Glass Ionomer (GI) Sealers

Advantages:

• Improved adherence (via dentin bonding)
• Increased resistance to root fracture
• Enhanced resistance to solubility in tissue fluids
• Antimicrobial properties
• Sustained fluoride release[16][17]

Disadvantages:

• Technically difficult to prepare deep aspects of root canal (potentially limits dentin-bonding ability)
• Difficult to remove in cases of endodontic retreatment[16][17]

Resin Sealers

Advantages:

• Improved adherence
• Enhanced tolerability of apical tissues[19][20]

Disadvantages:

• Technically difficult to prepare deep aspects of root canal (potentially limits dentin-bonding ability)
• Difficult to remove in cases of endodontic retreatment
• Potential for allergic and mutagenic bodily responses
• Shrinkage upon setting[19][20]

Bioceramic Sealers

Advantages:

• Biocompatible
• Ability to promote biomineralization
• Enhanced bonding between dentin and core material
• Dimensional stability upon setting[21][22]

Disadvantages:

• Technically difficult to manipulate
• Expensive
• Shorter shelf-life (cv. other sealers)[21][22]

Core Obturation Materials 

Gutta Percha

Advantages:

• Ease of manipulation
• Radiopaque
• Low toxicity and plasticity
• Relatively inexpensive[1][5][25]

Disadvantages:

• No dentin-bonding properties
• Material shrinkage upon setting[1][5][25]

Bioceramic Core Obturation Materials

Advantages:

• Biocompatible
• Ability to promote biomineralization
• High fracture resistance
• Antimicrobial
• Dimensional stability upon setting[26][27]

Disadvantages:

• Technically difficult to manipulate
• Expensive
• Shorter shelf-life (cv. other core materials)[26][27]

Indications

Several key characteristics should be kept in mind when selecting sealers and core obturation materials. Firstly, most sealers are soluble and subject to dissolution when they come into contact with tissue fluids.[32] Additionally, most sealers and core materials irritate the periapical tissues if extruded past the apex, except bioceramic materials, as these materials are biocompatible and rarely act as irritants.[20][33] Next, sealers differ in their ability to adhere to prepared dentinal walls, with sealers such as ZOE not being adherent, whereas resin-based sealers are.[9][34] 

Lastly, most sealers experience shrinkage upon setting and can result in void space being created between the core material and the dentinal wall, with the exception of bioceramic sealers exhibiting expansion upon setting.[9][22] Due to the enhanced biocompatible and physical properties of bioceramic materials, they have been indicated and utilized in most cases in modern endodontics.[5][21] However, the use of a specific sealer or core material depends entirely on the operator's preference and can be case-specific.

Contraindications

There are few true contraindications for sealers and core obturation materials. One major contraindication is an allergy to any component of the material.[35] Allergies to constituent materials are rare but possible in every sealer and core material discussed, and the operator should take care to review all of a patient's possible allergies and be able to respond should a medical emergency arise. Other non-absolute contraindications will be case-specific and depend on the treatment goal of the operator.

Technique

Root Canal Sealers

Several techniques are used to insert the sealer into the root canal and ensure that it evenly coats the prepared surface. One technique utilizes a screw-like file that attaches to a slow-speed handpiece called lentulo spiral.[36] The lentulo spiral is dipped into the sealer and inserted into the root canal. When the handpiece is activated, it causes the lentulo spiral to spin and evenly distribute the sealer on the walls through centrifugal force.[37] There are several downsides to its use, including the risk of instrument fracture due to the instrument being very fragile and easily breakable.[38][36] Additionally, it has been shown to accelerate the setting of resin-based sealers due to the stirring effect.[19] Other popular techniques for inserting sealer include the use of GP cones. The operator 'butters' the pre-fit paper point or GP cone in the sealer and then inserts it in the canal using a pumping action to ensure even coating.[1]

Core Obturation Materials

Since there are numerous types of obturation materials, there are several ways to insert them into the root canal, and as a result, this section will focus primarily on the insertion technique of gutta-percha (GP). GP has two main obturation techniques, cold lateral condensation (CLC) and warm vertical compaction (WVC).[5][39]

The cold lateral condensation technique involves the selection of a master GP cone, the use of spreaders, and the insertion of secondary GP cones.[5][40] The master cone is a GP cone with a taper and diameter that matches the prepared root canal at the working length. This master cone is measured to the working length and inserted into the canal, where it will display slight resistance to removal, aptly referred to as "tug back" if the correct size cone was selected.[5] After confirming with a radiograph, the canal is irrigated and dried, and sealer is applied to the walls. At this point, a finger spreader or a hand spreader is pre-fit in the canal to reach within 1 to 2 mm short of the working length, and appropriately sized secondary GP cones are selected that closely match the size of the spreader.[39] 

The master cone is inserted into the canal to the working length, followed by the spreader, which, if properly selected, should reach 2 mm shorter. The spreader is then rotated and withdrawn from the canal, and the matching GP cone is inserted into the vacant space.[39] The process is repeated until the operator is unable to insert the spreader deeper than the coronal third of the prepared canal. At this point, the excess GP is seared off with a heated instrument and compacted using a root canal plugger.[5][41][39]

The warm vertical compaction technique also uses a master GP cone, Schilder pluggers, and a heat source.[42][40][42] The master cone in this method should fit just short of the working length and have a slight tug back. After confirming with a radiograph, the canal is irrigated and dried, and sealer is applied to the walls. The master cone is reinserted, and the coronal portion is removed with a heated instrument.[42] Next, a heated plugger is used to gradually and sequentially remove portions of the coronal GP while softening the cone below.[42] The heated plugger is removed from the canal, a non-heated plugger is inserted, and the warm GP is compacted in order to force the now-plasticized material apically.[42] This process is repeated until 4 to 5 mm of adapted GP remains at the apex, forming an effective seal.[40] At this point, small sections of GP cones are inserted into the canal, heated with a heated plugger, and then compacted with a non-heated plugger. This is repeated until the canal is fully obturated with plasticized GP.[5][42][5][39]

Clinical Significance

The goal of root canal obturation is to ensure that any residual microorganisms remaining after canal cleaning and disinfection are completely entombed, thus preventing future proliferation and subsequent reinfection.[5] A well-made obturation also prevents bacteria or possible nutrients from the oral cavity and periodontal and periapical fluids from reaching the root canal system.[5]

Enhancing Healthcare Team Outcomes

A well-sealed root canal system provides the patient with the best possible treatment outcome. Ensuring an adequately filled root canal requires collaboration between dentists, endodontists, dental radiologists, and dental assistants. Dental nurses/assistants play a central role, as they are in charge of mixing and manipulating the sealers and core filling materials and handing them to the dentist. Root canal therapy is full of intricacies that require excellent interprofessional communication. This ensures that clinicians arrive at a proper diagnosis, generate clear imaging, and determine the best technique to perform the procedure, including which material to use to fill the root canal. [Level 3]