Purpose of Root Canal Obturation and method of root canal obturation

Introduction
Root canal obturation involves the three-dimensional filling of the entire root canal system and is a critical step in endodontic therapy. There are two purposes to obturation: the elimination of all avenues of leakage from the oral cavity or the periradicular tissues into the root canal system; and the sealing within the root canal system of any irritants that remain after appropriate shaping and cleaning of the canals, thereby isolating these irritants. Pulpal demise and subsequent periradicular infection result from the presence of microorganisms, microbial toxins and metabolites, and the products of pulp tissue degradation. Failure to eliminate these etiologic factors and to prevent further irritation as a result of continued contamination of the root canal system are the prime reasons for failure of nonsurgical and surgical root canal therapy.

The importance of three-dimensional obturation of the root canal system cannot be overstated, with the ability to achieve this goal primarily dependent on the quality of canal cleaning and shaping as well as clinical skill. Other factors that influence the ultimate success or failure of each case include the materials used and how they are used. The ultimate coronal restoration of the tooth following canal obturation may loom as the most important goal, for there is reasonable evidence that coronal leakage through improperly placed restorations after root canal treatment and failure of the restorative treatment or lack of health of the supporting periodontium are the final determinants of success or failure in treatment.

Factors influencing complete obturation
  • Quality of the cleaning and shaping of the canal system
  • Skill and experience of the clinician
  • Materials and their usage
  • Restoration of the tooth
  • Health of the supporting periodontium
Characteristics of an Ideal Root Canal Filling
An ideal root canal filling three-dimensionally fills the entire root canal system as close to the cemento-dentinal junction as possible. Minimal amounts of root canal sealers, most of which have been shown to be biocompatible or tolerated by the tissues in their set state, are used in conjunction with the core filling material to establish an adequate seal. Radiographically, the root canal filling should have the appearance of a dense, three-dimensional filling that extends as close as possible to the cemento-dentinal junction. These standards should serve as the benchmark for all clinicians performing root canal therapy, and it is only through a knowledgeable approach to root canal treatment that quality assurance can be continually demonstrated in the obturation of root canal systems.

The ideal root canal filling
3-D filling of the entire root canal space as close as possible to the cemento-dentinal junction

Radiographically dense fill with absence of voids
 
Shape reflecting a continuously tapered funnel that is approximately the same as the external root morphology


While a plethora of materials has been advocated over the last 150 years for root canal obturation, historically, gutta-percha has proven to be the material of choice for successful filling of root canals from their coronal to apical extent. Although it is not the ideal filling material, gutta-percha has satisfied the majority of criteria for an ideal root filling material. The disadvantages of gutta-percha – specifically, its adhesiveness, lack of rigidity, and ease of displacement under pressure – do not overshadow its advantages. In light of its shortcomings, a sealer/cement is always used with gutta-percha. However, regardless of the delivery system or technique used, neither gutta-percha nor sealer/cement alone enables standard-ofcare canal obturation. In addition, the available materials and techniques do not routinely provide for an impervious seal of the canal system; all canals leak to a greater or lesser extent.
It is recommended that clinicians master multiple obturation techniques and become competent with various root canal sealers/cements, to be able to manage the diversity of anatomical scenarios that may be encountered.

Contemporary Sealers/Cements

The use of a sealer during root canal obturation is essential for success. Not only does it enhance the possible attainment of an impervious seal, it also serves as a filler for canal irregularities and minor discrepancies between the root canal wall and core filling material. Sealers are often expressed through lateral or accessory canals and can assist in microbial control should there be microorganisms left on the root canal walls or in the tubules.Sealers can also serve as lubricants, enabling thorough seating of the core filling material during compaction. In canals in which the smear layer has been removed, many sealers demonstrate increased adhesive properties to dentin in addition to flowing into the patent tubules. A good sealer should be biocompatible and well tolerated by the periradicular tissues, and although all sealers exhibit toxicity when freshly mixed, their toxicity is greatly reduced on setting and all are absorbable when exposed to tissues and tissue fluids. Subsequent tissue healing or repair generally appears unaffected by most sealers, provided there are no adverse breakdown products of the sealer over time. In particular, the breakdown products may have an adverse action on the proliferative capability of periradicular cell populations. Some clinicians consider that a small puff of root canal filler extending beyond the working length is indicative of a fully obturated canal space with a well-sealed apical constriction. Excessive sealer should not be routinely placed in the periradicular tissues as part of an obturation technique.

Sealers/cements can be grouped based on their prime constituent or structure, such as zinc oxide-eugenol, polyketone, epoxy, calcium hydroxide, silicone, resin, glass ionomer, or resin-modified glass ionomer. However, many of the sealers/ cements are combinations of components, such as zinc oxideeugenol and calcium hydroxide,43 with the addition of calcium hydroxide claimed to create a therapeutic material that can be inductive of hard tissue formation.Epoxy-based and methacrylate-based resin sealers that can be bonded to the root canal dentin (but not to gutta-percha) are also now available.
Sealers should be mixed to a creamy consistency, allowing them to adhere to the master cone and not ball up at the shaft of the cone, leaving the gutta-percha exposed. The sealer should adhere to the cone evenly along its length and at the end of the cone. Clinicians should read the product insert and material safety data sheet for each product chosen before using it.

Contemporary Core Filling Materials
Gutta-percha is the standard material of choice as a solid core filling material for canal obturation. It demonstrates minimal toxicity and minimal tissue irritability, is the least allergenic material available when retained within the canal system,and in cases of inadvertent gutta-percha cone overextension into the periradicular tissues, is well tolerated provided the canal is clean and sealed. Chemical solvents have been used for almost 100 years to soften gutta-percha, with methods ranging from merely dipping the gutta-percha cones into the solvent for one second for better canal adaptation, to creating a completely softened paste of gutta-percha with the solvent. Solvents used have included chloroform, halothane, rectified white turpentine, and eucalyptol. Periradicular tissues may be irritated if the solvent is expressed beyond the canal or significant amounts of softened gutta-percha are inadvertently placed into the periradicular tissues. Failure to allow for dissipation of chemical solvents, if volatile, or the removal of excess solvent with alcohol can result in significant shrinkage and possible loss of the apical seal. The use of chemical solvents has been both praised and questioned, but with the advent of thermoplasticized gutta-percha, the need to consider the use of solvents at any time must be questioned. The use of solvents, however, may still be considered for a number of challenges the clinician may face in daily practice, such as the custom fitting of master cones in irregular apical preparations or following apexification.

Gutta-percha Cones
The composition of gutta-percha cones is approximately 19% to 22% Balata and 59% to 75% zinc oxide, with the remainder a combination of various waxes, coloring agents, antioxidants, and metallic salts.58 The specific percentages for components varies by manufacturer, with resulting variations in the brittleness, stiffness, tensile strength, and radiopacity of the individual cones attributable primarily to the percentages of gutta-percha and zinc oxide. The antimicrobial activity of gutta-percha is also primarily due to the zinc oxide.

The cones are manufactured in both standardized and nonstandardized sizes. The standardized sizes coordinate with the ISO root canal files sizes 15 through 140 and are used primarily as the main core material for obturation. They generally have a 2% taper, but can have a 4 or 6% taper or more. The non-standardized sizes are more tapered from the tip or point to the top. With some obturation techniques these cones have been used primarily as accessory or auxiliary cones during compaction, being matched with the shape of the prepared canal space or the compaction instrument.
Non-standardized cones began to assume a greater role as the primary core material in the more contemporary obturation techniques, and with the development of more predictable shapes with current nickel titanium (NiTi) rotary and hand instruments, cones tapered from 4% to 10% have gained use.

Tapered gutta-percha cones



In particular, for techniques that use vertical compaction of heat-softened gutta-percha, both the non-standardized and more tapered cones have become quite acceptable. Custom cones can also be developed for canals with irregular or large apical anatomy, Over time, numerous methods have been advocated for obturating the prepared root canal system, each with their own claims of ease, efficiency, or superiority. Most contemporary techniques still rely on gutta-percha and sealer to achieve their goal. Four basic techniques exist for the obturation of the root canal system with gutta-percha and sealer

(1) the cold compaction of gutta-percha; (2) the compaction of heat-softened gutta-percha with cold instruments

Cold compaction carrier              Heated compaction              Presoftened core

until it has cooled; (3) the compaction of gutta-percha that has been thermoplasticized, injected into the system, and compacted with cold instruments; and (4) the compaction of gutta-percha that has been placed in the canal and softened through the continuous wave technique (Calamus). A multitude of variations on these four basic themes exists. For injectable thermoplastic obturation techniques, gutta-percha may come in either pellet forms or in cannulae.

Cold compaction carrier Heated compaction Presoftened core

No single technique has proven to have statistically significant superiority when considering both in vitro and in vivo studies, as the success of all techniques is highly dependent on the cleaning and shaping of the canals and the clinician’s expertise in the use of a particular technique.While many have advocated the use of the lateral compaction technique or a single cone fill (monocone) to achieve a quality apical seal, the technique in itself does not necessarily favor the filling of canal irregularities.63 Recognizing this, use of a softened gutta-percha technique with heat or chemical softening is required to achieve a thorough obturation.

In addition, while filling the entire root canal system is the major goal of canal obturation, a major controversy exists as to what constitutes the apical termination of the root canal filling material. Working length determination guidelines often cite the cemento-dentinal junction or apical constriction as the ideal position for terminating canal cleaning and shaping procedures and placing the filling material. However, the cemento-dentinal junction is a histologic and not a clinical position in the root canal system and, in addition, the cemento-dentinal junction is not always the most constricted portion of the canal (yellow arrows) in the apical portion of the root

Contemporary practices of obturation favor material softening even this does not guarantee that an impervious seal of the root canal system will be established. Also, with softened gutta-percha obturation techniques there has been a greater incidence of material extrusion beyond the confines of the canal. While softening of gutta-percha may be viewed as routinely desirable, the selective use of this technique solely or in combination with a solid core of gutta-percha must be at the discretion of the competent clinician when anatomy dictates this approach.

Apical constriction of root canal


Recent research conducted at Nova Southeast University using micro CT scanning technology has shown the effectiveness of scanning for imagery and the greater precision observed compared to standard radiographs. In one example, a mesiobuccal canal was filled using GT® Series X™ obturator and the mesiolingual canal was filled using a single cone technique (ActiV GP). It appeared from one angle that all canals were equally filled.

Radiograph of filled canals

However, closer examination subsequent to filling showed voids using the single cone technique throughout the length of the root filled using this technique. The single cone technique did not produce a monoblock obturation. The gutta-percha from the GT obturator flowed into the canal isthmus and filled it.

Voids using single cone technique

Cross-section with ActiV GP and GT Series X obturation

                                          
                                Activ V GP             GT Series X obturator

Canal isthmus filled with GT obturator gutta-percha


Differences in obturation techniques and results are also more observable using CT scanning than using traditional radiographs.Contemporary techniques include the use of bonded root canal filling materials. Recent developments in resin-bonding have led to the availability of resin cones and pellets similar in shape and size to gutta-percha materials.
Resin-based cones containing methacrylate resin, fillers, bioactive glass, and polymers are available that can be handled similarly to gutta-percha and can be used with a lateral or vertical compaction technique. Resin-based materials can also be delivered via a heated syringe (Obtura gun, Spartan Obtura). Since resin-based materials require a slightly moist environment, it is important to avoid using any dessicating solutions, such as alcohol, during root canal preparation. Further, if sodium hypochlorite or peroxide was used during root canal preparation, this must be thoroughly removed prior to using a resin-based material as it would reduce the ability of the resin material to bond. Similarly, the smear layer must also be thoroughly removed.

Prefabricated Obturators

Gutta-percha can also be formed on a plastic carrier or corecarrier. Prefabricated obturators were first described in 1978 by William Ben Johnson.79 The prototype for the obturator had been prenotched K-files wrapped in gutta-percha (hand formed) that were then heated over a flame until the surface glistened and expanded. These prenotched “obturators” were inserted into the canal and apical pressure applied while the handle was twisted off.

Prototype obturator

Prefabricated obturators were introduced in 1988 (Thermafil) using first a stainless steel and subsequently a titanium core, coated with gutta-percha. Plastic obturators were first offered in 1992. Since then, a number of prefabricated obturator systems have been introduced, including one that does not involve thermosoftening of the gutta-percha (SimpliFil, Discus Dental) but instead is used cold with only the apical area coated in gutta-percha, and after placement the carrier itself is removed.
A recently developed prefabricated obturator utilizes a resin-based system (RealSeal One, Sybron Endo) and is used with, and bonded to, methacrylate resin-based sealer material and is first held in its custom oven and thermosoftened. Other systems use thermosoftened gutta-percha, including Calamus® (Tulsa Dental, Dentsply) , Successfil® (Hygienic-Coltene-Whaledent, Inc.), Gutta- Flow®, System B Obturation System, Thermafil, Thermafil Plus, ProTaper® Universal and ProSystem GT® Obturators (Dentsply, Tulsa Dental), and Soft-Core® (Soft-Core® Texas, Inc.).


Current plastic obturators are available in a nonvented shape with a taper of around 0.04 (Densfil) and a vented shape with the same taper (Thermafil Plus). Both are biologically inert. The carrier is thick with a thinner outer coating of gutta-percha, which helps to reduce material shrinkage as the gutta-percha cools in the canal. A vented prefabricated obturator helps the flow of gutta-percha during placement and also aids in retrieval of the obturator should retreatment be necessary.
For sizes 40 and below in the Thermafil series, an insoluble liquid crystal plastic is used. For size 45 and above soluble polysulfone polymer is used. All of these use a size verifier to help select the correct size obturator, as do ProTaper Universal carriers, which start at a .04 taper. Systems that do not use a size verifier include the ProSystem GT carriers and GT Series X carriers, which are made in a variety of tapers between 0.04 and 0.12.
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