After reading this chapter, the student will be able to:
State reasons and describe strategies for enlarging the cervical portion of the canal to promote straight-line access.
Define how to determine the appropriate size of the master apical file.
Describe objectives for biomechanical cleaning and shaping and explain how to determine when these have been achieved.
Illustrate shapes of differently created preparations and draw these both in longitudinal and cross-sectional diagrams.
Describe techniques for shaping canals that have irregular shapes, such as round, oval, hourglass, bowling pin, kidney bean, or ribbon.
Distinguish between apical stop, apical seat, and open apex, and discuss how to manage obturation in each.
Describe appropriate techniques for removing the pulp.
Characterize the difficulties of preparation in the presence of anatomic aberrations that make complete débridement difficult.
Describe techniques for negotiating severely curved, “blocked,” “ledged,” or constricted canals.
Discuss the properties and role of intracanal, interappointment medicaments.
Successful long-term outcomes of root canal treatment are based on establishing an accurate diagnosis and developing an appropriate treatment plan; applying knowledge of tooth anatomy and morphology (shape); and performing débridement, disinfection, and obturation of the entire root canal system while maintaining the strength of the tooth. Historically, emphasis was on obturation and sealing the radicular space. However, no technique or material provides a seal that is completely impervious to moisture from either the apical or coronal aspects. Early studies on prognosis indicated failures were attributable to incomplete obturation. This proved fallacious as obturation merely reflects the adequacy of the cleaning and shaping. Canals that are poorly obturated may be incompletely cleaned and shaped. Adequate cleaning and shaping and establishing a coronal seal are essential elements for successful treatment, with obturation being less important for short-term success. Elimination (or significant reduction) of inflamed or necrotic pulp tissue and microorganisms are the most critical factors. The role of obturation in long-term success has not been established but may be significant in preventing recontamination either from the coronal or apical direction. Sealing the canal space after cleaning and shaping will help to entomb any remaining organisms and, with the coronal seal, prevent or at least delay recontamination of the canal and periradicular tissues. However, some bacterial species have been shown to survive entombment.
These classic concepts define success in endodontics by healing of apical periodontitis if present preoperatively or the prevention of its occurrence in case that began with normal periapical tissues. However, in recent years, it has been demonstrated that vertical fracture or other nonendodontic causes are the major reasons for the eventual loss of root canal–treated teeth. As a result, a more patient-centered outcome as increased susceptibility to fracture should be considered. Functional retention of the endodontically treated tooth may serve as a relevant endpoint in endodontic treatment, which may compliment but should not replace the traditional focus on healing or prevention of apical periodontitis.
Principles of Cleaning and Shaping
Nonsurgical root canal treatment is a predictable method of retaining a tooth that otherwise would require extraction. Success of root canal treatment in a tooth with a vital pulp is higher than that of a tooth diagnosed with necrotic pulp and periradicular pathosis.
The reason for this difference in outcome is the persistent presence of microorganisms and their metabolic byproducts. The most significant factors affecting the clinician’s inability to completely remove intracanal microorganisms are tooth anatomy and morphology. Instruments are believed to contact and plane the canal walls to débride the canal ( Figs. 14.1 to 14.4 ), aided by irrigating solutions. Morphologic factors include lateral (see Fig. 14.2 ) and accessory canals, canal curvatures, canal wall irregularities, fins, cul-de-sacs (see Fig. 14.1 ), and isthmuses. These aberrations render full wall contact and therefore complete débridement virtually impossible. Consequently, a practical objective of cleaning is to significantly reduce the irritants, not totally eliminate them. Frequent and effective irrigation is necessary to achieve this goal. At the same time, root canals need to be enlarged to allow irrigants to properly clean the canal and to remove contaminated dentin. Irrigants readily remove microorganisms from the coronal third of a root canal, but further shaping is necessary to eliminate bacteria in less accessible canal areas. Meanwhile, the mechanical action of instruments generates debris that is typically pushed into accessory anatomy and may block the access to subsequent irrigation. This debris also needs to be flushed and removed. Therefore it is imperative to use mechanical shaping and irrigation in synergy to maximize antibacterial efficacy of endodontic procedures.
Apart from enhancing cleaning procedures, another purpose of shaping is to provide space for an effective filling of the root canal space. The main mechanistic objective of shaping is to maintain or develop a continuously tapering funnel from the canal orifice to the apex. Conceptually, the degree of enlargement is partly dictated by the method of obturation. For lateral compaction of gutta-percha, the canal should be enlarged sufficiently to permit placement of the spreader to within 1 to 2 mm of the working length (WL). For warm vertical compaction techniques, the coronal enlargement must permit the placement of pluggers to within 3 to 5 mm of the WL.
However, the more dentin is removed from the canal walls, the less resistant to fracture the root becomes. Micronized gutta-percha points for vertical compaction that allows melting at larger distances from the heat source are now available in order to allow proper obturation of more conservative preparations. New materials based on the concept of so-called hydraulic obturation techniques are also marketed for the same purpose. However, as always, clinicians should select an obturation technique judiciously, based on available evidence.
Ideally, the degree of shaping should be tooth dependent and not depend on the obturation technique. As an example, narrow thin roots, such as mandibular incisors, may not permit the same degree of enlargement as more bulky roots, such as the maxillary central incisors.
Apical Canal Preparation
Termination of Cleaning and Shaping
Although the concept of cleaning and shaping the root canal space appears to be straightforward, there are areas where consensus does not exist. The first is the extent of the apical preparation. Early studies identified the dentinocemental junction as the area where the pulp ends and the periodontal ligament begins. Unfortunately, this is a histologic landmark and the position (which is irregular within the canal) cannot be determined clinically.
Traditionally, the apical point of termination, also known as WL , has been 1 mm from the radiographic apex. A classic study described the apical portion of the canal with the major diameter of the foramen and the minor diameter of the constriction ( Fig. 14.4 ). The apical constriction is defined as the narrowest portion of the canal, and the average distance from the foramen to the constriction was found to be 0.5 mm. Another study found the classic apical constriction to be present in only 46% of the teeth and, when present, varied in shape and in relation to the apical foramen. Variations from the classic appearance consist of the tapering constriction, multiple constrictions, and a parallel apical canal part. To complicate the issue, the foramen is rarely located at the anatomic apex. Convincing micro–computed tomography data provide a more realistic portrait of apical canal morphology ( Fig. 14.5 ).
Apical anatomy has also been shown to be quite variable (see Figs. 14.4, B , and 14.5 ). A study found no typical pattern for foraminal openings and that no foramen coincided with the apex of the root. The same group reported the foramen to apex distance to range from 0.20 to 3.8 mm.
It has also been noted that the foramen to constriction distance increases with age, and root resorption may destroy the classic anatomic constriction. Resorptive processes are common with pulp necrosis and apical bone resorption. Therefore root resorption is an additional factor to consider in length determination.
In a prospective study, significant adverse factors influencing success and failure were the presence of a perforation, preoperative periradicular disease, and incorrect length of the root canal filling. , The authors speculated that canals filled more than 2.0 mm short harbored necrotic tissue, bacteria, and irritants that when retreated could be cleaned and sealed. A meta-analysis evaluation of success and failure indicated a better success rate when the obturation was confined to the canal space. A review of several studies on endodontic outcomes confirms that extrusion of materials decreases success. , , In one study examining cases with pulp necrosis, better success was achieved when the procedures terminated at or within 2 mm of the radiographic apex. Obturation shorter than 2 mm from the apex or past the apex resulted in a decreased success rate. In teeth with vital inflamed pulp tissue, termination between 1 and 3 mm was acceptable. Two larger studies confirmed that overfill was associated with inferior outcomes. ,
At the same time, working short presents higher risks of accumulation and retention of debris, which in turn may result in apical blockage and may contribute to procedural errors in the first place; furthermore, infected debris, bacteria, and their byproducts can remain in the most apical portion of the canal in cases with pulpal necrosis jeopardizing apical healing and contributing to a persistent or recurrent apical periodontitis , or posttreatment disease. ,
Most publications on outcomes that extrapolate the effect of apical termination are retrospective. On the other hand, a recent prospective study demonstrated that not only the maintenance of apical patency, but also the apical extent of canal cleaning, is a significant prognostic factor for root canal treatment, recommending extending canal cleaning as close as possible to its apical terminus. In that study, the odds of success were reduced by 12% for every 1 mm of the canal short of the terminus remaining “uninstrumented.”
Therefore the exact clinical point of apical termination of the preparation and obturation remains a matter of debate. The need to compact the gutta-percha and sealer against the apical dentin matrix (constriction of the canal) is important in creating a seal. The decision of where to terminate the preparation is based on knowledge of apical anatomy, tactile sensation, radiographic interpretation, apex locators, apical bleeding, and the patient’s response. To prevent extrusion, the cleaning and shaping procedures should be confined to the radicular space. Canals filled to the radiographic apex are actually slightly overextended.
Degree of Apical Enlargement
Generalizations can be made regarding tooth anatomy and morphology, although each tooth is unique. Length of canal preparation is often emphasized with little consideration given to important factors such as canal diameter and shape. Because morphology is variable, there is no standardized apical canal size. Traditionally, preparation techniques were determined by the desire to limit procedural errors and by the method of obturation. Small apical preparation reduces the incidence of preparation errors (as is discussed in the following section) but may decrease antimicrobial efficacy of cleaning procedures. It appears that with traditional hand instruments, apical transportation occurs in many curved canals enlarged beyond a No. 20 stainless steel file.
The criteria for cleaning and shaping should be based on the ability to adequately deliver sufficient amounts of irrigant and not on a specific obturation technique. The ability of irrigants to reach the apical portion of the root canal depends on canal’s size, taper, and the irrigation device used.
Larger preparation sizes have been shown to provide adequate irrigation and debris removal and significantly decrease the number of microorganisms. However, any removal of dentin has the potential to weaken radicular structure and therefore the use of an irrigation adjunct designed to promote irrigation efficacy in smaller canals may be advantageous. ,
In principle there may to be a relationship between increasing the size of the apical preparation and canal cleanliness and bacterial reduction. , Instrumentation techniques that advocate minimal apical preparation may be ineffective at achieving the goal of cleaning and disinfecting the root canal space. , However, this concept reaches its limits when too large a preparation leads to procedural errors , and when modifications created in the hard tissue block the very anatomy that was to be cleaned ( Fig. 14.6 ).
A variety of microbial species can penetrate deep into dentinal tubules. These intratubular organisms are sheltered from endodontic instruments, the action of irrigants, and intracanal medicaments. Dentin removal appears to be the primary method for decreasing their numbers. However, it may not be possible to remove bacteria that are deep in the tubules, regardless of the technique. There is a correlation between the number of organisms present and the depth of tubular penetration ; in teeth with apical periodontitis, bacteria may penetrate the tubules to the periphery of the root. ,
Elimination of Etiology
The development of nickel-titanium (NiTi) instruments has dramatically changed the techniques of cleaning and shaping; these instruments have been rapidly adopted by clinicians in many countries. The primary advantage to using these flexible instruments is related to shaping, specifically a significant reduction in the incidence of preparation errors.
Neither hand instruments nor rotary files have been shown to completely débride the canal. , , Mechanical enlargement of the canal space dramatically decreases the presence of microorganisms present in the canal but cannot render the canal sterile. Therefore antimicrobial irrigants have been recommended in addition to mechanical preparation techniques. There is currently no consensus on the most appropriate irrigant or concentration of solution, although sodium hypochlorite (NaOCl) is the most widely used irrigant.
Unfortunately, solutions such as NaOCl that are designed to kill bacteria , , are often toxic for the host cells, and therefore extrusion beyond the canal space is to be avoided. , A major factor related to effectiveness is the volume of irrigant used during the procedure. Increasing the volume produces cleaner preparations.
Apical patency is a technique that advocated the repeated placement of small hand files to or slightly beyond the apical foramen during canal preparation ( Fig. 14.7 ). A benefit of this technique during cleaning and shaping procedures is to ensure that WL is not lost and that the apical portion of the root is not packed with tissue, dentin debris, and bacteria (see Fig. 14.6, A ). The patency concept has historically been controversial; indeed, concerns regarding possible extrusion of dentinal debris, bacteria, and irrigants have been raised, a condition often considered to result in postoperative pain and possibly delayed healing.
However, a large retrospective study identified the presence of apical patency as a factor possibly associated with higher success rates. Moreover, at least in vitro, microorganisms do not appear to be transported beyond the confines of the canal by patency filing. Small files are not directly effective in débridement (see Fig. 14.3 ) but achieving patency may be helpful in enhancing irrigation efficacy, determining electronic WL, minimizing the risk of loss of length, reducing shaping mishaps such as canal transportation and ledges, better maintaining the anatomy of the apical constriction, and improving clinicians’ tactile sense during apical shaping.
Studies evaluating treatment failure have noted, besides several other factors, presence of bacteria outside the radicular space, and bacteria have in some cases been shown to exist as plaques or biofilms on the external root structure. It has been shown in vitro that maintaining patency may be connected to small amounts of irrigant to reach the periodontium, but this did not appear to increase the chance of irrigation accidents in the clinic.
Moreover, a recently published systematic review concluded that maintaining apical patency was not associated with postoperative pain in teeth with either vital or nonvital pulp. For all these reasons the benefits of maintaining apical patency seem to outweigh possible risks.
Before treatment, each case should be evaluated for its degree of difficulty. The American Association of Endodontists has developed The Endodontic Case Difficulty Assessment Form , a practical tool that helps practitioners to identify the complexities that should not be overlooked before starting a root canal treatment. All three, patient, diagnostic, and treatment aspects, are considered to identify the level of difficulty of a specific case and help in 3 minutes to both anticipate problems the clinician may have during the treatment and determine whether the complexity of the case is suitable for the clinician’s level of expertise and comfort.
Normal anatomy and anatomic variations are determined, as well as variations in canal morphology. The mishandling of natural difficulties will lead to procedural mishaps that will be even more difficult to manage. A root canal that seems to be straight in a radiograph may have multiple curvatures in three dimensions that are not captured using a two-dimensional film. Therefore more than one preoperative radiograph might be needed for a proper assessment; a cone beam scan or cone beam computed tomography (CBCT) may also help to determine the best strategy to shape the most difficult canal anatomies.
Specifically, the longer a root, the more difficult it is to treat; apically, a narrow, curved root is susceptible to perforation; in multirooted teeth, a narrow area midroot could result in a stripping perforation toward the root concavity. The degree and location of curvature are determined. Canals are seldom straight, and curvatures in a buccolingual direction are normally not visible on the radiograph. Sharp curvatures or dilacerations are more difficult to manage than a continuous gentle curve. Roots with an S-shape or bayonet configuration are very difficult to treat. Intracanal mineralization will also complicate treatment. Such mineralization generally occurs in a coronal to apical direction, thus a large tapering canal may become more cylindrical with irritation or age.
The presence of resorption also will complicate treatment. With internal resorption, it is difficult to pass instruments through the coronal portion of the canal and the resorptive defect and into the apical portion. Also, files will not remove tissue, necrotic debris, and bacteria from such a resorptive defect. External resorption may perforate the canal space and present problems with hemostasis and isolation. Restorations may obstruct access and visibility, as well as change the orientation of the crown in relation to the root.
Principles of Cleaning and Shaping Techniques
Cleaning and shaping are separate and distinct concepts but are performed concurrently. The criteria of canal preparation include developing a continuously tapered funnel, maintaining the original shape of the canal, maintaining the apical foramen in its original position, keeping the apical opening as small as possible, and developing glassy smooth walls. The cleaning and shaping procedures are designed to maintain an apical matrix for compacting the obturating material regardless of the obturation technique.
Knowledge of a variety of techniques and instruments for treatment of the myriad variations in canal anatomy is required. There is no consensus or clinical evidence on which technique or instrument design or type is clinically superior ( ). ,
NiTi files have been incorporated into endodontics because of their flexibility and resistance to cyclic fatigue. The resistance to cyclic fatigue permits these instruments to be used in a rotary handpiece, which gives them an advantage over stainless steel files. NiTi files are manufactured in both hand and rotary versions and have been demonstrated to produce superior shaping compared with stainless steel hand instruments ( ). , ,
NiTi instruments are available in a variety of designs, many with increased taper compared with .02 mm standardized stainless steel files. The superelasticity of NiTi alloy enabled the manufacturing of more tapered instruments still flexible enough to properly shape canals with different angles and radius of curvature. The increase in the taper provides better and more continuous shapes with the use of fewer instruments and in a shorter period of time. Common tapers are .04 and .06, and the tip diameters may or may not conform to the traditional manufacturing specifications. The file systems can vary the taper while maintaining the same tip diameter or they can employ varied tapers with International Organization for Standardization (ISO) standardized tip diameters; some NiTi instruments have multiple tapers along their cutting portions, with more recent instruments featuring smaller maximum fluted diameters of less than 1 mm at the end of the fluted instrument portion.
A rational concept of root canal preparation using current instruments unfolds in stages. Classically, Stage 1 is a defined preflaring, before bringing any hand file to the apical third of the canal. Depending on the expected canal difficulty, instruments may reach WL during Stage 2; for example, if there is only one curvature. If preoperative assessments have indicated that an S-shape or multiple curvatures are present, it may be useful to introduce a Stage 3 that finally reaches the estimated WL, whereas Stage 2 provides additional enlargement into the secondary curvature.
However, the appearance of proprietary thermal treatments of NiTi alloys with different series of heating and cooling treatments has led to the enhancement of the mechanical properties of contemporary rotary instruments by optimizing the microstructural characteristics of the alloy. The higher flexibility and cyclic fatigue resistance of these new instruments provides better clinical behavior and allows dentin preservation in the coronal third of many cases with a minimal orifice modification in Stage 1. Scientific evidence suggests that preserving pericervical dentin (4 mm above and below the crestal bone) is crucial for the distribution of functional stresses and the maintenance of the strength of the tooth and long-term survival. Computational simulations with finite element analysis showed that masticatory stresses are reduced even when small amounts of this pericervical dentin are preserved.
The authors strongly believe in this minimally invasive component of the technique because overflaring is liable to reduce dentinal wall thickness and structural strength and perhaps overall restorability (see Fig. 14.8 ). On the other hand, when it is performed with an instrument that allows a selective removal of dentin because of a limited maximum fluted diameter (MFD) that provides a conservative coronal preparation, early coronal flaring is also beneficial for the earlier access of disinfecting irrigation solutions, better tactile control of hand instruments during negotiation, and the easier placement of files in the delicate apical third.
In general, the use of NiTi rotary instruments to WL should be preceded by a manual exploration of the canal to the desired preparation length, also known as glide path verification. This step is performed with one or more small K-files that are not precurved. In recent years, NiTi rotary instruments have been specifically designed to simplify the process of glide path preparation after a negotiating file has previously reached WL. If it is possible to predictably reach WL without precurving, rotary instruments may be used to the desired length. However, caution should be exercised in S-shaped canals, canals that join within a single root, and canals with severe dilacerations. Canals in which ledge formation is present, and very large canals where instruments fail to contact the canal walls, do not lend themselves to rotary preparation.
Instrument fracture can occur as a result of torsional loading or cyclic fatigue. Torsional forces develop because of frictional resistance; therefore as the surface area increases along the flutes, the greater the friction and the more potential for fracture. Torsional stress can be reduced by limiting file contact, using a crown-down preparation technique, by verifying a glide path to WL, and with the presence of liquid irrigants such as NaOCl during shaping procedures.
Cyclic fatigue occurs as a file rotates in a curved canal. At the point of curvature the outer surface of the file is under tension while the inner surface of the instrument is compressed. As the instrument rotates, the areas of tension and compression alternate, crack initiation begins, ultimately leading to fracture. There is often no visible evidence that fracture is imminent.
Endodontic treatment failures result from all except which of the following?
Inadequate root canal débridement
Use of hand instrumentations
Bacteria that are more resistant to treatment protocols
Use of antimicrobial irrigants is an optional component of root canal débridement because new technologic advancements in file design allow better adaptation to canal irregularities.
Precise location of the apical constriction may be difficult to identify because of which of the following?
The location of the apical foramen may vary
The apical constriction can be altered by inflammatory changes
The canal terminus is not always located at the root apex
All of the above
Iatrogenic root perforation can adversely affect endodontic treatment outcome.
Maintenance of pericervical dentin contributes to the resistance to fractures that may be caused by masticatory forces.
Although new thermomechanical processed alloys have improved lifespan of instruments, it is still advised that the use of NiTi instruments be monitored and limited to a reduced number of cases. For difficult or calcified or severely curved canals, it is recommended the instruments be used only once.
Smear Layer Management
During cleaning and shaping, organic components of pulp tissue and inorganic dentinal debris accumulate that are not only pressed into accessory canals, fins, and isthmuses ( Fig. 14.6 ) but also deposited on the radicular canal wall, producing an amorphous, irregular smear layer ( Fig. 14.9 ). With pulp necrosis, the smear layer may be contaminated with bacteria and their metabolic byproducts. The smear layer is superficial, with a thickness of 1 to 5 μm, and debris can be packed into the dentinal tubules to varying distances.
There is not a consensus on removing the smear layer before obturation. , , The advantages and disadvantages of the smear layer removal remain controversial; however, evidence generally supports removing the smear layer before obturation. , The organic debris present in the smear layer might constitute substrate for bacterial growth, and it has been suggested that the smear layer prohibits sealer contact with the canal wall, which permits leakage. In addition, viable microorganisms in the dentinal tubules may use the smear layer as a substrate for sustained growth. When the smear layer is not removed, it may slowly disintegrate with leaking obturation materials, or it may be removed by acids and enzymes that are produced by viable bacteria left in the tubules or that enter via coronal leakage. The presence of a smear layer may also interfere with the action and effectiveness of root canal irrigants and interappointment disinfectants.
With smear layer removal, filling materials adapt better to the canal wall. , Removal of the smear layer also enhances the adhesion of sealers to dentin and tubular penetration , and permits the penetration of all sealers to varying depths. Removal of the smear layer reduces both coronal and apical leakage ( ). ,
The ideal properties for an endodontic irrigant are listed in Box 14.1 . Currently, no solution meets all the requirements outlined. In fact, no techniques appear able to completely clean the root canal space. , Frequent irrigation is necessary to flush and remove the debris generated by the mechanical action of the instruments. At the same time, preparation of radicular wall creates hard tissue debris that is typically pushed into accessory anatomy, blocking access for subsequent irrigation. Therefore it is imperative to use mechanical shaping and irrigation in synergy to maximize antibacterial efficacy of endodontic procedures.
Organic tissue solvent
Inorganic tissue solvent
Low surface tension
The most common irrigant is NaOCl, which is also known as household bleach. Advantages to NaOCl include the mechanical flushing of debris from the canal, the ability of the solution to dissolve vital and necrotic tissue, the antimicrobial action of the solution, and the lubricating action. In addition, it is inexpensive and readily available.
Free chlorine in NaOCl dissolves necrotic tissue by breaking down proteins into amino acids. There is no proven appropriate concentration of NaOCl, but concentrations ranging from 0.5% to 5.25% have been recommended. A common concentration is 2.5%, which decreases the potential for toxicity yet still maintains some tissue dissolving and antimicrobial activity. , Because the action of the irrigant is related to the amount of free chlorine, decreasing the concentration can be compensated by increasing the volume. Warming the solution can also increase the effectiveness of the solution. , However, NaOCl has limitations to tissue dissolution in the canal, because of limited contact with tissues in all areas of the canal.
Because of toxicity, extrusion is to be avoided. , , The irrigating needle must be placed loosely in the canal ( Fig. 14.10 ), and the use of side-vented needles specifically designed for endodontic irrigation is recommended in order to avoid accidents. Insertion to binding and slight withdrawal minimizes the potential for possible extrusion and an NaOCl accident ( Fig. 14.11 ). Special care should be exercised when irrigating a canal with an open apex. To control the depth of insertion the needle is bent slightly at the appropriate length or a rubber stopper is placed on the needle.
Most likely, any irrigant does not travel apically more than 1 mm beyond the irrigation tip, so deep placement with small-gauge needles enhances irrigation (see Fig. 14.10 ). During rinsing, the needle is moved up and down constantly to produce agitation and prevent binding or wedging of the needle.
Chelating Irrigants: Ethylenediaminetetraacetic Acid, Citric Acid, Hydroxyethylidene Bisphosphonate
As described previously, NaOCl is the most effective irrigant for organic tissue dissolution and elimination of bacteria biofilm; however, it does not remove inorganic tissue. For this reason, it needs to be combined with a chelating agent, such as ethylenediaminetetraacetic acid (EDTA), citric acid, or the more recently suggested hydroxyethylidene bisphosphonate (HEPB), also called etidronate. The chelating activity is directed toward removal of the smear layer because, in fact, these chelators have minimal tissue dissolution capacity.
EDTA is the most frequently used irrigant for this purpose. However, chemical interactions between EDTA and NaOCl have been described, and when combined, tissue dissolution ability of NaOCl may be affected as a result of a reduction in the active chlorine content. , For this reason, when using EDTA, the irrigation protocol recommended includes an irrigation with 17% EDTA for 1 minute at the end of the shaping procedure followed by a final rinse with NaOCl. Chelators such as EDTA remove the inorganic components and leave the organic tissue elements intact. NaOCl is then necessary for removal of the remaining organic components; however, the additional use of NaOCl after chelating agents may lead to excessive demineralization of radicular wall dentin.
Demineralization results in removal of the smear layer and plugs and enlargement of the tubules. , The action is most effective in the coronal and middle thirds of the canal whereas the effect is diminished in the apical third. ,
Reduced efficacy may be a reflection of canal size or anatomic variations such as irregular or sclerotic tubules. , The variable structure of the apical dentin presents a challenge during endodontic obturation with adhesive materials.
The recommended time for removal of the smear layer with EDTA is 1 minute. , , The small particles of the smear layer are primarily inorganic with a high surface to mass ratio, which facilitates removal by acids and chelators. EDTA exposure over 10 minutes causes excessive removal of both peritubular and intratubular dentin. A 10% solution of citric acid has also been shown to be an effective method for removing the smear layer, although it also reduces available chlorine in NaOCl solutions. ,
A possible alternative to citric acid or EDTA recently suggested is HEBP. HEBP is a weak and biocompatible chelator that prevents bone resorption and is used systemically in patients suffering from osteoporosis or Paget’s disease. In contrast with EDTA or citric acid, HEBP appeared to reduce NaOCl active chlorine content after 1 hour and reduction continued over time, but the mixture seemed not to interfere with the dissolving properties of NaOCl. Therefore it may be mixed and used in combination with NaOCl, reducing the formation of smear layer during the mechanical preparation of the root canal. It seems that 7% to 10% HEBP could be mixed chair-side with NaOCl, without fearing any loss of NaOCl activity and administered during the whole course of root canal preparation.
Chlorhexidine possesses a broad spectrum of antimicrobial activity, provides a sustained action, , and has little toxicity. Two percent chlorhexidine has similar antimicrobial action as 5.25% NaOCl and is more effective against Enterococcus faecalis. NaOCl and chlorhexidine are synergistic in their ability to eliminate microorganisms. A disadvantage of chlorhexidine is its inability to dissolve necrotic tissue and remove the smear layer. Moreover, clinical studies do not confirm that the use of chlorhexidine is associated with better outcomes.
Moreover, the interaction between chlorhexidine and NaOCl produced a precipitate that may have detrimental consequences for endodontic therapy; among them it may produce discoloration and potential toxic substances for periradicular tissues. At the same time, when chlorhexidine interacted with EDTA a precipitate was also produced. ,
An alternative method for disinfecting while at the same time removing the smear layer employs a mixture of a tetracycline isomer, an acid, and a detergent (MTAD) as a final rinse to remove the smear layer. The effectiveness of MTAD to completely remove the smear layer is enhanced when low concentrations of NaOCl are used as an intracanal irrigant before the use of MTAD. A 1.3% concentration is recommended. MTAD may be superior to NaOCl in antimicrobial action. , MTAD has been shown to be effective in killing E. faecalis , an organism commonly found in failing treatments, and may prove beneficial during retreatment. It is biocompatible, does not alter the physical properties of the dentin, and enhances bond strength. Although there are encouraging in vitro data, MTAD has not been shown to be clinically beneficial at this point.
A chlorhexidine-based mixture, marketed as QMix, employs a similar underlying strategy with the potential to not only remove smear layer but also to provide antibiofilm activity. QMix consists of a proprietary mix of chlorhexidine, EDTA, and a surface-active agent. Nothing is known about its contribution to clinical outcomes, but it appears that smear layer removal is similar to 17% EDTA, and antimicrobial effects are adequate. , However, tissue dissolution with prior canal shaping and use of NaOCl are still required.
Irrigants for Cryotherapy
A new use of irrigant solution has been recently described in root canal treatment. Posttreatment pain is a very common situation, especially in teeth presenting with preoperative pain, pulp necrosis, and symptomatic apical periodontitis. Postoperative pain has traditionally been controlled with paracetamol, nonsteroidal antiinflammatory medication, opioids, and/or corticosteroids. In other fields of medicine, other alternatives have been suggested in search of a greater efficacy for pain control while avoiding secondary effects, cryotherapy among them. A controlled irrigation with cold saline after cleaning and shaping procedures has been recently suggested to reduce incidence and intensity of postoperative pain in those patients presenting symptomatic apical periodontitis. The authors suggested a final irrigation after cleaning and shaping with cold (2.5°C) sterile saline solution, also using a cold (2.5°C) sterile microcannula attached to the Endovac negative pressure irrigation system for 5 minutes. Recently different cryotherapy applications have also resulted in lower postoperative pain levels (intracanal, intraoral, and extraoral).
There are many uses of ultrasonics in root canal treatment; for example, refinement of access cavity preparations for a more conservative approach, orifice location, pulp stone removal, removal of materials from the inside of the root canal (including posts, separated instruments, silver cones), enhancing irrigation, thermoplastic obturation, and root-end preparation during surgery; however, shaping curved root canals with ultrasonic instruments has been shown to create preparation errors and is no longer recommended.
In terms of enhancing irrigation, agitation techniques allow an irrigation solution to reach the apical third and irregularities in the root canal system, and hence improve cleaning efficiency. The use of ultrasonics, sonic devices, or apical negative pressure irrigation has been recommended. Many other devices or instruments are continuously being marketed for further disinfection of root canal system; however, cost-effectiveness still needs to be scientifically demonstrated.
The main mechanism of adjunctive cleaning with ultrasonics is acoustic microstreaming, which is described as complex steady-state streaming patterns in vortex-like motions or eddy flows that are formed close to the instrument. Agitation of the irrigant with an ultrasonically activated instrument after completion of cleaning and shaping has the benefit of increasing the effectiveness of the solution. ,
Lubricants facilitate manipulation of hand files during cleaning and shaping. They are an aid in initial canal negotiation, especially in small and constricted canals without taper. The use of lubricants during negotiation helps to avoid pulp tissue blockage. Especially in vital teeth, pulp tissue may block the root canal during negotiation. This type of blockage is difficult to bypass but very easy to prevent by filling the pulp chamber with viscous lubricants that will enhance the advancement of the small file without apically pushing the pulp tissue remnants.
Glycerin is a mild alcohol that is inexpensive, nontoxic, aseptic, and somewhat soluble. A small amount can be placed along the shaft of the file or deposited in the canal orifice. Counterclockwise rotation of the file carries the material apically. The file can then be worked to length using a watch winding motion.
Paste lubricants can incorporate chelators. One advantage to paste lubricants is that they can suspend dentinal debris and prevent apical compaction. One proprietary product consists of glycol, urea peroxide, and EDTA in a special water-soluble base. It has been demonstrated to exhibit an antimicrobial action. Another type is composed of 19% EDTA in a water-soluble viscous solution.
A disadvantage to these EDTA compounds appears to be the deactivation of NaOCl by reducing the available chlorine and potential toxicity. The addition of EDTA to the lubricants has not proved to be effective. In general, files remove dentin faster than the chelators can soften the canal walls. Aqueous solutions, such as NaOCl, should always be used instead of paste lubricants when using NiTi rotary techniques to reduce torque.
Regardless of the technique used in root canal preparation, procedural errors can occur (see Chapter 18 ). These include loss of WL, apical transportation, apical perforation, instrument fracture, and stripping perforations.
Loss of WL has several causes, including failure to have an adequate reference point from which the WL is determined, packing tissue and debris in the apical portion of the canal, ledge formation, and inaccurate measurements of files.
The selection of an adequate coronal reference point is very important. Some clinicians advocate for using the same coronal reference for all root canals in the same tooth to ease the procedure; however, the proper determination of a straight and stable reference localized in the original path of the instrument when shaping each canal will avoid procedural mishaps during canal preparation. Moreover, the more visible the reference point, the less stress for the rotary instrument when the clinician checks the proper shaping length.
On the other hand, the most predictable method to prevent any kind of blockage in the apical portion of the canal is to regularly use the so-called patency file during cleaning and shaping procedures. Not only does it minimize the risk of loss of length, but it also reduces further mishaps occurring when trying to force an instrument to go back to the initial length.
And lastly, the reconfirmation of the WL electronically with an apex locator after preparation of the coronal third will also help to maintain the correct length during the whole shaping procedure.
Apical transportation and zipping occur when relatively inflexible files are used to prepare curved canals. The restoring force of the file (the tendency to return to the original straight shape of the file) exceeds the threshold for cutting dentin in a curved canal ( Figs. 14.12 and 14.13 ). When this apical transportation continues with larger and larger files, a “teardrop” shape develops, and apical perforation can occur on the lateral root surface (see Fig. 14.12 ). Transportation in curved canals already begins with a No. 25 file. Enlargement of curved canals at the WL beyond a No. 25 file can be done only when an adequate coronal flare is developed. Moreover, when shaping a difficult root canal, the most challenge anatomy is often located in the apical third. The potential of avoiding accidents in this delicate portion starts with a proper negotiation after removing the restrictive dentin in the coronal and middle third if the root canal presents great curvatures or S-shaped root canals. Choosing flexible and resistant rotary instruments is very important not to deform the apical third of root canals with complicated anatomy.