Termination of Cleaning and Shaping

Although the concept of cleaning and shaping the root canal space is simple, consensus has not been reached on some points. One example 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 its position (which is irregular within the canal) cannot be determined clinically.

Traditionally, the apical point of termination, also known as the working length (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 (see Fig. 16.4 ). The apical constriction is defined as the narrowest portion of the canal; 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 studied. Also, when present, it varied in shape and in relationship 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. Recently, micro-computed tomography data have provided a more realistic portrait of the apical canal morphology ( Fig. 16.5 ).

A, Micro-computed tomography reconstruction of an unprepared root canal system of a maxillary molar. B, Prepared canal system, enlarged to an apical size 30 in the palatal and 25 in the mesiobuccal and distobuccal canal. C, Magnified view of the initial canal configuration for all three apices.

Apical anatomy has also been shown to be quite variable (see Fig. 16.4, B , and Fig. 16.5 ). A study found no typical pattern for foraminal openings; it also found that no foramen coincided with the apex of the root. The same group reported the foramen-to-apex distance to range from 0.2 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 mm short harbored necrotic tissue, bacteria, and irritants and that when retreated, these canals 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.

The exact clinical point of apical termination of the preparation and obturation remains a matter of debate. Compacting the gutta-percha and sealer against the apical dentin matrix (constriction of the canal) is important in creating a seal. The decision on where to terminate the preparation is based on a 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. The 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. A small apical preparation reduces the incidence of preparation errors (discussed later) but may decrease the 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 the canal’s size and 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; 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. 16.6 ).

A, Individual micro-computed tomography scans from the apical root third of a typical specimen before and after root canal preparation and irrigation with sodium hypochlorite (NaOCl), subsequent irrigation with ethylenediaminetetraacetic acid (EDTA), and final passive ultrasonic irrigation using again NaOCl (from left to right). B, The corresponding three-dimensional reconstructions of the whole canal system are depicted in the bottom panel.

(Modified from Paqué F, Boessler C, Zehnder M: Accumulated hard tissue debris levels in mesial roots of mandibular molars after sequential irrigation steps, Int Endod J 44[2]:148, 2011.)

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 instruments has dramatically changed the techniques of cleaning and shaping; these instruments have been rapidly adopted by clinicians in many countries. The primary advantage of these flexible instruments is seen in shaping, specifically a significant reduction in the incidence of preparation errors.

Neither hand instruments nor rotary files have been shown to completely debride the canal. Mechanical enlargement of the canal space dramatically reduces the number of microorganisms present in the canal, but it cannot render the canal sterile. Therefore, the use of antimicrobial irrigants has 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 to the host cells ; therefore, extrusion beyond the canal space is to be avoided. A major factor in the effectiveness of irrigants is the volume of irrigant used during the procedure. Increasing the volume produces cleaner preparations.

Apical Patency

In the apical patency technique, small hand files are repeatedly placed to or slightly beyond the apical foramen during canal preparation ( Fig. 16.7 ). A benefit of this technique during cleaning and shaping procedures is that it ensures that the working length is not lost and that the apical portion of the root is not packed with tissue, dentin debris, and bacteria (see Fig. 16.6, A ). This technique has raised some concerns about extrusion of dentinal debris, bacteria, and irrigants. However, in a recent large, retrospective study, the apical patency technique was identified 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 debridement (see Fig. 16.3 ), but achieving patency may be helpful in enhancing irrigation efficacy and in electronic length determination.

A small file (No. 10 or 15) is placed beyond the radiographic apex to maintain patency of the foramen. Note that the tip extends beyond the apical foramen .

Studies evaluating treatment failure have noted the presence of bacteria outside the radicular space, in addition to several other factors, and in some cases bacteria have been shown to exist as plaques or biofilms on the external root structure.

Sodium Hypochlorite

As mentioned, the most common irrigant is NaOCl, also known as household bleach. The advantages of an NaOCl solution include mechanical flushing of debris from the canal, the ability to dissolve vital and necrotic tissue, antimicrobial action, and lubricating action. In addition, NaOCl 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%, a concentration at which the potential for toxicity is reduced, yet some tissue dissolving and antimicrobial activity is maintained. Because the action of the irrigant is related to the amount of free chlorine, an increase in volume can compensate for a decrease in concentration. Warming the solution can also increase its effectiveness. However, NaOCl is limited in its ability to dissolve canal content because of limited contact with tissues in all areas of the root canal system.

Because of toxicity, extrusion is to be avoided. The irrigating needle must be placed loosely in the canal ( Fig. 16.9 ). Insertion to binding and slight withdrawal minimizes the potential for extrusion and a sodium hypochlorite accident ( Fig. 16.10 ). Special care should be taken 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.

For effective irrigation, the needle must be placed in the apical one third of the root and must not bind.

A sodium hypochlorite accident during treatment of the maxillary left central incisor. Extensive edema occurred in the upper lip and was accompanied by severe pain.

The irrigant does not move apically more than 1 mm beyond the irrigation tip, so deep placement with small-gauge needles enhances irrigation (see Fig. 16.9 ). During rinsing, the needle is moved up and down constantly to produce agitation and prevent binding or wedging of the needle.

EDTA

Ethylenediaminetetraacetic acid (EDTA) is another frequently used irrigant; its activity is directed toward removal of the smear layer. Irrigation with 17% EDTA for 1 minute, followed by a final rinse with NaOCl, is a recommended method. 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 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 and 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 and have 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. Citric acid has also been shown to be an effective means of removing the smear layer.

Chlorhexidine

Chlorhexidine has a broad spectrum of antimicrobial activity, provides a sustained action, and has little toxicity. Two percent chlorhexidine has an antimicrobial action similar to that of 5.25% NaOCl, and it 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.

MTAD

An alternative method for disinfecting while at the same time removing the smear layer uses a mixture composed of a tetracycline isomer, an acid, and a detergent (MTAD) as a final rinse to remove the smear layer. The effectiveness of MTAD in completely removing the smear layer is enhanced when low concentrations of NaOCl are used as an intracanal irrigant before the use of MTAD. A 1.3% NaOCl concentration was 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 it may prove beneficial during retreatment. It is biocompatible, does not alter the physical properties of the dentin, and enhances bond strength. Although there is encouraging in vitro data, MTAD has not been shown to be clinically beneficial at this point.

QMix

Use of a recently introduced irrigant, marketed as QMix, follows an underlying strategy similar to that for MTAD; in addition, QMix has the potential not only to remove the smear layer, but also to provide antibiofilm activity. QMix consists of a proprietary mix of chlorhexidine, EDTA, and a surface-active agent. Although this is a new material and nothing is known about its contribution to clinical outcomes, it appears that smear layer removal is similar to that seen with 17% EDTA, and antimicrobial effects are adequate. However, tissue dissolution with prior canal shaping and use of NaOCl are still required.

Ultrasonics

Ultrasonic activation is used to enhance irrigation and to remove materials from the canal, including posts and silver cones. Ultrasonically powered instruments may also be used for 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.

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.