General Considerations – A Review CRITERIA FOR EVALUATING CLEANING AND SHAPING
Phenols and aldehydes
Objective of temporization
Routine access cavities
Extensive coronal breakdown
Successful root canal treatment is based on: establishing an accurate diagnosis and developing an appropriate treatment plan; applying knowledge of tooth anatomy and morphology (shape); and performing the debridement, disinfection, and obturation of the entire root canal system. Initially emphasis was on obturation and sealing the radicular space. However no technique or material provides a seal that is impervious to moisture either from the apical or coronal areas. Early prognosis studies indicated failures were attributed to incomplete obturation.1 This proved fallacious as obturation only reflects the adequacy of the cleaning and shaping. Canals that are poorly obturated are often incompletely cleaned and shaped. Adequate cleaning and shaping and establishing a coronal seal are the essential elements for successful treatment with obturation being less important for short term success.2 Elimination (or significant reduction) of the 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 areas. Sealing the canal space following cleaning and shaping will entomb any remaining organisms3and, with the coronal seal, prevent re-contamination of the canal and periradicular tissues.
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 that is necrotic with periradicular pathosis. The difference is the persistent irritation of necrotic tissue remnants, and the inability to remove the microorganisms and their by-products. The most significant factors affecting this process are tooth anatomy and morphology, and the instruments and irrigants available for treatment. Instruments must contact and plane the canal walls to debride the canal (Figure 16-1, 16-2, 16-3). Morphologic factors such as lateral (Figures 16-2) and accessory canals, canal curvatures, canal wall irregularities, fins, cul-de-sacs (Figures 16-1), and ishmuses make total debridement virtually impossible. Therefore the goal of cleaning not total elimination of the irritants but it is to reduce the irritants.
Currently there are no reliable methods to assess cleaning. The presence of clean dentinal shavings, the color of the irrigant, and canal enlargement three file sizes beyond the first instrument to bind have been used to assess the adequacy; however, these do not correlate well with debridement. Obtaining glassy smooth walls is a preferred indicator.4 The properly prepared canals should feel smooth in all dimensions when the tip of a small file is pushed against the canal walls. This indicates that files have had contact and planed all accessible canal walls thereby maximizing debridement (recognizing that total debridement usually does not occur).
Principles of Shaping
The purpose of shaping is to 1) facilitate cleaning and 2) provide space for placing the obturating materials. The main objective of shaping is to maintain or develop a continuously tapering funnel from the canal orifice to the apex. This decreases procedural errors when cleaning and enlarging apically. The degree of enlargement is often 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 1-2 millimeters of the corrected working length. There is a correlation between the depth of spreader penetration and the apical seal.5 For warm vertical compaction techniques the coronal enlargement must permit the placement of the pluggers to within 3 to 5 mm of the corrected working length.6
As dentin is removed from the canal walls the root is weakened.7 The degree of shaping is determined by the preoperative root dimension, the obturation technique, and the restorative treatment plan. Narrow thin roots such as the mandibular incisors cannot be enlarged to the same degree as more bulky roots such as the maxillary central incisors. Post placement is also a determining factor in the amount of coronal dentin removal.
APICAL CANAL PREPARATION
Termination of Cleaning and Shaping
While the concept of cleaning and shaping the root canal space is a simple concept, 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 has been one millimeter from the radiographic apex. In a classic study it was noted the apical portion of the canal consisted of the major diameter of the foramen and the minor diameter of the constriction (Figure 16-4).8 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 millimeters. One study found the classic apical constriction to be present in only 46% of the teeth and when present varied in relation to the apical foramen.9 Variations from the classic appearance consist of the tapering constriction, the multiple constriction and the parallel constriction.9 Based on the variations in apical morphology, the term apical constriction is misnomer. To complicate the issue the foramen is seldom at the apex. Apical anatomy has also been shown to be quite variable (Figure 16-4). A recent study found no typical pattern for foraminal openings and that no foramen coincided with the apex of the root. 10 The foramen to apex distance can range from .20 to 3.8 mm.10 It has also been noted that the foramen to constriction distance increases with age8 and root resorption may destroy the classic anatomical constriction. Resorption is common with pulp necrosis and apical bone resorption and this can result in loss of the constriction11 therefore root resorption is an additional factor to consider in length determination.
In a recent prospective study evaluating prognosis, significant factors influencing success and failure were perforation, preoperative periradicular disease, and adequate length of the root canal filling.12 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. 12 A meta-analysis evaluation of success/failure indicated a better success rate when the obturation was confined to the canal space.13 A review of a number of prognosis studies confirms that extrusion of materials decreases success.14 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 2-3 mm was acceptable.
While the guideline of 1.0-2.0 mm from the radiographic apex remains rational, the point of apical termination of the preparation and obturation remains empirical. The need to compact the gutta-percha and sealer against the apical dentin matrix (constriction of the canal) is essential for success. The decision of where the minor diameter of the canal lies 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 must be confined to the radicular space. Canals filled to the radiographic apex are actually overextended.10 Degree of Apical Enlargement
While generalizations can be made regarding tooth anatomy and morphology, each tooth is unique. Length of canal preparation is often emphasized with little consideration given to important factors such as canal diameter and shape. Since 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 limits canal transportation and apical “zipping”, but decreases the efficacy of the cleaning procedure. It appears that, with traditional hand files, apical transportation occurs in most curved canals enlarged beyond a size #25 stainless steel file.15 The criteria for cleaning and shaping should be based on the ability to adequately remove the tissue, necrotic debris, and bacteria and not a specific obturation technique.
Irrigants are unable to reach the apical portion of the root if the canal is not enlarged to a size #35 or #40 file.16-18 The larger preparation sizes have been shown to provide adequate irrigation and debris removal as well as significantly decreasing the number of microorganisms.19-22 Thus there appears to be a relationship between increasing the size of the apical preparation and canal cleanliness23 and bacterial reduction.24, 25 Instrumentation techniques that advocate minimal apical preparation may be ineffective at achieving the goal of cleaning and disinfecting the root canal space.26, 23 Bacteria can penetrate the tubules of dentin. These intratubular organisms are protected from endodontic instruments, the action of irrigants, and intracanal medicaments. Dentin removal appears to be the primary method for decreasing their numbers. In addition 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;27 in teeth with apical periodontitis, bacteria penetrate the tubules to the periphery of the root.28, 29
Elimination of Etiology
The development of nickel titanium instruments has dramatically changed the techniques of cleaning and shaping. The primary advantage to using these flexible instruments is related to shaping. Neither hand instruments nor rotary files have been shown to completely debride the canal.30-32 Mechanical enlargement of the canal space dramatically decreases the presence of microorganisms present in the canal33 but cannot render the canal sterile.19 To improve the mechanical preparation techniques antimicrobial irrigants have been recommended.34 There is no consensus on the most appropriate irrigant or concentration of solution, although sodium hypochlorite is the most widely used irrigant.
Common irrigants include sodium hypochlorite and chlorhexidine.35-39 Unfortunately solutions designed to kill bacteria are often toxic for the host cells,40-43 so extrusion beyond the canal space therefore is to be avoided.44, 45 A major factor related to effectiveness is the volume. Increasing the volume produces cleaner preparations.46 Apical Patency
Apical patency has been advocated during cleaning and shaping procedures to ensure working length is not lost and that the apical portion of the root is not packed with tissue, dentin debris and bacteria (Figure 16-5). Concerns regarding extrusion of dentinal debris, bacteria and irrigants have been raised.47 Seeding the periradicular tissues with microorganisms may occur.48 Studies evaluating treatment failure have noted bacteria outside the radicular space,49, 50 and bacteria have been shown to exist as plaques or biofilms on the root external root structure.51
The apical patency concept also has been advocated to facilitate apical preparation. Extending the file beyond the apex increases the diameter of the canal at working length consistent with the instrument taper. The value of maintaining patency to prevent transportation is questionable52 and it does not result in bacterial reduction when compared to not maintaining patency.53 Small files are not effective in debridement (Figure 16-3).
Prior to treatment, each case should be evaluated for degree of difficulty. Normal anatomy as well as anatomic variations are determined as well as variations in canal morphology (shape).
A parallel preoperative radiograph or image is assessed. The longer a root, the more difficult it is to treat. Apically, a narrow curved root is susceptible to perforation; in multi-rooted teeth a narrow area mid root could lead to a lateral stripping perforation. The degree and location of curvature is determined. Canals are seldom straight and curvatures in a facial-lingual direction will not be 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 difficult to treat. Calcifications will also complicate treatment. Calcification generally occurs in a coronal to apical direction (See Chapter 15, Figure 15-14). A large tapering canal may become more cylindrical with irritation or age. This presents problems when the tapered instruments are used in the coronal third.
Resorption also will complicate treatment. With internal resorption it is difficult to pass instruments through the coronal portion of the canal, through the defect and into the apical portion. Also files will not remove tissue, necrotic debris and bacteria from this inaccessible area. External resorptions 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
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 walls6. The cleaning and shaping procedures are designed maintain an apical matrix for compacting the obturating material regardless of the obturation technique.6
Knowledge of variety of techniques and instruments for treatment of the myriad variations in canal anatomy is required. There is no consensus on which technique or instrument is superior.30 Nickel-titanium files have been incorporated into endodontics due to their flexibility and resistance to and cyclic fatigue.54 The resistance to cyclic fatigue permits the instruments to be used in a rotary handpiece, an advantage over stainless steel. The instruments are manufactured in both hand and rotary versions. Both have been demonstrated to produce superior shaping when compared to stainless steel hand instruments.55, 56 The instruments are designed with increased taper when compared to .02 mm standardized stainless steel files. Common tapers are .04, .06, .08, .10, and .12 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 ISO standardized tip diameters. They may incorporate cutting or non-cutting tips.
In general the nickel titanium rotary instruments are not indicated in S-shaped canals, canals that join within a single root (Type II configuration), in canals with severe dilacerations, canals in which ledge formation is present, and very large canals where they fail to contact the canal walls. Straight line access to the canal is essential and the instruments should be used passively.
Instrument fracture can occur due to torsional forces or cyclic fatigue. Torsional forces develop due to frictional resistance, therefore as the surface area increases along the flutes the greater friction and more potential for fracture. Torsional forces may produce an unraveling of the flutes prior to fracture and inspection of the instruments after each use is critical. Torsional stress can be reduced by limiting file contact, by using a crown down preparation technique, and by lubrication. Cyclic fatigue occurs as the file rotates in a curved canal.57 At the point of curvature the molecules on the outer surface of the file are under tension while the molecules on the inner surface of the instrument are compressed. As the instrument rotates the areas of tension and compression alternate and eventual fracture occurs. There is no visible evidence that fracture is imminent. Therefore it is advised that the use of nickel titanium instruments be monitored58 and limited to one to five cases. For difficult cases or calcified canals it is recommended the instruments be used only once.