Figure 9-50 Diagram of coronal enlargement in a maxillary anterior tooth. After preparation of the access cavity (Fig. 9-47) and copious irrigation, Gates-Glidden burs are used in a step-down manner to enlarge the orifice and provide straight-line access into the middle third of the canal. Prebent size #10 K-files are used to explore the canal path and dimension.
General agreement exists that hand files should be used for the balanced force technique. Roane et al223 described this technique as a series of rotational movements for Flex-R files, but it can also be used for K-files and other hand instruments, such as GT hand files. Many different explanations have been offered for the obvious and undisputed efficacy of the balanced force approach58,150,223; however, general agreement exists that it provides excellent canal centering ability, superior to other techniques with hand instruments.16,42,153,225
The balanced force technique involves three or four steps. The first step (after passive insertion of an instrument into the canal) is a passive clockwise rotation of about 90 degrees to engage dentin (Fig. 9-53). In the second step, the instrument is held in the canal with adequate axial force and rotated counterclockwise to break loose the engaged dentin chips from the canal wall; this produces a characteristic clicking sound. Classically, in the third step the file is removed with a clockwise rotation to be cleaned; however, because files used with the balanced force technique are not prebent, every linear outward stroke essentially is a filing stroke and may lead to some straightening of the canal path. Therefore, in many cases the clinician may advance farther apically rather than withdrawing the file, depending on the grade of difficulty.
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Figure 9-51 Diagram of coronal enlargement in a more complicated maxillary posterior tooth. This maxillary molar presents several difficulties, including a narrow mesiobuccal canal that exits the pulp cavity at an angle. A possible approach in a case involving difficult entry into the root canal system is to use a small orifice shaper (OS1) after ensuring a coronal glide path with a K-file. Use of a sequence of orifice shapers (OS3 to OS1) then allows penetration into the middle third of the root canal. Wider canals can accept a second sequence of orifice shapers. Copious irrigation and securing a glide path with a size #10 K-file are prerequisites for use of NiTi rotary instruments.
NiTi rotary instruments are an invaluable adjunct in the preparation of root canals, although hand instruments may be able to enlarge some canals just as efficiently when used in appropriate sequences (Fig. 9-54). Hand instruments should be used only after coronal preenlargement (e.g., with Gates-Glidden drills). After preenlargement, the access cavity and canals are flooded with irrigant, and a prebent scouting file is advanced into the canal. A lubricant can help prevent apical blockage in this early stage. Once the working length has been established (aided by an electronic apex locator and radiographically verified), apical enlargement to the desired size begins (Fig. 9-55). As stated previously, various apical preparation designs exist, and the choice is driven mostly by the desired obturation technique, whether an apical stop or an apical taper is prepared. Finally, canal taper is increased by decreasing the working length of larger instruments in 1 or 0.5 mm increments, producing #0.05 and #0.10 mm tapers, respectively.
Copious irrigation and frequent recapitulations with a smaller file to working length may be required, and in some instances clinicians must devise creative strategies using small crown-down and/or step-back sequences.
In many cases hand instrumentation produces adequate shapes, but clinicians often choose NiTi rotary instruments either to enlarge curved canals or to produce wider tapers. Fig. 9-56 illustrates the development of these shapes in the mesial root canals of a mandibular molar, clearly showing that substantial areas of the root canal surface are not instrumented, even when apical size #50 or #.09 tapers are reached (red areas in Fig. 9-56, G and I).
Since the introduction of LightSpeed instruments, the manufacturer's guidelines have changed17; this section presents the current version248 (Figs. 9-57 and 9-58).
After coronal preenlargement with the instrument of choice, working lengths are obtained and apical enlargement is done with at least a loose-fitting size #15 K-file. Apical canal diameters are then gauged by the insertion of LightSpeed instruments of increasing size until one binds just before reaching the working length. This instrument, which is then used in the handpiece, is the first LS instrument size to bind before reaching the working length (the FLSB).
All LightSpeed instruments are used in the following way: a slow, continuous apical movement is used until the blade binds; after a momentary pause, the blade is advanced to the working length (WL) with intermittent ("pecking") motions.248 The number of pecks required to reach the WL increases as instrument size increases, because more wall dentin is cut. The instrument size that requires 12 or more pecks (12-peck rule)248 to advance from the point of first binding to the WL is the master apical rotary size (MAR).
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Figure 9-52 Clinical example of the importance of straight-line access to the middle third of the root canal. A, Preoperative radiograph of tooth #30, diagnosed with irreversible pulpitis. This tooth serves as a retainer for a metal-free, fixed partial denture. Note the prominent dentin shelves (arrows).B, Working length radiograph with hand instruments inserted into the mesial and distal canals. C, Cone-fit radiograph showing tapered preparations after removal of the dentin shelves. D, Posttreatment radiograph after thermoplastic compaction of gutta-percha. E and F, Follow-up radiographs at 2 and 4 years. The tooth is clinically symptom free, and the periodontal ligament appears to be within normal limits.
An instrument one size larger than the MAR then is used to instrument to a length 4 mm short of the working length. This shapes the canal for subsequent obturation with SimpliFill (LightSpeed Technologies). The middle third of the canal is instrumented with sequentially larger full-size instruments until a size is reached that cannot be easily advanced beyond the coronal third. The midroot area usually is prepared with three or four LightSpeed instruments. Finally, the MAR is used to recapitulate to the working length (see Fig. 9-58).
Many different techniques have been advocated for the ProFile,242 but the general pattern remains a crown-down approach with varying tapers and tip diameters. The ProFile therefore can be used as an example for systems with this basic design (e.g., the HERO 642, K3, and FlexMaster). It must be noted that the manufacturers' instructions for those systems are somewhat different, and the instructions for GT rotary and RaCe files vary even more. Therefore the clinician should always read the manufacturers' instructions for details on working with those instruments. That being said, it also must be noted that the merits of specific instructions have not been scientifically elaborated.
Figure 9-53 Diagram of handle movements during balanced force hand preparation. Step 1: After pressureless insertion of a Flex-R or NiTiFlex K-file, the instrument is rotated clockwise 90 degrees using only light apical pressure.
Figure 9-53 Step 2: The instrument is rotated counterclockwise 180 to 270 degrees; sufficient apical pressure is used to keep the file at the same insertion depth during this step. Dentin shavings are removed with a characteristic clicking sound.
Figure 9-53 Step 3: This step is similar to step 1 and advances the instrument more apically.
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Figure 9-53 Step 4: After two or three cycles, the file is loaded with dentin shavings and is removed from the canal with a prolonged clockwise rotation.
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Figure 9-54 Root canal instrumentation with hand files: Part I. After the orifice has been accessed (see Figs. 9-47 and 9-52) and copious irrigation performed (1), the working length (WL) is determined. A size #10 and/or #15 K-file is advanced to the desired apical preparation endpoint, aided by an electronic apex locator (2). The apical canal areas are then enlarged with K-files (3) used in the balanced force technique (see Fig. 9-53). Frequent, copious irrigation with sodium hypochlorite is mandatory to support antimicrobial therapy. Frequent recapitulation with fine K-files is recommended to prevent blockage (4). Apical enlargement is complete to the desired master apical file (MAF) size (5), which depends on preoperative canal sizes and individual strategy. Typically, size #40 or larger may be reached in anterior teeth, as in this example. File sizes larger than #20 may be used with NiTi instruments (e.g., NiTiFlex).
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Figure 9-55 Root canal instrumentation with hand files: Part II. Frequent irrigation with sodium hypochlorite (1) is more efficient after the working length (WL) is reached, because irrigation needles may penetrate deeper into the canal. Canal taper is increased to further improve antimicrobial efficiency and to simplify subsequent obturation. Hand instruments are set to decreasing working length in 0.5-mm increments (step-back) from the master apical file (2 to 3). A fine K-file is used to recapitulate to WL during the procedure (4), and the MAF is used as a final recapitulation (5) to ensure that remaining dentin chips have been removed.
As with other instruments, coronal preenlargement is mandatory (see Figs. 9-51 and 9-52). The working length then is determined as described previously, and an open glide path is secured with K-files up to size #15 or #20, depending on the canal anatomy. If canal size permits, canal preparation begins with #.06 taper instruments in descending tip diameters36 (Fig. 9-59). In more difficult small canals, #.06 tapers are followed by #.04-tapered instruments, also with descending tip diameters (Fig. 9-60). Apical preparation is performed either with multiple shaping waves, as suggested for GT rotary files,47 or in a step-back manner.242 Because of their superior resistance to cyclic fatigue, #.02-tapered ProFile instruments are useful for abrupt apical curves. Preparation is complete once a continuous #.06 taper with an adequate apical size is achieved. Recapitulation during the preparation with a small hand file is recommended.