Chapter 16 Cleaning and Shaping

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Routine Access Cavities

Most access cavities involve only one surface and are surrounded by dentin walls or by porcelain or metal (if the restoration is retained). The temporary must last from several days to several weeks. Numerous types are available, including premixed cements that set on contact with moisture (Cavit), reinforced zinc oxide-eugenol cements (such as IRM), glass ionomer cements and specially formulated light-polymerized composite materials (such as TERM^®, temporary endodontic restorative material)¹⁶⁴. Ease of use and good sealing ability make Cavit an excellent routine material, but low strength and rapid occlusal wear limit its use to short-term sealing of simple access cavities. IRM and TERM provide improved wear resistance, although their sealing ability is probably marginally less than that of Cavit^{165, 166}. More durable restorative materials, especially glass ionomer cements, tend to provide the best seal. A double seal of GIC over Cavit will provide a durable and effective barrier to microbial leakage. It is not known whether experimental leakage differences based on bacterial leakage or dye penetration are significant clinically, especially if thermocycling and occlusal loading are not part of the testing procedure¹⁶⁷. Clinically, 4mm of Cavit provided an effective seal against bacterial penetration for 3 weeks¹⁶⁸. Most critical are the thickness and placement of the material.

Techniques of Placement - The quality of the coronal seal depends on the thickness of the material, how it is compacted into the cavity, and the extent of contact with sound tooth structure or restoration. A minimum depth of 3 to 4 mm is required around the periphery, preferably 4 mm or more to allow for wear. In anterior teeth, the access is oblique to the tooth surface; care must be taken to ensure that the material is at least 3 mm thick in the cingulum area.
Cavit (or a similar material) is placed as follows: Chamber and cavity walls should be dry. Cavit can be placed directly over the canal orifices, or more commonly a thin layer of cotton is placed over the canal orifices to prevent canal blockage¹⁶⁹. (Figure 16-28) Care must be taken not to incorporate cotton fibers into the restorative material, which can promote rapid leakage¹⁷⁰.Cavit is packed into the access opening with a plastic instrument in increments from the bottom up and pressed against the cavity walls and into undercuts (Figure 16-29). Excess is removed, and the surface smoothed with moist cotton. The patient should avoid chewing on the tooth for at least an hour.
Subsequent removal using a high speed bur requires care to avoid damage to the access opening. Alternatively, an ultrasonic tip can be used.
Extensive Coronal Breakdown
Teeth without marginal ridges or with undermined cusps require a stronger filling material (high-strength glass ionomer cement), taking care to ensure an adequate thickness and good marginal adaptation proximally. The temporary filling material should extend well into the pulp chamber deep to the proximal margin to ensure a marginal seal. Reducing the height of undermined cusps well out of occlusion reduces the risk of fracture. For severely broken-down teeth, a cusp-onlay amalgam or a well-fitting orthodontic band cemented onto the tooth (restored with glass ionomer cement) provides a durable temporary restoration and strengthens the tooth against fracture¹⁷¹. At the next appointment, access is prepared through the restoration.
Provisional Post Crowns
The use of a provisional crown with an incorporated resin post may be required, particularly when a cast post and core is being fabricated for a visible tooth with little remaining coronal tooth structure. However, the use of such a provisional crown retained with a post (preformed aluminum post, safety pin wire, paper clip, or a sectioned large endodontic file) has inherent problems. Using the canal space for a provisional post precludes use of an intracanal medicament, and the coronal seal depends entirely on the cement. The coronal seal is generally inadequate with a loosely fitting and mobile provisional post and crown¹⁷². However, in spite of these potential difficulties, such provisional restorations may be required while cast posts and cores are being fabricated. Due to the potential problems, it is prudent to cement the definitive post as soon as possible.
When such a provisional crown-post combination is being used, the post should fit the canal snugly (not binding) and extend apically 4 to 5 mm short of working length and coronally to within 2 to 3 mm of the incisal edge. A polycarbonate shell is trimmed to a good fit; autopolymerizing material then is added to the inside of the shell to mold to the root face and attach to the post. A provisional luting cement (Temp Bond or similar cement) is placed on the coronal 3 to 4 mm of the post and root face, and the unit is cemented into place. A provisional removable partial overdenture is a useful alternative; access remains excellent, and there is little chance of disturbing the coronal seal between appointments.

Long–term Temporary Restorations

Few indications exist to justify delaying the final restoration, and endodontic procedures (other than trauma management) rarely require prolonged treatment. If a temporary restoration has to last more than a few weeks, then a durable material such as amalgam, glass ionomer cement, or acid-etch composite should be used. The pulp chamber is filled with Cavit to provide a good coronal seal, and covered with a sufficient thickness of the restorative material to ensure strength and wear resistance. Subsequent access to the canal space is readily achieved without damage to remaining tooth structure because the layer of Cavit can be easily removed.

Figures
Figure 16-1 Cross-section through a root showing the main canal (C) and a fin (arrow) and associated cul-de-sac after cleaning and shaping, using files and sodium hypochlorite. Note the tissue remnants that remain in the fin.
Figure 16-2 The main canal (C) has a lateral canal (arrow) extending to the root surface. After cleaning and shaping with sodium hypochlorite irrigation, tissue remains in the lateral canal.
Figure 16–3 A. A size #15 file in the apical canal space. Note the size is inadequate for planning the walls. B. A size #40 file more closely approximates the canal morphology (Courtesy of Dr. Randy Madsen).
Figure 16-4 A. The classic apical anatomy consisting of the major diameter of the foramen and the minor diameter of the constriction. B. An irregular ovoid apical canal shape and external resorption. C. A bowling pin apical morphology and an accessory canal. D. Multiple apical foramina.
Figure 16-5 A small file (#10 or #15) is placed beyond the radiographic apex to maintain patency of the foramen. Note the tip extends beyond the apical foramen (arrow).
Figure 16-6 For effective irrigation the needle must be placed in the apical one-third of the root and must not bind.

Figure 16-7 A sodium hypochlorite accident during treatment of the maxillary left central incisor. Extensive edema occurred in the upper lip accompanied by severe pain.

Figure 16-8 A. A canal wall with the smear layer present. B. The smear layer removed it 17% EDTA.
Figure 16-9 Procedural errors of canal transportation, zipping and strip perforation occur during standardized preparation when files remove dentin from the outer canal wall apical to the curve and from the inner wall coronal to the curve. This is related to the restoring force (stiffness) of the files. Note in the apical portion the transportation takes the shape of a tear drop as the larger files are used.
Figure 16-10 The canals have been transported and there is an apical perforation.
Figure 16-11 A. A size #35 file fractured in the mesiobuccal canal. B. SEM examination reveals torsional fatigue at the point of fracture. Note the tightening of the flutes near the fracture and the unwinding of the flutes along the shaft.
Figure 16-12 A. The furcal region of molars at the level of the curvature (danger zone) is a common site for stripping perforation. B. Note the distal concavity (arrows) in the furcation area of this mandibular molar.
Figure 16-13 Straight line access can result in stripping perforations in the furcal areas of molars. A. The use of large Gates Glidden drills and overpreparation has resulted in the stripping perforation. B. Note that the perforation is in the concavity of the furcation.
Figure 16-14 The step-back preparation is designed to provide a tapering preparation. The process begins with one file size larger than the master apical file with incremental shortening of either .5 or 1.0 mm.
Figure 16-15 As an example of step-back preparation in a moderately curved canal. A. The size #25 master apical file at the corrected working length of 21.0 mm. B. The step-back process begins with the #30 file at 20.5 mm. C. #35 at 20.0 mm. D. #40 file at 19.5 mm. E. #45 file at 19.0 mm. F. #50 file at 18.5 mm. G. #55 file at 18.0 mm. H. #60 file at 17.5 mm. I. #70 file at 17.0 mm
Figure 16-16 Passive step-back. Smaller to larger files are inserted to their initial point of binding and then rotated 180 to 360º and withdrawn. This process creates slight taper and coronal space. This permits larger instruments to reach the apical one third.
Figure 16-17 The anti-curvature filing technique. Instruments are directed away from the furcal “danger zone” toward the line angles (safety zone) where the bulk of dentin is greater.
Figure 16-18 Straight line access in a maxillary left first molar with Gates-Glidden drills used in a slow speed handpiece using a step-back technique. A. The #1 Gates is used until resistance. B. This is followed by the #2 which should not go past the first curvature. C. The #3 Gates is used 3-4 mm into the canal. D. Followed by the #4 instrument.
Figure 16-19 A maxillary first molar following straight line access with the Gates Glidden Drills.
Figure 16-20 The mesiobuccal canal is prepared using nickel-titanium rotary files using a crown-down technique. In this sequence each instrument exhibits the same .06 taper with varied ISO standardized tip diameters. Instrument were used to resistance. A. The process begins with a .06\45 file to resistance at 16.0 mm. B. This is followed by a .06\40 instrument at 17.0 mm C. The .06\35 file is used to 18.0 mm. D. The .06\30 at 19.0 mm. E. The .06\25 at 20.0 mm. F. The .06\20 file is to the corrected working length of 21.0mm.
Figure 16-21 Nickel-titanium rotary files with a standardized ISO tip diameter and variable tapered files can be used in canal preparation. In this sequence, the instruments have a standardized tip diameter of .20 mm. A. Initially a 10/.0 file is used. B. This is followed by 08/.20. C. The third instrument is a .06/.20. D. The final instrument is a 04/.20 file to the corrected working length of 21.0 mm.
Figure 16-22 Final Apical Enlargement A. The master apical file of size #25 at the corrected working length of 21.0 mm. B. Enlargement with a #30 file to the corrected working length of 21.0 mm. C. Further enlargement with a #35 file. D. Final enlargement to a size #40 file. The final instrument used becomes the Final Apical File.
Figure 16-23 Recapitulation is accomplished between each instrument by reaming with the Master Apical File or a smaller instrument. This minimizes packing of debris and loss of length.
Figure 16-24 Following their use, the Gates Glidden drills should be removed from the handpiece to prevent injury. This #3 drill was accidentally driven into the palm of the dentist.

Figure 16-25 Following straight line access in this maxillary molar, the Master Apical File is determined by successively placing small to larger files to the corrected working length. A. A #15 stainless steel file is placed to 21.0 mm without resistance. B. A #20 is the placed is placed to 21.0 mm without resistance. C. The #25 file reaches 21.0 mm with slight binding. D. A size #30 file is then placed and does not go the corrected working length indicating the initial canal size in the apical portion of the canal is a size#25

Figure 16-26 The coronal taper is assessed using the spreader or plugger depth of penetration. A. With lateral compaction a finger spreader should fit loosely 1.0 mm from the Corrected Working Length with space adjacent to the spreader. B. For warm vertical compaction, the plugger should go to within 5.0 mm of the Corrected Working Length.
Figure 16-27 Calcium hydroxide placement. A. Calcium hydroxide mixed with glycerin to form a thick paste. B. Placement with a lentulo spiral. C. Injection of a proprietary paste. D. Compaction of calcium hydroxide powder with a plugger.
Figures 16-28 and 16-29 are provided by Dr. Harold Messer
Figure 16-28. Techniques for temporization. On the left are the correct techniques; either minimal space is occupied by cotton or no cotton pellet is used, particularly if the proximal is to be restored. It is wrong to pack most of the chamber with cotton, which leaves inadequate space and strength for the material (3-4 mm are required), and cotton fibers may promote bacterial leakage. (Courtesy of Dr L Wilcox)
Figure 16-29. Techniques for placing temporary material. A, A single large “blob” placed in the access opening will not seal the walls. B, The incremental technique, which adds successive layers, pressing each against the chamber walls, is correct. (Courtesy of Dr L Wilcox)

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