The clinician must choose the strategies, instruments, and devices to deal with these challenges and to control the preparation shape, length, and width precisely. This allows the practitioner to use endodontic therapy to address acute (Fig. 9-15) and chronic (Fig. 9-16) forms of the disease processes described previously. Recall radiographs taken at appropriate intervals will demonstrate longevity and favorable outcomes (see Figs. 9-1 to 9-4, 9-6, and 9-16) if clinical objectives are maintained (Box 9-1).
CLEANING AND SHAPING: TECHNICAL ISSUES
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Figure 9-10 Spectrum of strategies for accomplishing the primary aim of root canal treatment: elimination of infection. A, Schematic diagram of minimally invasive therapy using the noninstrumentation technique. B, Example of teeth cleaned in vitro using NIT. Note the clean intracanal surface, which is free of adhering tissue remnants. C and D, Examples of teeth cleaned in vivo and later extracted to investigate the clinical effects of NIT. Note the relatively clean, tissue-free canal space in C and the significant tissue revealed by rhodamin B staining in D. E and F, Course of maximally invasive therapy; apically involved tooth #30 was extracted, effectively removing the source of periradicular inflammation. (AandBcourtesy Professor A. Lussi;CandDcourtesy Professor T. Attin;EandFcourtesy Dr. T. Kaya.)
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Figure 9-11 Panel of 36 anatomic preparations of maxillary molars from the classic work by Professor Walter Hess of Zurich. Note the overall variability of root canal systems and the decrease of canal dimensions with age. (From Hess W: The anatomy of the root canals of teeth of the permanent dentition, London, 1925, John Bale, Sons & Danielsson.)
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Figure 9-12 Micro-computed tomographic scans of dental anatomy (36 μm resolution). A, Clinical view of tooth #9 shows two accessory canals and an apical bifurcation. B, Mesiodistal view of the tooth shown in A. C, Working length radiograph with files placed in both apical canal aspects.
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Figure 9-13 Micro-computed tomographic scans of more complicated dental anatomy (36 μm resolution). A, Clinical view of tooth #3 shows a fine mesiobuccal and distobuccal canal system with additional anatomy in all three roots. B, Mesiodistal view of the tooth shown in A.
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Figure 9-14 Micro-computed tomographic scan of anatomy of the apical 5 mm of a mesiobuccal root (8 μm resolution). A and B, Three-dimensional reconstruction of outer contour and root canal systems. C, Cross sections 0.5 mm apart.
Box 9-1 Basic Objectives in Cleaning and Shaping
The primary objectives in cleaning and shaping the root canal system are to:
Give disinfecting irrigants access to the apical canal space
Create space for the delivery of medicaments and subsequent obturation
Retain the integrity of radicular structures
Because several technical issues arise with the instruments and devices used for cleaning and shaping, a short review of these products is provided here (also see Chapter 8). A vast array of instruments, both hand-held and engine-driven, is available for root canal preparation. Up to the last decade of the past century, endodontic instruments were manufactured from stainless steel. With the advent of nickel-titanium,250 instrument designs began to vary in terms of taper, length of cutting blades, and tip design. Files traditionally have been produced according to empiric designs, and most instruments still are devised by individual clinicians rather than developed through an evidence-based approach. Similar to the development of composite resins in restorative dentistry, the development of new files is a fast and market-driven process. With new versions rapidly becoming available, the clinician may find it difficult to pick the file and technique most suitable for an individual case. Practitioners must always bear in mind that all file systems have benefits and weaknesses. Ultimately, clinical experience, handling properties, usage safety, and case outcomes, rather than marketing or the inventor's name, should decide the fate of a particular design.
Hand and Engine-Driven Instruments
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Figure 9-15 Sinus tract as a sign of a chronic apical abscess and effect of routine root canal treatment. A, Intraoral photograph of left maxillary region with draining sinus tract (arrow) periapical to tooth #14. B, Preoperative radiograph with gutta-percha point positioned in the sinus tract, pointing toward the distobuccal root of #14. C, Finished root canal fillings after 2 weeks of calcium hydroxide dressing. D, Intraoral photograph of the same region as in A, showing that the sinus tract had closed by the time obturation was performed.
Hand instruments have been in clinical use for almost 100 years, and they still are an integral part of cleaning and shaping procedures. A norm established by the American Dental Association (ADA) and the International Standards Organization (ISO)13,131 sets the standards for broaches, K-type files and reamers, Hedström files, and paste carries; however, the term ISO-normed instruments currently is used mainly for K-files (Fig. 9-17). One important feature of these instruments is a defined increase in diameter of 0.05 mm or 0.1 mm, depending on the instrument size (Fig. 9-18).
Barbed broaches are produced in a variety of sizes and color codes. They are manufactured by cutting sharp, coronally angulated barbs into metal wire blanks. Broaches are intended to remove vital pulp from root canals, and in cases of mild inflammation, they work well for severing pulp at the constriction level in toto. The use of broaches has declined since the advent of NiTi rotary shaping instruments, but broaching occasionally may be useful for expediting procedures and for removing materials (e.g., cotton pellets) from canals.
K-files were manufactured by twisting square or triangular metal blanks along their long axis, producing partly horizontal cutting blades (Fig. 9-19). Noncutting tips, also called Batt tips, are created by grinding and smoothing the apical end of the instrument (see Fig. 9-19). Roane and Powell223 introduced a modified shape, the Flex-R file, which was manufactured fully by grinding so that the transitional angles were smoothed laterally between the tip and the instrument's working parts. Similar techniques are required to manufacture NiTi K-files,281 such as the NiTi-Flex (Dentsply Maillefer, Ballaigues, Switzerland). NiTi K-files are extremely flexible and are especially useful for apical enlargement in severe apical curves. They can be precurved but only with strong overbending; this subjects the file to excess strain and should be done carefully. Because of their flexibility, the smaller NiTi files (sizes up to #25) are of limited use.
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Figure 9-16 Relationship of radicular anatomy and endodontic disease as shown by filled accessory canals. A, Working length radiograph of tooth #13 shows lesions mesially and distally but not apically. B, Posttreatment radiograph shows the accessory anatomy. C, Six-month recall radiograph before placement of the restoration. D, Two-year recall radiograph after resection of the mesiobuccal root of tooth #14 and placement of a fixed partial denture. Excess sealer appears to have been resorbed, forming a distal residual lesion. E, Four-year recall radiograph shows almost complete bone fill. F, Seven-year recall radiograph; tooth #14 is radiologically sound and clinically within normal limits.
Cross-sectional analysis of a K-file reveals why this design allows careful application of clockwise and counterclockwise rotational and translational working strokes. ISO-normed K- and Hedström files are available in different lengths (21, 25, and 31 mm), but all have a 16 mm long section of cutting flutes (Fig. 9-17). The cross-sectional diameter at the first rake angle of any file is labeled D0. The point 1 mm coronal to D0 is D1, the point 2 mm coronal to D0 is D2, and so on up to D16. The D16 point is the largest diameter of an ISO-normed instrument. Each file derives its numeric name from the diameter at D0 and is assigned a specific color code (see Fig. 9-17).
Another aspect of ISO files is the standard taper of 0.32 mm over 16 mm of cutting blades, or 0.02 mm increase in diameter per millimeter of length (#.02 taper) (see Fig. 9-17). Thus a size #10 instrument has a diameter of 0.1 mm at D0 and a corresponding diameter of 0.42 mm at D16 [0.1 mm + (16 × 0.02 mm)]. For a size #50 instrument, the diameters are 0.5 mm at D0 and 0.82 mm at D16.
The tip size increases by 0.05 mm for file sizes #10 to #60; for sizes #60 to #140, the absolute increase is 0.1 mm (see Fig. 9-18). Recalculation of these diameter increments into relative steps (in percentages) reveals dramatic differences: the step from size #10 to #15 is 50%, whereas the increase from size #55 to #60 is less than one fifth of that change (Fig. 9-18).
In very small files (sizes #6 to #10), the problem is partly resolved by several key points: (1) apical dimensions are such that a size #6 file does not significantly remove dentin other than in severely calcified cases; (2) a size #8 file taken 0.5 to 1 mm long, to establish patency (discussed later in the chapter), contacts the desired endpoint of the preparation with a diameter approaching the tip size of a #10 file; (3) similarly, placing a size #10 file just minutely through the foramen eases the way for passive insertion of the subsequent #15 file to full length.227
The ISO specifications inadvertently complicated the cleaning and shaping of root canal systems. The ISO-normed design is a simplification that has specific disadvantages, and it may explain the clinical observation that enlarging from size #10 to #15 is more difficult than the step from size #55 to #60. The introduction of the Golden Medium files (Dentsply Maillefer), which have tip sizes between the ISO-stipulated diameters, seemed to solve the problem. However, their use is not that important clinically, because the approved machining tolerance of ± 0.02 mm negates the intended advantage. Moreover, although ± 0.02 mm tolerance is stipulated by the ISO norm (see Fig. 9-17), many manufacturers do not adhere to it.139,245,274,335
A subsequent modification involved tips with a constant percentage of diameter increments, the Series 29. The first ProFile instruments (Dentsply-Tulsa, Tulsa, OK) followed this design with a nominal diameter increase of 29%. This sizing pattern creates smaller instruments that carry less of a workload. However, the intended advantage is offset by larger diameters, because the 29% increase between successive files is actually greater than the percentage change found in the ISO file series.