Figure 9-33 Physical factors (torque, axial force, and insertion depth) that affect root canal instrumentation documented with a torque-testing platform. A, ProFile size #45, #.04 taper used in a mildly curved canal of a single-rooted tooth, step-back after apical preparation to size #40. B, FlexMaster size #35, #.06 taper used in a curved distobuccal canal of a maxillary first molar, crown-down during the initial phase of canal preparation.
Newer motors have been developed for rotary instruments since the simple electric motors of the first generation in the early 1990s (Fig. 9-35, A). Electric motors with gear reduction are more suitable for rotary NiTi systems because they ensure a constant rpm level; however, they also deliver torques much higher than those required to break tips. Some authors believe that torque-controlled motors (Fig. 9-35, B to D), which have been used for several years, increase operational safety.95 However, others have suggested that torque-control motors may be helpful mainly to inexperienced clinicians.328 These motors probably do not reduce the risk of fracture caused by cyclic fatigue; also, even if the torque is below the fracture load at D3, a fracture at the smaller diameter (D2) is still possible.
To complicate matters further, an obvious differential exists between torque at failure at D3 and the working torque needed to operate an instrument effectively (Fig. 9-36 and Box 9-2).37,125,199,204,232 In many cases the working torque is greater than the torque required to fracture the instrument's tip. However, the tip will not break if a passive glide path has been verified.
This differential is especially large with files with a taper greater than #.06; therefore, these files are rather ineffective in most torque-controlled motors. Most motors allow adjustment of torque for the instrument used, either with a key or a system card that is inserted into the box.
Box 9-2 Instrument Breakage with Torsional Load (MacSpadden Factor)
For rotary instrument tips, susceptibility to breakage is governed by the quotient of torque needed to fracture divided by working torque. Simply put, the larger the value, the safer the file.
Other factors that may influence the incidence of fracture in motor-driven NiTi rotary instruments are lubrication, specific instrument motion, and speed of rotation. It cannot be overemphasized that nickel-titanium rotary instruments should be used only in canals that have been flooded with irrigant. Although lubricants such as RC-Prep (Premier, Norristown, PA) and Glyde (Dentsply Maillefer) have also been recommended, their benefit has not been proved conclusively.201 In fact, because of chemical interactions between NaOCl and ethylenediamine tetra-acetic acid (EDTA), alternating irrigants and using lubricants that contain EDTA may even be counterproductive. Moreover, no data have been produced linking the use of lubricants to reduction of torque during root canal preparation.
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Figure 9-34 Scanning electron micrographs of deformed or separated nickel-titanium rotary instruments. A, Lateral view of a ProTaper F3 instrument after application of torsional load. (×25.) B, Lateral view of a size #35, #.04 taper FlexMaster instrument after more than 500 rotations in a 90-degree curve with a 5 mm radius (see Fig. 9-31). (×30.) C, Cross section of the ProTaper instrument in A. Note signs of ductile fracture near center of the instrument core. (×140.) D, Cross section of FlexMaster instrument in B. (×100.)
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Figure 9-35 Examples of motors used with rotary nickel-titanium endodontic instruments. A, First-generation motor without torque control. B, Fully electronically controlled second-generation motor with sensitive torque limiter. C, Frequently used simple torque-controlled motor. D, Newest-generation motor with built-in apex locator and torque control.
For instrument motion, most manufacturers recommend a pecking, up and down motion. This not only prevents screwing in of the file, it also distributes stresses away from the instrument's point of maximum flexure, where fatigue failure would likely occur.154,214 Oscillating movements did not significantly enhance the life span of ProFile size #.04 or GT rotary instruments rotated around a 5 mm radius cylinder with a 90-degree curve.199,202 Furthermore, large variations were noted in the lengths of the fractured segments,125,299 which suggests that ductile fractures may originate at points of surface imperfections.
Rotational speed may also influence instrument deformation and fracture. Some studies indicated that ProFile instruments with ISO-norm tip diameters failed more often at higher rotational speed,79,93 whereas other studies did not find speed to be a factor.76,137
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Figure 9-36 Diagram comparing fracture loads at D3 (upper section of graph) to torques occurring during preparation of root canals (lower section of graph). Filled columns represent the largest file in each set, and open columns show the scores of the most fragile file (see text and Box 9-3 for details).*
Box 9-3 Factors Governing the Potential for Nickel-Titanium Rotary Instrument Fractures
Clinician's handling (most important)
Combination of torsional load, bending, and axial fatigue
Root canal anatomy
Clinicians must fully understand the factors that control the forces exerted on continuously rotating NiTi instruments (Box 9-3). To minimize the risk of fracture and prevent taper lock, they should not try to force motor-driven rotary instruments in an apical direction. Similarly, acute apical curves limit the use of instruments with higher tapers because of the risk of cyclic fatigue. The incidence of instrument fracture can be reduced to an absolute minimum if clinicians use data from well-designed torque and stress studies. Adequate procedural strategies, a detailed knowledge of anatomic structures, and specific instrumentation sequences may also improve shaping results.
Specific procedures have been developed for removing fractured instruments from root canals (Fig. 9-37); these are discussed in detail elsewhere in this book (see Chapter 25). Most of those methods require the use of additional equipment, such as a dental operating microscope and ultrasonic units. However, the best way to deal with instrument fracture is prevention. An understanding of the anatomy of the root canal system, together with a clear plan for selecting, sequencing, and using shaping instruments, can certainly help prevent procedural mishaps.
Disinfectants, Dentin Surface Modifiers, and Lubricants
Studies have demonstrated conclusively that mechanical instrumentation cannot sufficiently disinfect root canals, regardless of whether stainless steel52 or nickel-titanium75 instruments are used (Fig. 9-38). Irrigation solutions are required to eradicate microorganisms, and over time a variety of chemicals have been promoted for this purpose. The ideal irrigant or combination of irrigants kills bacteria, dissolves necrotic tissue, lubricates the canal, removes the smear layer, and does not irritate healthy tissues.103,129 Some formaldehyde-containing materials are no longer recommended for clinical use, but many irrigating solutions and varying concentrations of commonly used materials are described in the literature. Some solutions used in the past were sterile saline, NaOCl, and detergents (e.g., quaternary ammonium compounds, chlorhexidine, citric acids, and EDTA).271 This section describes current materials and gives some recommendations for their clinical use.
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Figure 9-37 Removal of a separated NiTi instrument from a mesiolingual canal of a mandibular molar. A, Fragment located in the middle third of the root. B, Clinical aspect of the fragment after enlargement of the coronal third of the root canal with modified Gates-Glidden drills, visualized with an operating microscope. (×25.) C, Radiograph taken after removal of the fragment; four hand files have been inserted into the canals. D, Final radiograph shows slight widening of the coronal third of the mesiolingual canal and fully sealed canal systems. A full crown was placed immediately after obturation. E, Recall radiograph 5 years after obturation shows sound periradicular tissues. F, Removed fragment and separated file (gradation of ruler is 0.5 mm).
A 0.5% solution of sodium hypochlorite was used effectively during World War I to clean contaminated wounds.74 Also, NaOCl at varying concentrations has been in use in root canal therapy for many decades.303 NaOCl is effective against endodontic microorganisms (Table 9-1), including those difficult to eradicate from root canals, such as Enterococcus, Actinomyces, and Candida organisms.*
In root canal treatment, NaOCl solutions are used at concentrations ranging from 0.5% to 5.25%. In infected dentin blocks, a 0.25% solution of NaOCl was sufficient to kill Enterococcus faecalis in 15 minutes; a concentration of 1% NaOCl required 1 hour to kill Candida albicans.247 NaOCl dissolves organic material, such as pulp tissue and collagen. Lower concentrations (e.g., 0.5% or 1%) dissolve mainly necrotic tissue.332 Higher concentrations allow better tissue dissolution but dissolve both necrotic and vital tissue, which is not always a desirable effect. In some cases full-strength NaOCl (5.25%) may be indicated; however, although higher concentrations may increase antibacterial effects in vitro,329 enhanced clinical effectiveness has not been demonstrated conclusively for concentrations stronger than 1%.