Vitality testing requires the measurement of pulpal blood flow. Several devices are used in the medical field to evaluate circulatory changes, and a number of these have been used experimentally to evaluate pulpal health. Several studies have reported successful use of laser Doppler flowmetry to study human pulpal blood flow.99,100,294,295 The value of this method has been well documented, but its high cost and difficulty of use in clinical situations have prevented widespread use.135,136,225,238
The pulse oximeter offers a nondestructive means of monitoring pulp vitality by recording the oxygenation of pulpal blood flow. A preliminary investigation found that this device performed satisfactorily as a clinical tool.251 Special sensors have been developed to study blood flow and blood oxygenation in vitro.70,207 However, further attempts to apply the technology to a clinically useful device have been disappointing.141 Photoplethysmography of pulpal blood flow also has been evaluated for assessment of pulp vitality.71
Doppler techniques, pulse oximetry, and photoplethysmography are all used in medicine and in dental research. However, they have been less successfully applied to routine endodontic care, because the circulatory system of the pulp is encased in a rigid structure and therefore is difficult to study without the removal of hard tissue. Consequently, the need for an absolute rigid observation point in the Doppler technique and the interference of extrapulpal circulatory systems in pulse oximetry and photoplethysmography have limited the introduction of these methods to endodontic practice.
Mechanical Probing Test
Running an explorer across dentin or into a carious lesion is a mechanical test of pulpal status. Some pain is expected; however, an extremely sharp pain or a lingering pain that does not resolve with removal of the stimulus is considered abnormal.
Periapical Tissue Testing Materials
Although the materials used to test the periapical tissues are "low tech," they can provide some of the most important diagnostic data. Percussion and palpation are tests of periapical tissues (see Chapter 1).
In percussion, pressure is applied to the tooth to try to stimulate the periapical tissues. If inflammation is present, the patient feels discomfort. Traditionally a mirror handle has been used for the percussion test, but other instruments also may be used. Tapping must be done in a vertical direction with the long axis of the root. An equivalent tooth (i.e., a control) must be tested first to establish a normal response for that particular patient. Before testing the problematic tooth, the clinician should instruct the patient to raise a hand if the sensation is different from that felt in the control tooth. If the patient is in acute pain, it is best to avoid tapping the tooth with a mirror. The clinician (or even the patient) can use a finger to tap the tooth to help determine the source of pain. However, symptoms often are difficult to reproduce using this technique.
Having the patient chew on the wooden end of a cotton-tip applicator can be a great diagnostic technique; cotton rolls also have been suggested for this purpose. The patient is asked to chew on the stick while moving it around the quadrant. When a sensitive area is detected, the patient should leave the stick in position so that the clinician can document the tooth. Practitioners should not be fooled by referred pain; the problematic tooth may be in the maxilla or the mandible (also see Chapter 3). The Tooth Slooth (Fig. 8-6) has proved very useful for the differential diagnosis of various stages of incomplete crown fractures. The design of the device permits chewing force to be applied selectively to one cusp at a time, allowing the clinician to evaluate weaknesses in defined areas of a tooth. This device can be effective when cotton rolls or wooden sticks are not helpful.
In palpation, the fleshy (ventral) part of the clinician's index finger becomes the diagnostic instrument. One means of locating the apex is to place the finger on the marginal gingiva and run it up the root eminence until the apex is located. The patient should be asked to raise a hand if the sensation is different from that felt in the control tooth. Palatal and lingual tissues should be palpated, along with lymph nodes, muscles, and trigger points.
Although most instruments used in general dentistry also can be used for endodontic therapy, some hand instruments are designed specifically for endodontic procedures. In addition, many different types of instruments have been designed for procedures performed inside the pulp space. These include manually operated instruments for root canal preparation, engine-driven and energized instruments for root canal preparation, instruments for root canal obturation, and rotary instruments for post space preparation.
Standardized specifications have been established to improve instrument quality. For example, the International Standards Organization (ISO) has worked with the Fédération Dentaire Internationale (FDI) through the Technical Committee 106 Joint Working Group (TC-106 JWG-l) to define specifications. These standards are designated with an ISO number. The American Dental Association (ADA) also has been involved in this effort, as has the American National Standards Institute (ANSI); these standards are designated with an ANSI number. However, new instrument designs have resulted in a need for reconsideration of the standards.
Two ISO standards pertain to endodontic instruments. ISO no. 3630-1 deals with K-type files (as does ANSI no. 28), Hedström files (ANSI no. 58), and barbed broaches and rasps (ANSI no. 63). ISO no. 3630-3 deals with condensers, pluggers, and spreaders (ANSI no. 71).
Traditional hand instruments sometimes are modified for endodontic uses. A typical set of endodontic instruments might include a mouth mirror, a D-5 explorer, a D-16 endodontic explorer, cotton pliers, a spoon excavator, a series of pluggers, a plastic instrument, a hemostat, a periodontal probe, and a ruler. The endodontic explorer has two straight, very sharp ends that are angled in two different directions from the long axis of the instrument.
Several types of endodontic spoons are available. These spoons have a much longer offset from the long axis of the instrument (for better reach inside constricted pulp chambers) than regular dental spoons. The spoons are used to remove carious material and to excise pulp tissue; therefore, they should be kept well sharpened (Fig. 8-7). The exact type and number of instruments usually depend on the techniques used and clinician's preference
Instruments for Pulp Space Preparation
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Figure 8-7 Endodontic explorer (A) and endodontic spoons (B). (From Boyd LR: Dental instruments: a pocket guide, ed 2, St. Louis, 2005, Saunders.)
The purposes of this section are to provide and consolidate the principles the clinician needs both to understand the design of instruments and to choose and use current and future instruments to the greatest effect. Most instructional materials mistakenly attempt to teach step-by-step techniques rather than explain the physics of instruments. However, an increasing number of new products and their advocates has created confusion in the selection process, and products become obsolete before they can be thoroughly evaluated. For these reasons, the clinician must understand the scientific principles of instrumentation.
The two primary goals of root canal instrumentation are (1) to provide a biologic environment (infection control) conducive to healing and (2) to develop a canal shape receptive to sealing. Historically most instruments used to shape the canal were designed to be used by hand. Although not universally used, rotary instrumentation has gained considerable interest and most often is used in combination with hand instruments. The information in the following sections should facilitate the most efficient use of rotary instruments, minimizing the chance of failure and allowing the clinician to achieve treatment ideals.
An understanding of the physics of rotary technology can provide financial rewards, save time and, most important, enhance the quality of treatment while avoiding inherent risks. However, one point must be strongly emphasized: these improvements are not derived from quickness or ergonomics; rather, they are the result of increased control and of the ability both to anticipate the optimal approach and to eliminate the less-than-optimal, the unnecessary, and the sometimes counterproductive elements of a technique.
Classification of Instruments Used for Pulp Space Preparation
Endodontic instruments for root canal preparation can be divided into three groups:
Group I: Hand- and finger-operated instruments, such as barbed broaches and K-type and H-type instruments.
Group II: Low-speed instruments on which the latch type of attachment is part of the working section. Typical instruments in this group are Gates-Glidden (GG) burs and Peeso reamers.
Group III: Engine-driven instruments similar to the hand- and finger-operated instruments. However, the handles of these engine-driven instruments have been replaced with attachments for a latch type of handpiece. In the past, few instruments were included in this group because rotary root canal files were rarely used. In recent years, however, the use of nickel-titanium rotary instruments has become popular, and although not standardized, these instruments are included in this category.
Terminology for the Physical Properties of Instruments
Successful use of an instrument depends on the ways in which the material, design, and technique relate to the forces exerted on the instrument. The following terms quantify the actions and reactions to these forces.
Stress: The deforming force measured across a given area.
Stress concentration point: An abrupt change in the geometric shape of a file, such as a notch, which results in a higher stress level at that point than along the surface of the file where the shape is more continuous.
Strain: The amount of deformation a file undergoes.
Elastic limit: A set value representing the maximal strain that, when applied to a file, allows the file to return to its original dimensions. After the strain is removed, the residual internal forces return to zero.
Elastic deformation: The reversible deformation that does not exceed the elastic limit.
Shape memory: A condition that exists when the elastic limit is substantially higher than is typical for conventional metals. It allows an instrument to regain its original form after being deformed.
Plastic deformation:Permanent bond displacement, which occurs when the elastic limit is exceeded. The file does not return to its original dimensions after strain is removed.
Plastic limit: The point at which a plastic-deformed file breaks.
Manually Operated Instruments
Manually operated instruments are all instruments that are generically called files. Defining endodontic instruments by function, files are instruments that enlarge canals with reciprocal insertion and withdrawal motions. Reamers cut and enlarge canals with rotational motions. Before using either instrument, the clinician must make sure that the canal is patent.
The first mechanical rotary files were formed from straight piano wire, which were ground and then twisted, producing the file configuration still used today. Files were first mass-produced by the Kerr Manufacturing Co. of Romulus, Michigan, in the early 1900s, hence the name K-type file (or K-file) and K-type reamer (K-reamer).
K-files and K-reamers originally were manufactured by the same process. Three or four equilateral, flat surfaces were ground at increasing depths on the sides of a piece of wire, producing a tapered pyramidal shape; the wire then was stabilized on one end and the distal end was rotated to form the spiral instrument (Fig. 8-8). The number of sides and the number of spirals determine whether the instrument is best suited for filing or reaming. Generally, a three-sided configuration with fewer spirals is used for reaming; a three- or four-sided configuration with more spirals is used for filing.
At first, root canal instruments were manufactured from carbon steel. However, chemicals (e.g., iodine, chlorine) and steam sterilization caused significant corrosion (Fig. 8-9). Subsequently, the use of stainless steel greatly improved the quality of instruments (Fig. 8-10).205,290 More recently, the introduction of the nickel-titanium (NiTi) alloy in the manufacture of endodontic instruments has resulted in significant changes in the specialty (this metal is described later in the chapter).
Barbed Broaches and Rasps
Dating from the early to mid-nineteenth century, broaches and rasps were the earliest endodontic instruments used to extirpate the pulp and enlarge the canal (Fig. 8-11). Still used today, these instruments are manufactured by hacking a round, tapered wire with a blade to form sharp, projecting barbs that cut or snag tissue. Specifications have been set for both the barbed broach and the rasp (ANSI/ADA standard no. 63, ISO/FDI standard no. 3630/1).
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Figure 8-8 The two ends of a blank, tapered pyramidal wire are stabilized (top), and then one end is rotated to create a spiral shape on the file's working surface (middle). Multiple rotations produce the final spiral shape (bottom).(Courtesy John T. McSpadden, Lookout Mountain, GA.)
Although similar in design, broaches and rasps show some significant differences in taper and barb size. The broach has a taper of #.007 to #.01 taper and the rasp has a taper of #.015 to #.02 taper. Barb height is much greater in the broach than in the rasp; because the barb derives from the instrument's core, the broach therefore is a much weaker instrument than the rasp. A barbed broach does not cut or machine dentin; however, it is an excellent tool for removing cotton or paper points that have accidentally become lodged in the root canal. These instruments are mostly used to engage and remove soft tissue from the canal, and they frequently are used in sonic or reciprocating handpieces to enlarge canals. They also have the potential, as demonstrated by prototypes, to become effective NiTi rotary instruments. The evolutionary development of endodontic instruments is far from over.
The K-file and K-reamer are the oldest useful instruments for cutting and machining dentin (Figs. 8-12 and 8-13). As mentioned previously, traditionally they have been made from a steel wire that is ground to a tapered square or triangular cross section and then twisted to create either a file or a reamer. During this process the steel is work hardened. A file has more flutes (see Components of a File) per length unit than a reamer.
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Figure 8-9 Effect of chlorine and steam sterilization on carbon steel file. A, Untreated file. B, Five minutes' exposure to 5% sodium hypochlorite (NaOCl) followed by water rinse, drying, and autoclave sterilization. C, Same treatment as in B but repeated three times. Note the severe damage to the file. (Courtesy Dr. Evert Stenman, Umea, Sweden.)
K-type instruments are useful for penetrating and enlarging root canals. The instrument works primarily by compression-and-release destruction of the dentin surrounding the canal.263 Generally, a reaming motion causes less transportation than a filing motion.309 (Transportation is the excessive loss of dentin from the outer wall of a curved canal in the apical segment. This procedural error can lead to perforation of the root canal system.) A stainless steel K-file can be precurved to a desired form to facilitate insertion and minimize transportation. Permanent deformation occurs when the flutes become wound more tightly or opened more widely (Fig. 8-14). Instruments fracture during clockwise motion after plastic deformation121,260; this occurs when the instrument becomes bound while the force of rotation continues. Interestingly, although the force required for failure is the same in both directions of rotation,54,162,167 failure occurs in the counterclockwise direction at half the number of rotations required for failure in the clockwise direction. Therefore K-type instruments should be operated more carefully when pressure is applied in a counterclockwise direction.
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Figure 8-10 Effect of chlorine and steam sterilization on a stainless steel file. A, Untreated file. B, Five minutes' exposure to 5% NaOCl followed by water rinse, drying, and autoclave sterilization. C, Same treatment as in B but repeated three times. The file shows no damage. (Courtesy Dr. Evert Stenman, Umea, Sweden.)
An H-type instrument has spiral edges arranged to allow cutting only during a pulling stroke. An example is a Hedstrom file. H-type instruments are better for cutting than a K-type instrument because it has a more positive rake angle (see Components of a File) and a blade with a cutting rather than a scraping angle. Bending a Hedström file results in points of greater stress concentration than occurs with K-type instruments. These concentration points can lead to the propagation of cracks and fatigue failure.121 Clinically, fatigue happens without any external physical signs of stress, such as the flute changes seen in K-type instruments (Fig. 8-14).
Currently all H-type instruments are ground from a tapered blank. Hedström files are formed by grinding a single continuous flute. Computer-assisted machining technology has allowed the development of H-type instruments with very complex forms. This process, called multiaxis grinding, allows adjustment of the rake angle, helix angle, multiple flutes, and tapers (Figs. 8-15 and 8-16).
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Figure 8-11 Barbed broach (Union Broach, York, PA).
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Figure 8-12 K-file with a blunt tip. An instrument fresh from the box shows a significant amount of debris. This is not an uncommon finding with many brands of instruments, therefore new instruments should be cleaned before they are sterilized and used.
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Figure 8-13 K-file #40 (Maillefer Instruments SA, Ballaiques, Switzerland). Note the clean surface and rounded tip.
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Figure 8-14 K-files stressed to deformation by clockwise and counterclockwise twisting (arrow indicates deformed areas). These instruments are close to fracture.
H-files cut the canal wall when pulled or rotated clockwise; the file is relatively ineffective when pushed or rotated counterclockwise. Because the H-file generally has sharper edges than the K-file, it has a tendency to screw into the canal during rotation, particularly if the instrument's blades are nearly parallel. Awareness of screwing-in forces is important for avoiding instrument failure (Fig. 8-17).
Instrument Design Modifications
K-files and H-files can be modified into numerous designs. Often the instruments can be improved for more effective instrumentation by changing the geometric dimensions created by computerized multiaxis grinding machines. For example, changing the cross-sectional geometry of a K-type instrument from square to rhomboid enhances the instrument's flexibility and rake angle (Fig. 8-18). However, the possible geometries can complicate adherence to ISO and ANSI standards (Figs. 8-19 to 8-24).
Studies have shown that tip design can affect file control, efficiency, and outcome in the shaping of root canal systems.194,195,315 The tip of the original K-file resembled a pyramid (Fig. 8-25). The file can break if the clinician applies excessive torque while attempting to enlarge a canal with a smaller diameter than the noncutting portion of the file tip. Instrument tips have been described as cutting, noncutting, and partially cutting, although no clear distinction exists among the three types.
The instrument tip has two functions: to enlarge the canal and to guide the file through the canal. A clinician who is unfamiliar with the tip design of a particular instrument is apt to do either of the following: (1) transport the canal (if the tip is capable of enlarging the canal and remains too long in one position) or (2) encounter excessive torsion and break the file (if a noncutting tip is forced into a canal with a smaller diameter than the tip). Transportation of the original axis of the canal can occur by remaining too long in a curved canal with a tip that has efficient cutting ability. On the other hand, with the H-file the clinician need not remain too long in one position, and the instrument's efficient cutting can facilitate enlargement or negotiation of constricted or blocked canals.
Figure 8-16 Hedström file #50 (Antaeos, Vereinigte Dentalwerke, München). This file has a steeper helix angle than the file in Fig. 8-15. Note the blunted tip, a common result of force used during attachment of the handle.
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Figure 8-17 Hedström file #100 (Roydent Dental Products, Rochester Hills, MI). The rake angle is close to neutral (arrow), which makes this instrument very efficient for machining strokes.
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Figure 8-18 K-Flex file #35 (Kerr Manufacturing Co.). This file resembles a classic K-file with its twisted pattern (compare to the file shown in Fig. 8-16). The cross section of the blank is rhomboid, giving the instrument a small and a large diameter that can be clearly seen. Note the untwisted tip.
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Figure 8-19 Flex-R file (Union Broach), a milled K-type file. The flutes are sharper and have a less negative rake than a traditional twisted K-file. The tip is rounded.
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Figure 8-20 FlexoFile (Maillefer Instruments SA, Ballaiques, Switzerland), a milled K-type file. Note the smooth surface and well-formed tip.
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Figure 8-21 Ultra Flex #30 (Zipperer, VDW, Munich, Germany), a milled K-type NiTi file. Note the coarse surface, a typical result of milling in NiTi alloy. The flutes are less sharp than in a steel counterpart and often are rolled over the edge.
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Figure 8-22 Sureflex #30 (Caulk/Dentsply, Milford, DE), a milled, K-type NiTi file has a greater helix angle than the Ultra Flex (Fig. 8-21). Compare the design with that of the FlexoFile (Fig. 8-20).
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Figure 8-23 Hyflex X-file (Hygenic Corp.), a Hedström-type NiTi file with a double helix.
The angle and radius of its leading edge and the proximity of the flute to its actual tip end determine the cutting ability of a file tip. Cutting ability and file rigidity determine the propensity to transport the canal. The clinician must keep in mind that as long as the file is engaged 360 degrees, canal transportation cannot occur. Only with overuse does the file begin to cut on one side, resulting in transportation. Most instrumentation occurs when the file tip is loose in the canal, which gives it a propensity to transport the canal.
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Figure 8-24 Mity Turbo (JS Dental, Ridgefield, CT), a Hedström-type NiTi file with a tighter double helix than the Hyflex X-file. This file is much less efficient at machining a substrate than the Hyflex X-file.139
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Figure 8-25 K-type instrument with a pyramidal tip.
A good beginner's rule is this: If the canal is smaller than the file, a cutting tip is more efficient. If the canal is larger than the tip, using a less effective cutting tip can help prevent transportation (Fig. 8-26). Much has been written about the importance of various sophisticated tip modifications to prevent such ledging, but no scientific proof exists that any one design is better than another for clinical work.148,222,223,229,232
The development of nitinol, an equiatomic alloy composed of nickel and titanium,310 has proved a significant advancement in the manufacture of endodontic instruments. Nickel-titanium is called an exotic metal because it does not conform to the normal rules of metallurgy. Because it is a superelastic metal, the application of stress does not result in the usual proportional strain seen in other metals. When stress is initially applied to nickel-titanium, the result is proportional strain; however, the strain remains essentially the same as the application of additional stress reaches a specific level, forming what is called a loading plateau. Eventually, of course, application of more stress results in more strain, which increases until the file breaks. This unusual property is the result of a molecular crystalline phase transformation. External stresses transform the austenitic crystalline form of nickel-titanium into a martensitic crystalline structure that can accommodate greater stress without increasing the strain. As a result of its unique crystalline structure, a nickel-titanium file has shape memory, or the ability to return to its original shape after being deformed. Simply stated, nickel-titanium alloys currently are the only readily available, affordable materials with the flexibility and toughness for routine use as effective rotary endodontic files in curved canals.
One study reported that stainless steel was more resistant to fracture than nickel-titanium when angular deflection (fracture by twisting) was measured.145 Attempts to improve the nickel-titanium alloy continue, and it has been shown that the surface characteristics can be greatly improved by treating instrument surfaces. Electropolishing, surface coatings, and surface implantation (Fig. 8-27) have been used for this purpose.170,226