Radiographs are essential to all phases of endodontic therapy. They inform the diagnosis and the various treatment phases and help evaluate the success or failure of treatment. Because root canal treatment relies on accurate radiographs, it is necessary to master radiographic techniques to achieve films of maximum diagnostic quality. Such mastery minimizes retaking of films and avoids additional exposure of patients. Expertise in radiographic interpretation is essential for recognizing deviations from the norm and for understanding the limitations associated with endodontic radiography
Functions, Requirements, and Limitations of the Radiograph in Endodontics
The primary radiograph used in endodontics is the periapical radiograph. In diagnosis this film is used to identify abnormal conditions in the pulp and periradicular tissues. It is also used to determine the number of roots and canals, location of canals, and root curvatures. Because the radiograph is a two-dimensional image (a major limitation), it is often advantageous to take additional radiographs at different horizontal or vertical angulations when treating multicanal and multirooted teeth. Taking additional radiographs is also helpful when treating teeth with severe root curvature. These supplemental radiographs enhance visualization and evaluation of the three-dimensional structure of the tooth.
Technically, for endodontic purposes, a radiograph should depict the tooth in the center of the films. Consistent film placement in this manner will minimize interpretation errors, because the center of the films contains the least amount of distortion. In addition, at least 3 mm of bone must be visible beyond the apex of the tooth. Failure to capture this bony area may result in misdiagnosis, improper interpretation of the apical extent of a root, or incorrect determination of file lengths for canal cleaning and shaping. Finally, the image on the film must be as anatomically correct as possible. Image shape distortion caused by elongation or foreshortening may lead to interpretive errors during diagnosis and treatment.29,62
The bite-wing radiograph may be useful as a supplemental film. This film normally has less image distortion because of its parallel placement, and it provides critical information on the anatomic crown of the tooth. This information includes the anatomic extent of the pulp chamber, the existence of pulp stones or calcifications, recurrent decay, the depth of existing restorations, and any evidence of previous pulp therapy.60 The bite-wing also indicates the relationship of remaining tooth structure relative to the crestal height of bone. Thus it can aid in determining the restorability of the tooth.
In addition to their diagnostic value, high-quality radiographs are mandatory during the treatment phase. Technique is even more critical, however, because working radiographs must be exposed while the rubber dam system is in place. Visibility is reduced, and the bows of the clamp often restrict precise film positioning. During treatment, periradicular radiographs are used to determine canal working lengths; the location of superimposed objects, canals, and anatomic landmarks (by altering cone angulations); biomechanical instrumentation; and master cone adaptation (see Fig. 5-1, C to F). After completion of the root canal procedure, a radiograph should be exposed to determine the quality of the root canal filling or obturation. Follow-up radiographs exposed at similar angulations enhance assessment of the success or failure of treatment (see Fig. 5-1, I and J).
The astute clinician can perceive that precise radiographic interpretation is undoubtedly one of the most valuable sources of information for endodontic diagnosis and treatment, but the radiograph is only an adjunctive tool and can be misleading. Information gleaned from proper inspection of the radiograph is not always absolute and must always be integrated with information gathered from a thorough medical and dental history, clinical examination, and various pulp-testing procedures (see Chapter 1).
Use of the radiograph depends on an understanding of its limitations and its advantages. The advantages are obvious: the radiograph allows a privileged look inside the jaw. The information it furnishes is essential and cannot be obtained from any other source, and its value is not diminished by a critical appraisal of its limitations.
One major limitation of radiographs is their inability to detect bone destruction or pathosis when it is limited to the cancellous bone. Studies56 have proved that radiolucencies usually do not appear unless there is external or internal erosion of the cortical plate. This factor must be considered in evaluating teeth that become symptomatic but show no radiographic changes. In most cases, root structure anatomically approaches cortical bone and, if the plate is especially thin, radiolucent lesions may be visible before there is significant destruction of the cortical plate. Nevertheless, inflammation and resorption affecting the cortical plates must still be sufficiently extensive before a lesion can be seen on a radiograph.
Principles of Endodontic Radiography
Film Placement and Cone Angulation
For endodontic purposes, the paralleling technique produces the most accurate periradicular radiograph. Also known as the long-cone or right-angle technique, it produces improved images. The film is placed parallel to the long axis of the teeth, and the central beam is directed at right angles to the film and aligned through the root apex (Fig. 5-3, A and B). To achieve this parallel orientation it is often necessary to position the film away from the tooth, toward the middle of the oral cavity, especially when the rubber dam clamp is in position.62 The long-cone (i.e., 16 to 20 inches) aiming device is used in the paralleling technique to increase the focal spot-to-object distance. This has the effect of directing only the most central and parallel rays of the beam to the film and teeth, reducing size distortion.46,48,62 This technique permits a more accurate reproduction of the tooth's dimensions, thus enhancing a determination of the tooth's length and relationship to surrounding anatomic structures.29 In addition, the paralleling technique reduces the possibility of superimposing the zygomatic processes over the apices of maxillary molars, which often occurs with more angulated films, such as those produced by means of the bisecting-angle technique (see Fig. 5-3, C and D). If properly used, the paralleling technique will provide the clinician with films with the least distortion, minimal superimposition, and utmost clarity.
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Figure 5-3 A, Paralleling, or right-angle, technique. B, Projection of the zygomatic process above the buccal root apices with the right-angle technique, allowing visualization of the apices (arrows). C, Bisecting-angle technique. D, Superimposition of the zygomatic process over the buccal root apices of the maxillary first molar with the bisecting-angle technique.
Variations in size and shape of the oral structures (e.g., shallow palatal vault, tori, or extremely long roots) or gagging by the patient can make true parallel placement of the film impossible. To compensate for difficult placement, the film can be positioned so that it diverges as much as 20 degrees from the long axis of the tooth, with minimal longitudinal distortion. With maxillary molars, any increase in vertical angulation increases the chances of superimposing the zygomatic process over the buccal roots. A vertical angle of not more than 15 degrees should usually project the zygomatic process superiorly and away from the molar roots. To help achieve this, a modified paralleling technique18 that increases vertical angulation by 10 to 20 degrees can be used. Although this orientation introduces a small degree of foreshortening, it increases periradicular definition in this troublesome maxillary posterior region. The Dunvale Snapex System (Dunvale Corporation, Gilberts, IL), a film holder and aiming device originally designed for the bisecting-angle technique, has been altered for the modified paralleling technique.18 In conjunction with this technique, a distal angulated radiograph (i.e., a 10- to 20-degree horizontal shift of the cone from the distal, with the beam directed toward the mesial) tends to project buccal roots and the zygomatic process to the mesial, thus enhancing anatomic clarity.18
The bisecting-angle technique is not preferred for endodontic radiography. However, when a modified paralleling technique cannot be used, there may be no choice because of difficult anatomic configurations or patient management problems.18,46,48,62 The basis of this technique is to place the film directly against the teeth without deforming the film (see Fig. 5-3, C and D). The structure of the teeth, however, is such that with the film in this position, an obvious angle exists between the plane of the film and the long axis of the teeth. This causes distortion, because the tooth is not parallel to the film. If the x-ray beam is directed at a right angle to the film, the image on the film will be shorter than the actual tooth (i.e., foreshortened). If the beam is directed perpendicularly to the long axis of the teeth, the image will be much longer than the tooth (i.e., elongated). Thus, by directing the central beam perpendicular to an imaginary line that bisects the angle between tooth and film, the length of the tooth's image on the film should be the same as the actual length of the tooth.
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Figure 5-4 A, With the paralleling technique, the tube head is positioned at a 90-degree angle to the film. The hemostat aids in film placement and in cone alignment. Note that the hemostat is resting on the mandibular anterior teeth so that the film is parallel with the long axis of the maxillary central incisors. B, Releasing a corner of the rubber dam aids in hemostat placement so that the film can be properly aligned. (ACourtesy Dr. Eddy Tidwell.BCourtesy Dr. Michelle Speier.)
Although the projected length of the tooth is correct, the image will show distortion because the film and object are not parallel and the x-ray beam is not directed at right angles to both. This distortion increases along the image toward its apical extent. The technique produces additional error potential, because the clinician must imagine the line bisecting the angle (an angle that, in itself, is difficult to assess). In addition to producing more frequent superimposition of the zygomatic arch over apices of maxillary molars, the bisecting-angle technique causes greater image distortion than the paralleling technique and makes it difficult for the operator to reproduce radiographs at similar angulations to assess healing after root canal treatment27 (see Fig. 5-3, C and D).
Film Holders and Aiming Devices
Film holders and aiming devices are required for the paralleling technique because they reduce geometric distortion caused by misorientation of the film, central beam, and tooth.18,46,48,60,62 They also minimize cone cutting, improve diagnostic quality, and allow similarly angulated radiographs to be taken during treatment and at recall. By eliminating the patient's finger from the x-ray field and thus the potential for displacing the film, these devices help to minimize retakes and make it easier for the patient and clinician to properly position the film.
A number of commercial devices are available that position the film parallel and at various distances from the teeth, but one of the most versatile film-holding devices is the hemostat. The clinician positions a hemostat-held film, and the handle is used to align the cone vertically and horizontally. The patient then holds the hemostat in the same position, and the cone is positioned at a 90-degree angle to the film (Fig. 5-4, A). When taking working radiographs, a radiolucent, plastic, rubber dam frame, such as an Ostby or Young frame, should be used and not removed. To position the hemostat or other film-holding device, a corner of the rubber dam is released for visibility and to allow the subsequent placement of the device-held film (Fig. 5-4, B). The Stabe disposable film holder (Dentsply Rinn Corporation, Elgin, IL) (Fig. 5-5) is another film-holding device that is ideal for taking preoperative and postoperative films.
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Figure 5-5 Examples of XCP® Film Holding Devices. Left to right, XCP Bite-Block, Stabe Bite-Block, EZ-Prop mouth prop, bite-wing loops, and adhesive bite-wing tabs. (Courtesy Dentsply Rinn, Elgin, IL.)
Besides the Dunvale Snapex System mentioned earlier, the major commercial film-holding and aiming devices include the XCP (extension cone paralleling) instruments, the Endo Ray II endodontic film holder, the Uni-Bite film holder, the Snap-H-Ray film holder, the Snapex System film holder with aiming device, and the Crawford Film Holder System (Figs. 5-6 to 5-10).
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Figure 5-6 XCP instruments hold the radiograph film packets and aid in cone alignment. Cone cutting is prevented, and consistent angulation can be achieved. (Courtesy Dentsply Rinn, Elgin, IL.)
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Figure 5-7 Snapex® film holder and aiming ring. The biting portion of the instrument is reduced to make it easier to place the instrument around the rubber dam. (Courtesy Dentsply Rinn, Elgin, IL.)
Variations in the use of the XCP system, for example, can prevent displacement of the rubber dam clamp and increase periradicular coverage during endodontic procedures. The film is placed off center in the bite block, and the cone is placed off center with respect to the aiming ring. This allows for placement of the bite block adjacent to the rubber dam clamp without altering the parallel relation of the cone to the film. A customized hemostat (with rubber bite block attached) can also be made to assist film placement during the taking of working radiographs. Other specialized film holders, such as the EndoRay and the Crawford Film Holder System, have been designed to help the dentist secure parallel working films with the rubber dam clamp in place. Generally these holders all have an x-ray beam-guiding device for proper beam/film relationship and a modified bite block and film holder for proper positioning over or around the rubber dam clamp (Fig. 5-10).
Figure 5-8 Snap-H-Ray® Film Holder. (Courtesy Dentsply Rinn, Elgin, IL.)
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Figure 5-9 Crawford Film Holder System. Components include Kelly hemostat with aiming rod (attached), aiming ring, and bite block. (Courtesy Dr. Frank Crawford.)
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Figure 5-10 Endo Ray II®: a film-holding device designed specifically for endodontic radiography. It fits over files, clamps, and dental dams without touching the subject tooth. (Courtesy Dentsply Rinn, Elgin, IL.)
The intricacies of proper kilovoltage, milliamperage, and time selection serve as examples of how the diagnostic quality of a film may be altered by changes in the film's density and contrast.48,62 Density is the degree of darkening of the film, whereas contrast is the difference between densities. The amount of darkening depends on the quantity and quality of radiation delivered to the film, the subject thickness, and the developing or processing conditions. Milliamperage controls the electron flow from cathode to anode; the greater the electron flow per unit of time, the greater will be the quantity of radiation produced. Proper density is primarily a function of milliamperage and time. Kilovoltage also affects film density by controlling the quality and penetrability of the rays. Higher kilovoltage settings produce shorter wavelengths that are more penetrating than the longer wavelengths produced at lower settings.48,62 The ability to control the penetrability of the rays by alterations in kilovoltage affects the amount of radiation reaching the film and the degree of darkening or density. Altering exposure time or milliamperage or both for each respective unit can control variations in density.48,62
Contrast is defined as the difference between shades of gray or the difference between densities. Most variation observed in endodontic radiography is because of subject contrast, which depends on the thickness and density of the subject and the kilovoltage used. Thus kilovoltage is really the only exposure parameter under the clinician's control that directly affects subject contrast.29,48,62 Exposure time and milliamperage only control the number of x-rays; therefore they mainly influence the density of the film image. A radiographic film may exhibit a long scale, or low, contrast (i.e., more shades of gray or more useful densities); high-kilovoltage techniques (e.g., 90 kVp) produce this long scale of contrast as a result of the increased penetrating power of the rays. This results in images with many more shades of gray and less distinct differences. Films exposed at low kilovoltage settings (e.g., 60 kVp) exhibit short-scale, or high, contrast, with sharp differences between a few shades of gray, black, and white.48,62 Although they are perhaps more difficult to read, films exposed at higher kilovoltage settings (e.g., 90 kVp) make it possible to discriminate between images, often enhancing diagnostic quality; films exposed at a lower kilovoltage (e.g., 70 kVp) have better clarity and contrast between radiopaque and radiolucent structures, such as endodontic instruments near the root apex. Nevertheless, the optimal kilovoltage and exposure time should be individualized for each radiograph unit and exposure requirement.