Endodontics requires radiographic imaging for diagnosis, treatment planning, therapy, and follow-up. Dental radiography allows for the identification of pathologic changes in the periradicular tissues that cannot be visualized by clinical inspection. For the precise execution of endodontic therapy, regular radiographic verification of individual treatment steps is necessary. As a review for clinicians, normal and pathologic findings relevant to Endodontics are presented. Key radiographic imaging techniques, such as the paralleling and bisecting techniques, as well as horizontal and vertical eccentric radiographs, are discussed. The increasing utilization and impact of cone-beam computed tomography providing 3-dimensional volume imaging are reviewed.
Key points
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Periapical radiographs are the most commonly used modality in Endodontics. The paralleling technique is preferred.
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Angulated radiographs are frequently used to identify superimposed objects or fractures. The SLOB rule (same lingual opposite buccal) is a standard technique.
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Cone-beam computed tomography is increasingly used in Endodontics. Its benefit derives from the ability to provide 3-dimensional image volumes.
Introduction
Radiography is an integral part of Endodontics. Radiographs are used for prevention, diagnostics, therapy, and follow-up. Shortly after Dr Otto Walkhoff took the first dental radiograph of his teeth in 1895, Dr Edmund Kells first determined endodontic working length by using dental x-rays in 1899, and in 1900, Dr Weston Price first suggested the use of radiographs to evaluate the adequacy of root canal fillings.
Periapical radiographs are the most frequently used type of radiographs for endodontic treatment. Bitewing radiographs are often taken to evaluate restorability before initiating treatment or to check for coronal leakage and decay. Occlusal and lateral cephalometric radiographs are used after dental and facial trauma to identify root or alveolar fractures by providing additional views compared with periapical or panoramic radiographs. The introduction of cone-beam computed tomography (CBCT) provides a 3-dimensional (3D) assessment of oral structures. It is now widely used in addition to periapical radiographs or instead of some traditional imaging techniques, such as occlusal radiographs.
This article provides a practical guide for the most common clinical applications for radiology in Endodontics, focusing on digital periapical radiographs and CBCT imaging.
Periapical radiographs
Periapical radiographs are a type of 2-dimensional (2D) radiograph imaging, arguably the most important and widely used radiographic technique in Endodontics. For decades, conventional films had been used until the modern digital era in dental radiography started with the introduction of the RadioVisioGraphy system in 1989. The digital systems rely on electronic detection of an radiograph-generated image processed and then reproduced on a computer screen. Various digital imaging modalities are currently available.
Digital radiography is firmly established as an indispensable diagnostic tool in endodontic practice. It demonstrated to be an excellent asset for Endodontics because of the number of radiographs indicated before, during, and after an endodontic procedure. Its benefits include a significant reduction in overall radiation dosage, increased speed of obtaining high-resolution digital images, the possibility of digital enhancement and ease of transmissibility, the elimination of manual processing steps and chemical waste, as well as digital data storage. The introduction of digital radiography allowed for a variety of image enhancements and modifications, including inversion, contrast, flashlight, magnification, pseudo colors, and digital measurements of root lengths and curvature angles.
However, periapical radiographs have limitations because of the 2D nature of the images produced, geometric distortions, and anatomic noise. A 2D projection of a 3D object can only provide suggestive and not final evidence in judging a clinical problem. The most common clinical problem is the difficulty of assessing any bucco-lingual dimensions, which can only be indirectly assessed by periapical radiographs through eccentric radiographs. Also, the bacterial status of hard and soft tissues cannot be determined, and inflammatory tissues cannot be differentiated from healed fibrous scar tissue. Last, radiographs do not provide information about the true nature of the tissue that replaced the bone. Abscesses, granulomas, or cysts resemble radiographically identical osteolytic lesions in a great majority of situations. Lesions in the medullary bone are undetected in the radiographs until there are substantial bone loss and cortical bone involvement. For a hard tissue lesion to be evident on a radiograph, there should be at least a mineral bone loss of 6.6%.
Paralleling and Bisecting Techniques
The paralleling technique ( Fig.1A ) is primarily recommended for endodontic periapical radiographs. It allows for projections with minimal geometric distortions and has a high level of reproducibility, which is beneficial for comparison with other radiographs throughout a procedure. Briefly, a sensor is placed parallel to the long axis of the tooth undergoing treatment and exposed using radiographs perpendicular to the sensor surface. Special sensor holding devices, such as radiograph holders or hemostats, are required to align the sensor precisely with the radiograph tube. In the maxilla, the sensor may have to be placed at the palatal vault’s height in the midline and in the mandible must displace the tongue toward the midline. Compromises may be necessary for patients with limited mouth opening, a severe gag reflex, or poor tolerance to the sensor.
The bisecting angle technique ( Fig.1B ) lets radiographs pass perpendicular to the angle bisector of the angle formed by the tooth’s long axis and the radiograph sensor. No holding devices are required. For conventional periapical and bitewing radiography, this technique is unreliable to achieve geometric accuracy, and distortions of anatomy are common. It is difficult to use rectangular collimation with this technique, as extension cone paralleling techniques are used with rectangular collimation to achieve maximum geometric sharpness and increased contrast of the resultant image. Although, when done with proper technique, it produces an only minimal distortion of the tooth length on the resultant images, the superimposition of adjacent anatomic landmarks or pathologic features may lead to difficulties in interpretation. For example, the superimposition of the maxilla’s zygomatic process over the root apices of molar teeth will often occur, which results in a characteristic radiopacity that renders interpretation difficult.
Although similar diagnostic results are achievable with either technique, more studies favor the paralleling technique for effectiveness and superior diagnostic quality. , Whether a radiograph with good quality can be obtained will depend on the proper sensor placement in the patient’s mouth and the correct angulation of the radiograph cone in relation to the sensor and oral structures. Proper exposure time and intensity of the radiograph beam must be chosen.
Adjustments in Vertical and Horizontal Angulation
Changes in horizontal radiograph angulation are employed by using the same lingual opposite buccal rule (SLOB), a technique helpful in identifying the relative spatial or buccal-lingual location of an object within the tooth or alveolus ( Fig. 2 ). It combines an orthoradial periapical radiograph taken at zero horizontal angulation with additional mesial and/or distal eccentric radiographs. The orthoradial radiograph may lead to the superimposition of buccal and palatal objects on the sensor ( Fig. 3 A). In Endodontics, these objects include roots, canals, instruments, or foreign objects. A secondary radiograph is then acquired with a slightly altered horizontal angulation of the radiograph beam ( Fig. 3 B). An object closest to buccal will appear to move in the opposite direction of the movement of the radiograph tube head, when compared with the orthoradial image. In turn, an object closest to lingual will appear to move in the direction of the movement of the radiograph tube head. For example, in the case of a working length film, if the eccentric direction of the radiograph beam is directed from mesial, a lingual canal will appear mesially on the image. In contrast, if the radiograph beam is directed from distal, the lingual will appear distally. Other than its utilization during root canal treatment, the SLOB rule can be applied to verify the presence or absence of foreign bodies or periapical lesion if radiopaque or radiolucent shadows are superimposed on an orthoradial radiograph.
In trauma, both vertical and horizontal angulations are of importance. Angulation changes may reveal a fracture line that otherwise could be concealed by hard tissue structures if the radiograph beam hits the fracture line within a ±3° angle. Horizontal angulation changes of 10° to 15° from the orthoradial direction should be used to identify a vertical crown, root, or alveolar fracture. Similarly, changes in vertical radiograph beam angulation are used for horizontal fractures. However, it must be appreciated that increases in vertical angulation will lead to a shortening of a tooth’s length. Buccal roots will appear shorter than lingual roots in multirooted teeth, as they are at a greater distance from the sensor. A more accurate visualization of lingual roots and their apices is possible by increasing the vertical angulation. Increasing the vertical angulation also alters the vertical relationship of anatomic landmarks and root apices. This effect can determine whether anatomic landmarks lie buccally or lingually, an assessment that has benefit during endodontic surgery. Last, identifying a “double periodontal ligament” by using a 20° horizontal angulation may hint at additional canals in a root with an hourglass cross-section.
Sequence of Periapical Radiographs in Root Canal Therapy
Periapical radiographs are essential for endodontic therapy. They are used for diagnosis, preoperative assessment and patient communication, interpretation of root and root canal system morphology, verification of procedural steps, postoperative assessment of the root filling (obturation), as well as the long-term evaluation of the treatment outcome (follow-up).
Diagnostic and preoperative radiographs
Periapical radiographs intended for endodontic therapy must include the complete area of interest with the full length of the root and at least 3 mm of periapical bone ( Fig. 4 A, B). Ideally, these radiographs should be taken using the paralleling technique, which provides a consistently high quality without shortening or elongation. One radiograph may be sufficient for a single-rooted tooth. For a multirooted tooth, roots and the root canal system may become superimposed (see Fig. 4 A). A second radiograph with the radiograph beam shifted mesially or distally following the SLOB rule should be taken. A bitewing image may be necessary to assess restorability (see Fig. 4 B).
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