Interpreting dental radiographs in dogs and cats can be challenging, particularly when assessing teeth for possible endodontic disease. The classic radiographic signs of endodontic disease include: 1) a wider than normal root canal, 2) periapical resorption and/or internal resorption of the root and 3) lucency of the bone surrounding the root apex, a.k.a. “periapical lucency.” Let’s discuss each of these radiographic signs individually: 1) A wide canal A wider than normal root canal is a reliable and specific sign of an endodontically diseased tooth, but the sensitivity of this finding is low, because it takes a long time for an endodontically diseased tooth to show evidence of this radiographic sign. Think of a tooth as a tree, except the rings of the tree are laid down internally. The diameter of the tooth does not change, but the width of the canal gets narrower over time as a vital, living tooth continues to age. The hard tissue, dentin, is produced throughout the life of the tooth by cells lining the pulp called odontoblasts. Odontoblasts produce primary dentin when the tooth is developing. Secondary dentin is produced after eruption and throughout the life of the tooth. Odontoblasts can even produce a reparative dentin (tertiary dentin) in response to wear of the tooth. If the cells within the pulp die due to endodontic disease, the lack of odontoblasts results in a cessation of maturation of the tooth and a wider canal than the contralateral tooth. It takes months for this discrepancy in canal width to be radiographically evident. 2) Tooth resorption Resorption of the hard tissue of the root can occur in the area of the apical delta or anywhere within the canal as a result of infection and inflammation. When it occurs within the canal, internal resorption manifests radiographically as a focal widening of the canal, often in the mid-root area. When resorption occurs around the apex of the root, it results in an irregular surface of the normally smooth, dense hard tissue of the tooth root. 3) Periapical lucency Periapical lucency can be evidence of periapical infection/inflammation. Normally, the apex of the root is surrounded by a narrow periodontal ligament space, which is radiolucent on radiographs. Adjacent to the periodontal ligament space, there is a thin layer of radiodense bone called lamina dura. When infection becomes established in the apical portion of the root, the bone of the lamina dura is lost, resulting in a periapical lucency that is often wider than the diameter of the tip of the root. Though this appearance is often referred to as a “tooth root abscess,” in reality, abscess is only one of the many possible reasons for a periapical lucency. Perhaps the most common cause of periapical lucency is a granuloma rather than a true abscess. Granulomas can sometimes be seen as a large ball of soft tissue attached to the root when extracting an endodontically diseased tooth (Figure 1). Periapical cysts can also cause a periapical lucency, but unlike abscesses or granulomas, a cyst often has a corticated border around the periapical lucency. Figure 1. Granuloma at the root tip (apex) of an endodontically diseased molar. Photo courtesy Dr. John R. Lewis One of the most commonly mistaken normal variants that can mimic a pathologic periapical lucency is the chevron radiolucency. Frequently seen at the apices of canine teeth, molars and maxillary incisors of dogs, the chevron lucency represents trabecular bone and vascular channels rather than true periapical lysis.1 Other normal jaw structures may appear on radiographs and mimic pathology. In the maxillary canine and premolar region in dogs, the junction of the vertical body of the maxilla with its palatine process and the maxillary conchal crest—can mimic a cystic structure and interfere with evaluating apical structures. In the mandible, the canal and various mental foramina are normal radiolucent structures that may superimpose over a root, depending on the angle and exposure. Distinguishing normal variants from true endodontic disease How can we determine if a lucency is pathologic or anatomic? Sometimes, even for board-certified dentists who interpret dental radiographs daily, it can be difficult to determine. Chevron lucencies usually extend apically at the same width of the apex instead of expanding beyond the dimensions of the apex, and are surrounded by a distinct periodontal ligament and lamina dura. Figure 2. Chevron lucencies seen at the distal root of tooth 108 and palatal root of 110. Note the lucency is not wider than the diameter of the root tip. Photo courtesy Dr. John R. Lewis Normal anatomical variations often present with regular, well-defined margins. Pathologic changes, by contrast, tend to be mottled, irregular, or “moth-eaten” in character. Normal variants are typically bilaterally symmetrical or follow predictable patterns related to jaw or tooth anatomy. Disease processes usually involve asymmetric destruction or expansion of bone or tooth structure. Clinical clues matter. When trying to determine if radiographic findings are truly significant, evaluate for signs such as tooth abrasion, discoloration (intrinsic staining), fracture, pulp exposure, swelling over the roots, and draining tracts. Normal anatomical variants often occur in teeth that have no other signs of trauma. If there are no clinical signs of disease, consider monitoring the tooth with serial radiographs to look for changes over time, which will distinguish stable anatomical variants from progressive pathologies. I hope this helps you with some of your future radiographic challenges. In next month’s column, we will continue to discuss diagnostic imaging modalities of the mouth, head, and neck. John R. Lewis, VMD, DAVDC, FF-OMFS practices at Veterinary Dentistry Specialists and teaches at Silo Academy Education Center, both in Chadds Ford, Pa. Reference DuPont GA, DeBowes LJ. Atlas of Dental Radiography in Dogs and Cats. Saunders Elsevier; St. Louis (2009): p. 150.
