Figure 1: Tooth resorption of tooth 107 in a cat. IMAGES COURTESY HTTPS://AVDC.ORG/AVDC-NOMENCLATURE Cats almost never get cavities, but they are prone to get cavity-like tooth defects called tooth resorption. Tooth resorption has had many different names through the years, including cervical line lesions, neck lesions, feline odontoclastic resorptive lesions (FORL) and “feline odontolysis.”1 Recent studies have helped clarify this disease starts on the root surface and not specifically at the cervical portion of the tooth, so we now simply call it “tooth resorption” (TR). TR is common in cats and less common in dogs. Studies have found a prevalence rate of 20 to 70 percent in cats, depending on the population of cats and the investigative methods utilized. Tooth resorption can be classified as internal resorption or external resorption. Internal resorption occurs within the endodontic system, which can be seen radiographically as a focal widening of the root canal system. When we are speaking of tooth resorption in cats and dogs, the type of resorption that most commonly occurs is external resorption. It occurs on the root surface and may extend onto the crown of the tooth, and if deep enough, into the endodontic system of the tooth. Diagnosing tooth resorption The lesions are noted clinically at the cervical portion, or the “neck” of the tooth (Figure 1), or they are noted to be affecting the roots on dental radiographs. Histological studies have found these lesions begin on the root surface, and radiographic changes can often be seen before a clinical lesion is obvious. When a lesion develops at the gingival margin, the adjacent gingiva often covers these lesions with a combination of hyperplastic gingiva and granulation tissue. A fine-tipped explorer (ODU 11/12 or Tufts 17 explorers are more sensitive tools than the traditional Shepherd’s hook 23) should be used to check for irregularities where the explorer catches on a rough defect with distinct borders. Dental radiographs of these teeth are needed to evaluate the severity of resorption and to guide treatment. Radiographic types of tooth resorption There are three radiographic types of external tooth resorption, and learning the distinction between these types is clinically important. On a radiograph of a tooth with type 1 appearance, a focal or multifocal radiolucency is present in the tooth with otherwise normal radiodensity and normal periodontal ligament space. A radiograph of a tooth with type 2 appearance shows narrowing or disappearance of the periodontal ligament space in at least some areas and decreased radiodensity of part of the tooth (Figure 2). A radiograph of a tooth with type 3 appearance shows features of both type 1 and type 2 present in the same tooth. For example, the mesial root of tooth 308 may show a focal type 1 lesion whereas the distal root shows clear evidence of root replacement suggestive of type 2 tooth resorption. Radiographic appearance of tooth resorption. Type 1 radiographic appearance of resorption describes a focal area of loss of tooth structure with normal root density. Type 2 radiographic appearance shows a generalized loss of root density, loss of periodontal ligament space and decreased ability to discern the endodontic system within the roots. Type 3 radiographic appearance shows evidence of a type 1 lesion on one area and type 2 appearance of an adjacent root of the same tooth. Stages of tooth resorption Know there is a staging system developed to describe the severity of the resorption, but clinically this staging system need not be memorized. (It can be obtained at https://avdc.org/avdc-nomenclature.) The important thing to remember is careful assessment of how close to the gingival margin the resorption is. If the resorption is breaking out at the gingival margin when the tooth is evaluated with an explorer, the tooth definitely needs to be removed. If resorption cannot be detected at the gingival margin, but if radiographically the resorption seems to be occurring very close to the alveolar crest, this tooth should probably be removed. If resorption is not detectable at the gingival margin, but radiographically is so severe the entire root has been replaced, radiographically evident tooth structure should be removed. What about dogs? Some dentists feel the prevalence of tooth resorption in dogs is on the rise, though it may just be we are detecting it more frequently due to the increasing popularity of dental radiography. One study found tooth resorption in 120 of 224 (53.6 percent) dogs and 943 of 8,478 (11.1 percent) teeth when evaluating full-mouth radiographs. There may also be a breed predilection, with older large-breed dogs being more commonly affected.2 The most common radiographic type of resorption in dogs is external root replacement resorption.2 I find middle-aged and older dogs with a history of being heavy chewers will often have widespread root replacement resorption of their premolar teeth as an incidental finding on dental radiographs. Next month, we will delve into possible causes and treatments of tooth resorption in cats and dogs. John Lewis, VMD, DAVDC, FF-OMFS practices and teaches at Veterinary Dentistry Specialists and Silo Academy Education Center, both located in Chadds Ford, Pa. References Reiter AM. Feline “odontolysis” in the 1920’s: the forgotten histopathological study of feline odontoclastic resorptive lesions (FORL). J Vet Dent. 1998;15(1):35-41. Peralta S, Verstraete FJ, Kass PH. Radiographic evaluation of the types of tooth resorption in dogs. Am J Vet Res. 2010;71(7):784-93.
