Resorption of tooth structure occurs through the action of odontoclasts—cells that are virtually identical to osteoclasts. It can occur on the external root surface or on the internal tooth surface lining the pulp cavity (the pulp chamber and root canal). Odontoclast activity can be stimulated by inflammation, by pressure from adjacent structures, as a result of normal processes such as exfoliation of deciduous teeth, or in the absence of these processes. This idiopathic tooth resorption occurs sporadically in many species (including people), but it is the most frequently seen dental lesion in domestic cats.
Etiology and Pathogenesis
Tooth resorption begins with focal damage to the cementum that covers the root surface. Microscopic areas of root resorption often repair uneventfully in cats. Tooth resorption from any cause occurs through the action of osteoclasts that remove tooth structure, creating a resorption lacuna. In many but not all lesions, concomitant osteoblast activity replaces the lost tooth with bone. Regardless, the resorption progresses through the dentin and can undermine crown enamel to cause clinically apparent defects on the tooth surface. Inflammation from periodontitis is known to cause external resorption and is most likely responsible for tooth resorption in areas of periodontal disease. However, the etiology of idiopathic tooth resorption has not yet been proved. Theories include abfraction (ie, abnormal horizontal forces on the teeth during chewing that cause microflexure and trauma in the cervical region) and nutritional causes (eg, excessive dietary vitamin D), among others.
Clinical Findings and Lesions
Clinical appearance varies. In cats, mandibular third premolars (the first premolar behind the canine tooth) is often the first tooth affected. In dogs, premolar and molar teeth are most commonly involved. Small lesions on the enamel of the tooth crown (“intraoral” lesions) usually begin at the gingival margin, appearing as marginal gingival inflammation or upgrowth of the gingiva onto the tooth crown. Larger lesions appear as obvious defects in the tooth that are filled with granulation tissue. The margin of the defect has a sharp ledge of enamel. At this stage, the visible lesion represents the “tip of the iceberg” with most of the defect affecting the roots or deeper dental tissues. Tooth resorption is characterized by severity (stage) and radiographic appearance (type). Stage 1 lesions affect the cementum but have not yet cavitated into the dentin. Stage 2 lesions affect the dentin but have not progressed to the pulp. Stage 3 lesions affect the pulp. Stage 4 lesions have significant crown or root damage with loss of integrity of the tooth. Stage 5 lesions have complete loss of the entire crown with intact gingiva over the site of the missing crown.
Lesions are categorized radiographically as type 1 when the root of the affected tooth generally retains its normal radiopacity except for the focal resorption lesions themselves, or type 2 when there is general loss of root radiopacity compared with that of roots of adjacent teeth. In severe cases, the roots “disappear” radiographically, or look like “ghosts” of their normal anatomy. This is consistent with replacement of the root with bone or cementum-like tissue.
Tooth resorption lesions that are exposed to the oral cavity may cause discomfort. Lesions that are limited to root surfaces are unlikely to cause discomfort or other clinical signs. Resorption caused by inflammation from periodontal or endodontic disease is associated with signs typical of those problems. Both of these types are accompanied by inflammation and infection, and those caused by endodontic disease also have the potential for causing discomfort.
Marginal gingivitis of individual teeth in the absence of periodontitis may indicate an early subgingival lesion. Lesions under the gingival margin can be identified by sharp dental exploration. Larger lesions are identified by their typical appearance on the tooth surface. Extraoral lesions affecting the root or the internal tissues of the crown are only identifiable radiographically and are visible as areas of decreased radiopacity.
Treatment and Prevention
Most teeth affected with resorptive lesions should be extracted. Surgical crown amputation can be performed on teeth that are radiographically confirmed to be type 2 lesions, and only in patients with no periodontitis or endodontic disease and no evidence of caudal stomatitis (see Diseases of the Mouth in Small Animals). Oral hygiene prevents inflammatory lesions caused by marginal periodontitis, and root canal treatment or extraction of endodontically involved teeth prevents resorption caused by apical periodontitis. Idiopathic lesions cannot be prevented because the etiology is unknown. If abfraction plays a role, then a diet with a consistency similar to that of birds or small rodents would be expected to prevent these lesions, but this has not been shown.
Last full review/revision March 2012 by Jack Easley, DVM, MS, DABVP (Equine); Gregg A. DuPont, DVM, Fellow AVD, DAVDC