Resorption of tooth structure occurs through the action of odontoclasts—cells virtually identical to osteoclasts. It can occur on the external or internal tooth surface (external or internal resorption). Odontoclast activity can be stimulated by inflammation, pressure from adjacent structures, orthodontic tooth movement, as a result of normal processes such as exfoliation of deciduous teeth, or in the absence of these processes (idiopathic). This idiopathic tooth resorption occurs sporadically in many species (including people), but it is most frequently seen in domestic cats.
Tooth resorption in cats begins with a loss of the normal periodontal ligament architecture and 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 odontoclasts that remove tooth structure, creating a resorptive lacuna. In many but not all lesions, concomitant osteoblast and cementoblastic activity replaces the lost tooth with bone or cementum. If repair does not occur, the resorption progresses into dentin and extends coronally into the crown of the tooth where it undermines the enamel to cause clinically apparent defects on the tooth surface (at the "neck" of the tooth). 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 affecting multiple (possibly all) teeth in cats has not yet been proved. Excessive intake of dietary vitamin D has been hypothesized as one possible cause among many others.
The clinical appearance of tooth resorption greatly varies. In cats, the mandibular third premolar (the first cheek 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 usually begin somewhere on the root surface but can progress coronally and then appear at the gingival margin as inflamed granulation tissue filling a defect. The margin of the defect has a sharp ledge of enamel. At this stage, the visible part of the lesion is small, with most of the defect affecting the roots. Tooth resorption is characterized by severity (stage) and radiographic appearance (type).
Stage 1 lesions affect the cementum or cementum and enamel but have not yet progressed into the dentin.
Stage 2 lesions affect the dentin but have not yet progressed into the pulp cavity.
Stage 3 lesions affect the pulp cavity, but most of the tooth retains its integrity.
Stage 4 lesions have significant crown or root damage, with most of the tooth having lost its integrity.
Stage 5 lesions have remnants of dental hard tissue visible only as irregular radiopacities, and gingival covering is complete.
Lesions are categorized radiographically as type 1 when a focal or multifocal radiolucency is present in the tooth with otherwise normal radiopacity and normal periodontal ligament space (inflammatory resorption), type 2 when there is narrowing or disappearance of the periodontal ligament space (dentoalveolar ankylosis) in at least some areas and decreased radiopacity of part of the tooth (replacement resorption, moth-eaten "ghost" roots), or type 3 when features of both type 1 and type 2 are present in the same tooth.
Tooth resorption lesions exposed to the oral cavity may cause discomfort. Lesions limited to root surfaces only are unlikely to cause discomfort or other clinical signs unless they are associated with resorption of bone adjacent to the tooth resorption (eg, resorption caused by painful inflammation from periodontal or endodontic disease).
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. Internal resorption may sometimes appear as pinkish discoloration of the crown but usually is only identifiable radiographically as round to oval-shaped areas of decreased radiopacity.
Most teeth affected with resorptive lesions should be extracted. Surgical crown amputation with intentional retention of already resorbing dental tissues can be performed on teeth that are radiographically confirmed to be type 2 lesions in the absence of periodontitis, endodontic disease, and stomatitis (see Diseases of the Mouth in Small Animals). Oral hygiene prevents inflammatory lesions caused by marginal periodontitis, and root canal therapy or extraction of endodontically involved teeth prevents resorption caused by apical periodontitis. Idiopathic lesions cannot be prevented, because their etiology is unknown.