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41 MAY/JUNE 2017 ■ TVPJOURNAL.COM CONTINUING EDUCATION involved permanent teeth to periodontitis and malocclusion. This is a common condition in dogs, especially toy breeds, but uncommon in cats. The treatment of choice is extraction of the persistent deciduous teeth. Although the initial diagnosis is clinical, radiographs are necessary to document the orientation of the root relative to the permanent counterpart and/or other teeth in the immediate vicinity, as well as the degree of root resorption (if present). This information is essential to minimize potential collateral damage when extracting a persistent deciduous tooth. Persistent deciduous teeth without a permanent counterpart ( Figures 1C and 1D ) are usually smaller and slightly more radiolucent than the contralateral, adjacent, and/ or opposing permanent teeth. Clinicians should keep in mind that the morphology of a persistent deciduous premolar resembles that of the permanent tooth immediately distal to it (eg, the deciduous fourth premolar resembles the permanent first molar) to it but is significantly smaller. Clinicians should be also familiar with normal deciduous tooth exfoliation times and know which teeth have a deciduous predecessor (ie, deciduous dental formula). 5,6 Retained (unerupted or embedded) teeth ( Figures 2A and 2B ) are important because they can result in dentigerous cyst formation (see Jaw Lesions of Developmental Origin ) 7 and are, therefore, considered pathologic. Although deciduous teeth may be retained, most retained teeth are permanent. The cause of tooth retention is not always apparent. If a physical barrier (eg, bone, another tooth) did not allow the tooth to erupt, the tooth can be referred to as impacted. Retention of the first premolar tooth appears to be relatively common in brachycephalic dogs, suggesting possible genetic mechanisms. 7 In some cases, historical or radiographic findings may suggest a traumatic origin (eg, local trauma during odontogenesis); some retained teeth may be dysplastic. As retained teeth are not visible clinically, radiographs are necessary to establish a diagnosis. Dental radiographs are indicated whenever there are missing teeth with no obvious cause (eg, previous tooth loss, extraction). Congenitally missing teeth ( Figure 2B ) are considered an incidental finding. The term FIGURE 1. Persistent deciduous teeth. FIGURE 1A shows a persistent right maxillary deciduous canine tooth in a 7-month-old dog; the permanent counterpart is present. FIGURE 1B shows a persistent right maxillary deciduous canine tooth in a 6-month-old cat; the permanent counterpart is present. FIGURE 1C shows a persistent deciduous left maxillary second premolar tooth in a 1-year-old dog; the permanent counterpart is not present. FIGURE 1D shows a persistent deciduous left mandibular fourth premolar tooth in the same dog as Figure 1C; the permanent counterpart is not present. A C B D FIGURE 2. Retained (unerupted or embedded) teeth. FIGURE 2A shows a retained right mandibular first premolar tooth in a 6-year-old dog. FIGURE 2B shows retained left and right mandibular canine teeth in a 4-year-old dog. Note that the left and right mandibular premolar teeth are missing; based on history and breed, the absence of these teeth was considered congenital in origin. A B