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43 MAY/JUNE 2017 ■ TVPJOURNAL.COM CONTINUING EDUCATION Dens-in-dens (dens invaginatus; Figure 5B ) is a rare malformation in which the enamel and underlying dentin invaginate towards the pulp cavity, sometimes resulting in a direct or indirect communication and, in some cases, secondary endodontic disease. The malformation may or may not be clinically evident. Radiographically, it may appear as a small tooth-like structure within the pulp cavity, and endodontic disease (see Endodontic Findings ) is often present. Double teeth ( Figures 6A and 6B ) appear to have two crowns due to gemination or fusion. Gemination occurs when two crowns originate from a single root; fusion occurs when the roots of two independent teeth fuse. Clinically, these conditions are indistinguishable; radiographs are necessary to determine if double teeth are due to gemination or fusion. Regardless, double teeth are most often an incidental finding. Concrescent or fused roots ( Figure 7A ) represent nonpathologic anatomic variations of clinical relevance because they may affect the surgical approach if a tooth requires extraction. The roots of multirooted teeth are usually slightly divergent with alveolar bone in between. However, in some cases, concrescence occurs when the roots of a tooth converge and are only separated by cementum. In other cases, actual fusion of the roots occurs. Dilacerated roots ( Figure 7B ) have an acute angulation at their apical third. Although this is considered a developmental abnormality, it is not usually of clinical significance, unless the tooth has to be extracted for any reason. The extraction may require additional root exposure to avoid fracturing the tooth. Peg teeth are relatively small, permanent mandibular premolar teeth with only one root ( Figure 8 ). These are usually considered an anatomic variation of little or no clinical significance. Jaw Lesions of Developmental Origin Dentigerous cysts, by definition, are associated with unerupted teeth. The most commonly associated tooth is the first mandibular or maxillary premolar in dogs 7 ; dentigerous cysts have not been reported in cats. The cystic lesion is usually visible radiographically as an area of geographic bone loss (see Jaw Lesions ) of varying size; in some cases the lesion involves adjacent teeth ( Figure 9A ). FIGURE 6. Double teeth. FIGURE 6A is a clinical image of a right maxillary first incisor tooth in a 4-year-old dog with seemingly two crowns. FIGURE 6B shows the occlusal radiograph of the same dog; note that the double maxillary incisor corresponds to fusion. A B FIGURE 7. Fused and dilacerated roots. FIGURE 7A shows fused roots at the left mandibular second molar tooth in a 5-year-old dog. FIGURE 7B shows dilacerated roots at the left mandibular first molar tooth in another 5-year-old dog. A B FIGURE 5. Odontodysplasia and dens-in-dens. FIGURE 5A shows a dysplastic root of the right maxillary canine tooth in a 9-year-old dog. Periodontitis of varying severity is present at the incisors, canine, and premolar teeth, as well as external inflammatory tooth resorption at the second premolar. FIGURE 5B shows dens-in-dens affecting the right mandibular first molar tooth of a 5-year-old dog; note the abnormal appearance of the crown just above the furcation area and extending mesially. A B