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42 IMAGING ESSENTIALS PEER REVIEWED hypodontia is applied when several teeth are absent; the term oligodontia is a relative term that can be used when only a few teeth are present. 8 Congenitally missing teeth should be suspected if dental radiographs do not show retained and/or resorbing roots, unerupted teeth, or vacated and/or remodeling alveoli. Crowding and rotation ( Figure 3A ) may be considered normal or part of the standard in certain breeds (eg, brachycephalic dogs). Teeth present in severely crowded areas may be rotated owing to the lack of space; this is particularly common with the maxillary third premolar tooth of dogs with maxillary brachygnathia. Teeth that are in close proximity represent a plaque- retentive area and may therefore predispose an animal to focal periodontitis. Although crowding and rotation can be appreciated clinically, dental radiographs are useful to document the periodontal status of the teeth involved ( Box 2 ). Supernumerary teeth (polydontia) ( Figure 3B ) can be present at any location in the dental arches. More than one supernumerary tooth can be present in the same area. Other than creating a plaque- retentive area and, therefore, predisposing the animal to periodontal disease, supernumerary teeth are usually considered a normal anatomic variation. Malformations Enamel hypoplasia ( Figures 4A and 4B ) is considered pathologic; affected areas are plaque retentive and predispose affected teeth to caries and periodontal disease. The defects can be seen clinically and radiographically as irregularities in the enamel. Teeth with enamel hypoplasia can also have dysplastic roots that are only detectable radiographically. 9 Dysplasia (odontodysplasia) ( Figure 5A ) of the crowns of erupted teeth is evident clinically; however, malformation of roots or of unerupted teeth is only detectable radiographically. If only one or a group of adjacent teeth are malformed, local trauma or infection during odontogenesis is suspected as the cause. If odontodysplasia is generalized or semigeneralized, systemic acquired or congenital causes are suspected. BOX 2. Determining Periodontal Status Radiographs are useful to establish the severity of periodontitis. In general, periodontitis is classified as: • Mild if <25% of alveolar bone has been lost • Moderate if 25% to 50% of alveolar bone has been lost • Severe if >50% of alveolar bone has been lost In many cases, the severity and extent of periodontitis on radiographs determine prognosis and treatment choice (ie, extraction, periodontal surgery, or conservative treatment). As a general rule, if severe periodontitis is present, extraction of the affected tooth is indicated. FIGURE 4. Enamel hypoplasia. FIGURE 4A shows the clinical appearance of enamel hypoplasia affecting the maxillary incisors and canine teeth in a 7-month-old dog. FIGURE 4B is a lateral projection of the left maxillary canine tooth on the same dog; note the defective enamel at the mesial aspect of the cusp. A B FIGURE 3. Crowding and supernumerary teeth. FIGURE 3A shows crowding of premolars with rotation and palatoversion of the right maxillary third premolar tooth in a 6-year-old dog. FIGURE 3B shows a supernumerary right maxillary first premolar tooth, as well as crowding of premolars with rotation and palatoversion of the right maxillary third premolar tooth in a 3-year-old dog. A B