Odontogenic cysts are derived from the oral epithelium associated with the development of the dental apparatus. In human oral pathology, cysts are histologically classified, depending on differentiation and development of cells and/or tissues, into primordial, dentigerous (follicular), periodontal, gingival, calcifying odontogenic cyst, or odontogenic keratocyst. Unfortunately, this classification has not been satisfactory for clinicians.

The dentigerous (follicular) cyst is the most common type of cyst in humans. This cyst is associated with the crown of an impacted permanent tooth. An eruption cyst is frequently associated with erupting deciduous or permanent teeth in children. Histologically, it is defined as epithelialized cavities containing fluid or semisolid material, associated with a developing dental apparatus. It is believed that the epithelium associated with odontogenic cysts can be derived from the tooth germ, the reduced enamel epithelium, the epithelial rests of Malassez, remnants of the sheath of Hertwig, remnants of the dental lamina, or possibly the basal layer of oral epithelium.

In small animal dentistry, odontogenic cysts have not been well documented. In dogs, dentigerous cysts (including eruption cysts) are the most common odontogenic cysts. However, cysts may be less frequently reported than other tumors in the canine oral cavity because the incidence is lower, or clinically, they were misdiagnosed as missing teeth with subtle or no clinical signs.

Dentigerous cysts, including eruption cysts, were observed in 12 dogs since 1996. All the cases were found in small breeds: one Toy Poodle, one Maltese, one Miniature Schnauzer, and nine brachycephalic breeds (e.g., Pug, Pekingese, Shih Tzu) or their crossbreeds. No dentigerous cysts have been found in middle or large breeds dogs, or in dolicocephalic breeds of any size. Most of the dentigerous cysts associated with mal-oriented teeth were found in dogs over five years of age, and two cases of eruption cysts were found in puppies less than one year of age. Many cases were found during routine prophylaxis with subtle or no clinical signs, while some showed gingival swelling, discomfort, and/or anorexia. Constant bleeding from a fistulated ruptured cyst and interrupted swellings were observed. The lower first molar was the most commonly affected, followed by anterior deciduous teeth and permanent canines.

All cysts were filled with mucous fluid, containing crystalline, hemosiderin/hemoglobin, blood and/or cell clusters. The amount of cystic fluid present depended on the extent of bone resorption and the presence of drainage. Histological findings include a thickened gingival epithelium and 2–4 layers of non-keratinized epithelium lining the cystic wall, with or without inflammation. The Schiff positive basement membrane was observed as a boundary between the epithelial lining and the connective tissue. Brachycephalic breeds also had remarkably thickened gingiva.

Radiological findings showed the crown of an unerupted deciduous or permanent teeth contained in the radiolucent area while the roots were anchored in alveolar bone. Severe bone resorption around unerupted teeth
was often associated with mal-orientation of impacted teeth. In some cases, the radiolucent area extended to the level of the mental foramen and just distal to the lower canines. Eruption cysts associated with the deciduous dentition were not associated with bone resorption, but deciduous root resorption did occur. In one puppy, asymmetric development of the maxilla and mandible was observed, resulting in a wry occlusion.

The dentigerous cysts were treated by gingivoplasty, surgical orthodontic treatment, and extraction with debridement of the cyst wall. The cystic cavities that were a result of severe bone resorption were filled with perioglass (Consil®, Nutromax). The prognoses were satisfactory. Recurrence did not occur.

Premolars and incisors are often missing in small breed dogs. This study suggests that clinically missing teeth, especially the first premolar in small breed dogs, should be examined radiologically. If any impacted teeth are found, gingival resection, extraction, or orthodontic correction is indicated. Predisposition of dentigerous cysts in brachycephalic breeds is not clear from this survey. One could speculate that their mandibular and gingival anatomical morphology may be contributing factors.

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