Head and Neck Imaging

Odontogenic cyst

jaw cyst arising from a cellular component of the enamel organ. Their general radiographic appearance is a radiolucent area, demarcated by a well corticated border. Large cysts may cause bone expansion, but the cortical plates usually remain intact. Most cysts display a similar behaviour (see bone cyst head and neck). Rarely a malignant tumour may arise from a cyst.

The following types of odontogenic cysts can be distinguished:

radicular cyst (also called dental cyst): most common jaw cyst. It arises at the apex of a nonvital tooth, representing a step in the progressive inflammatory events associated with bacterial invasion, death and degradation of the dental pulp. Radiographically, it resembles very closely a periapical granuloma: the larger the lesion, the more likely it is to be a cyst; lesions with a diameter of more than 2 cm are virtually always a radicular cyst.

follicular cyst (also called dentigerous cyst): forms around the crown of an unerupted, developing tooth. Usually it involves the crown of an unerupted mandibular third molar, maxillary canine or maxillary third molar. Radiographically, an unilocular cyst including the crown is seen (Fig.1).

residual cyst: may develop after partial removal of another odontogenic cyst or after removal or loss of a tooth.

odontogenic keratocyst: develops from the dental lamina, and is characterized by a high recurrence rate after removal. Radiographically, it frequently mimics another odontogenic cyst. These cysts may show undulating borders and an appearance suggesting multilocularity, and may spread into the surrounding soft tissues (keratocyst (VI:2), Fig. 1). Multiple odontogenic keratocysts are seen in basal cell naevus syndrome.

lateral periodontal cyst: arises in the periodontium and extends into the bone between the tooth apex and the alveolar crest (the bony crest between two teeth).

calcifying odontogenic cyst: uncommon, slowly growing benign entity. It is not always cystic and contains proliferating epithelium. Radiographically, a smooth-contoured or irregularly defined radiolucent lesion, that may contain calcifications, is seen; it may mimic several odontogenic tumours.

RH

Fig.1

Axial CT images of mandible. Expansile cyst in the right mandibular angle, containing an unerupted third molar (arrow, a); the cyst extends into the vertical mandibular ramus (b). Follicular cyst.

Odontogenic cyst, Fig.1 (a)		Odontogenic cyst, Fig.1 (b)