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A unilocular radiolucency associated with an endodontically treated tooth should make one think of a periapical cyst. Another cyst to consider would be an odontogenic keratocyst. Lack of expansion would be more consistent with OKC but periapical cyst would be certainly considered given the endodotically treated tooth present in the area. Pain is described in both cysts if they are infected.
Given the radiographic findings of a unilocular and well-defined radiolucency one has to consider a cyst(s) on the differential diagnosis. Being closely associated with the apex of an endodontically treated tooth #20, a radicular cyst would be a reasonable consideration. The histology is not supportive of a periapical cyst.
Apical periodontal cyst is also known by a variety of other names, including radicular and periapical cyst. It is an inflammatory cyst and is the most common odontogenic cyst. It is the result of pulpitis/pulp necrosis, which are mainly caused by a badly decayed tooth but can also be the result of tooth fracture, failed endodontic treatment or an old filling with secondary caries. It can occur at any age but is more common in adults in their fourth and fifth decade of life. It is usually present at the apex of a tooth but can be apical lateral to a tooth. It is usually asymptomatic and small (around 0.5-1.5 cm) in size but can occasionally reach large sizes. It can be symptomatic, i.e. painful and expansile, if infected. The infected cysts may break through the cortical bone in the form of a fistula, usually buccal or labial in the mandible and buccal or palatal in the maxilla. It is more common in males than females and slightly more common in the maxilla, especially the anterior maxilla. Radiographically, it presents as a well-demarcated or corticated unilocular radiolucency at the apex of a tooth or apical and lateral of a tooth. Occasionally, the radicular cyst can be multilocular. It can also be associated with root resorption, but rarely. Histologically, it is made up of a cystic structure lined by epithelium and supported by a connective tissue wall. The latter is usually chronically inflamed and frequently associated with abscess. Treatment ranges from conventional endodontic treatment to apicoectomy to extraction of the tooth with curettage of the cystic structure. It has a good prognosis.