Periapical Cyst (Radicular Cyst, Root-End Cyst)
A periapical cyst arises at the root-end of a tooth whose pulp in inflamed or necrotic. Common causes of pulp inflammation and necrosis are dental caries, blunt trauma, and thermal injury. Regardless of cause, inflammation is originally confined to the interior of the tooth. If untreated, inflammation exits the tooth and spreads to the apical periodontal membrane and adjacent bone. With time, bone is resorbed, creating a periapical lesion consisting of inflamed granulation tissue. At this stage, the lesion is referred to as a periapical granuloma. If there is suppuration, it is a peria-pical abscess. If epithelial rests of Malassez lie within this smoldering lesion, they may proliferate to form a ball of epithelium that eventually becomes hollow in the center. The result is a periapical cyst. Most periapical cysts are discovered on routine dental radiographs. Radiographs reveal a round to oval, sharply circumscribed radiolucent lesion usually centered over the root tip of the offending tooth. In some cases, the cyst may lie along the side of the root. The tooth is not responsive to thermal and electrical stimulation. If the tooth responds normally to stimulation, the lesion is not a periapical cyst but another disease masquerading as one. The histolo-gic features are rather constant. If the cyst is removed intact, the pathologist identities a central cavity, the lumen, which contains the detritus of inflammation, hemorrhage, and sloughed epithelium. The cyst wall is of fibrous connective tissue exhibiting a polymorphic inflammatory infiltrate, plasma cells, and lymphocytes. Apicular clefts identify the areas occupied by cholesterol crystals. Foreign body giant cells and hemosiderin pigment are found between these clefts. In a minority of cases, brightly eosinophilic Rushton bodies are seen in the squamous epithelium that lines the cyst lumen. The treatment of periapical cyst is tied to the treatment of the offending tooth. If the tooth is amenable to endodontic treatment, the cyst may resolve without surgical intervention. Large cysts are ordinarily removed in conjunction with the endodontic procedure. The alternative is extraction of the offending tooth accompanied by cystectomy.
Dentigerous Cyst (Follicular Cyst)
A cyst that occurs around the crown of an unerupted tooth is called a dentigerous cyst or follicular cyst. They are thought to arise as a result of the accumulation of fluid between the crown of the tooth and the dental follicular tissue that surrounds the crown. Why this happens is unknown. A cyst may develop around any impacted tooth but they are more commonly encountered around impacted third molar teeth. The risk of developing a dentigerous cyst around an unerupted tooth has been placed at approximately 1.0%. Dentigerous cysts range in size from those that are so small they may appear to be a hyperplastic follicle to those that are many centimeters, so large that they occupy large regions of the bone. Large cysts may remain asymptomatic but will cause expansion of the bone. On radiographs, they are purely radiolu-cent and usually are unilocular, rarely multi-locular. The associated tooth may be inverted and displaced far from its normal location. Histologically, the dentigerous cyst is lined by unremarkable stratified squamous epithelium. The cyst wall is of fibrous connective tissue and it is attached to the neck of the associated tooth. It may or may not exhibit inflammation. The treatment consists of cystectomy along with the offending tooth. Large cysts respond to marsupialization. The radiographic differential diagnosis is odontogenic keratocyst (OKC), adenomatoid odontogenic tumor, and cystic ameloblastoma.
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