What is a periapical cyst?

• A. Pathologic cavity filled with fluid, lined with epithelium, and surrounded by a definite connective tissue wall.
• B. Localized accumulation of pus.
• C. Proliferating granulomatous tissue.
• D. Inflammation of the periodontal ligament.

Answer: (D)

A periapical cyst is a true cyst that forms at the apex of a tooth with pulpal necrosis. The key idea is that a cyst is a pathologic cavity that is lined by epithelium and surrounded by a connective tissue wall, often containing fluid. In this situation, inflammatory signals from the nonvital pulp stimulate the epithelial rests of Malassez in the periodontal ligament to proliferate and create a cystic lining, producing a well-defined cavity at the tooth root tip. Radiographs typically show a well-circumscribed radiolucent area at the apex, reflecting bone loss around the cyst. This differs from an abscess, which is a localized pus collection; from a granuloma, which is proliferating granulation tissue without a true epithelial lining; and from general periodontal ligament inflammation, which is inflammation of the supporting ligament but may not form a cystic cavity. So the description that best fits a periapical cyst is a fluid-filled, epithelium-lined cavity with a connective tissue wall surrounding it.