<prompt> Answer this question truthfullyWhat are some characteristics and causes of periapical cyst or radicular cyst, which is a type of odontogenic cyst?  <response>: <reason> Periapical cyst or radicular cyst is the most common type of odontogenic cyst that results from inflammation caused by dental caries or trauma. It is caused by cystic degeneration around a periodontal ligament and is typically located at the apex of a nonvital tooth. The cyst is usually round with a well-corticated border and is less than 2 cm in size. 