Bone resorption caused by metabolism of acidic substances produced by cysts contributes to cyst growth. Such substances include Prostaglandin-2 and Interleukin-1 which are both produced by the cyst itself. Nutritional deficiency theory
Epithelial cells will form a mass inside the cavity and the innermost cells become deprived of nutrients because they are far from the source of nutrients (the blood vessels). The innermost cells die and form an aggregate of dead tissue. The inner cells undergo ischemic liquefactive necrosis which creates the cavity space surrounded by growing epithelial cells. This theory is unlikely in the absence of malignant transformation of epithelial cells as it does not follow the existing relationship between connective tissue and epithelium. Abscess theory
Epithelial cells have an inherent quality to reproduce and cover any connective tissue that is not already lined with epithelia. Formation of an abscess must precede the epithelial proliferation in order for the cells to carry out this tendency. This theory explains why cysts are lined in epithelia but not why the initial cysts itself forms. Diagnosis
A non-vital tooth is necessary for the diagnosis of a periapical cyst, meaning the nerve has been removed by root canal therapy. Oral examination
The surrounding intraoral anatomical structures should be palpated to identify the presence of bone expansion or displacement of tooth roots as well as crepitus noises during examination, indicating extensive bone damage. Bulging of the buccal or lingual cortical plates may be present.
Scattered ciliated cells
Cholesterol clefts in the cyst lining. Classification
Periapical cysts exist in two structurally distinct classes:

Periapical true cysts - cysts containing cavities entirely surrounded in epithelial lining. Resolution of this type of cyst requires surgical treatment such as a cystectomy. Periapical pocket cysts - epithelium lined cavities that have an opening to the root canal of the affected tooth. Resolution may occur after traditional root canal therapy. Differentiation
Radiographically, it is virtually impossible to differentiate granuloma from a cyst. If the lesion is large it is more likely to be a cyst. Radiographically, both granulomas and cysts appear radiolucent. Many lesions of the mandible in particular appear cystlike in appearance. It is often necessary to obtain a biopsy and evaluate the tissue under a microscope to accurately identify the lesion. Treatment
The infected tissue of the periapical cyst must be entirely removed, including the epithelium of the cyst wall; otherwise, a relapse is likely to occur. Root canal treatment should be performed on the tooth if it is determined that previous therapy was unsuccessful. Removal of the necrotic pulp and the inflamed tissue as well as proper sealing of the canals and an appropriately fitting crown will allow the tooth to heal under uninfected conditions.Surgical options for previously treated teeth that would not benefit from root canal therapy include cystectomy and cystostomy. This route of treatment is recommended upon discovery of the cyst after inadequate root canal treatment. A cystectomy is the removal of a cyst followed by mucosa and wound closure to reduce chances of cyst regeneration.