The apical cyst is a significant dental lesion. Among the periapical disorders only the acute apical periodontitis and the chronic apical periodontitis occur more frequently. Most agree that the apical cyst is a true cyst. i.e. .a pathologic cavity which is tined with epithelium and often fluid-filled.

The term apical cyst is substituted (again for mnemonic reasons) in place of the time-honored "radicular cyst. " The prefix "apical" directs attention to the tocation of the cyst and also suggests the etiology as endodontic in nature. The adjective "radicular", white it implies root, also signifies that the cyst may be at any location along the root. Thus a "lateral cyst" and the "apical cyst" are both "radicular cysts".

The presence of epithelium within apical inflamma tory lesions has been repeatedly confirmed. This epithelium. with few exceptions. is derived trom the epithelia rests of Malassez. The apical cyst develops within such apical lesions and from this epithelium. Actually a lined cystic cavity can develop at the site of either the chronic or the suppurative form of periodontitis. The latter metamorphosis (suppurative periodontitis to apical cyst) is. of course, less often seen.

Why and how does the cystic cavity form? The consensus views inflammation as the primary stimulus. Excited by inflammation. the epithelial cells in the apical area proliferate widely. The reticular pattern of their growth at this stage is quite dramatic. Mitosis occurs in the basal layer of cells along the periphery of the epithelial cords and clusters. More and more layers of squamous cells are produced. Eventually, the central cells of this epithelial mass die, because they have become too far removed from the connective tissue which is their source of nourishment. The common death of central epithefial cells leads to necrosis; necrosis to tiquefaction, liquefaction to the apical cyst a fluidfilled cavity within epithelium. There remains unanswered, ot course, an important question. Why do only some lesions of chronic apical periodontitis become apical cysts when epithelium and inflammation are present in all?

Growth of the apical cyst is a slow process. At the outset the cyst can be likened to an epithelial parasite within the apical "granuloma. " Once established, the apical cyst will occasionally press on its own into new territory far beyond the "granuloma" into the cavity of the maxiliary sinus. for example. In such an instance the original connective tissue lesion and additional alveolar bone give way before the cyst. Most apical cysts remain small. Many, in fact, stay within the limits of the inflammatory lesion which preceded them. For others. the process of local expansion continues, but at an extremely slow rate.

How does the minute cyst become a larger one? A continued apical inflammation, the unending mitosis of epithelial cells of the cyst wall, the necrosis and liquefaction of cells, epithelia and other, which are desquamated into the cyst cavity, the resultant increase in cyst content, and finally, the resorption of bone in response to pressure from this growing volume of cyst fluid - all contribute to the expansion.

Apical cysts have in common (1) epithelium. (2) a central lumen lined by the epithelial sheet, (3) a fluid or semi-fluid substance within the lumen, and (4) an outer capsule of connective tissue. When the cyst is young, its epithelium is continuous with the network of epithelium which has already ramified at the apex of the tooth. Later the accessory fingers and cords seem to "retract", leaving a consolidated cyst wall. Round cells often permeate both the epithelium and the immediate connective tissue. They also enter the cyst fuid in quantity. Should inflammation dominate, the epithetial lining may be thinned or interrupted.

A curious fluid fills the cavity of the cyst. Its slippery yet crystalline feel, and the often amber tint, render this fluid unique. Erythrocytes are rarely found when the fluid is aspirated from a cyst in situ. Free-floating epithelial cells and leukocytes, however, are often detected. Cholesterol is sometimes abundant and always unmistakable when encountered.