Infection from the pulp chamber induces inflammation and proliferation of the epithelial rests of Malassez. If infection can be eliminated from the root canal, small radicular cysts (up to 1 or 2  cm diameter) may occasionally regress without surgery.

That radicular and many other cysts expand in balloon-like fashion, wherever the local anatomy permits, indicates that internal pressure is a factor in their growth. The hydrostatic pressure within cysts is about 70  cm of water and therefore higher than the capillary blood pressure.

Cyst fluid is largely inflammatory exudate and contains high concentrations of proteins, some of high molecular weight which can exert osmotic pressure. Consistent with the inflammation usually present in cyst walls, cyst fluid may contain cholesterol, breakdown products of blood cells, exfoliated epithelial cells and fibrin.

Low-molecular-weight proteins are present in similar concentrations to those in the plasma, but there are smaller amounts of high-molecular-weight proteins. The capillaries in the cyst wall are more permeable as a result of inflammation and contribute varying amounts of immunoglobulins and other proteins. The net effect is that pressure is created by osmotic tension within the cyst cavity.

Experimentally, cyst tissues in culture release bone-resorbing factors. These are predominantly prostaglandins E2 and E3. Different types of cysts and tumours may produce different quantities of prostaglandins but, if so, it is unclear whether this affects the mode of growth of the cyst. Collagenases are present in the walls of keratocysts, but their contribution to cyst growth is also unclear.

The epithelial lining consists of stratified squamous epithelium of variable thickness. It lacks a well-defined basal cell layer and is sometimes incomplete. Early, active epithelial proliferation is associated with obvious chronic inflammation and may then be thick, irregular and hyperplastic or appear net-like (Fig. 7.5), forming rings and arcades (Fig. 7.6). Hyaline bodies may be seen in the epithelium (Fig. 7.7) and mucous cells may be present as a result of metaplasia (Fig. 7.8). Longstanding cysts typically have a thin flattened epithelial lining, a thick fibrous wall and minimal inflammatory infiltrate.

The capsule consists of collagenous fibrous connective tissue. During active growth the capsule is vascular and infiltrated by chronic inflammatory cells adjacent to the proliferating epithelium. Plasma cells are often prominent or predominant and are a response to antigens leaking through the tooth apex.

In the bony wall, there is osteoclastic activity and resorption. Beyond the zone of resorption there is usually active bone formation. The net effect is that a cyst expands but retains a bony wall, even after it has extended beyond the normal bony contours. This bony wall, nevertheless, becomes progressively thinner since repair is slower than resorption, until it forms a mere eggshell, then ultimately disappears altogether. The cyst then starts to distend the soft tissues and appears as a soft bluish swelling (see Fig. 7.1).

Within the cyst capsule there are often areas split up by fine needle-shaped clefts (Figs 7.9 and 7.10). These are left by cholesterol dissolved out during preparation for sectioning. The cholesterol is derived from breakdown of blood cells. Small clefts are enclosed by foreign body giant cells and extravasated red cells and blood pigment are associated. Clefts may also be seen extending into the cyst contents but are formed in the cyst wall.

The fluid is usually watery and opalescent but sometimes more viscid and yellowish, and sometimes shimmers with cholesterol crystals. A smear of this fluid may show typical notched cholesterol crystals microscopically (Fig. 7.11). Histologically, the protein content of the fluid is usually seen as amorphous eosinophilic material, often containing broken-down leucocytes and cells distended with fat globules.

Benign (odontogenic) tumours or, occasionally, an ameloblastoma may resemble a simple cyst radiographically. Resorption of adjacent teeth suggests a neoplasm rather than a cyst but is not diagnostic in itself. It is essential that the whole of a cyst lining be available for examination, since part of the lining, even though neoplastic, may appear as a thin layer of flattened stratified squamous epithelium like that of a simple cyst.

Rarely, a metastasis in the jaw may cause a sharply defined area of radiolucency. More often it causes a lesion with a hazy outline and irregular shape. Tumours also tend to be painful and to grow more rapidly than cysts. Nevertheless, it may be difficult or impossible to distinguish them from infected cysts radiographically, but at operation the solid nature of a tumour will be obvious and histological examination will confirm the diagnosis.

Enucleation is the usual method and usually entirely effective (Box 7.7). The affected (dead) tooth may be extracted or root-filled and preserved. A mucoperiosteal flap over the cyst is raised and a window is opened in the bone large enough to give adequate access. The cyst is then carefully separated from its bony wall. The entire cyst is removed intact and should be sent in fixative for histological examination. The edges of the bone cavity are smoothed off, free bleeding is controlled and the cavity is irrigated to remove debris. The mucoperiosteal flap is replaced and sutured in place. The sutures should be left for at least 10 days.

Any disadvantages are largely theoretical and in competent hands even very large cysts can be enucleated safely. Recurrence is remarkably rare. There are few contraindications: they are relative rather than absolute.

Marsupialisation is a largely outmoded treatment for radicular cysts. The cyst is opened essentially as for enucleation, but the lining is sutured to the mucous membrane at the margins of the opening. The aim is to produce a self-cleansing cavity which becomes, in effect, an invagination of the oral tissues. However, considerable aftercare is needed to keep the cavity clean. The cavity is initially packed with ribbon gauze and, after the margins have healed, a plug or extension to a denture is made to close the opening. Food debris has to be regularly washed out but, once the cavity has filled up from the base and sides sufficiently to become self-cleansing, the plug can be removed. The cavity usually becomes closed by regrowth of the surrounding tissue and restoration of the normal contour of the part. However, the orifice may close and the cyst re-form. Also, the complete lining is not available for histological examination.

The main application of marsupialisation is for temporary decompression of exceptionally large cysts where fracture of the jaw is a risk. When enough new bone has formed, the remaining cyst can be enucleated. Occasionally, retention of the tooth in a dentigerous cyst is needed and marsupialisation may allow it to erupt.