Cysts
Cysts are pathlogic cavities in the bone tissue.
A true cyst represent a pathologic cavity in bone. This cavity is lined by epithelium and may contain a small amount of fluid drawn from adjacent cells and tissues. The epithelial lining may be derived from odontogenic or nonodontogenic sources. Odontogenic sources include the dental lamina, the reduced enamel epithelium, and the epithelial rests of Malassez, or remnants of Hertwig epithelial root sheath. Nonodontogenic sources may include respiratory epithelium or remnant epithelial rests within areas of tissue fusion. A thin connective tissue layer separates the base of the epithelium from the adjacent bone.
Pseudocysts are a group of cysts that may not be lined by epithelium or they may not be cavities in the bone at all. Rather, pseudocysts may have some but not all of the radiologic features of a true cyst.
The patophysiology
The proliferation of remnant epithelial cells within the bone is the initiating event in the pathogenesis of a true cyst.
Epithelial proliferation may be the result of a genetic mutation or due to the release of a signaling molecule or mediator from an adjacent cell or tissue that stimulates proliferation. As the cells proliferate, they begin to displace the adjacent bone, so that space is made available for their increasing numbers. Cells closer to the center of the developing mass become more distant from the periphery as the population proliferates and differentiates, and there is a reduction in the nutrient supply they receive from the adjacent vasculature and tissues. With time, intraluminal epithelial cell debris accumulates at the center of the cavity, and the increasing osmotic gradient that develops across the developing cyst draws fluid in from adjacent cells and tissues. With time, the developing cavity increases in size as more fluid is drawn in and more debris accumulates centrally.
In addition, a thin connective tissue layer develops between the basal epithelial cells and the adjacent bone, and osteoclasts are recruited to accommodate the growing cyst within the bone. The result is a cavity within bone that expands much as a balloon would expand as it filled with water.
Clinical characteristics
The most common clinical feature is a firm to bony-hard swelling that may be accompanied by pain if the cyst is related to a tooth with a nonvital pulp or has become secondarily infected.
Diagnostic imaging
Diagnostic imaging has many important roles in the assessment and management of a cyst.
The role of imaging
Evaluating the extent within bone: first, imaging may aid in the initial diagnosis of a cyst and may describe its extent within bone.
Evaluating the extent of extraosseus involvement: radiologic investigations may also help to determine the extent of extraosseous involvement
Assisting in determining the best site for biopsy.
Intraoral images May reveal subtle changes occurring around the teeth, including the effects of the cyst on the periodontal ligament space and lamina dura.
Panoramic imaging Can provide an overall assessment of the osseous structures of the jaws and reveal relevant changes such as alterations to the borders of the maxillary sinus.
Computed tomography Cone beam computed tomography (CBCT) may be useful to demonstrate the three-dimensional involvement of bone
MDCT and MRI Multiderector computed tomography(MDCT) and magnetic resonance imaging (MRI) may show the involvement of adjacent tissue.
Location
Cysts may occur anywhere centrally within the maxilla or mandible but are rare in the condylar and coronoid processes of the mandible. Odontogenic cysts are most commonly found in the tooth-bearing areas of the jaws; in the mandible, they develop superior to the inferior alveolar canal. Some nonodontogenic cysts can originate within the antrum, whereas others can develop in the soft tissues around the face and neck.
Periphery
Cysts that arise centrally from within bone have a well-defined periphery and a cortex characterized by a fairly uniform, thin, radiopaque line. When a cyst becomes secondarily infected, the cortex may become less discrete and take on a thicker, somewhat sclerotic and less radiopaque appearance. In addition, some cysts may insinuate themselves around the roots of teeth or against a bone cortex, creating a series of contiguous arcs; this is referred to as scalloping.
Shape
Cysts have curved borders that may give them an overall round or oval shape. Sometimes resembling a fluid-filled balloon, a cyst is often described as having a hydraulic shape or hydraulic pattern of expansion within the bone.
Internal Structure
Cysts are most often totally radiolucent; however, long-standing cysts may contain cellular debris, including cholesterol granules or dystrophic calcification, both of which can give them a sparse, particulate radiopaque appearance. In some cysts, the epithelium can proliferate differentially, with some areas showing slower or faster proliferation. This type of differential proliferation may give the cyst a multilocular appearance and result in the development of bony walls or septa between the loculations. Occasionally scalloping of an adjacent bone by the cyst can produce ridges within the endosteal surface of the bone cortex, which may give the false impression that these are septa.
Effects on Surrounding Structures
Cysts develop and increase in size slowly, and when the periphery encounters an adjacent bone border, the bone border thins where it engages the cyst's periphery. With time, bone will expand to accommodate the cyst, and the pattern of expansion produces a smooth, curved surface. Cysts may displace the inferior alveolar nerve canal in an inferior direction or displace the floor of the maxillary sinus, maintaining a thin layer of bone separating the cyst cavity from the antrum.
Effects on Adjacent Teeth
The slowly growing nature of cysts can cause displacement of adjacent teeth as well as external resorption of a tooth's roots, producing an often sharp, curved border that mirrors the curvature of the cyst's border. This is referred to as “directional external resorption.” Also, the lamina dura and the periodontal ligament space may be lost.