The following odontogenic cysts and some related lesions will be presented here:

a)      Radicular cyst
b)      Residual cyst
c)      Paradental and mandibular infected buccal cysts
d)      Mandibular Infected Buccal Cyst
e)      Lateral periodontal cyst
f)       Glandular odontogenic cyst
g)      Odontogenic keratocyst
h)      Gorlin syndrome

A cyst can be defined as a benign pathologic cavity within bone or in soft tissues, generally formed by a connective tissue wall. The cavity, within the oral regions, is almost always lined by epithelium. Some cyst-like lesions, without epithelial lining, also can be seen in the maxillo-facial regions. The cyst's lumen usually contains fluids, keratin or cellular debris.

In this schematic drawing arrow A points to the connective tissue wall that forms the cyst. Arrows B point to the various types of epithelium that can line a cyst developing within the oral regions. It is important to remember that these differentiated epithelia are not normally found within bone. Therefore, when treating these cysts all the epithelium must be removed in order to prevent recurrences.

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RADICULAR CYST (Periapical cyst, apical cyst)

The most frequent cyst of the teeth bearing areas is the radicular, also called periapical or apical cyst. Around 60% of all jaw cysts are radicular or residual cysts. Radicular cysts can occur in the periapical area of any teeth, at any age but are seldom seen associated with the primary dentition. This cyst is classified as inflammatory, because in the majority of cases it is a consequence to pulpal necrosis following caries, with an associated periapical inflammatory response. Other causes include any event that may conduce to pulpal necrosis such as tooth fracture and improper restorations, among others. The first line of defense to pulpal necrosis in the periapical area is the formation of a granuloma. A granuloma is a highly vascularized tissue containing a profuse infiltrate of immunological competent cells i.e., lymphocytes, macrophages, plasma cells, etc.

Arrow A, in both, the drawing and the photomicrograph, points to an initial process of caries which has already invaded the dentin. Arrow B indicates the localized area of inflammatory reaction present in the coronal pulp in response to caries.

The epithelial rests of Malassez are remnants from the root sheath of Hertwig which are found in large numbers within the periapical area of all teeth. These epithelial cells derive from the ectoderm that gave rise to the tooth germ and they preserve their metaplastic embryonal potential. Therefore, they can differentiate into any type of epithelium, under the proper stimuli. These rests play a central role in the formation of radicular cysts. In the midst of the rich vascular area provided by the periapical granuloma, the rests of Malassez proliferate and eventually form a large tridimensional mass of cells. With continuous growth the inner cells of the mass are deprived of nourishment and they undergo necrosis by liquefaction. This conduces to the formation of a cavity which is located in the center of the granuloma, giving rise to a radicular cyst.

The radiological image of the radicular cyst is a peri- or para-apical, round or oval radiolucency of variable size which is generally well delineated and most likely with a marked radiopaque rim. Other lesions, such as: granulomas, neoplasms of various origin and some diseases of bone can also present a similar radiolucent periapical appearance. Therefore, a periapical radiolucency can not be automatically assumed to be a cyst. Several studies have indicated that it is not possible to rely on the radiographic size of a periapical radiolucency to establish the diagnosis of either cyst or granuloma unless the lesion is larger than 2 cm in diameter. Rarely radicular cysts will induce resorption of the root of the affected tooth.

This is a typical appearance of apical radiolucency. Note the well delineated cavity with a marked radiopaque rim. The biopsy proved this to be a radicular cyst. Note the radicular rest corresponding to the roots of a first mandibular molar.

This is another example of a radicular cyst as a result of pulpal necrosis. Note the large periapical radiolucency which is very close to the nasal cavity.

Periapical radiolucencies are a frequent finding in endodontically treated teeth. Microscopic examination of those radiolucencies can demonstrate either a residual granulomatous tissue, a collagenous scar as a consequence to the endodontic treatment or a radicular cyst. As explained before it is not possible, based only on radiographs, to ascertain the proper diagnosis. It is estimated that around 10% of periapical radiolucencies in endodontically treated teeth are cysts.

These radiographs illustrate examples of periapical radiolucencies. The diagnosis of radicular cyst or granuloma can only be made after histologic examination of the lesion. The size of these radiolucencies is not indicative of their diagnosis, either lesion can present great variation in its size, reflected by the amount of bone resorption as a result of pressure applied by the growing lesion into the surrounding bone.

Islands of squamous epithelium which have developed from odontogenic rests of Malassez can also be found in a periapical granuloma without cystic transformation. Endodontists refer to these granulomas as "bay cyst".

Microscopically a radicular cyst is formed by a mature collagenous connective tissue wall. This connective tissue is the basic framework (stroma) of most cysts found in the maxillofacial regions. Abundant fibroblasts, the basic cell of the connective tissue, can be identified within the cystic wall and characteristically present dark staining nuclei in the center of the cytoplasm. The fibroblasts are seen within the undulating collagen fibers.

The wall generally presents an inflammatory infiltrate of variable degree. Lymphocytes are generally the most prominent cells in the infiltrate and are characterized by their darkly stained nucleus, which occupies most of the cytoplasm. Plasma cells are also abundant in cysts' walls and mostly seen in long standing (chronic) cysts. They are characterized by an eccentric nucleus with a cart-wheel arrangement of the nuclear chromatin. Plasma cells are considered factories of immunoglobulins.

Other histological findings within the cystic wall are: erythrocytes (Arrow 1) and areas of intratissular hemorrhage, occasional spicules of dystrophic bone, multinucleated giant cells and cholesterol crystals.

These microscopic sections are from the same pariapical cyst. The one to the left is a low view where Arrow 1 points to hemorrhage within the cystic cavity and Arrow 2 points to a capillary within the connective tissue wall. The section to the right is a higher power view demostrating the stratified squamous epithilial lining of the cyst. Also note the underlaying connective tissue wall.

The cavity of a radicular cyst is generally lined by stratified squamous epithelium, these cysts can be lined by respiratory epithelium, especially if they are in the vicinity of the maxillary sinus. Rarely radicular cysts may be lined by mucus producing epithelium in either maxillary or mandibular locations. The mucous epithelium is the result of metaplastic transformation of the epithelial rests of Malassez which are totipotential.

This lateral incisor clinically shows a cavity previously occupied by a mesial composite restoration, which was done 4 years ago, and recently fell off. The patient relates a history of occasional pain in that tooth as well as tenderness in the periapical area. She also states that about 2 years ago she had an episode of marked swelling and intense pain in that area which was treated elsewhere with antibiotics. Further treatment at the time, as indicated by the attending dentist, was not performed because the patient failed to maintain the appointment. Note the caries on the distal of this tooth and the large periapical radiolucency. The lateral incisor was treated endodontically and with a retrofill and on the basis of the biopsy the periapical lesion was diagnosed histologically as a radicular cyst

The central incisor also presents a smaller periapical radiolucency. Note the improper endodontic treatment. The patient did not have any symptomatology associated with that tooth. The radiolucency could be either a cyst, a granuloma or a residual scar.

Radicular cysts are generally asymptomatic unless they are secondarily infected, in which case they will be accompanied by pain and the other signs and symptoms of inflammatory-infectious processes. Radicular cysts may vary in size from 1/2 to 2 centimeters or more in diameter. When a cyst reaches a large size it may produce intraoral or facial asymmetry and even paresthesia due to compression of nerves. Occasionally a large cyst may erode the bone cortical plate or invade the maxillary sinus or the nasal fossae. Around 60% of all radicular cysts occur in the maxilla and rarely extend palatally. Patients with extremely large radicular cysts may be at risk of accidental secondary fractures of bone.

This intraoral radiograph is from a 39 year old male who complained of a dull ache at the level of the first right mandibular molar. The molar has been endodontically treated and crowned 3 years prior. In this radiograph the endodontic treatment does not seem to completely fill the canal. In this case the large periapical radiolucency could have more than one etiologic factor. In addition to the previous endodontic treatment there is marked resorption of the alveolar bone at various points. In spite of its large size this lesion, radiographically, must be interpreted as a periapical radiolucency. Histologic examination after surgical removal rendered the diagnosis of periapical cyst.

This radiograph is of a molar that had an aberrant root canal which opened on the side of the root. Hence, the lateral location of this periapical lesion. Vitality tests demonstrated the tooth to be non-vital. The tooth was eventually extracted and the photograph to the right shows the molar with a soft tissue mass attached to the para-apical area of the mesial root. Biopsy of the soft tissue mass proved it to be a cyst. A careful study of the extracted molar demonstrated that the root canal of the mesial root exited on the lateral aspect of the root and not at the apex. Therefore, the cyst was an apical cyst with a para-apical location.

The treatment of the radicular cyst is surgical ablation. When the affected tooth is extracted the cyst generally comes attached to the root. If the cyst has been secondarily infected the cystic wall may have strong collagen bundles deeply inserted into the adjacent bone. When that occurs, portions of the cyst may remain in the bottom of  the socket after extraction of the tooth. A smooth curettage is recommended after extraction in order to eliminate any possible cystic remnant.

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RESIDUAL CYST

A residual cyst arises as a consequence of an improper surgical elimination of a radicular cyst. Its clinical and histological characteristics are identical to those of a radicular cyst. Radiologically it will be seen as a radiolucency of variable size at the site of a previous tooth extraction. Large residual cysts may be treated by marsupialization.

This large residual cyst has been present for many years in the mandible of a 67 year-old man. Arrow A points to the mandibular canal. Arrow B points to the expansion of the labial cortical plate produced by the cyst. Arrow C points to a radicular rest.

The radiograph to the left shows a well delineated radiolucency with a markedly radiopaque periphery. This lesion is not related to the neighboring premolar. Note the roof of the cyst elevating the floor of the maxillary sinus. After surgical removal and biopsy the lesion proved to be a cyst. This cyst was a consequence to caries in the first maxillary molar. That molar was extracted and portions of the cystic wall were left within bone. Those rests gave rise to a so-called residual cyst. Therefore, any cyst must be carefully removed in order to avoid recurrences.

The radiograph to the right is another example of a residual cyst. It is important to remember that on X-ray this lesion is a radiolucency and that the radiographic differential diagnosis should include a variety of lesions which can present as a radiolucid image such as: non-odontogenic benign neoplasms (i.e.: hemangiomas, neurolemmomas, etc.), odontogenic benign neoplasms (i.e. unicystic ameloblastoma, adenomatoid odontogenic tumor, etc.) or other lesions primary to bone like Langerhans cell histiocytosis. Therefore a biopsy is imperative to establish the proper diagnosis.

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PARADENTAL CYST

The paradental cyst is an inflammatory cyst which develops on the lateral surface of a tooth root. Histologically the paradental cyst can not be differentiated from a radicular cyst. Some authors refer to this cyst as an inflammatory periodontal cyst or collateral cyst. This cyst is of rare occurrence and must be radiographically differentiated from the lateral periodontal cyst. It is treated by surgical ablation and does not have a tendency to recur.

The arrows point to the periphery of a paradental cyst associated to the distal wall of a 3rd mandibular molar. This cyst is also considered inflammatory in etiology.

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