Читать книгу Shear's Cysts of the Oral and Maxillofacial Regions - Paul M. Speight - Страница 31

The characteristic radiological feature of all cysts of the jaws is a well‐demarcated radiolucency with a well‐defined and often corticated margin. Further features that assist in diagnosis include the shape and size of the lesion and the site, but in most cases it is the relationship to the teeth that provides the best indication of the type of cyst. A conventional plane radiograph is usually sufficient to determine the extent and relationships of jaw cysts, but computed tomography (CT) and magnetic resonance imaging (MRI) are often useful and may be essential for planning surgery of larger lesions. These relationships are discussed and illustrated in each chapter, but here we present an overview of characteristic radiological signs and the basic principles of an approach to interpreting the radiology. Table 2.2 shows the cyst types that have characteristic radiological features, provides a cross reference to the figures in each chapter, and summarises the diagnostic utility of each feature.

Table 2.2 Characteristic radiological features that assist in the diagnosis of cysts of the jaws.

Cyst type	Radiological sign	Diagnostic utility	Figure references
Radicular cyst	A radiolucency at the apex of a tooth	All radicular cysts are located at the opening of the root canal, almost always at the apex. This feature can be considered diagnostic of radicular cyst if the tooth is also non‐vital. The radicular cyst also lies within the lamina dura. If the tooth is vital then other lesions must be considered, but are rare. This may include cemental lesions (cementoblastoma, cemento‐osseus dysplasia) or, in the anterior maxilla, nasopalatine duct cyst	Figures 2.2 and 3.4
Paradental cyst	A radiolucency superimposed over the distobuccal aspect of an impacted third molar. The distal follicular space and lamina dura are intact	This feature is diagnostic of paradental cyst. If an intact follicular space cannot be seen, then the radiolucency may be due to a hyperplastic follicle or pericoronitis	Figures 4.2 and 4.5
Dentigerous cyst	Radiolucency surrounds the crown of an unerupted tooth	All dentigerous cysts show this feature. However, it is not specific, since it may be seen in about 30% of keratocysts, up to 50% of orthokeratinised odontogenic cysts, and occasionally in calcifying odontogenic cysts	Figures 5.5–5.12, 5.14 (dentigerous cyst), 7.7 (odontogenic keratocyst), 12.3 (orthokeratinised odontogenic cyst)
Odontogenic keratocyst	Mesiodistal extension with minimal buccolingual expansion	This appearance is almost pathognomonic for keratocysts in the mandible. Note however that glandular odontogenic cyst may also show this growth pattern (see below). Other cyst types and ameloblastomas show ballooning expansion. The feature is best visualised on computed tomography (CT) scans	Figures 7.6 (odontogenic keratocyst), 10.4, 10.5 (glandular odontogenic cyst, ameloblastoma)
	A well‐demarcated unilocular radiolucency in the ascending ramus not associated with a tooth	Such a radiolucency is most likely to be an odontogenic keratocyst. If the cyst is associated with an unerupted tooth a dentigerous cyst cannot be excluded, and if it is multilocular an ameloblastoma must be considered. A keratocyst is even more likely if there is little buccolingual expansion (see above)	Figure 7.11
Lateral periodontal cyst	Well‐defined, round corticated radiolucency lateral to the tooth root. Periodontal space and lamina dura are intact	This feature is characteristic of lateral periodontal cyst. Lesions are rarely greater than 10 mm in diameter. If the lamina dura surrounds the cyst, or cannot be seen, a lateral radicular cyst must be considered. If the radiolucency is larger than 10 mm or is multilocular, an alternative diagnosis must be considered: possibly botryoid odontogenic cyst, keratocyst, or glandular odontogenic cyst	Figure 8.2
Glandular odontogenic cyst	Large multilocular radiolucency crosses the midline of the mandible in a symmetrical pattern	This is not diagnostic, but is typical of the glandular odontogenic cyst. In some reports up to 85% of cases are located in the anterior mandible. Keratocysts and ameloblastoma may be multilocular, but are more often located in the posterior mandible	Figures 10.3 and 10.4
Calcifying odontogenic cyst	A cystic radiolucency associated with irregular calcifications	About 25% of calcifying odontogenic cysts are associated with an odontoma and show irregular radiopacities either in or adjacent to the cyst. Note that simple bone cyst is occasionally associated with calcifications, but these are usually multiple and represent florid cemento‐osseous dysplasia (Chapter 17)	Figures 11.4 and 11.5
	A cystic radiolucency with a peripheral band of calcifications	About 50% of calcifying odontogenic cysts contain dentinoid in the wall or show dystrophic calcification in the lining. A peripheral band of calcification is characteristic and is best seen on CT scans
Nasopalatine duct cyst	A radiolucency in the midline of the anterior maxilla	Almost diagnostic of nasopalatine duct cyst. Very rarely a radicular cyst may be in the midline. Occasional nasopalatine duct cysts are displaced laterally, in which case a radicular cyst must be considered. Note that the nasopalatine duct cyst is not associated with the periodontal ligament and the lamina dura may be intact	Figures 13.7 and 13.8
	A heart‐shaped radiolucency in the midline of the anterior maxilla	This appearance is diagnostic of nasopalatine duct cyst and is seen in about 20% of cases	Figure 13.7
Nasolabial cyst	An upward or posterior convexity of the inferior margin of the nasal aperture or anterior floor of the nose	Nasolabial cyst is a soft tissue cyst, but it may distort the margin of the nasal aperture. This ‘distorted anchor appearance’ is diagnostic of nasolabial cyst. It is only seen on an anterior occlusal radiograph	Figure 14.3
Simple bone cyst	A scalloped margin at the superior aspect of a mandibular cyst, which rises up and embraces the roots of multiple teeth	This feature is typical and almost diagnostic of simple bone cyst and is seen in 50% or more of cases. It has been described as the tooth roots ‘hanging’ into the cyst cavity (Chapter 17)	Figure 17.1
	A cone‐shaped margin at the anterior aspect of a mandibular cyst	This feature is specific to simple bone cyst. The margins converge at a 45° angle to form a cone. However, it is only seen in about 10% of cases	Figure 17.1
Stafne bone cavity	A corticated unilocular radiolucency at the angle of the mandible below the inferior dental (ID) canal	85% of Stafne bone cavities are located in the posterior mandible and are always below the ID canal. This excludes a lesion of odontogenic origin. The feature can be regarded as diagnostic	Figure 17.5
	A radiolucency that, in a coronal view, is open on the lingual aspect of the mandible	This feature is best visualised on CT and is diagnostic of Stafne bone cavity	Figure 17.6

Figure 2.1 In the posterior region of the mandible, the course of the inferior dental (ID) canal (hashed lines) allows the tooth‐bearing areas (the alveolar bone) to be clearly distinguished from the basal bone of the mandible. Radiolucencies below the ID canal are not odontogenic in origin (see text for details).

In the first instance, the site of the cyst in the jaws can suggest an initial diagnosis. Odontogenic cysts arise in the tooth‐bearing areas of the jaws in the alveolar bone and in the mandible are always situated above the inferior dental (ID) canal (Figure 2.1). The cyst displaces the ID canal downwards towards, and sometimes beyond, the lower border of the mandible. Examples of this feature can be seen in Figures 5.5, 5.11 (dentigerous cyst), 7.6, 7.7 (odontogenic keratocyst), 10.4 (glandular odontogenic cyst), 11.3 (calcifying odontogenic cyst), 12.2, and 12.3 (orthokeratinised odontogenic cyst). A cystic radiolucency located below the ID canal is not an odontogenic cyst and when such a feature is seen, an alternative diagnosis must be considered. Figure 17.5 shows a Stafne bone cavity presenting as a radiolucency below the ID canal, excluding the possibility of an odontogenic origin. In the posterior region of the mandible, this judgement is easy to make (Figure 2.1), but in the anterior mandible and in the maxilla, the distinction between alveolar bone and basal bone is less clear. Although radiolucencies below the ID canal cannot be odontogenic, the converse is not true and a number of radiolucent lesions of non‐odontogenic origin may arise above the ID canal. Figure 17.1 shows an example of a simple bone cyst that is not odontogenic, but characteristically lies within the alveolar bone and embraces the roots of multiple teeth. Other lesions that may arise in the alveolar bone and be associated with tooth roots include giant cell granuloma, Langerhans cell histiocytosis, and ossifying fibroma. Non‐cystic odontogenic lesions, including periapical granulomas, odontogenic tumours, cemento‐osseous dysplasias, and cementoblastoma, must also be considered in the differential diagnosis of lesions in the tooth‐bearing area. Overall, however, odontogenic cysts and in particular radicular cysts are by far the most common.

Figure 2.2 Diagrammatic representation of a radicular cyst. The cyst develops from rest cells of Malassez within the periodontal ligament, and lies within the lamina dura. The corticated margin of the cyst is continuous with the lamina dura (see text for details).

The defining feature of the radicular cyst is of a radiolucency associated with the apex of a non‐vital tooth (Table 2.2; Figure 3.4). The radicular cyst arises within the periodontal ligament from the rest cells of Malassez, and an important sign is that the cyst lies within the lamina dura that surrounds the root of the tooth. Furthermore, the corticated margin of the cyst is continuous with the lamina dura (Figure 2.2). Although this feature is helpful in diagnosing a radicular cyst, it is of more value in excluding a radicular cyst when another cyst type appears to be associated with a tooth root. If a cystic radiolucency is associated with the root of a tooth, but the lamina dura is intact, then a radicular cyst can be excluded and another diagnosis must be considered. This feature is especially helpful in the diagnosis of inflammatory collateral cysts (Figures 4.2 and 4.3), lateral periodontal cyst (Figure 8.2), nasopalatine duct cyst (Figures 13.7 and 13.8), surgical ciliated cyst (Figure 16.2), and simple bone cyst (Figure 17.1).

It must be noted, however, that other lesions arise within the periodontal ligament and may lie within the lamina dura. In particular, a periapical granuloma may have an identical radiological appearance to a radicular cyst, and it is not possible to reliably distinguish between a granuloma and a cyst (discussed in detail in Chapter 3). Although cysts are often larger (see Table 3.1), when a radiolucency is encountered at the apex of a tooth there is an equal chance that the lesion is a periapical granuloma or a radicular cyst (Jones and Franklin 2006a ,b ; Koivisto et al. 2012 ; discussed in Chapter 1). Cemental lesions also arise within the periodontal ligament and, especially when small and not fully calcified, may present as a radiolucency identical to radicular cyst. This feature may be seen in lesions of cemento‐osseous dysplasia, cemento‐ossifying fibroma, and cementoblastoma. The dentigerous cyst embraces the crown of an unerupted tooth and cannot be confused with radicular cyst, but the corticated margin is continuous with the lamina dura (Figures 5.5 and 5.6).