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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

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