Anatomy

Adults

The teeth consist of dentin, which is capped by enamel over the crown and by cementum over the root. Due to the difference in the degree of mineralization between the enamel and the dentin, the dentino-enamel junction is discernible radiographically (Fig. 3). Since the cementum is very thin and its mineral content is approximately the same as that of the dentin, cementum and dentin are radiographically indistinguishable.

The centre of the tooth is occupied by the pulp, which contains the nerves

	Figure 4. Incisive foramen is visible as a rounded radiolucency (arrows) between the apices of the central incisors. The anterior nasal spine, and part of the nasalfossa, appear above the teeth. Loss of alveolar bone due to periodontal disease.
	Figure 5. The superior foramina of the incisive canal are seen on both sides of the nasal septum.

and vessels that support the dental tissues. It has a wider coronal part, the pulp chamber, and a more narrow root portion, the root pulp.

The wall of the alveolar tooth socket forms a thin layer of dense bone, the lamina dura, that parallels the root surface but is separated from the root by the periodontal ligament. The ligament is represented by a thin, radiolucent line. Normally, the radiopaque lamina dura is continuous around the root and with the crest of the alveolar ridge. General absence of lamina dura may be a sign of systemic bone loss such as occurs in for example hyperparathyroidism, Cushing's syndrome, leukaemia, myeloma, osteomalacia caused by adult celiac disease (sprue), chronic glomerular and tubular dysfunction, and scleroderma.

The maxilla

In the maxilla, the alveolar bone has a uniform trabecular appearance with small marrow spaces except for the tuberosity region. The incisive foramen is located anteriorly in the midline of the palate. In radiographs of the central incisors it appears between their apices (Fig. 4). It varies in size, shape, and visibility. In radiographs of the lateral incisors, and in some instances the cuspids, the image of the incisive foramen may be superimposed on the images of the central incisors, mimicking a periapical 

Figure 6. The inferior border of the maxillary sinus projects as a downward, convex, radiopaque line (arrow). The floor of the nasal fossa is represented by a thin horizontal radiopaque line (double arrows). The maxillary zygomatic process, which is aerated, forms a radiopaque U (triple arrows) from which the zygomatic bone extends posteriorly (open arrow). The radiolucent band (double open arrows) is caused by grooves in the lateral sinus wall containing the posterior superior alveolar nerves and vessels.

Figure 7. The maxillary sinus is extensively pneumatized following early tooth extraction. The complex pattern of radiolucent bands (arrows) illustrates the branching of the posterior superior alveolar nerves and vessels. The coronoid process is also visible (1).

lesion. If the central incisors are radiographed with an excessive vertical angulation of the x-ray beam, the superior foramina of the incisive canal can become visible on both sides of the nasal septum (Fig. 5).

The anatomy of the maxillary sinus is described elsewhere. Certain aspects of the antral anatomy will be considered here as they pertain to intraoral periapical radiographs. Before 4-5 years of age, the maxillary sinus does not appear in these radiographs. In the adolescent and in the adult, the sinus usually extends from the premolar area to the tuberosity. The inferior border is defined by a thin, downward, mostly convex, radiopaque line (Fig. 6). The floor of the nasal fossa may appear as a horizontal, radiopaque line superior to the sinus border. Frequently, narrow, curved radiolucent bands running a posteroanterior course are seen within the image of the maxillary sinus. They are caused by grooves in the lateral sinus wall containing the posterior superior alveolar nerves and vessels that are supporting the maxillary teeth and their surrounding tissues (Figs. 6, 7). Early removal of the molars usually results in

Figure 8. The mandibular canal (arrow) extends from the mandibular foramen (1) to the mentalforamen (2). The two radiopaque objects on the left side of the maxilla are caused by excessive root filling material forced through the root canals into the sinus.

expansion of the maxillary sinus in the alveolar process (Fig. 7).

The maxillary zygomatic process extends laterally from the surface of the maxilla in the region of the first and second molars. In the periapical radiograph it frequently gives rise to a U-shaped radiopacity (Fig. 6). Posterior to the zygomatic process, the zygomatic bone appears as a radiopaque structure. The coronoid process of the mandible may show in the posterior part of the periapical radiograph of the maxillary molars (Fig. 7).

The mandible

In the mandible, the trabecular pattern varies much more than in the maxilla. Overall, there is a coarser trabecular network with much larger marrow spaces. In some instances, particularly in the posterior regions, there are areas where the trabecular pattern may be missing.
The mandibular canal, which contains the inferior alveolar nerve and vessels, runs anteriorly from the mandibular foramen which lies on the medial aspect of the mandibular ramus (Fig. 8). In most individuals, it is radiographically visible as a radiolucent band defined by thin radiopaque lines running as far as, or slightly anterior to, the premolar region. Here, the mental nerve and vessels emerge from the mandibular canal through the bucally located mental foramen, which is discernible

Figure 9. a) Periapically, at the second preomolar, a well defined radiolucency (arrow) mimics a granuloma. b) Vertical change in projection relative to the radiograph in a). The radiolucency (double arrows), caused by the mental foramen, is now located below the apex of the second premolar.

in about 50 % of periapical radiographs. The image of the mental foramen may be superimposed upon the apex of one of the premolars, mimicking a periapical lesion (Fig. 9).

Frequently the borders of the mandibular canal are not discernible. The distance between the canal and the apices of the posterior teeth varies. At times the canal runs very dose to the roots of the third molar. In order to prevent injury to nerves and vessels, this relationship should be determined before extraction of the third molars is attempted. Expansile benign lesions of the mandible frequently cause a displacement of the mandibular canal. Local widening of the canal may be caused by neural tissue tumours. Increased width over longer distances, combined with lack of definition and widening of the mandibular foramen, occurs when malignant tumors are spreading via the perineural lymphatic tissues.

The submandibular gland fossa is found on the lingual aspect of the mandible in the molar region below the mylohyoid ridge (Fig. l). The fossa exhibits a considerable variation in length, height and depth. Sometimes it appears as a well-defined radiolucency which can be mistaken for an osteolytic lesion.

Children

The radiographic anatomy of the jaws in children differs markedly from that in adults. The younger the children, the greater the difference. In young children, bone trabeculae are obscured by the germs of the

	Figure 10. Posterior part of the left maxillary alveolar process from a young child. The tooth germs of the permanent teeth obscure the bone structure.
	Figure 11. The follicle surrounding the crown of the maxillary right cuspid is wide, mimicking a dentigerous cyst.
	Figure 12. The radiolucent area (arrow) derives from the incompletely formed apex and should not be mistaken for a periapical lesion.

permanent teeth and their follicles (Fig. 10), which limits the diagnosis of bone disease. The rounded tooth follicle is defined by a cortical lining. At an early stage of tooth formation, before mineralization of the crown has begun, the uniformly radiolucent image of the tooth follicle may mimic an osteolytic lesion. The follicles of the mandibular third molars are most likely to cause such mistakes. When the crowns are mineralized, a radiolucent area remains between the cortical border of the follicle and the tooth enamel. The width of this area also varies under normal circumstances. Around certain teeth, such as the maxillary cuspids the follicle prior to eruption may have a considerable width, mimicking a follicular cyst (Fig. 11). When tooth formation is nearly complete the

Figure 13. A small invagination of dentin (arrow) and enamel (arrows) in the right maxillary lateral incisor. Due to infection of the pulp and the periapical bone, after eruption of the incisor, the root formation ceased and a large radicular cyst developed.

apical tip of the root canal is shaped like an inverted V. The radiolucent area between the mineralized portion of the forming apex and the radiopaque line caused by the lamina dura, should not be mistaken for a pathological lesion (Fig. 12).

Lars Hollender and Karl-Åke Omnell