Intraoral Radiographic Anatomical Landmarks
During the process of interpreting a radiograph the radiologist is expected to identify normal radiographic anatomy, thereby distinguishing normal features from pathology. These normal radiographical anatomical landmarks are unique to each area of the maxilla or mandible and appear either radiopaque or radiolucent. They may be used to identify a specific area of the jaw. For example, the intermaxillary suture is unique to the radiographs of the maxillary central incisors, mental foramen is typically associated with the radiographs of the mandibular premolars etc. On occasions, these landmarks may exhibit some minor variations (such as a bifid mandibular canal) or may not be very evident on a faulty radiograph.
Enamel appears almost snow white and is seen extending from the neck of the tooth, i.e. the cementoenamel junction from one side to the other covering the tooth like a cap (enamel cap). It is the most mineralized structure on the radiograph (90% mineralized) and appears more radiopaque than other structures (Figure 1).
In a developing tooth, the pulp canal diverges and the walls of the root taper to a knife edge. A small round area of radiolucency is seen at the root tip which is surrounded by a thin layer of hyperostotic bone which is the dental papilla enclosed by the bony crypt (Figure 2).
The periodontal ligament space is seen as a radiolucent area between the root surface and the lamina dura. It is seen all around the root extending from the alveolar crest on one side to the other (Figure 3). The width of the periodontal ligament space varies from 0.15 to 0.36 mm; it is generally thinner in the middle third of the root and wider near the alveolar crest and the apical region of the root. However due to projection errors, the periodontal ligament space may not be discernible on some radiographs.
Lamina dura is a radiographic term used to describe alveolar bone proper (cortical bone), which forms the sockets of teeth. It is also considered to be a ‘specialized’ continuation of the cortical plate. Radiographically, lamina dura is seen as a well-defined radiopaque line that surrounds the roots of teeth in health (Figure 4). The appearance of lamina dura on a radiograph is because of the attenuation of the X-ray beam as it passes through the thin layer of bone tangentially.
Usually lamina dura is well-defined. However, on occasions, even in a healthy tooth lamina dura may appear indistinct and diffuse because of an obliquely directed X-ray beam. Lamina dura is generally more radiodense and thick around the roots of teeth under heavy occlusal forces. A double lamina dura is seen when the surfaces of the mesial and distal root are in the path of the central beam of the X-ray. Loss of lamina dura either partially or generalized may indicate the presence of a local periapical pathology or an underlying systemic disturbance.
Crest of the alveolar bone is a radiopaque structure. It is seen as a continuation of the lamina dura (cortical bone). The junction between the alveolar crest and lamina dura is seen as a sharp well-defined angle. Generally the crest of the alveolar ridge is 1.5 mm apical to the cementoenamel junction. In the anterior teeth, the alveolar crest terminates as a pointed projection and in the posterior teeth it appears flat and parallel to the cementoenamel junction (Figures 5 and 6).
It is present between the buccal and lingual cortical plates in the maxilla and the mandible and accounts for most of the bulk of the alveolar bone. The cancellous bone is made up of a network of radiopaque trabeculae that enclose radiolucent bone marrow spaces.
In the maxillary anterior region, the trabeculae are fine and dense and in the posterior region the bone marrow spaces are relatively larger and oriented irregularly (Figure 7A). In the anterior region of the mandible, the trabeculae are fewer in number and oriented horizontally. In the posterior region of the mandible the bone marrow spaces are larger (Figure 7B).
The intermaxillary suture runs from the alveolar crest between the maxillary central incisors to the posterior aspect of the hard palate. It is seen when an intraoral periapical radiograph (IOPAR) is taken for the upper central incisors.
It is seen as a linear radiolucency bounded by radiopaque lines extending from the crest of the alveolar bone between the maxillary central incisors to the anterior nasal spine on an IOPAR. Occasionally it is seen as a V-shaped enlargement at the alveolar crest.
Seen on an IOPAR at the junction of the nasal septum and the floor of the nasal fossa. When an IOPAR is taken for the upper central incisors it is located between and roughly 2 cm beyond the periapices of the central incisors.
The inferior portion and floor of the nasal fossae are seen on an IOPAR beyond the periapical regions of the maxillary anterior teeth, when an IOPAR is taken for the upper central and lateral incisors.