Dental radiology
Interpretation of normal radiographic anatomy
This is a descriptive overview of normal radiographic anatomy. You should come back to this basic anatomy any time during the course or after the course if you have questions.
A radiograph is a two-dimensional representation of a three-dimensional structure. There is always superimposition of anatomical structures and sometimes the projection of anatomy into areas where you wouldn't expect it to be normally. You must reconstruct the anatomy in your mind to understand what you are seeing and understand how the angle of the x-ray beam can affect the location of structures on the film. Being able to identify "normal" anatomy on x-ray images is critical-otherwise, how would you recognize an abnormality?
X-ray images rely on the physical attenuation of the x-ray beam to form an image on film. The denser the tissue, the more radiation it will absorb. The more radiation absorbed by the tissues, the less reaches the film. The more radiation that passes through the tissue and reaches the film, the darker the area will be on the image.
The most dense area of a normal tooth is the enamel cap, which typically appears more radiopaque (white) than the other tissues. The dentin is less dense and appears as a uniform grey area. The junction between the enamel and dentin is very distinct. The layer of cementum on the root surface is nearly the same density as the dentin, thus it is usually not apparent radiographically. The soft tissues of the pulp are much less dense than the other tooth structures and typically appear radiolucent. In normal, fully-formed teeth the root canal may be apparent extending to the apex of the root with a recognizable apical foramen.
Supporting structures of the tooth that are visible radiographically include the lamina dura, the alveolar crest, the periodontal ligament space, and the cancellous bone. When the x-ray beam is projected directly through the long axis of the lamina dura, it is seen clearly as a thin, white line. If the beam passes through at an angle, the lamina dura may appear more diffuse or not be visible at all. The radiographic appearance of the alveolar crest varies from a dense layer of cortical bone to a smooth surface without cortical bone. The level of the bony crest is considered normal when it is not more than 1.5 mm from the cementoenamel junction of the adjacent teeth. The periodontal ligament space appears as a radiolucent space between the root and lamina dura, beginning at the alveolar crest, extending around the portion of the root within the alveolus, and returning to the alveolar crest on the opposite side. The width of the PDL varies from tooth to tooth, although it is typically thinner in the middle of the root and wider near the alveolar crest and root apex. Cancellous bone lies between the cortical plates of both jaws and shows many small radiolucent pockets of marrow which create the trabecular pattern we see on the film. The trabecular pattern varies considerably from patient to patient and even within the same patient.
The trabeculae in the maxilla are typically small and form a dense granular pattern while the trabecular pattern of the mandible is larger and coarser.
The borders of the maxillary sinus are formed of thin cortical bone which appear as thin radiopaque lines on periapical radiographs. The size of the maxillary sinus varies considerably although the right and left sinuses are typically symmetrical. The floor of the sinus is seen on periapical radiographs near the apices of the molars and premolars, and may extend down as far as the crest of the alveolar ridge, particularly in edentulous areas. Radiopaque lines traversing the sinus either horizontally or vertically are septae, bony projections from the floor and wall of the antrum. Septae give the sinus the appearance of being divided into compartments, although this is not the case. The radiolucent compartments formed by the septae sometimes mimic periapical pathoses.
Occasionally small bone nodules on the floor of the maxillary sinus may imitate root tips. One way to differentiate the two is to look for a trabecular pattern; a nodule will show a trabecular pattern while a root tip will not.
The zygoma appears as a U-shaped radiopaque line with the round portion superimposing the area of the first and second molars. Depending on the angle in which the x-ray beam passes through the zygoma, it will vary in size, width, and definition.
The nasolabial fold may appear as an oblique line traversing the premolar region. The line of contrast is well-defined and the area of increased radiopacity is caused by the superimposition of the cheek tissue. This feature increases with age and can be used to identify the side of the maxilla if the area is edentulous.
The medial and lateral pterygoid plates lying immediately posterior to the maxillary tuberosities have a variable appearance(another view of the pterygoid plates), often not being visible at all. Typical appearance is a single radiopaque shadow with no trabecular pattern. The hamulus may be seen extending inferiorly from the medial pterygoid plate and does show trabecular pattern.
On periapical radiographs of the central incisors the mental fossa appears as a radiolucent depression extending laterally from the midline and between the alveolar ridge and the mental ridge. Due to the thinness of the bone in the area, the mental fossa appears slightly radiolucent compared to adjacent bone and may be mistaken for periapical disease.
The mental foramen is seen on some periapical radiographs and has a varying appearance; sometimes round or oblong, sometimes slitlike. Typically it is positioned halfway between the lower border of the mandible and the alveolar crest, in the region of the apex of the second premolar. It may appear over the apex of a tooth, mimicking periapical pathoses. A second radiograph from another angle will likely cause the appearance of the foramen to shift in relation to the apex and confirm its identity.
The mandibular canal appears inconsistently and is seen as a dark linear shadow with thin radiopaque borders. The canal extends radiographically from the mandibular foramen to the mental foramen.
Nutrient canals appear in a small number of patients as radiolucent lines extending vertically from the inferior dental canal to the interdental space between the mandibular incisors. Occasionally the canals may appear as small round radiolucencies perpendicular to the cortex and can be mistaken for pathology.
The mylohyoid ridge appears as a radiopaque line running from the area of the third molars to the premolar region, occasionally superimposing the molar roots. The margin of the ridge is varies and is often not well defined.
The submandibular gland fossa is located below the mylohyoid ridge in the molar area and appears as a radiolucent area with a sparse trabecular pattern.
The external oblique ridge is the continuation of the anterior border of the mandibular ramus which disappears in the area of the first molar. On periapical radiographs it appears superior to the mylohyoid ridge, running nearly parallel to it. Radiographically it appears as a radiopaque line with varying width, density, and length.
The coronoid process is often seen in the molar region and appears as a triangular opacity superimposed on the area of the third molar. Trabecular pattern may or may not be visible.
The multiple superimpositions of anatomy on a panoramic radiograph make the identification of normal anatomy difficult.
The mandible should be symmetrical, although poor positioning of the jaw can cause the mandible to appear otherwise. The cortical bone should be continuous from one condylar head down along the border of the mandible and up to the opposite condylar head and of a consistent thickness. The density of the bone should be fairly consistent throughout, with an increase in opacity along the midline. The region of the submandibular gland will appear more radiolucent. The external oblique ridge is visible as a radiopaque line along the superior edge of the mandible. The mandibular canal and mental foramen are typically seen, although the clarity varies. The mandibular canal extends from roughly the upper third of the ramus and follows the curve of the jaw down to the area of the second premolar, where it terminates in the mental foramen. The mental ridge is apparent in the center of the mandible, between the foramen. The hyoid bone often superimposes the mandible bilaterally in the region of the angle of the mandible, extending horizontally towards the ends of the film.
The nasal septum should appear as a vertical opacity in the relative center of the film beginning in the area of the hard palate and extending upward. On either side of the nasal septum the inferior turbinates are seen, typically as oval opacities. The nasal cavity appears as a radiolucency between the turbinates and the nasal septum. The borders of the maxillary sinus are seen as thin radiopaque lines. The sinuses should be roughly symmetrical and comparable in density, although they will appear more opaque in the region of the zygoma. The zygoma arises over the first or second molar area. The zygomatic arch begins at the inferior portion of the zygoma and extends posteriorly to the articular eminence and glenoid fossa (another view of zygomatic arch). The zygomaticotemporal suture is often seen in the center of zygomatic arch and may be confused with a fracture site.
The inferior border of the orbit is commonly seen in the upper third of the maxillary sinus. Between the inferior medial border of the orbit and the turbinates is the radiolucent ethmoid sinus.
The hard palate appears as a horizontal radiopaque line above the maxillary teeth. Depending on patient positioning, the hard palate will sometimes superimpose the apices of the teeth. Often the floor of the nasal cavity is superimposed with the hard palate.
The pterygomaxillary fissure is seen as a tear-drop shaped area with a radiopaque outline. Air trapped between the tongue and the hard palate creates a radiolucent area beneath the hard palate in the oropharyngeal air space , superimposing the apices of the maxillary teeth. If the radiolucency is dark enough, the apices of the teeth may not be visible. To prevent this, the patient must press their entire tongue against the hard palate throughout the exposure.
Air passing through the trachea and maxillary sinuses (nasopharyngeal air space) also appears as a band of radiolucency, although it does not superimpose normally erupted maxillary teeth. The soft tissue shadow of the ear lobe may be visible lateral to the rami. Superior to the ear lobe the external auditory meatus may be visible as an oval radiolucency. Occasionally the styloid process is visible in the area of the ear lobe and angle of the mandible. Along the outer edges of the film a portion of the cervical spine is often seen and does not superimpose the mandible unless the patient has been poorly positioned.
The median palatal suture appears as a thin radiolucent line between the central incisors extending roughly from the alveolar crest to the apices of the central incisors. The incisive foramen is seen as an oval radiolucent area between the apices of the central incisors. The nasal septum is seen as a radiopaque area extending vertically down the center of the image from the posterior of the image to the apices of the central incisors. It is bordered on each side by the nasal fossa which appear as radiolucent lines parallel to the nasal septum. If the maxillary sinus appears in the image it is seen as a radiolucent area in the posterior lateral aspect of the image.
The inferior border of the mandible is seen in a mandibular occlusal radiograph. Typically the cortical plate can be seen as a radiopaque line along the border of the mandible. The mental ridge also appears as a radiopaque line parallel to the cortical plate. The genial tubercle appears as a small pointed protrusion extending downward from the cortical plate.
The PA Skull view is used to detect abnormalities caused by trauma or developmental delays, particularly in the mediolateral dimensions of the skull such as asymmetric growth. It demonstrates the frontal and ethmoid sinuses as well as the orbits, nasal anatomy, coronoid process of the mandible, and a frontal view of the mandible.
Areas of cortical bone that are thinner than other areas may mimic disease by appearing more radiolucent than the surrounding bone. Typically, cortical bone is thinner in the area of the mandibular incisive fossa and submandibular fossa. In the maxilla, the incisive fossa in the globulomaxillary region appears radiolucent.
The radiopaque outline of the orbit is seen bilaterally, traversed by the petrous portion of the temporal bone mid-orbit. The radiopaque innominate line traverses upward from the upper outer portion of the orbit bilaterally. The radiolucent frontal sinus is visualized between the upper inner area of the orbits and may vary greatly in size and shape. The sphenoid and ethmoid sinuses are seen as radiolucent areas located between the mid and lower inner portions of the orbit.
In the center of the image is the nose and its associated anatomy: inferior nasal turbinates, nasal septum, and the nasal turbinates. The nasal septum appears as a vertical radiopaque line floored by the radiopaque nasal fossa and hard palate and sided by the middle nasal turbinates superiorly and the inferior nasal turbinates laterally.
Radiolucent areas lateral to the orbits are the mastoid air cells. Just below the mastoid air cells begins the radiopaque coronoid process of the mandible which extends downward to the mandible. The mandibular canal is visible as it runs most of the length of the rami, ending in the mental foramen at the area of the second mandibular premolar. The shadow of the cervical vertebrae begins in the area of the nasal anatomy and extends downward past the mandible to the bottom of the film, where they are seen much more clearly.
This view is used to demonstrate the skull and facial bones, visualizing the hard palate, nasopharyngeal soft tissues, and paranasal sinuses. In orthodontics this view is used to assess facial growth.
This view shows all of the sinuses which appear as large cluster radiolucent areas. The frontal sinus is the smallest and most superior, with the maxillary sinus below it, bordered by the hard palate which runs consistent with the floor of the maxillary sinus. The ethmoid and sphenoid sinuses are visualized posterior to the maxillary sinus. Adjacent to the posterior wall of the maxillary sinus is the pterygomaxillary fissure which appears triangular in shape and slightly radiopaque. The pterygoid plates are posteriorly adjacent to the fissure.
The soft palate is an ill-defined radiopaque area between the hard palate and the mandible.
Other radiolucent areas include the oropharyngeal airspace running from the oral cavity to the trachea, and the nasopharyngeal air space running from the maxillary sinuses to the trachea.
The cervical vertebrae are visible and well-defined. The mastoid air cells appear as a slightly radiolucent area in the skull above the process of the second vertebra.
This view is particularly useful for evaluating the maxillary sinuses which appear radiolucent. Other radiolucent structures seen include the ethmoid air cells, orbits, nasal turbinates, frontal sinus, and nasal cavity. Radiopaque structures seen include the nasal septum, body of the zygoma, frontal process of the zygoma, zygomatic arch, coronoid process of the mandible, and crista galli.
Radiolucent structures demonstrated on this view include the ethmoid sinus, mastoid air cells, and maxillary sinus. Radiopaque structures demonstrated include the base of the skull, mandibular condyles, coronoid process of the mandible, lateral and medial pterygoid plates, nasal septum, and zygomatic arch. Structures of a medium density that are commonly seen include the carotid canal, foramen spinosum, foramen ovale, and foramen magnum.
Normal radiographic anatomy of the tempomandibular joint
A panoramic image of the TMJ typically includes 2 views of each condyle; one with the mouth wide open and one with the mouth closed. Radiopaque anatomy demonstrated typically includes the mandibular condyle, articular eminence, soft palate, and maxillary tuberosity. Radiolucent structures seen include the maxillary sinus, external auditory meatus, and pterygomaxillary fissure.