Cross-Sectional Imaging of Third Molar-Related Abnormalities

Abstract

Third molars may be associated with a wide range of pathologic conditions, including mechanical, inflammatory, infectious, cystic, neoplastic, and iatrogenic. Diagnosis of third molar-related conditions can be challenging for radiologists who lack experience in dental imaging. Appropriate imaging evaluation can help practicing radiologists arrive at correct diagnoses, thus improving patient care. This review discusses the imaging findings of various conditions related to third molars, highlighting relevant anatomy and cross-sectional imaging techniques. In addition, key imaging findings of complications of third molar extraction are presented.

FIG 1.

Anatomy of a normal molar tooth. Sagittal CBCT image shows the 3 mineralized hard tissues (enamel, dentin, cementum) and the pulp. The enamel is the outermost layer of the crown, and the root is enveloped by a thin layer of cementum. The dentin is isoattenuating to the cementum and hypoattenuating to the enamel. The pulp contains the neurovascular elements.

FIG 2.

Horizontal unerupted impacted third molar. Sagittal CBCT (A) and MDCT (B) images show an unerupted third molar impacted in the second molar. A close relationship between the third molar and the roof of the mandibular canal, hypercementosis around the roots (arrows), and mild reduction of the caliber of this canal (dotted lines) are observed. The CBCT image (A) has considerably higher spatial resolution than the MDCT image (B) (voxel size, 0.09 mm versus 0.625 mm).

FIG 3.

Pericoronitis. Axial contrast-enhanced CT (A) image shows thickening and enhancement of pericoronal tissues around the left mandibular third molar (long arrow), thickening of the adjacent buccinator–buccal mucosa complex (short arrow), and stranding of the left buccal space (arrowheads). Coronal CT (B) image shows that this tooth is partially erupted and covered by thickened gingiva (star). The “puffed-cheek” technique distends the oral cavity with air, helping to detect these abnormalities more accurately.

FIG 4.

Panoramic reformatted CT image shows impacted unerupted maxillary third molars causing external root resorption of the maxillary second molars (arrows).

FIG 5.

Pericoronitis. Panoramic reformatted contrast-enhanced T1-weighted MR image shows enhancement of the soft tissues (short arrows) around the crown of the horizontally impacted right mandibular third molar (long arrow) as well as in the right mandibular canal (arrowhead).

FIG 6.

Odontogenic sinusitis. Oblique sagittal CT image shows mucosal thickening of the maxillary sinus associated with a bone defect (short arrow) that communicates the sinus floor with periapical disease of a third molar with a carious lesion (long arrow).

FIG 7.

Odontogenic sinusitis following maxillary third molar extraction. Coronal contrast-enhanced T1-weighted MR image shows complete opacification of the right maxillary sinus with mucosal thickening and enhancement (arrow) associated with a bone defect in its floor (third molar socket) (arrowhead).

FIG 8.

Dentigerous cyst. Oblique sagittal CT image shows an expansile well-defined lesion surrounding an unerupted mandibular third molar that is partially attached in the cementoenamel junction (arrow). Hyper-cementosis is observed around the roots of this tooth (arrowhead).

FIG 9.

Dentigerous cyst. Panoramic reformatted FLAIR MR image shows an expansile and homogeneous well-defined lesion (arrow) surrounding the crown of the unerupted right maxillary third molar (arrowhead), superiorly displacing the maxillary sinus floor.

FIG 10.

Odontogenic keratocyst. Oblique sagittal CT (A) image shows an expansile, well-defined lesion (arrow) in the left mandibular angle in contact with the distal surface of the unerupted third molar, inferiorly displacing the mandibular canal (arrowhead). Axial T1-weighted (B) and T2-weighted (C) MR images of the same patient show the lesion to be heterogeneous but predominantly isointense to hyperintense on T1WI and hyperintense on T2WI (arrows). Axial contrast-enhanced T1-weighted (D) image shows thin peripheral enhancement (arrow). Diffusion-weighted image (E) and the corresponding ADC map (F) reveal restricted diffusion (arrows).

FIG 11.

Ameloblastoma. Oblique sagittal (A) and axial contrast-enhanced (B) CT images show a unilocular, expansile lesion in the right mandibular angle (arrow) with marked cortical thinning, buccolingual expansion, and internal solid mural nodules (arrowheads). (Case courtesy of Eloisa S. Gebrim, MD, PhD, InRad, University of São Paulo, Brazil.)

FIG 12.

Abscess after dental extraction. Axial contrast-enhanced CT image shows an abscess (arrow) near the right mandibular third molar socket (arrowhead).

FIG 13.

Acute osteomyelitis of the mandible after extraction of the left first and third molars. Axial T1-weighted (A), fat-saturated T2-weighted (B), and contrast-enhanced T1-weighted (C) MR images show signal intensity abnormalities and enhancement of the bone marrow in the left mandibular body (arrows).

FIG 14.

Fracture of the maxillary sinus walls after third molar extraction. Axial (A) and coronal (B) CT images show fractures of the anterior and lateral walls of the left maxillary sinus (arrows) with complete opacification of the sinus.

FIG 15.

Oroantral fistula after third molar extraction. Sagittal CT image shows an air-filled gap (arrow) between the maxillary sinus and the oral cavity through the tooth socket.

FIG 16.

Right inferior alveolar nerve injury after third molar extraction. Coronal MR neurography image of the lingual (long arrows) and inferior alveolar (short arrows) nerves shows a discontinuity in the right inferior alveolar nerve (arrowhead). The remaining nerves have a normal caliber and signal intensity.

FIG 17.

Active bleeding in the tooth socket. Coronal contrast-enhanced CT image shows a focus of active extravasation of contrast material in the tooth socket after extraction (arrow). The patient is biting a gauze pad (arrowhead).

FIG 18.

Accidental third molar displacement into the maxillary sinus. Oblique sagittal CT image shows a third molar displaced into the maxillary sinus (arrow) and rupture of the maxillary sinus floor (arrowhead) with nearby small bone fragments.

FIG 19.

Accidental displacement of a foreign body into the maxillary sinus. Coronal CT image shows a dental bur (arrow) displaced into the right maxillary sinus. Fragmentation of the right maxillary third molar is also observed (arrowhead).

FIG 20.

Emphysema following a dental procedure. Axial CT images show extensive deep emphysema in the left face and neck (arrows) as well as in the retropharyngeal or danger space (arrowheads).

FIG 21.

Sagittal CT image shows overeruption of the maxillary third molar (arrow) caused by previous extraction of the opposing third molar.