Head and Neck ImagingRetromolar trigone
1. AnatomySmall mucosal area on the mandibular ramus, behind the last molar, continuous with the maxillary tuberosity. The pterygomandibular raphe, just under the retromolar trigone mucosa, connects the pterygoid process of the sphenoid bone with the myloid ridge on the mandible; on this raphe, the buccinator muscle and superior pharyngeal constrictor muscle attach. By virtue of its location, the retromolar trigone is at the crossroads of the oropharynx, nasopharynx, buccinator space, floor of the mouth and parapharyngeal space.
2. Pathology
A retromolar trigone lesion requiring imaging is nearly always a squamous cell carcinoma head and neck. Cancer originating at this location rapidly spreads to neighbouring structures, such as medially to the anterior tonsillar pillar in the oropharynx, anteriorly to the buccinator space, posteromedially to the parapharyngeal space via the pterygomandibular space, inferiorly to the floor of the mouth, and superiorly to the nasopharynx and maxillary tuberosity. As the retromolar trigone overlies the mandible, early bone invasion may occur. Much of the tumour spread may occur subclinically, but is visualized with dedicated CT or MR imaging (Fig.1).
The retromolar trigone is fairly commonly involved by lateral spread of oropharyngeal cancer (see oropharynx cancer).
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a. Axial contrast-enhanced CT-image in a patient with a ulcerative lesion in the left retromolar trigone. A fairly large, inhomogeneous soft tissue mass (arrows) is seen, centred on the retromolar trigone, eroding the underlying mandibular bone.
b. The lesion (arrows) is extending superiorly along the anterior margin of the mandible.
c. Three-dimensional reconstruction shows extensive bone defect (arrows) in left side of mandible. Squamous cell carcinoma.
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Retromolar trigone, Fig.1 (a) | | Retromolar trigone, Fig.1 (b) | | Retromolar trigone, Fig.1 (c) |