Radiology · patient case question
Each of the following belongs in the differential diagnosis EXCEPT one. Which is the EXCEPTION?
Patient case: Corticated radiolucency around an unerupted third molar
Patient: Female, 24 years old
Chief Complaint: “No symptoms — my dentist wanted a panoramic film before I get my wisdom teeth out.”
Background: No systemic disease, no medications. No swelling, no pain, no altered sensation.
Vital Signs: BP 114/70 mmHg · HR 68 bpm
Findings: Panoramic radiograph shows a WELL-DEFINED, CORTICATED RADIOLUCENCY associated with the UNERUPTED lower LEFT third molar. The lucency surrounds the crown and appears to attach in the region of the cementoenamel junction. Mild displacement of the tooth. No root resorption of the second molar. No cortical perforation.
Show answer and explanation
Correct answer: C. Primordial cyst
- A. A DENTIGEROUS CYST is the single commonest cause of this appearance and is the leading diagnosis — a corticated pericoronal lucency attaching at the CEMENTOENAMEL JUNCTION of an unerupted tooth, arising from the reduced enamel epithelium. The mandibular third molar is its favourite site.
- B. A unicystic AMELOBLASTOMA classically presents exactly this way — a corticated pericoronal lucency around an unerupted mandibular third molar in a young adult. Missing it is the reason every pericoronal lucency of significant size is sent for histology.
- C. CORRECT as the exception. A PRIMORDIAL CYST develops from the enamel organ IN PLACE OF a tooth — the tooth germ degenerates before any hard tissue forms, so the cyst occupies the position where a tooth SHOULD have been and NO TOOTH IS PRESENT. This patient has an unerupted third molar sitting inside the lucency, which excludes the diagnosis by definition. The term is largely historical in any case; most lesions once called primordial cysts are now recognised as odontogenic keratocysts.
- D. An ODONTOGENIC KERATOCYST frequently presents in a pericoronal relationship in the posterior mandible and ramus, mimicking a dentigerous cyst radiographically. It belongs in every differential of this appearance and is separated only histologically.
Note: The measurement worth carrying: a normal follicular space is up to about 3 mm from the crown. Beyond roughly 4–5 mm, treat it as a pathological pericoronal lucency and biopsy rather than assume. The three-way origin map is also examinable — reduced enamel epithelium to DENTIGEROUS cyst; rests of Malassez to RADICULAR cyst; rests of Serres (dental lamina) to KERATOCYST and lateral periodontal cyst.