Both mandibular third molars are mesially impacted against the second molars. If they are not removed, what is the most likely consequence?

Correct answer: C. Distal caries and a distal periodontal defect on the second molars

• A. The pulp of the second molar is not devitalised by the impaction itself, though deep distal caries can eventually reach it.
• B. Ankylosis of the second molar is not a recognised consequence of an adjacent impaction.
• C. CORRECT. A MESIALLY impacted third molar creates a plaque-retentive contact against the DISTAL surface of the second molar that cannot be cleaned. Two consequences follow: DISTAL CARIES on the second molar — often deep and sometimes untreatable by the time it is found — and a DISTAL INFRABONY periodontal DEFECT from chronic inflammation and the absence of attachment on that surface. Pericoronitis and follicular pathology are additional risks.
• D. The role of third molars in late incisor crowding has been extensively studied and is NOT supported. Crowding is multifactorial and continues in patients whose third molars were removed.

Note: The two well-documented consequences are distal caries and the distal infrabony defect — both often cited as the strongest indications for prophylactic removal of mesioangular impactions.