When designing a flap for surgical removal of a mandibular third molar, the flap margins should:

Correct answer: B. Rest on sound, intact bone when the flap is closed

• A. Placing the incision over the defect is precisely the error the principle exists to prevent.
• B. CORRECT. A suture line lying over a bony void has nothing to support it, collapses into the defect, and dehisces. Margins supported by intact bone heal predictably by primary intention. This is a fundamental principle of flap design.
• C. Flaps should be adequate for access. An undersized flap tears, is traumatized by retraction, and heals worse than a properly designed larger one. Flap size does not affect healing time when the base is broad and blood supply is preserved.
• D. Lingual flaps and lingual releasing incisions in the third molar region place the lingual nerve at direct risk and open the submandibular space. Access is obtained buccally.

Note: Core flap design rules: base wider than the free margin to preserve blood supply; full thickness mucoperiosteal elevation; margins on sound bone; avoid vital structures — mental nerve, lingual nerve, greater palatine artery; and never cross the papilla in the middle.