When designing a flap for removal of an impacted third molar, the incision margins should:

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

• A. Crossing directly over the defect is the error the principle is meant to prevent.
• B. CORRECT. A fundamental principle of flap design: the incision line must be supported by INTACT BONE at closure. If the suture line lies over the bony void created by the extraction, the flap collapses into the defect, the wound margins separate, and dehiscence with delayed healing follows. This is why envelope and triangular flaps are extended beyond the anticipated bony window.
• C. Lingual incisions in the third molar region risk injury to the LINGUAL NERVE, which may lie at or above the crest — this is precisely what to avoid.
• D. Terminating at the mucogingival junction is not a requirement and would compromise the base of the flap.

Note: Other flap rules: broad base for blood supply, no vertical release over the mental foramen or in the palate, and full-thickness reflection.