A full-thickness flap is being raised to remove an impacted tooth #17. Which principle governs where the flap margins are placed?

Correct answer: A. The flap margins must rest on sound bone when the flap is repositioned

• A. CORRECT. The incision line must lie over intact bone once the flap is closed. If the margin sits over the osseous window created during bone removal, it has nothing to support it, the wound dehisces and healing is delayed. Plan the flap AFTER deciding how much bone you will remove, not before.
• B. Reversed. The BASE must be broader than the free margin so the flap keeps its blood supply. A narrow base starves the flap and causes necrosis.
• C. Releasing incisions are placed at the line angles, in the concavity BETWEEN root prominences. Over a prominence the bone is thin and the tissue recedes.
• D. This is the error the principle exists to prevent. A margin over a defect has no underlying support, collapses into the void and breaks down.

Note: The full set of flap rules worth memorising: broad base, adequate size (a bigger flap heals no slower than a small one), full thickness including periosteum, margins on sound bone, avoidance of vital structures such as the mental nerve and lingual nerve, and releasing incisions at line angles rather than over papillae or root prominences.