When placing a post, how much gutta-percha should remain apically to preserve the apical seal?

Correct answer: C. 4–5 mm

• A. 8–10 mm would leave inadequate post length for retention in most teeth.
• B. 1–2 mm is insufficient and carries a high risk of losing the apical seal and causing periapical breakdown.
• C. Correct. A minimum of 4–5 mm of gutta-percha must remain apically to maintain the APICAL SEAL — this is the single most important consideration in post space preparation. Additional rules: the post should not exceed one-third of the root diameter, at least 1 mm of circumferential dentin must remain, post length should be at least equal to the crown height, and the FERRULE (1.5–2 mm of circumferential tooth structure engaged by the crown) is what actually confers fracture resistance.
• D. Removing all gutta-percha destroys the apical seal and guarantees failure.