How much gutta-percha should remain in the apical portion of the canal when preparing space for a post?

Correct answer: D. 4–5 mm

• A. 6–10 mm would generally leave insufficient canal length for a post of adequate retention. Post length should ideally equal the crown height or three quarters of the root length.
• B. 1–2 mm is inadequate. Studies consistently show that leaving less than about 3–4 mm significantly increases apical leakage.
• C. Clearing the canal entirely destroys the apical seal altogether and would leave the root canal treatment functionally undone.
• D. CORRECT. A minimum of 4–5 mm of gutta-percha must remain APICALLY to preserve the apical seal. Removing more than this compromises the seal and allows coronal microleakage to reach the periapex — the leading cause of endodontic failure after post placement. This requirement is balanced against the need for adequate post length, and the post should not exceed two thirds of the root length or leave less than 4 mm of apical fill.

Note: Best practice is to remove the gutta-percha with heat (System B or a heated plugger) rather than a rotary drill, since heat is more controlled and far less likely to perforate or strip a curved root.