A tooth with a failing root canal treatment requires a post, core and crown, and the margin lies subgingivally. What is the correct sequence?

Correct answer: B. Endodontic retreatment, then crown lengthening, then post and core, then crown

• A. Omitting retreatment leaves the cause of failure untreated beneath a new, expensive restoration.
• B. CORRECT. The endodontic problem is addressed FIRST — there is no point building a restoration onto a tooth with untreated apical disease. CROWN LENGTHENING comes next, to establish the biologic width and expose sound tooth structure for a ferrule; this must precede the post and core because the surgery changes where the margin will sit and requires 6–8 weeks for tissue maturation. The post and core is then placed against a known margin position, and the crown last.
• C. Cementing the crown before crown lengthening leaves the margin violating biologic width, producing chronic inflammation, and the surgery would then destroy the crown margin.
• D. Placing the post and core before crown lengthening means the core is built to a margin that surgery will then move, requiring the core to be redone.

Note: The FERRULE is the reason crown lengthening precedes everything restorative: 1.5–2 mm of circumferential sound tooth structure coronal to the margin, encircled by the crown, is the single most important determinant of survival for a post-restored tooth — more important than the post material or design.