What is the most important factor determining the prognosis of a post-retained restoration?

Correct answer: D. The quantity of remaining coronal tooth structure (ferrule)

• A. A wider post does not strengthen the tooth. It requires removal of more radicular dentine, thinning the root walls and INCREASING fracture risk. Post diameter should not exceed about one third of the root width.
• B. Canal diameter is an anatomical given that constrains post selection, but it is not what determines prognosis.
• C. Post material influences the failure MODE — a fibre post flexes with dentine and tends to fail retrievably, whereas a rigid metal post transmits stress and is associated with catastrophic root fracture — but it is secondary to ferrule.
• D. CORRECT. REMAINING CORONAL TOOTH STRUCTURE is the single strongest predictor of survival. What protects an endodontically treated tooth is the FERRULE — a 1.5–2 mm circumferential band of sound dentine encircled by the crown, which resists the wedging forces that split roots. Studies consistently show that teeth with an adequate ferrule survive well regardless of post type, and teeth without one fail regardless of post type.

Note: Reminder of the underlying principle: a post does NOT reinforce a tooth. Its only function is to retain the core when insufficient coronal structure remains.