What is the most critical factor for the long-term success of an endodontic post?

The ferrule effect is the most critical determinant of post and core success. A ferrule is a 1.5-2 mm circumferential band of sound tooth structure coronal to the finish line of the crown preparation. It provides resistance to fracture by distributing functional forces and preventing root fracture and crown dislodgement. Posts do NOT provide strength to the root; they simply provide retention for the core material. Without adequate ferrule (less than 1.5 mm of sound dentin), the risk of catastrophic root fracture is very high. The diameter of the post should be conservative (no more than one-third of root diameter) to preserve root dentin.