The “ferrule effect” refers to the height of the intact, uncut vertical wall above the gingival margin of a tooth to be crowned. Mechanically, an ideal ferrule effect requires at least 1.5 to 2 mm of intact tooth tissue above the gingival margin, and at least 1 mm of thickness all around. Just like an iron ring around a barrel, the crown must firmly grip this 2 mm of intact tooth tissue. If the tooth is completely destroyed due to caries and there is no ferrule effect, all chewing loads are placed on the fiber post and root embedded in the tooth; this can lead to the post dislodging quickly or the root cracking vertically. If the ferrule effect is insufficient, this distance must be artificially achieved by surgically lowering the gingival and bone levels (crown lengthening).
