Ferrule is a feature of:
Retention form
Resistance form
Conservation of tooth structure
Antirotational design
Although there is evidence that preserving as much coronal tooth structure as possible will enhance prognosis, it is less clear whether the prognosis will improve by creating a ferrule in an extensively damaged tooth by surgical crown-lengthening. In this latter circumstance, although the crown-lengthening allows a ferrule, it also leads to a much less favorable crown to root ratio and therefore increased leverage on the root during function.