Biomechanical principles of rational root preparation
Anterior teeth that treated endodontically do not automatically require crowns. If a moderate sized anterior tooth is intact except for the endodontic access and small proximal lesion, composite resin restorations will be suffice. Placement of a post in such a tooth is more likely to weaken it than to strengthen it. But if there is severely damage crown a post-core is needed.
Posterior teeth that been treated endodontically must have occlusal coverage. Whenever enough tooth structure left that can withstand restoration retaining, no post is needed.
Premolars post can be used only if the roots are adequately long, bulky and straight.
On molars if there is one sound cusp the core may be retained by extension of amalgam into the pulpal chamber alone or with pins, no post needed.
Post is used when molars have little or no remaining coronal tooth structure.
Post is placed to provide retention for a crown. In order to use post tooth, canal should be obturated with gutta-percha.
The post length should be equal to the height of anatomical crown or 2/3 of root length to give good retention and stress (the longer the post the greater its retention). The canal should be prepared to the length and width and it should not be bigger in diameter than 1/3 of root cross-section in this area, with the root walls at least 1mm thick less than that will cause to weakening of root walls which can lead to fracture. Any undercuts in canal should be removed.
3-5mm of canal apex should be left with gutta-percha to prevent dislodgment and subsequent leakage.
In single canal teeth make anti-rotational elements - not more than 2mm in apical direction.
Crown margins must be placed on tooth structure to protect it from fracture. If the crown margin is not placed onto solid tooth structure, the risk of root fracture is increased.