The purpose of an endodontic post is to retain a core that is necessary due to extensive loss of coronal tooth structure, according to the newsletter of the American Association of Endodontists. If other anatomic features are available to retain a core, posts should be avoided. In the case of molar teeth, the pulp chamber and canals will usually retain a core, and a post may not be needed. If a post is required for molar teeth, it should be placed in a distal canal for mandibular molars and the palatal canal for maxillary molars. In the case of anterior teeth that have extensive loss of coronal tooth structure, a post is usually needed. When using a post, there is attendant risk of root fracture, especially if sound dentin has been removed during preparation. When a post is needed, little if any dentin beyond what is needed to perform root canal treatment should be removed. A minimum of 4 mm of gutta-percha should be retained apically when placing a post. A post should be placed to extend apical to the crest of the bone, since the crest is where forces are concentrated during function. A guideline is that a post should extend, "into bone" at least as far as it protrudes "out of bone."
(Source: Endodontics: Colleagues for Excellence, Spring/Summer 2004)