Post and core
[1][2] The role of the post is firstly to retain a core restoration and crown, and secondly to redistribute stresses down onto the root, thereby reducing the risk of coronal fracture.
The post does not play any role in reinforcing or supporting the tooth and can in fact make it more likely to fracture at the root.
However, there are also disadvantages, during the preparation for the post space there is a risk of perforation, a post can also make a tooth more likely to fracture, it makes future orthograde root canal treatment much more difficult and finally it is very destructive and requires excessive removal of tooth tissue.
A better alternative to posts on a posterior tooth is core restoration which extends down into the entrance of the root canal through the Nayyar technique using an amalgam dowel–core.
[7] In this technique, retention for the amalgam-core is derived from the remaining pulp chamber and the prepared canals by extending amalgam to these areas.
Gutta percha can be removed mechanically (use of Gates Glidden),[9] thermally (use of System B Tip),[10] and chemically (use of chemical solvents, however this method is not advocated nowadays due to difficulty in controlling the depth of softening)[9] The space that exists coronal to the remaining gutta percha, called the post space, is now available within which to place a post.
This procedure does not even require local anesthesia as the tooth has long been dead after the root canal treatment and no pain is felt.
[10] Overall, non-metallic posts show a lower incidence of root fracture due to their ability to resist high stress without permanent deformation and breaking.
Zirconium posts can't be etched, thus leading to difficulty in retention of composite core.
However, they can only be used when there is sufficient tooth tissue remaining as they can delaminate upon exposure to moisture and this can lead to post failure.
[12] Carbon fibre posts show very little deformation and are able to absorb and transfer forces similarly to dentine.
However, it is difficult to produce this type of material as fibre bundles require infiltration and wetting with resin.
[10] Traditionally metal posts have been used due to the good corrosion resistance and high yield strength.
This inherent drawback is taken into account when the prognosis of the finished restoration is determined and explained to the patient prior to the onset of treatment.
It is because of this increased risk of failure inherent in the use of post and core restorations that, when all of the independent failure rates of the many procedures needed for the restoration of the tooth are considered together (endodontic treatment, crown lengthening (when indicated), post and core and prosthetic crown), the patient is sometimes advised to have the tooth extracted and an implant placed.
Its use displaces the gingival tissues long enough to allow the impression material to flow down below the margin preparation lines and into the orifice.
[10] Once the cord has been left in place for the required length of time, it is removed allowing the sulcus to be rinsed and dried in order to prevent the haemostatic agent from creating problems with the set of the impression material.
