Post Design
There are many types of post designed available for cast post and cores, utilizing various combinations of the following properties:
- parallel vs. tapered
- smooth-sided vs. serrated vs. threaded
- post only vs. with additional single-circle ring
In a classic case one of the best design for a post to decrease the risk of failure is the narrowest & longest smooth, parallel post that one can fit into the post space. Utilizing the longest possible post ensures that the forces transmitted from the crown are distributed over as much of the root as possible. Using the appropriate diameter post ensures that as much natural tooth structure as possible is left to support the post and absorb the transmitted forces. A parallel post ensures the greatest retention of the post within the canal, and is perhaps utilized with only the slightest loss of tooth structure to the internal wall of the canal. A smooth-surfaced post, although less retentive than either serrated or threaded post surfaces, transmits the least amount of force to the root structure. While both smooth and serrated posts are passive, in that they simply lie within the post space after being cemented, threaded posts actively engage the internal walls of the root canal as they are screwed in, and, while being the most retentive by far, produce such a force on the brittle root structure that they are contraindicated in most situations.
The use of a post and core does not strengthen the tooth prior to restoration with a crown; rather, it may contribute to the weakening of the tooth structure, as the forces placed upon the future prosthetic crown and core are now transmitted along virtually the entire length of the brittle, endodontically treated tooth. 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.
The post with additional single-circle ring increases the contact surface area between the core and involved hard tissue of tooth significantly, therefore this system does not require a post with at least 2/3 of the root canal depth as its indication.
Read more about this topic: Post And Core
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