Post and core
From Wikipedia, the free encyclopedia
A post and core is a dental restoration used to sufficiently build-up tooth structure for future restoration with a prosthetic crown when there is not enough tooth structure to properly retain the crown, due to loss of tooth structure to either decay or fracture. Post and cores are therefor referred to as foundation restorations.
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[edit] Treatment with a post and core
Because a post and core requires a prophylactic endodontic procedure, other, less obtrusive methods are sometimes employed in building a tooth up for future restoration with a crown. These methods would include an amalgam build-up, as well as those with resin materials, such as composite or materials dedicated to tooth build-up that may or may not release fluoride, such as Compcore and Fluoracore. These methods can incorporate the use of a retentive pin that is drilled into the remaining tooth structure.
If a pin-retained directly restored build-up will not suffice, a post and core system can be implemented. There are two types of post and core systems: prefabricated and cast. Both of these systems employ a metal post that is placed within the root canal or one of the root canals of the tooth being restored, and thus requires that the tooth be endodontically treated. After this procedure has been completed, and the root canal(s) is/are filled with the inert gutta percha root canal filling material, the gutta percha is removed from the entire length of the canal except for the apical 5 mm or so. The space that exists coronal to the remaining gutta percha, called the post space is now available within which to place a post.
[edit] Post space and associated length of post
In post and core fabrication, it is important that the post descend as far down the root canal as possible in order to provide as much retention as possible to the post and core system. However, it is important to leave at least 5 mm of gutta percha at the apex of the root canal, even at the expense of a longer post, because it is within the apical 5 mm of the root canal that 95% of lateral accessory canals split off from the main canal and anastomose with the exterior surface of the root. Should these lateral canals not be blocked with the gutta percha and the cement used to place the gutta percha, the chances of microleakage and percolation of microbes is drastically increased, thereby increasing the likelihood of an endodontic failure.
It is also important to keep in mind that it is not necessarily the length of the post within the root canal that provides for retention of the core, and thus the eventual prosthetic crown, but rather the length of post that will exist within root structure that exists within surrounding alveolar bone. If the post is 16 mm long, but only extends 4 mm into root structure that is surrounded by solid bone, the restoration has a poor prognosis. This consideration is very important to keep in mind when evaluating the tooth for a crown lengthening procedure.
[edit] Prefabricated post and cores
Prefabricated post and cores take less time to place, as they do not involve any lab work and can be inserted immediately upon the decision to utilize them, once the endodontic therapy has been compeleted and the post space cleared of gutta percha. They consist two separate entities, each composed out of its material, that are joined by the dentist at the time of the procedure. A universal metal post, sized for the particular diameter of the root canal, is placed as far down as possible into the post space and cemented with a permanent cement, such as ZOP, or zinc-oxide phosphate. The ZOP should be mixed to a slightly more fluid consistancy than when it is being used to cement a prosthetic crown, and can be more evenly distributed along the length of the post space if placed with a Lentulo spiral. After the prefabricated metal post is properly cemented into the post space, a core material, such as many of the same materials mentioned above that can be used with a pin-retained core build-up, can be packed around the cemented post. After the material has been cured or has had a chance to set and properly formed into a crown preparation, an impression can be taken for the fabrication of a prosthetic crown.
[edit] Cast post and cores
If added strength is required, a cast post and core can be custom fabricated for the tooth. A resin pattern is produced by placing a preformed plastic post into the post space as though it were a prefabricated metal post, and a resin material, such as GC pattern resin, is used to build up the tooth to the proper dimensions. When this is completed, the pattern resin/plastic post is removed from the tooth structure and attached to a sprue former, much in the same way as a is done with the wax pattern of a prosthetic crown, and a single-unit cast post and core can thus be fabricated out of gold, titanium or another metal using the lost-wax technique.
[edit] Post design
There are many types of post designed available for cast post and cores, utilizing various combinations of the follwing properties:
- parallel vs. tapered
- smooth-sided vs. serrated vs. threaded
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 narrowest diameter post ensures that as much natural tooth structure as possible is left to support the post and absorb the transmitted forces; the largest ideal diameter for a post is ⅓ the diameter of the root at the most apical portion of the post space. 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.
[edit] Posts weaken teeth
There is much confusion surrounding the post and core restoration, even among dental practitioners themselves. The use of a post and core does not strengthen the tooth prior to restoration with a crown; rather, it unquestionably contributed 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 inherant 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 inherant in the use of post and core restorations that, when all of the independant 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 & prosthetic crown), the patient is sometimes advised to have the tooth extracted and an implant placed.
[edit] References
Herbert T. Shillingburg. FUNDAMENTALS OF FIXED PROSTHODONTICS, 1979.
