Treatment with Post & Core

There are numerous post and core systems. They are divided into different groups according to their functions, structures, methods of production and names of creators.

According to the method of production post and cores divide into two main groups: 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.

Post space and associated length of post

In post and core fabrication, it is important that the post descend at least two third of the length of root canal (or not less than the height of the crown) in order to provide a maximum possible retention for the using system. However, in the case of having a problem to reach the essential canal depth, then it is required to check the possibility of using the Nankali Post-system.

Basically, 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.

The post on the left is a tapered post, the one on the right is a parallel post.

It is not necessarily the length of the post within the root canal that provides for retention of the core, and thus the eventual crown, but rather the length of post that will exist within root structure that exists within surrounding bone. If the post is 16 mm long, but only extends 4 mm into root structure that is surrounded by solid bone, the restoration will have a poor prognosis. This consideration of crown-to-root ratio is essential when evaluating the tooth for a crown lengthening procedure.

In the picture at right, the two teeth on the extreme left and right are the ones under discussion. The two teeth in the middle have been endodontically treated, but do not have post and cores.

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 completed 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 zinc-oxide phosphate (ZOP). The ZOP should be mixed to a slightly more fluid consistency 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.

Maxillary posterior teeth restored with prefabricated screw posts.

Metal prefabricated post systems are being superseded by fibre-reinforced composite resin post systems.

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 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.

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

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.

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 & 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

:Post and Core Restored Tooth after Endodontic Treatment

Recommended by many well-known lecturers, the Ribbond endodontic post and core technique minimizes the chance for root fracture and has the following advantages.

• Compared to preformed posts, there is no additional tooth removal after endodontic treatment. This maintains the natural strength of the tooth.
• Eliminates the possibility of root perforation.
• Because it is made when the Ribbond is in a pliable state, it conforms to the natural contours and undercuts of the canal and provides additional mechanical retention.
• There are no stress concentrations at the tooth-post interface.
• The Ribbond post and core is passive and highly retentive.

Furthermore, because Ribbond's translucent fibers take on the color characteristics of the composite it allows for the natural transmission of light through teeth and crowns. This provides an exceptionally esthetic result.

Technique

Other than the normal endodontic treatment, further shaping is not required to accommodate the size and shape of a preformed post. The following is a brief description of the procedure. The Ribbond instruction manual, included with a purchase, clearly describes this technique in full detail.

Prepare the canal for normal dentin bonding. Inject a dual cured moderately filled composite such as a luting composite into the canal. Use the special Ribbond post and core instrument to carry the wetted Ribbond through the luting composite in the canal to the apical end of the canal.

Apply composite to the protruding Ribbond ends and roughly form them into the shape of the core and cure. Build up the core with composite, cure it and shape it