Core Fabrication

The core of a post-and-core restoration replaces missing coronal tooth structure and thereby forms

Prefabricated Post And Core
Fig. 12-36. Post-and-core patterns made by adding wax to prefabricated plastic posts.

the shape of the tooth preparation. It can be shaped in resin or wax and added to the post pattern before the assembly is cast in metal. This prevents possible failure at the post-core interface. The core can also be cast onto most prefabricated post systems (although there is then some concern that the casting process may unfavorably affect the physical properties of wrought metal posts). A third alternative is to make the core from a plastic restorative material such as amalgam, glass ionomer, or composite resin.

Plastic Filling Materials. The advantages of amalgam, glass ionomer, or resin include the following:

1. Maximum tooth structure can be conserved because undercuts do not need to be removed.

2. Treatment requires one less patient visit.

3. There are fewer laboratory procedures.

4. Testing generally shows good resistance to fatigue testing71 and good strength characteristics^ possibly because of the good adapta tion to tooth structure. However, these plastic restorative materials, especially the glass ionomers, have lower tensile strength than do cast metals.

Disadvantages include the following:

1. Long-term success may be affected by corrosion of amalgam cores, the low strength of glass ionomer' 73 or the continued polymeriza tion'' and high thermal expansion coefficients of composite resin cores.

2. Microleakage with temperature fluctuations (thermocycling) is greater under composite resin and amalgam cores than under conven tional crown preparations'-' (however, the extent of leakage under cast cores has yet to be determined).

3. Difficulty may be encountered with certain operative procedures such as rubber dam or matrix application (particularly on badly damaged teeth).

Amalgam cores are suitable for restoring posterior teeth, particularly when some coronal structure remains. The procedure described by Nayyar et a1, 42 with amalgam also used for the posts, is conservative of tooth structure. The cores are placed during the same appointment as the root canal obturation, because then the teeth are still isolated by the rubber dam, the root canal morphology is still fresh in the practitioner's mind, and the cores can serve as a support for the provisional restoration (Fig. 12-37).

Step-by-Step Procedure for Amalgam (see also Chapter 6).

1. Apply the rubber dam and remove guttapercha from the pulp chamber as well as 2 to 4 mm into each root canal if less than 4 mm of coronal height remains. Use a warmed endodontic instrument.

2. Remove any existing restoration, undermined enamel, or carious or weakened dentin. Establish the cavity form using conventional principles of resistance and retention form. Even if cusps are missing, pins are not normally required because adequate retention can be gained by extending the amalgam into the root canals.

3. If you suspect that the floor of the pulp chamber is thin, protect it from condensing pressures with a cement base.

4. Fit a matrix band. Where lack of tooth structure makes the application of a conventional matrix system difficult, an orthodontic or annealed copper band may be used.

5. Condense the first increments of amalgam (select a material with high early strength)

into the root canals with an endodontic plugger.

6. Fill the pulp chamber and coronal cavity in the conventional manner.

7. Carve the alloy to shape. The impression can be made immediately. Alternatively, the amalgam can be built up to anatomic contour and later prepared for a com-plete crown. Under these circumstances, avoid forces that would fracture the tooth or newly placed restoration.

Cast Metal. Cast metal cores have the following advantages:

1. They can be cast directly onto a prefabricated post, providing a restoration with good strength characteristics.

2. Conventional high-noble, metal-content alloys can be used.

3. An indirect procedure can be used, making restoration of posterior teeth easier.

Direct Procedure for Single-rooted Teeth

Direct patterns can be formed by combining a prefabricated post with autopolymerizing resin. Alternatively, a thermoplastic material can be used to create a post pattern ' 76 and the core portion can be developed in either autopolymerizing resin, light polymerized resin, or wax.

Pattern Fabrication with Autopolymerizing

1. Use a prefabricated metal or custom acrylic resin post.

2. Add resin by the "bead" technique, dipping a small brush in monomer and then into

Dental Indirect Post Pattern Fabrication
Fig. 12-37. Retention for an amalgam core can be obtained from the root canal system, preserving as much tooth structure as possible. (B to D courtesy Dr. M. Padilla)
Amalgam Core
Fig. 12-38. Direct pattern for a single-rooted tooth.

polymer and applying it to the post. Some experts recommend light-cured resin to facilitate this step."

3. Slightly overbuild the core and let it polymerize fully (Fig. 12-38, A).

4. Shape the core with carbide finishing burs or paper disks (Fig. 12-38, B). Use water spray to prevent overheating of the acrylic resin. Correct any small defects with wax.

5. Remove the pattern (Fig. 12-38, C; sprue and invest it immediately.

Direct Pattern for Multirooted

A direct pattern can be used for multirooted posterior teeth, although limited access may make the indirect approach easier. A single-piece core with auxiliary posts is used, as opposed to the multisection core recommended for indirect posterior cast post-and-cores. The core is cast directly onto the post of one canal. (The other canals already have prefabricated posts that pass through holes in the core.)

Nayyar Core Amalgam

Fig. 12-39. A direct post-and-core for posterior teeth can be made by cementing a prefabricated post through a casting. Here the two buccal canals had a common path of withdrawal and could be incorporated into the core casting. More typically, only one canal has a fixed post, and the others are cemented through the core.

Fig. 12-39. A direct post-and-core for posterior teeth can be made by cementing a prefabricated post through a casting. Here the two buccal canals had a common path of withdrawal and could be incorporated into the core casting. More typically, only one canal has a fixed post, and the others are cemented through the core.

The procedure is simple, as long as smooth parallel-sided or tapered posts are used.

1. Fit prefabricated posts into the prepared canals. One post is roughened; the others are left smooth and lubricated. All posts should extend beyond the eventual preparation.

2. Build up the core with autopolymerizing resin, using the bead technique.

3. Shape the core to final form with carbide finishing burs.

4. Grip the smooth, lubricated posts with forceps and remove them.

5. Remove, invest, and cast the core with the roughened single post. When this has been done, the holes for the auxiliary posts can be refined with the appropriate twist drill.

6. After verifying the fit at try-in, cement the core and auxiliary posts to place.

Indirect Pattern for Posterior

1. Wax the custom-made posts as described previously.

2. Build part of the core around the first post.

3. Remove any undercuts adjacent to other post holes and cast the first section.

4. Wax additional sections and cast them.

Using dovetails to interlock the sections makes the procedure more complicated and is probably of li mited benefit, especially because the final buildup is held together by the fixed cast restoration.

provisional restorations (see Chapter 15)

To prevent drifting of opposing or adjacent teeth, an endodontically treated tooth requires a proper provisional restoration immediately following completion of endodontics (Fig. 12-41). Of particular importance are good proximal contacts to prevent

Cast Post And Core
Fig. 12-40. A to D, Multipiece post-and-cores can be made by the indirect technique, waxing each section to ensure that no undercuts are created. E to H, Alternatively, interlocking sections can be made, but this complicates the laboratory phase.

tooth migration leading to unwanted root proximity. If a cast post-and-core is made, an additional provisional restoration is needed while the post-and-core is being fabricated. This can be retained by fitting a wire (e.g., a paper clip or orthodontic wire) into the prepared canal. The restoration is then conveniently fabricated with autopolymerizing resin by the direct technique.

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  • efrem
    How to use dental post core?
    8 years ago

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