Post And Core Fabrication

CAST POST & CORE Fabrication Technique

Preoperative condition of the tooth. Tooth may be fractured and/or contain various amounts of temporary or permanent restorative material.

The tooth will probably have temporary (or permanent) restorative material in the endodontic access opening. There may be a cotton pellet under the temporary.

Reduce the tooth axially and incisally. Tooth should receive full reduction commensurate with final crown preparation. Preparation of axial and incisal tooth structure facilitates access and makes gutta percha removal easier. Final margination need not be done at this time.

Alternate First Step: You may have an exact recorded measurement of canal length relative to a known landmark (e.g. incisal edge). If you are totally confident of the measurement (e.g. you personally did the endodontic treatment on this tooth) you may remove the gutta percha prior to axial and incisal reduction (refer to technique below). Be careful not to rely solely on measurement of radiographs (since there is usually a magnification error) or someone else's measurements (which may be recorded in error or may have been made relative to a different landmark).

Remove all old permanent and/or temporary restorative material (including material in the access opening and pulp chamber). Remove all caries.

Remove any undercuts created by removal of the restorative material and/or caries. Remove any undercuts from the pulp chamber. All undercuts must be either removed with a diamond or blocked out with a base (do not use eugenol containing material that will inhibit set of acrylic). If undercuts cannot be successfully eliminated, a technique other than the one described below must be employed to fabricate the post/core pattern.

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Remove any weak, thin, and/or unsupported tooth structure. Do not remove any more sound tooth structure than is necessary to accomplish the above steps.

If additional tooth structure has not been removed from parts of the incisal edge (beyond the initial reduction), then reduce the incisal in these areas an additional 1.0 mm so that the entire incisal edge of the tooth will ultimately be covered by metal of the cast post/core. Use a shallow contrabevel on the incisal edge.

With a hot instrument (Glick heated over a Bunsen burner) remove gutta percha from the coronal portion of the canal to better visualize the direction of the canal. Experienced operators may opt to go directly to removal of gutta percha with a Pesso reamer, but care should be taken not to get "off-line" and perforate the root.

Using progressively larger diameter Pesso reamers remove the gutta percha to the predetermined initial depth (established by measurement of a radiograph). Use a rubber stop placed on the shank of the instrument. Stop short of the intended final depth and confirm the amount of gutta percha removal with another radiograph. Repeat this process as necessary leaving 4-5 mm of gutta percha as an apical seal. Make a final radiograph when gutta percha removal has been completed. Do not arbitrarily enlarge the canal in diameter. A #3 Pesso reamer is usually more than adequate (remember a #3 Pesso = #4 Gates = #110 file).

If there are no irregular features in the tooth preparation you may need to add a keyway to resist rotation of the post/core. Place the keyway in the bulkiest part of the remaining tooth structure. Use a small diameter tapered diamond or bur. A single keyway is sufficient (two shown in diagram*).

*Shillingburg 2nd edition p. 151

Be sure that you have a path of draw for the coronal portion of the tooth preparation that is consistent with the direction of the canal. You may need to "flare" the access opening slightly at this point so the Duralay acrylic does not get locked-in during the fabrication of the post/core pattern. Do not use high speed instrumentation in the canal itself (pulp chamber only).

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Be sure there is a positive vertical stop for the post/core so that the casting does not act as a wedge (which may split the tooth). This may be a flat area (90 degrees relative to the path of draw) or a slight contrabevel around the perimeter of the preparation (creating a "ferrule" effect). Fabricate a temporary crown before making the post/core pattern.

Try-in the preformed plastic post and be sure it goes all the way down the prepared canal without binding (a totally passive fit). Trim it as necessary or custom make a post from a plastic sprue pin. Measure the post relative to the depth of the canal with a periodontal probe if you are not sure.

Lubricate the canal with Duralay lubricant. Use a perio probe, paper point, or cotton on a barbed broach to carry the lubricant to place. A thin coating is all that is necessary. Do not leave the canal filled with lubricant. Also be sure a thin layer of lubricant covers all coronal surfaces of the tooth that will come into contact with the Duralay resin. Be sure you check the fit of the post before you lubricate the canal so the lubricant does not interfere with the bonding of the Duralay to the plastic post.

Using the "bead brush" technique, fill the canal completely with Duralay resin. Use the bristles of a brush (bent at a 45 degree angle for easier access) to force acrylic down the canal and express any trapped air.

Immediately, dip the plastic post in acrylic liquid (to soften post and enhance bond of acrylic) and seat it in the canal to its full depth. Allow the Duralay to set completely. Some clinicians advocate moving the post up and down in the canal 1-2 mm (only after the Duralay is nearly set) to avoid getting it "locked in." If this method is used, do not remove the post from the canal until the Duralay is completely set.

Using hemostats, carefully remove the post pattern and inspect it to be sure it is fully formed (with no voids). If the post should break it can usually be removed by heating an instrument and "searing it" to the plastic post in the canal. After cooling, the plastic post will be stuck to the instrument and can be removed. Great care should be exercised if an attempt is made to remove the post with a rotary instrument (e.g. Pesso reamer) since the instrument may easily cut the lateral wall of the canal (instead of the plastic post) and cause a perforation. After removal of a broken post, be sure there are no undercuts (you may want to lightly resurface the walls of the canal again with a Pesso reamer) and relubricate before proceeding.

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If the post portion of the pattern is acceptable, place it back into the canal (it is easier to remove with hemostats, but easier to replace with your fingers). Be sure it goes completely to place. If unacceptable, attempts to add acrylic to a deficient post usually result in a post that cannot be reseated completely. It is generally easier to make a new post before continuing.

Cut off the top of the plastic post so that your patient can close completely (the patient has probably had his/her mouth open a good while at this point). Do not try to maintain the top of the plastic post to use as a casting sprue.

Use the "bead brush" technique to build the core portion of the pattern to full contour. Build in small increments so that the Duralay does not "slump and run" making the subsequent preparation step more difficult. The core portion should be slightly overbuilt (avoid grossly overbuilding which will only necessitate additional preparation later).

Using a large diameter coarse diamond (high speed handpiece at "near stall" speed with water spray), shape the pattern to ideal preparation form (on the tooth). If the handpiece is used at high speed the diamond will tend to burn the acrylic. Have patient close in order to check lingual clearance. Be sure there is at least a 1.0 mm "cap" of acrylic over the entire incisal of the tooth preparation.

Using hemostats (held mesiodistally), remove the pattern from the tooth. The pattern should not be removed (except one time to check that the post portion is fully formed) until it is completed. There is the risk of breakage each additional time the pattern is removed.