CAST POST & COREFabrication Technique

Preoperative condition of the tooth. Tooth maybe fractured and/or contain various amounts oftemporary or permanent restorative material.

The tooth will probably have temporary (orpermanent) restorative material in the endodonticaccess opening. There may be a cotton pelletunder the temporary.

Reduce the tooth axially and incisally. Toothshould receive full reduction commensurate withfinal crown preparation. Preparation of axial andincisal tooth structure facilitates access andmakes gutta percha removal easier. Finalmargination need not be done at this time.

Alternate First Step: You may have an exactrecorded measurement of canal length relative toa known landmark (e.g. incisal edge). If you aretotally confident of the measurement (e.g. youpersonally did the endodontic treatment on thistooth) you may remove the gutta percha prior toaxial and incisal reduction (refer to techniquebelow).

Be careful not to rely solely on measurement ofradiographs (since there is usually amagnification error) or someone else'smeasurements (which may be recorded in error ormay have been made relative to a differentlandmark).

Remove all old permanent and/or temporaryrestorative material (including material in theaccess opening and pulp chamber). Remove allcaries.

Remove any undercuts created by removal of therestorative material and/or caries. Remove anyundercuts from the pulp chamber. All undercutsmust be either removed with a diamond orblocked out with a base (do not use eugenolcontaining material that will inhibit set ofacrylic).

If undercuts cannot be successfully eliminated, atechnique other than the one described belowmust be employed to fabricate the post/corepattern.

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Remove any weak, thin, and/or unsupportedtooth structure. Do not remove any moresound tooth structure than is necessary toaccomplish the above steps.

If additional tooth structure has not beenremoved from parts of the incisal edge(beyond the initial reduction), then reduce theincisal in these areas an additional 1.0 mm sothat the entire incisal edge of the tooth willultimately be covered by metal of the castpost/core. Use a shallow contrabevel on theincisal edge.

With a hot instrument (Glick heated over aBunsen burner) remove gutta percha from thecoronal portion of the canal to better visualizethe direction of the canal. Experiencedoperators may opt to go directly to removal ofgutta percha with a Pesso reamer, but careshould be taken not to get "off-line" andperforate the root.

Using progressively larger diameter Pessoreamers remove the gutta percha to thepredetermined initial depth (established bymeasurement of a radiograph). Use a rubberstop placed on the shank of the instrument.Stop short of the intended final depth andconfirm the amount of gutta percha removalwith another radiograph.

Repeat this process as necessary leaving 4-5mm of gutta percha as an apical seal. Make afinal radiograph when gutta percha removalhas been completed. Do not arbitrarily enlargethe canal in diameter. A #3 Pesso reamer isusually more than adequate (remember a #3Pesso = #4 Gates = #110 file).

If there are no irregular features in the toothpreparation you may need to add a keyway toresist rotation of the post/core. Place thekeyway in the bulkiest part of the remainingtooth structure. Use a small diameter tapereddiamond 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 thecoronal portion of the tooth preparation that isconsistent with the direction of the canal. Youmay need to "flare" the access opening slightlyat this point so the Duralay acrylic does notget locked-in during the fabrication of thepost/core pattern. Do not use high speedinstrumentation in the canal itself (pulpchamber only).

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Be sure there is a positive vertical stopfor the post/core so that the casting doesnot act as a wedge (which may split thetooth). This may be a flat area (90degrees relative to the path of draw) or aslight contrabevel around the perimeter ofthe preparation (creating a "ferrule"effect). Fabricate a temporary crownbefore making the post/core pattern.

Try-in the preformed plastic post and besure it goes all the way down the preparedcanal without binding (a totally passivefit). Trim it as necessary or custom makea post from a plastic sprue pin. Measurethe post relative to the depth of the canalwith a periodontal probe if you are notsure.

Lubricate the canal with Duralaylubricant. Use a perio probe, paper point,or cotton on a barbed broach to carry thelubricant to place. A thin coating is allthat is necessary. Do not leave the canalfilled with lubricant. Also be sure a thinlayer of lubricant covers all coronalsurfaces of the tooth that will come intocontact with the Duralay resin.

Be sure you check the fit of the postbefore you lubricate the canal so thelubricant does not interfere with thebonding of the Duralay to the plastic post.

Using the "bead brush" technique, fill thecanal completely with Duralay resin. Usethe bristles of a brush (bent at a 45 degreeangle for easier access) to force acrylicdown the canal and express any trappedair.

Immediately, dip the plastic post inacrylic liquid (to soften post and enhancebond of acrylic) and seat it in the canal toits full depth. Allow the Duralay to setcompletely. Some clinicians advocatemoving the post up and down in the canal1-2 mm (only after the Duralay is nearlyset) to avoid getting it "locked in." If thismethod is used, do not remove the postfrom the canal until the Duralay iscompletely set.

Using hemostats, carefully remove thepost pattern and inspect it to be sure it isfully formed (with no voids). If the postshould break it can usually be removedby heating an instrument and "searing it"to the plastic post in the canal. Aftercooling, the plastic post will be stuck tothe instrument and can be removed. Greatcare should be exercised if an attempt ismade to remove the post with a rotaryinstrument (e.g. Pesso reamer) since theinstrument may easily cut the lateral wallof the canal (instead of the plastic post)and cause a perforation. After removal ofa broken post, be sure there are noundercuts (you may want to lightlyresurface the walls of the canal again witha Pesso reamer) and relubricate beforeproceeding.

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If the post portion of the pattern is acceptable,place it back into the canal (it is easier to removewith hemostats, but easier to replace with yourfingers). Be sure it goes completely to place. Ifunacceptable, attempts to add acrylic to adeficient post usually result in a post that cannotbe reseated completely. It is generally easier tomake a new post before continuing.

Cut off the top of the plastic post so that yourpatient can close completely (the patient hasprobably had his/her mouth open a good while atthis point). Do not try to maintain the top of theplastic post to use as a casting sprue.

Use the "bead brush" technique to build the coreportion of the pattern to full contour. Build insmall increments so that the Duralay does not"slump and run" making the subsequentpreparation step more difficult. The core portionshould be slightly overbuilt (avoid grosslyoverbuilding which will only necessitateadditional preparation later).

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

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