Restoration of Endodontically Treated Teeth

Lecture Twelve------Operative dentistry

Restoration of endodontically treated teeth

An endodontically treated tooth should have a good prognosis. It can resume full function and serve satisfactorily as an abutment for a fixed or removable partial denture. However, special techniques are needed to restore such a tooth. Usually a considerable amount of tooth structure has been lost because of caries, endodontic treatment, and the placement of previous restorations. The loss of tooth structure makes retention of subsequent restorations more problematic and increases the likelihood of fracture during functional loading. Two factors influence the choice of technique: the type of tooth (whether it is an incisor, canine, premolar, or molar) and the amount of remaining coronal tooth structure. The latter is probably the most important indicator when determining the prognosis.

Before restoration, existing

Endodontically treated teeth need to be assessed carefully for the following:

• Good apical seal
• No sensitivity to pressure
• No exudates
• No fistula
• No apical sensitivity
• No active inflammation

CONSIDERATIONS FOR ANTERIOR TEETH

Endodontically treated anterior teeth do not always need complete coverage by placing a complete crown, except when plastic restorative materials have limited prognosis (e.g., if the tooth has large proximal composite restorations and unsupported tooth structure). Many otherwise intact teeth function satisfactorily with a composite resin restoration. Discoloration in the absence of significant tooth loss may be more effectively treated by bleaching than by the placement of a complete crown, although not all stained teeth can be bleached successfully.

Resorption can be an unfortunate side effect of nonvital bleaching. However, when loss of coronal tooth structure is extensive or the tooth will be serving as an FPD or RPD abutment, a complete crown becomes mandatory. Retention and support then must be derived from within the canal because a limited amount of coronal dentin remains once the reduction for complete coverage has been completed.

Coupled with the loss of internal tooth structure necessary for endodontic treatment, the remaining walls become thin and fragile often requiring their reduction in height.

CONSIDERATIONS FOR POSTERIOR TEETH

Endodontically treated posterior teeth are subject togreater loading than anterior teeth because of theircloser proximity to the transverse horizontal axis.

This, combined with their morphologic characteristics(having cusps that can be wedged apart), makesthem more susceptible to fracture. Careful occlusaladjustment will reduce potentially damaging lateral forces during excursive movements. Nevertheless,endodontically treated posterior tooth should receivecuspal coverage to prevent biting forces fromcausing fracture. Possible exceptions are mandibularpremolars and first molars with intact marginalridges and conservative access cavities not subjectedto excessive occlusal forces (i.e., posterior disclusionin conjunction with normal muscle activity).

Complete coverage is recommended on teethwith a high risk of fracture. This is especially truefor maxillary premolars, because complete coveragegives the best protection against fracture, since thetooth is completely encircled by the restoration.However, considerable tooth reduction is required,particularly when a metal-ceramic restoration is tobe used. When significant coronal tooth loss has occurred, a cast post-and-core or an amalgamfoundation restoration is needed.

Tooth preparation for endodontically treated teethcan be considered a three-stage operation:

1.Removal of the root canal filling material tothe appropriate depth.

2.Enlargement of the canal.

3.Preparation of the coronal tooth structure

PRINCIPLES OF TOOTHPREPARATION

1. CONSERVATION OF TOOTH STRUCTURE:
2. Preparation of the Canal:When creatingpost space, great care must be used to removeonly minimal tooth structure from the canal. Excessiveenlargement can perforate or weaken the root,which then may split during cementation of the postor subsequent function. The thickness of the remainingdentin is the prime variable in fracture resistanceof the root. Nevertheless, it isdifficult to enlarge a root canal uniformly and tojudge with accuracy how much tooth structure hasbeen removed and how thick the remaining dentinis. Most roots are narrower mesiodistally than faciolinguallyand often have proximal concavities thatcannot be seen on a standard periapical radiograph.
3. Preparation of Coronal Tissue.

Endodonticallytreated teeth often have lost much coronal toothstructure as a result of caries, of previously placedrestorations, or in preparation of the endodontic accesscavity. However, if a cast core is to be used, furtherreduction is needed to accommodate a completecrown and to remove undercuts from thechamber and internal walls. This may leave very littlecoronal dentin. Every effort should be made tosave as much of the coronal tooth structure as possible,because this helps reduce stress concentrationsat the gingival margin." The amount of remainingtooth structure is probably the single mostimportant predictor of clinical success. If more than2 mm of coronal tooth structure remains, the postdesign probably has a limited role in the fracture resistance of the restored tooth.

2. RETENTION FORM

Anterior Teeth:

Dislodgment of a post-retainedanterior crown is frequently seen clinically andresults from inadequate retention form of the preparedroot. Post retention is affected by the preparationgeometry, post length, diameter, surface texture,and by the luting agent.

Posterior Teeth:

Relatively longposts with a circular cross section provide good retentionand support in anterior teeth but should beavoided in posterior teeth, which often have curvedroots and elliptical or ribbon-shaped canals. Forthese teeth, retention is better provided by two ormore relatively short posts in the divergent canals.

When amalgam is used as the core material, it canbe condensed either around cemented metal postsor directly into short, prepared post spaces. If morethan 3 to 4 mm of coronal tooth structure remains,use of the root canals for retention is not necessary,and this avoids the chance of perforation. Usingthe canals for retention can provide good results, although the strength of the tooth once a completecrown has been provided is not dramatically influencedby differences in technique.

Mandibular premolars and molars with a reasonableamount of remaining coronal tooth structure,when coupled with a circumferential cervical bandof tooth structure with restricted taper of about 2mm, can often be restored with amalgam directlycondensed into the chamber. Core buildups in molarswith one or more missing cusps will benefitfrom one or more cemented posts around which theamalgam can be condensed. The posts provide the additional retention, which was compromised becauseof the missing tooth structure. In mandibularmolars, the larger distal canal is recommended forpost placement. In maxillary molars, the palatalcanal is used.

Types of posts:

A- Prefabricated Posts

1.Enlarge the canal one or two sizes with a drill,endodontic file, orreamer that matches theconfiguration of the post. When using rotary instruments, alternatebetween the Peeso-Reamers and twist drillsthat correspond in size. In the case of athreaded post, the appropriate drill is followedby a tap that prethreads the internalwall of the post space. Parallel-sided posts aremore retentive and distribute stresses betterthan tapered posts, but they do not conformwell to the shape of a canal that has beenflared to facilitate condensation of gutta-percha.

In this situation, it may not be possible toenlarge the canal sufficiently toprovide adequateretention for the post; in that case, a taperedcustom-made post is preferred.

2. Use a prefabricated post that matches standard endodontic instruments. A tapered post will conform better tothe canal than a parallel-sided post and requiresless removal of dentin to achieve anadequate fit. However, it will be slightly lessretentive and will cause greater stress concentrations,although retention may be improvedby controlled grooving.

3. Be especially careful not to remove moredentin at the apical extent of the post spacethan is necessary.

NOTE: If careful measurement techniques havebeen followed, radiographs are not normally requiredto verify the post space preparation.

Most of the time a preformed parallel-sided postwill fit only in the most apical portion of the canal.Modified posts are available with tapered ends, andthese conform better to the shape of the canalalthough they have slightly less retention than parallelsided posts do, particularly the shorter ones.

In the absence of a vertical stop on sound toothstructure, such posts can also create an undesirablewedging effect.

B-Custom-made Posts:

1.Use custom-made posts in canals that have anoncircular cross section or extreme taper.Enlarging canals to conform to a preformedpost may lead to perforation. Often very littlepreparation will be needed for a custom-madepost. However, undercuts within thecanal must be removed, and some additionalshaping usually is necessary.

2. Be most careful on molars to avoid rootperforation. In mandibular molars the distalwall of the mesial root is particularly susceptible. In maxillary molars the curvature of themesiobuccal root makes mesial or distal perforationmore likely.

Direct Procedure

1.Lightly lubricate the canal and notch a loose fittingplastic dowel. It shouldextend to the full depth of the prepared canal.

2.Use the bead-brush technique to add resin to the dowel andseat it in the prepared canal. This should bedone in two steps: Add resin only to the canalorifice first. An alternative is to mix someresin and roll it into a thin cylinder. This is introducedinto the canal and pushed to placewith the monomer-moistened plastic dowel.

3.Do not allow the resin to harden fully withinthe canal. Loosen and reseat it several timeswhile it is still rubbery.

4.Once the resin has polymerized, remove thepattern.

5.Form the apical part of the post by addingadditional resin and reseating and removingthe post, taking care not to lock it in the canal.

6.Identify any undercuts that can be trimmedaway carefully with a scalpel.The post pattern is complete when it can be insertedand removed easily without binding in thecanal. Once the pattern has been made, additionalresin or light-polymerized resin* is added for thecore.

Indirect Procedure

Any elastomericmaterial will make an accurate impressionof the root canal if wire reinforcement is placed toprevent distortion.

1.Cut pieces of orthodontic wire to length andshape them like the letter J.

2. Verify the fit of the wire in each canal. Itshould fit loosely and extend to the full depthof the post space. If the fit is too tight, the impressionmaterial will strip away from thewire when the impression is removed.

3.Coat the wire with tray adhesive. If subgingivalmargins are present, tissue displacementmay be helpful. Lubricate the canals to facilitate removal of the impression without distortion(die lubricant is suitable).

4. Using a lentulo spiral, fill the canals withelastomeric impression material. Beforeloading the impression syringe, verify thatthe lentulo will spiral material in an apicaldirection (clockwise). Pick up a smallamount of material with the largest lentulospiral that fits into the post space. Insert thelentulo with the handpiece set at low rotationalspeed to slowly carry material into theapical portion of the post space. Then increasehandpiece speed and slowly withdrawthe lentulo from the post space. Thistechnique prevents the impression materialfrom being dragged out. Repeat until thepost space is filled.

5. Seat the wire reinforcement to the full depthof each post space, syringe in more impressionmaterial around the prepared teeth, and insert the impression tray.

6. Remove the impression, evaluate it, and pour the working cast.

NOTE: Access for waxing is generally adequatewithout placement of dowel pins or sectioning ofthe cast.

7. In the laboratory, roughen a loose-fittingplastic post (a plastic toothpick is suitable)and, using the impression as a guide, makesure that it extends into the entire depth ofthe canal.

8.Apply a thin coat of sticky wax to the plasticpost and, after lubricating the stone cast, add soft inlay wax in increments. Startfrom the most apical and make sure that thepost is correctly oriented as it is seated toadapt the wax. When this post pattern hasbeen fabricated, the wax core can be addedand shaped.

9. Use the impression to evaluate whether thewax pattern is completely adapted to the postspace.

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