Post length is unique and individualized for each case.
The clinician should have a thorough knowledge of root morphology before placing a post. The effect of the em-bedded depth of posts on retentive capacity has been shown to be significant.20 The longer the post, the greater the retention. A guideline of one half to three quarters of the root length is often followed but may not be reasonable for extremely long, short, narrow, or curved roots. At a minimum, research has established that the post length should be equal to or greater than the crown length of the restored tooth (see Figure 9-3).
Retrospective clinical data on 1273 endodontically treated and restored teeth demonstrated few failures for posts that were as long as the crown or longer.'° At least 4 to 5 mm of gutta-percha should be left in the apical portion of the canal to ensure an adequate seal. If this guideline cannot be followed and the post is shorter than the crown, extraction should be considered. The width of the post should not be greater than one third of the
13 6
Ism
width of the root at any point along the dowel. Main-taining a minimum of 1 mm of sound dentin around the post is also advisable. Tjan 21 demonstrated that root Valls with 2 or 3 mm of buccal dentin were less likely to fracture than those having only 1 mm. Because of the two-dimensional nature of radiographs, buccal-lingual dimensions cannot be assessed and therefore the actual amount of dentin present in the mesio-distal direction may be smaller than the amount perceived. The clinician should keep this in mind and carefully consider teeth that have roots with fluting or depressions along the mesial and distal surfaces. Smaller posts not only con-serve tooth structure, but also provide increased resis-tance to fracture compared with larger posts. 22
Preparation o f the Post Space As It Relates to the Apical Seal and Microleakage
The post space may be prepared immediately after root canal treatment is completed . 2 , 23 This procedure is a two-step process. Gutta-percha should be removed incre-mentally with a heated instrument or solvents before canal enlargement. This method reduces the chance of root perforation and microleakage 23 ( Figure 9-4), a common sequela of gutta-percha removal during post space preparation. The use of a heated instrument results in significantly less leakage at both the 3 and 5 mm lev-els of remaining gutta-percha, compared with rotary in-strument removal . 23 Whenever possible, post space should be prepared with rubber dam isolation to prevent bacterial contamination of the root space. If the space becomes contaminated during post space preparation or before cementation, an antibacterial solution such as 2.5% sodium hypochlorite can be used to disinfect the space before treatment continues.
Color Atlas of Endodontics
B
FIGURE 9-3 A, Tooth #12, which exhibited loss of periodontal support, was restored with a post to a level unsupported by bone. B, The maxillary premolar eventually demonstrated a root fracture at the apex of the post.
Excessive removal of gutta-percha may contribute to failure of the root canal treatment (Figure 9-5). A retrospective study of 200 patients concluded that the increased apical periodontitis associated with teeth with posts may result from the loss of the apical seal or the improper removal of root canal obturating mate-ria1. 24 Observation of teeth with less than 3 mm of remaining root filling indicated a statistically signifi-cant increased frequency of periapical radiolucencies than teeth where more than 3 mm of root filling re-mained. 25 An in vitro study by Nixon 26 also demon-strated that leaving more than 3 mm of filling mate-rial greatly decreases the level of leakage; 5 mm or more is optimal. Numerous other authors support leaving 4 to 5 mm of undisturbed gutta-percha after post preparation.1,27-34
Anti-Rotation and Positive Stop
Anti-rotation features are required in all post and core restorations. Techniques for applying these features in-clude using pins or keyways or preparing the remaining coronal tooth structure. In a ribbon-shaped canal anti-rotation is achieved in the core. The rounder the canal, the greater the need to provide anti-rotation in the preparation. 3 A positive stop of the core onto sound dentin is required to prevent a wedging effect that could contribute to root fracture.
Ferrule Effect
The ferrule effect is achieved by encircling the remaining tooth structure with a cast band of metal. This effect has been shown to significantly increase the fracture resis-tance of an endodontically treated tooth by counteract-ing functional stresses such as lever forces and wedgcounteract-ing A
Chapter Nine Restoration o f the Endodontically Treated Tooth 13 7
A B
FIGURE 9-4 A, The inadequate length of this post, which was shorter than the crown, may have contributed to the failure of the restoration. B, The prepared tooth exhibits a lack of ferrule on the
mesial side of the tooth.
FIGURE 9-5 A rotary drill was used instead of a heated instru-ment to remove gutta-percha. This technique resulted in perfora-tion of the root.
FIGURE 9-6 Key features of an endodontically treated incisor restored with a crown include antirotation, ferrule, positive stop, and a minimum of 5 mm of remaining gutta-percha.
effects (Figure 9-6). 28 Opinions vary regarding the amount of ferrule needed, 35,36 but 1.5 to 2 mm should be considered the clinical minimum. 28 Surgical crown lengthening is often used to increase ferrule. This option should be considered with caution. Gegauff 37 demon-strated that decoronated premolar teeth that received surgical crown lengthening to provide a 2-mm ferrule
were more likely to fracture because of the altered crown-to-root ratio than teeth receiving no crown lengthening. Orthodontic extrusion can be used to in-crease ferrule without compromising the crown-to-root ratio. 38 Incorporation of a ferrule is perhaps the single most important factor in maintaining the endodontically treated tooth restored with a crown.
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Core Materials
The core should provide an ideal preparation form for the final restoration. Similar to a preparation in nat-ural tooth, the core provides retention and a resistance form for the fabricated crown. A number of direct core materials are commercially available for use with pre-fabricated posts. Categories of core materials include amalgam alloy, reinforced resins, and glass ionomer.
Advantages of alloy include its strength and stability.
However, it requires a bulk of material and therefore is more frequently applicable to molars and larger pre-molar teeth. Amalgam demonstrated no microleakage in a study of various core materials placed under crowns and should therefore be considered for a core material whenever the crown margin cannot be ex-tended more than 1 mm from the core junction. 39 Resin cores are often placed in anterior teeth because of their ability to mimic natural tooth color, which can be ad-vantageous under all-ceramic crowns. This matching color can be a disadvantage if it makes detecting the lo-cation of the resin with respect to the crown margin dif-ficult. An opaque white resin core material precludes a graying effect and allows detection of its location by the clinician. In vitro research on glass ionomer has demonstrated low strength compared with other core materials. Glass ionomer cements should probably be reserved for use in very selected anterior and posterior teeth because of their insufficient adhesion when used alone with pins or posts and their questionable long-term fatigability.1
PREPARATION AND FABRICATION TECHNIQUES
Corono-Radicular Amalgam Foundations The corono-radicular amalgam foundation, also called
anamalcoreor chamber-retained amalgam, iscondensed into the pulp chamber and occasionally the coronal root
canal space to gain retention. It can be used for poste-rior teeth with adequate coronal structure (two remain-ing walls) that exhibit undercut chambers large enough to receive a bulk of amalgam. This foundation can be placed in one visit and can be as strong as other types of foundations provided adequate bulk is created.40,41 Kane 4 2 demonstrated that 4 mm of remaining pulp chamber height negated the need to place amalgam into the canals. Amalgam cores placed in the chamber can easily be removed if retreatment becomes necessary. Re-moval is more difficult if the amalgam has been placed into the radicular space.
1. Place a rubber dam if possible.
2. Remove the provisional restoration and all remain-ing coronal restorative materials, caries, and unsup-ported tooth structure.
3. Plan for at least a 2-mm thickness of amalgam over tooth structure that is going to be covered by amalgam.
4. Remaining gutta-percha in the pulp chamber can be removed with a warmed endodontic plugger.
5. Remove 2 mm of gutta-percha from each root canal only if this is necessary for additional reten-tion (i.e., retenreten-tion in chamber is less than ideal [4 mm]).
6. Inspect chamber for undercuts, and enhance reten-tion if necessary by placing addireten-tional undercut ar-eas into the chamber walls. An inverted cone or di-amond bur is useful for this step.
7. Place a stainless steel or copper matrix band if needed.
An existing provisional crown can be converted to a matrix by creating a large opening in the occlusal sur-face through which amalgam can be condensed.
8. Incrementally condense amalgam into the pulp chamber, then continue to fill the matrix to the level dictated by remaining tooth structure and predeter-mined height.
9. Begin to carve the alloy to the appropriate height while the matrix is in place.
10. Remove the matrix after initial setting has occurred and carefully check occlusion if the tooth has not al-ready been prepared for a crown.
11. If the tooth has already been prepared for a crown, the amalgam may be carved to the contours of the desired crown preparation. In this situation the clin ician should select a fast set amalgam alloy that will allow careful preparation of the core at the same visit.
12. The final prepared core should be well condensed and exhibit good retention and resistance form (Fig-ure 9-7).
13. The clinician must take care when fabricating or re-lining a provisional crown not to fracture the newly placed core.
Pre fabricated Posts
The tooth type, root length, and root morphology dic-tate the type of post to be used. Two basic categories of prefabricated posts are available: passive and active. Ac-tive posts primarily gain their retention by threads that engage the intraradicular tooth structure. Passive posts gain their retention from the luting agent.
A large number of prefabricated posts are available in lengths ranging from 8 to 22 mm and diameters rang-ing from 0.5 to 1.9 mm. A majority of current prefabri-cated posts are a passive design, are just as likely to be parallel as tapered, and tend to be made of stainless steel or titanium. Newer compositions include the carbon fiber post (C-Post, Bisco Dental Products, Schaumburg, IL).
The manufacturer of the C-post claims that it has the same elasticity as dentin and prevents the root fractures associated with cast and prefabricated metallic posts. The newer zirconium oxide posts (CosmoPost, Ivoclar North America, Inc., Amherst, NY) are tooth-shaded posts to
Chapter Nine Restoration o f the Endodontically Treated Tooth 13 9
FIGURE 9-7 A, Preparation for an amalcore foundation. B, Restora-tion with an amalcore foundaRestora-tion.
FIGURE 9-8 Preformed post systems usually contain calibrated drills to assist with post space preparation.
which a tooth-colored resin or ceramic core may be at-tached. Zirconium oxide posts are becoming popular be-cause of the increased use of all-ceramic crowns. They are advocated for use with the more translucent all-ceramic crowns (such as IPS Empress) where a metallic post and
core may render a graying effect.43
Prefabricated post systems often come with cali-brated drills that correspond to the available diameters;
these allow for ease of final post space shaping and re-move minimal tooth structure (Figure 9-8). The fol-l owing method is used for preparation and pfol-lacement of a parallel-sided prefabricated post (ParaPost XP, Coltene/Whaledent, Mahwah, NJ).
1. Prepare remaining tooth structure. Undercuts do not need to be removed.
2. Plan for the length and diameter of the post using a parallel, distortion-free radiograph as a guide.
3. Remove gutta-percha to the planned length desired for the post using a heated instrument or solvents.
Adequate removal can be confirmed by exposing a radiograph.
4. Using Gates Glidden drills, enlarge the post space according to the size ParaPost desired, ending with the size that corresponds to the size of the planned ParaPost (Table 9-2).
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*#1 (0.50) and #2 (0.70) drills may be used to enlarge a small canal to accommodate a #3 drill.
5. To parallel the post space, select a ParaPost X drill that is smaller than the last Gates Glidden drill used in step 4. Place a rubber stop on the drill to the de-sired depth or use the drill markings as a guide.
( ParaPost X drills can be used manually with the Universal Hand Driver or with a slow-speed contra-angle [750 to 1000 rpm]. A new drill is indicated when manual drilling is performed.)
'' 6. Sequentially step up to the next larger ParaPost X drill until the predetermined diameter and depth are achieved.
7. Select the ParaPost XP post that corresponds to the last drill used to prepare the post space.
8. Before cementing the post, confirm that the length of the post corresponds to the depth of the newly created post space. Reinsert the last drill used into the post space to confirm the depth of the preparation. Place the post next to the drill so the base of the post head is slightly above the final depth measurement identified on the drill. Using a cutting disc, remove the portion of the post that extends beyond the apical end of the drill. Re-chamfer the end of the post to its original shape. A radiograph can be made to confirm complete seating of the post before cementation.
9. Use a cylindrical diamond or carbide bur to prepare an anti-rotational box.
0. Place the post into the post space. Check for oc-clusal clearance. Remove the post and further ad-j ust the length by trimming the apical end if needed.
Re-chamfer the apical end to its original shape and debride the post.
11. Air abrade the post with 50 micron aluminum ox-ide before cementation to enhance micro-mechan-ical retention
12. Use the dental cement of your choice, according to the manufacturer's instructions, to cement the post.
Place the cement in the canal with a lentulo spiral.
This results in an even coating of cement. However, this technique should be avoided when using a
ce-ment that sets by oxygen inhibition such as Panavia 21 (Kuraray America, Inc., New York, NY). For this type of cement, the luting agent should be placed only on the post.
13. Remove excess cement and fabricate the rest of the amalgam or composite core. This is most easily done with the aid of a matrix (Figure 9-9).