Written by Michael Lazaros, DMD Tuesday, 31 May 2005 19:00
“I don’t get no respect!” This was the recurring lament of the late, great Rodney Dangerfield. I often feel the same can be said for post-cores. Dentists will line up for courses designed to improve their technique in endodontics or crown and bridge, but often these same dentists feel their current method of post-core fabrication has no need for any tweaking.
To many, the post-core is just the simple step that follows the root canal treatment and precedes the crown. However, the post-core plays a vital, dual role in the restoration of the badly decayed tooth. It should be viewed as the critical, final seal in root canal therapy as well as the foundation and blueprint for the crown that follows.
“I have been doing it this way for 25 years, and it has worked just fine,” many will say in defense of their current technique. But there are no total failures or total successes in dentistry, only degrees in between. Therefore, our goal with post-cores, endodontic therapy, or any procedure must be continually to add degrees of success. Today, implants have more than a 90% success rate that creates a high standard for the “root canal, core, and crown” option to match. With that in mind, let’s look at 10 ways to improve the post-core.
KEY NO. 1: POST LENGTH DETERMINATION
A post that is too short is nonretentive. A post that is too long, however, may require that the tooth be extracted. Numerous rules of thumb exist for proper post length, such as “two thirds of the root” or “half the root that’s in bone.” The point that is most critical is to avoid the last 4 to 6 mm of the root.
(1) to leave enough gutta-percha to maintain an effective apical seal;
(2) to avoid the narrowest portion of the root; and
(3) to avoid the curved portion of the root.
Traditionally, the way dentists gauge their post depths is to utilize the endodontist’s (or their own) working lengths, measured from reference point to apex. My suggestion is that in lieu of this measurement the doctor performing the endodontic therapy records the orifice-to-apex measurement.
(1) This measurement is not subject to change if a cusp/reference point is reduced or breaks off.
(2) It is not practical to hold the post drills in the same orientation as the flexible root canal file that took the measurement.
(3) An orifice-to-apex measurement tells us exactly what we wish to know about root length, while a reference point-to-apex measurement is greatly influenced by the irrelevant height of surrounding tooth structure. For example, an orifice-to-apex measurement of 7 mm tells us instantly that no useful post can be constructed without violating the gutta-percha seal.
|Figure 1. Master cone marked at orifice to determine orifice-to-apex measurement.|
Recording the orifice-to-apex measurement is very simple. Just before cementing the master cone, take the endodontic explorer and scratch the master cone at the orifice level. Measure from the mark made to the end of the point (Figure 1). In the majority of molars and premolars this length will be 12 to 14 mm. For these canals I recommend a standard post hole of 8 mm, which is exactly the depth of flutes on a standard peeso No. 3 reamer. The 8-mm post hole is adequately retentive even in very long roots. For canals that measure 10 or 11 mm I recommend the short post of 6-mm depth. In this case, a colored periodontal probe is utilized to avoid over cutting. For canals of 9 mm or less, any post hole of practical use will likely violate the endodontic seal and is thus inadvisable. In those cases, a bonded core with undercuts may be the best one can do.
KEY NO. 2: SELECTION OF POST TYPE
Prefabricated posts are preferred over custom posts in most cases because they are more retentive, more conservative, and more practical than custom posts. Among prefabricated posts there has been a recent trend toward using metal-free bonded posts. However, research shows that at this point in time, dentinal bonding techniques cannot be relied upon to provide long-term retention. This has been most evident in class V rest-orations in nonretentive preparations. Therefore, it is prudent to utilize a post with mechanical retention. Screw-type posts are most retentive, but if poorly de-signed they may wedge the root apart.
The post I favor is the Flexipost (Essential Dental Systems).
|Figure 2. The Flexipost is highly retentive and stable due to its second tier.|
(1)It has sharp retentive threads that cut into the root rather than expand the root.
(2)It has a countersink: a second tier that provides lateral stability at the coronal aspect.
(3) It has a vent that allows the post to compress as it is inserted to diminish any lateral forces (Figure 2).
In most cases, I find that size No. 1 (red) is the best fit for the root width. The post is designed for a 10-mm depth post hole but can be shortened easily using a wide diamond in a high-speed handpiece.
KEY NO. 3: ISOLATION OF THE OPERATING FIELD
|Figure 3. The sectional dam isolates 3 to 5 teeth.|
As with so many other dental procedures, the rubber dam is highly recommended for post-core fabrication, especially in the molar region. In many cases, a sectional dam that isolates 3 to 5 teeth works best (Figure 3).
(1)Saliva contamination in the post preparation can jeopardize endodontic success and diminish bond strengths.
(2)Posts and wrenches can fall down the throat without a rubber dam.
(3)A rubber dam provides retraction for easier operating.
KEY NO. 4: CORD PACKING
|Figure 4. Isolated tooth with cord in place.|
Cord packing is advisable for any area of the tooth that is subgingival or equigingival after removing old restorations. If cord is kept cut and presoaked, it should only add a minute to the procedure and add tremendously to overall success (Figure 4).
(1)It will expose unseen areas of decay for more thorough excavation.
(2)It will prevent resin flash from adhering to the root.
(3)It will prevent voids in the core resin near the gumline.
(4)It will prevent bleeding for improved bonding.
(5)It will give the clinician a chance to evaluate the ferrule and the biologic width (see Key No. 6).
KEY NO. 5: CARIES DETECTOR SOLUTION
|Figure 5. Caries-detecting solution applied inside the tooth.|
Now is the last chance to remove any trace of undetected decay. There are several brands of caries-detecting solutions on the market, but in my experience the red dyes work best (Figure 5). Stain the tooth, rinse, excavate, and repeat until clean and firm.
KEY NO. 6: EVALUATE THE FERRULE AND BIOLOGIC WIDTH
Posts work best when they are not needed. That is to say that the crown should engage a 2-mm band of tooth structure (not core material) all the way around. To omit this “ferrule” is to overstress the post and invite failure. However, periodontal health requires that the future crown margin be 3 mm (the biologic width) from the osseous crest. If after cord packing and excavating you find you are approaching the osseous crest, it is advisable to refer the case to a periodontist for evaluation before beginning the crown preparation. A simple gingivectomy or electrosurgery may not be sufficient to restore proper biologic width.
KEY NO. 7: USE A MATRIX BAND
|Figure 6. The retainerless matrix sytem in place.|
It is tempting to rationalize that since cores are meant to be prepared, there is no need for a finicky measure like using a matrix band. In truth, utilizing a matrix is advisable any time there is a lack of healthy tooth structure circumferentially. In some cases, a thin shell of amalgam can be left intact to confine the core material. However, without removing all of the old restoration one risks incomplete decay removal and cannot evaluate the need for crown-lengthening surgery. In order to begin the crown preparation with a nearly ideal tooth size and/or shape, one must avoid the use of premade core formers. This author prefers to use a retainerless band system, such as the Supermat matrix (KerrHawe, Figure 6).
(1)The band protects the adjacent tooth from etchant and bonding resin.
(2)The band prevents overhangs, which may irritate tissues if crown preparation must be postponed.
(3)The band helps delineate the tooth-gum interface for more accurate crown preparation.
KEY NO. 8: USE AN IRRIGATION SYRINGE
|Figure 7. An endodontic irrigation syringe is indispensable for the post-core procedure.|
To many readers this may seem superfluous, but I have observed many clinicians who feel they can flush out canals with a 3-way syringe! The endodontic syringe is useful in both “forward” and “reverse” (aspirating) modes (Figure 7).
The following are uses for the irrigation syringe:
(1)flush out gutta-percha grindings from the canal;
(2)rinse etchant from the canal;
(3)remove water from the canal without desiccating it; and
(4)remove excess pooled bonding agent before curing.
KEY NO. 9: BOND CORES WITH A DUAL-CURE BONDING AGENT
|Figure 8. A bonding agent suitable for dual-cure resin cores is applied.|
Bonding the core material is not only for retention, but perhaps even more importantly, it is to provide a coronal seal to the root canal system. Endodontists debate whether the post itself should be bonded, as this makes its removal for retreatment far more difficult. It is important to note that many dentinal bonding agents are ineffective when using self-cure or dual-cure core composites. One acceptable bonding agent is ONE-STEP (Bisco Dental, Figure 8). This product has the added virtue of not requiring any mixing. Care must be taken that the dentin is very moist when using this and similar products.
KEY NO. 10: CREATE PROPER CONTOURS AND OCCLUSION
|Figure 9. The completed post-core should have contours similar to the final crown.|
As stated earlier, one of the goals of the well-executed post-core is to create a blueprint for the final crown. It is critical that the post-core be created with occlusion and contours as ideal as possible, not chopped down far from the opposing occlusion (Figure 9).
(1) This allows for an early assessment of interocclusal distance. Reducing the opposing tooth at this stage will be more acceptable to the patient than at the crown preparation visit.
(2) If the crown preparation visit will be postponed, a post-core should prevent occlusal drifting of the opposing dentition.
(3) The provisional crown can be made from a preoperative impression if the post-core is made to proper contours.
(4) Depth cuts for precise crown preparation must begin with a core of proper contour and occlusion.
A meticulous execution of the post-core visit will provide a final seal to the root canal system as well as a blueprint for precise crown preparation and temporization.
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