A Modern Approach to Post and Core Restoration: An Evolving Paradigm

Dr. Foroud Hakim

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INTRODUCTION
“The foremost purpose of a dowel is to provide retention for a core build-up; it should do so without compromising the tooth.” This statement reflects the consensus from some of the early editions of Pathways of the Pulp, edited by Stephen Cohen and Richard C. Burns, the text all of us know to be the Bible of endodontic education. What this statement essentially translates to is that the evidence supports the notion that rigid dowels are known to introduce potential liabilities to the root structure of an endodontically treated tooth. Because of this, best practices indicate a risk-benefit assessment by a practitioner when considering a core build-up procedure. If the absolute requirement of increased core retention overrides the risks associated with post preparation and placement, utilization is supported. However, if remaining tooth structure, supplemented by retention form and adhesive technology, provides a restorative platform that includes adequate ferrule, a post is deemed unnecessary.1

In other words, the decision to place a post for added core retention comes down to the practitioner’s interpretation of the specific clinical circumstances and how he or she frames this decision with the overlay of his or her individual background or experiences and knowledge of available evidence.

Contemporary Approach
The 2 most important advancements in restorative dentistry related to post and core delivery are the improvements in dentin bonding chemistry/technique and the development of fiber post systems. The paradigm shift from traditional rigid post cementation provides 2 advantages. First, the opportunity presents to bond a fiber post and resin complex to intraradicular dentin as well as cervical and coronal dentin. If this can be achieved to a particular threshold, then the practitioner has the opportunity to reinforce teeth through the internal splint, thus improving fracture resistance.2-4 Simple fixation of rigid metal posts with traditional luting cements cannot yield this benefit. Second, fiber posts exhibit a flexural modulus very similar to dentin. This equates to a post complex that can bend with the natural tooth under load and also distribute forces along the entire post space rather than focalize them at the apical post terminus.4-6

Figure 1. A PFM was removed from tooth No. 8, exposing a carious tooth and a
failing buildup.
Figure 2. Fitting a fiber post after a metallic post and core and caries were removed.
Figure 3. A completed mono-block fiber post and core buildup. Figure 4. Four sizes of flexible fiber post clusters (Rebilda Post GT [VOCO]) are shown here.
Figure 5. The clusters are flexed to demonstrate fiber count and distribution. Figure 6. Tooth No. 20, after endodontic treatment and provisional glass ionomer (GI) restoration.
Figure 7. Tooth No. 19 was maintained with a provisional crown after the permanent PFM was removed and endodontic treatment was completed. Figure 8. A preoperative PA radiograph of tooth No. 20.

To address the intraradicular bonding challenge, laboratory evidence indicates that a successful dentin bond can be achieved with a variety of techniques across the various adhesive generational classes (fourth generation through contemporary universal adhesives). This said, the predictability, longevity, and standard deviation that impacts clinical outcomes varies largely and often boils down to the product and technique used, as well as the condition of the dentin. While we can eliminate a single step and possibly save a modicum of time with a simple injection of self-adhesive resin cement into the canal chamber to place a post, it is important to understand that we would not approach the dentin bonding thresholds that maximize our opportunity for long-term success.7,8 When faced with the challenge of bonding to a mixed substrate (enamel and dentin), I choose to employ a total-etch or selective-etch technique. I know that predictable etch patterns are not consistently achieved on enamel under microscopic evaluation unless phosphoric acid is used. However, successful intraradicular dentinal bonds are demonstrated with both total-etch classes of adhesives (fourth and fifth generations) as well as self-etching adhesive classes (sixth generation through universal).8,9 Since we are only bonding to dentin during a post and core procedure, my preference is to migrate to a self-etching adhesive protocol and, specifically, to a universal adhesive that has the added chemistry to allow for a dual/dark cure. This provides 2 valuable benefits. First, I eliminate the challenge of removing 100% of the phosphoric acid (which is much easier to confirm visually in direct intracoronal preparations compared to deep canal chambers), which, if left behind, can devastate the bond. Second, the dual/dark cure capabilities allow for accelerated conversion in the light-accessible areas of the core while the chemical conversion gives me confidence about the terminal aspects of the post and core. The critical thing to remember when using a universal self-adhesive agent for intraradicular bonding is to follow manufacturer’s directions for use. These will invariably include actively scrubbing in the adhesive for a prescribed amount of time. This active application improves bond values.10

Figure 9. A preoperative PA radiograph of tooth No. 19. Figure 10. Tooth No. 19’s clinical presentation upon removal of the provisional crown, exposing a large GI core with remaining caries.
Figure 11. Tooth No. 20, after GI removal and the exposure of coronal gutta-percha obturation. Figure 12. Tooth No. 19, after caries and GI removal and with the gutta-percha exposed.

Post Selection and Space Preparation
With the caveat that long-term success hinges on good dentin bonding, the next challenge in the equation is fiber post selection and best practices in post space preparation. Consultation with the endodontic specialty and academic community, as well as investigation of the literature, makes it abundantly clear that deploying anatomically sensitive posts, which enable the operator to maintain as much healthy remaining dentin as possible during post space preparation, is critical.11,12

Figures 1 to 3 demonstrate the post and core rehabilitation of a badly broken-down central incisor, utilizing an anatomic fiber post, a dual-cure core material, and a universal adhesive (Rebilda Post, Rebilda DC, and Futurabond U [VOCO]). This particular tooth’s history included a failing PFM crown with recurrent caries, which was supported by a stable endodontic obturation. However, the short metallic post and core were failing and were thus removed, along with all of the affected tooth structure. Since the canal space had a history of traditional shaping with cylindrical instrumentation and post drills, a more traditional fiber post was selected that closely fit the apical portion of the post space. However, efforts to get closer adaptation of the post to the coronal half of the canal space would have meant further cylindrical instrumentation; stepping up to a wider diameter post; and, most negatively, removing additional sound dentin from an already compromised tooth. The decision was made to simply supplement the fiber post volume with a woven quartz splint (RTD Germany) and deliver a mono-block core buildup where the dual-cure core material not only performed the role of adhesive cement for both the activated dentin wall and the post/mesh complex, but also formed the coronal core. This notion of a core material that has a low film thickness, allowing the seating of a post without displacement or creation of dangerous hydrostatic forces, while also demonstrating favorable compressive strength and stacking behavior, facilitates the desirable mono-block. Other strategies to reduce resin volume (ie, shrinkage and C-factor) and provide a more robust measure of fiber without removing additional dentin include the addition of accessory fiber cones to a master fiber post. These slender “needles” of fiber fill in the spaces in flared canals as we move coronally from the apex of the master fiber post, much like the role accessory cones play around a master gutta-percha cone in endodontic lateral obturation. However, neither the master fiber post nor the accessory cones have flexibility, which is a limiting factor to canal navigation and maximum dentin preservation.

Material Evolution
Recently, a novel variation in post technology was introduced to the market that utilizes bundled clusters of glass fiber-reinforced resin posts (Rebilda Post GT [VOCO]). Figures 4 and 5 display the 4 bundled sizes (4, 6, 9 and 12 fiber post counts, respectively) that are held together by a color-coded carrier sleeve to aid in handling. Bundles of fine posts are easily visualized when fanned out by pressing them against a working surface. This system provides 3 advantages. First, the individual fiber flexibility allows for navigation around gentle curves in canal architecture where rigid fiber systems cannot without risk of canal or root violation. Second, the bundled grouping allows for 3 distributions of the individual fine posts across asymmetrical canal spaces that can range in shape (ie, oval, ribbon, or C-shaped). Finally, as the bundled fine posts emerge from the post space orifice and the carrier sleeve is removed, they automatically distribute themselves three-dimensionally within the coronal portion of the remaining tooth like a bouquet, providing an optimal scaffolding for the core buildup.

Figure 13. Tooth No. 20, with post space preparation. Note the optimal dentin preservation primarily due to removing gutta-percha and sealer while maintaining the canal profile. Figure 14. Tooth No. 19, showing a dentin preservation approach to post space preparation in the distal canal. Note that a small plug of gutta-percha was removed from the mesial canals for an added retention of core.
Figure 15. Tooth No. 20, with matrix in place, showing flexible fiber distribution after adhesive steps were accomplished and post space was loaded with dual-cure core material (the carrier was removed and fibers were sectioned to size). Figure 16. Tooth No. 19, after adhesive steps were completed and flexible fibers were seated into the post space loaded with dual-cure core material (Rebilda DC [VOCO]) (the carrier was removed and fibers were shortened to size).
Figure 17. Tooth No. 20, following the core buildup to full contour and being fully cured. Figure 18. Tooth No. 19’s core buildup to full contour, taking advantage of this “stackable” material.
Figure 19. Tooth No. 20’s buildup, completed and adjusted out of occlusion for future crown preparation. Figure 20. The completed tooth No. 19 buildup and gross crown preparation.

CASE REPORTS
The following 2 cases were restored with the Rebilda Post GT System. Figures 6 and 7 show teeth Nos. 19 and 20 in their clinical presentation after endodontic therapy and provisional restoration. Tooth No. 20 contained an interim DOBL glass ionomer (GI) restoration, while tooth No. 19 presented with a provisional crown seated over a GI provisional buildup. Figures 8 and 9 show the radiographic presentation of these teeth. Figure 10 shows tooth No. 19 after provisional crown removal. Figures 11 and 12 show the remaining viable tooth structure after the GI and affected tooth structure were removed. It was clear that both teeth were candidates for post and core restoration and reinforcement prior to final crown delivery.

Figures 13 and 14 show gutta-percha removal from the prescribed length and the resulting irregular post space created in deference to dentin preservation. Figures 15 and 16 show the initial fiber post bundle seated in place after application of the universal adhesive (Futurabond U) and the flooding of the post space with the dual-cure core material (Rebilda DC). Carrier sleeves were removed and fiber posts were shortened and teased into a passive 3-D distribution. Only after this step was light-curing initiated to tack down the complex. A traditional matrix was utilized for tooth No. 20, and a free stack core buildup was performed on tooth No. 19 due to its irregularity. Figures 17 and 18 show the completed bulked mono-block buildups. Figures 19 and 20 show the teeth after adjustments were done in preparation for future restoration. Tooth No. 20 was simply adjusted to hypo-occlusion as the crown preparation procedure was to occur the following week. Tooth No. 19 underwent preparation and final impression on the same visit. It is important to point out that both cases were completed under optimal isolation: A rubber dam was used when restoring the post and core for tooth No. 20, while isolation for tooth No. 19 was accomplished with an Isolite (Zyris/Isolite Systems) due to its terminal tooth position and difficulty in access when clamped.

CLOSING COMMENTS
I am a big fan of dental implants, and I regularly prescribe and restore them in my practice. The cases shown in this article might raise the question, “Why not extract and implant?” I think the answer is a complicated one. I do believe my decision to save and restore teeth (like the ones shown here) may have certainly been altered if I was limited to historically used materials. However, if the latest materials and technology offer the opportunity to save teeth with reasonable prognostication, we must include that option in our differential treatment planning. This is not possible if we are unaware of such advances. Flexible post and core systems can be categorized as products that contemporary practitioners should have in their tool belts so they can offer the most appropriate care for a given clinical circumstance.


References

  1. Zogheib LV, Saavedra Gde S, Cardoso PE, et al. Resistance to compression of weakened roots subjected to different root reconstruction protocols. J Appl Oral Sci. 2011;19:648-654.
  2. Torres-Sánchez C, Montoya-Salazar V, Córdoba P, et al. Fracture resistance of endodontically treated teeth restored with glass fiber reinforced posts and cast gold post and cores cemented with three cements. J Prosthet Dent. 2013;110:127-133.
  3. Guldener KA, Lanzrein CL, Siegrist-Guldener BE, et al. Long-term clinical outcomes of endodontically treated teeth restored with or without fiber post-retained single-unit restorations. J Endod. 2017;43:188-193.
  4. Alarami N, Sulaiman E, Al-Haddad A. Fracture resistance of endodontically-treated mandibular molars restored with different intra-radicular techniques. Am J Dent. 2017;30:197-200.
  5. Habibzadeh S, Rajati HR, Hajmiragha H, et al. Fracture resistances of zirconia, cast Ni-Cr, and fiber-glass composite posts under all-ceramic crowns in endodontically treated premolars. J Adv Prosthodont. 2017;9:170-175.
  6. Kubo M, Komada W, Otake S, et al. The effect of glass fiber posts and ribbons on the fracture strength of teeth with flared root canals restored using composite resin post and cores. J Prosthodont Res. 2018;62:97-103.
  7. Migliau G, Piccoli L, Di Carlo S, et al. Comparison between three glass fiber post cementation techniques. Ann Stomatol (Roma). 2017;8:29-33.
  8. Amiri EM, Balouch F, Atri F. Effect of self-adhesive and separate etch adhesive dual cure resin cements on the bond strength of fiber post to dentin at different parts of the root. J Dent (Tehran). 2017;14:153-158.
  9. Ferrari M, Mannocci F. A ‘one-bottle’ adhesive system for bonding a fibre post into a root canal: an SEM evaluation of the post-resin interface. Int Endod J. 2000;33:397-400.
  10. Gruber YL, Bakaus TE, Gomes OMM, et al. Effect of dentin moisture and application mode of universal adhesives on the adhesion of glass fiber posts to root canal. J Adhes Dent. 2017;19:385-393.
  11. Bhagat A, Mittal L, Mogla S, et al. Impact of root dentin thickness on the in vitro compressive strength of teeth treated with recent post and core systems. J Contemp Dent Pract. 2017;18:1065-1070.
  12. Belli S, Eraslan Ö, Eraslan O, et al. Effect of restoration technique on stress distribution in roots with flared canals: an FEA study. J Adhes Dent. 2014;16:185-191.

Dr. Hakim is a 1991 graduate of the University of the Pacific and practices in San Ramon, Calif. He is an associate professor and a former vice chair of the department of restorative dentistry at his alma mater. He is also a member of Catapult Education, a leading speaker’s bureau comprising many of the profession’s prominent key opinion leaders. Dr. Hakim lectures internationally and also serves as a consultant to the dental industry in the development of new materials, products, and technologies. He can be reached at his practice’s website or via email at sanramondentalexcellence@gmail.com.

Disclosure: Dr. Hakim receives honoraria from VOCO.

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