Biomimetic Perspective on the Restoration of Endodontically Treated Teeth
Protecting the structural integrity of endodontically treated teeth
Randall G. Cohen, DDS
The loss of structural integrity associated with caries, lost tooth structure, and access preparation leads to higher occurrence of fractures in endodontically treated teeth compared to vital teeth.1 Accordingly, restorations need to strengthen endodontic teeth and improve their prognosis when subjected to heavy masticatory forces.2 Following endodontics, the clinician must evaluate remaining tooth structure so that the final restoration seals the root canal system (preventing “crown down” re-infection) and restores the tooth’s natural contour and long-term function. Preservation of the dentin in the crown portion of the tooth as well as in the occlusal third of the canals helps in preventing fracture.
Restorations Following Endodontics
Once the root canal treatment has been completed, prompt restoration—within a few weeks of the endodontic treatment—is necessary to avoid microleakage that could cause an endodontic failure, and/or fracture of the remaining tooth structure. The topic of posts and full-crown coverage often arises as an issue in restoring endodontically treated teeth. Many doctors believe that a post and full crown strengthen the endodontically treated tooth against fracture.3 Most contemporary studies, however, have suggested that a post weakens a tooth rather than reinforces it.4 Following endodontics, many restorative dentists routinely place a post into the largest canal available despite evidence that preparation of this root space may lead to fracture.5 Following this post-and-core buildup, many clinicians routinely restore the tooth with a full cut-down porcelain-fused-to-metal crown.
The Biomimetic Strategy
Biomimetic dentistry uses modern materials to replicate the tooth’s biologic and mechanical function. This strategy is a different approach that avoids post placement as well as the 360° preparation in teeth that have not already been cut down for a full crown. Biomimetic techniques strengthen the dentin–resin bond and prevent stress and strain within the restoration. This technique involves using dental adhesives and sometimes fiber reinforcement to reproduce the tooth’s natural hard tissues to facilitate a secure bond. These biomimetic restorations work harmoniously with the tooth substrate so that normal occlusal forces are less likely to disrupt the tooth–restorative bond. Further, any failures are easily repaired.
Buildups Without Posts
Passive posts transfer stress to the remaining root structure, but not as much as active posts do.6 Post space preparation is similarly a challenge because root diameter may vary facial-lingually and mesio-distally. Coronal enlargement of the canal weakens the dentin and contributes to fracture; complex root anatomy risks perforations; and disrupting the apical seal of the gutta-percha can occur if the post preparation comes too close to the root end. Other factors to consider during preparation of the canal space include root taper, proximal root invaginations, root curvatures, and angle of the crown to the root.8
Molars typically have sufficient volume of tooth structure and pulp chamber to retain the core buildup, so a post is unnecessary.9 Accordingly, the anchorage provided by a well-placed core utilizing the pulp chamber is considerable and posts should be avoided. Low-shrink composite is effective provided optimal polymerization is assured in deeper layers by using either an auto-cured composite core material or incremental insertion of a photo-polymerized one.
The canal space is difficult to cleanse and dry prior to cementation of a post, a particular concern when using resin cements. Accordingly, Cheung8 describes the proper, albeit complicated, preparation of the post space:
• Remove gutta-percha and root canal sealer from dentinal walls
• Cleanse the walls using a Peeso brush with pumice slurry
• Thoroughly rinse with a 3-in-1 syringe and an irrigation syringe
• Remove demineralized collagen with a proteolytic agent such as sodium hypochlorite
• Remove the smear layer through acid treatment
The dentin bond within the post space is also compromised by the polymerization shrinkage of the resin cement that clinicians often use. Further, stresses from polymerization shrinkage and problems with adequate access to the root canal complicate the formation of high-strength bonds when cementing endodontic posts with resin cements.10
Determining the correct post length can also be problematic. While longer posts are more retentive, increased post length also increases the risk of fracture and root perforation.11 Further, it is generally accepted that the apical 3 mm to 6 mm of gutta-percha must be preserved to maintain the apical seal.
Posts can fracture and sometimes they cause the retaining root to fracture as well, condemning the tooth to extraction. Posts also complicate canal access should re-treatment become necessary.
Stockton et al12 concluded that variations in the use of posts to improve crown retention or other restorations are not acceptable on endodontically treated teeth. There are too many complications that could occur, including root perforations during post space preparations, and adhesive properties of modern resin-based materials.13 The placement of posts could damage the structure of the tooth, emphasizing the need to reevaluate the use of posts in endodontically treated teeth.12
The Biomimetic Alternative
This strategy is different in that it involves using the intact tooth as the guide for restoration rather than cutting the tooth down for a full crown.13 The stiffness of the porcelain/metal crown impedes proper function of the underlying dentin substrate.13 There are six main concepts (Six Lessons14) of the biomimetic strategy, all of which combine to create a low-stress, strong bonded restoration that is in harmony with the surrounding dental tissues:
• Caries removal and disinfection of dentin
• Removal of structural compromises
• Creating a secure dentin bond
• Low C-factor layering
• Preparation
• Occlusal harmony
Biomimetic techniques are employed in the restoration of endodontically treated teeth for several important purposes:
• Preventing crown and root fracture
• Restoring occlusion and contours of the original or the idealized tooth
• Maintaining strong bond to dentin
• Sealing dentin to prevent re-infection
• Preventing occlusal forces from dislodging the bond
• Reducing stress/strain within the restoration.
Full crowns with posts do not fulfill these criteria because a stiffer crown impedes the normal flexure of the tooth substrate, resulting in a push-pull taking place at the gingival margin. Continued bending may result in cement washout and recurrent caries. Occlusal forces also may interact unfavorably with a cemented post, increasing the chance of dislodgment and fracture of either the post or root.
Clinical Case Presentation
With minimal tooth structure remaining, a matrix helps retain the composite “biobase” and can be accomplished as follows.
The pretreatment view shows deep caries (Figure 1). Following the completion of endodontic treatment (Figure 2 and Figure 3), debridement of dentin is completed. Next, a 4-mm channel is prepared into the gutta-percha using a No. 2 bur (long shank); the structure is then isolated and the canal is cleaned with ethanol, rinsed, and dried.
The dentin surface is then conditioned using bottle No. 1 of the self-etch, dual-cured resin (CLEARFIL™ SE Protect with Activator, Kuraray Noritake, www.kuraraynoritake.eu). Then, bottle No. 2 (SE Protect and Activator) is applied, lightly air-dried, and light-cured for 20 seconds. The 16-mm length of Ribbond (www.ribbond.com) is soaked in bottle No. 2, mixing well. A small amount of Ribbond Securing Composite is deposited into the prepared canal. Using the Ribbond Placement Instrument, the middle of the 16-mm length of Ribbond is inserted into the prepared and cleaned canal, leaving the ends to extend outward (Figure 4). The bonding resin is then light-cured.
The biobase is then constructed. First, a small amount of flowable composite (N’Durance® Flow, Septodont, www.septodont.com) is deposited in and around the Ribbond extensions, and light-cured. The flowable composite is used to seal the gingival margins; then a putty composite (N’Durance Universal, Septodont) is used to build up 2-mm proximal walls, and light-cured. The large internal portion of the biobase is built using a dual-cured composite (N’Durance Dimer Core, Septodont), and light-cured for 1 second as the material is deposited into the deeper layers of the preparation. Allow 5 minutes for complete polymerization. Then reduce the occlusal, and place a 360° bevel (Figure 5). Seal the biobase using light-cured SE Protect Bond, and cover the preparation with clear glycerin gel, light-cure again, and clean the preparation with alcohol. The impression can then be made. The lithium-disilicate biomimetic casting (e.max, Ivoclar Vivadent, www.ivoclarvivadent.com) is bonded into place (Figure 6) using a dual-cured luting cement (Panavia V5, Kuraray Noritake).
Conclusion
Biomimetic dentistry is focused on using restorative materials to replace lost tooth structure (enamel, dentin, and dento-enamel junction) and to replicate the biologic and mechanical aspects of natural tooth function. Biomimetic techniques and materials give a result where the restored tooth functions just as an intact one does, avoiding the stress and strain that can occur when an intracanal post is used in the restoration.
The placement of a post followed by a crown is not without complications such as re-treatment difficulties and fracturing of the root, crown, or post. Posts do not strengthen teeth; they only retain the composite base that supports the overlying casting, which contemporary bonding materials will accomplish. If there is minimal dentin remaining, polyethylene ribbon, soaked in bonding resin and light-cured, can provide a satisfactory matrix for the low-shrink composites that will comprise the biomimetic base.
This biobase has incorporated minimal or no contraction stress, provides a better bond, minimizes complications, and supports a tooth-conserving casting that functions more like a natural tooth than does a post-and-core buildup and a full-coverage crown.
Disclosure
The author has no relevant financial relationships to disclose.
References
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Randall G. Cohen, DDS
Private Practice
Yardley, Pennsylvania
