The Endodontic-Restorative Interconnection Interdisciplinary Integration

By Nathan Blitz, B.Sc., DDS
and Kenneth S. Serota, DDS, MMSc

The susceptibility of a person's back teeth to fracturing means that crowns or onlays are appropriate. This article describes a procedure that conserves the maximum amount of natural tooth structure following root canal treatment. This technique avoids the use of conventional posts which could increase the chances of root fracture.

The past decade has been witness to a revolution in the art and science of endodontics. Predictable clinical success in 100% of cases has become an imminent reality as a function of the following; acceptance of the "treating the apex last" approach as the logical sequence for cleaning and shaping,1 development of super-elastic rotary files whose design configuration planes the root canal space, enhancing the rheologic characteristics of the system,2 and the introduction of instruments capable of simultaneously thermosoftening and compacting gutta-percha.3

In a similar vein, adhesion dentistry has opened the door to restorative possibilities heretofore unimaginable. Advancements in the science and the materials involved have produced stronger porcelains that are no longer traumatic to the opposing natural dentition. Bonding agents have been developed that manifest bond strengths to dentin that approach that of enamel.4

These two disparate but uniquely linked disciplines have become increasingly confluent by virtue of these dynamic changes. The elimination of the anachronistic post/core complex as a component of the restoration of the endodontically treated tooth except in the most extreme cases of debilitation is now possible. Modified endodontic access preparation and innovative restorative concepts have evolved into the atypical porcelain onlay crown which incorporates minimalistic preparation with maximal preservation of tooth structure thereby enhancing the surface area available for the adhesive interface (Fig. 1,2).

Fig. 1 This sagittal view demonstrates the interconnection of the finished endodontic procedure in maxillary and mandibular bicuspids with the minimalistic preparation for the atypical porcelain crown. Composite resin is used to create a subfloor as well as the elimination of axial undercuts. The flat subfloor surface facilitates the creation of a restoration with a positive seat which eliminates the possibility of fracture.

Fig. 2 This sagittal view of maxillary and mandibular molars demonstrates the degree of occlusal clearance required for these restorations. 2 mm in support cusp areas is essential and a minimum of 1.5 mm elsewhere is required. Of important clinical note, there is no such thing as a maximum of porcelain presence.

Posts, regardless of their design or composition, morphometrically weaken teeth and significantly increase the risk of root fracture.5,6,7 Traditionally, the design parameters of a full coverage restoration have involved the reduction of intact, healthy, coronal tooth structure. The ability to bond an atypical porcelain onlay crown using the pulp chamber for retention has obviated the need for this reduction, thereby eliminating the need for a traditional post/core complex in the vast majority of cases.8 The purpose of this article is to graphically demonstrate the interconnection between the intrachamber anatomy of the root canal system and the design of the adhesion-enhanced restoration.

Discussion

The applicability of the bonded atypical porcelain onlay crown is not universal. The inherent limitations of the bonding process make it impossible to place this restoration where the operator cannot maintain a dry field, gain access for finishing procedures, and where occlusal forces are formidable. This may include second molars and preparations with subgingival margins of equal to or greater than 1 mm. Extreme parafunctional occlusal forces obviate the use of this modality as well. Esthetic concern regarding the visible interface of tooth and restoration on the facial middle-third of the clinical crown can constitute a contraindication for those who are unable to accept this result thereby necessitating the use of a full coverage restoration with gingival or subgingival marginal placement. However, the incorporation of a 15 degree slope or a chamfer into the marginal finish line can improve the esthetic blending of this marginal interface (Fig. 3).

Fig. 3 Image A illustrates a butt joint reduction used for bonded restorations of the endodontically treated tooth.

Image B reflects a modification of this reduction with the incorporation of a 15 degree slope on the facial aspect.

Image C demonstrates a chamfer finish as an alternative to the slope. In both cases, the modifications have to be short vertically and broad horizontally in order to avoid fragile, thin margins.

Until recently, the porcelains used in reconstructive dentistry were of the feldspathic variety. These are extremely hard and difficult to finish after final adjustments for occlusion as well as abrasive to the opposing natural dentition. The introduction of castable leucite-reinforced porcelains such as IPS Empress TM (Ivoclar-Williams, Amherst NY) and Fortress TM (Chameleon, Kansas City KS) has established new horizons for porcelain usage. These materials exhibit intrinsic and antagonistic wear rates that lie midway between that of amalgam and enamel. As such, they are not damaging to the natural dentition in the manner of the feldspathic porcelain used in PFM crowns.

The use of a leucite-reinforced atypical porcelain onlay crown can obviate the need for a conventional post/core by establishing an internal porcelain extension of the crown within the confines of the pulp chamber.(8) In addition, porcelain onlay crowns enable the operator to keep the majority of the tooth/restoration margins away from the gingival areas enhancing hygiene maintenance. The minimal removal of sound tooth structure, both internal and external, produces a cumulative effect resulting in a stronger, more retentive structure that is less likely to fail than would a conventional restoration (Fig. 4,5).

Fig. 4 In this graphic of the maxillary quadrant, the slope on the bicuspid is extended into the mesial proximal area to facilitate the camouflaging of the marginal interface. A butt joint was used on the facial aspect of the molar to ensure maximum strength of the porcelain in this region.

Fig. 5a In this graphic of the mandibular quadrant, the composite subfloor is shown. The subfloor ensures positive seating of the atypical porcelain onlay crown. The depth of the chamfer on the facial aspect is maintained throughout in order to prevent weakness in the porcelain casting. Longitudinal evaluation of this technique will require patience and prudence. Case selection, as in all facets of dentistry, will remain the focal criteria for success. With exacting standards and knowledge of the limitations of the materials involved, clinical excellence can be achieved.

Conclusion

Interdisciplinary treatment can approach biologic perfection provided we can mimic natural variables. The concepts of the past focused on optimizing post length and shape as a means of ensuring core retention while reducing the risk of root fracture. With the advent of adhesion dentistry, retention of the crown can be provided by the pulp chamber itself and the need for a post/core complex becomes unnecessary in most situations. Extrapolation of the current research on bonding agents and resin cements substantiates this treatment approach. With time, research should validate what is transactionally apparent.

Fig. 5b Clinical view of a mandibular molar and bicuspid after the completion of endodontic treatment. Note how the Class I inlay access preparation design initiates the final preparation for the atypical porcelain onlay crown.

Fig. 5c The composite resin subfloor is shown. The authors strongly advise using composite to block out undercuts in the preparation rather than having the technician block out undercuts on the working model. This invariably results in a greater thickness of luting composite which increases polymerization shrinkage and has negative implications for long term success.

Fig. 5d The modified chamfer margin used on the facial aspect of the bicuspid is demonstrated on the working model.

Fig. 6a The leucite-reinforced atypical porcelain onlay crowns have been bonded to place. The crown/tooth interface is located in areas that are optimally maintained by routine hygiene.

Fig. 6b The interconnection of the endodontic and restorative components are dramatically evident in this radiograph.

References

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Guzy GE, Nichols JI. In vitro comparison of intact endodontically treated teeth with and without endo-post reinforcement. J Prosthet Dent 1979; 39-42.
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