Adaptation of a Fiber-Reinforced Restorative System to the Rehabilitation of Endodontically Treated Teeth

by Nathan Blitz, B.Sc., DDS

New all-porcelain crowns avoid the dark line that is visible at the gum line of old-fashioned crowns. However, metal or dark posts cannot be used with all-porcelain crowns or the entire crown will appear lifeless. This article outlines a method developed by Dr. Blitz for the fabrication of a light-coloured, metal-free post to obtain optimum aesthetic results.

The Challenge

A 47-year-old patient presented for treatment of the maxillary right central incisor. Clinical examination revealed a porcelain-fused-to-metal crown restoration with mobility due to coronal fracture Figures 1 and 2). The tooth had received inadequate endodontic treatment and had been previously restored with a paste of indeterminate origin. Following completion of the diagnostic evaluation, it was evident that the extreme coronal debilitation of tooth #8 required interdisciplinary endodontic and prosthodontic rehabilitation.

Historically, restoration of endodontically treated anterior teeth has mandated the use of cast or prefabricated metal posts. This procedure was acceptable for porcelain-fused-to-metal restorations; however, with the introduction of all-porcelain crown restorations, aesthetic difficulties frequently develop. Since translucency and natural optical characteristics are desirable in all-porcelain crown restorations, metal posts or substructures must be masked or opaqued. This requisite applies to dark carbon fiber posts as well. In the highly visible anterior region, restorations fabricated with a metal substructure do not achieve the aesthetic expectations of the patient.

Traditional tapered dowel restorations have frequently experienced a loss of retention that has caused the failure of the treatment.1,2 It has also been demonstrated that tapered posts induce extensive shoulder stresses and a wedging effect,3,4 which further compromises their application in contemporary treatment. Retention failure has also been attributed to the ineffective performance of zinc phosphate cements.5 Due to these functional concerns the patient declined treatment with conventional prefabricated metal posts. The challenge presented by the rehabilitation of the endodontically treated tooth is the disparity and the complexity of the interfaces between a variety of restorative materials. Tension at the interface of the tooth, cement, and post will induce bond failure with resultant clinical difficulties, due to differing thermal and elastic properties and varying degree of deformation when subjected to load (modulus of elasticity).

The ideal post must provide superior retention, post and root resistance to fracture, and a degree of translucency that accommodates the aesthetic demands of the patient. A decision had to be made on how to treat the patient without utilizing traditional treatment modalities.

The Solution

A new fiber-reinforced, light-activated composite material (Targis/Vectris, Ivoclar Williams, Amherst, NY) has been developed for posterior bridgework and achieves the criteria required for the post. The material ensures stability in circumstances where permanent loads are applied and lightweight is required. The fibers demonstrate high tensile strength and tensile modulus, while the matrix exhibits improved durability. These components are chemically bonded to form a material with enhanced aesthetic and functional properties. A treatment plan that utilized this material was developed accordingly. The patient was advised of its experimental nature and consented to proceed with the treatment.

Post Fabrication

Following the completion of conventional endodontic treatment and post preparation, a parallel-sided post analog was selected to provide the optimal passive fit in the tapered canal.6 Post length was determined and adjusted at this time. A silicone index of the post was fabricated. The analog was separated and the index was subsequently removed. A silicone index was fabricated around a second analog, which was removed from the index. The diameter of the second post was smaller than the first in order to leave foom for wrapping with the fiber-reinforced frame.

The pontics exhibited high flexural strength and were densely packed into the narrower die of the second post. All fibers in this design were densely packed in a unilateral (longitudinal) direction, which provided maximum fracture resistance and indicated its applicability as a post in endodontically treated teeth.

The loaded second index was heat and pressure processed to fabricate a densely packed restoration with enhanced flexural strength. The post was separated from the second index and the excess was then removed. In order to cover the post, a frame was cut to size and placed on the original silicone index. The post was placed onto the frame-lined index. The frame resisted torsional movements and evenly distributed the masticatory forces throughout the post and subsequently the entire tooth. The pontic and frame reinforced one another and strengthened the resultant structure. The frame wrapping ensured that the individual pontic components would not separate in the future.

The entire index structure was cured, the post was removed from the index, and the excess was reduced. The post was placed into the oven unit where it was tempered to complete the curing process. The laboratory completed the fabrication process by customizing the core to the post.

Post Cementation

The root canal was cleansed and air-dried, at which time the fit of the post, core, and crown was evaluated. In order to initiate the chemical bonding procedure, a silane-coupling agent (Monobond-S, Ivoclar Vivadent, Amherst, NY) was applied to the post for a 60-second interval. Acid etching of the tooth was performed with 32% phosphoric acid for 15 seconds. The tooth and the post were subsequently treated with adhesive materials (Syntac, Ivoclar Vivadent, Amherst, NY) to facilitate retention in the treatment site. The post was then seated using dual-cure cement (Figure 3). The all-ceramic crown (IPS Empress, Ivoclar Vivadent, Amherst, NY) was placed simultaneously and affixed with light-cured resin cement (Figure 4).

The restoration was spot-cured and the excess resin was removed. Polymerization was completed and occlusion was evaluated and adjusted as required. The resin cement/tooth/restoration interface was polished with rubber cups and polishing pasted (Figure 5). The completed all-ceramic crown restoration exhibited enhanced aesthetics and soft tissue health (Figure 6).

Unlike metal posts, the fiber-reinforced post has been designed to establish a chemical bond with the resin cement and the unit bonds to the canal in the same manner as conventional bonding to dentin. While the long-term efficacy of the procedure remains to be determined, excellent results were achieved in this case.

Acknowledgment

The author wishes to acknowledge the assistance and support of Dr. Ken Serota (Toronto, Canada) and David Moore (Baluke Dental Studios).

References

1,2 Turner CH. Post-retained crown failure: A survey. Dent Update 1982; 9(4): 221-229. Lewis R, Smith BGN. A clinical survey of failed post-retained crowns. Brit Dent J 1988; 165(3): 95-97.
3,4 Standlee JP, Caputo AA, Collard EW, Pollack MH. Analysis of stress distribution by endodontic posts. Oral Surg 1972; 33(6): 952-960. Cooney JP, Caputo AA, Trabert KC. Retention and stress distribution of tapered-end endodontic posts. J Prosthet Dent 1986; 55(5); 540-546.
5 Nathanson D, Ashayeri N. Effect of cementing system on retention of cast posts in vitro. J Dent Rest 1989: 68(Special Issue): 3157 (Abstract No. 1066).
6 Sorensen JA, Martinoff JT. Clinically significant factors in dowel design. J Prosthet Dent 1984; 52(1): 28-35.

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