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Zirconium Oxide Post and Core System for the Restoration of an Endodontically Treated Incisor

The utilization of ceramic restorations to improve anterior aesthetics has dramatically increased due to their enhanced translucency. The underlying dentin of endodontically treated teeth is, however, often stained. In affected teeth, the coronal aspect of the tooth must be replaced, discolorations caused by pulpal necrosis must be counteracted, and support must be obtained from the root. This article presents the use of a contemporary post and core system for the restoration of endodontically treated anterior teeth.

Components of Root-Filled Teeth

Restoration of an endodontically treated tooth is dependent upon five components: proper selection of a restorative material as determined by the nature of the definitive restoration, the remaining tooth structure, core selection, post selection, and the luting mediums used for their cementation.1 The successful restoration of an endodontically treated tooth depends, to a significant extent, on the clinician's ability to address the individual requisites of these five components in the comprehensive treatment.

           

Zirconium Oxide Posts

The introduction of the zirconium oxide intraradicular post has provided clinicians with the means to achieve natural-looking aesthetic prostheses in root-filled teeth. This ceramic post obviates the need to utilize metal in the root canals of anterior teeth. The physical properties of zirconium dioxide include improved fracture resistance, a flexural strength greater than unalloyed steel, and a modulus of elasticity of 55 GPa.2 Furthermore, zirconium oxide is stable at human body temperature, is radiopaque, and does not promote adverse reactions from biological tissues.3 In addition, the metal-free posts do not interact with the cement, and corrosive products are not deposited in the hard and soft tissues surrounding the teeth.

Clinical Procedure

A 23-year-old male presented with overcontoured composite resin fillings that had restored a fractured maxillary left central incisor (Figure 1). The tooth was nonvital, and transillumination revealed decay below the existing composite filling (Figure 2). Due to the tooth's aesthetically prominent position in the maxillary arch, an all-ceramic crown restoration was selected prior to initiating root preparation.

Following isolation with a rubber dam, an access cavity was prepared to facilitate root canal therapy (Figure 3). Once an apical seal was achieved with gutta-percha and calcium hydroxide paste, the palatal access cavity was provisionally sealed with a composite resin filling material.

The DSARP Technique

The double-shoulder axial root preparation technique (DSARP) involves cutting an external 360° shoulder of uniform width for the definitive crown, and a second interior countersink axial shoulder for the post and core complex.4 The margins for the definitive crown restoration are defined following the completion of the root filling.

The external shoulder for the definitive restoration was fabricated by utilizing a 1.4-mm round-end straight cylindrical bur to place depth cuts buccally, palatally, and interproximally (Figure 4). In order to ensure accurate uniform reduction, the tooth was prepared in mesial and distal halves. This process prepared the coronal aspect of the tooth for the retention of the provisional crown and ensured that the external shoulder of the definitive restoration was clearly discernible.

The completed distal section of the preparation was used as a guide for the mesial half. The external shoulder was placed 0.5 mm below the gingival margin and followed its circumferential contour (Figure 5). The shoulder was subsequently extended to the mesial aspect of the tooth and completed with a uniform 1.4-mm width reduction. The margins were perpendicular to the enamel prisms -- not to the long axis of the tooth. In the coronal region of the tooth, the enamel prisms were aligned at 90° angles to the long axis of the tooth, but apically parallel the root surface. The perpendicular shoulder preparations also increased the surface area of exposed enamel, which reinforced the bond between the crown margins and the tooth (Figure 6).

A provisional crown restoration was then fabricated with an accurate emergence profile to allow gingival healing and permit assessment of the definitive aesthetics (Figure 7). Any alterations can be performed on the acrylic crown prior to the fabrication of the definitive restoration.

Upon healing and verification of aesthetics, the provisional crown restoration was removed to assess the interim composite core and remaining tooth structure (Figure 8). Since dentin can only be retained if it has a minimum thickness of 2 mm, this composite layer was removed until only healthy supported dentin remained. A second, internal countersink shoulder was subsequently prepared axially to the root to accommodate the ceramic core buildup. The preparation was a 360° shoulder with a depth of 2 mm into the root surface to allow for an adequate bulk of porcelain, while its peripheral limit was dictated by the previously prepared external shoulder of 1.4-mm width (Figure 9).

Utilizing a canal reamer, the root filling was extracted to 4 mm from the apical seal, and a root bur was selected to prepare the canal to the dimensions of the 1.7-mm zirconium oxide post (Figure 10). The post was passively inserted and was marked with an indelible pen at the most coronal aspect of the root surface (Figure 11). A radiograph was taken to confirm the length and proper post placement in the root canal (Figure 12). This radiograph indicated that 4 mm of the root filling remained and the length of the post from the most coronal aspect of the root surface to the root filling was 12 mm. The length of the definitive crown restoration from the free gingival margin to the incisal edge would be 11 mm.

Ceramic Core Buildup

Following radiographic confirmation, an impression was taken. The coronal aspect of the post was coated with an impression tray adhesive to prevent slipping following removal from the mouth. Once the post was recorded in the impression (Figure 13), care was taken not to disrupt its position, as this could alter the predetermined length and affect proper seating.

In the laboratory, light pressure was applied to ensure that the zirconium oxide post was fully seated in the impression. Isolation liquid was applied, and models were poured utilizing a low expansion die stone. The post was then loosened from the model and replaced to ensure proper engagement into the root canal preparation. The blue spot marked intraorally on the post should coincide with the most coronal aspect of the root surface and measure 12 mm. The silver-colored die spacer corresponds to the fitting surface of the pressed all-ceramic core surrounding the zirconium oxide post (Figure 14). A nonresidual wax was utilized to construct the wax core. The post protruding beyond the wax core must not be cut at this stage.

The ceramic core was attached to the zirconia post using all-ceramic ingots that establish a ceramic-to-ceramic bond. The zirconium oxide posts maintain an adequately rough surface and do not require additional treatment in the laboratory. The bond creates a monolithic structure between the post and core complex. The all-ceramic stain technique burns out the mold to press the all-ceramic ingots around the post, which was subsequently cut to approximate the all-ceramic core buildup (Figure 15). The fitting surface of the core that engages the second shoulder was etched with hydrofluoric acid prior to clinical evaluation of fit and the cementation procedure.

Once intraoral fit of the post and core complex had been verified, its apical region was reduced by 0.5 mm to allow passive seating in the canal. The post was cemented with an adhesive technique that increased the stability of the post and core/dentin complex.5 The post and fitting of the core were cleansed with phosphoric acid and treated with silane and a bonding agent. The intraradicular internal shoulder was then treated with 0.2% chlorhexidine solution, etched, and coated with the luting agent. A resin cement was spiraled into the root canal and placed onto the fitting surface of the ceramic post and core. Passive insertion seated the core until the internal shoulder was engaged, and any excess cement was subsequently removed (Figure 16).

Definitive Crown

Following cementation, an impression was taken. Transillumination of the definitive all-ceramic crown through its translucent core allowed visualization of the intrinsic and extrinsic characterizations and stains in the prosthesis, which, faithfully reproduced, approximated those of the natural right central incisor (Figure 17). Uniformity of reduction and the smooth line angles were clearly evident. The crown was cemented utilizing an adhesive technique similar to that utilized for the ceramic post and core (Figure 18).

Conclusion

The use of a zirconium oxide post can provide a suitable option for the restoration of endodontically treated teeth. This therapeutic modality satisfies material requisites for masking underlying discolorations while simultaneously using a highly translucent all-ceramic material for the definitive crown restoration to ensure aesthetics postoperatively. The definitive result is indistinguishable from the adjacent natural teeth.

References:

  1. Chalifoux PR. Restoration of endodontically treated teeth: Review, classification, and post design. Pract Periodont Aesthet Dent 1988;10(2):247-254.
  2. Schweiger M, et al. Microstructure and properties of a pressed glass-ceramic core to a zirconia post. QDT 1998;21:73-79.
  3. Ichikawa Y, Akagawa Y, Nikai H, Tsuru H. Tissue compatibility and stability of a new zirconia ceramic in vivo. J Prosthet Dent 1992;68(2):322-326.
  4. Ahmad I. Yttrium-partially-stabilised zirconium dioxide (YPSZ): An approach to restoring coronally compromised non-vital teeth. Int J Periodont Rest Dent 1998;18(5):455-465.
  5. Mendoza DB, Eakle WS. Retention of posts cemented with various dentinal bonding cements. J Prosthet Dent 194;72(6):591-594.
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