From Gold to Zirconia

A 30-year-old patient reported to the dental office with a chief complaint of pain in the periapical area of the left central incisor. The central incisors had been treated with gold-ceramic crowns years ago, and tooth No. 9 had endodontic treatment and gold cast post and core (Figure 1 through Figure 4).

The endodontic examination showed a periapical lesion at tooth No. 9. The tooth was endodontically retreated and a new cast post individualized with a resin core was fabricated by the dental laboratory.

Due to the different width of the old crowns, it was decided to widen tooth No. 10 so that it would match the width of tooth No. 7, allowing for the fabrication of two symmetrical zirconia crowns for teeth Nos. 8 and 9. To accomplish this, a procedure of adding direct resin composite was the treatment of choice, and the composite was added mesially to No. 10 in one appointment before the impression for the zirconia crowns was taken.

The color difference of the two prepared teeth was noticeable, caused mainly by the endodontic treatment of tooth No. 9 and the newly laboratory-fabricated cast post with resin on tooth No. 9. Subsequently, the esthetics and the optical properties of the two laterals were observed: great depth illusion, great translucency, reduced opacity, and yellow/pink translucencies at the cervical area due to the color absorption of the soft tissue² (Figure 5). These observations are critical to properly attain a deep understanding of what would match well in the patient's oral environment and allows the ceramist to strategically place layers of ceramics in a manner that would yield esthetic and natural results.

After evaluation of the surrounding characteristics, the focus was averted to the underlying substructures. To achieve the appropriate color(s) between the two centrals, normally the zirconia cores or copings would be made using an opaque zirconia blank. However, this material choice would downgrade the natural appearance of the centrals by reducing their depth illusion and their translucency. (The higher the opacity value, the shorter the light wavelength it will exhibit, leading the restorations to have poor internal light diffusion and perhaps not achieving the proper color[s] as described above for the laterals.) This was deemed as the most important issue that should be taken into account.³ For this reason, the decision was made to use the highly translucent multi-layered zirconia, KATANA Zirconia UTML (Kuraray Noritake; kuraraynoritake.com). The final shade of choice for this case was selected as A2. A1 was milled for No. 8, while A3 was milled for No. 9 in an effort to balance the color differences between the two preparations. Furthermore, zirconia liner in shade A1 was used for No. 8, and a similar zirconia liner in shade A3 was used for No. 9 (Figure 6). In these two ways, the difference in color was balanced.⁴ Omitting this critical step in the process would have resulted in shade imbalance and the final color result would be different for the two centrals. The technique described provides for the color sum of the stump and the zirconia framework to be similar for the centrals: light zirconia coping on the dark stump (No. 8) and dark zirconia coping for the light stump (No. 9).

The next step was to address the adjustment of the opacity level. This was accomplished by using opaque dentin in shade A2 at a thickness of approximately 0.2 mm for both centrals, but for No. 8, A2 was mixed with 20% white. This formula is generated every time, dependent on the desired outcome and the ceramist's experience, and is case-specific. This way the opacity level of both centrals would be balanced, while the color of No. 8 was lightened (Figure 7 and Figure 8).

By utilizing different multi-layered ceramic powders in different colors for the cores, the different liner colors, different opaque porcelain masses, and different cement colors produced a collective effort to achieve and correct the value discrepancy by balancing the different-colored central stumps.⁵ A segmental build-up, using multiple ceramics to layer the restorations would achieve the predetermined desired shade(s), colors, and effects. This segmented build-up was done in typical fashion and shown in Figure 9 through Figure 11. Building up as such would provide tooth shade harmony without facing the challenges previously addressed in the core. After the application of secondary-fluorescence dentin, a bluish opalescence was added interproximally in order to further increase the value (Figure 12 and Figure 13). This was followed by T-Blue opalescence, which was used to increase the depth illusion in the middle of the teeth with light refraction (Figure 14).

The next step was to incorporate incisal enamel and mamelon ceramics (Figure 15). Pure transparent was used to simulate the dentin-enamel junction (Figure 16), followed by bright enamels which were added at the transitional angles (Figure 17). Finally, a secondary clear cervical translucent was used to further enhance the 3D effects of the restoration (Figure 18) and high-value enamels were layered to prevent the final result from being too low in value (Figure 19 through Figure 21). Ceramic multi-layered stratification using the author's “One-Bake Technique” followed,⁶ using Cerabien ZR (CZR ceramic from Kuraray Noritake) in order to simulate the esthetics of the natural teeth of the patient (Figure 22).

This build-up technique is a process the author uses to control the numerous firing cycles which may negatively affect feldspathic porcelain bonding to the zirconia core. Multiple firings may cause undesired consequences due to the cooling rate and the coefficient of thermal expansion during heating and cooling while firing in a porcelain furnace. The core design and build-up process of the One-Bake Technique was developed by the author for solving such issues and creating esthetic long-term restorations.⁶ The One-Bake Technique necessitates the underlying core or zirconia substructure to be designed with a uniform cut-back of anatomical design, using and exposing the emergent crests and ridges, as well as the concavities and convexities of each tooth on the substructure. This leads to a homogeneous layer of ceramic when layered, providing for the proper esthetic and ceramic distribution on the zirconia substructure.⁶

Although different zirconia materials, different liners, and different opaque dentins were used, the result between the centrals is a perfect match. Most importantly, the centrals and laterals have similar reflectivity and illusion of depth (Figure 23 through Figure 28).

New technologies, methods, and materials combined with technical knowledge have improved the predictability and longevity of prosthetic restorations. What we have learned from the past is that the human factor—the maturity and increased knowledge of dentists and dental technicians—is as important to the final outcome as all these technologies combined. Individual development, observation, and the ability to imitate nature, coupled with the understanding and use of new materials and methods that meet the esthetic and functional expectations of the patient, will reward the dental team with patients who love their new smiles. These factors, plus the passion for the work, are the secrets of success.

This article was double-blind peer reviewed by members of IDT's Editorial Advisory Board

Nondas Vlachopoulos
AestheticLab
Athens, Greece

Disclosure: The author had no disclosures to report.