Making the Most of Full-Contour Zirconia
Creating exceptionally strong and esthetic single-tooth restorations
By Bob Cohen, CDT
Over the last decade, monolithic, monochromatic materials have become very commonly proscribed and fabricated in most dental laboratories. Some of the advantages of using these materials are a lower cost of manufacturing, reduced failures due to delamination of layered ceramics, and higher-strength restorations with fewer flaws in the structure of the ceramic. However, these advantages come with some compromises, including a lack of enamel and dentin layering, which generally leads to difficulty in obtaining a multi-shaded restoration that matches the shade gradations of adjacent dentition. Thus, manufacturing with a digital workflow will increase efficiencies, but it inherently adds difficulty in fabricating life-like restorations.
The fastest-growing monolithic material in North America, full-contour zirconia, provides some unique properties and offers opportunities for improved esthetics. Zirconia is the strongest all-ceramic material ever put into mainstream manufacturing for dental, and tooth preparation design is far less critical with this material than with other all-ceramics. Zirconia facilitates automated manufacturing, thus keeping production costs low. The cost of materials is also comparatively economical. For the purpose of this article, we will examine how to maximize excellence through automation, in addition to some analog processing for full-contour zirconia restorations.
The process begins with scanning and designing the restoration. The CAD process enables the dental technician to scan and design a full-contour single tooth in less than 10 minutes. Through advances in digital and anatomic libraries, it is now possible to achieve best-in-class tooth form for cases with adequate room for restorative materials. Seeing the form of the initial proposal in any CAD software will play a large role in the quality of the output and the time needed to complete the design. Doing a comparative analysis of an anatomical library can be accomplished through the initial proposal of multiple libraries (Figure 1 and Figure 2).
Once the design is complete, the .stl file is loaded into the CAM software. Though the combination of high-definition CAM milling strategies and a 0.3-mm tool, it is possible to mill very intricate anatomic detail (Figure 3). Many of the currently used mill strategies do not incorporate milling the high-definition detail from the CAD design.
Post processing of the green-state milled zirconia can also provide an opportunity for product enhancement. This is done after the milled zirconia restoration is removed from the disk. In the green state, zirconia is quite soft and easily trimmed, and adjustment can be facilitated with rubber wheels and high- or slow-speed diamond burs. Green-state adjustments or detailing, such as subtle re-contouring and touching up occlusal anatomy and surface texture, can be easily added (Figure 4). Once final contouring is complete, it is critically important to remove all surface debris from the restoration. Surface particulate or powder left on the zirconia restoration will reduce light transition and may cause clouding.
Today, most monolithic ceramic materials are monochromatic. The nature of green-state zirconia provides the unique opportunity to shade the material prior to sintering. In this state, this pre-sintered material is naturally absorbent. For the past decade, most standard zirconia coloring protocols included dipping the pre-sintered zirconia restoration in a coloring liquid to obtain a final monochromatic shade that mimicked a particular VITA® shade. Through the advent of Zirkonzahn’s Prettau® Aquarell coloring liquids (www.zirkonzahn.com), it has become easy to paint on several different colors that penetrate the zirconia restoration (Figure 5). As a result, technicians can pre-color or multi-shade restorations.
Sintering zirconia is a critical step for both the integrity of the material and the final esthetics, and sintering beads are a key factor in that step. If sintering daily, technicians should check the sintering beads on a weekly basis for color absorption from the shading liquids used in previous firings. The used beads should be checked against new beads to ensure that the used ones have not absorbed too much color. Failure to do this periodically can cause the new restorations to absorb the colorants from the beads during firing. For sintering furnaces, it is best to purchase a furnace that has bottom entry, because furnaces with a door in the front have no heating elements and are therefore much more prone to inconsistency of temperature throughout the muffle.
Once sintered (Figure 6), the crowns are checked on the die. If the CAD parameters are set properly, 99% should require no fitting. For the 1% that do not fit properly, use of a fit check spray can quickly identify the point of premature contact. All adjustments can be made with a high-speed diamond, using water for irrigation. Subsequently, the margins are thinned and the restoration is checked to the cast for proximal and occlusal contacts. Once complete, GC Initial™ IQ Luster Paste (GC America, www.gcamerica.com) is used to fine-tune the color and achieve a final stain and glaze (Figure 7).
Bob Cohen, CDT is the co-founder of Custom Automated Prosthetics in Stoneham, MA.
For more information, contact:
Custom Automated Prosthetics
The preceding material was provided by the manufacturer. The statements and opinions contained therein are solely those of the manufacturer and not of the editors, publisher, or the Editorial Board of Inside Dental Technology.