Advancements in Millable Materials for Today's Laboratory

Through CAD/CAM technology, laboratories now have the ability to digitally design full-contour anterior and posterior restorations. During the design process, areas of importance like occlusion, contour, and function are all developed in the software. This allows skilled designers the ability to fine tune even the toughest processes, like anterior contouring, in the virtual world. 

Previously, cutback techniques were required to be applied to increase the esthetics of zirconia, especially in the case of anterior restorations. This caused valuable clinical diagnostic information, already established in the design stage, to be lost when the ceramics had to be reapplied by hand. The cut-back technique, while more efficient than full ceramic build-up processes on support coping structures, still required hand techniques that brought the human element to fabrication. 

Until now, laboratories needed to utilize standard porcelain build-up techniques to gain the appropriate level of esthetics. The need to layer for esthetics hindered the ability to design and mill the restoration in full contour. Valuable information already established by the designer in CAD had to be removed during a cut-back technique and ultimately had to be replaced by the application of ceramics. By not utilizing the design technology to its fullest, dental laboratories could miss out on the efficiency that it can bring.

Multilayer Zirconia

Fixed prosthetics have benefited greatly from the advancement of zirconia as a restorative material. Initially, zirconia lacked the ability to mimic natural esthetics without the additional application of labor-intensive manual techniques. This material limitation led to several techniques being developed to increase esthetics, but they required a lot of work and were not predictably replicable. Manufacturing advancements in zirconia have led to today's innovative offerings. Multilayer zirconia discs are unique in the fact that they are pre-shaded and offer a color gradient to produce lifelike esthetics (Figure 1). Manufacturers are now able to produce zirconia discs with a natural built-in transition from dentin to incisal. This natural gradient within the disc produces results similar to simple hand-built ceramic techniques. With multilayer zirconia as the restorative material, laboratories are able to utilize CAD/CAM technology to its fullest ability (Figure 2).

Post-process finishing has now become very predictable and replicable when implemented correctly in the modern laboratory. The restoration is designed in full contour, then milled, and green stage finished by applying surface texture and characterization (Figure 3 and Figure 4). While not required under most manufacturer recommendations, this is truly where the technician's artistry helps bring the true potential of the material to life.

The global acceptance of zirconia in dentistry has been a revolution. This is partly due to limited tooth preparation requirements, allowing dentists to utilize preparation techniques they have become accustomed to over many years (Figure 5). After sintering, the restoration is final adjusted and checked for accuracy on the die. Once the restoration is verified as accurate, a simple stain-and-glaze technique easily achieves individual characterization of the final restoration (Figure 6). The implementation of multilayer zirconia allows for a predictable digital workflow from diagnostic wax-up to final restoration. These esthetic results were previously only possible with labor-intensive layering techniques (Figure 7 and Figure 8) but are now achievable in single-unit cases to full-mouth rehabilitations without the need for ceramic application (Figure 9).

Multilayer zirconia is currently offered by multiple manufacturers, and new variations continue to be launched onto the market. Each manufacturer's products differ slightly, so it is important to research and evaluate these products to identify the one that meets your individual needs.

There is a new family of zirconia that is considered gradient zirconia. The yttria elements in the zirconia matrix have varying levels of concentrations within the same disc, providing the laboratory technician the opportunity to place the restoration in the best possible location while nesting for lifelike esthetics. Placing the CAD design for anterior monolithic crowns in the top side of the disc will lead to more translucent restoration with lower MPa values, while placing the restoration towards the lower side of the puck will provide lower concentrations of yttria, and therefore producing a stronger (higher MPA), more opaque zirconia, appropriate for substructures and frames. These gradient zirconia varieties do not have delineation between the different layers, but rather have a gradient or range of Y5 at the very top, transitioning to Y4 in the middle and ultimately Y3 at the bottom. With multilayer zirconia as the restorative material, laboratories are able to utilize CAD/CAM technology to its fullest ability.

PMMA

Historically, laboratory-fabricated provisionals were extremely labor intensive and not a profitable product offering for dental laboratories. With the development of PMMA as a millable option, laboratories can now fabricate incredibly lifelike and highly esthetic provisionals that are efficient, predictable, and cost effective to produce.

Through CAD, the utilization of PMMA provisionals or prototypes is an invaluable aid to full-arch, full-mouth rehabilitations. In the prosthetic division of the author's laboratory, the results of larger and more complex cases have become much more predictable since the introduction of this interim step (Figure 10). Using PMMA provisionals, or transitional temporaries, has greatly helped to improve customer satisfaction and meet or exceed patient expectations.

CAD technology allows the dental laboratory to design and process the suggested final restoration in a lifelike copy of the proposed prosthesis. This gives the laboratory the ability to try in the final suggested restoration design in a transitional material that is easy to adjust. Requested changes can easily be made at this time, which greatly reduce costly remakes and unhappy patients. When dealing with hybrid implant-supported solutions for edentulous patients, these transitional PMMA materials eradicate the need for costly, time-consuming tooth set-ups. The material provides a predictable digital workflow from PMMA try-ins to final zirconia restorations.

Furthermore, some of today's PMMA materials offer built-in gradient color that provides the ability to create truly lifelike transitional prosthetics (Figure 11 and Figure 12). Most of these materials are approved for medium-term use in the mouth, which allows patients to function with their temporary prosthetics for an extended period before transitioning to the final prostheses (Figure 13).

The provisional prototype has become the master record able to help accurately capture and verify occlusion, test function, set mid-line, establish tooth form and shade, and even determine gingival architecture (Figure 14). All modifications can be transferred predictably to the final restoration. This simple digital process provides the ability to have an extremely accurate restoration ready for the final delivery appointment.

Other Materials

Beyond the latest zirconia and PMMA, many other millable material options exist for various indications and situations in the dental laboratory.

Millable lithium disilicate is available in multiple translucencies and is indicated for crowns, veneers, inlays, onlays, implant abutments, screw-retained crowns, and bridges up to three units. Feldspathic porcelain is available in monochromatic and multichromatic blocks for crowns, veneers, inlays, and onlays; leucite-reinforced glass ceramic can be used for those same indications. Various composites, including hybrids, offer solutions for veneers, inlays, and onlays that are strong but less taxing on milling burs. Lithium silicate hybrids with either zirconia or alumina can be used for crowns, veneers, inlays, onlays, and sometimes for implant abutments, screw-retained crowns, and three-unit bridges. A number of high-performance polymers have hit the market for partial denture frameworks, crown-and-bridge substructures, and implant suprastructures. Various metals also are available for these indications and for crowns. Several manufacturers also have introduced millable denture pucks, some for the base only and some with either a tooth-colored layer or teeth bonded into the disc.

Conclusion

The advancements in millable materials are in direct correlation with the adoption of digital technology by laboratories. The importance of accurate and esthetic millable materials to complete the digital link cannot be overstated. Today's laboratory has no choice but to accept and engage in digital fabrication methods. In turn, the acceptance of digital technology is driving the demand for better millable material options.

Continued manufacturer research and development has increased the number of options available. This has allowed laboratories to offer new and innovative products to their clinicians by utilizing better workflow solutions. Using these new millable materials solves yesterday's problems and greatly increases efficiency, predictability, and profitability. The future is bright.

Acknowledgements

The author would like to thank Jack Marrano, CDT, of Absolute Dental Services; Mark Scurria, DDS, of Triangle Restoration Dentistry; and Christopher Barwacz, DDS, of the University of Iowa, for their contributions to this article.

About the Author

Conrad J. Rensburg, ND, NHD, is the co-owner of Absolute Dental Services, which has four locations in North Carolina and South Carolina.