Dental Materials for Esthetic Implant-Supported Restorations

The dental industry has seen exponential advancements in technology in recent years, and with these advances has come a subsequent evolution in modern-day clinical workflows.¹ As technology has progressed, materials have also evolved. As a result, the question of which materials to prescribe for dental restorations has also become more complex.

Contemporary dental materials are no longer judged only by their esthetic capabilities or strength, but also by how well they adapt within a digital processing workflow. At the outset of this digital evolution, compromises in esthetics and sometimes even in strength had to be made in the name of progress.² Today the dental industry is finally seeing a synergy develop between digital design, processing equipment, and materials. One of the greatest benefits brought about by these digital workflows is increased clinical predictability.¹This is proving to be especially relevant in larger and complex implant-supported cases. The ability to prototype a case before proceeding to the final prosthesis is instrumental in simplifying final delivery of these once very complicated cases.

In today's high-technology dental environment, restoring implant-supported cases efficiently requires simplified digital workflows and superb final materials, coupled with exceptional temporary and transitional prototyping options.

Modern-day dental materials can be segmented into three distinct fabrication processes: hand-processed, additive/printing, and reductive/milling. Each of these processes has unique advantages and disadvantages.

A decade ago, restorative materials were judged mainly by their esthetics and strength. Today, with a shortage in skilled dental laboratory labor and an increasing demand for more efficient digital workflows, accompanied by more predictable materials, a third crucial criterion has emerged: digital compatibility. If implemented correctly, technology can enhance artistry and eliminate unnecessary hand processes. This combination of factors can lead to the need for less human interpretation and enhanced digital communication with improved clinical data gathering, resulting in more predictable deliveries.³

In implant-supported applications, in lieu of a disposable printed try-in, transitional polymethyl methacrylate (PMMA) materials (eg, TempEsthetic^™ double cross-linked PMMA, Harvest Dental, harvestdental.com; ArgenPMMA^™ Multilayer, Argen^® Corporation, argen.com; Dentsply^® Multilayer PMMA, Dentsply Sirona, denstplysirona.com) can be used for a more esthetic and functional solution.⁴ This affords the patient a more realistic trial prosthesis and adequate time to evaluate both function and esthetics (Figure 1 and Figure 2). After patient approval is established, the transitional hybrid is digitally indexed with an intraoral scanner or analog impressed and digitized for final hybrid fabrication. The laboratory will then model match the approved hybrid data with the original design, make adjustments to incorporate any required changes, and copy-mill the final prosthesis from the approved data (Figure 3).

This highly accurate digital "jump" between patient-approved try-in and final hybrid (Figure 4 and Figure 5) allows for efficient, streamlined deliveries of these often-complex cases. The need for adjusting on post-sintered zirconia, which has been proven to reduce longevity of zirconia hybrids,⁵ is greatly reduced with this workflow.

This transitional PMMA hybrid, which is an exact copy of the final restoration, also can be delivered as a long-term emergency hybrid with the definitive prosthesis (Figure 6). Transitional hybrids are digitally produced and, therefore, relatively inexpensive, making this device an excellent cost-effective option in case of a future emergency.

Although very different technologies, additive and subtractive manufacturing methods have revolutionized the way products are produced in the dental industry. Additive manufacturing is a process that adds successive layers of material to create an object; this process is often referred to as 3D printing. Subtractive manufacturing, as the name suggests, is the opposite. Rather than adding layers, subtractive manufacturing involves the removal of sections of a material by machining.

Additive processes generally are more efficient than reductive processes and generate products in finer detail, especially with regard to complex contours and undercuts (Figure 7).⁶ In general, printers require less maintenance than milling machines and are not as prone to component wear, resulting in less equipment fatigue and progressive product quality loss.⁷ Presently, it appears the dental industry is in the early stages of a material development focus shift toward additive technology.

Historically, PMMA materials comparable to today's photopolymers were primarily available in a millable puck.⁸ Improvements in 3D printing technology, combined with material development, have now opened a pathway to functional intraoral polymer materials, and a future shift away from milling seems inevitable.

Currently, millable double cross-linked PMMA is still the most esthetic and strongest material available for transitional prototypes and temporary application,⁹ but newer glass-infiltrated polymers (eg, Permanent Crown Resin, Formlabs, formlabs.com; VarseoSmile Crown plus, Bego, bego.com ) are starting to compete in this arena (Figure 8).

As the dental industry further progresses into this exciting uncharted future and technology continues to rapidly advance, the importance of processing in a fully comprehensive, technical, and clinical digital workflow cannot be overstated. Thus, it is crucial to minimize hand processes like layering ceramics by utilizing materials that support these digital processes.

Since the introduction of the All-on-4^® technique in 2004, the weakest link in this otherwise highly successful protocol has been the denture tooth prosthesis supported by a titanium substructure, wrapped with acrylic.¹⁰ This traditional restorative option requires extensive hand processing and offers no digital workflow or archiving of the data. Furthermore, flexural differences and lack of chemical bonding between the acrylic and alloy components can be problematic. Moreover, when compared to a zirconia hybrid these acrylic options seem to be more susceptible to fluid absorption and discoloration.

Although 21st century zirconia products have been proven to withstand strong occlusal forces, even in patients with bruxism, many clinicians still choose to prescribe a prosthesis that offers a softer occlusion.¹¹ This is especially true when restoring against weakened opposing dentition or even against another zirconia hybrid. Because of this, it is important to be able to offer a nontraditional ceramic that is yet still a functional and esthetic monolithic solution.

Designing the support substructure with a technopolymer¹² (eg, TriLor^®, Preat, preat.com; Trinia^™, Shofu, shofu.com; Pekkton^®, Anaxdent, anaxdentusa.com; JUVORA^® PEEK, Invibio, invibio.com) in lieu of a titanium frame allows the structure to closely replicate the microflexing in natural bone (Table 1). Furthermore, this material combination allows for far better flexural compatibility between the acrylic and technopolymer base. This matched flexural compatibility, combined with the added advantage of chemical bonding between the materials, contributes to a well-stabilized base to support the tooth structure.

As an alternative to setting individual denture teeth, which are prone to debonding,¹³ the tooth overlay is digitally designed to perfectly key into the technopolymer as a monolithic structure (Figure 9). The tooth overlay is milled from a nanoceramic (also known as a resin-matrix ceramic) (eg, Crystal^® Ultra, Digital Dental, digitaldental.com; Cerasmart^®, GC America, gcamerica.com) or tooth-colored PMMA material. This allows for a highly esthetic monolithic tooth overlay, custom supported by the stronger technopolymer material (Figure 10). Depending on the design, technopolymers are generally available in a pink or tooth-colored millable puck.

A nanoceramic material can also be used in implant clip-bar or LOCATOR^®-supported type digital dentures. This monolithic, glass-like, resin-matrix material, processed into a printed denture base, can shade match an opposing zirconia material more effectively than even some current hybrid composite denture teeth(Figure 11).

Digital design intimately mates the support and overlay components. The combination of a monolithic tooth structure supported by a custom-designed technopolymer frame and wrapped with an injectable high-impact acrylic provides a true contemporary acrylic-polymer hybrid dental restoration.

To facilitate a proprietary connection into the implant interface, the technopolymer is custom milled to receive a titanium-base abutment. This proprietary titanium interface is cemented into the substructure frame on a verified cast. This step is the only part of this restorative protocol that cannot yet be completed in a digital workflow. Therefore, it is important to save the verified analog cast in case a future remake is needed.

The substructure and tooth overlay files are digitally archived for potential future remakes. This feature greatly reduces additional post-delivery clinical appointments and allows for streamlined remakes.

With a long, rich history, zirconia only started competing with ceramic materials like aluminum oxide and lithium disilicate once it became more esthetic.¹⁴ For dental application, zirconia is a yttria-stabilized tetragonal zirconia polycrystalline material and is classified by "mole % yttria (%Y)." The values of 3Y, 4Y, and 5Y define both the mechanical and physical properties of the material.¹⁵

Initial zirconia products were mostly classified as 3Y products (3 mole % yttria to partially stabilize the tetragonal phase) and displayed high fracture toughness and flexural strength of between 1200 MPa to 1500 MPa.¹⁶ Although these pioneering products were extremely strong, they were exceptionally opaque. Because of this opacity they could not compete esthetically with the aluminum oxides and lithium disilicates of the early 2000s, even when layered with nanofluorapatite powders.¹⁷

To create more esthetic solutions, manufacturers drove the market toward 4Y and eventually 5Y products. Although these products were more translucent, the materials displayed a loss in flexural strength and fracture toughness,¹⁸ with original 4Y zirconia materials having a flexural strength between 600 MPa and 900 MPa and 5Y materials exhibiting a flexural strength of 700 MPa to 800 MPa.

Like most other evolutionary material cycles, the future of zirconia would be driven by demand. The market demanded a monolithic material capable of eradicating layering ceramics.¹⁹ Furthermore, it required a more esthetic material that offered a natural transition from gingiva to incisal edge and a high (≥1000 MPa) flexural strength for bridge application.¹⁶

Today, zirconia material processing techniques have given way to expanded product classifications. No longer are materials only classified and marketed by the mole percentage yttria. Modern-day materials, with multi-layer, high-strength, and high-translucency characteristics, are now classified with a greater emphasis on transitional properties and light-distribution characterization.²⁰

Regarding zirconia product development, when changing alloy properties, metallurgists manipulate the composition of the alloy, according to Paul Cascone, senior vice president of Research and Development at Argen Corporation, whom the author interviewed. It is important to understand how different metals interact when alloyed. When attempting to manipulate the light-distribution and refraction properties of zirconia, the handling, and not only the composition, of the raw zirconium-oxide material ultimately determines the mechanical and physical properties of the final product. To create present-day, high-strength (≥1000 MPa), high-translucent materials, the raw material is dissolved into hydrochloric acid, and yttria chloride is added.²¹ After spray-drying, oxide forms and the yttria becomes intimately engaged to form dental zirconia. This process balances the crystallites to better align and reduces grain boundaries that are responsible for opacity in zirconia.²²

By manipulating the raw material handling of a 4Y material, it is now possible to produce a monolithic, high-translucent 4Y zirconia with 1200 MPa to 1300 MPa of flexural strength (eg, Argen HT+, Argen; Cercon^™ ht, Dentsply Sirona). In single-unit, titanium abutment-supported applications, these materials offer an exceptional ability to block out the underlying abutment color while still offering excellent monolithic esthetics (Figure 12).²³

Other philosophies in today's dental marketplace are focused on pressing or combining different layers within the puck. Multilayer zirconia (eg, IPS e.max^® ZirCAD Prime, Ivoclar Vivadent, ivoclarvivadent.com; Argen HT+ML, Argen) is a novel approach to combining strength and esthetics, and manufacturers have been developing highly innovative products.²⁰ This is done mainly by combining 5Y for translucency and 3Y for strength.

Contemporary zirconia development has evolved into combining proprietary chemistry, specialized equipment, and secret pressing techniques. Currently, zirconia (in its many forms) remains a favorite restorative material in the industry (Figure 13), but new product development always seems to be lurking just around the corner.

Although the focus of this article is the strength and esthetics of the materials presented, the importance of material biocompatibility cannot be overstated. Materials presented in this article have been US Food and Drug Administration-approved for intraoral use and have undergone extensive studies, and biocompatibility has been established and well documented.

As technology continues to advance, it will render many of these materials discussed obsolete and replace them with even more advanced options. Staying abreast of these changes is crucial for clinicians. The dental laboratory of the future must evolve to become a full solutions partner, helping its customers navigate this ever-changing landscape. Ultimately, the true value in these state-of-the-art processes is found in clinical workflow upgrades. These improvements help clinicians restore more efficiently and provide patients with higher-quality products.

The author would like to acknowledge the technical contributions from the Absolute Dental Lab, Signature ART team, Durham, North Carolina; Dries van Aarde, ND, NHD, digital design team oversight; and Jack Marrano, CDT, handcrafted prosthetic artistry. He also acknowledges the superb photography and clinical work of Drs. Mark Ludlow, Barry Goldenberg, Mark Scurria, Chris Barwacz, and Ace Jovanovski that helped make this article possible.

Conrad J. Rensburg, ND, NHD
Chief Executive Officer and Head of the Dental Implant Division, Absolute Dental Services, Durham, North Carolina

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