Novel Block Materials and Processing Techniques for Machined Restorations
Scientific developments continue to offer laboratories new options
By Russell Giordano II, DMD, CAGS, DMSc
As the use of digital applications for fabrication of dental prostheses expands, so have the material options. While dentists still lag behind in the use of digital technology for restorations, dental laboratories have embraced it as the new standard for production of dental prostheses. Material options may be broken down into several categories: porcelain, glass-ceramics, interpenetrating phase ceramics, polycrystalline ceramics, composite resins, and polymers. The range of materials available for machining has grown in these categories, but various forms of zirconia have become the most popular choice for machined indirect restorations and frameworks.
Several advantages exist for block materials as opposed to conventionally fabricated materials. The reliability of the material may be enhanced due to the reproducibility of the block manufacturing process. The blocks are produced in the same manner from day to day, year in and year out, resulting in a dense, high-quality material. Conventionally processed restorations, although also of high quality, are generally fabricated by hand, which may affect the reliability of the restoration both with respect to mechanical and esthetic properties. Porosity is generally present in pressed and hand-fired restorations, whereas dense block materials are almost pore-free. For fully dense blocks such as IPS Empress CAD (Ivoclar Vivadent, ivoclarvivadent.com), VITABLOCS® Mark II (VITA North America, vitanorthamerica.com), CEREC Blocs (Dentsply Sirona, sirona.com), Lava™ Ultimate (3M, 3m.com), VITA ENAMIC® (VITA North America), and CELTRA® DUO (Dentsply International, dentsplyceltra.com), only minimal post-machining processing may be needed, such as polishing and possible stain and glaze.
Other materials such as zirconia and lithium disilicate require significant post-machining processing. These processes can significantly affect the accuracy and mechanical properties of the materials. Furnaces need to be properly calibrated and the manufacturer-recommended cycles must be followed. For glass ceramics such as lithium disilicate, crystallization cycles are crucial to achieving both the proper mechanical properties and shade. A cycle that is too fast or a temperature too low can lead to less crystallization, improper shade, and weak and soluble materials. A temperature that is too high might lead to distortion of the restoration and improper crystallization as well. It is extremely important to follow guidelines for support of lithium disilicate materials during crystallization as the glass transition temperature (akin to the onset of melting) is very close to the crystallization temperature.
With zirconia, altering the recommended cycle affects the crystal structure and the mechanical properties. The zirconia category is broad and includes a variety of types with very different cycles. Firing a certain type of zirconia at the wrong temperature can cause significant weakening of the material due to enlarged crystal growth.
Materials may be classified as porcelains, glass-ceramics, composite resins, interconnected ceramics, and polycrystalline materials. Porcelains refer to those materials with a glassy phase surrounding a crystal phase. These are represented by materials such as IPS Empress CAD, VITABLOCS Mark II, and CEREC Blocs.
Glass-ceramics such as CELTRA DUO, IPS e.max (Ivoclar Vivadent), and VITA SUPRINITY (VITA Zahnfabrik, vita-zahnfabrik.com) start as homogenous glass. They may be machined in the glass state and then subjected to a heat treatment to grow crystals and achieve proper color, crystal density, and mechanical properties. IPS e.max also has been available as a block with a “hole” in it, designed to be used in the TiBase implant system (Dentsply Sirona). It may be used for an abutment only or for a screw-retained combined abutment/crown restoration.
CELTRA DUO was developed by Dentsply International jointly with VITA Zahnfabrik and the Fraunhofer Institute for Silicate Research in Germany (Figure 1). With this material, the manufacturer provides two options. The first is to machine (single crown approximately 14 minutes) in the crystal state, stop, polish, and cement. Mechanical properties decrease from approximately 430 MPa to 200 MPa, indicating significant effects of machining on fracture resistance. The alternative is a second heat cycle after machining for another 14 minutes, producing a material with a strength of approximately 370 MPa. VITA SUPRINITY, a similar product, has only a crystallization cycle and does not require a refractory support material because its glass transition temperature is not close to the crystallization temperature. Both of these materials have similar compositions and rely in part on dissolved zirconia to create their unique crystal structure.
Interpenetrating Phase Ceramics
Interpenetrating phase materials such as VITA ENAMIC may be the new area of development for advancing machined materials that are resistant to damage and easy to machine (Figure 2). This first-generation material has a unique microstructure. It is easy to machine (four minutes for a crown), is bur kind, and requires only polishing. It is chipping resistant. It is more fracture resistant at lower thicknesses than conventional glass ceramics such as lithium disilicate and has a minimum thickness of only 1.0 mm, almost matching that of zirconia (approximately 0.8 mm). New products in this area include a block for implant abutments or one-piece implant/abutment crowns as well as discs in addition to blocks. Ongoing research has produced additional formulations with higher toughness and load-bearing capacity that may allow for even thinner restorations, saving valuable tooth structure. These are still in the development stage.
Research in the author’s laboratory regarding combinations of various abutment materials with machined, bonded crowns has shown that a VITABLOCS Mark II crown/VITA ENAMIC abutment produces a restoration with a failure load equivalent to that of a VITABLOCS Mark II crown/zirconia abutments or a lithium disilicate crown/zirconia abutment. This potentially could allow single-visit implant restorations without any need for a sintering or crystallization step.
These types of materials have drawn increasing interest due to their resistance to machining damage and the ability to layer using light-cured composite resin materials. Two composite resin block materials are available: Lava Ultimate, a “nano” particle-filled polymer with approximately 65% glass and zircon filler by volume, and CERASMART™ (GC America Inc., gcamerica.com), a 55%-by-volume nano-particle glass-filled polymer. Both of these materials had indications for inlays, onlays, and crowns. However, due to bonding issues, the crown indication for Lava Ultimate was discontinued. This is possibly due to the highly cured polymer making it difficult to adhere to resin bonding cements as well as the lower modulus/higher flexibility of these composite resins.
Families of polymers under the general category of polyaryletherketones (PAEKs) are thermoplastic materials with different properties relating to their exact chemical composition. Three basic types are being used increasingly for frameworks upon which composite resins or ceramics may be bonded. These include poly-ether-ether-ketone, or PEEK (PEEK-Optima®, Invibio Biomaterial Solutions, invibio.com) and poly-ether-ketone-ketone, or PEKK (Pekkton®, Cendres+Métaux, cmsa.ch). PEEK is an amorphous material that may be compounded with fibers to create a fiber-reinforced polymer. PEKK is a crystalline material with higher mechanical properties. Flexural strengths are 140 MPa (PEEK), 200 MPa (PEKK), and up to 300 MPa for fiber-reinforced PEEK. An increasingly popular use for these materials is implant-supported frameworks. Researchers continue to explore optimizing these materials’ bonding characteristics with cements for luting to natural teeth as well as to veneer ceramics and composite resins. Although these materials may have high strength values, they still are highly flexible with lower elastic modulus values (approximately 5-8 GPa) compared with those of composite resin blocks (approximately 10-14 GPa). Only the fiber-reinforced materials have modulus values similar to the composite resins. The higher flexibility could lead to deboning of veneer materials over time.
The dominant machinable material, zirconia (Y-TZP, or yttria partially stabilized zirconia), is a polycrystalline ceramic. Zirconia (ZrO2) is the oxidized form of zirconium (Zr), just as alumina (Al2O3) is an oxide of aluminum (Al). Some important developments have occurred in the material type and processing of the zirconia family over the past year.
Zirconia exists in three major phases: monoclinic, tetragonal, and cubic. Monoclinic is the largest, tetragonal intermediate, and cubic the smallest. As the temperature goes from room temperature to above 2370°C, the material changes into a cubic phase. In pure zirconia ceramics, the cubic-to-monoclinic phase transformation occurs during cooling with an approximately 5% volumetric expansion (causing cracks), which
may then fracture zirconia at room temperature. Therefore, biomedical and structural/functional applications of zirconia typically do not use pure zirconia. The addition of other ceramic components may stabilize the monoclinic phase at room temperature. If the correct amount of component is added, such as calcia (CaO), magnesia (MgO), yttria (Y2O3), and ceria (CeO2), then a fully stabilized material can be created. The addition of smaller amounts (5% by weight) produces a partially stabilized zirconia. Although stabilized at room temperature, the tetragonal zirconia phase may change under stress to monoclinic with a subsequent 3% volumetric increase. This is a property called transformation toughening.
Dentistry typically has used Y-TZP with approximately 5% yttria by weight. Another key component is a small amount of alumina to help prevent uncontrolled transformation that would result in cracking and failure of the material; the “standard” zirconia has approximately 0.25% by weight. Why do these numbers matter? The yttria content is responsible for the zirconia’s ability to resist damage and stop cracks. The alumina prevents wholesale transformation leading to failure of the material under aging. “Ultra/super/mega” translucent zirconia has entered the marketplace in the past year. It has only approximately
0.05 % alumina by weight and approximately 9.0% yttria by weight. Testing at Boston University has revealed that the crack-stopping/damage-resistant property in a standard zirconia is not present in the high-translucent material. In fact, typical procedures that might be performed in the laboratory or by the dentist when adjusting this material can reduce the strength from an untouched value of approximately 700 MPa to 400 MPa after using a 125-micron wheel and only 300 MPa after sandblasting with 50-micron alumina. Thus, caution must be used when selecting this material for various clinical applications and particularly if adjustments need to be made chairside. Studies into the fatigue resistance of this material are necessary to fully determine its proper clinical use.
Advances in the production of single-unit standard zirconia have taken two approaches. One is by Glidewell Laboratories (glidewelldental.com) with the release of BruxZir Now (Figure 3). This is a traditional Y-TZP zirconia material that is already fully dense as opposed to the porous blocks that require sintering. The block and bur are supplied together for single use. Machining time is about 45 minutes. The standard six-hour sintering cycle is eliminated. Only polishing or polishing and glazing is required before placement of the restoration.
Dentsply Sirona has taken a different approach. A porous zirconia block is machined to produce a crown. The crown is then placed in a special furnace, CEREC SpeedFire (Dentsply Sirona), that allows for sintering and glazing of the crown in approximately 15 minutes. In a recently completed double-blind study performed at Boston University, the strength of the speed-fired zirconia was found to be statistically the same as that of conventionally fired zirconia (six-hour cycle). In other research using fast firing with a different furnace and different zirconia, the zirconia was significantly weaker. Additional experiments are planned to examine speed-fired zirconia under fatigue to determine if there are any differences in long-term use.
Advances in material types and processing continue at a rapid pace. New materials are designed for improved machinability, ease of fabrication, and resistance to stress and damage that occur in the mouth. These advances will continue to transform the laboratory’s ability to provide optimal patient care with the best quality restorations.
Russell Giordano II, DMD, CAGS, DMSc, is an Associate Professor in the Department of Restorative Sciences & Biomaterials at Boston University’s Henry M. Goldman School of Dental Medicine.