Millable Materials for the Modern Laboratory

Milling is a subtractive process, performed either under wet or dry conditions, where a restoration is cut from a solid block of prefabricated, industrially produced, uniform material. Because there is less potential for technique variations in how a milled material is manufactured, the mechanical properties of milled final restorations are much more predictable and standardized. Overall, milling allows the restorative team to provide consistent, high quality prosthetics to patients.

While the subject of biomaterials may not be the hottest topic in dentistry, both clinicians and technicians need to make great efforts to stay abreast of the latest materials on the market in order to ensure optimal treatment outcomes for their patients. This article aims to provide a review and update of the millable materials currently being used in dentistry so that the restorative team can feel comfortable knowing exactly what is best to prescribe for a given patient.

Not all commercially available zirconia powders are the same. Differences between products in grain size and additives greatly control the material’s strength, long-term stability, and translucency.¹ Furthermore, the varying processes by which dental zirconia powders are formed into milling blanks significantly influences the quality of the final product. Unidirectional axial pressing creates a milling blank shape that is very precise but lacks material consistency and therefore is not ideal for larger restorations.² On the other hand, cold isostatic pressing (CIP) uses a liquid medium, like water, to apply pressure uniformly in all directions to the zirconia powder enclosed in a mold to maintain its shape.³ The very high pressures involved with CIP minimize and remove voids in the powder to increase material density and produce a green-state (unsintered) zirconia with excellent homogeneity throughout the entire material. The green-state zirconia is then pre-sintered to allow it to be easily milled and post-processed by the technician. The final production step involves sintering the zirconia at very high temperatures (1350°C to 1500°C) whereby the final restoration hardens and shrinks linearly by 20% to 25% to achieve the desired strength and optical properties.

The first generations of high-strength CIP zirconia milling materials resulted in monochromatic and densely opaque restorations that exhibited limited esthetics unless layered. However, over the past 5 years, newer iterations of zirconia materials have evolved into milling blocks that are trending toward ever higher translucency and pre-shaded or multilayered blocks that greatly increase the esthetic outcome while reducing production processes. A great benefit to these high-translucency materials is that they require less reduction than monolithic glass-ceramic restorations and are kinder to natural opposing dentition.

In the past year, new formulations of zirconia milling blocks have included those that can be sintered in as few as 2 hours. There are also fully sintered hot isostatic pressing (HIP) iterations that require only polishing after the milling process and therefore are suitable for chairside milling. Since an HIP milling block is milled at a ratio of 1:1 (as there is no post-mill sintering shrinkage), the zirconia’s dimensional stability makes it ideal for large-span restorations. The drawbacks, however, include that milling HIP zirconia is very slow and taxing on the milling unit and burs, resulting in added expense and the need for more machine maintenance. Additionally, grinding dense sintered zirconia can introduce surface defects that may negatively impact the strength of the final restoration.⁴

The first CAD/CAM-produced inlay was made in 1985 using a feldspathic ceramic block.⁵ Feldspathic porcelain is currently available in millable form in VITABLOCS™ (VITA,

vitanorthamerica.com) and CEREC™ Blocs (Dentsply Sirona, dentsplysirona.com). Vita™ Mark II blocks are fabricated using extrusion molding from feldspar porcelain particles embedded in a glass matrix, resulting in a reported flexural strength of 100 MPa to 160 MPa when glazed. They are available in multiple monochromatic shades, but have excellent translucency characteristics that allow for seamless transition with natural dentition. The newer VITABLOCS, like TriLuxe™, Triluxe™ Forte, and RealLife™ blocks, come in color and translucency gradations that better match natural teeth for superb esthetics. CEREC Blocs are similar to VITABLOCS but use a different shading system. The indications for millable feldspathic ceramic materials include veneers, inlays, onlays, and anterior crowns.

IPS Empress® CAD is a millable leucite-reinforced glass-ceramic first produced by Ivoclar Vivadent (ivoclarvivadent.com) in 2006. Leucite-reinforced glass-ceramics provide excellent blending results due to their natural translucency and chameleon-like effect. With more than 20 years of clinical data and an advertised flexural strength of 160 MPa, IPS Empress CAD is predictably indicated for veneers, inlays, onlays, and single-unit crowns. The blocks come in two translucency options in the traditional A-D shades as well as 4 bleach shades with the option to be further cut back for layering with IPS Empress Esthetic Veneer ceramic.

However, as monolithic restorations became the preferred choice of dentists and laboratories, IPS Empress CAD Multi blocks were developed to provide a seamless shade transition from the higher-chroma cervical area to a lifelike translucency at the incisal with optimal esthetics. Once milled, these restorations are ready to be delivered by simply requiring the practitioner to polish the restoration prior to bonding. The option to stain and glaze the restoration also exists, and firing milled leucite-reinforced restorations has been shown to increase the material’s strength. Other leucite-reinforced glass milling blocks include the GC Initial LRF BLOCK (GC America Inc., gcamerica.com), which provides natural light transmission with a well-balanced translucency, fluorescence, and opalescent character, and the Paradigm C™ block by 3M ESPE (3m.com), which offers all the above-mentioned benefits and is indicated for inlays, onlays, crowns, and veneers.

Lithium disilicate (LS2) is available as a millable glass-ceramic produced by Ivoclar Vivadent under the product name IPS e.max® CAD. Known for its enhanced esthetics, IPS e.max CAD is milled in a pre-crystalized blue state that needs to be glazed and fired before delivery. This glass-ceramic displays moderate strength (360 MPa) with indications that range from veneers to inlays, onlays, single-unit anterior and premolar crowns, and 3-unit bridges up to the second premolar as the terminal abutment. Blocks are offered in different sizes to accommodate the span of the desired restoration and come in up to 20 different shades in four different translucency options. IPS e.max CAD can be CAD designed with a cutback or manually cut back so the glass ceramic can be layered for enhanced esthetics. Specialized blocks with pre-milled screw-access holes are also available for milling implant abutments.

Millable lithium silicate blocks have offered restorative solutions with similar results. Dentsply Sirona’s Celtra® Duo is a fully crystalized block that provides well-balanced translucency and opalescence for crowns, inlays, onlays, and veneers. These blocks can be milled and polished or milled and glazed. VITA Suprinity® and Glidewell’s Obsidian® millable blocks (Glidewell Laboratories, glidewelldental.com) are two lithium silicate ceramic blocks that can be milled in the laboratory or chairside using a variety of available mills. Suprinity claims to exhibit a fine-grain homogeneous structure that provides excellent material quality and consistency as well as a high load capacity and long-term reliability with an approximate strength of 420 MPa. Moreover, the material also offers outstanding processing characteristics, including easy milling and polishing⁸. Glidewell’s Obsidian lithium silicate exhibits an average flexural strength of 385 MPa and is indicated for full-contour crowns, 3-unit anterior bridges, veneers, inlays, and onlays. All lithium disilicate and lithium silicate prostheses can be cemented or adhesively bonded.

Resin-ceramic CAD/CAM millable materials, like VITA Enamic®, Lava™ Ultimate (3M ESPE), and Cerasmart™ (GC America Inc.) are the newest and most exciting millable innovations as they combine the desirable properties of ceramics and composites. The ceramic characteristics of these materials provide added strength and improved adhesive bonding, while the composite characteristics minimize wear on milling tools and allow for smoother margins, faster milling times, and easier finishing. Hybrid ceramics do not require crystallization firing after being milled, and instead are finished by simple hand polishing. The indications for these materials include veneers, inlays, onlays, crowns, and implant-supported crowns.

3M™ Paradigm™ MZ 100 by 3M ESPE is a traditional millable composite. As it requires little post-processing other than polishing, it is considered a fast and easy alternative to ceramic blocks. This radiopaque material is available in six common shades, has a flexural strength of about 150 MPa, and is indicated for veneers, inlays, onlays, and crowns. Some advantages to milled composite are that the material is kind to milling tools as well as opposing dentition and is easier to add to after milling if needed.

The milling of metal is particularly appealing in dentistry because it eliminates the risk of mis-casting the desired restoration. Two different types of metals are used for milling dental restorations: titanium and chrome cobalt. Both of these metals are biocompatible and corrosion-resistant, making them ideal for use in dentistry. Chrome cobalt (CrCo) lends itself to inexpensive milled crown-and-bridge frameworks onto which porcelain can be built. There are two ways by which a user can process chrome cobalt substructures, implant bars, and telescope retainers in the laboratory. One is by utilizing a robust mill that is capable of milling CrCo pucks in solid form (solid state); the second involves using a conventional benchtop mill and a softer dry CrCo mill material such as Ceramill Sintron by Amann Girrbach (amanngirrbach.com). The softer material mills like wax and is sintered in an argon gas environment to produce a solid-state CrCo metal. Titanium blanks with the appropriate machined implant interface are available to mill custom abutments. Titanium can also be anodized in the desired color to prevent metal show-through, even under soft tissue and zirconia restorations.

Polyether ether ketone (PEEK) has a long history of use in medicine in the fabrication of implant devices like artificial cranial plates and spine implants.⁶ PEEK is a high-performance polymer with many desirable properties. It has a high strength-to-weight ratio and a bone-like modulus of elasticity. Therefore, PEEK frameworks are tough (170 MPa) but lightweight and flexible to maximize patient intraoral comfort. It has a zero corrosion rate and is hypoallergenic in nature, making it ideal for patients with metal allergies. It also displays a high resistance to wear and abrasion.^6,7 Given its exceptional biocompatibility and superior mechanical properties, PEEK is now being adapted for long-term use in dentistry.

Pekkton (thermoplastic polyaryletherketone), distributed by anaxdent North America (anaxdentusa.com), offers a wide range of benefits, particularly for the making of implant-supported bars, abutments, and cement-retained prosthetics. The high-performance polymer has compression abilities and acts as a shock absorber; this ability to absorb shock with some compression in implant prosthetics exhibits physical properties more like natural teeth.

PEEK and Pekkton are used in the digital dental CAD/CAM workflow to mill removable partial denture frameworks, and fixed restorations including crowns, 3-unit bridges, custom implant abutments, implant-supported superstructures, and telescopic copings. Milling these plastics does not alter their mechanical properties, and they are much easier to mill than metals, so production is quicker and contributes to considerably longer tool life and less machine maintenance.

High-performance polymers used in conjunction with CAD/CAM technologies provide multiple solutions for restorative challenges. Implant solutions like PEEK, Pekkton, or Shofu’s Trinia (shofu.com) present both shock-absorbing ability and flexibility in a very rigid restorative environment.

Trinia, unlike PEEK and Pekkton, is a fiber-reinforced resin that is pink in color. This provides the ability to embed an implant-supported bar that produces an esthetic outcome, while having the biocompatibility, flexibility and strength for a fixed implant-supported hybrid prosthesis. The material is available in block and/or puck form.

High-performance polymers also offer great advantages in the removable partial denture space. Acetal (polyoxymethylene)—a product used heavily in manufacturing but adapted to dental use—is offered in millable puck form and used to provide a lightweight, metal-free, esthetic partial denture. Acetal is sold in the U.S. as Duracetal® from Myerson Dental (myersontooth.com) and Zirlux Acetal® from Zahn Dental, a Henry Schein Laboratory Division (henryschein.com). A new entrant into the removable denture arena is Solvay Dental 360 (solvaydental360.com) with their Dentivera™ milling discs made from Ultaire™ AKP. Ultaire AKP is a high-performance polymer that reduces the need for uncomfortable and unappealing metal removable partial dentures.⁹

The traditional practice of hand-waxing dental restorations is both skill-demanding and very time-consuming. With millable wax incorporated into the CAD/CAM workflow, the process is streamlined and automated to reduce manufacturing costs while greatly improving the quality, fit, and precision of the final product. Wax can be milled very thin without cracking or chipping at the margins, and it displays minimal pattern deformation so the end result is an improved replica of the desired final restoration. The precise wax pattern is then finished through a metal casting or pressed ceramic technique. This material is available through many different manufacturers in disc form for endless indications, from veneers, copings, and full-contour restorations, to bridge frameworks, implant-supported bridge frameworks, and more.

Polymethyl methacrylate (PMMA) is available from many manufacturers in large pucks that can be milled to produce long-term provisionals from single-unit crowns to full-arch bridges. These resins have flexural strengths comparable to feldspathic porcelain (about 130 MPa) and great color-stability for extended life. Multilayered PMMA blocks provide for a natural shade gradient, and while the milled product can be finished with simple polishing, the option to stain and glaze milled PMMA further enhances esthetics. PMMA also provides an inexpensive method for producing “prototype restorations,” which patients can try in to confirm marginal fit and esthetics prior to the laboratory milling the same design file in the final desired restorative material.

Recently a great deal of excitement has surrounded PMMA material as it has become the material of choice for digital denture solutions. These solutions provide the ability to mill polymethyl methacrylate (PMMA) in a pink-colored puck to produce the denture base plate and mill the denture teeth from a tooth-colored disc. The two are bonded together to produce a full digital denture solution. The puck material is the same basic material that has been used for years in conventional full denture fabrication, and polishing processes are identical. Furthermore, should there be a need for adjustments or relining the denture, the same clinical protocols remain. Amann Girrbach, VITA, Ivoclar Vivadent, and Merz Dental (en.merzdental.de) all offer in-house millable digital denture solutions utilizing this material.

Alan Jurim, DDS, and Barbara Jurim, DDS, have their own digital dental practice, integratedDENTAL, as well as a CAD/CAM digital dental laboratory, Jurim Dental Studio, both in Woodbury, New York.