Addition by Subtraction
Learn the pluses and minuses of today’s variety of millable materials
Darin Throndson, BS
In the modern dental laboratory, it is difficult to imagine not utilizing subtractive manufacturing to some degree. More commonly referred to as "milling," this process of creating dental products by removing material from a solid block to the desired final shape and size has become the norm in our profession. Whether it is done in-house or outsourced, almost every laboratory today uses this technology and milled materials to some degree. Machines that accomplish this task are multifaceted and can have many features, including the ability to dry-mill material with fluted instruments; wet-grind material with diamond-coated burs; rotate the tools and holders in three, four, or five axes; cut the designs out of pucks and blocks in different sizes, shapes, and thicknesses; and do a variety of these tasks all with the same one piece of equipment.
The armamentarium of millable materials available today has vastly expanded ever since the digital revolution in our industry. No longer are laboratories limited to a few indications and material options. Companies are constantly releasing new materials that expand the options for computer-aided manufacturing (CAM). Gone are the days of taking advantage of CAM for only crown-and-bridge substructures. Today laboratories can mill inlays, onlays, veneers, crowns, bridges, implant abutments, bars, models, dentures, denture teeth, removable partial frames, etc; the list continues to grow. Below is a listing and description of some of the most common and significant millable materials available today.
Feldspathic Porcelain
Feldspathic porcelain milling blocks were first introduced in 1991 and feature a homogenous, fine-grained particle size of 4 μm that lends itself to easy polishing.1 This porcelain is available in a vast array of shades and translucencies. The blocks are sold in both monochromatic and multi-chromatic form by both VITA North America (vitanorthamerica.com) and Dentsply Sirona (dentsplysirona.com). The multi-chromatic blocks come in a three-layer translucency, a four-layer translucency, and a unique block that has a gradient of color and translucency from the internal quarter-sphere portion to the external portion that simulates the transition from a dentinal core to enamel veneer. Some CAM softwares will even precisely position anterior restorations to take advantage of this innovative design. Feldspathic porcelain restorations must be etched with hydrofluoric acid and bonded to dentition, and they can be finished with either polishing or staining and glazing. This material can be used for inlays, onlays, veneers, and crowns.
Leucite-Reinforced Glass Ceramic
Ivoclar Vivadent (ivoclarvivadent.com) and GC America Inc. (gcamerica.com) both make leucite-reinforced glass-ceramic blocks. Ivoclar Vivadent's ProCAD was the precursor to what is known today as IPS Empress, which is made of 35% to 45% leucite crystals with a particle size of 1 μm to 5 μm.2 It comes in high-translucency, low-translucency, and multi-translucency versions. The multi-translucency variety has a gradient of eight different opacities built in from the cervical to incisal edge. GC Initial LRF also is available in high-translucency and low-translucency versions. Both companies' blocks can be either polished or stained and glazed. The restoration must be etched before being bonded to the tooth. Indications for leucite-reinforced glass-ceramics include inlays, onlays, veneers, and crowns.
Composites
Paradigm MZ100 composite (3M; 3m.com) for CAD/CAM was first introduced in 2000. It is a preshrunk, non-discoloring composite that is very wear friendly to burs and opposing dentition.3 The blocks are easily distinguishable by their round shape. Although monochromatic restorations cannot be characterized using a traditional porcelain oven, light-cured stains and glazes are available for added esthetics. In addition, the restorations can be repaired intraorally using traditional dental composite after air abrasion. MZ100 blocks can be used for inlays, onlays, veneers, and crowns. They are available in six shades.
Hybrid Composites
Recent years have seen a rapid expansion of the number of companies offering hybrid composite blocks for milling dental restorations. The popularity of these blocks is aided by the fact that they are easy to machine by extending bur life over traditional ceramics and mill very cleanly at margins, leaving behind little chipping. These materials combine both resin and floating ceramic particles to create inlays, onlays, and veneers. Some have indications for crowns; others do not. Current producers of these blocks and pucks include Shofu Dental Corporation (shofu.com) with its Shofu Block & Disk HC material, Coltene's Brilliant Crios (nam.coltene.com), GC America Inc.'s Cerasmart, and 3M's Lava Ultimate. Air abrasion is recommended before the final seating of these restorations.
One hybrid composite that is formulated differently is the Enamic material made by VITA. Enamic is unique in that instead of having floating ceramic particles in resin, it has a ceramic "skeleton" or interconnected structure of ceramic (much like the appearance of a sponge) infused with resin.4 Another distinguishing feature of Enamic is that the restoration should be hydrofluorically etched before seating. Like its composite cousins listed above, Enamic cannot be characterized in a porcelain furnace but instead can be customized with light-cured stains and glazes or simply polished. It is indicated for inlays, onlays, veneers, crowns, implant abutments, and screw-retained crowns.
Lithium Silicate Hybrids
Lithium silicate hybrids are materials that have a mixture of lithium silicate and either zirconia or alumina. Celtra Duo from Dentsply Sirona is a block that has suspended particles of zirconia floating in its matrix. It is milled in a tooth-shaded color and can be taken directly from the mill, polished, etched, and bonded in the mouth. However, if the material is put into an oven, it can reach approximately double its initial milled strength. This gives the laboratory the option of spending a little more time to characterize and cook or polish and send off depending on the occlusal forces of the case. Celtra Duo is indicated for inlays, onlays, veneers, and crowns and comes in high-translucency and low-translucency blocks.
Obsidian from Glidewell Dental (glidewelldental.com) is lithium silicate that also has zirconia in its makeup. Unlike Celtra Duo, this material must be fired in a furnace prior to seating in a patient's mouth.5 It can be cemented or bonded after firing and is used for inlays, onlays, veneers, and crowns, as well as anterior three-unit bridges.
Another lithium silicate hybrid available for milling is the n!ce block offered by Straumann (straumann.com). Instead of having zirconia in its makeup, the n!ce block is a lithium silicate matrix with aluminum added. Unlike the aforementioned materials, n!ce does not get any added strength from the firing process in an oven; it is at full strength directly from the mill. The restoration can then simply be polished or stained and glazed on either a fast-cool or slow-cool oven cycle. n!ce blocks have indications for inlays, onlays, veneers, crowns, implant abutments, and screw-retained crowns in both high and low translucencies.
Lithium Disilicate
Until recently, the only lithium disilicate milling block on the market was IPS e.max from Ivoclar Vivadent. Known by its characteristic blue hue, it comes in three translucencies—high, medium, and low—and is available only in block form for milling purposes. It is milled in a semi-crystallized state and must be put into a ceramic furnace to reach its final strength and shade. It is available in a variety of sizes and can be either cemented or bonded once completely crystallized. The indications for e.max include inlays, onlays, veneers, crowns, implant abutments, and screw-retained crowns, as well as three-unit anterior bridges.
This month, Hass BIO America Inc. (hassbioamerica.com) launched the Amber Mill lithium disilicate milling block, and the company plans to launch the Amber Mill milling puck in the near future. The transparent blocks can achieve four different levels of translucency (HT, MT, LT, and MO) by adjusting the heat-treatment temperature levels when fired.
PMMA
Commonly known as PMMA, polymethyl methacrylate is a versatile, medical-grade plastic that has many dental applications and suppliers. It can be used in situations as varied as long-term temporaries of final restorations, nightguards, surgical guides, and milled denture teeth. It comes in a variety of colors including monochromatic tooth shades, multi-chromatic tooth shades, and clear, and is available in both puck and block forms.
Wax
Dental waxing no longer has to be the arduous process that it was just a few years ago. Millable wax gives the laboratory an option of milling out a wax pattern in the hopes of saving time and labor. A technician can then either carve into it more or add to it using conventional methods as desired. Millable wax comes in a wide variety of colors and hardnesses. It can be pressed, cast, or used for diagnostic purposes. When used properly, it helps bridge the gap between digital and analog processes.
Denture Materials
There has been a surge of interest in the digitization of dentures. What was once the domain of crown and bridge has spread over into removables. Through the use of CAD/CAM software, it is now possible to design either a digital denture base and mill it out of pucks of denture acrylic or to mill specialized pucks with different-shaped teeth setups already embedded in them. Both options are gaining steam, and the race for the complete digitization and milling of dentures is on. Whether accomplished using a milled base with bonded teeth or a monolithic structure from a proprietary puck, denture milling is advancing rapidly.
High-Performance Polymers
There is no shortage of players in the high-performance dental polymer market, and the list is continuing to grow. These strong yet flexible plastics have been used in medicine for orthopedics and neurosurgery and are now being adapted for dental use. These materials can be used for many indications including partial frameworks, crown-and-bridge substructures, and implant superstructures. PEEK (polyether ether ketone), Pekkton (anaxdent North America; anaxdentusa.com), Ultaire AKP (Solvay Dental 360; solvaydental360.com), acetal resin, TRINIA (TRINIA; trinia.com), TRILOR (Bioloren; bioloren.com), and Crystal Tritan (Digital Dental; crystalultra.com) are just some of the impactful products in this product category. They demonstrate a high strength-to-weight ratio, are more easily milled, and are kinder on milling equipment than the metals they replace.
Metals
Gone are the days of casting being the only way to fabricate metal dental frames, substructures, and full-contour metal crowns. Today, we can mill titanium bars and abutments using pucks and blanks, cobalt chrome substructures, non-precious and precious substructures, and full-contour gold restorations in varying degrees of purity. Milling metal requires more industrial-grade machinery than many laboratories currently possess. However, the possibility of outsourcing this task is highly appealing when one considers the labor and inventory involved in not using CAD/CAM for these processes.
Zirconia
One would be hard pressed to find a material that has advanced as far and fast in the dental laboratory world as zirconia. First introduced as a substrate-only material, zirconia has advanced to the point today where it is also used for cosmetic veneers. It is typically milled in a chalk-like state and then sintered in a furnace where it shrinks and compacts by 18 to 26%. Each lot of zirconia will have slight variances in its shrinkage rate, and this information will have to be input to the CAM software so the milling machine can compensate. There are even zirconias being made now that have varying levels of color, translucency, and strength from cervical to incisal edge, known as multilayer and multi-strength. It is up to the purchaser to do their homework and know the properties of the material they are using.
Unfortunately, there is no uniform wording to date that can tell you how strong or translucent your zirconia is. Terms such as "high translucency," "ultra translucency," "high strength," and "super strength" are used by many manufacturers, but there is no agreed-upon distinction between them.
Shading of zirconia can also be tricky. Some laboratories will buy pre-shaded zirconia, some choose to dip their zirconia in coloring liquids, and yet others use acid or water-based stains to pre-color their zirconia prior to sintering. Finding a consistent shading solution has proven challenging to the industry. Even so, some manufacturers now advertise their shade accuracy.
Some of the advantages of zirconia include its ease of milling, relatively low cost, and high strength in comparison to other dental ceramics. The material can be used for many applications from single-unit anteriors to full roundhouse hybrid bridges.
Conclusion
The list of millable materials is long and growing every year as new advancements in materials engineering and machining lead us further to the digitization of the dental laboratory industry. The pace of advancement has not yet slowed and should continue to increase in the years ahead. It is up to the laboratories and technicians to keep themselves informed of these rapidly changing options so they can offer the most effective solutions to their clients and their patients.
About the Author
Darin Throndson, BS, is President of Innovative Dental Technologies in Memphis, Tennessee.
References
1. Vitablocs Mark II for CEREC. Materials Science and Clinical Studies. Brea, CA: Vident; September 2003.
2. IPS Empress CAD. Scientific Documentation. Lichtenstien: Ivoclar Vivadent; October 2011.
3. 3M Paradigm MZ100 Block. Technical Product Profile. St. Paul, MN: 3M ESPE; 2000.
4. VITA Enamic. Technical and Scientific Documentation. Germany: VITA; August 2016.
5. Glidewell. Obsidian Milling Blocks User Manual. Irvine, CA: Glidewell.
