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An overview of the various types of machinable blocks for laboratory-based CAD/CAM systems.
By Russell A. Giordano II, DMD, CAGS, DMSc
A wide variety of block materials are available for CAM-milling full-contour and framework aplications, ranging from glass-based ceramics to high-strength zirconia and metals.
This article provides a general overview of the materials available for laboratory-based CAD/CAM systems and addresses some of the issues the industry is facing with respect to material types and clinical uses.
Full-Contour Block Materials
Most full-contour block materials are based on a glass matrix with a dispersed crystal component and are used for milled veneers, inlays, onlays, and crowns. These restorations have traditionally been milled chairside, using materials primarily indicated for the CEREC® CAD/CAM system (Sirona Dental Systems, www.sirona.com). However, these types of materials are now available for a variety of laboratory CAM systems and may become increasingly used instead of pressable ceramics as digital impressions become more widely used by dentists. Also, as the perception of CAD/CAM quality has improved, there have been more requests for milled restorations fabricated using physical impressions.
The most widely used material over the past 25 years has been the VITABLOCS Mark II® (Vident, www.vident.com), a ceramic specifically designed for CAD/CAM. CAD/CAM materials must be able to resist damage from machining, be easily milled, and easily finished after machining. Fabricating materials with fine particle sizes is one important processing mechanism used to achieve these objectives. At the time VITA Mark II was developed, porcelains were very coarse—with crystal sizes up to 200 to 300 μm. This often created excessive wear on the opposing dentition and made it difficult to polish these materials. VITA Mark II was developed with approximately 5- to 10-μm size crystals that resisted machining damage, improved finishing, and vastly increased wear kindness of the ceramic. VITA Mark II is a combination of feldspar materials with a fine crystalline component. It has a long history of clinical success with about a 95% success rate over 5 to 7 years for inlays, onlays, and crowns.1
Another fine crystal material, IPS Empress® CAD (Ivoclar Vivadent, www.ivoclarvivadent.com), is based on the IPS Empress Esthetic press-ceramic. Both have the same microstructure, a 45 wt% leucite in a feldspathic glass matrix. The leucite crystals are approximately 10 μm in size, which improves machineability, wear kindness, and polishability.
Both the VITA Mark II and IPS Empress CAD come in a variety of shades as well as a multicolor block, in which there are layers of increasing chroma and decreasing translucency. VITA Mark II, VITABLOCS TriLuxe (Vident), and IPS Empress CAD may be stained, glazed, or veneered with the proper porcelain. However, most of these restorations are simply polished with polishing wheels, diamond paste, and then bonded into place.
The most recent advance in the area of multicolor blocks is the VITABLOCS RealLife® block (Vident). This is a truly layered block similar to a hand-layered porcelain crown. It consists of an internal dentin shade that is overlaid with an enamel shade. The crown may be positioned within the block, with the Sirona CEREC system to vary the degree of enamel-to-dentin, depending upon the tooth being matched. This produces a milled, layered crown (Figure 1, Figure 2 and Figure 3).
The Paradigm™ C (3M ESPE, www.3MESPE.com) is available in a limited number of shades but has a structure similar to IPS Empress CAD. The above materials all have relatively low strength values, ranging from 120 to 160 MPa, depending on surface finish and exact block type. Even though they are at the low end of the strength chart, when bonded to tooth structure, they have excellent clinical survivability. The material itself has a modulus or stiffness value close to that of enamel. Bonding reinforces the tooth and the material at the same time. This illustrates one very significant issue with respect to clinical success; it is not just strength values that are important. Clinically, dental professionals are concerned about the load-bearing capacity of the tooth-restoration complex as a whole and its stability and resistance to long-term stress, not just the starting strength.
Based on the IPS Empress II structure, the IPS e.max CAD (Ivoclar Vivadent)—also known as a "blue block"—is a glass-ceramic that begins life as a bluish-violet glass and then after milling is heat-treated to grow crystals in the glass matrix. The glass matrix consists of a lithium silicate with 60% to 70% lithium-disilicate micron-sized crystals and submicron lithium orthophosphate crystals. The strength after heat-treating increases to a range of approximately 360 to 400 MPa. Unlike the previous blocks, this material must be heat-treated to grow the crystals and needs to be supported with a refractory material to prevent warping of the milled restoration. It is critical that the proper crystallization procedure is used; otherwise, the material may be under- or over-crystallized, which can weaken it and lead to unexpected failure. In one clinical trial published using IPS e.max CAD, there was a 100% success rate after 1 year using bonded restorations.2 Even though the strength is higher than VITA Mark II or IPS Empress CAD, some literature shows that if these materials are bonded to tooth structure, they have the same load-bearing capacity as IPS e.max at the point that fracture starts.3
Designed as a permanent restoration, the Paradigm MZ100 block (3M ESPE) is a composite resin material. Bioloren (www.bioloren.com) also offers a few fiber-reinforced resin materials in block/disc form. There have been limited studies on these materials with respect to longevity.
Provisional materials, such as CAD-Temp (Vident) and Telio CAD (Ivoclar Vivadent), are available in a variety of shades and are touted as long-term provisionals. Their esthetic quality is excellent and margins are superior to chairside-fabricated provisional restorations; however, there is no data on exact long-term survivability in the oral environment.
What advantages, if any, do CAM-milling blocks possess versus pressed or hand-layered restorations? The milling blocks often have the same chemical composition and microstructure, so why use a restoration milled from a block? A major advantage of milling block materials is that they are processed using machines that apply consistent pressure to compact the ceramic powder and have a regular firing schedule. As a result, the blocks are much more homogeneous in structure. This fact, in the author's opinion, makes them superior to pressed ceramics and hand-layered porcelain, which always contain bubbles and result in restorations that vary widely due to handling conditions.
Framework Block Materials
The first all-ceramic framework material, the VITA In-Ceram® (Vident), belongs to a class of materials known as interpenetrating phase composites. They consist of at least two phases that are intertwined and extend continuously from the internal to the external surface. Porous blocks of In-Ceram materials are milled to produce a framework. The blocks are then infused with a glass in different shades to produce a 100% dense material, which is then veneered with porcelain. In-Ceram is available in three types, each designed for a specific region of the mouth. With moderately high strength (350 MPa) for anterior crowns, In-Ceram SPINELL is the most translucent, followed by In-Ceram ALUMINA, which has high strength (450 to 600 MPa) and moderate translucency for anterior bridges and anterior/posterior crowns. In-Ceram ZIRCONIA is a very high-strength (700 MPa) framework material with lower translucency for three-unit anterior/posterior bridges and anterior/posterior crowns.
The most widely used all-ceramic framework material is "pure zirconia." Millable zirconia is not exactly pure, but is predominately zirconia with stabilizing components (ie, yttria) and minor components (ie, alumina), which are important for long-term stability. Zirconia (ZrO2) may exist primarily in the tetragonal phase at room temperature by adding components such as calcia (CaO), magnesia (MgO), yttria (Y2O3), and ceria (CeO2). The addition of 3 wt% to 5 wt% yttria 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 dimensional change diverts energy from crack formation and can stop the crack from progressing further. This is called transformation toughening. This material characteristic helps zirconia frameworks resist catastrophic failure. Even though a crack may exist in the material, the phase change prevents it from proceeding throughout the restoration.
Most restorations with zirconia frameworks are fabricated by machining a porous or partially fired block of zirconia. The framework is milled oversize and then fired at approximately 1500°C to fully densify the zirconia, producing a translucent material and strength values from 900 to 1200 MPa. Most blocks have barcodes that communicate the density of the milling block to the computer in order to properly mill the framework oversize.
Zirconia-based restorations represented approximately 50% of the total number of all-ceramic restorations produced in 2010. As the popularity has increased, the number of machinable block suppliers has too. There are many "brand name" manufacturers with reliable products, but there are also many questionable blocks being sold on the market. Differences among zirconia ceramics can occur with the level of zirconia purity, grain size, trace elements, and stabilizing compounds. Variations exist among zirconia powders and the processing of those powders into blocks. The high cost of blocks is not necessarily because of the material, but because of the processing required to produce a reliable, homogeneous block. Just because it pops out of the mold does not mean it should be sold. Poor-quality zirconia blocks may warp and have significant porosity (Figure 4, Figure 5, Figure 6 and Figure 7). Consequently, clinical outcomes, such as fit, translucency, strength, and long-term stability, can be significantly affected. Zirconia may degrade over time and minor components, such as alumina, help prevent this degradation.4
One concern that is spurring research and controversy is the issue of the chipping of veneering porcelain. In tests of zirconia itself, the zirconia does not seem to fail; however, there have been numerous reports of veneer chipping. In several published studies, the veneer-chipping rate was about 15% after 3 to 5 years.5-7 A review of various clinical trials on zirconia restorations showed a wide range of chipping at 1 to 5 years from 5% to 25% for low-fusing porcelains, such as Cercon® ceram (DENTSPLY Prosthetics, prosthetics. dentsply.com), Lava™ Ceram (3M ESPE), and IPS e.max Ceram (Ivoclar Vivadent). Chipping rose to 54% after 1 year for Triceram® (B&D Technologies, www.origincadcam.com) porcelain on dense zirconia.8,9 However, others report minor amounts of chipping problems.10,11 It is important to note that there appears to be a correlation with the firing cycle and peak firing temperature of the veneer. Laboratory studies have shown that low-fusing porcelains may be less resistant to cracking than high-fusing porcelains, and the porcelain density is lower. Zirconia is a good thermal insulator, which prevents heat transfer to the veneer porcelain, hindering it from becoming fully dense if fast-fired or fired at a low temperature. Also, fast cooling may create stress in the porcelain, which could lead to cracking. Any surface adjustment—grinding, sandblasting, and even polishing—can change the phase on the surface of the zirconia and may affect the stability and strength of the zirconia, as well as the veneer porcelain.12-14
Another area of interest is the use of zirconia as a full-contour restoration. Although zirconia has a fine microstructure, there are varied reports as to its properties of abrasion, and there are no published clinical studies on the longevity of full-contour zirconia and the wear of opposing dentition. Some research shows that zirconia is abrasive to the opposing dentition. Studies using natural teeth have shown that zirconia causes excessive wear of the tooth structure. If glazed, the glaze wears off, exposing a rough surface that then accelerates tooth wear. If the zirconia is not polished to a mirror finish then it may cause excessive wear.15 Other studies in progress demonstrate that polished zirconia is "wear-kind."16 Recent studies using an enamel substitute, steatite (soapstone), tend to show that highly polished zirconia is wear-kind.17 If one wants to provide zirconia crowns, the surface must be highly polished and remain intact. If the occlusion needs to be adjusted and the dentist cannot achieve a high polish, it is the author's opinion that the restoration should be sent back to the lab for finishing. Furthermore, many of the "high-translucency zirconias" have low amounts of alumina. Alumina is an important stabilizer, and lower amounts may cause excessive transformation, cracking, and tooth abrasion.9 This also may be accelerated by adjustment of the zirconia. Ongoing clinical trials are expected to provide more definitive answers.
Combo Milled Framework and Veneer Restorations
The Rapid Layer Technique from VITA is an interesting solution to the veneer-chipping problem, as well as a method for quickly fabricating the veneer for frameworks. It is presently limited to the Sirona inLab system, in which two files are created when fabricating a full-contour bridge. One file is used to mill the zirconia framework, and a second file mills a corresponding full-contour veneer from a fully dense TriLuxe block. This milled veneer is then bonded onto the zirconia framework using composite resin cement (Figure 8). This milling concept holds the possibility of completely eliminating physical casts for three-unit bridges. Combining the CEREC digital impression of the preparation, opposing teeth, and bite registration may produce a virtual model, in which the proper occlusal contacts are generated in the milled veneer. Thus, no physical articulation is needed to develop the correct occlusal contacts.
Ivoclar Vivadent also recently introduced a similar product in their IPS e.max line. The CAD-on technique combines a substructure milled from IPS ZirCAD zirconia and a veneer milled from IPS e.max lithium disilicate. The two components are joined using CAD Crystall./Connect fusion glass-ceramic.
A third milled veneer system, the DVS (Digital Veneer System), is available from 3M ESPE for single-unit restorations. In this case, the zirconia coping is milled, and a data file to produce the milled veneer is also made. However, the veneer is milled from a porous porcelain block. The veneer is attached to the zirconia with a special dentin-fusing powder, and the combined components are fired to densify the porcelain veneer and fuse it to the zirconia.
Other Millable Ceramics
Zirconia blocks have been developed by Sirona to produce custom abutments for a variety of implant systems. The TiBase, an intermediate element between the implant and zirconia, is used. This element screws into the implant, and the milled zirconia abutment is bonded to the abutment part of the TiBase.
Alumina blocks/discs are available from VITA, Sirona, and other manufacturers to produce frameworks as well. These structures have strength values of about 600 MPa and use the same "shrink to fit" concept as the porous zirconia approach. These materials may be supplied in various shades just like zirconia blocks but are limited in use as anterior bridge frameworks and single units. There are also several manufacturers of titanium and chromium-cobalt discs for the production of frameworks for porcelain-fused-to-metal restorations. Lastly, millable plastics to produce patterns for metal castings and pressable ceramic restorations are widely available.
Regardless of what type of material the laboratory professional chooses, it is important to evaluate the supplier of that material. Currently, it seems as if every manufacturer is fabricating and selling zirconia milling blocks. This trend will probably continue and eventually include full-contour ceramic blocks as well.
Unfortunately, it is impossible to look at a zirconia milling block and determine the quality. Therefore, it becomes the responsibility of the laboratory and the dentist to ensure the quality of the materials being used in patients' mouths. One warning sign is if the company website or in-print advertisement refers to the blocks as the best "zirconium" on the market. Understand that zirconium is a metal, not a ceramic. An outrageous claim of strength (1400 to 1600 MPa), with no real data to support it, is another red flag.
As an automotive repair company used to say about using "substandard" parts, "You can pay me now, or pay me a lot more later." In the dental technology industry, if laboratory professionals are not careful about the materials they choose, they could up paying a lot more later—in quality, reputation, and perhaps, even legal fees.
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About the Author
Russell A. Giordano II, DMD, CAGS, DMSc, is the director of biomaterials at Boston University's Henry M. Goldman School of Dental Medicine in Boston, Massachusetts.