CAD/CAM Materials: An Overview

Irina Singh^*1, Kavipal Singh² and Nimish Sethi³

¹Empanelled Consultant – Dental Sciences, Fortis Escorts Hospital, Amritsar, Punjab

²Principal, Professor and Head, Department of Prosthodontics and Crown & Bridge, Sri Guru Ram Das Institute of Dental Sciences and Research, Amritsar, Punjab

³Professor, Department of Prosthodontics and Crown & Bridge, Sri Guru Ram Das Institute of Dental Sciences and Research, Amritsar, Punjab

INTRODUCTION

The use of multiple dental materials is a major goal in dental CAD/CAM technology. Commitment to a specific system includes the decision for a certain framework material and the compatible veneering materials available. Regardless of the manufacturing system (CAM) and the materials used, the CAD program is able to design and simulate the final outcome (Li, R. W. K. et al., 2014). To make the user of one specific system use only the materials offered by the company, the machine and materials are linked by a code system. Such CAD/CAM systems can process materials tested and offered by the company. Scanner, CAD, CAM, and materials are only usable as a “package.” Some companies work with an account system by the unit or by a basic fee, providing a license for the user to run the machine with specific materials (Liu, P. R., & Essig, M. E. 2008).

DISCUSSION

For some systems, new materials were announced several years ago but have not yet been launched. Other materials, like zirconium dioxide (ZrO₂) ceramics, are now being introduced because new CAM technologies have made an application possible (Denry, I., & Holloway, J. A. 2010). The CAD/CAM systems of Precident DCS and Digi-Dent (Girrbach; manufactured by Hint-ELs,Griesheim, Germany) deserve special attention regarding the wide range of materials (titanium, alloys, acrylics, and ceramics) that can be applied. Industrial dense polycrystalline ceramics such as alumina, zirconia and alumina-zirconia composites are currently available with the application of CAD⁄ CAM technology using a networked machining centre (Witkowski, S. 2005).

The material groups available for the various CAD/CAM systems are the following:

1. Silicate ceramics

2. Aluminium oxide ceramics with glass infiltration

3. Aluminum oxide ceramics with dense sintering

4. Zirconium dioxide ceramics (ZrO2 Y-TZP Zirconia [yttria-tetragonal zirconia polycrystal]) with dense sintering, manufactured as green stage, presintered stage, and completely sintered stage

5. Titanium

6. Precious alloys

7. Non precious alloys

8. Acrylics of improved strength

9. Castable acrylics (Witkowski, S. 2005; & Beuer, F. et al., 2008).

Zirconium dioxide ceramics have been introduced into dentistry as a framework material for various indications. Initial clinical tests for crowns, implant abutments, and root canal posts are underway. The ZrO₂ frameworks for crowns and FPDs are made by milling in the green stage the presintered stage and the completely sintered stage. The frameworks made of the green stage and presintered ZrO₂ are post-sintered in special furnaces after milling (about 1500°C) (Silva, N. R. et al., 2010). ZrO₂ that belongs to the green-stage group can be individualized by coloring of the framework according to the Vita shade concept (Apholt, W. et al., 2001). There are two types of zirconia blocks currently available for distinct CAD⁄CAM applications. The first application is the use of fully sintered dense blocks for direct machining using a dental CAD⁄CAM system with a grinding machine. The second application is the use of partially sintered blocks and green blocks for CAD⁄CAM fabrication followed by post-sintering to obtain a final product with sufficient strength (Lambert, H. et al., 2017).

The former application has a superior fit because no shrinkage is involved in the process, but is disadvantaged by inferior machinability associated with heavier wear on the milling tool. In addition, micro crack formation on the material during the milling procedure might deteriorate the mechanical durability of the restoration. The latter application has the advantage of easy machinability without as much wear on the tools or chipping of the material (Miyazaki, T., & Hotta, Y. 2011). However, because of extensive shrinkage during the post-sintering process, the fit of the frameworks must be compensated for by the dimensional adjustment of CAD procedures involving the frameworks. a new hybrid structure of CAD⁄CAM porcelain crowns adhered to the CAD⁄CAM zirconia framework (PAZ) has been proposed. In this system, zirconia frameworks are digitized and porcelain crowns are also fabricated by the CAD⁄CAM process (Kunii, J. et al., 2007). Milled porcelain crowns are adhered to zirconia frameworks using adhesive resin cements and the final restoration is completed. Manipulation of the structure is reproducible and reliable without conventional manual porcelain work. Adhesive treatments reinforce the durability of porcelain (Daou, E. E. 2014).

Product examples of ZrO₂ materials and the groups according to the milling/grinding technology are:

1. Milling at green stage: Cercon base, Cercon; Lava Frame, Lava; Hint-ELs Zirkon TZP-G, DigiDent; ZirkonZahn, Steger; Xavex G 100 Zirkon, etkon

2. Grinding at presintered stage: In-Ceram YZ Cubes,Cerec InLab; ZS-Blanks, Everest; Hint-ELs Zirkon TZP-W, DigiDent; DC-Shrink, Precident DCS

3. Grinding at completely sintered stage: DC-Zirkon, Precident DCS; Z-Blanks, Everest; Zirkon TM, Pro 50, Cynovad; Hint-ELs Zirkon TZP-HIP, DigiDent; HIP Zirkon, etkon (Li, R. W. K. et al., 2014).

Yttrium partially stabilized tetragonal zirconia polycrystalline (YTZP) has a very high fracture toughness from 5 to 10 MPa. When a crack initiates on the YTZP, the concentration of stress at the top of the crack causes the tetragonal crystal to transform into a monoclinic crystal with volumetric expansion. This prevents further crack propagation. Zirconia nano-composites developed in Japan are very tough with a fracture toughness of 19 MPa and a bending strength of 1400 MPa. Zirconia is available for fabricating frameworks of bridge restorations instead of metal bonded restorations because of its higher fracture toughness (Witkowski, S. 2005; & Beuer, F. et al., 2008).

A number of categories of materials are available for chair side CAD/CAM restorations that have demonstrated predictability and longevity. Each category of materials has unique features designed for specific clinical applications. Restorations fabricated with chair side CAD/CAM systems are monolithic-the entire restoration is a single homogenous material rather than a bilayered restoration consisting of a coping and a second veneer layer. Monolithic materials for chair side CAD/CAM systems have several unique features. The industrial material fabrication process provides a homogenous, dense material without porosity or voids, which maximizes the material's physical properties. The material is manufactured in a solid block form that is mounted on a milling mandrel unique to the specific CAD/CAM milling system (Beuer, F. et al., 2008; & Marchesi, G. et al., 2021).

Commercial systems employ a subtractive wet-grinding process for shaping or milling the restoration from the preformed blocks based on the 3-dimensional (3-D) volumetric design created with the systems' software programs. The restoration should be able to be milled in an efficient time period suitable for delivery at the same appointment, generally less than 20 minutes-without damaging the material, as compromised material could lead to early failure of the restoration. Additionally, post-milling processing time is a key consideration. The need for significant handling time or for procedures that may be required to create the final strength and surface characteristics of the restoration can detract from a material's ability to be used in a one-appointment restorative procedure (Beuer, F. et al., 2008; & Fasbinder, D. J. 2012).

Esthetic Ceramics

Esthetic ceramics are glass-containing materials with very good translucency and moderate ñexural strength. The presence of the glass component permits the materials to be etched with hydrofluoric acid, treated with a silane coupler, and adhesively bonded to the tooth. The adhesive bonding not only provides retention for the restoration, but it also contributes to the restoration's clinical strength to resist fracture (Kim, Y. R. et al., 2021; & Lee, H. Y. et al., 2017). Vitablocs* Mark II (Vident) and CEREC Blocs (Sirona Dental Systems) are feldspathic glass ceramics. Both materials are fine-grained, homogeneous feldspathic porcelain with an average particle size of 4 μm. The small particle size allows for a high-gloss finish and minimizes abrasive wear of the opposing dentition. Introduced in 1991, Vitablocs Mark II is available in the 10 most common Vita 3D-Master" shades. CEREC Blocs became available in 2007 and are also manufactured by Vita Zahnfabrik. Blocks are available in six shades and three different degrees of colour saturation (chroma): translucent (T), medium (M), and opaque (O). Both of these feldspathic ceramic materials are unique to the CEREC system and are not available on milling mandrels for the E4D system. Vitablocs Mark II is also available in several multicoloured blocks. Triluxe blocks (Vident) contain three different bands of color to recreate the shade and translucency of the tooth from cervical to incisal with increased fluorescence and chroma in the cervical area (Shenoy, A., & Shenoy, N. 2010; & Fasbinder, D.J. 2012).

The most recently introduced RealLife block (Vident) has an innovative 3-D radial gradient of colour and translucency from the internal portion of the block to the external part of the block to simulate the natural transition from dentin core to enamel veneer. CEREC Blocs also come in multicoloured block form. The CEREC Bloc PC features a three-layered structure; the bottom (cervical) layer has the highest pigmentation and lowest translucency, and the top (incisal) layer has the highest translucency and lowest colour intensity. It is available in three different gradient shades. Multicolour blocks offer an enhanced esthetic result compared to conventional monochromatic blocks. The application that offers the maximum benefit for a polychromatic block is a crown, because an onlay tends to have a limited vertical dimension for visualizing the gradient of colour. Further customization of either the monochromatic or polychromatic blocks can be accomplished by shade characterization and glazing using the Vita Shading Paste Assortment Kit. The kit is compatible with both the Vitablocs Mark II and

CEREC Blocs (Fasbinder, D.J. 2012; & Fasbinder, D.J. 2010)

The first leucite-reinforced glass-ceramic CAD/CAM block, ProCAD (Ivoclar Vivadent) was introduced in 1998. It evolved to become the current IPS Empress CAD (Ivoclar Vivadent) and is a 35% to 45% leucite-reinforced glass-ceramic similar to IPS Empress 1 but with a finer particle size of 1 μm to 5 μm. The blocks are available in nine common shades in either a high-translucency (HT) or low-translucency (LT) version. The HT version has an increased translucency, while the LT version has a brighter value and is also available in four bleach shades. IPS Empress CAD is also available in a multicolour block form. Offering five popular colours, the block has a gradient of colour and translucency ranging from cervical to incisal to simulate the transition of colour and translucency in the natural dentition. Individual shade customization of either the monochromatic or multicolour blocks can be accomplished using IPS Empress Universal Stains. Paradigm C (3M ESPE) was second leucite reinforced glass-ceramic mill block, but it is no longer available for either the CEREC or E4D system (Fasbinder, D.J. 2012; & Fasbinder, D.J. 2012a).

High-Strength Ceramics (Charlton, D. G. et al., 2008;& Vivadent, I. 2009)

IPS e.max CAD (Ivoclar Vivadent) was introduced in 2006 as a lithium-disilicate CAD/CAM material with flexural strength (360 MPa) two to three times that of esthetic ceramic materials. The increased strength affords the opportunity to either etch and adhesively bond the material to the tooth or use a conventional cementation technique. Lithium disilicate was initially developed as a substructure material that offered greater translucency compared to other high-strength ceramic core materials. However, it has gained popularity for use as a monolithic restoration in chair side CAD/CAM systems due to its enhanced strength.

The CAD/CAM block form is available in nine A-D shades, two translucencies, and four bleach shades. IPS e.max CAD blocks consist of 0.2-nm to 1-nm lithium metasilicate crystals with approximately 40% crystals by volume. The block is a blue-violet colour, which accounts for the commonly used "blue block" description. This partially crystallized soft state allows the block to be easily milled without excessive diamond bur wear or damage to the material. After the restoration is milled, it must undergo a two-stage firing process in a porcelain oven under vacuum to complete the crystallization of the lithium disilicate. The choice of glazing media affects the firing time; the spray-on glaze requires a 21 minute firing cycle, while the paint- on glaze demands a 28-minute firing cycle due to the presence of more organic compounds in the paste. The crystallization firing also converts the blue shade of the precrystallized block to the selected tooth shade and results in a glass ceramic with a fine grain size of approximately 1.5μm and 70% crystal volume incorporated in a glassmatrix.

Nanoceramics (Fasbinder, D.J. 2012 a; & Rusin, R. P. 2001)

A recently introduced unique CAD/CAM block is based on the integration of nanotechnology and ceramics. This nanoceramic material is purported to offer the ease of handling of a composite material with the surface gloss and finish retention similar to a porcelain. Lava™ Ultimate (3M ESPE) contains three ceramic filler particles. Silica particles of 20 nm, zirconia particles of 4 nm to 11 nm, and agglomerated nanoparticles of silica and zirconia are all embedded in a highly cross-linked polymer matrix. The aggregated clusters are comprised of 20-nm silica and 4-nm to 11-nm zirconia particles, with approximately an 80% ceramic load. The manufacturer has reported a flexural strength of 200 MPa, which is greater than the flexural strength of feldspathic and leucite-reinforced porcelain blocks (140 MPa to 160 MPa), as well as that of veneering porcelains for porcelain-fused-to-metal (PFM) crowns (generally less than 100 MPa).

Manufacturer testing indicates that the fracture toughness of the nanoceramic material is statistically greater than feldspathic porcelain and direct composite materials while being less brittle than fcldspathic glass ceramics, and, therefore, it is less prone to cracking during try-in and function. The manufacturer recommends an axial reduction of 1 mm and cuspal reduction of 1.5 mm, with both dimensions being about 0.5-mm less than what is conventionally recommended for porcelain restorations. The inclusion of nanoparticles in the Lava Ultimate block offers the potential for easy contour adjustment and creation of a high gloss surface finish. A purported improvement in the nanoceramic material is the ability to retain a high-gloss surface finish over time, which tends to be a limitation of CAD/CAM composite blocks. In vitro studies by the manufacturer indicate that Lava Ultimate has resistance to toothbrush abrasion along with retention of the initial glossy surface finish similar to glass-ceramics. Long-term clinical evaluation is needed to confirm this desirable property of the material. Lava Ultimate restorative will be available in eight shades in both low- and high-translucency forms for both CEREC AC and E4D chairside CAD/CAM systems.

Composite Resin for Permanent Restorations (Paradigm™ MZ100 Block. 2000; Leibrock, A. et al., 1999;& Appeldoorn, R. E. et al., 1993)

Paradigm™ MZ100 (3M ESPE), introduced in 2000, is a polymer composite block based on the Z100 composite chemistry and relies on a proprietary processing technique to maximize the degree of cross-linking. Paradigm MZ100 has zirconia-silica filler particles and is 85% filled by weight with an average particle size of 0.6 μm. It is radiopaque and available in six shades as well as a more translucent enamel shade. No working die is created with the in-office CAD/CAM process to use in refining the margins and proximal contacts prior to delivery of the restoration. Often times, there is a need to adjust and repolish these areas and refine the occlusal contacts since the lateral guidance cannot be replicated on the software design programs. The polymer chemistry of Paradigm MZ100 makes it easier to adjust and polish intraorally. Repair of porcelain restorations intraorally has not proven to be more than a moderately effective temporary technique. With Paradigm MZ100, the surface of the restoration is air-abraded with 50μm silicon dioxide, and a hyhrid composite can be bonded to the abraded surface. Although untested for clinical longevity, this affords an easy, efficient intraoral repair procedure for Paradigm MZ100 restorations.

Composite Resin for Temporary Materials (Fasbinder, D.J. 2012; 2010; 2012a)

Not all clinical situations are amenable to treatment with chairside CAD/CAM restorations. In an effort to complement the laboratory fabrication process, CAD/CAM temporary blocks have been introduced for chairside fabrication of long-term temporary crowns and fixed partial dentures (FPDs). The CAD/CAM process avoids the air-inhibited layer found with conventional self-cure or visible light-cure (VLC) acrylics as well as polymerization shrinkage. Vita CAD-Temp' (Vident) is a highly cross-linked, microfilled polymer that is available in extended block sizes, including lengths of 40 mm and 55 mm to accommodate multiple-unit FPDs. It is offered in a monocolour block that comes in four shades or a multicolour form with four shade layers for increased esthetics. Telio CAD (Ivoclar Vivadent) has been introduced as a millable cross-linked polymethylmethacrylate (PMMA) block for temporary crowns and FPDs. The block is part of the Telio system that includes a self-curing composite, desensitizer, and cement. It is available in 40-mm and 55-mm size blocks and in six shades.

CONCLUSION

Dental restorations produced with computer assistance have become more common in recent years. Most dental companies have access to CAD/CAM procedures, either in the dental practice, the dental laboratory or in the form of production centers. The many benefits associated with CAD/CAM generated dental restorations include: the access to new, almost defect-free, industrially prefabricated and controlled materials; an increase in quality and reproducibility and also data storage commensurate with a standardised chain of production; an improvement in precision and planning, as well as an increase in efficiency.

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