Dentistry’s Evolution Ushers in New Materials, Workflows
Traditionally, indirect restorative dental materials were utilized by the dental laboratory to fabricate restorations, which the dentist inserted into the patient's mouth. With the evolution of dentistry becoming what is now a highly digital industry, workflows and materials have changed.
Key Material Developments
Today, while dental providers continue to work with indirect materials that were used traditionally, they are also utilizing new materials that require new equipment and different workflows. Most of these newer indirect materials are now used with computer-aided design/computer-aided manufacturing (CAD/CAM) fabrication methods. One of the most significant of these materials is zirconia, a ceramic that is used for many different kinds of restorations.
Zirconia is currently available in white, preshaded, and even multilayered blanks. Blanks are the "ready-to-use" zirconia products after the material has gone through intensive production finishing steps to be made usable. A simplified explanation is that zirconia powder is pressed into a negative form and sintered to produce a ready-to-mill blank.1 These blanks are made to be milled into restorations. Restorations are milled about 28% larger than the final result needs to be, because the milled object has to get sintered to achieve its final stability, color, and size. After milling, the restorations can be finished with a stain-and-glaze finish, or porcelain can be layered. The advantage of zirconia is that it has an enormous indication spectrum, from fixed to removable restorations, from tooth-supported to implant-supported. Its disadvantages are the sensitivity with which it has to be handled so that stability and esthetics are not compromised.
Another important material development for indirect dental restorations is lithium disilicate. Interestingly, this material can be utilized with traditional fabrication methods as well as with CAD/CAM, and in combined workflows. This also can be highly advantageous as the technician is therefore not limited. The disadvantage of lithium disilicate is the relatively limited indication spectrum versus zirconia. It cannot be used for bridges or certain implant situations that require the masking of different materials, as the flexural strength range of 350 MPa to 400 MPa would require bridge connectors in a size that would hinder an esthetic outcome and the overall translucency of the material would show metal color underneath. The bond ability of lithium disilicate, however, makes it a great material for size-compromised cases with regard to retentive surfaces and for veneers.
On the polymer and acrylic side of the indirect material aisle, there has been a boom in developments. Digital denture materials are extremely versatile since the fabrication of digital dentures can be done by milling or 3-dimensional (3D) printing, or a combination of both. Available options are printable resins for 3D printing and polymethyl methacrylate (PMMA) blanks for milling.
Other polymer types are the so-called high-performance polymers, which are polyetheretherketone (PEEK) and polyetherketoneketone (PEKK). Like zirconia, these materials are millable. However, they are not esthetically relevant. They are being used purely as framework materials to support ceramics and composites as veneering materials. The advantages of PEEK and PEKK are their light weight, density, great stress distribution, biocompatibility, and stability, which allow them to be used for reconstructions ranging from small to large.2
When reviewing indirect materials for use in digital dentistry, fabrication methods must be considered. As mentioned earlier, many of the materials are available as millable blanks. Milling is one of the oldest digital fabrication methods known. Computer numerical control (CNC) milling has been used in many other industries. Dentistry started milling multiple decades ago, which involved the milling of chairside restorations after an intraoral scan of the patient.3 Interestingly enough, since CAD/CAM is utilized in dental practices, indirect materials are not exclusively used in dental laboratories. The reality is, although CAD/CAM is still used mostly in dental labs, a lot of change has occurred over the past few years, including milling becoming increasingly common in the dental office.
After starting with three-axis milling machines, labs are now using five-axis machines to mill the most complex geometries possible. Because of the availability of many different materials, milling machines are now capable of milling in dry or wet modes, with some machines able to mill both dry and wet. This allows a smaller footprint to be utilized because of the need for just one machine to mill a wide range of materials.
Besides milling, 3D printing is one of the most prolific fabrication methods for indirect materials. 3D printing in dentistry is experiencing fast, significant growth, as it is compatible with all available materials and is expected to be compatible with newly developed materials as well.4 Dental labs are already printing for almost all aspects of dentistry. As a purely additive process as opposed to a subtractive process like milling, one of the main benefits of 3D printing is the minimization of waste. When first introduced to dentistry, 3D printing started with model printing; now a wide range of items are being printed, including models, surgical guides, dentures, crown-and-bridge provisionals, and implant provisionals. Definitive crown-and-bridge and implant restorations are the latest development in 3D printing and are expected to be available soon.
Presently, neither milling nor 3D printing is used as a replacement method for fabricating dental restorations with indirect progressive materials. Time will tell how both methods will be utilized in the future. One may succeed over the other as a major process in general, or combinations of both may emerge.
When considering these newer materials, their indications in indirect dental restorations, and the methods of fabrication, it is becoming increasingly important for the treatment team to work together and communicate openly from the beginning, allowing all team members to be "in the loop" and contribute their specialty to ensure a successful finished restoration. CAD/CAM materials certainly represent a forward step when considering the wide indication spectrum, enhanced esthetics, and process efficiency. These materials, however, are also sensitive to work with, so verifications, evaluations, and confirmations are crucial throughout the restorative process. As an example, zirconia or polymers cannot be soldered or easily repaired or corrected like traditional materials such as alloys or acrylics can. With this in mind, as long as the team works together, outstanding results can be achieved.
About the Author
Alexander Wünsche, CDT, ZT
Owner, Zahntechnique Dental Laboratory,
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