Advances in Materials and Digital Technologies: Keeping Up With the Many Changes

One of the most impactful changes-and one that arguably has started a new era in dentistry-is the introduction of zirconia as a crown-and-bridge material. When zirconia was first introduced in dentistry in the early 2000s, it was intended as an alternative to materials that were proven for many decades. The benefits of utilizing this highly popular restorative material, therefore, are worth examining.

Esthetics: Before its use in dentistry, the first-generation zirconia was not a highly esthetic material; it was just opaque white. The white color, however, was what made it interesting to the dental profession. Upon being used in dentistry, zirconia was able to be colored before sintering, a characteristic that was considered a "game changer" in dental materials. This enabled improved control over the final esthetic.

Over the years different generations of zirconia have been introduced, and esthetic properties have changed with each generation. Currently, there are many different translucencies and color schemes available, from higher opacity to super-high translucency, from just white to multilevel pre-colorings. This variety allows technicians to fabricate dental restorations in varying styles. Technicians are now able to design either a full anatomic restoration or an anatomic-reduced restoration, allowing for either just staining and glazing or layering with porcelain before staining and glazing.

Strength: While zirconia is tremendously strong, it is not unbreakable. The more opaque, high-strength zirconia materials have 1100 MPa to 1500 MPa, and even the super-high-translucent zirconias are around 700 MPa, which is still three times the strength of natural tooth, which is about 200 MPa to 300 MPa.¹

 However, because zirconia does not flex, it is brittle, which allows the material to break under force if not properly supported. Therefore, bridges on teeth and/or implants have to be absolutely passive in fit, which can be achieved with proper verification. Cantilevers are contra-productive and should be kept as short as possible with proper connector thickness.

Mechanical properties: The high density of zirconia allows for an extremely highly polishable surface, which is beneficial for natural opposing dentition. Studies have demonstrated that the amount of wear of the opposing natural dentition is the lowest when compared with all other restorative fixed materials, such as metals and ceramics.²

The mechanical properties of zirconia, especially its heat sensitivity, also makes this material highly sensitive³; adjusting a sintered zirconia restoration can cause micro-cracking, and over time the restoration can break due to expansion of these cracks. The same can happen when zirconia receives improper technical handling through firing or glazing in a porcelain furnace; rapid heating or cooling can create shock cracks causing the material to break either immediately or over time.⁴

Besides zirconia, the digital evolution has enabled the use of many different materials, such as high-performance polymers (HPPs) like polyetheretherketone (PEEK) and Pekkton^® HPP.⁵ The author considers HPPs to be at the top of the polymer pyramid with regard to performance. They are very dense, light, and fully biocompatible materials. Their structure is similar to bone, which is especially advantageous for implant-supported reconstructions. The good flexural strength of HPPs allows for a strong and shock-absorbing material.⁵ HPPs are well suited as a framework material, as their opaque grey or beige color limits their esthetic capabilities.⁶ Just beneath HPPs in terms of performance are commodity polymers, polymethyl methacrylates (PMMAs), and industrial polymers.

These material advancements go hand in hand with the evolution in dental technology. To work with these materials dental laboratories require such equipment as milling machines, desktop scanners, 3D printers, and more. In the modern dental practice new equipment like intraoral scanners are at the forefront, and CT scanners and digital photography are now commonly used.

These digital devices offer a valuable advantage over analog methods in that communication can be achieved instantly. Records can be sent and received among the dental team and laboratory while the patient is still sitting in the dental chair. Regardless of where in the world the dental laboratory is located, the technician is able to look at the records and discuss the case with the doctor and patient via video call. For many dental professionals, this is one of the most enjoyable and productive aspects of the progression in technology.

The quality of scans from both intraoral and desktop scanners has improved exponentially. Scans are as accurate as 5 µm to 8 µm,^7,8 which is difficult to accomplish with any traditional method of record-taking and fabrication of a dental restoration. Both digital scans and analog impressions require the operator to take all necessary precautions and tend to details to receive a highly accurate result. As a digital record, the intraoral scan has the significant advantage of not deteriorating over time and can be saved for later reproduction or documentation.

Before embracing digital impressions completely, however, clinicians should understand that any technology has limitations. Patients are human beings, and no two are the same. In some cases, if a crown preparation is very sublingual and the area cannot be scanned clearly, a digital impression likely will not be as accurate as a polyvinyl siloxane impression whereby the material can be physically injected into the deep, hidden area and impress the preparation. Another example may be a veneer case in which the clinician is not opening interproximal contacts; the fabrication of the veneers would be much easier if an analog impression was taken.⁹

For implant cases, the author's experience with digital impressioning has been very positive in regards to accuracy of individual implant-retained crowns. For full-arch or multiple connected implants with restorations, care is needed to avoid digital distortions. An effective way to accomplish full-arch reconstructions in a fully digital manner is to scan using photogrammetry, where an extraoral scan is taken of scan flags that are applied to the implants in the mouth. This offers a better depth of field to accomplish a higher rate of accuracy than regular intraoral scanning.¹⁰ Unfortunately, the use of photogrammetry does not replace an intraoral scanner, which is still needed to finalize the record-taking; however, it is a technology that may play a much more significant role in the near future of dentistry.

After record-taking comes the design and fabrication of restorations. Again, the dental industry has made huge strides in efficiency, accuracy, and consistency. While milling is the oldest digital fabrication method, over the years technicians have learned much about which materials to use for milling, and which not to use. The most common milling materials today are zirconia, glass ceramics, hybrid ceramics, wax, PMMAs, HPPs, and metals.¹¹ Digital dentures generally fall under the umbrella of PMMAs.

The technology of milling machines has also developed greatly to be able to mill all of these materials. Today, three-, four-, five-, and even hybrid six-axes milling devices are available, along with dry and wet milling machines or hybrid solutions that allow for dry and wet milling in one machine. Machines are available with a single blank holder or a multiple blank changer. Chairside milling machines generally offer smaller-block milling to allow the dental office team to switch between materials quickly and easily depending on each patient's needs.

Perhaps the fastest-growing fabrication method in dentistry is 3D printing. This is due not just to the explosive expansion of intraoral scanning, but because of 3D printing's highly efficient workflow, whereby analog steps in the laboratory and/or dental practice may be eliminated. Dental models seem to be reaching a higher level of accuracy with each new device or print material released on the market. Also, tools for verification or evaluation purposes and to help finish final restorations are being printed; examples include the printing of seating jigs to shorten chairtime, and 3D-printed prototypes that are being utilized before finalizing a restoration in ceramic.

It seems that it is just a matter of time before 3D printing in dentistry will be a primary fabrication method for almost all materials. Resins and metals are already being printed. Other industries are now printing ceramics and polymers, with some dental companies experimenting with these materials. The rapid growth of 3D printing is also impacting digital dentures. This was the last pillar of analog dentistry, but the digitization of removable dentures is experiencing a push similar to what was seen at the development of zirconia. This push, however, is moving even faster, because errors that were made in the early stages of the digital evolution can be avoided.

Although there may be skepticism or even fear among many dental professionals about the digitization of dental laboratories and practices, steps can be taken, as outlined above, to maintain control over this evolution. Patients must be treated as people, with each one being different. Dentistry appears to be on the right track as it continues to embrace technology and all of its benefits.

Alexander Wünsche, CDT, ZT
President, Zahntechnique Dental Laboratory,
Miami, Florida