Chairside Digital Dentistry: A Review of Current Technologies

Computer-aided design/computer-aided manufacturing (CAD/CAM) systems were initially developed in 1950 by the defense arm of the US Air Force for aircraft and automotive manufacturing, but it took three more decades until they were applied in dentistry. In the early 1980s in France, Dr. Francois Duret developed a dental CAD/CAM device that included an optical impression of the abutment tooth and a numerically controlled milling machine.² The first commercial CAD/CAM system that could fabricate indirect same-day ceramic dental restorations based on an optical impression in the dental office (CEREC^®, Sirona Dental Systems GmbH) was later developed by Dr. Walter Mörmann and first used for a patient in 1985 at the University of Zurich in Switzerland.³

Laboratory-based CAD/CAM systems that included a scan of a master cast, digital restoration design, and a CAM system either in the dental laboratory or a centralized milling center were introduced soon thereafter. In general, digital workflows provide high accuracy and precision, predictability, efficiency, and cost-effectiveness while offering a wide range of materials with physical, optical, and biological properties that oftentimes exceed those fabricated conventionally. Digital tooth design and treatment planning tools facilitate "digital wax-ups" and digital smile design to create highly esthetic smiles, independent of the wax-up skills of the clinician or dental technician.

The types of dental restorations that can be fabricated with current CAD/CAM technologies are virtually limitless, from single-unit inlays, onlays, crowns, veneers, implant abutments and restorations to fixed and removable dental prostheses for partially and completely edentulous patients. For chairside systems, constraining factors include the size of the milling machine and material blocks as well as the type and extent of the restoration. However, most chairside systems allow the clinician to send an intraoral scan file to any manufacturing site or dental laboratory for more complex reconstructions.

Chairside CAD/CAM Systems

The number of intraoral scanners (IOSs), milling machines, sintering furnaces, 3D printers, and other CAD/CAM equipment specifically designed for chairside application is increasing exponentially. Several manufacturers offer various components of the chairside digital workflow individually, such as scanners, chairside milling machines, and ceramic furnaces with small, office-friendly footprints. This article summarizes the current state of technological resources used for chairside digital dentistry.

Intraoral Scanners

In recent years, scanning technologies have been constantly updated and improved to make IOSs faster, more accurate, versatile, smaller, as well as user- and patient-friendly. Current IOSs do not require any anti-reflective powders used in the past. Some also scan tooth shades and employ new technologies such as near-infrared imaging, which can detect demineralizations in dental hard tissues. Such capabilities allow capturing of additional information already in the diagnostic phase, making IOSs even more attractive for the general practice. Patient comfort is an advantage for intraoral scanning, as it eliminates uncomfortable aspects of conventional impressions. Also, the digital impression can be inspected immediately and areas that have not been captured adequately can simply be rescanned without having to remake the entire impression.

An IOS projects structured light (white, red, or blue), which is recorded as individual images or videos and compiled by the software after recognition of specific points of interest. The time for making a digital impression is shorter than for a conventional one, while accuracy and precision are comparable if not better. A current limitation may be the use of the IOS for full-mouth reconstructions on teeth or implants, not because of the accuracy and precision at a tooth level but because of slight deviations in cross-arch accuracy. However, since chairside-fabricated CAD/CAM restorations are typically limited to single teeth or limited units, this may not be a factor in most cases. Nevertheless, new scanner developments seem to limit and may ultimately eliminate this shortcoming. What does impact IOS accuracy is scanning technique and sequence, so proper technique is important.

CAD/CAM Design Software

The chairside digital workflow allows for visualization and analysis of a digital impression immediately after scanning, and corrections to the scan or preparation can be made on the spot. Special software can detect tooth preparation errors, such as insufficient occlusal clearance, undercuts, uneven preparation margins, sharp corners, and rough surfaces.

Definitive restorations can be delivered in one visit, eliminating temporary restorations and multiple appointments. Current high-strength ceramic materials and implant prosthetic solutions even facilitate chairside production of fixed dental prostheses and implant-supported restorations. Dental restoration design software has become significantly faster, intuitive, and user-friendly. Features like preparation finish line detection and digital wax-ups are becoming increasingly automated, oftentimes through application of artificial intelligence tools. Natural tooth shapes and smiles can be selected from digital libraries and customized in any way necessary, providing truly natural and esthetic results based on the individual needs and preferences of the patient. Current face scanning technologies and automated image analyses further assist with providing optimized, patient-centered esthetics.

Clinicians who prefer to delegate the design steps can simply send their intraoral scan as well as photographs or face scans to a laboratory or smile design center. With most systems, CAD data is handled and transmitted in an STL format, which has become the standard file format in 3D printing and rapid prototyping. Other formats currently used include PLY, DCM, and UDX. To communicate with a milling machine, these file formats are "translated" into "millable" data file formats (CNC).

Chairside Milling Machines and Furnaces

Numerous milling machines with a small footprint for use in the dental office are available for milling a wide variety of restorative materials. Many of these units are four-axis mills, where the milling bur moves in the three axes (x, y, and z) while the material block can rotate in one additional axis (also called 3+1 axis milling machines). More advanced systems employ two or more burs on separate motors with the ability to mill a restoration from a block in only a few minutes. Precision of fit and milling time of a restoration depend on several parameters, such as the number of axes and spindles, bur size and abrasiveness, milling speed, and the type of material. Different materials may have to be milled under different conditions, either wet or dry. Compact chairside milling units typically accommodate material blocks of up to 20 mm, 40 mm, and 85 mm. Laboratory units can mill large or multiple restorations from discs that are up to 98.5 mm in diameter and 30 mm in thickness.

Other digital manufacturing technologies include laser milling and 3D printing. While promising, 3D printing technologies are not yet ready to produce definitive restorations in a reasonable amount of time and with the same quality as milling.

A furnace is needed for materials that require sintering or ceramic glazing. Several options are offered for dental offices for sintering zirconia, glazing ceramics, and crystallization/processing of lithium disilicates. Specific speed sintering cycles allow for timely finalization of all-ceramic restorations, even zirconia, within minutes.

Chairside CAD/CAM Materials

Chairside CAD/CAM technologies fabricate indirect restorations from acrylics, indirect resin-based composites, and various ceramic materials. Proper selection of these materials based on indication as well as patients' individual esthetic and functional needs is essential for clinical success and longevity. Clinical studies indicate very high long-term success rates of chairside CAD/CAM restorations.^1,4

Crossed-linked polymethyl methacrylate CAD/CAM blocks are typically used for provisional restorations. Some manufacturers offer them with increased physical and optical properties via polychromatic layers of different shades and translucency levels.

Composite resin blocks are offered for a variety of definitive, mainly partial-cov­erage restorations and are easy to use, because they only require polishing or, if needed, application of a light-cure stain.

Resin-matrix ceramics (RMCs) were at first specifically developed for chairside digital dentistry to combine the advantages of composite resins and silicate ceramics. RMCs are divided into two subgroups: resin-based ceramics and hybrid ceramics. Because they only recently have been introduced to the market, long-term clinical studies on these materials are not available. Like composite resins, RMC restorations only require polishing. They can be glazed and customized with light-cure stains, making finishing simple and fast without the need for firing in a furnace.

Silica-based ceramics are extremely popular for chairside CAD/CAM restorations and can be divided into feldspathic and silicate ceramics. Traditional feldspathic ceramics are highly translucent and esthetic but lack flexural strength and, therefore, require resin bonding for final insertion. Many CAD/CAM blocks are also available in polychromatic, multi-layer versions for enhanced esthetics to simulate the different shade and translucency layers of natural teeth. Despite the low physical properties of feldspathic ceramic ("porcelain"), several clinical studies indicate excellent success.¹

With increased strength over traditional feldspathic ceramics and high translucency, leucite-reinforced feldspathic ceramics are indicated especially for anterior crowns and posterior inlays/onlays. However, over the years, they have been largely replaced by lithium silicates. Lithium-silicate ceramics have become extremely popular for several indications, especially monolithic crowns, inlays, and onlays. With a biaxial flexural strength of around 407 MPa, they are considered the strongest silica-based ceramics in dentistry. These materials must be crystalized after milling and can be stained and glazed in a sintering furnace. Excellent success rates are well documented in the literature for CAD/CAM lithium silicates, especially single-unit restorations.⁵

High-strength polycrystalline metal oxide-based CAD/CAM ceramics, such as zirconium dioxide (zirconia), are characterized by excellent mechanical properties, which are significantly greater than those of silica-based ceramics. Their inherent strength allows for conventional cementation of adequately dimensioned full-coverage restorations. More recent zirconia generations offering significantly greater light transmission are available in pre-shaded multi-layer chairside blocks as well. With a special chairside furnace and speed sintering programs, the sintering of a single crown can be accomplished within minutes.

In combination with 3D printers, chairside CAD/CAM systems can provide an even greater range of clinical applications and appliances. These include nightguards, occlusal splints, orthodontic appliances, surgical guides, provisional restorations, study models, and more. In light of current developments in metal and ceramic printing, 3D printing may well become the preferred manufacturing method in the future. At this time, however, milling from homogenous, industrially fabricated material blocks is still preferred.

Conclusions

Chairside CAD/CAM technologies are versatile, predictable, and highly accurate. With proper technique, current IOS technologies are at minimum as accurate and precise as conventional impressions, at least for single or short-span multi-unit restorations. Scan or preparation errors can be corrected immediately. Design software is fast and user-friendly, able to create natural esthetics and function through digital smile design tools. Current smaller footprint milling machines and sinter furnaces allow for fabrication of restorations from a variety of materials in a single patient visit.

Markus B. Blatz, DMD, PhD
Professor of Restorative Dentistry, Chair, Department of Preventive and Restorative Sciences, Assistant Dean for Digital Innovation and ProfessionalDevelopment, University of Pennsylvania School of DentalMedicine, Philadelphia, Pennsylvania; Editor-in-Chief,Compendium of Continuing Education in Dentistry