CAD/CAM Dentistry: What you need to know

3 October 2019

In August 2017, the FDI World Dental Federation (FDI) published a Policy Statement on CAD/CAM dentistry, which it defined as the "Process of designing and manufacturing a custom-made dental device, or a patient-specific dental device from an industrialized product, with the aid of a computer." (1) An excellent review of digital dentistry including CAD/CAM by Blatz and Conejo (2) has been published recently in Dental Clinics of North America.

The first commercial CAD/CAM system (CEREC, Dentsply Sirona,Bensheim, Germany) was developed by Mormann in 1985 at the University of Zurich, and enabled the production of an in-office same-day ceramic restoration (CEREC: CEramic REConstruction). The early CAD/CAM systems were only able to produce inlays,
onlays and single crowns. However, today’s systems can produce a wide range of restorations.

The first requirement of using a CAD/CAM system is capturing a digital impression of the tooth preparation(s) and surrounding tissues.

All parts of the preparation must be captured; current systems no longer require the application of an anti reflective powder and some are able to determine tooth shades. One of the principal benefits of intra-oral scanning is that a defective scan can simply be redone, in contrast to having to retake a time-consuming and potentially uncomfortable impression. The scanner software manipulates the data to produce a 3D image, and the resulting digital impression may be more accurate than a conventional impression. The 3D image is often linked to the computer wirelessly, rather than by cable.

There is a range of software available for image analysis and manipulation. Defects in the tooth preparation, such as undercuts, inadequate occlusal clearance and unclear margins can be identified. Restoration design software has become easier to use: margins can be detected; occlusal morphology can be designed, matched to a natural tooth in the same mouth or selected from a digital library; face scans can be included to optimise the aesthetics of anterior restorations.

If the restoration is to be manufactured (milled) in the laboratory, the digital impression can be sent electronically. Various file formats are available, e.g. STL, PLY, DCM and UDX. Translation to computer numerical control (CNC) format is required to program the milling machine to produce the restoration. (2)

Several compact, bench top, milling machines are now available, which can mill a range of materials. Typically such machines are 4-axis (or ‘3 + 1 axis’), with the milling tool being able to move in three axes and the material block being able to rotate in an additional axis. Some units employ two simultaneous cutting tools, allowing a milling time in the order of eight minutes with an accuracy of 25 μm. (2)

Five-axis machines can be purchased, allowing complex designs to be fabricated from a range of materials. Laser-milling is also available; the small laser spot allows for high resolution, and there is a cost-saving in not having to replace cutting tools or provide water cooling. Whereas typically the restoration is fabricated by the ‘subtractive’ process of milling the restoration from a block of raw material, ‘additive’ systems such as 3D printing are appearing on the market. There are laser-based devices that can perform additive and subtractive manufacturing, however due to the high cost and their inherent complexity they remain in the realm of large dental laboratories.

As indicated above, a range of materials can be milled to produce the restoration. (2) Such materials include poly-methyl methacrylate (for provisional restorations), resin-based composites and various ceramics (resin matrix, silicate, feldspathic, lithium silicate and oxide). Each has advantages and disadvantages and may require the availability of a furnace for sintering and/or characterisation and glazing. (2)

In summary, CAD/CAM involves three processes: an optical scanner to capture the intra-oral (and possible extra oral) image(s); software to convert the image(s) to a ‘digital model’ to design the prosthesis; and a device to manufacture the prosthesis. The device is customarily a milling machine. However, 3D printing of the restoration is under development. In Australia, the Therapeutic Goods Administration (TGA) is reviewing the registration of 3D printing materials.*

Dentists planning to fabricate 3D printed restorations, both in their own practices or by prescription from a laboratory, are advised to contact the TGA for up-to-date information. (Either email [email protected] or call 1800 141 144.

References are supplied on request at [email protected]

Martin Tyas (on behalf of the Dental Instruments, Materials and Equipment Committee)