CADCAM in dentistry. Materials and methods: an overview for the dental team

Digital workflow

A workflow consists of a repeatable pattern of activity, enabled by the systematic organization of resources into processes that transform materials, provide services, or process information. Making a cup of tea is a workflow. In the context of digital dentistry, an example of a digital workflow might be digital impressions (DI) (Figure 1).

Digital workflows in dentistry will usually start with some form of imaging. It may be a cone beam CT (CBCT) that forms part of an implant workflow, or it may be an optical impression. A simple DI workflow could involve entering patient demographics into a system, image acquisition and image transmission to a third party with a suitable prescription (Figure 2).

The image data is transferred in the form of an STL file. Many people believe this means ‘standard tessellation language’ or ‘standard triangulation language’. However, Chuck Hull, regarded as the inventor of 3D printers, in a recent interview, stated that he formulated this as an abbreviation for ‘stereolithography’.²

Numerous software packages, including the Connect SW package (Dentsply Sirona, Benzheim, Germany), are available and ensure the transmission of data, in this case impressions, literally in seconds, to a laboratory. Alternatively, after image acquisition, the images may be used to produce a crown or bridge in-house, as a milling workflow, or it may be used to plan an orthodontic treatment and in the production of aligners, by means of a 3D printer.

Digital spectrum

The range of technology and workflows is substantial. Whether it be milling, printing, DI, heat forming, CBCT or workflow planning, solutions and technology are available from numerous manufacturers.

The early days of CEREC (Dentsply Sirona),^3,4 as described by Werner Mörmann and Marco Brandestini in the 1980s, is a familiar starting point for CADCAM in dentistry. The first CEREC patient was treated in September 1985 (Figure 3).

3D printing has been vital in all forms of manufacturing from the early 1990s. The emerging technologies and the vast increase in the range of materials has seen an accelerating uptake in printing technology, both in laboratories and at practice level in recent years.

Production modalities

Within the dental practice and laboratory environment, we can divide the production of parts into subtractive and additive manufacturing.

Subtractive manufacturing is a term for controlled machining and material removal processes that start with bars, blocks, discs or rods. The materials could be metal, acrylic, porcelain, composite, bone, hybrid, zirconium or others.

Grinding and milling

Chairside milling was first described by Werner Mörmann and Marco Brandestini,⁵ when reviewing chairside milling of inlays and onlays with the CEREC 1 (Dentsply Sirona).

Since the original concept, we have seen a rapid development in scanning and milling technology. The progress made from the early days of CEREC 1 (Dentsply Sirona) to Primescan (Dentsply Sirona) shows incredible advances in image acquisition, data processing and analysis, and the advent of the PrimeMill (Dentsply Sirona)⁶ enables us to produce a wide variety of high-quality, and highly aesthetic restorations (Figures 4–6).

All manufacturers, including 3shape (Copenhagen, Denmark), iTero (Align Tech, San Jose, USA), Carestream (Carestream, Atlanta, USA), Planmeca (Planmeca, Helsinki, Finland), Medit i700/i500 (Medit Corp, Seoul, Korea) and others, have shown continuous development in terms of scanning accuracy and data processing.⁷ The introduction of artificial intelligence (AI) into workflow algorithms has also added to the operator experience and again improved the quality of the final product. The time needed to produce the restorations has also reduced considerably. The reality of producing a crown, or a number of crowns in a single visit, is now a routine workflow in a digital practice (Figures 7 and 8).

Milling materials

Materials used in chairside milling include feldspathic porcelain, lithium disilicate, zirconia, acrylic, resin composite and hybrid materials (Table 1).

Table 1. Examples of milling blocks for CAD/CAM in dentistry. DS: Dentsply Sirona; GC: GC Corporation, Tokyo, Japan; IOCV: inlays/onlays/crowns/veneers; I: Ivoclar Vivadent; KN: Kuraray Noritake, Tokyo, Japan; STr: Straumann, Basel, Switzerland; V: Vita Zahnfabrik, Bad Sackingen, Germany.

Material	Brand	Indication	Block size/approximate cost	Flexural strength (MPa)
Leucite-reinforced ceramic	IPS Empress (I)	IOCV	Single unit, £15–25	100–170
	Vita Mark 2 (V)
	Cerec blocks (DS)
	Cerec polychromatic blocks (DS)
Lithium disilicate	IPS e.maxCAD (I)	IOCV/abutments/bridges	Single, £20Three-unit, £45Implant, £45	500
Zirconia-reinforced lithium silicate	Celtra Duo (DS)	IOCV	£20	210–370
	Vita Suprinity (V)	IOCV/abutments		420
	NICE! (STr)	IOCV/abutments		350
Lithium disilicate with virgilite	Tessera (DS)	Just launched	To be confirmed	700
Resin nano ceramic (hybrid)	Lava Ultimate (3M)	Inlays/onlays	All single unit, £20–25	170
	Enamic (V)	Inlays/onlays		150
	Cerasmart (GC)	IOCV		230
	Katana Avencia (KN)	IOCV		230
Acrylic (PMMA)	TelioCAD (I)	Temporary crowns	£20	n/a
	Vita CAD_temp (V)	Temporary crowns
	CEREC Block (DS)	Drilling guides
	TetricCAD (I)	IOCV
Composite	TetricCAD (I)	IOCV	£20	272
Zirconia (full contour)	Cerec Zirconia (DS)	IOCV/bridges/abutments	£20–150	900
	In Coris (DS)	IOCV/bridges/abutments
	emaxZirCAD (I)	IOCV/bridges/abutments/full arch		850–1200
	KatanaZirconia (KN)	IOCV/bridges/abutments		763

Even in the early days of chairside manufacturing, the profession was able to produce well-fitting restorations. However, we have all seen the YouTube videos of a workflow, with motivational music, producing well-fitting restorations with fantastic marginal fit, that looked pretty average and were best assigned to posterior teeth.

It is the introduction of materials like IPS e.max (Ivoclar Vivadent) and newer lithium disilicates, such as Tessera (Dentsply Sirona), that has transformed our ability to produce highly aesthetic, strong, definitive restorations in as little as an hour or so. We can produce implant-retained crowns, abutments and implant bridges in a matter of minutes. When combined with a CBCT, we can also mill drilling guides or endodontic guides as part of a DI workflow.^8,9,10 With the latest generation of mill, we are literally on the ‘cusp’ of being able to mill in-house, customized titanium abutments (should we wish).

Having said all this, some practitioners will have no desire to mill in-house and the easy workflow, using DI, enables them to send very accurate impressions^11,12,13 to their laboratory of choice, in a timely and environmentally conscious manner. The absence of postage, laboratory collections, stone models and silicone impression materials, all contribute to a reduction in the potential for inaccuracies (Figure 9).

3D printing

Additive manufacturing, or 3D printing, builds objects one layer at a time, by means of bonding each new layer to the previous layer, until the object is complete (Figure 10).

The workflow for 3D printing is exactly the same as described above, whether it be in-house production, or via a laboratory. An STL file (DI) is generated and used in design software, either in a practice or laboratory environment. There are numerous forms of 3D printer (Table 2).

Selective laser sintering (SLS)

This method of production uses a high-energy laser to cause fusion of a powdered material. Surgical guides, study models, metal frameworks and implant bars can be produced using SLS. Recent studies have shown printed metal partial dentures to be harder than those produced using traditional methods.¹⁵ Other studies also demonstrate better biocompatibility for appliances made with SLS.¹⁶ Ceramics and resins can also be used in SLS techniques. The high temperatures involved in this process mean that the end products are autoclavable, and they are also very strong.

The metal framework in Figure 11 has been printed directly from a digital impression. The models, from the same digital impressions, were not used in the production process and met for the first time after completion of the framework. The quality of fit and finish is clear. The design was checked via a mobile phone app, by the author, prior to manufacture.

Choice of technique

The choice of manufacturing technique and apparatus is usually self-evident, and driven by the materials, or end product desired. There is a degree of overlap. The accuracy of the final product needs to be considered, as does the cost of its manufacture. When considering surgical guides, it has been demonstrated that milled and printed guides show similar levels of accuracy and may be more accurate than thermoplastic guides.^10,20 Milled guides can only be used for single fixtures. The relative materials cost is approximately £3 for a printed guide, but £50 for a milled guide (Figure 12).

While there are limitations to all aspects of 3D printing and milling, the willingness of our profession to adapt and explore makes this a significant addition to any laboratory or practice.²¹

The numbers game

Having seen that it is possible to produce high-quality restorations, prostheses and adjunctive products digitally, we need to establish whether it makes financial sense to work digitally. High street practices, academic institutions and laboratories need to justify and live with their investment in technology. It makes little or no sense commercially to have attractive looking equipment, producing good quality products, resulting in insolvency. If we look at the costs of individual items, and the time frames involved, it is possible to see whether there is a business case for digital dentistry.

When considering that the cost of an e.max CAD (Ivoclar Vivadent) block is around £15 for a single crown or £45 for a 3–4-unit bridge, it becomes more straightforward to calculate the financial efficacy of working this way. With the continuous improvement in CADCAM software, the design process is a matter of minutes in most cases. The materials cost to produce prostheses in-house is as little as 10%, or less, of a typical laboratory fee. This principle also applies to drill guides, aligners, and orthotics. An in-house, screw-retained, implant crown can be produced for approximately £100 using lithium disilicate, zirconia or any other material available in block form for this purpose.

This absolutely does not mean an end to the dental laboratory. Clinicians working digitally, are able to choose to produce some, or all, of their routine prostheses and adjuncts in-house, or with their laboratory. Larger crown and bridge cases, larger implant cases, and removable prosthetics all fit the DI workflow, using improved communications, efficiencies and the skills of the technician.

The digital state of play in the UK

So, if the technology and workflow for digital dentistry is so good, why is it still an infrequent clinical partner in the high street and institutional practice? Is it that the system that many of our colleagues work within precludes investment? Is the initial investment perceived as a barrier?

There are numerous myths around digital dentistry. Many still believe that highly aesthetic restorations are not possible, and well-fitting ‘soft mints’ are the result of in-house milling. This is clearly not the case; however, the industry and those of us who are advocates for digital dentistry have not, until recently, been effective at convincing our colleagues of the quality that is possible with digital dentistry. With the latest communication software packages, open systems, digital photography and webcam meetings, it is possible to communicate with high levels of accuracy and discuss cases in real time.

Another major factor is the perceived level of investment required to follow full or partial digital workflows. However, for many practices performing a moderate level of activity, they are more profitable when working digitally. Whether they are using DI to communicate with their laboratory or full chairside workflows, producing their own prostheses and other items, the increased accuracy, the reduction in material use, the reduction in error-forming procedures, all lead to better profit margins, even after the cost of the investment is taken into account.

Single practices may choose to go ‘in-house’ for a proportion of their prosthetic items, because all the above advantages and the benefits outlined below, are highly persuasive (Figure 13). Alternatively, a group of practices may find the concept of a ‘print centre’ or ‘milling centre’ attractive.

The most important consideration in the discussion about digital dentistry is the welfare of patients. Many patients dislike impressions. Patient acceptance of digital impressions is considerably better than traditional impression methods.¹³ The improved accuracy, smoother workflow and, in a number of cases, single rather than multiple visits, makes digital dentistry far more appealing.

We are in an age when patients are actively looking for something better, more efficient, more comfortable and less invasive, and the digital dental experience is increasingly sought.

Other benefits include those experienced by the DCPs involved in the process of treating a patient. DI compared to mixing alginate or taking conventional impressions with the scanner and being actively involved in the production of restorations and other appliances, significantly elevates job satisfaction. The cross-infection benefits in our COVID-19 world are significant, whether it be reduced patient contact, or lack of the ‘physical’.

When considering dental undergraduate education, there has been an increasing uptake in the delivery of curricula by means of virtual reality (VR) in UK dental schools. However, the lack of digital workflows in the clinical environment is a concern. Of course, the logistics involved in delivering digital dentistry in an educational institution is complicated²² by volume, data protection and academic rigour. While accepting the necessity to fully educate dental students in all manner of analogue workflow, we must also recognize that our pending and new graduates are of a generation where digital is normal. There is an expectation that technology and efficiency should be commonplace. This expectation is increasingly mirrored by our patients. Our European and North American colleagues are introducing digital workflow into their fourth- and fifth-year curricula.^23,24

Conclusion

The relatively low, current level of digital dental practice in the UK is set to expand. Whether it is the high street or the need and expectation of graduates to use this technology, patients' wishes or our academic institutions driving this change, only time will tell.

One thing is certain. If you are going to be working in clinical dentistry in 10 years' time, you will need to be working digitally, and you will need to be doing it very well, because everyone else will be doing it.