Abstract
Objectives
Major changes are taking place in dental laboratories as a result of new digital technologies. Our aim is to provide an overview of these changes. In this article the reader will be introduced to the range of layered fabrication technologies and suggestions are made how these might be used in dentistry.
Methods
Key publications in English from the past two decades are surveyed.
Results
The first digital revolution took place many years ago now with the production of dental restorations such as veneers, inlays, crowns and bridges using dental CAD–CAM systems and new improved systems appear on the market with great rapidity. The reducing cost of processing power will ensure that these developments will continue as exemplified by the recent introduction of a new range of digital intra-oral scanners. With regard to the manufacture of prostheses this is currently dominated by subtractive machining technology but it is inevitable that the additive processing routes of layered fabrication, such as FDM, SLA, SLM and inkjet printing, will start to have an impact. In principle there is no reason why the technology cannot be extended to all aspects of production of dental prostheses and include customized implants, full denture construction and orthodontic appliances. In fact anything that you might expect a dental laboratory to produce can be done digitally and potentially more consistently, quicker and at a reduced cost.
Significance
Dental device manufacturing will experience a second revolution when layered fabrication techniques reach the point of being able to produce high quality dental prostheses. The challenge for the dental materials research community is to marry the technology with materials that are suitable for use in dentistry. This can potentially take dental materials research in a totally different direction.
1
Introduction
Dentistry has had a love affair with new materials and new technologies that goes back decades. Soon after the discovery of anesthetics the dental drill was invented, which meant that filling materials such as silicates and amalgams became widely used. In the early 20th century Dr. William H. Taggart introduced the loss-wax casting process to dentistry for the construction of crowns and bridges, which was adapted from the method then used in the jewellery business. The developments in new polymers during the 1940s and 1950s resulted in the use of acrylic resins for dentures, acidic polymers for restorative cements and monomers for composite resin restorative materials. The lasting contributions of Michael Buonocore, Dennis Smith, Raphael Bowen, John McLean, Alan Wilson and many others in this respect are well known. The discovery by Branemark of the special properties of titanium metal did not take long to be translated into an explosion in dental implantology. Thus dentistry has shown itself to lead the medical disciplines in embracing new materials and new technologies. And so it has also proved in making use of new technologies such as CAD/CAM (computer aided design/computer aided manufacture).
CAD/CAM began its dental life in 1970s with the first worker to explore its application in dentistry being Duret and Preston . This was followed by the work of Moermann in the 1980s, which led to the development of the CEREC ® system. CAD/CAM has now become a well accepted technology in most modern dental laboratories and for some enterprising clinicians at the chairside .
2
The development of CAD/CAM in dentistry
The development of CAD/CAM is based around three elements, namely: (1) data acquisition, (2) data processing and (3) manufacturing ( Fig. 1 ). The exponential increase in power of computers has resulted in major advances in all of these areas. This is particularly exemplified by the recent introduction of intra-oral scanners of which there are now a number on the market ( Fig. 2 : Lava™ C.O.S from 3M, Trios™ from 3Shape and iTero™ from Cadent .) For example the Lava™ C.O.S. system captures approximately 20 3D data sets per second and models the data in real time . Thus it is possible to create a 3D model of the oral cavity directly with such a system, without the need to take an impression, pour a model and then digitize the model with one of the many laser scanner that are now available. The digital model can now be used to design the restoration and there are now many software packages available for the design of dental restorations such as crowns, bridges and partial denture frameworks. Some software providers are now able to claim that their partial denture software can survey, design and wax a partial denture framework in less than 20 min .
A further development in the CAD/CAM technologies used in dentistry is the transition from closed to open access systems. Whereas in the past the digitizing, designing and manufacturing came as a closed system (e.g. CEREC ® ), more and more the technology is being opened up and the component part of a CAD/CAM system can be purchased separately . This creates much greater flexibility in that data can be acquired from a range of sources (intra-oral scanner, contact or laser model digitizer, CT, MRI), appropriate design software can be matched to the object to be manufactured (e.g. crown and bridge frameworks, partial denture frameworks, customized implant and implant abutments). Another very important consequence of the transition from closed to open systems is that this opens up access to a much wider range of manufacturing techniques such that the most appropriate manufacturing processes and associated materials can be selected. Thus, one is no longer constrained by the computer numerically controlled machining technologies that are currently used in most dental CAD/CAM systems.
2.1
Subtractive manufacturing
If we look at where we are today then CAD/CAM in dentistry is primarily based around the process of subtractive manufacturing. The technology most people will be familiar with is computer numerically controlled machining, which is based on processes in which power-driven machine tools, such as saws, lathes, milling machines, and drill presses, are used with a sharp cutting tool to mechanically cut the material to achieve the desired geometry with all the steps controlled by a computer program. Thus one starts out with a block of the material and the machine cuts away the bits that are not wanted. It has been demonstrated that by using this method the overall production time will reduce considerably and complex models, which are otherwise difficult and/or impossible to make by the conventional dental processes could be build up rather easily. These technologies have achieved a high degree of sophistication with new technologies such as electrical discharge machining, electrochemical machining, electron beam machining, photochemical machining, and ultrasonic machining . Nowadays all these processing routes come under the umbrella of subtractive machining. However, as one might imagine this method of manufacturing is very wasteful as more material is removed compared to what is used in the final product. This can readily be seen from the example shown in Fig. 3 ( Fig. 3 : block of material used in the fabrication of a mandibular onlay).
In the aerospace industry there is currently much talk about the weight to flight ratio of an airplane. What this refers to is the weight of material that has to be used in relation to the weight of material in the final product such as an airplane. The aerospace industry uses expensive materials and is thus concerned about saving cost by reducing the weight to flight ratio. For example, a 1 kg reduction in weight can save $3000 in fuel per annum and this means potential savings in the longer term of many billions of dollars . Similarly this is a concern in the automotive industry. As a consequence of this drive for saving costs there has been a major transition from making parts by subtractive manufacturing to what is referred to as additive manufacturing. Using additive methods for manufacturing is more advantageous as many problems associated with milling can be readily overcome. The main advantage of this type of manufacturing is the ability of the technique to create fine detail such as undercuts, voids, and complex internal geometries. Another limitation of the current dental CAD–CAM systems is that the process does not easily lend itself to mass production such as crowns and bridges, since only one part can be machined at any one time.
2.2
Additive manufacturing
So what is additive manufacturing and what are its benefits? Additive manufacturing is defined by the American Society for Testing and Materials (ASTM) as:
The process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies.
In principle the process works by taking a 3D computer file and creating a series of cross-sectional slices. Each slice is then printed one on top of the other to create the 3D object. One attractive feature of this process is that there is no waste. Traditionally additive manufacturing processes started to be used in the 1980s to manufacture prototypes, models and casting patterns. Thus it has its origins in rapid prototyping (RP), which is the name given to the rapid production of models using additive layer manufacturing. Today additive manufacturing describes technologies that can be used anywhere throughout the product life cycle from pre-production (i.e. rapid prototyping) to full scale production (also known as rapid manufacturing) and even for tooling applications or post production customization. It is a remarkably rapidly changing field with a huge investment in developing enhanced manufacturing technologies and it is changing the way we make things. Today, additive manufacturing is used for a much wider range of applications and is even used to manufacture production-quality parts in relatively small numbers. Some sculptors use the technology to produce complex shapes for fine arts exhibitions .
Thus additive manufacturing is transitioning from rapid prototyping models to manufacturing real parts for use as final products. The equipment is becoming competitive with traditional manufacturing techniques in terms of price, speed, reliability, and cost of use. This, in turn, has led to the expansion of its use in industry and there has been explosive growth in the sales and distribution of the equipment. In addition a new industry is emerging to create software to enable more effective use of the technology. The use of the technology is likely to grow especially as it is now possible to purchase a 3D printer for less than $3500.00 . Consequently centers providing 3D printing services are springing up around the globe. One interesting development is RepRap, which is a desktop 3D printer able to print plastic objects. RepRap has been described as a self-replicating machine, since many parts are made from plastic and RepRap can print those parts .
Alongside these developments the number of materials that the industry uses has increased greatly and modern machines can utilize a broad array of polymers, metals and ceramics. As the industry makes the transition from prototypes to functional devices the materials available will begin to play a much bigger role. When producing a prototype it is enough for it to look good, but as we move to functional objects such as customized implants and oral prostheses the materials and their properties become much more important.
It is worth noting that the process of additive manufacturing is in fact ideally suited to dentistry, which has a tradition of producing customized parts made to fit the patient and not the other way around. This is a great opportunity for dentistry and there is already a huge array of additive manufacturing technologies that we can use and these include:
- •
Stereolithography (SLA).
- •
Fused deposition modeling (FDM).
- •
Selective electron beam melting (SEBM).
- •
Laser powder forming.
- •
Inkjet printing.
This list is by no mean exhaustive and every day something new is added. Below some of these technologies are described as to how they work and how they are or might be used in dentistry.
2
The development of CAD/CAM in dentistry
The development of CAD/CAM is based around three elements, namely: (1) data acquisition, (2) data processing and (3) manufacturing ( Fig. 1 ). The exponential increase in power of computers has resulted in major advances in all of these areas. This is particularly exemplified by the recent introduction of intra-oral scanners of which there are now a number on the market ( Fig. 2 : Lava™ C.O.S from 3M, Trios™ from 3Shape and iTero™ from Cadent .) For example the Lava™ C.O.S. system captures approximately 20 3D data sets per second and models the data in real time . Thus it is possible to create a 3D model of the oral cavity directly with such a system, without the need to take an impression, pour a model and then digitize the model with one of the many laser scanner that are now available. The digital model can now be used to design the restoration and there are now many software packages available for the design of dental restorations such as crowns, bridges and partial denture frameworks. Some software providers are now able to claim that their partial denture software can survey, design and wax a partial denture framework in less than 20 min .
A further development in the CAD/CAM technologies used in dentistry is the transition from closed to open access systems. Whereas in the past the digitizing, designing and manufacturing came as a closed system (e.g. CEREC ® ), more and more the technology is being opened up and the component part of a CAD/CAM system can be purchased separately . This creates much greater flexibility in that data can be acquired from a range of sources (intra-oral scanner, contact or laser model digitizer, CT, MRI), appropriate design software can be matched to the object to be manufactured (e.g. crown and bridge frameworks, partial denture frameworks, customized implant and implant abutments). Another very important consequence of the transition from closed to open systems is that this opens up access to a much wider range of manufacturing techniques such that the most appropriate manufacturing processes and associated materials can be selected. Thus, one is no longer constrained by the computer numerically controlled machining technologies that are currently used in most dental CAD/CAM systems.
2.1
Subtractive manufacturing
If we look at where we are today then CAD/CAM in dentistry is primarily based around the process of subtractive manufacturing. The technology most people will be familiar with is computer numerically controlled machining, which is based on processes in which power-driven machine tools, such as saws, lathes, milling machines, and drill presses, are used with a sharp cutting tool to mechanically cut the material to achieve the desired geometry with all the steps controlled by a computer program. Thus one starts out with a block of the material and the machine cuts away the bits that are not wanted. It has been demonstrated that by using this method the overall production time will reduce considerably and complex models, which are otherwise difficult and/or impossible to make by the conventional dental processes could be build up rather easily. These technologies have achieved a high degree of sophistication with new technologies such as electrical discharge machining, electrochemical machining, electron beam machining, photochemical machining, and ultrasonic machining . Nowadays all these processing routes come under the umbrella of subtractive machining. However, as one might imagine this method of manufacturing is very wasteful as more material is removed compared to what is used in the final product. This can readily be seen from the example shown in Fig. 3 ( Fig. 3 : block of material used in the fabrication of a mandibular onlay).
In the aerospace industry there is currently much talk about the weight to flight ratio of an airplane. What this refers to is the weight of material that has to be used in relation to the weight of material in the final product such as an airplane. The aerospace industry uses expensive materials and is thus concerned about saving cost by reducing the weight to flight ratio. For example, a 1 kg reduction in weight can save $3000 in fuel per annum and this means potential savings in the longer term of many billions of dollars . Similarly this is a concern in the automotive industry. As a consequence of this drive for saving costs there has been a major transition from making parts by subtractive manufacturing to what is referred to as additive manufacturing. Using additive methods for manufacturing is more advantageous as many problems associated with milling can be readily overcome. The main advantage of this type of manufacturing is the ability of the technique to create fine detail such as undercuts, voids, and complex internal geometries. Another limitation of the current dental CAD–CAM systems is that the process does not easily lend itself to mass production such as crowns and bridges, since only one part can be machined at any one time.
2.2
Additive manufacturing
So what is additive manufacturing and what are its benefits? Additive manufacturing is defined by the American Society for Testing and Materials (ASTM) as:
The process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies.
In principle the process works by taking a 3D computer file and creating a series of cross-sectional slices. Each slice is then printed one on top of the other to create the 3D object. One attractive feature of this process is that there is no waste. Traditionally additive manufacturing processes started to be used in the 1980s to manufacture prototypes, models and casting patterns. Thus it has its origins in rapid prototyping (RP), which is the name given to the rapid production of models using additive layer manufacturing. Today additive manufacturing describes technologies that can be used anywhere throughout the product life cycle from pre-production (i.e. rapid prototyping) to full scale production (also known as rapid manufacturing) and even for tooling applications or post production customization. It is a remarkably rapidly changing field with a huge investment in developing enhanced manufacturing technologies and it is changing the way we make things. Today, additive manufacturing is used for a much wider range of applications and is even used to manufacture production-quality parts in relatively small numbers. Some sculptors use the technology to produce complex shapes for fine arts exhibitions .
Thus additive manufacturing is transitioning from rapid prototyping models to manufacturing real parts for use as final products. The equipment is becoming competitive with traditional manufacturing techniques in terms of price, speed, reliability, and cost of use. This, in turn, has led to the expansion of its use in industry and there has been explosive growth in the sales and distribution of the equipment. In addition a new industry is emerging to create software to enable more effective use of the technology. The use of the technology is likely to grow especially as it is now possible to purchase a 3D printer for less than $3500.00 . Consequently centers providing 3D printing services are springing up around the globe. One interesting development is RepRap, which is a desktop 3D printer able to print plastic objects. RepRap has been described as a self-replicating machine, since many parts are made from plastic and RepRap can print those parts .
Alongside these developments the number of materials that the industry uses has increased greatly and modern machines can utilize a broad array of polymers, metals and ceramics. As the industry makes the transition from prototypes to functional devices the materials available will begin to play a much bigger role. When producing a prototype it is enough for it to look good, but as we move to functional objects such as customized implants and oral prostheses the materials and their properties become much more important.
It is worth noting that the process of additive manufacturing is in fact ideally suited to dentistry, which has a tradition of producing customized parts made to fit the patient and not the other way around. This is a great opportunity for dentistry and there is already a huge array of additive manufacturing technologies that we can use and these include:
- •
Stereolithography (SLA).
- •
Fused deposition modeling (FDM).
- •
Selective electron beam melting (SEBM).
- •
Laser powder forming.
- •
Inkjet printing.
This list is by no mean exhaustive and every day something new is added. Below some of these technologies are described as to how they work and how they are or might be used in dentistry.
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