Dr Andrew Dawood explains 3D printing and its uses and the options. Julian English looks his work and the applications for dental technicians.
As far back as 2015 Andrew Dawood and colleagues published a paper in the British Dental Journal. It looked at the applications of 3D printing in dentistry and talked about the different technologies therein.
At the time of publishing 3D printing was hailed as a disruptive technology – about to change production forever. The technology has a particular resonance with dentistry. Also, with advances in 3D imaging and modelling technologies such as cone beam computed tomography and intraoral scanning, and with the relatively long history of the use of CAD/CAM technologies in dentistry, it will become of increasing importance.
Uses of 3D printing include the production of drill guides for dental implants, the production of physical models for prosthodontics, orthodontics and surgery, the manufacture of dental, craniomaxillofacial and orthopaedic implants, and the fabrication of copings and frameworks for implant and dental restorations (Dawood et al, 2015).
3D printing technology
From a mechanical perspective, 3D printers are often quite simple robotic devices.
The apparatus would be nothing without the computer-aided design (CAD) software. This allows objects, and indeed whole assemblies to be designed in a virtual environment. CAD software is commonplace in industrial design, engineering, and manufacturing environments. It is also common in the dental laboratory; it is even becoming a feature of many dental surgeries (Dawood et al, 2015).
3D printing process
Computer Aided Design (CAD) software allows us to create objects from scratch. However, in dentistry and surgery we also have ready access to volumetric data in the form of computed tomography (CT) data, cone beam computed tomography (CBCT) data, and intraoral or laboratory optical surface scan data. These powerful technological tools are at the disposal of dental technicians.
Technicians often employ a broad level of creativity and an understanding of technology. This includes engineering and materials skills that extend well beyond that of many others working in individual fields of endeavour (Dawood et al, 2015).
Dental technicians know very well subtractive manufacturing in the form of milling. CAD/CAM for the milling of crown copings and bridge frameworks is common place. This technology facilitates the use of materials, which would otherwise be hard to work with. It eliminates labour intensive artisanal production techniques. This allows the dental technician to focus his manual skills on more creative aspects of the manufacturing process. For example the aesthetic layering of porcelain (Dawood et al, 2015).
Crown copings and partial denture frameworks
With the use of laboratory scanners, it is possible to develop a precise virtual model of the prepared tooth. Also, implant position, and the dental arch, says Dawood et al (2015). In fixed and removable prosthodontics, treatment may be planned and restorations designed in CAD software. This scan data and CAD design may be used to mill or print crown or bridge copings, implant abutments, and bridge structures.
3D printing may be harnessed for the fabrication of metal structures either indirectly by printing in burn-out resins or waxes for a lost-wax process, or directly in metals or metal alloys. The advantage of printing in resin/wax and then using a traditional casting approach is that there is much less post-processing involved than in the direct 3D printing of metals; casting alloys and facilities are also familiar and widely available.
Printing directly in metals requires the use of more costly technologies which have their own very specific health and safety requirements, and demand a great deal of post-processing before components may be ready for use (Dawood et al, 2015).
When printing elaborate implant bridge structures 3D printing may be used in conjunction with milling/machining technologies to produce a high precision mechanical connection to the implant – combining the best attributes of printing – complex geometry with little waste – with milling – high precision mechanical connecting surfaces.
While it may be somewhat wasteful in material, milling has the advantage that the material used is intrinsically homogeneous and unaffected by operating conditions. There is little need for post-processing, and the equipment is considerably less costly (Dawood et al, 2015).
3D printing technologies and materials
Steriolithography (SLA, SL): a stereolithography apparatus uses a scanning laser to build parts one layer at a time, in a vat of light-cured photopolymer resin. Each layer is traced-out by the laser on the surface of the liquid resin. At this point a ‘build platform’ descends, and another layer of resin is wiped over the surface, and the process repeated.
Supports must be generated in the CAD software, and printed to resist the wiping action and to resist gravity. They must later be removed from the finished product. Post-processing involves removal of excess resin and a hardening process in a UV oven.
The process is costly when used for large objects.
Photopolymer jetting (PPJ)
Inkjet print heads jet liquid photopolymers onto a build platform. The material is immediately cured by UV lamps and solidified which allows to build layers on top of each other.
Advantages:
- Multiple materials can be jetted together allowing multi-material and multi-colour parts
- Functionally graded materials are possible
- Multi-material and/or multi-colour parts
- Can achieve good accuracy and surface finishes.
Disadvantages:
- Does not work with standard materials but with UV-active photopolymers which are not durable over time (thermoset)
- Works with UV-active photopolymers. Therefore, parts are not durable over time and have limited mechanical properties.
A variety of materials may be printed including resins and waxes for casting, as well as some silicone-like rubber materials. Complex geometry and very fine detail is possible, as little as 16 microns resolution.
Powder binder printers (PBP)
Binder jet 3D printing is known variously as “Powder bed and inkjet” and “drop-on-powder” printing. It is a rapid prototyping and additive manufacturing technology for making objects described by digital data such as a CAD file. Binder jetting is one of the seven categories of additive manufacturing processes according to ASTM and ISO.
A model is built up in layers as the powder bed drops incrementally, and a new fine layer of powder is swept over the surface. The model is supported by un-infiltrated powder, and so no support material is required. Post-processing to infiltrate the delicate printed model with a cyanoacrylate or epoxy resin will improve strength and surface hardness (Dawood et al, 2015).
The resulting models are useful as study models or visual prototypes, but accuracy is limited and the models are rather fragile despite the post-processing. A particular excitement of this technology lies in its ability to print models in full colour; from a surgical perspective the drawback is that the models may not be sterilised or directly manipulated at operation.
Accuracy is inadequate for prosthodontic applications. The machines and materials cost less, but still not inexpensive. As the material is mostly plaster of Paris, there is some compatibility with having the apparatus situated in a dental laboratory plaster room (Dawood et al, 2015).
Selective laser sintering (SLS)
Selective laser sintering (SLS) is an additive manufacturing (AM) technique. It uses a laser as the power source to sinter powdered material (typically nylon or polyamide), aiming the laser automatically at points in space defined by a 3D model, binding the material together to create a solid structure. It is similar to selective laser melting; the two are instantiations of the same concept but differ in technical details. SLS (as well as the other mentioned AM techniques) is a relatively new technology that so far has mainly been used for rapid prototyping and for low-volume production of component parts.
Materials available include nylon, which is perhaps the most versatile, flexible elastomeric materials, and metal-containing nylon mixtures. An interesting possibility for medical implants is the use of polyether ether ketone (PEEK). Although, this requires high temperatures and complex control – and a great deal of wastage.
There are a broad range of metals and metal alloys available including titanium, titanium alloys, cobalt chrome alloys, and stainless steel. 3D printed partial dentures and prosthesis frameworks are already being made in this way. For implant bridge frameworks, technology may be combined with milling processes to provide high precision connections. The technology is broadly the same as that described for polymers above. Although, these apparatus may also be described by different manufacturers as, ‘selective laser melting’, or ‘direct metal laser sintering’ (Wikipedia).
Fused deposition modelling (FDM)
Fused filament fabrication (FFF), also known as fused deposition modelling (with the trademarked acronym FDM), or called filament freeform fabrication, is a 3D printing process that uses a continuous filament of a thermoplastic material. Filament is fed from a large spool through a moving, heated printer extruder head, and is deposited on the growing work.
The print head is moved under computer control to define the printed shape. Usually the head moves in two dimensions to deposit one horizontal plane, or layer, at a time; the work or the print head is then moved vertically by a small amount to begin a new layer.
The speed of the extruder head may also be controlled to stop and start deposition and form an interrupted plane without stringing or dribbling between sections. “Fused filament fabrication” was coined by the members of the RepRap project to give a phrase that would be legally unconstrained in its use, under the mistaken belief that a trademark protected the term “fused deposition modelling”.
Fused filament printing is now the most popular process (by number of machines) for hobbyist-grade 3D printing.[3] Other techniques such as photopolymerisation and powder sintering may offer better results, but they are much more costly (Wikipedia).
This is the process that most low cost ‘home’ 3D printers use. It allows for the printing of crude anatomical models without too much complexity. For example, printing an edentulous mandible might be possible. Although, printing a detailed maxilla would be a tall order. More costly, more accurate FDM printers are available. They have application in anatomical study-model making, but little else in dentistry or in surgery.
Conclusion
3D imaging and modelling, and CAD technologies are hugely impacting on all aspects of dentistry. 3D printing makes it possible to accurately make one-off, complex geometrical forms from this digital data, in a variety of materials, locally or in industrial centres. Even now, you can make nearly everything for your patients using a 3D printer. Although, no single technology is sufficient for all our patient’s needs.
The technology is already widely used in orthodontics. High-resolution printing in resin is already an entirely practical proposition. Similar technology is being used to print models for restorative dentistry and patterns for the lost wax process. This is becoming increasingly important with the rise of intraoral scanning systems.
In maxillofacial and implant surgery, it is becoming commonplace and prerequisite to use anatomical models made by any number of different 3D printing techniques to assist with the planning of complex treatments. It is widely acknowledged that surgery is less invasive and more predictable with the use of surgical guides printed in resins (commonly) or autoclavable nylon.
For many, the real excitement will be in the direct production of metal-based restorations for implants and teeth. This is yet to become routine in the dental laboratory in the UK.
Although 3D printers are becoming more affordable, you must carefully consider the cost of running, materials, maintenance, and the need for skilled operators. As well as the need for post-processing and adherence to strict health and safety protocols. Despite these concerns it is clear that 3D printing will have an increasingly important role to play in dentistry.
The congruence of scanning, visualisation, CAD, milling and 3D printing technologies, along with the professions innate curiosity and creativity makes this an exceptionally exciting time to be in dentistry (Dawood et al, 2015).
Reference
Dawood A, Marti B, Sauret-Jackson V et al (2015) 3D printing in dentistry. Br Dent J 219: 521–529. https://doi.org/10.1038/sj.bdj.2015.914
This article first appeared in Laboratory magazine. You can read the latest issue here.
