Gypsum models vs 3D models that can be printed: characteristics and accuracy factors compared
3D printing is an additive process (AM – Additive Manufacturing) that enables the physical construction of digital models by printing layer upon layer.
It was invented in 1986 when Charles Hull filed the first patent for stereolithography (SLA), which consisted in creating objects in layers by solidifying liquid resin sensitive to the wavelength of ultraviolet light (400 nm). The layers are thus printed one on top of the other to create the 3D object.
Since then, the layered printing process has evolved with different techniques and a plethora of increasing varied and high-performance materials[2,3].
3D printing in the dental sector
With the advent of digital technology and intraoral scanners, 3D printing started to become more widely-utilised in dentistry with a number of uses. Of these, we can certainly recognise the construction of dental models as one of the main applications, in which it is essential for obtaining a physical model from an intraoral scanner STL file.
In the past, preparing gypsum models cast using dental impressions made with specific materials (polyvinylsiloxanes, condensation silicones, alginates, polyethers, polysulphides) was the only way to obtain a positive physical model of the patient’s arches[6,7].
The dental technician would then use these analogue models to build the prosthetic framework or the orthodontic/gnathological devices that were subsequently fitted in the patient’s mouth.
Gypsum models and models that can be printed
The workflow described above is still valid and is still the gold standard for a number of clinical applications[9,10].
As a matter of fact, while intraoral scanners have achieved a level of accuracy and precision that almost matches, in some cases, those of impression materials, the same cannot be said of 3D printed models, which are generally less accurate than gypsum models[11,12].
Several factors influence the accuracy of 3D printed models and, to date, many of them are still in the study phase.
Errors and accuracy factors of 3D models
The errors introduced in 3D printing can result from:
- digital data acquisition;
- the processing of the images of the hard and soft tissues of the oral cavity;
- the myriad of printing parameters;
- the post-processing that is carried out on each printed object .
Indeed, various scientific papers state that the thickness or depth of the layer of resin to be printed, the spacing between the various curing sections, the power of the light, the translucency of the colour of the resin used and the magnitude of overcuring, strongly influence the accuracy of 3D printing[13,14].
The angle of construction of the object, its geometry and the parameter settings of the supports, which are always necessary to prevent the deformation of the objects being printed, are also important factors.
Furthermore, the models acquired through the curing of liquid resin are all subject to contraction during the curing phase[16,17].
The contraction of 3D printing resins is usually significant, given the almost total absence of fillers that is necessary to allow the resin to flow easily between the base of the platform and the bottom of the tray with each printing cycle. The magnitude of the contraction is always a function of the amount of resin used.
As a matter of fact, one study on 3D printed dental models concluded that 3D dental models with a hollow base design are more accurate than those with a honeycomb or solid design.
The scarcity of fillers in these resins also affects the mechanical properties of the object, which are almost solely determined by the structural rigidity of the monomer composing the resin[18,20,21].
3D printing post-processing
Any 3D printed object then needs to be further processed in order to be usable.
Indeed, immediately after the initial printing, the object will still be part-cured and with rough surfaces covered in semisolid material residues.
The post-processing steps therefore consist in:
- physically removing the printed device from the construction platform;
- cleaning the surface of the object and removing the uncured resin by immersing it in an organic solvent such as isopropyl alcohol (IPA);
- the final curing step to complete the curing of the device using a UV ray machine;
- removing the supporting structures using a cutting device, a diamond disc or an ultrasonic tip.
During these procedures, the 3D printed object may be improperly handled by the dental technician and undergo changes that could affect both its accuracy and final mechanical properties.
One solution to this problem was provided by Dentsply Sirona with PrimePrint[22,23] in which the automation of post-processing allows the technician/practitioner to handle the object only once it is completely cured, in order to remove the supports.
Accuracy factors of gypsum models
With respect to all the variables described above, the production of gypsum models is certainly more standardised, although their accuracy is affected by factors such as[24,25]:
- the proportions of water/powder used;
- the mixing of the gypsum (automatic under vacuum or manual);
- the contact time between the impression material and the gypsum;
- the exposure of the gypsum to temperature and humidity conditions that are such to determine the conversion of sulphate bihydrate into hemihydrate.
- the thickness of the light impression material, which has lower resistance during the volumetric expansion of the gypsum.
The costs of the different models
A direct comparison between the two types of models must, of course, necessarily also consider their costs.
These are some of the main disadvantages of 3D printing technologies that, compared to the analogue techniques (gypsum) still have very high costs, not only with regard to the machines/printers, but also with regard to the materials (resins in particular).
Even amongst the CAD-CAM technologies, making a rapid comparison with classic numerical control (CNC) milling, although 3D printing is certainly the most promising technology, it is still very expensive.
It is also important to consider the initial cost of the software and the learning curve for mastering the user of the slicer (the function used by the software to convert an STL file into the 3D printing specifications in order to produce the object, including its position in the construction platform, the direction of printing and the supports).
In this sense, the use of gypsum material is very simple and inexpensive, relinable, and dimensionally stable even for long periods of time[26,27], another characteristic in respect of which 3D resins have significant limits, especially three weeks after printing.
The clinical application of analogue and 3D models
Amongst the various models that can be produced with 3D printing or gypsum it is also important to consider their clinical applications; dental models for orthodontics can have discrepancy levels of up to 500 µm compared to the corresponding STL file.
Values that are decidedly unacceptable when fabricating restorations on natural teeth or implants[29,30]. And, considering the differences in terms of the accuracy and precision of the 3D printers on the market, it is fundamental to know and choose those that are best suited to the clinical requirements.
However promising and interesting these new technologies may be from the clinical application perspective, at the current time (2022), the traditional analogue workflow involving the fabrication of gypsum models is still the gold standard because it is predictable and has been clinically validated for many years.
It will undoubtedly be interesting to see whether and how this might change in the future with the evolution of increasingly high-performance technologies.
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