Dental News - A guide to evidence-based clinical evaluation of dental 3D printing

Dr Elisa Praderi

Thu. 2. February 2023

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Technologies are producing a paradigm shift in all areas of healthcare. Dentistry has seen significant changes in this context. Alongside new patient digitalisation tools, manufacturing processes and materials are evolving to such an extent that they are changing the way we understand new data and redefining how we approach our workflows. The fast-paced development of these new dental materials complicates the decision-making process for dental professionals, how research is being conducted and how fast evidence is being produced to back up these new solutions and protocols. This article provides some guidelines for dental professionals and educational institutions to understand, adopt and shape the future of digital dentistry using 3D printing.

Taking an evidence-based approach to digital dentistry

Forward-looking dental professionals who seek detailed results on newly available 3D-printing materials may find themselves disappointed if they ask the same questions they would for traditional materials. Instead, they should consider the following avenues of enquiry when evaluating a new 3D-printing solution.

Compare manufacturing options and categories

Understanding new materials starts with understanding the different manufacturing methods for which they are used. Here, three key areas of distinction are presented in increasing order of specificity.

Comparing conventional and digital manufacturing

Today, the digital clinical process includes four major steps: patient digitalisation (via intra-oral scanning, CBCT, photography, and static and dynamic occlusion), design (via CAD software), manufacturing (milling or 3D printing) and final steps for delivery of the appliance (thermoforming or other). Just like we understand the traditional workflow, we need to understand how the intra-oral scanning techniques and CAD can affect the printed outcome.

Some aspects remain the same. For instance, the consequences of not following protocols and manufacturer’s instructions are the same for both traditional and digital workflows: a poor intra-oral scan will lead to a defective virtual model, just like a poor impression will yield an inaccurate model.

Figs. 1a & b: Surface analysis of occlusal splint (a) and surgical guide (b).

Some aspects are different. With the digital workflow, precision and accuracy can be quantitatively measured, such as via surface scanning and comparison with the digital impression (Fig. 1). This is fundamental for predicting how parts will adapt intra-orally and has the added benefit of peace of mind for both clinician and patient that the prostheses and appliances are suitably accurate for the patient’s condition.

Additionally, the timing of the delivery of digital products may have implications for the patient’s appointment schedule. For example, for a complete denture, the digital workflow requires half the number of appointments compared with traditional workflows. Moreover, in case of loss or fracture, a new denture can be easily refabricated using the digital files and only a seating appointment is required.

Comparing milling and 3D printing

Milling has been used in the dental profession for longer than 3D printing. The most significant difference between the two technologies is how they are conducted. Milling, or subtractive manufacturing, begins with a solid block of material (such as PMMA or zirconia) and uses cutting tools to remove material until the final shape is achieved. In contrast, 3D printing, or additive manufacturing, uses various processes to build up the raw material into its final shape, one layer at a time. Dental 3D printers typically use a viscous liquid resin that reacts with light to become solid, using a laser or other light source to selectively expose the material to light and polymerise it in place.

Given the difference in the nature of the manufacturing processes, the design parameters to bear in mind are different. In milling, we acknowledge limitations such as milling radius compensation during the design and preparation of our restoration or splint for its specific output. In 3D printing, there are fewer design limitations, but file preparation for manufacturing requires more attention, such as the orientation of the part relative to the printer (Fig. 2) and the use of supporting structures to hold the part securely during the printing process (Fig. 3).

The costs of operation and maintenance of each manufacturing option are relevant when analysing investment. In milling, it is important to acknowledge the total waste generated in production, as well as the long-term maintenance costs. In printing, the waste is reduced owing to the nature of the manufacturing process, but some equipment requires nearly as much maintenance as milling machines, and consumables like resin tanks can be expensive.

Lastly, the materials available and developed for each manufacturing system and clinical indication are important to analyse and understand, as well as the clinical scenarios for which dental professionals will opt for one technology or the other.

Fig. 2: Design of a model for clear aligner and retainer production printed horizontally with the base directly on the build platform or vertically with supports.

Fig. 3: Design of an occlusal splint with supporting structures.

Comparing 3D-printing technologies

As mentioned, most dental 3D printers use a liquid resin in a technology called vat photopolymerisation, but several other technologies exist, including powder bed fusion (such as selective laser sintering), material extrusion (such as fused deposition modelling) and material jetting. Within the category of vat photopolymerisation are the techniques of low-force stereolithography, stereolithography, digital light processing and liquid crystal display (Fig. 4). When choosing a 3D-printing technology, it is important to consider several aspects, the most important of which are accuracy, production time and material availability.

Accuracy can be measured in multiple ways. Trueness and precision refer to how closely the produced part matches the digital file. This can be achieved through a combination of equipment specifications, such as resolution in the x- and y-axes, layer height (resolution in the z-axis) and laser spot size. Post-processing methods, such as post-polymerising the printed part, can also impact accuracy.

With the fast pace of clinical dentistry, timing is critical. However, printing time is only one step of the process. File preparation, post-processing and final clean-up of parts may be more time-consuming, and crucially, these are the steps that take up valuable technician or assistant time. Be sure to evaluate the entire production time, not just the printing speed, when looking at production speed.

Fig. 4: Different 3D-printing categories. SLA=stereolithography; DLP=digital light processing; LFS=low-force stereolithography.

Fig. 5: Complete denture printed in Denture Base Resin and Denture Teeth Resin (both Formlabs) in 50 µm.

Fig. 6: Restorative model printed in Model Resin (V3, Formlabs) in 50 µm.

Lastly, material availability varies. Consider whether it is more important to you to have highly versatile equipment or to have equipment that is specialised for a certain use case. Research your intended use case and look for material availability, often categorised by whether it is biocompatible or not—if biocompatible, whether it is Class I or Class IIa—and the intended application. It is also important to look into certifications or product approvals (U.S. Food and Drug Administration, EU medical device regulations, and others) and the implied suitability for the intended application (Figs. 5 & 6).

Carefully read and follow protocols

In the absence of in vivo results, following the right protocols according to the manufacturer’s instructions is key (Fig. 7). Manufacturers conduct testing according to International Organization for Standardization (ISO) standards to guarantee that the mechanical and biocompatible properties of parts made with their materials are suitable for their intended application, and they publish these protocols in their instructions for use documents. Following these protocols precisely is mandatory to ensure biocompatibility and optimal performance of the printed part.

Many published research studies have explored how the mechanical properties of a printed part can vary when the post-processing steps are altered, but it is important to bear in mind that altering these parameters means that the protocol provided in the instructions is not being followed and safety and biocompatibility could thus be at stake.

Fig. 7: Following the right protocols according to the manufacturer’s instructions is key. IPA=isopropyl alcohol.

Consider in vitro and technical data

Randomised controlled clinical trials and systematic reviews provide the strongest evidence, but to generate these, we need one fundamental thing: time. In light of the fast-paced development of new dental materials, simulated in vitro data is extremely helpful and a good starting point for understanding material performance. Manufacturers of dental materials need to understand that conducting in vitro studies will help build preliminary trust of materials. These need to be focused on answering key questions, such as how long this material will last, what the cementation protocol of these new materials is, and what clinical considerations need to be taken into account based on this material.

ISO standards are a valuable resource when analysing technical data sheets. The introduction of these specifications is beneficial to the dental community, as we have benchmarks and standardised tests available to measure the performance and quality of dental materials. As mentioned in Phillips’ Science of Dental Materials: “Dentists are provided with criteria for selection that are impartial and reliable. Awareness by dental laboratory technicians and dentists of the requirements of these specifications is essential in recognising the limitations of the dental materials with which they are working.” Current and future generations need to be instructed on ISO standards as part of traditional dental education to guide their clinical decisions. Moreover, it is an opportunity to test and define if these standards are enough or need to be updated in order to cover any upcoming new categories of materials.

Manufacturers of 3D-printing materials publish safety data sheets and often marketing brochures that may contain additional technical data. Be sure to evaluate these documents carefully to glean important information about how the material has performed against ISO standards, to gain important insights into material performance in the absence of in vivo research.

Implications for research

Of course, evidence cannot end with in vitro data. On the contrary, these are the foundations and evidence for institutions to start conducting clinical studies. The industry and research institutions need to work closely to help build this data and start executing in vivo studies as soon as the material is available. New research strategies will need to be developed in order to produce relevant and significant clinical evidence in an appropriate time frame. Since new 3D-printing technologies and materials are introduced to the market so quickly—multiple new innovations are introduced commercially every year—clinicians and researchers need to be creative in their strategies to deliver useful clinical results before the products they are testing become obsolete. For instance, researchers can conduct multicentre studies to maximise the sample size of tests using the same protocol. Forward-looking educational institutions should also consider more collaborations with industry to rigorously test the next generation of materials while they are still in development.

3D printing is not the future—it is the present

3D printing is here, and clinicians, researchers and manufacturers are pushing forward the capabilities of this technology together. It is time for the dental community as a whole to start understanding and learning about this technology and adapting to this fast pace of innovation if we want to be able to benefit from the advantages that technologies bring while providing safe treatment options to patients. Expanding dental materials knowledge to include these new procedures and protocols while promoting more cross collaboration among research institutions worldwide is what will allow the dental profession to continue to innovate and thrive using up-and-coming technology.

Editorial note:

This article was published in 3D printing—international magazine of dental printing technology, issue 2/2022.