A review of additive manufacturing in conservative dentistry and endodontics part 1: basic principles

Search strategy

An extensive electronic search was performed of articles on PubMed from 1990 to 2017. Key words such as ‘Additive printing’, ‘3D-bioprinting’, ‘CAD CAM’, ‘Rapid prototyping’ and ‘Restorative dentistry’ were used alone or in combination to search the database. The option of ‘related articles’ was also utilized. Finally, a search was performed of the references of review articles and the most relevant clinical research papers.

Introduction

In recent times, the use of CAD/CAM in dentistry has been synonymous with the rapid production of dimensionally accurate prostheses thereby omitting tedious laboratory procedures. Most of the current methods using CAD/CAM fabrication techniques in dentistry have concentrated on milling from a solid block of material (ie subtractive manufacturing). However, this method of manufacturing comes with some inherent limitations such as:

Considering the limitations, in the near future one can expect a major transition from making prostheses by subtractive manufacturing to what is referred to as additive manufacturing. Additive manufacturing processes were traditionally used to make prototypes or models and thus it had its origin in rapid prototyping (RP).

Rapid prototyping (RP)

The first form of creating layer by layer three-dimensional objects using computer-aided design (CAD) was rapid prototyping, developed in the 1980s for creating models and prototype parts. Rapid prototyping is used to describe the customized production of solid models using 3-D computer data.² This technology was created to help the realization of what engineers have in mind. Rapid prototyping is one of the earlier additive manufacturing (AM) processes. It allows for the creation of printed parts, not just models. Among the major advances that this process facilitates are time and cost reduction, human interaction and, consequently, a shorter product development cycle and also the possibility to create almost any shape that could be very difficult to make by machine.³ It operates on the principle of depositing material in layers or slices to build a model (additive technique). The primary advantage of this process is that the model created directly retains all the details of the internal geometry rather than just the outer surface contours.⁴

Data used by RP machines can be obtained by medical imaging techniques such as computed tomography (CT), magnetic resonance imaging (MRI), or laser imaging scans. These digitized technologies allow parts of the subject to be serially recorded, which can then be 3-dimensionally rendered and manipulated to generate accurate models.⁵ Several studies have analysed the dimensional accuracy of anatomic models produced by rapid prototyping systems (Figure 1).^{6, 7, 8}

The basic methodology for all current rapid prototyping techniques

Summary of various RP techniques

Stereolithography (STL)

The STL file was created in 1987 by 3D Systems Inc when they first developed stereolithography, and the STL file stands for this term. It is also called Standard Tessellation Language. This is a liquid-based process that consists of curing or solidification of a photosensitive polymer when an ultraviolet laser makes contact with the resin. The process starts with a model in a CAD software and then it is translated to a STL file in which the pieces are ‘cut in slices’ containing the information for each layer. The thickness of each layer as well as the resolution depends on the equipment used. A platform is built to anchor the piece and support the overhanging structures. Then the UV laser is applied to the resin, solidifying specific locations of each layer. When the final layer is polymerized, the platform is lowered and the excess resin can be reused.^{9, 10, 11} A newer version of this process has been developed with a higher resolution and is called micro stereolithography, a process that means that a layer thickness of less than 10µm can be achieved.¹² The basic principle of this process is photo polymerization, which is the process where a liquid monomer or a polymer converts into a solidified polymer by applying ultraviolet light, which acts as a catalyst for the reactions. This process is also called ultraviolet curing. It is possible to have powders suspended in the liquid-like ceramics (Figure 2).¹³

Disadvantages

Errors can be introduced to the final piece from the process of stereolithography, like over curing, which occurs to overhanging parts.¹⁴

LASER scanning of the object has some inherent limitations, called scanned line errors, which lead to dimensional inaccuracies of the object being printed. These limitations include optically reflective surfaces, such as machined steel or aluminium, and often lead to second-order reflections (optical noise) superimposed on the true laser projections, making it difficult to analyse captured images. This is countered by applying an anti reflective coating before scanning, but it cannot be applied in every scenario. Scanning errors also occur due to inaccuracies in relative positions and orientations of LASER sensors and target surfaces, or the digitization of a scanned image where precision depends on resolution of the LASER being used (blue LASER being higher resolution than the red variant).¹⁵

The resin is a high-viscosity liquid, controlling the layer thickness is highly unpredictable and this introduces an error in the border position control, ie overhangs.¹⁴

Final surface finish is carried out manually, hence is vulnerable to human error.¹⁴

There is the possibility of using different materials while building a piece, through a process called multiple material stereolithography. In order to print with different materials, all previous material has to be drained and filled with the new material when the process reaches the layer where the change is going to take place. Hence, this process is tiresome and the software scheduling process has to be specified.¹⁶

Selective laser sintering (SLS)

This is a three-dimensional printing process in which a powder is sintered or is fused by the application of a carbon dioxide laser beam. The chamber is heated to almost the melting point of the material. The laser fuses the powder at a specific location for each layer specified by the design. The particles lie loosely in a bed, which is controlled by a piston, which is lowered the same amount of the layer thickness each time a layer is finished. This process offers a great variety of materials: plastics, metals, combination of metals, combinations of metals and polymers, and combinations of metals and ceramics (Figure 3).^12,17,18

Advantages

Wide range of materials that can be used;

Unused powder can be recycled.

Disadvantages

Accuracy is limited by the size of particles of the material;

The process needs to be executed in an inert gas chamber to prevent oxidation and to maintain constant temperature near the melting point of the metal;

Direct metal laser sintering process requires binder (eg polyvinyl alcohol) for adhering consecutive sintered particle layers,¹⁰ which can be later removed via heat application.

Fused deposition modelling

Fused deposition modelling (FDM) is an additive manufacturing process in which a thin filament of plastic feeds a machine where a print head melts it and extrudes it in a thickness typically of 0.25 mm. Materials used in this process are polycarbonate (PC), acrylonitrile butadiene styrene (ABS), etc (Figure 4).

Advantages

No chemical post-processing required;

No resin curing is involved hence polymerization overhangs and over curing is absent;

Availability of equipment and feeder material renders it a cost-effective process.^{9, 10}

Disadvantages

Resolution on the z-axis (depth) is low compared to other additive manufacturing processes (limit 0.25 mm); for a smooth surface a finishing process is required;

It is a slow process, sometimes taking days to build large complex parts. To save time, some models permit two modes; a fully dense mode and a sparse mode that saves time but compromises the mechanical properties.¹⁹

3D printing (3DP)

The 3DP process is a Massachusetts Institute of Technology licensed process in which a water-based liquid binder is supplied in a jet onto a starch-based powder to print the data from a CAD drawing. The powder particles lie in a powder bed and they are glued together when the binder is jetted. This process is called 3DP because of the similarity with the inkjet printing process that is used for two-dimensional printing on paper (Figure 5).

Advantages

This process can handle a high variety of polymers;^{10, 12}

Variation of this includes the polyjet range of printers from Objet^TM (a commercially available inkjet printing technology). They build the model, layer by layer by depositing droplets of various polymers (Figure 6) and, as each layer is formed, it is cured by UV light (as opposed to the use of binder).²⁰

In a study conducted by Ibrahim et al, the capacity of Selective Laser Sintering, 3D Printing and polyjet models to reproduce mandibular anatomy and their dimensional error was analysed.²⁰ The authors concluded that the SLS prototype had a greater dimensional accuracy than the PolyJet and 3DP models (Figure 6).

3D bioprinting

3D bioprinting systems can be laser-based, inkjet-based, or extrusion-based. The inkjet-based bioprinting is most common.²¹ This method deposits ‘bioink’ droplets of living cells or biomaterials onto a substrate according to digital instructions to reproduce human tissues or organs.²¹ Multiple print heads can be used to deposit different cell types (organ specific, blood vessel and muscle cells), a necessary feature for fabricating whole heterocellular tissues and organs.²¹

A process for bioprinting organs

Create a blueprint of an organ with its vascular architecture;

Generate a bioprinting process plan;

Isolate stem cells;

Differentiate the stem cells into organ-specific cells;

Prepare bioink reservoirs with organ-specific cells, blood vessel cells and support medium and load them into the printer;

Bioprint;

Place the bioprinted organ in a bioreactor prior to transplantation;²¹

LASER printers have also been employed in the cell printing process, in which laser energy is used to excite the cells in a particular pattern, providing spatial control of the cellular environment (Figure 7).²¹

Benefits of 3D printing

Challenges in traditional organ transplant protocols

Although still in its infancy, 3D bioprinting offers additional important advantages beyond this traditional regenerative method (which essentially provides scaffold support alone) such as, highly precise cell placement and high digital control of speed, resolution, cell concentration, drop volume and diameter of printed cells.^{21, 28}