References

Van Noort R. The future of dental devices is digital. Dent Mater. 2012; 28:3-12
Wu G, Zhou B, Bi Y, Zhao Y. Selective laser sintering technology for customized fabrication of facial prostheses. J Prosthet Dent. 2008; 100:56-60
Kaufui V, Wong KV, Hernandez A. A review of additive manufacturing. Mech Eng. 2012;
Zemnick C, Woodhouse SA, Gewanter RM, Raphael M, Piro JD. Rapid prototyping technique for creating a radiation shield. J Prosthet Dent. 2007; 97:236-241
Barker TM, Earwaker WJ, Lisle DA. Accuracy of stereolithographic models of human anatomy. J Med Imaging Radiat Oncol. 1994; 38:106-111
Choi JY, Choi JH, Kim NK, Kim Y, Lee JK, Kim MK, Lee JH, Kim MJ. Analysis of errors in medical rapid prototyping models. Int J Oral Maxillofac Surg. 2002; 31:23-32
Bill JS, Reuther JF, Dittmann W, Kübler N, Meier JL, Pistner H, Wittenberg G. Stereolithography in oral and maxillofacial operation planning. Int J Oral Maxillofac Surg. 1995; 24:98-103
Mankovich NJ, Samson D, Pratt W, Lew D, Beumer J. Surgical planning using three-dimensional imaging and computer modeling. Otolaryngol Clin North Am. 1994; 27:875-889
Sood AK, Ohdar RK, Mahapatra SS. Parametric appraisal of mechanical property of fused deposition modelling processed parts. Mater Des. 2010; 31:287-295
Cooper KG. Rapid Prototyping Technology: Selection and Application.USA: CRC Press (Taylor & Francis Group); 2001
Kruth JP. Material incress manufacturing by rapid prototyping techniques. CIRP Annals (Elsevier). 1991; 40:603-614
Halloran JW, Tomeckova V, Gentry S, Das S, Cilino P, Yuan D Photopolymerization of powder suspensions for shaping ceramics. J Eur Ceram Soc. 2011; 31:2613-2619
Pham DT, Ji C. A study of recoating in stereolithography. Proc Inst Mech Eng C J Mech Eng Sci. 2003; 217:105-117
Kim H, Choi JW, Wicker R. Scheduling and process planning for multiple material stereolithography. Rapid Prototyp J. 2010; 16:232-240
Isheil A, Gonnet JP, Joannic D, Fontaine JF. Systematic error correction of a 3D laser scanning measurement device. Opt Lasers Eng. 2011; 49:16-24
Szilvśi-Nagy M, Matyasi GY. Analysis of STL files. Math Comput Model. 2003; 38:945-960
Tang HH, Chiu ML, Yen HC. Slurry-based selective laser sintering of polymer-coated ceramic powders to fabricate high strength alumina parts. J Eur Ceram Soc. 2011; 31:1383-1388
Salmoria GV, Paggi RA, Lago A, Beal VE. Microstructural and mechanical characterization of PA12/MWCNTs nanocomposite manufactured by selective laser sintering. Polym Test. 2011; 30:611-615
Morvan S, Hochsmann R, Sakamoto M. ProMetal RCT (TM) process for fabrication of complex sand molds and sand cores. Rapid Prototyp. 2005; 11:1-7
Ibrahim D, Broilo TL, Heitz C, de Oliveira MG, de Oliveira HW, Nobre SM Dimensional error of selective laser sintering, three-dimensional printing and PolyJet™ models in the reproduction of mandibular anatomy. J Craniomaxillofac Surg. 2009; 37:167-173
Ozbolat IT, Yu Y. Bioprinting toward organ fabrication: challenges and future trends. IEEE Trans Biomed Eng. 2013; 60:691-699
Banks J. Adding value in additive manufacturing: researchers in the United Kingdom and Europe look to 3D printing for customization. IEEE Pulse. 2013; 4:22-26
Mertz L. Dream it, design it, print it in 3-D: what can 3-D printing do for you?. IEEE Pulse. 2013; 4:15-21
Ursan ID, Chiu L, Pierce A. Three-dimensional drug printing: a structured review. J Am Pharm Assoc. 2013; 53:136-144
Schubert C, Van Langeveld MC, Donoso LA. Innovations in 3D printing: a 3D overview from optics to organs. Br J Ophthalmol. 2014; 98:159-161
Gross BC, Erkal JL, Lockwood SY, Chen C, Spence DM. Evaluation of 3D printing and its potential impact on biotechnology and the chemical sciences. ACS Publications Anal Chem. 2014; 86:3240-3253
Wong JY, Pfahnl AC. 3D printing of surgical instruments for long-duration space missions. Aviat Space Environ Med. 2014; 85:758-763
Cui X, Boland T, D'Lima DD, Lotz MK. Thermal inkjet printing in tissue engineering and regenerative medicine. Recent Pat Drug Deliv Formul. 2012; 6:149-155

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

From Volume 46, Issue 2, February 2019 | Pages 125-132

Authors

Peddi Shanmukh Srinivas

Postgraduate Student, Department of Conservative Dentistry and Endodontics, JSS Dental College and Hospital, Jagadguru Shree Shivaratheeshwara University, Mysuru, India

Articles by Peddi Shanmukh Srinivas

TS Ashwini

Postgraduate Student, Department of Conservative Dentistry and Endodontics, Maratha Mandal's Nathajirao G Halgekar Institute of Dental Sciences, Belgaum, Karnataka, India

Articles by TS Ashwini

MG Paras

Faculty, Department of Conservative Dentistry and Endodontics, JSS Dental College and Hospital, Jagadguru Shree Shivaratheeshwara University, Mysuru, India

Articles by MG Paras

Abstract

Abstract: The field of science and research is dynamic and the scientific discipline of restorative dentistry and endodontics is no exception. The practice of dentistry and the technology involved has evolved tremendously from the traditional to the contemporary. As a result of continual developments in technology, newer cutting edge methods in production and treatment have evolved. This paper explores the scope of additive manufacturing technology in restorative dentistry and endodontics, progress achieved in this field, practicality hurdles, and a promising future that this technology might provide if harnessed to its full potential.

CPD/Clinical Relevance: This paper gives an update on current concepts of additive manufacturing being employed in the field of restorative dentistry and endodontics for clinical practice, academic progress and translational research.

Article

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:

  • Economically it's a wasteful process as more material is removed compared to what is used in the final product;
  • It's time consuming, as only one prosthesis can be machined by a milling unit at any given time compared to a 3D printer which can print multiple custom prostheses in one production cycle.1
  • 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

    Figure 1. Schematics of 3D printing.

    Literature on dimensional accuracy of RP

    Barker et al compared measurements between anatomic landmarks on a dry skull with an anatomic replica and noted an absolute mean deviation of 0.85 mm.5 Choi et al found the absolute mean deviation between 16 linear measurements made on a dry skull and an RP model to be 0.62 mm.6 Bill et al determined that an accuracy of 0.5 mm can be reached for anatomic models based on CT data.7

    The basic methodology for all current rapid prototyping techniques

  • A CAD model is constructed which is then converted to STL (standard tessellation language) format;
  • The RP machine processes the STL file by creating sliced layers of the model;
  • The first layer of the physical model is created. The model is then lowered by the thickness of the next layer, and the process is repeated until completion of the model;
  • Supporting structures are removed from the final model, followed by surface cleaning of the model.
  • 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

    Figure 2. Stereolithography.

    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

    Figure 3. Selective LASER sintering.

    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).

    Figure 4. Fused deposition modelling.

    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).

    Figure 5. 3D Inkjet printing.

    Advantages

  • This process can handle a high variety of polymers;10, 12
  • Variation of this includes the polyjet range of printers from ObjetTM (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
  • Figure 6. 3D Polyjet printing.

    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
  • Figure 7. 3D Bioprinting.

    Benefits of 3D printing

    Customization and personalization

    The greatest advantage that 3D printers provide in medical applications is the freedom to produce custom-made medical products, equipment and treatment planning such as:

  • Facial prosthetics;22
  • 3D printed customized made-to-order jigs and fixtures for use in operating rooms;23
  • Custom-made implants, fixtures, and surgical tools can have a positive impact in terms of the time required for surgery patient recovery time, and the success of the surgery or implant.23
  • It is also anticipated that 3D printing technologies will eventually allow drug dosage forms, release profiles and dispensing to be customized for each patient.24
  • Increased cost efficiency

    3D printing offers the ability to design and manufacture items cheaply.25 Traditional manufacturing methods remain less expensive for large-scale production, however, the cost of 3D printing is becoming increasingly competitive for small production runs.25 This is especially true for small-sized standard implants or prosthetics, such as those used for spinal, dental or craniofacial disorders.22The cost to custom-print a 3D object is minimal, with the first item being as inexpensive as the last.25 This is especially advantageous for companies that have low production volumes or that produce parts or products that are highly complex or require frequent modifications.23 3D printing can also reduce manufacturing costs by decreasing the use of unnecessary resources.24 For example, a pharmaceutical tablet weighing 10 mg could potentially be custom-fabricated on demand as a 1-mg tablet.24

    Enhanced productivity

    ‘Fast’ in 3D printing means that a product can be made within several hours.23 That makes 3D printing technology much faster than traditional methods of production, which require milling, forging and a long delivery time.22 In addition to speed, 3D printing is also holistically improving in resolution, accuracy, reliability and repeatability.22

    Democratization and collaboration

    Another beneficial feature offered by 3D printing is the democratization of the design and manufacturing of goods. An increasing array of materials are becoming available for use in 3D printing, at reduced price.23 This allows people from various sectors to employ 3D printing with imagination to design and produce novel products for personal or commercial use.23 The nature of 3D printing data files also offers an unprecedented opportunity for sharing among researchers.26 Researchers can standardize their methodology through access to downloadable ‘.stl’ files that are available in open-source databases.26 By doing so, they can use a 3D printer to create an exact replica of a medical model or device, allowing the precise sharing of designs.26 Hence, 3D printing is instrumental in international scientific collaboration and, towards this end, the National Institutes of Health established the 3D Print Exchange (3dprint.nih.gov) in 2014 to promote open-source sharing of 3D print files.

    Bioprinting tissues and organs

    Tissue or organ failure due to ageing, diseases, accidents and birth defects is a critical medical problem.27

    Challenges in traditional organ transplant protocols

  • Current treatment for organ failure relies mostly on organ transplants from living or deceased donors.28 However, there is a chronic shortage of human organs available for transplant.25, 28
  • Organ transplant surgery and follow-up is also expensive, costing more than $300 billion in 2012.28
  • An additional problem is that organ transplantation involves the often difficult task of finding a donor who is a tissue match.25 This problem is likely to be eliminated by using cells taken from the organ transplant patient's own body to build a replacement organ.21, 25 This would minimize the risk of tissue rejection, as well as the need to take lifelong immunosuppressants.21, 25
  • 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