REDEFINING PROSTHODONTICS WITH 3D PRINTING

Dentistry is amidst a digital revolution and patients are the definitive recipients of these innovative technological advancements. Three-dimensional (3D) printing is no more considered the future, but isthe reality for daily clinical practice. The term 3D printing, additionally referred as rapid prototyping, is commonly used to depict an additive manufacturing method which adds numerous layers under computerized control in order to create a three-dimensional object. Using this procedure, 3-Dimensional printed restorations, crowns, bridges, surgical guides and implants can be manufactured rapidly with extreme accuracy and precision. The benefits of this innovative technique exceed its drawbacks. 3D printing has prompted a change in digital dentistry with its broad learning, penetrating opportunities and a wide scope of applications. This article will facilitate an understanding of the digital workflow, methods and current uses of 3D printing in prosthetic dentistry.

Dentistry is amidst a digital revolution and patients are the definitive recipients of these innovative technological advancements. Threedimensional (3D) printing is no more considered 'the future', but is'the reality' for daily clinical practice. The term '3D printing', additionally referred as 'rapid prototyping', is commonly used to depict an additive manufacturing method which adds numerous layers under computerized control in order to create a three-dimensional object. Using this procedure, 3-Dimensional printed restorations, crowns, bridges, surgical guides and implants can be manufactured rapidly with extreme accuracy and precision. The benefits of this innovative technique exceed its drawbacks. 3D printing has prompted a change in digital dentistry with it's broad learning, penetrating opportunities and a wide scope of applications. This article will facilitate an understanding of the digital workflow, methods and current uses of 3D printing in prosthetic dentistry.
There has been a long relationship of dentistry with subtractive manufacturing-commonly referred as "milling". In subtractive manufacturing, a solid block of material is cut successively in order to construct a 3D object (Azari and Nikzad, 2009). This innovative technology utilizes power driven machines to precisely cut a block of material into an ideal shape and geometry. However, subtractive manufacturing carries with it disadvantage of increased wastage due to removal of excess material and failure of mass production (Beuer et al., 2008).
Rapid prototyping (RP) is a great transformative technology that has advanced swiftly in different fields of dentistry since it has the ability to overcome the known limitations of the subtractive techniques (Wang and Shaw, 2006) .Through this technique, the clinicians are able to create physical objects out of liquid resins in a futuristic The harmonious use of scanning, visualization, CAD, milling and 3D printing, along with professional creativity has resulted in new approaches to manufacture dental restorations (Jain et al, 2016). It is promising to view these innovations, however the current need is-assessment of printing accuracy, evaluation of properties of materials utilized, and extensive clinical use (Ciobota, 2012), (Alharbi et al., 2017). Digital Workflow:-Firstly digital acquisition of the data is obtained through the use of an "Intraoral Scanner" which consists of -a mini camera, integrated software, and a computer. The Intraoral Scanner possesses extreme precision and after capturing the 3D images of the object, stores the data acquired as STL (Standard Tessellation Language) file. This data is encoded by the tessellation process which involves the stitching of these files ( Revilla and Özcan, 2019). After the desired support structures are designed through the software, "slicing" of the structure is done in order to create a stack of layers. The sliced data is then sent to the printer, where successive layers of material are deposited to create a three dimensional object by the 3D printing process (Dawood et al., 2015). The post-processing of the printed object is then achieved which involves -removal of support material, sandblasting and jet-washing. This essential workflow is employed for various 3D printing processes, incorporating the use of materials such as polymers, metals or ceramics (Dawood et al., 2015).

3D Printing Technologies:-
Additive manufacturing includes multiple fabricating techniques which differ with respect to the material and the technology utilized in the manufacture of components (Prakash et al., 2018). The most widely used additive manufacturing methods are as follows: Stereolithography (SLA) is a method which involves curing of thin layers of photopolymer resins such as, epoxy and acrylic resins through the use of laser beams produced by a UV laser source (Wang et al., 2017) The laser beam is activated and falls on the scanning mirror ( Fig. 1). This laser beam is directed by the mirror and is made incident on the resin reservoir. Firstly, the photocurable resin polymerizes to form a 2-dimensional patterned layer. After each layer is cured, the lowering of curing platform takes place in order to facilitate another photocuring cycle for deposition on the solidified freshly printed layer. Hence this process involves layer-by-layer curing (  Fused deposition modelling (FDM), is an extrusion-based technique involving the use of thermoplastic materials, such as-Polylactic acid (PLA) , polypropylene (PP) and polyethylene (PE), which are exposed to high temperature to form filaments (Chameettachal et al, 2019). The nozzle of the 3D printer expels the molten filaments by either pneumatic or mechanical forces (Fig. 2). The filament is continuously fed through the extruder and nozzle by means of two rollers which rotate in opposite directions. Deposition of material is accomplished on the build plate layer-bylayer until the desired dimensions of the product are achieved . FDM is the most widely used and cost effective additive manufacturing technology which is used by "home" 3D printers (Dawood et al., 2015). This technique is the most economical method of manufacturing custom thermoplastic parts and prototypes. The disadvantages of this technique are-lack of dimensional accuracy and precision, hence it is unsuitable for parts with minute details. Warpage occurs when the extruded material cools during solidification. Moreover, there is presence of noticeable layer lines, so post-processing is needed to attain a smooth finish (Varotsis, 2019).   Photopolymer Jetting is a technology which involves jetting of light sensitive polymer from an inkjet type print head on a build platform. A UV lamp is used to solidify the resin or wax after each layer is jetted (Fig. 5). Layer by layer the cured material is thus laid down incrementally on a descending platform .

Conclusion:-
The various innovations of 3D printing which facilitate visualization of treatment outcomes, have dramatically transformed the way products are designed, manufactured and delivered, thereby bringing an unfolding revolution in dentistry. Additive manufacturing has a promising task to carry out explicitly in prosthodontics. This driving edge technology has been utilized to reduce the tedious task of a dental technician and provide a more precise framework contrasted with the conventional techniques.
Educational programs using 3D-printed models invigorate development of dental skills amongst dental students who get the chance to foster better proprioceptive skills. 3D printing technologies thus possess the ability to transform research, treatment methods and educational streams of dentistry enhancing oral health care.

Financial Support and Sponsorship:-
No financial support received for his article.

Conflicts Of Interest:-
There are no conflicts of interest.