MODERN TRENDS IN THE DEVELOPMENT OF THE TECHNOLOGIES FOR PRODUCTION OF DENTAL CONSTRUCTIONS

The aim of the present paper is to make a review of the modern trends in the development of the technologies for production of dental constructions. Three are the main trends in production technologies in dentistry last 30 years: digitalization, simulation and implementation of additive technologies. The simulation occurred first and due to the computers development it underwent fast progress from the mathematical calculations and analytical analysis to the 3D modeling and visualization. Thus Computer Aided Engineering (CAE) was developed, allowing dental constructions with optimal design to be produced by optimal technological regimes. The first Computer Aided Design (CAD) – Computer Aided Manufacturing (CAM) systems were created in 1970s as a result of the digitalization. In this mode of operation at first virtual 3D model is generated by CAD, which then is used for production of the real construction by CAM. The CAD-CAM systems allow fabrication of dental restorations which is difficult or impossible to be manufactured by conventional technologies. The development of CAD unit runs from indirect scanning of the plaster model for obtaining data for the 3D model to direct scanning of the prosthesis area. While the development of CAM unit leads to direct manufacturing of the real dental construction using subtractive or additive technologies. The future development of the CAD-CAM systems as a whole characterizes with transition from closed to open access systems, which make them more flexible. In the late 1980sthe new approach to the production of constructions appeared – by addition of material layer by layer. The additive technologies were developed. They characterize with building of one layer at a time from a powder or liquid that is bonded by means of melting, fusing or polymerization. Stereo lithography, fused deposition modeling, selective electron beam melting, laser powder forming and inkjet printing are the methods, mostly used in dentistry. Due to the great variety of the additive manufacturing processes various materials can be used for production of different dental constructions for application in many fields of dentistry. The simulation, digitalization and implementation of additive technologies in dentistry led to fast development of the technologies for production of dental constructions last decade. As a result many of manual operations were eliminated, the constructions’ accuracy increased and the production time and costs decreased.


INTRODUCTION
The technologies for production of dental constructions undergo fast development last 30 years.This process characterizes with three main trends: digitalization, simulation and implementation of additive technologies.Historically, the simulation occurs first.Computers, facilitating the mathematical calculations, led to the Computer Aided Engineering (CAE), which is intended to simulate the performance of the construction in order to improve its design.As a result of the digitalization the first Computer Aided Design (CAD) -Computer Aided Manufacturing (CAM) systems were created in 1970s.Implementation of CAD-CAM systems in dentistry led to elimination of many manual operations, increase of the constructions' accuracy and decrease of the production time.In the late 1980sthe new approach to the production of constructions appeared -by addition of material layer by layer.The additive technologies were developed as alternative of subtractive ones.Their main advantages are: production of complex objects by different materials -polymers, composites, metals and alloys; manufacturing of parts with dense structure and predetermined surface roughness; controllable, easy and relatively quick process.
The aim of the present paper is to make a review of the main modern trends in the development of the technologies for production of dental constructions.

Simulation
The first simulations occurred in aviation, military and automotive industry in the beginning of the last century.The military was a major impetus in the transfer of modeling and simulation technology to medicine.The definition of medical simulation as "an imitation of some real thing, state of affairs, or process" describes its historical roots [1].Physical models of anatomy and disease were constructed long before the advent of modern plastic or computers.But the computers facilitated the mathematical description of the human physiology and pharmacology, the worldwide communication, and the design of virtual worlds [2].Nowadays mathematical modeling and 3D visualization are used in learning of dental anatomy, morphology of the teeth and analysis of the state of the teeth and the surrounding tissues in different impacts [3][4][5][6].3D digital models of selected anatomy are intended to support activities such as proper diagnosis, preoperative planning, orthodontic and surgical simulations, leading to successful treatment, risk reduction and increased patient safety [3,[7][8][9].
The simulation is very often used in manufacturing of dental constructions, because it gives fast and standardized results on the prognosis of prosthetic restorations in comparison with the clinical trials [10,11].Laboratory simulation is used in two ways: 1) for investigation the biomechanical behavior of dental constructions to understand the failure behavior of complex structures [12, 13]; 2) to optimize the experiments through the mathematical simulation and selection of the best design to perform the test and the manufacturing process [11,14].The complex process of simulation is called Computer Aided Engineering (CAE).It consists of three stages: simulation, validation and optimization.
The simulation stage, when simplified specimens like disks and micro-bars are used, is fast, but with low accuracy because the influence of the restoration geometry on the stress distribution is not taken into account.When the models are with the shape of the crown and bridges, the mechanical behavior is closer to the clinical situation, but the evaluation of the stress distribution within complex geometries is limited [11].In this case the Finite Element Analysis (FEA) -fast and a relatively low cost method is used.In the FEA, a large structure is divided into a number of small simple shaped elements, for which individual deformation (strain and stress) can be more easily calculated than for the whole undivided large structure [15].During FEA the model should be created, material properties and software limitations should be input, the mesh and convergence analysis should be done, loading and boundary conditions should be applied.The computer software solves a set of simultaneous equations with thousands of variables to achieve the desired results.Finally, the graphical presentation of results, including qualitative and numerical results, is given [11,12,15].Using FEA biomechanical behavior of different dental constructions such as crown, bridges, implants, implant-bone interfaces etc., made of different materials or combination of materials -metal alloys, porcelain, metal-ceramic, composites and polymers can be investigated.The main advantage of FEAis the virtual simulation of real structures that are difficult to be clinically evaluated which makes it a low cost alternative in compari-son to the other in vitro methods.But there are some limitations of the FEA models concerning mainly to the specific patient anatomy, the wet environment and the damage accumulation under repetitive loading [12].
For investigation of complex structures by FEA usually 2D or 3D models are used depending on the complexity of the geometry, the type of analyses required, expectations of accuracy as well as the general applicability of the results.The main limitation of the 2D models is the lower accuracy and reliability comparing to the 3D ones.In contrast, a 3D model has acceptable accuracy/reliability while properly capturing the geometry of complex structures.However, the higher the complexity of 3D models the higher the difficulty in generating appropriate mesh refinement for simulation [11,12].The 3D models can be generated by two ways depending on the structure of interest and the purpose of study.The first one is manually by using the appropriate software such as AutoCad, SolidWorks, Pro/Engineer or Rhino 3D.The second is the imaging approach which involves transformation of available medical imaging files from computed tomography (CT) scans, magnetic resonance images (MRI), ultrasound, and laser digitizers into wire frame models that are then converted into FE models [11,12,13].
Validation is the second stage of CAE which means comparing the behavior of the model with data of the analytical and experimental in vitro or in vivo investigations.In vitro validation allows the loads and boundary conditions to be carefully controlled in order to assess the validity of the model's geometry and elastic properties [12].A combination of in vitro and in vivo experimentation potentially offers the best validation which leads to the third stage of CAE process -optimization.As a result dental constructions with optimal design can be produced with optimal technological regimes.
The digitalization and the computers development played leading role in the CAE implementation into the biomechanical investigation of dental constructions.CAE let to make the engineering calculations and analysis without manufacturing of physical model, resulting in optimal design of dental constructions produced with optimal technological regimes in considerably reduced time and costs.

Digitalization
As a result of the digitalization the first CAD-CAM systems for application in dentistry were created in 1970s [16,17].In the first CAD stage the geometry of the parts, which should be produced, is defined.While during the second, CAM stage, the information for the production is merged and mostly the control of the production machines is made [18].So, the all CAD-CAM systems have three functional components: 1) a digitalization tool (scanner) that transforms geometry into digital data that can be processed by a computer; 2) software which processes scanned data and produces a data set readable by a fabrication machine; 3) a manufacturing technology that takes the data set and transforms it into the desired product by fabricating the restoration [17,19].On the first stage of the CAD development the data for the 3D model are obtained by indirect scanning of the plaster model (Fig. 1).In this case the first operations for manufacturing of a dental construction are manually done by the dentist (taking an impression) and the dental technician (pouring the plaster model).After that the plaster model is scanned by contact [13,15] or the more modern contact less scanning methods [11,12].Using scanned data the virtual 3D model is generated with specialized software which is then transferred to the CAM unit for manufacturing.Thus, the real dental construction can be made of porcelain by milling, or the polymer casting model -by stereo lithography (3D printing) [17].
On the next stage of the CAD development the data are obtained by direct scanning of the prosthesis area in the patient's mouth (Fig. 2).This stage is a result of the recent introduction of the intra-oral scanners.Thus the process of generating the 3D model is shortened several times as well as the accuracy of dental restoration arises, because the first manual operations are eliminated.Now there are many software packages available for the design of dental crowns, bridges and partial denture frameworks, as some of them can survey, design and wax a partial denture framework in less than 20 min [17].
The development of CAM unit leads to direct manufacturing of the real dental construction (Fig. 2).It can be done by milling of sintered porcelain and metal alloys with ultrasonic machine which ensures very smooth surfaces and high accuracy, or by selective laser melting machine, guaranteeing predetermined surface roughness.The further development ofthe CAD-CAM systems as a whole is the transition from the closed to open access systems, which means that the componentparts of a CAD-CAM system can be purchased separately.As a consequence the following advantages can be ensured: 1) the data can be obtained from different sources; 2) appropriate design software can be used for 3D modeling of different dental constructions;3) the mostappropriate manufacturing techniques can be selected for wide range of materials [17].
Due to the digitalization and exponential development of computers the CAD-CAM systems were implemented in dentistry.Their development from closed to open access gives opportunities for fast production of dental restorations from direct scan of the prostheses field, generating of 3D model and direct manufacturing of the real construction.This approachleads to more flexibility ofCAD-CAM systems, elimination of manual operations, increase of the constructions' accuracy and decrease of their production time and cost.

Subtractive/additive technologies
The first CAD-CAM systems in dentistry are based on the process of subtractive manufacturing in which the restoration is produced from a prefabricated block with guaranteed properties by cutting or milling with bursor diamond disks on conventional numerical controlled milling machine [17,19,20].Subtractive processes characterize with carefully planned tool movements to cut the material.This technology is used in dentistry for manufacturing of metallic and ceramic crowns.The main advantage of the subtractive fabrication is that the complexshapeswhich are difficult or impossible to make by the conventional dental processes can be effectively created in reduced time.Nowadays new highly sophisticated technologies are developed: electrical discharge machining, electrochemical machining, electron beam machining, photochemical machining, ultrasonic machining and so called laser "milling" (material removal by laser ablation) [17,21,22].
But there are still some limitations, concerning to: 1) the precision fit of the inside contour of the restoration which depends on thesize of the smallest usable tool; 2) the waste of considerable amount of raw material, as in some cases approximately 90 percent of the initial bock could be removed [17]; 3) abrasion wear of the milling tool and its short running cycles; 4) microscopic cracks on the ceramic surfaces due to machining of the brittle material [19].
They can be overcome by using of Additive Manufacturing (AM).American Society for Testing and Materials (ASTM) defined additive manufacturing as: "the process of joining materials to make objects from 3Dmodel data, usually layer upon layer, as opposed to subtractive manufacturing methodologies" [17,23].
AM technologies produce parts by polymerisation, fusing or sintering of materials in predetermined layers without need of tools, thus enable production of geometries that arealmost impossible to produce using other machining or moulding processes and nearly with no waste.The layers of all AM parts are created by slicing CAD data with specialized software.Each sliceis then printed one on top of the other to create the 3D object -the so called "3 Dimensional printing" process [17,24].These processes are also known as "layered manufacturing", "freeform fabrication", "rapid prototyping", "rapid manufacturing" [23,25].
Additive manufacturing processes started to beused in the 1980s when Hull invented stereo lithography (SLA) -the first 3D printing technology [26].The earliest applications of these technologies were mainly for manufacturing of prototypes, models and casting patterns.That is why this process was called "Rapid Prototyping"(RP).But today the additive manufacturing technologies are used throughout the whole product cycle: from pre-production, i.e. rapid prototyping, to full scale production, known as Rapid Manufacturing (RM) [17,23,24,27].
The ASTM International committee, dedicated to the specification of standards for AM, formed in 2009 (known as ASTM F42) created a categorization of all 3D printing technologies into seven major groups [25].According to it the 3D printing technologies with application in biomaterials are as follows: 3D plotting/direct ink writing, laser-assisted bioprinting, selective laser sintering, stereo lithography, fused deposition modeling, robotic assisted deposition/robocasting.
During the stereo lithography process (Fig. 3) a concentrated beam of UV light is focused onto thesurface of a tank filled with liquid photopolymer and, as the light beam draws the object onto the surface of the liquid,each time a layer of resin is polymerized or cross linked.Thedetail is built up layer by layer, to give a solid object [17, 19,24,28].At first SLA was used in medicine and dentistry for production of physical models of the human anatomy,for planningof surgical procedures and as a means of constructing customized implants such as cranioplasties, orbital floors andonlays.Nowadays the application of SLA processes is extended to manufacturing of surgical guides for placement ofdental implants, temporary crowns and bridges as well as resin models for loss wax casting [17,19].The process of fused deposition modeling characterizes with extruding of the thermoplastic materials through heated nozzle or the material is fed from a reservoir through a syringe.Depending on the strength and thermal properties several types of polymers are used: acrylonitrile butadiene styrene (ABS) polymer, polycarbonates, polycaprolactone, polyphenylsulfonesand waxes.During manufacturing process a water-soluble material is used for temporary supports.This technology is mostly used for production of wax patterns for subsequent casting [19,29].
By selective electron beam meltingnear net shape metal parts can be produced.Theparts are manufactured by melting of metal powder layerper layer with an electron beam in a high vacuum [17].This technology permits production of porous structures from biocompatible metals and alloys such as commercially pure Ti, Ti-6Al-4V alloy and Co-Cr alloys.But the accuracy of SEBM is in the range of 0,3-0,4 mmand the surface finish tends to be rough with an Ra value in therange of 25 µm.So, this technology is applied mainly in production of customized implants for orthopedics and maxillofacial surgery and is not good enough for crown and bridges frameworks [17].
Laser powder forming technique consists of two processes -Selective Laser Sintering (SLS) and Selective Laser Melting (SLM).In this technology layers of particular powder material are fused into a 3D model by adopting a computer-directed laser [19].When processing polymers and ceramic the term of"selective laser sintering" is used, whereas the processes for manufacturing of metals and metal alloys are known as "selective laser melting" (Fig. 4) or "Direct Metal Laser Sintering"(DMLS) [17,24].This technology is very attractive for dentistry, particularly for prosthodontics, because a large variety of materials can be used as building materials -thermoplastic polymers, investment casting wax, metallic materials (Ti and its alloys, Co-Cr alloys, stainless steel), ceramics and thermoplastic composites.When using polymers and composites, facial prosthesis, functionally graded scaffolds and customized scaffolds for tissue engineering can be produced by SLS.While using of metals and alloys, orthopedic and dental implants even with porous surface [17,30], dental crowns and bridges as well as partial denture frameworks can be manufactured by SLM [31][32][33][34].During production process many constructions can be packed in one powder bad (Fig. 4), thus ensuring high productivity of the laser powder forming technique.In inkjet printing technology an extremely small ink droplet is ejected towards the substrate.Different substances can be used as ink -aqueous solution of coloring agents and binders to a ceramic suspension, such as used in some studies to produce zirconiadental restorations [17,19,35].Another variant of the inkjet printing technology is thatbuilds up layer by layer by depositing droplets of a polymer and each formed layer is cured by UV light [17].This technology found wide range of dental applications for reproduction of dental models, orthodontic bracketguides, surgical guides for implant placement, mouth guards and sleep apnea appliances.
Additive manufacturing technologies offer a number of advantages over subtractive technologies and traditional methods of production: 1) the objects with complex geometry can be produced, without need of any complex machinery setup; 2) the method of production is a controllable, easy and relatively quick process; 3) the objects can be made of the same or different materials and depending on the technological regimes the desired properties can be obtained; 4) due to the great variety of the additive manufacturing processes various materials can be used for production of different dental constructions for application in many fields of dentistry.The undoubted advantages of the additive technologies led to their exponential development and wide application in dentistry last decade.

CONCLUSION
Three are the main trends in the development of the technologies for production of dental constructions last 30 years -digitalization, simulation and implementation of the additive technologies.
As a result of the digitalization the first CAD-CAM systems were created in 1970s.In this mode of operation at first virtual 3D model is generated by CAD, which then is used for production of the real construction by CAM.On the first stage of development the data for the 3D model are obtained by indirect scanning of the plaster model.On the next stage the data are in result of direct scanning of the prosthesis area in the patient's mouth.Implementation of CAD-CAM systems in dentistry led to elimination of many manual operations, increase of the constructions' accuracy and decrease of the production time and costs.
Computer aided engineering is an intermediate unit in the CAD-CAM system, which is intended to simulate the performance of the construction in order to improve its design.It consists of three processes: simulation, validation and optimization.Using the CAE helps dentists and dental technicians to create the optimal construction concerning to mechanical properties and accuracy and to manufacture it by the optimal process.
In the late 1980sthe new approach to the production of constructions appeared -by addition of material layer by layer.The additive technologies were developed, which characterize with building of one layer at a time from a powder or liquid that is bonded by means of melting, fusing or polymerization.Stereo lithography, fused deposition modeling, selective electron beam melting, laser powder forming and inkjet printing are the methods, mostly used in dentistry.Their advantages include: production of complex objects of various materials -polymers, composites, porcelains, metals and metal alloys; manufacturing of parts with dense structure and predetermined surface roughness; controllable, easy and relatively quick process.Due to the great variety of the additive manufacturing processes and the various materials used, different dental constructions can be produced for application in many fields of dentistry.

Fig. 2 .
Fig. 2. Second stage of CAD-CAM development: direct scan with intraoral scanner -a), virtual 3Dmodel -b), selective laser melting machine -d); dental bridge made of Co-Cr alloy by SLM process -e).

Fig. 3 .
Fig. 3. 3D printer and stereo lithography process -a) and various dental constructions manufactured by it -b) and c).

Fig. 4 .
Fig. 4. Possibilities of the SLM technology: dental constructions made of Co-Cr alloy -a) and skull with porous structure made of Ti -b).