The tooth loss due to caries, periodontal disease, injuries or primary absence of teeth due to dental pathologies, it is currently estimated as a serious health problem. A wide range of methods and techniques for replacing missing teeth appeared in dentistry. They are constantly improved with the evolution of the scientific base and at the same time new methods, materials and technologies are found to minimize or eliminate the inherent deficiencies in some methods of treating edentate. The selective laser sintering technology allows the execution of dental prosthesis considering all individual particularities of the patient’s anatomy to realize them more comfortable and supportable. In addition, this process has considerable advantages compared with traditional methods used in prosthetic dentistry.
Keywords: Dental prostheses, Selective laser sintering, Selective laser deposition, Biomaterials.
1. Introduction
The tooth is the hardest tissue in the human body which has a characteristic shape and structure, occupies a definite position in the dentition, and has its own nervous apparatus, blood and lymphatic vessels. Teeth are located in the alveoli of the jaws, take part in the mechanical processing of food, articulation of speech and perform an aesthetic function 1, 2. Tooth loss is very common and it happens as a result of disease and trauma; therefore, the use of dental prostheses to replace missing teeth is indispensable and has a long and multifaceted history 3.
Modern prosthetic dentistry has taken the best ideas form antiquity and multiplied them into current technologies and materials. As a result of this symbiosis the current prostheses that are worthy of admiration have appeared. Dental prostheses are constructions designed to restore the anatomy and physiology of the dental system 4. In the figure below are presented dental prostheses realized by selective laser deposition 5.
Fig. 1. Stainless steel frameworks produced by SLS (a, b) and Ti6Al4V (c, d). d shows the framework coming out from the powder
Among the main conventional techniques for making dental prostheses are casting, plastic deformations of metallic materials, polymerization, galvanoforming and others 6 – 9.
Each process has its advantages and disadvantages, but with the evolution of the scientific level appeared the so-called CAD-CAM technique with the help of which are obtained prostheses with the highest performances. Application of CAD/CAM systems have become a real revolution in the field of dental prosthetics due to the transition to a fundamentally new level of accuracy and consistency in the manufacture of crowns and bridges. Modern restoration is unthinkable without computer technology, and CAD/CAM in dentistry is a system that is one of the last and best achievements in this domain. CAD/CAM is a new technology for manufacturing dental prostheses with the help of computer modeling and milling on CNC machines or selective laser deposition on special equipment 10.
Dental treatment with prostheses has a great interest among specialists and attracts a growing number of patients. Currently, the high efficiency of dental prostheses, predictability and long-term reliability after treatment has been demonstrated. It has been established that on average 92% of orthopedic dental prostheses can be used for more than 10 years.
Fig. 2. Factors that influence the duration of prosthesis functioning
It results from the above figure that according to the modern concepts of prosthetic biomechanics, only 3 of the 8 factors that determine long-term stability in the human organism are of medical nature, most of which are of engineering nature 11.
2. Experimental
2.1. SLS process
The laser is used in many medical fields such as surgery, therapy, dermatology, ophthalmology, and has recently gained much ground in dentistry through appearance of selective laser deposition technique 12-16.
Selective laser sintering (SLS) is a layer manufacturing process that allows the generation of 3 D complex pieces by consolidating the successive layers of the pulverized material over one another. The consolidation is achieved by processing the selected areas using the thermal energy provided by a focused laser beam. With a beam deviation system (Galvano mirrors), each layer is scanned according to the correspondence cross section calculated from de CAD model. The deposition of successive layers of powders with a typical thickness of 20 to 150 µm is achieved by a powder deposition system. Fig. 3 shows a schematic example of an SLS system. Commercial installations differ, for example, by way of deposition of powder, atmosphere (Ar or N2) and the type of laser used (CO2 laser, Nd: YAG, laser fiber).
Fig.3. Scheme of the SLS process 17
Table 1 Process parameters of SLS/SLM technology
Material Laser Scan Environment
Composition
Mode
Speed
Pressure
Density of the powder Wavelength Layer thickness O2 level
Morphology
Power Impulse distance
Type of gas
Diameter of the granules
Frequency
Scaling factor
Preheating
Distribution
Pulse width Operating surface
Thermal properties
Rheological properties
Unlike other 3D printing methods, SLS requires a small number of additional tools once an object is printed, which means that objects do not usually have to be polished or otherwise modified once they are out of the SLS.
SLS doesn’t require the use of additional supports to keep an object together while it is being printed. Such additional supports is often needed with other 3D printing methods, such as stereo lithography or others, which makes these methods more time consuming that SLS.
SLS is a complex thermo – physical process and the determination of parameters is very important to achieve high precision. Table 1 divides the process parameters into four groups: material, laser, scan and environmental parameters. The optimal parameter setting can be found through the combination of empirical research and numerical simulation. Energy density is an absolute process parameter for a particular type of powder. This parameter represents the energy supplied by the laser beam to a volumetric unit of the powder material and combines some important laser parameters and scanning:
? E?_density=P_laser/(v_scan?s_oper?t_layer ) (1)
Fig.4. Comparison of prosthesis manufacturing methods 5
2.2. Material
Choosing the right material is one of the most important factors in the process of realizing dental prostheses. All materials used in dentistry should be evaluated for biocompatibility in order to protect patient health and safety. The need for biomaterials for use in restorative dentistry has generated a requirement for cytotoxicity assays to examine compounds and to characterize the potentially harmful effects of a material on oral tissues prior to clinical use. A biocompatible material can be defined as a material that does not cause significant adverse reactions with adjacent tissues or better said biocompatibility is the ability of a restorative material to induce an appropriate and advantageous response of the host during its clinical use 18
Nowadays biomaterials are divided into four general categories of metals, ceramics, polymers and composites as shown in fig. 5:
Fig.5. Classification of biomaterials in dentistry 19, 20
In this study the powder of the Co-Cr metallic alloy was used as the material with the following component: Co 59%, Cr 25%, W 9.5%, Mo 3.5%, Si 1%, C, Fe, Mn, N
