Lj. Slokar et. al Metallic Materials for Use in

ISSN 1848-0071 616.314+546.82=111 Recieved: 2015-11-17 Accepted: 2015-12-18 Review

METALLIC MATERIALS FOR USE IN DENTISTRY

LJERKA SLOKAR, JOSIP PRANJIĆ, ANDREJA CAREK¹

Faculty of Metallurgy, University of Zagreb, Sisak, Croatia ¹School of Dental Medicine, University of Zagreb, Croatia e-mail: [email protected]

Dentistry as a branch of medicine has an important role in the health of the human, while applying new technologies the maintaining of oral health is much more effective. For that purpose many dental products are used, such as crowns, bridges, implants, which are made of various metallic materials the mostly by casting or forging. They have to possess an adequate properties among which a biocompatibility, hardness and strength are the most important. Metal as an individual element with its properties does not meet certain conditions, therefore, the process of alloying results in different alloys that may be applied in oral environment. The most common classification of metallic materials for use in dentistry divides them in: noble and non-noble alloys and dental amalgames. Further, development of new technologies provides a new metallic materials with more adequate properties. Among them, titanium and titanium-based alloys stand out as materials of future in dentistry. Also, progress in computer technology results in faster production of economic dental products of much more quality. Key words: metallic materials, dentistry, structure, properties.

Metalni materijali za primjenu u stomatologiji. Stomatologija kao grana medicine ima značajnu ulogu u zdravlju čovjeka, a primjenom novih tehnologija odrţavanje oralnog zdravlja je puno učinkovitije. U tu svrhu upotrebljavaju se dentalni proizvodi, poput krunica, mostova, nadomjestaka, koji se izrađuju od različitih metalnih materijala najčešće lijevanjem ili kovanjem. Oni moraju posjedovati odgovarajuća svojstva, među kojima su najvaţnija: biokompatibilnost, tvrdoća, čvrstoća. Metal kao zaseban element sa svojim svojstvima ne zadovoljava u potpunosti traţene uvjete, te se stoga postupkom legiranja dobivaju različite legure koje se mogu primjenjivati u oralnoj sredini. Najčešća podjela metalnih materijala koji se primjenjuju u stomatologiji je na: plemenite i neplemenite legure, te dentalne amalgame. Nadalje, razvojem novih tehnologija omogućeno je dobivanje novih metalnih materijala s još pogodnijim svojstvima. Tu se svakako ističu titan i legure na bazi titana kao materijali budućnosti u stomatologiji. Također, napredak u računalnoj tehnologiji rezultira brţom proizvodnjom kvalitetnijih i ekonomičnijih dentalnih metalnih proizvoda. Ključne riječi: metalni materijali, stomatologija, struktura, svojstva.

INTRODUCTION

Metallic materials in dentistry are desirable materials in dentistry. In addition, applied as internal and/or external structural if they are implanted into the body, or if they component of many prosthetic restorations. come into contact with the tissue, must have They have wide indication field due to their other properties such as: biocompatibility, good mechanical properties contribute to the resistance to corrosion, high static and strength of the structure, and the ability to dynamic strength and toughness, and must resist the deformation under the not release metal ions in the environment in applied load. Metals and their alloys have which they are located. The most important properties, such as: a high elastic modulus mechanical properties in clinical practice and load carrying capacity that make them

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are: elastic modulus, tensile strength and increased demands of patients for aesthetics hardness. [1] in regard to service life, knowing and The primary goal of dentistry is to understanding of the structure and properties maintain or improve the oral health of the of cast metal and alloys is necessary to patient, which includes a variety of dental ensure the quality and characteristics of materials. Dental casting alloys must satisfy dental products. certain criteria, which must be a balance of So far, the main application of dental factors that influence its application. It is casting alloys was for production of partial therefore important to know the properties of denture frameworks, base for crowns and dental materials, in order to understand their bridges for metal-ceramic restorations as the behavior in the oral cavity and predict the most durable aesthetic substitute for natural outcome of the patient's health. [2] teeth. [3] Although, use of dental casting alloys is decreased in the last few years, due to the

HISTORICAL DEVELOPMNET OF METALLIC MATERIALS IN DENTISTRY

In the 7th century BC Etruscans used Although many methods for the ivory and -rimmed bone supported producing the dental materials are available, as core of the teeth. In 1800s, metallic the best and most popular method, especially substitutes were made by pressing of for making crowns, is casting. The aluminum, , gold made metallic impression of the prepared is replicated substitutes, , and in in a refractory mold and the required sample dental cavities. Although and other is made of wax. Then the samples are dental restorations were made from metals invested in the investment material and for centuries, technology of precision casting burned. In such prepared mold, molten metal was not possible until the 20th century, or is casted under pressure by a precisely in 1907. when Taggot introduced centrifugal force. the method of precision casting for the The historical development of production of crowns and bridges. Electric metallic materials in dentistry was furnaces and equipment for casting began to influenced by many factors, and the most be used in the early 1900s, and soon after important are: the technological changes of that, casting technologies rapidly progressed. dentures, progress in metallurgy and price However, successfully casting of titanium, changes of the precious metals. one of the most biocompatible metal for use An important, recent event in the in dentistry, was not achieved until the 1970s field of dental metallic materials is the because of its high reactivity with oxygen development of composites materials and sensitivity of casting technique. produced by the powder metallurgy and Subsequently several technologies were sintering respectively. These composites developed, such as CAD/CAM (Computer consist of sintered high precious alloy in the aided design/computer aided manufac- form of a "sponge" in which almost pure turing), laser sintering, laser welding to gold is infiltrated. The resulting composite facilitate production of metallic materials for of two metallic gold materials is not casted use in dentistry. [4]. but sintered in a refractory mold. [2]

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METALLURGY OF DENTAL METALLIC MATERIALS

All metallic materials used for compounds. For elements of similar atomic making the dental restorations are produced charge, diameter and crystal structure there by mixing in the molten state and casting is no limit of solubility of one element in into a mold in which solidify. Some another and, therefore, they solidify as a substitutes may be made of cast or molded single phase. For example, and nickel parts into a form that is very close to the are completely soluble. Melting point of final, while others are subjected to a series of nickel is higher than that of copper, so in the thermomechanical procedures from an initial first solidified phase the more of nickel will ingot to produce the final product. be represented, and the later solidified phase Differences in the microstructures can have a will be richer in copper. Thus, the casted significant impact on the rate of wear and implants can have a pronounced dendritic corrosion. In order to understand why the structure, with differences in chemical particular alloy is used and how it is able to composition on the macroscopic level. behave in vivo, it is crucial to understand the Casted products can be subjected to the physical metallurgy of metallic materials subsequent heat treatment known as used in dentistry. [4] homogenization or solution annealing in During the cooling of molten metal order to ensure an uniform chemical in the mold, solidification begins usually at composition by atomic diffusion. Small the surface. If the mold is very hot, there are differences in the diameter of atoms of two few places where the metal begins to stiff, or (or more) elements in the single-phase or where it starts to form the nucleus and two-phase alloys allow the strengthening. grows. If the mold is cold, there is much The presence of large atoms in the lattice of room for the development of the nucleus. smaller ones causes localized stress in the For most metallic systems, solid phase lattice, so that the atoms are under the increases as the advancing front with side influence of local compression. Similarly, branches, which is reminiscent of the front, several small-sized atoms in the lattice will called dendritic growth. Solidification be influenced by localized stress. This stress continues to encounter two nucleuses, which can increase the strength of the metal by a then form the border. Each of these places is mechanism known as a solid solution called a growth or crystal or grain, and every strengthening. Elements with very different boundary is boundary of crystal or grain properties and crystal structures have limited boundaries. At the microscopic level, clearly solubility. In many alloys systems, the visible areas can be identified as dendrites, precipitation of secondary phase is used as the interdendritic area and grain boundaries. the strengthening mechanism, known as To achieve a fine grain and nondendritic precipitation hardening. In some alloys such structure of castings used in dental medicine, as cobalt alloys, carbides have beneficial it is necessary to add various alloying effect on the wear and strength. In contrast, elements. Thus, alloys are mixtures of they have an adverse effect on the corrosion several elements in a solid solution, resistance of stainless steel. [4] sometimes with the precipitated intermetallic

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Production of dental metallic materials

Some metallic products for use in plasma or ion nitrating, or coating by the dentistry can be produced by casting in hard ceramic materials in order to improve nearly final form, and used in as cast or heat- the wear resistance. Since the 1980s a treated state. However, castings are often number of methods are used to modify the subjected to the mechanical methods of surface to ensure the biological bone growth rolling or drawing, and then are heated with from the surface of the porous metal the aim of yielding the stress. This results in material. In most cases, the final stage is the formation of new crystals with passivity in nitric acid, which increases the dislocations. By controlling the temperature thickness of the passive film, and any and time the soft metals with fine grains that impurities from the surface are removed. are suitable for cold working can be [4,6] achieved. Dental metallic products can be As in many other industries, the also produced by cold or hot forging and by production phases are increasingly powder metallurgy. In the latter case, the automated in dental technology too. Since fine powder is usually produced by melting the cost of laboratory work has become a the alloy and atomizing it. Thereafter is major factor in the planning of treatment and compacted into the almost final shape, and is , automation would allow a more subjected to the controlled high temperature competitive production and profits. and pressure in the sintering process. [4] Therefore, advances in computer technology The characteristics of the surface are enable cost-effective production of very important factor in the use of materials individual pieces. In recent years, the dental in the oral cavity, as they may affect the products, especially titanium and its alloys, ability of polishing, the appearance of and cobalt-chromium alloys, are increasingly scratches etc. Therefore, surface treatment produced or processed, with the help of during production has a major influence on computers, especially by CAD/CAM the wear and corrosion resistance. After technique. It provides a new, almost without casting, casted product has a matted surface faults, industrially processed and controlled originating from the investment material, so materials, increased the quality and it has to be abrasive blasted or sandblasted reproducibility, as well as increased the with purpose of removing the particles of efficiency and precision of the finished investment material from the castings product. Due to the continuous development surface. For example, on the steel castings of computer technology, development of that would be used as implants, first the new methods of production and processing, mechanical then the electrolytic polishing is which will further reduce the costs is carried out. However, the surface of metallic expected. [7] materials can be treated by ion implantation,

METALS IN DENTAL MEDICINE

Metals and alloys have an important preparation and manipulation of teeth. role in dental medicine. The different metals Although the latest trends go to metals free are used in all aspects of dentistry, including restorations, they remain the only clinically dental laboratories, direct and indirect dental proven material for the base of the long span restorations and instruments for the

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bridge and for partial denture framework in mechanical properties and resistance to dental medicine. [2,8] tarnishing. Gold (Au). Gold is used in dentistry Silver (Ag). Although silver is a because of its good properties and aesthetics. valuable element, it is not considered as It is corrosion stable, with no change in noble in the oral cavity. Silver decreases the color, and has a sufficient hardness, melting range, improves viscosity, helps to elasticity and oligodynamic action. Also, it is control the coefficient of thermal expansion chemically inert, non-allergenic and it is of the gold and palladium based alloys. easy to work with. Gold is noble metal that Silver has a high affinity for the absorption provides high resistance to corrosion and of oxygen, which can cause porosity during wear, and slightly increases the melting casting. However, by the addition of small range of the alloy. Gold improves the amounts of or indium this absorption workability, the ability of polishing, can be controlled. In the presence of sulfur, increases the density and ductility of alloy to silver corrodes and darks. which is added. Also increases the Aluminum (Al). Aluminum is a metal coefficient of thermal expansion of that is added to nickel based alloys to reduce palladium alloys. However, if it is present in the melting range. It solidifies the alloy and sufficient quantities, gold gives a pleasant affects on the formation of oxides. It is one golden yellow color to alloy, which can be of the elements, besides the Co-Cr alloys, easily neutralized by adding a "white" metal, which is "etched" on the surface of the alloy such as palladium or silver. with the aim of creating the micro- Palladium (Pd). Palladium is a noble mechanical retention. metal of the platinum group in the periodic (Be). Such as aluminum, table of elements. If it is added to the gold beryllium decreases the melting range of alloys, palladium increases the strength, nickel-based alloys, improves castability and hardness, resistance to corrosion and the ability of polishing and it helps to control tarnishing. In addition, it increases the elastic the formation of oxides. However, biological modulus, strength and hardness of gold properties, as health safety for the patients alloys and reduces their density. Palladium and technicians, are questionable when increases melting range and improves its working with alloys containing the resistance to sag. It has a great ability of beryllium. bleaching, so that the alloy with 90% gold Boron (B). Boron is deoxidation and 10% of palladium has platinum color. agent. It increases the hardness of nickel- Platinum (Pt). Platinum is a precious based alloys, reduces the surface tension of metal that belongs to the platinum group of the molten alloy, improves its castability and elements. Platinum increases the melting reduces the ductility of the alloys to which is range, strength, hardness and elastic added. modulus of gold alloys, and improves their Chromium (Cr). Chromium is a resistance to corrosion, tarnish and sag. It metal that improves solid solution hardening. gives the white color to alloys, and because Because of its passivating effect it of the high density platinum increases the contributes to the corrosion resistance in density of non-gold based alloys. nickel and cobalt based alloys and provides Iridium (Ir) and Ruthenium (Ru). their resistance to tarnishing. However, Iridium and ruthenium are precious metals chromium forms a brittle sigma phase, belonging to the platinum group. If added to which to the breaking of the castings. the gold or palladium based alloys they For this reason, its content in these alloys is reduce the grain size in order to improve the limited to 28 - 29%.

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Cobalt (Co). Cobalt is added to Manganese (Mn). Manganese binds palladium alloys to increase the coefficient the oxygen in cobalt and nickel based alloys. of thermal expansion. Furthermore, it However, if present in high concentrations, it strengthens the alloy, increases the hardness is health risky for the patient, and can cause and modulus of elasticity. Cobalt-based the Parkinson's disease. alloys are an alternative to nickel-based Molybdenum (Mo). Molybdenum alloys, but they are more difficult to process. improves the corrosion resistance, forms the Copper (Cu). Copper is added for oxides, and has a beneficial effect on thermal hardening and strengthening of the alloy and expansion coefficient of nickel-based alloys. can reduce the melting range. In the Nickel (Ni). Nickel has a coefficient interaction with platinum, palladium, silver of thermal expansion close to that of gold, and gold, it provides the ability of thermal and therefore it forms the basis of many processing of alloys based on gold, silver alloys. In addition, it provides the corrosion and palladium. Copper slightly decreases the resistance and allows the elasticity and density, and can improve the passivity of Pd- toughness to alloys. However, nickel is not Cu alloys. completely safe for the patient's health, Gallium (Ga). Gallium is added to especially for the female population, because the non-silver ceramics to compensate the it can cause allergies and dermatitis. reduced coefficient of thermal expansion (Sn). Tin helps the hardening of that is the result of the silver absence. alloys and decreases the melting range. It is Indium (In). Indium has different one of the key trace elements for oxidation functions in metal-ceramic gold-based of Pd-Ag alloy. alloys. It decreases the melting range and Titanium (Ti). Titanium is non- density of alloy, improves viscosity and has precious metal. Numerous studies prove its beneficial effect on strengthening and good biocompatibility and resistance to hardening. Indium is added to alloys with a corrosion. It reduces the melting range and high silver content to increase the resistance improves the castability. Also, it increases to tarnishing. the hardness of the alloy and forms the (Fe). Iron is added to the gold oxides at high temperatures. alloys for metal-ceramic system for Zinc (Zn). Zinc enhances castability hardening and strengthening and for bonding and contributes to the hardness when the oxygen to oxides. Also, the iron is the combined with palladium. It helps to reduce base of several types of metallic alloys. It the melting range of the alloy and reduces has good complements with nickel and the gaseous porosity. [1-3,9] together enhance the ability of processing of the alloy in cold state.

CLASSIFICATION OF METALLIC DENTAL MATERIALS

Metallic materials used in dentistry, group of metals (ruthenium, palladium, particularly alloys, can be divided in several osmium, rhodium and iridium). Precious ways (e.g. regarding to application, the main alloys may contain non-precious elements element, producing method etc.) (about 10%), but the content of the precious Metals classified as noble are: gold, metals may not be less than 75%. If the copper, , platinum and platinum content is still lower, the alloys are named as

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reduced precious alloys or alloys of reduced as biocompatibility, minimal reactivity with gold content. Corrosion resistance and the material of the mold, must be easily stability in the mouth are the main melted, casted, joined, polished, must have characteristics of noble alloys.[9] high strength, good wear, corrosion and Today, the most used alloys in tarnishing resistance, and should be inert in practice are obtained by casting (Table 1). the oral cavity. [2] They have to possess certain properties, such

Table 1. Classification of casting alloys for metal ceramic prosthesis and partial dentures [2] Table 1. Klasifikacija lijevanih legura za zubarske proteze [2]

Metal ceramic Partial denture Metal Type All-metal prostheses prostheses frameworks Au-Ag-Pd Pure Au (99.7%) Au-Ag-Cu-Pd Au-Pd-Cu-Ag Au-Pt-Pd HN metal ceramic Au-Pd-Ag (5-12 wt. High Noble (HN) alloys % Ag) Au-Pd-Ag (>12 wt. % Ag) Au-Pd Ag-Pd-Au-Cu Pd-Au Ag-Pd Pd-Au-Ag Noble (N) Noble metal Pd-Ag ceramic alloys Pd-Cu-Ga Pd-Ga-Ag CP Ti CP Ti CP Ti Ti-Al-V Ti-Al-V Ti-Al-V Ni-Cr-Mo-Be Ni-Cr-Mo-Be Ni-Cr-Mo-Be Predominantly Ni-Cr-Mo Ni-Cr-Mo Ni-Cr-Mo Base metal (PB) Co-Cr-Mo Co-Cr-Mo Co-Cr-Mo Co-Cr-W Co-Cr-W Co-Cr-W Cu-Al

Noble alloys

Gold alloys. Ternary Au-Ag-Cu alloy is added to improve casting of the alloy. is the oldest one used in dentistry. Copper is Small amounts of ruthenium or iridium added to gold for increasing the strength and (0.0005 - 1%) are added to promote the silver for workability. This alloy with 18- nucleation centers development. In addition, carat gold (75 wt.%) is very resistant to they form a fine-grained structure in the corrosion. Palladium and platinum are added alloy. to improve the mechanical properties. Zinc

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Figure 1. Metallic gold [10] Slika 1. Metalna zlatna krunica [10]

Gold alloys can be classified as: of palladium, which significantly increases - (1) alloys with high gold content; they are the melting range, especially the solidus golden yellow colored; show good temperature; castability and solderability; used for metals - (4) reduced gold alloys without silver; they fused to veneers system; have a white color; used for metal-ceramic - (2) alloys with reduced gold content; they systems; silver was replaced with palladium are light yellow colored; show good because it causes discoloration of ceramics; castability and solderability; portion of the however, the palladium addition increases gold is compensated by the addition of the melting temperature of the alloy, so palladium or silver; palladium’s tendency to indium is added to decrease it.[4,9] changing the color of alloy is compensated The chemical compositions of these by adding a larger copper content; types of alloys are showed in Table 2. [9] - (3) reduced gold alloys with silver; higher gold content is compensated by the addition

Table 2. Chemical compositions of gold alloys, wt. % [9] Table 2. Kemijski sastav slitina na bazi zlata, teţ. % [9]

Alloy/element Au Pt Ag Cu Pd Sn In

(1) 70-80 1-5 10-15 5-10 - - -

(2) 55-60 - 0-25 10-12 5-10 - -

(3) 0-50 - 0-20 - 0-30 0-5 -

(4) 50 - - - 0-40 - 0-10

Palladium alloys. Melting point of should be added to reduce the melting pure palladium is 1552 °C and is too high for temperature of the alloy to 1200 - 1400 °C. dental casting devices. Certain larger These base metals are used for the formation percentage of silver or copper, and other of oxide on the surface of the metallic part of elements such as gallium, indium and tin, crowns or bridges that are necessary for

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binding of the alloy and the ceramic after alloys are Pd-Ag and Pd-Cu alloy. Their appropriate heat treatment. In most cases, the chemical compositions are listed in Table 3. alloys that do not contain copper are more [4,9] resistant to corrosion. Most used palladium

Table 3. Chemical composition of the alloy of palladium, wt. % [9] Table 3. Kemijski sastav slitina na bazi paladija, teţ. % [9]

Alloy/element Pd Ag Cu Sn+In Ga

Pd-Ag 0-60 0-30 - 10-12 -

Pd-Cu 80 - 10 - 10

Pd-Ag alloys are economical, unlike compared to palladium is decreased. The the Pd-Cu alloys that have the highest maximum content of non-precious melting temperature among the all noble components (Zn, Sn, Cu, Ni, Mn, Si) is alloys, and they are difficult to cast and limited to 10% to achieve better mechanical solder. Both types of alloys are used for properties. Regarding to microhardness, metal-ceramic systems. [9] precious alloys can be divided into 4 types: Silver-palladium alloys. Pd-Ag (53 - soft alloy, medium-hard, hard and extremely 61% Pd and 28 - 40% Ag) were the first hard alloys. [2,9] precious alloys on the market that did not The most important physical and have gold in its composition. In these alloys mechanical properties of noble metal alloys the content of the precious metal is the include: lowest among the all other precious alloys. - biocompatibility Indium and tin are added to form the oxide - good corrosion and tarnishing resistance needed for the binding of the ceramic and to - melting temperature at 1000 °C, molding increase the hardness, while the ruthenium is temperature higher for 75-150 °C added to improve the castability. Because of - density of about 15 g/cm3 the higher content of palladium the - hardness, from soft to extremely hard coefficient of thermal expansion is reduced. - elongation, as a measure of ductility, 20 - This can be prevented by increasing the 39% content of silver that also reduces the - coefficient of linear thermal expansion in melting range. These alloys are gray colored, the range of 14 - 18 ⋅ 10-6 / °C and by the alloying the color durability and - tensile strength ranging from 103 to 572 the hardness compared to the silver are MPa. [2] increased, and the melting point when

Non-precious alloys

This group of alloys is based on the titanium. These alloys are introduced in non-precious metals, such as chromium, dentistry as an alternative to gold alloys after nickel, iron, tin, lead, magnesium and significant increase in gold price. Generally,

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non-precious alloys, besides they are more numerous. For example, they are lighter than economical, they are characterized by: high gold alloys, and mechanical properties and hardness, resistance to discoloration and corrosion resistance are the same or even corrosion, high rigidity, low elasticity, low better. The big advantage that ensures the thermal conductivity, low density, platinum wide application in economically weaker color. The alloys that are commonly used are countries is their low price. However, due to Ni-Cr and Co-Cr based alloys. [2,9] the complex procedures of technical Ni-Cr alloys. Nickel content in these appliances, high melting temperatures alloys is 70 to 90% and chromium from 13 requiring the use of special casting devices, to 20%. The balance is iron, aluminum, and high hardness, their application is still molybdenum, beryllium, silicon and copper. difficult. Co-Cr based alloys are mostly used The alloys based on Ni-Cr have great for base frameworks of removable partial strength and hardness, and due to a large dentures, as well as for base of crowns and melting range it is necessary to use the high- bridges in metal-ceramic systems.[9] frequency melting furnace. Despite the local Cobalt possesses a good corrosion fracture corrosion, these alloys are resistance and can be used in multi-phase considered to be stable in the mouth. Due to alloys to improve their mechanical the nickel content, the elasticity and properties. Chromium is the primary processing in the cold state are increased. alloying element in a wide variety of cobalt However, nickel is considered as an allergen superalloys, and is preferably added to and it is increasingly replaced by cobalt. Ni- improve corrosion resistance. In addition, it Cr alloys are characterized by high elastic contributes by a solid solution hardening. [4] modulus, high hardness and tensile strength, The first cast cobalt-chromium alloy lower density, relatively low cost and high (30% Cr, 5% Mo with small portions of Ni ductility when compared to the Co-Cr alloys. and C) dates back to the year 1911. and [2,9] Haynes named it "stellite". Later, in 1926 Co-Cr alloys. The discovery of alloy of similar chemical composition is proper casting techniques and materials for patented under the name vitallium, and it has making models resistant to high melting become one of the major cobalt alloy for temperatures was enabled the use of cobalt- dental implants. The typical microstructure chromium alloys in dental medicine. After of cast and wrought Co-Cr-Mo alloys are introducing, their application in dentistry is shown in Figure 2. [4] constantly growing, and the reasons are the

Figure 2. Microstructure of as cast and wrought Co-Cr-Mo alloy, 200x [11] Figure 2. Mikrostruktura lijevane i kovane Co-Cr-Mo slitine, 200x [11]

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The microstructure of the cast alloy a better resistance to wear and fatigue. As consists of carbides and large grains, as cast Co-Cr-Mo alloys are used for complex opposed to the wrought alloy that possesses designs and applications (Figure 4).[11]

Figure 3. Applications of Co-Cr-Mo alloys [11] Figure 3. Primjena Co-Cr-Mo slitine [11]

In general, Co-Cr alloys are very increases the corrosion resistance. The most hard and brittle and they are not easy to important properties of these alloys are solder and weld. Due to the high hardness summarized in Table 4. Co-Cr alloys are they are difficult to cut, grind and polish. cheaper than gold alloys, and therefore more The mechanical properties and castability economical. [2,9] are determined by cobalt, while chromium

Table 4. The properties of Co-Cr alloys [2,11] Table 4. Svojstva Co-Cr slitina [2,11]

Melting Density, Yield strength, Tensile Hardness, Elongation,

temp., C g/cm3 MPa strength, MPa HVN %

1250 8,8 470 – 710 685 – 870 264 – 432 1,6 – 3,8

Because of the high melting point of 15% Ni, 15.8% Fe, 7% Mo, 2% Mn, 0.16% the Co-Cr alloy, melting can be performed C and 0.04% Be. They have excellent only by an induction technique. These alloys resistance to corrosion and tarnishing. are not viscous so the casting is preferably Hardness, yield strength and tensile strength performed in the furnace under pressure. are the same as for stainless steel, while the Due to their high hardness a method of ductility is higher in softened condition, but electrolytic method for polishing must be it is less after hardening. [2] employed. [2] Co-Cr-Ni alloys. These alloys have Ni-Ti alloys. These alloys typically been launched in the 1950's. Their typical contain 54% Ni, 44% Ti and 2% or less of chemical composition is: 40% Co, 20% Cr, cobalt. The crystal structure at high

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temperature is cubic volume-centered and . However, they are difficult to corresponds to the austenitic phase. When shape and cannot be joined by soldering or cooled through a critical temperature range welding. [2,12] these alloys show a significant change in the Titanium and titanium-based alloys. elastic modulus, yield strength and the Titanium is a very popular metal in dental electrical resistance as a result of changes in medicine and it is an integral part of dental the electron bonds. Cooling through this area treatment since the 1960s due to its good leads to changes in the crystal structure, properties, such as low density (4.51 g/cm3), known as a martensitic transformation. Ni-Ti low elastic modulus (110 GPa), and alloys show two closely related and unique compatibility with the tissues of the mouth. properties, such as: shape memory effect and The high price of precious metals and the superelasticity. Cobalt helps to reduce the potential health harmfulness of some metals temperature of the phase transition, which have led to the widespread use of can be close to the temperature in the oral commercially pure titanium (CP Ti) and its cavity. Shape memory effect is achieved by alloys, especially for dental implants and establishing a shape at a temperature near crowns (Figure 4). The main advantage of 482 °C, and then by cooling and forming of the titanium implant is based on good the second form. Superelasticity can be with the bone of the jaw. achieved when the phase transition is Due to the low density it is applied to induced by stress. These alloys have a produce the high-strength and lightweight variety of applications, particularly in dentures. [2,13-15]

Figure 4. Titanium dental implants and crowns [16-18] Figure 4. Titanski zubni implantati i krunice [16-18]

Titanium is considered as the most which remains stable up to the melting biocompatible among the all metals. When temperature of 1672 ° C, then occurs. the patient is allergic to nickel, titanium is an There are 4 types of CP titanium for excellent choice. At room temperature it is dental applications, which contain small stable as a ductile metal, but of low-strength, amounts of iron, nitrogen and oxygen. As while during the heating to a temperature their content increases from Type 1 to 4, so above 883 °C titanium transforms into the does the strength. In order to improve its hard, brittle form. Namely, the primarily mechanical properties titanium is transformation of the hcp lattice of alpha alloyed by different elements. The most used phase (α) in the bcc lattice of beta phase (β), alloy is Ti-6Al-4V containing 90% Ti, 6% Al and 4% V. However, because of proven

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health damaging of aluminum and composition: 17-19% Cr, 13-15% Ni and 2- vanadium, titanium is nowadays alloyed by 3% Mo for improving the corrosion other elements, such as Nb, Ta, Zr, Mo or resistance and the carbon content is below Sn, resulting in new alloys with a lower 0.03%. Besides the mechanical strength, elastic modulus, a higher fatigue strength corrosion resistance is the most valuable and a better biocompatibility and more feature of stainless steel. The addition of a aesthetical acceptability as well. In addition, minimum of 12% Cr makes steel a stainless, the melting temperature is decreased by due to the formation of stable and passive alloying. [2,4,9,19-25] oxide film. Since chromium has a cubic Titanium is electrochemically very volume-centered structure (bcc), if added, it active. The consequence of this is its high stabilizes a bcc structure of iron. However, reactivity to oxygen resulting in a stable carbon has a high affinity to the chromium, passive oxide film that makes titanium and forms carbides of Cr23C6 type. This resistant to electrochemical corrosion. leads to the precipitation of carbon in the Titanium and its alloys are produced by carbides area wherein the chromium centrifugal or vacuum-pressure casting. concentration decreases as it enters the Although these are cost-effective, carbide, resulting in decreased the corrosion biocompatible and easily available materials, resistance around carbide. If the content of there is a need to find appropriate techniques chromium is reduced to <12%, it is not for easier casting, processing, welding, in enough for repassivation and stainless steel order to obtain a more diverse applications. becomes susceptible to corrosion. Stainless [4,14,15] steel is the most commonly used for crowns (Figure 5) and in the orthodontics. [4,9] Stainless steel. Stainless steel, which is used in dental medicine, has chemical

Figure 5. Stainless steel crowns [26] Figure 5. Krunice od nehrđajućeg čelika [26]

Dental implants made of stainless main advantage of stainless steel is steel are produced by casting or punching. availability in sufficient quantities, while its The costs are low because the complex main disadvantage is a low resistance to methods of shaping are not required. The edge corrosion. [4,9]

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Dental amalgams

Dental amalgam, an alloy of mercury - possible toxicity due to the high mercury and one or more metals, e.g., silver, tin, content. [9,27-29] copper, is the oldest filling material for the Since there are ongoing concerns posterior teeth. Mercury has to be very clean about the toxicity of dental amalgams, they and free of harmful ingredients. Silver are constantly evolving and improving. contributes to the total resistance of the alloy Among the most important advances in the and reduces spillage edge of parts fillings. In development of amalgams is discovery of addition, silver facilitates the handling of so-called "Non-γ2" amalgam and amalgam amalgam and coordinates the expansion of packaging in capsules in exactly dosed the volume. Amalgams with lower silver ingredients. According to the number of content are hardened slowly and they are chemical elements, dental amalgams may be: less resistant to compression, but they are binary, ternary, quaternary, while according more contracted during the hardening when to the copper content they are divided into: compared to the amalgam with a greater conventional dental amalgams and amount of silver. Tin takes up a quarter of amalgams with a high portion of copper. the amalgam alloy, reduces its mechanical Proportion of copper in the resistance and expansion. Unlike silver, tin conventional dental amalgams is up to 3%. increases spilling amalgam in the cavity. However, because of insufficient strength Copper contributes to the hardness of the and tendency to corrosion due to γ2 phase amalgam alloy, and prevents its excessive (Sn8Hg), in the amalgam alloys the content expansion in the cavity. Although the of copper is increased, leading to elimination tendency in recent times is put it out of use, of this phase. These amalgams are called because of environmental reasons and the "non-γ2" amalgams. In the dental amalgams growing use of modern materials, amalgams with a high content of copper γ2 phase is are still used for filling in primary and young completely removed. This type of amalgam permanent teeth. Dental amalgams are can be classified, depending on the copper characterized by good mechanical content, as: mixed amalgam alloys and properties, while some modern amalgams dental amalgams with a very high portion of show antibacterial activity too. Advantages copper. Mixed amalgam alloys contain about of the amalgams when compared to the other 9% copper. The γ2 phase is formed in the materials include: process of amalgamation, but the final alloy - easy to handle, i.e. the dry work area is not does not contain it. Dental amalgams with a required and time of manipulation is short very high portion of copper contain 9-28% - easy production, which excludes several of this element, and the γ2 phase in this type working phases in contrast to composites of amalgam is not formed. [9,28] - excellent resistance to masticatory forces According to investigations, the - ease of use in ordination conditions amalgam alloys are more resistant to - affordable price. compression then to tensile and bending However, they have drawbacks such as: loads. For this reason, the cavity must be - bad aesthetics designed in such a way that fulfills are - chemical and micromechanical bond to exposed to compression forces. The average enamel and is not possible resistance to pressure of the amalgam alloy - extensively grinding of healthy hard tissue is about 275 MPa, and some even up to 550 due to retention of the amalgam is necessary MPa. In comparison, the resistance of these alloys to tensile load is 55 - 62 MPa.[9]

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PROPERTIES OF METALLIC DENTAL MATERIALS

For use of metal in dental medicine it - the possibility of arc and induction melting, is very important to know its properties, - the achievement of a low viscosity melts by which depend on its crystal structure. simple casting technique, Properties are determined by the strength of - the ability of processing by standardized bond (metallic) that links the atoms, and in technique, dental metals a strong metal bond is present. - a wide tolerance of processing with a The physical properties are very important in minimal risk dental researches, because they provide - economic viability. information necessary to evaluate the For use of metal or alloy in clinical characteristics of materials and their applications it has to: improvement during development. If some -possess the hardness, strength and ductility alloy could be used in dentistry, it has to determined by a specific clinical indication, meet a certain requirements, such as: -have an aesthetically acceptable color, appropriate physical and chemical -be a biocompatible. [6,9,30] properties, technical workability and Hardness. One of the most important suitability for clinical use. Physical and properties of dental materials is hardness. It chemical properties include: represents the resistance of a material to - homogeneous fine-grained structure penetration of another much harder indenter. - self-hardening after casting, There are different procedures for testing the - high strength values, hardness, and in dentistry the most common - the possibility of soldering are Knoop and Vickers methods. In this - corrosion resistance. case, the hardness is expressed by Knoop Technical workability of dental metal (HKN) (Table 5) or Vickers (HVN) number. or alloy means: [6,9,31]

Table 5. Knoop hardness of dental materials [9,15] Table 5. Knoopova tvrdoća zubarskih materijala [9,15] Dental material HKN

Dentin 68

Enamel 350

Amalgam (Ag-Sn-Hg) 110

22-carat gold alloys 85

Titanium 170

Co-Cr alloys 420

Composite material 90

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It should be noted that even small final tensile strength of non-precious alloys differences in the composition of cobalt- varies ranging from 640 MPa to 825 MPa. based alloys have an effect on their hardness The hard gold alloys for partial dentures values. Cobalt-chromium alloys are for one have similar values of tensile strength. third harder than gold alloys used for the Elongation is an important property same purpose. This enables a production of a of a metallic material that is applied in smaller and thinner denture bases and other dentistry. The percentage of elongation is an dental elements. However, because of the indicator of the relative brittlement or high hardness these alloys are more difficult implant’s flexibility. This property is to handle. [9] particularly significant for a clasp in the Strength. The strength is the partial dentures. Elongation and ultimate resistance of a solid material to plastic tensile strength affect the ductility of each deformation and fracture. It contains a material. Also should be noted that the small number of mechanical properties such as: micro-porosity of alloy significantly changes viscosity, coefficient of strengthening, its ductility. Percentage of elongation of tensile and bending strength. Strength alloy in large extent depends on the casting characteristics depend on the nature of the conditions. It is therefore very important to atomic bonds in the crystal and the crystal control the temperature of melting and lattice defects. casting of these alloys. [9,32] The tensile and yield strength are the Elastic modulus. This property is most important mechanical properties that very important when selecting materials in value the resistance of the dental materials. dentistry. In order to ensure the positive These properties are very important in terms therapeutic effect, dental material must of the development of partial dentures possess a elastic modulus value equal or especially clasps and sublingual arches similar to the modulus of dentin (15 - 25 because they indicate the permanent GPa) or enamel (83 GPa) depending on deformation. If that property is poor, clasp application (Table 6). [9,33] would be broken during the activation. The

Table 6. Elastic moduli of dental materials [9,33] Table 6. Elastični moduli zubarskih materijala [9,33] Dental material Elastic modulus, GPa Dentin 15 – 25 Enamel 83 Amalgam (Ag-Sn-Hg) 27,6 22-carat gold alloys 96,6 Titanium 110 Co-Cr alloys 228 16,6

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Higher value of the elastic modulus depends on the combination of used alloys, indicates a higher stiffness and lower their chemical composition, concentration of flexibility of the cast construction of partial electrolytes, oral hygiene, diet, activity of denture or other prostheses. Alloys with a proteins, microorganisms and temperature in high elastic modulus allow the production of the mouth. The most common cause of smaller restorations. The implant becomes a biological corrosive action is based on the lighter by reducing the thickness. Stiffer and release of component ions in the corroding inflexible denture base framework ensures alloy. The eluted ions from the alloy are the uniform distribution of masticatory and harmful to the surrounding tissues in the parafunctional loads on the stronghold of form of allergic reactions, toxic and teeth and denture bearing area. carcinogenic effect. Biocompatibility of the Elastic modulus of non-precious alloy and its components determines the metallic alloys is at least twice higher than ability of the implanted material to perform the modulus of gold dental alloys. This its therapeutic function and physically should be considered when selecting the replace the damaged or destroyed tissue. So, alloys for partial dentures frameworks and biocompatible metallic dental material other restorations. [9,21,30]. should not cause irritation in the mouth or be Biocompatibility. Biocompatibility of subjected to biodegradation, must not be a material is one of the determining toxic or carcinogenic, and may not cause an characteristics for its application in clinical allergic reactions. practice. It depends on the mechanical and In general, the metals can be a stable corrosion properties of materials and tissues. passive, meaning that if broken the Surface chemistry of dental material, its protective oxide layer will be spontaneously topography, roughness and types of renew, or may be unstable when the passive integration with tissue are correlated with the layer cannot be renewed, which results in response of the host, i.e. the oral cavity. exposing of metal to active corrosion. That Biocompatibility of the metallic implant and depends on the oxidation or reduction its structure are important for the proper potential of the environment. functioning of the prosthesis and other A problem that might occur in some restoration in oral cavity. [4,16] alloys for basic dental parts containing Indicators of biological compatibility beryllium is its evaporation. Namely, vapor of dental materials are: biocompatibility, is harmful for dental technicians exposed to biofunctionality and biodegradation. A dust and fumes during the various processes biological inert behavior of material in the of casting and machining of alloys. Exposure body indicates its biocompatibility. Dental to vapors of beryllium can lead to acute or materials should not be a toxic for the chronic infection (dermatitis, pneumonia). patient, therapist or technician and may not Furthermore, the problem may be patient’s cause the irritation of oral and other tissues, allergy to the nickel. Exposure to nickel allergic reactions and may not be a leads to various diseases, such as dermatitis, cancerous. Biofunctionality of material is lung and sinus carcinoma, loss of sense of expressed by its efficiency and performance smell, asthma, pneumoconiosis. of functions over time, while changes that Dental metallic materials are occur in the material under the influence of constantly exposed to saliva, to the changes the surrounding environment indicate a of conditions in the oral environment with biodegradation. frequent changes of alkaline and acidic The corrosion of dental metallic conditions caused by intake of various food materials is the result of various factors. It ingredients and liquids. It is therefore very

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important to choose an alloy that is the metallic construction or rupture. From chemically resistant to their effects, and is the above it can be concluded that the not susceptible to electrochemical corrosion. metallic material used for dental therapy In addition, dental alloys are exposed to should not be chemically, electrochemically masticatory forces in different directions and or mechanically dissipated in the mouth. intensity. Therefore they have to be very [9,15] strong in order to prevent the deformation of

CONCLUSION

In dentistry pure metals are apply -only proven materials produced by known rarely because they cannot meet required manufacturers should be used high demands. For example, gold has too -only alloys resistant to corrosion should be low hardness, silver easily oxidizes and used, which is proved by investigations palladium has a high melting point. -long-term clinical performance should be in Therefore metals are alloyed by other metals focus or non-metals resulting in alloy of metallic -finally, it is important that the dentist takes character. By alloying is therefore exploited over responsibility and be responsible for the the good properties of each, and the reduced safety and effectiveness of any intervention. negative properties of alloy components. By knowing the basic properties of Since there are on the market a lot of the alloy such as: strength, hardness and different materials, the dentists have melting range the best suited alloy for the difficulties to choose the right one. Hence patient and certain application could be they should keep the guidelines, such as: determined. Today, of course, advances in - it is necessary the thoroughly knowledge of technology, enable the production of alloys the alloy with improved properties. Taking into -it is necessary to avoid a choosing the alloy account all metallic materials, which have so based on the color, unless other factors are far been applied in dentistry and which are the same currently in use, the biggest advantage may -it is necessary to know the chemical be given to titanium and titanium based composition of the alloy and to avoid the alloys due to their superior properties, the elements which could cause the allergic economy factor and the most important fact reaction of the patient that they are not harmful to the patient. -if it is possible, single-phase alloys should be used

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