CONTENTS: INTRODUCTION HISTORY OVERVIEW REGARDING CERAMICS ADVANCES IN METAL CERAMICS ADVANCES IN ALL CERAMIC RESTORATIONS CONCLUSION REFERENCES
INTRODUCTION: Esthetics, durability of the material and its biocompatibility in oral environment has always been and will be the foremost concern of the dentist. Esthetically ceramic can be considered as “better than the best�. Its color, translucency and vitality cannot as yet be matched by any materials except other ceramics further material is durable and biocompatible thus it holds a special place in dentistry and is widely used. Ceramic substances are not new. Infact our ancestors discovered ceramics thousands of years ago. Examples are many, earthen pots, bricks, crackers, wash basins, tiles sanitary etc. Term ceramic is defined as any product made essentially from a nonmetallic material by firing at high temperature to achieve desirable properties. Porcelain refers to a family of ceramic materials composed essentially of Kaolin, Quartz, Feldspar also fixed at high temperatures. TERMINOLOGIES: Dental ceramic: An inorganic compound with non metallic properties typically consisting of oxygen and one or more metallic or semimetallic elements (eg. aluminum, calcium, lithium, magnesium, potassium, silicon, sodium, tin, titanium and livconium) that is formulated to produce the whole or part of a ceramic based dental prosthesis. Glass ceramic: A ceramic consisting of a glass matrix phase and at least one crystal phase that is produced by the controlled crystallization of the glass.
Ceramic: An inorganic compound with non metallic properties typically composed of metallic and nonmetallic elements. HISTORY: 700 BC : Etruscans made teeth of Ivory and bone that were held in place by a gold framework. 1774 : French pharmacist – Duchateau. Procelain material called mineral paste teeth. 1789 : de Chemant-French dentist in collaboration with Duchateau – improved version of mineral paste teeth – but not used to produce individual teeth because there was no effective way at that time to attach the teeth to a denture base material. 1808 : Fronzi – Italian dentist “Ferrometallic” porcelain tooth that was held in place by a platinum pin or frame. 1817 : Plateau-French dentist introduced porcelain teeth to USA. 1822 : Peale on artist developed, a baking process in Philadelphia for these teeth. 1825 : Commercial production of these teeth began in Stockton. 1903 : Dr. Charles land introduced first ceramic crown to dentistry in 1903. Using platinum foil matrix and high fusing feldspathic porcelain low flexural strength – increased incidence of failure. 1962 : Weinstein and Weinstein – formulation of fedlspathic porcelain that allowed systematic control of the sintering temperature and thermal expansion coefficient – patient. 1962 : Weinstein et al components that could be used to produce alloys that bonded chemically to and were thermally compatible with feldspathic porcelain – patient. 1963: first commercial porcelain developed by “Vita Zahnafabrik”.
1965 : Mc Lean and Hughes – Aluminous core ceramic Al2O3–40 to 50 wt %. 1984 : Adair and Grossman – Dicor glass ceramic consisting of tetrasilicic fluoramica crystal in glass matrix. Machinable glass ceramic : Dicor MGC which has tetrasilicic fluoromica crystal volume of 70%. Early 1990 : Pressable glass ceramic i.e. IPS Empress 34 Vo. % leucite. IPS empress 2 – Containing 70% volume of Lithia disilicate crystals could be used for 3 unit FPD upto II premolar – fracture toughness was 3.3 MPa M1/2 3 times more than Empress 1. 1992 : Duceram LFC – low fusing ceramic – 3 unique feature. a) Hydrothermal glass – in which water is incorporated into the silicate glass structure to produce non bridging hydroxyl groups that disrupts the glass network, thereby increases the glass transition temperature, Viscosity and firing temperature increasing the thermal expansion coefficient to allow its use a veneer for certain low expansion metals. b) Self healing: By forming 1 µm thick hydrothermal layer along the ceramic surface. c) Extremely small size of the crystal particles (400-500 nm) enhances opalescence of ceramic by reflecting blue light hues from the surface and yellow hues from the interior of the ceramic. Other ultralow fusing ceramics now referred as low fusing ceramics have been introduced as veneering glass – are claimed to be kinder to opposing tooth enamel – due to glass phase or very small crystal particles. 1980’s: Level 1 System introduced and improvements in software and hardware led to level 2 and level 3. 3 system for production of ceramic inlays, onlays and veneers.
The future of dental ceramic is bright because the increased demand for tooth colored restorations will lead to an increased demand for ceramic based and polymer based restorations and the reduced use of amalgam and traditional cast metals. OVERVIEW OF DENTAL CERAMICS: Classification of dental ceramics: 1) Use or indications – anterior, posterior, crowns, veneers, post and cores and glaze ceramic. 2) Composition (Pure alumina, pure Zirconia, silica glass, leucite based glass ceramic and Lithia based glass ceramic). 3) Processing methods : Sintering, partial sintering and glass infiltration, CAD-CAM, and copy milling. 4) Fusing tamperature : low fusing 800-10500C : Medium fusing 1050-17000C : high fusing 1200-14000C 5) Microstructure : Glass, crystalline and crystal containing glass. 6) Transluscency : Opaque, transluscent and transparent. 7) Fracture resistance or abrasiveness Firing temperature: High fusing 13000C Medium fusing 1101-13000C Low fusing 850-11000 C Ultra low fusing 8500C
Crown and bridge construction
Production of denture teeth
Craig 11th edition Fabrication Machined
All ceramic
Crystalline phase Alumina Feldspar
Ceramic-metal Denture teeth
Slip Cast
Mica Alumina
Heat pressed
Spinel Leucite
Sintered
Lithium disilicate Alumina
Sintered Manufactured
Leucite Lucite Feldspar
Ceramics – Greek word “Kerami Kos” – meaning burnt earth. Porcelain-portuguese = Porcelong “little pig” = “Cowrie shell – Shell of molusca. The shape of cowrie shell and little pigs back are alike. The smoothness of cowrie shell and this earthen material were alike. So the name porcelain. Kaolin – from mountain Kao-ling in China. Kao-high
Ling – ridge.
Vitrous latin – Vitrous = Glass (amorphous) Devitrification – gradual formation of very small crystals in a glass and minerals so that glass becomes less transparent. Transparent – latin – trans = through. Parora = to appear = able to see through
Transsluscent – latin – lucis = lighting shine = Shining through. Veneer – French fournir = to fumish = to overlay a thin sheet of fine wood or other substance. Vitrification – converting into glass. Feldspar – is the name given to a group of rocks containing aluminosilicate along with sodium, potassium or calcium. It is found in igneous rock eg. granite and metamorphic rocks. High potassium containing feldspar is potaspar high sodium containing feldspar is sodaspar. High fusing
13000C
Medium fusing
1101 – 1300
Low fusing
850-1100
Ultra low
< 8500C
Denture teeth Crown and bridge construction
Used for titanium alloys because of their low contraction coefficient that closely match those of these metals and because the low firing temperatures reduce the risk for growth of metal oxide. Advance reduction in sintering times, decrease in sag deformation of FPD frame work, less thermal degradation of ceramic firing ovens, and less wear of opposing enamel surfaces. Porcelain manufacture: is produced from a blend of quartz, Feldspar and other oxides. During manufacture the materials are heated to high temperature to form a glassy mass and then rapidly cooled by quenching them in water, which causes the glassy mass to fracture. The resulting product is called frit. This process may be repeated several times, after which the frit is ball milled until the desired particle size distribution is obtained. Because fritting takes place at temperature much higher than those used in the fabrication of dental restoration, most of the chemical
reactions between raw materials occur before they are used in the dental laboratory. Most formulations designed for metal ceramic consists of mixture of two frits, a low fusing glass frit and a high expansion frit consisting of crystalline leucite with tetragonal geometry. This combination overcomes the two principal difficulties in veneering metal with ceramic-having a porcelain firing temperature well below the melting range of the metal and having a sufficiently high thermal expansion compatible with the metal. After firing in the laboratory, dental ceramic consists of about 20 volume percent tetragonal leucite crystals dispersed in a glassy matrix. The structure of this glassy matrix. The structure of this glassy matrix is a random Si-O network. The silicon atom combines with four oxygen atoms in a tetrahedral configuration. These tetrahedral may be linked into a chain with both covalent and atomic bonds, leading to stable structure. However such a Si-O network would have a very high melting point. Usually potassium and sodium are added to the glass composition to help break down the Si-O network and are therefore known as glass modifiers. In dental ceramics, potassium and sodium are initially provided by the feldspars. Two desirable consequences result 1) The softening temperature of the glass is reduced 2) the coefficient of thermal expansion is increased. The manufacturer adjusts the oxide constant so that the dental ceramics coefficient of thermal expansion will be close to the corresponding value for the alloys used to make the substructure. If the composition of the glass is not properly adjusted, extensive breakdown and reorganization of the Si-O network may occur leading to the crystallization / devitrification. This phenomenon may occur partially
in dental ceramics if a ceramic restoration is fired too often, and it is typically associated with an increase in the coefficient of thermal expansion and opacity. Feldspar also contains alumina which acts as an intermediate oxide to increase the viscosity and hardness of the glass. As a result, dental porcelain has a good resistance to slump or pyroplastic flow, which is necessary to obtain the desired configuration of the restoration. Dental porcelain are available as fine powders to be used with liquid water or a water based glycerine to form a paste of workable consistency. This is then shaped / moulded into a desired shape. It is then fired at a high temperature in order to fuse the particles together to form a ceramic body which is esthetically like a natural crown. Mode of supply: Supplied as a kit containing 1) Fine ceramic powders in different shade â&#x20AC;&#x201C; Dentin â&#x20AC;&#x201C; Enamel â&#x20AC;&#x201C; Opaquer 2) Special liquid / distilled water 3) Stains or color modifier 4) Glaze. Feldspar: Naturally occurring mineral and is a double silicate of potassium and aluminium. Basic glass former during firing feldspar fuses and acts as a matrix and binds silica and kaolin. Feldspar has two important properties:
1) When fused at higher temperature it retains its form without rounding â&#x20AC;&#x201C; due to increase content of potash which is more viscous. 2) Between 11500 and 15300C it undergoes incongruent melting and forms crystals of leucite in a liquid glass. Leucite is a potassium aluminium silicate mineral with a large coefficient of thermal expansion compared with feldspar glasses. Incongruent melting is a process by which one material melts to form a liquid plus a different crystalline material. Kaolin:
White clay like material Hydrated aluminum silicate Binder Increase opacity of mass
Silica: Obtained by grinding pure quartz. Silica acts as a refractive skeleton and provides strength and hardness to porcelain during fusing. It remains unchanged during firing. Aluminum oxide: Replaces some silica in glass matrix. It brings strength and opacity to the porcelain. It alters softening point and increases the viscosity of porcelain during fixing. Coloring frits: These are produced by fusing various metallic oxides with fine glass and feldspar and then grinding to powder. They are added to dental porcelain to obtain various shades to match natural tooth color. They are oxides of tin, nickel, cobalt, titanium, iron or gold.
TYPES OF PORCELAIN: • Following porcelain blends are produced for the different roles they play in the fabrication of metal ceramic restoration: opaque, body and incisal porcelains.
Opaque porcelain: Applied as I ceramic coat and performs two major functions. • Masks the color of the alloy • Responsible for metal ceramic bond. The density of the oxides is greater than that of glass matrix. Consequently, the oxides of tin, titanium and zirconium have a high refractive index than the components of the glass matrix. (Feldspar and
-> 2.01 to 2.61 and 1.52 to 1.54) when a specific
range of oxide particle size is used. Most of the incident light is scattered and reflected rather than transmitted through the porcelain, effectively masking the color of the alloy substrate. A SEM of alloy-porcelain interface for a high noble alloy. When a region around the interface is examined for specific elements – a technique called elemental mapping the concentration of aluminium and titanium can be identified. These appear as dense region in the figure, indicating that discrete oxide particles of Al and Ti are in opaque porcelain. Body porcelain:
Fired onto the opaque layer, usually in conjunction with the incisal porcelain. It provides some translucency and contains oxide that aid shade matching. Body porcelains are available in a wide selection of shades matching. Body porcelains are available in a wide section of shades to match adjacent natural teeth. Most porcelain manufactures provide an opaque shade for each body shade. Incisal porcelain: Usually translucent. As a result, the perceived color of the restoration is significantly influenced by the color of the underlying body porcelain. Porcelain – alloy bonding: • Oxide layer formation degassing conditioning bake oxidation • The linear coefficients of thermal expansion for the metal and ceramic must closely match to achieve a strong interfacial bond. αm = 13.5 to 14.5 x 10-6/0C αl = 13.0 to 14.0 x 10-6/0C The slightly higher coefficient for the metal causes the ceramic to be in a beneficial state of residual compressive stress at room temperature. Interfacial bond should be strong enough so that fracture occurs entirely with in porcelain. • Metal ceramic bond should exceed (28 mpa) to have cohesive failure through the porcelain rather than failure at interface. • Another philosophical change in the recommended method for evaluating the metal-ceramic bond appears to be underway with the introduction of ISO standard No 9693 for dental porcelain
fused to metal restoration, which contains a three point bending test. â&#x20AC;˘ Oxidation step for the alloy can be performed in air or by using the reduced atmospheric pressure (approximately 0.1 atm) available. A much thinner oxide layer will be formed if the alloy is oxidized at this reduced atoms pressure compared to the thickness for oxidation in air.
Porcelain alloy bonding Bond failure
ADVANCES IN DENTAL CERAMIC: Coping for metal ceramic prosthesis: 1. Electrodeposition of gold or other metal on a duplicate die.
2. Burnishing and heat-treating metal foils on a die. 3. CAD-CAM processing of a metal ingot. 4. Casting of pure metal or an alloy through lost wax technique.
Metal ceramics: Metal and porcelain veneer exhibit similar contraction curves and an average contraction coefficient difference of 0.5 ppm/ 0C or less fracture is unlikely to occur except in cases of extreme stress concentration or extremely high intraoral forces. These metal-ceramic combinations are known as thermally compatible system. 1984 ADA classification of dental casting alloys: Alloy type High noble
Most contain ≥ 40 wt% Av and ≥ 60 wt% of noble metal elements / Au, Pt, Pd, Rh, Ru, Iv, OS) Must contain ≥ 25 wt% of noble metal
Noble Predominantly
elements. base Contain a 25wt% of noble metal elements.
metal The term precious and semiprecious should be avoided because they are imprecise terms. Rather, term high noble, noble and predominantly base metal should be used. The principal reason that alloys for all metal restorations cannot be used for metal-ceramic restorations are as follows.
1. The alloys may not form thin, stable oxide layers to promote atomic bonding to porcelain. 2. Their melting range may be too low to resist. Sag deformation or melting at porcelain firing temperatures. 3. Their thermal contraction coefficient may not be close enough to those of commercial porcelain. For metal ceramic prosthesis: Metal type High noble
Metal ceramic prosthesis Pure Au (99.7 wt %) Au-Pt-Pd Au-Pd-Ag (5-12 wt% Ag) Au-Pd-Ag (>12 wt% Ag)
Noble
Au-Pd Pd-Au Pd-Au-Ag Pd-Cu-Ga Pd-Ga-Ag
PBM
Pd-Ag CPTi Ti-Al-U, Mi-Cr-Mo-Be, Mi-Cr-Mo, Co-Cr-Mo, Co-Cr. Base metal alloys were introduced originally during the upward
spiral of gold prices in the late 1970s and early 1980s, base metal alloys have been developed to the point where they are superior to high noble and noble alloys in social respects. Presently 40% of metal ceramic prosthesis are made of base metal alloys. When an alloy is identified according to the elements it contains, the components are listed in decreasing order of concentration with
largest constituent first followed by II largest and so on. An exception to this rule is the identification of certain alloys by elements that significantly affect physical properties or that represent potential biocompatibility concerns or both. E. g. : Ni-Cr-Mo be are after designated as Ni-Cr-Be alloys because of the contribution of beryllium to the control of castability and surface oxidation at high temperatures and because of the relative toxicity potential of beryllium compared to other metals. ALLOY SYSTEMS: High noble metal alloys: • Au-Pt-Pd I casting alloys Strengthened by precipitates of Fe-Pt intermetallic compound as copper could not be used due to color concern. Tin + indium for porcelain bonding undergo dimensional changes and not recommended for multiple unit FPD restorations. • Au-Pd-Ag : First lower gold content alternative alloys to be widely used in the 1970s. Platinum was eliminated from the alloy composition, gold content was reduced to about 50%, with corresponding increase in palldium and silver. • Tin + Indium oxidation • Green discoloration has been reported resulting from diffusion of silver atoms into the porcelain. The discolored region can be ground away but this involves an additional processing step.
• Silver vapor generated in the porcelain furnace during processing can contaminate the muffle, and periodic purging of the furnace with carbon block is required. • Green discoloration has been apparently been eliminated in some porcelain composition by substituting K
–
for Ma- the larger
potassium ions impede the diffusion of silver into the porcelain Discussion: Au-Pd and Au-Pd-Aq alloys have higher values of Y.S and MOE along with lower density than Au-Pt-Pd thus FPD will be more resistant to masticatory forces and undergo less bending. Noble metal alloys: Minimum of 25% wt. of noble metal. Pd-Aq – Presence of palladium, silver appears to assume noble metal character which is beneficial for corrosion resistance. • Increases silver can cause porcelain greening and furnace contamination. • Gold metal conditioners or ceramic coating agents may minimize this effect. • Compared to high noble they have similar values of yield strength, and modulus of elasticity and much lower density values. • Some of these alloys may form internal rather than external oxides. Instead of formation of the desired external oxide. Pd-Ag nodules may develop on the surface which enhance retention by mechanical then chemical.
Pd-Cu-Ga: Increases than 70% wt. palladium Melting point Pa + 15550C, Gold + 10640C.
Gallium melting 300C • Multi orifice torches are required to fuse high palladium alloys. Ceramic crucibles dedicated to individual alloys is recommended. • Carbon containing investments – cause incorporation of very small amount of carbon degrading the bond strength with porcelain. • Some Pd-Cu-Ga have increases hardness than enamel-casting from these alloy difficult to finish in lab. Substituting indium for tin yields Pd-Cu-Ga alloys with much lower hardness VHN = 270. Yield strength 690.0 MOE 140 YS 970, Elong 20, VH N 340. Pd-Ga : 1980 to provide lower hardness than that of initial Pd-Cu-Ga formulations. The hard Pd5GO2 phase absent Pd-Ga-CO alloy has particular dark oxide that is more difficult to mask with dental porcelain Legacy (Jelenko)
Pd 85.2
Ga 10.0
In 1.1
Au 2.0
YST 720, MOE 130, TS 970, Elong 25, VHN 280, Density 10.9 Protocol (Jelenko)
Pd 75.2
Ga 6.0
In 6.0
Au 6.0
Ag 6.5
Y.S 500, MOE 100, TS 760, Elong 34, VHN 240, Das 11.0 Predominantly base metal: Ni-Cr • ↑ MOE – so ↓ flexure • ↑ technique sensitive and difficult to cast compared to base metal.
Yield strength, modulus of Elasticity, can greatly be affected by small differences in weight percentages of minor elemental components among composition of these alloy. • Many Ni-Cr has 2% by weight berrylium – it ↓↓ the melting range and to decrease the viscosity of molten alloy ↑↑ its castability also effects strengthening and thickness of oxide layer. • Berrylium vapor exposure and dust exposure is more significant especially to lab technicians. • Potential hazard of berrylium is based on atomic considerations than its wt % in alloy. I.P % wt Be = 10.7% Be on an atomic basis Risk is greatest during alloy melting adequate exhaust and filtration system. OSHA – recommendation 24g/m3 of air determined from an 8 hr time weighted average. •
Physiological response vary from contact dermatitis to severe domical pneumonitis, which can be fatal symptoms also range from coughing, chest pain, general weakness to pul dysfunction.
Nickel - OSHA 15µg/m3 for 10hr / day Occupational exposure in referring markers has led to lung and nasal cancer. Acute effects – skin sensitization that can lead to chronic eczema therefore operator should neat mask and use efficient suction when grinding and finishing a nickel base alloy. Ni – carcinogenic - ↑ for dental technician ADA states that Ni alloys should not be used in individuals with known nickel sensitivity.
20 patient clinical study 10 control – no sensitivity to nickel – reresponse 10 – known sensitivity P showed positive dermal response to alloy and on nearing intraoral appliance 30% manifested an allergic response within 48 hrs Rexillium III (Penton)
Ni
Cr
MO
Al
Be
76
14
6
2.5
1.8
YS – 830, MOE 180, TS 1100, Elong 15, VHM 360, Rens 7.8. Microband MP2 (Astenal)
Mi
Cr
Ga
Mo
Fe
Si
Mn
Be
66.6
13
7.5
7.0
5
0.75
0.15
0
YS – 260, MOE 170, TS 640, Elong 19, VHM 170, Density 8.6 For Mickel allergy take proper history Co-Cr – The potential health problems associated with the Berrylium and Nickel containing alloys have led to the development of CoChromium. MOE is highest, very less elongation So decreases elongation and increases hardness suggest finishing restorations made with it may be difficult. Genesis II (Jelenko)
Co 53
Cr 27
W 10
Ru 3
Ga 3
Cu 1
Ys – 520, MOE 170, TS 760, Elong 15, VHM 350, D 8.8 Movavex (Pentron)
Co 55
Cr 25
W 10
YS – 620, MOE 220, TS 760, Elong 7, VHM 350, D 8.8. Ti – Titanium based alloy adv.
Mb 1
Ta 1
Excellent biocompatibility, corrosion resistance due to thin passivating surface layer i.e TiO2 Low density 4.5 gm/cm3 + results in lighter and potentially less expensive restoration. Alloys used are : Cp (Commercially pure) Ti Ti-6Al-4V. Casting problem due to increasing melting point 16680C Strong tendency to oxidize with other materials. Special casting machine: With vacuum / argon pressure Argon are melting / induction melting. A very hard near surface region that can exceed 50 µm in thickness is also present on the lasting due to reaction of titanium alloy with investment and perhaps residual atmosphere in casting machine called as α case. To overcome – A system developed by manufacturers copings from blocks of pure titanium by machine duplication and spark erosion procera. Because of presence of α case – special surface modification of last titanium using castic mooh based solutions or silicon nitride coatings have been employed to improve bond between cast Ti and dental porcelain. Alternatives: 1) Sintering / diffusion bonding of burnished metal foil Most commonly used i.e. captek requires 3 pairs of material to form composite metal structure.
i)
Captek P and Captek G which are used to fabricate crown coping and fixed partial denture abutments.
ii)
Capcon and capital which are used to connect copings
iii)
Captek repair paste and copfil which are used to add material to captek structures.
The copings thickness 0.25mm – anterior crown 0.35 mm – posterior crown Coping contains 88.2 wt % Au 9 wt % platinum group metals 28 wt % Ag. Captek p layer is adapted first to the die and find at a temperature of 10750C. During this firing cycle adhesive and binders are eliminated, and the Pd and Pt particles becomes interconnected by sintering to form 3 dimensional network of capillary channels. Capteck G is applied over captek P .Captek G is drawn by capillary action into the network structure of captek P coping vacated by the adhesive binder. 0.35 mm thick layer of procelain is applied to the coping, which may or may not require capbond bonding agent. Advantage: very low metal thickness can be achieved thus minimal tooth reduction / increase esthetics Ex : Metal margin of captek coping can be ground to a thickness of 50 µm and total thickness of PFM can be as low as 0.3 mm. Although anterior 0.7 – 1 mm for optimal esthetics Posterior crown 1.2 mm to resist fracture. CAD CAM processing:
•
Provides an alternative method to produce metal, ceramic or composite restoration without need for 2 / more appointment.
•
Although CAD CAM for metal crown is not commonly used.
•
Milled or ground metal block more widely used
•
Electrolytic or electrical discharge removal of metal.
Copy milling: This process is based on the principle of tracing the surface of a pattern that is then replicated from a blank of ceramic, composite or metal that is ground, cut or milled by a rotating wheel whose motion is controlled by a link through the tracing device. The process is similar to that associated with cutting a key blank using a tracing of a master key. Commercial
system
celay
(1991).
Mikrona
technologies,
Switzerland. Electroforming: •
Helioform HF 600 system – thin pure gold coping.
Step by step procedure: •
A master cost of the prepared tooth is prepared and coated with a special die spacer to facilitate separation of the duplicating material.
•
Dies are duplicated with a gypsum product and having setting expansion 0.1% to 0.2%.
•
Apply an even coat of the silver spacer to the preparation and allow it to dry.
•
Insert the dies into the plating equipment a magnetic stirrer ensures circulation of the cyanide free gold sulfite solution.
•
Turn on the electric current and gold will be deposited on the die at an approximate rate of 0.02 mm/hr.
•
Remove the plated copings by heating the dies and remove the silver spacer with nitric acid or air abrasion.
•
Seat the coping on the die
•
Air abrade the surface and apply special bonding paste before porcelain application.
Ceramic for PFM: •
The major drawback with conventional PFM restorations was poor bond strength between the metal substrate and the veneering porcelain due to difference in co-efficient of thermal expansion of the two.
•
The crystalline mineral leucite is included in procelain for metal ceramic restorations to elevate their thermal expansion coefficient to match that of casting alloys.
•
Glass modifies - Bonds between the silica tetrahedra can be broken by the addition of alkali metal ions such as Na, K and Ca. These ions are associated with the oxygen atoms at the corners of the tetrahedra and interrupt the oxygen silicon bonds. As a result, the 3 dimensional silica network contains many linear chains of silica tetrahedra that are able to move more easily at lower temperature than the atoms that are locked into the 3 dimensional structure of silica tetrahedra. This ease of movement is responsible for the increased fluidity, decrease softening temperature and increase thermal expansion conferred by glass modifiers. Too high a modifier
concentration, decrease chemical durability that is resistance to attack by water acids and alkalis, if too many tetrahedral are disrupted the glass may crystallize during porcelain firing. •
Another important glass modifier is water the hydronium ion H3O+ can replace sodium or other metal ions in a ceramic that contains glass modifiers. This fact accounts for the phenomenon of “slow crack growth” of ceramics that are exposed to tensile stresses and moist environments. It also may account for the occasional long term failure of porcelain restoration after several yrs of service.
Metal substructure design: Porcelain labial margins: Many patient object to the grayness at the margin associated with metal ceramic restorations. If esthetics is of prime importance a collarless metal-ceramic crown should be considered. Advantages: •
Esthetic improvement
•
Plaque removal also is easier when gingival tissues are in contact with vaccum fired glazed porcelain than when they are contacting highly polished gold. Therefore porcelain material of choice for restorations that will contact gingival tissues.
Disadvantages: •
Difficulty in fabrication
•
Marginal adaptation of these restoration is slightly inferior to cast metal
•
Fracture of margin during handling
•
Increase time and so increase cost.
Indications: •
Indicated when conventional metal ceramic restoration will not create the desired esthetic result.
Contraindications: When an extremely smooth, 1 mm-wide shoulder cannot be prepared in the area of ceramic veneer and in this respect conventional metal ceramic restoration is somewhat more forgiving.
Methods of fabrication: •
Porcelain labial margins
•
Metal-ceramic restorations
•
Platinum foil matrix
•
Direct lift / cyanoacrylate resin
•
Procelain wax
Platinum foil matrix technique: Procedure: a) Wax the metal substructure and cast it in the conventional manner. Some technicians prefer to cast a conventional restoration and trim the collar off the casting; others wax the substructure exactly as desired and cast it.
b) To prevent the foil from becoming distorted on removal block out undercuts apical to the margin, modelling compound is suitable for this step. Amy excess that may have covered the shoulder of tooth preparation must be carefully cleaned off. c) Burnish a small piece of platinum foil onto the facial portion of the die where the porcelain margin is to be placed and extend it a few millimeters onto the axial wall of the preparation. d) After burnishing trim it so there is a 2 to 3 mm skirt lying cervical to the margin. e) Carefully place the coping over the foil and if necessary, scrape the die in the cervical area to allow the casting to seat. To help stabilize the foil, a drop of sticky wax may be applied. f) Remove the casting from the die, together with the foil, and position the assembly between the electrodes of an orthodontic spot welder. The foil can now be welded to the framework, which should be done as close as possible to the edge of the metal. Four or five welds are usually adequate to attach the foil to the substructure. The restoration is then fabricated in a conventional manner, although the facial margin is similar to that for the porcelain jacket crown. The foil itself is not covered with opaquer at this time. However, The restoration is build to contour, and the marginal portion is ditched as for the porcelain jacket crown, some technicians prefer to pain a thin coat of separating liquid on foil instead of ditching. g) When the coronal portion has been shaped to a satisfactory contour, burnish the foil and fill in the ditched portion with cervical porcelain. h) When the desired contour has been obtained after-timing, trim the platinum skirt.
i) When satisfied with characterization, staining and glazing, remove the foil and cement the restoration after verifying the fit one more time. Direct lift / cyanoacrylate technique: â&#x20AC;˘
Less time consuming
â&#x20AC;˘
Easy to perform Difficulty associated during staining and glazing firing because
porcelain is not supported as in the platinum foil technique, the margin tends to round off slightly, therefore, special shoulder powders are needed. Procedure: a) Apply cyanoacrylate resin to the labial margin area of the die. This acts as a sealant of the porous stone, compressed air should be used to minimize the thickness of the film. b) Apply porcelain release agent to the shoulder of the prepared die. c) Seat the opaqued casting on the die. d) Mix shoulder porcelain and apply it directly to the die and the opaque porcelain. Light tapping will assists in condensation and should be done before separating the dry buildup from the die. e) After the first firing of the shoulder porcelain reseat the crown on the die. At this time, the restoration should be examined for margin discrepancies. A second shoulder firing is usually necessary. f) Relubricate the die, reseat the crown and apply a thinner mix of shoulder powder to the margin. Vibration will help the porcelain fill the defect completely. After blotting, the restoration can be separated from the die.
g) After firing is completed, proceed with the conventional build up of body and incisal porcelains, followed by glazing of the final restoration. Porcelain wax technique: A mixture of body porcelain and wax (6:1 by wt) is applied to the die for final adaptation of porcelain labial margin. Procedure: a) After coating the substructure with opaque porcelain, lubricate the die with a porcelain release agent. b) Apply the porcelain-wax mixture to the cervical shoulder use an electric waxing instrument to flow it into the proper areas. c) With a conventional wax-carving instrument, shape the material and blend it into the opaque. It will separate easily from the die and can be fired in the conventional manner. d) A second application will be needed using the electric waxing instrument, keep the mixture liquid long enough so that capillary action can draw it into the marginal discrepancies. The restoration is completed in the conventional manner. If rounding of the margins is experienced during glazing, using a shoulder porcelain rather than a body porcelain in the wax mixture is recommended. Advantages and disadvantages: Method Platinum foil
â&#x20AC;˘
Advantages No shoulder â&#x20AC;˘ porcelain esthetics
and
best
Disadvantages Time consuming
â&#x20AC;˘
Difficult
•
Good marginal adaptation
• Wax suspension
Smooth
surface, increase plaque Separates easily
•
Shoulder porcelain needed
• Direct left
Least time consuming
Less
accurate fit • Shoulder porcelain needed •
Rougher margins
ALL – CERAMIC RESTORATIONS: •
The chief disadvantage of the early restorations was their low strength, which limited their use to low stress situation, such as anterior teeth. Even so, fracture was a fairly common occurrence, which prompted the development of higher-strength material.
•
These development have followed two path o Two ceramic material a high strength but nonesthetic core material is veneered with esthetic porcelain but color of ceramic core is easily masked o Development of a ceramic that combines good esthetics with high strength.
All ceramic systems Aluminous core ceramics: •
McLean and Hughes 1965.
â&#x20AC;˘
Advocated using aluminous porcelain, which is composed of aluminum oxide crystals dispersed in a glassy matrix. Their recommendation was based on the use of alumina reinforced porcelain in the electrical industry and the fact that alumina has increase fracture toughness and hardness.
â&#x20AC;˘
Method consists of bonding aluminous porcelain to platinum foil copings. Attachment of the porcelain is secured by electroplating the platinum foil with a thin layer of tin and then oxidizing it in the furnace to produce a continuous film of tin oxide for porcelain bonding.
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The rationale - foil act as inner skin decrease subsurface porosity and formation of micro cracks in the porcelain, increase strength.
Procedure: a) Core porcelain buildup. b) Thin blade is used to form a cervical ditch which will prevent the matrix from becoming distorted during the first firing. c) The fired cure should be checked with a thickness guage. d) Foil is readapted to the margin and the ditch is filled with additional porcelain. Core should be much thinner on the facial surface for esthetic reason. e) Core is seated on the working cast before the application of body and incisal porcelains. Advantages: Better in esthetics compared to metal ceramic that employs a metal coping.
Disadvantages: Strength is inadequate to be used for posterior teeth Mclean has reported a fracture rate of molar aluminous porcelain crown of approximately 15% after 54 years. Slip cast ceramics – In- Ceram Alumina, In- Ceram Spinell and In- Ceram Zirconia a) Slurry of one of these materials is slip-cast on a porous refractory die and heated in a furnace to produce a partially sintered coping or framework. b) The partially sintered core is infiltrated with glass at 1100 for 4 hrs to eliminate porosity and to strength slip cast core. c) Decreased shrinkage thus good fit. d) Flexural strength ICS –350 Mpa, ICA—500 Mpa, ICZ—700 Mpa
STEPS FOR FABRICATION OF IN-CERAM PROSTHESIS: 1) Prepare teeth with an occlusal reduction of 1.5-2mm and a heavy circumferential chamfer(1.2). 2) Make an impression and pour 2 dies 3) Apply aluminium oxide on a porous duplicate die 4) Heat at 120 for 2 hrs to dry aluminium oxide 5) Sinter the coping for 10 hrs at 1120 6) Apply a sodium lanthanum glass slurry mixture on the coping 7) Fire for 4 hrs at 1120 to allow infiltration of glass 8) Trim excess glass from the coping with diamond burs 9) Build up the core with dentin and enamel porcelain and fire .
Hot pressed ceramic â&#x20AC;&#x201C; Glass ceramic is a material that is formed into desired shape as a glass , then subjected to heat treatment to induce partial devitrification. The crystalline particles, needles or plates formed during the ceramming process serve to interrupt the propagation of cracks in the materials. --Glass ceramic in dentistry was first proposed by Mac Culloch in 1968. Pressable ceramic uses a piston to force a heated ceramic ingot through a heated into a mould, where the ceramic form cools and hardens to the shape of the mould. when the object has solidified , the refractory mold is broken apart and ceramic piece is removed. Then it is stained , glazed or veneered with one or more layer of thermally compatible ceramics. eg IPS Empress
1â&#x20AC;&#x201C; contains increased concentration of leucite 35
volume % that increases the resistance to crack propagation. Advantage â&#x20AC;&#x201C;translucent ceramic core , moderately increased flexural strength and excellent fit and esthetics. IPS Empress 2 --- Lithia disilicate crystals reinforces glass ceramic.Veneering ceramic contains apatite crystals causes light scattering in a way that resembles the scattering by the structure and components of tooth enamel Both IPS Empress 1 and 2 have low to moderately increase flexural strength and fracture toughness. These properties limit their use to conservative designs on low and moderate stress environment.
Procedure: a) Wax the restoration to final contour. Sprue and invest as with conventional gold castings. If veneering technique is used only the body porcelain shape is waxed. b) Heat the investment to 8000C to burn out the wax pattern. c) Insert a ceramic Ignot of the appropriate shade and alumina plunger in the sprue and place the refractory in the special pressing furnace. d) After heating to 11500C, the softened ceramic is slowly pressed into the mold under vacuum. e) After pressing, recover the restoration from the investment by airborne particle abrasion, remove the sprue and refit it to the die. Esthetics can be enhanced by applying an enamel layer of matching porcelain or by adding surface characterization.
Fabrication of restoration.
DICOR AND DICOR MGC – •
Life expectancy of dicor crowns in high stress areas is not as good as that of PFM crown.
•
Two veneering materials to improve color of dicor are. Dicor plus Consisted of pigmented feldspathic porcelain veneer
Willi’s Glass Veneer of vitadur N aluminous porcelain
Tooth preparation: Same as PFM except that for I and II molars a reduction of 2mm is recommended. •
Axial surfaces should be reduced a minimum of 1mm preparation should be either a shoulder with a rounded gingivoaxial line angle / heavy chamfer.
CAD-CAM: •
Mormann and Brandestini for the first time used a CADCAM device to digitize and store cavity parameters, and a computer milling device to then shape a restoration out of the ceramic block.
•
Commercially available as integrated CAD-CAM unit for dental use in 1988 by Siemens known as CEREC. This came to known as CEREC 1 when in sept 1994 an improved version CEREC 2 was introduced.
•
The unit consists of 3 dimensional video cameras (Scan head). An electronic image processor memory unit and a processor (computer) which is connected to miniature milling machine.
•
Cerec 1 cannot prepare the occlusal anatomy
•
Cerec 2 is also equipped with a cylindrical diamond stone which as able to finish off undercuts at buccal extensions, curved shoulders at cusp preparations and the proximal areas.
Cavity consideration for CAD-CAM inlays: •
Requires
conventional
cavity
design
with
slight
modifications to accommodate computerized milling device. •
No convexities should be present on pulpal and gingival floor – Either flat / concave buccolingually.
•
Occlusal step should be prepared 1.5-2 mm in depth and any isthmus or groove extension should be 1.5mm wide to decrease the possibility of the fracture of the restoration.
•
Buccal and lingual walls of occlusal portion of the preparation may converge towards the occlusal. This feature is unique to CEREC system as it can automatically block out any undercuts during the optical impression. A more conservative cavity preparation is therefore permissible along the occlusal aspect especially when replacing old amalgam restorations where undercuts were purposely given for retention. The facial and lingual walls in the proximal box are prepared in the usual fashion with slight divergence towards the occlusal, convergence is not given there so as to avoid excessively thick composite cement lines.
•
Axial wall straight and not follow convex contour of the proximal surface of the tooth.
•
No cavosurface or marginal bevels.
Five steps are basically involved in any type of CAD-CAM system: 1) Computerized surface digitization
After tooth preparation a scanning device is used to collect information on the shape of the preparation called as “Optical impression”. •
An image of the preparation is displayed on the monitor. Repeated optical impressions are taken until the most ideal is found and stored in the computer.
•
Scanning devices can be either Mechanical - By profilometer / pin point sensors are very precise. However they have shortcoming - Scanning tip produces errors in measuring steep flanks and distort easily. - Undercut areas and marrow gaps like fissure also cannot be explored and have to be blocked out.
•
Optical (Infrared video camera) - Not able to measure highly transparent reflective surfaces and so enamel has to be covered with powder or water soluble colour. -
Can be prepared directly / indirectly
Advantages: •
From cavity preparation to bonding one session.
•
Adjustment done in mouth obviating the need for opposing cast and articulator mounting, lab facilities not required.
•
Interim restoration not necessary, no lost temporary, sensitiveness and microleakage.
Disadvantages: •
Waste of expensive chair time in case of difficulty during milling or designing
•
Time required is increased.
•
Multiple
practitioners
sharing
same
CAD-CAM
simultaneous appointment cannot be given. 2) Computer aided designing: involves three dimensional image processing. •
The operator enters data and confirms the features of the preparation like boundaries of the restoration, position of the gingival margins, proximal contacts and contours, buccal and lingual extensions etc.
•
The collected data is further processed by curve smoothening and if necessary by data reduction. Undercuts can be blocked at this stage.
3) Computer aided manufacturing (CAM) •
The cavity surface of inlays, onlays and crowns are milled to the dimensions of the scanned image with diamond disks or other instruments that are electrically driven and lubricated with water.
•
Cerec 1 occlusal surface cannot be ground – is completed later by the operator using Diamond burs.
•
Cerec 2 can form occlusal surface also. Controlled cutting of the ceramic is done by rotation of the block, horizontal movement of the block into the wheel and vertical movement of the cutting wheel. The fit of the restoration is confirmed in the patients mouth and any necessary adjustments made.
•
The inlay is then prepared for bonding which includes preparation of the restoration and the tooth. Etching is done on the cavity surface of the inlay either with a microetcher and / or ammonium bifluride or hydrofluoric acid. After etching, a silane
bonding enhancer is painted onto the surface. Depending on the bonding system used, the appropriate primer and bonding agent are applied to the tooth surface and then cemented. •
Glazing may not be required in these restoration as they are easily polished.
• Disadvantages CAD-CAM: 1) Initial high cost for the purchase of unit 2) Time and cost must be invested to master the technique. 3) Contouring of the occlusal surface may still have to be carried out by the clinician. Cerce 2 system: 3 materials can be used with this system. Vita mark II : Contains sanidine (KAISi3O8) as a major crystalline phase within a glassy matrix. Dicor MGC: Mica-based machinable glass ceramic that contains 70 vol % of crystalline phase “House of cards” Microstructure found in Dicor MGC is due to the interlocking of the small platelet. Shaped mica crystals with an average size of 1 to 2ùm. This particular microstructure leads to multiple crack deflection and ensures a greater strength than leucite-containing ceramics. •
Procad is leucite-containing ceramic designed for making machine restorations.
•
Fabrication time for crown is about 20 min
Copy milling technique:
•
Best known copy grinding system is celay (Mikrons Switzerland) 1991 commercially
•
Originally, the system was developed to produce inlays and onlays, however recent crown copings, bridge substructure and veneers.
•
Celay blocks, inceram spinel and inceram used.
•
Copy milling technique is based on the idea of first fabricating a prototype inlay-called pro-inlay which is then copied using a scanning tool or micropalpation method. The final restoration is then milled from a preformed ceramic block. It is highly necessary to stress here that any carving / tooth preparation should be free of undercuts.
•
Conservative approach is the blocking of undercuts in the cavity by using a resin modified glass ionomer on the die if an indirect method of manufacturing is used.
•
Proinlay is fabricated with a blue resin based composite (celay tech) made directly on a prepared tooth or indirectly on a die made from the impression. The prototype is fixed into the celay unit. As the surface of the proinlay is scanned with a tracing tool (smooth disc), a coarse diamond coated disc (124 µm grain size) simultaneously roughs out the shape of the ceramic restoration.
•
A true white powder is applied to the proinlay and the scanning is again done using a smooth disc and fissured and tapered burs. Matching fine diamond disc and burs (60-70 µm grain size) refine the shape of the gross ceramic restoration.
•
Once the white powder is completely traced off, milling of ceramic inlay is considered complete.
•
Average time taken is 20-30 min.
•
One problem frequently encountered is difficulty in obtaining proinaly.
•
Marginal accuracy seems to be good, a little better than CEREC 2 system.
Cerce 3: new •
View all surface of the restoration from all angles and make any modifications required with the possibility to immediately viewing and assessing the effects on the entire restoration.
•
Has ability to superimpose optical impressions of the preparation and the antagonists permits optimal adjustments of the proximal and occlusal contacts to the neighboring and opposing teeth.
•
Professor Korda from university of Greifswald, provided a break through in which a crown with suitable occlusal surface erupts into the occlusal surface of the antagonists until it has reached a stable position with maximum intercuspation called Biomimetic process.
•
CEREC shows a list of available tooth database after selection of the most appropriate database, CEREC automatically fits the crown between the neighboring teeth, and use them to adjust the position and height of the occlusion. Thus above mentioned biomimetic procedure take place.
•
CEREC turns and shifts the occlusal surface of the selected database crown in all direction until it fits optimally to the opposing teeth.
•
CEREC 3D – process not only copied from the nature but also is much faster and more efficient than all existing method for developing occlusion.
Other advantages: •
Second restoration can be designed while first is being milled.
•
The grinding unit is also equipped with a laser scanner and can also be used for indirect application.
•
Equipped with intraoral video camera / digital radiography unit for patient education and for user training.
PROCERA
ALL
CERAM
SYSTEM:
Involves
an
industrial
CAD/CAM process. The Die is mechanically scanned by the technician, and the data are sent to a work station where an enlarged die is milled using a computer controlled milling machine. This enlargement is necessary to compensate for the sintering shrinkage. •
Aluminium oxide powder is then compacted onto the die, and the copying is milled before sintering at very high temperature (>15500C).
•
The coping is further veneered with an aluminous ceramic with matched thermal expansion.
•
Good clinical performance and marginal adaptation.
•
May be suitable for posterior crowns and FPD although long term data are needed.
Procedure: •
Tooth preparation
•
Cast made in conventional way but the die is ditched to make the margin easier to identify during scanning.
•
Die is mapped using contact scanner.
•
Design of the restoration is transferred to the manufacturer via computer line.
•
Production process starts with milling an enlarged die to compensate for the sintering shrinkage
•
An enlarged high-alumina coping is milled that shrinks to the desired shape after sintering.
•
The coping is returned to the laboratory, and bondy and incisal porcelain are applied in the conventional manner.
Cicero Dental Systems B.V. Cicero-computer
integrated
ceramic
reconstruction
--HORN
Netherlands. •
Uses optical scanning, ceramic sintering and computer assisted milling techniques to fabricate restorations with maximum static and dynamic occlusal contacts relation.
•
Technique consists of optically digitizing a gypsum die, designing the crown layer build up, and subsequent pressing, sintering and milling consecutive layers of shaded high strength alumina core material, a layer of dentin porcelain, and a final layer of incisal
porcelain. Final finishing is performed in the dental
laboratory. •
Allows efficient production of all ceramic restorations without compromising on esthetic or function.
CONCLUSION
Although predictably stronger materials have been developed the universal use of all ceramic restorations is not yet a reality. However clinical evaluation of recent developments may indicate their wider usage. It is no exaggeration to state that the last century saw a revolution in dental esthetics and is expected to continue, which will be influential in determining the range of ceramic products made available.
REFERENCES
1) Phillips science of dental material --- Anusavice, XI edition 2) Restorative dental material—Craig,Powers XI edition. 3) Contemporary fixed prosthodontics—Rosenstiel, Land and Fujimoto 4) Fundamentals of fixed prosthodontic—Shillingberg III edi. 5) Advances in clinical prosthodontics—Gent, Geneve. 6) Fixed prosthodontics.DCNA2004 vol48,no 2. 7) Art and science of dental ceramic---John Mclean.