DIE MATERIALS IN FIXED PARTIAL PROSTHESIS
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INDIAN DENTAL ACADEMY Leader in continuing dental education www.indiandentalacademy.com
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INTRODUCTION Accurate duplication of prepared teeth and maintaining their relationship to one another are important in constructing fixed partial prosthesis . For this it is necessary to obtain an accurate reproduction of prepared tooth, adjacent opposing tooth and surrounding soft tissues. Since direct fabrication in the mouth is inconvenient, difficult, time consuming and virtually impossible all wax patterns for extracoronal restorations are made by indirect technique in the laboratory. www.indiandentalacademy.com
An accurate working cast with removable dies is essential to make a well fitting restoration . Detailed reproduction of die materials for fixed prostheses affects the accuracy of working casts and is related to the compatibility between the die and impression materials. A working cast is the replica of the prepared teeth, ridge areas and other parts of the dental arch. www.indiandentalacademy.com
Die may be defined as : ďƒ˜It is the positive reproduction of the form of the prepared tooth in any suitable substance. (GPT 7)
ďƒ˜A reproduction of a prepared tooth made from a gypsum product, epoxy resin, a metal or a refractory material. Anusavice ( 11th edition ) www.indiandentalacademy.com
ďƒ˜A die is a model of a single tooth, again prepared from an impression. (E.C.Combe) ďƒ˜The die is a positive reproduction of the prepared tooth and consists of a suitable hard substance of sufficient accuracy usually an improved stone, resin or metal. (Rosenstiel et.al) ďƒ˜The die is a model of the individual prepared tooth on which the margins of the wax patterns are finished. (Schillingburg)
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A cast and die system captures the necessary information so that it can be transferred to the laboratory. There are two basic working casts and die systems : A working cast with a separate die and a working cast with a removable die.
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MATERIALS USED FOR MAKING DIES IN FIXED PARTIAL PROSTHESIS 1. Gypsum products-
Type IV Dental Stone Type V Dental stone
2 Die stone -
Investment combination
3. Electroformed dies-
Silver plated Copper plated
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4. Epoxy resins 5. Polyurethane
Alternative die materials 6.Silicophosphate cement 7.Amalgam 8.Metal sprayed dies 9.Ceramic die materials www.indiandentalacademy.com 10.Flexible die materials
DESIRABLE
QUALITIES
OF
DIE
MATERIALS They should accurately reproduce all fine details in the impression. They should be dimensionally stable. Setting expansion, contraction and dimensional variation in response to setting or change in temperature should be minimal. The die should have a smooth surface that is sufficiently strong to withstand the subsequent www.indiandentalacademy.com
manipulative procedures without abrasion of the surface that is it should have the ability to resist abrasion. ďƒ˜They should be compatible with impression materials and there should be no interaction between the surface of the impression and cast or die. ďƒ˜Toughness to resist breakage during fabrication or burnishing of fine edges. www.indiandentalacademy.com
Colour of the die should be in contrast to the colour of wax. This helps to facilitate the manipulative procedures that will be carried out. It should be reasonably easy to use without excessive manipulative procedures. It should be relatively inexpensive.
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THE REQUIREMENTS FOR DIE AND WORKING CASTS It must reproduce the prepared tooth exactly. All surfaces must be accurately duplicated, and no bubbles or voids should be present. A die must be made of a material that is dense, hard and capable of being used in production of wax patterns and in fitting and finishing of casting without undue risk of damage to its surface. www.indiandentalacademy.com
Die must be given a form that allow easy handling during waxing and other procedures. The die must have root like extension that serves as a handle. The
remaining
unprepared
tooth
structure
immediately cervical to finish line should be easily distinguished on the die, ideally with 0.5 to 1mm visible. Adequate access to the margin is necessary. www.indiandentalacademy.com
GYPSUM PRODUCTS It is a mineral mined in various parts of the world chemically applications
the is
gypsum pure
produced
calcium
sulfate
for
dental
dihydrate
[CaSo4.2H2O]. Gypsum products are used in dentistry for the preparation
of
study
maxillofacial structures
models
for
oral
and
and as important auxillary
materials for dental laboratory operations that are www.indiandentalacademy.com involved in the production of dental prosthesis.
International standard ISO 6873:1998 “ Dental gypsum products“This standard identifies five types of materials.
TYPES OF GYPSUM PRODUCTS : 1. Impression plaster [Type I]: Used to make replicas of the soft tissues when making prosthesis. 2. Mounting plaster [Type II]: used to mount casts on an articulator and make casts of oral structures when strength is not an important consideration. Both Type I and Type II are white plaster [plaster of paris ]. www.indiandentalacademy.com
3.Dental stones [Type III]: These are products used to make yellow coloured casts used as models of the hard and soft tissues of the mouth. 4.Dental stone high strength [Type IV] and 5.Dental stone, high strength, high expansion [Type V] These special stones are much harder and denser then regular stones. In general these are used when extreme accuracy in the models is demanded, such as in the dies used to make crowns, bridges and frame works for prostheses. www.indiandentalacademy.com
PRODUCTION OF STONE : Plaster and stone products are produced by calcining calcium sulphate dihydrate or gypsum. Gypsum is ground and subjected to temperatures of 1100 to 1200c to drive off part of the water of crystallization and is converted to calcium sulphate hemihydrate[ CaS04 .1/2 H2O]. : www.indiandentalacademy.com
The temperature is further raised and following products are formed
1100-1300c
1300-2000c
CaSO4.2H2O → CaSO4.1/2 H2O → Calcium sulphate Dihydrate
Calcium sulphate hemihydrate
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2000-10000C
CaSO4 → CaSO4 Hexagonal orthorhombic Anhydrite Anhydrite
The principal constituent of gypsum – based products such as dental plasters and stones is calcium sulphate hemihydrate. Depending on the method of calcinations, different forms of the hemihydrate can be obtained They are - α hemihydrate - α -modified hemihydrate - β -hemihydrate The differences between alpha & beta hemihydrates are a result of differences in crystal size, surface area www.indiandentalacademy.com & degree of lattice perfection.
The ι-hemihydrate is called artificial stone, die stone or improved stone and it requires much less water when it is mixed than does the βhemihydrate. The β-hemihydrate particles absorb more water because the crystals are more irregular in shape and are porous in character.
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The beta form, which is known as dental plaster, consists of largely irregularly shaped orthorhombic crystal particles with capillary pores. The alpha form consists of smaller, regularly shaped crystalline particles in the form of rods or prisms. The alpha modified hemihydrate is made by boiling gypsum in a 30% aqueous solution of calcium chloride & magnesium chloride. This process yields the smoothest, most dense powder particles of the three types, and thewww.indiandentalacademy.com powder is used primarily for dies.
The amount of mixing water required depends on Particle size Total surface area Particle size distribution Adhesion between the particles Grinding the particles after the preparation of the hemihydrate can eliminate needle like crystals and provide
better
packing
characteristics.
lowering the amount of mixing water required. www.indiandentalacademy.com
Thus
Adhesion between the particles of hemihydrate is also a factor in determining the amount of water required to produce a product that can be poured. Small amounts of surface active materials, such as gum arabic plus calcium carbonate, added to the
hemihydrate
can
reduce
the
water
requirements of both plaster and dental stone. www.indiandentalacademy.com
SETTING REACTIONS:The setting reaction of gypsum occurs by dissolution of calcium sulfate hemihydrate, formation of a saturated solution of calcium sulfate, aggregation of less soluble calcium sulfate dihydrate and precipitation of dihydrate crystals. The crystallization
of calcium sulfate dihydrate
occurs while most of the remaining hemihydrate particles dissolve. Hemihydrate is four times more soluble in water than is the dihydrate near room temperature (20째c). www.indiandentalacademy.com
Setting reactions can be understood as follows: 1. When the hemihydrate is mixed with water, a suspension is formed that is fluid and workable. 2. The hemihydrate dissolves until it forms a saturated solution. 3. This saturated hemihydrate solution, supersaturated in dihydrate precipitates out dihydrate. www.indiandentalacademy.com
4. As the dihydrate precipitates, the solution is no longer saturated with the hemihydrate, so it continues to dissolve. Dissolution of the hemihydrate and precipitation of the dihydrate proceeds as either new crystals form or further growth occurs
on the crystals already
present. The reaction is continuous and it continuous until no further dihydrate precipitates out of solution. The anhydrite is not formed in aqueous media. www.indiandentalacademy.com
As the amount of gypsum increases during the setting period, the mass thickens because of the formation of needle-like crystals. When a lower W/P ratio is used the crystals grow and through intergrowth they form a strong solid mass. At a W/P ratio near the theoretical limit of 0.18, some of the hemihydrate crystals do not fully dissolve, but they hydrate and still tend to harden the structure. www.indiandentalacademy.com
PROPERTIES: Water /Powder ratio: The ratio of the water to the hemihydrate powder is usually expressed as the W/P ratio. It is an important factor in determining
the physical and
chemical properties of the final gypsum product, As the W/P ratio increases, the setting time increases, the strength of the gypsum product decreases, and the www.indiandentalacademy.com setting expansion decreases.
This is the reason why plaster of Paris which has the highest water powder ratio has the least compressive strength and Type V dental stone which has the least water: powder ratio has the maximum strength. When a lower water: powder ratio is used the crystals become broader and through intergrowth they form a strong solid mass. www.indiandentalacademy.com
According to A.D.A specification No, 25 the W/P ratio for: Type I
0.40- 0.75
Type II
0.45- 0.50
Type III
0.28- 0.30
Type IV
0.22- 0.24
Type V
0.18 -0.22
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Mixing time: Mixing time is defined as the time from the addition of the powder to the water until the mixing is completed. Mechanical mixing of stones and plasters is usually completed in 20 to 30 sec. Hand spatulation requires at least a minute to obtain a smooth mix. www.indiandentalacademy.com
Working Time: Working time is the time available to use a workable mix, one that maintains a uniform consistency to perform one or more tasks. Generally, a 3 min working time is adequate. Setting Time: The powder mixed with water, and the time that elapses from the beginning of mixing until the material hardens is known as the setting time. According to A.D.A specification No.25 the setting time for the various die materials is Type IV dental stone 12 minÂą4 minutes. Type V dentalwww.indiandentalacademy.com stone 12 minÂą4 minutes
Control of setting time : Three basic methods are used to control the setting time -The solubility of the hemihydrate can be increased or decreased. If the solubility of the hemihydrate is increased super saturation of the calcium sulfate increases, and the rate of crystallization deposition is also increased . -The number of the nuclei of crystallization can be increased or decreased .The greater the number of nuclei of crystallization the faster the gypsum crystals form and the sooner the hardening of the mass occurs www.indiandentalacademy.com because of crystalline intermeshing .
-The setting time can be accelerated or retarded by increasing or decreasing the rate of crystal growth.
Operator can vary the setting time within reason by changing the W/P ratio and mixing time .
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Factors effecting the setting time. Effect of spatulation : Within practical limits, the longer and the more rapidly the plaster is mixed, the shorter is the setting time. When the powder is placed in water the chemical reaction starts and some calcium sulphate dihydrate is formed. During spatulation the newly formed calcium sulfate dihydrate breaks down to smaller crystals and starts new centers of nucleation, around which the calcium sulfate dihydrate can be precipitated. www.indiandentalacademy.com
Because an increased amount of spatulation causes more
nuclei centers to be formed, the
conversion of calcium sulfate hemihydrate to dihydrate requires somewhat less time. Thus the setting time is decreased. The longer & more rapidly the plaster is mixed, the shorter is the setting time. www.indiandentalacademy.com
Effect of Humidity: During the calcination process
most of the
gypsum particles are changed to the hemihydrate, although a small portion may remain as the dihydrate, and possibly some particles may further dehydrate completely to form anhydrous soluble calcium sulfate. Soluble calcium sulfate to a greater degree and plaster to a lesser degree are hygroscopic materials by nature and can easily absorb water vapor from a humid atmosphere to form calcium sulfate dihydrate, which www.indiandentalacademy.com
changes the original proportion of calcium sulfate.
The presence of small amounts of calcium sulfate dihydrate on the surface of the hemihydrate powder provides additional nuclei of crystallization. Thus if the calcination is not complete and gypsum particles remain, or if the manufacturer adds gypsum, the setting time is shortened because of the increase in potential nuclei of crystallization. For best results all gypsum products should be kept in a closed container and well protected from the atmospheric humidity. www.indiandentalacademy.com
Fineness: The finer the particle size of the hemihydrate, the faster the mix hardens, particularly if the product has been ground during manufacture. Fineness results in -Increased rate of dissolution of the hemihydrate. -Gypsum nuclei are more numerous and thus a more rapid rate of crystallization occurs. www.indiandentalacademy.com
According to ADA specification no. 25 the fineness of the various dental stone is as follows: Type IV dental stone : 98% passes through 150mm 90% passes through 75mm Type V dental stone : 98% passes through 150mm 90% passes through 150mm www.indiandentalacademy.com
Water: powder ratio: The more water used for mixing, the fewer nuclei there are per unit volume .Thus the setting time is prolonged but the set material is weak. Temperature: The effect of temperature on the setting time is likely to be erratic and may vary from one plaster [stone] to another, little change occurs between 0ยบc and 50ยบc . If the temperature of the plaster- water mixture exceeds a 50ยบc gradual retardation occurs . As the temperature approaches 100ยบc no reaction www.indiandentalacademy.com takes place.
Retarders & accelerators : Setting time can be controlled by the addition of certain chemical modifiers to the mixture of plaster or dental stone. If the chemical added decreases the setting time it is known as an accelerator, if it increases the setting time it is known as retarder . Retarders generally act by forming an adsorbed layer on the hemihydrate to reduce its solubility and on the gypsum crystals present to inhibit growth. Many inorganic salts in smaller concentration may act as accelerators, but when the concentration is increased www.indiandentalacademy.com they can become retarders .
Sodium chloride [NaCl] is an accelerator upto about 2% but at higher concentration acts as a retarder. The setting time can be accelerated by adding gypsum[<20%], potassium sulfate, or sodium chloride[<28%]. Organic materials such as glue, gelatin and some gums act as retarders. Another type of retarder consists of salts that form a layer of a calcium salt that is less soluble than is the sulfate .These may include borax, potassiumwww.indiandentalacademy.com citrate, and sodium chloride.
Setting expansion: Crystallization process is pictured as an outgrowth of crystals from nuclei of crystallization. On the basis of entanglement of the dihydrate crystals, crystals growing from the nuclei can intermesh with and obstruct the growth of the adjacent crystals. If this process is repeated by thousands of the crystals during growth, an outward stress or thrust develops that produces an expansion of the entire mass. Thus a setting expansion will take place. www.indiandentalacademy.com
This expansion of the die material is a desirable quality as it is one of the means of compensation of the thermal shrinkage of casting alloy. As far as the dentist is concerned only the setting expansion that occurs after the initial set is of interest. Any expansion or contraction that occurs before this time can be overcome by friction between the mold surface against which the fluid mixture is poured. At the time of initial set the crystalline framework is sufficiently rigid that it can overcome for most part, such frictional retention. www.indiandentalacademy.com
Any initial contraction that occurs during the induction period does not affect the accuracy, because the mix is fluid at this stage and the contraction occurs at the free surface.
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According to ADA specification no.25 the setting expansion of Min Max TYPE IV stone - 0.00 0.10 TYPE V stone 0.10 0.30 Control of setting expansion: Setting expansion must be controlled to obtain the desired accuracy in dental applications. -A lower W/P ratio and a longer mixing time increases the setting expansion. Each of these factors increases the nucleiwww.indiandentalacademy.com density.
At higher Water powder ratio, fewer nuclei of crystallization per unit volume are present than with the thicker mixes because it can be assumed that the space between the nuclei is greater in such a case, it follows that there is less growth interaction of the dihydrate crystals and less outward thrust. The most effective method of controlling setting expansion is through the addition of chemicals. The setting expansion can be reduced by adding either potassium sulphate, sodium chloride or borax. www.indiandentalacademy.com
Strength The strength of the gypsum products is generally expressed in terms of compressive strength. The strength of plaster or stone increases rapidly as the material hardens after the initial setting time. But the free- water content of the set product affects its strength. For this reason two strength properties of gypsum are reported: wet strength. dry strength www.indiandentalacademy.com
The wet strength is the strength obtained when the water in excess of that required for hydration of the hemihydrate is left in the test specimen. When the excess water in the specimen has been driven off by drying, the strength obtained is the dry strength. The dry strength may be two or more times as high as wet strength. According to the ADA specification no 25 is at 1 hour: Type IV - 5000 psi Type V - 7000 psi www.indiandentalacademy.com
FACTORS AFFECTING STRENGTH: Water powder ratio: Greater the water powder ratio the less is the dry strength of the set material because the greater the porosity, the fewer crystals are available per unit volume for a given weight hemihydrate . Spatulation time: With an increase in mixing time the strength is increased to a limit that is approximately equivalent to that of hand mixing for 1min.If the mixture is over mixed the gypsum crystals formed are broken up and less crystalline interlocking results in the final product. www.indiandentalacademy.com
Chemical modifier : Addition of an accelerator or retarder lowers both the wet and the dry strengths of the gypsum product.
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TYPE IV DENTAL STONE These are high strength dental stone. The principal requisites for a die material stone are strength hardness resistance to abrasion minimum setting expansion To obtain these properties one should use an α-hemihydrates of the “densite”-type .The cuboidal shaped particles and the reduced surface area produce such properties . Surface detail reproduction is acceptable with type www.indiandentalacademy.com IV and type V gypsum products.
The materials are capable of reproducing a 20Îźm-wide line as prescribed by A.D.A specification No.19. A hard surface is necessary for a die stone because the cavity preparation is filled with wax that is carved flush with the margins of the die. A sharp instrument is used for this purpose therefore the stone must be resistant to abrasion. Since the surface dries more rapidly the surface hardness
increases
more
rapidly
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compressive strength.
than
does
the
Even though the surface of the type IV stone is harder, care should be observed
when the
pattern is being carved. It has been recommended to wait for 12-24 hrs to avoid the stone fracturing during removal from the type IV dental stone high strength.
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W: P ratio
-
0.22 -0.24
Setting time
-
12 Âą4
Setting expansion
-
0.10
Compressive strength
-
5000 psi
Hardness
-
92 RHN
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TYPE
V
DENTAL
STONE,
HIGH
STRENGTH,
HIGH
EXPANSION:
The Type V dental stone gypsum product exhibits an even higher compressive strength than does the type IV dental stone. In addition setting expansion has been increased from a maximum of 0.10% to 0.30%. This is because certain newer alloys such as base metal alloys have a greater casting shrinkage than noble metal alloys. Thus higher expansion is required in the stone used for the die to aid in compensating for the alloy www.indiandentalacademy.com
solidification shrinkage.
Type V stone is indicated when inadequate expansion may have been achieved during the fabrication of cast crowns. It should be avoided in the production of dies for inlays since the higher expansion may lead to unacceptably tight fits. www.indiandentalacademy.com
W: P ratio
-
0.18 -0.22
Setting time
-
12 Âą4
Setting expansion
-
0.30
Compressive strength
-
7000 psi
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Advantages of Type IV & Type V stones : They are relatively inexpensive Easy to use Compatible with all impression material Disadvantages of Type IV gypsum die Susceptibility to abrasion during carving of wax pattern.
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Gypsum dies are sometimes modified to: To make them more abrasion-resistant To change the dimensions of the dies. To increase the refractoriness of the dies. To produce a combination of these effects. www.indiandentalacademy.com
Several means are used to increase the abrasion resistance •Including silver plating, •Coating the surface with cyanoacrylate • Adding a die hardener to the gypsum However each of these methods may increase the die dimensions slightly thus reducing accuracy.
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Special gypsum products ; Since 1991, a plethora of new dental stones have been introduced One type is extremely fast- setting and ready to use in 5min,but it as little working time another product changes color to help denote when it is ready for use . Most recently another trend is the addition of a small amount of plastic or resin ,which reduces brittleness and improves resistance to abrasion during the carving of wax patterns. www.indiandentalacademy.com
Usually in the production of gypsum products when one feature is improved another feature is sacrificed. A faster set may be accepted in return for less working time. An improved resistance to carving may be gained in return for greater difficulty in manipulation and decrease in detail reproduction www.indiandentalacademy.com
Methods of altering die dimensions To reduce the setting expansion of the type IV die stone to less than 0.1% additional accelerator such as potassium sulphate and retarders (Borax) can be added to the gauging water this will reduce the diameter of the die. To produce relief space for cement, a die spacer with a stone die is used. Die spacer : An agent applied to a die to provide space for the luting agent in the finished casting. www.indiandentalacademy.com
Types of die spacers : Resins (Commonly used) Paint or liquids Model paint Coloured nail polish Thermoplastic resins dissolved in volatile solvents.
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Spacers are applied in several coats to within 0.5mm of the preparation finish line to provide relief for the cement luting agent and to ensure complete seating of an otherwise precisely fitting casting or coping.
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DIE STONE INVESTMENT COMBINATION : A technique has been developed which the die material and the investing medium have a comparable composition . A commercial gypsum- bonded material called divestment is mixed with a colloidal silica liquid. The die is made from this mix, and the wax pattern is than constructed on it then the entire assembly [die and pattern] is invested in a mixture of divestment and water thereby eliminating the possibility of distortion of the pattern on removal from the die or during the setting of www.indiandentalacademy.com the investment.
When it is heated to 677째c the setting expansion of the material is 0.9% and the thermal expansion is 0.6% because divestment is a gypsum- bonded material it is not recommended for high-fusing alloys that are used for metal-ceramic restorations but it is a highly accurate technique for use with conventional gold alloys especially for extra coronal preparations. Divestment phosphate recommended for high fusing alloys. www.indiandentalacademy.com
ELECTROFORMED DIES : Besides resins, electroplating can be used to overcome the poor resistance of gypsum. The metal dies that are produced when an impression material is electroplated have moderately high strength, adequate hardness and excellent abrasion resistance. Detail reproduction of a line
4Âľ-m or less is readily
attainable on an electroplated die when an nonaqueous elastomeric impression material is used. www.indiandentalacademy.com
The first step in the procedure is to treat the surface of the impression material so that it conducts electricity. This process is referred to as metalizing This technique has been used for many years and involves the deposition of a coat of pure silver or copper on the impression. The areas to be coated are first coated with finely powdered silver or graphite to make them conduct electricity, and the impression is than placed in an electroplating bath. A layer of pure metal is deposited on the impression and is supported with type IV stone or resin. www.indiandentalacademy.com
Silver formed dies : (Silver plating) Copper plated compound dies began in the early 1930â&#x20AC;&#x2122;s and the silver plated dies became more popular in later years. Polysulphide and silicone impression materials can be silver plated. In this process A thin layer of metal such as silver powder is deposited on the surface of the impression material. Various metallizing agents are available, including bronzing powder and aqueous suspensions of silver www.indiandentalacademy.com powder and powdered graphite.
These agents can be deposited on the surface of the impression with the camel hair brush. The electroplating bath is a solution of silver cyanide. Chemical deposition of silver from a silver nitrate solution can be used if greater surface detail reproduction is required . The greater the concentration of silver in the bath, the faster the silver is deposited. The acid content increases the throwing power , a term that refers to the penetration of current into a concave structure, www.indiandentalacademy.com such as an impression for a full crown.
An electrical contact is made with the metallized surface of the impression, which is the cathode in the electroplating bath. A plate of silver is used as the anode. A direct current is applied for approximately 10hr. Electroformed dies made from polysulphide rubber impressions are clinically acceptable when a silver cyanide bath is used, but they are generally slightly less accurate than a properly constructed stone www.indiandentalacademy.com die .
Polysulphide
rubber
impressions
are
cleaned
thoroughly and dried . They are then metallized
with a fine silver powder
although other metallizing agents can be used, the silver powder results in a superior surface on the electroformed die. A anode of pure silver, at least twice the size of the area to be plated, is employed and the electroplating is carried out as before for approximately 10hrs, using www.indiandentalacademy.com
5- 10mA/cm2 of cathode surface.
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An impression that contains the electroformed die surface is then filled with dental stone. When the stone hardens. It is mechanically locked into the rough interior of the electroformed metal shell. The impression material is then removed to provide a die with greater surface hardness and resistance to abrasion. The model and die are prepared in the normal manner and margins of the die are trimmed with a finishing disk. www.indiandentalacademy.com
Composition of solution for silver forming bath Ingredients Quantity Silver cyanide 36gms Potassium cyanide 60gm Potassium carbonate 45gms Water(distilled) 1000ml Advantages Moderately high strength Adequate hardness Excellent abrasionwww.indiandentalacademy.com resistance
DISADVANTAGES : The electroplating bath is a solution of silver cyanide. This solution is poisonous, and extreme care should be taken that the hands workbench area and clothing not become contaminated. The addition of acid to the solution produces hydrogen cyanide, an extremely poisonous gas and for this reason copper-forming solution should be kept far away. The plating bath should have a cover that can be in placed at all times to control evaporation and dissipation of fumes. Because of this risk, www.indiandentalacademy.com silver plating is not often used.
COPPER PLATING OR COPPER FORMED DIES : Metal dies can be made by copper plating compound or silicon such a die is tough and has good strength characteristics and metal inlays restorations can be finished and polished on these dies . The following process is involved : -The surface of the impression is coated with fine particles of copper or graphite to make them conduct electricity. www.indiandentalacademy.com
When the impression is in compound, a colloidal dispersion of graphite is painted on the surface to be plated and allowed to dry before it is placed into the plating bath . When the impression is a silicone rubber ,finely divided copper powder is brushed on the surface to be plated before placing the impression into the bath. -The coated impression is made the cathode of a plating bath, with an anode of copper. www.indiandentalacademy.com
- The electrolyte is an acid solution of copper sulphate together with organic constituents [ex: alcohol or phenol ]which help in increasing the hardness of the deposited metal. Sulphuric acid increases the conductivity of the solution and the phenol sulphonic acid assists the penetration of copper ions to the deeper parts of an impression and improves the throwing power of the solution . -A current is passed, causing slow dissolution of the anode and movement of copper ions from anode to cathode, so plating the impression. www.indiandentalacademy.com
About 15mA is a suitable current to start plating a single tooth impression . Once a thin layer of copper has covered the entire surface of the impression, the current can be increased to as much as two or three times the initial current. If too high a current setting is used, the copper deposit will be granular and friable in nature, and the die will be unsatisfactory. www.indiandentalacademy.com
High current densities also produce a heavier deposit on the areas of the impression nearest to the anode, and sometimes rapid plating results in a failure to adequately cover the deeper areas of an impression. Plating is allowed to proceed for 12 to 15 hours; overnight usually is a convenient period of time. In copper forming, the distance between the anode and impression to be plated is important in relation to plating the deeper areas of an impression. The greater the anode to impression distance the more even is the quantity of copper deposited over the impression the morewww.indiandentalacademy.com readily are the deep areas plated.
Dental stone is than cast into the plated impression when the stone has set the metal covered die can be removed from the impression This technique is often not considered suitable for the elastomeric materials because they are not dimensionally stable in an acid solution. www.indiandentalacademy.com
Composition for solution for copper forming bath
Ingredients
Quantity
Copper sulfate
200 gms
Sulphuric acid ( conc )
30 ml
Phenosulphonic acid
2 ml
Water ( distilledwww.indiandentalacademy.com )
1000 ml
Problems in Electroplating Variable degrees of distortion commonly occur and hence the technique must be performed slowly. Otherwise distortion in the metal will subsequently stress the impression. The time required to produce a cohesive film of metal(typically 8 hrs) is ample for the development of dimensional changes in the impression. Not all the impression materials are suitable for plating because of their low surface energies, silicone impression materials are difficult to electroplate evenly. www.indiandentalacademy.com
Polyether
impressions
because
of
their
hydrophilic nature imbibe water & become distorted. Therefore they cannot be plated accurately. Poly sulfide polymers can be silver plated but it is difficult to copper plate them Drawback of silver plating is the use of a cyanide
solution,
which
requires
special
precautions because of its extreme toxicity. www.indiandentalacademy.com
Epoxy resins Resins are used as die materials to overcome the low strength and abrasion resistance of die stones. Most available resin die material is epoxy resin but polyurethane is also used.
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Epoxy materials until recently where supplied in the form of a paste to which a liquid activator was added to initiate hardening. Since the activators are toxic they should not come into contact with the skin while mixing and manipulating of the unset material Shrinkage of 0.1% has occurred during hardening, which may take up to 24 hours.The hardened resin is more resistant to abrasion and stronger than a high strength stone die. www.indiandentalacademy.com
The viscous paste is not so readily introduced into the details of a large impression as high strength dental stone .Recently fast setting epoxy materials have been supplied in automixing systems . The epoxy resin is in one cartridge and the catalyst is in the other. Forcing the two pastes through the static mixing tip thoroughly mixes the epoxy material, which can be directly injected into a rubber impression. www.indiandentalacademy.com
The fast setting epoxy hardens rapidly so that dies can be waxed up in half an hour after injecting into the impression .Epoxy resins cannot be used with water containing agar and alginate impression materials because the water retards the polymerization of the resin and thus are limited to use with rubber impression materials . Working time â&#x20AC;&#x201C; 15 min Setting time _ 1-12 hrs (depending on product) Shrinkage _ 0.03%-0.3% Compressive strength _ 9500-14200psi Hardness www.indiandentalacademy.com _ 83Rhn
Advantages: -It can be cured at room temperature without expansive or complicated equipment. -It is dimensionally stable . -Its abrasion resistance is many times greater than gypsum products. -high strength. -Suitable for fabrication of precision dies . -Detail reproduction is better than die stone Hence prostheses fabricated on resin dies will fit more tightely than those made on gypsum. -Good results are achieved with silicone and polyether www.indiandentalacademy.com
Disadvantages: -It undergoes shrinkage during polymerization. The amount of shrinkage is approximately equal to
the expansion with gypsum. -It is more expansive than gypsum. -Not compatible with impression materials such as www.indiandentalacademy.com
polysulfide and hydrocolloid.
Polyurethane These resins compared to epoxy resins were inexpansive and easily manipulated. Unfilled polyurethane resins demonstrated considerable shrinkage during polymerization, but these materials then appeared to swell slightly. Long and narrow tooth preparations must be reproduced with high transverse-strength working cast materials to avoid fracturing;thus filled polyurethane resins could be indicated for these conditions . www.indiandentalacademy.com
Silicophosphate cement: This is similar to the filling and cementing material. Advantage : Harder than die material. Disadvantage: -Shrinkage on setting. -Loss of water on standing. www.indiandentalacademy.com
Amalgam: Advantage: -Produces a hard die -Reproduces fine details and sharp margins. Disadvantages: -Can only be packed into a rigid impression . -Long time to reach a maximum hardness. -High thermal conductivity hence can cool a wax pattern rapidly which can lead to distortion of the pattern. This can be overcome by warming the die , a separating agent is needed as with stone dies. www.indiandentalacademy.com
Metal sprayed dies: A bismuth-tin alloy, which melts at 138 0c, can be sprayed directly on to an impression to form a metal shell, which can then be filled with dental stone. Advantage -A metal coated die can be obtained rapidly from elastomeric impression materials. Disadvantage -The alloy is rather soft, care is needed to prevent abrasion of the die. www.indiandentalacademy.com
Ceramic die materials: Two ceramic die materials are available -A material for the production of dies on which porcelain restorations are to be fabricated, without the use of a platinum foil matrix. To form the dies heating to over 10000c is necessary. -A ceramic material, supplied as a powder and liquid, and mixed to a putty like consistency. After 1 hour the material is removed from the impression and fired at 6000c for 8 minutes to produce a hard strong die. www.indiandentalacademy.com
Flexible die material : -Similar to silicone or polyether impression material. -Used to make provisional restorations or indirect composite resin inlays . eg; polyvinyl medium viscosity impression material Advantages: -More rapid setting -ease of removal of provisional or inlay Disadvantages: -expensive www.indiandentalacademy.com
Compatibility with impression materials : Dental stone
Electroplated Cu Electroplated silver
-Impression compound -Zinc oxide eugenol -Agar-agar -Alginate -Impression plaster when used with separator -Rubber base material - Rubber base material
-Polysulphide -Polyether www.indiandentalacademy.com -Addition silicone
Polymer
-Rubber base material
Epoxy Resin
-Polyether -Addition silicone -Polysulphides(Separator)
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Comparisons of the various die materials : According to Craig: The epoxy resin dies reproduce detail the best followed by metal die and high strength dental stone . Chaffe et al in 1997 reported that epoxy resins reproduce a single die with a degree of accuracy similar to gypsum material used. Derriem et al in 1995 reported that the detail reproduction of stone was inferior epoxy and polyurethane resins stone cannot reproduce details smaller than 20 nm as the gypsum crystal size ranged formal 15 towww.indiandentalacademy.com 25 nm.
Bailey et al in 1984 found no difference in the accuracy of stone, epoxy resins and electroplated silver dies . Nomura et al in 1980 concluded that the detail reproduction of epoxy resin dies was comparable to stone . Moser et al in 1975 found that the epoxy resins reproduced detailswww.indiandentalacademy.com better than stone.
Dimensional stability: Chaffee et al in 1997 concluded that the improved dental stone dies were significantly larger in occluso-gingival dimension than epoxy resins. Derriem et al in 1995 were able to show that the dies exhibited slight shrinkage as compared to stone , which exhibited slight expansion. This required the epoxy dies to be coated with several layers of spacers . Nomura et al in 1980 and Mose et al in 1975 demonstrated thatwww.indiandentalacademy.com the epoxy dies were under sized
Abrasion resistance and strength Chaffee et al in 1997, Derriem et al 1995 ,Nomura et al in 1980 and Moser et al in 1975 concluded that metal formed dies have superior abrasion resistance ,epoxy dies have good resistance and the high strength dental stones have the least resistance to abrasion www.indiandentalacademy.com
Philip Duke et. al in 2000 conducted a study of the physical properties of type IV gypsum, resin containing and epoxy die materials. The results were all gypsum products expanded where as the epoxy resin material contracted during setting. The epoxy resin exhibited much better detail reproduction, abrasion resistance, transverse strength than gypsum materials. In general the epoxy resin exhibited the best properties of the materials studied www.indiandentalacademy.com
Jacinthe M et al in 2000 conducted a study on the dimensional accuracy of an epoxy resin die material using two setting methods and concluded that retarding the setting reaction of an epoxy resin die material improved its accuracy. Epoxy resin die materials had a net shrinkage but the gypsum based materials had a net expansion . www.indiandentalacademy.com
Summary Detailed reproduction of die materials for fixed partial prosthesis affects the accuracy of working casts and is related to the compatibility between the die and impression materials. Hence an accurate working cast and die are essential in making successful cast restoration . Different types of materials and techniques are available that will give an extremely precise reproduction of prepared teeth . Dental stone is compatible with majority of www.indiandentalacademy.com impression materials.
This material is inexpensive easy to use and generally compatible with all impression material. Resistance to abrasion of the material can be effectively increased by using gypsum hardner, such as aqueous colloidal silica or soluble resin . Epoxy resin and electroplated dies are good alternatives www.indiandentalacademy.com
Conclusions: A good impression and an accurate die are the first step towards the fabrication of an accurate restoration whether its inlay, onlay or crown. Proper selection of the die material and its manipulation are paramount to achieve accuracy in the die. In conclusion, a wise choice of material, combined with proper handling and meticulous approach to the details of each step of fabrication will bring us that much closer to perfection . www.indiandentalacademy.com
References: Fundamentals of fixed Prosthodontics:Shillingberg. Contemporary fixed Prosthodontics
:
Stephen F.Rosenstiel Philipâ&#x20AC;&#x2122;s science of dental materials
:Anusavice
Dental material -properties and manipulation : Craig ,powers www.indiandentalacademy.com
Notes on dental materials- E C Combe Restorative dental materials- Robert G Craig Philip Duke et al ;Physical properties of type IV gypsum, resin containing and epoxy die materials JPD April 2000 vol 83, no. 4 p-466-73. Jacinthe M et al in 2000 dimensional accuracy of an epoxy resin die material using two setting methods. JPD March 2000 vol 83 no3 p 301-305 www.indiandentalacademy.com
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