CASTING PROCEDURES

CASTING: casting is the process by which the wax pattern of a restoration is converted to a replicate in a dental alloy. The casting process is used to make dental restorations such as inlays, onlays, crowns, bridges and removable partial dentures.

 

CASTING PROCEDURE: It involves both the clinical and the laboratory steps:

Step 1: Mouth preparation

Step 2: Direct wax pattern is done on the tooth or the impression of the prepared tooth is taken and the die is made for indirect wax pattern.

Step 3: Preparing the wax pattern for investing.

Spruing the pattern:

The sprue former or the sprue pin acts as the channel or passage for the entry of the liquid metal into the mold in an investing ring after wax elimination. The diameter and length of the sprue former depend to a large extent on the dimensions of the flask or ring in which the casting is to be made.

Sprue former gauge selection is often empirical, is yet based on the following five general principles:

1.                  Select the gauge sprue former with a diameter that is approximately the same size as the thickest area of the wax pattern. If the pattern is small, the sprue former must also be small because a large sprue former attached to a thin delicate pattern could cause distortion. However if the sprue former diameter is too small this area will solidify before the casting itself and localized shrinkage porosity may result.

2.                  If possible the sprue former should be attached to the portion of the pattern with the largest cross-sectional area. It is best for the molten alloy to flow from the thick section to the surrounding thin areas. This design minimizes the risk of turbulence.

3.                  The length of the sprue former should be long enough to properly position the pattern in the casting ring within 6mm of the trailing end and yet short enough so the molten alloy does not solidify before it fills the mold.

4.                  The type of sprue former selected influences the burnout technique used. It is advisable to use a two-stage burnout technique whenever plastic sprue formers or patterns are involved to ensure complete carbon elimination, because plastic sprues soften at temperatures above the melting point of the inlay waxes.

5.                  Patterns may be sprued directly or indirectly. For direct sprueing the sprue former provides the direct connection between the pattern area and the sprue base or crucible former area. With indirect spruing a connector or reservoir bar is positioned between the pattern and the crucible former. It is common to use indirect spruing for multiple stage units and fixed partial dentures.

Reservoir:

Reservoir is the piece of wax that is attached to the sprue former approximately 1mm from the pattern as an added precaution to prevent localized shrinkage porosity. When the liquid metal in the mold solidifies first and shrinks the liquid metal in the reservoir will flow into the mold and thus overcomes that shrinkage. Reservoir is necessary only with sprue formers of very small diameter.

Wax pattern removal:

Sprue former can be used to remove the pattern. If not the pattern is removed with a sharp probe. Then the sprue former is attached to it. The pattern should be removed directly in line with the principle axis of the tooth or the prepared cavity. Any rotation of the pattern will distort it. Hollow sprue pin is advisable because of its greater retention to the pattern.

Crucible former:

It is also known as the sprue base. It is like a stand to hold the sprue former along with the pattern within the casting ring while the pattern is being invested with the investment material. The shape of the crucible former is such that when it is removed after the investment is set it forms a funnel like shape which is most suitable to pour liquid metal into it. Crucible former can be made of metal, rubber or resin.

Casting ring: It is a hollow tube fitted over the crucible former encircling the wax pattern to a height of ¼” or so above the edge of the pattern. The ring and the crucible former provide a seal and so the investment material can be poured inside the ring to surround the wax pattern and sprue former.

Casting ring liners:

For the setting and hygroscopic expansion of an investment to take place more uniformly, some allowance must be made for the lateral expansion of the investment. Solid rings do not permit the investment to expand laterally during the setting, hygroscopic and the thermal expansions of the mold. To overcome this lateral restriction a liner is placed inside the ring. With the metal casting ring is used it must be lined with a liner of moistened paper made of glass fibre. This liner provides a cushion for the hardening investment material to expand into, during the setting reaction. The ceramic paper liner is cut to fit the inside of the metal ring and is and held in place with the fingers. The ring containing the liner is then soaked into the water until it is completely wet.  The liner is moistened because a dry liner would absorb water from the investment and minimize the setting expansion.

The liner is done in two layers inside the ring, and the thickness must be not less than 1mm, so that ring can accommodate more expansion. The liner is placed somewhat short of ends of the ring to enable the investment to obtain a grip and provide a seal. And this also restricts the longitudinal expansion, so that a more uniform expansion takes place and less distortion of the wax pattern.

Step 4: INVESTING

Mixing investment with distilled water is done according to the manufacturers ratio in a clean dry bowl without entrapment of the air into the mix.

Mixing methods:

1. Hand mixing and the use of the vibrator to remove air bubbles.
2. Vacuum mixing- This is the better method because it removes air bubbles as well as gases that are produced and thus produces a smoother mix.

Methods of investing:

1. Hand investing
2. Vacuum investing

Hand investing:

First the mixed investment is applied on all the surfaces of the pattern with a soft brush. Blow off any excess investment gently, thus leaving a thin film of investment over the pattern, then apply again.

Then the coated pattern can be invested by two methods;

1.      Placing the pattern in the ring first and then filling the ring full with investment.

2.      Filling the ring with the investment first and then force the pattern through into it.

Vacuum investing :

Vacuum investing unit: This consists of the chamber of small cubic capacity from which air can be evacuated quickly and in which casting ring can be placed.

Evacuation of air can be done by electrically or water driven vacuum pump.

Procedure:

The ring filled with investment is placed in the vacuum chamber. Air entry tube is closed. Then the vacuum is applied. The investment will rise with froth vigorously for about 10-15 sec and then settles back. This indicates that air has been extracted from the ring. The pressure is now restored to atmospheric by opening the air entry tap gradually at first and then more rapidly as the investment settles back around the pattern. Then the ring is removed from the chamber and the investment is allowed to set. Modern investing unit does both mixing and investing under vacuum and is considered better than hand mixing and pouring.

Then there are two alternatives to be followed depending upon what type of expansion is to be achieved in order to compensate for metal shrinkage. They are:

1. If hygroscopic expansion of the investment is to be achieved then immediately immerse the filled ring in water at the temperature of 37C.

Or “under controlled water adding technique”. A soft flexible rubber ring is used instead of usual lined metal ring. Pattern is invested as usual. Then specified amount of water is added on top of the investment in the rubber ring and the investment is allowed to set at room temperature. In this way only enough water is added to the investment to provide the desired expansion.

2.                  If thermal expansion of the investment is to be achieved, then investment is allowed to set by placing the ring on the bench for 1 hour or as recommended by the manufacturer.

Step 5: WAX BURNOUT AND HEATING THE RING

After the investment has set hard, the crucible former and the metal sprue former is removed carefully, and any loose particles at the opening of the sprue hole are removed with small brush.

The purpose of the wax burnout is to make room for the liquid metal. The ring is placed in the oven at 250C with the sprue end down, thus allowing the melted wax to flow, out for 30min or even up to 60min may be a good procedure to ensure complete elimination of the wax and the carbon.

Heating the ring: The object is to create a mold of such dimension, condition and temperature so that it is best suited to receive the metal.

Hygroscopic Low-Heat Technique. 

After the wax elimination the temperature of the same furnace can be set to a higher temperature for heating or else, the ring can be transferred to another furnace, which has already set to the higher temperature. In any case accurate temperature control is essential and therefore these furnaces have pyrometer and thermocouple arrangement. The ring is placed in the furnace with the sprue hole down and heated to 500C and kept at this temperature for 1 hour. In this low heat technique the thermal expansion obtained is less but together with the previously obtained hygroscopic expansion the total expansion amounts to 2.2 percent, which is slightly higher than what is required for gold alloys.

So this technique obtains its compensation expansion from three sources:

(1)   The 37º C water bath expands the wax pattern

(2)   The warm water entering the investment mold from the top adds some hygroscopic expansion

(3)   The thermal expansion at 500' C provides the needed thermal expansion.

High-Heat Thermal Expansion Technique. 

After the wax elimination, the ring should be placed in the furnace which is at room temperature and then the temperature is gradually raised, until it comes to 700C in 1 hour. Then the ring is heat soaked at this temperature for ½ hour. This slow rise in temperature is necessary to prevent 

This approach depends almost entirely on high-heat burnout to obtain the required expansion, while at the same time eliminating the wax pattern.  Additional expansion results from the slight heating of gypsum investments on setting, thus expanding the wax pattern, and the water entering the investment from the wet liner, which adds a small amount of hygroscopic expansion to the normal setting expansion.

 

Step 6: CASTING THE METAL

Casting Machines:

Alloys are melted in one of the three following ways, depe­nding on the available' types of casting machines:

Centrifugal Casting Machine. 

This method makes use of centrifugal force to thrust the liquid metal into the mold. The aim is to force the liquid metal under sufficient pressure, so that the pressure can be maintained for at least four seconds after the metal has been cast. Pressure is necessary because the liquid with high surface tension will not enter the mold on its own.

Centrifugal casting machine also known as broken arm casting machine has an arm which is supported in the middle by a rotating spindle. One side of the arm has the weights to balance the machine. Other side of the arm has crucible to melt the metal and an arrangement to hold the casting ring. The spindle is spring loaded.

Procedure:

1. The force exerted by the machine is adjusted by turning 3-4 turns of the arm to wound the spring and kept in that wounded position with the help of a stop rod.
2. Balancing the machine should have been done before the ring is heated by placing the ring on the casting machine so that the arm is balanced to compensate for the weight of the ring and the investment.
3. Preheating the alloy to its melting point is done by using the reducing zone of the torch flame in ceramic crucible attached to the broken arm of the casting machine.                                                  Use of reducing zone only is necessary to avoid carburization of the metal and because it is the hottest part of the flame. Reducing zone is blue in color.

During the heating of the alloy reducing flux such as borax is sprinkled over the alloy as soon as it is hot enough for the flux to adhere to it. Applying flux removes the oxide skin on the surface of the alloy and reduces its surface tension so that the liquid metal becomes fluidy.

4. Then the ring is immediately taken out of the heating furnace and place firmly against the back plate of the machine. Then the crucible is moved up against the sprue hole end of the ring. The crucible also has a hole in it. Thus both the holes are up against each other.
5. The alloy is reheated again until it spins, and looks bright red hot (1100C) with shiny mirror like surface. This indicates its proper fusion.
6. At this stage torch flame is removed and arm of the machine is released by dropping the stop rod simultaneously.

 The machine begins to spin and stops on its own. This act will throw the metal through the hole and directly through the sprue hole into the mold cavity, in the investment material.

Two things are important during this final step—one is metal must be in full liquid state- that means flame must be held at the metal until the arm of the machine is released.

     Secondly, there must be enough rotational force to fill the mold cavity quickly before the metal solidifies in the sprue area.

As the metal fills the mold there is a hydrostatic pressure gradient develops along   the length of the casting. The pressure gradient from the tip of the casting to the bottom surface is quite sharp and parabolic in form, reaching zero at the button surface.  Ordinarily, the pressure gradient at the moment before solidifica­tion begins reaches about 9.21 to 0.28 MPa (30 to 40 psi) at the tip of the casting.  Because of this pressure gradient, there is also-a gradient in the heat transfer rate such that the greatest rate of heat transfer to the mold is at the high pressure end of the gradient (i.e., the tip of the casting).  Because this end also is frequently the sharp edge of the margin of a crown, there is further assurance that the solidification progresses from the thin margin edge to the button surface.

Electrical Resistance-heated Casting Machine. 

In this instance there is an auto­-6iatic melting of the metal in a graphite crucible within a furnace rather than by use of a torch flame.  This is an advantage, especially for alloys such as those used for metal -ceramic restorations, which are alloyed with base metals in trace amounts that tend to oxidize on overheating. 

Another advantage is that the crucible in the furnace is located flush against the casting ring.  Therefore, the metal button remains molten slightly longer, again ensuring that solidification progresses completely from the tip of the casting to the button surface.  A carbon crucible should not be used in the melting of high palladium or palladium-silver alloys, where the temperature exceeds 1504' C or with nickel-chromium or Cobalt-Chromium base metal alloys.

Induction Melting Machine. 

With this unit, the metal is melted by an induction field that develops within a crucible surrounded by water-cooled metal tubing. Once the metal reaches the casting temperature, it is forced into the mold by air pressure, vacuum, or both, at the other end of the ring.  The device has become popular in the casting of jewelry but has not been used as much as the other two techniques for noble alloy castings.  It is more commonly used for melting base metal alloys.

There is little practical difference in the properties or accuracy of castings made with any of the three types of casting machines.  The choice is a matter of access and personal preference

Casting Crucibles.

Generally, three types of casting crucibles are available: clay, carbon, and quartz (including zircon-alumina).  Clay crucibles are appropriate for many of the crown and bridge alloys, such as the high noble and noble types.  Carbon crucibles can be used not only for high noble crown and bridge alloys but also for the higher-fusing, gold-based metal-ceramic alloys.

Quartz crucibles are recommended for high-fusing alloys of any type.  They are especially suited for alloys that have a high melting range and are sensitive to carbon contamination.  Crown and bridge alloys with a high palladium con­tent, such as palladium-silver alloys for metal-ceramic copings, and any of the nickel-based or cobalt-based alloys are included in this category.

Step 7: CLEANING THE CASTING

After the casting has been completed, the ring is removed and quenched in water as soon as the button exhibits a dull-red glow.  Two advantages are gained in quenching: (1) the noble metal alloy is left in an annealed condition for burnishing, polishing, and similar procedures, and

(2) when the water contacts the hot investment, a violent reaction ensues.  The investment becomes soft and granular, and the casting is more easily cleaned.

Often the surface of the casting appears dark with oxides and tarnish.  Such a surface film can be removed by a process known as pickling, which consists of heating the discolored casting in an acid.  Probably the best pickling solution for gypsum-bonded investments is a 50% hydrochloric acid solution.  The hydrochlo ric acid aids in the removal of any residual investment as well as of the oxide coating. 

The disadvantage of the use of hydrochloric acid is that the fumes from the acid are likely to corrode laboratory metal furnishings.  In addition, these fumes are a health hazard and should be vented via a fume hood.  A solution of sulfuric acid is more advantageous in this respect.  Ultrasonic devices are also available for cleaning the casting, as are commercial pickling solutions made of acid salts.

The best method for pickling is to place the casting in a test tube or dish and to pour the acid over it.  It may be necessary to heat the acid, but boiling should be avoided because of the considerable amount of acid fumes involved.  After pickling, the acid is poured off and the casting is removed.  The pickling solution should be renewed frequently because it is likely to become contaminated with use.

Step 8: FINISHING AND POLISHING:

Finally the sprue is removed and the restoration may be stoned and polished on the external surfaces except at the edges, in the laboratory. Edges are finished in the clinic after cementing.

Finishing tools and polishers:

1. mandrels, abrasive disks.
2. Rubber cup polishers, bristle brushes.
3. Pumice
4. Wool mop

 

Casting procedure for chrome cobalt removable partial denture:

As usual impression of the jaw is made and the master model an dental stone is made. This stone model is then duplicated to make a refractory cast of the casting investment. Wax pattern is then made on this refractory cast.

-Wax sprue formers are are used and more than one are necessary because of the large size of the pattern. Vents are made by attaching very thin sprues at the strategic areas before the pattern is invested.

-The pattern is not removed from the model instead the whole model along with the pattern and sprue formers is invested in a large ring or the casting flask.

The investment material is either silica bonded or phosphate bonded. This is necessary for 2 reasons:

1. Investment must withstand the high temperature of melting chrome-cobalt alloy that is above1250C.
2. investment must have sufficient expansion to compensate for the high casting shrinkage of the metal.

Both of these investment give high thermal expansion of an average 1.5 to 2%.

Even then this value may be less considering the casting shrinkage of chrome cobalt which is around 2.2 %. However other factors like shape of the casting, method of spruing etc, also contribute to this and provide adequate compensation for  the shrinkage. Casting temperature of the investment to achieve this much thermal expansion is between 800 to 1100C in any case above 1000C.

Chrome – cobalt alloy is melted using an oxy-acetylene gas flame or by an electric source. As usual the centrifugal casting machine is used for the casting. The flask is cooled slowly after casting and the casting is separated form the investment. The surface of the appliance is smoothened by sand blasting and highly polished.

 

DEFECTIVE CASTINGS

Defects in castings can be classified under four I-leadings: (1) distortion; (2) surface roughness and irregularities; (3) porosity; and (4) incomplete or missing detail.  Some of these factors have been discussed in connection with certain phases of the casting techniques.  The subject is summarized and analyzed in some detail in the following sections.

Distortion:  Any marked distortion of the casting is probably related to a distor­tion of the wax pattern. This type of distortion can be minimized or prevented by proper manipulation of the wax and handling of the pattern.

Unquestionably, some distortion of the wax pattern occurs as the investment hardens around it.  The setting and hygroscopic expansions of the investment may produce an uneven movement of the walls of the pattern.

This type of distortion occurs in part from the uneven outward movement of the proximal walls.  The gingival margins are forced apart by the mold expansion, whereas the solid occlusal bar of wax resists expansion during the early stages of setting.

Surface Roughness, Irregularities, and Discoloration:

The surface of a dental casting should be an accurate reproduction of the surface of the wax pattern from which it is made.  Excessive roughness or irregularities on the outer surface of the casting necessitate additional finishing and polishing whereas irregularities on the cavity surface prevent a proper seating of an otherwise accurate casting.

Causes of these surface defects:

Air Bubbles:  Small nodules on a casting are caused by air bubbles that become attached to the pattern during or subsequent to the investing procedure.  Such nodules can sometimes be removed if they are not in a critical area.  However, for nodules on margins or on internal surfaces, removal of these irregularities might alter the fit of the casting.

Prevention:

-By vacuum investing

-using of mechanical mixer

Water Films:  Wax is repellent to water, and if the investment becomes separated from the wax pattern in some manner, a water film may form irregularly over the surface.  Occasionally, this type of surface irregularity appears as minute ridges or veins on the surface.

Prevention:

-Use wetting agent on the pattern before investing.

Rapid Heating:  It results in fins or spines on the casting or characteristic surface roughness may be evident because of flaking of investment when the water or steam pours into the mold.  Furthermore, such a surge of steam or water may carry some of the salts used as modifiers into the mold, which are left as deposits on the walls after the water evaporates

Prevention:

As previously mentioned, the mold should be heated gradually; at least 60 minutes should elapse during the heating of the investment-filled ring from room temper­ature to 700º C. The greater the bulk of the investment, the more slowly it should be heated.

Underheating:  Incomplete elimination of wax residues may occur if the heating -time is too short or if insufficient air is available in the furnace.  These factors are particularly important with the low-temperature investment techniques.  Voids or porosity may occur in the casting from the gases formed when the hot alloy comes in contact with the carbonaceous residues.  Occasionally, the casting may be covered with a tenacious carbon coating that is virtually impossible to remove by pickling.

Liquid:Powder Ratio:  The amount of water and investment should be measured accurately.  The higher the L: P ratio, the rougher the casting.  However, if too little water is used, the investment may be unmanageably thick and cannot be properly applied to the pattern.  In vacuum investing, the air may not be suffi­ciently removed.  In either instance, a rough surface on the casting may result.

Prolonged Heating:  When the high-heat casting technique is used, a prolonged -heating of the mold at the casting temperature is likely to cause a disintegration of the investment, and the walls of the mold are roughened as a result.  Further­more, the products of decomposition are Sulfur compounds that may contami­nate the alloy to the extent that the Surface texture is affected.  Such contamina­tion may be the reason that the surface of the casting sometimes does not respond to pickling.  When the thermal expansion technique is employed, the mold should be heated to the casting temperature-never higher than 700º C – and the casting should be made immediately.

Temperature of the Alloy:  If an alloy is heated to too high a temperature before casting, the surface of the investment is likely to be attacked, and a surface roughness of the type described in the previous section may result.  As previously noted, in all probability the alloy will not be overheated with a gas-air torch when used with the gas supplied in most localities.  If other fuel is used, special care should be observed that the color emitted by the molten gold alloy, for example, is no lighter than a light orange.

Casting Pressure:  Too high a pressure during casting can produce a rough surface on the casting.  A gauge pressure of 0.10 to 0.14 MPa in an air pressure casting machine or three to four turns of the spring in average type of centrifugal casting machine is sufficient for small castings.

Composition of the Investment:  The ratio of the binder to the quartz influences -the surface texture of the casting.  In addition, a coarse silica causes a surface roughness.  If the investment meets ADA Specification No. 2, the composition is probably not a factor in the surface roughness.

Impact of Molten Alloy:  The direction of the sprue former should be such that the molten gold alloy does not strike a weak portion of the mold surface.  Occasionally, the molten alloy may fracture or abrade the mold surface on impact, regardless of its bulk. Such a depression in the mold is reflected as a raised area on the casting, often too slight to be noticed yet sufficiently large to prevent the seating of the casting. Prevention:

This type of surface roughness or irregularity can be avoided by proper spruing so as to prevent the direct impact of the molten metal at an angle of 90 degrees to the investment surface.

Porosity:

Porosity may occur both within the interior region of a casting and on external surface.  The latter is a factor in surface roughness, but also it is generally a manifestation of internal porosity.  Not only does the internal porosity weaken the casting but if it also extends to the surface, it may be a cause for discoloration.  If severe, it can produce leakage at the tooth-restoration interface, and secondary caries may result.  Although the porosity in a casting cannot be prevented entirely, it can be minimized by use of proper techniques.

Porosities are classified as :

-Those caused by solidification shrinkage

-Localized shrinkage porosity

-Micro porosity

Those caused by gas

Pinhole porosity

-Gas inclusions

-Sub surface porosity

Those caused by air trapped in the mold(back pressure porosity)

Shrink spot or localized shrinkage porosity:

 These are large irregular voids usually found near the sprue casting junction. It occurs when the cooling sequence is incorrect and the sprue freezes before the rest of the casting. During the correct cooling sequence the sprue should freeze last. This allows more molten metal to flow into the mold to compensate for the shrinkage of the casting as it solidifies. If the sprue solidifies before the rest of the casting no more molten metal can be supplied from the button. The subsequent shrinkage produces voids or pits known as shrink spot porosity.

Avoid by:

Using sprue of correct thickness

Attach sprue to the thickest portion of the pattern.

Flaring the sprue at the point of attachment or placing a reservoir close to the pattern.

Suck back porosity:

It is  the variation of the shrink spot porosity. This an external void seen in the inside of the crown opposite to the sprue. A hot spot is created by the hot metal impinging on the mold wall near the sprue.

 The hot spot causes this region to freeze last. Since the sprue has already solidified no more molten material is available and the resulting shrinkage causes a peculiar type of shrinkage called suck back porosity. It is avoided by reducing the temperature difference between the mold and the molten alloy.

Microporosity: these are fine irregular voids within the casting. It is seen when the casting freezes too rapidly. Rapid solidification occurs when the mold or casting temperature is too low.

Pin hole porosity:

The voids are spherical and small in size. Gases like oxygen and hydrogen are dissolved in the liquid metal. Then during solidification these gases will be expelled, and cause pinpoint holes known as pin hiole porosity.

Gas inclusion porosity:

The voids are spherical but large in size. This is due to gas mechanically trapped by the molten metal in the mold or carried in during the casting procedure.

Subsurface porosity:

This occurs just beneath the surface. This may be due to simultaneous nucleation of the solid grains and gas bubbles at the first moment that the metal freezes at the mold walls.

Prevention:

By controlling the rate at which the liquid metal enters the mold.

Back pressure porosity:

This is seen as surface irregularity on the fitting surface of the casting. But may also be seen on the outside surface. Is due to inability of air in the mold to escape out due to non-porous investment. Air in the mold must be eliminated first and then only the liquid metal is made to enter. It is because no two things occupy the same space at one and the same time.

This is also due to very low casting or mold temperature leading to solidification before the entrapped air can escape.

Prevention:

Proper burnout.

Adequate proper mold and casting temperature.

Adequate casting pressure.

High w/p ratioMaking sure that the thickness of the investment between the tip of the pattern and the end of the ring is not more than ¼”.

Incomplete or missing detail:   Causes:

a.       Due to inhibition of the entry of the liquid metal into the mold. This is in turn is due to insufficient venting or due to high viscosity of the liquid metal.

Prevention:

There must be sufficient casting pressure and that pressure must be maintained at least for few after casting. The metal must be heated to its correct fusion temperature so that it is less viscous and flows readily into the mold. Since it takes less than a second for the liquid metal to solidify, the casting must be done immediately done when the metal is fused.

b.      Due to incomplete elimination of the wax.

Prevention:

Proper time and temperature adapted during burnout.

Too large size casting is due to excessive mold expansion and this is prevented by the use of correct type of investment and correct temperature.

Too small casting is due to, too little mold expansion and it is prevented by heating the mold sufficiently.

REFERENCES:

1. Kennth J Anusavice, Philips science of dental materials 11^th edition W B Saunders publication 2003
2. Rossenstiel, Land, Fujimoto : Contemporary Fixed prosthodontics 3^rd edition Missouri Mosby 2001
3. Shilingburg, Herdert : Fundamentals of fixed prosthodontics ;3^rd edition Chicago