The history of the lost wax technique in dentistry

Ministry of Public Health of Ukraine

Higher State Educational Establishment of Ukraine

"Ukrainian Medical Stomatological Academy"

"Approved"

at the meeting of the department

propedeutics of Prosthodontics

Head of the department

Associate Professor, Korol D.M. _________

"_____"_______________ 20___

RECOMMENDATIONS

FOR STUDENTS’ independent WORK

during the preparation to practical classes

Poltava 2011

Actuality of the theme: Clear knowledge of construction features indications and contra-indications to their application, clinical and laboratory stages of making largely determine end-point of treatment – restoration of a form and function of masticatory system.

Specific targets:

- to analyse the composition of base alloys;

- to analyse the composition of casting gold alloys;

- to distinguish the sequence of the stages of casting;

- to analyse the composition of investment material;

- to explain defects in the casting.

2. Base level of training:

Tasks for independent work during preparation to practical classes.

Recommendations for students’ independent work and album for independent work of students.

4.2 Theoretical questions to the lesson:

1. Classification of the casting alloys.

2. Composition of base alloys.

3. Composition of casting gold alloys.

4.Technique of casting.

5. Composition of investment material.

6. Defects in the Casting.

4.3. Practical works (tasks) that are executed at the lesson:

By means of diagnostic models, thematic patients demonstratively examined students:

1. indication and contra-indication to cast bridge making.

2. technique of casting.

3. sequence and maintenance of the laboratory stages of cast bridge making.

Content of the topic:

The history of the lost wax technique in dentistry

No one knows when or where the first casting was made. It was probably in ancient Egypt or China. Someone someplace conceived the idea of making a wax replica of an item he wanted to replicate in gold, embedding it in an investment material made of plaster, and then "burning out" the wax. He did this by placing the hardened plaster containing the wax replica into a fire. The wax would melt and burn away leaving a hole in the plaster in the shape of the original replica. By drilling a small hole (called a sprue hole) from the surface of the plaster down to the hole, he could now pour melted gold into the sprue hole, filling the space left behind when the wax replica was burned out. When the gold solidified, the now solid casting would be broken out of the mold and the gold replica would be retrieved.

Prior to the introduction of cast gold restorations in 1907, dentistry consisted mostly of extracting teeth and replacing them with various forms of false teeth. There were fillings available, but they were haphazard, painful and generally unsuccessful affairs until 1855 when a rather laborious and expensive technique consisting of hammering tiny pieces of gold foil into an equally laboriously prepared cavity preparation was invented. In 1895, G.V. Black standardized the formula for dental amalgam, which was made out of powdered silver mixed with mercury, and made it possible for the average person to save a decayed tooth rather than having to extract it, but not all dentists were on board with the mercury, so they remained wedded to the gold foil technique. Finally, in 1907, William H. Taggart invented a simple casting machine for use with the lost wax technique. He actually worked up the procedures for the technique and patented it. However he eventually lost the patent when it was discovered that a Dr. Philbrook of Denison, Iowa had published a paper on the subject twenty five years before. Taggart's procedure was as follows: He laboriously drilled out the decay from a tooth, being careful that there were no undercuts in the preparation. (Bear in mind that there were no high speed drills or local anesthetics at this time.)

Then he pressed softened wax into the cavity preparation--the hole he had prepared in the tooth, and then removed the wax. Now he had, in his hand, a wax "filling" (in fact a replica of a filling) that if reinserted back into the tooth, would fit the preparation exactly. He then attached a wax sprue (simply a small wax rod) to the wax replica and invested the entire thing in a small cylinder filled with plaster of Paris. The wax replica of the filling was completely immersed in the plaster, but the wax sprue was left partly sticking up and out of the plaster.

Once the plaster set, he placed it in a very hot oven and when the wax had burned away, the plaster in the ring now contained a space in the shape of the original wax filling with a ready made sprue hole now exiting the surface of the plaster where the sprue had also burned away. He then proceeded to fill the hole with molten gold using his newly invented centrifugal casting machine.

By immersing the still hot plaster with its gold innards in water, the plaster shattered away leaving behind the filling still attached to its now gold sprue.

After removing the sprue, he polished up the gold which had formed itself neatly into the shape of the original wax replica and cemented it into the original cavity preparation in the tooth.And the reaction from the rest of the dental profession was resounding silence. The profession was still just getting used to the introduction of dental amalgam and various members were taking sides in the ongoing amalgam wars. They had enough to fight about and didn't want any new "advances" to get in the way of their arguments. Besides, Martin's formula produced castings which were not quite an exact fit for the cavity preparation, so it could be argued that gold foil was still superior to the results achieved with cast gold.

It wasn't until the 1920's that the commercial advantages of gold castings had caught on. Wealthy people wanted high class dentistry and were willing to pay more for the privilege of not having to sit around while the dentist hammered gold into the cavity preparation. Finally, in 1929, Coleman and Weinstein invented cristobalite investment to replace the plaster of Paris, eliminating most of the shrinkage and distortion problems which had plagued the production of gold castings up to that point. In fact, it was not until the 1940's that investment materials were formulated that compensated for all of the distortions encountered in the lost wax technique.To replace the wax by metal, wire rods 1.0 to 2.0 mm in diameter and 2.0 to 3.0 cm in length are attached to the pattern of the denture body and secured to a wooden cone. The wax teeth are lowered into a casting ring filled with cream-mix investment (Fig. 1).

Fig 1. The rods attached to the wax pattern of the denture body and secured to the wooden cone

When the invest­ment has set (in 10 to 15 minutes) the cone is removed and the ring placed into a muffle furnace to melt out the wax. As soon as the ring is warmed a little the rods are removed from the investment leaving sprue canals through which the wax will escape (melted metal is later also poured through them into the ring). After the rods have been removed heating is continued till all the wax is melted out and moisture disappears from the investment mate­rial. For final heating of the ring the temperature in the muffle furnace is raised to 800°C at which the investment expands, thus compensating for shrin­kage of the metal during casting. Various apparatus are used to melt the metal. Gold alloys are melted by means of gas burners which produce a tempera­ture of up to 1200°C. The melted metal is forced into the mould by means of machines utilizing centrifugal force or steam pressure. Steel is melted by an electric arc in a Kryptolyte furnace or in a special high- frequency furnace

Fig.2 Preparing the casting ring loadedwith fire-proof material to replace the wax by metal: the wax pattern of the denture body is invested in the fire-proof material

Fig.3 Metal is placed into a funnel in the investment to be melted and driven into the mould

Recovery of the Casting.

After the red glow has disappeared from the button, the casting ring is plunged under running cold water into a large rubber mixing bowl.

Gypsum-bonded investments quickly disintegrate, and elimination of residue is easily accomplished with a toothbrush. Final traces can be removed ultrasonically. Oxides are removed by pickling in 50% hydrochloric acid (or preferably a nonfuming substitute). Phosphate-bonded investments do not disintegrate and must be forcibly removed from the casting ring. They can be handled as soon as they are sufficiently cooled under running water. Care must be taken to prevent scratching of the internal surface of the casting or damage to the margins.

Evaluation.

The casting is never fitted on the die until the inner surface has been carefully evaluated under magnification; even tiny imperfections can cause damage to the stone die. A die may be rendered useless in a matter of seconds if a casting is fitted

prematurely.

SELECTING AN INVESTMENT MATERIAL

After the choice of casting alloy has been made, the investment material can be selected.

Ideal Properties. An ideal investment should incorporate the following features:

1. Controllable expansion to compensate precisely for shrinkage of the cast alloy during cooling

2. The ability to produce smooth castings with accurate surface reproduction without nodules

3. Chemical stability at high casting temperatures

4. Adequate strength to resist casting forces

5. Sufficient porosity to allow for gas escape

6. Easy recovery of the casting

Several investment materials are available for fabricating a dental casting mold. Typically these consist of a refractory material (usually silica) and a binder material, which provides strength. Additives are used by the manufacturer to improve handling characteristics.

GYPSUM-BONDED INVESTMENTS

Gypsum is used as a binder, along with cristobalite or quartz as the refractory material, to form the mold. The cristobalite and quartz are responsible for the thermal expansion of the mold during wax elimination. Because gypsum is not chemically stable at temperatures exceeding 650° C (1200° F), these investments are typically restricted to castings of conventional Type II, III, and IV gold alloys.

PHOSPHATE-BONDED INVESTMENTS

Phosphate- bonded investment materials offer certain advantages over gypsum-bonded investments. They are more stable at high temperatures and thus are the

material of choice for casting metal-ceramic alloys.

They expand rapidly at the temperatures used for casting alloys, and their size can be conveniently controlled.

Because most metal-ceramic alloys fuse at around 1200° C (2300° F) (as opposed to conventional gold alloys at 925° C [1700° F]), additional shrinkage occurs when the casting cools to room temperature.

To compensate for this, a larger mold is necessary. The added expansion can be obtained with phosphate-bonded investments.

The principal difference between gypsum-bonded and phosphate-bonded investments is the composition of the binder and the relatively high concentration of silica refractory material in the latter. The binder consists of magnesium oxide and an ammonium phosphate compound. Contrary to gypsum-bonded products, this material is stable at burnout temperatures above 650° C (1200° F), which allows for additional thermal expansion. Most phosphate-bonded investments are mixed with a specially prepared suspension of colloidal silica in water. (Some, however, can be mixed with water alone.)

Some phosphate-bonded investments contain carbon and therefore are gray in color. Carbon- containing materials should not be used for casting base metals because the carbon residue affects the final alloy composition. They may be used for casting high-gold or palladium content alloys.

CASTING MACHINES

A casting machine requires a heat source to melt the alloy and a casting force. For a complete casting, the casting force must be high enough to overcome the high surface tension of the molten alloy as well as the resistance of the gas within the mold. The heat source can be either the reducing flame of a torch or electricity. Conventional alloys can be melted with a gas-air torch (Fig. 22-23, A and B), but the metal-ceramic alloys in a higher melting range need a gas-oxygen torch (Fig. 22-23, C.)Fig. 22 Casting machines

A , Kerr Broken-Arm.

Base metal alloys need a multiorifice gas-oxygen (Fig. 22-23, D) or oxyacetylene torch. Electric heating can occur by convection from a heating muffle or by generation of an induction current in the alloy. Advocates of the latter maintain that heating can be more evenly controlled, preventing undesirable changes in alloy composition caused by volatilization of the lower- melting point elements. In general, the electric machines are expensive and more appropriate for larger dental laboratories, whereas a torch may be the method of choice for smaller laboratories and dental offices.

Present-day casting machines still use either air pressure or centrifugal force to fill the mold, which were first proposed in the early days of lost-wax castings. Some machines evacuate the mold before it is filled with metal, and vacuum has been shown to improve mold filling, although it is not clear if the difference is clinically significant.

Date: 2015-12-24; view: 2156