Historical Articles

October, 1952 issue of Plating

 

Your Dentist’s A Plater, Too*

Walter J. Krumbeck, Lieutenant, Dental Corps, U.S. Navy

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*The opinions or assertions contained herein are the private ones of the writer and are not to be construed as official or reflecting the views of the Navy Department or the Naval Service at large.
**Distributed by Metals Disintegrating Corporation, New Brunswick, N. J.

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INTRODUCTION
How many times have you left your plating plant and gone to see your friendly dentist? Perhaps you thought that you were getting away from plating. However, you might be surprised to learn that dentists are platers, too. Current density’ throwing power, anodes, etc., are familiar terms to them since they utilize all the advantages of electroforming to provide certain necessary-dental restorations.

PRE-PLATING TECHNIQUE
Crowns (the dental term used to denote the artificial replacement of the portion of the tooth projecting out beyond the gum) and inlay techniques require great accuracy. Electroforming provides the desired accuracy, and as with all plating operations, the correct pre-plating technique pays dividends in the production of quality finished items.

The initial operation in the formation of a gold crown is in the preparation of the tooth. Reshaping and alteration of the tooth structure is performed so that all of its surfaces are very nearly parallel but tapering very slightly toward the biting surface of the tooth. The tooth is also shortened about one-sixteenth of an inch to allow for the thickness of gold over the biting surface. There can be no undercuts in the preparation since the crown must slip on the tooth much as a thimble would slip on the finger of a seamstress. After the shaping preparations are completed an impression is taken. First a copper band that will just fit over the prepared tooth is trimmed so that it is a quarter of an inch longer than the tooth at one end and contoured to the shape of the gum at the other end. Impression compound, warmed and softened is then inserted in the copper band. The banded impression compound is placed on the tooth and gently pressed downward until the prepared tooth is covered. The band is held in place until the compound in contact with the tooth cools and hardens thus forming an indentation in the impression compound which is an exact negative replica of the prepared tooth, just as a footprint in wet concrete is a negative replica of the foot.

To make a positive replica the use of poured plaster or a similar material in the impression is suggested. However, the resulting replica or model would lack accuracy and would not have the hard non-porous surface so desirable in such a model. To overcome these shortcomings an electroformed copper model is made from the impression of the tooth. This requires the sensitization of the impression surface of the compound with a suitable electrical conductor.

One method consists of dusting a very finely divided copper powder into the impression and blowing off the excess thereby leaving a film of copper dust. A second consists of painting a thin film of colloidal graphite in the impression. Another method involves the chemical application of silver to the impression surface. The recent development of (Silbrite)** a material that forms a highly conductive silver film which may be applied with a brush is replacing other materials for surface treatment of impression compound. Through the use of this material a more nearly equal conductive cathode surface is obtained, resulting in a more even deposit of copper. The sensitizing step is carried out so that the conductive surface contacts the copper band to which a short piece of copper wire is connected as the cathode contact. Then the exposed surface of the band and the copper wire are given a coating of soft wax or stop-off lacquer as insulation, to restrict the deposition of copper to the conductive surface of the impression.

PLATING PROCEDURE
A neat compact dental electroforming apparatus is commercially available. Such a unit with its built-in rectifier operates directly from a 110-volt a-c line and is capable of electroforming several models at one time Although this is a desirable unit the need for an electroforming apparatus usually begins in dental school or early in practice when the commercial unit is prohibitively expensive. To meet the need and stay with in a limited budget, many dentists assemble their own unit from inexpensive materials (See Fig. 1). A popular method consists of connecting two large dry cells, a rheostat and a milliammeter in series. A long glass ice box dish serves as the plating tank with a strip of copper used as the anode at one end of the tank. The end of the copper is bent over the top of the tank and connected into the circuit with a battery clip. A plastic strip is notched to fit the sides of the tank and two holes are drilled in this strip so that the back ends of the two alligator clips will fit snugly in the holes. These clips serve as cathode holders and are cemented or sealed in place in the plastic. A standard acid copper sulfate solution is used with about 8 drops of liquid glue per gallon as an addition agent.

Fig. 1. Home-made copper electroforming unit

When the preliminary steps have been completed a glass medicine dropper is used to flush out the impression with a debubblizer to make sure that there are no air bubbles. The sensitized banded impression compound is then plated at a current density of approximately 15 milliamperes for each square centimeter of surface being electroformed (14 amp/ft2) for about six hours. However, sometimes at the end of a busy day it may be forgotten and is plated all night. Since building up the crown takes place toward the inside, no harm is done to the crown; it forms a thicker shell, and weaker batteries. To effect a more uniform deposit of copper, slight agitation is employed along with an addition agent such as glue. Proper bath maintenance is carefully observed. All standard electroforming procedures for obtaining a hard and a smooth deposit are generally followed.

After the electroforming process has been completed a core of hydrocal (a substance similar to but much harder than plaster) is poured into the electroformed copper shell. When the core has set, the band is dipped into hot water to soften the impression material and thus facilitate removal of the band and impression material. The remaining electroformed copper shell with its hydrocal core is an exact duplicate of the tooth. Hot wax is then built up on the model of the tooth to form a crown of wax and is then used to make a gold casting by the lost wax process. The resulting gold casting is then put back on the copper model to check- the fit. If all went well it fits and is then finished and polished on the copper model. The making of the copper model is the same for a gold crown, a gold inlay, a porcelain jacket crown or a plastic jacket crown. Fitting of the crown to the patient’s tooth at a return appointment is the culmination of the process which involved the plating steps detailed above. At that time the crown is cemented ‘through the use of a special dental adhesive. Since crowns and inlays rely upon frictional fit for their retention, the better the fit and the more nearly parallel the prepared surfaces of the teeth are, the greater the retention. However, they must have a slight taper to permit their seating and removal. The material used for cementing of crowns and inlays is non-cohesive when it sets. It merely fills in the microscopic gap between the material and the tooth. The cement is slightly soluble in saliva. Therefore, the more accurate the fit of the crown or inlay, the thinner the layer of cement between it and the tooth. The thinner the cement layer the less surface area to be attacked by saliva and associated mouth bacteria and the longer the life of the cement and thus the tooth and crown or inlay.

It becomes apparent that an accurate fit of the crown or inlay is essential for permanency. Without an accurate model an accurate fit is impossible. By using the electroforming process it is possible to have a model more accurate than that produced by any other technique known today.

Fig. 2. Electrolyzer with electrode in root canal of tooth.

ELECTRO-STERILIZATION
Electrolysis is another electro-chemical technique used in the dental office. Electrolysis is used to sterilize the inside of teeth in root canal therapy (endodontia). To better understand endodontia and thus the need for the electrolysis technique a glimpse at the anatomy of a tooth is necessary (See Fig. 2). The part of the tooth projecting through the gum is the crown. It is covered with enamel, a pearly substance harder than steel. Below the gum is the root which is about two-thirds the length of the tooth. The root is covered by cementum, an intermediate substance between the tooth and surrounding tissue. Inside the enamel and cementum is the dentin, a substance somewhat similar in general appearance to hard dense bone. Starting at the root end of the tooth or apex and running down the center into the crown is a canal which is called the root canal. The root canal contains the pulp, a collective term for the soft connective tissue, the blood vessels, and the nerves. The opening of the canal at the apex of the tooth while not completely formed in a child may be a sixteenth of an inch in diameter. However, in an adult the diameter would be measured in hundredths of an inch.

Dental caries or decay travels through a defect in the enamel to the dentin. When caries gets to the dentin it progresses rapidly in all directions. If the tooth is not cared for, the infection reaches the pulp, and causes a swelling. Since the pulp is confined the process strangles the blood vessels at the narrow apex of the tooth. The pulp, thus devoid of a blood supply cannot get oxygen and nutrition and therefore dies. If the condition is still not cared for the infection travels the length of the tooth and out into the bone at the apex of the tooth. Another cause of death to the pulp is a- sharp blow to the tooth. When this injury occurs the apex moves laterally against the bone and the narrow filament of the pulp is sheared off and dies from lack of nutrition and oxygen.

A patient with a dead pulp either must have the tooth extracted to prevent further spread of the infection or the tooth must receive root canal therapy. When the dentist decides that root canal therapy is the best treatment the tooth will be retained and treatment begun. First an opening is made in the back of the crown-that projects down into the root canal. The remaining pulp is removed with barbed broaches. The canal is then cleaned and enlarged with tiny round tapering files in conjunction with sulfuric acid. The length of the tooth is determined through algebraic interpretation of X-ray pictures.

The most difficult part, that of sterilizing the tooth is done by electrolysis. Electrolysis for sterilization of root canals is not a new technique since it was first used in 1893. A commercial instrument for electrosterilization that operates from 110 volts may be purchased. However, both the cost of the unit and possibility of electrocution of the patient make an easily constructed battery unit more desirable in the opinion of many dentists. The electrosterilizer consists of: a 10 volt battery, a switch, a 0-3 milliampere direct current meter, a 0-3500 ohm rheostat, a negative arm electrode, and a positive platinum-iridium tooth electrode. The components are all connected in series. The electrolyte consists of Appelton’s iodine solution composed of 15.0 grams zinc iodide, 0.6 grams iodine and 50 cubic centimeters distilled water. When the root canal has been dried the electrolyte is pumped to the apex with a file. The platinum-iridium wire electrode is inserted until it reaches to the apex of the tooth. A gauze napkin moistened in saline solution is placed on the arm and the arm-electrode is fastened securely in place, making a good electrical contact with the body. The current is turned on and increased until a slight tingling sensation is felt around the tooth. The current is then reduced until no sensation is felt. Then a reading is taken on the milliammeter. The formula first determined by Ziegler in l900 is used to determine the sterilization time. Ziegler’s formula is:

in minutes. Assuming the patient tolerates 1.5 milliamperes, the sterilization time will be 20 minutes. The patient usually tolerates between 1.0 and 2.0 milliamperes. Since bacteria have a negative charge they migrate away from the microscopic crevices and pits of the tooth and travel toward the positive platinum iridium electrode. The electrolyte also undergoing electrolysis forms nascent iodine and iodates. The nascent iodine released at the positive electrode kills the bacteria which migrate toward it. The iodates kill the bacteria which cannot break away from the inside and apex of the canal. When the canal is sterile the entire length of the root canal is filled.

CONCLUSION
The dental profession has been using electrochemical techniques for over half a century and through their use has improved the quality of the service it is able to render. Just recently dental laboratories have explored the possibilities of electro-polishing and they: are using it more and more for the treatment of stainless steel frameworks of partial dentures. Such electropolished dentures will not harbor as many bacteria and are easier to keep clean and sanitary. The valued use by the dental profession of techniques developed in the laboratories and shops devoted to plating is a tribute to that industry. What new dental techniques will be developed in the next half century only time will tell. Very probably though, judging from the past, some of them will come from the workbenches of the plating industry.

REFERENCES
C. B. Frankel, “A Scientific Approach to the Solution of Practical Problems Encountered in Electroforming Copper Dies”, J. A. D. A. 32, 1131138 (Sept., 1945). ;
C. B. Frankel, “Fundamental Principles of Electroforming as Applied to Copper Dies in Indirect Inlay Work”. N. Y.
D. J. 17:7, 319-322 (Aug. Sept., 1951).

 

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