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Gold as an restorative material

INTRODUCTION

             Several types of dental restorative materials are currently available. They are generally grouped into categories such as silver amalgam materials, cast gold, tooth-coloured materials, dental porcelains, Porcelains fused to metal and direct gold.

             Direct Golds are those gold restorative materials that are manufactured for compaction directly into prepared cavities. Direct gold restorations can last for a lifetime if attention is paid to details of restorative technique and proper home care. The longevity of direct gold restoration is a result of both superb biocompatibility of gold with oral environment and its excellent marginal integrity.

CLASSIFICATION

1. Foil

a)     Sheet

–       Cohesive

–       Non-cohesive

b)     Ropes

c)     Cylinders

d)     Laminated

e)     Platinized

2. Electrolyte Precipitate

a)     Mat gold

b)     Mat foil

c)     Gold-calcium alloy

3. Powdered Gold

 

GOLD FOIL

Gold foil is the oldest of all restorative materials.

Gold Foil is Supplied as

Flat square sheets of varying thickness.

- Standard No. 4 wt. 4 grains (0.259 gm) 0.51 µm thick

- Standard No. 3 wt. 3 grains (0.194 gm) 0.38 µm thick

These sheets may be cut into eighths, sixteenths, sixty-fourths, etc. and then compressed into pellets or cylinders fig I.

After pellets of gold are rolled, they may be conveniently stored in a gold foil box, which is divided into labeled sections for various sizes of pellets.

PLATINIZED GOLD FOIL

             Platinized gold foil is manufacture by laminating a machined sheet of platinum foil between two sheets of gold. This sandwich is thinned by rolling to a desired thickness. This material creates a restoration surface that is harder than conventional gold foil after compaction, and it is use for restoration of tooth surfaces subject to heavy occlusal wear such as tips of cusps of posterior teeth.

PREFORMED GOLD FOIL

             Nowadays ropes and cylinders are available in preformed shapes. Both are made from No. 4 foil that has been "carbonized".

             By placing the gold foil in between sheets of paper and igniting it in a closed container, carbonized foil can be obtained. On igniting, the paper gets destroyed, but the gold foil is left unharmed except that it becomes "carbonized". This is because of the shriveling of the paper while oxidizing in the air-tight safe container. After the carbon is removed it is found that the gold exhibits superior welding property.

GOLD SHEET

Cohesive Gold Sheet

             For cold-welding, gold should have a clean surface free from impurities. Gold attracts gases, e.g. oxygen, to its surface and any absorbed gas film prevents cohesion of individual increments of gold, during their compaction. The manufacturer, therefore, supplies the gold foil essentially free of surface contaminants. This type of gold is known as cohesive gold foil.

Non-Cohesive Gold Sheet

             The manufacturer subjects the foil to a volatile agent such as ammonia which is absorbed on the surface of the gold. This acts as a protective film to prevent adsorption of non-volatile gases and premature cohesion of pellets in their container. Ammonia-treated foil is called as non-cohesive gold foil.

             Non-cohesive gold can also have adsorbed agents like iron salt or an acidic gas (sulfur or phosphorous containing groups) on its surface. The volatile film is readily removed by heating, thereby restorating the cohesive character of the foil.

             Non-cohesive gold is rarely used nowadays, but may be used to build up the bulk of a direct gold restorations.

INDICATIONS

• Class I small carious lesions in pits and fissures of posterior teeth and lingual surfaces of anterior teeth.
• Class V carious lesions on facial surfaces of teeth.
• Class III small carious lesions on proximal surface of anterior teeth.
• Class II in small cavitated proximal surface carious lesions in posterior teeth in which marginal ridges are not subjected to heavy occlual forces.
• Class VI restoration on incisal edge or cusp tips.
• Repair of cast gold restorations.

CONTRAINDICATIONS

• It can not be used in teeth with very large pulp chambers.
• It can not be used in severely periodontally weakened teeth.
• Contraindicated in handicapped patients who are unable to sit for the long dental appointments required for the procedures.
• Root canal filled teeth because these teeth are brittle.

ADVANTAGES

• Gold is resistant to tarnish and corrosion.
• Gold has good biocompatibility.
• Gold has good mechanical properties.

DISADVANTAGES

• Direct filling gold restorations are not adhesive to tooth structure. Therefore, they can not function to reinforce tooth structure in a manner similar to that for composites or ceramic restorations.
• Gold foil has poor esthetic restorative material.
• Gold foil cause pulpal sensitivity to thermal stimulus, because of gold's high thermal conductivity.
• Manipulation of gold foil is a difficult procedure.
• Gold is a highly costly metal.

PROPERTIES

• Direct gold is essentially 100% gold. Pre alloying with other elements would reduce the weldability and malleability at room temperature. However, other element may be incorporated (platinum or calcium) indirectly into final structure by layering them onto the gold in forms such as gold foils.
• Restoration of gold foil are more dense than mat gold or powdered gold and newer gold foil have lesser flexural strengths than mat gold and powdered gold.
• Density of gold foil ranges from 14.3-15.9 gm/cm3.
• Transverse strength of gold foil averages from = 265-296 Mpa.
• Hardness of gold foil is 69 KHN.

CLINICAL CHARACTERISTICS

• It retains a good surface finish.
• Restoration can be completed in one appointment.    

MANIPULATION

             There are two processes involved in manipulation of direct filling gold restorations. (1) Degassing (2) Compaction

Degassing or Annealing

             The direct filling gold are received by the dentist in cohesive condition, except for the non-cohesive gold. However, during storage and packaging, they absorb gases from the atmosphere. Adsorbed gases prevent gold from fusing. Hence it is necessary to heat the foil or pellet immediately before it is carried into the prepared cavity. This heating process which removes surface gases (oxygen, nitrogen, ammonia, moisture or sulfur dioxide) and ensures a clean surface is called desorbing or degassing.

             Gold foil is stored in air tight containers therefore it is advised that the operator should wear Chamoi's finger tips to protect the gold from contamination.

             Degassing is accomplished by heating gold foil

–       in a bulk on a mica tray over a flame or on an electric annealer, or

–       by heating each piece of gold over a pure ethanol flame.

Electric Annealing

The electric "annealer" is maintained at a temperature between 340oC and 370oC. The time required varies from 5 to 20 mins depending on the temperature and the quantity of gold on the tray.

Disadvantages of electric annealing are :

• Pellets may stick together, if the tray is moved.
• Air currents may effect the uniformity of heating.
• Greater exposure to contamination.

Flame Desorption

The fuel for the flame is pure ethanol. Each piece is picked individually, and passed into blue inner core of the flame on the tip of a foil passing instrument and held just until the gold becomes Dull Red; the instrument is withdrawn from the flame. After a few seconds are allowed for cooling, the gold is placed in the prepared cavity.

Advantages of flame desorption are:

• Ability to select a piece of gold of desired size.
• Less exposure to contamination between time of degassing and use.

Underheating: is to be avoided. Because it does not adequately remove impurities and results in incomplete cohesion.

Overheating: is also to be avoided, because it may cause the gold to become brittle or melt and render it unstable.

Compaction

Direct-filling golds must be compacted during insertion into tooth preparations. The compaction takes the form of malleting forces that are delivered either by a hand mallet used by the assistant or by an Electro-mallet or a pneumatic mallet used by the dentist. Successful malleting of the gold foil may be achieved with any of the currently available equipment. Some operators prefer the electromallet or pneumatic mallet because a dental assistant is not required for the procedure.

Compaction begin when a piece of gold is placed in a tooth preparation. The gold is first pressed to place by hand; then a condenser of suitable size is used to begin malleting in the center of the mass (often this is done while this first increment is held in position with a holding instrument). Each succeeding step of the condenser overlaps (by half) the previous one as the condenser is moved towards the periphery. The gold moves under the nib face of the condenser, effecting compaction as malleting proceeds.

             The most efficient compaction occurs directly under the nib face. Some compaction also occurs by lateral movement of the gold against surrounding preparation walls. The result of compaction is to remove most of the void space from within each increment of gold, to  compact the gold into line and point angles and against walls, and to attach it to any previously placed gold via the process of cohesion. The line of force is important when any gold is compacted. The line of force is that direction through which the force is delivered (i.e., the direction in which the condenser is aimed).

             The Electro-Mallet is an acceptable condenser if the maufacturer's instructions for mallet intensity are followed. Correct hand malleting technique requires a light, bouncing application of the mallet to the condenser, rather than the delivery of heavy blows.

Gold foil compacts readily because of its then form and produces a mass with isolated linear channels of microporosity. Because the thin folds of the gold pellet weld to each other, the remaining channels of microporosity do not appear to be entirely confluent with one another.

About the Author

Dr. Rohit Anand
M.D.S (Pedodontics with preventive dentistry)
Lucknow (U.P.), INDIA.