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.

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.

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 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.

             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.

             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.

             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

CONTRAINDICATIONS

ADVANTAGES

DISADVANTAGES

PROPERTIES

CLINICAL CHARACTERISTICS

             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.

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 :

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:

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.

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.

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