ABRASIONABRASION

DEFINITION:DEFINITION:

The wearing away of a The wearing away of a substance or structure through a substance or structure through a mechanical process, such as mechanical process, such as grinding, rubbing or scraping.grinding, rubbing or scraping.

TYPES OF ABRASIONTYPES OF ABRASION

ABRASION MAY BE: A two body process e.g. action of a diamond bur on

enamel. A three body process e.g. Pumice applied with a

bristle brush.

SUPPLIED AS :SUPPLIED AS :

In dentistry the abrasive is applied to the work by a In dentistry the abrasive is applied to the work by a variety of tools.variety of tools.

► paper / plastic coated:paper / plastic coated: The abrasive particles may be glued The abrasive particles may be glued

on to a paper or plastic disc that can be attached to on to a paper or plastic disc that can be attached to a handpiece .sand paper belongs to this category.a handpiece .sand paper belongs to this category.

► Stainless steel coated strips:Stainless steel coated strips: The abrasive (e.g.diamond ) may The abrasive (e.g.diamond ) may

be attached to stainless steel or plastic strips. This be attached to stainless steel or plastic strips. This category is similar to the above. category is similar to the above.

Electroplating bonding :Electroplating bonding : In case of diamond rotary instruments the In case of diamond rotary instruments the

diamond chips are attached to steel wheels,discs diamond chips are attached to steel wheels,discs and cylinders by electro-plating with nickel based and cylinders by electro-plating with nickel based matrix. matrix.

Bonded stones :Bonded stones : In grinding wheels and dental stones, the In grinding wheels and dental stones, the

abrasive particles are mixed with a bonding agent abrasive particles are mixed with a bonding agent that holds the particles together. before hardening, that holds the particles together. before hardening, the matrix material with the abrasive is moulded to the matrix material with the abrasive is moulded to form tools of desired shapes.form tools of desired shapes.

Powder form :Powder form : An abrasive may also be mixed with water or An abrasive may also be mixed with water or

glycerine and to form a paste or slurry. It is applied glycerine and to form a paste or slurry. It is applied with felt cone, rubber cup or brush and used for with felt cone, rubber cup or brush and used for smoothening irregularities, e.g. pumise powder. smoothening irregularities, e.g. pumise powder.

CAKE FORM : They are also available in the form of cakes. RUBBER IMPREGNATED : Abrasives can be incorporated into rubber or

shellac discs or cups for “soft grade” abrasion. PASTE FORM : The abrasive is made into a paste and supplied

in a tube, e.g.Ivoclar polishing paste, tooth paste, etc. Abrasion is affected by properties of the

abrasive as well as the material being abraded. The important properties are hardness, strength, ductility and thermal conductivity.

MECHANISM OF ABRASIVE ACTION

The action of an abrasive is essentially a cutting action.Each tiny particle presents a sharp edge that cuts through the surface similar to a chisel.A shaving is formed which crushes to a fine powder.This powder clogs the abrasive tool and frequent cleaning is required.

STRESS, STRAIN AND HEAT PRODUCTION DURING

ABRASION

In denture resins,too rigorous abrasion introduces stresses.

The generation of heat during abrasion partially relieves such stresses, but if it is too great, it may relieve processing stresses and a warpage may result.

The resin surface may even melt. Similarly ,high speed cutting of tooth structure

generates excessive heat which can lead to pulpal damage.

Therefore, it is very important to control the heat by air/water spray and intermittent cutting.

RATE OF ABRASION

The rate of abrasion of a given material by a given abrasive is determined primarily by three factors.

SIZE OF THE PARTICLES : Larger particles cause deeper scratches in the

material and wear away the surface at a faster rate. The scratches caused by the coarse abrasive

must then be removed by finer ones.

PRESSURE : Heavy pressure applied by the abrasive will

cause deeper scratches and more rapid removal of material. Heavy pressure is not advisable as it can

fracture or dislodge the abrasive from the grinding wheel.

SPEED

The higher the speed, the greater the frequency per unit of time the particles contacts the surface.

Increasing the speed increases the rate of abrasion.

CLASSIFICATION Finishing abrasive : finishing abrasives are hard, coarse abrasives

which are used initialy to develop contour and remove gross irregularities. E.g. coarse stones.

Polishing abrasives : polishing abrasives have finer particle size

and are less hard than abrasives used for finishing. They are used for smoothening surfaces that have been roughened by finishing abrasives. e.g. polishing cakes, pumice, etc.

Cleansing abrasives : cleansing abrasives are soft materials with

small particle sizes and are intended to remove soft deposits that adhere to enamel or a restorative material.

TYPES OF ABRASIVES : EMERY : 1)Emery consists of a natural oxide of

aluminium called corundum. 2)There are various impurities present

in it such as iron oxide, which may also act as an abrasive.

3)The greater the content of alumina, the finer the grade of emery.

4)Pure alumina is also used as a polishing agent.

ALUMINIUM OXIDE :

1) pure aluminium is manufactured from bauxite.

2) it can be produced in fine grain sizes and has partially replaced emery for abrasive purpose.

GARNET :

1) The mineral comprises of silicates of aluminium, cobalt, magnesium, iron, and manganese.

2) garnet is coated on paper or cloth with glue.

3) it is used on discs which are operated on handpieces.

PUMICE : It is a highly siliceous material of

volcanic origin and is used either as an abrasive or polishing agent.

KIESELGURH : 1) consists of siliceous remains of

minute aquatic plants known as diatoms. 2) it is excellent as a mild abrasive

and polishing agent.TRIPOLI : This mild abrasive and polishing

agent.

ROUGE : 1) Rouge is a fine red powder composed of

iron oxide. 2) It may be impregnated on paper or

cloth known as “crocus cloth” 3) It is an excellent polishing agent for

gold and noble metal alloys. TIN OXIDE : 1) Putty powder used as polishing agent

for teeth and metallic restorations in the mouth. 2) It is mixed with water, alcohol or

glycerin and used as paste. CHALK : 1) It is calcium carbonate prepared by

precipitation method. 2) There are various grads and physical

forms available for different polishing techniques. 3) It is sometimes used in dentifrices.

CHROMIC OXIDE : 1)A relativity hard abrasive capable of polishing a

variety of metals. 2) used as a polishing agent for stainless steel. SAND : Sand as well as other forms of quartz is used as sand

paper or as powder in sandblasting equipment. CARBIDES : 1) The silicon carbide is sintered, or pressed with a

blinder, into grinding wheels or discs. 2) Most of the stone burs used for cutting tooth

structure are made of silicon carbide. DIAMOND : 1) It is the hardest and most effective abrasive for

tooth enamel. 2) The chips are impregnated in a blinder or plated

on to a metal shank to form the diamond ‘stones’ and disks so popular with the dental profession.

• ZIRCONIUM SILICATE : 1) Occurs in nature as zircon. 2) This mineral is ground to various

particle sizes and used as polishing agent.• ZINC OXIDE : 1) Zinc oxide in alcohol can be used

for polishing amalgam restorations.• DESIRABLE CHARACTERISTICS OF AN

ABRASIVE : 1) Abrasive should be harder than

the work it abrades. 2) If it cannot indent the surface to

be abraded, it cannot cut it and the abrasive dulls or wears out.

POLISHING• It is production of a smooth mirror like surface without

much loss of any external form.• If the particle size of an abrasive is reduced sufficiently,

the scratches become extremely fine and may disappear entirely.

• The surface then acquires a smooth shiny layer known as a polish.

• The polishing agents remove material form the surface molecule by molecule.

• The microcrystalline layer is referred to as polish layer or Beilby layer.

• A polishing agent is employed only after an abrasive obliterates or eliminates most of the fine scratches, leaving a smooth finish.

NON –ABRASIVE POLISHING

Polishing is usually achieved by an abrasive process. However , a smooth shiny surface can also be achieved through non-abrasive means. These include :

COMPOSITE GLAZING : A layer of glaze or gloss is applied over the

restoration and cured. This results in a smooth highly glossy surface.

GLAZING CERAMICS : Ceramics are difficult to polish conventionally. The

finished restoration is subjected to high temperatures. At this temperature the surface layer and flows to produce a smooth glass-like surface.

ELECTROLYTIC POLISHING : Electrolytic polishing is not truly an abrasive

process. Although material is removed, it is removed through an electrochemical process rather an abrasive process.

BURNISHING

It is related to polishing in that the surface is drawn or moved. Instead of using many tiny particles, only one large point is used.

If a round steel point is rubbed over the margins of a gold inlay, the metal is moved so that any gap between the inlay and the tooth can be closed. A special blunt bur revolving at high speed can also be used.

DENTRIFICES

• These are agents used with a toothbrush to cleanse and polish natural teeth.

• They should have maximum cleansing efficiency with minimum tooth abrasion.

• Highly abrasive dentrifices should not be used especially when dentin or cementum is exposed.

FUNCTION

• Assists the toothbrush to mechanically remove stains, debris and soft deposits from the teeth.

• To impart a polished surface to the tooth.

• Thus, they help to reduce caries, maintain healthy gingiva, improve esthetics and reduce mouth odors.

PROPHYLACTIC PASTE

• Oral prophylaxis is a widely used procedure in the dental office.

• Prophylactic paste may be available commercially or can be made in the dental office.

• They are usually employed in paste form by mixing with a suitable vehicle.

FUNCTION :FUNCTION :

o Remove extrinsic stains, pellicle, materia-Remove extrinsic stains, pellicle, materia-alba and oral debrisalba and oral debris

o Impart a highly polished and esthetic Impart a highly polished and esthetic appearance.appearance.

Different types of abrasives may be Different types of abrasives may be employed, e.g. zirconium silicate, employed, e.g. zirconium silicate, magnesium silicate, silica, pumice, etc. In magnesium silicate, silica, pumice, etc. In addition, some may contain fluoride in addition, some may contain fluoride in order to reduce caries. order to reduce caries.

PROPHYJET :

The prophyjet is a relatively new dental prophylaxis system of removing intraoral stains.

An abrasive blasting process is used to mechanically remove extrinsic stains as well as light supragingival

adherent plaque the calculas.COMPOSITION :

Consists of sodium carbonate, hydrophobic modified silica and some flavoring agent. The

abrasive is available as powder in a small sachet.CLINICAL CONSIDERATIONS :

The prophyjet is directed at 45 angles to the tooth surface. For obvious reasons it is less effective in proximal areas. The chances of soft

tissue injury exist especially if the tissue is inflamed and friable.

DENTURE CLEANSERS

BRUSHING : a) The dentures may be brushed using a soft

bristle brush and gentle abrasive or cream. b) Hard abrasive and stiff bristles should be

avoided because they may produce scratches on the denture surface.

SOAKING : Chemical cleaners are an alternative to

brushing especially among very old or handicapped persons.

a) Alkaline perborates b) Alkaline peroxides c) Alkaline hypochlorites d) Dilute acids.

DR .K.JESUDOSS M.D.S

DEPT OF PROSTHODONTICS