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1. ABRASIVES AND POLISHING AGENTS
Introduction
It is imperative that all dental appliances and restorations be smooth to
the maximum. Absense of the smooth surface leads to
1. Discomfort to the patient
2. Deposition of plaque and food debris
Rough surfaces are not only discomfortable but may also leads to
constant irritation to the Oral mucosa. Plaque/ food deposition will lead to
tarnish and corrosion of the metallic restoration, nidas for microbial flora,
secondary caries and periodontal deterioration.
Hence both dental surgeon and dental technician must know the science
of dental abrasives and polishers. The knowledge includes.
1. Various abrasives and polishing agents
2. Availability of the same
3. Method of using the same
4. Mechanism of the abrasion, wear and tear and polishing.
The difference between an abrasive agent and polishing agent is some
what difficult to define agent having large particle size may acts as an abrasive,
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2. producing scratches. The same abrasive with smaller particles size may be used
as polishing agent.
Abrasion : The wearing away of the substance or structure through some
unusual or abnormal or abnormal mechanical process.
Abrasives are rough gritty and wear producing particles. abrasion is a
substance which is used to remove excess material by cutting. Substances used
to wear off or grind objects in order to give then the desired size, shape or
finish.
Attrition: Attrition may be defined as the physiologic wearing away of a tooth
as a result of tooth to tooth contact, as in mastication. This occurs on occlusal
incisal and proximal surfaces of the teeth. This process is physiologic rather
than pathologic.
Erosion is defined as a loss of tooth substances by a chemical process that does
not involve known bacterial action.
Grinding is the gross reduction to the surface of the subtrate by the process of
abrasion. It is usually performed with large particle sized abrasives.
Polishing: Polishing is the process of making a rough surface smooth to the
touch and glossy. Polishing is usually performed with small sized particles.
(Submicron sized abrasives).
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3. Abrasives: The material that causes the wear or abrasion of another material.
Substrates: The material being abraded is called substrate, while tooth
preparation. Diamond will cut the tooth, and tooth is called substrate and the
diamond is called Abrasive. While polishing denture, denture is called substrate
and pumice or french chalk is called abrasive.
Wear: Wear is common phenomenon is dentistry that occurs when two
surfaces undergo slipping or sliding movements as a load is applied. Wear is
defined as progressive loss of substance from the surface of a body brought
about by mechanical action.
Mechanical causes that can cause wear include rubbing impact scapping
and erosion, which may result in the progressive removal of a substance from
the surface of the material that is undergoing wear.
The types of wear
1. Adhesive wear
2. Abrasive wear
3. Erosive wear
4. Surface fatigue wear
Mechanism of action
Abrasion occurs when a hard rough surface slides along a soften surface
and cut or plows a series of grooves. It may be either two body process or three
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4. body process. An example of three body process would be the action of pumice
as it is applied to tooth enamel by rubber cup during oral prophylaxis
procedure. An example of two body process is when the diamond bur is used to
cut tooth enamel.
The action of an abrasive is essentially a cutting action. Abrasive tools
or slurries however differ from dental cutting instruments in that the cutting
edge or a point of the abrasive are not arranged in any particular pattern. Each
point or edge of an abrasive acts as an individual cutting blade. The process of
abrasion is affected by the physical and mechanical properties of the material
being abraded.
The surface of the work is penetrated by the abrasive and material is
removed by a process of shear, in the from of plastically deformed chips. In
case of polishing crystalline form will change into amorphous form and
polished layer is called “Beilby layer”. Melting point is important. The solid
with high melting point polishes, the one with lower melting point, even if it is
not hard.
Rotary cutting instrument
Burs were introduced in 1891. Steel burs at slow speed efficiently cut
the dentin but at high speed when used on enamel become dull. Dull burs
should not be used as cutting efficiency is less and more heat is produced.
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5. Carbide burs were introduced in 1947. Now mainly used for tooth
preparation.
Steel burs are now-a-days used for finishing procedures. Carbides are
superior at higher speed. Carbide bur head is made up of tungsten carbide
particle held together by a matrix of cobalt and nickel. Carbide is stiffer and
stronger than steel but more brittle (hence sudden blow or shock will lead to the
fracture of the carbide bur).
Both steel and carbide are available in different shape (shape of the head
means general contour or silhouette of the head).
Common shapes are round, inverted cone, straight fissure and taper
fissure.
Hand cutting instruments:
They are made up of carbon steel or stainless steel.
High grade carbon steel Stainless steel
- contains carbon, manganese, silicon
and iron
- harder than stainless steel but rusts
and corrodes
- contains carbon, chrome and iron
- cutting edge is lost earlier than the
carbon steel
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6. Usually cutting edge is made up of carbon steel, carbon steel or stainless
steel are subjected to heat treatment by the manufacturer.
a. Hardening heat treatment, hardens the alloy and makes it brittle.
b. Tempering heat treatment relieves the strain and increase the toughness.
Cutting: Cutting is generally understood to mean the removal of a part of
structure surface by means of shearing action. The difference between
industrial cutting and dental cutting is the method of applying the cutting tool.
In metal work the work is usually carried or fed to the tool at a constant rate,
whereas dentist carries the tool to the work.
Dental burs: Dental burs are miniature milling cutters as used in industry.
Many shape and sizes are available in dentistry for various purposes.
General design of dental bur
a. Bur Tooth: This terminate the cutting edge or blade. It has two surfaces the
tooth face which is the side of the tooth on the leading edge; and the back or
flank of the tooth which is the side of the tooth on the trailing edge.
b. Rake angle: The rake angle is the angle that the faces of the bur tooth
makes with the radial line from the centre of the bur to the blade. The angle
can be negative if the blade is beyond or leading the radial line (referring to
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7. the direction of rotation). It can be ‘o’ if the radial line and the tooth face
coincide with each other (radial rake angle).
c. The angle can be positive if radial line leads the face so that the rake angle
is on the inside of the radial line.
d. Land: The plane surface immediately following the cutting edge.
e. Clearance angle: The angle between the back of the tooth and the work.
If the land is present on the bur, the clearance angle is divided into
primary clearance which is the angle the land will make the work and
secondary clearance, which is the angle between the back of the bur tooth and
work. When the back surface of the tooth is curved, the clearance angle is
called radial clearance.
f. Tooth angle: This is measured between the face and the back. If a land is
present, it is measured between the face and the land.
g. Flute or chip space: The space between successive teeth. The number of
teeth in dental cutting bur is 6-8.
Each bur having three portions
1. Head: the portion carrying cutting blades.
2. Shank: The portions connecting the head to the attachment part.
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8. 3. Shaft or the attachment part: The portion which will be engaged
within the hand piece.
Factor affecting the rate of abrasion
1. A large difference in harness between the abrasive and substrate allows the
most efficient grinding to take place.
Surface hardness ability to result scratching or resistance to indentation.
Tests are Brinell, Rockwell, Wickers and Knoop, surface hardness of the
diamond is more than other materials, so it is most commonly used in dentistry
as abrasive.
Knoop harness numbers of abrasives
Abrasive
Sand
Emery
Silicon carbide
Boron carbide
Diamond
Number (KHN)
800
2000
2500
2800
7000
Brinell and Knoop hardness values are function of materials resistance
to indentation whereas moh’s value indicate the resistance to scratching of one
material by another.
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9. 2. The particle size of an abrasive: Larger abrasive particles will abrade a
surface more rapidly than will smaller particles, however they tends to leave
coarser scratches in the abraded surfaces than do fine particles.
The particle size of an abrasive may be expressed in micrometers. By
convention, parties are classified as fine (0-10um), medium 10-100um) and
coarse (100-500um) according to the average particles size of the sample.
3. The particles shape also has an effect on the rate of abrasion. Sharp
irregularly shaped particles will abrade a surface more rapidly than will
more rounded particles having duller cutting angle.
4. The greater the speed which the abrasive travels across the surfaces being
abraded the greater the rate of abrasion. The greater friction at higher
speeds, however tends to create higher temperatures at the surface of the
substrate.
5. The greater the pressure applied, the more rapid will be the abrasion for a
given abrasive. Greater pressure produces deeper and wider scratches and
creates higher temperature.
6. Lubricants: (e.g., silicon grease, water spray or glycerol) are used during
abrasion for two purpose to reduce heat build up and to wash away debris to
prevent clogging of the abrasive instrument.
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10. Ideal requirement of an abrasive
1. It should be harder than the work it abrades.
2. It should have high impact strength.
Strength is the maximal stress required to fracture a structure.
Impact strength: It is the energy required to fracture a material under an
impact force.
3. It should have high attrition (wearing) resistance.
4. Shape of the abrasive: It should be with irregular shape with sharp edges.
Particles of bigger size and cutting edges make deeper groove and abrade
quickly. Edges must be sharper.
5. Rate of movement of abrasive increases rate of abrasion only upto a certain
speed.
6. Greater pressure greater is the abrasion.
7. Brittle surface can be abraded more quickly. During abrasion or polishing
generation of heat is to be minimized by spraying water. In some cases
excessive heat cause stress relief and distortion.
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11. Abrasion Resistance: abrasion resistance is the ability of a material to resist
wearing of the surface when it is rubbed across the surface of another material.
Abrasion resistance depends upon following factors.
1. Surface hardness of the material.
2. Temperature of the surface layer (AR decreases with increase in
temperature).
3. Frictional force.
4. Rate and amount of slip.
5. Thermal conductivity of the material Hardness is related to some extent to
abrasion resistance. Diamond is the hardest of material and has got greater
abrasion resistance. Hardness is not only a factor in selecting abrasives,
because steel is more harder than rubber but has much lower abrasion
Resistance.
An understanding of A.R. is possible only from a knowledge of
mechanism of abrasion.
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