Before any dental restoration or appliance is placed in the mouth is
should be highly polished.
Not only a rough surface on restoration, denture, orthodontic
appliance and so forth uncomfortable, but also food and other debris cling
to it. Such a restoration or appliance becomes dirty and in some cases
tarnish or corrosion may occur.
Rough surface are likely to occur unavoidably during the fabrication
of an appliance.
The finished restoration has three benefits the oral health, function
and esthetics, the well contoured and polished restoration promotes oral
health by resisting the accumulation of food debris and pathologic bacteria.
This is accomplished through a reduction in total surface area and
reduced roughness of restoration surface. Another surfaces are easer to
maintain in a hygienic state when preventive home care is practiced.
Oral function is enhanced with a well polished restoration, because
food glides more freely over occlusal and embrasure surface during
Smooth restoration contacts minimize wear rates on opposing and
adjacent teeth. This is particularly true for restorative materials such as
ceramics that contain phases that are harder than tooth enamel and dentin.
Rough material surfaces lead to the development of high contact
stresses that can cause the loss of functional and stabilizing contacts
Finally, esthetics demands may require the dentist to handle highly
visible surfaces of restorations differently than those that are not
Although a high mirror like polish is preferred for previously
mentioned reasons this type of surface may not be aesthetically compatible
with adjacent teeth in highly visible areas such as labial surfaces of
maxillary anterior teeth. Fortunately, these surfaces are not subject to high
contact stresses, and they are easily accessible for cleaning.
Finishing, cutting, grinding and polishing process:
The finishing process usually removes material such that:
a) The surface finishes and imperfections are removed.
The material is shaped to an ideal form and the outermost surface of
the material is developed to a desired state.
The particles of substrate material are recovered the action of hard
material that comes into frictional contact with the substrate.
In this process the tensile and shear stress are induced within both
the substrate and abrasive instrument but for the abrasive to cut the
substrate, the stresses induced within the abrasive should not exceed the
stresses induced into the substrate. If the stress in the instrument exceeds
which developed in the substrate, than in such cases the instrument fails to
cut, grind or polish and the blade edges will become dull and abrasive will
fracture or tear away from their binder.
A cutting operation usually refers to the use of bladed instrument or
use of any instrument in a blade like fashion. The substrate may divided
into large separate pieces or may sustain large deep grooves by a cutting
operation. The cutting blades are regularly arranged. A separating wheel
can be used in a blade like a fashion.
A separating wheel does not contain individual blades but its shape
allows it to be used in a rotating blade like fashion, to slice through the
sprue and die stone materials.
A grinding operation remove small particles of a substrate through
the action of bounded or coated abrasive instrument. Each particle may
contain several sharp points that run along the substrate surface and
remove particles of the material.
These grinding instruments contain many randomly arranged
abrasive particles, because these particles are randomly arranged
innumerable unidirectional scratches are produced on material surface.
Cutting and grinding are both considered to be predominantly
unidirectional in their course of action.
A polishing operation is considered at the most refined of finishing
process, produces the finest of the particles. It acts on an extremely thin
region of the substrate surface.
The polishing is considered to be multidirectional in its course of
action. The ideally polished surface should be atomically smooth with no
surface imperfections. This condition is virtually impossible to achieve
because most restorative materials are brittle and easily acquire surface
flaws during cutting and grinding procedures.
Types of abrasives:
Abrasives can be broadly divided into natural abrasives and
Natural abrasives include : Arkansas stone, chalk, corundum, diamond,
Emery, garnet, pumice, quartz sand, Tripoli and zirconium silicate cuttle
and kieselguhr are derived from the remnants of living organisms.
Manufactured abrasives are synthesized materials that are generally
preferred because and their predictable physical properties.
Silicon, carbide, aluminum oxide, synthetic diamond, rough and tin
oxide are example of manufactured abrasives.
Arkansas stone is semi translucent, light gray, siliceous sedimentary
rock mixed in Arkansas. It contains microcrystalline quartz and is dense
hard and uniformly textured. Small pieces of this material are attached to
metal shanks and trued to various shapes for fine grinding of tooth enamel
and metal alloys.
Chalks : One of the mineral forms of calcite is called chalk. Chalk is white
abrasive composed of calcium carbonate. It is used as a mild abrasive paste
to polish tooth enamel gold foil, amalgam and plastic materials.
Corundum : This mineral form of aluminum oxide is usually white, its
physical properties are inferior to those of manufactured aluminium oxide,
which has largely replaced corundum in dental applications, corundum is
used primarily for grinding metal alloys and is available as a bonded
abrasive is several shapes. It is most commonly used in an instrument
knows as a white stone.
Diamond : Diamond is transparent, colorless mineral composed of carbon.
It is the hardest substance known. Diamond is called a super abrasive,
because and its ability to abrade any other diamond abrasives are supplied
in several forms, including bonded abrasive rotary instruments, flexible
metal backed abrasive strips, and diamond polishing pastes. They are used
in ceramic and resin-based composite materials.
Emery : Thus abrasive is a grayish black corundum that is prepared in a
fine ground forms. Emery is used predominantly in the form of coated
abrasive disks and is available in a variety of grit size. It may be used for
finishing metal alloys or plastic material.
Garnet : The term garnet includes a number of different minerals that
possess similar physical properties and crystalline forms. These minerals
are the silicates of aluminium, cobalt, iron, magnesium and manganese.
The garnet abrasive used in dentistry is usually dark red. Garnet is
extremely hard and when fractured during the grinding operation, forms
sharp, chisel-shaped plates, making garnet a highly effective abrasive.
Garnet is available on coated disks and arbor bands. It is used in grinding
metal alloys and plastic materials.
Pumice : Volcanic activity produces this light gray, highly siliceous
material. It is used mainly in grit form but can be found in some rubber-
bonded abrasives. Both forms are used on plastic materials. Flour of
pumice is an extremely fine grained volcanic rock derivative from Italy and
is used in polishing tooth enamel, gold foil, dental amalgam, and acrylic
Quartz : The most commonly used form of quartz is very hard, colorless
and transparent. It is the most abundant and widespread of minerals. Quartz
crystalline particles are pulverized to form sharp, angular particles that are
useful in making coated abrasive disks. Quartz abrasives are used mostly to
finish metal alloys and may be used to grind dental enamel.
Sand : Sand is a mixture of small mineral particles predominantly
composed of silica. The particles represent a mixture of colors, making
sand abrasives distinct in appearance. Sand particles have a rounded to
angular shape. They are applied under the air pressure to remove refractory
investment materials from base metal alloy castings. They are also coated
onto paper disks for grinding of metal alloys and plastic materials.
Tripoli : This abrasive is derived from a light weight, friable siliceous
sedimentary. It can be white, gray, pink, red or yellow. The gray and red
type are most frequently used in dentistry. The rock is ground into very
fine particles and is formed with soft binders into very fine particles and is
formed with soft binders into bars of polishing compound. It is used for
polishing metal alloys and some plastic materials.
Zirconium silicate : Zircon or zirconium silicates is supplied as an off-
white mineral. This material is ground to various particle sizes and is used
to make coated abrasives disks and strips. It is frequently used as a
component of dental prophylaxis pastes.
Cuttle : Cuttle fish, cuttle bone, or cuttle are the common names for this
abrasive. It is a white, calcareous powder made from the pulverized
internal shell of a Mediterranean marine mollusk of the genus sepia. It is
available as a coated abrasive and is useful for delicate abrasion operations
such as polishing of metal margins and dental amalgam restorations.
Kieselguhr : This material is composed of the siliceous remains of minute
aquatic plants known as diatoms. The coarser form is called diatomaccous
earth, which is used as a filler in many dental materials, such as the
hydrocolloid impression materials. It is excellent mild abrasive. The risk of
respiratory silicosis caused by chronic exposures to airborne particles of
this material is significant and appropriate precautions should always be
Silicon carbide : This extremely hard abrasive was the first of the
synthetic abrasives to be made. Both green and blue black types are
produced and have equivalent physical properties. The green form is often
preferred because substrates are more visible against the green color.
Silicon carbide is extremely hard and brittle. Particles are sharp and break
to form new sharp particles. This results in highly efficient cutting of a
wide variety of materials, including metal alloys ceramics, and plastic
materials. Silicon carbide is available as an abrasive in coated disks and
vitreous bonded and rubber bonded instruments.
Aluminium oxide : Fused aluminium oxide was the second synthetic
abrasive to be developed after silicon carbide. Synthetic aluminium oxide
(alumina) is made as a white powder. It can be much harder than corundum
(neutral alumina) because of its purity. Alumina can be processed with
different properties by slight alteration of the reactants in the
manufacturing process. Several grain sizes are available and alumina has
largely replaced emery for several abrasive uses. Aluminium oxide is
widely used in dentistry. It is used to make bonded abrasives, coated
abrasives, and air-propelled grit abrasives. White stones are made of
sintered aluminium oxide and are popular for adjusting dental enamel and
for finishing both metal alloys and ceramic material.
Pink and ruby variations of aluminium oxide abrasives are made by
adding chromium compound to the original melt. Those variations are sold
in a vitreous bonded form as non-contaminating mounted stones for the
preparation of metal-ceramic alloys to receive porcelain. Any remnants of
these abrasives should not interfere with porcelain bonding to the metal
alloy. A review by Yamamoto (1985) suggested that carbide burs are the
most effective instrument for finishing this type of alloy because they do
not contaminates the metal surface with entrapped abrasive particles.
Synthetic diamond abrasives : Manufactured diamond is used almost
exclusively as an abrasive and is produced at five times the level of natural
diamond abrasive. This abrasive is used in the manufacture of diamond
saws, wheels, and burs. Block with embedded diamond particles are used
to true other types of bonded abrasives. Diamond polishing pastes are also
produced from particles smaller than 5mm in diameter and are useful in
polishing ceramic materials. Synthetic diamond abrasive are used primarily
on tooth structure, ceramic materials and resin-based composite materials.
Rouge : Iron oxide is the fine, red abrasive component of rouge. It is
blended, like Tripoli, with various soft binders into a cake form. It is used
to polish high noble metal alloys.
Tin oxide : This extremely fine abrasive is used extensively as a polishing
agent for polishing teeth and metallic restorations in the mouth. It is mixed
with water, alcohol, or glycerin to form a mildly abrasive paste.
Factors influencing the efficiency of abrasives:
1. The hardness of the abrasive particles. To be effective abrasive and
polishing agents should possess certain characteristic. First of all the
cutting or abrading particle must be able to scratch or cut grooves on
the surface of the material to be treated. It must therefore be harder
than the surface against which it is to be used. It must also be
sufficiently hard to retain an effective cutting edge.
The harder the material within the particle limits the longer the agent
remains effective as an abrading agent.
2. The shape of the abrasive particles. Particles with sharp edges are
more efficient than those with abture angle.
3. The particle size of the abrasive the particle size must be enough to
remove sufficient amount of mateials.
4. Mechanical properties of the particles. Brittleness can be an
advantage. If the material breaks in the process, then it should be
able from another sharp edge.
5. The pressure applied to the abrasive. The pressure should be light,
because great pressure may fracture an abrasive cutting instrument
and also causes frictional heat.
6. Properties of the material that is being abraded.
Brittle material can be abraded rapidly where as the mollable and
ductile material (e.g. pure gold), will flow and instead of being removed
7. Rate of movement. Slower the movement of the abrasive instrument
deeper will be the scratches.
The ideal abrading and polishing speeds respectively are for acrylic
resin, it is 2000 and 3000 rpm for gold alloy – 1500 rpm to 3000 rpm.
During abrading polymeric materials excessive heat must be
avoided as it can cause stress relief and warpage.
Finally, the range of particle size as well as distribution and position
of the abrading particles on the holding surface plays an important role in
effectiveness of an abrasive agent.
For example if the particles are to close to each other and the debris
from the cutting can’t escape and becomes entrapped between the abrasive
particles and an it loses its effectiveness.
ABRASIVE AND EROSIVE WEAR
Abrasive wear, wear is a material removal process that can occur
whenever surfaces slide against each other. The process of finishing a
restoration involves abrasive wear through the use of hard particles.
The dentistry the outermost particles or surface material of an
abrading instrument is referred to as the abrasive. The material being
finished is called as substrate.
Substrate wear is further divided into process of two body and three
body wear. The body occurs when abrasive particles are firmly bonded to
the surface and abrasive instrument and no other abrasive particles are
used. E.g. diamond bur.
Three body wear occurs when abrasive particles are free to translate
and rotate between two surfaces.
E.g.: Dental prophylaxis, which involves the use of a rotating
rubber cup and abrasive paste on tooth or material surface.
These two process are not mutually exclusive. The diamond
particles may debond from a diamond bur and cause three body wear.
Likewise some abrasive particles in the abrasive paste can become trapped
in the surface of a rubber cup and cause two body wear.
Lubricants are often used to minimize the risk for these
unintestional shifts from the body wear and vice versa.
Erosive wear : Erosive wear is caused by hard particles impacting a
substrate surface, carried either by a sit of ream of air or liquid.
Most dental laboratories have air driven grit blasting units that
employ hard particle erosion to remove surface material. A distinction
must be made between this type of erosion and chemical erosion, which
involves chemicals such as acids and alkalis instead of hard particles to
remove substrate material.
Acid etching is a familiar term that is used, more commonly
chemical erosion, chemical erosion is not used as a method of finishing
dental materials it is used primarily to prepare surfaces to enhance
bounding or coating.
Hardness of abrasive:
A stated previously the inherent strength of cutting blades or
abrasive particles on a dental instrument must be great enough is remove
particles of substrate material without becoming dull or fracturing too
rapidly. The strength of an abrasive is often measured by the hardness of its
particles or surface material to plastic deformation. The first hardness was
published by Eriedrich Mohs- A German microbiologist in 1820 the ranked
10 minerals by their relative earth resistance, mineral received a score of 1
and most scratch resistance. Mineral received the score of 10 (diamond).
The Mohs scale was later exposed in 1930s to accommodate several new
abrasive materials that received scores in a 9 to 10 range.
Knoop and Vickers hardness tests are based on indentation methods
that quantity the hardness of materials. The tip of a knoop diamond indents
had an elongated pyramid shape. Where as the Vickers diamond indenter
has an equilateral pyramid design. Both tests involve the application and
indenter to a test surface under a known load (usually 100W). The depth of
surface penetration is reported as hardness in units of force per unit area.
Hardness values for dental materials and abrasives
Abrasive Instrument Design
Abrasive grits – abrasive grits are derived from materials that have been
crushed and passed through a series of mesh screens to obtain different
particle size ranges. Dental abrasive grits are classified as course, medium
and superfine according to particle size ranges.
In finishing process rate of materials is not the only important
factor. The surface finish obtained with each abrasive is as important. If too
hard abrasive is used, or the grain size is too course for use on a given
material, deep acratches result in substrate that can’t be removed easily in
subsequent finishing process. Additionally. If an abrasive does not have the
proper particle shape of does not break down in a manner that creates or
exposure new sharp edges particle, it will tend to gauge the substrate.
1. Bonded abrasive : Bonded abrasive consists of abrasive particles
that are incorporated through a binder to form grinding tools such as
points, wheels, separating discs and wide variety and other abrasive
shapes. Particles are bonded by four general methods.
2. Vitreous bonding (glass or ceramic.)
3. Resinold bonding (usually phenolic resin).
4. Rubber bonding (usually silicone rubber).
Sintered abrasives are the strongest type because the abrasive
particles are fused. The vitreous bonded abrasives are mixed a glass or
ceramic matrix material, cold pressed and then are heated to cure the resin.
Rubber bonded abrasives are made in a manner similar to that for resin
The type of bounding method employed for the abrasive greatly
affects the grinding behaviour of the tool on the substrate, bounded
abrasives that lead to disintegrate rapidly against a substrate are too weak
and result in increased abrasive costs because the reduced instrument life.
Those that tend to degrade too slowly clog with grinding debris and
result in the loss of abrasive efficiency increased heat generation and
increased finishing time. An ideal binder holds the abrasive particles in tool
sufficiently long enough to cut, grind, or polish the substrate, yet release
the either before its cutting efficiency is lost or before heat build up causes
thermal damage to the substrate. Besides are specially formulated for
substrate specific applications.
A bounded abrasive instrument should always be treed and dressed
before its use. Truing is a procedure through which the abrasive instrument
is run against a harder abrasive block until the abrasive instrument rotates
in the handpiece without accealricty or ran out when placed on substrate.
The dressing procedure like is used to shape the instrument but
accomplishes two different purposes. First dressing procedure reduces the
instrument to its correct working size and shape. Second it is used to
remove clogged debris from the abrasive instrument to restore grinding
efficiency during the finishing operation.
The clogging of the abrasive instrument with debris is called
‘abrasive blinding’. Abrasive binding occurs when the debris generated
from grinding or polishing occludes the small spaces between the abrasive
particles on the tool and reduces the depth that particles can penetrate into
the substrate. As a result, abrasive efficiency is lost and greater heat is
A blinded abrasive appears to have a coating of the substrate
material on its surface. Frequent dressing of the abrasive instrument during
the finishing operation on truing instrument, maintain the efficiency of the
abrasive in removing the substrate material.
Blinders for diamond abrasive are manufactured specifically to
resist abrasive particle loss rather than no degrade at a certain point and
release particles. One reason for this is that diamond is the hardest material
known, so that diamond abrasive particles do not loose their cutting
efficacy against substrate. There is no need for new abrasive particles to be
exposed during the grinding process.
Another reason is that diamond grits are expensive and must be used
in limited quantities for instrument manufacture special bonding processes
have been designed to allow for extended instrument life by keeping the
abrasive particles firmly bound to the instrument shank yet with maximum
particle exposure. Diamond particles are bonded to metal wheel and to bur
with special heat resistant resins such as polymides. The supercourse plated
with a refractory metal film such as nickel titanium nitride coatings are
used as an additional layer on some of the recent diamond abrasive
instruments to further extend their longevity.
Finishing diamond used for resin based composites contain diamond
particles 40mm or less in diameter, and many are not nickel plated.
Therefore they are highly susceptible to abuse and should always be used
with light force and copious water spray to preserve the very fine diamond
Coated abrasive disks and strips
Coated abrasives are fabricated by securing abrasive particles to a
flexible backing material (heavy weight proper or mylar) with a suitable
adhesive material. These abrasives typically are supplied as disks and
finishing strips disks are available in different diameter and with thin and
very thin backing thickness.
Finishing and polishing for cast restoration
Finishing and polishing for cast restoration are finished and polished
using rotary instruments. The speed of the instruments (revolutions) differs
form one alloy to another, generally increasing, going from Class I to Class
IV alloys. This should be done on the die.
According to the following sequences:
1. Gross finishing
This should remove roughness impacted on the casting from the
investment mold walls, eliminate surface discolouration and eliminate
fine excess and obliterate minute defects. This step is always done
using abrasive stones and discs, ranging from sand and silicone carbide
to alumina, until no surface discrepancies are detected.
2. Removal of scratches and irregularities
This will eliminate undetected fine scratches by removing the alloy
substance between these scratches or locally melting the alloy at the
surface, obliterating these irregularities. Also, the conversion stage of
the finishing process is initiated. This step is done using wire metal
brushes, usually brass or steel, to be followed by rubber wheels and
cones. There is a large amount of heat produced in this stage, and it
may be a factor in fulfilling is objective. The alloy surface should have
a satin like surface by the end of this step.
Conversion will create the most biologically compatible surface.
This step is done using a rubber wheel or cone, followed by brushes
carrying extremely fine abrasives, such as Tripoli or specifically
compounded abrasive pastes. The alloy surface should be shiny by the
end of this stage.
4. This creates a reflective, non-adhesive surface.
It is done using rouge on a felt cone or wheel. The alloy surface
should be extremely shiny by the end of this stage. The entire oral
surface of cast alloys should be finished and polished.
FINISHING AND POLISHING THE PORCELAIN JACKET AND
Porcelain adjustment kits are used for contour reduction of porcelain
restorations. These kits have a variety of stones and polishing wheels of
different abrasiveness. Since, there is some vibration produced as the rotary
instrument abrades from the surface, the porcelain crown should be
adjusted while being held between the thumb and fingers, not on the die.
A porcelain surface adjusted with disks of stones has had the surface
glaze removed. The resultant surface is rough and dull in appearance.
Restoring the polished porcelain surface is accomplished by employing one
of a variety of polishing sequences.
There may be a perforation for a polished porcelain surfaces, rather
than a highly glazed surface. Polished porcelain has a thin like surface than
in some cases simulates the surface of natural tooth structure. A highly
glazed porcelain surface at times will appear to reflective. In some patients
whose natural tooth surfaces are highly reflective, a glazed surface is,
naturally, indicated. The surface finish is a matter of careful evaluation for
each patient. From the standpoint of soft tissue compatibility, the porcelain
surfaces in the gingival crevices are best allowed to retain the natural glaze
produced by baking.
Porcelain surfaces that have been adjusted can be reglazed in a
porcelain oven, polished, by using a series of porcelain polishing
instruments, or polished using a series of rubber impregnated abrasives
followed by a creamy mix of the leviated alumina and water. More recently
specially compounded porcelain polishing pastes have gained popularity
for final polishing of a porcelain surface. Reglazing the porcelain does
require an oven, whereas the other polishing procedures can be
accomplished with rotary instruments.
In order to avoid fracture of a margin by vibration, the porcelain
jacket is polished also while being held between the fingers rather tan the
die. No matter which polishing procedure is used, it is important to follow
the recommended sequence to obtain the optimal surface finish on the
porcelain. The levigated alumina is used with a lead-center unstiched
muslin wheel on a lathe motor. A stitched wheel is undesirable since it
tends to tear the crown out of the finger grasp. Further protection of the
crown is secured by lining the polishing cubicle with dental towels. The
crown and adjacent thumb surfaces are loaded with the creamy levigated
alumina and the crown is pressed firmly against the rotating wheel, turning
the crown constantly in many directions assures the production of a good
luster. In the areas where cervical excess has been removed, a final polish
is also achieved with the lead center wheel and creamy levigated alumina.
The wheel should turn away from the cervical margin, not into it. This is
sequence with the lead center wheel is completed in 3 to 4 minutes.
Polishing material is removed by scrubbing the crown with a soft bristled
tooth brush under a stream of water. All moisture is removed with
compressed air. The crown is cleansed with alcohol, redried and the tested
in the patient’s mouth. The patient is asked to wet the anterior teeth,
including the crown, with saliva. An evaluation of the finish can now be
made by the patient and the operator. If a high reflective luster is desired,
the crown is returned to the laboratory and polished with a creamy mix of
tin oxide. This tends to add even further highlight effect to the porcelain
Finishing, Cutting, Grinding and Polishing Process
Factors Influencing the Efficiency of Abrasives
Hardness of Abrasive
Abrasive Instrument Design
Finishing and Polishing of Cast Restoration
Finishing and Polishing of the Porcelain Jacket and Ceramic