Rotary instrument/cosmetic dentistry course

ROTARY INSTRUMENTS
IN OPERATIVE
DENTISTRY
INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com

CONTENTS
 INTRODUCTION
 HISTORY OF ROTARY INSTRUMENTATION
 CHARACTERISTICS OF ROTARY
INSTRUMENTATION
 POWERED ROTARY INSTRUMENTS
 ROTARY CUTTING INTRUMENTS
Common design characteristics
Bur classification systems
Modifications in bur design
Factors influencing cutting efficiency of burs

 ROTARY ABRASIVE INSTRUMENTS
Diamond abrasives
Other abrasives
Factors influencing abrasive efficiency
 CUTTING MECHANISMS
Evaluation of cutting
Bladed cutting
Abrasive cutting
Cutting recommendation
 HAZARDS WITH ROTARY INSTRUMENTS
Pulpal precautions
Soft tissue precautions
Eye precautions
Ear precautions
Inhalation precautions

 DENTAL CONTROL UNIT AND HANDPIECE ASEPSIS
 STERILIZATION OF ROTARY EQUIPMENT AND
HANDPIECE
 CONCLUSION

INTRODUCTION

HISTORICAL REVIEWHISTORICAL REVIEW
 In 350 BC, Hippocrates described a
drill driven by chord around the shaft.
 1858 – 1862: First rotary instrument
was introduced by Dr. Jonathan Taft
and called them “bur drills”. Scranton’s
drill - rotated in either direction to
perform cutting action. Drill ring which
was adapted to the middle or index
finger with a socket that fitted against
the palm. Chevalier’s drill stock
designed to bear bur in different
directions.
 1871: Morrison - dental foot engine,
700 rpm.

 1874: the electrical dental engine - 1,000
rpm.
 1910: the belt driven handpiece
 1914: Electrical engine was incorporated
into the dental unit - 5,000 rpm.
 1942: Diamond abrasive points were
introduce to perform at 5,000 rpm.
 1947: Tungsten carbide bur– 12,000
rpm.
 1949: Walsh and Symmons - removal
tooth structure with diamond points
70,000 rpm.
 1950: Early 50s ball bearing hand piece
was introduced, closely followed by the
ball-bearing contra-angle. – 25,000 rpm.
 1953: Fluid-turbine handpiece,50,000
rpm with moderate torque.

 1954: the air-driven handpieces were introduced.
 1955: A continuous belt-driven contra-angle, 1,50,000
rpm.
 1960s: Air-bearing handpiece, 5,00,000 rpm. Ultrasonics
method for hard tissue removal
 1961: Air turbine straight handpiece – 25,000 rpm.
 1994: Contemporary air turbines handpiece– 3,00,000
rpm.
The small size of the turbine head limits the power
output. Speeds drop to 2,00,000 rpm or less under lateral
workloads. This tendency to stall at high loads is a safety
feature, since excessive pressure cannot be applied.

CHARACTERISTICS OF ROTARY
INSTRUMENTATION
SPEED:
 Number of revolutions per unit time. Surface feet per unit time
contact that the tool has with the work.
Revolutions per minute
Size of the working tool
 Large diameter when working on slow speed and vice-versa
maximum cutting efficiency.
In dentistry speed may be classified as:
Ultra-low speed 300-3000 RPM
Low speed 3000-6000 RPM
Medium high speed 20,000- 45,000 RPM
High speed 45,000- 1,00,000 RPM
Ultra-high speed 1,00,000 < RPM
Some equipments can produce as high as 5,00,000 Rpm.

PRESSURE:
 Pressure = F/A.
 Same force, a smaller tool will apply more pressure than a larger
tool. To keep the pressure constant with both tools, it is
necessary to vary the force application. It has been clinically
seen that low speed requires 2-5 pounds force, high speed
requires 1 pound and ultra-high speed requires only 1-4 ounces
of force.
HEAT PRODUCTION:
 Heat proportional to: Pressure, RPM, Area of tool contact
 permanent pulp damaged at 130°F
 inflammatory changes are seen at 113°F. mandatory to employ
coolants (air, water or both during cutting.)

VIBRATION:
 A major annoying factor, also causes
operator fatigue.
 Vibration is a product of the equipment used
and the speed of rotation.
Amplitude:
 A vibration - frequency and amplitude.
 Slow speed- large amplitude but frequency
is small. Amplitude affects both the patient’s
attitude and the instrument used.
 Vibration wave is measured in cycles per
second.
6000 RPM - fundamental vib of 100 cycles/
s
1,00,000 RPM- fundamental vib of 1600
cycles/ s
 Studies have shown that wave vibrations of
1300 cyc/s are totally imperceptible to the
patient.

Undesirable modulating frequency:
 Caused by poorly maintained
handpiece.
 Each part vibrates- amount of
wear or eccentricity of the
moving parts.
 Sets up a modulating frequency
+ fundamental vibration wave.
Perceived by the patient and
operator.

PATIENT REACTION:
 The factors that cause primary apprehension to the
patient are;
Heat production
Vibration produced in the handpiece
Length of operating time
Number of visits
 Proper understanding of the instruments and the
speed at which it is being used can help in
counteracting potential irritating stimuli. Proper use
of coolants, intermittent tool application, sharp
instruments etc, can minimize patient discomfort.

OPERATOR FATIGUE:
 The major causes are:
Duration of operation
Vibrations of the handpiece
Forces needed to control the rotating instrument
Apprehension on the part of the dentist
Lack of patient co-operation.
SOURCES OF POWER:
 After its introduction in the 1950s, air turbine has
been main power source in dental practice.

INSTRUMENT DESIGN:
 This should be evaluated in two parameters;
Handpiece
The cutting tool
 Handpiece:
Friction: occurs in the moving parts of the handpiece.
Becomes critical for high speed as it generates heat.
bearing- ball bearings, needle bearings, glass and resin
bearings.
Torque: It is the ability of the handpiece to withstand
lateral pressure on the revolving tool without decrease its
speed or cutting efficiency. Dependant -bearing and the
amount of energy supplied to the handpiece.

POWERED CUTTING EQUIPMENT
A Handpiece is a device for holding rotating
instruments, transmitting power to them and for
positioning them intra-orally.
Classification of Handpieces
There are a no. of ways to classify a
handpiece. A few are;
1. Based on speed
2. Based on power source
3. Based on angulations
4. Based on cutting tool retention

Principle of contra-angulation

AIR-TURBINE
 This has high speed but reduced
torque.
 Mechanism
 High speed revolutions causes
wear of supporting bearings. So,
the rotating turbine and cutting bur
suspended in air bearings. When
over-powered, these bearings
crash due to the lateral forces.
 Miniature ball-races to suspend
the rotor. Provides improved
torque abilities and hence cannot
be stalled.

 Air pressure requirements:
Air-suspension bearing – 0.35 to 0.5 MPa (50
to 70 psi)
Ball-race bearing – 0.2 to 0.35 MPa ( 30 to 50
psi)
 Speed of handpiece is rapidly reduced to 1,00,000
rpm under load.
 Cutting must be performed just above the stalling
speed to improve tactile sensation.
 Modern handpieces lubrication by a cleaning,
lubricating spray from an aerosol dispenser prior to
sterilization

MICRO-MOTORS
 It is necessary to have a slower speed motor
to remove soft caries, finishing and polishing
(500 rpm to 1,00,000 rpm).
 High torque with low speed is essential to
prevent the instrument from stalling during
work.
Micro-motors fall into two categories:
Air driven – cheaper and robust
Electric driven – versatile but expensive.

Air motor:
 Two patterns are in common use:
Rotary Vane type
Swash-plate type
Rotary vane drive air motor:
 Such motors run smoothly and can
develop considerable torque.
 Torque dependant on length and
diameter of motor and the pressure
of the drive air.
 These can be easily autoclaved.

Swash-plate drive air
motor:
 It cannot operate at
high speeds
 It is losing popularity
when compared with
the rotary vane air
motor.

ELECTRIC MICRO-MOTOR
 These are dc motors and are designed with an armature sitting
within a permanent magnet assembly. The performance is
dependent on;
Design and power of field magnets.
Design and number of armature coils.

Auto- Regulation
Load put on an electric motor
↓
Motor slows down which causes a
drop in voltage.
↓
Voltage stabilizer will re-establish the
voltage and hence the current
↓
Restoration of the actual speed
experienced at the beginning of the
work.

COUPLINGS
 A number of couplings are available to
connect the air-turbine and micro-motor
to the hoses of the instrument delivery
units. Two of the commonest fitting used
are:
Borden two-hole connector – one for
compressed air and smaller hole for
water coolant
Mid-west 4 holes connector – one
for compressed air
One for exhaust airlet
Other two smaller holes for air and water
coolant.

HANDPIECES
 Handpieces are mechanical link between
the micro-motor and cutting bur. Head and
shank.
 Handpiece fitted with gear systems for
effective torque control.

Gear & Speed Reduction Systems

 Handpiece is attached to the motor by means of an
‘E-coupling’ which is a snap-on alignment of the
parts.
 Coolant spray connecting systems may be internal
or external. Their purpose is to deliver air and water
in the form of an aerosol.
 Color coding for handpieces:
These indicate the relative gear ratio of each
component and are present in the form of dots or
rings.
Blue – no change in speed
Green – speed reduction
Red – speed increase

HANDPIECES FOR ORAL SURGERY
 Fast rotary vane motors have
been developed for effective cutting
in impaction surgeries.
 Cutting of bone for implant
insertion requires cool cutting to
avoid damage to the bone due to
heat. Extremely slow speed and
efficient coolant flow are needed.
Speed is controlled by geared
handpieces. Special bur are
available which drill through axially to
allow coolant to be piped in directly
to the cutting edges.

ROTARY CUTTING INTRUMENTS
The individual instruments intended for use with
handpieces are available in different shapes and sizes. The
number of instruments essential for use with particular
handpiece is small, especially in case of high-speed turbine
handpieces.
 Common design characteristics
 Bur classification systems
 Modifications in bur design
 Factors influencing cutting efficiency of burs

COMMON DESIGN CHARACTERISTICS
 Certain common design features, each instrument consists of;
Shank, Neck and Head.
 SHANK DESIGN:
The shank is that part that fits into the handpiece, accepts
the rotary motion from the handpiece, and provides bearing
surface to control the alignment and concentricity of the
instrument. ADA specification no.23 for dental excavating burs
includes 5 classes of instrument shanks.
Straight handpiece shank:
 The shank portion – cylinder, held by a metal chuck that accepts
a range of shank diameters. Straight handpieces are now used
for finishing and polishing completed restorations.

Latch-type handpiece shank:
 Complicated - method by which they are held in handpiece.
 Overall dimensions - small, easy access in mouth.
 The handpiece has a metal tube within which the instrument fits
 The posterior portion of shank is flattened on one side, end fits into
a D-shaped socket at the bottom of the bur tube.
 Retained by a latch that slides into D-shaped socket
 Used in slow and medium speed.
 Wobble due to the clearance between instrument and bur tube -
controlled by the lateral pressure during cutting procedures.

Friction-grip shank design:
 Developed for its use in high speeds.
 The overall dimensions are smaller thus increasing
access in posterior teeth. Simple cylinder
manufactured very close to dimensional tolerances.
 Held in handpiece by friction between the metal chuck.
 Minor variations in shank diameter can cause
substantial vibration in the instrument performance and
problems with insertion, retention and removal

NECK DESIGN:
 The neck is the portion that connects the head to the shank.
 It tapers from the shank to the head.
 Main function is to transmit rotational and transitional force to
head.
 It also provides visibility and ease of operation.
 Neck diameter is a compromise between strength and
improved access and visibility.
HEAD DESIGN:
 It is the working part of the instrument- cutting edges or points
 The shape and composition of the head in dependent on its
intended use. Head design forms the basis of instrument
classification, such as; bladed instrument or abrasives, shape
of the head, material of construction, etc.

Composition & Manufacture of Burs
 Steel Burs:
They are cut from a still block by a rotary cutter that cuts
parallel to the long axis of the bur. The bur is then hardened and
tempered till the VHN is 800. It performs well on slow speed, but
dulls at higher speed. Once dulled, the reduced cutting
effectiveness creates increased heat and vibration.
 Carbide Burs:
This is a product of powdered metallurgy - powder of tungsten
carbide mixed with powdered Co or Ni under pressure and sintered
in vacuum. A blank is then formed and a diamond rotary cutter is
used to form the head design. VHN is in the range of 1650 – 1700.
Carbide burs are harder than steel burs and are less subjected to
dulling during cutting.
In most burs, carbide head is welded or brazed to a steel
shank. This offers following features:
 Increased life of the bur.
 Economical.
 Reduced chances of fracture during working.

BUR CLASSIFICATION SYSTEMS
 ADA system of classification was most preferred, but
newer design features made inclusions difficult
 Classifications systems developed by FDI and ISO to
use separate designations for shape head and head
diameter measured in tenths of an mm
SHAPES: It is the contour of the head and is
basically;
 ROUNDROUND:: tooth entry, extension of preparation,
preparation of retentive features and caries removal.
 INVERTED CONEINVERTED CONE:: Rapidly tapered towards the neck.
Head length = diameter. Undercuts.

 PEAR-SHAPEDPEAR-SHAPED: Tapered cone towards shank. The end
of the head may be continuously tapered or may be flat
with rounded corners. A normal length bur - gold foil. A
long-length pear bur - amalgam.
 STRAIGHT FISSURESTRAIGHT FISSURE: It is an elongated cylinder -
amalgam preparations.
 TAPERED FISSURETAPERED FISSURE: Head tapered away from shank.
Indirect tooth preparations, prevent undercut generation.

Bur Blade Design

 Design modifications simplify the technique and
reduce the effort needed for optimal results.
 Three modifications: ↓use of crosscuts, extended
heads on fissure burs & rounding of sharp tip
angles.
 Crosscuts effective at slow speeds. At high
speeds → rough surface
 Non-crosscut versions of the original
classification are available.

 Carbide fissure burs with extended head lengths
2-3 times those of normal tapered fissure burs
effective at high speed with light pressure.
 Markley and Sockwell proposed roundening the
sharp tip corners of the bur. Conventional bur →
sharp angles in tooth preparation tooth → fracture.
Rounded corners of bur → flat preparation &
rounded internal line angles, preserves vital dentin
& easy adaptation of restoration. This feature also ↑
life of the bur

Additional features in head design
 Head length: long to reach full depth of
preparation.
 Taper angle: generate necessary
occlusal divergence.
These factors otherwise do not
affect the performance of the bur.
 Neck diameter: a small neck →
weakening of instruments against
lateral forces & vice-versa hampers
visibility during preparation. Length ↑ -
neck diameter also ↑ to minimize the
moment arm exerted by the lateral
forces.
 Spiral angle: produces smooth wall. In
high speed, smaller angle is preferred
to improve efficient cutting.

 Cross-cutting: notches in the blade edges to improve
cutting effectiveness at low and medium speeds.
Crosscuts effectively increase both cutting pressure
resulting from rotation and perpendicular pressure holding
the blade edge against the tooth.
As each crosscut blade cuts, it leaves behind small
ridges on the tooth surface. Since notches of two
successive blades do not line up with each other, these
ridges formed from one blade are removed by the
successive blade.
However, at high speeds, the contact of bur with the
tooth is not continuous. Here, high cutting rate of crosscut
is maintained but the ridges are not removed. This leaves
behind a rough cut surface.

1. Rake angle
2. Clearance angle
3. Number of blades & distribution
The number of teeth is restricted
to 6 – 8. As the number of blades
decreases;
 The magnitude of force on each
blade↑and the thickness of chip
removed also ↑
 Tendency of clogging decreases.
↓ in heat production with straight
flutes because large chip resulting
from a straight flute will carry some
heat energy with it. Burs with10 – 12
or even 40 blades - finishing and
polishing.

4. Run-out
Refers to the eccentricity or
maximum displacement of the bur
head from its axis of rotation while
the bur turns. Acceptable run-out is
0.023mm. Run-out ↑ with ↑ in bur
length.
During a run-out process all the
blades will not cut equally. This
results in disagreeable vibration and
heat production. Such a method of
tooth removal is inefficient and
inaccurate.
5. Finish of the flutes
6. Heat treatment

7. Design of flute ends:
The dental burs are formed in two different types of flute ends;
Revelation cut – where the flutes come together at two
junctions near the diametrical cutting
edge.
Star cut – where the flutes come together at a common junction
at the axis of the bur.
It is seen that the revelation cut is more efficient in direct cutting.
However, in lateral cutting both proved to be the same.
8. Bur diameter:
The volume of the material removed directly depends on the bur
diameter.

9. Influence of load:
Load signifies the force exerted by the dentist of the tool
head and not the pressure or stress induced in the bur
during cutting. The load or force exerted is dependent
on the speed of the handpiece.
 Slow speed – 1000 gm or 2 pounds
 High speed – 60-120 gm 2-4 ounces.
10. Influence of speed:
At constant load, rate of cutting increases with increase in
speed, but this increase is not directly proportional. A
minimum rotational speed exists.

ROTARY ABRASIVE
INSTRUMENTS
The second major rotary cutting instruments
involve abrasive cutting. Small, angular particles of
hard substance held in a softer matrix. Cutting
occurs at a large number of points rather than along
a continuous blade edge.
Diamond abrasives
Other abrasives

Diamond abrasives
Great clinical impact due to long life and effectiveness in
cutting enamel and dentin. Introduced before carbide
burs. Popular as grinding and finishing agents.
TERMINOLOGY:
Diamond instruments consists of three parts:
Metal blank
Powdered diamond abrasive
Metallic bonding material
The metal blank is comparable to that of the carbide burs.

Parts Bur blank
Diamond blank
Shank
dimension
Same Same
Neck
dimension Gradual taper from
shank to head
Similar except in large
disks/ abrasives where it
may not be reduced
below the shank
Head
dimension It corresponds to
the diameter of bur
It is undersized to
accommodate uniform
thickness of abrasive
layer. Some have a
mandrel and detachable
head suited for abrasives.

 Diamonds employed are either natural or
synthetic. The shape of individual particle
decides the cutting efficiency and durability of
the instrument. The diamond particles are held
against the blank while it is being electroplated
with a metal.

CLASSIFICATION
 Various shape and sizes, comparable with burs, due to simplicity
in manufacture.
 Smallest diamond not as small in diameter as the smallest bur.
 It is necessary to inspect the point for proper size and shape.
DIAMOND PARTICLE FACTORS:
The clinical performance of a diamond abrasive is
dependent on size, spacing, uniformity, exposure and bonding.
Diamond particle size is commonly categorized as;
Coarse – 125-150 µm
Medium – 88-125 µm
Fine – 60-74 µm
Very fine – 38-44 µm

 When larger particle are used, the area for the particles
is reduced and are widely spaced. During cutting only
few particles come in contact with the tooth surface,
which increases the pressure of each particle. When the
pressure of cutting is increased there is increased depth
of engagement resulting in a rough surface.
 Factors determining the service life are speed and
pressure. Most often the only cause of failure of diamond
instruments is the loss of diamond particles. This occurs
when increased pressures are applied to improve cutting
rate at inadequate speeds.

OTHER ABRASIVES
Many types of abrasive were used even in tooth
preparation but are now restricted to shaping, finishing
and polishing restorations
CLASSIFICATION:
In these instruments, the head is composed of
abrasive particles held in a continuous matrix of softer
material. These abrasive can be broadly divided as:
molded instruments
coated instruments

Molded abrasive instruments
 Manufacture: by molding or pressing a uniform
mixture of abrasive around a roughened shank or by
cementing a pre-molded head.
 Matrix is soft and tends to wear with use thus
exposing fresh abrasive particles.
 Hard and rigid heads use rigid polymers or ceramics
materials for matrix
 These heads are used for grinding and shaping
procedures.
 Other molded heads use flexible matrix materials
like rubber, which are used for finishing and
polishing procedures.

Methods of obtaining molded
abrasive
 Sintering:Sintering: strongest.
 Vitreous-bonding:Vitreous-bonding: Abrasive + ceramic matrix
material, molded to the shape & fired
 Resinous-bonding:Resinous-bonding: Cold/ hot pressed then
heated to cure the resin.
 Rubber-bonding:Rubber-bonding: similar to resinous bonded
but the binder is latex or silicone based rubber.

 Molded abrasives are available as;
 Mounted Stones or points: They are
used to shape or cut metals.
Eg., Carborundum (SiC)
Aluminum oxide ( Alumdum) white
 Unmounted discs or wheelstones:
Held by a screw to the mandrel. This
permits easy change of abrasives and
is also economical.
Eg., Heatless stone ------ 3/32’ or 3/16”
Carborundum disk – ‘Joe Dandy disk’----
0.022”
Ultra-thin separating disk (carborundum) --
0.010”
Porcelain grinding wheel – Busch silent stone
-- 2mm

Coated abrasive instruments
 These are discs that have a thin layer of abrasive
cemented to a flexible base. It conforms to the
surface contour of the tooth or restoration.
 Disks are available in sizes ranging from ¼” to 7/8”
diameter.
 Mandrels for these may be snap-on type or screw
type. They are used in;
 Finishing certain enamel margins/walls for indirect
restorations
 Most often for finishing procedures for restorations

 Abrasive instruments are softer
and easily lose their sharp edges
and cutting efficiency with usage.
Coated instruments need to be
discarded. However, molded
instruments partially regenerate
the loss because the abrasive
particles are present throughout
the matrix. But these may also
require to be shaped against a
truing or shaping stone in order
to improve instrument
concentricity.
Abrasive Life

MATERIALS USED
The matrix materials used are phenolic resins or rubber. Some
molded abrasives may be sintered but most are resin bonded.
A rubber matrix is flexible and allows ease of polishing. Non-
flexible rubber matrix is used for molded SiC discs.
Following are the commonly used abrasives:
 Silicon carbide ( Carborundum):
Usually are molded in forms of rounds, bud-shapes,
wheels and cylinders of various sizes. They are gray-green in
color suited for fast cutting except on enamel. They produce
moderately smooth surface.
Unmounted discs, popularly called as carborundum
discs, are black or dark in colour. They have a soft matrix and
wear easily. They produce moderately rough surface.

 Aluminum oxide:
It is used for the same instrument design as SiC.
Points are white, rigid, fine textured and less porous.
They produce smoother surface than SiC.
 Garnet (reddish) and Quartz (white):
They are used for coated discs and are available in
a series of particle sizes ranging from coarse to
medium-fine. They are used for initial finishing. They
are hard enough to cut tooth and other restorative
materials except some porcelain.
 Pumice:
It is a powdered abrasive produced by crushing
foamed volcanic glass into thin glass flakes. It cuts
effectively but breaksdown rapidly. It is used for initial
polishing procedure.

 Cuttlebone:
It is derived from cuttlefish and is a soft white
abrasive. It is becoming scarce and is being replaced
by synthetic products. It is used only in coated discs for
final finishing and polishing. It is so soft that it reduces
the potential for tooth damage due to its abrasive
action.

Finishing & Polishing of restorationsFinishing & Polishing of restorations
 Resin composites:Resin composites:
- Most difficult due to difference in wear of matrix & filler.
- sequential use of abrasive grades
- Direction of use
 Dental Amalgam:Dental Amalgam:
- Same appt: non-ribbed prohy-cup with prophylaxis
paste at slow speed.
- Next appt: Contour – slow speed green stone/ diamond
burs
Polish – fine pumice with water/ -OH on
rotary brush/ felt wheel

Finishing & Polishing of restorationsFinishing & Polishing of restorations
 Gold alloys:Gold alloys:
- contour with TC burs, SiC stones in slow speed
- finish with Al2O3 medium grade abrasive.
- fine abrasive on rubber cups/ wheels. Polish-tripoli/ rouge
 Ceramic restorations:Ceramic restorations:
- Contour with flexible diamond disk, heatless stone
- Finish with abrasive impregnated rubber cups
- Polish with fine grit abrasive or diamond paste on felt
wheel.
- overglaze layer

1. Irregularity in shape of abrasive particles:
An abrasive must be irregular with a sharp edge. This
improves the cutting efficiency. Cubicle or smooth round
particles are less effective.
2. Hardness of the abrasive relative to that of work:
The harder the abrasive when compared with that of the
work, the more is the abrasive efficiency. Otherwise will
result in dulling of the abrasive.
3. Impact strength of abrasive material:
During abrasion abrasive particles must fracture so that a
sharp tip is always maintained. If abrasive particle does not
fracture, it will result in dulling resulting in inefficient cutting.
When diamond point cuts, it does not fracture but loses the
particle at the tip.

Diamond points have a tendency to get clogged when
they cut through ductile material like dentine. They are more
effective for use on enamel.
4. Size of abrasive particle:
The larger the particles, deeper will be the depth of
engagement resulting in faster removal of tooth structure.
5. Pressure and RPM:
The load or force exerted is dependent on the speed of
the handpiece.
Slow speed – 1000 gm or 2 pounds
High speed – 60-120 gm 2-4 ounces.
With a given load, the rate of cutting increases with
increase in speed, but this increase is not directly
proportional.

CUTTING MECHANISMS
 For cutting, it is necessary to apply some pressure
so that the cutting tool will dig into the surface. The
process of rotary cutting is complex and the
following will help in understanding it better
Evaluation of cutting
Bladed cutting
Abrasive cutting
Cutting recommendation

EVALUATION OF CUTTING
 Cutting can be measured in both effectiveness and
efficiency.
 Cutting effectiveness is the rate of tooth structure
removal (mm/min or mg/min).
 Cutting efficiency is the percentage of energy actually
producing the cutting. It is reduced when energy is
wasted as noise or heat.
 It is possible to increase effectiveness while decreasing
the efficiency.
 In general both effectiveness and efficiency can be
increased by increasing the speed.

BLADED CUTTING
 Tooth structure undergoes brittle and ductile fracture.
Brittle fracture is associated with crack propagation,
usually by tensile loading. Ductile fracture involves
plastic deformation of the material proceeding shear.
 Speed:
low speed – plastic deformation before tooth structure
fracture
High speed – proceeds brittle fracture
 Strain rate: faster the rate of loading, greater will be the
strength, hardness, modulus of elasticity and brittleness
of the material.

BLADED CUTTING
 The blade must be sharp, harder
with high modulus of elasticity than
the material being cut. This helps in
exceeding the shear strength of the
material . The sheared segments of
the surface get accumulated in the
clearance face.
 Mechanical distortion of the tooth
surface can generate heat in both
the surface and the cutting tool, and
may vary with varying speeds.

ABRASIVE CUTTING
 Abrasive cutting is similar to bladed cutting in many ways, but
key differences result from the properties, size and distribution
of the abrasive. Hardness of diamond provides superior
resistance to wear and these particles tend to have a very
high negative rake angle.
 When diamond particle cuts through a ductile material,
material will flow laterally around the cutting point and be left
as a ridge of deformed material on the surface. Repeated
deformation work hardens the distorted material until irregular
portion become brittle and breaks off. This is less efficient
than bladed cutting; therefore bur are preferred to cut through
ductile material like dentin.

 When diamond cuts through brittle material, most cutting
results from tensile fractures that produces subsurface
cracks. Hence they are most efficient to remove enamel
the burs. They are also preferred for use in tooth
preparations for bonded restoration, since they increase
the surface area.
Ductile material
Brittle material

CUTTING RECOMMENDATION
 The requirements for effective and efficient cutting
include using
 Contra-angle handpiece
 High operating speed
 Air water spray for cooling
 Light pressure
 Carbide or diamond instrument
 Carbide burs are effective for punch cuts to enter tooth
structure, intra-coronal tooth preparation, amalgam
removal, small preparations and secondary retentive
features. Diamonds are more effective than burs for both
intra and extra coronal tooth preparation, beveling
enamel margins and enameloplasty.

HAZARDS WITH ROTARY INSTRUMENTS
Pulpal precautionsPulpal precautions
 Injury to the pulp - mechanical vibration, heat generation,
desiccation of the dentin and transaction of the
odontoblastic process. The Pulpal sequelae, take 2 wks
to 6 months, depending on degree of trauma.
 The remaining tissue is effective in protecting theThe remaining tissue is effective in protecting the
pulp in proportion to the square of it thickness.pulp in proportion to the square of it thickness.
 Factors that produce heat:
 Steel burs > than carbide burs
 Tools plugged with debris
 Used without a coolant, diamond abrasives > carbide burs.

 Air coolantAir coolant in itself is insufficient. It absorbs less
unwanted heat & also desiccates the dentin.
Used for finishing procedures only.
 Air-water sprayAir-water spray is universally used to cool,
moisten and clear the operating site. It also,
cleans and cools the cutting tool thus increasing
tool life.
 During cutting procedures, a smear layer is
formed which acts as a bandage. However,
smear layer is still porous. Air spray produces
desiccation. Air is applied only to the extent ofAir is applied only to the extent of
removing excess moisture, leaving a glisteningremoving excess moisture, leaving a glistening
surface behind.surface behind.

Soft tissue precautionsSoft tissue precautions
 The lips, tongue and cheek
 Rubber dam.
 Good access for handpiece use.
 Retraction of soft tissues – assistant / retraction
type saliva ejector
 Never remove a rotating handpiece from mouth.
 Patient’s reactions - gagging, swallowing or
coughing during cutting.
 Control hemorrhage with pressure pack first aid
in case of accidents.

 The chance of mechanical pulp involvement during caries
excavation is more with hand instruments than with rotary
instruments. Residual caries can be removed using a bur at
low speed and light intermittent forces. High-speed hand
pieces must be used just above the stalling speed to improve
tactile sensation
Eye precautionsEye precautions
 Should wear protective glasses - airborne particles during
cutting procedures.
 High-volume evacuation - removes particles of old
restorations, tooth structure, bacteria and other debris.
 Airborne particles - matrix failure of molded abrasives
 Soft abrasive may increase in temperature during use,
causing the rubber matrix to debond from the abrasive into
fine particles.

Ear precautions:Ear precautions:
 Air-turbine handpieces produce a high-pitched can
cause hearing loss.
 Potential damage to hearing depends on:
 Intensity or loudness (decibels- db)
 Frequency (cps)
 Duration of the noise
 Susceptibility of the individual
 Auditory threshold, temporary threshold shift,
permanent threshold shift

 Air turbine handpieces at 30 pounds → 70 – 94 db
at high frequency. Noise levels > 75 db @ of 1000 –
8000 cps→ hearing damage.
 Protective measures are recommended for 85 db
@ 300 – 4800 cps.
 Protection is mandatory at 95 db.
 Use of handpiece less than 30 minutes per day.
 Earplugs, sound proof rooms with absorbing
materials on walls and floor Anti-noise devices can
be used to cancel the unwanted sounds as well.

Inhalation precautionsInhalation precautions
 Aerosols and vapors
 Aerosols are fine dispersion in air of water, tooth
debris, micro-organisms and / or restorative
materials.
 Cutting amalgams or composite resin produce both
sub-micron particles and vapors.
 Vapors from cutting amalgam - mercury & that from
composite resins -monomers.
 Inhalation can produce alveolar irritation & tissue
reactions.
 A face mask filters out bacteria and fine particulate
matter but not mercury or monomer vapors.

Dental control unit water systems &Dental control unit water systems &
handpiece asepsishandpiece asepsis
 Handpiece surface contamination control
 Turbine contamination control
 Water retraction system correction
 Inherent water system contamination
 Control of contamination from spatter &
aerosol

 Handpiece surface contamination control
contamination occurs thro’ blood & saliva.
Disinfection alone cannot provide infection control,
sterilization is must.
 Turbine contamination control
contaminated oral fluids may enter the turbine by
negative pressure
Cross contamination can be prevented by flushing the
handpiece if it is not sterilized.
 Water retraction system correction
a device in the dental unit retracts water from the line
when handpiece is stopped, this also retracts bacteria
operate the handpiece for 20 sec bet patients.

 Inherent water system contamination
Bacteria tend to grow as a biofilm on the inner walls of
dental unit water lines.
Main inhabitants are Flavobacteria, opportunistic gram –
ve bact.
Regular cleaning combined with disinfection or
sterilization of equipment.
 Control of contamination from spatter & aerosol
these can be inhaled causing respiratory infection.
M.TB aerosol may result in the spread of MDRTB.
Universal use of personal barriers, HVEs becomes
mandatory.

Infection Control
 Latch angles, burs and rotary stones must be
cleaned & sterilized.
 Handpieces are semicrtical instrumentation
requiring sterilization
 Motor-end of micro-motor must be covered with
a single used disposable plastic bag. Scrub and
disinfection of the end may also be performed

Sterilization of BursSterilization of Burs
 Presoak: burs placed in soap water to loosen debris
 Cleaning: Stainless brush under water or ultrasonic
systems
 Sterilization done by:
Dry-clave - 160°C for 30min
EO gas – best method for delicate instruments.
Autoclave – 121°C for 15min @ 15 lbs.Tendency of
corrosion at the neck region, hence soak in 2% Sodium
nitrite prior to autoclaving.
Chemiclave – chemical vapor under pressure: 131°C @
20 pounds pressure. Best suited for corrosion prone
instruments.

Sterilization of HandpieceSterilization of Handpiece
 With metal bearing: Scrub the metal bearing with water
and soap. Lubricate and place in sterilization bag &
autoclaved.
 Lube-free ceramic bearing turbine handpieces must not
be chemically sterilized – damage to internal parts.
 Chemical vapor pressure sterilization
 Ethylene oxide gas provides both internal & external
sterilization due to penetrating capacity. But takes long
time for sterilization.
 Dry heat for handpiece is generally not recommended

RECENT ADVANCES
 Single patient use burs:
Devloped by CDC & ADA to minimise cross-
contamination & prolonged sterilization protocol
 Turbo diamond:
these have diamond free zone or continual
spiral of blank space. The diamond free zone breaks
surface contact with the tooth, thus allowing cooler &
cleaner cutting. The continual spiral design leaves a
smooth wall.
 Fissureotomy burs:(carbide)
the tip of the but is smaller
than no.1/4 round bur.
Helpful in conservative preparations

RECENT ADVANCES
 Fiber-optic handpieces:
provide light at the working site. Shut off delay –
allows illumination even after release at foot control
 Cellular optic handpiece:
Handpiece can be repeatedly sterilized without
light degradation.
 Lube free ceramic bearing handpiece:
do not require lubrication
But care should be taken
against chemicals

CONCLUSION
We are fortunate to belong to the
millennium which has advanced rotary
instrumentation to improve the quality &
quantity of treatment. These advances have
enabled us to move from operative dentistry
to conservative dentistry.
Proper understanding of speed and its
implication in clinical use will provide a
cutting edge over time and expertise.

References
 Art & science of operative Dentistry –
Sturdevant 4th
edn
 Operative Dentistry – Marzouk
 Operative Dentistry – Baum, Philips & Lund
 A Practical Guide to technology in Dentistry –
Nicholas, M. Jedynakiewicz
 Science of Dental Materials – Philips 11th
edn

Thank you
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