rotary instrument/cosmetic dentistry course

ROTARY INSTRUMENTS IN

OPERATIVE DENTISTRY

INDIAN DENTAL ACADEMY

Leader in continuing dental education www.indiandentalacademy.com

www.indiandentalacademy.com

CONTENTS INTRODUCTION HISTORY OF ROTARY INSTRUMENTATION CHARACTERISTICS OF ROTARY

INSTRUMENTATION POWERED ROTARY INSTRUMENTS ROTARY CUTTING INTRUMENTS

Common design characteristicsBur classification systemsModifications in bur designFactors influencing cutting efficiency of burs


ROTARY ABRASIVE INSTRUMENTSDiamond abrasivesOther abrasivesFactors influencing abrasive efficiency

CUTTING MECHANISMSEvaluation of cuttingBladed cuttingAbrasive cuttingCutting recommendation

HAZARDS WITH ROTARY INSTRUMENTSPulpal precautionsSoft tissue precautionsEye precautionsEar precautionsInhalation 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 RPMLow speed 3000-6000 RPMMedium high speed 20,000- 45,000 RPMHigh speed 45,000- 1,00,000 RPMUltra-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/ s1,00,000 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 productionVibration produced in the handpieceLength of operating timeNumber 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 operationVibrations of the handpieceForces needed to control the rotating

instrumentApprehension on the part of the dentistLack 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;

HandpieceThe 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 EQUIPMENTA Handpiece is a device for holding

rotating instruments, transmitting power to them and for positioning them intra-orally.

Classification of HandpiecesThere are a no. of ways to classify a

handpiece. A few are;1. Based on speed2. Based on power source3. Based on angulations4. 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 robustElectric driven – versatile but expensive.


Air motor: Two patterns are in common use:

Rotary Vane typeSwash-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 airOne for exhaust airletOther 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 speedGreen – 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


Modifications in bur 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


Modifications in bur design


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.


Factors influencing cutting efficiency of burs1. Rake angle

2. Clearance angle3. 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.


Factors influencing cutting efficiency of burs4. 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 flutes6. Heat treatment


Factors influencing cutting efficiency of burs7. 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.


Factors influencing cutting efficiency of burs

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 abrasivesOther abrasivesFactors influencing abrasive efficiency


Diamond abrasivesGreat 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 blankPowdered diamond abrasiveMetallic bonding material

The metal blank is comparable to that of the carbide burs.


Parts Bur blankDiamond 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 µmMedium – 88-125 µmFine – 60-74 µmVery 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 ABRASIVESMany 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 USEDThe 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 Finishing & Polishing of restorationsrestorations 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 Finishing & Polishing of restorationsrestorations

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


Factors influencing abrasive efficiency1. 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.


Factors influencing abrasive efficiencyDiamond 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 poundsHigh 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 cuttingBladed cuttingAbrasive cuttingCutting 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

fractureHigh 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 the The 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 of Air is applied only to the extent of removing excess moisture, leaving a glistening removing 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 Dental control unit water systems & handpiece asepsissystems & handpiece 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 controlcontamination 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 correctiona device in the dental unit retracts water from the line

when handpiece is stopped, this also retracts bacteriaoperate the handpiece for 20 sec bet patients.


Inherent water system contaminationBacteria 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 30minEO 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

For more details please visit www.indiandentalacademy.com


rotary instrument/cosmetic dentistry course

Education