Rotary cutting instruments in conservative dentistry

BYV.VASUNDHARA

2ND YEAR PG

contents

Introduction Evolution of rotary cutting instruments Recent developments Classification of rotary cutting instruments Classification of dental handpiece Adv and dis adv of speed Characteristics of rotary instruments Dental handpiece Dental burs History development of burs Classification of burs Bur blade design Abrasive instruments

Cutting mechanism Evaluation of cutting Factors influencing cutting effectiveness and

efficiency of the bur Hazards of rotary cutting instruments references

introduction

WHAT DOES ROTARY MEAN?

Instruments which turn on an axis to perform work.

Work may be :- 

CuttingAbradingBurnishing

Finishing and polishing

EVOLUTION OF ROTARY EQUIPMENTS:

Historical Development:-

Prehistoric man used sharp pieces of flint for trephineing holes in bones.

Hippocrates in 350 B.C. described a drill driven by a cord wound around

a shaft. Celsus (25 B.C. –50 A.D) described two kinds of drillers or

“Terebra”. One with a guard to prevent it from sinking deep into the

tissues and the other one was similar to a carpenter’s drill.

In 2A.D. Cladius Galenius a celebrated physician reports of Archigenes

an eminent surgeon of Asia minor and practicing in Rome successfully

treated tooth ache by opening the tooth with a trephine.

Galen (130 –200 A.D) modified Celsus’s “Terbra” and called it

“Terebraabatista” or “Modiolus”. Lubrication was done with olive oil

or milk or by dipping in cold water.

Abulcasis (936 – 1013 A.D) described a boring instrument “Incisura”.

Perre Fauchard “Father of Dentistry” in his book “The Chirurgien

Dentiste “ in 1728 described the first dental rotary instrument of modern

times. It was known as the “Bow Drill” could be rotated at 300rpm and

was later on modified into the “Scranton’s drill” which could cut by

rotating in either direction.

In 1831 dental chair was introduced.

In 1838 John Levis made a hand held drill.

Dr. West Cott in 1846 used “Fingerings” with drills. Taft called them

“Bur Drills”.

drill stocks, bur chucks or bit holders – forerunner of the present dental handpiece.

eg: -Cheavlier drill stock

-Merry’s drill stock

‘Chevalier drill stock” was hand

powered like an egg-beater.

Tomes in 1859 described three types of burs.

1. Rose head: a short shank bur inserted in a crutch rotated between

thumb and index finger supported at the base of the thumb.

2. Long hand bur: teeth are cut for same distance along the shaft and it

is mounted in a handle.

3. Long steel shaft with too cutting blades.

Charles Merry in 1862 used a “Drill Stock” which had a flexible cable

drive. This also was a type of angle handpiece

George Fellows Harrington in 1865 used “Clock work drill” or

“Harrington’s Erado” which is the first motor driven drill. At first burs were

hand cut and ground and were expensive.

America in 1860s began mass production of burs from carbon

steel. The earliest burs had limited lateral and end cutting action. The

diameter varied form 1/32” to 1/5”. These were particularly used for

small and medium sized varieties. These carbon steel burs were called

“Small milling cutters”.

In 1871 Morison’s foot engine was introduced. Morrison modified and

adapted the dental foot engine from the singer sewing machine.

Cutting procedure was carried out with a power source

A speed of 700 rpm was obtained. 

In 1873 Coxeter used an electric engine with a speed of 1000 rpm. This is

the predecessor of the modern micromotor. This was held in hand and

connected to a coil. The motor was open and the spindle of the motor was

connected with the hand piece.

In 1874 the electric motor hand piece was invented by S.S white and later

he also pioneered the invention of various carbon steel burs and hand pieces.

In 1883 rotary power from an electric engine was transferred to the straight hand

piece by a belt that ran over a series of pulleys and a three-piece extension cord arm. A

variable rheostat was used as a foot control.

Rotary cutting instruments were inserted into the chucking mechanism at the

front of the handpiece.

The desired angle handpiece is attached to the front of the straight hand piece

and a shaft and gears inside the angle section produce rotation of the working

instrument.

GEAR DRIVEN HANDPIECE

In 1891 Edward G. Acheson an American invented and produced

carborundum and carborundum tools were introduced.

In 1901 hand piece with forward (clockwise) and reverse

(anticlockwise) direction of rotation and burs for each type movement were

brought into use.

In 1910 Emile Huet a Belgian perfected an electric engine to give a

speed of 10,000 rpm.

In 1935 diamond abrasives were introduced and W.H Drendel

introduced the process of galvanized bonding of diamond powder to copper

blanks and used at a speed of 5,000 rpm.

In 1947 Tungsten carbide was introduced and

S.S White in 1948 made tungsten carbide burs which were

used at a speed of 12,000 rpm.

In 1949 Walsh and Symons used diamond points at a speed

of 70,000 rpm.

In 1950 ball bearings were used in contra angel handpieces.

In 1951 air abrasive technique was introduced.

In 1953 Nelson produced a Hydraulic driven turbine angle handpiece

of speed, 60,000 rpm.

In 1955 Page-chayes introduced first belt-driven angle handpiece to operate

successfully at speeds over 100,000 rpm. All gears were eliminated by having a small belt run inside the handpiece

sheath over ball bearing pulleys in the angle sections.

Improved models -Page-Chayes 909 and the Twin 909

In 1955 Turbo-jet was designed as a compact mobile unit that required no outside

plumbing or air connections.

Only a source of electricity was need.

A sound proof cabinet contained a motor, water pump, water reservoirs and

necessary plumbing for water circulation. Water was conveyed to and from the hand

piece by co-axial type plastic tubing

. The small inner tube carried water under high pressure to rotate

a turbine in the handpiece head and the larger outer tube returned the

water to the reservoir for re circulation.

In 1960 ultrasonics were used for hard tooth structure removal.

In 1961 air turbine straight handpiece was introduced.

In 1962 air turbine angle handpiece with air bearings were

introduced.

A small compact unit consists of a handpiece, control box, foot control and various connector hoses.

When the foot control is activated, compressed air flows through the control box and is carried by a flexible hose to the back of the handpiece.

From here the air is directed to the head of the handpiece and is blown against the blades of a small turbine to produce Rotation, while the greater part is exhausted at the back of the handpiece or returned to the control box.

Most modern angled handpieces also include fireoptic lighting of the

cutting site.

- earlier units were water driven and later came the air driven units.

- have free running speed of 300,00 rpm but lateral loads during cutting can reduce it to 200,00 rpm.

( excellent safety feature )

- ADV: simple, ease of control, patient acceptance, versatility.

- DISADV: low torque & power output makes them unsuitable for finishing & polishing purposes.

- SOLUTION: Straight handpieces which provided high torque & low speed operation.

RECENT DEVELOPMENTS:  Allow repeated sterilization

Smaller head size More Torque Lower noise Better chucking mechanism Fiber-optic lighting of the cutting site. (contemporary air turbine handpiece)

CLASSIFICATION OF ROTATING INSTRUMENTS:

According to speed : ( According to Sturdevant )

High speed range or ultra- 100,000 to 300,000 rpm Intermediate speed range- 12000 to 100,000 rpmLow speed range- Below 12000 rpm

(Acc. to Marzuok)

Ultra low speed : 300-3000 rpm Low speed : 3000-6000 rpm Medium high speed : 20,000-45,000 rpm High speed : 45,000-100,000 rpm Ultra high speed : 1000,000 and above

(Acc. to Charbonneau)

Conventional or low speed : below 10,000 rpm

Increased or high speed : 10,000-150,000 rpm

Ultra speed : Above 150,000 rpm

(Gilmore- 3 Edition )

Regular speed Ultra speed 500-6000 rpm 200,000 rpm

SO MANY SPEEDS – WHICH IS THE RIGHT ONE TO USE ??? 

Low speeds:

cleaning teeth caries excavation finishing & polishing

ADV: better tactile sensation less chances of overheating cut surfaces.

DISADV:

more heat productionvibrationtime consuminginefficient for cuttingburs have tendency to roll out proximal margin or tooth surface.carbide burs do not last long.

High speeds:

tooth preparation removing old restorations

ADV: faster tooth structure removal less heat production less vibration better control & ease of operation time saving diamond & carbide instruments stay longer

Characteristics of rotary instruments

Speed refers not only to revolutions per minute, but also to the surface feet per unit time of contact that the tool has with the work to be cut.

It is important to consider the size of the working tool in relation to the speed of operation.

A rotary tool should be large in diameter when used with low speeds to approach the optimum surface feet per unit time.

In ultra high speed range, the diameter of cutting tool should be reduced to approximate the limits of maximum cutting efficiency.

speedspeed

Pressure is resultant effect of two factors under the control of dentist.

1. force : the gripping of the handpiece and its positioning and application to the tooth.

2. Area : the amount of surface area of cutting tool in contact with the tooth surface during a cutting operation.

P=F/AIt has been observed that low speed requires 2-5 pounds force, high speed

requires less force i.e 1 pound and ultra high speed still less force i.E 1-4 ouncesHigher speed-less fatigue to operator-greater comfort to patient

pressure

At is directly proportional to;1. Pressure2. RPM3. Area of tooth in contact with the tool

At temp of 130 F tooth pulp is permanently damagedAt 113 F inflammatory responses that could result in pulpitis and

eventual pulp necrosis is seen Higher speed call for less force and if coolants are used, heat production

could be eliminated or at least minimized

Heat production

Vibration is not only a major annoying factor for the patient, but it also causes fatigue for operator, excessive wear of instruments and most importantly, a destructive reaction in the tooth and supporting tissue.

Vibration is a product of the equipment used and speed of rotation.

The equipment primarily the hand pieces and the revolving cutting tools

The deleterious effects of vibration are two fold in origin1. Amplitude2. Undesirable modulating frequencies

Vibration

1. Amplitude

a wave of vibration consists of frequency and amplitude. at low speed, the amplitude is large but the frequency is small. At

higher speeds the reverse is true

By increasing the operating speed the amplitude and its effects are reduced as well as its sequelae.

Higher RPM s produce less amplitude and greater frequency of vibrations. As a result, perception will be lost in the ultra high speed ranges of 1,00,000 RPM or more.

2. Undesirable modulating frequency

The second deleterious effect of vibration is caused by improperly designed, or poorly maintained equipments.

Improper equipment use or care allows modulating frequencies to be established so that a series of vibrations are perceived by the patient and the dentist. The end result is again apprehension in the patient, fatigue for dentist and accelerated wear of cutting instruments

To eliminate these, The operator should supply himself with true running energy source, centrically cutting tools and handpieces that run at high speed.

The factor that cause patients apprehension are Heat production Vibrational sensation Length of operating time Number of visits

The proper understanding of the instruments and the speed at which it is being used,

the use of coolants, intermittent application of a tool to the tooth; sharp instruments aid in minimizing patients discomfort and

unnecessary irritant to oral tissue

Patients reaction

the major cause of fatigue isDuration of operationVibration produced in the handpiece

High speed rotary instrumentation minimizes fatigue by decreasing both the vibrations and the time of the operation.

Operator fatigue

Instrument design for rotary instrumentation should be evaluated in 2 parameters .

1. Handpiece2. Cutting tool itself

Instruments design

Dental Handpieces:

- Device for holding rotating instruments, transmitting power to them & for positioning them intra-orally.  Types: a) Straightb) Angled

Classification of Dental Hand Pieces

According to Speed: Conventional <10,000 rpmIntermediate 10,000-1,00,000 rpmHigh /Ultra high >1,00,000 rpm

According to Applications: Straight Contra angle

Prophylaxis

Type of power mechanism: Belt devices Gear devices Direct motor devices Water driven Air driven

The following criteria should be used in evaluating handpieces :

a. Friction:

Friction will occur in the moving parts of handpiece; especially the turbine.

heat from friction is prevented by handpieces equiping with bearings: ball bearing, needle bearings, glass and resin

bearings, etc

b. Torque

Torque is the ability of the handpiece to withstand lateral pressure on the revolving tool without decreasing its speed or reducing its cutting efficiency.

Torque is dependent upon the type of bearing used and the amount of energy supplied to the handpiece.

C. Vibration

While some vibration is unavoidable, care should be taken not to introduce it unnecessarily.

Excessive wear of the turbine bearings, will cause eccentric running which creates substantial vibration .

DENTAL BURS

All rotary cutting instruments that have-bladed cutting heads.

This includes instruments intended for such purposes as finishing metal restorations and surgical removal of bone, & tooth preparation.

COMMON DESIGN CHARACTERISTICS

three parts:

(1)shank, (2) neck, and(3) head.

Shank Design

part that fits into the handpiece, accepts the rotary motion from the hand-piece, and provides a bearing surface to control the alignment and concentricity of the instruments .

straight handpiece shank, latch-type angle handpiece shank, friction-grip angle handpiece shank,

Neck Design

intermediate portion of an instrument that connects the head to the shank.

The main function of the neck is to transmit rotational and translational forces to the head.

Head Design

working part of the instrument . the cutting edges or points that perform the

desired shaping of tooth structure.

bladed instruments abrasive instruments

Historical Development of Dental Burs

earliest burs : expensive variable in dimension and performance

• first machine made burs introduced in 1891

Carbide burs

introduced in 1947 .

All carbide burs have heads of cemented carbide in which microscopic carbide particles, usually tungsten carbide, are held together in a matrix of cobalt or nickel.

• In most burs, the carbide head is attached to a steel shank and neck by welding or brazing.

• Although most carbide burs have the joint located in the posterior part of the head, others are sold that have the joint located within the shank and therefore have carbide necks as well as heads .

Bur Classification Systems

BASED ON MODE OF ATTACTMENT TO THE HANDPIECE:

latch-type friction-grip

COMPOSITION:

o Stainless steelo Tungsten carbide burso combination

MOTION:

o Right buro Left bur

LENGTH OF THEIR HEAD:

o Longo Shorto regular

USES:

o Cutting burso Finishing & polishing burs

1.Head shapes

o round,o inverted cone,o pear,o straight fissure, o tapered fissure ,o Wheel shaped,o End cutting bur

Round bur

spherical . initial entry into the tooth , extension of the preparation, preparation of retention features/and caries

removal. numbered from ¼, ½, 1, and 2 to 10 .

Inverted cone bur

providing undercuts in tooth preparations .

numbered from 33¼, 33½, 34, 35 to 39.

Pear-shaped bur A normal-length pear bur (length slightly

greater than the width) is advocated for use in Class I tooth preparations for gold foil .

long-length pear bur (length three times the width) is advocated for tooth preparations for amalgam.

They are numbered from 229 to 333 and mainly used in pedodontics.

Straight fissure bur

Elongated cylinder. amalgam tooth preparation. They are numbered from 555, 556 to 560 .

Tapered fissure bur

slightly tapered cone with the small end of the cone directed away from the bur shank.

Tooth preparations for indirect restorations.

They are numbered from 168, 169 to 172.

Wheel burs

They are numbered as 14 and 15.

They are wheel shape and are used to place grooves and for gross removal of tooth structure.

End cutting burs

They are cylindrical in shape, with just the end carrying blades.

They are very efficient in extending preparations without axial reduction.

They are numbered from 900 to 904.

Modifications in Bur Design

Reduced use of crosscut ,

extended heads on fissure burs ,

rounding of sharp tip angles.

Bur Blade Design The actual cutting action of a bur (or a diamond) takes

place in a very small region at the edge of the blade (or at the point of a diamond chip). In the high-speed range, this effective portion of the individual blade is limited to no more than a few thousands of a centimeter adjacent to the blade edge

The optimal angles are dependent on such factors as the mechanical properties of the blade material, the mechanical properties of the material being cut, the rotational speed and diameter of the bur

force applied by the operator to the handpiece, and thus to the bur.

ABRASIVE INSTRUMENTS

Abrasive instruments

Head consists of small angular particles of hard substance embedded in a softbinder(ceramic, metal, shellac, rubber)

1.Diamond abrasive

2.Other abrasives – boron carbide,pumice,silicon, garnet

The second major category of rotary dental cutting instruments involves abrasive rather than blade cutting.

Abrasive instruments are based on small, angular particles of a hard substance held in a matrix of softer material.

Cutting occurs at a large number of points where individual hard particles protrude from the matrix, rather than along a continuous blade edge

ADV:

Effective in cutting enamel & dentin Long life of instruments.

Diamond particle size is commonly categorized as:

Coarse (125- 150 um); Medium (88- 125um); Fine ( 60- 74um); Very fine (38- 44um).

Factors influencing the abrasive efficiency and effectiveness

Size of the particle Shape of the particle Density of the particle Hardness of the particle Clogging of abrasive surface

Other abrasive instruments

Moulded abrasive instrument – heads that manufactured by pressing a uniform mixture of abrasive and matrix around roughened end of shank; points and stones; finishing &polishing

Coated abrasive instrument – disks that have a thin layer of abrasive cemented to a flexible backing ;surface contour, finishing

Diamond bur preferred over tungsten carbide because- {Sharon Siegel 1996;JADA vol.127 }

* Greater resistance to abrasion * Lower heat generation * Longer life

Disadvantages : Heterogeneity of grain shapes Difficulty of automation during fabrication. The decrease of cutting effectiveness due to repeated sterilization Short lifetime Potential release of nickel ions from the metallic binder into the body fluids.

CUTTING MECHANISMS

For cutting, it is necessary to apply sufficient pressure to make the cutting edge of a blade or abrasive particle dig into the surface. Local fracture occurs more easily if the strain rate is high (high rotary instrument surface speed) because the surface being cut responds in a brittle fashion.

When the bur is used a compressive stress is first induced as the blade forces its way into the work. If cutting results, the blade shears the surface it gradually becomes parallel with the surface of work and it pushes the material ahead along the tooth face to form the chip. The chip is subjected to compressive stress.

On the other hand , if the force exerted by the blade is insufficient to induce stress to exceed the elastic limit of the material, it cause’s only elastic deformation of the surface , without chip formation.

There is general agreement that increased rotational speed results in increased effectiveness and efficiency.

Adverse effects associated with increased speeds are heat, vibration, and noise

EVALUATION OF CUTTING

Cutting can be measured in terms of both effectiveness and efficiency

Cutting effectiveness is the rate of tooth structure removal (mm/min or mg/sec). Effectiveness does not consider potential side effects such as heat or noise.

Cutting efficiency is the percentage of energy actually producing cutting. Cutting efficiency is reduced when energy is wasted as heat or noise. It is possible to increase effectiveness while decreasing efficiency

Factors influencing the cutting effectiveness and efficiency of the bur :

Rake angle, Clearance angle and Blade angle

Neck diameter

Heat treatment

Influence of load

Number of teeth

Concentricity is a direct measurement of the symmetry of the bur head itself. It measures how closely a single circle can be passed through the tips of all of the blades. Thus, concentricity is an indication of whether one blade is longer or shorter than the others. It is a static measurement not directly related to function.

Runout, on the other hand, is a dynamic test measuring the accuracy with which all blade tips pass through a single point when the instrument is rotated. It measures not only the concentricity of the head, but also the accuracy with which the center of rotation passes through the center of the head

The runout is the more significant term clinically, because it is the primary cause of vibration during cutting and is the factor that determines the minimum diameter of the hole that can be prepared by a given bur. The average acceptable run out is 0.023mm.

BLADED CUTTING

Brittle fracture is associated with crack production, usually by tensile loading.

Ductile fracture involves plastic deformation of material, usually proceeding by shear.

Extensive plastic deformation also may produce local work hardening and encourage brittle fracture as well.

The rate of stress application (or strain rate) affects the resultant properties of materials. In general, the faster the rate of loading, the greater will be the strength, hardness, modulus of elasticity, and brittleness of a material.

A cutting instrument with a large diameter and high rotational speed produces a high surface speed, and thus a high stress (or strain) rate.

Many factors interact to determine which cutting mechanism is active in a particular situation. The mechanical properties of tooth structure, the design of the cutting edge or point, the linear speed of the instrument‘s surface, the contact force applied, and the power output characteristics of the handpiece influence the cutting process in various ways.

In order for the blade to initiate the cutting action, it must be sharp, must have a higher hardness and modulus of elasticity than the material being cut, and must be pressed against the surface with sufficient force. The high hardness and modulus of elasticity are essential to concentrate the applied force on a small enough area to exceed the shear strength of the material being cut

Heat thus produced is dissipated-

1. By conduction through the tool 2. By conduction through the work 3. By the chip itself 4. By the coolant

HAZARDS WITH ROTARY CUTTING INSTRUMRENTS:

classified into:

Pulpal dangersSoft tissue dangersEye, ear, inhalational dangers.

Pulpal precautions:

Steel burs produce more heat than carbide burs

Burs/diamond points clogged with debris produce more heat. When used without coolant, diamond instruments are more damaging than carbide burs.

Prevention: use of air-water coolant.

Use of air alone is not advised as it can cause desiccation of the dentinal tubules.

Soft tissue precautions:

Lip, tongue, cheeks most common areas of injury.

Prevention: use rubber dam, cotton rolls, mouth mirrors, evacuator tips etc.

Eye precautions:

Patient, operator, dental assistant ALL should wear protective eye glasses.

Ear precautions:  Noise levels more than 75db may cause hearing

damage.

Prevention:

o Use ear plugso Sound proofing of rooms

Inhalational precautions:  Aerosols & vapors produced while cutting with

rotary instruments all hazardous.  May cause alveolar irritation & tissue reactions.  Prevention:

o Use rubber dams,o Mouth masks etc.

References Art And Science Of Operative Dentistry, Sturdevent..5th Edition

Principles and practice of operative dentistry second edition Gerald T. Charbencau.

Operative dentistry, modern theory and practice. 1st edition Marzouk.

Textbook of operative dentistry. 3rd edition vimal k sikri.

Text Book of operative Dentistry. Second edition Baum Philips hand. CUTTING EFFICIENCY OF THREE DIAMOND BUR GRIT SIZES. SHARON

CRANE SIEGEL, D.D.S., M.S.; J. ANTHONY VON FRAUNHOFER, M.SC., PH.D 1996;JADA vol.127

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