HIGH SPEED CUTTING INSTRUMENTS IN PROSTHODONTICS

Introduction

In order to perform the intricate and detailed procedures associated with

restorative dentistry, the dentist must have a complete knowledge of the

purpose and application of the many instruments required. During each day of

his clinical experience the dentist operates on vital tissues within the oral

cavity where a millimeter or a fraction there of, is a very significant

dimension. A skillful application of sharp hand and rotary instruments requires

ability and coordination gained only by extensive training.

Before the advent of rotary instruments, removal of tooth tissue was

accomplished with sharp – edged chisels, hatchets, and hoes. These hand

instruments possessed a cutting capability, which was used for clearing away

unsupported and undermined enamel resulting from dental caries. Walls and

floors of the cavity were formed by a planning and lateral scraping action of

these sharp edged instruments. At best, such efforts were crude, time

consuming and often difficult.

The first, rotary instruments for cutting tooth tissue were modified hand

instruments. These, drill or bur heads could be twisted in the fingers to

produce a cutting or abrading action. In 1846 the finger ring was introduced

with a drill socket attached for adapting a series of long bundled burs or drills.

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This was the primitive application of the rotary principle. The first drill having

flexible cable drive and the first angle hand piece were introduced by Charlee

Merry between 1858 and 1862. In 1871, Morrison modified and adapted the

dental foot engine from the Singer Sewing machine. This was followed by the

introduction of the electric dental engine utilizing a cable arm in 1883. In 1910

the endless cord on a jointed arm was made available. The earlier dental hand

pieces were capable of speeds from 4500 to 6000 rpm.

In 1940 the use of diamond abrasive paints became widespread. The

diamond point is compared of a number of small diamond particles bound on a

rotary blank.

In 1945 Dr. G.V. black, published a report on the non mechanical

preparation of cavities and in doing so introduced the air abrasive technique.

The impact of Dr. Black’s revolutionary cutting technique on the dental

profession was considerable. This was the first significant break in the long

established traditional method of cavity preparation. The airabrasive principle

utilized particles of aluminium oxide propelled against the tooth surface by a

carbon dioxide stream under the pressure of 110 psi, and funneled through a

tungsten carbide nozzle with a lumen of 0.018 inch. The penetration of enamel

and dentin was rapid but some what difficult to control.

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In 1949 Walsh and Symons published their initial findings relating to

the removal of tooth tissue with diamond points at rotational speeds upto

70,000 rpm. This report indicated the use of lighter forces and a resulting

increased cutting efficiency at these higher speeds.

In early 1950, the ball-bearing hand piece was introduced.

In 1963, following the work of Nelson the first fluid turbine type

handpiece was introduced. This instrument was capable of rotational speeds of

approximately 50,000 rpm and was limited to diamond instruments operated at

one speed only. In 1954, air-driven hand pieces were developed. A continuous

belt-driven contra-angle which utilized a friction grip chuck and bur was

introduced, making possible cutting speeds of upto 150,000 rpm.

By 1957, many dentists were using rotational speeds upto 3,00,000

rpm. The introduction of air-bearing hand piece in the early 1950 made

possible greater rotational speeds of approximately 5,00,000 rpm.

In 1953, an ultrasonic method of tooth tissue removal was also

introduced, which used suitably shaped tips vibrating at frequencies ranging

from 2,50,000 to 3,00,000 cycles – per seconds.

This brief historical back ground reveals that the profession has been

searching for a suitable method of tooth tissue removal. Only during last 30

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years, this hunt has slowed down still the profession is trying to refine the

procedure and instruments.

Review of literature

A search through literature reveals various methods used in the past for

removal of tooth tissue. The continuous development of newer methods till

1960, indicates , that the earlier instruments had some disadvantages. Inspite

of the introduction of numerous tooth reduction instruments, and procedures,

the principles and the biologic objectives have remained the same. These are

as follows.

1. The operator should remove the least amount of tooth tissue consistent

with necessary mechanical retention.

2. This should be done with the least barm to the periodontal tissues and the

pulp.

3. It should be done with the least discomfort to the patient.

4. No pathologic reactions should be initiated in the pulp.

Advantages of high speeds

1. Smaller stones can be used at the increased speeds.

2. Less fatigue results both for the patient and operator.

3. Due to high speed, very light pressure is required.

4. Less vibrations are felt by the patient.

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5. The chairside time for a given preparation is considerably reduced.

6. Trauma to the pulp is reduced.

7. The efficiency and life of the cutting tools is increased.

8. Because of small tools, control is easy.

9. Removal of old amalgam and gold restorations is easy.

Disadvantages of high speeds

1. The increased speed creates increased temperatures in the tooth. Therefore

some method of cooling the tooth more efficiently is required not to injure

the pulp. This necessitates additional equipment.

2. When a dentist changes from the lower speeds, which utilize a pressure in

pounds, to high speeds which need only a pressure in ounces, he must

develop a new technique and retrain himself to a new tactile sense.

3. To operate at high speeds good visibility of the cutting instrument is

necessary to avoid over cutting.

4. Due to the ease with which tooth tissue is removed, caution must be taken

not to injure the proximal enamel of the adjacent healthy tooth and the

gingiva.

5. High speeds result in greater wear on the working parts of the handpiece,

necessitating more frequent repairs and replacements.

6. Unless used properly, high speeds have a tendency to create striations on a

tooth surface.

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7. The ideal preparation for any type of restoration cannot be accomplished

by using high speed equipment alone. The final exactness and finishing

line can best be established by instruments revolving at moderate speeds.

Types of high speed instruments

Hand piece can be divided into four types depending upon their speeds

as follows.

1. Low speed – upto 10,000 rpm.

2. Intermediate speed – 25,000 to 45,000 rpm.

3. High speeds – 50,000 to 1,00,000 rpm.

4. Ultra high speeds 1,00,000 rpm and over.

Kilpatric has further classified the ultra high speed handpiece into three

classes.

Type I – the gear driven centre-angle handpiece, upto1,25,000 rpm.

Type II – the belt driven contra-angle handpiece upto 2,00,000 rpm.

Type III – turbine driven air contra-angle handpiece upto 3,00,000 rpm and

higher.

Heat generation:

Knowledge of the physics tells us that, whenever there is friction

between two surfaces, heat is generated, which may bring about rise in

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temperature of either or both the surfaces. The same applies in the tooth

reduction procedures. Here the rotating cutting tools come in contact with the

tooth surface and the heat is generated.

It was not until 1930 that the workers began to investigates the heat rise

in the dental pulp.

There are many factors that influence the rise in temperature which

takes place in cutting operations. The greater the speed of rotation of the

cutting tool, the faster the tool revolves, the higher the resultant temperature. It

has been found that the temperature rise develops within 10-12 seconds, after

the cutting operation is started. Size of the cutting instrument has an important

bearing on heat generation, since, its diameter affects the cutting speed at its

periphery. Larger the size of the cutting tool more the host generation.

A third factor is the pressure applied by the dentist during cutting

operation. As the pressure increases, greater will be the rise in temperature.

Hudson and associates in 1954 conducted a study on temperature

developed in dental cutting instruments from their study they have concluded

that,

1. The temperatures resorb by dental burs in cutting human dentin ranged

from 125°F to 275°F. Since these temperatures are above those, said to be

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tolerated by normal human dentin, it would seen adivsable to use some

form of coolant.

2. A significant decrease in time required to accomplish a given operation is

apparent, when high operating speeds are used.

3. The amounts of heat transferred to the tooth from the bur decreases, at

speeds above 12000 rpm, since cutting time at these speeds is reduced and

bur temperature remains.

Substantially constant and there is less heat trauma to the vital structures.

Coolants:

From the study of Hudson and Sweeney, it is evident that the

temperatures reached during tooth reduction procedures are above those said

to be tolerated by normal human dentines. This indicates that, some form of

coolant must be used, during the cutting operations, particularly when high

speeds are used.

Every means should be employed to keep the temperature down as

much as possible during cutting operations. Coolants must employed which, to

be effective, should be applied at the point of contact between the cutting

instrument and the tooth tissue. There are three types of coolants usually

employed in dental practice.

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1. Water.

2. Spray of air and water

3. Air alone.

Peyton has shown that at speeds ranging from 30000 rpm to 170000

rpm and with an application of four ounces of pressure, a temperature rise

within the tooth of less than 15°C occured when water or air-water sprays

were employed. He also found that even with a water coolant, excessive

temperatures developed, when large diameter instruments or excessive

pressure were applied with increased operating speeds. This indicates that the

were use of a coolant, does not eliminate the danger of excessive temperature

rise.

A reduction in concentration of the amount of water used during cutting

procedure shows the significant temperature rise of the dental bur.

The minimum volume of water to be applied was estimated at 1.5 per

minute.

The question whether water in spray form should be used at mouth or

temperature seems to have no significance as far as temperature rise in the

tooth was concerned. Tylmon is of the opinion that if the water reservoir is

kept at 100°F, it is most comfortable to the patient, less liable to be harmful to

the pulp and still reduces the heat of friction during cutting.

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There are certain other problems associated with the use of the

highspeed cutting tools. Most of the hand pieces are so designed that a spray

or stream of water is directed from the head of the handpiece directly onto the

cutting operation. Where the water strikes the tooth and the cutting tool

directly, full benefit is obtained from the coolant. Where however, the abrasive

on the cutting tool, is on the surface away from the stream of water, water does

not flood the tooth surface being cut, resulting in excessive temperature rise.

The overcome this difficulty perforated disks have been developed, which

permit the water to go through the openings and lubricate the disk and tooth on

the cutting side. The use of perforated disk results in less temperature rise.

Consequently when disks are non perforated, and when the stream of water

cannot be directed to the cutting contact areas, they should be used at speeds

not exceeding 10,000 rpm.

Another advantage of a water coolant lies in the foot that the tooth

debris from the cutting is removed rapidly, preventing the elegging of the

cutting tools. This results in greater cutting efficiency of the stone. Also, it

prolongs the life and effectiveness of the instrument. It is essential that the

water be in intimate contact with the revolving instrument and the tooth tissue.

To do this more effectively, Nelson recommended the addition of a wetting

agent to the water spray.

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Because the high speed technique requires a larger quantity of water as

a coolant, there is the problem of removing this water from the mouth. To have

the dentist stop frequently to allow the patient to spit out the excess water is

time consuming. The constomary saliva ejector has insufficient removal

capacity.

To solve this problem, Thompson has suggested a washed field

technique.

This technique adapts the suction or vacuum principle. It established

and maintains a powerful but gentle negative pressure of air in the mouth,

close to the field of operation.

Accompanying the air stream, is a flow of isothermal water which is

projected copiously onto the operative field. This water is entrained into the

vacuum air stream, which draws it rapidly across the operative area. The

irrigant pulls away with it tooth cuttings and debris. These are taken into the

vacuum air stream and disposed off in a filter system. A clean, clearly visible

operative field is provided. This technique has the distinct advantages that it

facilitates the use of high speed instruments, maintains visibility during

capious irrigation of the operative field, reduces operating time, improves the

patient’s well being and introduces a new concept of cleanliness. Human

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tissues are maintained in their natured wet safe pain, trauma and postoperative

complication, which may arise due to ingestion of tooth debris are reduced.

Desiccation of hard and soft tissues is avoided. Heat is eliminated thus

preserved the tissues.

Vibration:

Cutting a tooth may be very annoying and unpleasant to the patient but

still not be painful. In pain there is usually an involvement of the nerve

endings, either by trauma or extreme irritatino, resulting in an acute, painful

reaction. Most patients associate the sensation of vibration, noise, pressure and

the slight increase in cutting temperature with the sensation of pain.

Consequently, if the factors of vibration, heat and pressure are reduced to a

minimum, the patient usually experience reduced or no pain.

One mechanical factor that influence vibration is the dental handpiece,

whether it is friction-bearing, ball bearing, high speed belt driven or turbine

ultra speed. When the friction bearing, conventional type of handpiece is used

at a speed of 4500 rpm to 6000 rpm. It is connected by the conventional belt

and pulley system of the dental engine. In this case one may expect a high

order of vibration depending upon the condition of various mechanical parts,

their adjustment and speeds of their operation.

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Pulleys, that are worn, a worn belt, or an improperly adjusted belt will

cause vibrations that are transmitted down to the cutting tool. Similarly hand

piece which do not hold the cutting tool properly, which have worm bearing or

are cut of adjustment will also cause vibration.

The investigations of Walsh and Symmoss showed that vibration, when

applied to tooth, produced the most unfavourable response when the frequency

was between 100 cps and 200 cps. When the frequencies were above 1000 cps,

they were generally beyond the upper threshold of perception of the average

patient. It is the lower frequencies, in the range of 100 – 200 cps, that are

usually developed at the lower speeds, especially or the equipment is worm

and maladjusted.

Hudson and Sweeney have reported the importance of having contricity

in the cutting tool. They found that eccentric burs when rotated at 6000 to

10000 rpm produced a lower frequency in the range of 100-200 cps, whereas a

true running bur at 10000 rpm produced vibrations in the frequency range

above the upper threshold.

Tamner pointed out that only a part of an eccentric cutting tool is used

as it rotates, thus causing unfavorable impects and vibrations, which fall into

the most annoying frequency range. The disks and stones that are unmounted

and are screwed onto a mandrel very frequently are eccentric and therefore

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should not be used in high speed cutting operations. The permanently mounted

instruments are indicated in preference to unmounted type.

Poorly built burs with blades not evenly cut or chipped will likewise

cause vibration. In using now carbide burs, it is very important the see that

none have chipped blades.

Correct adjustment of the belt is important in the reduction on and

elimination of vibration. A belt that is too loose increases the vibration pattern

transmitted directly to the tooth.

In the ultra highspeed hand pieces the metal chuck holding the cutting

instrument often is replaced by a rubber or plastic chuck. This lessens the

vibration transmitted to the cutting instrument and facilitates the more rapid

cutting action.

In cutting with a water turbine handpiece at 45,000 rpm the intensity of

vibrations was well tolerates by the patient.

Morrison and Grinnel made the following observations.

The deliferious effects of vibration are two fold in origin.

1. Amplitude.

2. Undesirable modulating frequencies.

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If we minimize or aliminate these factors, we can then reduce the

undesirable effects of vibration.

Amplitude: The wave of vibration consists of frequency and amplitude.

At conventional speeds, amplitude is greater but frequency is less. At

higher speeds the reverse is true. The greatest harm is caused by the amplitude

of vibration which is the factor, most destructive of instruments and which

causes the most apprehension in the patient and the greatest fatigue in the

dentist.

By increasing operating speeds, the amplitude and its effects are

reduced and a more satisfactory result is attained.

Vibration waves are measured in cycles per second. It has been shown

that rotation of approximately 6000 rpm sets up a vibrational wave of

approximately 100 cps. As the rpm is increased the cps of the fundamental

vibration wave are increased until, at ranges of 100000 rpm, we have a

vibration wave of 1600 cps. It has been demonstrated that at wave of vibration

of over 1300 cps, vibration is practically imperceptible to the patient. The

reason for this is not fully understood, but there are two theories for this

phenomenon.

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(1) The wedensky inhibited phenomenon helds that where stimulation occurs

during the refractory period of recovery of the perceptor mechanism nor

further reaction will be evident.

(2) Vibratory perception depends upon the product, the amount of stimulation

i.e. pressure multiplied by the frequency of stimulation necessary for a

reaction. This is called chronaxis. As the speeds above 1,00,000 rpm, due

to light pressure and high speeds, chronaxis is attained, which is necessary

for reaction.

Thus it can be concluded that, the more the rpm, the less the amplitude,

and the greater the frequency. Vibratory perception will be lost in the ultra

highspeed range of 1,00,000 rpm or more.

Spread of pathogenic organisms by Ultra speed cutting procedures:

Atmospheric contamination through the spread of oral organisms

particularly from air turbine action has been a concern of the dental profession

for some time. Dental procedures tend to expose the operator to infectious

diseases. Recent studies suggest that the extent of aerosol produced by air

turbine may increase the normal hazard. A report involving patients with

pulmonary tuberculosis cultures were demonstrated on all petri dishes exposed

during cutting procedures, with the heaviest concentration being at 2 feat in

distances from the patients mouth. This indicates that the dentist and his

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assistant are exposed to a serious health hazard when operating with an ultra

speed exposed instrument on patients having such pathogens in their oral flora.

When a patient’s history suggests the existence of tuberculosis, pneumonia,

influenza, infections hepatitis or any infectious diseases including the common

cold, a protective face mask should be worn by both dentist and assistant.

During all ultraspeed cutting procedures, protective eye-glasses should be

worm routinely.

Size of cutting instrument and cutting speeds:

It has been pointed out by Peyton, and Nelson that, the important factor

of increased operating speeds is the instrument surface speed in fact per

minute rather than the revolution per minute of the instrument.

The larger the diameter of the cutting instrument, the slower the speed

required at the spindle. The specific phase in preparation of an abutment

should determine the size of te cutting instrument and the rpm that should be

used. Employing superspeeds for all operations places unnecessary strain upon

the patient and equipment. If the same effect can be accomplished by using a

larger instrument at a lower speed, but still remaining above the threshold of

perception, this should be done. However, oversized cutting tools should not

be used at super speeds due to the difficulty of instrument control and

accuracy of cutting.

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Biologic response of dentin and pulp to high speed cutting:

Dentin: As the contents of the dentinal tubules are in direct continuity with the

odontoblasts, and pulp, cutting or grinding the dentin causes a reaction in the

pulp and this may lead to changes in the dentin.

An early experimental investigation into the effect of cavity preparation

on the dentin and pulp was carried out by Fish in 1932. He out cavities in the

teeth of dogs and left the cavities open to the saliva. by sealing dyes into the

pulp chambers of the treated teeth he has shown that one of two reactions is

produced in the dentin.

In some cases there was sclerosis of the cut dentinal tubules which

forms a protective some sealing off the pulp from the injury and underneath

this region, there is a further growth of tubular dentine. These reactions are

produced by the stimulation of the odontoblasts. The other reaction that

resulted was the formation of dead tracts. With this lesion some or all of the

odontoblasts, that are in connection with the cut tubules die. On the pulpal

aspect of these tubules, hyaline mineralized barrier, secondary dentine is laid

down, thereby sealing the lesion from the pulp.

Pulp: The changes in the pulp have been studied by Langeland and Morslard

and Shovelton. They state that the damage to the pulp is to a large extent due

to the heat generated. They have shown that when precautions are taken to

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minimize heat production by using burs rotated slowly in a speed reducing

handpieces, the only evidence of pulp damage was a slight dasmption of the

odontoblast layer with the displacement of a small number of odontoblasts into

the dentinal tubules. When speeds upto 5,000 rpm were employed, there was

more extensive displacment of odontoblasts associated with marked

vacuolization of the odontoblast layer, and local haemorrhages may be seen in

the pulp. As the speed was increased, the changes became more severe. When

tooth reduction was done under a streamor spray of water, the damage to the

pulp was markedly reduced.

Pulp changes associated with tooth reduction using the airabrasive

technique have been studied by Kennedy and using ultrasonic technique by

Mitchell and Jenson. These changes in both the cases are similar to those

produced at the speeds of 5,000 rpm.

The effects on the pulp of using high speed rotary instruments such as

the air turbine have been investigated by Marsland and Shovelton. The

changes found are no severe than those produced at lower operating speeds

provided that adequate cooling of the cutting instrument by water jet or

air/water spray is ensured.

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Alterations in the hard tissues of tooth cut by air turbines have been

observed. The enamel over a wide area of crown may show minute cracks and

the dentin shows altered staining reactions as a result a local overheating.

Discussion

It is for more than 125 years, that rotary instruments have been in use,

for tooth reduction operations, in different forms, from a hand rotary

instrument to ultra sonic instruments, which have the rotational speeds ranging

from very low speeds in case of band rotary instruments to 5,00,000 rpm in

case of air bearing hand piece. These remarkable advances in the instruments

have greatly reduced fatigue in the operator because of the physical case of

manipulation and have considerably increased the comfort to the patient by

reducing the actual working time and pressures required for tooth reduction,

thereby minimizing the factors of heat generation and pain. Though high speed

techniques have been a born to the dental profession, they have their can

limitations. It is interesting to not that, inspite of considerable improvments in

tooth reduction procedures and the instruments used for the same, the

principles and biologic objectives have not changed.

These improved methods of tooth tissue removal have a potential to

damage the healthy teeth and surrounding structures, if they are used without

proper understanding of their working and if they are used without taking

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proper care. Improper handling of those modern equipment may also be

different to the longevity and working capacity of the instruments themselves.

For successful and efficient use of those cutting tools, certain factors

should be given consideration. Heat that is generated, while the tooth tissue is

being removed must be kept, down to the minimum and at the sometime,

whatever beat is generated, must be eliminated as efficiently and as quickly as

possible by employing coolants, in any one of three forms commonly used i.e.

water, air/water spray or air alone. Simultaneously with the coolant, if water or

air/water spray is used, an efficient mechanism for remove of the water from

the oral cavity must be employed. Otherwise, the clinical procedure is delayed,

if the patient has to spit out the water, every now and then. By eliminating the

water evacuation equipment, we are losing one of the advantages of these high

speed instruments i.e. reduced working time for a particular preparation. Use

of efficient coolants, not only eliminate the heat generated, but at the same

time, keeps the operating area clean and free of any debris.

High speed cutting methods have a further advantages in that, they

reduce the annoyance that may be caused to the patient, when low speeds are

used with the modern high speed cutting devices, the vibration produced is of

a frequency that is generally beyond the upper threshold of perception of the

average patient.

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Pressures that have to be employed in the use of high speeds are

considerably reduced, in comparison with those needed for low speeds.

Thus, when the factors of pressure, temperature and vibration are kept

within the tolerance limits, the patient comfort is certainly improved.

Size of the cutting tool to be used for particular tooth reduction

procedure is an important consideration, particularly while using high speeds.

Oversized cutting tools should be avoided, as they are difficult to control and

at the same time, the accuracy of tooth preparation on procedure is also

adversely affected.

Biologic reactions of the tooth tissues, particularly dentin and pulp,

should not be over locked, when high speeds are employed for tooth reduction

operation. These responses have been studied by a number of people and they

have shown that, the response are not significantly different from those, when

low speeds are used, provided, effective coolants are employed.

Thus it can be concluded that, high speed equipments for tooth

reduction if used with proper understanding and due care, provide definite

advantages over the conventional low speed cutting procedures. This fact

places the high speed devices at definitely a higher level as against their low

speed counterparts.

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Conclusions

1. High speed cutting devices, if used with a thorough understanding of their

mechanism and due care to the biologic integrity of teeth and surrounding

structure, are a boon to dentistry.

2. In the process of tooth reduction using high speeds considerable amount of

heat is generated and an effective coolant is a must for preservation of

tooth integrity and patient comfort.

3. Vibration is increased with the increase in speed, but it is beyond the

threshold of prerception of the normal human beings and hence not

harmful.

4. Biologic reactions of the dentin and pulp, to high speed cutting, cannot be

overlooked.

Summary

A brief history of rotary instruments has been presented. A critical

evaluation of the high speed cutting devices, as to their advantages,

disadvantages, and precautions to be taken during their use, has been assessed

at length. Biologic reactions of dentin and pulp, to high speed cutting have

been discussed in brief.

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Contents

I. Introduction

II. Review of Literature

a. Advantages of high speeds

b. Disadvantages of high speeds

c. Types of high speed instruments

d. Heat generation

e. Coolants

f. Vibration

g. Spread of pathogenic organisms

h. Size of cutting instruments and cutting speeds

i. Biologic responses of dentin and pulp to high speed cutting

III. Discussion

IV. Conclusion

V. Summary

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