ROTARY CUTTING INSTRUMENTS

• Rotary cutting instruments are those instruments which�rotate on an axis to do the work of abrading and cutting on�tooth structure.��

Types of Rotary Cutting

�• Handpiece: It is a power device.�• Bur: It is a cutting tool.��

Dental Burs

�• “Bur is a rotary cutting instrument which has bladed cutting head”.

�• Burs remove tooth structure either by chipping it away or�by grinding. (Bladed cutting/abraded cutting)��

Materials used

Stainless steel

• First developed
• Slow speed
• Gets dulled easily,does not have long life span

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Tungsten carbide

• Designed to withstand heavy stress and long shelf life
• Used in high speed

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• According to their mode of attachment to the handpiece:�

– Latch type�– Friction grip type

�• According to their composition:

�– Stainless steel burs�– Tungsten carbide burs�– A combination of both��

• According to their motion:�– Right bur: A right bur is one which cuts when it revolves�clockwise.�– Left bur: A left bur is one which cuts when revolving�anticlockwise.

�• According to the length of their head:�– Long�– Short�– Regular��

• According to their use:�– Cutting burs�– Finishing burs�– Polishing burs�

• According to their shapes:�– Round bur�– Inverted cone�– Pear-shaped�– Wheel shaped�– Tapering fisure�– Straight fisure�– End cutting bur��

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• Shank : part of the bur that fits into the handpiece,
• Neck: neck connects the shank to the head
• Head: It is working part of the instrument. Based upon their head characteristics, the instruments can be bladed or abrasive.

Bur Design��

• Bur head consists of uniformly spaced blades with concave�areas in between them.
• These concave depressed areas are called chip or flute spaces�

DESIGN OF A BUR

• Bur blade: clearance face,rake face
• Rake angle:
• Radial line
• Land
• Clearance angle – Primary and secondary clearance angle
• Radial clearance
• Blade angle��

Factors Affecting Cutting Effiency of Bur��

• Clearance angle, rake angle and blade angle:

Clearance angle reduces the friction between cutting edge and the

work. It also prevents the bur from digging excessively into

the tooth structure. But an increase in rake angle decreases the blade angle which in turn decreases the bulk of bur blade

• End-cutting or side-cutting bur: According to particular task, choice of bur can be end cutting, side cutting or combination of both.

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• For example, it is preferred to make entry to enamel by end cutting bur, while for making preparation outline, use side cutting bur.

Neck diameter of bur:

If neck diameter of bur is large, it may interfere with accessibility and visibility. But if diameter is too short, it will make bur unable to resist the lateral forces.�

Linear surface speed: Within the limit, faster the speed of cutting instrument, faster is the abrasive action and more effient is the tooth cutting instrument. Bur speed should be increased in limits because with ultrahigh speed,

centrifugal force comes into the play.�

• Application of load: Load is force exerted by operator on tool head. Normally for high speed instruments, load should range between 60 and 120 g and for low rotational speeds, it should range between 1000 and 1500 g.
• Cutting effiency decreases when load is applied. Thre is increase in temperature at work face which results in greater wear and tear of handpiece bearings.

• Concentricity and runout: Th average clinically acceptable runout is 0.023 mm. Increase in runout causes increase in vibrations of the bur and excessive removal of tooth structure.
• Lubrication: Lubricant/coolant applied to tooth and bur during cutting increases the cutting effiency and decreases the rise in temperature during cutting. Absence of coolant can result in increase in surface temperature which may produce deleterious effcts on pulp.

• Heat treatment of bur: Heat treatment of bur preserves the cutting edges and increases shelf life of the bur.
• Number of blades: Usually a bur has 6 to 8 number of blades. Decrease in number of blades reduces the cutting effiency but causes faster clearance of debris because of larger chip space
• Visual contact with bur head: For effient tooth cutting, it is mandatory to maintain visual contact with bur head while working

• ABRASIVE INSTRUMENTS

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The head of these instruments consists of small angular particles of a hard substance held in a matrix of softer material called as the binder.

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Diffrent materials used for a binder are ceramic, metal, rubber, shellac, etc.����

• Abrasive instruments are generally grouped as diamond or other instruments.
• Diamond instruments -long life and great effectiveness in cutting enamel and dentin.

• Abrasive instruments can be divided into:�• Diamond abrasives�• Other abrasives.��

• Diamond Abrasive Instruments��1. A metal blank.�2. Powdered diamond abrasive�3. metallic bonding material that holds the diamond
• powder onto the blank ��

• • Coarse grit diamonds burs (125–150 µ particle size)�• Medium grit diamond burs (88–125 µ particle size)�• Fine grit diamond burs (60–74 µ particle size)�• Very fie grit diamond burs (38–44 µ particle size).��

Cutting mechanism

Abrasive cutting

• Individual diamond particles have very sharp edges, are randomly oriented on the surface and tend to have large negative rake angles.

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• Diamond instruments cutting ductile materials (dentin):

When diamond instruments are used to cut ductile materials, some material is removed as debris, but much material flows laterally around the cutting point and is left as a ridge of deformed material on the surface

• Repeated deformation work hardens the distorted material until irregular portions become brittle, break off and are removed.

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Diamond instruments cutting brittle materials (enamel):

• Diamonds cut brittle materials by a different mechanism.
• Most cutting results from tensile fractures that produce a series of subsurface cracks

Factors Inflencing the Abrasive Effiency and Effectiveness Hardness�

1. Size of the abrasive particle

• Larger the particle size, more deeper is the penetration on the surface of the work, hence rapid removal of the material occurs.

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2. Shape of the particle

• Should be irregular in shape for greater efficiency.

• Irregular particles – sharp edge

• So cuts better than round smooth or cuboidal particles which have a flat edge.

3. Density of abrasive particles

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• Refers to the no. of abrasive particles per unit area.

• High density : closely spaced

• Low density : widely spaced

• Therefore, greater force will be exerted on each particle with low density when the particles are widely spaced increasing grinding efficiency.

• Coarse grit have low density compared to fine grit.

4. Hardness of abrasive particles

• To be effective, hardness of abrasive particle should be greater than that of the work.

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5. Clogging of the abrasive surface

• Clogging of debris between the spaces of the abrasive particles affects grinding because this partially blocks the penetration of the abrasive particles into the surface.

• Clogging is enhanced when particles are close together.

• Use of coolant washes away the debris and prevent clogging.

• Speed and Pressure

• Usual cause of failure of abrasive instruments is when excessive pressure is applied onto them to increase cutting efficiency at inadequate speeds.

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• This results in loss of diamonds decreasing their cutting efficiency. 

Hazards with Rotary Instruments

• Pulpal Precautions

Injury to the pulp caused by:

• Mechanical vibration

• Heat generation

• Desiccation of the dentin

• Transection of the odontoblastic process.

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• The Pulpal sequelae, take 2 weeks to 6 months, depending on degree of trauma.

• The remaining tissue is effective in protecting the pulp in proportion to the square of its thickness.

•  Heat is produced by:

• Steel burs than carbide burs

• Tools plugged with debris

• When used without a coolant, diamond abrasives > carbide burs.

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Air-water spray must be used as

• Acts as a coolant

• Moisten the tissues, lubricates

• Cleans and cools the cutting tool thus increasing tool life

• Clear the operating site

• Soft Tissue Precautions

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• Injury to lips, tongue and cheek.

• Rubber dam used to isolate soft tissues

• Use good accessibility and visibility to the operative field

• Patient instructed not to make sudden movements.

• If accident occurs, control haemorrhage with pressure pack

• 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.

• Eye Precautions

• Use of protective eye wear

• Eye damage from airborne particles

• High volume evacuation is advised

Ear Precautions

• High pitched sound by some air-turbine handpieces at high speeds.

• Potential damage to hearing depends on: • Intensity or loudness (decibels- db) • Frequency (cps) • Duration of the noise • Susceptibility of the individual

• Earplugs, sound proof rooms with absorbing materials on walls and floor

• Anti-noise devices can be used to cancel the unwanted sounds as well.

• Inhalational Precautions

• 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 vapours.

• Vapours 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 vapours.

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