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SPEEDS IN DENTISTRY INTRODUCTION The term “ROTARY” when applied to tooth cutting instruments describes a group of instruments that turn on an axis to perform any kind of work which in dentistry extends from cutting, abrading, finishing or polishing of tooth or any restorative material. Archaeological evidence suggests dental treatment dating back to 5000 B.C. Early instruments were activated by either the operator’s hands or feet. Early drills were powered mostly by the operator’s fingers. Dr Jonathan Taft (1868) said in his book, “Textbook of Operative Dentistry” that “hand cutting instruments were of good steel, well wrought and thoroughly tempered. Every process in the manufacturing process should be most perfectly executed so as to ensure an edge that will cut not only but also enamel (which is the hardest animal substance).” These instruments were heavily handled and as wide as 0.25 inch at the cutting edge. Taft suggested that “a heavy instrument with a sharp point and a lateral curve is often efficient in opening up cavities and cutting down strong projections of enamel.” Therefore it can be assumed that only very large carious lesions can permit the entry or give access to such instruments. Thus it can be said that conservative dentistry was not practiced at that time. All actions from cutting walls to refining the cavity were done by hand instruments. Also for access to proximal regions, separators were used and for a bulky instrument, enough separation was required that caused deleterious effects to the tooth and supporting tissues. Therefore the development of efficient cutting instruments was mandatory. The first rotary instruments developed were drills or bur heads that could be twisted in the fingers for a crude cutting or abrading action. The first rotary instruments used for cutting were drills or bur heads that could be twisted in the fingers for a crude cutting or abrading action. Taft described them as “bur drills”. These simple rotary instruments were capable of a very limited lateral cutting action. Their diameters ranged from 1/32 to 1/5 of an inch and were used for 1


cavity preparations, providing a more regular and precise orifice than was formerly possible. The Scranton drill was made which was an improvement on previous bur drills since it could be rotated in either direction. Modifications thus made at that time resulted in number of hand drills: 1. SCRANTON’S DRILL: could be rotated in either direction. 2. DRILL RING: could be adapted to the middle finger with a socket that fitted against the palm of the hand. 3. CHEVALIER’S DRILL STOCK: bur could be in various directions used similar to an egg beater. 4. MERRY’S DRILL STOCK: had a flexible cable. Earlier these drills were called as drill stocks, bur chucks or bit holders. These were the forerunners modern “Dental Handpiece”. HANDPIECE: is a device used for holding rotating instruments, transmitting power to them and for positioning them intraorally. The modern handpiece came into existence after pioneering work was done by Morrison in 1871 when he adapted the dental foot engine from the Singer Sewing Machine. This addition was important since it had a power source other than the operator. In 1883 an electric dental engine was linked to the handpiece by a flexible cable arm. In 1910 a belt driven handpiece was made. In 1950 the first air turbine was introduced. LANDMARKS IN EVOLUTION OR ROTARY INSTRUMENTATION 1860 to 1870

-

Hand powered bur drills

1871

-

Dental foot engine attached

1883

-

Electric dental engine

1950

-

First air turbine.

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Although by 1910 the necessary speed was achieved but the required cutting efficiency was not available. Steel burs of that time could not efficiently cut enamel even though speed was increased, it only resulted in heat production and instrument wear. Therefore diamond cutting instruments were developed in 1947. These burs worked very nicely at 15000 rpm. By 1960 the speed increased from 10000 to 60000 rpm. Major developments that improved the efficiency of rotary instrumentation: 1) Contra angling. 2) Development of air powered turbines from previously used water powered turbines. ADVANCEMENT IN MODERN ROTARY INSTRUMENTATION 1942

-

Introduction of Diamond points

1947

-

Introduction of Carbide Burs

1950

-

Ball Bearing Handpiece developed

1953

-

Fluid Turbine Type Handpiece by Nelson

(50,000 rpm)

1954

-

Air driven Handpiece

(150000 rpm)

-

Contra Angle Handpiece

1957

-

Increased speed of Air Turbine Friction Grip

(300000 rpm)

1960

-

Air Turbine

(600000 rpm)

# 1n 1953 ultrasonic method was developed for removing hard tissue with a frequency of 15000 to 30000 cycles per second.

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CLASSIFICATION OF SPEED 1. According to CHARBENEAU: a) Conventional or Low speed

-

below

10000 rpm

b) Increased or High speed

-

10000 to 150000 rpm

c) Ultra Speed

-

Above

150000 rpm

2. According to STURDEVANT: a) Low or Slow Speeds

(below 12000 rpm)

b) Medium or Intermediate Speeds

(12000 to 200000 rpm)

c) High or Ultrahigh Speeds

(above 200000)

3. According to MARZOUK: a) Ultra Low Speed

(300 - 3000 rpm)

b) Low Speed

(3000 – 6000 rpm)

c) Medium High Speed

(20000 – 45000 rpm)

d) High Speed

(45000 – 100000 rpm)

e) Ultra High Speed

(100000 rpm & more)

CHARACTERISTICS OF ROTARY INSTRUMENTATION: 1. Speed 2. Pressure 3. Heat production 4. Vibration 5. Patient reaction 6. Operator fatigue 7. Sources of Power 8. Instrument Design

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1. SPEED: Speed refers to revolutions per minute Or Surface feet per unit time of contact the tool has with the work to be cut. # according to industrial investigation, maximum cutting efficiency of a tool of uniform width ranges between 5000-6000 surface feet per minute. 2. PRESSURE: Mathematically,

pressure = force / unit area

Force: It is the gripping of the handpiece and subsequent transmission to the tooth. Area: Area of bur in contact with tooth surface. Pressure is inversely proportional to Area. Pressure x Area = Constant Therefore a smaller tool will apply greater pressure. This implies that if pressure kept constant for a larger tool, to remove as much tooth structure with a smaller one, the force has to be increased. Therefore for best efficiency and convenience of the operator, a smaller tool with increased revolutions is best i.e operator will have to use minimum force. As speed keeps on increasing pressure required keeps on reducing. 1. Low Speed

-

2 to 5 pounds

2. High Speed

-

1 pounds

3. Ultra High Speed

-

0.0625 to 0.25 pounds ( 1 to 4 ounces)

3. HEAT PRODUCTION: Heat is directly proportional to the

a) Pressure b) RPM c) Area of tooth in contact

Pulp damage occurs at

130 F (54.4 C)

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Inflammatory changes at

113F (45.0 C)

# Therefore a coolant is required for all practical purposes involving high speeds. 4. VIBRATION: It leads to

a) Patient discomfort b) Excessive wear of instruments c) Destructive reaction in the tooth and the supporting tissues

Deleterious effects of vibrations are due to: 1) Amplitude: Frequency is inversely proportional to amplitude. Therefore an increase in one will cause a decrease in the other. Amplitude causes greater harm when it is high. This implies that speed should be increased to in order to reduce amplitude. At 6000 rpm,

vibration = 100 s-1 (most annoying to the patient).

At 100000 rpm,

vibration = 1600 s-1(imperceptible to the patient). (Human perception ends at 1300 s-1).

Thus, high speeds are best for patient comfort since amplitude is lesser and greater the frequency is imperceptible to the patient. 2) Undesirable modulating frequency: Old and poorly maintained equipment will have a fundamental vibration when in rotation. A superimposed vibration caused due to eccentricity and wear & tear of the individual components of the equipment will produce a modulated frequency which will have deleterious effects. 5. PATIENT REACTION: Patient reaction consists primarily of heat production, sensation to vibrations and number of visits. 6. OPERATOR FATIGUE: Causes :

a) Duration of procedure. b) Vibrations produced.

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c) Force needed to control the rotating instrument. d) Lack of patient cooperation. Remedies :

a) High speed instrumentation. b) Proper balancing of handpieces (contraangling) c) Reduction of weight of handpiece minimizes forces needed

to control it. 7. SOURCE OF POWER: Depending upon the source of power:

a) Air turbine b) Water driven c) Belt driven

8. INSTRUMENT DESIGN: For rotary instrumentation requirements are: I)

Handpiece: A device used for holding rotating instruments, transmitting power and positioning them.

II)

Cutting Tool: It can be a bur, stone, rubber cup or rubber disc.

I) HANDPIECE: Classification of Handpieces based on DESIGN: a) Straight b) Right angled c) Contra angled Classification of Handpieces based on source of POWER: a) Belt drive b) Gear driven c) Water driven d) Air driven e) Micromotors: Air motor

(rotary vane & swash plate type)

Electric motor

(D.C motor & Electric induction motor)

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Classification of Handpieces based on RETENTION OF CUTTING TOOL: a) Screw-In Type b) Latch Type c) Friction Grip Type (wear & tear is the least) Classification of Handpieces based on DESIGN: a) STRAIGHT HANDPIECE: Simplest and most stable but not used intraorally because of access problems. It is used primarily in the laboratory because of less wear & tear. b) RIGHT ANGLED HANDPIECE: Bur is placed almost 90 degree to the handpiece, hence it is difficult to handle. While in use it causes a displacing force which is 90 degree to the axis of rotation. The bur is offset from the handpiece axis, forming a lever arm and leading to instability. c) CONTRA ANGLED HANDPIECE: Principle of contraangling introduces a second angle into the handpiece to return the tip of the bur to the long axis of the handpiece. This removes the lever arm of consequent rotation of the handpiece.

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Engine driven instruments can be used in three types of contra-angle handpieces: i)

FULL ROTATING HAND PIECE: It can be latch or friction grip

type and requires that instruments mounted should be used only in a straight line. For curved canals handpieces with NiTi files should be used e.g. Medidanta/Micro Mega MM 324 reduction gear handpiece, Quantec ETMElectric Torque Motor. Some have torque and speed control facility. Some also have the provision to stall and ‘reverse� if torque is exceeded. The Tri Auto-ZX works in gets activated only in moist canals and stops as soon as the file is withdrawn. It has a built in apex locator as well. ii)

RECIPROCATING HANDPIECE: e.g Giromatic Handpiece, M4

Safety Handpiece. GIROMATIC HANDPIECE accepts only latch type instruments. It gives a quarter turn motion @ 3000 times per minute. M4 SAFETY HANDPIECE gives a 30 degree reciprocating action and accepts regular hand files. ENDO-GRIPPER HANDPIECE has a 10:1 gear ratio and a 45 degree turning motion. It uses regular hand instruments. iii)

VERTICAL STROKE HANDPIECE: It is driven by air or electricity

that delivers a vertical stroke ranging from 0.3 to 1mm. The more freely the instrument moves in the canal, longer is the stroke. It has a quarter turn reciprocating motion. It stalls if the canal is too tight.

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MICROMOTORS: (500 to 100000 rpm) Micromotors are slow speed motors are used for removal of soft caries, finishing and polishing. Advantages: 1) High torque 2) Does not stall because of good torque at low speed It is of two types: I) Air Motors II) Electric Micromotors I) AIR MOTORS: Two types: 1) Rotary Vane Type Motor: A number of sliding vanes hold the central core to the cylinder wall when air is forced into the chamber at high pressure. Expansion of air within the chamber will drive it towards the low pressure side where is vented out of the system. Advantage: 1) Runs smoothly. 2) Develops considerable torque. Disadvantage: Wear & Tear is high on the sealing edges of the vane. 2) Swash Plate Type Drive Air Motor: It operates by a series of pistons pressing sequentially against a disc which is offset against its axis of rotation. Pistons move in tandem sequentially, i.e. as one gets over with its movement and the other piston starts off. The rotation of the disc operates a rotary valve which feeds air to the pistons sequentially. Advantage: High speed Disadvantage: It is very noisy.

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II) ELECTRIC MICROMOTORS: Two types: 1) D.C Motors: These are D.C motors designed with an armature coil within a permanent magnet. Performance depends upon design and power of the field magnets and number of armature coils. Speed can be altered by varying distance between the coil and magnet. More the coils, smoother it is. Electric control system facilitates operation and control of speed. Resistance while cutting causes the speed to decrease and thus to compensate for this decrease, voltage is increased by a stabilizer leading to an increase in speed. Advantage:

1) Speed 2) Great torque control

Disadvantage:

1) Complexity of the overall drive mechanism. 2) Entire apparatus can not be sterilized

2) Electric Induction Motors: (40000 rpm) Motor uses a series of overlapping fields which are energized sequentially around the armature. The iron armature contains copper rods within which eddy currents create the magnetic fields and these react with the magnetic field produced from the armature coil which is in motion. Advantage: they are maintenance free except for the bearings which need to be changed # Both motors require air cooling as heat generation is high. COUPLINGS: These are used to connect air turbines and Micromotors to the hoses of the instrument delivery units. Two types: 1) 2 Hole Connector

–

1 for Water (smaller bore) 1 for Air

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(larger bore)


2) Mid-West 4 Hole Connector

–

1 for Drive Air In 1 for Exhaust Drive Air Out 1 for Coolant Water In 1 for Coolant Air In

E – FITTING / COUPLING It allows interchange of handpieces between motors of different manufacturers. It is standardized, reliable and an effective method of transferring the drive to the handpiece. CONTROL OF SPEED & TORQUE Operating speed can be optimized by correct selection of handpieces and corresponding gear ratio. Most common method for gearing handpieces is by the use of an Epicyclic Ball Race Gear System which is incorporated into the shank of the handpiece. It can either increase or decrease the speed of rotation depending upon whichever way it is mounted. It is a modification of a ball race bearing. Advantage: 1) Relatively smooth and quiet in operation. 2) High torque can be transmitted without slipping of the ball bearing. 3) Easy to clean and lubricate. # Latest modification is Epicyclic Gear Boxes with toothed gears. REDUCTION GEAR HANDPIECES Reduction gear handpieces reduce the speed of the drive while increasing the torque. They are necessary for large diameter instruments like bristle brushes and prophylaxis heads.

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COLOUR CODING: It indicates relative gear ratio of each component. Head of handpiece is marked with a colored dot or ring and the handpiece shank is marked with one or more colored rings. Blue - No change in speed. Green - Speed reduction. Red

- Speed increase.

# Two or more rings indicate a large change. SPECIALIST HANDPIECES: 1) Endodontic Handpieces: They reciprocate, oscillate or do both. 2) For Oral Surgery:

(a) Fast rotary vane motors for cutting impacted teeth. (b) Extremely slow speeds for bone cutting & drilling. (c) Highly geared and slow handpieces for implants.

CRITERIA TO BE EVALUATED WHILE USING HANDPIECES: 1) Friction:

Present in the gears, ball bearings etc.

2) Torque:

Ability to withstand lateral pressure on the revolving tool without

decreasing its speed or reducing its cutting efficiency. It is dependant on the type of bearing used and amount of energy supplied. 3) Vibration: It causes patient discomfort, operator fatigue and excessive wear & tear. II) CUTTING TOOL: These include dental burs and dental stones. - A dental bur is applied to all rotary cutting instruments that have bladed heads. - Stones are used for abrading. Burs are used for cutting and abrading. They are designed to operate at a minimum speed and for a specific type of handpiece. Optimum speed is dependant on the nature of the cutting material and also the diameter of the instrument.

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Earliest burs were hand made and operated by fingers. With advancement burs were made with steel although they wore out rapidly. In 1947, carbide burs and diamond points were introduced which proved to be a very great success. CLASSIFICATION OF DENTAL BURS I) Classification based upon Composition II) Classification based upon the Shape of the Bur III) Classification based upon the Length of the Head IV) Classification based upon Function

I) Classification based upon COMPOSITION: 1) Steel Burs: Steel burs are manufactured from a steel blank. Steel is hardened and tempered until Vicker’s hardness number is approximately 800. Advantage: Better for smoothening of the cavity, making retention grooves and in laboratory procedures. Disadvantage: Due to the hardness of enamel and heat produced while cutting enamel, they get dulled, fracture, discolor and become ineffective. Steel burs are only effective for cutting dentine. 2) Tungsten Carbide Burs: Tungsten Carbide Burs are produced by a product of powder metallurgy, a process of alloying in which complete fusion of the constituents does not occur. Tungsten Carbide powder is mixed with powdered cobalt under pressure and heated in a vacuum. A blank is formed and cut accordingly. Sometimes only the cutting head is formed with Tungsten Carbide or welded to a steel blank. A 5-10% mixture of Cobalt and Tungsten Carbide increases strength. Its Vicker’s hardness number is 1650-1700. # Carbide burs do not last long because they are brittle and easily brake at low speeds.

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# Carbide burs are better for end cutting procedures, produce lower heat and have more blade edges per diameter for cutting. 3) Diamond Points: These fall under the category of abrasives. # Diamond points are more effective for both intracoronal and extracoronal tooth preparations, beveling enamel margins on tooth preparations and enameloplasty.

II) Classification based upon the SHAPE OF THE BUR: 1. Round Bur (001) 2. Wheel Bur 3. Inverted Cone Bur (010) 4. Plain Cylindrical Fissure Bur 5. Cross Cut Cylindrical Fissure Bur 6. Plain Tapered Fissured Bur 7. Cross Cut Tapered Fissured Bur 8. Pear Shaped Bur 9. End Cutting Bur 10. Round Nose Fissured Bur III) Classification based upon the LENGTH OF THE HEAD: 1. Long 2. Short (pedominiature) 3. Regular IV) Classification based upon FUNCTION: 1. Cutting Burs. 2. Polishing & Finishing Burs.

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BASIC DESIGN OF THE BUR: It has three basic parts: I) Shank: Shank is the portion that fits into the handpiece and accepts rotary motion from the handpiece (Sturdevant). It has three designs types: 1) Straight:

for straight handpiece for finishing and polishing

2) Latch type:

shorter overall, improves access to posteriors. A Latch type

bur fits into a D-shaped socket, for use at low / medium speed as wobble is controllable at such a speed. 3) Friction grip: for high speed, shorter in length than latch- type and has better access in posterior regions. II) Neck or Shaft: intermediate portion that connects the head and the shank. Functions: 1) Transmit of rotation from shank to the head. 2) Tapering provides visibility of the cutting head. 3) Provides freedom of movement to the head. III) Head: It is the working end of the instrument. Head is designed in two ways1) Bladed ends 2) Abrasive ends GENERAL DESIGN OF A BUR 1) BUR TOOTH: It terminates in the cutting edge or blade. It has two faces:

(a) Leading Edge / Tooth Face: Face that faces the tooth. (b) Trailing Edge / Back / Flank: Face that is away from tooth

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2) RAKE ANGLE: Rake angle is the angle that the face of the bur tooth makes with the radial line from the center of the bur to the blade. It can be

Negative: if face / leading edge is beyond the radial line. Positive: if the radial line leads the face. “0”

: if radial line and tooth face coincide.

3) LAND: Plane surface immediately following the cutting edge. 4) CLEARANCE ANGLE: Clearance Angle is the angle between the back of the tooth and the work. It provides clearance between the work and the cutting edge to prevent the tooth back from rubbing on the work. 5) TOOTH ANGLE: It is the angle between the face and back. If land is present, it is measured between the face and land. 6) FLUTE / CHIP SPACE: It is the space between successive teeth. Number of teeth in dental cutting burs is 6 – 8.

BUR CLASSIFICATION SYSTEM: According to Federation Dentaire Internationale [FDI] and International Standards Organization [ISO], The name of a bur has

(i) Shape name (ii) Size - (1/10thof a mm). E.g: Round 010 means 010 x 1/10 = 1 mm (head).

1)

Shapes: round, inverted cone, pear shaped, straight fissure etc.

2)

Sizes : S.S White Company gave 9 shapes and 11 sizes Cross - Cut version = add 500 to the original bur no. e.g. No. 57 cross-cut is no. 557 Prefix of 900 for end cutting burs.

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FACTORS INFLUENCING CUTTING EFFICIENCY OF BURS : Influence of design and manufacturing a) Rake Angle b) Clearance Angle c) No: of teeth & their distribution d) Run-out e) Finish of the flutes f) Heat treatment g) Design of the flute ends h) Bur diameter i) Depth of engagement j) Influence of load k) Influence of speed A) RAKE ANGLE: Rake angle is the angle that the face of the bur tooth makes with the radial line from the center of the bur to the blade. It can be

Negative: if face / leading edge is beyond the radial line Positive: if the radial line leads the face. “0�

: if radial line and tooth face coincide

The Cutting efficiency is more for positive rake angle than for a negative angle. However, with a negative rake angle the cut chips move away from the blade angle and often fracture into small dust or bits. Where as in a positive rake angle the chips are larger and tend to clog the chip space or flute.

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Positive rake angles can’t be used with steel burs since bulk of the bur tooth gets decreased, increasing possibility of the bur tooth getting deformed and fractured. Tungsten carbide burs are more suitable for this feature. B) CLEARANCE ANGLE: Clearance Angle is the angle between the back of the tooth and the work. It provides clearance between the work and the cutting edge to prevent the tooth back from rubbing on the work. A large clearance angle will prevent premature dulling of the bur tooth. C) NUMBER OF TEETH / BLADES & THEIR DISTRIBUTION: Usual number of blades is 6-8. Lesser the blades, magnitude of forces at each blade and the size of the chip removed increases. Lesser the teeth, lesser will be the clogging tendency. Also lesser the blades, lesser is the temperature rise seen because a cut chip takes away some of the heat. Burs with very large number of blades (up to 40) are designed for polishing since they will cut very fine chips of material. D) RUN-OUT: Run-Out refers to the eccentricity or maximum displacement of the bur head from its axis of rotation while the bur turns. Clinically acceptable run out is = 0.023mm. Run out depends upon the precision of the handpiece. A newer handpiece will have lesser vibration and play between its gears. Run-out will be greater for a longer bur than a shorter bur. Run out causes vibration, unequal cutting, heat generation and patient discomfort. E) FINISH OF THE FLUTES: When bur flutes are cut from a metal blank, the cutter passes over the blank producing flutes. As the number of passes over the flute increases, the flute keeps on getting refined and the efficiency also increases. A bur that has had six passes with a cutter is the most refined and efficient.

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F) HEAT TREATMENT: It is used to harden a bur a bur. It preserves the cutting edge for longer duration. G) DESIGN OF FLUTE ENDS: Flute ends are cut in two different styles: a) Revelation Cut: Flutes come together at two junctions near a diametrical edge. It has greater cutting efficiency in direct cutting only. b) Star Cut:

Flute ends come together in a common junction at the

axis of the bur. H) BUR DIAMETER: It follows that because the length of the cut is constant the volume of the material removed will vary directly with bur diameter as will as the torque. I) DEPTH OF ENGAGEMENT: As the depth of engagement is decreased, the force intensity on each small portion of the bur still cutting is correspondingly increased and accordingly the average per flute revolution should also be increased. This increase is so great that the volume of a material removed by a shallow cut exceeds that of deeper cuts. J) INFLUENCE OF LOAD: Load signifies the force exerted by the operator on the bur head. It is related to the rotational speed of the bur. 1. Low Speed

-

2 to 5 pounds

2. High Speed

-

1 pounds

3. Ultra High Speed

-

0.0625 to 0.25 pounds ( 1 to 4 ounces)

K) INFLUENCE OF SPEED: Rate of cutting increases with the rotational speed but this increase is not proportional. It has been found that at a speed of 150000 rpm, the time required for the removal of the same weight of tooth structure is very nearly the same as at still higher speeds. However, a minimum rotation speed

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for a given load below which the tool will not cut. Greater the load, lower is the minimum rotation speed. MODIFICATIONS IN BUR DESIGNS 1. Smaller bur heads are being used as they are more effective. 2. Increased effectiveness of carbide burs at high speeds. LATEST TRENDS IN BUD DESIGNS 1. REDUCED USE OF CROSS CUT BURS: Crosscuts are needed on fissure burs to obtain adequate cutting effectiveness at low speeds, but at high speeds they are not needed. As cross cut burs used at high speeds tend to produce unduly rough surfaces. 2. EXTENDED HEADS ON FISSURE BURS: Extended head lengths for carbide fissure burs help in reaching deeper regions. Carbide burs can be used at high but only with an extended head. (5000-6000 rpm) 3. ROUNDING OF SHARP TIP ANGLES: Bur heads with rounded corners result in lower stresses in restored teeth, enhance the strength of the tooth by preserving dentine and facilitate in adaptation of restorative materials. ADDITIONAL FEATURES IN HEAD DESIGN These features include: 1. Head Length: It is determined according to area of use. 2. Taper Angle: Like the head length it is determined according to function. 3. Neck Diameter: Too small a diameter will result in a weak instrument and too large a diameter will cause visibility problems. # as length or diameter of head of a bur increases, moment arm exerted by lateral forces also increases, and the neck diameter needs to be larger. 4. Spiral Angle: For efficient high speed cutting the spiral angles are reduced. 5. Crosscutting: In order to increase cutting pressure from rotation of the blade and perpendicular pressure holding the blade edge against the tooth,

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the length of blade length in contact with tooth needs to be decreased. Crosscutting decreases the length and also provides space for the chips to fly off. 6. Flutes: These are depressed areas present between the blades. 7. Concentricity: It is the direct measurement of the symmetry of the bur head. Its a measure if all blades of a bur circumscribe the same circle or not 8. Runout: It is a dynamic test that measures the accuracy with which all blade tips pass through a single point when the instrument is rotated. DENTAL ABRASIVES STONES: Abrasive particles are held together by means of a binder substance which can be ceramic (for diamond chips), metallic, rubber or shellac. The binder is impregnated throughout with abrasive particles of a certain grade so that even after wear and tear the instrument stays even. Abrasive particles should be spaced so that debris does not clog the flutes. According to the abrasive particles, dental stones can be classified as follows: 1. Diamond Points

-

most efficient

2. Carbides

-

Silicon carbide(carborundum) or Boron carbide

3. Sand

-

Forms of quartz

4. Aluminum Oxide

-

Natural or pure aluminum oxide

5. Garnet (reddish)

-

Used for polishing and finishing

6. Quartz (white)

-

Same as garnet

7. Pumice

-

Formed by crushing volcanic glass

8. Cuttlebone

-

Derived from cuttlefish used for finishing

Abrasive instruments can either be Molded Abrasive instruments or Coated Abrasive Instruments. 1. Molded Abrasive instruments: manufactured by molding or pressing a uniform mixture of abrasive and matrix around the roughened end of the shank, or cementing a premolded head to the shank

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2. Coated Abrasive Instruments: have a thin layer of abrasive coated onto a flexible backing. They are used primarily for finishing or polishing. Factors influencing the abrasive efficiency of dental stones: 1. Irregularity in shape of abrasive particles 2. Hardness of the Abrasive material 3. Impact strength of the abrasive material 4. Size of the abrasive particles 5. Pressure and RPM DIAMOND ABRASIVE INSTRUMENTS These were introduced in 1942. The instrument consists of three parts; metal blank, powdered diamond abrasive and a metallic bonding material that holds the diamond powder onto the blank. The metal blank is similar to the regular bur since it has a head, neck and shank. Diamond and diamond powder used for these instruments may be synthetic or natural in origin. Diamonds are generally attached to the metal by electroplating a layer of metal on the blank while holding the diamonds in place. Diamond points conform to all shapes and sizes. There are over 200 shapes and sizes currently in use. Diamond points are classified according to the diamond particle size used on them: 1. Coarse

-

125 to 150 µm

2. Medium

-

88 to 125 µm

3. Fine

-

60 to 74 µm

4. Very Fine

-

32 to 44 µm

5. Super Fine

-

10 to 38 µm

Fine versions are used primarily for polishing and finishing. # Diamond instruments have a very long life until the binder gives way. This occurs due to excessive pressure applied while cutting. The heat produced due to

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excessive pressure causes the binder or electroplated metal as in this case to disintegrate and exposing the underlying metal blank to be exposed. INDICATIONS FOR DIAMOND ABRASIVES: 1) For tooth preparation for bonded restorations since they produce a rougher surface. 2) Most efficient to cut brittle materials like enamel but not so efficient for dentine since they cause more plastic deformation. 3) For finishing and polishing. CUTTING MECHANISMS Cutting Effectiveness: It is the rate of tooth structure removal (mm/min or mg/ sec. Cutting Efficiency:

It is the percentage of energy actually producing

cutting. Cutting is of two types: Bladed cutting and Abrasive cutting. 1. Bladed Cutting:Bladed cutting includes both rotary and hand instrument cutting. Rotary cutting causes Brittle and Ductile fracture. Brittle Fracture is associated with crack production due to tensile loading at higher speeds. Ductile fracture involves plastic deformation of material usually proceeding shear at low speeds. As the blades work along the work, they cause shearing of the segments and accumulation along the rake face until they fracture. 2. Abrasive Cutting: It is affected by the properties of the abrasive like hardness, size and distribution. While using diamond abrasives, most of the material cut will be removed as chips but some material will flow laterally around the cutting point and be left as a ridge of deformed material on the surface. Continuous deformation will ultimately lead to fracture of chips and subsequent removal.

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SONICS AND ULTRASONICS Ultrasonic refers anything that moves faster than the speed of sound. Ultrasonics Endodontics is based on a system in which sound as an energy source activates an endodontic file resulting in three dimensional activation of the file in the surrounding medium. 1. ULTRASONICS -

operate at 25 to 30khz -

Due to the alternating current passing

through the ultrasonic tip, two types of effects are seen Two types:

a) Magnetostrictive – The pattern of vibration of the tip is elliptical in all directions. It requires a cooling system due to large heat production b) Piezoelectric

– The pattern of vibration of the

tip is linear or back and forth. It does not require a cooling system. The action of ultrasonic is attributed to two features: Acoustic Streaming & Cavitation. Acoustic Streaming: due to the movement of the ultrasonic tip of the instrument, circular fluid movement is created in the fluid around the tip. Cavitation: while the instrument oscillates in a liquid it creates negative and positive pressure areas. When tensile strength of fluid is exceeded during positive phase of, a cavity is formed in the negative phase. In the next positive phase the cavity implodes with greats force like bubbles bursting resulting release of energy 2. SONICS

-

operate at 2 to 3 khz

STERLIZATION & INFECTION CONROL HANDPIECE STERILIZATION According to the Food and Drug Administration (FDA) & American Dental Association (ADA) recommended that dental handpiece must be sterilized after each use. The recommended method of sterilization for handpieces is steam under pressure-Autoclave.

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• STEAM JET AUTOCLAVE: this works on the principle of steam under pressure. Temperature 134 to 138 oC Time 3 minutes Pressure 32 psi • KAVOKLAVE 2100: based on the principle of steam under pressure • HISTRON N1100: It is a new compact ultraviolet dental handpiece sterilizer. It sterilizes 50 times higher than regular sterilizers Time 60 seconds Source UV radiation of 10000 rpm Handpiece sterilization has been considered to be a potential source of cross infection during dental treatment and accordingly sterilization between has been strongly recommended. Since the handpiece comes in contact with soft tissues, blood and saliva causing internal and external contamination, sterilization is very essential. DENTAL BURS STERILIZATION TYPE OF BUR STERILIZATION METHOD (PREFERED & ACCEPTED) 1) Carbon Steel • Dry Heat Oven • Chemical vapor • Ethylene oxide 2) Tungsten carbide • Dry Heat Oven • Ethylene oxide 3) Steel Burs

• Ethylene oxide

RECOMMENDATION

Steam autoclave (NOT)

Steam autoclave (NOT PREFERED) Chemical vapor (NOT PREFERED) Steam autoclave & Chemical vapor acceptable (BUT NOT PREFERED)

• Dry Heat Oven • Chemical vapor BIOLOGICAL CONSIDERATIONS Both the patient and the doctor are vulnerable to risk during dental procedures. Therefore precautions should be taken for the benefit of both of these groups of people.These is:

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1. Pulpal Precautions: Pulp is sensitive to vibration, heat, desiccation and loss of tubular fluid. Heat and desiccation can lead to pulpal abscess and death of pulp. These sequelae can take from 2 weeks to 6 months depending upon the degree of insult. A young pulp is more prone to injury since pulp chambers are large. Enamel and dentine are good insulators of heat. Remaining tissue is effective in protecting the pulp in proportion to the square of its thickness. To protect the pulp from thermal insult, air and water spray is required. Water is much more effective as it has higher heat capacity and carries away more heat. Air is better for finishing procedures and slower speeds. Air water combination is best. 2. Soft Tissue Precautions: Lips, tongue, cheeks, gingiva and mucosa are frequent areas of injury. Therefore precautions like adequate retraction, visibility and sharp reflexes of the dentist are required. Rubber dam, cheek retractors, mouth mirrors, evacuation tips etc should be used. While using air turbine it should be kept in mind that the bur does not stop rotating immediately after the foot control is released. Large discs should be used with great care. 3. Eye Precautions: The patient, operator and the assistant should always wear protective eye gear during rotary instrumentation since there is risk of injury from aerosol, particles of old restorations, bacteria and tooth structure. Protective eye gear (with special filters like UV filters) should be worn to prevent injury from light from Curing units and LASERS. 4. Ear Precautions: The dental team is most prone to this kind of injury because of continuous exposure to high pitched noise arising from air turbines. It depends upon the intensity, duration and frequency of noise. Auditory Threshold: Minimum level of sound before the ear can detect it. Temporary Threshold Shift: When subjected to a loud

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noise, the threshold increases as a protective mechanism. Permanent Threshold Shift: Continuous or permanent exposure results in a continuous shift. Air turbines produce noise up to 70 to 94 db at high frequencies. Noise levels in excess of 75 db in frequency ranges of 1000 to 8000 cycles per second may cause hearing damage. Protective measures are required when the noise levels reaches 85db. Intermittent use of handpieces is recommended so that the ear gets time to recuperate. Handpiece use should not exceed 30 minutes 5. Inhalation Precautions: Disposable masks should be worn to protect from aerosol of water, microorganisms, tooth debris and/or restorative materials. Aerosols also contain amalgam and composite particles. Mercury is the major constituent of amalgam vapors. It has been observed that aerosol dissemination occurs as far as 10 feet from the operating site.

REFERENCES 1. STURDEVANT’S ART & SCIENCE OF OPERATIVE DENTISTRY, 4TH EDITION 2. OPERATIVE DENTISTRY – MARZOUK 3. PRINCIPLES

&

PRACTICE

OF

OPERATIVE

DENTISTRY

CHARBENEAU 4. A PRACTICAL GUIDE TO TECHNOLOGY IN DENTISTRY 5. OPERATIVE DENTISTRY BY WILLIAM GILMORE. 6. COHEN 8TH EDITION 7. INGLE 5TH EDITION 8. D C N A --- MORDEN ENDODONTIC PRACTICE :2004; VOL28 9. CARRANZA’S CLINICAL PERIODONTOLOGY 9TH EDITION.

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