cutting instruments

7/28/2019 Cutting Instruments

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CUTTINGINSTRUMENTSPRESENTED BY:

PAYAL GUPTA (64)3rd prof.


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CUTTING INSTRUMENTS:instrument(s) : a tool or implement, especially one used fordelicate or scientific work.(e.g., knife).


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Cutting Instrument : an instrument used to cut, cleave, orplane the walls of a cavity preparation; the blade ends in a sharp, bevelededge.Materials used in the manufacture of hand instruments:Carbon steel : alloy made by addition of small percentage of carbon (0.5-1.5%) to iron.Other ingredients, Mn (0.2%), Si (0.2%), rest is Fe (98.4-98.6%)Melting point: 1500-1600 degree C.Stainless steel: it is composed of Carbon(6-10%), Cr (18%) and Fe(81-81.4%)Heat treatments of steel :There are two methods,Hardening heat treatment : steel is heated to 1500 degree F (815 degree C)in oxygen free enviornment, then quenched in oil. It makes alloy harder butalso makes steel brittle.


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Tempering heat treatment :steel is reheated for one hour at 350 degree F(176 degree C) and thenquenched in oil. This relieves the strain and increases the toughness.Methods of sterilization:Sporicidal cold disinfectionBoilingSteam under pressure( autoclave)Chemical vaporHot air (dry heat)Methods to prevent discoloration, rust and corrosion caused bysterilization:Electroplate the instruments ( used by manufacturers)Use of rust inhibitors (soluble alkaline compounds)remove the instruments promptly at the end of the recommendedsterilizing period, dry them thoroughly

BLACK is credited with the first acceptable nomenclature for andclassification of hand instruments.


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CLASSIFICATION:Cutting:1. Excavator

Ordinary hatchets Hoes Angle formers spoons2. Chisels Straight chisels Curved chisels Binangle chisels Enamel hachets Gingival marginal trimmers

Non cutting :1. Amalgam condensers2. Mirrors3. Explorers4. probes

Others:1. Knives2. Files3. Sclares4. carvers


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INSTRUMENT DESIGN:Most hand instruments are compsed of three parts:1. Handle2. Shank3. blade

BLADE:The blades are of many shapes and sizes depending on the function theyare to perform.


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Some instruments have a blade on both ends of the handle and areknown as double ended instruments.

When instrument blade is on one side only so known as single endedinstrument.

DOUBLE-ENDED INSTRUMENT

SINGLE-ENDED INSTRUMENT


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Handle:Early hand instruments had handles of quite large diameterIt is not always conductive to delicate manipulation.Now handles with small diameter (5.5mm) and which are light are used.They are commonly eight sided and knurled to facilitate control.Shanks: it serve to connect the handle to the working ends of theinstruments.They are normally smooth, round and tapered.They often have one or more bends to avoid instrument having atendency to twist in use when force is applied.INSTRUMENT SHANK ANGLES:Black classification based on number of shank angles as:1. Mon-angle (one)2. Bin-angle (two)3. Triple-angle (three) Instruments with short blades may be easily designed in mon-angleform. And with longer blades or more complex orientations may require twoor three angles in shank to bring the cutting edge near to long axis ofthe handle.


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INSTRUMENT NAMES:Blacks classification system by instrument name categorised instruments by:1. Function (e.g., scaler, excavator)2. Manner of use (e.g., hand condenser)3. Design of working end (e.g., spoon excavator, sickle scaler)4. Shape of the shank (e.g., mon-angle, bin-angle, contra-angle.These names were combined to form the complete description of theinstrument (e.g., bin-angle spoon excavator)OPERATIVE CUTTING INSTRUMENT FORMULAS: Describes the dimensions and angle of the working end. Placed on the handle using a code of three or four numbers separated bydashes. (e.g., 10-8.5-8-14). First number indicates the width of the blade or primary cutting edge in

tenths of a millimeter (0.1 mm) (e.g., 10=1mm). Second number indicates the primary cutting angle, measured from a lineparallel to the long axis of the instrument handle in clockwisecentigrades.The angle is expressed as a percent of 360 degrees (e.g., 85=85% x360degree = 306 degrees).

Instrument is positioned so that this number always exceeds 50.


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Third number indicates blade length in millimeters (e.g., 8=8mm).Fourth number indicates the blade angle, relative to the long axis of thehandle in clockwise centigrade (e.g., 14=50 degrees).

CUTTING INSTRUMENT BEVELS:Single bevelled instruments: instruments which have a bevel on only oneside.

Distally bevelled: if the bevel is on the side away from the shaft.Mesially bevelled: if the bevel is on the side towads the shaft.Right sided instruments: if the bevel is on the right side of theinstrument blade.Left sided instruments: if the bevel is on the left side of theinstrument blade.

Bibevelled instruments: which have two bevels on the opposite side of theblade which meet to form cutting edge.Triple-bevelled instruments: bevelling the blade laterally, together withthe end, forms three distinct cutting edges.Circumferentially bevelled instruments: these are usually double-planedinstruments where the blade is bevelled at all peripheries (e.g., spoonexcavator).


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HAND CUTTING INSTRUMENTS:1. CHISELS:- these are intended for planning and cleaving. characterised by a blade that terminates in cutting edge formed by one-sided bevel. Cutting edge is at right angle to the shaft.TYPES:a. Straight chisel: straight blade in line with the handle and shank. cuttingedge is on one side.b. Monangle chisel: blade is placed at an angle to the shaft. may be mesiallyor distally bevelled.c. Binangle chisel: have two angles between the shaft and blade. May bemesially or distally bevelled.d. Triple-angle chisels: have three angles in the shank and used to flattenthe pulpal floor. May be mesially or distally bevelled.


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CHISELSShank and the blade ofchisel is slightly curved(Wedelstaedt design).


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2. EXCAVATORS: used for excavation and removal of caries andsharpening or refinement of internal parts of cavity preparation.TYPES:a. Hatchet: has cutting edge of the blade directed in the same plane asthat of long axis of handle and is bibeveled.b. Hoe excavator: primary cutting edge of the blade perpendicular tothe axis of the handle.c. Spoon excavator: used for removing caries and carving amalgam ordirect wax patterns. Discoid excavator: have circular edge. Cleoid excavator: blade resembles the claw.3. OTHERS:a. KNIVES: nibs of these carry knife-edged faces onone of their sides only.a. FILES: nibs in files can be foot shaped, hatchetshaped or paralleogram shaped with serrations.


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HAND INSTRUMENT TECHNIQUES:There are four grasps used with hand instruments:1. Modified pen2. Inverted pen3. Palm-and thumb4. Modified palm-and thumbModified Pen: similar to that used in holding a pen. Pads of the thumb, index and middle fingers contact the instrument Tip of the ring finger (or little finger) is placed on nearby tooth surface

on the same arc as a rest. Palm of the hand is facing away from the operator. Pad of the middle finger is placed near the topside of the instrument. This finger working with wrist and forearm generates pressure on theblade for cutting and cleaving. This position appears comfortable as it limits the application of

pressure.Inverted Pen Grasp: Finger positions are same. Hand is rotated, so that the palm faces more towards the operator. Used mostly for tooth preparations employing the lingual approach on

anterior teeth.


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Palm-and-Thumb Grasp: similar to that used for holding a knifewhile paring the skin from an apple.Handle is placed in the palm of the hand and grasped by all thefingers. thumb is free of the instrument.Rest is provided by supporting the tip of the thumb on a nearby toothof the same arch or on a firm, stable structure.Modified Palm-and-Thumb Grasp: used when it is feasible to restthe thumb on the tooth being prepared or the adjacent tooth.Handle is held by all four fingers.Pads press the handle against the distal area of the palm and the padand first joint of the thumb.This grip fosters control against slippage.RESTS: a firm rest to steady the hand during operating procedures,rest is established by placing the ring (or little) finger on the tooth (otteeth) of the same arch and as close to the operating site as possible.Closer the rest , more reliable it is.GUARDS: are the hand instruments or other items, used to protect softtissues from contact with sharp cutting or abrasive instruments. Suchas interproximal wedges.


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SHARPENING OF HAND INSTRUMENTS:Instruments with dull cutting edges :-Causes more painProlong operating timeMore difficult to controlreduce quality and precision in tooth preparation.So all cutting instruments should be sharp.Resharpening requires little time.PRINCIPLES OF SHARPENING:1. Sharpen instruments only after they have been cleaned and sterilized.2. Establish the proper bevel angle (usually 45 degrees) and desired angle ofthe cutting edge to the blade before placing the instrument against thestone, and maintain these angles while sharpening.3. Use a light stroke or pressure against the stone to minimize frictionalheat.4. Use a rest or guide whenever possible.5. Remove as little metal from the blade as possible.6. Lightly hone the unbeveled side of the blade after sharpening, to removethe fine bur that may be created.7. After sharpening, resterlize the instruments along with other items onthe instrument tray setup.8. Keep the sharpening stones clean and free of metal cuttings.


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EQUIPMENTS USED FOR SHARPENING:1. Stationary Shapening Stones2. Mechanical Sharpeners3. Handpiece Sharpening StonesStationary Sharpening Stones: Most frequently used. Consists of a block or stick of abrasive material called a stone. Stone is supported on a firm surface, n instrument is oriented and heldby hand while being stroke against the stone surface. Stationary stones are often are often called oilstones. Application of a coating of oil to aid in sharpening process. Available in coarse, medium, or fine grit. Fine grit for final sharpening. Obtained in various shapes, including flat, grooved, cylindrical, andtapered. Flat stones are preferred for instruments with straight cutting edges. Other shapes are used for curved cutting edge. Four types of materials are in common use:a. Arkansas stone.b. Silicon carbide (SiC).c. Aluminum oxide.d. Diamond.


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MECHANICAL SHARPENERS:E.g. :The Rx Honing Machine (Rx Honing Machine corp, Mishawaka, IN).This instrument moves a hone in a reciprocating motion at a slow speedInstrument is held at the appropriate angulation and supported by a rest.

HANDPIECE SHARPENING STONES:Mounted SiC and aluminum oxide stones for use with straight and anglehandpieces.Available in various sizes and shapes.Speed is variable.Lacks rest or guide for the instrument.SHARPENING TECHNIQUES:MECHANICAL SHARPENING TECHIQUE:While using reciprocating honing sharpener (mechanical sharpeners) theblade should be placed against the steady rest and the proper angle of thecutting edge to the blade should be established before activating the motor.Used for instruments like: chisels, hatchets, hoes, angle former, or gingivalmargin trimmers.


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STATIONARY STONE SHARPENING TECHIQUES:Size of Arkansas oilstone should be at least 2 inches wide and 5 incheslong.Before using, a thin film of light oil should be placed on the workingsurface.Stone should be laid on a flat surface and should not tilt while sharpening.Instrument should be grasped with a pen grip, to avoid rotation or changein angles while sharpening.Third and fourth fingers are used as rest and guide along a flat surface toprevent rolling of instrument and to ensure stability.

SHARPNESS TEST:sharpness of a instrument can be tested :By lightly resting the cutting edge on a hard plastic surface.If cutting edge digs in during an attempt to slide the instrument forwardover the surface, the instrument is sharp.If it slides, the instrument is dull.Very light pressure is exerted in testing for sharpness.


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POWERED CUTTING EQUIPMENT:ROTARY CUTTING INSTRUMENT: it refers to a group of instruments that turnon an axis to perform a work such as cutting, abrading, burnishing,finishing or polishing tooth tissues or a restoration.HANDPIECE:Its a device for holding rotating instruments, transmitting power to them,and for positioning them intraorally.Dental handpiece are classified according to the driving mechanism:Gear Driven HandpieceWater Driven HandpieceBelt Driven HandpieceAir Driven Handpiece

HANDPIECE


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ROTARY SPEED RANGES:Measured in revolutions per minute (rpm).Three speed ranges are:Low or slow speeds (200,000 rpm)Most useful instruments are rotated at either low or high speed.Low Speed Range is used for:Cleaning teethOccasional caries excavationFinishing and polishing proceduresHigh Speed Range is used for:Tooth preparationRemoving old restoration


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COMMON DESIGN CHARACTERISTICS:1. Shank2. Neck3. HeadShank : shank is the part that fits into the handpiece, attempts the rotarymotion from the handpiece, and provides a bearing surface to controlthe alignment and concentricity ot the instrument. Its design and dimensions vary with the handpiece for which is intended. ADA Specification No. 23 for dental excavating burs includes five classesof instrument shank. Three of these are:

The straight handpiece shank The latch-type angle handpiece shank Friction-grip angle handpiece shank

Neck: the neck is the intermediate part of the instrument that connects theshank to the head. It tapers from the shank diameter to a smaller size adjacent to the head Size should be adjusted that it allows greatest possible visibility andmanipulation. Function: to transmit rotational and translational forces to the head.


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Head: is the working part of the instrument whose cutting edges performthe desired shaping of the tooth structure.Shows great variation in design and construction, size and shape.Based on head characteristics, the inatrument can be bladed or abrasive.DENTAL BURS:Term bur is applied to all rotary cutting instruments that have bladedcutting heads.Includes instruments intended for:Finishing metal restorationsSurgical removal of boneTooth preparation


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DENTAL BURS


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HISTORICAL DEVELOPMENT OF DENTAL BURS:Earliest burs were hand made and they were expensive and variable indimensions and performance.Machine-made were introduced in 1891.Early burs were made of steel.Steel burs: perform well, cutting human dentin at low speeds, but dullrapidly at higher speeds or when cutting enamel.Creates increased heat and vibration.

Carbide burs: were introduced in 1947.These are harder than steel burs.Less subject to dulling during cutting.these have heads of cemented carbide in which microscopic carbideparticles, usually tungsten carbide, are held together in a matrix of cobalt ornickel.


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EARLY STEEL BURS


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BUR CLASSIFICATION SYSTEM:According to :ShapessizesSHAPES:Bur shape refers to the contour of the head.Basic head shapes are:RoundInverted conePearStraight fissureTapered fissureRound Bur:It is spherical.Used for initial entry into the tooth, extension of the preparation,preparation of retention features and caries removal.Inverted Cone Bur:Portion of a rapidly tapered cone with apex of the cone directed towardsthe bur shank.Used for providing undercuts in tooth preparation.


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Pear-shaped Bur:Portion of a slightly tapered cone with the small end of cone directedtowards the bur shank.Used in class I tooth preparation.Long-length pear bur is advocated for tooth preparations for amalgam.Straight Fissure Bur:Is an elongated bur.It is advocated for amalgam tooth preparation.Tapered Fissure Bur:Portion of a slightly tapered cone with the small end of the cone directedaway from the bur shank.Used for tooth preparations for indirect restorations.SIZES:numbering system was developed by SS White Dental ManufacturingCompany in 1891.Burs were grouped by 9 shapes and 11 sizes.The and were added with the introduction of small sized instruments.Crosscut burs were indicated by adding 500 to the equivalent noncrosscut size, e.g., no. 56 bur with crosscuts was designated as 556.End cutting bur was indicated by adding 900 to the equivalent size.


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MODIFICATIONS IN BUR DESIGN:Modifications were seen because of introduction of high speed hand pieces.3 major changes included :Reduced use of crosscutsExtended heads on fissure bursRoundening of the sharp tip anglesCrosscuts are notches present on the blade of the instrument to obtainadequate cutting effectiveness at low speeds.These tend to produce unduly rough surfaces at high speeds. So crosscutburs designed to be used at low speeds have now been replaced byequivalent non crosscuts burs to be used at high speeds.Extended head lengths, two to three times the normal length of similardiameter, as such design is not practical at low speeds using a brittlematerial such as carbide.Roundening of sharp angles enhances the strength of the tooth and also theinstrument lasts longer because there are no sharp corners to chip and wear.


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DESIGN OF A DENTAL BUR:Cutting action of a bur takes place at the edge of the blade present on thebur head.Head consists of uniformly spaced blades with depressed areas in betweenthem known as flute or the chip spaces.Blades are usually 6 to 8 to 10.On finishing bur are usually 12 to 40.Greater the no. of blades, smoother is the cutting action at low speeds.Only one blade cuts effectively at one time at high speeds.BLADE:Its Projection on the bur head.Terminates in the cutting edge.It has two surfaces:

The blade face/rake face(towards the direction of cutting i.e. thesurface of bur blade on the leading edge).The blade back/flank/clearance face(is the surface of bur blade on thetrailing edge).

Three important angles:The rake angleThe edge angleClearance angle


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Rake Angle: it is the angle between the rake face and the radial line ( lineconnecting the centre of the bur and the b lade).It is the most important design characteristic of a bur blade.It can be positive, negative or a zero rake angle.It is negative when the rake face is ahead of the radial line.It is positive when the rake face trails the radial line.It is zero or the radial rake angle, seen when the rake face and radial linecoincide with each other.Land: the plane surface immediately following the cutting edge.Clearance Angle: it is the angle between the clearance face and the work.if land is present on the bur, the clearance angle is divided into two:

Primary clearance angle (i.e. angle between the land and the work)Secondary clearance angle (i.e. the angle between the clearanceface and the work)

Radial clearance: If the clearance face is curved.Blade angle/tooth angle: angle between the rake face and the clearance face.If land is present, it is the angle between the rake face and the land.


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ABRASIVE INSTRUMENTS:These are based on small, angular particles of a hard substance held in amatrix of softer material called as binder.Materials used for a binder are:

CeramicMetalRubberShellac

Abrasive instruments are grouped inDiamond abrasive instrumentsOther abrasives

Diamond instruments have clinical impact because of their long life andgreat effectiveness in cutting enamel and dentin.Other abrasives instruments includes use of:Silicon carbide

Boron carbideAluminum oxideGarnetSand

These are used for producing grinding wheels, discs or stones etc.


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DIAMOND ABRASIVE INSTRUMENTS:Introduced in United States in 1942.It consists of three parts:

A metal blankPowered diamond abrasiveA metallic bonding material

Metal blank : it resembles a bur without blades.It has same parts: head, neck and shank.Head of the blank is slightly smaller then the final dimensions of theinstrument head.Neck is normally a tapered section of reduced diameter that connects theshank to the head.Shank is similar to that of a bur.Abrasive Diamond:May be natural or synthetic.These are crushed to a powder of desired particles, in size and shape.Manufactured in multiple layers by electrodeposition, sintering ormicrobrazing.Provides a continous regeneration of the cutting surface as wear occurs.


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OTHER ABRASIVE INSTRUMENTS:They are divided in two distinct groups:Molded instrumentsCoated instrumentsMolded abrasive instruments:they have heads that are manufactured by molding or pressing a uniformmixture of abrasive and matrix around the roughened end of the shank orcementing a premolded head to the shank.They have much softer matrix and wear during use.Abrasive is distributed throughout the matrix so that new particles areexposed by the wear.Mounted heads are termed points and stones.Coated abrasive instruments: are mostly discs that have a thin layer ofabrasive cemented to a flexible backing.This type of construction allows the instrument to conform to the surfacecontour of a tooth or restoration.May be used in finishing and smoothening procedures of certain enamelwalls (and margins) of tooth preparation.


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CUTTING TECHIQUES:Cutting can be measured in terms of effectiveness.Cutting Effectiveness: is the rate tooth structure removal in mm/min ormg/s.Does not consider potential side effects, such as heat or noise.Cutting efficiency: is the percentage of energy actually producing cutting.Efficiency is reduced when energy is wasted as heat or noise.

Increase in rotary speed causes:HeatVibrationNoiseHeat causes pulpal injury.

Rotary instruments can cut by two mechanisms:Bladed cuttingAbrasive cutting


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Bladed Cutting:It refers to the use of bladed instrument or the use of any instrument in ablade like fashion.Blade must be sharp.Should have higher hardness and modulus of elasticity than the materialbeing cut.The sheared segment form a distorted layer.This move up the rake face until it breaks of until the blade disengagesfrom the surface as it rotates.These chips accumulate in the clearance space between the flutes untilthey are washed away or removed by centrifugal force.Abrasive Cutting: or grindingRefers to the use of bonded or coated abrasive instruments for removingsmall particles of the substrate.These instruments contain randomly arranged abrasive particles that arevery sharp.They have large negative rake angles.


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cutting instruments

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