Rotary Cutting Instruments 443wy

ROTARY CUTTING INSTRUMENTS

1ST

SHAMNAH C USMAN,BDS BATCH

Introduction A group of instruments that turns on an axis

to perform cutting, abrading, burnishing, finishing or polishing tooth tissues or restoration

Speed ranges Low/ conventional speed

Below 6000 rpm

High/intermediate speed

6000 – 100,000 rpm

Ultra/ super speed

Above 100,000 rpm

Low speed – excavating caries with round

burs, refining cavity preparations, polishing procedures High speed – cavity preparation, finishing procedure such as placement of retentive groves and bevels ; preferred when vision is poor, more positive sense of touch required or both üWater coolant system üFiber optic lighting üFriction grip locking system 

Ultra speed – bulk reduction, obtaining

outline form, remove metal restoration

Common design charecteristics 1.Shank 2.Neck 3.Head   Head – working part of instrument, cutting edge

perform desired shaping of tooth structure. Instruments are bladed or abrasive  Neck – connects shank to head, transmit rotational and translational forces to head Ø Tapers from shank diameter , dimension a compromise between need for large CS (provide strength) & small CS (improve access and visibility)

1. 

Shank – part that fits into the handpiece,

accepts rotary movement ,controls alignment and concentricity of instrument 1.Straight handpiece shanks 2.Latch type handpiece shank 3.Friction grip handpiece shank

Dental burs Burs – all rotary instruments that have

bladed cutting heads Can be steel bur or tungsten carbide burs Steel burs cut human dentin at low speeds, but dulls rapidly at higher speeds or when cutting enamel Carbide burs (1947) performs better. Have heads of cemented carbide in which microscopic WC particles are held in a matrix of cobalt or nickel Modern burs – combination of the two ; freedom in design and economical, but steel necks bends easily causing vibration

Classification Mode of attachment – latch type, friction

grip type Composition – stainless steel, carbide burr or combination Length of head – long, short, regular Use – cutting , finishing , polishing Shape – round, inverted cone ,pear shaped ,tapering fissure, straight fissure, end cutting 

Round bur: caries removal, initial tooth

preparation, extension, placing retentive grooves Inverted cone: wall angulation, creating undercuts, smoothening floors Pear shaped bur: class I cavity for gold foil Straight fissure: amalgam preparation Tapering fissure: inlay and crown End cutting bur: preparation apically without axial reduction

Modifications In bur design Reduced use of crosscuts Extended heads on fissure burs Rounding of sharp tip angles – Markley &

Sockwell 

Design of a dental bur

Blade - projection on bur head Flute/chip space – depressed areas in

between the blades Rake angle – angle between rake face and radial line Land – plane surface immediately following cutting edge 

Factors affecting cutting efficiency of bur Rake angle Spiral angle and cross cuts Concentricity and runout Neck diameter No of teeth/blades

Abrasive instruments Head consists of small angular particles of

hard substance embedded in a soft binder(ceramic, metal, shellac, rubber) 1.Diamond abrasive 2.Other abrasives – boron carbide, pumice,Al2O3, garnet

Diamond abrasives Deposited by Electroplating, sintering or

microbrazing Classification 1.Coarse grit(125 – 150u) 2.Medium grit(88- 125) 3.Fine grit(60 – 74u) 4.Very fine grit(38 – 44u)

Burs, Discs, Mandrel, Stones, and Wheels

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Other abrasive instruments Moulded abrasive instrument – heads that

manufactured by pressing a uniform mixture of abrasive and matrix around roughened end of shank; points and stones; finishing &polishing Coated abrasive instrument – disks that have a thin layer of abrasive cemented to a flexible backing ;surface contour, finishing 

Different methods of cutting Cutting effectiveness – rate of removal of

tooth structure in mm/min or mg/sec Cutting efficiency – percentage of enegy producing cutting

Hazards Pulpal precautions Soft tissue precautions Eye Ear Inhalation precaution 

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