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INTRODUCTION The removal and shaping of tooth structure are essential aspects of restorative dentistry. In order to perform the intricate and detailed procedures associated with operative dentistry, the dentist must have a complete knowledge of the purpose, availability and application of many instruments required. The early hand-operated instruments, with their large, heavy handles and inferior metal alloys in the blades, were cumbersome, awkward to use, and ineffective in many situations. Likewise, there was no uniformity of manufacture or nomenclature. Dr G V Black, among his many contributions to dentistry, is also credited with the first acceptable nomenclature for the classification of hand instruments.

Modern hand instruments, when properly used, produce beneficial results for both the operator and the patient. It should be noted that some of these results can only be achieved with hand instruments. The introduction of rotary powered cutting equipment was one of the truly major advances in dentistry. Modern high-speed equipment has eliminated the need for many hand instruments for tooth preparation. The need for a distinctive instrument in every single step of each treatment procedure is responsible for the huge armamentarium we have today. The judicious usage of 1


hand cutting instruments along with mechanical one’s is the key for a successful conservative therapy. The trend has been to put greater emphasis on efficiency while minimizing trauma during tooth tissue removal by rotary instruments.

HISTORY OF DENTAL INSTRUMENTS Prehistoric man used sharp pieces of flint for trephining holes in bones. Hippocrates in 350 B.C. described a drill driven by a cord wound around a shaft. Celsus (25 B.C. –50 A.D) described two kinds of drillers or “Terebra”. One with a guard to prevent it from sinking deep into the tissues and the other one was similar to a carpenter’s drill. In 2A.D. Cladius Galenius a celebrated physician reports of Archigenes an eminent surgeon of Asia minor and practicing in Rome successfully treated tooth ache by opening the tooth with a trephine. Galen (130 –200 A.D) modified Celsus’s “Terbra” and called it “Terebraabatista” or “Modiolus”. Lubrication was done with olive oil or milk or by dipping in cold water. Abulcasis (936 – 1013 A.D) described a boring instrument “Incisura”. Perre Fauchard “Father of Dentistry” in his book “The Chirurgien Dentiste “ in 1728 described the first dental rotary instrument of modern times. It was known as the “Bow Drill” could be rotated at 300rpm and was later on modified into the “Scranton’s drill” which could cut by rotating in either direction.

Scranton’s drill

In 1831 dental chair was introduced. In 1838 John Levis made a hand held drill. Dr. West Cott in 1846 used “Finger rings” with drills. Taft called them “Bur Drills”. ‘Chevalier drill stock” was hand powered like an egg-beater.

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Tomes in 1859 described three types of burs. 1.

Rose head: a short shank bur inserted in a crutch rotated between thumb and index finger supported at the base of the thumb.

2.

Long hand bur: teeth are cut for same distance along the shaft and it is mounted in a handle.

3.

Long steel shaft with too cutting blades.

Charles Merry in 1862 used a “Drill Stock” which had a flexible cable drive. George Fellows Harrington in 1865 used “Clock work drill” or “Harrington’s Erado” which is the first motor driven drill.

At first burs were hand cut and ground and were expensive.

America in 1860s began mass production of burs from carbon steel. The earliest burs had limited lateral and end cutting action. The diameter varied form 1/32” to 1/5”. These were particularly used for small and medium sized varieties. These carbon steel burs were called “Small milling cutters”.

In 1871 Morison’s foot engine was introduced. Rotation of cutting instrument was made possible by a long belt running over a series of pulleys to the back of a straight hand piece. When the angle hand piece was needed it would be attached to the shaft of the straight hand piece. A speed of 700 rpm was obtained.

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In 1873 Coxeter used an electric engine with a speed of 1000 rpm. This is the predecessor of the modern micromotor. This was held in hand and connected to a coil. The motor was open and the spindle of the motor was connected with the hand piece. In 1874 the electric motor hand piece was invented by S.S white and later he also pioneered the invention of various carbon steel burs and hand pieces. In 1883 rotary power from an electric engine was transferred to the straight hand piece by a belt that ran over a series of pulleys and a three-piece extension cord arm. A variable rheostat was used as a foot control. Rotary cutting instruments were inserted into the chucking mechanism at the front of the handpiece. The desired angle hand piece is attached to the front of the straight hand piece and a shaft and gears inside the angle section produce rotation of the working instrument.

In 1891 Edward G. Acheson an American invented and produced carborundum and carborundum tools were introduced. In 1901 hand piece with forward (clockwise) and reverse (anticlockwise) direction of rotation and burs for each type movement were brought into use. In 1910 Emile Huet a Belgian perfected an electric engine to give a speed of 10,000 rpm. In 1935 diamond abrasives were introduced and W.H Drendel introduced the process of galvanized bonding of diamond powder to copper blanks and used at a speed of 5,000 rpm. In 1947 Tungsten carbide was introduced and S.S White in 1948 made tungsten carbide burs which were used at a speed of 12,000 rpm. In 1949 Walsh and Symons used diamond points at a speed of 70,000 rpm.

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In 1950 ball bearings were used in contra angle handpieces. In 1951 air abrasive technique was introduced. In 1953 Nelson produced a Hydraulic driven turbine angle handpiece of speed, 60,000 rpm. In 1955 Page-chayes introduced first belt-driven angle handpiece to operate successfully at speeds over 100,000 rpm.

In 1955 Turbo-jet was designed as a compact mobile unit that required no outside plumbing or air connections. Only a source of electricity was need. A sound proof cabinet contained a motor, water pump, water reservoirs and necessary plumbing for water circulation. Water was conveyed to and from the hand piece by co-axial type plastic tubing. The small inner tube carried water under high pressure to rotate a turbine in the handpiece head and the larger outer tube returned the water to the reservoir for re circulation.

In 1960 ultrasonics were used for hard tooth structure removal. In 1961 air turbine straight handpiece was introduced. In 1962 air turbine angle handpiece with air bearings were introduced.

Most modern angled handpieces also include fireoptic lighting on the cutting

site. Borden Airotor handpiece

Contemporary Air-tubine handpiece

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Rapid improvements in technology have spawned several computerized devices for use in conservative dentistry such as 1. CAD/CAM system: They are (Computer aided design/computer aided manufacturing) laboratory based or chairside based (CEREC 1 & CEREC 2) which design and fabricate porcelain restorations. 2. Air Abrasive: Newer equipments have been introduced in the market utilizing this technology. 3. LASER: Various studies are still being done to utilize LASER in a very useful way without any inherent disadvantages.

CLASSIFICATION The variety and complexity of instruments used in operative dentistry make it necessary to classify them according to their purpose or function. 1. According to charbenaeu: Operative instruments can be conveniently classified into 6 categories. 1) Cutting instruments Hand- Hatchets, Chisels, Excavators, Others Rotary- Burs, Stones, Disks, Others 2) Condensing instruments Pluggers- Hand, Mechanical 3) Plastic instruments Spatulas, Carvers, Burnishers, Packing instruments 4) Finishing and polishing instruments Hand- Orange wood sticks, Polishing points, Finishing strips Rotary- Finishing burs, Mounted brushes, Mounted stones, Rubber cups, Impregnated disks and wheels 5) Isolation instruments Rubber dam frame, Clamps, forceps, punch, Saliva ejector Cotton roll holder, Evacuating tips and equipment 6) Miscellaneous instruments

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Mouth mirrors, Explorers, Probes, Scissors, Pliers, Others 2. According to Marzouk: Instruments for operative dentistry procedures can be generally classified as 1) Those used for exploration 2) Those used for removal of tooth structure. 3) Those used for restoration of teeth. 1. Those used for exploration A) Drying the area on the tooth: This necessitates the use of an 1) Air syringe. 2) Pair of tweezers (pliers) 3) Cotton pellets to dry the tooth. 4) Cotton rolls – to isolate the area around the tooth. B) Illuminate the area: A source of light could be either an overhead fixture supplying non-reflecting light or an intra-canal light. They can be 1. Battery operated lights. 2. Built in lights attached to dental unit. 3. Light attached to mirror or hand piece. Light can be introduced directly or indirectly by reflecting it on the filed via a mirror. C) To retract the soft tissues: 1. The hand mirror is used to move the tongue and cheek away. 2. Blunt plastic instruments may help in retraction. 3. Tongue depressor or retractors are sometimes helpful for this procedure. D) To probe the potential lesion: Explorers are used for this purpose. These are 5 types of explorers. 1) Straight explorer. 2) Right angled explorer( No. 6) 3) Arch explorer( No. 23 or shepherd’s crook/hook) 4) Interproximal explorer.(No.17)

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5) Cow horn explorer(No.2)

2. Those used for tooth structure removal: 1. Hand cutting instruments. 2. Rotary cutting and abrasive instruments. 3. Ultrasonic instruments 3. Those used for restoring: 1. Mixing instruments 2. Spatulas 3. Plastic instruments 4. Condensing instruments 5. Burnishing instruments 6. Carvers 7. Files 8. Knives 9. Finishing and polishing instruments. 3 .According to sturdevant, instruments are categorized as: i.

Hand 1. Cutting – excavators, chisels and others 2. Non-cutting – amalgam condensers, mirrors, explorers, probes.

ii.

Rotary or powered instruments.

4. According to alloy used for manufacturing:

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- Stainless steel, carbon steel, tungsten carbide - anodized titanium or plastic HAND CUTTING INSTRUMENTS: Modern hand instruments, when properly used, produce beneficial results for both the operator and the patient. Materials: Hand cutting instruments They are manufacture from two main materials. Carbon steel – is an Iron carbide binary alloy which contain less than 2.1% carbon. The major classes of carbon are based on three possible crystal structures that occur for iron-carbon alloys. 3. Ferritic , 2. Austenitic and 3. Martensite. The cutting edge carbon steel instruments are ordinarily martensitic because the high hardness of this structure allows the grinding of a sharp edge that is retained in use. Carbon steel is harder than stainless steel but when unprotected it will corrode Stainless steel - it is composed of carbon (0.2-1.2%), chromium 18% (12-30%) and Iron (81-81.4%). The resistance to passivating effect tarnish and corrosion is associated with passivating effect of chromium. There are three major types of stainless steelFerritic, martensitic and Austenitic. - Some are made with carbide inserts to provide more durable cutting edges. - Stainless steel remains bright under most conditions but loses a keen edge during carbide, while hard and weak resistance, is brittle and cannot be used in all designs. - Other alloys of nickel, cobalt, chromium like – Monel metal, Nichrome , Stellite and Tarno are used in the manufacture of hand instruments, but they are usually restricted to instruments other than those used for cutting tooth structures.

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Hardening and tempering heat treatments: To gain maximal benefits from carbon or stainless steel, the manufacturer must submit them to two heat treatments. The hardening heat treatment hardness the alloy, but also makes it brittle, especially when the carbon content is high. The tempering heat treatment relieves stains and increases toughness. Effects sterilization: Sterilizing carbon steel instruments by using 1. Sporicidal cold disinfection. 2. Boiling water 3. Steam under pressure (autoclave) Causes of discoloration. – Rust and Corrosion Methods of protection: 1) Electroplate the instrument and affords protection except on the blade, where use and sharpening remove the plating. 2) Use of rust inhibitors (soluble alkaline compounds). 3) Minimizing the effect of moisture is to remove the instruments promptly at the end of the recommended sterilizing period, dry them thoroughly and place them in the instrument cabinet. The boiling water or autoclave methods of sterilization do not produce discolouration rust or corrosion of stainless steel instruments. Prolong immersion in cold disinfectant solution may cause rust. Dry heat sterilizers do not rust and corrode carbon steel instruments, but the high heat may reduce the hardness of the alloy. Instrument categories: Hand instruments in the dental operatory may be categorized as 1) Cutting -Excavators, chisels and others. 2) Non cutting

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-Amalgam condensers, mirrors, explorers, probes. Excavators- may be further be divided in ordinary hatchets, Hoe, angle formers and Spoons. Chisels- primarily used for cutting Enamel and may be further divided into Straight, curved ,Binangle chisels, enamel hatchets and Gingival marginal trimmers. Other cutting instruments may be subdivided as knives, files, scalers and carvers. Design characteristics of Hand instruments: Most hand cutting instruments are composed of 3 parts. -Handle, Shank and Blade.

HANDLE or shaft is straight, usually without variations in size, serrated to increase friction for hand gripping and should be light and preferably small in diameter. In North America , most instruments are small in diameter(5.5mm) and light. They are commonly eight-sided and knurled to facilitate grip or control. In Europe the handles are often larger and taper in diameter. 11


SHANK is that which connect the shaft with the blade, is normally smooth, round and tapered. It is here where any angulation in the instrument can be placed and can have one or more bends to avoid the instrument having a tendency to twist in use when force is applied. Hand instruments should be balanced to allow for the concentration of force onto the blade without causingrotation of instruments in grasp. Balancing is accomplished by designing the angles of the shank so that the cutting edge of the blade lies within the projected diameter of the handle and co-incide with the projected axis of handle. For optimal antirotational design, the blade edge must not be off-axis by more than 1-2 mm. Instrument shank angle: The functional orientation and length of the blade determines the number of angles necessary in the shank to balance the instrument. G.V. Black classified instruments based on the number of shank angles as 1. Mono-angle (one) 2. Bin-angle (two) or 3. Triple-angle Instruments with longer shanks require 2 or 3 angles in the shank to bring the cutting edge near to the long axis of the handle such shanks are termed contra-angled. BLADE or working point is the part, which bears the cutting edge.

It begins at the

termination of the shank or at the last angle in the shank. Double-ended instruments have a blade on both ends of the handle. For non-cutting instruments the parts corresponding to the blade is termed – the nib. The end of the nib or the working surface is known as the – face. The blade or nib is connected to the handle by the shank. CUTTING EDGE is the working point of the instrument. It is usually in the form of a bevel with different shapes. BLADE ANGLE : is defined as the angle formed between the long axis of the blade and long axis of shaft.

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CUTTING EDGE ANGLE: it is defined as an angle between the margin of cutting edge and long axis of the shaft.

INSTRUMENT NOMENCLATURE In 1908 G.V. Black in his “Book of Operative Dentistry “Volume II, wrote about the classification, establishment of formula, nomenclature of various instruments used in operative dentistry. This is widely accepted even today and used. G.V. Black described a way to name dental instruments. 1) Order: The order denotes the purpose or function of the instrument. E.g. Excavator, scaler. 2) The sub-order denotes the position or manner of use of the instrument. E.g Push, pull. 3) The class describes the form of the blade. Eg. Hatchets, chisel. 4) Shape of the shank – subclass. Eg. Mon-angle, Binangle. Naming of instruments usually moves from subclass to order eg. Binangle hatchet push excavator. INSTRUMENT FORMULA Cutting instruments have formulas describing the dimensions and angles of working end. In order to describe the parts accurately, it is necessary to give three measurements all expressed in the metric system and are placed on the handle using a code of numbers separated by dashes or spaces. Eg. 10-8.5-8-14

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The basic instrument formula consist of three units whose measurements are based upon metric system. 1st number indicates: Width of the blade or primary cutting edge in tenths of a mm (0.1mm).

2nd number of indicates: the length of the blade in millimeters. The 3rd number indicates: -the angle the blade forms with the axis of the handle. This angle is expressed in “hundredths� of a circle or centigrade (in clockwise). For these measurements, the instrument is positioned so that its number is always 50 or less.

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When the cutting edge or face of an instrument is at an angle other than right angle to the length of the balde a fourth unit is added to basic unit formula. The 4th number indicates the: 4. angle formed between cutting edge and central axis of the shaft. It is placed in second position of the formula. It is measured from a line parallel to the long axis of instrument shaft in clockwise centigrade. The angle can be expressed as a percentage of 360 degrees. (eg. 85=85% X 360 degree= 306 degree).The instrument is positioned so that this number always exceeds 50.

Three unit

Four unit formula

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INSTRUMENT DESIGN The main principle of cutting with hand instruments is to concentrate forces on a very thin cross section of the instrument, at the cutting edge. Thus the thinner this cross section, the more the pressure is concentrated and more efficient is the instrument.

1. Direct cutting and lateral cutting instrument •

A direct cutting instrument is one in which the force is applied in the same plane as that of the blade and handle and is called a “single planed” instrument.

A lateral cutting instrument is one is which the force is applied at a right angle to the plane of the blade and handle and is called a “double-planed” instrument with a curved blade.

The cutting is done such that •

For direct cutting acts the non-beveled side of the blade should be in contact with the wall being shaved.

For lateral cutting acts, move the instrument in a scarping motion from the beveled side to the non-beveled side of the blade.

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Right and left side instruments •

Direct cutting instruments are made either right or left by placing a bevel on one side of the blade. The instrument must be made in pairs, having the bevels on opposite sides of the blades.

For determining whether the instrument has a right or left bevel, the primary cutting edge is held down and pointing away-if the bevel appears on the right side of the blade, it is the right side instrument of the pair and vice versa for left side instrument.

A ring on the shank identifies the right of the pair, or the letter ‘L’ or ‘R’ is added to the instrument formula.

2. Contra angling Instruments with longer blades or more complex orientations may require two or three angles in the shank to bring the cutting edge near to the long axis of the handle-such shanks are termed contrangle instruments. The instruments are designed to have one or more angles in the shank placing the working point within 3 mm from the axis of the handle. The advantages of contrangling are balance and also providing better access and clearer view of the field.

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3. Single bevelled instruments •

These are single planed with cutting edge at a right angle to the long axis of the shaft. If are bevelled on the side away from the shaft, they are called “distally bevelled” and if bevelled on the side of the blade towards the shaft, they are called “mesially bevelled” (i.e. reverse bevels). A ringe in the shank identifies reverse bevel.

If the shank angle is 12° or less, they are used in push and scraping motion and if exceeds 12°, used in pull (distally bevelled) and push (mesially bevelled) motions.

single-beveled hatchet 4. Bi bevelled instruments •

Only Hatchet and straight chisels can be bibivelled

The blade is equally bevelled on both sides; they cut by pushing them in the direction of the long axis of the blade.

Bibevelled ordinary hatchet

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5. Tribe bevelled instruments •

Beveling the blade laterally, together with the end, forms three distinct cutting edges, which afford an additional cutting potential. Eg. Angle former

Chisel with triple cutting edge 6. Circumferentially bevelled instruments •

It is done in double planed instruments where the blade is bevelled at all peripheries. eg.Spoon excavator

7. Single and Double ended instruments •

Single ended instruments are confined now to those instruments having only one specific function. They may be safer to use, but double sided instruments are efficient because they reduce the time of instrument exchange.

Double ended instruments have blades and shanks on both ends of the handle. The right instrument of the pair is on one end of the handle and the left is on the other end. Similar blades of different widths are placed on either end.

TYPES OF HAND CUTTING INSTRUMENTS FOR CUTTING Hand cutting instruments can be classified into 3 groups: 1. Excavators 2. Chisels 3. Special forms of chisels.

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1. Excavators They are desined for the excavation and removal of carious dentin andfor the shaping of internal parts of cavities. There are 5 types ; i. Ordinary hatchet ii. Hoe iii. Spoon excavator iv. Discoid v. Cleoid i. •

Ordinary hatchet:

Blade is bi-bevelled directed in the same plane as that of long axis of the handle.

Cut by push or pull motion, in the direction of the blade.

Used in anterior teeth for preparing retentive areas and sharpening internal line angles, particularly in preparations of direct gold restorations

ii. Hoe •

Primary cutting edge of the blade is perpendicular to the long axis of the handle.

Single planed instrument with four possible cutting moments vertical, pull (push), right and left.

Are distally or mesially bevelled and used for cutting mesial and distal walls of premolars and molars. 20


Hoe-pull(distal) •

Hoe-push(mesial)

Hoe excavator is commonly used for planning tooth preparation walls and forming line angles. It is used in class III and class V preparations for direct filling gold restorations.

Both hatchet and hoe are used to remove harder varieties of caries as well as to give form to the internal parts of the cavity preparation.

iii. Spoon excavators •

Are made in pairs with the blade of one curved to the rights and the other to the left.

Cutting edge is ground to as semi-circular circumferential bevel and sharpened to a thin edge.

Double planed instrument with lateral cuttings action

Bin-angled or triple angled for better accessibility.

Used for removal of decayed dentin, carving amalgam or direct was pattern.

Formula 12-8-12 R & L

iv.Discoid excavator •

Has a circular blade, with cutting edge extending around the periphery except where it is joined to the shank and with the blade placed at an angle with the shaft.

Double planed instrument with right or left cutting movement.

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v. Cleoid ( claw-like) excavator •

Similar to the spoon excavator except for the blade is pointed resembling a claw, hence the name “cleoid”

Double planed instrument with lateral cutting movement.

Discoid and cleoid excavator function similar to the spoon excavator and may be used to trim or burnish inlay – only

2. Chisels These are instruments designed after ordinary carpenters tools and are intended for cutting enamel. They are usually beveled on one side only and are of four types: i. Straight chisel •

A straight blade in line with the handle and shank

Cutting edge is on one side only, with the bevel of the blade running at a right angle to the shaft.

They are single planed instruments.

ii. Mono angle chisel Similar to straight chisel, except the blade is placed at an angle to the shaft with mesial or distal bevel.

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iii .Binangle chisel •

These have two angles in the shank, between the shaft and the blade.

The primary cutting edge is in a plane perpendicular to the axis of the handle and may have either a distal bevel or a mesial bevel. (reverse bevel)

The straight, mono angle and binangle chisels are used to cleave or split undermined enamel.

iv.Triple angle chisel •

Has three angles in its shank, used to flatten pulpal floors.

All the chisels are single planed instruments and posses vertical, right and left cutting movements, with mesial bevel chisel cutting in push motions and distal bevel cutting in pull motions.

3. .SPECIAL FORMS OF CHISELS These chisel are designed to perform specific functions. i. Enamel hatchet •

A chisel similar in design to the ordinary hatchet except for a large heavier blade, which is bevelled on one side.

The cutting edge is in a plane parallel to the axis of the handle with vertical, push, pull and lateral cutting movements.

They are designed as right or left type for use on opposite sides of the cavity and used for cutting enamel.

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They are used for splitting or cleaving undermined enamel in proximal cavities and on buccal and lingual walls , where chisels cannot be used.

Formula: 10-6-12 or 15-8-12.

ii. Gingival margin trimmer •

Similar in design to spoon excavators in both their curves and the dimensions of their blades.

It is a modified form of hatchet. Two distinct modifications of the basic hatchet design are observed 1. First, whereas the cutting edge of a hatchet is at right angle to the axis of the blade, the cutting edge of a Gingival margin trimmer is at an angle other than a right angle of the blade.

2. Second, while the hatchet has a straight blade, the blade of the gingival margin trimmer is curved.

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It is primarily lateral cutting instrument.

They are designed as right and left types and paired such that they form a mesial pair or distal pair.

Usually the instrument is used in lateral scrapping motion, the right instrument is used form right to left and the left instrument is used for operation left to right.

In distal gingival margin trimmer the cutting edge makes an acute angle with that edge of the blade furthest from the handle and the second number in the formula is 90 to 100.

In mesial gingival margin trimmer the cutting edge makes an acute angle with that edge of the blade nearer to the handle and the second number in the formula is 85 to 75.

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They are designed to a proper bevel in gingival enamel of proximocclusal preparations; 100/75 pairs are for inlay/onlay preparations with steep gingival bevels; 90/85 pairs are for amalgam preparations with gingival enamel bevels that decline gingivally only slightly.

Other use is rounding or beveling of the axiopulpal line of two surface preparations and placement of gingival lock in dentine.

Distal GMT 10-92-6-12 mesial GMT 10-80-6-12.

iii. Angle formers •

These instruments are made by grinding the bevel at an angle of 80˚ with the shaft, thus forming an acute angle with the long axis of the blade. It may be described as a combination of a chisel and GMT.

It is monoangled , available in pairs i.e. left and right.

Used in vertical, pull and push cutting motions.

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Used primarily for sharpening line angles and creating retentive features in dentin in preparation for gold restorations and for placing bevel on enamel margins.

Formula 7-80-21/2-9

iv. Wedelstaedt chisel •

Has the primary cutting edge in a plane perpendicular to the axis of the handle and may have either a distal bevel or a mesial bevel.

It is like a straight chisel, but with a slight vertical curvature in its shank.( the blade does not make a real angle with the shaft)

The blade with a distal bevel is designed to plane a wall that faces the blades inside surface and the mesial bevel is designed to plane a wall that faces the blade’s outside surface.

They are single planed instruments with three cutting motions; vertical , right and left.

Used mainly to plane Class II and V cavities with curved walls.

v. Off –set hatchet •

Similar to regular hatchet, except the whole bade is rotated a quarter of a turn forward or backward around its long axis.

They may be right or left

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Useful in creating shape specific angulation for cavity walls, especially in areas of difficult access.

vi. Triangular chisel •

The blade is triangular in shape with base of the triangle away from the shaft.

Has a terminal cutting edge like the straight chisel

vii. Hoe chisel •

Similar to a hoe excavator but has a sturdier blade.

Blade angle is more than 12.5°, distally bevelled and mesially bevelled are available.

viii. Jefferey Hatchets •

Similar to off-angle hatchets but have their blades more nearly at right angles to the shaft.

Used in preparation of maxillary anterior cavities form the lingual side of the teeth

Other cutting instruments: These are used for trimming restorative material rather for cutting tooth structure. Knife: used for trimming excess filling material on the gingival facial or lingual margins of proximal restoration or trimming and contouring the surface of class-V restoration. E.g. Blacks knives, wilson’s knife, stein’s knife.

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Files: can be used to trim excess filling material. They are particularly useful at the gingival margins. Blades of files are very thin and teeth on the cutting surface are short. The teeth of the instrument are designed to make the file either a push or a pull instrument. The discoid-cleiod instrument may be used to trim or burnish inlay- onlay margins. (It is principally used for carving occlusal anatomy).

HAND INSTRUMENTATION TECHNIQUE Most prior experiences in school that require digital expertise require only light forces of barely an ounce of pressure. Operative dentistry on other hand, requires controlled pressure of 6-8 pounds in some procedures. Moreover, these forces must be applied without tiring the hand. This is important so as to utilize the instrument to gain the maximum advantage out of it. It has been found that movement directed by shoulders and wrists are better oriented and can be given for a longer time than movements given by fingers which fatigue easily, thus leading to operator’s failure quickly. So fingers are used merely to grip the instrument and to prevent it from slipping and rotating. When force is applied along the long axis the force acts on tooth surface with fingers preventing slipping of the instrument. But if the force is applied at right angles to the tooth, but in the plane of the cutting edge, the resisting tooth structure tries to rotate the instrument. This is prevented by the thumb and the finger, which prevents rotation, thereby shearing of the tooth structure and allowing the instrument to move in the direction of the force. This is 29


of lever action type-I, where the mechanical advantages always less than 1. Hence the mechanical advantage and contra angling must be considered for a better orientation of force. Dental instrument grasps must be learned and used to achieve success in operatory. GRASPS A proper instrument grasp is essential for performing operative dental procedure. It provides for control of the instrument while allowing flexibility of motion and prĂŠcised adaptation. It aids in prevention of muscle fatigue to the fingers, hand and arm while also allowing application of controlled pressure to the instruments. The grasps are variable, non-specific and variable among operators. There are 4 types of grasps. 1. Modified pen grasp 2. Inverted pen grasp 3. Palm and thumb grasp 4. Modified palm and thumb grasp

The modified pen grasp offers the greatest control during the performance of procedures in the oral cavity. The instrument is held near the junction of the handle and the shank between the first finger and the thumb. The first finger rests on top of the instrument opposite the thumb in the underside. The pad of the second finger is placed on the top of the shank closer to the working end than the thumb and the finger. The handle is rested at the junction of the finger and the thumb.

The grasp should be firm and not rigid to allow for maximal

maneuverability of the instrument.

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Difference between pen grasp and modified pen grasp In the inverted pen grasp the finger positions are the same as for the modified pen grasp, but the palm faces upwards while the instruments is held between the first finger and the thumb and this is used for the maxillary arch, mainly the anterior teeth.

In the palm and thumb grasp the instrument is held in the palm of the hand with the first, the second, third and fourth fingers wrapped round it. The thumb remains free and serves as a rest. It is used only in anterior region owing to lack of tactile sense and flexibility of the movement.

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In the modified palm and thumb grasp, the instrument is held by the pulps of the first, second and third finger on one side and the mesial phalange of the fourth finger and with the tip of the resting on the adjacent tooth surface and very close to the cutting edge. Here the hand is only half closed and permits easy and free movement with greater precision of control. The modified pen and inverted pen grasps are practically universal. FINGER REST •

A finger rest or fulcrum is essential for the controlled movement of an instrument. It is the point of support from which the hand moves to activate the instrument and also provides control and prevents injury to the soft tissues from an instrument which inadvertently slips.

The third finger is the digit of choice for the finger rest. Additional support is achieved by the second finger, which rests against the third, and the fourth fingers and the entire hand moves as a unit. This additional support is applied when increased pressure is required.

Pressure placed on the fulcrum finger is usually directed to the working end of the instrument. Depending on the pressure needed on the working end of the instrument light or heavy pressure is applied on the finger rest. This creates a balance between the working end and the finger rest.

Wherever possible the finger rest is placed close to the area of operation. As it moves away from the working area precise manipulation and control becomes difficult.

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The finger rest is usually placed on the occlusal, facial or on the lingual surface. Tooth surface provides the most stable and firm support. Soft tissue is flexible and mobile and therefore not suitable as rests.

Rests can be Intraoral or extra-oral. Intraoral rest are1. Conventional 2. Cross-arch 3.

Opposite arch

4. Finger on finger Extraoral rests are – 1. Palm-up and 2. Palm down fulcrum, less commonly used.

Variations in finger rest placement may be necessary because of the position of the matrix and wedge, the absence of adjacent teeth, lack of access, or difficulty in attaining the desired angle f the instrument blade. SHARPENING HAND INSTRUMENTS: Instruments with dull cutting edges cause more pain, prolong operating time, are less controllable and reduce quality and precision in cavity preparation. It is essential therefore that all cutting edges be sharp. Resharpening requires little time and is very rewarding. The sharpening equipments include 1) Stationary sharpening stones. 2) Mechanical sharpeners. 3) Stones that are used in hand piece. 1.Stationary sharpening stones: •

The most frequently used sharpening equipment consists of a block or stick of abrasive material called a stone.

33


They are sometimes called as oil stones (because of the common practice of applying a coating of oil to them as an aid to the sharpening process).

The oil stones are available in coarse, medium, or fine grit. They can be obtained in a variety of shapes like -Flat , Grooved, Cylindrical and Tapered.

Types of materials are in common use for sharpening stones •

Arkansas stone (Semi-translucent, white or gray) - is naturally occurring mineral containing microcrystalline quartz and traditionally has been the preferred material for fine sharpening stones. It is semi-translucent, white or gray in color and hard enough to sharpen steel, but not carbide instruments.

Silicon carbide (Black or greenish black) - is widely used as an industrial abrasive. it is most commonly used material for grinding wheels and “sand papers” , as well for sharpening stones. It is hard enough to cut steel effectively, but not hard enough to sharpen carbide instruments.

Aluminium oxide- is increasingly used to manufacture sharpening stones. They are commonly produced in various textures from different particle sizes of abrasives.

Diamond – is the hardest available abrasive and most effective for cutting and sharpening hard materials. It is the only material routinely capable of sharpening carbide as well as steel.

2. Mechanical sharpeners: •

One type of mechanical sharpener is represented by the treatment honing machine. Basically this instrument moves hone in a reciprocating motion at a slow speed while the instrument is held at an appropriate angulation and supported by a rest.

Advantages include restoration of cutting edge more easily and in less time. This type of sharpener is very versatile.

3.Handpiece sharpening stones:

34


Mounted silicon carbide (SiC) and aluminium oxide stones for use with both straight and angle handpieces are available in a variety of sizes and shapes.

Because of their curved periphery, it is difficult to produce a flat surface using any of these instruments.

These stones may produce inconsistent results because of the speed variables and the usual lack of a rest or guide for the instrument.

Principles of sharpening: In use of any equipment there are several basic principles of sharpening that should be followed. 1) Sharpen instruments after they have been cleaned and sterilized. 2) Establish the proper bevel angel (usually 45°) and the desired angle of the cutting edge to the blade before placing the instrument against the stone and maintain these angles while sharpening. 3) Use light stroke or pressure against the stone to minimize frictional heat. 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 remove the fine bur that might be created. 7) After sharpening, resterilize the instrument along with other instruments on the tray setup. 8) Keep sharpening stones clean and free of metal cuttings.

TECHNIQUE OF SHARPENING 1. MANUAL METHOD It is a good technique, provides good control over the instrument less risk of removing excess metal but its time consuming. Usually Arkansas stone 1” x 2” is used. This is well

35


lubricated by sterile machine oil and stabilized on firm surface. The instrument is held and modified pen grasp is used with third and fourth finger providing rest. The correct bevel angulation is established and entire hand and arm is moved with the fingers holding the instrument firmly. After few times the blade is wiped well and checked and redone if necessary. Sometimes the instruments are held steadily while the stone is moved up and down.

2. MOUNTED STONE METHOD Cylindrical or conical mounted stones are used in a straight handpiece and instrument sharpened. The disadvantage is that it is difficult to control the angulation of the instrument against rotation of the stone.

3.MECHANICAL SHARPENERS Here lathe type mounted stones are used. It requires greater control and precision, has chance of removing more metal and change of angulation may occur.

36


Rotary type sharpener

4.SHARPENING MACHINE. -are available with moving sharpening stones in a reciprocating movement. The instrument to be sharpened is held fixed and supported by an angle guide that maintain the angle of the bevel.

Oscillating type sharpener Sharpness test: •

Sharpness of an instrument can be tested by lightly resting the cutting edge on a hard plastic surface. If the cutting edge digs in during an attempt to slide the instrument forward over the surface, the instrument is sharp. If it slides the instrument is dull.

Also by looking at cutting edge in a bright light; the presence of a “glint” indicates that the edge is dull or rounded.

A specially made sterilizable sharpness testing stick is also available.

STERILIZATION AND STORAGE: Sterilization in dental office can be accomplished by •

Autoclaving

Dry heat procedure

37


Ethylene oxide equipment

Chemical vapour sterilizers.

Boiling and chemical solution will not sterilize and should be considered only as a disinfection procedure. Advantages of Hand cutting instruments 1. Self-limiting in cutting 2. Can remove larger pieces of undermined enamel quickly, saving time and effort 3. No vibration or heat accompany cutting, making it painless 4. They are most effective means for precise intricate cutting 5. Create smoothest surface of all cutting instruments 6. Have longest life span of all cutting instruments.

ROTARY CUTTING INSTRUMENTS Historical Development •

The availability of some method of cutting and shaping of tooth structure is essential for the restoration of teeth. Although archeological evidence of dental treatment dates from as early as 5000 BC, little is known about the equipment and methods used then.

Early drills powered by hand. Much of the subsequent development leading to present powered cutting equipment can be seen as a search for improved sources of energy and means for holding and controlling the cutting instrument.

This has culminated in the use of replaceable bladed or abrasive instruments held in a rotary handpiece, usually powered by compressed air.

38


•

A handpiece is a device for holding rotating instruments, transmitting power to them, and for positioning them intraorally. Handpieces and associated cutting and polishing instruments developed as two basic types, straight and angle.

•

Most of the development of methods for preparing teeth has occurred within the last 100 years. Effective equipment for removal (or preparation) of enamel has been available only since 1947, when speeds of 10,000 rpm were first used, along with newly marketed carbide burs and diamond instruments.

•

Since 1953, continued improvements in the design and materials of construction for both handpieces and instruments have resulted in equipment that is efficient and sterilizable, much to the credit of manufacturers and the profession alike.

39


One of the most significant advancements was the introduction of the electric motor as a power source in 1874. It was incorporated into a dental unit in 1914. The initial handpiece equipment and operating speeds (maximum of 5000 rpm) remained virtually unchanged until 1946. •

The steel burs used at the time could not cut enamel effectively, even when applied with great force. With steel burs, increased speed and power resulted only in increased heat and instrument wear.

Further progress was delayed until the development of instruments that could cut enamel. Diamond cutting instruments were developed in Germany around 1935, but were scarce in the United States until after World War II.

In a 10-year period, starting in late 1946, cutting techniques were revolutionized. Diamond instruments and tungsten carbide burs capable of cutting enamel were produced commercially.

Both instruments performed best at the highest speeds available and that prompted the development of higher speed handpieces. Obtaining speeds of 10,000 to 15,000 rpm was a relatively simple matter of modifying existing equipment by enlarging the drive pulleys on the dental engine.

By 1950, speeds of 60,000 rpm and above had been attained by newly designed equipment employing speed-multiplying internal belt drives . They were found to be more effective for cutting tooth structure and for reducing perceived vibration.

The major breakthrough in the development of highspeed rotary equipment came with the introduction of contra-angled handpieces with internal turbine drives in the contra-angle head. Early units were water driven but subsequent units were air driven .

Although most current air-turbine handpieces have free-running speeds of approximately 300,000 rpm, the small size of the turbine in the head limits their power output. The speed can drop to 200,000 rpm or less, with small lateral workloads during cutting, and the handpiece may stall at moderate loads. This tendency to stall under high loads is an excellent safety feature for tooth preparation, since excessive pressure cannot be applied.

Air-driven handpieces continue to be the most popular type of handpiece equipment because of the overall simplicity of design, ease of control, versatility, and patient

40


acceptance. The external appearance of current handpieces is very similar to the earliest models. •

The low torque and power output of the contra-angle turbines made them unsuitable for some finishing and polishing techniques, where large heavy instruments are needed. The application of the turbine principle to the straight hand-piece eliminated the necessity of having an electric engine as part of a standard dental unit. The design of the straight handpiece turbine provided the desirable high torque for low-speed operation .

Increasing concern about patient-to-patient transfer of infectious agents has put emphasis on other aspects of handpiece performance. Recent advancements in both straight and angle handpieces allow repeated sterilization by several methods. However sterilization produces some damage to parts of the hand-piece, thus necessitating more frequent service and repair.

Other improvements of the angle handpiece include smaller head sizes, more torque, lower noise levels, and better chucking mechanisms. Since 1955, angle handpieces have had an air-water spray feature to provide cooling, cleansing, and improved visibility. Most modern-angled handpieces also include fiber-optic lighting of the cutting site.

CHARACTERISTICS OF ROTARY CUTTING INSTRUMENTS.

1. Rotary speed ranges An explanation of the term speed is needed for a more complete understanding of rotary instrumentation. Speed refers not only to the revolutions per minute but also to the surface feet per unit time of contact that the tool has with the work to be cut. Considering the essential requirements of operative dentistry it is necessary to assess the appropriate speed for each activity. Most manufacturers’ colour code handpiece. SPEED  Low speed i. 500 – 1000rpm 1000-25000rpm

HANDPIECE COLOUR Green band Blue band

41


 Intermediate high speed i. 20,000 – 80,000 rpm Red band ii. 20,000 – 1,20,000 rpm Orange band  Ultra high speed 2,50.000 – 4,00,000rpm Air turbine only is used According to Marzouk: Speeds are classified as a) Ultra low speed (300-3000rpm) b) Low speed (3000-6000rpm) c) High sped (20,000-45,000rpm) d) Ultra-high speed (1,00,000rpm) Some dental equipment can actually produce upto 5,00,000. According to Sturdevant, 3 speed ranges are generally recognized. 1) Low or slow speeds (below 12,000 rpm) 2) Medium or intermediate speed 12,000 to 2,00,000rpm. 3) High or ultra-high speeds above 2,00,000rpm. According to Charbeneau, speeds are – Conventional or low speed- below 10,000 rpm Increased or high speed- 10,000- 150,000 rpm Ultra speed- above 150,000 rpm Low speed Low-speed cutting is ineffective, time consuming and requires relatively heavy force application. This results in heat production at the operating site and produces vibrations of low frequency and high amplitude. The low speed range is used for cleaning teeth, occasional caries excavation and finishing and polishing procedures. 42


High speed At high speed, the surface speed needed for efficient cutting can be attained with smaller and versatile cutting instruments. This speed is used for 1) Tooth preparation 2) Removal of old restoration Other advantages are 1) Diamond and carbide cutting instruments remove tooth structure faster with less pressure vibration and heat generation. 2) The number of rotary cutting instruments need is reduced because smaller sizes are more universal in application. 3) The operator has better control and greater ease of operation. 4) Instruments last longer 5) Patients are generally less apprehensive because annoying vibrations and operating time are decreased. 6) Several teeth in the same arch can and should be treated all teeth same appointment. SPEED Low

High

    

USE ADVANTAGES Caries excavation  Better tactile sensation Cleaning teeth  Less chance for overheating Finishing and polishing procedures For tooth preparation  Efficient cutting with Remove old restorations smaller and versatile instrument  Faster, less load, vibration and heat generation  Better control and ease of operation  Several teeth can be treated in the same appointment  Instruments last longer  Patient comfortable.

43


2. Pressure(p) - is a resultant effect of two factors under the control of the operator i.e. P = F/A. •

Force (F) gripping of the handpiece and its positioning and application to the tooth.

Area (A) amount of surface area of the cutting tool in contact with the tooth surface during a cutting procedure.

It has been observed clinically for efficient cutting. i.

Low speed require 2-5 lbs Force

ii.

High speed require 1 lb Force

iii.

Ultra –high speed require 1-4 ounces Force

3.Heat production •

Heat production is directly proportional to Pressure (P), revolutions per minute (rpm) and area(a) of tooth in contact with the tool.

It has been shown that when the area of cutting tool is reduced but the speed of rotation is increased, it is an absolute necessary that coolants (copious stream of water or airwater spray) be employed to eliminate pulpal damage.

It has been shown that cutting dentin with no lubricant may result in a temperature rise at the surface of the tooth in contact with the bur of up to 136°C in only 2 seconds.

Using an air-water spray with a water flow rate of 35-50ml/min, the temperature rise can be limited to 20-30°C and water alone is more effective with a flow of 10ml/min, the temperature rise limited to 10°C. However combined air water spray is more effective.

4. Vibration: It is an annoying factor for the patient, causes fatigue for operator, excessive wear of instruments and a destructive reaction in the tools and supporting tissues. The equipment primarily the handpieces, various revolving cutting tools, the speed of rotation all contributes to the quantity and quality of vibration.

44


The deleterious effects of vibration are two fold in origin : 1. Amplitude 2. Undesirable modulating frequencies Minimizing or eliminating these factors can reduce the undesirable effects of vibration. 1. Amplitude •

A wave of vibration consists of frequency and amplitude. At low speed the amplitude is large but the frequency is small. At higher speeds the reverse is true.

The greater harm is caused by the amplitude; it is the factor most destructive to instruments and it not only causes the most apprehension in the patient but also the greatest fatigue for the dentist. By increasing the operating speed the amplitude and its effects are reduced as well as its sequelae.

Vibration waves are measured in cycles per second. It has been shown that rotation of approximately 6000 RPM sets up a fundamental vibrational wave of approximately 100 cycles per second. This has been demonstrated to be the range most annoying to the patient and the dentist.

As the RPMs are increased, the cycles per second of the fundamental vibration waves are increased until at 100,000 RPM there is an average vibration of 1600 cycles per second. In experiments conducted in vivo, it has been demonstrated that at a wave vibration over 1300 cycles per second, vibrations are practically imperceptible to the patient. This is due to the fact that stimulations occur during the refractory period of recovery of the perception mechanism. In other words, at vibration cycles of 1300 cycles/sec, and more, no apparent stimulation will be evident.

45


•

Thus it can be concluded that higher RPMs produce less amplitude and greater frequency of vibrations. As a result, perception will be lost in the ultra-high speed ranges of 100,000 RPM or more.

2.Undesirable modulating frequency •

The second deleterious effect of vibration is caused by improperly designed, or poorly maintained, equipment. Although there must be a fundamental vibrational wave, improper equipment use or care allows modulating frequencies to be established so that a series of vibrations (in different directions) are perceived by the patient and the dentist. The end result is again apprehension in the patient, fatigue for dentist and accelerated wear of cutting instruments.

•

The fundamental vibration wave is set up when the handpiece turbine is running. Each piece of the remaining attachment (handpiece) will vibrate, depending upon the amount of wear or eccentricity in. its moving parts. Each will set up a modulating frequency, or "overtone", accompanying the fundamental vibrational wave, so that the patient and the dentist are subjected not only to the basic wave but also to other accompanying vibrations. It should be the objective of the operator to eliminate these by having the equipment free from any such defects

5. Patient reaction 46


The factors that cause patient apprehension consist primarily of heat production, vibrational sensation, length of operating time, and number of visits. The proper understanding of the instrument being used and the speed at which it is being used allows the operator to counteract these potentially irritating stimuli.

The use of coolants, intermittent applications of a tool to the tooth; sharp instruments, etc., all aid greatly in minimizing both patient discomfort and unnecessary irritation to the oral structures. In instances where irritation is unavoidable (e.g., drilling pin channels in vital teeth), the patients should be forewarned and properly anesthetized.

6. Operator fatigue •

The major causes for fatigue are: duration of operation, vibration produced in the handpiece, forces needed to control the rotating instrument, apprehension on the part of the dentist regarding the possibility of producing a pulp exposure or injuring adjacent oral, intra- and paraoral tissues, and lack of patient cooperation.

High speed rotary instrumentation minimizes fatigue by decreasing both the vibrations and the time of the operation. Proper balancing (contrangling) of the handpiece and reduction of its weight will minimize the forces needed to control the instrument.

7. Sources of power The introduction in the 1950's of the air-turbine as a power source changed the shape of dental practice. The belt driven handpiece was rendered obsolete for operatory use. The air turbine remains the main power source.

8. Instrument design Instrument design for rotary instrumentation should be evaluated in two parameters. 1. The Handpiece – which holds and provide power for the cutting tool 2. The cutting tool- eg. Bur, stone , diamond etc.

47


HANDPIECE They are either high speed or slow speed i.

They are either straight, contra-angled or right angled at the working end and its use depend on the type of work.

ii.

The cutting tool is retained in the handpiece by 3 major type of attachment •

Screw in type (e.g. air motor handpiece straight)

Latch type (e.g. Micromotor contraangle handpiece)

Friction grip (e.g. Airotor handpiece)

iii.

They may have a fiber optic light attachment

iv.

Available in normal head (adult) size and miniature head (Paediatric) sizes.

The following criteria should be used in evaluating handpieces: •

Friction – will occur in moving parts of the handpiece especially turbine, hence heat generated should be prevented or counteracted. Hence handpiece are equipped with bearings, (ball bearings, glass or resin bearings).

Torque – is the ability of handpiece to withstand lateral pressure on the revolving tool without decreasing speed and cutting efficiency. It depends on the bearing and energy supplied. There was no significant difference in the applied force between plain and cross-cut

burs, cutting wet or cutting dry, but there was a significant difference between high and lower torque handpieces. The higher torque handpiece was used to a mean cutting force of 1.44 N and lower torque handpiece at 1.20 N. The overall generated mean force observed was 1.30 N. (Kanaan Elias et al) •

Vibration – unnecessary vibrations are deleterious, hence excessive wear of the turbine bearings is avoided by maintaining the handpiece according to the manufacturer’s instructions.

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ROTARY CUTTING INSTRUMENTS: The individual instruments intended for use with dental hand pieces are manufactured in hundreds of sizes, shapes and types. Common design characteristics: Inspite of the great variation that exists among rotary cutting instruments, they have certain design features in common. Each instrument consists of 3 parts (i)

Shank

(ii)

Neck

(iii)

Head

(Note- There is a difference in the meaning of the term “shank” as applied to rotary instruments and to hand instruments.)

Shank design: The shank is the part •

That fits into the hand piece

Accepts rotary motion from the hand piece.

Provides a bearing surface to control the alignment and concentricity of the instrument.

The shank design and dimensions vary from the hand piece for which it is intended. The ADA specification no. 23 for dental excavating burs include 5 classes. Three of these •

Straight hand piece shank.

49


Latch type angle hand piece shank.

Friction grip angle hand piece shank.

1. Straight hand piece shank: (0.520” length / 0.0925”dia) The shank portion of the straight handpiece instrument is a simple cylinder, which is held in the handpiece by a metal chuck that accepts a range of sank diameters. They are commonly used for finishing and polishing completed restorations. 2. Latch type angle handpiece shank (0.520” length / 0.0925” diameter) •

Latch type instruments are retained in the handpiece by a retaining latch that slides into the groove found at the shank end of the instrument.

The posterior portion is flattened on one side so that the end of the instrument fits into a D-shaped socket at the bottom of the bur tube and it is thus that the instrument is rotated.

The handpieces have a metal bur tube within which the instruments fit as closely as possible while still permitting easy interchange.

They are used in low and medium speed ranges where the small amount of potential wobbles inherent in the clearance between the instrument and the handpiece bur tube is controlled by the lateral pressure excited during cutting procedures.

Their shorter overall length permits substantially improved access to posterior regions of the mouth.

50


They are used for finishing procedures.

3. Friction grip shank (0.500 length / 0.0628” diameter) •

Were originally designed to be held in the handpiece by friction between the shank and a plastic or metal chuck. Newer handpiece designs have metal chucks that close to make a positive contact with the bur shank.

The shank is a simple cylinder manufactured to very close dimensional tolerances.

Design is smaller in overall length and provides very good access for posterior instrumentation

They are used for high-speed handpieces

Neck design: •

The neck is the intermediate portion of an instrument that connects the head to the shank.

It normally tapers form the shank diameter to a smaller size immediately adjacent to the head.

The main function of the neck is to transmit rotational and translation forces to the head. At the same time it is desirable for operator to have the greatest possible visibility of the cutting head and greatest manipulative freedom.

The neck dimensions represent a compromise between the need for a large cross section to provide strength ad a small cross section to improve access and visibility

Head design: •

The head is the working part of the instrument, the cutting edges or points of which perform the desired shaping of tooth structure.

The heads of instruments show great variation in design and construction than either of the other main portions. For this reason the characteristics of the head for the basis on which rotary instruments are classified.

E.g.,. Type: bladed or abrasive instrument , Material of construction: tungsten carbide, steel or diamond abrasives, Head size: regular or long ,Head shape: round, straight, tapering etc

51


Dental cutting Burs :The term bur is applied to all rotary cutting instruments that have bladed cutting heads. This includes instruments intended for such purposes as finishing metal restorations and surgical removal of bone, as well as those primarily intended for tooth preparation. HISTORICAL DEVELOPMENT OF DENTAL BURS: The earliest burs were hand made. They were both expensive and variable in dimension and performance. The shapes and dimensions of modern burs are directly related to those of the machine made burs first introduced in 1891. Early burs were made of steel. Carbide burs which were introduced in 1947, have largely replaced steel burs for tooth preparation. COMPOSITION AND MANUFACTURE The dental burs are of two types according to composition: Steel burs Are cut from blank steel stock by means of a rotary cutter that cuts parallel to the long axis of the bur. The bur is then hardened and tempered until its Vicker’s hardness number is approximately 800. They perform well in cutting human dentin at low seeds, but dull rapidly at higher speeds or when cutting enamel. Steel burs now are used mainly for finishing procedures. Tungsten carbide burs It is a product of powder metallurgy i.e. a process of alloying in which complete fusion of the constituents does not occur. The tungsten carbide powder is mixed with powdered cobalt under pressure and heated in a vacuum. A partial alloying or sintering of the metals takes place. A blank is then formed and the bur is cut from it with a diamond tool. The Vicker’s hardness number is in the range of 1650-1700. Carbide is stiffer and stronger than steel, but it is also more brittle.

52


BUR CLASSIFICATIONS SYSTEMS In order to facilitate description, selection and manufacture of burs it is highly designation, which represents all of the variables of a particular head design by some simple code. 1. According to their mode of attachment to handpiece, they can be classified as i) Latch-type ii) Friction-grip 2. According to composition i)

Stainless steel

ii)

Tungsten carbide

iii)

Combination

3. According to their motion of cutting i. Right ( revolves in clockwise direction) ii. Left bur ( cuts when revolving in anticlockwise direction) 4. According to their use i) Cutting burs ii) Polishing and finishing burs 5. According to Shapes: The term bur shape refers to the contour or silhouette the head. The basic head shapes are

53


Round bur – is spherical. The shape has been used for initial entry into the tooth, extension of the preparation , preparation of retention pin holes and caries removal. Inverted cone bur- It is a portion of a rather rapidly tapered cone with the apex of the cone directed towards the bur shank. It is particularly suitable for providing undercuts in cavity preparation. Pear-shaped bur- It is a portion of a slightly tapered cone with the small of the cone directed towards the bur shank. The Normal length( length slightly greater than width) pear bur is advocated for use in Class I cavity preparation for gold foil. Long length pear bur is advocated for cavity preparations for amalgam. Straight fissureA straight fissure bur is an elongated cylinder. This shape is advocating some for amalgam cavity preparation. Tapered fissure bur-Is a portion of a slightly tapered cone with the small end of the cone directed away from the bur shank. This shape is useful for inlay and crown preparations where freedom from undercuts is essential for successful withdrawal of patterns and final seating of cast restoration. Among these basic shapes, variations are possible 6. According to Sizes: In the United States the number designating bur size also has traditionally served as a code for head design.

54


The numbering system for burs was originated by the S.S White Dental Manufacturing Company in 1891 bur their first machine – made burs.

The original

numbering system grouped burs by 9 shapes and 11 sizes. The ½ and Ÿ designations were added later when smaller instruments were included in the system. All original bur designs had continuous blade edges. The crosscut modification was indicated by adding 500 to the number of the equivalent non cross cut size.

(eg.No.57 with crosscut was designated

No.557). Similarly a 900 prefix was used to indicate a head design intended for end cutting only. Except for differences in blade design e.g. a No.957; No.557 and No.57 bur all had the same head dimensions. In the United States dental burs traditionally have been described in terms of an arbitrary numerical code for head size and shape, eg.2 = 1.00 mm diameter round bur, 34=0.8 mm diameter inverted cone bur. Despite the complexity of the system, it is still in common use.

55


7. FDI and ISO System of classification There has been much confusion over bur sizes with American , British and European numbering systems failing to coincide. Newer classification systems, such as that developed by the Federation Dentaire Internationale (FDI) and International Standards Organization (ISO) tend to use separate designations for shape, usually a shape name, and size, usually a number giving the head diameter in tenths of a mm. E.g. round 010, inverted cone 008. The International Standards Organization has developed a classification ISO 6360. The essential dimension of burs, including the material, shank shape, overall length, shape and type of finish of the working head and the size of the head have been ordered numerically.

56


ISO number has 15 digits. Every three numbers provide the details. No.

Interpretation

A. 1 3 digits

310 or 330

Steel

Type of material

500

Tungsten carbide

615, 625,635

Aluminium oxide

806 or 807

Diamond

B.2nd 3 digits

103,104,105

Standard straight handpiece

Shank form

124

Special heavy duty laboratory handpiece

202,204,205,206

Conventional contra angle handpiece

313,314,315,316

Airotor handpiece

900

Unmounted stones

001 &002

Round head

010-020

Inverted cone head

030-039

Double cone head

040-099

Wheel head

100-150

Cylinder head (St. fissure)

160-229

Cone head (tapering)

243

Pear shaped head

260

Flame shaped head

284

Bud shaped head

320-329

Torpedo shaped head

st

3rd digits bur head

Torpedo shaped head Discs th

4 3 digits

494

Superfine (15µ)

504

Extra fine (30µ)

514

Extra fine (50µ)

524

Medium (100- 120µ)

534

Coarse (135-140µ)

544

Extra coarse (180µ)

001

Straight

006

Right hand twist

007

Cross cut twist

019

Double diagnol

071

Coarse flute

072

Fine flute

140

Fine diamond cut

175

Medium tooth lab-bur

215

Coarse tooth lab-bur

Surface finish & grain coarseness of abrasive points

Machine cut steel or tungsten carbide bur

5th set of 3 digits

Maximum diameter of the bur head in one tenth of a millimeter E.g., 307, 314, 107, 534, 018

57


Modifications in bur design: There has been a reduction in the number of standard sizes that have continued in use. As effectiveness of small burs has increased they have replaced larger burs in many procedure. 3 other major tends in bur design are used 1. Reduced use of crosscuts. 2. Extended heads on fissure burs. 3. Rounding of sharp tip angles. Crosscuts :are needed on fissure burs to obtain adequate cutting effectiveness at low speeds but at high speeds they are not needed. Because crosscut burs used at high speeds tend to produce unduly rough surfaces many of the cross cut sizes originally developed for low speed use have been replaced by non-crosscut instrument of the same dimensions for high-speed use. Extended head lengths: Carbide fissure burs have been introduced that have extended head lengths 2 or 3 times those of the normal tapered fissure burs of similar diameter. Rounding of the sharp tip corner : Early contributions to this trend were made by Markley and Shockwell. Because teeth are relatively brittle, the sharp angles produced by conventional burs can result in high stress concentration and include the tendency of the tooth to fracture. Additional features in head design: A large number of factors other than head size and shape are involved in determining the clinical effectiveness of a bur design. They include 1. Neck diameter 2. Head diameter 3. Head length 4. Taper angle

58


5. Blade spiral angle 6. Crosscut size and spacing

Head length and taper angle are primarily descriptive and may be varied within limits consistent with the intended use of the bur. The taper angle therefore is intended to approximate the desired occlusal divergence of the lateral walls of the preparations, and the head length must be long enough to reach the full depth of the normal preparation. Neck diameter •

is important functionally because a neck that is too small will result in a weak instrument unable to resist lateral forces.

•

Too large a neck diameter may interfere with visibility and the use of the part of the bur head next to the neck and may restrict access for coolants.

•

As the head of a bur increases in length or diameter, the moment arm exerted by lateral forces increases, and the neck needs to be larger.

In comparison with these factors, two other design variables, the spiral angle and crosscutting, have considerably greater influence on bur performance. Spiral angle 59


There is a tendency toward reduced spiral angles on burs intended exclusively for highspeed operation where a large spiral is not needed to produce a smoother preparation and a smaller angle, which produces more efficient cutting.

Crosscut bur design •

A certain amount of perpendicular force is required to make a blade grasp the surface and start cutting as it passes across the surface.

The harder the surface, the duller is the blade, and the greater its length, the more force that is required to initiate cutting.

By reducing the total length of bur blade that is actively cutting at any one time, the crosscuts effectively increase both the cutting pressure resulting from rotation of the bur and the perpendicular pressure holding the blade edge against the tooth.

As each crosscut blade cuts, it leaves small ridges of tooth structure standing behind the notches. Because the notches in two succeeding blades do not line up with each other, the ridges left by one blade are removed by the following one at low or medium speeds.

However, at the high speed attained with air-turbine handpieces, the contact of the bur with the tooth is not continuous, and usually only one blade cuts effectively. Under these circumstances, although the high cutting rate of crosscut burs is maintained, the ridges are not removed and a much rougher cut surface results.

Bur blade design: The dental bur is a small milling (cutting) instrument. The actual cutting action of a bur takes place in a very small region at the edge of blade. In high speed range, this effective portion of the individual blade is limited to no more than few thousands of a centimeter adjacent to the blade edge. 1. Bur tooth: terminates in the cutting edge or blade. It has two surfaces, the tooth face, which is the side of the tooth on leading edge; and the back or flank (clearance face), which is the side of the tooth on trailing edge.

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2. Rake angle: is the angle the face of the bur tooth makes with the radial line from the center of the bur to the blade. Referring to the direction of rotation, the angle can be. Negative: If the face is beyond or leading the radial line. Zero: If the radial line and the tooth face coincide with each other i.e. radial rake angle. Positive: If the radial line leads the rake face, so that the rake angle is inside the radial line. 3. Land: The plane surface immediately following cutting edge. 4. Clearance angle: The angle between the back of the tooth and the work. If a land is present on the bur, the clearance angle is divided into: Primary clearance: The angle the land will make with work and

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Secondary clearance: The angle between the back of the bur tooth and work. When the back surface( Clearance face) is curved , the clearance is called radial clearance 5. Tooth angle: This is measured between the face and back. If a land is present, it is measured between face and land. 6. Flute or chip space: The space between successive teeth/blades, which is a depressed area, is known as flutes. The number of blades on a bur is always even because even numbers are easier to produce in the manufacturing process, and instruments with odd numbers of blades cut no better than those with even numbers. The number of blades on an excavating bur may vary from 6 to 8 to 10. Burs intended mainly for finishing procedures usually have 12 to 40 blades. FACTORS INFLUENCING THE CUTTING EFFICIENCY OF BURS 1. RAKE ANGLE •

The more positive that the rake angle is, the greater is the bur's cutting efficiency . Also, burs with radial rake angles cut more effectively than designs with negative rake angles.

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However, with a negative rake angle the cut chip moves directly away from the blade edge and often fractures into small bits or dust. This is in contrast to burs with a positive rake angle where the chips are larger and tend to clog the chip space.

The negative rake is used for steel burs. It minimizes fractures of the cutting edge. The positive rake is used with tungsten carbide burs.

2. CLEARANCE ANGLE •

As its name implies, this angle provides clearance between the work and the cutting edge to prevent the tooth back from rubbing on the work.

There is always a component of frictional force on any cutting edge as it rubs against the surface, following the dislodgement of the chip .

Any slight wear of the cutting edge will increase the dulling perceptibility. However, it is possible that large clearance angle may result in less rapid dulling of the bur.

3. EDGE ANGLE •

Increasing the edge angle reinforces the cutting edge and reduces the likelihood for the edge of blade to fracture.

Carbide burs blades have higher hardness and a more wear resistant, but they are more brittle than stainless steel blades and require greater edge angle.

The three angles cannot be varied independently of each other. An increase in clearance angle for example, causes a decrease in edge angle.

4. NUMBER OF TOOTH OR BLADES AND THEIR DISTRIBUTION •

The number of teeth in a dental bur is usually limited to 6-8.

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Since the external load is distributed among the blades actively cutting, as the number of blades is decreased, the magnitude of forces at each blade increases and the thickness of the chip removed by each flute correspondingly increases.

Fewer blades provide increased space between the teeth, reducing clogging tendency.

Burs intended mainly for finishing procedures usually have 12-40 blades.

5. RUN OUT AND CONCENTRICITY Run out: •

Refers to the

eccentricity or maximum displacement of the bur head from its axis of

rotation while the bur turns. The average value of clinically acceptable run out is about 0.023mm.

Run-out will depend on eccentricity of bur and also on the precision of handpiece.

Run out is a dynamic test measuring the accuracy with which all blade tips pass through a single point when the instrument rotates.

It measures the concentricity and also the accuracy with which the center of rotation passes through the center of the head.

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Concentricity is a direct measurement of the symmetry of the bur head. It measures how closely a single circle can be passed through the tips of all of the blades. It indicates whether one blade is longer or shorter that the others and is a static measurement not directly related to function. •

Even a perfectly concentric head will exhibit substantial run out if: 1. The head is off center on the axis of the bur 2. The bur neck is bent 3. The bur is not held straight in the handpiece chuck 4. The chuck is eccentric relative to the handpiece bearings

6. FINISH OF THE FLUTES The dental bur is formed by cutting each flute into the bur blank with a rotating cutter while it progresses nearly parallel to the axis of the bur. Tests for cutting efficiency were done on different types of burs undergoing two, four, and six flute cuts. Those cut six times were the most efficient while those cut two times were the least efficient.

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7. HEAT TREATMENT Is used to harden a bur that is made of soft steel. It preserves the edge placed on the bur flute and hardens the bur to increase cutting life. 8.DESIGN OF FLUTE ENDS Dental burs are formed with two different styles of end flutes: •

The revelation cut, where the flutes come together at two junctions near a diametrical cutting edge.

The star cut, where the ends of flutes come together in a common junction at the axis of the bur.

The revelation type shows superiority in cutting efficiency during direct cutting but in lateral cutting both are equal. 9. BUR DIAMETER With the length of cut as a constant the volume of materials removed will vary directly with the bur diameter.

10. DEPTH OF ENGAGEMENT( Depth of cutting) •

As the depth of engagement is decreased, the force intensity on each small portion of the bur tooth still cutting is correspondingly increased and accordingly the average displacement per flute revolution should also be increased. 66


This increase is so great that the volume of material removed by a shallow cut exceeds that of deeper cuts.

11.INFLUENCE OF LOAD •

Load signifies the force exerted by the dentist on the tool head and not the pressure or stress induced in the tooth durins cutting. The force or load exerted is related to the rotational speed of the bur of a given design.

The exact amount of force generally employed is not known, but is has been estimated as being equivalent to a maximum of 1000 gm (2 pounds) for low rotational speed and from 60-120 gm (2~-4 ounces) at high rotational speed.

The minimum and maximum loads for: o Low speed 1000-1500 gm o High speed 60-120 gm

DENTAL ABRASIVE INSTRUMENTS: The second major category of rotary dental cutting instruments involves abrasive rather than blade cutting. Abrasive instruments are based on small angular particles of a hard substances held in a matrix of softer material. Cutting occurs at a large number of points where individual hard particle protrude from the matrix rather than along a continuous blade edge. Abrasive instruments are generally grouped as diamond or other instruments.

Diamond instruments: Have a great clinical impact because of their long life and great effectiveness in cutting enamel and dentin. Diamond instruments for dental use were introduced in the United States in 1942, before carbide burs were available and at a time when interest in increased rotational speeds was beginning to expose the limits of steel burs.

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Terminology: Diamond instruments consist of 3 parts. Metal blank, Powdered diamond abrasive, A Metallic bonding material that hold the diamond powder onto the blank. The blank has the same parts a) Head b) Neck c) Shank

The diamonds employed are industrial diamonds either natural or synthetic that have been crushed to powder and then carefully graded for size and quality. Method of manufacture: Diamond particles are held together by means of a “binder� (base) of variable nature. A ceramic binder is used in many cases, particularly for binding diamond chips. Also

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electroplating a layer of metal on the blank while holding the diamonds in place against it can be used. Classification 1.Head shapes and sizes: Diamond instruments are available in a wide variety of shapes and in sizes, which corresponded to all except the smallest diameter burs.

More than 200 shapes and sizes of diamond are currently marketed. 2.Diamond particle factors: The clinical performance of diamond abrasive instruments depend on the Size, Spacing, Uniformity, Exposure and Bonding of diamond particles. Diamond particle size is commonly categorized as coarse (125 to 150μm), medium (88 to 125μm), fine (60 to 74μm) and very fine (38 to 44μm) for diamond excavating instruments. Diamond finishing instruments use even finer diamond particles (10-38μm) to produce a relatively smooth surface. When using large particle size, the number of abrasive particles that can be placed on a given area of the head is decreased. Almost the only cause of failure of diamond instruments is the loss of diamonds from the critical area. Other abrasive instruments: Many types of abrasives instruments are used in dentistry in addition to diamond instruments.

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Their use is primarily restricted to shaping , finishing and polishing restorations, both in the clinic and in the laboratory. They may be divided into 2 distinct groups Moulded instruments Coated instruments Moulded abrasive instruments: They have heads that are manufactured by moulding or pressing a uniform mixture of abrasive and matrix around the roughening end of the shank or cementing a pre-moulded head to the shank. In contrast to diamond instruments, moulded instruments have a much softer matrix and are expected to wear during use. The mounted heads are often termed as points or stones. Other molded instrument heads use flexible matrix materials, such as rubber, to hold abrasive particles. Coated abrasive instruments: They are mostly discs that have a thin layer of abrasives cemented to a flexible backing. They are used in finishing / smoothening procedure of certain enamel walls of cavity preparations for indirect restoration as well as in finishing procedures for restoration. Materials: The matrix materials usually are phenolic resins or rubber.some points may be sintered but most are resin bonded. A rubber matrix is used primarily to obtain a flexible head on instruments to be used for polishing. A harder, non-flexible rubber matrix is often used for moulded silicon carbide discs. Synthetic or natural abrasives may be used including- Silicon carbide (carborundum), Aluminium oxide, Garnet, Quartz, Pumice and Cuttlebone. Cutting mechanism:

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The process by which rotary instruments cut tooth structure is complex and not fully understood. For cutting it is necessary to apply sufficient pressure to make the cutting edge of a blade or abrasive particle dig into the surface. EVALUATION OF CUTTING: Cutting can be measured both in terms of effectiveness and efficiency. Cutting effectiveness is the rate of tooth structure removal (mm/min or mg/sec). It does not consider the potential side effects such as heat or noise. Cutting efficiency is the percentage of energy actually producing cutting. Cutting efficiency is reduced when energy is wasted as heat or noise. It is possible to increase effectiveness while decreasing efficiency. There is a general agreement that increased rotational speed results in increased effectiveness and efficiency. Bladed cutting: Tooth structure, like other materials, undergoes both brittle and ductile fracture. Brittle fracture is associated with crack production, usually by tensile loading. Ductile fracture involves plastic deformation of material, usually proceeding by shear. Extensive plastic deformation also may produce local work hardening and encourage brittle fracture as well. a. Low speed cutting tends to proceed by plastic deformation before tooth structure fracture. b. High speed cutting , especially of enamel proceeds by brittle fracture.

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Many factors interact to determine which cutting mechanism is active in a particular situation. •

In order for the blade to initiate the cutting action, it must be sharp, must have a higher hardness and modulus of elasticity than the material being cut, and must be pressed against the surface with sufficient force.

The high hardness and modulus of elasticity are essential to concentrate the applied force on a small enough area to exceed the shear strength of the material being cut.

The sheared segments accumulate in a distorted layer that slides up along the rake face of the blade until it breaks or until the blade disengages from the surface as it rotates. These chips will accumulate in the clearance space between blades until washed out or thrown out by centrifugal force.

Mechanical distortion of tooth structure ahead of the blade produces heat. Frictional heat is produced by both the rubbing action of the cut chips against the rake face of the blade and the blade tip against the cut surface of the tooth immediately behind the edge.

This can produce extreme temperature increases in both the tooth and the bur in the absence of adequate cooling. The transfer of heat is not instantaneous, and the reduced temperature rise observed in teeth cut at very high speeds may, in part, be caused by removal of the heated surface layer of tooth structure by a following blade before the heat can be conducted into the tooth.

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Abrasive cutting: The cutting action of diamond abrasive instruments is similar in many ways to that of bladed instruments but the key differences result from the properties size and distribution of the abrasive. When diamond instruments are used to cut ductile materials, some material will be removed as chips, but much material will flow laterally around the cutting point and be left as a ridge of deformed material on the surface.

Ductile material

Brittle material

Repeated deformation work hardens the distorted material until irregular portions become brittle, break off, and are removed. This type of cutting is less efficient than that of a blade; therefore burs are generally preferred for cutting ductile materials such as dentin. Diamonds cut brittle material by a different mechanism. Most cutting result from tensile fractures that produce a series of subsurface cracks. Diamonds are more efficient when used to cut brittle materials and are superior to burs for the removal of dental enamel. Because diamond prepares a rougher tooth surface, diamond may be preferred for use in tooth preparation for bonded restorations. Diamond abrasives are commonly used for milling disks or instruments of CAD / CAM or copy milling applications. CUTTING RECOMMENDATIONS: The requirements for effectiveness and efficient cutting include using a contra-angle handpiece, air-water spray for cooling, high operating speeds, light pressure, and a carbide or diamond instrument.

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Carbide burs are better for- End cutting, Produce lower heat, Have more blade edges per diameter for cutting.they are more effectively used for punch cuts to enter tooth structure. Intracoronal cavity preparation, Amalgam removal,Small preparations and secondary retention features.

Diamond instruments have higher hardness and coarse diamonds have very high cutting efficiency. They are better for- Extra-coronal cavity preparation, Beveling enamel margins on cavity preparation and Enameloplasty.

HAZARDS WITH CUTTING INSTRUMENTS: Almost everything done in a dental office involves some risk to the patient, dentist and / or auxiliaries. The patient has pulpal dangers from the cavity preparation and restoration procedure. There are also soft tissue dangers. Everyone is potentially susceptibility to eye, ear and inhalation dangers. However, careful adherence to normal precautions can eliminate or minimize most risks associated with cutting instrument use. PULPAL PRECAUTIONS: •

As the thickness of the remaining dentin decreases, the pulpal insult from heat or desiccation increases.

•

Steel burs produce more heat than carbide burs because of insufficient cutting. Burs and diamond instruments that are dull or plugged with debris do not cut efficiently, resulting in heat production. When used without coolants, diamond instruments generate more damaging heat than carbide burs.

•

The most common instrument coolants are air or air water spray. Air alone as a coolant is not effective in preventing pulpal damage since it needlessly desiccates the dentin and damages the odontoblasts.

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The use of a water spray and its removal by an effective high volume evacuator are especially important when old amalgam restorations are removed in order to decrease mercury vapour release and include visibility. A study by J Anthony Von Fraunhofer et al, indicated that a 20% addition of mouthwash to water coolant increased the cutting rate by more than 130% for dental diamond bur and by more than 200% for carbide burs, thus prolonging the cutting life of the bur. Also a study done by same authors concluded that the use of higher coolant flow during tooth preparation produced increased production of cutting efficiency. Soft tissue precautions: •

The lips tongue and cheeks of the patients are the most frequent areas of soft tissue injury.

A rubber dam is very helpful in isolating the operating site. If not used then a mouth mirror, cotton roll or evacuator tip may be used to retract the tissues.

The dentist must always be aware of the patients response during the cutting procedure. A sudden reflex movement by the patient, such as gagging, swallowing or cough could result in serious injury.

Eye precautions: •

When using high speeds, particles of old restorations, tooth structure, bacteria, and other debris are discharged at high speeds from the patients mouth.

Sufficiently strong high-volume suction should be applied to alleviate this problem with protective glasses are always indicated.

If an eye is injured it must be covered by a clean gauze pad until medical attention can be obtained. Precautions from unusual light sources (VLC, laser) should be taken.

Ear precautions •

An objectionable high-pitched whine is produced by some air-turbine handpieces at high speeds. Aside from the annoying aspect of this noise, there is some possibility that hearing loss can result from continued exposure.

Potential damage to hearing from noise depends on:

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o the intensity or loudness (decibels[db]) o frequency (cps), and o duration (time); of the noise, as well as o the susceptibility of the individual, increased age, existing ear damage, disease, and medications are other factors that can accelerate hearing loss. •

Normal ears require that the intensity of sound reach a certain minimum level before the ear can detect it. This is known as the auditory threshold. It can vary with frequency and exposure to other sounds. When subjected to a loud noise of short duration, a protective mechanism of the ear causes it to lose some sensitivity temporarily. This is described as a temporary threshold shift. If sufficient time is allowed between exposures, recovery will be complete. Extended or continuous exposure is much more likely to result in a permanent threshold shift with persistent hearing loss.

Turbine handpieces with ball bearings, free running at 30 pounds air pressure, may have noise levels as high as 70-94 db at high frequencies. Noise levels in excess of 75 db in frequency ranges of 1000 to 8000 cps may cause hearing damage.

Protective measures are recommended when the noise level reaches 85 db with frequency ranges from 300 to 4800 cps. Protection is mandatory in areas where the level transiently reaches 95 db.

Normal use of a dental handpiece is one of intermittent application that generally is less than 30 minutes per day.

• •

Earplugs can be used to reduce the level of exposure but have several drawbacks. Room soundproofing helps and can be accomplished with absorbing materials used on walls and floors.

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

Inhalation precautions: •

Aerosols and vapors are created by cutting tooth structure and restorative materials.

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•

Both aerosols and vapors are a health hazard to all present. The aerosols are fine dispersions in air of water, tooth debris, microorganisms, and/or restorative materials.

•

Cutting amalgams or composites produce both submicron particles and vapor. The particles that may be inadvertently inhaled have the potential to produce alveolar irritation and tissue reactions. Vapor from cutting amalgams is predominantly mercury and should be eliminated, as much as possible, by careful evacuation near the tooth being operated on. The vapors generated during cutting or polishing by thermal decomposition of polymeric restorative materials (sealants, acrylic resin, composites) are predominantly monomers. They may be efficiently eliminated by careful intraoral evacuation during the cutting or polishing procedures.

•

A rubber dam protects the patient against oral inhalation of aerosols or vapors, but nasal inhalation of vapor and finer aerosol may still occur. Disposable masks worn by dental office personnel filter out bacteria and all but the finest particulate matter. However, they do not filter out either mercury or monomer vapors.

INSTRUMENTS FOR RESTORING PROCEDURES Each restoring instrument is composed of three parts the handle, the shank and nib, which is the working end of the instruments. They may be either a hand, rotary or mechanized instruments.

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The various types of restoring instruments are: 1. MIXING INSTRUMENTS •

Most common are the spatulas (hand instruments).

They have flat and wide nibs with blunt edges and straight shank or a double-ended instrument with other end sharp.

Are of different sizes with different degrees of stiffness in their nibs to suit various sizes.

May be made of stainless steel, ivorine or plastic.

Stainless steel is used for mixing lining cements and intermediate base restorative materials whereas agate spatulas are used for mixing glass ionomer and composite resin materials in order to prevent incorporation of metal and corrosive particles in the cement.

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2. PLASTIC INSTRUMENTS •

Used for carrying and handling materials after mixing while the materials are in plastic stage in the prepared cavities or tooth.

Usually double ended hand instruments, with a flat sided nib with blunt edges and corners, but may differ in their nib shapes and angulation or curvature of shanks.

May be made of stainless steel, ivorine or plastic.

They can also be plated with Teflon or anodized aluminum titanium nitrate coated to minimize material adhesion (composite manipulation) and facilitate easy cleaning.

3. AMALGAM CARRIER •

Single ended hand instrument with a cylindrical hollow working tip, with a curved tip or straight tip with different tip diameter (1.5mm , 2mm, 2.5 mm or 3 mm).

They have a spring action handle and may be made of stainless steel or plastic.

Used to carry the mixed silver amalgam into the prepared cavity or calcium hydroxide powder to the exposure site in pulp capping procedures.

Modified amalgam carries are available for retrograde filling after apicoectomy (Hill carrier, Messing carrier and Dimaskieh carrier).

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4. CONDENSING INSTRUMENTS •

Used to condense filling materials into the prepared cavity for better adaptation without any voids.

They can be hand condensing instruments or mechanical condensers.

Depending upon the material (amalgam, direct gold, composite) they will differ in the surface configuration of the nib-face: (smooth or serrated) and have different shapes: round, triangular or diamond and of different sizes for each shape and shank angulation. AMALGAM CONDENSER

Double ended, cylindrical working end with smooth or serrated face of various size and shapes, hand instruments are available.

Serrated face is said to increase the surface area and also to provide a mechanical interlock between the already condensed amalgam to the freshly introduced plastic mass and better adaptation to cavity and prevent spillage of amalgam.

With condenser of 2mm diameter the ideal condensation pressure would be 1.4 and 1.8 kg and spherical alloys requires less condensation pressure than lathe cut and admix alloy. COHESIVE GOLD CONDENSERS

Available as single end or double end instrument, with straight or angular shank with serrated nib.

Mechanical condenser are also available wherein the handpiece is adjusted so that it delivers 360-3600 blows / minute. COMPOSITE INSTRUMENTS

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With the use of posterior composite special hand instruments made of special materials such that the composite does not stick to instruments are available with round, rectangular or rhombic end, which help in insertion and condensation of composite material.

5. CARVERS •

Cutting instruments with their blades either beveled on knife-edged [e.g., Hollenback, wards , cleoid discoid carver, Frahm’s carver (diamond cravers)].

Hollenback carver possess double-side knife edged point edged nibs with curved monoangled or binangled shanks-efficient in carving amalgam and wax.

Cleoid discoid carver used for direct gold restorations.

Other carvers with triangular nibs or diamond shaped nibs are also available.

6. BURNISHERS •

Double and hand instrument with smooth working ends of various shapes such as ball shaped, beaver tail shaped, conical, egg-shaped, apple shaped etc., are available.

They can be in the form of burs with perfectly smooth heads to perform a burnishing operation by rotary action.

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7. FILES •

Used for margination of restorations.

Nibs can be foot-shaped, hatchet-shaped or parallelogram shaped.

Serrations can be directed away from the handle: push file or directed towards the handle: pull file.

8. KNIVES •

Nibs carry knife edged faces on one of their sides only. E.g., Bard-Parker knife, Black’s knife, Wilson’s knife and Stein’s knife

Black’s knife has the nibs at various angulations from acute to obtuse, may be push or pull knife used for various purposes.

Wilson’s knife has the nib in a plane a right angle to the shaft, introduced interproximally for proximal and gingival manipulation of restorative tips.

Stein’s knife trapezoidal nib is used mainly for direct gold contouring and margination.

9. APPLICATOR TIPS •

Various types of applicator tips made of synthetic bristles are available for pinpoint take-up, transfer and application of Etchant, conditioners or bonding agents to the prepared tooth surface for adhesive restorations.

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CONCLUSION

In order to properly prepare a cavity, the tooth tissues, enamel and dentin must be excised with specific instruments in an efficiently ordered sequence. Most cavity preparations require the use of both rotary and hand instruments. With the evolution of tooth reduction methods, today most of the cavity preparation is performed with the air turbine handpiece, but hand cutting instruments are still an integral part of any dental armamentarium. Almost everything in dental office involves some risk to the patients, dentist and/ auxiliary. However, careful adherence to normal precautions can eliminate or minimize most risk associated with cutting instruments.

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REFERENCES: 1. Sturdevant’s Art and Science of Operative Dentistry.4th edition. 2. Operative dentistry. Modern theory and practice. M A Marzouk, A L Simonton and R D Gross. 1st edition 3. Principles and practice of operative dentistry. Gerald Charbeau. 3rd edition 4. Text book of Operative dentistry. Baum, Philips and Lund. 3rd edition. 5. Clinical dentistry. Alexander Rowe. Vol.2 6. Kanaan Elias, Andrew a Amis, Derick J Setchell. The magnitude of cutting forces at high speed. J Prosthet Dent 2003; 89(3):286291) 7. J Anthony Von Fraunhofer. Enhanced Dental cittung through chemomechanical

effects.

Journal

of

American

Dental

Association.131; 2000:pg 165 8. J A Von Fraunhofer, S C Siegel and F Feldman. Handpiece coolant flow rates and cutting. Oper Dent 2000; 25:544-548

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Operative instrumentsf/ dental implant courses by Indian dental academy