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    Instruments and instrumentation Instruments and instrumentation Document Transcript

    • INTRODUCTION: The removal and shaping of tooth structure is an essential part of restorative dentistry. Initially this was a difficult process accomplished entirely by the use of hand instruments. The introduction of rotary, powered cutting equipment was one of the truly major advances in 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 the many instruments required. CLASSIFICATION: 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. Those used for exploration A) Dry the area on the tooth: - This necessitates the use of an a) Air syringe. b) Pair of tweezers (pliers) c) Cotton pellets to dry the tooth. d) Cotton rolls – to isolate the area around the tooth. 1
    • 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 - Battery operated lights. - Built in lights attached to dental unit. - 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: The hand mirror is used to move the tongue and cheek away. - Blunt plastic instruments may help in retraction. - Tongue depressor or retractors are sometimes helpful for this procedure. D) To probe the potential lesion: Explorers are used for this purpose. These are 4 types of explorers. 1) Straight explorer. 2) Right angled explorer 3) Arch explorer 4) Interproximal explorer. I) Those used for tooth structure removal: - Hand cutting instruments. - Rotary cutting and abrasive instruments. - Ultrasonic instruments 2
    • II) Those used for restoring: - Mixing instruments - Spatulas - Plastic instruments - Condensing instruments - Burnishing instruments - Carvers - Files - Kives - Finishing and polishing instruments. 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 3
    • 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 Mouth mirrors Explorers Probes Scissors Pliers Others 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. 4
    • - Carbon steel - Stainless steel - Some are made with carbide inserts to provide more durable cutting edges. - Carbon steel is harder than stainless steel but when unprotected it will corrode. - 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 are used in the manufacture of hand instruments, but they are usually restricted to instruments other than those used for cutting tooth structures. Hardening and tempering heat treatments: To gain maximal benefits from carbon or stainless steel, the manufacturer must submit them to two heat treatments. - Hardening and tempering - 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 - Sporicidal cold disinfection. - Boiling water - Steam under pressure (autoclave) - Causes discolouration. Rust Corrosion 5
    • 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 case 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 - Amalgam condensers, mirrors, explorers, probes. Instrument design: Most hand cutting instruments are composed of 3 parts. - Handle, Shank and Blade. 6
    • 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. Instruments which have blades on both sides of the handle are known as double – ended instruments. Shafts and handles are available in various shapes and sizes. It may be serrated to increase friction grip. Shanks serve to connect the handles to the working ends of the instruments. They are; Smooth, rounded and tapered. They often have one or more bends to avoid the instrument having a tendency to twist in use when force is applied. Hand instruments must be balanced to allow for the concentration of force onto the blade without causing rotation of the instrument in the grasp. This balance 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 nearly coincides with the projected axis of the handle. For optimal anti-rotational design the blade edge must not be off axis by more than 1 to 2 mm. All dental instruments and equipment need the satisfy this principle of balance. 7
    • 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 - Mono-angle (one) - Bin-angle (two) or - Triple-angle Instruments with small, short blades may be easily designed in mono-angle form while confining the cutting edge within the required limit. 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. Instrument name: 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. These names were combined to form in complete description of the instrument (eg. Binangle spoon excavator). 8
    • Operative cutting instrument formulas: Cutting instruments have formulas describing the dimensions and angles of the working end. 1st number indicates: Width of the blade or primary cutting edge in tenths of a mm (0.1mm). 2nd number of indicates: The primary cutting edge angel (measured from a line parallel to the long axis of the instrument handle in clockwise centigrades). The instrument is positioned so that this number always exceeds 50. If the edge is located angular to the blade then this number is normally omitted resulting in a 3 number code. The 3rd number indicates: - blade length in mm. The 4th number indicates the: Blade angle relative to the long axis of the handle in clockwise direction. the angle is expressed in hundredths of a circle or centigrade. It is always 50 or less than 50. Cutting instrument bevels: Most hand cutting instruments have on the end of the blade a single bevel that forms the primary cutting edge. Two additional edges, called secondary cutting edges, extend from the primary edge for the length of the blade. Bibeveled instruments, such as ordinary hatchets, have 2 bevels that form the cutting edge. 9
    • Certain single beveled instruments such as spoon excavators and GMT’s are used with a scraping or lateral cutting motion. Others as enamel hatchets may be used with a planing or direct cutting motion for such single- beveled designs the instruments must be made in pairs. To determine whether the instruments has a right or left bevel, the primary cutting edge is held down and pointing away, and if the bevel appears on the right side of the blade, it is the right instrument of the pair and vice versa. Most instruments are available with blades and shank on both ends of the handle. Such instruments are termed double-ended. Instruments having cutting edges angled the axis of the handle such as bin-angle chiles are single beveled and not designated as right or left but have a mesial bevel or distal bevel. If when one observes the inside of the blade carvature, the primary bevel is not visible, the instrument has a distal bevel and if the bevel can be seen then the instrument has a reverse or mesial bevel. APPLICATIONS OF CUTTING INSTRUMENTS: The cutting instruments are used to cut hard or soft tissues of the mouth. Excavators are used for removal of caries and refinement of the internal parts of the cavity. The four subdivisions of excavators are 1) Ordinary hatchets 10
    • 2) Hoes 3) Angle formers 4) Spoon Ordinary hatchet excavator (3-2-28) Has the cutting edge of the blade directed in the same plane as that of the long axis of the handle and is bibeveled. These instruments are used primarily on anterior teeth for preparing retentive areas and sharpening internal line angles, particularly in preparations for direct gold restoration. The hoe excavator (4½ - 1½ -22) has the primary cutting edge of the blade angle to the axis of the handle. Used for planing cavity walls and forming line angles. Used in Class III and V preparations for direct gold restoration. Some sets contain hoes with longer and heavier blades with shanks contra-angled. They are intended for use on enamel or posterior teeth. Angle former (12-85-5-8) (Monangled): It is primarily used for sharpening line angles and creating retentive feature in dentin in preparations for gold restoration. May be used for placing bevels on enamel margins. It is described as a combination of a chisel and GMT. It is available in pairs (right and left). Spoon excavator (15-8-14): Used for removing caries and carving amalgam or direct wax patterns. The blades are curved and cutting edges are either circular (discoid) or claw like – cleoid. 11
    • The shanks may be binangled (13-7-14) or triple angled, to facilitate accessibility. Chisels: Are intended primarily for cutting enamel and may be grouped as - Straight, slightly curved or binangle. - Enamel hatchets - Gingival margin trimmers. Straight chisel (12-7-0): The straight chisel has a shank and blade with the bevel on only one side. The shank and blade of the chisel may be slightly curved (Wedelstaedt design) or may be bin-angled. The force used with all these chisels is essentially a straight thrust. Enamel hatchet (10-7-14): Is a chisel similar in design to the ordinary hatchet except that the blade is larger, heavier and is beveled on only one side. It is used for cutting enamel and comes as right or left types for use on opposite sides of the cavity. Offset hatchets: The offset is like the regular hatchet or backward except the whole blade is rotated a ¼ turn from the long axis. Gingival marginal trimmer (12 ½ -100 – 8-14) (12 ½ -75-8-14) 12
    • It is similar in design to the enamel hatchet, except the blade is curved, and the primary cutting edge is at an angle to the axis of the blade. It is made as right and left types. When the second number in the formula is 90 to 100 the pair is used on the distal gingival margin. When this number is 85 to 75 the pair is used to bevel, the mesial margin. The GMT is designed to produce a proper bevel on the gingival enamel margins of proximal occlusal preparations. The 100 and 75 pairs are for inlay / onlay preparation with steep gingival bevels. The 90 and 85 pairs are for amalgam preparation with gingival enamel bevels that decline gingivally only slightly. Among other uses is the rounding or beveling of the axiopulpal line angle of the 2-surface preparations. Other cutting instruments: Other hand cutting instruments include 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. Files: can be used to trim excess filling material. They are particularly useful at the gingival margins. 13
    • The discoid-cleiod instrument may be used to trim or burnish inlay- onlay margins. (It is principally used for carving occlusal anatomy). HAND INSTRUMENT TECHNIQUE: There are 4 grasps used with hand instruments. 1) Modified pen. 2) Inverted pen. 3) Palm and thumb. 4) Modified palm and thumb. Modified pen grasps: It is similar to that used in holding a pen but not identical. Pads of the thumb, index and middle fingers contact the instrument, while the tip of the ring finger or tips of the ring and little finger is placed on a nearly tooth surface of the same arch as a restoration. The palm of the hand generally is facing away from the operator. The pad of the middle finger is placed near the top side of the instrument. Inverted pen grasps: The finger position of the inverted pen grasp are the same as for the modified pen grasp, but the hand is rotated so that the palm faces more towards the operator. This grasp is used mostly for cavity preparations utilizing the lingual approach on anterior teeth. The modified pen and inverted pen grasps are practically universal. 14
    • Palm and thumb grasp: The handle is placed in the palm of the hand and grasped by all the fingers, while the thumb is free of the instrument and the restoration is provided by supporting the tip of the thumb on a nearby tooth of the same arch or on a firm, stable structure. It may be useful on maxillary teeth especially the right side when working from the right rear position. This form of grasp is usually necessary where the rest support for the thumb is at some distance from the point of operation, so that the thumb itself must be extended and hence cannot be used for gripping. Modified palm and thumb grasp: In this, the handle of the instrument is in contact with the tips of the 4 fingers on one side, opposed to which are contacts with the mesial end of the 1st phalanx of the thumb. The hand is only half closed instead of being fully closed. The end of the thumb is used as a rest. Rests: A proper instrument grasp must include a firm rest to steady the hand during operating procedures. The closer the rest areas are to the operating area, the more reliable they are. In cases where it is impossible to establish a rest on tooth structure then soft tissue may be used. Occasionally, it is impossible to establish normal finger rests with the hand holding the instrument, then instrument control may be gained using the forefinger of the opposite hand on the shank of the instrument or using an indirect rest (i.e. operating hand rests on the opposite side which rests on a stable oral structure). 15
    • Guards- are hand instruments or other items such as interproximal wedges, used to protect soft tissue from contact with sharp cutting or abrasive instruments. 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. Stationary sharpening stones: The most frequently used sharpening equipment consists of a block or stick of abrasive material called a stone. 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 - Tapered 16
    • 4. Types of materials are in common use for sharpening stones - Arkansas stone - Silicon carbide - Aluminium oxide - Diamond 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. This type of sharpener is very versatile. Handpiece sharpening stones: Mounted silicon carbide and aluminium oxide stones for use with both straight and angle handpieces are available in a variety of sizes and shapes. 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 17
    • 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. 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. STERILIZATION AND STORAGE: Sterilization in dental office can be accomplished by - Autoclaving - Dry heat procedure - Ethylene oxide equipment - Chemical vapour sterilizers. Boiling and chemical solution will not sterilize and should be considered only as a disinfection procedure. 18
    • POWERED CUTTING EQUIPMENT: Development of rotary equipment: Much of the subsequent history 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. A handpiece is a device for holding rotating instruments, transmitting power to them and for positioning them intraorally. Handpiece and associated cutting and polishing instruments developed as 2 basic types straight and angle. One of the most significant advancement was the introduction of the electric motor as a power source in 1874. It was incorporated into a dental unit in 1914. Steel burs that were used that time were not able to cut the enamel effectively even when applied with great force. It only resulted in increased heat and instrument wear. Diamond cutting instrument were developed in Germany around 1935. In 1946 diamond instruments and tungsten carbide burs capable of cutting enamel were produced commercially. By 1950 speeds of 60,000 rpm and above have been attained. 19
    • The major break through came with the introduction of contra-angled handpieces with internal turbine drives in contra angle heads. Air driven hand pieces continue to be the most popular type of hand piece equipment because of the overall simplicity of design ease of control, versatility and patient acceptance. Rotary sped ranges: The rotational speed of an instrument is measured in revolution pr minute (rpm). 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 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. The crucial factor for some purpose is the surface speed of the instrument (i.e.) the velocity at which the edges of the cutting instrument pass across the surface being cut. This is proportional to both the rotational speed and diameter of the instrument. 20
    • 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. 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. 21
    • LASER EQUIPMENT: The word laser is an acronym for “Light amplification by stimulated emission of radiation”. They are devices, which produce beams of very high intensity light. The lasers range from Long wavelengths (infrared) Visible wavelengths Short wavelengths (Ultraviolet) Excimers are special ultraviolet lasers. At present time CO2 and Nd:YAG lasers have shown the most promises. Lasers are regulated by food and drug administration (FDA) for safety and efficacy. They have been approved for soft tissue surgery but not for tooth preparation. Other equipment: In the mid 1950’s air abrasive cutting was tested but there were several clinical problems. There was no tactile sense associated with air abrasive cutting of tooth structure. At present air abrasive equipment is being promoted for stain removal, debriding pit and fissures prior to sealing and micromechanical roughening of surfaces to be bonded (enamel, porcelain). ROTARY CUTTING INSTRUMENTS: The individual instruments intended for use with dental hand pieces are manufactured in hundreds of sizes, shapes and types. 22
    • 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 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. - Latch type angle hand piece shank. - Friction grip angle hand piece shank. Are commonly encountered. Neck design: The neck is the intermediate portion of an instrument that connects the head to the shank. 23
    • 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. Head design: The head is the working part of the instrument, the cutting edges or points of which perform the derived 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. Dental 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 introduced in 1891. Steel burs: They perform well, cutting human dentin at low speeds, but dull rapidly at higher speeds or when cutting enamel. Once dulled the reduced cutting efficiency creates heat and vibration. 24
    • Carbide burs – Introduced in 1947, have largely replaced steel burs for cavity preparation. Steel burs now are used mainly for finishing procedures. They perform better than steel burs at all speeds and their superiority is greatest at high speeds. All carbide burs have heads of cemented carbide in which microscopic carbide particles, usually of tungsten carbide are held together in a matrix of cobalt or nickel. Carbide is stiffer and stronger than steel but it is also more brittle. Bur classification systems: In the United States dental burs traditionally have been described in terms of an arbitrary numerical code for head size and shape. 2 = 1mm diameter round bur. 57 = 1mm diameter straight fissure bur. 34 = 0.8mm diameter inverted cone bur. Newer classification systems like the FDI and 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. Eg. Round 010; inverted cone 008. Shapes: The term bur shape refers to the contour or silhouette the head. The basic head shapes are Round bur – is spherical. The shape has been used for 1) Initial entry into the tooth. 2) Extension of the preparation 3) Preparation of retention potholes. 4) Caries removal. 25
    • 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. (Normal length) It is advocated for use in Class I cavity preparation for gold foil. Long length pear bur is advocated for cavity preparations for amalgam. Straight fissure: A 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 1) 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. Sizes: In the United States the number designating bur size also has traditionally served as a code for head design. The original numbering system grouped burs by 9 shapes and parallel sizes. 26
    • 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 - Reduced use of crosscuts. - Extended heads on fissure burs. - 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. 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. The third major trend in bur design has been toward rounding of the sharp tip corner. Because teeth are relatively brittle, the sharp angles produced by conventional burs can result in high stress cone 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. 27
    • They include - Neck diameter - Head diameter - Head length - Taper angle - Blade spiral angle - Crosscut size and spacing Bur blade design: The actual cutting action of a bur takes place in a very small region at the edge of the blade. Each blade has 2 sides - The rake face (towards the direction of cutting) - Clearance face and Three important angles: - Rake angle (angle made between the rake force and the line connecting the edge to the axis of the bur) - Edge angle (is the internal angle at the edges followed by the forces of the bur blade). - Clearance angle (is the angle between the clearance face immune behind the edge and a tangent to the path of rotation). The rake angle is the most important design characteristic of a bur blade. For cutting hard, brittle materials a negative rake angle minimizes fractures of a cutting edge, thereby include the tool life extreme positive rake angles are used. When relatively soft and weak materials are cut. A rake angle is said to be negative when the rake face is ahead of the reduce (from cutting edge to axis of bur). 28
    • Increasing the edge angel reinforces the cutting edge and reduces the likelihood for the edge of the blade to fracture. The 3 angles cannot be varied independently of each other. At high speeds a radial rake angle that is either negative or positive may be equally effective. (An increase in the clearance angle causes a decrease in the edge angle. The clearance angle eliminates rubbing friction of the clearance face, provides a stop to prevent the bur edge from digging into tooth structure extensively and reduces the radius of the blade back of the cutting edge to provide adequate flute space for the chips formed ahead of the following blade). Carbide burs normally have blades with slight negative rake angles and edge angles of about 90 degrees. DIAMOND 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. 29
    • Terminology: Diamond instruments consist of 3 parts. A metal blank, powered diamond abrasive, Metallic bonding material that hold the diamond powder onto the blank. The blank has the same parts - Head - Neck - 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: The diamonds are attached to the blank by electroplating a layer of metal on the blank while holding the diamonds in place against it. 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. Diamond particle factors: The clinical performance of diamond abrasive instruments depend on the - Size - Spacing - Uniformity - Exposure and - Bonding of diamond particles 30
    • Diamond particle size is commonly categorized as coarse (125 to 150m), medium (88 to 125m) fine (60 to 74m) and very fine (38 to 44m) for diamond excavating instruments. When using large particle size, the number of abrasive particles that can be placed on a given area of the head is decreased. Almost of 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. 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. 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. 31
    • Materials: The matrix materials usually are phenolic resins or rubber. 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 - Cuttlebone Cutting mechanism: 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. Bladed cutting: Tooth structure, like other materials, undergoes both brittle and ductile fracture. Brittle fracture is associated with crack production, usually by tensile loading. 32
    • Ductile fracture involves plastic deformation of material, usually proceeding by shear. 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. 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. The very high hardness of diamonds provides superior resistance to wear. Diamonds are more efficient when used to cut brittle materials and are superior to burs for the removal of dental enamel. 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. Carbide burs are better for - End cutting - Produce lower heat - Have more blade edges per diameter for cutting. - For punch cuts to enter tooth structure - Intercoronal cavity preparation - Amalgam removal - Small preparations - Secondary retention features 33
    • 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 - 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. 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 in sufficient 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 needle by desiccates the dentin and damages the odontoblasts. 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. 34
    • Soft tissue precautions: 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. Eye precautions: 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 must be taken. Inhalation precautions: Both aerosols and vapours are a health hazard to all present. Rubber dam should be used, Disposable masks should be worn. REFERENCES:  Operative dentistry: Modern theory and practice – MA Marzouk, I Edition.  The art and science of operative dentistry – Sturdevent, III Edition.  Principles and practice of Operative Dentistry – Charbenaeu. 35