ROTARY CUTTING INSTRUMENTS • Introduction • Historical development • Rotary speed ranges • Bur Classification • Instrument design • General Design of Dental Burs & associated terminology • History of dental burs • Modification of dental burs • Dental Abrasive Stone • Rotary cutting instruments & their common design characteristics • Cutting mechanisms • Evaluation of cutting • Cutting recommendations • Hazards with rotary cutting instruments • References • Conclusion
ROTARY CUTTING INSTRUMENTS Introduction The term "rotary" applied to tooth cutting instruments describes a group of instruments that turn on an axis to perform work. Applied to dental procedure, the character of work performed is primarily cutting, abrading, burnishing, finishing or polishing tooth tissues or various restorative materials. Many treatment procedures in restorative dentistry substantially involve rotary instrumentation. While the great majority of cutting procedures on enamel & dentin once employed hand instruments, the bulk of tooth tissue removal is now accomplished using rotary instruments. Historical development When Dr. Jonathan Taft wrote his "Textbook of Operative dentistry" in 1868, cutting procedures on tooth enamel and dentin were carried out using thick, bulky chisels and excavators, which by Dr. Taft's an description, were "of good steel" well wrought and thoroughly tempered. Every step in the process of manufacturing was perfectly executed so as to ensure an edge that would cut not only dentin but also enamel, which is the hardest animal substance. Those instruments were heavy-handled and as wide as 1/4 inch at the cutting edge. Taft suggested that "a heavy instrument with a sharp point and a lateral curve is often efficient in opening up cavities and cutting down strong projections of enamel." It can be assumed that many carious lesions treated by such procedures were extensive enough to permit access with these instruments with a gross carious lesion; the chisel was used to gain entrance through the carious dentin and make its removal possible by hand excavators. Walls and floors of the cavity were then defined by planing by latest scraping action. Access to same interproximal lesions was gained by separation of teeth with an assortment of wedges or mechanical separators. Damage caused by overzealous tooth movement was, and is today a potential hazard. The first rotary instruments used for cutting tooth tissue were actually drills or bur heads that could be twisted between the fingers for a crude cutting or abrading action. Taft described them as "bur drills". He suggested that they be made by best steel, forged close to their proper size and finally 2
shaped on a lathe. The bulb was then cut into basic shapes by hand with a sharp-edged file. These simple rotary instruments, twisted with the fingers were capable of very limited lateral - and end-cutting action. The early bur drills ranged in diameter from 1/32" to about 1/5" and were used in opening cavity preparations, formerly possible; they were particularly adapted to small and medium size cavities. In addition it was suggested that they be used for making "retaining points" for fillings. One of the refinements of those drills was Scrantonâ€™s drill. This called be rotated in either direction to achieve cutting action. The next notification was the drill-ring, which was adapted to the middle or index finger with a socket that fitted against the palm, providing a seat for the blunt end of the bur drill. Other types of drill stocks, bur chucks or bit holders as they were variously called, were the forerunners of what is presently termed as "dental handpiece". Early examples of these were "Chevaliers drill stock", which was designed to bring the bur in various directions and was hand powered in a manner similar to that in an eggbeater, and Merry's drill stock, which boasted a flexible cable. This was also a type of angle handpiece. A handpiece is a device for holding rotating instruments, transmitting power to them and for positioning them intra-orally. Handpiece and associated cutting and polishing instruments developed as two basic types: straight and angle. Most of the development for preparing teeth has occurred within the last 100 years. Effective equipment for removal of enamel has been available only since 1947, when speeds of 10,000 pm were first used, along with newly marketed carbide burs and diamond instrument. Since 1963, certain improvements in the design & materials of construction of both handpiece & instruments have resulted in equipment that is efficient and steralizable, much to the credit of manufacturers and the profession alike. One of the most significant advancements was the introduction of the electric motor as power source in 1984. It was incorporated into a dental unit in 1914. The initial handpiece equipments and operating speeds (more of 5000 rpm) remained unchanged until 1946. The steel beers used at that time could not enamel efficiently, 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 the World War II. In the 10 years period, starting from late 1946, cutting techniques were revolutionized. Diamond instruments and tungsten carbide burs capable of cutting enamel were produced commercially. Both instruments performed best at high speeds available and that promoted the development of higher speed handpieces. Obtaining speeds of 10,000 15000 rpm was relatively simple matter of modifying existing equipment by enlarging the drive pulleys on the dental engine. By 1950, speeds of 60000 pm and more had been attained by newly designed equipments employing speed multiplying internal belt drives. They were known to be more effective for cutting tooth structure and for reducing perceived vibrations. The major breakthrough in the development of high speed rotary equipment came with the introduction of contra-angled handpieces with internal turbine drives in the contra-angled head. Early unit were water driven but subsequent units were air-driven. Although most current airturbine handpiece have free-running speeds of approx 300,00 rpm, the small size of the turbine in the head limits their power output. The speed can drop to 200,00 rpm or less, with small workloads during cutting and the handpiece may stall at moderate loads. This tendency to stall at high loads is an excellent safety feature for tooth preparation, since excessive pressure cannot be applied. Air-driven handpiece continue to be the most popular type of handpiece equipment because of the overall simplicity of design, case of control, versatility and patient acceptance. The external appearance of current handpiece is very similar to the earliest models. The low torque and power outputs 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 handpiece eliminated the necessity of having an electric engine as part of a standard dental unit. The design of straight handpiece turbine provided the desirable high torque for low speed operation. The most important function of rotary instrument in operative dentistry is the action of cutting and abrading. Cutting instruments used in density consists basically of a six bladed bar fabricated from a blank by special cutter. Prior to 1947 carbon steel was used to make dental cutter and their rotational speeds ranged up to 6,500 rpm. During rotation as each
cutting edge and blade face contacts enamel or dentin as fragment of tissue is removed. In 1947, tungsten carbide burs were introduced to the dental profession. This "Carbide bur was characterized by its hardness, being more than twice that of steel bur. In design and cutting potential as well as it efficiency and life expectancy it surpassed its predecessor. Other types of rotary instruments are those which abrade. Abrasion is the action of wearing away by friction. Typical of the abrasives used in dentistry are diamond, silicon carbide, aluminum and silicon dioxide. The first two of these abrasives are usually bonded to a blank from shaped to meet certain dental applications. The alumina and silica types are frequently impregnated on disk or strips or used in the form of thin water slurry on a soft wheel or brush. The use of diamond abrasive points, which became widespread in the 1940s, also was an important milestone, together with the carbide bur, in making possible higher and more efficient speeds for rotary instruments. The diamond point is composed of a number of small diamond particles bound to a rotary blank. In 1945 Dr. G.V.Black published a report on the â€œnon-mechanical preparationâ€? of cavities and introduced the air abrasive technique. The impact of Dr. Blackâ€™s revolutionary cutting technique on the dental profession was considerable. This was the first significant break with the long-established, traditional method of cavity preparation. While it did not stand the test of time as a practical method of removal of hard tooth tissue, it did serve to send the profession and the equipment manufacturers into an intense search for more effective and efficient methods of cutting tooth tissue. The air abrasive principle utilized particles (30 TO 50 u) of aluminum oxide propelled against the tooth surface by a carbon dioxide stream (110 psi) and funneled through a tungsten carbide nozzle with a lumen of 0.018 inch. The penetration of enamel and dentin was rapid but some what difficult to control. Precise, sharply defined cavity from was not achieved by the air abrasive method alone. In 1949 Walsh and Symons published their initial findings relating to the removal of tooth tissue with diamond points at rotational speeds up to 70,000 rpm. This report indicated the use of lighter forces and resulting increased cutting efficiency at these higher speeds. 5
Superimposed on this rapidly development era of rotary instrumentation was the unique ultrasonic method of tooth tissue removal. This unit, introduced in 1953, was designed so that suitably shaped tips vibrating at frequencies ranging from 15000 to 30000 cycles per second were used to remove tissue. Abrasive slurry of water and fine aluminum oxide, placed between the tip of the instrument and the tooth, produced an abrading action on dentin and enamel. A number of disadvantages of this method of cavity preparation limited its acceptance by the profession. ROTARY SPEED RANGES:Because of the vast range of rotational speeds currently available to the dental profession, some arbitrary classification is required to simplify terminology. Acc. to Charbonneau:Conventional of low speed
Below 10,000 rpm
Increased or high speed
10,000 to 150,000 Rpm (maximum range of bellDriven equipment)
Above 150, 00 rpm
According to Sturtevant three speed ranges are generally recognized: low or slow speeds (below 12000 rpm) medium or intermediate speeds (12,000 to 200000 rpm) and high or ultrahigh speeds (above 200000 ramp). The terms low-speed medium-speed, and light-speed are used. Most useful instrument is rotated at either low or high speed. Acc. to Marzuonk:a. Ultra-low speed (300-3000 RPM) b. Low speed (3000-6000 RPM) c. Medium high speed (20,000-45,000 RPM) d. High speed (45,000 - 100,000 RPM) e. Ultra-high speed (10,000 RPM and more) B. Pressure (P)
It has been observed clinically that low speed requires 2-5 pounds force, high speed requires less force (1 pound) and ultra-high speed still less force (1-4 ounces) for efficient cutting. One of the most desirable features of high speed rotary operation is better control with less fatigue on the part of the operator, and greater patient comfort. All these are due to reduction of forces on the tool and more efficient removal of tooth structure. C. Heat production Heat is directly proportional to: 1. Pressure 2. RPM 3. Area of tooth in contact with the tool Therefore, if any of these factors is increased, heat production is increased. Since heat production will cause pulps of teeth to be permanently damaged if a temperature of 1300 F is reached, heat must be carefully controlled. Even a temperature of 1300 F within the pulp can produce inflammatory responses that could result in pulpitis and eventual pulp necrosis. It has been shown that when the area of the cutting tool is reduced, but the speed to rotation is increased, it is an absolute necessity that coolants be employed to eliminate pulpal damage. This can be accomplished by various coolants such as flowing water a water-air spray or air. D. Vibration Vibration is not only a major annoying factor for the patient, but it also cause fatigue for the operator, excessive heat of instruments and, most importantly, a destructive reaction in the tooth and supporting tissue. Vibration is a product of the equipment used and the speed of rotation. The equipment, primarily the handpieces and the various revolving cutting tools, all contribute to the quantity and quality of vibration. E. Patient Reaction The factors that cause patient apprehension consist primarily of heat production, vibration 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 operation to counteract these potentially
irritating stimuli. The use of coolants, intermittent applications of a cool to the tooth; sharp instruments, etc. all aid greatly in minimizing both patient discomfort and unnecessary irritation to the oral structure. In instances where irritation in unavoidable (.e.g. drilling pin channels in vital teeth), the patients should be forewarned and properly anesthetized F. 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 pararoral 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. Operatorâ€™s confidence and patient cooperation can be enhanced only by proper training and experience on the part of the operator. G. Sources of Power The belt driven handpiece was rendered obsolete for operatory use. The air turbine remains the main power sources. H. Instrument design Instrument design for rotary instruments should be evaluated in two parameters: one the handpiece, which will hold and provide power for cutting tool and two the cutting tool itself (bur, stone, etc.) 1. Handpiece Handpieces come in a variety of sizes and shapes: straight, contraangled, and right-angled. Each is designed for a specific range of functions. They will retain the cutting tool by a screwing, latch, or friction grip type of attachment. The following criteria should be used in evaluating handpieces. a. Friction Friction will occur in the moving parts of handpiece; especially the turbine. This becomes critically important at speeds of high speed or above.
If the heat from friction is not prevented or counteracted, the handpiece will be unsuitable for dental use. For this reason, handpieces are equipped with bearings: ball bearing, needle bearings, glass and resin bearings, etc., the life spans of which will vary. b. Torque Torque is the ability of the handpiece to withstand lateral pressure on the revolving tool without decreasing its speed or reducing its cutting efficiency. Torque is dependent upon the type of bearing used and the amount of energy supplied to the handpiece. C. Vibration Vibration is a very deleterious aspect of rotary instrumentation. While some vibration is unavoidable, care should be taken not to introduce it unnecessarily. Excessive wear of the turbine bearings, for example will cause eccentric running which creates substantial vibration. It is best to always follow the manufacturerâ€™s recommendations for use and maintenance of handpieces to minimize turbine wear. 2. The rotary tools for the removal of tooth structure These are the units actually responsible for the removal of tooth structure and may be one of two types burs, which are cutting tools, and stones, which are abrading tools. a. Dental cutting burs i. Composition and manufacture Dental burs can be classified by their composition into two types: steel burs and tungsten carbide burs. Steel burs are cut from blank steel stock by means of a rotary cutter that cuts parallel to the long axis of the bur. Tungsten carbide burs are the product of powder metallurgy. i.e. a process of alloying in which complete fusion of the constituents does not occur. ii. General design of dental bur The dental burs are small milling (cutting) instruments.
A. Bur tooth: This terminates on the cutting edge, or blade. It has two surfaces, the tooth face, which is the side of the tooth on the leading edge; and the back or flank of the tooth, which is the side of the tooth on the trailing edge. B. Rank angle. The rank angle is the angle that the face of the bur tooth makes with the radial line from the centre of the bur to the blade. This angle can be negative if the face is beyond or leading the radial line (referring to the direction of rotation). It can be 0 if the radical line and the tooth face coincide with each other (radial rake angle). The angle can also be positive if the redial line leads the face, so that the rake angle is on the side of the radial line. C. Land: The plane surface immediately following the cutting edge. D. 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 which is the angle the land will make with work, and secondary clearance which is the angle between the back of the bur tooth and work. When the back surface of the tooth is curved, the clearance is called radial clearance. E. Tooth angle: This is measured between the face and back. If a land is present, it is measured between the face and land. F. Flue or chip space. The space between successive teeth. The number of teeth in dental cutting burs in usually 6-8. Every bur will have three parts the head - the portion carrying the cutting blades, the shank - the portion connecting the head to the attachment part, and the shaft or the attachment part - the portion which will be engaged within the handpiece. According to their mode of attachment to the handpiece dental burs can be classified as either latch type of friction grip type. Also they may be classified according to the handpiece they are designed for i.e. a contrangle bur or a straight handpiece they are designed for i.e. a contrangle bur or a straight hand piece bur. They can also be classified as right and left. The most common ones are the right, which cut when they revolve clockwise.
Burs can be classified as long, short (pedominiature) or regular, according to the length of the head. They may be classified as cutting burs or those used to finish and polish restorations. According to their shapes and sizes, they may be classified as follows: Round burs They are numbered from ¼, ½, 1, and 2 to 10. They are round is shaped and used for initial tooth penetration and for the placement of retention grooves. Wheel burs They are numbered as 14 and 15. They are wheel shape and are used to place grooves and for gross removal of tooth structure. Inverted cone burs They are numbered from 33¼, 33½, 34, 35 to 39. As the name indicates, they are an inverted cone shape, used mainly for cavity extension and occasionally for establishing wall angulations and retention forms. Plain Cylindrical fissure bur. They are numbered from 55 to 59. The bur teeth can be cut parallel to the long axis of the bur, which are designated straight or cut obliquely to the long axis of the teeth (for better unclogging), which are called spiral. Cross cut cylindrical fissure bur They are numbered from 555, 556 to 560. They arc numbered from 555. 556. to 560. Their teeth can also be cut parallel to the long axis of (he bur (straight) of obliquely (spiral). All four types of cylindrical fissure burs are used for gross cutting, cavity extension and creation of walls. Plain tapered fissure bur They are numbered from 168. 169, to 172. They have a tapered cylindrical head their teeth can be straight or spiral Cross-cut tapered fissure bur
They are numbered from 699, 700, to 703. They also can be straight or spiral. The four types of tapered fissure burs arc the most universally used burs in operative dentistry All eight types of fissure burs can be round-ended. The number I will be added to previous numbering to denote round nosing, e.g. Round plain cylindrical fissure burs will have the number from 156 to 159 Round crosscut cylindrical fissure burs will have (the numbers from 1555 to 1559, etc. Round-nose tapered fissure burs will have numbers from 1169 to 1172 and from 1700 to 1702. (It is impossible to round-nose the smaller tapered fissure burs. Pear-shaped burs As the name indicates, they are shaped like pears. They are numbered from 229 to 333 and arc mainly used in pedodontics End cutting burs They are cylindrical in shape, with just the end carrying blades. They are very efficient in extending preparations without axial reduction. They are numbered from 900 to 904. 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. Any slight wear of the culling edge will increase the dulling perceptibility. However, it is possible that large clearance angle may result in less rapid dulling of the bur. Number of teeth or blades and their distribution. The number of teeth in a dental bur is usually limited to 6-8. 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. Under certain conditions, nearly the same amount of material can be removed by cither 8, 7 or 6-flutcd burs. I.e. the product of the chip thickness removed by each tooth and the number of flutes may be nearly a constant. The reason for constructing burs with a fewer number of bur teeth arises from the possible increased space between the bur teeth 12
which decreases their clogging tendency. Inasmuch as each bur tooth is removing more material, the tendency for bur tooth wear should be greater and the cutting life reduced. Furthermore, it has been shown that a fissure bur with straight flutes produces less temperature rise than one with spiral flutes. This may be due to the formation of large chips by the straight bur. That chip then carries some heat energy with it. As might be expected, the fewer the number of bur teeth, the greater the tendency for vibration. However, if there arc two or more blades in contact with the work at one time this effect would not be of great importance Run-out 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 0.023 mm. Run-out will depend not only on the eccentricity of the bur itself, but also on the precision of the dental handpiece. If the shaft or collar holding the bur wobbles during rotation, the effect will be magnified at bur head according to the length of the bur shank. The efficiency in cutting of the bur is definitely affected by its run-out. If the bur moves away from the tooth periodically, all of the blades will not cut equally. If the operator senses this lack of cutting, he will probably exert greater force on the bur. The result will be that at one stage of revolution the bur and the work tend to be pushed apart, only to be driven together at the next half-revolution, resulting in disagreeable vibration. As far the rate of removal of tooth structure is concerned, although it may appear to be increased during such vibration, the structure is removed by a shattering rather than cutting process. Such a method of tooth removal is inefficient, inaccurate, and increases heat generation. Finish of the flutes The dental bur is formed by cutting each flute into the bur blink with a rotating cutter while it progresses nearly parallel to the axis of the bur. During the first cut or pass of the cutter the flute is roughly formed. The second cut places a cutting edge on the bur flute. However, considerable roughness all long the flute will remain. This roughness may be removed by making subsequent passes or cuts on the bur flute. Tests for cutting efficiency were done on different types of burs undergoing two, four, and six
5 lute cuts. Those cut six times were the most efficient while those cut two times were the least efficient. Heat treatment Heat treatment is used to harden a bur that is made of soft Heel. This operation preserves the edge placed on the bur flute by the cutter, and hardens the bur to increase its cutting life. (This operation is not needed with tungsten carbide burs).
Design of flute ends Dental burs are formed with two different styles of end flutes revelation cut where the flutes come together at two junctions near a diametrical cutting edge and the star where the end flutes come together in a common junction of the bur. The revelation type shows some superiority in cutting efficiency, but in direct cutting only lateral cutting both types prove to be of equal efficiency. Bur diameter Generally the forces on each bur tooth from external load do not depend on the diameter of the bur, but rather on the number of flutes or teeth and their rotational position. The average linear displacement per revolution and length of cut docs not depend upon the diameter of the bur. It follows that because the length of the cut is constant the volume of material removed will vary directly with the bur diameter as will as the torque and the amount of mechanical energy that the power source is required to supply.
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. This increase is so great that the volume of material removed by a shallow cut exceeds that of deeper cuts.
Influence of load 14
Load signifies the force exerted by the dentist on the tool head and not the pressure or stress induced in the tooth during cutting. 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. Actually the dentist operates according to the variables encountered every range of speed at which the bur is rotating has a minimum force or load under which the cutting efficiency of the bur used is decreased It also has a maximum force or load over which the cutting efficiency of the bur is decreased too because of torque. The minimum and maximum loads for low speed arc 1000-1500 gm, while the minimum and maximum loads for high speed arc 60-120 gm. Influence of speed With a given load the rate of cutting increases with the rotational speed, but this increase is not in direct proportion. The rate of increase in cutting at rotational speed above 30000 RPM is greater than that below this speed. b. Dental abrasive stones i. Designs Abrasive particles arc held together by means of a â€œbinderâ€? (base) of variable nature. The type of binder is intimately related to the life of the tool in use. With most abrasives, the binder is impregnated through- out with abrasive particles of a certain grade so that as a panicle is wrenched from the binder during use. Another will take its place as the binder wears. Furthermore, the abrasive should be distributed so that the surface of the tool wears evenly. The wide spacing between the panicles provides room for the resultant debris with less chance for packing or clogging. Some abrasives are glued to paper to form discs that can be attached to a handpiece via certain mounting tools. According to the composition of the abrasive panicles, dental stones can be classified as follows: A. Diamond stones They are hardest and most efficient abrasive stones for removing tooth enamel. B. Carbides They may be silicon carbides or boron carbides. 15
C. Sand Sand and other forms of quartz (cuttle) can be bound and mounted into different shapes of discs, stones, and strips. D. Aluminum oxide Natural or extracted pure aluminum oxide is one of the most efficient abrasives for stones in fine cutting. E. Garnet These particles contain a number of different minerals which possess similar physical properties and crystalline forms. Stones made of these can be used for finishing and polishing of dental appliances H. Factors Influencing the Abrasive Efficiency of Dental Stones A. Irregularity in shape of abrasive particles An abrasive should be irregular in shape so that it presents a sharp edge, i.e. a round smooth panicle would possess poor abrasive properties. Similarly, cubical panicles which would always present a flat face to the work would not be as efficient in abrasion as would irregularly shaped panicles. Therefore, the more irregular the particles, the greater the abrasive efficiency of the stone. B. Hardness of the abrasive material relative to that of the work will dull or wear. The harder the abrasive material relative to the hardness of the work the more the abrasive efficiency of the stone C. Impact strength of the abrasive material During rotation the abrasive particles strike the work suddenly. The abrasive should fracture rather than dull so that sharp edges arc always present. Fracture of the abrasive is also helpful in shedding the debris accumulated from the work. Although diamond stones will cut almost any type of tooth structure or restorative material, the diamond particles do not fracture, but rather lose substance at the lip. Furthermore, they arc likely to become clogged when ductile or soft substances arc abraded. They arc most efficient if they are used in removal of the very hard and brittle tooth enamel. 16
D. Size of the abrasive panicles The larger the particles, the deeper the scratches on the surface of the work and the faster the work will be worn away. E. Pressure and RPM The same factors arc involved here as discussed with rotary cutting instruments. iii. Types of dental stones Dental stones can be mounted, i.e., the abrading head (similar to the cutting head in a bur) is permanently welded to the shank and attachment part. They can also the unmounted, i.e. the abrading head is supplied separately and may be mounted on an appropriate mandrel. Mounted dental stones are provided in short (miniature), Regular or long lengths. They also may be cither in latch or friction grip form (attachment part) Dental stones can be produced in countless shapes: cylinder, wheel, and cone. inverted cone, tapered, doughnut, round, filamentous, V-shaped, hour glass, etc. At the present there is no standardized numbering system for dental stones; rather, every manufacturer has his own nomenclature according to size and shape. Although intact tooth structure can be removed by an-instrument rotating at low speeds, it is a traumatic experience for both the patient and the dentist. 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. Heat and vibration are the main sources of patient discomfort. At low speeds, burs have a tendency to roll out of the tooth preparation and mar the proximal margin or tooth surface. In addition, carbide burs do not last long because their brittle blades are easily broken at low speeds. Many of these disadvantages of low-speed operation do not apply when the objective is some procedure other than cutting tooth structure. The low-speed range is used far cleaning teeth, occasional caries excavation and finishing and polishing procedures. At low speeds, tactile sensation is better and there is generally less chance for overheating cut surfaces. The availability of a lowspeed option is it valuable adjunct for many dental procedures.
At high speed, the surface speed needed for efficient cutting can be attained with smaller and more versatile cutting instruments. This speed is used for tooth preparation and removing old restorations. Other advantage are : (1) diamond and carbide cutting instrument remove tooth structure faster with less pressure, vibration, and heat generation (2) the number of rotary cutting instruments needed 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; and (6) several teeth in the same arch can and should be treated at the same appointment. ROTARY CUTTING INSTRUMENTS The individual instruments intended for use with dental handpieces are manufactured in hundreds of sixes, shapes, and types. This variation is in part a result of the need for specialized designs for particular clinical applications or to fit particular handpieces, but much of the variation also results from individual preferences on the part of dentists. Since the introduction of high-speed techniques in clinical practice, a rapid evolution of technique and an accompanying proliferation of new instrument designs have occurred. Nevertheless, the number of instruments essential for use with any one type of handpiece is comparatively small, especially in the case of high-speed turbine handpieces. COMMON DESIGN CHARACTERISTICS In spite of the great variation among rotary cutting instruments, they have certain design features in common. Each instrument consists of three parts: (1) shank, (2) neck, and (3) head. Each has its own function, influencing its design and the materials used for its construction. Note that there is a difference in the meaning of the â€œshankâ€? as applied to rotary instruments / and to hand instruments.
Shank Design: The shank is the part that fits into the/ handpiece, accepts the rotary motion from the hand-piece, and provides a bearing surface to control the
alignment and concentricity of the instruments The shank design and dimensions vary with the handpiece for which it is intended. The American Dental Association Specification No. 23 for dental excavating bursâ€™ includes five classes of instrument shanks. Three of these the straight handpiece shank, the latch-type angle handpiece shank, and the friction-grip angle handpiece shank, are commonly encountered. The shank portion of the straight handpiece instrument is a simple cylinder. It is held in the handpiece by a metal chuck that accepts a range of shank diameters. Therefore precise control of the shank diameter is not as critical is for other shank designs. Straight handpiece instruments arc now rarely used for preparing teeth, except for caries excavation. However, they are commonly used for finishing and polishing completed restorations. The more complicated shape of the latch-type shank reflects the mechanisms by which these instruments are held in the handpiece. Their shorter overall length permits substantially improved access to posterior regions of the mouth in comparison with straight handpiece instruments. Handpieces that use latch-type burs normally have a metal bur tube within which the instruments fit as closely as possible, while still permitting easy interchange. The posterior portion of the shank 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, causing the instrument to be rotated. Latch-type instruments are not retained in the handpiece by a chuck, but rather by a retaining latch that slides into the groove found at the shank end of the instrument. This type of instrument is used predominantly at low and medium speed ranges for finishing procedures. At these speeds the small amount of potential wobble inherent in the clearance between the instrument and the handpiece bur tube is controlled by the lateral pressure exerted during cutting procedures. At higher speeds, the latch-type shank-design is inadequate to provide a truerunning instrument head, and as a result, an improved shank design is required for these speeds. The friction-grip shank design was developed for use with high-speed handpieces. This design is smaller in overall length than the latch-type instruments, providing a further improvement in access to the posterior regions of the mouth. As the name implies, friction-grip instruments originally were designed to be held in the handpiece by friction between the shank and a plastic or metal chuck. Careful dimensional control on the shanks of these instruments is important, because for high-speed use, even
minor variations in shank diameter can cause substantial variation in instrument performance and problems with insertion, retention, and removal. Neck Design Neck is the intermediate portion of an instrument that connects the head to the shank. It corresponds to the part of a hand instrument called the shank. Except in the case of the larger, more massive instruments, the neck normally tapers from the shank diameter to a smaller size immediately adjacent to the head. The main function of the neck is to transmit rotational and translational forces to the head. At the same time, it is desirable for the operator to have the greatest possible visibility of the cut-ting head and the greatest manipulative freedom. For this reason the neck dimensions represent a compromise between the need for a large cross-section to provide strength and 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 that perform the desired shaping of tooth structure. The shape of the head and the material used to construct it are closely related to its intended application and technique of use Many characteristics of the heads of rotary instruments could be used for classification. Most important among these is the division into bladed instruments and abrasive instruments. Material of construction, head size, and head shape are additional characteristics that are useful for further subdivision. Bladed and abrasive instruments exhibit substantially different clinical performances, even when operated under nearly identical conditions. This appears to result from differences in the mechanism of cutting that are inherent in their design DENTAL BURS The term bur .is applied to all rotary cutting instruments that havebladed 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, they were both expensive and variable in dimension and performance. The shapes, dimensions, and nomenclature of modern burs are directly related to those of
the first machine made burs introduced in 1891. Early burs were made of steel. Steel burs perform well, cutting human dentin at low speeds, but dull rapidly at higher speeds or when cutting enamel. Once dulled, the reduced cutting effectiveness creates increased heat and vibration.
Carbide burs. Which were introduced in 1947, have largely replaced steel burs for tooth preparation. Steel burs now are used mainly for finishing procedures. Carbide burs 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 tungsten carbide, are held together in a matrix of cobalt or nickel. Carbide is much harder than steel and less subject to dulling during cutting. In most burs, the carbide head is attached to a steel shank and neck by welding or brazing. The substitution of steel for carbide in those portions of the bur where greater wear resistance is not required has several advantages. It permits the manufacturer more freedom of design in attaining the characteristics desired in the instrument and at the same time allows economy in the cost of materials of construction. Although most carbide burs have the joint located in the posterior part of the head, others are sold that have the joint located within the shank and therefore have carbide necks us well as heads. Carbide is stiffer and stronger than steel, but it is also more brittle. A carbide neck subjected to a sudden blow or shock will fracture, whereas a steel neck will bend. A bur that is even slightly bent produces increased vibration and over cutting as a result of increased run-out. Thus, although steel necks reduce the risk of fracture during use, if bent they may cause severe problems Bur Classification Systems the United-States, dental burs traditionally have been described in terms of an arbitrary numerical code for head size and shape Newer classification system such as those developed by the International Dental Federation (Federation Dentaire Internationale (FDI) and International 21
Standards Organization (ISO) tend to use separate designations for shape (usually shape name) and size (usually a number giving the head diameter in tenths of a millimeter.
Shapes. The term bur shape refers to the contour or silhouette of the head. The basic head shapes are round, inverted cone, pear, straight fissure, and tapered fissure A round bur is spherical. This shape customarily has been used for such purposes as initial entry into the tooth, extension of the preparation, preparation of retention features/and caries removal. An inverted cone bur is a portion of a rather rapidly tapered cone with the apex of the cone directed toward the bur shank. Head length is approximately the same as the diameter. This shape is particularly suitable for providing undercuts in tooth preparations. A pear-shaped bur is a portion of a slightly tapered cone with the small end of the cone directed toward the bur shank. The end of the head either is continuously curved or is flat with rounded corners where the sidesâ€™ and flat end intersect. A normal-length pear bur (length slightly greater than the width) is advocated for use in Class I tooth preparations for gold foil. A long-length pear bur (length three times the width) is advocated for tooth preparations for amalgam. A straight fissure bur is an elongated cylinder. Some advocate this shape for amalgam tooth preparation Modified burs of this design with slightly curved tip angles are available. A 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 used for tooth preparations for indirect restorations Sizes.
In the United States, the number designating bur size also has traditionally served as a code for head design. This numbering system for burs was originated by the S.S. White Dental Manufacturing Company in 1891 for their first machine-made burs. The original numbering system grouped burs by 9 shapes and 11 sixes. The Â˝ and Â˝ designations were added later when smaller instruments were included in the system. All original bur designs had continuous blade edges. Later, when crosscut burs were found to be more effective for cutting dentin at low speeds, crosscut versions of many bur sizes were introduced. This modification was indicated by adding 500 to the number of the equivalent noncrosscut size. Thus, a 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 a No. 957 No. 557 and No.57 bur all the same head dimensions. Modifications in Bur Design. As available hand-piece speeds increased after 1950, particularly after the high-speed turbine handpieces were introduced, a new cycle of modification of bur sizes and shapes occurred. As the effectiveness of small burs has increased, they have re-placed larger burs in many procedures. Three other major trends in bur design are discernible reduced use of crosscut, extended heads on fissure and rounding of sharp tip angles. Crosscuts are needed on fissure burs to obtain adequate cutting effective at low speeds, but at high speeds they are not needed. Because crosscut burs used at high speeds tend to produce unduly rough surfaces. Carbide fissure burs, with extended head lengths two to three times those of the normal tapered fissure burs of similar diameter have been introduced. Such a design would never have been practical using a brittle material-such as carbide if the bur were to be used at low speed. The applied force required to make a bur cut at speeds of 5000 to 6000 rpm would normally be sufficient to fracture such an attenuated head. The extremely light applied pressures, needed for cutting at high speed, however permit many modifications of burs that would have been impractical at low speed.
The third major trend in bur design has been toward rounding of the sharp tip corners because teeth are relatively brittle, the sharp angles produced by conventional burs can result in high stress concentrations and increase the tendency of the tooth to fracture. Bur heads with rounded corners result in lower stresses in restored teeth, enhance the strength of the tooth by preserving vital dentin, and facilitate the adaptation of restorative materials. Both carbide burs and diamond instruments of these designs last longer because there are no sharp corners to chip and wear. Such burs facilitate tooth preparation with desired features of a flat preparation floor and rounded internal line angles. CUTTING MECHANISMS: For cutting, it is necessary to apply sufficient pressure to make the cutting edge of a blade or abrasive particle dig into the surface. The process by which rotary instruments cut tooth structure is complex and not fully understood. EVALUATION OF CUTTING: Cutting can be measured in terms of both effectiveness and efficiency. Certain factors may influence one, but not the other. Cutting effectiveness is the rate of tooth structure removal. Effectiveness does not consider potential side effects such as heat or noise. It is possible to increase effectiveness while decreasing efficiency. A dull bur, for example, may be made to cut faster than a sharp bur by applying a greater pressure, but the experience indicates that this results in great increase in heat production, thus reduces efficiency. There is general agreement that increased rotational speed results in increased effectiveness and efficiency. Adverse effects associated with increased speeds are heat, vibration, and noise. Heat has been identified as a primary cause of pulpal injury. Air-water sprays do not prevent the production of heat, but do serve to remove it before it causes a damaging rise in temperature within the tooth. BLADED CUTTING: Tooth structure like any other brittle material undergoes both brittle and ductile fracture. Brittle fracture involves plastic deformation of a 24
material usually proceeding with shear. Low speed tends to proceed by plastic deformation before tooth structure fractures. High speed cutting, especially of enamel proceeds with brittle fracture. Many factors interact to determine which cutting mechanism is active in particular situation. The mechanical properties of the tooth structure, the design of the cutting edge or point, the linear speed of the instrumentâ€™s surface the contact force applied and the power output characteristics of the handpiece influence the cutting process in various ways. In order for the blade to initiate the cutting action, it must be sharp, have a higher hardness and modulus of elasticity than the material and must be pressed against the surface with sufficient force. Sheared segments accumulate in a distorted layer as it rotates. The chips that accumulate in the clearance space between blades until washed out or thrown out by centrifugal force. Mechanical distortion of the tooth surface ahead of the blade produces heat. Frictional heat is produces 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 increase 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 is observed in teeth cut at very high speeds may, in part, be caused by the removal of the heated surface layer of tooth structure by a following blade before the heat can be conducted into the tooth. ABRASIVE CUTTING: The cutting action of the diamond abrasive instruments, but the key differences results from the properties, size, and distribution of the abrasive. The very high hardness of the diamonds provides them superior resistance to wear. A diamond instrument that is not abused has little or no tendency to dull with use Individual diamond particles have a very sharp edge, are randomly oriented on the surface and tend to have large negative rake angles.
When diamond instrument are used to cut ductile materials, some material will be removed as chips, but much will flow laterally, around the cutting point and be left as a ridge of deformed material on the surface. Repeated deformation work hardens the distorted material until irregular portions become brittle and break of before being removed. This type of cutting is less effective than that of the blade; therefore burs are generally used for cutting ductile materials such as dentin. Diamonds cut brittle materials by a different mechanism. Most cutting results from tensile fractures that produce a series of subsurface cracks. Diamonds are most efficient when used to cut brittle materials and are superior to burs for the removal of dental enamel.
CUTTING RECOMMENDATIONS: Overall, the requirements for effective and efficient, cutting include using a contra-angle handpiece, air-water spray for cooling, high operating and a carbide bur or diamond instrument. Carbide burs are better for endcutting, produce lower heat and have more blade edges per diameter for cutting. They are effectively used for punch cuts to enter tooth structure, intracoronal preparations, amalgam removal, small preparations and secondary retentive features. Diamond instruments have higher hardness and coarse diamonds have very high cutting effectiveness. Diamonds are more effective than burs for both intracoronal and extracoronal tooth preparations, beveling enamel margins on tooth preparations and enameloplasty. HAZARDS WITH CUTTING INSTRUMENTS: Almost everything done in a dental office involves some risk to the patient, dentist and/or auxiliaries. For the patient, there are pulpal dangers from the tooth preparation and restoration procedures. There are also soft tissue dangers. Everyone is potentially susceptible to eye, ear and inhalation dangers. However, careful adherence to normal precautions can eliminate or minimize mot risks associate with cutting instruments. PULPAL PRECAUTIONS:
The use of cutting instruments can harm the pulp by exposure to mechanical vibration, heat generation, desiccation and loss of dentinal tubule fluid and/or transaction of odontoblastic processes. As the thickness of the remaining dentin decreases, the pulpal insult (and response) from heat or desiccation increases. Slight to moderate injury produces a localized, protective pulpal response in the region of the cut tubules. In severe injury, destruction extends beyond the cut tubules, often resulting in pulpal abscess and death of the pulp. These pulpal sequel (recovery or necrosis) take from 2 weeks to 6 months or longer, depending on the extend and degree of the trauma. A young pulp is more prone to injury; it also recovers more effectively when compared with older pulp, in which the recuperative powers are slower and less effective. Enamel and dentin are good thermal insulators and will protect the pulp if the quantity of heat is not too great and the remaining thickness of tissue is adequate. The longer the time of cutting and the higher the local temperature produced, the greater is the threat of thermal trauma. Steel burs produce more heat than carbide burs because of inefficient cutting. Burs and Or plugged with debris do not cut efficiently, resulting in heat production. When used with 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 because it needlessly desiccates the dentin and damages the odontoblasts. An air coolant be used only when visibility is a problem, such as the finishing procedures of tooth preparations. Air-water spray is universally used to cool, moisten and clear the operating site during normal cutting procedures. In addition, the spray lubricates, cleans and cools the cutting instrument, thereby increasing its efficiency and service life. A well-designed and properly directed air-water spray also helps keep the gingival crevice open for better vision when gingival extension is necessary. The use of a water spray and its removal by an effective high-volume evacuator are especially important when old amalgam restorations are removed because they decrease mercury vapor release and increase visibility. During normal cutting procedures a layer of debris, described as a smear layer, is created that covers the cut surfaces of the enamel and dentin. The smear layer on dentin is moderately protective because it occludes dentinal tubules and inhibits the outward flow of tubular fluid and the 27
inward penetration of microleakage contaminants. However, the smear layer is still porous. When air alone is applied to dentin, local desiccation may produce fluid flow and affect the physiologic status of the odontoblastic processes in the underlying dentin. SOFT TISSUE PRECAUTIONS: The lips tongue and cheeks of the patient are the most frequent areas of soft tissue injury. The handpiece should never be operated unless there is good access and vision to the cutting site. A rubber dam is very helpful in isolating the operating site. When the dam is not used, the dental assistant can retract the soft tissue on one side with a mouth mirror, cotton roll and / or evacuator tip. The dentist can usually manage the other side with a mirror And a cotton roll. With air turbine handpiece, the rotating instruments do not stop immediately when the foot control is released. The operator must either wait to stop or be extremely careful when removing the handpiece from the mouth so as not to lacerate soft tissues. The large disc is one of the most dangerous instruments used in the mouth. Fortunately such dics are seldom indicated intraorally. They should be used with light, intermittent application and with extreme caution. The dentist and the assistant must always be aware of the patientâ€™s response during the cutting procedures. A sudden reflex movement by the patient such as gagging, swallowing or coughing could result in serious injury. If an accident does occur in which soft tissue is damaged, the operator should remain calm and control any hemorrhage with a pressure pack. The patient should be told what has happened and medical assistance should be obtained if needed. The chance of mechanical pulpal involvement may be greater if a hand excavator is used to remove the last portions of soft caries in a deep preparation. When the remaining dentinal wall is thin, the pressure exerted on the excavator may be sufficient to break into the pulp chamber. Therefore a round bur may be used at a low speed with light, intermittent pressure for caries removal. Air-turbine hand pieces should be operated just above stalling speed to improve tactile for caries removal. EYE PRECAUTIONS:
The operator, assistant and patient should wear glasses with side shields to prevent eye damage from airborne particles during operative procedures utilizing rotary instrumentation. Protective glasses are always indicated when rotary instrumentation is being used. The dentist is more likely to receive injury than is the assistant or the patient, because of being in a more direct path of such particles, if an eye is injured, it should be covered by a clean gauze pad until, and medical attention can be obtained. Furthermore, precautions must be taken for prevention of eye injury from unusual light sources, such as visible light-curing units and laser equipment. Dental personnel and patients should be protected from high intensity visible light using colored plastic shields (attached to the fiberoptic tip). Laser light can be inadvertently reflected from many surfaces in the dental operatory, therefore the dental operatory should be closed and everyone should wear protective goggles. EAR PRECAUTIONS: An objectionable high-pitched whine is produced by some air-turbine handpiece 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:1. The Intensity of loudness (decibels (db}), 2. Frequency 3. Duration (time) of the noise, as well as 4. The susceptibility of the Individual A certain amount of unnoticed noise (ambient noise level} is present even in a quiet room (20 to 40 db). An ordinary conversation averages 50 to 70 db in a frequency range of 500 to 2500 hertz. Turbine handpiece with ball bearings, free running at 30 pounds air pressure, may have noise levels as high as 70-94 b at high frequencies. Noise levels in excess of 75db in frequency ranges from 1000- 8000 hertz may cause hearing damage. There is considerable variation in the noise levels among handpiece by the same manufacturer. Handpiece wear and eccentric rotating instruments can cause increased noise. Protective 29
measures are recommended when the noise level reaches 85 db with frequency ranges from 300 to 4800 hertz. Protection is mandatory in areas where the level transiently reaches 95 db. The effect of excessive noise levels depends on exposure times. Normal use of the 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 sound proofing helps and can accomplished with absorbing materials used on walls and floors, Anti noise devices can be used to cancel unwanted sounds as well. INHALATIONAL PRECAUTIONS: Aerosols and vapors are created by cutting tooth structure and restorative materials. Both aerosols and vapors are a health hazard to all present. 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, and composites) as 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 aerosol 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 fitter out either mercury or monomer vapors. Conclusion: Many treatment procedures in restorative dentistry substantially involve rotary instrumentation. While the great majority of cutting procedures on enamel & dentin once employed hand instruments, the bulk of tooth tissue removal is now accomplished using rotary instruments. The use of these rotary cutting instruments have made the job a lot easier for the practitioner as well as a lot comfortable for the patient besides saving precious time. 30
Nevertheless, their use demands a sound knowledge of the equipment and instruments without which the optimum effect of these instruments will be not achievable.
References 1. ADA: Council of dental research adopts standards for shapes and dimensions for excavating burs and diamond instruments, J Am Dent Assoc 67:943,1963. 2. Eames WB, Nale JL: A comparasion of the cutting efficiency of air driven fissure burs, J Am Dent Assoc 86;412-415,1973. 3. Hartley JL, Hudson DC: Modern rotating instruments: burs and diamond points, DCNA 737, Nov 1958. 4. ISO 2157: head and neck dimensions of designated shapes of burs, Geneva, Switzerland 1972, ISO. 5. Leonard DL, Charlton DG: Performance of high speed dental handpieces, J Am Dent Assoc 130:1301,1999. 6. Morrant GA, Burs and rotary instruments: introduction of a new standard numbering system, Brit Dent J 147(4):97-98,1979. 7.
Peyton FA: Effectiveness of water coolants with rotary cutting instruments. J Am Dent Assoc 56: 664-675,1958.