HAND CUTTING INSTRUMENTS
The removal and shaping of tooth structure are essential aspects of restorative dentistry. Initially this was a different process accomplished entirely by the use of hand instruments. The introduction of foot operated engine in 1871 and rotary powered cutting equipments was one of the truly major advances in the dentistry. Modern high speed equipments have eliminated the need of many hand instruments for tooth preparation nevertheless; hand cutting instruments remain an essential part of armamentarium for quality restorative dentistry. The early hand-operated instruments were with their large heavy handle, inferior metal alloys in the blades, were cumbersome and awkward to use, and ineffective in many situations. Like wise there was no uniformity in manufacturing and nomenclature of the instruments. As the commercial manufacture of hand instrument increased, the various dentists began to express their ideas of tooth preparation. G.V. Black among many contributors to modern dentistry is credited with the acceptable nomenclature and classification of hand cutting instruments. The hand cutting instruments are used to split and plane enamel along its direction of enamel rods or they are used to split the dentin by sculpturing the internal parts of the cavity.
Rotary burs, or diamonds are used for gross reduction, angulation and refinement, and hand cutting instrument are used to place the intricate details of the cavity preparation. Material used / Manufacturing process: Hand cutting instruments are mainly made up of carbon steel and stainless steel. In addition some instruments are made by carbide inserts to provide more durable cutting edges. Carbon steel alloy contain alum and 0.5% - 1.5% carbon. Carbon steel is harder than stainless steel and known for its sharpness but disadvantage is that is susceptible to corrosion when sterilized in metallic salt solutions. Also if dropped can fracture. In manufacturing process, blank steel is bent to the degree of angulation, needed in the shank and blade. The edges are then milled to produce cutting edges and structural design. The steel is then heated to 1500째F to 1600째F and then quenched to harden the working edge. Not more than 1 to 2
is heated for
hardening purpose because since beyond this dimension the instrument will loose balance after the heated treatment. Cutting edges are usually tempered to produce additional hardness and remove some brittle effects. To accomplish this tip is reheated at lower temperature and quenched in solutions of oil, acid or mercury. Hard steel is capable of being tempered, and additional treatment supplements the quality of alloy. After the cutting edges are (heat tempered)
To gain maximum beneficial effects from carbon steel and stainless steel, manufacturers must submit two heat treatments. Hardening and Tempering: Hardening heat treatment hardens the alloy but also makes it brittle especially when the carbon content is more. Tempering heat treatment relieves strain and increases toughness. After cutting edges are tempered, the shanks of the instruments are swaged into handles and pressure welded. Because of cost factor and type of a different metal than that for handle is used for cutting edge. The handle and shank are chrome plated to prevent unsightly tarnish and corrosion. Thus the cutting edges are only sight that undergo tarnish and corrosion in sterilizing solutions. This corrosion is caused by attack on the grain boundaries of carbon steel by sterilizing solution. Periodic sharpening and steam alcohol sterilization prevent the running of cutting edges. Stainless steel alloy that used to make the hand cutting instruments Cr = 15 to 25%, carbon = 1% and rest Fe. Cr gives the corrosion resistance but loses keen edges much more quickly than the carbon steel. Carbide although is hard and wear resistant is brittle. Stellite another alloy used for manufacturing of the instruments. It consists of 65% to 90% Co and 10% to 35% Cr and trace elements amounts of tungsten molybedum and iron. There is high resistance acid of hardness.
CLASSIFICATION: General classification of operative instruments:1) Cutting instruments:
Hand Hatchet Chisel Hoes Excavator Other Rotary burs Stones Disks and other
2) Condensing instrument: Pluggers hand Mechanical 3) Plastic instruments:
Spatulas Carvers Burnishers Packing instruments
4) Finishing and polishing instruments: Hand:
Orange wood sticks Polishing Finishing strips
5) Isolation instruments:
Rubber dam frames Clamps, forceps, punch Saliva ejector Cotton roll holder
6) Miscellaneous instruments:
Mouthmirror Explorer Probes Scissors Pliers, Others
According to Margouk instruments for the tooth removal can be classified as:1) Hand cutting instruments 2) Rotary instruments an abrasive instruments 3) Ultrasonic instruments According to Sturdwent hand instruments are:1) Cutting instruments a. Excavator. E.g. Ordinary hatchet, hoe, angle former, spoon b. Chisel. E.g. Straight, triangle, binangle, 3-angle, 4 curved, enamel hatchet, gingival margin trimers c. Others: Knifes, files, scalers and carvers 2) Non-cutting instruments According to Sturdevant all hand cutting instruments are excavators. Basic Design of Hand Cutting Instruments: Most hand instruments regardless of their use can be classified as, a) Handle or shaft b) Shank c) Blade Handle or Shaft: small, medium or large It can be smooth, knurled or serrated. Serration to increase the friction for hand gripping. Earlier large handle with large diameters were used which were grasped in the palm of the hand. These large handles were not always conducive of the delicate manipulations.
In north America most handles are small in diameter 5.5mm and light 1/4 th inch. In Europe the handles are often larger in diameter and tapered. Shank: Which connects shaft to the blade or working point. It usually tapers from the connection with the shaft to where the blade begins. Thus they are smooth rand and tapered. The shank has often one or more bents to avoid the instrument having a tendency to twist in use when a force is applied. The shank can be straight, monoangled, binangled and triple angled. The shank joins the blade to the handle and is angled to keep the working end within the 2.0 to 3.0 mm of the long axis of the handle. This angulation in the shank is intended to provide balance so that when force is applied or exerted on the instrument, no torque should be produced i.e. instrument is no likely to rotate, decreasing the effectiveness of the blade and causing damage to the tooth structure. Thus balance allows the concentration of the force on to the blade without allowing rotation of the instrument in the grasp. When working end is not aligned with the handle, the instrument is said to be out of balance. Such an instrument is also useful in tooth preparation. The blade of these type of instruments is relatively short and is used with the minimal force. Sharpness concentration of he forces at the small area of the edge producing high stress.
Blade or rib: Blade is the part of the instrument bearing the working end or cutting edge. This is the functional end of the instrument. The blade is used to cleave and smooth enamel and dentin. Cutting edge forms a 45° with the blade thus having maximum bulk and sharpness on the edge are developed. This angle is maintained during the sharpening. Nib is found on condensers and are used to impact and adapt the materials in the cavity forms. They can be smooth and serrated. Amalgam and cohesive fold are serrated but others are not. Nomenclature of hand cutting instruments: G.V.Black gave the nomenclature of hand cutting instruments similar to biological classification:1) Order – purpose of instrument (excavator) 2) Suborder - position and manner of use (push) 3) Class – form of working end (hatchet) 4) Subclass – shape / type of shank (binagle) Naming from 4 to 1 2 number is variable and non-specific and for practical purposes it is omitted. Blacks instrument formula: Cutting instruments have a formula describing the dimension of the blade and angles of the working ends. They are placed on the handle using a code of 3 or four numbers separated by the dashes or spaces. 7
There can be three numbered formula and four numbered formula. 1st number describes the width of the blade in 1/10th of the millimeter. 2nd number describes the length of blade in millimeter 3rd number describes the blade angle (The angle between the long axis of the blade and long axis of the handle. It is represented in centigrade. (% of 360째) e.g. 85. If the cutting edges are not at right angle to the long axis of the blade. Then the fourth number comes into existence which represents the primary cutting edge angle. Primary cutting edge angle is the angle between the long axis of the cutting edge and long axis of the handle. The instrument is so positioned that the primary cutting edge angle always exceeds no.50. it is kept in 2nd position. e.g. of the 3 number instrument: Hatchet, chisel, hoe e.g. of the 4 number instrument: Angle former, gingival margin trimmer Cutting Instrument Bevels: Most 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 extends from the primary edge for length of the blade. Bibevelled instrument such as ordinary hatchet have two bevels that form the cutting edge. Principle of hand cutting instruments: The main principle of hand cutting instruments is to concentrate the forces on the very thin cross section of instrument at he cutting edges. Thus thinner the
cross section more pressure that is concentrated and more efficient the instrument will be. Direct cutting and lateral cutting instruments: A direct cutting instrument is one in which the forces applied in the same plane as that of the blade and handle. It is a single planed instrument. Lateral cutting instrument is one in which the force is applied at the right angles to the blade and handle. These instruments have generally curved blades and called double planed instruments. The single planed instrument may have two or more curves or angles in their shanks, all in the same plane to handle. Double planed instruments have an angle in the shank angle to the handle. Single planed instruments can be used for direct and lateral cutting. Double planed instruments can be used for only lateral cutting. Contrangling: In order to gain access, many instruments have the shank bent at one or more points to the angle the blade relative to the handle. The extent of this arrangement generally depends upon the length of the blade and degree of the angulation of the shank. If the working point is moved out of the line of the axis of the handle and if it is more than 3 mm the instrument is said to be out of balance. To solve this problem in the modern instrument the shank is angled in such a way that the working end is placed within 3 mm of the long axis of the blade. This principle design is called contrangling. 9
Short blades and smaller blade angles require binangle contrangling. Larger blades and large blade angles require triple angle contrangling. The length of the blade is determined by the depth of the cavity and blade angle is determined by the accessibility requirements. It follows that greater blade angles are required for posterior teeth and incisal portion of proximal cavities of anterior teeth so in addition to balancing contrangling provides better access and clearer for field of operation. Right and left instrument: Direct cutting Instruments can be right or left depending upon the side the bevel is placed. To determine whether the instrument is right or left. The primary cutting edge is held downwards poorly away if the bevel is pointing towards the left side than it left sided instrument, if it is forcing towards the right side it is right sided instrument. In single planed instruments and for direct cutting acts as the non-beveled side of the blade should be in contact with the wall being shared. For lateral cutting acts always more the instrument scraping action from beveled side to nonbeveled side of the blade. Single beveled instruments: Single beveled instruments are single planed instruments with cutting edges perpendicular to the long axis of the blade. They are all designed as the straight chisel. If the bevel is placed away from the shaft it distal beveled. If the bevel is placed towards the shaft then it is mesial beveled. If one observes the inside of the
blade (or the inside of the angle at the junction of the blade and shank) the primary bevel is not visible, the instrument has a distal bevel. Conversely if the bevel is seen than it is the mesial bevel or reverse bevel. Bibeveled instrument: Ordinary hatched and straight chisel can be bibeveled. The blade is equally beveled on both sides, and they cut by pushing them in the direction of the long axes of the blade. Triple beveled instruments: Beveling the blade laterally together with the end forms the three distinct cutting edges. Quadri-beveled instruments: mainly occurs in the double planed instruments where the blade is beveled at all the peripheries. Push and pull motion: If the bevel is on the side of the blade towards the shaft they are called mesially beveled. If these type of instruments has no angle in the shank or angle less than 12째 or less, they are used in push or direct cutting and scraping motion (beveled to non-beveled side). If the bevel is on the side of the blade away from the shaft they are called distally beveled. If these types of instruments has angle more than 12째 on the shank they are used in pull motion. Single-ended and double ended instruments: Most modern instruments are double ended incorporating the right and left or mesial and distal form of the instrument of the same handle. Single planed instruments with no angle in the shank has potential of fine cutting movements. 11
Vertical (parallel to long axis of blade), right, left push and pull. For single planed right and left instruments vertical push and pull, right or left. For mesial and distal beveled instrument – vertical, right, left, push or pull. The hand cutting instruments: 1) Excavators:- Excavators are designed for the excavation and removal of carious dentin and shaping the internal parts of the cavity. a) Ordinary Hatchet: • Cutting edges are directed in the same plane to the long axis of the handle. • Cutting edges are perpendicular to the long axis of the blade. • 3 number formula • Bibevelled • Paired right and left. The right of the pair is identified in the pair by indented ring on the shaft. • Bibeveled: Two bevels of the equal length placing the cutting edge in the center. -
These are single ended
Uses: Preparing the retentive areas and sharpening the internal line and point angles in the direct gold restorations.
• Bibeveled their blade length is small and used in the anterior teeth. • They are used in the chopping motion to create incisal dentin retention in class III cavity form. They can share the wall by vertical action and may be used laterally with scraping motion. 12
b) Hoe:- The primary cutting edge of the blade is perpendicular to the long axis of the handle. Difference between the hoe and chisel:- Hoes blade has an angle greater than the 125° centirgrates to the long axis of the handle. Chisel blade has an angle lesser than 12.5 centigrade to the long axis of the handle. Uses: This type of instrument is mainly used for the planning the walls of the cavity and refining and sharpening of the cavity internal line angles. Used in class III and V direct gold restorations. Hoes are doubled planed instruments. Hoe is used with pull motion. Chisel is used with push motion. Some sets of the hoe contains larger and heavier blades with the shanks contraangled. They are intended for use for enamel on posterior teeth. 3) Angle former:The primary cutting edge is at an angle other than 90° to the long axis of the blade. The angle of the cutting edge to the blade axis of the angle former is usually 80 to 85°. This creates a pointed and linear cutting edge. Being single planed instruments with right and left beveling they can have three cutting potentials – vertical, push and pull. • they can be large, medium and small angle former • It is modified chisel and be described as the combination of the chisel and gingival margin trimmers. • Right and left according to acute angle being directed.
â€˘ Angle former used to accentuate line angle and point angle of internal line angle form of the cavity for cohesive gold restoration. â€˘ Angle former has primary cutting edges but can also have secondary cutting edges. Beveling of the blade laterally together with the end bevel form three distinct cutting edges. This is helpful in additional lateral cutting quality of the instrument. 4) Spoon excavator:The spoon excavator is a double planed instrument (curved blade) that is a modified hatchet. Unlike the slight cutting edges of the hatchet the spoon excavator has rounded cutting edges or cutting edges at the end of the blade is in the form of semicircle. This gives instrument on outer convexity and inner concavity that makes it somewhat look like spoon. It can be left sided or right sided. This shape of cutting tip to some degree, the action of the instrument in spooning and scooping of the carious material. Like double planed instrument for lateral scraping. It is always a paired instrument with curved blade directed right. Used for the caries removal and amalgam cavity. The cutting edges are circular than it is discoid and claw like than it is cleoid. The shank binangle and triple angle to facilitate accessibility:Discoid:- The cutting edges are a periphery and are used with right and left cutting movements.
Cleoids:- Lateral scraping movements and used in areas which are difficult to accessible. Chisel: A chisel is an excavator primarily used for planning and cleaving enamel and dentin. It is characterized by blade that terminates in a cutting edge formed by one sided bevel. The cutting edge of the chisel as at perpendicular to the long axis of the handle of the instrument. Different types of chisel:Straight chisel:- have a straight blade in line with shank and handle. They are single planed instruments with five possible cutting movements. Monoangle chisel:- Similar to straight chisel except the blade is placed at same angle to the shaft. It may be mesially or distally beveled. Binangle chisel:- has two angles in the shank. It is also mesially and distally beveled. Used to clear the undermined enamel. Triple angle chisel:- Three angle in the shank and usually often used to plane the pulpal floor. The monoangle, binangle and triple angle chisels are single planed instruments. All possible cutting movements are vertical, right and left. The mesially beveled chisels can cut in push movement and distally beveled chisel can cut in pull movement.
Weldstaldt chisel: Weldstaldt chisel is a chisel with a slight vertical curvature in the shank. It is beveled on one side. If the bevel is towards the curvature than it is mexially beveled if the bevel is away from the curvature then it is distally beveled. Used for cleaving of the undermined enamel and shaping of enamel walls. Single planed instruments, three cutting motions â€“ vertical, right and left. Mesial beveled can be use for push motion and distal beveled can be used for pull motion. Straight chisel available in sizes 1.0 and 2.0 nm. Binangle chisel available in these widths 1.0, 1.5 and 2.0 mm. Weldstaldt chisel available in these widths 1.0, 1.5 and 2.0 mm. Enamel Hatchet: The cutting edge is in the same plane to the long axis of the handle. Paired right or left sided. Similar to design of one ordinary hatchet except the blade is larger, heavier and single beveled. Used for cutting enamel. Gingival Marginal Trimmer: Gingival marginal trimmer is an modified hatchet. The two distinct modifications of the basic design of the hatchet are seen. 1) In the hatchet cutting edge is at the same plane to the long axis of the blade but in gingival margin trimmer the cutting edge is at angle other than 90Â°. 2) The hatchet is single planed instrument as it has a straight blade. Whereas the gingival margin trimmer has a curved blade thus it is double planed instrument. 16
3) The curved blade accentuates the lateral scrapping ability of the instrument. Like hatchet GMT is also paired right and left, but it is also paired according to the bevels mesial and distal. Thus GMTs are two paired instruments and double ended instruments, one instrument mesial GMT (with left and right cutting ends) and distal GMT (with left and right cutting ends). The cutting edge of one pain making an angle with the edge of the blade furthest from the handle. These are distal GMT. The cutting edge of the other pan making an acute angle with the edge of the blade nearer to the handle. This is mesial GMT. 4) Numbered instrument:When the 2nd number in the formula is 90 to 100, the instrument is used in the distal gingival margin. When the 2 nd number on the formula is 85 to 75 the instrument is used in the mesial gingival margin (formula of mesial GMT). The 100 and 75 pair are for inlay or onlay to prepare steep gingival bevels 90 and 85 pairs for amalgam preparation with gingival enamel bevel that decline gingivally only slightly. Used to give the bevel on the gingival enamel margins of inlay and amalgam restorations. Offset Hatchets: The offset hatchets are the set of the regular hatchets with the whole blade rotated quarter of a turn forward or backward around its long axis. They are single
planed instrument. The rotation and angulation provides accessibility the difficult areas. • Quenching • Heat treatment • Age hardening • Work hardening • Temperature In equilibrium phase diagram: The temperature at which the first solid begins to form is called the temperature. The temperature at which the last liquid is solidus temperature. Heat treatment is the thermal processing of an alloy for a length of time above room temperature put below the solidus temperature. Quenching:- This is the rapid cooling from an elevated temperature to room temperature or below. This is done for basically two reasons. 1) To preserve at room temperature a phase ordinarily stable only at elevated temperatures. 2) To rapidly terminate a process that only occurs at elevated temperatures. HAND INSTRUMENT TECHNIQUES: Instrument Grasp: There are four grasps for hand cutting instruments. a) Modified Pen Grasp:- The grasp that permits the greatest delicacy of tough is modified pen grasp. Pads of thumb, index and middle finger contact the
instrument, while the tip of ring finger (or tips of ring and little finger) is placed on a nearby tooth surface of the same arch as a rest. The pad of middle finger is placed near the topside of the instrument. The index finger tip is positioned well above the middle finger on same side of the handle. The pad of the thumb is placed midway between the index and middle finger on the opposite side of handle. This creates a triangle of forces or tripoding effect that enhances control because it counteracts the tendency of instrument to turn uncontrollably between fingers. b) Inverted pen grasp:- The finger positions of the inverted pen grasp are same as modified pen grasp. However the hand is rotated so that palm faces towards the operator. This grasp is used mostly for tooth preparation utilizing lingual approach on anterior teeth. c) Palm and thumb grasp:- The palm and thumb grasp is similar to that used for holding a knife. The handle placed in the palm of the hand and grasped by all the fingers, while the thumb is free of instrument and the rest is provided by supporting the tip of the thumb on a nearby tooth of the same arch or on a firm stable structure. Appropriately used in holding hand piece for cutting incisal retention in class III preparation. d) Modified palm and thumb grasp:- It may be used when it is feasible to rest the thumb on the tooth being prepared or adjacent tooth. The handle of instrument is held by all four fingers whose pads press the handle against the distal area of the palm as well as the pad of first joint of the thumb. e) Palm-Thrust grasp:- The end of the large handle is grasped in the center of palm, the shank being held between balls of thumb and first and second finger. 19
No rest is utilized. A powerful thrust is thus obtained such as needed in condensing non-cohesive gold foil. Rests: A proper instrument grasp must include a firm rest to steady the hand cutting procedures. The closer the rest area to operating site more reliable it is. Intra oral finger rest can be divided into:a) Conventional:- Established on tooth adjacent to working area. b) Cross arch:- Established on tooth of other side of same arch. c) Opposite arch:- Established on opposite arch. d) Finger on finger:- Established by finger or thumb of non-operating hand (indirect rest). Guards: Guards are hand instrument or other items such as interproximal wedges, used to protect soft tissues from contact with sharp cutting or abrasive instruments. SHARPENING OF HAND CUTTING INSTRUMENTS: Sharp instruments can cut more precisely and quickly than dull instruments. Instruments with dull cutting edges causes more pain, prolonged operating time, are less controllable and reduce quality and precision in tooth preparation. Resharpening requires less time and is very rewarding. Principles of Sharpening: 1) Sharpen instrument only after they have been cleaned and sterilized. 2) Establish the proper bevel angle (usually 45째) and the desired angle of cutting edge to the blade before placing the instrument against the stone and maintain these angles while sharpening. 20
3) Use of light stroke or pressure against the stone to minimize frictional heat. 4) Use of rest or guide whenever possible. 5) Remove as little metal from blade as possible. 6) Lightly hone the unbeveled side of blade after sharpening to remove the fine bur that may be created. 7) After sharpening, resterilize the instrument along with other items on the instrument tray setup. 8) Keep the sharpening stones clean and free of metal cuttings. Types of Sharpener: There are three types of sharpeners. a) Stationary sharpening stones b) Mechanical sharpeners c) Hand piece sharpening stone a) Stationary Sharpening Stone:The most frequently used sharpening equipment consist of a block or stick of abrasive material called ‘stone’. The stone is supported on a firm surface and instrument is oriented and held by hand while being stroked against the stone surface. Stationary stones are also called ‘Oilstones’ because of common practice of applying a coat of oil to them as an aid to sharpening process. Stationary stones available in fine, medium and coarse grit. Fine grit is suitable for final sharpening. Coarse and medium grit used for initial reshaping of badly damaged instruments.
Stationary stones can be obtained in various shapes including flat, grooved, cylindrical and tapered. Flat are preferred for straight cutting edges. Other shapes are useful in sharpening curved edges. The properties of the stone depends on the volume and size of pores as well as composition and size of abrasive. Four type of stones are available for sharpening instruments. 1) Arkansas Stone:- It is a naturally occurring mineral containing microcrystalline quartz. It is semi-translucent, white, gray in colour and hard enough to sharpen steel but not carbide instruments. Arkansas stone are available in hard and soft varieties. These stone should be lubricated with light machine oil before being used. This assists in fineness of sharpening, prevent clogging of stone pores and avoid the creation of heat. When the stone appears to be dirty it should be wiped with woolen cloth and when extremely dirty wiped with cloth soaked in alcohol. 2) Silicon Carbide (SiC):- It is widely used as an industrial abrasive. It is chard enough to cut steel efficiently but not hard enough to sharpen carbon steel. SiC is available in many shapes in coarse and medium grits but not fine grits. SiC are normally of black colour often greenish black. These stones are moderately porous and require lubrication with light oil to prevent clogging. 3) Aluminium Oxide:- Aluminium oxide stones commonly are produced in various textures from different particle sizes of abrasive. Coarse and medium grit stones appear brownish. Fine grit stones are usually white have superior properties and are less porous so require less lubrication. Either water or light oil is adequate as lubricant. 22
4) Diamond:- It is the hardest available abrasive and is most effective for cutting and shaping hard materials. It is the only material routinely capable of sharpening carbide as well as steel instruments. Diamond hones are small blocks of metal with fine diamond particles impregnated in the surface. The diamonds are held in place by an electroplated layer of corrosion resistant metal. These hones are non-porous but the use of lubricant will extend the life of hones. Stationary Stone Technique:- The stationary sharpening stone should be at least 2 inches wide and 5 inches long. It should be medium grit for hand cutting instrument. Before the stone is used, a thin film of light oil should be placed on working surface. Several fundamental rules apply using stationary stone:• Lay the stone on a flat surface and do not tilt while sharpening • Grasp the instrument firmly • To ensure stability during sharpening strokes, use the ring and little fingers as a rest. • Use a light stroke to prevent the creation of heat and scratching of the stone. • Use different areas of stone’s surface while sharpening because this helps to prevent the formation of grooves on stone that impair efficiency and accuracy of sharpening procedure. • When sharpening chisel, hatchets or hoes on the stationary stone, grasp the instrument with a modified pen grasp, place the blade perpendicular to the stone and then tilt the instrument to establish the correct bevel. The motivating 23
force should be from shoulder so that relationship of hand to plane of stone is not changed during the stroke. â€˘ The procedure for sharpening angle former is essentially the same as that used for chisel, hatchet, hoes except allowance must be made for angle f cutting edge. â€˘ Gingival margin trimmer require more orientation of cutting edge to the stone before sharpening than regular hatchet. Proper bevel angle and cutting edge angle should be maintained. It may be expedient to use palm and thumb grasp when sharpening a trimmer with 95 or 100 centigrade cutting edge angle. â€˘ Span excavator and discoids are most difficult instruments to sharpen on a flat stone. Only the rounded outside surface of spoon can be honed on a flat stone and this involves the rotatory movement accompanied by pull stroke to main curvature of the edge. The spoon is placed on the far end of the stone and held to that the handle is pointing towards the operator. As the instrument is pulled along the stone towards the operator, the handle is rotated gradually away from the operator at the end of the stroke. The instrument is picked up and placed at the far end of stone and the motion is repeated until the edge is honed. The stone is either may be placed on a flat surface or held in the hand for this procedure. b) Mechanical Sharpeners:The treatment honing machine is an example of a mechanical sharpener. Basically this instrument moves a hone in a reciprocating motion at a slow speed, while the instrument is held at appropriate angulation and supported by a rest.
Interchangeable Al2O3 hones of different shapes and coarseness are available to accommodate the various instrument sizes, shapes and degree of dullness. Restoration of cutting edge is accomplished more easily and in less time than by other sharpening methods. The mechanical sharpener is easily mastered with a little practice and is a quick method of sharpening hand instruments. c) Handpiece Sharpening Stones:Mounted SiC and Al2O3 stones for use with both straight and angle hand pieces are available in various sizes and shapes. Those intended for use in straightsided silhouettes, are more useful for sharpening hand instruments than are the smaller points intended for intraoral use in angle hand pieces. These stones also may produce somewhat inconsistent results because of speed variables and usual lack of a rest or guide for the instruments. EFFECTS OF STERILIZATION: Methods of sterilization of hand cutting instrument are sporicidal cold disinfection, boiling water, steam under pressure (Autoclave), chemical vapour and hot air (dry heat). Sterilization of carbon steel method by first three methods causes discolouration, rust and corrosion. These problems can be minimized by:1) To electroplate the instrument 2) Protection is by use of rust inhibitors, which are soluble alkaline compounds. 3) Minimizing the effect of moisture is to remove the instrument promptly at the end of the recommended sterilizing period, dry them thoroughly.
Boiling water or autoclave methods of sterilization do not produce discolouration, rust and corrosion. Prolonged immersion in cold disinfectant can cause rust. Dry heat sterilization do not rust and corrode carbon steel instruments, but the high heat may reduce the hardness of alloy, which may reduce the ability of instrument to retain a sharp cutting edge.
CONCLUSION: As high-speed rotary cutting instruments have been improved and their use has increased, the use of hand cutting instruments and the need for resharpening has decreased. Nevertheless, hand cutting instrument remain an essential part of armamentarium for quality restorative dentistry.
Published on Apr 28, 2014
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