Cutting Instruments PDF

Cutting Instruments INTRODUCTION Heat Treatment of Materials A wide range of specific instruments hand/rotary are required for preparation and cutting of tooth, and for other operative procedures. Rotary instruments help in gross cutting and final refining of the preparation whereas hand instrume...

Cutting Instruments INTRODUCTION Heat Treatment of Materials A wide range of specific instruments hand/rotary are required for preparation and cutting of tooth, and for other operative procedures. Rotary instruments help in gross cutting and final refining of the preparation whereas hand instruments are used for examination, producing minor details of the tooth preparation and for insertion, compaction and finishing of the restoration. One must be able to use both hand and rotary instruments judiciously so as to perform the operative procedures accurately. In this chapter we will discuss different types of instruments and instrumentation techniques used in operative dentistry. In Lilian Lindsay’s English translation of 1946, it has been shown that most preparation was carried out by hand instruments. Fauchard advocated the use of the manually operated bow drill, an unwieldy device widely used in the early 18th century and adapted by dentists from the workshops of jewellers, silversmiths and ivory turners. George Greenwood, modified spinning wheel for use as footoperated dental engine in 1790. e first commercially manufactured footpowered engine was patented by Morrison in 1871. Black described hand instruments such as chisels, hatchets, hoes, excavators and margin trimmers—terms which might have been taken from wood working and gardening. For gaining maximum benefits from instruments made from carbon or stainless steel, they are subjected to two heat treatments—hardening and tempering heat treatment. • Hardening heat treatment: In this, instrument is heated to 815°C in oxygen free environment and then quenched in a solution of oil. By hardening treatment, the alloy becomes brittle. • Tempering heat treatment: In this, instrument is heated at 176°C and then quenched in solutions of oil, acid or mercury. Tempering heat treatment is done to relieve the strains and increase the toughness of alloy. Metals Used for Manufacturing Cutting Instruments Carbon steel or stainless steel are most commonly used for manufacturing of cutting instruments. The Carbon Steel Carbon steel alloy contains 0.5 to 1.5 percent carbon in iron. Instruments made from carbon steel are known for their hardness and sharpness. But disadvantages with these instruments are their susceptibility to corrosion and fracture. ey are of two types: 1. Soft steel: It contains < 0.5% carbon 2. Hard steel: It contains 0.5 to 1.5% carbon Stainless Steel Stainless steel alloy contains 72 to 85 percent iron, 15 to 25 percent chromium and 1 to 2 percent carbon. Instruments made from stainless steel remain shiny bright because of deposition of chromium oxide layer on the surface of the metal which reduces the tendency to tarnish and corrosion. Problem with stainless steel instruments is that they tend to lose their sharpness with repeated use, so they need to be sharpened again and again. CLASSIFICATION OF HAND CUTTING INSTRUMENTS Classification Given by GV Black is classification is based according to use of the instrument. • Cutting instruments – Hand – Rotary i. Hatchets i. Burs ii. Chisels ii. Stones iii. Hoes iii. Others iv. Excavators v. Others • Condensing instruments Pluggers – Hand – Mechanical • Plastic instruments – Plastic filling instrument – Cement carriers – Carvers – Burnishers – Spatulas • Finishing and polishing instruments – Hand – Rotary i. Orangewood sticks i. Finishing brushes ii. Polishing points ii. Mounted brushes iii. Finishing strips iii. Mounted stones iv. Rubber cups • Isolation instruments – Rubber dam frame clamps, forceps and punch – Saliva ejector – Cotton roll holder – Evacuating tips and equipment • Miscellaneous instruments – Mouth mirrors – Explorers – Probes 103 104 Textbook of Operative Dentistry – Scissors – Pliers – Others Classification Given by Marzouck is classification is based upon different procedures performed by different instruments. • Exploring instruments – Tweezers/cotton pliers – Retractors: Mouth mirror, blunt bladed restoring instruments, plastic instruments, tongue depressors – Probes/Explorers: Straight, right angled, arch explorer, interproximal explorer. – Separators – Instruments for tooth structure removal – Hand cutting instruments i. Excavators: Hatchet, hoes, spoon, discoid, cleoid, angle formers. ii. Chisels: Straight, monoangle, biangle and triple angle. iii. Special forms of chisel: Enamel hatchets, gingival marginal trimmers, Wedelstaedt chisel, offset hatchets, triangular chisel and hoe chisel. – Rotary cutting and abrasive instruments i. Handpieces ii. Burs iii. Ultrasonic instruments • Restoring instruments – Mixing instruments: Stainless steel or plastic spatulas – Plastic instruments – Condensing instruments: Rounded, triangular, diamond, or parallelogram condensers. – Burnishing instruments: Ball/egg/conical-shaped burnishers – Carvers: Hollenback’s discoid and cleoid, diamond shaped carvers – Files: Hatchet/parallelogram-shaped – Knives: Bard parker knife and Stein’s knife • Finishing and polishing instruments: Burs, stones, brushes, rubber (wheel, cups or cones), cloth or felt ese names are combined to give a complete description of the instrument. Naming of an instrument generally moves from 4 to 1. Sometimes, the suborder is omitted due to variable and nonspecific use of the instrument. For example, the instrument will be named according to the classification as biangle enamel hatchet or biangle spoon excavator. PARTS OF HAND CUTTING INSTRUMENTS ough there is great variation among hand cutting instruments, they have certain design features in common. Each hand instrument is composed of three parts (Figs 6.1A and B)    Handle or shaft Shank Blade or nib. Handle or Shaft e handle is used to hold the instrument. e handle can be small, medium or large, smooth or serrated for better grasping and developing pressure (Figs 6.2A to C). Earlier, instruments had handles of quite large diameter that were to be grasped in the palm of the hand. Now-a-days, instrument handles are smaller in diameter for ease of their use. On the handle, there are two numbers; one is the instrument formula, which describes the dimensions and angulation of the instrument, the other number is the manufacturer’s number which is used for ordering purposes. Nomenclature for the Instruments Dr GV Black has given a way to describe instruments for their easier identification similar to biological classification.  Order: Function or purpose of the instrument, e.g. excavator, condenser  Suborder: Position, mode or manner of use, e.g. push, pull  Class: Design or form of the working end, e.g. hatchet, spoon excavator  Subclass: Shape of the shank, e.g. binangle, contra-angle. Figures 6.1A and B: 105 Cutting Instruments Shank Shank connects the handle to the blade. It tapers from the handle down to the blade and is normally smooth, round or tapered. e shank may be straight or angled. e angulation of instrument is provided for access and stability. Closer the working point to the long axis of the handle, better will be the control on it. For better control, the working point should preferably be within 3 mm of the center of the long axis of the handle (Figs 6.4A and B). Classification base on number of shank angles (Figs 6.3A to E)      Straight: Shank having no angle Monoangle: Shank having one angle Binangle: Shank having two angles Tripleangle: Shank having three angles Quadrangle: Shank having four angles Advantages of contrangling of instrument    Better access and stability Better balance Clear view. Blade or Nib e blade is the last section. It is the working part of the instrument. It is connected to the handle by the shank. For non-cutting instruments, the working part is termed the nib and is used to place, adapt and condense the materials in the prepared tooth. Depending on the materials being used, the surface of the nib may be plain or serrated. To cleave and smoothen the enamel and dentin, the working point of the instrument is beveled to create the cutting edge. If instrument has blade on both the ends of the handle, it is known as ‘double-ended’ instrument. In such cases, one end is for the left side and other for the right. In some instruments, there are three bevels. Two are on the side and one is at the end. e edge on the end is called the primary cutting edge and the edges on the sides are called the secondary cutting edges. Figures 6.2A to C: Figures 6.4A and B: Figures 6.3A to E: 106 Textbook of Operative Dentistry Instrument Formula GV Black established an instrument formula for describing dimensions of blade, nib or head of instrument and angles present in shank of the instrument (Fig. 6.5). e formula is usually printed on the handle consisting of a code of three or four numbers separated by spaces. e first number of the formula indicates width of the blade or primary cutting edge in tenths of a millimeter (Fig. 6.6). e second number represents the angle formed by the primary cutting edge and long axis of the instrument handle in clockwise centigrade. e instrument is positioned in such a way that the number always exceeds 50 and is measured in clockwise centigrades. If the cutting edge is at right angle to the length of the blade, then this number is omitted. e third number (second number in three number code) represents the length of the blade in millimeters, that is, from the shank to the cutting edge (Fig. 6.7). e fourth number (third number in three number code) represents the angle which the blade forms with the long axis of the handle or the plane of the instrument in clockwise centigrade. To calculate the measurement of the angle, place the instrument on the center of the circle and move it until the blade lines up with one line on the ruler. is measurement represents the angulation of the blade from the long axis of the handle. To keep balance during working, tip of blade is brought in the line of the long axis of the handle. Example of three number formula  An instrument having instrument formula of 15-8-14 (Figs 6.8A and B) indicates following:  15 represents the width of the blade in tenths of a mm, i.e. 1.5 mm  8 represents the length of the blade in millimeters, i.e. 8 mm  14 represents the blade angle in centigrades. Figure 6.5: Figure 6.7: A Figure 6.6: B Figures 6.8A and B: Cutting Instruments Example of four number formula  Instrument with formula 15-95-8-12 (Figs 6.9A and B) represents the following:  15 represents width of the blade in tenths of a millimeter, i.e. 1.5 mm  95 represents the cutting edge angle in centigrades  8 represents length of the blade, i.e. 8 mm  12 represents blade angle in centigrades  5 no. formula on handle. Want to Know More Manufacturer’s number: This number is found on the handle of the instrument. This number is used when ordering the instrument and indicates the instrument’s placement in a set of instruments. Examples of three number formula instruments are chisels, hatchets and hoes. Examples of four number formula instruments are angle formers and gingival marginal trimmers. DIFFERENT INSTRUMENT DESIGNS Bevels in Cutting Instruments During use, move the instrument from right to left in right beveled instrument and from left to right in left bevel instrument. Identification of bevel: Hold the instrument in such a way that the primary cutting edge faces downwards and pointing away from operator. If bevel is on the right side of the blade, the instrument is right sided and if bevel is on the left side of the blade the instrument is left sided. Mesial and distal bevel instrument: If we observe the inside of the blade curvature and the primary bevel is not visible then the instrument has a distal bevel and if the primary bevel can be seen from the similar view point the instrument has a mesial or reverse bevel. Bibeveled Instrument (Fig. 6.10B) If two additional cutting edges extend from the primary cutting edges, then the instrument with secondary cutting edges is called bibeveled instrument. Triple-beveled Instrument If three additional cutting edges extend from the primary cutting edge, then the instrument is called triple-beveled instrument. Single Bevel Instruments Circumferential Bevel Most of the instruments have single bevel that forms the primary cutting edge. ese are called single beveled instruments. ese can be right or left bevel and mesial or distal bevel instruments (Figs 6.10A and B). Here instrument blade is beveled at all its peripheries for example spoon excavator. Right and left bevel instruments: Single-beveled direct cutting instruments such as enamel hatchets are made in pairs having bevels on opposite sides of the blade. These are named as right and left bevel instruments. Direct and Lateral Cutting Instruments Direct cutting instruments are those in which force is applied the same plane as that of blade and handle.ey A B Figures 6.9A and B: Figures 6.10A and B: 107 108 Textbook of Operative Dentistry are called as single planed instrument. ey can be used for both direct and lateral cutting. Lateral cutting instruments are those in which force is applied at their right angle to the plane of the blade and the handle. ey have curved blade, and also called double plane instrument. ey can be used lateral cutting only. Single and Double Ended Instruments In single ended instruments, working end is present on one side only. In double ended instruments, working end is present on both sides of the instrument. ey are used to give mesial and distal or right and left form of the instrument in the same handle. Instrument motions     Pulling: Here, instrument is moved towards operator’s hand Scraping: Here, instrument is moved side to side or back and forth on the tooth surface Pushing: Here, instrument is moved away from operator’s hand Cutting: Here, instrument is moved parallel to the long axis of handle EXPLORING INSTRUMENTS Mouth Mirrors • Rear surface reflecting mirror: It is most commonly used mirror. In this, coating is present on back side of the mirror • Plane or flat surface: It provides clear image without distortion • Concave surface: It is used to provide different degrees of magnification, but it causes image distortion • One sided: Image on one side • Two sided: Image on either side (Advantage—retraction with indirect vision simultaneously). Uses of Mouth Mirror • Direct vision: Retraction is done with mirror to enhance visibility in specific area. Illumination allows the dentist to use mirror as a spotlight to reflect light from dental light on to a specific area of oral cavity. For example, use in maxillary left palatal aspect • Indirect illumination: In this, mirror is placed in oral cavity that can not be seen directly without compromising dentist’s position. For example, use in maxillary right palatal area and for palatal surfaces of anterior teeth (Fig. 6.13) • Transillumination: Mirror is placed behind the teeth and direction of light perpendicular to long axis of teeth • Retraction: Retraction of soft tissue such as tongue and cheeks to aid in better visualization of the operating field (Figs 6.14A and B). Mouth mirrors are used as supplement to improve access to instrumentation (Figs 6.11A and B). Uses of mouth mirror  Types of Mirror Faces (Fig. 6.12) • Front surface reflecting mirror: Here the coating is pressent on front surface of the mirror to prevent image distortion    Direct vision Indirect illumination Retraction Transillumination. A B Figures 6.11A and B: Figure 6.12: 109 Cutting Instruments Explorer Explorer is commonly used as a diagnostic aid in evaluating condition of teeth especially pits and fissures (Fig. 6.15). • Interproximal explorer/Briault explorer/Back action probe: is explorer has two more angles in the shank with working tip-pointed towards the handle. Uses of interproximal explorer  Parts  • Handle of explorer is straight which could be plain or serrated • Shank of explorer is curved with one/more angle • Working tip of explorer is pointed. Types of Explorer (Figs 6.16A to C) • Straight explorer: It is bent perpendicular to the handle. is is used for examining occlusal surfaces of teeth. • Shepherd’s Hook or curved explore or arch explorer: It has semilunar shaped working tip perpendicular to the handle. is is used for examining occlusal surfaces. Examination of interproximal caries For assessing marginal fit of the restoration. Cow horn/pigtail explorer: It has smaller arch than curved explorer. Tweezers ese have angled tip and are available in different sizes (Figs 6.17A and B). ey are used to place and remove cotton rolls and other small materials. Probes ough they almost look like straight explorers but they have blunt end which is marked with graduations (Fig. 6.18). Uses of probes   Mainly used for measuring pocket depth To determine dimensions of tooth preparation Types • William‘s probe • PCP 12 probe • PSR (periodontal screening and recording probe). These probes differ in: • Diameter • Position of markings • Type of marking (painted/notched). Figure 6.13: A B Figures 6.14A and B: 110 Textbook of Operative Dentistry Figure 6.15: Figure 6.18: HAND CUTTING INSTRUMENTS Instrument families Chisels  Chisels  Straight chisel  Monoangle chisel  Binangle chisel  Wedelstaedt chisel.  Enamel hatchet  Gingival marginal trimmer. Figures 6.16A to C: Excavators  Hatchet  Hoe  Angle former  Spoon excavator. Others Knives  Files  Discoid-cleoid.  Chisels Chisels are used for cleaving, planing and lateral scraping. In other words, they are used to split tooth enamel, to smooth preparation walls and to sharpen the preparations. Chisels are used with a push motion. Straight Chisel Figures 6.17A and B: In straight chisel • e cutting edge of the chisel makes a 90° angle to the plane of the instrument 111 Cutting Instruments • It is used with straight thrust force, push motion. • It is used for gingival restoration of the anterior teeth (Fig. 6.19). Angled Chisel In angled chisel • e primary cutting edge is in a plane perpendicular to the long axis of the shaft and may have either a mesial or distal bevel. Distal bevelled chisel is also called as reverse bevelled or contra-bevelled. • ey are used with a push or pull motion for anterior proximal restorations, smoothening proximal walls and gingival walls for full coverage restorations (Figs 6.20A and B). • Two most common types used are the Wedelstedt and binangle chisels. • e Wedelstedts chisel is almost similar to straight chisel except for slight vertical curvature in its shank (Fig. 6.21). It can be mesially or distally bevelled. It is mainly used on anterior teeth. • e binangle chisel has two different angles—one at the working end and other at the shank. is design permits access to tooth structures which is not be possible with straight chisels. It is mesially or distally bevelled. It is used to cleave the undermined enamel. Hatchet • Any instrument where the cutting edge is parallel or close to parallel to the plane of the instrument is called a hatchet • Basically, a hatchet is the similar to an axe except that it is much smaller (Figs 6.22A and B) • Hatchet is a paired instrument in which blades makes 45 to 90° angle to the shank • In paired right and left hatchets, blades are beveled on opposite sides to form their cutting edges (Figs 6.23A and B) • Some hatchets have single cutting ends and some have cutting edges on both ends of the handle • Hatchets are used for cleaving enamel and planing the dentinal walls so as to have sharp outline of the preparation • Some hatchets are bibeveled, i.e. blade has two bevels with cutting edge in the center. ese bibeveled binangle hatchets are used in a chopping motion to refine line and point angles. Figure 6.19: Figures 6.22A and B: Figures 6.20A and B: Figure 6.21: Figures 6.23A and B: 112 Textbook of Operative Dentistry Figures 6.24A and B: Figures 6.25A and B: Gingival Margin Trimmer • e gingival margin trimmer (GMT) is a modified hatchet which has working ends with opposite curvatures and bevels (Figs 6.24A and B) • e gingival marginal trimmer is available in a set of two double ended styles and is used in pairs, constituting a set of four instruments (Figs 6.25A and B) • Distal gingival margin trimmer is used for the distal surface and the mesial GMT is used for the mesial surface • If the second number in instrument formula is 75 to 85, it is mesial GMT and if second number is 95 to 100, it is distal GMT • GMT is used for planing gingival cavosurface margin that is removal of unsupported enamel and to bevel axiopulpal line angle in the class II tooth preparation (Figs 6.26A and B). Figures 6.26A and B: Main difference between gingival marginal trimmer (GMT) and hatchet   GMT has a curved blade, hatchet has straight blade. The curved blade helps in the lateral scraping skill of the GMT The cutting edge of the GMT makes an angle with the plane of the blade whereas cutting edge of the hatchet makes a 90° angle to the plane of the blade. EXCAVATORS Ordinary Hatchet • An ordinary hatchet excavator is a bevelled instrument in which cutting edge of blade is directed in the same plane as that of long axis of the handle. • Mainly used for preparing and sharpening line angles in anterior teeth. Hoes • Dental hoe resembles a miniature garden hoe • By definition, the hoe is any instrument where the blade makes more than a 12.5° angle with the plane of the instrument (Figs 6.27A and B). Usually hoe blades make 45 to 90° angle to the long axis of handle • Its shank can have one or more angles (Figs 6.28A and B) • Hoe is used with a pulling motion • Hoe is used to shape and smoothen the floor and form line angles in class III and V restorations. 113 Cutting Instruments A B Figures 6.27A and B: Figures 6.29A and B: • Each instrument in the set is a double-ended instrument • e mesial angle former is used to plane the gingival cavosurface margin in the mesial proximal box. e distal angle former is used to plane the gingival cavosurface margin in the distal proximal box. Spoon Excavator Figures 6.28A and B: • e spoon excavator is a modified hatchet. It is a double-ended instrument with a spoon, claw, or diskshaped blade (Figs 6.30A and B) • Spoon excavator is used to remove caries and debris in the scooping motion from the carious teeth. Chisel vs Hoe By definition, chisel is an instrument where the blade makes up to 12.5° angle with the plane of the instrument, whereas in hoe, the blade is angled more than a 12.5° with the plane of the instrument. Two main differences between the spoon excavator and hatchet   The blade of a spoon excavator is curved to emphasize the lateral scraping motion The cutting edge of the spoon excavator is rounded. Angle Former (Figs 6.29A and B) Knives • Angle former is a type of excavator which is monangled with the cutting edge sharpened at an angle to the long axis of the blade • Angle of cutting edge to blade axis lies between 80 to 85 centigrades • Blade of angle former is beveled on sides as well as the end, this forms three cutting edges • It is used with a push or pull motion for accentuating line and point angles, to establish retention form in direct filling gold restoration • ere are two sets of angle formers, mesial and the distal angle former ey are known as finishing knives, gold knives or amalgam knives. ey have thin knife like blade and are used for removing excess material and contouring. Files Files are used for trimming excess material especially in the gingival margins. Cleiod-discoid • It is modified chisel with different shape of cutting edges (Figs 6.31A and B) 114 Textbook of Operative Dentistry Figures 6.31A and B: A B Figures 6.32A and B: A Figures 6.30A and B: B Figures 6.33A and B: • In cleoid, it is claw-like and in discoid it is disk-like. instead of a sharp edge, the edge is rounded • ese instruments have sharp cutting edges as spoon excavators but blade to shaft relationship is similar to chisels • ey are used for removing caries and carving amalgam or wax patterns. RESTORATION INSTRUMENTS Following are the commonly used instruments when temporary or permanent restoration is being done. Cement Spatulas • Several types of cement spatulas are available in the market differing in shape and size (Figs 6.32A and B) • On the basis of size, cement spatula can be classified into two types (Figs 6.33A and B): 1. Large cement spatula: Mixing of luting cements 2. Small cement spatula: Mixing of liner Cement spatula also can be classified on the basis of thickness rigid and flexible. eir use depends on viscosity of cement and personal preference. Plastic Filling Instrument ese instruments have a small metal ball at the working end. ey are used to mix, carry and place cements (Figs 6.34 and 6.35). 115 Cutting Instruments Condensers Condensers are use to deliver the restoration to the tooth preparation and properly condense it. • e hammer-like working end of condenser should be large enough to pack the restoration without sinking into it • Condensers come in single and double-ended designs • ey are available in differently shaped and sized working ends like round, triangular or parallelogram, which may be smooth or serrated • Condensers can be hand or mechanical in nature (Figs 6.36 and 6.37). Amalgam Carriers • Amalgam carriers carry the freshly prepared amalgam restorative material to the prepared tooth • Amalgam carriers have hollow working ends, called barrels, into which the amalgam is packed for transportation (Figs 6.38 and 6.39) • Carriers can be both single and double ended Figure 6.34: Figure 6.35: Figure 6.37: Figure 6.38: Figure 6.36: Figure 6.39: 116 Textbook of Operative Dentistry • Barrel of amalgam carriers comes in a variety of sizes viz; small, large and jumbo • Lever of amalgam carrier is located on the top of the carrier. When lever is depressed, the amalgam is expelled into the preparation • A poorly packed amalgam carrier may result in amalgam fall out before it is ejected into the prepared tooth • After restoration is completed, any remaining amalgam alloy is expelled out from the carrier into the amalgam well, otherwise carrier will no longer be serviceable if the amalgam is allowed to harden in the carrier. Carvers (Fig. 6.40) • Sharp cutting edges present in carvers are used to shape and form tooth anatomy from a restorations • Carvers come in different shapes and sizes in double ended designs (Fig. 6.41) • Many carvers are designed for carving specific tooth surfaces • For example, interproximal and hollenback carvers are used for carving proximal surfaces and discoid cleoid and diamond-shaped carvers are used for carving occlusal surfaces (Fig. 6.42). Burnisher (Fig. 6.43) • Burnishers are the kind of instruments which make the surface shiny by rubbing • ey are used to smoothen and polish the restoration and to remove scratches present on the amalgam surface after its carving • Burnishers have smooth rounded working ends and come in single and double ended types (see Figs 6.33A and B). Different types of burnishers are available but most commonly used are (Figs 6.44A to C): • PKT3—designed by Peter K omas —Rounded cone-shaped burnisher. • Beavetail condenser—narrow type of burnisher. • Ovoid burnisher—comes in various sizes such as 28, 29, 31. Uses of burnishers      Final condensation of amalgam Initial shaping of occlusal anatomy of amalgam Shaping of metal matrix bands Shaping of occlusal anatomy in posterior resin composite before polymerization of resin Burnishing margins of cast gold restoration. Figure 6.40: Figure 6.42: Figure 6.41: Figure 6.43: 117 Cutting Instruments other words, correct instrument grasps are important for achieving success in operative procedures. e correct grasp is selected according to the instruments being used, position of instrument being used, the operator, the area which is being operated and the specific procedure to be done. Commonly used instrument grasps in operative dentistry     Modified pen grasp Inverted pen grasp Palm and thumb grasp Modified palm and thumb grasp. Modified Pen Grasp (Fig. 6.46) Figures 6.44A to C: Figure 6.45: Composite Resin Instruments • For composite resin restorations, a wide range of double-ended instruments are used to transport and place resins • e working ends on these instruments range from small cylinders to angled, paddle like shapes (Fig. 6.45) • Composite resin instruments are made of plastic or titanium coating • Advantages of using plastic instruments are that they do not discolor or contaminate the composite restoration, and the composite resin material does not stick to the instrument. is is the most commonly used grasp. e greatest delicacy of touch is provided by this grasp. Normally, a pen is held with the thumb and index finger, with the middle finger placed under the pen. e modified pen grasp is similar to the pen grasp except the operator uses the pad of the middle finger on the handle of the instrument rather than going under the instrument (Figs 6.47A and B). e positioning of the fingers in this manner creates a triangle of forces or tripod effect, which enhances the instrument control. It is most commonly used in mandibular teeth. Here palm of the operator faces away from the operator. is position stabilizes the instrument and allows the middle finger to help push the instrument down. Inverted Pen Grasp In inverted pen grasp, finger positions are the same as for the modified pen grasp except that hand is rotated so that palm faces towards the operator (Fig. 6.48). is grasp is most commonly used for preparing a tooth in the lingual aspect of maxillary anterior and occlusal surface of maxillary posterior teeth (Fig. 6.49). INSTRUMENT GRASPS For accurate and precise control over the instrument certain instrument grasps are suggested which help in increasing efficiency of the operator, offer more flexibility of movements and decrease strain on the operator. In Figure 6.46: 118 Textbook of Operative Dentistry A Figure 6.49: B Figures 6.47A and B: Figure 6.50: Figure 6.48: Palm and Thumb Grasp (Fig. 6.50) is grasp is same as for holding the knife for peeling the skin of an apple. e palm and thumb grasp is commonly used for bulky instruments. In this, instrument is grasped very near to its working end so that thumb can be braced against the teeth so as to provide control during instrument movements. e shaft of the instrument is placed on the palm of the hand and grasped by the four fingers to provide firm control, while the thumb is free to control movements and provide rest on a adjacent tooth of the same arch. To achieve the thrust action with the fingers and palm, instrument is forced away from the tip of the thumb which is at the rest position. is grasp has limited use only while operating on maxillary anterior teeth. Since it offers application of heavy force with greater control, it is used for holding a handpiece while cutting incisal retention for a class III preparation in maxillary incisor. Modified Palm and Thumb Grasp (Figs 6.51A and B) e instrument is held like the palm grasp but the pads of all the four fingers press the handle against the palm and pad and first joint of the thumb. Here tip of the thumb rests on the tooth being prepared or the adjacent tooth. 119 Cutting Instruments Modified palm and thumb grasp provides more control to avoid slipping of instrument. is grasp is commonly used in maxillary anterior teeth. FINGER RESTS e finger rest helps to stabilize the hand and the instrument by providing a firm rest to the hand during operative procedures. Finger rests may be intraoral or extraoral. • Intraoral finger rests: – Conventional: In this, the finger rest is just near or adjacent to the working tooth (Fig. 6.52) – Cross-arch: In this, the finger rest is achieved from tooth of the opposite side but of the same arch (Fig. 6.53) – Opposite arch: In this, the finger rest is achieved from tooth of the opposite arch – Finger on finger: In this, rest is achieved from index finger or thumb of nonoperating hand. • Extraoral finger rest: It is used mostly for maxillary posterior teeth. – Palm up: Here rest is obtained by resting the back of the middle and fourth finger on the lateral aspect of the mandible on the right side of the face (Fig. 6.54) Figure 6.53: Figure 6.54: A B – Palm down: Here rest is obtained by resting the front surface of the middle and fourth fingers on the lateral aspect of the mandible on the left side of the face (Fig. 6.55). Figures 6.51A and B: Methods of use of instruments     Figure 6.52: The instruments are effectively used when they are used from the bevel side to the non-bevel side Instrument should be held in such a way that allows the cutting edge to remove any unsupported enamel from the preparation walls Instrument should always be held parallel to the wall being worked upon. Holding an instrument at this angle may increase its cutting but it may also cause damage or fracture of the tooth For the buccal wall, one side of the instrument is used and on the lingual wall, the other side of the instrument should be used. 120 Textbook of Operative Dentistry • Sharpening should be done with light stroke or pressure. Avoid excessive pressure • When sharpening is completed observe the cutting edge for wire edges. Wire edges should be removed. (Wire edges are unsupported metal fragments that extend beyond the cutting from the lateral side or face of blade) • Resterilize the sharpened instruments. Devices Used for Sharpening Figure 6.55: SHARPENING OF HAND INSTRUMENTS Instrument sharpening is a critical component of operative dentistry. It is impossible to carry out procedures with dull instruments. A sharp instrument cuts more precisely and quickly than dull instruments. erefore to avoid wasting time on using dull instruments, dentists must be thoroughly familiar with principles of sharpening. Goals of sharpening    To produce a functionally sharp edge Maintain the contour (shape) of instrument Maintain the life of instrument. Advantages of sharp instruments Use of well sharpened instruments results in:  Improved efficiency  Improved tactile sensations  Less pressure and force  Improved instrument control  Minimized patient discomfort  Less treatment time. Principles of Sharpening Some basic principles used during sharpening are: • Select the appropriate type of stone for type of instrument to be used • Instrument should be clean and sterile before sharpening • Establish proper angle between stone and surface of instrument on the basis of design • Lubricate the stone during sharpening as it reduces the clogging of sharpening stone and heat generated during sharpening • Stable and firm grip of both instrument and stone is required during sharpening. Maintain the proper angulation throughout sharpening strokes • Mechanical • Mounted stone • Handhold stones (Unmounted). Mechanical It is bench type piece of equipment in which honing disks are mounted. On top disk rotates up to 7,000 rpm. It saves time, e.g. honing machine. Mounted Stones In this, stones are mounted on metal mandrel and used with slow speed handpiece. Most common mounted stones are Arkansas and ruby. Various shapes such as cylindrical, conical or disk shaped are available. Mounted stones are not preferred in routine because they: • Tend to wear down quickly • Result in generation of frictional heat • Difficult to control during sharpening. Unmounted/Handhold Stones ese are commonly used for instrument sharpening. ese come in variety of sizes and shapes. Stone can be rectangular with flat, rectangular with grooved surfaces or cylindrical in shape. • Flat stone is ideal for moving technique • Cylindrical stone for removing wire edges. Stone type can come in natural or synthetic form: • Natural–Arkansas (preferred) • Synthetic: – India stone – Ceramic stone – Composition stone. Guidelines for Sharpening Operative Instruments • When sharpening GMT, chisels, hatchets and hoes, place the cutting edge against the flat stone and push or pull the instrument so that acute cutting angle moved forward (Fig. 6.56) Cutting Instruments Figures 6.57A to C: Figure 6.56: • Bevel of instrument should make 45° angle with face of blade. So, while sharpening, blade should make a 45° angle with the sharpening surface (Figs 6.57A to C) • While sharpening spoon excavators, cleoid and discoid carvers, rotate the instrument as the blade is moved on the sharpening stone • Move the instrument with bevel against the stone surface and cutting edge placed perpendicular to the path of movement (Fig. 6.58) • For curved or round cutting edge instrument, handle of edge instrument should be moved in an arc to keep the cutting edge perpendicular to direction of cutting stroke. Advantages of hand cutting instruments       Self-limited in cutting enamel They can remove large pieces of undermined enamel quickly No vibration or heat accompanies the cutting Efficient means of precise cutting Create smooth surface on cutting Long lifespan and can be resharpened. ROTARY CUTTING INSTRUMENTS Rotary cutting instruments are those instruments which rotate on an axis to do the work of abrading and cutting on tooth structure. Types of Rotary Cutting • Handpiece: It is a power device • Bur: It is a cutting tool. Figure 6.58: Handpieces e first rotary instruments were drill or bur heads that were twisted with the fingers for crude cutting of the tooth tissue. Drilling came as the modification (1728) where seat for the drill was provided by a socket fitting against the palm and the ring was adapted to the index or middle finger. In the mid 19th century, the invention and development of both mechanical and pedal powered handpieces occurred in 1864, British dentist George Fellows invented the “clockwork” drill. Here the bur was attached to it by a shaft with a rotating spindle inside. e drill was wound up like a clock, with a key inserted into the back. Drill used to spin for 2 minutes before needing to be rewound. 121 122 Textbook of Operative Dentistry In 1868, American dentist GF Green developed a pneumatic handpiece powered by pedal operated bellows. First “dental engine” to provide enough power to spin the bur with sufficient speed for tooth cutting was developed in 1871 by Dr James B Morrison. It was adapted from sewing machine concept. Between 1950s and 1960s, maximum developments occurred for improvements in design, mechanical operation and speed of the handpieces. In 1957, the Borden, Airotor was developed as the prototype for today’s modern air-turbine handpiece. It had speed up to 250,000 rpm. Air-turbine systems depend mainly on momentum to produce their power. Since they are powered by the speed of airflow, there is no physical or mechanically connection to the power source. us, under load, they tend to slow down. erefore “touch-and-go” rule should be followed with these air-driven turbines. is means touch the bur to tooth and start cutting by applying pressure. As the bur slows, release pressure until the bur resumes its speed. Because of their ease of use, simple design and patient acceptance air turbine handpieces are still very popular. Electrically driven handpieces were introduced in the 1970s. ey offer many advantages over their air driven predecessors. ough these handpieces are heavier and bigger than air driven handpieces, but they have the advantage of maintaining a constant speed during cutting, which does not decrease under load. Also these handpieces have ability to control the rpm rate. In general for better efficiency, the diameter and bend of the handpiece should be designed such that it fits optimally between thumb and forefinger and should have balanced center of gravity to make the handpiece feel lighter than its actual weight. Head diameter of handpiece should be smaller in size so as to allow greater visibility and maneuverability. Classification of Handpiece Dental handpiece are classified according to their driving mechanisms. • Gear driven handpiece: Rotary power is transferred by a belt which runs from an electric engine. Power is transferred from the straight handpiece by a shaft and gears inside the angle section. is handpiece is capable of working with wide speed range, though it works best at low speed because of so many moving parts with metal to metal contact. • Water driven handpiece: It was discovered in 1953. It operates at speed up to 100,000 rpm. In this handpiece, a small inner piece transports water under high pressure to rotate the turbine in the handpiece and the larger outer tube returns the water to the reservoir. Advantage of this handpiece is its quiet nature and highest torque. • Belt driven handpiece: Belt driven angle handpiece was made available in 1955. It runs at speed of > 100,000 rpm. is handpieces has excellent performance and great versatility. • Air driven handpiece: is handipiece became available in the later part of 1956. It runs at speed of approximately 300,000 rpm. Types of Handpiece • Contra-angle handpiece: In this, head of handpiece is first angled away from and then back towards the long axis of the handle. Because of this design, bur head lies 123 Cutting Instruments close to long axis of the handle of handpiece which improve accessibility, visibility and stability of handpiece while working. – Air-rotor contra-angle handpiece: It gets power from the compressed air supplied by the compressor. is handpiece has high speed and low torque (Fig. 6.59). – Micromotor handpiece: It gets power from electric micromotor or airmotor. is handpiece has high torque and low speed (Fig. 6.60). • Straight handpiece: In straight handpiece, long axis of bur lies in same plane as long axis of handpiece. is handpiece is commonly used in oral surgical and laboratory procedures (Fig. 6.61). Dental Burs “Bur is a rotary cutting instrument which has bladed cutting head”. Burs are used to remove tooth structure either by chipping it away or by grinding. e earliest burs were handmade. Before 1890s, silicon carbide disks and stones were used to cut enamel since carbon steel burs were not effective in cutting enamel. William and Schroeder first made diamond dental bur in 1897, modern diamond bur was introduced in 1932 by WH Drendel by bonding diamond points to stainless steel shanks. Diamond burs grind away the tooth. Diamond particles of < 25 µm size are recommended for polishing procedures and > 100 µm are used for cavity preparation. Figure 6.61: Diamond particles are attached to bur shank either by sintering or by galvanic metal bond. Degree of bonding and clearance of shavings determine the quantity and effectiveness of bur. Materials Used for Bur (Fig. 6.62) Figure 6.59: Figure 6.60: • Stainless steel burs: ese were the first developed burs. Stainless steel burs are designed for slow speed < 5000 rpm. Usually a bur has eight blades with positive rake angle for active cutting of dentin. But this makes steel burs fragile, so they do not have a long life. ey are used for cutting soft carious dentin and finishing procedures. • Tungsten carbide burs: With the development of high speed handpieces, tungsten carbide burs were designed to withstand heavy stresses and increase shelf life. ese burs work best beyond 3,00,000. ese burs have six blades and negative rake angle to provide better support for cutting edge. Tungsten carbide burs have head of cemented tungsten carbide in the matrix of cobalt or nickel. ese burs can cut metal and dentin very well but can produce microcracks in the enamel so weaken the cavosurface margins. Diamonds have good cutting efficiency in removing enamel (brittle) while carbide burs cut dentin (elastic material) with maximum efficiency. 124 Textbook of Operative Dentistry – Straight fissure – End cutting bur Part of a Bur (Fig. 6.63) Parts of bur    Figure 6.62: Classifications of Burs ere are various systems for the classification of burs. • According to their mode of attachment to the handpiece: – Latch type – Friction grip type • According to their composition: – Stainless steel burs – Tungsten carbide burs – A combination of both • According to their motion: – Right bur: A right bur is one which cuts when it revolves clockwise. – Left bur: A left bur is one which cuts when revolving anticlockwise. • According to the length of their head: – Long – Short – Regular • According to their use: – Cutting burs – Finishing burs – Polishing burs • According to their shapes: – Round bur – Inverted cone – Pear-shaped – Wheel shaped – Tapering fissure Shank Neck Head. • Shank: e shank is that part of the bur that fits into the handpiece, accepts the rotary movement from the handpiece and controls the alignment and concentricity of the instrument. e three commonly seen instrument shanks are: – Straight handpiece shank – Latch type handpiece shank – Friction grip handpiece shank. • Neck: e neck connects the shank to the hand. Main function of neck is to transmit rotational and translational forces to the head. • Head: It is working part of the instrument. Based upon their head characteristics, the instruments can be bladed or abrasive. ese are available in different sizes and shapes. Different designs of bur shank neck and head. • Shank design (Fig. 6.64): Depending upon mode of attachment to handpiece, shanks of burs can be of following types: – Straight handpiece shank – Latch type angle handpiece shank – Friction grip angle handpiece shank. i. Straight handpiece shank: Shank part of straight handpiece is like a cylinder into which bur is held with a metal chuck which has different sizes of shank diameter. Figure 6.63: Figure 6.64: 125 Cutting Instruments ii. Latch type angle handpiece shank: In this handpiece posterior portion of shank is made flat on one side so that end of bur fits into D-shaped socket at bottom of bur tube. In this, instrument is not retained in handpiece with chuck but with a latch which fits into the grooves made in shank of bur. ese instruments are commonly used in contra-angle handpiece for finishing and polishing procedures. iii. Friction grip angle handpiece shank: is was introduced for high speed handpiece. Here the shank is simple cylinder which is held in the handpiece by friction between shank and metal chuck. is design of shank is much smaller than latch type instruments. • Design of neck: Neck connects head and shank. It is tapered from shank to the head. For optical visibility and efficiency of bur, dimensions of neck should be small but at the same time it should not compromise the strength. • Design of bur head (Figs 6.65 to 6.67): e term ‘bur shape’ refers to the contour or silhouette of the bur head. – Round bur: Spherical in shape, used for removal of caries, extension of the preparation and for the placement of retentive grooves. – Inverted cone bur: It has flat base and sides tapered towards shank. It is used for establishing wall angulations and providing undercuts in tooth preparations. Figure 6.66: Figure 6.67: – Pear shaped bur: Here head is shaped like tapered cone with small end of cone directed towards shank. It is used in class I tooth preparation for gold foil. A long length pear bur is used for tooth preparation for amalgam. – Straight fissure bur: It is parallel sided cylindrical bur of different lengths and is used for amalgam tooth preparations. – Tapering fissure bur: It is tapered sided cylindrical but sides tapering towards tip and is used for inlay and crown preparations. – End cutting bur: It is used for carrying the preparation apically without axial reduction. Modifications in Bur Design Figure 6.65: Because of introduction of handpieces with high speed ranges, many modifications have been made in design of 126 Textbook of Operative Dentistry bur. Since cutting efficiency of carbide burs increase with increase in speed, the larger diameter carbide burs have been replaced by small diameter burs. Others modifications in bur design are as following: • Reduced number of crosscuts: Since at high speed, crosscuts tends to produce rough surface, newer burs have reduced number of crosscuts. • Extended head lengths: Burs with extended head length have been introduced so as to produce effective cutting with very light pressure. • Rounding of sharp tip corners: Sharp tip corners of burs produce sharp internal angles, resulting in stress concentration. Burs with round tip corners produce rounded internal line angles and thus lower stress in restored tooth. Bur Size Bur size represents the diameter of bur head. Different numbers have been assigned to burs which denote bur size and head design. Earlier burs had a numbering system in which burs were grouped by 9 shapes and 11 sizes. But later because of modifications in bur design this numbering system was modified. For example, after introduction of crosscut burs, 500 numbers was added to the bur equivalent to noncrosscut size and 900 was added for end cutting burs. us we can say that no. 58, 558 and no. 958 burs all have same dimensions of the head irrespective of their head design. Bur Design Bur head consists of uniformly spaced blades with concave areas in between them. ese concave depressed areas are 127 Cutting Instruments Figure 6.68: Figure 6.70: Figure 6.69: called chip or flute spaces. Normally, a bur has 6, 8, or 10 numbers of blades (Fig. 6.68). • Bur blade (Fig. 6.69): Blade is a projection on the bur head which forms a cutting edge. Blade has two surfaces: – Blade face/Rake face: It is the surface of bur blade on the leading edge – Clearance face: It is the surface of bur blade on the trailing edge. • Rake angle: is is angle between the rake face and the radial line (Fig. 6.70). – Positive rake angle: When rake face trails the radial line • • • • – Negative rake angle: When rake face is ahead of radial line – Zero rake angle: When rake face and radial line coincide each other. Radial line: It is the line connecting center of the bur and the blade. Land: It is the plane surface immediately following the cutting edge (Fig. 6.71). Clearance angle: is is the angle between the clearance face and the work (Figs 6.72 and 6.73). Significance: Clearance angle provides a stop to prevent the bur edge from digging into the tooth and provides adequate chip space for clearing debris. Blade angle: It is the angle between the rake face and the clearance face.