Rotary Cutting Instruments PDF

Rotary Cutting Instruments Classification of Dental Handpiece These are instruments are used for bulk tooth reduction may be  Ultra speed (fictional grip) tools for gross reduction or regular speed (latchet type), used for smoothening of preparation. CRotary speed ranges:  The rotational...

Rotary Cutting Instruments Classification of Dental Handpiece These are instruments are used for bulk tooth reduction may be  Ultra speed (fictional grip) tools for gross reduction or regular speed (latchet type), used for smoothening of preparation. CRotary speed ranges:  The rotational speed of an instrument is measured in revolutions per minute (rpm). Three speed ranges are generally recognized: Regular, medium and high. 1. Low speed (below 12,000 rpm). Low speed cutting is a traumatic procedure for both the dentist and the patient where it is: • Ineffective and time consuming. • Requires heavy force of application. • Heat production is accentuated. • Induces vibrations of low frequency and high amplitude. • The brittle carbide burs are easily broken at such speed. That is why the low- speed range is used for cleaning teeth, caries excavation and finishing and polishing procedures. High or ultra high speeds (above 200,000 rpm).  This speed is used for tooth preparation and removing old restorations, with the following advantages: • Remove tooth structure with less pressure, vibration and heat generation. • The number of cutting instruments used is reduced as the smaller sizes are more universal in application. • Better control and ease of operation. • Instruments last longer. • Patients are less apprehensive due to less vibration and decreased operating time. • Several teeth could be treated in the same visit. Classification of Cutting Tool itself BURS(Bladed) ABRASIVES(Stones and discks) *Supported with blades * Scattered hard materials (e.g. diamond) * May be different sizes and shapes * May be different sizes and shapes * Used: production of smooth cut surface *The smoothness of surface depend on fineness of abrasive. * Produce less heat generation *Produce more heat generation due to higher friction The cutting tool is Composed of three main parts:  1) Shank: Secured to the hand piece, which may be: a: straight hand piece. b: latched secured to angled hand piece. c: frictional grip angled hand piece shank used with ultra speed air turbine. 2) Shaft: Connects head of bur with shank. 3) Head: The working part which does the cutting by means of small blades/teeth located on the metal head Classification of burs 1.The material:  Steel bur: hardness is 600-700 V.H.N, its geometry and design is less perfect than carbide burs. Used with regular speed only as it could be damaged if used with high speed due to increase in temperature. Rust easily and shows less cutting efficiency than carbide burs.  Tungsten carbide burs: hardness number 1200 – 1700 VHN, more precise in geometry and design. It could be used with regular and ultra-high speed; it resists melting and bending, thus shows a high cutting efficiency with less heat production. Could be distinguished by its dark color.  Polymeric burs: Smart prep burs:  It is a method based upon constructing a mechanical device for dentin caries removal using a material much softer than carbide steel, and providing cutting elements that abrade or deflect upon encountering sound dentin. 2.Attachment to the hand piece: Frictional  grip: used with ultra-speed handpiece. Latch type: for conventional handpiece. 3.According to the number of blades and mode of use: Either cutting: 6-8 blades. Finishing burs: 12-16 blades. 4. According to the shape of working head : : . According to the shape of working head :  1. Rounded : → Used for :  - Gaining access during cavity preparation.  - Excavation of carious dentin.  - Placing retentive grooves and pinholes.  - Shaping of casted metallic restoration.  - Shaping tooth colored restoration.  2. Inverted cone : → Used for :  - Lateral extension of cavity walls by undermining the enamel.  - Placing the retentive mechanical undercuts. 3. Fissure : → Used for :   - Lateral extension of cavity walls.  - Finishing of cavity walls.  - Roundation of line angles of the prepared cavity.  - Tapered forms are used to place retentive grooves. 5.According to the size 6. Modifications in bur design :  (1) Reduced use of cross-cuts : Since cross -cuts are needed on fissure burs to obtain adequate cutting effectiveness at low speeds, at high speeds they are no longer needed as they produce unduly rough surfaces.  (2) Carbide fissure burs with extended head lengths two to three times those of the normal tapered fissure burs of similar diameter.  (3) Rounding of sharp tip corners. This results in lower stresses in restored teeth and enhance strength of tooth by reserving vital dentin. Bur Design: Each blade is composed of: Face: Meets the cut structure, it is the leading side. Back: Follow face, it is the trailing (drawn along behind)side. Blade angle: It is the angle between face and back of blade. Radius of the blade: It is an imaginary line pass through the tip of blade and center of rotation. Line of work: An imaginary line which is tangent to the lip of the blade and 1 to radius. Rake Angle It is the angle existing between the face of the blade and the radius of the bur may be in 3 positions: A) Positive: The radial line is ahead of the face of blade. Provides better cutting efficiency but on expense of the bulk of the blade i.e. blade is weak and may be more liable to bending / fracture in case of excessive positive angle. B) Negative: The radial line trails the face of the blade, where the face and back of the blade are equal. Producing minimal cutting efficiency. C) Zero / Radial angle The radial line is overlying the face of the blade. Moderate cutting efficiency Cutting efficiency: is the ability to remove maximum amount of tooth structure / tissue with minimum effort and time involved".  Factors affect cutting efficiency: 1. Bur • Design. • Material. • Condition. 2. Pressure. 3. Tissue to be cut. 4. Speed. 5. Coolant acts as a lubricant which enhance cutting efficiency.  "It Eccentricity / Run-out "It is the maximum displacement of the periphery of working point of an instrument / tool from its central axis." Eccentric bur will cause lateral displacement > 0.025 mm Heat generation  "It is the heat generated during the cavity preparation by rotary instrument."  Occur due to: • Friction between tool and tooth. • Friction between tool and hand piece. → Friction increases by: Excessive pressure, speed, size of the bur Cutting mechanisms: It is measured in terms of effectiveness and efficiency. Cutting effectiveness: It is the rate of tooth structure removal (mm/min or mg/sec). Cutting efficiency: It is the percentage of energy producing actual cutting. It is reduced when energy is dissipated as heat or noise. It is possible to increase effectiveness while reducing efficiency, e.g. a dull bur may cut faster than a sharp bur by applying pressure, but will result in an increase in heat generation, thus reduced efficiency. It is well known that increased rotational speed result in increased effectiveness and efficiency. Adverse effects may be associated with increased speed: heat, vibration and noise.