Rotary cutting instruments in conservative dentistry

A detailed explanation of rotary cutting instruments used in operative dentistry. This is based on the Textbook of preclinical conservative dentistry by Nisha Garg and Amit Garg

1. ROTARY CUTTING INSTRUMENTS SARA SULTANA II BDS

2. INTRODUCTION Rotary cutting instruments are those instruments which rotate on an axis to do the work of abrading and cutting on tooth structure TYPES: 1. Handpiece: power device 2. Bur: cutting tool

3. HANDPIECES First rotary instruments: drill or bur heads that were twisted with the fingers for crude cutting of the tooth tissue. Drilling—modification 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 Mid 19th century- invention and development of both mechanical and pedal powered handpieces. George fellow invented the clockwork drill—bur was attached to it by a shaft with a rotating spindle inside The 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.

4. Electrically driven handpieces Electrically driven handpieces—advantageous over the air driven predecessors. Heavier and bigger than air-driven handpieces Maintain constant speed during cutting which doesn't decrease under load Ability to control the rpm rate.

5. Design for better efficiency Diameter and bend of  handpiece should be designed such that it fits optimally between thumb and forefinger and also has 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 .

6. Classification of handpiece According to their driving mechanisms: gear driven, water driven, belt driven and air-driven 1. 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 Wide speed range Work best at low speed  because of so many moving parts with metal to metal contact 2. Water driven handpiece:  speed can be up to 100,000 rpm Small inner piece  transport water under high pressure to rotate the turbine in the handpiece and the larger outer tube returns the water to the reservoir Advantage: quiet, and have highest torque

7. 3. Belt-driven handpiece: Speed >100,000 rpm Excellent performance and great versatility 4. Air-driven handpiece: Speed= 300,000 rpm

8. Types of handpiece based on design Contra-angle handpiece Head is first angled away from and then backwards towards long axis of handle Bur head lies  close to long axis of the handle of handpiece which improve accessibility, visibility and stability of handpiece while working.  a. Air-rotor contra-angle handpiece: It gets power from the compressed air supplied by the compressor. This handpiece has high speed and low torque   b. Micromotor handpiece: It gets power from electric micromotor or airmotor . This handpiece has high torque and low speed Straight handpiece: Long axis of  bur lies in same plane as long axis of handpiece Used in oral surgical and lab procedures.

9. DENTAL BURS “Bur is a rotary cutting instrument which has bladed cutting head.” Used to remove tooth surface by chipping it away or by grinding. 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. Diamond particles are attached to bur shank either by sintering or by galvanic metal bond.

10. Materials used to make burs Stainless stee l: slow speed (<5000rpm)) Usually a bur has 8 blades with positive rake angle for active cutting of dentin This makes them fragile, therefore short life Used for  cutting soft carious dentin and finishing procedures Tungsten carbide burs: Withstand heavy stresses and increase shelf life Work best beyond 300,000 rpm. 6 blades and negative rake angle—better support for cutting edge. Head of  cemented tungsten carbide in the matrix of cobalt or nickel 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

11. CLASSIFICATION OF BURS: 1. ACCORDING TO THEIR MODE OF ATTACHMENT TO THE HANDPIECE: Latch type Friction grip type 2. ACCORDING TO THEIR COMPOSITION: Stainless steel burs. Tungsten carbide burs A combination of both

12. 3. ACCORDING TO THEIR MOTION: Right bur: cuts when it revolves clockwise Left bur: cuts  when revolving anticlockwise.  4.  ACCORDING TO THE LENGTH OF THEIR HEAD :  Long   Short  Regular  5. ACCORDING TO THEIR USE: :  Cutting burs  Finishing burs  Polishing burs

13. 6. ACCORDING TO THEIR SHAPES: Round bur Inverted cone Pear shaped Wheel shaped Tapering fissure Straight fissure End cutting bur

14. PARTS OF A BUR 1. SHANK 2. NECK 3. HEAD

15. SHANK 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.  The three commonly seen instrument shanks are: 1. straight handpiece shank 2. latch type handpiece shank 3. friction grip handpiece shank

16. NECK Connects the shank to the hand Main function- to transmit the rotational and translational forces to the head

17. HEAD It is working part of the instrument.  Based upon their head characteristics, the instruments can be bladed or abrasive

18. 1. SHANK DESIGN STRAIGHT HANDPIECE SHANK Shank part of straight handpiece is like a cylinder into which bur is held with a metal chuck LATCH TYPE ANGLE HANDPIECE SHANK 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 Instrument is not  retained in handpiece with chuck but with a latch which fits into the grooves made in shank of bur. In contra angle handpiece for finishing and polishing FRICTION GRIP ANGLE HANDPIECE SHANK Used in high speed handpiece Shank- simple cylinder held in the handpiece by friction between shank and metal chuck Smaller than latch type

19. 2. 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.

20. 3. DESIGN OF BUR HEAD 1. ROUND BUR Spherical in shape, used for removal of caries, extension of the preparation and for the placement of retentive grooves 2. 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. 3. PEAR SHAPED BUR 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

21. 4. STRAIGHT FISSURE BUR It is parallel sided cylindrical bur of different lengths and is used for amalgam tooth preparations. 5. TAPERING FISSURE BUR It is tapered sided cylindrical but sides tapering towards tip and is used for inlay and crown preparations. 6. END CUTTING BUR It is used for carrying the preparation apically without axial reduction.

22. MODIFICATIONS IN BUR DESIGN Larger diameter carbide burs have been replaced by small diameter burs 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: Since, 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

23. SIZES OF BUR bur size—diameter of the bur head Earlier burs  had a numbering system in which burs were grouped by 9 shapes and 11 sizes Later this numbering system was modified.

24. BUR DESIGN Bur head consists of uniformly spaced blades with concave areas in between them.  These concave depressed areas are called chip or flute spaces.  Normally, a bur has 6, 8, or 10 numbers of blades

25. 1. BUR BLADE: 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.

26. 2. RAKE ANGLE: The angle between the rake face and the radial line. • 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

27. 3. RADIAL LINE It is the line connecting center of the bur and the blade

28. 4. LAND It is the plane surface immediately following the cutting edge 

29. 5. CLEARANCE ANGLE The angle between the clearance face and the work. Significance:  Clearance angle provides a stop to prevent the bur edge from digging into the tooth and provides adequate chip space for clearing debris.

30. 6. BLADE ANGLE It is the angle between the rake face and the clearance face. Significance: Among these rake angle is one of the most important feature of bur blade design. Negative rake angle increases the life of bur by reducing fracture of cutting edges. Positive rake angle increases the cutting efficiency but since it reduces the bulk of bur blade, it becomes prone to fracture.  Positive rake angle also causes clogging of debris in the chip space.  If blade angle is increased, it reinforces the cutting edge and thus reduces their fracture. But clearance angle, blade angle and rake angle cannot be varied independent of each other. For example, increase in blade angle, decreases the clearance angle. Usually, the carbide burs have negative rake angles and 90° of blade angle so as to reduce their chances of fracture. For better clearance of debris, the clearance faces of carbide burs are made curved to provide adequate flute space.

31. 7. CONCENTRICITY It is a direct measurement of symmetry of the bur head. In other words, concentricity measures whether blades are of equal length or not. It is done when the bur is static.

32. 8. RUN-OUT It measures the accuracy with which all the tip of blades pass through a single point when bur is moving  It measures the maximum displacement of bur head from its center of rotation.  In case,  there is trembling of bur during rotation, this effect of run-out is directly proportional to length of bur shank Run-out occurs if: a. Bur head is off  center  on axis of the bur. b. If bur neck is bent. c. If bur is not held straight in handpiece chuck.  Run-out causes:  a. Increase in vibration during cutting.  b. Causes excessive removal of tooth structure.

33. FACTORS AFFECTING CUTTING EFFICIENCY OF A BUR

34. 1. CLEARANCE ANGLE, RAKE ANGLE AND BLADE ANGLE Clearance angle reduces the friction between cutting edge and the work. It also prevents the bur from digging excessively into the tooth structure.  But an increase in rake angle decreases the blade angle which inturn decreases the bulk of bur blade Positive rake angle increases cutting efficiency of bur, but increase in rake angle causes decrease in bulk of bur blade and clogging of flute space because of production of larger chips

35. 2. END CUTTING OR SIDE CUTTING BUR According to particular task, choice of bur can be end cutting, side cutting or combination of both. For example, it is preferred to make entry to enamel by end cutting bur, while for making preparation outline, use side cutting bur.

36. 3. NECK DIAMETER OF BUR: If neck diameter of bur is large, it may interfere with accessibility and visibility.  But if diameter is too short, it will make bur unable to resist the lateral forces

37. 4. SPIRAL ANGLE Burs with smaller spiral angle have shown better efficiency at high speeds.

38. 5. LINEAR SURFACE SPEED Within the limit, faster the speed of cutting instrument, faster is the abrasive action and more efficient is the tooth cutting instrument. Bur speed should be increased in limits because with ultrahigh speed, centrifugal force comes into the play.

39. 6. APPLICATION OF LOAD : Load is force exerted by a operator on tool head.  Normally for high speed instruments, load should range between 60 and 120 gm and for low rotational speeds, it should range between 1000 and 1500 gm. Cutting efficiency decreases when load is applied, there is increase in temperature at work face which results in greater wear and tear of handpiece bearings.

40. 7. CONCENTRICITY AND RUN-OUT The average clinically acceptable run-out is 0.023 mm. Increase in run-out causes increase in vibrations of the bur and excessive removal of tooth structure.

41. 8. LUBRICATION Lubricant/coolant applied to tooth and bur during cutting increases the cutting efficiency and decreases the rise in temperature during cutting.  Absence of coolant can result in increase in surface temperature which may produce deleterious effects on pulp.

42. 9. HEAT TREATMENT OF BUR Heat treatment of bur preserve the cutting edges and increases shelf life of the bur.

43. 10. NUMBER OF BLADES Usually a bur has 6-8 number of blades. Decrease in number of blades reduces the cutting efficiency but causes faster clearance of debris because of larger chip space

44. 11. VISUAL CONTACT WITH BUR HEAD For efficient tooth cutting, it is mandatory to maintain visual contact with bur head while working.

45. 12. DESIGN OF FLUTE ENDS There are two types of flute ends: • Star cut design: Here the flutes come together in a common point at the axis of bur. • Revelation design: Here the flutes come together at two junctions near diametrical cutting edge. It has better efficiency in direct cutting

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