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ContentsIntroduction
Historical development
Classification
Advantages
Maintenance of instruments
Cutting points
 Bu...
Introduction
The removal and shaping of tooth structure is
essential aspect of dentistry
The term “rotary” applied to to...
Practical application of rotary instruments can be:
 Penetration
 Extension
 Excavation
 Refinement
 Today cavity pr...
History
There is evidence that Maya (250 AD – 900 AD)
and other ancient cultures used primitive “bow
drills” and other de...
First rotary instruments – drill or bur heads,
twisted into fingers for crude cutting or abrading
action
Dr. Taft (1728)...
One of refinements – Scranton’s drill
Could be rotated in either direction to achieve cutting
Next modification – drill...
In 1850s another type of hand drill – developed by
Angle for indirect access preparations
Consists of handle and spring ...
During 1870s efforts were made to make designs
to bring the bur to bear in various directions and
was hand powdered
Merr...
Techniques were
improved significantly
when Morrison modified
and adapted the dental
foot engine from singer
sewing machi...
12 years later in 1883, the
electric dental engine linked
to the handpeice by flexible
cable arm was introduced
This was...
Dental foot engine was incorporated into dental
unit in 1914
In early 1950s, the ball bearing handpeice was
introduced
...
A continuous belt – driven contra-angle handpiece
was introduced in an attempt to increase rotational
speeds upto 1,50,00...
Evolution of rotary cutting equipment in
dentistry
Date Instrument Speed(rpm)
1728 Hand rotated instrument 300
1871 Foot e...
ClassificationAccording to design:
 Straight
 Contrangle
Straight handpiece:
Long axis of bur – same as long axis of ...
According to speed ranges:
 Strudavent:
Low or slow speed <12,000 rpm
Medium or intermediate 12,000 – 2,00,000 rpm
Hi...
AdvantagesTime: because of high speed time required very less
Energy: compared to early rotary instruments –
make use of...
Characteristics of rotary instruments
Speed:
Refers to not only revolutions pre minute, but also
surface feet per unit t...
Important to consider size of rotating tool in
relation to speed of operation
Rotary tool – used with low speed – should...
 Pressure:
 Resultant of 2 factors under control of dentist
1. Force (F) – gripping of handpiece in position and
applica...
Larger tools remove more tooth structure –
increase surface per minute contact of smaller
tools by increasing rpm
Clinic...
Heat production:
Heat is directly proportional to:
Pressure
RPM
Area of tooth in contact with tool
Cause pulps of te...
Vibration:
It is product of equipment used and speed of
rotation
Delirious effects of vibration two-fold in origin
 Am...
It is factor most destructive to
instrument , causes apprension
in patient and greatest fatigue
for dentist
Increasing o...
A wave of vibration over 1300 cycles /sec –
vibrations practically imperceptible to patient
Higher speed – less amplitud...
Undesirable modulating frequency:
Are series of vibrations in all directions perceived
by patient and dentist
End resul...
Each piece of remaining attachment will vibrate
Each will set up modulating frequency or
‘overtone’ accompanying the fun...
Friction:
Will occur in many parts of handpeice, in turbine
Critically important – high speed or above
Friction betwee...
Torque:
It is ability of handpeice to withstand lateral
pressure on revolving tool without decreasing its
speed or reduc...
Maintenance of instrument
Because of sensitivity of rotary equipment and
their parts , these are not sterilized as other
...
To reduce the friction of rotating parts of
handpeice, agents should be applied
These are usually lubricant sprays avail...
Cutting points
Burs
The term bur is applied to all rotary cutting
instruments that have bladed cutting edges
Historical development
The earliest burs were hand made
They were variable in dimension and performance
First machine m...
Carbide burs – 1947, have largely replaced steel
burs
Steel burs - now mainly used for finishing
Carbide burs – perform...
Parts:
Shank:
Part that is secured in hand piece to hold and
drive the bur
Shaft:
Connects shank to head of bur
Head...
General design
Bur tooth:
It is projections from the bur which aid in cutting
They terminate in cutting edge or blade
...
Rake angle:
Angle that face of bur tooth makes with radial
line from the center of bur to blade
Angle can be negative i...
Land:
The plane surface immediately following the
cutting edge
Clearance angle:
Angle back of tooth and work
If land is present – clearance is divided into two:
 primary clearance –...
Tooth angle:
Measured between the face and blank
If land present – measured between face and land
Flute or chip space:
The space between successive teeth
no. teeth in dental cutting cutting burs usually-
6-8
Burs classification system
To facilitate description, selection and
manufacture – highly desirable to have shorthand
desi...
Newer classification systems - Federation
Dentaire internationale (FDI) and international
standers organization (ISO)
Us...
Classification
According to mode of attachment:
Latch type
Friction grip
According to direction of motion:
Right – mo...
According to their shape and sizes:
Round burs:
Numbered from ¼, ½, 1,2, to 10
Round in shape
Uses: - initial tooth p...
Wheel burs:
Numbered – 14 and 15
Wheel in shape
Uses – placement of grooves
 gross removal of tooth structure
Invert...
Plain cylindrical fissure burs:
Numbered – 55 to 59
Bur teeth cut – parallel to long axis of teeth
– called straight
O...
Plain taper fissure:
Numbered – 168 – 172
Tapered cylindrical head
Teeth can be straight or spiral
Cross- cut tapered...
Round nose- fissure burs:
All eight types of fissure burs can be
round-ended
No 1 will be added to previous
numbering t...
Pear shaped burs:
As name indicates, shaped like pears
Numbered – 229 – 333
Uses - Mainly used in pedodontics
End cut...
Factors influencing cutting efficiency
Rake angle:
More positive – greater cutting efficiency
Burs with radial angles c...
Negative rake angle – cut chip moves directly
away from the blade edge and often fractures into
small bits
Positive rake...
As a result, greater possibility – bur teeth will be
curved, flattened, or even fractured during cutting
Positive rake a...
Clearance angle:
Provides clearance between the work and cutting
edge to prevent tooth back from rubbing on work
There ...
No. of teeth or blades:
No. of teeth in a bur is usually limited to 6-8
The external load is distributed among the blad...
Each bur tooth – removing more material-
tendency of tooth wear greater and cutting life
reduced
Bur with straight flute...
However, fewer the no. of bur teeth –
greater the tendency of vibration
If there are two or more blades in
contact with ...
Run – out:
Refers to eccentricity or displacement of bur
head from its axis of rotation while it turns
Average value of...
If bur moves away from tooth periodically – all
blades will not cut equally
Operator senses lack of cutting – greater fo...
Finish of flutes:
Bur formed – cutting each flute into bur blank with
rotating tool
During 1st
cut or pass of cutter- f...
Heat treatment:
Used to harden the bur made of soft steel
Done by heating the bur according to metal used
to a temperat...
Design of flute end:
Formed with 2 different types of end flutes:
Revelation cut – flutes come together at 2
junctions ...
Bur diameter:
Factor on which the volume of material removed
will vary
More the diameter head, more will be amount of
m...
Depth of engagement:
As depth of engagement is decreased – force
intensity on each small portion of bur tooth is
increas...
Influnce of load:
It signifies force exerted by dentist on tool head and
not pressure or stress induced in the tooth dur...
Influence of speed:
Rate of cutting increases with rotational speed,
but this increase is not directly proportional
Rat...
Dental abrasives
Designs:
The shape used for burs
are similar used of
abrasives
Only difference is bur
contains cutting...
Abrasive particle are held together by means of
“binder” (base)
Different binders are usually used
Ceramic binder – dia...
With most abrasives – binder is impregnated
throughout with abrasive particles of certain
grade
Abrasive distributed eve...
Parts
According to composition of abrasive particles,
are of following types
Diamond stones:
Hardest and most efficient abrasi...
Carbides:
May be silicon or boron carbides
Manufactured by heating silicon or boron at high
temperature to affect union...
Aluminum oxide:
Natural or extracted pure aluminum – most
efficient abrasive in fine cutting
Particles are sintered on ...
Factors influencing abrasive efficiency
Irregularity in shape of partices:
Irregular in shape – a sharp edge
Smooth par...
Hardness of abrasive material:
If abrasive cannot indent the surface, it cannot
remove any of that material
Abrasive wi...
Impact strength of abrasive material:
During rotation – abrasive particles strike work
suddenly
If it engages and doesn...
Size of abrasive particles:
Larger particles – deeper scratches on
surface – faster the material will be
removed
Pressu...
Limitations
Tactile sensitivity: it is sensation while cutting to
which control is necessary
For rotary instruments it i...
Source of power: at present source of power
applied to rotary instruments is other than human
power
Any deficiency or va...
Conclusion
In present operative procedures majority of the
work is carried out with rotary instruments
Although other al...
References
Art & science of operative dentistry
Strudavent ..5th
ed
Operative dentistry
M.A.Marzouk ..1st
ed
Principles...
Rotary instruments in operative dentistry
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Rotary instruments in operative dentistry

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Presentation describes about rotary instruments in conservative dentistry.

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Rotary instruments in operative dentistry

  1. 1. ContentsIntroduction Historical development Classification Advantages Maintenance of instruments Cutting points  Burs  Types  Design  Factors influencing cutting  Dental abrasive stones  Types  Factors influencing cutting Limitations of rotary instruments Conclusion
  2. 2. Introduction The removal and shaping of tooth structure is essential aspect of dentistry The term “rotary” applied to tooth cutting instruments, describe group of instruments that turn on its own axis to perform work Introduction of rotary powered cutting equipment was a truly major advance in dentistry
  3. 3. Practical application of rotary instruments can be:  Penetration  Extension  Excavation  Refinement  Today cavity preparation made with rotary instruments is 90% and only 10% tooth preparation accounts for hand instruments  This seminar will guide us the application and types of rotary equipment and instruments used
  4. 4. History There is evidence that Maya (250 AD – 900 AD) and other ancient cultures used primitive “bow drills” and other devices Used to prepare round ornamental cavities
  5. 5. First rotary instruments – drill or bur heads, twisted into fingers for crude cutting or abrading action Dr. Taft (1728) – “bur drills” Ranged in diameters – 1/32 " – 1/5 " Simple rotary instrument, twisted in fingers – very limited cutting
  6. 6. One of refinements – Scranton’s drill Could be rotated in either direction to achieve cutting Next modification – drill-ring (1800) Ring – adapted to middle or ring finger with a socket fitted against palm, providing seat for blunt end of drill Front end rotated with thumb and forefinger
  7. 7. In 1850s another type of hand drill – developed by Angle for indirect access preparations Consists of handle and spring loaded bur Bur activated- squeezing spring loaded handle
  8. 8. During 1870s efforts were made to make designs to bring the bur to bear in various directions and was hand powdered Merry’s drill stock is example of this modification This was a type of Angle handpeice
  9. 9. Techniques were improved significantly when Morrison modified and adapted the dental foot engine from singer sewing machine in 1878 For the first time cutting procedures were carried out with a power source other than operators hand
  10. 10. 12 years later in 1883, the electric dental engine linked to the handpeice by flexible cable arm was introduced This was first time, cutting made possible – power source other than human hands or feet Consists of – foot control with rheostst (w), belt driven straight handpeice (x), three piece adjustable extension arm (y), and electric motor (z)
  11. 11. Dental foot engine was incorporated into dental unit in 1914 In early 1950s, the ball bearing handpeice was introduced In 1953, following the work of Nelson, the first fluid turbine type of handpeice was developed Initially it was capable of rotational speed – 50,000 rpm, was operated at one speed only Soon after in 1954- air driven handpeices were developed
  12. 12. A continuous belt – driven contra-angle handpiece was introduced in an attempt to increase rotational speeds upto 1,50,000 rpm By introduction of air-driven handpiece , by 1957 many dentists capable of using rotational speeds upto 3,00,000 rpm The major breakthrough – development of handpiece with internal turbine drives By 1960s – possible to use greater rotational speed – 5,00,000 rpm
  13. 13. Evolution of rotary cutting equipment in dentistry Date Instrument Speed(rpm) 1728 Hand rotated instrument 300 1871 Foot engine 700 1874 Electric engine 1000 1914 Dental unit 5000 1942 Diamond cutting inst 5000 1946 Old units converted to increase speed 10,000 1947 Tungsten carbide burs 12,000 1953 Ball bearing handpieces 25,000 1955 Water turbine handpiece 50,000 1955 Belt-driven handpiece (Page-Chayes) 1,50,000 1957 Air turbine handpiece 2,50,000 1962 Experimental air bearing handpiece 8,00,000 1994 Contemporary air turbine handpiece 3,00,000
  14. 14. ClassificationAccording to design:  Straight  Contrangle Straight handpiece: Long axis of bur – same as long axis of handpiece More frequently used- lab , less frequently clinically Contra-angled handpiece: Primary hand piece in oral procedures Head angled – first away from and back towards long axis Ensures- working point within 2-3 mm of long axis, balance
  15. 15. According to speed ranges:  Strudavent: Low or slow speed <12,000 rpm Medium or intermediate 12,000 – 2,00,000 rpm High or ultra-high > 2,00,0000 rpm  Marzouk: Ultra-low speed 300 – 3000 rpm Low speed 3000 – 6000 rpm Medium high speed 20,000 – 45,000 rpm High speed 45,000 – 1,00,000 rpm Ultra-high speed 1,00,000 rpm and more
  16. 16. AdvantagesTime: because of high speed time required very less Energy: compared to early rotary instruments – make use of energy other than manual energy Mainly reduces operator fatigue Pressure: in case of hand instruments- pressure or force applied for removal of tooth substance – unwanted High speed of rotary- no pressure or negligible pressure Precision: by attaning good control over these instruments- tooth structure removed and shaped precisely
  17. 17. Characteristics of rotary instruments Speed: Refers to not only revolutions pre minute, but also surface feet per unit time of contact that tooth has with work to be cut Maximum cutting efficiency of a cutting tool of uniform width ranges- 5000-6000 surface feet pre minute Surface feet per minute – controlled mainly by rpm and surface feet per minute
  18. 18. Important to consider size of rotating tool in relation to speed of operation Rotary tool – used with low speed – should be larger in diameter to approach optimum surface feet per unit time Ultra-high speed range – diameter of tool reduced to limit cutting efficiency
  19. 19.  Pressure:  Resultant of 2 factors under control of dentist 1. Force (F) – gripping of handpiece in position and application to tooth 2. Area (A) – amount of surface area of cutting tool in contact with tooth surface in operation P F A  Same force F, smaller tools – apply more pressure to point of contact than larger tools  To have cut both smaller and larger tools at same pressure – necessary to reduce force applied with smaller ones = -
  20. 20. Larger tools remove more tooth structure – increase surface per minute contact of smaller tools by increasing rpm Clinically low speed- 2-5 pounds of force, high speed 1 pound, ultrs high sped 1-4 ounce for efficient cutting More desirable features of higher speed – better control , less fatigue, greater patient comfort
  21. 21. Heat production: Heat is directly proportional to: Pressure RPM Area of tooth in contact with tool Cause pulps of teeth permanently damaged if temp of 130°F is reached Absolute necessity – coolants may be employed to eliminate pulpal damage  various methods – flowinf water, water air spray, or air Pressure – resultant of applied force, reduction of force will minimize heat production
  22. 22. Vibration: It is product of equipment used and speed of rotation Delirious effects of vibration two-fold in origin  Amplitude  Undesirable modulating frequencies Amplitude: Wave of vibration – frequency and amplitude Low speed – amplitude larger, frequency smaller High speed – amplitude small, frequency large Greater harm - amplitude
  23. 23. It is factor most destructive to instrument , causes apprension in patient and greatest fatigue for dentist Increasing operating speed – amplitude and effects are reduced Vibrational waves – measured in cycles 6000 rpm – 100 cycles per sec As vibration increases, cycles per sec of vibrational waves are increased
  24. 24. A wave of vibration over 1300 cycles /sec – vibrations practically imperceptible to patient Higher speed – less amplitude and greater frequency As a result – perception will be lost in ultra high speed of >1,00,000 rpm
  25. 25. Undesirable modulating frequency: Are series of vibrations in all directions perceived by patient and dentist End result – apprehension in patient, fatigue for dentist and accelerated wear of cutting instrument Fundamental vibrational wave – set up when handpeice turbine is runing
  26. 26. Each piece of remaining attachment will vibrate Each will set up modulating frequency or ‘overtone’ accompanying the fundamental wave Patient and dentist – subjected to basic wave and other accompanying waves Objective of operator – eliminate these effects by having equipment free from any defects
  27. 27. Friction: Will occur in many parts of handpeice, in turbine Critically important – high speed or above Friction between moving parts – less reduce damage to instrument and obstruction in free running tool To reduce this – bearings incorporated Different bearings – ball bearings, needle bearings, glass in resin bearings have been uesd by different manufacturers
  28. 28. Torque: It is ability of handpeice to withstand lateral pressure on revolving tool without decreasing its speed or reducing cutting efficiency It is dependent on type of bearing used and amount of energy supplied to handpiece
  29. 29. Maintenance of instrument Because of sensitivity of rotary equipment and their parts , these are not sterilized as other instruments It is usually sterilized by using different method suitable for the instrument Chemical sterilizing agent through the hand piece in an compartment
  30. 30. To reduce the friction of rotating parts of handpeice, agents should be applied These are usually lubricant sprays available These are oily in nature and reduces the friction of the components
  31. 31. Cutting points Burs The term bur is applied to all rotary cutting instruments that have bladed cutting edges
  32. 32. Historical development The earliest burs were hand made They were variable in dimension and performance First machine made burs introduced in 1891 Early burs – steel Perform well – cutting human dentin at low speed But dull rapidly – higher speeds or cutting enamel Dulled burs – reduced cutting efficiency, increased heat and vibration
  33. 33. Carbide burs – 1947, have largely replaced steel burs Steel burs - now mainly used for finishing Carbide burs – perform better than steel burs at all speed and superiority is greatest at high speed Much harder than steel and less subjected to dulling during cutting Carbide head is attached to a steel shank and neck by welding or brazing Carbide is stiffer and stronger than steel, but it is also more brittle
  34. 34. Parts: Shank: Part that is secured in hand piece to hold and drive the bur Shaft: Connects shank to head of bur Head: Contains the blades that cut the tooth
  35. 35. General design Bur tooth: It is projections from the bur which aid in cutting They terminate in cutting edge or blade Has two surfaces:  Tooth face – side of the tooth on leading edge  Back or flank – side of the tooth on the trailing edge
  36. 36. Rake angle: Angle that face of bur tooth makes with radial line from the center of bur to blade Angle can be negative if face is beyond or leading the radial line Can be 0 if radial line and tooth face coincide with each other Can also be positive if radial line leads the face
  37. 37. Land: The plane surface immediately following the cutting edge
  38. 38. Clearance angle: Angle back of tooth and work If land is present – clearance is divided into two:  primary clearance – angle the land will make with work  Secondary clearance – angle between back of tooth and work  Back surface of tooth is curved – clearance angle is called radial angle
  39. 39. Tooth angle: Measured between the face and blank If land present – measured between face and land
  40. 40. Flute or chip space: The space between successive teeth no. teeth in dental cutting cutting burs usually- 6-8
  41. 41. Burs classification system To facilitate description, selection and manufacture – highly desirable to have shorthand designation In US – traditionally described in arbitrary numerical code for head size and shape eg.-2 = 1mm diameter round bur 57 = 0.8mm diameter inverted cone bur Despite complexity – common in use Other countries – similar arbitrary systems
  42. 42. Newer classification systems - Federation Dentaire internationale (FDI) and international standers organization (ISO) Use a separate designation for shape (shape name) and size (diameter in tenths of millimeter) are used Eg.- round 010, straight fissure plain 020 inverted cone 008
  43. 43. Classification According to mode of attachment: Latch type Friction grip According to direction of motion: Right – move clockwise Left – anti-clockwise According to length of shank: Short shank Long shank
  44. 44. According to their shape and sizes: Round burs: Numbered from ¼, ½, 1,2, to 10 Round in shape Uses: - initial tooth penetration  placement of retention grooves
  45. 45. Wheel burs: Numbered – 14 and 15 Wheel in shape Uses – placement of grooves  gross removal of tooth structure Inverted cone burs: Numbered – 33 ¼, 33 ½, 34, 35, to 39 Have an inverted cone shape Uses – in cavity extensions  establishing wall angulations and retention forms Flatten the pulpal floors
  46. 46. Plain cylindrical fissure burs: Numbered – 55 to 59 Bur teeth cut – parallel to long axis of teeth – called straight Or cut obliquely to long axis of teeth – called spiral, have better unclogging Cross-cut cylindrical fissure burs: Numbered from 555 – 560 Teeth can cut parallel to long axis (straight) or obliquely (spiral) Uses – gross cutting  cavity extension and creation of walls
  47. 47. Plain taper fissure: Numbered – 168 – 172 Tapered cylindrical head Teeth can be straight or spiral Cross- cut tapered fissure bur: Numbered – 699 – 703 Can also be straight or spiral Uses – same as that as straight fissure burs  mostly used in cutting cavities for inlays
  48. 48. Round nose- fissure burs: All eight types of fissure burs can be round-ended No 1 will be added to previous numbering to denote round nosing Eg.- round-nose plain cylindrical burs no- 156-159. - round-nose tapered fissure burs no from 1169-1172 Uses – for extension of cavities
  49. 49. Pear shaped burs: As name indicates, shaped like pears Numbered – 229 – 333 Uses - Mainly used in pedodontics End cutting burs: Cylindrical in shape Just the end carrying blades Numbered – 900- 904 Uses – in extending preparations apically without axial reduction
  50. 50. Factors influencing cutting efficiency Rake angle: More positive – greater cutting efficiency Burs with radial angles cut more effectively than designs with negative rake angles
  51. 51. Negative rake angle – cut chip moves directly away from the blade edge and often fractures into small bits Positive rake angle – chips are larger and tend to clog the chip space Positivity of rake angle – decreases size of bur tooth and tooth angle – decreasing its bulk
  52. 52. As a result, greater possibility – bur teeth will be curved, flattened, or even fractured during cutting Positive rake angle – can be used with tungsten carbide burs where there is greater hardness and strength of the material
  53. 53. Clearance angle: Provides clearance between the work and cutting edge to prevent tooth back from rubbing on work There is always a component of frictional force on ant cutting edge as it rubs against the surface Slight wear of cutting edge – increase the duling perceptibility Larger clearance angle – less rapid dulling of bur
  54. 54. No. of teeth or blades: No. of teeth in a bur is usually limited to 6-8 The external load is distributed among the blades As no. of blades decreased – magnitude of forces at each blade increases and thickness of chip removed by each flute increases Reason for construction of burs with fewer teeth- increased space between bur teeth - decreases the clogging tendency
  55. 55. Each bur tooth – removing more material- tendency of tooth wear greater and cutting life reduced Bur with straight flutes- less temperature than one with spiral flutes Formation of large chip by straight flutes- chip then carries some heat energy Bur with fewer flutes – cooler with operating
  56. 56. However, fewer the no. of bur teeth – greater the tendency of vibration If there are two or more blades in contact with work at one time, this effect is reduced If bur teeth are cross-cut – tendency is to increase no. of teeth , based on assumption that cross-cutting reduces friction in cutting and provides more chip space Burs – 10-12 or even up to 40 blades Used only for finishing and polishing
  57. 57. Run – out: Refers to eccentricity or displacement of bur head from its axis of rotation while it turns Average value of clinically acceptable- 0.023mm Also depends on precision of handpeice Shaft or collar, holding the bur wobbles – effect is magnified at bur head Efficiency in cutting is definitely affected by run- out
  58. 58. If bur moves away from tooth periodically – all blades will not cut equally Operator senses lack of cutting – greater force will be exerted on bur Structure will be removed by shattering rather than cutting Such removal of tooth structure – inefficient, inaccurate and increases heat generation
  59. 59. Finish of flutes: Bur formed – cutting each flute into bur blank with rotating tool During 1st cut or pass of cutter- flute is roughly formed 2nd cut – places cutting edge on bur flute Roughness will remain along the flute Roughness removed - making subsequent passes or cuts on the flute Those cut 6 times- more efficient, cut 2 times- least efficient
  60. 60. Heat treatment: Used to harden the bur made of soft steel Done by heating the bur according to metal used to a temperature below its melting temperature It is then allowed to come to room temp slowly This operation – preserves edge placed on bur flute by the cutter , and hardens the bur to increase the cutting life
  61. 61. Design of flute end: Formed with 2 different types of end flutes: Revelation cut – flutes come together at 2 junctions near a diametrical cutting edge Star cut – end flutes come together in common junction at axis of bur Revelation type shows more superiority in direct cutting Both show equal efficiency in lateral cuting
  62. 62. Bur diameter: Factor on which the volume of material removed will vary More the diameter head, more will be amount of material removed and vice versa Forces on each bur tooth, linear displacement and length of cut do not depend on diameter It is because length is constant
  63. 63. Depth of engagement: As depth of engagement is decreased – force intensity on each small portion of bur tooth is increased The average displacement per flute revolution should also be increased This increase can be such that volume of material removed by shallow cut exceeds that of deeper cuts
  64. 64. Influnce of load: It signifies force exerted by dentist on tool head and not pressure or stress induced in the tooth during cutting Load exerted is related to speed of the bur It is estimated that 2 pounds- for low speed and 2-4 ounces- high rotational speed Generally – every range of speed at which bur is rotating has a minimum force or load under which cutting efficiency of bur used is decreased So range do low speed- 1000-1500gm and that for high speed- 60-120gm
  65. 65. Influence of speed: Rate of cutting increases with rotational speed, but this increase is not directly proportional Rate of cutting is increased at a rotational speed above 30,000 than below this The time required for removal of same weight of tooth structure at a speed 0f 1,50,000 and above is nearly the same Conclusion – no time is saved by dentist when rotational speeds are employed higher than 1,50,000 rpm
  66. 66. Dental abrasives Designs: The shape used for burs are similar used of abrasives Only difference is bur contains cutting edges where as this contains abrasive particles
  67. 67. Abrasive particle are held together by means of “binder” (base) Different binders are usually used Ceramic binder – diamond chips Metallic binder is used in electro plating of particles Rubber or shellac – soft grade stones Type of binder – related to life of tool Later types wear rapidly, useful only in delicate cutting
  68. 68. With most abrasives – binder is impregnated throughout with abrasive particles of certain grade Abrasive distributed evenly - the surface of tool wears evenly Wide spacing between particles – room for resultant debris less chance of packing or clogging Some abrasives glued to paper that can be attached to handpeice through mounting tools
  69. 69. Parts
  70. 70. According to composition of abrasive particles, are of following types Diamond stones: Hardest and most efficient abrasive stones for removing tooth enamel Diamond chips are bind together with either ceramic binder or more efficient metallic binder
  71. 71. Carbides: May be silicon or boron carbides Manufactured by heating silicon or boron at high temperature to affect union with carbon Carbides are sintered or pressed with binder into grinding wheels, disks or stones Sand: Sand and other forms of quartz can be bound together with adhesives Mounted into different shapes of discs stone and strips
  72. 72. Aluminum oxide: Natural or extracted pure aluminum – most efficient abrasive in fine cutting Particles are sintered on the surface Garnet: These particles contain a no. of different minerals with similar physical properties and crystalline form Stones made from these used for finishing and polishing of dental appliances
  73. 73. Factors influencing abrasive efficiency Irregularity in shape of partices: Irregular in shape – a sharp edge Smooth particle – poor abrasive property Cubical particles- flat face and not efficient More irregular particles – greater abrasive efficiency
  74. 74. Hardness of abrasive material: If abrasive cannot indent the surface, it cannot remove any of that material Abrasive will dull in such a case Harder abrasive relative to hardness of material – more abrasive efficiency
  75. 75. Impact strength of abrasive material: During rotation – abrasive particles strike work suddenly If it engages and doesn't fracture – become dull The abrasive should fracture rather than dull so sharp edges are always present Fracture of abrasive – shedding of debris Diamond stones do not fracture but loose substance Because of their hardness – most efficient in removal of very hard and brittle tooth enamel
  76. 76. Size of abrasive particles: Larger particles – deeper scratches on surface – faster the material will be removed Pressure and rpm: Same factors are involved as discussed in rotary instruments Both factors are proportional to the abrasive efficiency
  77. 77. Limitations Tactile sensitivity: it is sensation while cutting to which control is necessary For rotary instruments it is less, especially at higher speeds Heat production: it is a major factor in rotary instrument application Due to high speed and friction heat production is more If not controlled may lead to damage to pukp tissue
  78. 78. Source of power: at present source of power applied to rotary instruments is other than human power Any deficiency or variation in power will affect the continuity of work Control of cutting: due to rotary nature of cutting there needs to be more control of instrument for the amount of removal to be controlled Sterilization: because of sensitivity of these instruments these equipments cannot be efficiently sterilized by regular methods
  79. 79. Conclusion In present operative procedures majority of the work is carried out with rotary instruments Although other alternatives have been evolving for removal of tooth structure, rotary cutting is most practically used For better dentistry it is then necessary to have a knowledge about these instruments and their applications
  80. 80. References Art & science of operative dentistry Strudavent ..5th ed Operative dentistry M.A.Marzouk ..1st ed Principles & practice of operative dentidtry G.T.Charbenau ..3rd ed Fundamentals of operative dentistry James B. Sumitt ..3rd ed Operative dentistry Gillmore ..4th ed Pickards manual of operative dentistry E.M.A.Kidd ..8th ed

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