20 Questions
What is the maximum allowed peripheral displacement from the central axis of a bur, known as eccentricity or run-out?
0.025mm
What is the purpose of crosscuts on fissure burs?
To obtain adequate cutting effectiveness at low speeds
Which of the following bur materials is prone to rust?
Steel
What is the term for the direct measurement of symmetry of the bur head?
Concentricity
What is the primary advantage of a negative rake angle on a bur?
Increased tool life
Which of the following bur design features is used for finishing and producing a smoother surface?
More blades
What is the result of excessive eccentricity or run-out in a bur?
Loss of efficiency and heat generation
What is the term for the rate of tooth structure removal during cutting?
Cutting effectiveness
Which of the following is a disadvantage of carbide burs?
Brittleness
What is the purpose of a latch-type shank design in burs?
To provide a secure grip in the handpiece
What is the primary advantage of high speed hand pieces in operative dentistry?
Reduced operating time
Which of the following types of rotary tools is most effective for extra-coronal tooth preparations?
Diamond point
What is the primary mechanism of cutting for dental abrasive tools?
Abrasation
What is the primary advantage of carbide burs over diamond points?
Better for end cutting
What determines the surface finish of a tooth preparation?
All of the above
What is the primary purpose of the shank design in a dental bur?
To lock into the hand piece
Which material is commonly used for dental burs?
Tungsten carbide
What determines the cutting effectiveness of a dental bur?
All of the above
What is the primary advantage of using diamond points for beveling enamel margins?
Improved surface finish
What is the primary purpose of the cross-cut design in a dental bur?
To increase cutting efficiency
Study Notes
Rotary Instruments for Tooth Preparation
- Rotary instrument shank fits into a hand piece and accepts rotary motion, unlike shank in hand-cutting instruments.
- Shank connects power to cutting end, while grasp is done on the shaft.
Bur Design
- Plain blades: gross cutting (6-8 blades) for less smooth surface, and finishing (above 12 blades) for smoother surface.
- Rake angle: negative rake angle (rake face ahead of radius) minimizes fractures of cutting edge, increasing tool life.
Types of Burs
- Crosscuts: needed for fissure burs at low speeds, but not at high speeds to avoid rough surfaces.
- Shank design:
- Latch type
- Friction grip
Material
- Steel: main disadvantage is rust
- Carbide: main disadvantage is brittleness (breaks)
Eccentricity or Run-out
- Maximum displacement periphery from central axis is 0.025mm
- Causes: loss of efficiency, heat generation, discomfort to the patient, and cracks in enamel
- Related to precision of handpiece
Cutting Effectiveness and Efficiency
- Cutting effectiveness: rate of tooth structure removal (mm/min or mg/s)
- Cutting efficiency: percentage of energy actually producing cutting
Rotary System Components
- Source of power:
- Electric driven
- Air driven
- Hand piece:
- Low speed
- High speed
- Cutting tool:
- Dental bur
- Abrasive stone
Hand Pieces
- Low speed: < 200,000RPM
- High speed: 400,000RPM
- Advantages of high speed HP:
- Faster cutting with less pressure and vibration
- Reduced operating time
- Better control and ease of operation
- Longer instrument life
- Less patient apprehension
Rotary Tools
- Dental cutting tools (burs):
- Blades: cut by chipping action
- Dental abrasive tools (stones):
- Abrasive particles: cut by abrasion
- Carbide bur:
- Better for end cutting
- Produces lower heat generation
- Used for punch cuts, intracoronal tooth preparation, amalgam removal, and secondary retention features
- Diamond points:
- Produce more friction due to large surface area
- High heat generation
- Used for extra-coronal tooth preparations, beveling enamel margins on tooth preparations
This quiz covers the components and design of rotary instruments and burs used in dentistry, including shank types and blade functions.
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