1. Msc. Operative dentistry
Rotary Instruments
Lec.3
Karam Ahmed
Msc. Operative dentistry

2. Handpiece Definition and function: it’s a device used for:
Holding rotating instrument (Burs).
Transmitting power to them.
Positioning them intraorally.
Form:
Straight
angle

3. Speed ranges: Measured by revolution per minute (rpm)
Low or Slow speed below 12,000 rpm.
Medium or Intermediate speed 12,000 – 200,000 rpm.
High or Ultra-high speed above 200,000 rpm.
The crucial speed factor is the surface speed:
The velocity at which the edges of the cutting instrument pass across the surface being cut.
Factors affecting surface speed:
Rotational speed (directly proportional).
Diameter of instrument (directly proportional).

4. Slow speed
Tooth structure can be removed by slow speed but this must be avoided,
Why?????????
Traumatic experience for both patient and dentist.
Ineffective.
Time consuming.
Require heavy force.
Excessive vibration and heat generation.
Burs have tendency to roll out of cavity and the cavity margins or outer tooth surface.
Carbide burs easily broken by slow speed and high pressure.
So it has limited uses for:
teeth cleaning.
caries excavation.
finishing and polishing.

5. OTHER NON HAND CUTTING EQUIPMENT:
high speed
Advantages over slow speed
Remove tooth structure much more faster.
Less pressure required.
Less vibration and heat generation.
Better controlled by operator and easier for use.
Instruments (burs) last longer.
Patient less apprehensive because annoying vibration and time reduced.
Permit operating several teeth within same arch at the same appointment.
OTHER NON HAND CUTTING EQUIPMENT:
Laser
Air abrasion

6. Rotary Cutting instruments (Burs)
Variable size, type, shape….. to:
Accommodate particular handpiece.
Accommodate particular clinical application.
Common design characteristics
Shank
Neck
Head

7. Classification of Burs
According to Material used
Steel burs:
Well cutting human dentin but Mainly used now for finishing procedures, Why???
Dull easily.
Heat production.
Vibration.
Carbide burs:
Stiffer and stronger than steel but unfortunately:
Rust easily,
Easily fractured at neck when subjected to sudden blow, so replaced by steel neck.
Again steel neck may bend and producing a risky eccentric rotation (run- out) vibration.

8. According to shank design
Grasped by metal chuck
Grasped by D-shape socket and retaining latch
Grasped by plastic or metal chuck
A = Straight.
B = Latch type.
C = Friction grip.

9. Neck Design
Neck permit better vision for the operation field and transmit the rotational force from the shank to head.
Larger diameter is stronger but interfere with vision.
Smaller permit better vision but its weak.

10. Head Design
Inverted cone
Round
Straight Fissure
Pear
Tapered Fissure

11. Round: Straight fissure: Inverted cone: Pear: Tapered fissure:
Initial cavity preparation.
Extension of preparation.
Preparation of retention grooves.
Removal of caries.
Straight fissure:
For amalgam cavity preparation.
Inverted cone:
Providing undercuts and planing cavity floors.
Pear:
For cavities of gold or amalgam.
Tapered fissure:
For indirect works (inlay/onlay, crown….) free of undercuts.

12. Blades Design Plain-cut (non cross-cut) Cross-cut: Advantages:
High quality surface
Durable.
Disadvantages:
Less cutting efficiency.
Cross-cut:
High cutting performance.
Low quality surface (rough).
Less durable.

13. Art and Science of Operative Dentistry (p 348).
Numbering system
Art and Science of Operative Dentistry (p 348).

14. Bur features affecting cutting efficiency
1- Cross-cut:
More pressure per unit area.
2- Neck diameter:
Larger permits more pressure without fracture
3- Head diameter:
Larger = more surface speed.
4- Spiral angle:
Excessive increase of the angle decrease cutting efficiency.
Blade direction
Axis

15. 5- Number of blades
6-10 Excavator
12-40 Finishing

16. 7- Head length, head tapering don’t have significant effects
6- Concentricity
Runout:
- Causes:
- Effects:
Vibration
Larger cavity than bur diameter
7- Head length, head tapering don’t have significant effects
Eccentric chuck
Bent Neck
Eccentric head

17. Diamond abrasive instruments

18. Bonding may be done by electroplating
Cutting based on large number of small angular particle that protrude from softer bonding matrix
Blank
Bonding may be done by electroplating

20. Diamond particles factors
Size:
Coarse ( µm), medium (88-125µm), fine (60-74µm), very fine (38- 44µm). Larger for excavation, smaller for finishing.
Spacing:
Increasing the size and number of particles decreasing spaces and thus increasing cutting efficiency as more particles contacting the surface to be cut at any one time.
Uniformity:
Affect number of particles that come in contact at any one time.
Exposure:
Related to spaces and uniformity, plus the operator skills and techniques.
Bonding:
Durability and cutting efficiency directly proportional with bonding strength.

21. OTHER ABRASIVE INSTRUMENTS
1- Molded
Blank
Matrix
&
Abrasives
Mold
Molded abrasives are durable as when the surface layer wear, the subjacent layer still possess the same characteristics

22. Molded (Mounted)
Mounted Disc

23. 2- Coated Thin layer of abrasive material Cemented Blank
Mostly used for polishing discs fabrication

24. Coated abrasive disc / Unmounted molded disc
Screw
Double sided coating
mandrel

25. Materials Matrix: Abrasives:
Phenolic resin (non flexible) or rubber (flexible).
Abrasives:
Silicon carbide (Carborundum™): gray-green (disc is black) in color, produce moderately smooth surface.
Aluminum Oxide: white in color and produce smoother surface.
Garnet (red) and Quartz (white): used for initial finishing procedures.
Pumice: powder abrasive formed by crushing foamed volcanic glass into thin glass flakes. Used with rubber discs for initial polishing.
Cuttlebone: driven from cuttlefish, only used with coated polishing discs for final fishing procedures.

26. The End