rotary insturments

* Lec.3 Karam Ahmed Msc. Operative dentistry

* Handpiece
Form: Straight

angle

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

* 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).
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.

* 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.

* 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

* Rotary Cutting instruments (Burs)

Variable size, type, shape….. to: Accommodate particular handpiece. Accommodate particular clinical application. Common design characteristics
Shank
Neck
Head

* 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.

* According to shank design

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

* 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.

* Head Design

Round
Straight Fissure
Inverted cone
Tapered Fissure
Pear



* Round: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.

* Blades Design

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

* Numbering system

Art and Science of Operative Dentistry (p 348).


* 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.
Bur features affecting cutting efficiency
Axis
Blade direction

* 5- Number of blades

6-10 Excavator
12-40 Finishing



* Runout: - Causes:- Effects:VibrationLarger cavity than bur diameter7- Head length, head tapering don’t have significant effects 6- Concentricity
Eccentric chuck
Bent Neck
Eccentric head


*

* Blank

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


*

*
Diamond particles factors

* Matrix & Abrasives

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

* Molded (Mounted)

Mounted Disc

* 2- Coated

Blank
Mostly used for polishing discs fabrication
Cemented
Thin layer of abrasive material

* Coated abrasive disc / Unmounted molded disc

Screw
mandrel
Double sided coating

* Materials

Matrix: 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.

The End

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