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Lec.3
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د.عبدالمنعم الخفاجي
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Powered cutting equipment
Handpiece:
Is a device for holding rotary instruments, transmitting power (Source of power is motor or air) to them, and for positioning them intraorally. Handpieces and associated cutting and polishing instruments developed as two basic types, straight and angle.
Straight Handpiece Contra-angle Handpiece Air Turbine Handpiece
Rotary speed ranges
Three speed ranges are generally recognized measured in revolutions per minute (rpm)
Low speed or slow speeds: (below 12,000 rpm.) Used for tooth cleaning, caries excavation, and finishing and polishing procedures. At low speeds, tactile sensation is a better and there is generally less chance for overheating cut surfaces.
Medium or intermediate speeds: (12,000 – 200,000 rpm).
High or ultra-high speeds: (above 200,000 rpm). Used for tooth preparation and removing old restorations.
* Most useful instruments are rotated at either low or high speed.
Rotary Cutting Instruments
The individual instruments intended for use with dental handpieces are manufactured in hundreds of sizes, shapes, and types. This variation is in part resulted from the need for specialized designs for particular clinical applications or to fit particular handpieces, but much of the variation also resulted from individual preferences on the part of dentists.
In spite of the great variation that exists among rotary cutting instruments (Burs), they have certain design features in common. Each instrument consists of three parts: shank, neck, and head.
Materials of dental burs
Stainless Steel: introduced in 1891 , perform well cutting human dentin at low speeds, but dull rapidly at high speed or when cutting enamel. Once dulled, reduced cutting effectiveness creates increased heat and vibration.
Carbide burs: usually tungsten carbide, they are introduced in 1947, replaced steel burs for cavity preparation, carbide is much harder than steel and less subjected to wear during cutting. But they are brittle, so they will fracture when subjected to sudden blow or shock.
Diamond burs: introduced in 1942, they belong to “abrasive instruments” are based on small particles of a hard substance held in a matrix of softer material.
Cutting occurs at a large number of points where individual hard particles protrude from the matrix. Diamond instrument have long life and great effectiveness in cutting enamel and dentin.
1- Round Burs: Spherical in shape, with different sizes ( 1/4 = 0.5 mm, 1/2 =0.6 mm , 1 = 0.8 mm, 2 = 1.0 mm, 3 = 1.2 mm, 4 = 1.4mm …etc) . Large round burs with slow speed used for caries removal which can not be removed by spoon excavator. Small round burs used for entry into the tooth and for preparation of retentive pinholes and grooves.
Elliptical Burs (Pear shaped): Elliptical or elongated round burs have became popular as a result of a trend toward conservative cavity design. Characterized by round corners with a reverse taper. They produce a conservative cavity preparation with rounded internal corners and a minimum removal of tooth structure.
3- Straight fissure burs: Elongated cylinder, cut enamel and dentin well at high speed, establishing outline form and have advantage of leaving smooth cut surface.
Numerical code: (56 = 0.8 mm, 57= 1.0 mm, 58 = 1.2 mm, 59 = 1.4 mm…..).
Tapered fissure burs: Tapered cone with the small end of the cone directed away form the shank. Used for inlay and crown preparation, where freedom of the undercut is essential.
Numerical code: (700 = 1.0 mm, 701 = 1.2 , 702= 1.6 mm, 703= 2.1mm)
Inverted cone bur: Tapered cone with the apex of the cone directed toward the bur shank. The head length is about the same as the diameter. Used for providing undercut in cavity preparation and flattening of the pulpal and gingival walls. Numerical code: ( 33 1/2 = 0.6 mm, 34= 0.8 mm, 35 = 1.0 mm…..).
Rotary finishing and polishing instruments
Used for finishing and polishing of filling material and tooth surface, they have different sizes and shapes and colors Examples: Silicon carbide (Carborundum) , Diamond stones, Aluminum oxide discs, silicon rubber wheel and points, brushes and rubber cups.
Control of operative instruments
Grasping instrument as well as resting or balancing them while in function
The pen grasp: Holding instrument similar to the pen grasp except that the pulps of the thumb and first and second fingers contact the instrument the third and fourth finger are placed on the adjoining teeth. The position of the second finger is important for obtaining thrust and preventing instrument from slipping during manipulation. It is used usually with mandibular arch; the action of the instrument is down and away from the operator.
Inverted pen grasp: The same as pen grasp , but the action of the instrument is up and toward the operator . usually applied in maxillary arch either left or right side , turning the wrist so that the palm and finger tips directed towards the operator positions (stands behind and slightly at right side of the patient).
Palm and thumb grasp: The handle is placed in a palm of the hand and grasped by the four fingers. The thumb remains free resting on the area other than that being operated on. Usually used for upper arch.
Modified palm and thumb grasp: Used when it is feasible to rest the thumb on the same tooth being working on or on a tooth immediately adjacent. The handle is held between the thumb and first and second fingers; the third and fourth finger held the instrument under the first joint of each finger (to act as stabilizer and prevent slippage of the instrument) and press the instrument against the distal area of the palm.
Instrument Sharpness
It is essential that all cutting instruments should remain sharp, dull instrument with dull cutting edge cause:
Excessive force required for cutting that may lead to pain.
Prolong operator time.
Less controllable.
Reduce quality in cavity preparation.
Sharpening of the instrument is necessary to maintain the original bevel inclination.
The majority of operative hand cutting instruments are sharpened on either Arkansas stone or the mechanical sharpeners.
With Arkansas stone the instrument should be grasped with pen grasp or thumb and palm grasp firmly maintaining the proper angle of the cutting edge to the stone. The instrument is pushed with heavy pressure and pulled with light pressure.
Testing sharpness: The sharpness of the cutting edge can be tested satisfactory on the thumbnail with very light pressure.
Actions
Speak
Louder than
Words
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