Principles Associated with the Use of High Speed Rotary Instruments

Abstract

Vibration contributes to the discomfort usually associated with a dental operation. Many factors influence and cause the vibration that is perceived by the patient, but it is known that the major source of vibration is from the equipment and not the cutting instrument. One of the major advantages to the use of higher speeds for rotary instruments is that the intensity of the vibration is decreased as the frequency is increased. Speeds are available that make it possible to increase the frequency of vibration beyond the threshold of perception by the patient. Evidence has been collected, both clinically and in the laboratory, that demonstrates the necessity for using a coolant when speeds of 10,000 rpm or above are used. Air is not a desirable coolant because it fails to control the generation of heat, and it dehydrates exposed dentin. A sufficient volume of water or the air-water spray are the most effective coolants. Water, in copious quantities, is the coolant of choice if an efficient aspirating unit is employed. Carbide burs are most effective for use with higher speeds than are steel burs. In dentin, the initial cutting rate of either carbide or steel burs is about the same, but the efficiency of the steel bur diminishes very rapidly. At the ultra-high speeds of 100,000 rpm and above, carbide burs cut enamel with ease; however, at the lower speeds they should be used primarily in dentin. Design is an important factor in the cutting effectiveness of a bur, and at the present time there is a great variation both in design and in efficiency amongst various manufacturers’ products. Diamond instruments offer conveniences in size and shape that are not afforded by carbide burs. Because of these conveniences diamond instruments are used almost exclusively by some operators even though their efficiency when cutting dentin is not as great as that of the carbide bur. Diamond points are most effectively used in enamel at light pressures and with an adequate amount of coolant. Instruments of inferior quality are a poor economic investment because their functional life at increased speeds of rotation is relatively short. All diamond instruments should not be operated at the same number of revolutions per minute, inasmuch as it is the peripheral speed that is important. A diamond point with a diameter of $$ inch would have to travel three and one-third times as fast as a point with a diameter of 5 / 16 inch in order to cut with the same efficiency. Clinically it is evident that the cutting rate of the smaller diamond instrument is increased tremendously at the ultra-high speeds; however, the cutting rate of opening wheels and other instruments of comparable or larger size appears to be increased only slightly at speeds in excess of 20,000 to 30,000 rpm. As with any new technique, the use of higher speeds for rotary instruments is in a state of flux and transition. Despite all that has been learned about its use during the last five or six years, much investigation and research are still needed, particularly in connection with the use of speeds above 100,000 rpm. The dentist who has not experienced the advantages to be gained by the judicious use of higher speeds of rotation is encouraged to begin by first gaining experience with the technique in the laboratory. Thought should be given to the use of higher speeds only after becoming thoroughly familiar with the characteristics of cutting instruments that are rotating at 10 to 15,000 rpm. There is now available a range of speeds from the conventional to 200,000 rpm. The choice of the highest speed to be used is one of individual concern. Its selection should be based upon sound biologic, physiologic and operative principles, and should offer the patient and the dentist confidence, efficiency and comfort.