19: Cutting instruments

Chapter 19 Cutting instruments

Dental Handpieces

Working at high speeds (in excess of 180 000 rpm)

Two types of dental handpiece (drill) are available to work at a high cutting speed, for example, to cut through enamel and to remove existing restorations.

• High-speed handpiece, also known as an air rotor or air turbine (Figure 19.1). As its name implies, it is driven by a supply of compressed air.

• Speed-increasing handpiece (Figure 19.2). These are generally driven by an electrical motor. They achieve their speed by using a gearing mechanism much like a gear box in a car.

Both the air driven high-speed instrument and the speed-increasing handpieces use friction grip (FG) burs, the bur being the cutting component of the system (Figure 19.3). The bur is slid into the jaws of the chuck while these are open and then the jaws are released to grip the bur shank. Box 19.1 illustrates how a friction grip bur should be inserted correctly into a high-speed or speed-increasing handpiece.

High-speed handpiece

Internal structure

The turbine is powered by compressed air, which passes up the central lumen of the instrument. The air pressure is usually 3 bar (43.5 psi) although this varies depending on the handpiece manufacturer’s advice. This column of air then strikes the blades of a windmill in the handpiece head, causing it to rotate. The chuck is at the centre of the windmill. As the windmill rotates, so does the bur. The internal components of a high-speed handpiece are shown in Figures 19.4 and 19.5.

Surrounding the chuck is the bearing housing, which is held centrally within the head of the handpiece by a plastic ring. This housing must be made to very precise tolerances to prevent the bur running eccentrically. Failure of the bur to run centrally will cause:

Inside the bearing housing are seven or eight ball bearings, which run freely inside a cage called a ball race. The ball bearings are surrounded by a phenolic resin that lubricates their movement in the same way as oil lubricates the moving parts of a car engine. The race holds the shank of the bur, allowing it to rotate smoothly along a central axis with minimal friction. These ball bearings are made of either stainless steel or a ceramic material. There has been a move in recent years towards ceramic ball bearings, primarily because ceramic, is a harder material than stainless steel and wears less. It is also lighter in weight.

The importance of torque

One of the critical properties for any dental handpiece is torque (along with concentricity and noise). This is the ability of the bur to continue to rotate and therefore cut when pressure (approximately 70 g) is applied to the substrate. The free running speed of a turbine is in the order of 300 000–400 000 revolutions per minute (rpm). As the bur is applied to the tooth the bur slows to a cutting speed of between 180 000 and 200 000 rpm. The optimum cutting speed is approximately one half of the free running speed.

To allow the bur to continue to rotate, the power must be maintained. The relationship of power, torque and speed are illustrated in Figure 19.9. Power is difficult to maintain with an air supply as the air pressure is not supplied at a constant level and may fluctuate depending on the draw down of air from the compressor. This will have an effect on the speed of the bur and hence the torque. The air pressure should be set at 2–3 bar (29–43.5 psi) and should be confirmed using an air gauge. There may be significant fluctuations in air pressure due to the various demands on the compressor due to demand from other clinics in a multi-clinic practice.

Speed-increasing handpiece

The speed-increasing handpiece (Figures 19.2 and 19.10) is driven by an electrical motor, also called a micromotor. The handpiece is placed onto the coupling of the micromotor on the dental unit (Figure 19.11).

The power to drive the handpiece is provided by the micromotor and the internal gearings in the handpiece cause the bur to rotate at a constant speed and torque.

Comparison of high-speed and speed-increasing dental handpieces

A comparison of the features of high and low speed handpieces are set out in Table 19.1.

Table 19.1 Comparison of the features of high-speed and speed-increasing handpieces

  High-speed Speed-increasing
Type of bur used Friction grip Friction grip
Power source Compressed air Electric micromotor
Torque Variable Constant
Motion of bur Rotation and pecking Rotation only
Balance Usually neutral Motor end heavy

Slow-speed handpieces

Handpieces that operate at a slower cutting speed, that is, between 600 and 250 000 rpm are available as either contra-angle or straight (Figure 19.13).

The internal workings of slow-speed handpieces are essentially the same as previously described for the speed-increasing handpiece as shown in Figure 19.11. The main differences between them are:

• In the internal gearings

• The slow-speed handpiece uses a latch grip bur (Figure 19.14) instead of the friction grip bur used in the speed-increasing handpiece.

Indications for slow-speed handpieces

Contra-angle handpieces are generally used for operative procedures such as the removal of dental caries and for polishing enamel and restorative materials intraorally. Straight handpieces are used in oral surgical procedures or for the extraoral adjustment and polishing of acrylic and metals. The speed of the handpiece will depend on the task. Table 19.2 lists the cutting speeds for common dental procedures and materials.

Jan 31, 2015 | Posted by in Dental Materials | Comments Off on 19: Cutting instruments
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