Читать книгу A Practical Approach to Operative Dentistry - Gordon B. Gray - Страница 16

These are often referred to as slow‐speed handpieces and are used for removing caries or refining the margins of a cavity. These handpieces require a motor to drive them. This can be an air driven motor or an electric version. The resultant speed of rotation differs between the two with an air motor providing a rotational speed of about 6000 rpm, whereas an electric version can rotate at speeds up to 40 000 rpm. The speed of electric motors can be controlled by rotating a ring at their lower end (see Figure 1.2).

Figure 1.1 The air turbine handpiece is well balanced and operates in excess of 400 000 rpm. It is driven by compressed air that passes over the blades of the turbine and rotates them.

Figure 1.2 An air driven or electric motor can be used to drive the slower‐speed handpieces. These can be either straight or contra‐angled, and both are designed to be easily held by the operator.

The handpiece connects over the motor shaft and is held in place with a locking device. Both straight and contra‐angled handpieces are available. A handpiece with one of more bends in the body allows the bur to lie at an angle to the shaft and is known as a contra‐angled handpiece. This allows the bur access to less accessible areas of the mouth. The burs are held within the head of the handpiece by mechanical means and are known as latch grip burs. The body of the handpiece contains a series of gears and is available in a number of versions: speed reducing (4 : 1), speed increasing (1 : 10), and direct (1 : 1).

The torque available with a motor driven handpiece is greater than that of an air turbine, but the speed is lower. In some countries, there is a preference for using a speed increasing handpiece on an electric micromotor because this can produce a speed of nearly 400 000 rpm but with much greater torque than is available with an air turbine. The disadvantage is that there is greater vibration experienced by the patient and a greater potential for rises in pulpal temperature.