Hand Instrument Parts
Even though there is great variation among hand cutting instruments, they have certain design features in common. Each hand instrument is composed of three parts: the handle, the shank and the blade. The handle can be small, medium, or large, smooth or serrated. On the handle are two numbers. One is the instrument formula, and in this case it is 15-8-12. The instrument formula describes the dimensions and angulation of the instrument. The other number is the manufacturer’s number which is used for ordering purposes.
The next section is the shank. It connects the handle to the blade. The shank may be straight or angled. If it is angled, the shank may be mono, bin, or triple angled. The angulation provides for access and stability.
The blade is the last section. It is the working part of the instrument. The blade of the instrument is beveled to create the cutting edge. In some instruments there are three bevels. Two are on the side and one is at the end. The edge on the end is called the primary cutting edge and the edges on the sides are called the secondary cutting edges.
The hand instrument can be either double ended if there are working ends at both sides or long handled if there is just one working end.
The Instrument Formula
The basic instrument formula consists of three units. The handle here has the instrument formula of 15-8-14. The number 15 indicates the width of the blade in tenths of a mm. The fifteen, therefore, represents 1.5mm. The second number, 8, represents the length of the blade in millimeters, that is, from the shank to the cutting edge. It is 8mm in this case. Fourteen represents the angle the blade makes with the long axis of the handle, or the plane of the instrument. This angle is expressed in "hundredths" of a circle or centigrade. To calculate the measurement of the angle, you place the instrument on the center of the circle and move it until the blade lines up with one line on the ruler. This measurement represents the angulation of the blade from the long axis of the handle, which is fourteen degrees in this case.
When the cutting edge of an instrument is at an angle other than a right angle to the length of a blade, a fourth unit is added to the basic three unit formula. This number is placed in the second position of the formula. The first number, 15, tells the width of the blade in tenths of a millimeter. The second number, 95, represents the angle that the cutting edge of the instrument makes with the long axis of the handle. The third number, 8, is the length of the blade; and the fourth number, 12, is the angle the blade makes with the plane of the instrument or the long axis of the handle. To measure the fourth number, the instrument is placed in the center of the measuring circle and moved to the left until the cutting edge lines up with one of the lines on the ruler; in this case, it is 95 degrees.
There are two families of instruments: the chisel family and the hatchet family. In the chisel family, we have the chisel, hoe, angle former, and cleoid-discoid.
In the hatchet family, we have the hatchet, gingival margin trimmer, and spoon excavator.
At one time all instruments were called excavators and were used for removal of carious lesions as well as shaping cavity walls and refining line angles and point angles. At the present time, excavators are used only for removal of carious lesions.
The Chisel Family
Chisels are used for cleaving, planing, and lateral scraping. The cutting edge of the chisel makes a 90 degree angle to the plane of the instrument. Chisels are used with a push motion.
The hoe is another member of the chisel family. By definition, the hoe is any instrument where the blade makes more than a 12.5 degree angle with the plane of the instrument. While the chisel is normally used with a push motion, the hoe modification is frequently used with...