For additional ancillary materials related to this chapter, please visit thePoint.
* Note: The information in this chapter is provided courtesy of Hu-Friedy Manufacturing Co., Inc.
After studying this chapter, the student will be able to do the following:
1. Explain the basic differences between carbon steel and stainless steel instruments.
2. Discuss the processes of passivation and electropolishing.
3. Summarize the problems or conditions that can affect instruments, including corrosion, rust, pitting, spotting, and stains.
4. Explain why it is important to inspect instruments.
5. Explain the reasons for sharpening instruments, and determine the appropriate time and frequency of sharpening.
6. Discuss the cleaning of instruments, both immediately after use and when timely cleaning is not possible.
7. Recall the advantages and disadvantages of the four methods of sterilizing instruments.
8. Design an instrument maintenance schedule or cycle that could be used routinely in a private practice office setting.
carbon steel instruments
stainless steel alloy
The previous chapters have discussed dental materials such as gypsum, cements, and impression and restorative materials, all of which are typically used in a dental practice. Other dental materials that are important to the dental hygienist but often overlooked are those that constitute dental instruments. It is a rare workday for a dental hygienist when a dental instrument goes unused. A hygienist’s instruments are some of the most important items necessary to fulfill his or her professional responsibility.
Dental instruments represent a significant financial investment. For this reason alone, it would benefit the student to have a clear understanding of the materials from which they are made and what is necessary to keep them in good working condition. The amount of care that is given to dental instruments directly affects the lifetime of those instruments. The longer an instrument lasts, the greater return it yields on the financial investment.
This chapter addresses the composition, undesirable conditions, inspection, and care of instruments.
I. Composition of Instruments
Dental instruments are usually made of either carbon steel or stainless steel alloys. Some instruments have resin handles.
A. Carbon Steel Alloy
Carbon steel instruments are known for their hardness and ability to hold sharp, cutting edges. They are more sensitive than stainless steel instruments to chemicals, are susceptible to corrosion, and require special handling.
B. Stainless Steel Alloy
Major components of stainless steel alloy include iron, chromium, and nickel. The amount of carbon in a stainless steel alloy is directly related to the alloy’s hardness and ability to hold a sharp, cutting edge. The addition of chromium enhances corrosion resistance, and nickel improves the mechanical properties of the metal. The problems with stainless steel instruments are discoloration, constant necessity of sharpening, and corrosion when exposed to certain chemicals. Some instruments are made with two kinds of stainless steel. One is hard and maintains a sharp edge; this is used to make the cutting edge or tip. The second is more resistant to corrosion and is welded or soldered to the first to form the handle.
The carbon in the alloy, which is necessary for hardness and a sharp edge, is the culprit that causes corrosion or rusting. Instrument manufacturers reduce surface corrosion by using two processes:
Passivation is a chemical process that creates a thin layer of chromium oxide on the surface of the instrument. This layer is transparent but tough, and it protects the underlying metal. After passivation, the instrument is much less likely to corrode.
Electropolishing produces a smooth, highly polished finish. A highly polished surface is less likely to corrode. Electropolishing is an efficient method for polishing complex shapes.
C. EverEdge 2.0 Technology
EverEdge 2.0 is the newest and most advanced scaler developed by Hu-Friedy Mfg. Co., Inc. EverEdge Technology was introduced about 10 years ago. This technology in metallurgy, heat treatment, and cryogenics is used in the manufacturing of instruments enabling their blades to stay sharper significantly longer when compared to other instruments. Recently, EverEdge 2.0 instruments have been developed. These instruments are designed to stay sharper even longer than the original EverEdge instruments. These instruments are not coated, but the long-lasting wear is present throughout the entire instrument tip. If the instrument stays sharper longer, there is less sharpening required and less hand fatigue. An EverEdge 2.0 instrument is illustrated in Figure 19.1.
FIGURE 19.1. An EverEdge 2.0 scaling instrument. (Courtesy of Hu-Friedy Mfg. Co., Inc.)
D. Resin Handles
Some instruments may have resin handles. Inside the resin is a full-length, steel inner core for added strength and tactile sensitivity. The handles may have grooves and knurling to increase rotational control and to provide a light (but secure) grasp. Resin instruments and items require specific care.
E. Separation of Stainless Steel and Carbon Steel Instruments
Stainless steel and carbon steel instruments should be kept separate throughout the cleaning and sterilization process. If processed together, the carbon steel instruments may create cross-corrosion on the stainless steel instruments.
Carbon steel instruments should be thoroughly dried before sterilization to prevent rusting or corrosion. Use of a protective rust inhibitor solution is recommended before sterilization.
II. Problems of Instruments
A. Forms of Corrosion
Corrosion is a process in which a metal is changed to a metal oxide. A common example is when iron is changed to iron oxide, or rust. Corrosion occurs because the metal oxide is the lower-energy form of the metal. Corrosion is increased in a warm, wet environment, such as in the mouth or an autoclave. An example of a corroded curette tip is shown in Figure 19.2B.
FIGURE 19.2. Photographs of A. a well-maintained instrument, B. a corroded curette tip, and C. a pitted instrument. (C. Courtesy of Hu-Friedy Mfg. Co., Inc.)
Tarnish is a chemical or electrochemical attack on a metal surface. Many times, corrosion starts as a surface discoloration called tarnish. If tarnish is a continuous film, it will protect the metal from the environment (like paint on metal) and prevent corrosion. Many films on metals are transparent and unseen, but they may still protect the surface from corrosion. If the film is not continuous and the surface not protected, corrosion may proceed, and loss of material will occur.
3. Galvanic Corrosion
Corrosion in a wet environment, such as saliva, is an electrochemical process called galvanic corrosion. Galvanic corrosion is the same process that produces electricity in a battery. In a battery, two dissimilar metals cause a current to flow. Corrosion may be caused by two dissimilar metals in contact, such as carbon steel and stainless steel, or by the same metal existing in two different environments. The two environments could differ in humidity, pH, oxygen concentration, or other chemical concentrations. As corrosion proceeds, the material is lost. The instrument or object becomes discolored and weakened.
Corrosion does not always occur uniformly over a metal surface. Many times, corrosion aggressively attacks small areas. As a result, surface staining and pitting occur. The base of a pit in a metal restoration or an instrument may have a different pH and oxygen concentration compared to the rest of the surface. The different environments at the base of the pit and the surface will encourage corrosion. Corrosion will continue in the pit, and the pit may become deeper and deeper. Therefore, removing pits and other surface defects by polishing reduces corrosion.
When two dissimilar metals are present in the mouth, galvanic corrosion may occur. Sometimes, this is called oral galvanism. The closer the two metals are physically, the greater the likelihood of galvanism. Several authors have stated that one should not place an amalgam restoration in contact with a gold crown, and vice versa. Galvanism is the alleged reason. However, such adjacent restorations frequently occur in patients with little or no ill effect on either restoration. Galvanism has been blamed for numerous health problems, but without a scientific basis.
5. Preventing Corrosion
In dentistry, we protect metallic restorations and instruments from corrosion by using two techniques.
a. The first technique is to make restorations with noble metals. Noble metals do not corrode; but they are expensive—too expensive to use for instruments.
b. The second technique is to use metals that form a tough, adherent oxide layer on the surface. This is called passivation, and it protects the metal surface from the environment. Stainless steel works this way. Unfortunately, the chromium oxide layer that protects stainless steel can break down in the presence of chloride ions. It is important to rinse off chloride-containing cleaning agents before sterilizing instruments. Residual cleaning chemicals can attack the protective film; instruments may then corrode, rust, stain, and pit.
Pitting is caused by a chemical and electronic attack on surfaces. Pitting is localized corrosion, and it is prevented by:
- Rinsing instruments thoroughly after use
- Avoiding long exposure to chlorides and acids
- Avoiding detergents with high pH
- Not mixing metals in ultrasonic cleaners
An example of pitting is shown in Figure 19.2C.
Slow or improper drying leaves mineral deposits that cause spotting. To prevent this, check the operation of the autoclave, and use chloride-free solutions for sterilizing, disinfecting, rinsing, and cleaning.
8″ TOPIC-CONTENT-TYPE=”summary. Summary
Table 19.1 summarizes several of the instrument problems discussed above.