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After studying this chapter, the student will be able to do the following:
1. Briefly define the following terms:
- Cleaning Agent
2. Recall six common abrasives that may be used for clinical or laboratory procedures.
3. Explain the difference between two-body and three-body abrasion. Provide an example of a polishing procedure that exemplifies each type of abrasion.
4. Summarize factors that may influence the rate of abrasion, and explain why the dental hygienist must have a clear understanding of these factors when providing patient care.
5. Discuss the reasons why tooth structure and restorations are polished.
6. Recall the details of the polishing process. Include the series of steps, scratches produced, and wavelength of visible light.
7. Describe the difference between selective polishing and essential selective polishing.
8. Describe the characteristics of an acceptable prophylaxis paste.
9. Describe the difference between a cleaning agent and a polishing agent.
10. Identify the types of restorations that cannot be polished with an air powder polisher.
11. Identify the restorative materials, dental tissues, and periodontal tissues that are compatible with the following air polishing powders: sodium bicarbonate, aluminum trihydroxide, calcium sodium phosphosilicate, calcium carbonate, and glycine.
essential selective polishing
traditional air powder polishing
tungsten carbide burs
One of the major responsibilities of the dental hygienist is the cleaning and polishing of teeth and restorations. This responsibility also includes any removable appliances, such as complete and partial dentures.
Before discussing polishing materials and abrasion, it is important to distinguish between the terms “cleaning” and “polishing.” Polishing, by definition, involves the abrasion of a surface by an abrasive agent that is harder than the surface to be abraded, or polished. When a dental hygienist is truly polishing, the paste or slurry containing an abrasive agent will microscopically alter the tooth or restorative surface. Section V of this chapter, “The Polishing Process,” addresses this topic in greater detail. Cleaning, sometimes referred to as plaque removal, is done with agents that do not contain abrasive particles. A cleaning agent is not abrasive and will not alter the surface characterization of enamel or esthetic restorative materials. The surface being cleaned is not altered or abraded as it would be in polishing.
The topic of polishing materials and abrasion will be easier to discuss if we first define the basic terms that are involved in these procedures.
Cutting refers to removing material by a shearing-off process. Examples of cutting would be milling, machining, or drilling. The process results in a somewhat smooth surface. In dentistry, cutting is done with metal burs and hand instruments to create cavity and crown preparations, which receive permanent restorations. An assortment of hand-cutting dental instruments is shown in Figure 16.1. When dental burs are used, the cutting process is affected by:
FIGURE 16.1. Examples of hand-cutting dental instruments (from left to right): Wedelstaedt chisel, spoon excavator, gingival margin trimmer, hoe, and hatchet.
1. Design of the Bur
Dental burs are available in many shapes that aid the dentist in creating the correct design of the cavity or crown preparation. Examples of typical burs are illustrated in Figure 16.2.
FIGURE 16.2. Names and shapes of dental burs. A. Egg or football diamond. B. Needle diamond. C. Flame diamond. D. Round. E. Pear. F. Inverted cone. G. Straight-fissure plain. H. Tapered-fissure plain. I.Tapered-fissure crosscut. J.Needle finishing. K.Egg or football finishing. (Courtesy of Brasseler USA.)
2. Sharpness of the Bur
The “lifetime” (or longevity) of a dental bur depends on the material from which it is made. Usually, cutting burs are made of carbon steel or tungsten carbide. Tungsten carbide burs are made by packing powdered metal constituents into a mold and then sintering (see Chapter 10, Materials for Fixed Indirect Restorations and Prostheses) at high temperatures. These burs are harder and maintain a sharper cutting edge than do carbon steel burs. Therefore, they last longer; however, they are more expensive.
Abrasion is the wearing away of a surface. It may also be referred to as grinding. Irregular grooves or scratches are produced on a surface as the result of abrasion.
The process of producing the final shape and contour of a restoration is termed finishing. After an amalgam restoration is placed, it may need to be finished and polished at a later appointment. Most other restorations are finished and polished when they are first placed. The instruments and armamentarium for finishing differ from those that are used for polishing. Examples used in finishing would be burs and stones.
Polishing is the process of abrading a surface to eventually reduce the size of the scratches until the surface appears shiny. This concept not only applies to dentistry in regard to tooth structure and restorative materials but also extends into everyday life as well. We know that jewelry is polished, and we “polish” our sinks and bathtubs with certain kinds of cleansers that are recommended for those surfaces. This kind of polishing is different from polishing shoes or furniture. The shoe and furniture polish acts as a surface coat, similar to that of car wax.
The material doing the “wearing” (abrading) is the abrasive. In nature, wind and water carry abrasive particles and can wear away the surface of rocks.
In dentistry, abrasive particles may be bound together onto burs, disks, stones, wheels, or strips or they may be used with liquids to form a paste or slurry. These are discussed later in this chapter.
Tribology is the science of interacting surfaces in motion; it incorporates the study and application of the principles of friction, lubrication, and wear. In dental polishing procedures, an abrasive agent creates friction and wear when it comes in contact with the surface being polished. The abrasive agent can be found embedded either in a surface such as a polishing wheel or in a moist paste; the moist ingredients in the paste serve as the lubricant. Within the science of tribology, polishing can be considered as two-body abrasion or three-body abrasion.
1. With two-body abrasion polishing, the abrasive agent particles are solidly fixed to a substrate, such as a dental bur, disk, wheel, strips, or in rubber cups impregnated with abrasive agents that do not require polishing paste.
2. Three-body abrasion occurs when abrasive particles move in a space between the surface being polished and the application device. The best example of three-body abrasion is polishing with a rubber cup and prophylaxis paste. The abrasive particles are mixed in the prophylaxis paste. The abrasive particles move in the space between the tooth surface being polished and the surface of the rubber cup. Dental hygienists primarily use three-body abrasion.
II. Types of Abrasives
Many types of abrasives and polishing agents are used in dentistry, and to mention all of them would go beyond the scope of this text. Listed below are some of the more common ones that may be used when performing typical clinical or laboratory procedures:
A mineral form of calcite is called chalk. It is also called whiting or calcium carbonate. Chalk is a mild abrasive and is used to polish teeth, gold and amalgam restorations, and plastic materials.
Pumice is a silica-like, volcanic glass that is used as a polishing agent on enamel, gold foil, and dental amalgam and for finishing acrylic denture bases in the laboratory. It is the abrasive agent in “Lava” hand cleaner and is used to remove dried or callused skin in the form of a “pumice stone.” Pumice, also found in clay, is the most common abrasive used in commercially prepared prophylaxis polishing pastes. Both chalk and pumice are illustrated in Figure 16.3.
FIGURE 16.3. Chalk (whiting) and pumice in bulk form.
Sand is a form of quartz and may be seen in various colors. Sand particles are rounded or angular in shape. They are typically bonded to paper disks for grinding metals and plastics.
As we know it today, cuttle is a fine grade of quartz. These particles are also bonded to paper disks and are beige in color. They are available in coarse, medium, and fine grits. In the past, it was manufactured from the inside shell of a Mediterranean marine mollusk. A “cuttle bone” mounted in a parakeet’s cage is made of the same material.
Garnet is usually dark red in color. Because it is very hard, garnet is a highly effective abrasive. It is found on coated disks and is used for grinding plastics and metal alloys. The term garnet refers to several different minerals that have similar properties. These minerals are the silicates of manganese, magnesium, iron, cobalt, and aluminum.
Emery is sometimes also called “corundum.” We are familiar with this abrasive because of “emery boards,” which are used to file our fingernails. Emery is a natural form of aluminum oxide, and it looks like grayish-black sand. It is commonly found on arbor bands that attach to a dental lathe for grinding custom trays and acrylic appliances. Sand, cuttle, garnet, and emery disks can be seen in Figure 16.4.
FIGURE 16.4. Kit of assorted disks coated with sand, cuttle, garnet, and emery.
Silex, a commercial product, is a silica-like material such as quartz and is used as an abrasive agent in the mouth. It is supplied as a powder and is mixed with various liquids to form a paste or slurry.
H. Tin Oxide
An extremely fine abrasive, tin oxide is supplied as a white powder and is used as a final polishing agent for teeth and metallic restorations. It is used as a paste or slurry in the same manner as Silex. Both Silex and tin oxide are illustrated in Figure 16.5.
FIGURE 16.5. Silex and tin oxide in bulk form.
I. Aluminum Oxide
Aluminum oxide is a common abrasive used in dentistry, and it has essentially replaced emery for several uses. This abrasive, shown in Figure 16.6, is widely used in the form of disks and strips. It is also impregnated into rubber wheels and points. It is the abrasive used in the popular “white stones” to adjust enamel or to finish metal alloys and ceramic materials.
FIGURE 16.6. Aluminum oxide–coated disks and strips. (Courtesy of 3M/ESPE Dental Products.)
III. Bonded and Coated Abrasives
To use the abrasives previously discussed, they must be attached to devices that permit an abrasive action. This action is usually rotary-powered, but in the case of finishing strips, it is accomplished by hand. Examples of these items include the following.
A. Diamond Burs
Diamond burs are actually very small diamond chips that are bonded to a shaft. Diamonds are very hard materials and make very good abrasives. Depending on the size of the chips, diamond burs can be used in many dental procedures.
Stones are available in various shapes, sizes, and grits, and they are made from a variety of materials. A “heatless stone” is illustrated on the right in Figure 16.7. Stones are used in clinical and laboratory procedures.
FIGURE 16.7. An assortment of bonded abrasive instruments (stones, rubber wheels, and rubber points) used in dentistry.
C. Rubber Wheels or Points
Molded rubber is impregnated with an abrasive into a wheel or point shape. The rubber acts as the matrix (or binder) of the abrasive agent. Examples are shown in Figure 16.7. Rubber wheels and points are designed for both clinical and laboratory procedures.
D. Rubber Cups
Abrasive agents are embedded in rubber cups intended for polishing. These rubber cups are available on disposable prophylaxis angles for use in polishing procedures during an oral prophylaxis, as shown in Figure 16.8. Rubber cups with embedded abrasives are not intended to be used with prophylaxis polishing pastes.
FIGURE 16.8. Disposable prophylaxis angle with abrasive particles embedded in the rubber cup. (Courtesy of Sunstar Americas, Inc.)
E. Disks and Strips
Abrasive particles are bonded to a paper, metal, or plastic backing to form disks or strips. Examples of coated disks and strips can be seen in Figure 16.9. They are used for intraoral and laboratory procedures.
FIGURE 16.9. An assortment of coated disks and strips used in dentistry.
F. Polishing Powders
Polishing powders, such as those illustrated in Figures 16.3 and 16.5, are used in conjunction with other agents and devices. These agents and devices include the following:
1. “Vehicles,” such as water, alcohol, glycerin, fluoride, or mouthwash, are used to make pastes or slurries for polishing.
2. Brushes, rubber cups, felt cones and wheels, and cloth wheels are used to move an abrasive or polishing agent over the surface to be polished. Figure 16.10 shows an assortment of cloth wheels, felt cones, bristle brushes, and rubber cups.
FIGURE 16.10. An assortment of cloth wheels, felt cones and wheels, brushes, and rubber cups used in dentistry.
Some powders are used for laboratory and clinical procedures, whereas others are used only in the laboratory.
IV. Factors Affecting the Rate of Abrasion
Chances are, even though a class of dental hygiene students is taught to polish the same way, each student probably polishes differently. Depending on how much paste is put into the cup, the size and type of cup, the amount of pressure used against the tooth, how fast the cup is rotating, and what type of abrasive is in the cup, the surface being polished (technically abraded) will be significantly affected. The following factors affect the rate of abrasion.
The abrasive particle must be harder than the surface being abraded if an acceptable rate of abrasion is to occur. Otherwise, the abrasive will be worn, and the surface will not be greatly affected. The abrasion rate can be “temperature dependent” (the abrasive heats up during use). The object being abraded could become heated, which may make it softer and affect the rate of abrasion.
Abrasives are usually made of very hard, ceramic materials. Table 16.1 lists Knoop and Mohs hardness of several restorative materials, abrasives, and tooth tissues.
TABLE 16.1. Mohs and Knoop Hardness Values of Restorative Materials, Abrasives, and Tooth Tissues
Adapted from Weast R., ed. Handbook of Chemistry and Physics. 64th ed. Boca Raton, FL: CRC Press; 1983; Anusavice KJ. Phillips’ Science of Dental Materials. 11th ed. Philadelphia, PA: Saunders; 2003:362; Callister WD. Fundamentals of Materials Science and Engineering. 2nd ed. Hoboken, NJ: Wiley; 2005:217.
Common sense tells us that larger abrasive particles will produce deeper scratches than will smaller particles. Deeper scratches result in a greater amount of surface material removed. Grit is a term that is used to describe the size of the abrasive particle. Finer abrasives such as powders or flours are graded F, FF, and FFF as the fineness increases. When the particles are bonded to paper, the grit is designated as O, OO, and OOO in the order of increasing fineness. When a prophy paste is labeled “coarse” or “fine,” the label is referring to the grit (or particle size) of the abrasive. It is important to note that there is no standardization in the definition of fine, medium, and coarse grit in prophy pastes among the manufacturers. The “fine” grit paste of one manufacturer may be nearly equal to the “coarse” grit paste of another manufacturer.
Most individuals would agree that spherically shaped particles would be less abrasive than irregularly shaped particles. The sharp edges on irregularly shaped particles tend to dig into the surface rather than roll across it as rounded abrasive particles would, thus increasing the rate of abrasion. Cleaning agents have very soft or flat particles and do not abrade.
Using excess pressure during finishing and polishing causes a higher abrasion rate because the abrasive particle cuts deeper into the surface. Increased pressure may also result in an increased temperature of the material being polished. An example of this would be using heavy pressure on an amalgam restoration. Raising the temperature of the amalgam could release mercury to the surface, which may increase corrosion and contribute to a marginal breakdown.
The term “speed” refers to the rate at which the polishing device is rotating. Like pressure, the speed at which the abrasive is applied will increase the rate of abrasion. Higher speed also results in a temperature increase. It is important to control the speed of the polishing cup or brush during polishing so that the abrasion rate and increase in temperature are kept to a minimum.
The most frequently used lubricant in dentistry is water. It is used with handpieces and burs to cool the tooth when cavity preparations are being made. During finishing and polishing, lubrication is also recommended to diminish the heat that is created by the abrasive action. This is accomplished by mixing lubricating agents, such as water, mouthwash, fluoride solutions (usually neutral sodium fluoride), glycerin, or alcohol, with the abrasive agent, which is usually in powder form. We refer to the resulting mixture as a “paste” or “slurry” depending on the liquid content.
V. The Polishing Process
A. Why We Polish
Tooth structure and restorative materials are polished for several reasons.
1. To Reduce Adhesion
As discussed in Chapter 6, Amalgam, a smooth surface inhibits adhesion. Plaque, stain, and calculus are less likely to adhere to a smooth surface. Polishing removes the acquired pellicle. When the acquired pellicle reforms, it provides a medium for the adherence for dental plaque. This applies to tooth surfaces and restorative materials.
2. To Make the Surface Feel Smooth
Patients expect a smooth surface on any permanent restoration that is placed in their mouths. In addition, they may comment on how they look forward to and value the smoothness that is produced after scaling and polishing during a routine dental hygiene recall appointment.
3. To Increase Esthetics
An unpolished amalgam or gold crown is not as attractive as the one that appears smooth and shiny. This also holds true for the tooth surfaces of a heavy cigarette smoker before and after polishing. Esthetics play a very important role in dentistry, and polishing helps to create an attractive dentition for the patient. For the smoker, the subject of stain removal becomes an opportunity for the dental hygienist to discuss the subject of smoking cessation with the patient.
4. To Reduce Corrosion
When metallic restorations are polished, it reduces the formation of tarnish and corrosion. In turn, this may extend the lifetime of the restoration.
B. Preparation for Polishing
Three steps should be taken prior to polishing.
1. First, the health history should be completed. No dental treatment procedures should ever be initiated without completion of the patient’s health history to confirm there are no contraindications for polishing.
2. The second preparatory step prior to polishing procedures is the completion and/or review of the patient’s chart of existing oral conditions and restorations. Some esthetic restorations are so artfully created and the colors so perfectly matched that detection of the restoration with the naked eye is almost impossible.
3. Finally, current radiographs should be reviewed and matched with the patient’s intraoral chart to confirm the presence or absence of esthetic restorations or any restorations or conditions not previously charted.