Materials and Processes for Cutting, Grinding, Finishing, and Polishing

Abrasive—A sharp, hard, natural or synthetic substance used for grinding, finishing, or polishing a softer surface.

Air-particle abrasion—Process of removing material, contaminants, stain, or carious tissue by use of air pressure and abrasive particles appropriate for the substrate being treated.

Buffing—Process of producing a lustrous surface through the abrading action of fine abrasives bound to a nonabrasive carrier with or without a liquid or paste medium.

Bulk reduction—Process of removing excess material (natural tooth or synthetic structure) by cutting or grinding with rotary instruments to provide a desired anatomic form.

Contouring—Process of producing a desired anatomic form by cutting or grinding away excess material.

Cutting—Process of removing material from the substrate by use of a bladed bur or an abrasive embedded in a binding matrix on a bur or disc.

Finished and polished restoration—A prosthesis or direct restoration whose outer surface has been progressively refined to a desired state of surface finish.

Finishing—Process of removing surface defects or scratches created during the contouring process through the use of cutting or grinding instruments or both.

Glaze ceramic—A specially formulated ceramic powder that, when mixed with a liquid, applied to a ceramic surface, and heated to an appropriate temperature for a sufficient time, forms a smooth glassy layer on a dental ceramic surface (see natural glaze).

Grinding—Process of removing material from a substrate by abrasion with relatively coarse particles.

Natural glaze—A vitrified layer that forms on the surface of a dental ceramic containing a glass phase when the ceramic is heated to a glazing temperature for a specified time.

Overglaze—Thin surface coating of glass formed by fusing a thin layer of glass powder that becomes a viscous liquid at a lower temperature than that associated with the ceramic substrate.

Polish—Luster or gloss produced on a finished surface.

Polishing—Process of providing luster or gloss on a material surface.

A Brief History of Abrasives

Abrasive processes have been used since prehistoric times. Hunting and gathering instruments were shaped and sharpened by chipping and abrading one surface against another over 10,000 years ago to produce sharp edges on hard natural materials. Spear points, arrowheads, scraper tools, and hoes were made by chipping, grinding, and honing the surfaces and edges of relatively hard rocklike materials. Sandstone was used to produce smoother surfaces on the Egyptian pyramids.

Grinding wheels of a primitive type were created over 4000 years ago by taking a cylindrical stone with an abrasive surface and spinning it against metals and ceramics to adjust their shapes, reduce rough areas, and produce smoother surfaces. These processes were refined over subsequent millennia to produce, by the Middle Ages, metal daggers, swords, spears, and shields of relatively high quality. The Chinese introduced the first coated abrasives in the thirteenth century by embedding seashell fragments in natural gums that were spread on a parchment backing.

In the early 1900s, abrasive technology advanced further through the development and use of alumina grains, diamond particles, and silicon carbide grit. New products in the form of powders, slurries, particle-embedded discs and wheels, and burs of different types soon emerged for use in dentistry. The further refinement of dental handpieces, air or abrasive technology, and methods of bonding abrasives to various binders has led to major processing breakthroughs that have rapidly advanced the quality of treatment in the current era of restorative dentistry, particularly with adhesive and esthetic dentistry.

Applications of Abrasives in Dentistry

The intraoral surfaces of virtually every direct and indirect restoration must be contoured by grinding, finishing, and polishing procedures. The goal of these procedures is to produce the smoothest surface possible in a limited time. A single type of abrasive cannot be used effectively for all types of dental materials. Different abrasives are used for the three major classes of materials: ceramics, metals, and resin-based composites.

Why are abrasives different? The abrasive instruments used for metals must be able to remove metal particles quickly and efficiently without generating excessive heat or becoming clogged with debris. Although the flexible discs used for resin composites can be used for metals, they are incapable of removing large amounts of metal quickly. Instead, silicon carbide discs are required for cutting metal sections such as casting sprues, and bonded abrasive wheels or points are used for rapid adjustment of surface contours. Likewise, although diamond burs have been developed for grinding and finishing zirconia frameworks, specialized Zir-Cut (Axis/SybronEndo, Coppell, Texas) coarse blue wheels with embedded diamond particles may be more effective because they may not wear out as fast as diamond burs and they can remove relatively large amounts of zirconia from framework surfaces rather efficiently. Therefore, suitable finishing and polishing instruments should be utilized for the respective dental materials.

The form of polishing instruments also affects the rate of material removal and the surface finish. For example, regular and extra-thin Sof-Lex (3M ESPE St. Paul, MN) contouring and polishing discs, useful for finishing and polishing resin-based materials, are provided in the stiff and flexible disc forms to allow either light or heavy pressure to be applied, depending on the amount of composite that needs to be removed. The polymer backing allows the discs to be used either in the dry or wet condition. The regular Sof-Lex discs are available in four grades of abrasiveness, coarse (black), medium (dark blue), fine (medium blue), and superfine (light blue). The extra-thin discs and gapped strips are also provided in these four abrasive grades, but their colors are different. A word of caution is important here. The abrasive discs and strips of other manufacturers may not follow the same convention for Sof-Lex discs (i.e., darker colors corresponding to coarser abrasives and lighter colors to finer abrasives).

The Astropol finishing and polishing system (Ivoclar Vivadent, Amherst, NY) for composites and ceromers is also provided in different forms and levels of abrasiveness. Astropol is a comprehensive finishing and polishing set that consists of four differently shaped polishers in three grit sizes for interdental and occlusal applications, small flame, large flame, cup, and disc forms. The grit sizes are designated as (1) finish (Astropol F for the removal of excess material and prepolishing); (2) polish (Astropol P, for polishing of restorations: especially those made from microfilled composite materials); and (3) high-gloss polish (Astropol HP, that is best suited for hybrid composites). Similar types of abrasive tools are available from other manufacturers and suppliers of dental products.

In summary, dental abrasives are used for tooth cleaning (dental prophylaxis), occlusal adjustment of tooth enamel and restoration surfaces, contouring of material (acrylic, composite, metal, and ceramic) surfaces, finishing and debris removal (grinding and air-particle abrasion), and fine polishing to produce glossy surfaces. The abrasives can be provided in the form of powders, pastes, diamond burs, and abrasive stones, discs, wheels, points, and cups. The best choice for any dental application depends on the initial surface quality, material type, and specific purpose or need. The specific need can vary from cutting or rough grinding to final polishing to achieve a desired luster or gloss.

Benefits of Finishing and Polishing Restorative Materials

Critical Questions

What are the six main benefits of finishing and polishing the surfaces of restorative materials? What are the goals of finishing and polishing?

Finished and polished restorations provide four benefits of dental care: better gingival health, chewing efficiency, patient comfort, and esthetics. Patients can detect a surface roughness change of less than 1 µm (Jines et al., Research Brit Dent J, 196:42–45, 2004) by tongue proprioception. Surface changes greater than 1 µm can also lead to increased bacterial adhesion as well as surface staining. A well-contoured and polished restoration promotes gingival health by resisting the accumulation of food debris and pathogenic bacteria. This is accomplished through a reduction in total surface area and reduced roughness of the restoration surface. Smoother surfaces have less retention areas and are easier to maintain in a hygienic state when preventive oral home care is practiced because dental floss and the toothbrush bristles can gain more complete access to all surfaces and marginal areas.

Tarnish and corrosion activity of some metallic materials can be significantly reduced if the entire metal restoration is highly polished. Oral function is enhanced with a well-polished restoration because food glides more freely over occlusal and embrasure surfaces during mastication. More importantly, smooth restoration surfaces minimize wear rates on opposing and adjacent teeth. This is particularly true for restorative materials such as ceramics, which contain phases that are harder than tooth enamel and dentin.

Rough material surfaces lead to the development of high, two-body contact stresses that can cause the loss of functional and stabilizing contacts between teeth or a reduction in the vertical dimension of occlusion. Rough surfaces on ceramics also act as stress concentration points. Finishing and polishing these surfaces can improve the strength of the restoration, especially in areas that are under tension, such as the perimeter of ceramic-ceramic crowns where unsupported areas of veneering ceramic are present. Finally, esthetic demands may require the dentist to finish and polish highly visible surfaces of restorations differently from those that are not accessible. Although a mirrorlike polish is preferred for previously mentioned reasons, this type of surface may not be esthetically compatible with adjacent teeth in highly visible areas, such as the facial surfaces of maxillary anterior teeth. Fortunately, these surfaces are not subject to high contact stresses and they are easily accessible for cleaning. Subtle anatomic features and textures may be added to these areas without affecting oral health or function.

In summary, the goals of finishing and polishing procedures are to obtain the desired anatomy, proper occlusion, and reduction of roughness and the depth of gouges and scratches produced by the contouring and finishing instruments. The instruments available for finishing and polishing restorations include fluted carbide burs, diamond burs, stones, coated abrasive discs and strips, polishing pastes, and soft and hard polymeric cups, points, and wheels impregnated with specific types and sizes of abrasive particles. The polished surface should be smooth enough to be well tolerated by oral soft tissues and to resist bacterial adhesion and excessive plaque accumulation. When plaque deposits exist on restorative material surfaces, they should be easily removable by brushing and flossing.

Critical Question

Principles of Cutting, Grinding, Finishing, and Polishing

Particles of a substrate material (workpiece) are removed by the action of a harder material that makes frictional contact with the substrate. This contact must generate sufficient tensile and shear stresses to break atomic bonds and release particles from the substrate. With rotary instrumentation, the blades of a carbide bur or the tips of abrasive particles transfer the force to the substrate. These tensile and shear stresses are induced in both the substrate and the rotary instrument. The instrument will fail to cut, grind, or polish if the stress that develops in any part of the cutting or grinding surface exceeds the strength of the instrument blade edges or particle bond strength to the binder compared with the strength of the substrate (workpiece). As a result, blade edges will become dull, and abrasive particles will fracture or tear away from their binder. Such degradation of finishing instruments is discussed in more detail in a later section.

Subtle differences distinguish the cutting, grinding, and polishing processes. A cutting operation usually involves the use of a bladed instrument or any other instrument in a bladelike fashion. Substrates may be divided into large separate segments, or they may sustain deep notches and grooves by the cutting operation. High-speed tungsten carbide burs have numerous regularly arranged blades that remove small pieces or segments of the substrate as the bur rotates. As shown in Figure 11-1, A, the unidirectional cutting pattern reflects the action of the regularly arranged blades on a tungsten carbide bur. The pattern produced by a diamond bur is shown in Figure 11-1, B. The surface of a coarse diamond bur is shown in Figure 11-1, C. When 30-fluted finishing burs have been used on a surface, the regular pattern of the cutting blades is discernible only if the surface is magnified for inspection. On the other hand, a separating wheel is an example of an instrument that can be used in a bladelike fashion. A separating wheel does not contain individual blades, but its thin blade design allows it to be used in a rotating fashion to grind through cast metal sprues and die stone materials.

FIGURE 11-1 A, Tooth cut by a carbide bur. Note the regular pattern of removal of tooth structure, which corresponds to the regular arrangement of blades on the bur. B, Tooth ground by a diamond bur. Note the multiple scratches formed by the random arrangement of abrasive particles on the diamond bur. C, Magnified image of the bonded diamond particles on a coarse diamond bur (×150).

A grinding operation removes small particles of a substrate through the action of bonded or coated abrasive instruments. Grinding instruments contain many randomly arranged abrasive particles. Each particle may contain several sharp points that run along the substrate surface and remove particles of material. For example, a diamond-coated rotary instrument may contain many sharp diamond particles (see Figure 11-1, C) that pass over a tooth during each revolution of the instrument. Because these particles are randomly arranged, many unidirectional scratches are produced within the material surface, as illustrated in Figure 11-1, B, which shows a tooth surface ground by a diamond bur. Cutting and grinding are both considered predominantly unidirectional in their action. This means that a cut or ground surface exhibits cuts and scratches oriented in one predominant direction.

Different types of burs have unique effects on surfaces. In general, a carbide bur with more blades will produce a smoother surface than a carbide bur containing fewer blades. For example, a 16-fluted carbide bur produces a smoother finish than an 8-fluted carbide bur, but the latter removes material more rapidly. Similarly, the coarsest diamond bur removes material more quickly but leaves a rougher surface. (See Figure 11-2 for scanning electron microscopy [SEM] images of carbide and diamond burs.)

FIGURE 11-2 Scanning electron microscopy (SEM) images. *Upper left*, 16-fluted (*left*) and 8-fluted (*right*) tungsten carbide finishing burs. *Upper right*, Fine diamond bur. *Bottom left*, Medium-grit diamond bur. *Bottom right*, Coarse-grit diamond bur.

Critical Question

How does a clinician know when the smoothest surface has been achieved?

Polishing

The purpose of polishing is to provide an enamel-like luster to the restoration. Smaller particles provide smoother and shinier surfaces. The speed of achieving a luster, however, depends on the hardness and size of the abrasive particles and the method of abrasion (e.g., two-body abrasion or three-body abrasion). Ideally, abrasive particles in the range of particle sizes up to 20 µm provide luster at a low magnification. At the end of this process, there should be no visible scratches. However, there will always be scratches that are detectable at higher magnification. The surface must be cleaned between steps because abrasive particles left on the surface from the previous step can cause deep scratches.

The quality of the surface finish and polish can be characterized by the measurement of the surface roughness using a profilometer, an optical microscope, or an SEM. In clinical practice, the surface luster is usually judged without magnification. Most of the time, surface smoothness is correlated with the luster, as in cases such as resin-based composite restorations. However, the smoothest surface does not necessarily provide the most lustrous surface. For industrial applications, reflectometers are used to measure the luster. However, it is difficult to use them successfully for dental applications because of the irregular contour and small size of dental restorations.

Polishing procedures, the most refined of the finishing processes, remove the finest surface particles. Each type of polishing abrasive acts on an extremely thin region of the substrate surface. Polishing progresses from the finest abrasive that can remove scratches from the previous grinding procedure and is completed when the desired level of surface smoothness is achieved. Each step is followed by the use of progressively finer polishing media until no further improvement in surface finish is observed. The final stage produces scratches so fine that they are not visible unless greatly magnified. Polishing should be terminated when no further change in surface luster or glossiness occurs during the application of the finest abrasive that is used for that application. Further attempts to improve the surface appearance may actually degrade the surface by generating heat and by smearing dislodged material across the surface.

Examples of polishing instruments are rubber abrasive points, fine-particle discs and strips, and fine-particle polishing pastes. Polishing pastes are applied with soft felt points, muslin (woven cotton fabric) wheels, prophylaxis rubber cups, or buffing wheels. A nonabrasive material should be used as an applicator while using polishing pastes. Felt, leather, rubber, and synthetic foam are popular applicator materials for buffing. A common feature of some of these materials is their porous texture, which allows fine abrasive particles to be retained during the buffing procedure.

Polishing is considered multidirectional, which means that the final surface scratches are oriented in many directions. Some examples of ground and polished surfaces are shown in Figure 11-3. Note that the differences in surface appearance are subtle because of the transitional nature of the grinding and polishing processes. If there were larger differences in the size of particles removed, the surface change would be more easily detected.

Heat generation during the cutting, contouring, finishing, and polishing processes of direct restorations is a major concern. To avoid adverse effects to the pulp, the clinician must cool the surface with a lubricant, such as an air–water spray, and avoid continuous contact of high-speed rotary instruments with the substrate. Intermittent contact during operation is necessary not only to cool the surface but also to remove debris formed between the substrate and the instrument. The effectiveness and the speed of the contouring, finishing, and polishing procedures will be greatly improved by the removal of debris.

Critical Questions

What precautions should be taken to minimize the generation of aerosols? What precautions should be taken to minimize exposure to and inhalation of aerosols?

Biological Hazards of Grinding, Finishing, and Polishing Processes

Dispersions of solid particles are generated and released into the breathing space of laboratories and dental clinics whenever finishing operations are performed. These airborne particles may contain tooth structure, dental materials, and microorganisms. Such aerosols have been identified as potential sources of infectious and chronic diseases of the eyes and lungs and present a hazard to dental personnel and their patients. Silicosis, also called grinder’s disease, is a major illness caused by inhalation of aerosol particles released from any of a number of silica-based materials that are used in the processing and finishing of dental restorations. Silicosis is a fibrotic pulmonary disease that severely debilitates the lungs and doubles the risk for lung cancer. The risk for silicosis is substantial, because 95% of generated aerosol particles are smaller than 5 µm in diameter and can readily reach the pulmonary alveoli during normal respiration. Additionally, 75% of airborne particles are potentially contaminated with infectious microorganisms. Furthermore, aerosols can remain airborne for more than 24 hours before settling and are, therefore, capable of cross-contaminating other areas of the treatment facility. Aerosol sources, in both the dental operatory and laboratory environments, must be controlled whenever finishing procedures are performed. A concise and informative source of information on aerosol hazards has been written by Cooley (see Selected Reading).

Aerosols produced during finishing procedures may be controlled in three ways: First, they may be controlled at the source through the use of adequate infection control procedures, water spray, and high-volume suction. Second, personal protective equipment (PPE) such as safety glasses and disposable face masks can protect the eyes and respiratory tract from aerosols. Masks should be chosen to provide the best filtration along with ease of breathing for the wearer. Third, the entire facility should have an adequate ventilation system that efficiently removes any residual particulates from the air. Many systems are also capable of controlling chemical contaminants such as mercury vapor from amalgam scrap and monomer vapor from acrylic resin.

Critical Question

How can the amount of lubricant either increase or decrease cutting efficiency?

Abrasion and Erosion

Abrasion

Wear is a material removal process that can occur whenever surfaces slide against each other. The process of finishing a restoration involves abrasive wear through the use of hard particles. In dentistry, the outermost particles or surface material of an abrading instrument is referred to as the abrasive. The material being finished is called the substrate. In the case of a diamond bur abrading a tooth surface, as illustrated in Figure 11-4, the diamond particles bonded to the bur represent the abrasive, and the tooth is the substrate. Also, note that the bur in the high-speed handpiece rotates in a clockwise direction as observed from the head of the handpiece.

FIGURE 11-4 The mechanics of high-speed rotary instrumentation. A, The blue circular arrow indicates that the high-speed diamond bur rotates in a clockwise direction when viewed from the head of the handpiece. The green arrows indicate the direction that the instrument should be drawn to counteract the rotational force of the bur and achieve the most rapid abrasive action of the bur. B, Incisal view of the forces generated during high-speed rotary tooth preparation. As the bur rotates in a clockwise direction, it generates a rotational force at the tooth surface, F_B (*large blue arrow* that represents schematically the force of tooth structure against the bur). The operator of the instrument must generate an opposing force, F_o (*green arrow*), which will exceed the rotational force of the bur, F_B, and carry the instrument against the tooth surface where the surface will be abraded.

The rotational direction of a rotary abrasive instrument is an important factor in controlling the instrument’s action on the substrate’s surface. When a handpiece and bur are translated in a direction opposite to the rotational direction of the bur at the surface being abraded, a smoother grinding action is achieved. However, when the handpiece and bur are translated in the same direction as the rotational direction of the bur at the surface, the bur tends to “run away” from the substrate, thereby producing a more uncontrolled grinding action and a rougher surface.

Abrasion is further divided into the processes of two-body and three-body wear. Two-body abrasion occurs when abrasive particles are firmly bonded to the surface of the abrasive instrument and no other abrasive particles are used. A diamond bur abrading a tooth represents an example of two-body wear. Three-body abrasion occurs when abrasive particles are free to translate and rotate between two surfaces. An example of three-body abrasion involves the use of nonbonded abrasives such as those in dental prophylaxis pastes. These nonbonded abrasives are placed in a rubber cup, which is rotated against a tooth or material surface. These two processes are not mutually exclusive. Diamond particles may debond from a diamond bur and cause three-body wear. Likewise, some abrasive particles in the abrasive paste can become trapped in the surface of a rubber cup and cause two-body wear. Lubricants are often used to minimize the risk of these unintentional shifts from two-body to three-body wear, and vice versa. Thus, the efficiency of cutting and grinding will be improved with the use of lubricants such as water, glycerin, or silicone. Intraorally, a water-soluble lubricant is preferred. Excessive amounts of lubricant may reduce the cutting efficiency by reducing the contact between the substrate and the abrasive. Too little lubricant results in increased heat generation and reduced cutting efficiency.

Erosive wear is caused by hard particles impacting a substrate surface, carried by either a stream of liquid or a stream of air, as occurs in sandblasting a surface. Figure 11-5 illustrates schematically the processes of two-body abrasion, three-body abrasion, and hard-particle erosion. Most dental laboratories have air-driven grit-blasting units that employ hard-particle erosion to remove surface material. A distinction must be made between this type of erosion and chemical erosion, which involves chemicals such as acids and alkalis instead of hard particles to remove substrate material. Chemical erosion, more commonly called acid etching in dentistry, is not used as a method of finishing dental materials. It is used primarily to prepare tooth surfaces to enhance bonding or coating.