Materials and Processes for Cutting, Grinding, Finishing, and Polishing
Key Terms
Abrasive A product that has a hard phase as an essential constituent that provides many individual particles with sharp cutting edges.
Air-particle abrasion The process of material removal by way of air-pressure–propelled abrasive particles. Typical applications are surface cleaning, cavity preparation, and surface preparation for bonding. The process is also known as erosion in the field of tribology.
Bonded abrasive An abrasive instrument containing a phase that holds abrasive particles in a more or less tight grip with spaces in between that allow air or liquids to pass. The strength of bonding in a product varies based on its intended use. Strong bonding is required for work on hard materials. Breaking away of grains from the bond of an abrasive is desirable as they become dull. Bond materials include organic (resin or rubber), vitrified (glass or glass-ceramic), and metal.
Buffing Polishing with a soft absorbent material such as cloth or leather, typically in combination with a medium containing very fine abrasive particles.
Bulk reduction and contouring An early process that precedes finishing and polishing, whereby excess restorative material is removed to develop the anatomical form of the final restoration.
Cutting Reduction of a material by use of an edged instrument, such as a bladed dental bur, or a bonded abrasive disk or wheel; implies material removal by a slicing action.
Dressing The use of a variety of tools to remove any debris that may be clogging the spaces between abrasive particles on a bonded abrasive instrument to expose fresh abrasive and restore grinding efficiency. Diamond-coated tools or bonded abrasive stones of aluminum oxide or silicon carbide are the most common devices used for dressing grinding wheels.
Erosion A process of material removal achieved by (1) air-pressure–propelled abrasives (air-particle abrasion), (2) pressurized liquid-abrasive mixtures (slurry erosion), or (3) chemical dissolution using strong acids or alkali ( chemical erosion or acid etching ).
Finishing The process of removing surface defects or scratches created during the contouring process through the use of cutting or grinding instruments or both.
Grinding A machining process that uses coated or bonded abrasive wheels or points to turn against the surface of a material workpiece in order to reduce the surface. Ground surfaces typically display linear scratch marks where abrasive particles have ground material away.
Polish, polishing A process that uses very fine abrasives to bring a material surface to a highly developed, finished, or refined state. A minimal amount of material is removed from the workpiece to generate a smooth, glossy appearance. The force per unit area for polishing is the lightest of all processes that use abrasives.
True/truing A process of correcting the concentricity and shape of a grinding wheel; truing keeps the grinding wheel rotating concentrically with the spindle axis of the motor or handpiece head without vibration or wobble.
As has been stated many times throughout this book, the intraoral surfaces of virtually every direct and indirect restoration must be contoured by various abrasive procedures. The goal of these procedures is to efficiently produce the desired surface contours, contacts, textures, and gloss on a restoration. These procedures involve removing varying volumes of material from the surface by a process called wear. More specifically, they employ abrasive materials and instruments to produce abrasive wear. This form of abrasive wear is intentional as opposed to the unintentional type of abrasive wear of composites ( Chapter 5, Wear ) and ceramics ( Chapter 10, Abrasiveness to Enamel ).
Abrasive processes have been used since prehistoric times. Over 10,000 years ago, hunting and gathering instruments such as spear points, arrowheads, scraper tools, and hoes were formed from hard, rock-like natural materials using primitive forms of abrasion, chipping, grinding, and honing. 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 the stone against metals and ceramics to adjust their shapes, reduce rough areas, and produce smoother surfaces. These processes were refined over subsequent millennia to produce metal daggers, swords, spears, and shields of relatively high quality. The Chinese introduced coated abrasives in the 13th century by embedding seashell fragments in natural gums that were spread on a parchment backing. The invention transformed loose abrasive particles into practical instruments.
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 emerged for use in dentistry. The further refinement of dental handpieces, air-abrasive technology, and methods of bonding abrasives to various binders 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. Figure 16-1 illustrates the series of abrasive procedures and instruments used to bring a rough removable dental prosthesis casting to the final polished state of the prosthesis. The procedures depicted are sandblasting, cutting, grinding, finishing, and polishing.
The focus of this chapter is to provide the reader with an awareness and broader background of the principles and mechanisms of tools available to produce optimal surface finish and integrity in dental restorations.
What are the benefits of finishing and polishing the surfaces of restorative materials? What are the goals of finishing and polishing?
Benefits of Finishing and Polishing Restorative Materials
Before any dental restoration or appliance is placed permanently in the mouth, it should be polished to a smooth surface. Not only is a rough surface on a restoration, prosthesis, orthodontic appliance, and so forth uncomfortable for the patient, but it can also cause food and other debris to cling to the surface. Such a restoration or appliance becomes unsanitary and, in some cases, tarnished or corroded.
Finished and polished restorations provide four benefits in dental care: patient comfort, better gingival health, chewing efficiency, and esthetics. One study showed that patients could distinguish a difference in roughness from between 0.25 and 0.50 μm by tongue proprioception. The value is much lower than 20 μm as previously reported. Further, surface roughness greater than 1 μm can lead to increased bacterial adhesion and unsightly surface staining. Smooth surfaces resist the accumulation of food debris and permit food to glide freely over occlusal and embrasure surfaces during mastication. Smoother restoration surfaces help patients maintain oral hygiene by facilitating preventive oral home care because dental floss and toothbrush bristles gain better access to all surfaces and marginal areas. Tarnish and corrosion activity can be significantly reduced on some metallic restorations if their entire surface area is polished ( Chapter 3, Concentration Cell Corrosion ).
Rough restoration surfaces, especially those of ceramic-based materials, can abrade and wear opposing dentition and restorations, resulting in the loss of functional and stabilizing contacts between teeth or a reduction in the vertical dimension of occlusion. Rough contact areas on brittle materials can be stress concentration points, leading to chipping and fracture of the restoration. Finishing and polishing of these surfaces will remove surface flaws and improve the restoration’s resistance to fracture, especially in areas that are under tension, such as the perimeter of layered all-ceramic crowns where unsupported areas of veneering ceramic are present.
How can the amount of lubricant either increase or decrease cutting efficiency?
Mechanics of Abrasive Procedures
Mechanical abrasion, as described in this section, is a more controlled form of the abrasive wear discussed in Chapter 4, Wear . In dentistry, the abrading instrument is referred to as the abrasive, and the restorative material that has been abraded is called the substrate or workpiece. In this section, we will discuss two mechanisms of abrasive wear used in dentistry: abrasion and erosion. We will also discuss the proper motion of abrasive instruments.
Abrasion
Abrasive wear occurs where a rough, hard object, or a soft surface containing hard particles, slides against a softer surface, digs into the surface, and plows a series of grooves. The material removed during the formation of the grooves, normally in the form of loose particles, is called wear debris. A typical example is a diamond bur abrading tooth structure during cavity preparation.
Abrasive wear can also arise in a somewhat different situation when hard abrasive particles are introduced between two sliding surfaces and abrade material from one or both surfaces. The mechanism of this form of abrasive wear seems to be that abrasive particles adhere temporarily to, or become embedded within, one of the sliding surfaces and plow grooves into the opposing surface. An example is 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.
The two forms of abrasive wear are (1) two-body wear ( Figure 16-2, A ), which involves a hard, rough surface; and (2) three-body wear, involving loose, hard abrasive grains ( Figure 16-2, B ). 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 oil. 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.
Erosion
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 16-2, C ). Therefore erosion as discussed here is technically a form of abrasive wear that differs only in the mechanism by which the load necessary for wear is delivered. 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. Chemical erosion removes substrate material through the chemical-energy dissolution effects of acids and alkalis versus the mechanical-energy effect of force-driven abrasive streams. Chemical erosion, more commonly called acid etching in dentistry, is not used as a method of finishing dental materials. This type of erosion is used primarily to prepare tooth surfaces and glass-phase ceramics to enhance bonding or coating.
Abrasive Motion
The motion of abrasive instruments is classified as rotary, planar, or reciprocal. In general, burs act in a rotary motion, flexible discs act in a planar motion, and reciprocating handpieces convert a cyclic motion into a reciprocal direction of motion of the abrasive tool insert. Different abrasive grit sizes can be incorporated with each motion. Reciprocating handpieces provide the special benefit of accessing interproximal and subgingival areas to remove overhangs, finish subgingival margins without creating ditches, and create embrasures.
In the case of a diamond bur abrading a tooth surface, as illustrated in Figure 16-3, A, 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. 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 ( Figure 16-3, B ). 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 less controlled grinding action and a rougher surface. This effect is more noticeable at lower rotational speeds.
What are the similarities and differences among the mechanisms responsible for the cutting action of carbide burs and abrasive wheels?
Abrasive Processes in Dentistry
Abrasion is a cutting action, but the topography of an abraded surface is distinctly different from that of a cut surface. In a high-speed tungsten carbide bur, the blades or cutting edges are regularly arranged and allow removal of small pieces or segments of the substrate as the bur rotates. As shown in Figure 16-4, A, the pattern of removal of material by the tool corresponds to the regular arrangements of the cutting blades. In contrast with cutting instruments, abrasive tools have many abrasive points that generally are not arranged in an ordered pattern. For example, a diamond rotary instrument may contain hundreds of sharp abrasive points that pass over the workpiece during each revolution of the instrument ( Figure 16-4, B ). Each sharp point acts as an individual blade and removes a chip or shaving from the material. Because these many cutting edges are randomly arranged, innumerable scratches are produced on the surface ( Figure 16-4, C ).
Bulk reduction can be achieved through the use of instruments such as diamond burs, tungsten carbide burs, steel burs, abrasive wheels, and separating discs. Whereas the action of diamond burs and abrasive wheels is often described as grinding, the action of hard blades of steel and carbide burs is described as cutting. Figure 16-5 illustrates two sizes of carbide burs and diamond burs of three grit sizes. Different types of instruments have unique effects on surfaces. 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. The scanning electron microscope (SEM) images shown in Figure 16-6 are the surfaces of a resin-based composite produced by five instruments: a coarse diamond, a 12-fluted carbide bur, a 16-fluted carbide bur, an abrasive-coated finishing disc, and an abrasive-coated polishing disc. The first three images represent surfaces of bulk reduction, and the last two represent surfaces of fine finishing.
Rough nodules on a denture base can be removed with an abrasive, such as sandpaper, an emery arbor, or a grinding wheel. In each case, the edges formed by the abrasive particles can remove the rough spots as they move over the surface. Abrasives are available in varying particle sizes. Coarse abrasives leave deep scratches in the surface, which must be removed with finer abrasives. Finally, an abrasive can be so fine that this results in a surface so smooth that the surface reflects light, and the surface is said to be polished.
According to Figure 16-1 , after sandblasting, an indirect prosthesis will go through a sequence of four abrasive procedures—cutting, grinding, finishing, and polishing—depending on the type of instruments used, the quantity of material removed from the workpiece, and the purpose. Direct restorations normally do not require cutting. We also use the term contour to refer to an abrasive procedure. Dental contouring is a process of removing small amounts of tooth enamel to change the shape, length, or surface of the tooth to improve esthetic appearance or gingival health. The term has evolved to comprise all procedures needed to shape the restoration before finishing and polishing. All four procedures perform abrasive wear, and there are only subtle differences among them. The purpose of the following discussion is not to make a distinction among the four procedures but to present these subtle differences.
Cutting
Cutting is generally understood as the removal of a part of a structure by means of a shearing action, as in the removal of diseased tissue with a bladed instrument or any other instrument in a bladelike fashion. Substrates may be divided into separate segments, or they may sustain deep notches and grooves by the cutting operation. Examples of cutting instruments are diamond burs, tungsten carbide burs, steel burs, abrasive wheels, and separating discs. On the other hand, a separating disc is an example of an instrument that can be used in a bladelike fashion. A separating disc does not contain individual blades, but the thin blade design allows the disc to be used in a rotating fashion to grind through cast metal sprues and die-stone materials.
Grinding
A grinding operation is considered to be cutting but on a smaller scale. Small particles of a substrate are removed through the action of bladed or abrasive instruments. Grinding instruments typically contain many randomly arranged abrasive particles. Each particle may contain several sharp points that run along the substrate surface and remove particles of the substrate. 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.
Finishing
Surface imperfections can be an integral part of the substrate’s internal structure or can be created by the instruments used for cutting and grinding. Finishing involves removing marginal irregularities, defining anatomical contours, and smoothing away the surface roughness of a restoration, with the aim of providing a relatively smooth, blemish-free surface. A good example is the removal of excess material at the junction of the tooth structure and the restorative material to establish a smooth, uniform, and well-adapted cavosurface margin.
How does a clinician know when the smoothest surface has been achieved?
Polishing
Polishing is a special form of abrasive wear characterized by the use of very small abrasive grains (5 μm or less) on an elastic backing. The purpose of polishing is to provide an enamel-like luster to the restoration. 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 abrasive procedure and is completed when the desired level of surface smoothness is achieved. 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. At the end of this process, there should be no visible scratches. However, there will always be scratches appearing at higher magnification. The instrument and the surface must be cleaned between steps because abrasives left from the previous step will continue making scratches. In clinical practice, the quality of the surface finish is usually judged by the surface luster without magnification.
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. Nonabrasive materials, such as felt, leather, rubber, and synthetic foam, are popular applicator materials for retaining polishing pastes during buffing procedures.
Polishing is considered multidirectional, which means that the final surface scratches, albeit invisible, are oriented in many directions. Some examples of ground and polished surfaces are shown in Figure 16-6 . 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 abrasive procedures used for 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.
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 Abrasive Procedures
Dispersions of solid particles are generated and released into the breathing space of laboratories and dental clinics whenever abrasive procedures 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 the inhalation of silica-based aerosols.
The green-state zirconia-based computer-aided design/computer-aided manufacturing (CAD-CAM) blocks for milling prostheses are made from condensed zirconia (3Y-TZP, 4Y-TZP, etc.) particles in the 40- to 90-nm particle-size range. The dust generated from milling is currently classified as an eye, skin, and respiratory irritant. The carcinogenic potential of this material has yet to be determined. The milling generally is conducted in machines equipped with high-efficiency particulate air (HEPA) filtration and is considered safe for the operator. However, technicians and dentists must de-sprue and smooth these prostheses at the workbench prior to the sintering procedure. Therefore, following the protocol of controlling aerosolized dust in the working place is of paramount importance.
Aerosol sources, in both the dental operatory and laboratory environments, must be controlled whenever finishing procedures are performed. Aerosols produced during finishing procedures may be controlled in three ways: (1) containing at the source through the use of adequate infection control procedures, water spray, and high-volume suction; (2) using personal protective equipment (PPE) such as safety glasses and disposable face masks that can protect the eyes and respiratory tract from aerosols; and (3) installing 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.
Why is it sometimes inappropriate to select the hardest abrasive to reduce the time required for finishing and polishing a prosthesis?
Abrasive Instruments
Abrasive particles should be irregular in shape so that they present multiple sharp edges. Most abrasive grits are derived from crushing hard materials and passing particles through a series of mesh screens (sieves) to obtain different particle-size ranges, namely, coarse, medium-coarse, medium, fine, and superfine. Table 16-1 lists grit and particle sizes for commonly used dental abrasives.
| Grit/Mesh (USA) | Aluminum Oxide, Silicon Carbide, and Garnet (μm) | Grade † | Coated Disc Diamond (μm) | Abrasive Descriptions for Diamond Burs and Diamond Polishing Paste |
|---|---|---|---|---|
| 120 | 142 | Coarse | 142 | Supercoarse–coarse |
| 150 | 122 | 122 | Coarse–regular | |
| 180 | 70–86 | 86 | Coarse–regular | |
| 240 | 54–63 | 60 | Fine | |
| 320 | 29–32 | Medium | 52 | Fine |
| 400 | 20–23 | 40 | Fine–superfine–coarse finishing | |
| 600 | 12–17 | Fine | 14 | Superfine–medium finishing |
| 800 | 9–12 | 8 | Ultrafine–fine finishing | |
| 1200 | 2–5 | Superfine | 6 | Milling pastes |
| 1500 | 1–2 | 4 | Polishing pastes (2–5 μm) | |
| 2000 | 1 | 2 | Polishing pastes (2–5 μm) |
An abrasive should be harder than the workpiece the abrasive is supposed to abrade. If the abrasive cannot indent the surface to be abraded, then the abrasive cannot possibly cut the surface. In such a case, the abrasive dulls or wears. The first ranking of the hardness of minerals was published in 1812 by Friedrich Mohs, who ranked 10 minerals by their relative scratch. The least scratch-resistant mineral, talc, received a score of 1, and the most scratch-resistant mineral, diamond, received a score of 10. Despite the lack of precision, a Mohs hardness scale kit can estimate how hard the workpiece is to enable one to select an appropriate instrument to abrade the material with. Knoop and Vickers hardness tests ( Chapter 4, Hardness ) quantify the hardness of materials. The farther apart a substrate and an abrasive are in hardness values, the more efficient is the abrasion process. On the basis of a comparison of hardness values for several dental materials listed in Table 16-2 , silicon carbide and diamond abrasives are expected to abrade dental porcelain more readily compared with garnet, even though the abrasive particles for all three materials have very sharp edge characteristics.
| Material | Mohs Hardness | Knoop Hardness (kg/mm 2 ) | Vickers Hardness (kg/mm 2 ) |
|---|---|---|---|
| Abrasives | |||
| Talc | 1 | – | – |
| Gypsum | 2 | – | 12 |
| Tripoli | 6 | – | – |
| Pumice | 6–7 | 460–560 | – |
| Porcelain | 6–7 | 560 | 430 |
| Tin oxide | 6–7 | – | – |
| Sand | 7 | – | – |
| Cuttle | 7 | 800 | – |
| Quartz | 7 | 820 | – |
| Zirconium oxide (Y-TZP) | 7 | – | 1200–1300 |
| Zirconium silicate | 7.5 | – | – |
| Garnet | 8–9 | 1350 | – |
| Emery | 7–9 | 2000 | – |
| Corundum | 9 | 2000 | – |
| Aluminum oxide | 9 | 2100 | 1200 |
| Tungsten carbide | 9.8 | 1900 | – |
| Silicon carbide | 9–10 | 2500 | – |
| Diamond | 10 | 7000–10,000 | – |
| Tooth | |||
| Cementum | – | 40 | – |
| Dentin | 3–4 | 70 | 62–70 |
| Enamel (apatite) | 5 | 340–431 | 294–408 |
| Restorative Materials | |||
| Denture base resin | 2–3 | 20 | – |
| Calcite | 3 | 135 | – |
| Metal-reinforced glass ionomer | 14–24 | 40 | |
| Type III gold alloy | 3 | – | 122–180 (soft)/155–250 (hard) |
| Type IV gold alloy | 4 | 220 | 150–194 (soft)/248–280 (hard) |
| Amalgam | 4–5 | 90 | 120 (Ag2Hg3 phase) |
| Rouge | 5–6 | – | – |
| Glass (glass–ceramics) | 5–6 | 360 | 420 |
| Composite (compomer) | – | – | 52 |
| Composite (nanohybrid) | – | – | 60–62 |
| Composite (nanofilled) | 5–7 | 50–60 | 73–76 |
| Composite (minifilled) | 5–7 | 50–60 | 80 |
| Composite (laboratory cured) | – | – | 86–124 |
| Titanium | – | – | 210 |
| Ti-6Al-4V | – | – | 320 |
| Nickel–chromium alloy (as cast) | – | 153–328 | 200–395 |
| Cobalt–chromium alloy (as cast) | – | – | 280–380 |
| Layered gold metal-ceramic alloy | – | – | 35 |
| Lithia-disilicate (milled) | – | – | 550 (milled), 591 (sintered) |
| Lithia-disilicate (pressed) | – | – | 601 |
| Zirconia-reinforced lithia silicate | – | – | ∼700 |
| Zirconia oxide (Y-TZP) | – | – | 1200–1300 |
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