- Before any restoration or appliance is placed permanently in the mouth it should be highly polished.
- In spite of all the care taken during processing, many restorations and prostheses usually require further trimming, smoothing and finally polishing.
A rough or unpolished surface may
- Be uncomfortable to the patient
- Cause food and other debris cling to it and make it unhygienic
- Lead to tarnish and corrosion.
Read And Learn More: Basic Dental Materials Notes
Abrasion
It occurs when a hard, rough surface slides along a softer surface and cuts a series of grooves.
Defined As
The wearing away of a substance or structure through a mechanical process, such as grinding, rubbing or scraping (GPT-8).
- (A) Abrasive polishing strip;
- (B) Electroplated diamond;
- (C) Bonded stone.
Types Of Abrasion
Abrasion may be
- A two-body process, for example, action of a diamond bur on enamel.
- A three-body process, for example, pumice applied with a bristle brush.
Abrasion Supplied as
- In dentistry, the abrasive is applied to the work by a variety of tools.
- Paper/plastic coated The abrasive particles may be glued onto a paper or plastic disk that can be attached to a handpiece. Sandpaper falls in this group.
- Coated strips The abrasive (for example diamond) may be attached to stainless steel or plastic strips (used for proximal stripping of teeth). This category is similar to the above.
- Electroplating bonding In the case of diamond rotary instruments the diamond chips are attached to steel wheels, disks, and cylinders by electroplating with nickel-based matrix.
- Bonded stones In grinding wheels and dental stones the abrasive particles are mixed with a bonding agent that holds the particles together. Before hardening, the matrix material with the abrasive is molded to form tools of desired shapes.
- Powder form An abrasive may also be mixed with water or glycerine to form a paste or slurry. It is applied with a felt cone, rubber cup, or brush and used for smoothening irregularities, for example, pumice powder.
- Cake form They are also available in the form of cakes.
- Rubber-impregnated Abrasives can be incorporated into rubber or shellac disks or cups for ‘soft grade’ abrasion.
- Paste form The abrasive is made into a paste and supplied in a tube, for example, Ivoclar polishing paste, toothpaste, etc.
Abrasion is affected by the properties of the abrasive as well as the target material. The important properties include hardness, strength, ductility, thermal conductivity, etc.
Mechanism Of Abrasive Action
- In a cutting tool, for example, carbide or steel bur, the blades or cutting edges are regularly arranged and the removal of material corresponds to this regular arrangement.
- An abrasive tool on the other hand has many abrasive points that are not arranged in an ordered pattern.
- Thus, innumerable random scratches are produced.
- The action of an abrasive is essentially a cutting action.
- Each tiny particle presents a sharp edge that cuts through the surface similar to a chisel.
- A shaving is formed which crushes to a fine powder.
- This powder clogs the abrasive tool and frequent cleaning is required.
Stress-Strain And Heat Production During Abrasion
- While abrading metals, the crystalline structure of the metal is disturbed to a depth of 10 µm.
- The grains become disoriented and strain hardening may occur. Thus, the surface hardness increases.
- In denture resins too, rigorous abrasion introduces stresses.
- The generation of heat during abrasion partially relieves such stresses but if it is too great, it may relieve processing stresses and a warpage may result.
- The resin surface may even melt.
- Similarly, high-speed cutting of tooth structure generates excessive heat which can lead to pulpal damage.
- Therefore, it is very important to control the heat by air/water spray and intermittent cutting (rather than continuous cutting).
Rate Of Abrasion
The rate of abrasion of a given material by a given abrasive is determined primarily by three factors.
- Size of the abrasive particle.
- The pressure of the abrasive agent.
- The speed at which the abrasive particle moves across the surface being abraded.
Rate Of Abrasion Size Of The Particles
- Larger particles cause deeper scratches in the material and wear away the surface at a faster rate.
- The use of course is abrasive is indicated on a surface with many rough spots or large nodules.
- The scratches caused by the coarse abrasive must then be removed by the fier ones.
Rate Of Abrasion Pressure
- Heavy pressure applied by the abrasive will cause deeper scratches and more rapid removal of material.
- However, heavy pressure is not advisable as it can fracture or dislodge the abrasive from the grinding wheel, thus reducing the cutting effiency.
- In addition, the operator has less control of the abrasion process because the removal of material is not uniform.
Rate Of Abrasion Speed
- The higher the speed, the greater the frequency per unit of time the particles contact the surface.
- Thus increasing the speed increases the rate of abrasion. In a clinical situation, it is easier to control speed than pressure to vary the rate of abrasion.
- Varying the speed has the additional advantage that low pressure can be used to maintain a high cutting effiency.
Rotational Speed and Linear Speed
- Rotational speed and linear speed should be diffrentiated.
- The speed with which the particles pass over the work is its linear speed or it is the velocity with which the particles pass over the work.
- The rotational speed is measured in revolutions per minute (RPM or r/min), whereas, the linear speed is measured in meters per second Linear speed is related to rotational speed according to the following formula
V = CR, where
V = linear speed
C = circumference of the bur or disc
R = revolutions per minute
Rotational and Linear Speed Classification
- Finishing abrasives
- Polishing abrasives
- Cleansing abrasives
Finishing Abrasives
Finishing abrasives are hard, coarse abrasives which are used initially to develop contour and remove gross irregularities, for example, coarse stones.
Polishing Abrasives
- Polishing abrasives have fiery particle sizes and are less hard than abrasives used for finishing.
- They are used for smoothening surfaces that have been roughened by finishing abrasives, for example, polishing cakes, pumice, etc.
Cleansing Abrasives
Cleansing abrasives are soft materials with small particle sizes and are intended to remove soft deposits that adhere to enamel or restorative material.
Types Of Abrasives
Emery
- Emery consists of a natural oxide of aluminum called corundum.
- There are various impurities present in it, such as iron oxide, which may also act as an abrasive.
- The greater the content of alumina, the faster the grade of emery.
Aluminum Oxide
- Pure alumina is manufactured from bauxite, an impure aluminum oxide.
- It can be produced in fine grain sizes and has partially replaced emery for abrasive purposes.
- Pure alumina is also used as a polishing agent. Alumina is used in sandblasting machines.
Garnet
- It is composed of different minerals which possess similar physical properties and crystalline forms.
- The mineral comprises of silicates of aluminum, cobalt, magnesium, iron, and manganese.
- Garnet is coated on paper or cloth with glue. It is used on disks, which are operated on handpieces.
Pumice
- It is a highly siliceous material of volcanic origin and is used either as an abrasive or polishing agent depending on particle size.
- Its use ranges from smoothening dentures to polishing teeth in the mouth.
Kieselguhr
- It consists of siliceous remains of minute aquatic plants known as diatoms.
- The coarser form (diatomaceous earth) is used as a filer in many dental materials.
- It is excellent as a mild abrasive and polishing agent.
- Tripoli
- This mild abrasive and polishing agent is often confused with kieselguhr.
- True tripoli originates from certain porous rocks, first found in North Africa near Tripoli.
Rouge
- Rouge is a fine red powder composed of iron oxide.
- It is used in cake form. It may be impregnated on paper or cloth known as ‘crocus cloth’.
- It is an excellent polishing agent for gold and noble metal alloys but is likely to be dirty to handle.
Tin Oxide
- The putty powder is used as a polishing agent for teeth and metallic restorations in the mouth.
- It is mixed with water, alcohol or glycerin and used as a paste.
Chalk
- It is calcium carbonate prepared by the precipitation method.
- There are various grades and physical forms available for different polishing techniques.
- It is sometimes used in dentifrices.
Chromic Oxide
- A relatively hard abrasive capable of polishing a variety of metals.
- It is used as a polishing agent for stainless steel.
SAND
Sand, as well as other forms of quartz, is used as sandpaper or as a powder in sandblasting equipment.
Carbides
- Silicon carbide and boron carbide are manufactured by heating silicon and boron at a very high temperature to effct their union with carbon.
- The silicon carbide is sintered or pressed with a binder into grinding wheels or disks.
- Most of the stone burs used for cutting tooth structures are made of silicon carbide.
Diamond
- It is the hardest and most effctive abrasive for tooth enamel.
- The chips are impregnated in a binder or plated onto a metal shank to form the diamond ‘stones’ and disks so popular with the dental profession.
Zirconium Silicate
- The mineral zircon is ground to various particle sizes and used as a polishing agent.
- It is used in dental prophylactic pastes and in abrasive-impregnated polishing strips and disks.
zINc OxIDe
Zinc oxide in alcohol can be used for polishing amalgam restorations.
Desirable Characteristics Of An Abrasive
- It should be irregular in shape so that it presents a sharp edge. Round smooth particles of sand are poor abrasives. Sandpaper with cubical particles presents flt faces to the work, and so they are not effctive. Irregular and jagged particles are more effctive.
- Abrasive should be harder than the work it abrades. If it cannot indent the surface to be abraded, it cannot cut it and the abrasive dulls or wears out.
- The abrasive should possess a high impact strength. The abrasive particle should fracture rather than dull out so that a sharp edge is always present. The fracture of an abrasive is also helpful in shedding the debris accumulated from the work. A diamond does not fracture,
it loses substances from the tip. They become clogged when used on ductile metals. They are most effctive when used on brittle tooth enamel. - They should have attrition resistance so that it does not wear.
Grading Of Abrasive And Polishing Agents
- Abrasives are graded according to the number of the last sieve it passed through.
- For example, silicon carbide is graded as 8, 10, 12, 14, 16, 20, 24, etc.
- Finer abrasives are designated as powder or flours and are graded in increasing fitness as F, FF, FFF, etc., and impregnated papers as 0,00,000, etc.
Binder
The abrasives on a disk and wheel are held together by a binder.
- Commonly used binders in dentistry are ceramic bonding is used for silicon carbide or corundum in a mounted abrasive point.
- Electroplating with nickel is often used to bind the diamond chips onto the diamond rotary instruments.
- For soft-grade abrasion, rubber or shellac may be used. These wear rapidly, but they are useful in some dental operations in which delicate abrasion is required.
- The type of binder is related to the life of the tool in use. In most abrasives, the binder is impregnated throughout with an abrasive of a certain grade so that, as a particle is removed from the binder during use another takes its place as the binder wears.
- Furthermore, the abrasive should be so distributed that the surface of the tool wears evenly, particularly if the disk or wheel is used for cutting along its periphery.
Diamond Burs
- Either synthetic or natural diamond chips are employed for dental rotary instruments.
- The cutting effiency of diamond rotary instruments depends on whether the diamonds used are natural or synthetic, the grit size, the distribution, and the extent of plating that attaches the particles to the instrument shank.
- The larger the grit size the greater the abrasion. Some companies indicate the grit size by color coding.
- Adequate water spray is essential not only to minimize heat but also to reduce clogging.
Polishing
- It is the production of a smooth mirror-like surface without much loss of any external form.
- If the particle size of an abrasive is reduced sufficiently, the scratches become extremely fie and may disappear entirely.
- The surface then acquires a smooth shiny layer known as a polish.
- The polishing agents remove material from the surface molecule by molecule.
- In the process fie scratches and irregularities are filled in by powdered particulates being removed from the surface.
- The microcrystalline layer is referred to as the polish layer or Beilby layer.
- A polishing agent is employed only after an abrasive obliterates or eliminates most of the fine scratches, leaving a smooth finish.
Difference Between Abrasion And Polishing
The difference between an abrasive agent and a polishing agent is difficult to define. The terms are generally interchangeable.
- Particle size A given agent having a large particle size acts as an abrasive, producing scratches. The same abrasive with a smaller particle size is a polishing agent.
- Material removed Very little of the surface is removed during polishing—not more than 0.005 mm (0.002 inches).
- Speed The optimum speed for polishing is higher than that for abrading. Linear speed as high as 10000 ft/min may be used. It varies with the polishing agent. The average speed is approximately 7500 ft/min.
Nonabrasive Polishing
Polishing is usually achieved by an abrasive process. However, a smooth shiny surface can also be achieved through nonabrasive means. These include
- Application of a glaze layer, for example
- Glazing of composites
- Glazing of ceramics
- Electrolytic polishing
- Burnishing.
Composite Glazing
A layer of glaze or gloss (a clear transparent liquid made of unfiled resin) is applied over the restoration and cured. This results in a smooth glossy surface.
Glazing Ceramics
- Ceramics are difficult to polish conventionally. The finished restoration is subjected to high temperatures.
- At this temperature, the surface layer melts and flows to produce a smooth glass-like surface (refer ceramics chapter for more details).
- Alternatively, a glaze layer can be applied and failed to obtain a shiny surface.
Electrolytic Polishing
- Electrolytic polishing is not true abrasion. Although the material is removed, it is removed through an electrochemical process rather than an abrasive process.
- This is the reverse of electroplating. The alloy to be polished is made of the anode of an electrolytic cell.
- As the current is passed, some of the anode is dissolved leaving a bright surface.
- This is an excellent method for polishing the fitting surface of a cobalt–chromium alloy denture.
- So little material is removed, that the fi of the denture is virtually unaltered.
Burnishing
- It is related to polishing in that the surface is drawn or moved.
- Instead of using many tiny particles, only one large point is used. If a round steel point is rubbed over the margins of a gold inlay, the metal is moved so that any gap between the inlay and the tooth can be closed. A special blunt bur revolving at high speed can also be used.
- Note The burnishing instrument should not adhere to or dissolve in the surface of the burnished metal, for example, brass instruments impregnate copper into the surface of a gold inlay.
Technical Considerations
Methods Of Abrasion
Abrasion may be carried out
- Manually, for example, proximal stripping of enamel using abrasive strips.
- Rotary instruments, for example, burs, wheels, cups, discs, cones, etc
- Blasting The object is blasted with a steady stream of abrasive, for example, prophy-jet polishing of enamel, sandblasting to remove investment of castings, shell blasting, etc.
- Smoothen work with a coarse abrasive or bur which leaves large scratches. These scratches are removed with fier abrasives but the difference in fitness should not be too great. The use of too fine an abrasive after a relatively coarse abrasive wastes time and may cause streaking.
- After changing to a finer abrasive, the direction of abrasion should be
changed each time if possible, so that new scratches appear at right angles to the coarser scratches, for uniform abrasion. - When the scratches are no longer visible to the eye, the preliminary polishing can be accomplished with pumice flur applied with a canvas buf wheel [used for resin dentures.
- The work is cleaned to remove all traces of abrasives and the particles of the material removed by the abrasive
- A paste is formed of pumice and water to a sticky ‘muddy’ consistency. The buf wheel is turned at high speed. Apply the paste to the work and carry it to the buff Hold the work fimly but without excessive pressure. Repeat this over the entire surface till the surface is bright and well-polished.
- Clean the work with soap and water. Change to a flannel buf wheel. Rotate at high speed. A polishing cake with grease is held against the buf wheel to impregnate with the agent. The work is held against the wheel and turned so that all of the surfaces are polished uniformly. Light pressure is used to avoid excess heat generation (especially in resins).
Dentifrices
- Popularly known as toothpaste these are agents used with a toothbrush to cleanse and polish natural teeth.
- They should have maximum cleansing effiency with minimum tooth abrasion.
- Highly abrasive dentifrices should not be used especially when dentin (abrades 25 times faster) or cementum (35 times faster) is exposed.
Dentifrices Function
- Assists the toothbrush to mechanically remove stains, debris, and soft deposits from the teeth.
- To impart a polished surface to the tooth. Thus, they help reduce caries, maintain healthy gingiva, improve esthetics, and reduce mouth odors.
Available As
Pastes, powders, liquid, and gels.
Classification Of Dentifrices
Dentifrices may classified based on their primary function.
Caries prevention and treatment
- Fluoride concentrations up to 1000 ppm
- Fluoride concentrations from 1000 to 2000 ppm
- Fluoride concentrations from 2500 to 5000 ppm
Periodontal disease prevention and treatment
- Natural antibacterial and antiseptic agents
- Synthetic antibacterial and antiseptic agents
Desensitizing pastes
- Analgesic
- Dentine tubules blocking
Whitening pastes
- Abrasive
- Bleaching
Pastes for specific purposes
- Toothpaste for xerostomia (for example olive oil, betaine, and xylitol)
- Antiviral pastes (for example larifan for herpetic infection or aphthae)
Dentifrices Composition
- The basic component of dentifrices is abrasive.
- However, pastes today contain numerous other components.
- Specialized dentifrices contain medicaments for various problems, such as sensitivity and gum diseases.
- The composition of typical dentifrices is presented in.
Herbal Toothpaste
- Some toothpastes are marketed as natural or herbal-based toothpastes.
- These contain extracts which are claimed to be beneficial to the gums and teeth.
- Some of these include extracts from the neem tree, miswak tree (Salvadore persica), zanthoxylum alatum (Tomar beej – Dabur red), propolis (resinous extract collected by bees), etc.
Whitening And Bleaching Toothpaste
- Although in several studies whitening toothpastes have shown the ability to improve tooth color, they have several side effects.
- The most significant is enamel and dentin abrasion, which in turn leads to increased tooth sensitivity.
- A key indicator of toothpaste abrasiveness is relative dentin abrasion (RDA).
- The larger the number, the greater the potential of dentin abrasiveness.
RDA
- A0 – 70 Low abrasive
- 70 – 100 Medium abrasive
- 100 – 150 Highly abrasive
- 150 – 250 is Regarded as a harmful limit
- Dentin abrasion significantly increases when the concentration of abrasive substances in toothpaste is increased.
- Teeth have a natural defense mechanism against abrasion, called the pellicle.
- Its presence on tooth surfaces reduces the abrasive effct of toothpaste on enamel.
- Therefore, it is best to avoid mechanical tooth cleaning after consuming acidic foods or drinks, as they may dissolve the pellicle and can combine abrasive and erosive defects.
Bleaching toothpaste
- Contains chemicals, most commonly hydrogen peroxide or calcium peroxide.
- When peroxides touch the tooth surface, they break down the stain molecule, providing a bleaching effct.
- Various bleaching systems for the home or professional use also contain these substances.
- When adding peroxides to toothpaste it should be noted that the concentration is small (usually 1% hydrogen peroxide or 0.5–0.7% calcium peroxide), and the exposure time is short.
- Therefore, there is a lack of evidence about whether such toothpaste can improve internal tooth color. They certainly bleach the pellicle on the tooth surface.
Desensitizing Pastes
Desensitizing pastes works through two methods—analgesic action and blocking of dentinal tubules.
Analgesic toothpaste
- Toothpaste containing potassium salts maintains a high K+ extracellular level, thus preventing repolarization of the nerve cell membrane and
- Inhibiting the transmission of impulses without causing changes in the pulp.
- Toothpaste containing 5% or 10% potassium nitrate, can decrease tooth sensitivity for up to 4 weeks.
Dentin tubule-blocking toothpaste
- Dentin tubule-blocking toothpaste are Also called tubular occludes.
- Examples include fluoride compounds (in high concentrations), strontium salts [10% strontium chloride (SrCl 2) and strontium acetate], arginine (with calcium carbonate), hydroxyapatite and calcium sodium phosphosilicate (Novamin).
- Fluoride compounds increase dentin resistance against acids by forming a protective layer on the tooth surface, increasing the microhardness and precipitating fluoride compounds that block dentin tubules.
Prophylactic Abrasives
- Oral prophylaxis is a widely used procedure in the dental office.
- Prophylactic polishing agents may be available commercially or can be made in the dental office.
- They are usually employed in paste form by mixing with a suitable vehicle.
Prophylactic Function
- They remove extrinsic stains, pellicle, materia-alba, and oral debris.
- Impart a polished and esthetic appearance.
- Different types of abrasives may be employed, for example zirconium silicate, silica, pumice, etc. In addition, some may contain fluoride in order to reduce caries.
- They are applied onto the teeth with the help of rubber cups or bristle brushes which are attached to a rotary instrument.
Prophylactic Prophyjet
- The Prophyjet is a relatively new dental prophylaxis system of removing intraoral stains.
- An abrasive blasting process is used to mechanically remove extrinsic (tobacco) stains as well as light supragingival adherent plaque and calculus.
- The powder is loaded into the device, which then directs the abrasive through a small nozzle in a steady stream of air and water.
- Composition It contains sodium carbonate, hydrophobically modified silica, and a flavoring agent. It is supplied as a powder in sachets or containers.
- Clinical considerations The Prophyjet is directed at 45º angles to the tooth surface.
- For obvious reasons, it is less effctive in proximal areas. The chances of soft tissue injury exist especially if the tissue is inflamed and friable.
Denture Cleansers
Dentures collect deposits in the same manner as natural teeth during their use. These can be removed by two ways.
- Professional repolishing in the laboratory or clinic.
- Soaking or brushing the dentures daily at home.
Brushing
The dentures may be brushed using a soft bristle brush and gentle abrasive or cream. Hard abrasives and stiff bristles should be avoided, because they may produce scratches on the denture surface.
Soaking
Chemical cleaners are an alternative to brushing especially among very old or handicapped persons.
- Alkaline perborates
- Alkaline peroxides
- Alkaline hypochlorites
- Dilute acids
- The dentures are soaked in these chemical solutions for a prescribed period of time.
- Their main requirements are that they should be nontoxic, non-irritant, and should not attack the materials used in denture construction.
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