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Wednesday, November 12, 2008
Monday, November 10, 2008
spectrophotometer
In physics, spectrophotometry is the quantitative study of electromagnetic spectra. It is more specific than the general term electromagnetic spectroscopy in that spectrophotometry deals with visible light,
near-ultraviolet, and near-infrared. Also, the term does not cover time-resolved spectroscopic techniques.
Spectrophotometry involves the use of a spectrophotometer. A spectrophotometer is a photometer (a device for measuring light intensity) that can measure intensity as a function of the color, or more specifically, the wavelength of light. There are many kinds of spectrophotometers. Among the most important distinctions used to classify them are the wavelengths they work with, the measurement techniques they use, how they acquire a spectrum, and the sources of intensity variation they are designed to measure. Other important features of spectrophotometers include the spectral bandwidth and linear range.
Perhaps the most common application of spectrophotometers is the measurement of light absorption, but they can be designed to measure diffuse or specular reflectance. Strictly, even the emission half of a luminescence instrument is a kind of spectrophotometer.
The use of spectrophotometers is not limited to studies in physics. They are also commonly used in other scientific fields such as chemistry, biochemistry, and molecular biology.[1]
Design
There are two major classes of spectrophotometers; single beam and double beam. A double beam spectrophotometer measures the ratio of the light intensity on two different light paths, and a single beam spectrophotometer measures the absolute light intensity. Although ratio measurements are easier, and generally more stable, single beam instruments have advantages; for instance, they can have a larger dynamic range, and they can be more compact.
Historically, spectrophotometers use a monochromator to analyze the spectrum, but there are also spectrophotometers that use arrays of photosensors. Especially for infrared spectrophotometers, there are spectrophotometers that use a Fourier transform technique to acquire the spectral information quicker in a technique called Fourier Transform InfraRed.
The spectrophotometer measures quantitatively the fraction of light that passes through a given solution. In a spectrophotometer, a light from the lamp is guided through a monochromator, which picks light of one particular wavelength out of the continuous spectrum. This light passes through the sample that is being measured. After the sample, the intensity of the remaining light is measured with a photodiode or other light sensor, and the transmittance for this wavelength is then calculated.
In short, the sequence of events in a spectrophotometer is as follows:
1. The light source shines through the sample.
2. The sample absorbs light.
3. The detector detects how much light the sample has absorbed.
4. The detector then converts how much light the sample absorbed into a number.
5. The numbers are either plotted straight away, or are transmitted to a computer to be further manipulated (e.g. curve smoothing, baseline correction)
Many spectrophotometers must be calibrated by a procedure known as "zeroing." The absorbency of some standard substance is set as a baseline value, so the absorbencies of all other substances are recorded relative to the initial "zeroed" substance. The spectrophotometer then displays % absorbency (the amount of light absorbed relative to the initial substance).[1]
UV and IR spectrophotometers
The most common spectrophotometers are used in the UV and visible regions of the spectrum, and some of these instruments also operate into the near-infrared region as well.
Visible region 400-700nm spectrophotometry is used extensively in colorimetry science. Ink manufacturers, printing companies, textiles vendors, and many more, need the data provided through colorimetry. They usually take readings every 20 nanometers along the visible region, and produce a spectral reflectance curve. These curves can be used to test a new batch of colorant to check if it makes a match to specifications.
Traditional visual region spectrophotometers cannot detect if a colorant has fluorescence. This can make it impossible to manage color issues if one or more of the printing inks is fluorescent. Where a colorant contains fluorescence, a bi-spectral fluorescent spectrophotometer is used. There are two major setups for visual spectrum spectrophotometers, d/8 (spherical) and 0/45. The names are due to the geometry of the light source, observer and interior of the measurement chamber. Scientists use this machine to measure the amount of compounds in a sample. If the compound is more concentrated more light will be absorbed by the sample; within small ranges, the Beer-Lambert law holds and the absorbance between samples vary with concentration linearly.
Samples are usually prepared in cuvettes; depending on the region of interest, they may be constructed of glass, plastic, or quartz.
IR spectrophotometry
Spectrophotometers designed for the main infrared region are quite different because of the technical requirements of measurement in that region. One major factor is the type of photosensors that are available for different spectral regions, but infrared measurement is also challenging because virtually everything emits IR light as thermal radiation, especially at wavelengths beyond about 5 µm.
Another complication is that quite a few materials such as glass and plastic absorb infrared light, making it incompatible as an optical medium. Ideal optical materials are salts, which do not absorb strongly. Samples for IR spectrophotometry may be smeared between two discs of potassium bromide or ground with potassium bromide and pressed into a pellet. Where aqueous solutions are to be measured, insoluble silver chloride is used to construct the cell.
Spectroradiometers
Spectroradiometers, which operate almost like the visible region spectrophotometers, are designed to measure the spectral density of illuminants in order to evaluate and categorize lighting for sales by the manufacturer, or for the customers to confirm the lamp they decided to purchase is within their specifications. Components:
1. The light source shines onto or through the sample.
2. The sample transmits or reflects light.
3. The detector detects how much light was reflected from or transmitted through the sample.
4. The detector then converts how much light the sample transmitted or reflected into a number.
Readmore »»
Spectrophotometry involves the use of a spectrophotometer. A spectrophotometer is a photometer (a device for measuring light intensity) that can measure intensity as a function of the color, or more specifically, the wavelength of light. There are many kinds of spectrophotometers. Among the most important distinctions used to classify them are the wavelengths they work with, the measurement techniques they use, how they acquire a spectrum, and the sources of intensity variation they are designed to measure. Other important features of spectrophotometers include the spectral bandwidth and linear range.
Perhaps the most common application of spectrophotometers is the measurement of light absorption, but they can be designed to measure diffuse or specular reflectance. Strictly, even the emission half of a luminescence instrument is a kind of spectrophotometer.
The use of spectrophotometers is not limited to studies in physics. They are also commonly used in other scientific fields such as chemistry, biochemistry, and molecular biology.[1]
Design
There are two major classes of spectrophotometers; single beam and double beam. A double beam spectrophotometer measures the ratio of the light intensity on two different light paths, and a single beam spectrophotometer measures the absolute light intensity. Although ratio measurements are easier, and generally more stable, single beam instruments have advantages; for instance, they can have a larger dynamic range, and they can be more compact.
Historically, spectrophotometers use a monochromator to analyze the spectrum, but there are also spectrophotometers that use arrays of photosensors. Especially for infrared spectrophotometers, there are spectrophotometers that use a Fourier transform technique to acquire the spectral information quicker in a technique called Fourier Transform InfraRed.
The spectrophotometer measures quantitatively the fraction of light that passes through a given solution. In a spectrophotometer, a light from the lamp is guided through a monochromator, which picks light of one particular wavelength out of the continuous spectrum. This light passes through the sample that is being measured. After the sample, the intensity of the remaining light is measured with a photodiode or other light sensor, and the transmittance for this wavelength is then calculated.
In short, the sequence of events in a spectrophotometer is as follows:
1. The light source shines through the sample.
2. The sample absorbs light.
3. The detector detects how much light the sample has absorbed.
4. The detector then converts how much light the sample absorbed into a number.
5. The numbers are either plotted straight away, or are transmitted to a computer to be further manipulated (e.g. curve smoothing, baseline correction)
Many spectrophotometers must be calibrated by a procedure known as "zeroing." The absorbency of some standard substance is set as a baseline value, so the absorbencies of all other substances are recorded relative to the initial "zeroed" substance. The spectrophotometer then displays % absorbency (the amount of light absorbed relative to the initial substance).[1]
UV and IR spectrophotometers
The most common spectrophotometers are used in the UV and visible regions of the spectrum, and some of these instruments also operate into the near-infrared region as well.
Visible region 400-700nm spectrophotometry is used extensively in colorimetry science. Ink manufacturers, printing companies, textiles vendors, and many more, need the data provided through colorimetry. They usually take readings every 20 nanometers along the visible region, and produce a spectral reflectance curve. These curves can be used to test a new batch of colorant to check if it makes a match to specifications.
Traditional visual region spectrophotometers cannot detect if a colorant has fluorescence. This can make it impossible to manage color issues if one or more of the printing inks is fluorescent. Where a colorant contains fluorescence, a bi-spectral fluorescent spectrophotometer is used. There are two major setups for visual spectrum spectrophotometers, d/8 (spherical) and 0/45. The names are due to the geometry of the light source, observer and interior of the measurement chamber. Scientists use this machine to measure the amount of compounds in a sample. If the compound is more concentrated more light will be absorbed by the sample; within small ranges, the Beer-Lambert law holds and the absorbance between samples vary with concentration linearly.
Samples are usually prepared in cuvettes; depending on the region of interest, they may be constructed of glass, plastic, or quartz.
IR spectrophotometry
Spectrophotometers designed for the main infrared region are quite different because of the technical requirements of measurement in that region. One major factor is the type of photosensors that are available for different spectral regions, but infrared measurement is also challenging because virtually everything emits IR light as thermal radiation, especially at wavelengths beyond about 5 µm.
Another complication is that quite a few materials such as glass and plastic absorb infrared light, making it incompatible as an optical medium. Ideal optical materials are salts, which do not absorb strongly. Samples for IR spectrophotometry may be smeared between two discs of potassium bromide or ground with potassium bromide and pressed into a pellet. Where aqueous solutions are to be measured, insoluble silver chloride is used to construct the cell.
Spectroradiometers
Spectroradiometers, which operate almost like the visible region spectrophotometers, are designed to measure the spectral density of illuminants in order to evaluate and categorize lighting for sales by the manufacturer, or for the customers to confirm the lamp they decided to purchase is within their specifications. Components:
1. The light source shines onto or through the sample.
2. The sample transmits or reflects light.
3. The detector detects how much light was reflected from or transmitted through the sample.
4. The detector then converts how much light the sample transmitted or reflected into a number.
Otolaryngology
Otolaryngology is the branch of medicine that specializes in the diagnosis and treatment of ear, nose, throat, and head and neck disorders.
The full name of the specialty is otolaryngology-head and neck surgery. Practitioners are called otolaryngologists-head and neck surgeons, or sometimes otorhinolaryngologists (ORL). A commonly used term for this specialty is ENT (ear, nose and throat). The term comes from the Greek ?t??a?????????a (oto = genitive for ear, laryngo = genitive for larynx/throat, logy = study), and it literally means the study of ear and neck. The full term ?t???????a?????????a (otorhinolaryngology), also includes rhino, which is the genitive of nose.
Explanation
Otolaryngologists are medical doctors (MD, MBBS, MBChB, etc.) or osteopathic doctors (DO) who complete at least five years of surgical residency training. This is composed of one year in general surgical training and four years in otolaryngology - head and neck surgery; in the past it varied between two and three years of each.
Following residency training some otolaryngologists elect to complete advanced subspeciality fellowship training which can range from as little as 2 weekend courses (Allergy)[1] or as long as 1-2 years in duration (pediatric otolaryngology)[2].[citat
Readmore »»
Explanation
Otolaryngologists are medical doctors (MD, MBBS, MBChB, etc.) or osteopathic doctors (DO) who complete at least five years of surgical residency training. This is composed of one year in general surgical training and four years in otolaryngology - head and neck surgery; in the past it varied between two and three years of each.
Following residency training some otolaryngologists elect to complete advanced subspeciality fellowship training which can range from as little as 2 weekend courses (Allergy)[1] or as long as 1-2 years in duration (pediatric otolaryngology)[2].[citat
Light Cure
Dental restorative materials are specially fabricated materials, designed for use as dental restorations (fillings),
which are used to restore tooth structure loss, usually resulting from but not limited to dental caries (dental cavities). There are many challenges for the physical properties of the ideal dental restorative material.
Restorative material development
The goal of research and development is to develop the ideal restorative material. The ideal restorative material would be identical to natural tooth structure, in strength adherence and appearance. The properties of an ideal filling material can be divided into four categories: physical properties, biocompatibility, aesthetics and application.
Physical properties
The physical properties include heat insulation, resistance to different categories of forces, and wear, bond strength, and chemical resistance. The material needs to withstand everyday forces and conditions on it without fatiguing.
Biocompatibility
Biocompatibility refers to how well the material coexists with the biological equilibrium of the tooth and body systems. Since fillings are in close contact with mucosa, tooth, and pulp, biocompatibility is very important. Common problems with some of the current dental materials include allergies, chemical leakage from the material, and pulpal irritation. Some of the byproducts of the chemical reactions during different stages of material hardening need to be considered.
Aesthetics
Filling materials ideally would match the surrounding tooth structure in shade translucency and texture.
Application
Dental operators require materials that are easy to manipulate and shape, where the chemistry of any reactions that need to occur are predictable or controllable.
Direct restorative materials
The chemistry of the setting reaction for direct restorative materials is designed to be more biologically compatible. Heat and byproducts generated cannot damage the tooth or patient, since the reaction needs to take place while in contact with the tooth during restoration. This ultimately limits the strength of the materials, since harder materials need more energy to manipulate.
Amalgam
Amalgam fillings, (also called silver fillings) are a mixture of mercury (from 43% to 54%) and powdered alloy made mostly of silver, tin, zinc and copper commonly called the amalgam alloy.[1] Due to the known toxicity of the element mercury, there is some controversy about the use of amalgams. see Amalgam controversy.
History
The Chinese were the first to use a silver amalgam to fill teeth in the 7th century; in 1816, Auguste Taveau developed his own dental amalgam from silver coins and mercury. This amalgam contained a very small amount of mercury and had to be heated in order for the silver to dissolve at an appreciable rate. Taveau's formula offered lower cost and greater ease of use compared to existing materials such as gold, but had many practical problems, including a tendency to significantly expand after setting. Because of these problems, this formula was abandoned in France. In 1833, however, two untrained Europeans, the Crawcour brothers, brought Taveau's amalgam to the United States under the name "Royal Mineral Succedaneum".[2]
Aesthetics
Most of the patients don't like the silvery glossy appearance of dental amalgams to be visible. So dental amalgams are mostly not used for the restoration of incisors or canines. Composites are given preference in such cases.
Gamma 2 phase amalgams
After widespread adoption and wildly varying standards, the multitude of formulas for making amalgams were standardised into the gamma-2-phase amalgam formula in 1895.
The gamma-2-phase amalgams contain approximately equal parts 50% of liquid mercury and 50% of an alloy powder containing:
* > 65% silver (Ag)
* < 29% tin (Sn)
* < 6% copper (Cu)
* < 2% zinc (Zn)
* < 3% mercury (Hg)
The resulting amalgam is composed of the gamma phase (the silver-tin eutectic Ag3Sn, which reacts with mercury, yielding the gamma-1 phase (Ag2Hg3) and gamma-2 phase (Sn7-8Hg). The gamma phase is prone to corrosion and its mechanical strength is low. The alloy tends to undergo crevice corrosion and form local galvanic cells.
Around 1970, the ingredients changed to the new non-gamma-2 form, with lower manufacturing cost, greater mechanical strength, and better corrosion resistance. The reduced-gamma-2 amalgams (sometimes referred to as "high-copper" amalgams) contain approximately equal parts 50% of liquid mercury and 50% of an alloy powder containing:
* > 40% silver (Ag)
* < 32% tin (Sn)
* < 30% copper (Cu)
* < 2% zinc (Zn)
* < 3% mercury (Hg)
The amalgam alloy is strengthened by presence of Ag-Cu particles. The gamma-2 phase reacts with the Ag-Cu particles to form eta phase Cu6Sn5 and gamma-1 phase.
The possible difference in toxicology between the two has not been studied conclusively. Amalgams continue to be used today because they are hard, durable and inexpensive.
Galvanic shock
When aluminium foil makes contact with some amalgam fillings, saliva in the mouth can act as an electrolyte. This can generate small electrical currents which are felt through the nerves in the tooth as (often extremely painful) electrical "jolts" or shocks.
Composite resin (also called white or plastic filling)
Composite resin fillings (also called white fillings) are a mixture of powdered glass and plastic resin, and can be made to resemble the appearance of the natural tooth. They are strong, durable and cosmetically superior to silver or dark grey colored amalgam fillings. Composite resin fillings are usually more expensive than silver amalgam fillings. Bis-GMA based materials contain Bisphenol A, a known endocrine disrupter chemical, and may contribute to the development of breast cancer. PEX-based materials do not.
Most modern composite resins are light-cured photopolymers. Once the composite hardens completely, the filling can then be polished to achieve maximum aesthetic results. Composite resins experience a very small amount of shrinkage upon curing, causing the material to pull away from the walls of the cavity preparation. This makes the tooth slightly more vulnerable to microleakage and recurrent decay. With proper technique and material selection, microleakage can be minimized or eliminated altogether.
Besides the aesthetic advantage of composite fillings over amalgam fillings, the preparation of composite fillings requires less removal of tooth structure to achieve adequate strength. This is because composite resins bind to enamel (and dentin too, although not as well) via a micromechanical bond. As conservation of tooth structure is a key ingredient in tooth preservation, many dentists prefer placing composite instead of amalgam fillings whenever possible.
Generally, composite fillings are used to fill a carious lesion involving highly visible areas (such as the central incisors or any other teeth that can be seen when smiling) or when conservation of tooth structure is a top priority.
Composite resin fillings require a clean and dry surface to bond correctly with the tooth, so cavities in areas that are harder to keep totally dry during the filling procedure may require a less moisture-sensitive filling. The use of a rubber dam is highly recommended.
Glass Ionomer Cement
See main article Glass ionomer cement
These fillings are a mixture of glass and an organic acid. Although they are tooth-colored, glass ionomers vary in translucency. Although glass ionomers can be used to achieve an aesthetic result, their aesthetic potential does not measure up to that provided by composite resins.
The cavity preparation of a glass ionomer filling is the same as a composite resin; it is considered a fairly conservative procedure as the bare minimum of tooth structure should be removed.
Conventional glass ionomers are chemically set via an acid-base reaction. Upon mixing of the material components, there is no light cure needed to harden the material once placed in the cavity preparation. After the initial set, glass ionomers still need time to fully set and harden.
Glass ionomers do have their advantages over composite resins:
1. They are not subject to shrinkage and microleakage, as the bonding mechanism is an acid-base reaction and not a polymerization reaction.
2. Glass ionomers contain and release fluoride, which is important to preventing carious lesions. Furthermore, as glass ionomers release their fluoride, they can be "recharged" by the use of fluoride-containing toothpaste. Hence, they can be used as a treatment modality for patients who are at high risk for caries. Newer formulations of glass ionomers that contain light-cured resins can achieve a greater aesthetic result, but do not release fluoride as well as conventional glass ionomers.
Glass ionomers are about as expensive as composite resin. The fillings do not wear as well as composite resin fillings. Still, they are generally considered good materials to use for root caries and for sealants.
Resin modified Glass-Ionomer Cement (Compomer)
A combination of glass-ionomer and composite resin, these fillings are a mixture of glass, an organic acid, and resin polymer that harden when light cured. (The light activates a catalyst in the cement that causes it to cure in seconds.) The cost is similar to composite resin. It holds up better than glass ionomer, but not as well as composite resin, and is not recommended for biting surfaces of adult teeth.
In general, resin modified glass-ionomer cements can achieve a better aesthetic result than conventional glass ionomers, but not as good as pure composites.
Indirect Restorative materials
Porcelain (ceramic)
Porcelain fillings are hard, but can cause wear on opposing teeth. They are brittle and are not always recommended for molar fillings.
Gold
Gold fillings have excellent durability, wear well, and do not cause excessive wear to the opposing teeth, but they do conduct heat and cold, which can be irritating. There are two categories of gold fillings, cast gold fillings (gold inlays and onlays) made with 14 or 18 kt gold, and gold foil made with pure 24 kt gold that is burnished layer by layer. For years, they have been considered the benchmark of restorative dental materials. Recent advances in dental porcelains and consumer focus on aesthetic results have caused demand for gold fillings to drop in favor of advanced composites and porcelain veneers and crowns. Gold fillings are usually quite expensive, although they do last a very long time. It is not uncommon for a gold crown to last 30 years in a patient's mouth.
Other historical fillings
Lead fillings were used in the 1700s, but became unpopular in the 1800s because of their softness. This was before lead poisoning was understood.
According to American Civil War-era dental handbooks from the mid-1800s, since the early 1800s metallic fillings had been used, made of lead, gold, tin, platinum, silver, aluminum, or amalgam. A pellet was rolled slightly larger than the cavity, condensed into place with instruments, then shaped and polished in the patient's mouth. The filling was usually left "high", with final condensation — "tamping down" — occurring while the patient chewed food. Gold foil was the most popular and preferred filling material during the Civil War. Tin and amalgam were also popular due to lower cost, but were held in lower regard.
One survey of dental practices in the mid-1800s catalogued dental fillings found in the remains of seven Confederate soldiers from the U.S. Civil War; they were made of:
* Gold foil: Preferred because of its durability and safety.
* Platinum: Was rarely used because it was too hard, inflexible and difficult to form into foil.
* Aluminum: A material which failed because of its lack of malleability but has been added to some amalgams.
* Tin and iron: Believed to have been a very popular filling material during the Civil War. Tin foil was recommended when a cheaper material than gold was requested by the patient, however tin wore down rapidly and even if it could be replaced cheaply and quickly, there was a concern, specifically from Harris, that it would oxidise in the mouth and thus cause a recurrence of caries. Due to the blackening, tin was only recommended for posterior teeth.
* Thorium: Radioactivity was unknown at that time, and the dentist probably thought he was working with tin.
* Lead and tungsten mixture, probably coming from shotgun pellets. Lead was rarely used in the 19th century, it is soft and quickly worn down by mastication, and had known harmful health effects.
* Amalgam: The most popular amalgam was a mixture of silver, tin and mercury. According to the authors of the article " It set very hard and lasted for many years, the major contradiction being that it oxidized in the mouth, turning teeth black. Also the mercury contained in the amalgam was thought at that time to be harmful." as explained in the pre-eminent dental textbook of that century, The Principles and Practice of Dental Surgery by Chapin A. Harris A.M., M.D., D.D.S..[3]
Replacement fillings
Fillings have a finite lifespan: an average of 12.8 years for amalgam and 7.8 years for composite resins.[4] Fillings fail because of changes in the filling, tooth or the bond between them.
Amalgam fillings expand with age, possibly cracking the tooth and requiring repair and filling replacement. Composite fillings shrink with age and may pull away from the tooth allowing leakage. As chewing applies considerable pressure on the tooth, the filling may crack, allowing seepage and eventual decay in the tooth underneath.
The tooth itself may be weakened by the filling and crack under the pressure of chewing. That will require further repairs to the tooth and replacement of the filling.
If fillings leak or if the original bond is inadequate, the bond may fail even if the filling and tooth are otherwise unchanged.
Readmore »»
Restorative material development
The goal of research and development is to develop the ideal restorative material. The ideal restorative material would be identical to natural tooth structure, in strength adherence and appearance. The properties of an ideal filling material can be divided into four categories: physical properties, biocompatibility, aesthetics and application.
Physical properties
The physical properties include heat insulation, resistance to different categories of forces, and wear, bond strength, and chemical resistance. The material needs to withstand everyday forces and conditions on it without fatiguing.
Biocompatibility
Biocompatibility refers to how well the material coexists with the biological equilibrium of the tooth and body systems. Since fillings are in close contact with mucosa, tooth, and pulp, biocompatibility is very important. Common problems with some of the current dental materials include allergies, chemical leakage from the material, and pulpal irritation. Some of the byproducts of the chemical reactions during different stages of material hardening need to be considered.
Aesthetics
Filling materials ideally would match the surrounding tooth structure in shade translucency and texture.
Application
Dental operators require materials that are easy to manipulate and shape, where the chemistry of any reactions that need to occur are predictable or controllable.
Direct restorative materials
The chemistry of the setting reaction for direct restorative materials is designed to be more biologically compatible. Heat and byproducts generated cannot damage the tooth or patient, since the reaction needs to take place while in contact with the tooth during restoration. This ultimately limits the strength of the materials, since harder materials need more energy to manipulate.
Amalgam
Amalgam fillings, (also called silver fillings) are a mixture of mercury (from 43% to 54%) and powdered alloy made mostly of silver, tin, zinc and copper commonly called the amalgam alloy.[1] Due to the known toxicity of the element mercury, there is some controversy about the use of amalgams. see Amalgam controversy.
History
The Chinese were the first to use a silver amalgam to fill teeth in the 7th century; in 1816, Auguste Taveau developed his own dental amalgam from silver coins and mercury. This amalgam contained a very small amount of mercury and had to be heated in order for the silver to dissolve at an appreciable rate. Taveau's formula offered lower cost and greater ease of use compared to existing materials such as gold, but had many practical problems, including a tendency to significantly expand after setting. Because of these problems, this formula was abandoned in France. In 1833, however, two untrained Europeans, the Crawcour brothers, brought Taveau's amalgam to the United States under the name "Royal Mineral Succedaneum".[2]
Aesthetics
Most of the patients don't like the silvery glossy appearance of dental amalgams to be visible. So dental amalgams are mostly not used for the restoration of incisors or canines. Composites are given preference in such cases.
Gamma 2 phase amalgams
After widespread adoption and wildly varying standards, the multitude of formulas for making amalgams were standardised into the gamma-2-phase amalgam formula in 1895.
The gamma-2-phase amalgams contain approximately equal parts 50% of liquid mercury and 50% of an alloy powder containing:
* > 65% silver (Ag)
* < 29% tin (Sn)
* < 6% copper (Cu)
* < 2% zinc (Zn)
* < 3% mercury (Hg)
The resulting amalgam is composed of the gamma phase (the silver-tin eutectic Ag3Sn, which reacts with mercury, yielding the gamma-1 phase (Ag2Hg3) and gamma-2 phase (Sn7-8Hg). The gamma phase is prone to corrosion and its mechanical strength is low. The alloy tends to undergo crevice corrosion and form local galvanic cells.
Around 1970, the ingredients changed to the new non-gamma-2 form, with lower manufacturing cost, greater mechanical strength, and better corrosion resistance. The reduced-gamma-2 amalgams (sometimes referred to as "high-copper" amalgams) contain approximately equal parts 50% of liquid mercury and 50% of an alloy powder containing:
* > 40% silver (Ag)
* < 32% tin (Sn)
* < 30% copper (Cu)
* < 2% zinc (Zn)
* < 3% mercury (Hg)
The amalgam alloy is strengthened by presence of Ag-Cu particles. The gamma-2 phase reacts with the Ag-Cu particles to form eta phase Cu6Sn5 and gamma-1 phase.
The possible difference in toxicology between the two has not been studied conclusively. Amalgams continue to be used today because they are hard, durable and inexpensive.
Galvanic shock
When aluminium foil makes contact with some amalgam fillings, saliva in the mouth can act as an electrolyte. This can generate small electrical currents which are felt through the nerves in the tooth as (often extremely painful) electrical "jolts" or shocks.
Composite resin (also called white or plastic filling)
Composite resin fillings (also called white fillings) are a mixture of powdered glass and plastic resin, and can be made to resemble the appearance of the natural tooth. They are strong, durable and cosmetically superior to silver or dark grey colored amalgam fillings. Composite resin fillings are usually more expensive than silver amalgam fillings. Bis-GMA based materials contain Bisphenol A, a known endocrine disrupter chemical, and may contribute to the development of breast cancer. PEX-based materials do not.
Most modern composite resins are light-cured photopolymers. Once the composite hardens completely, the filling can then be polished to achieve maximum aesthetic results. Composite resins experience a very small amount of shrinkage upon curing, causing the material to pull away from the walls of the cavity preparation. This makes the tooth slightly more vulnerable to microleakage and recurrent decay. With proper technique and material selection, microleakage can be minimized or eliminated altogether.
Besides the aesthetic advantage of composite fillings over amalgam fillings, the preparation of composite fillings requires less removal of tooth structure to achieve adequate strength. This is because composite resins bind to enamel (and dentin too, although not as well) via a micromechanical bond. As conservation of tooth structure is a key ingredient in tooth preservation, many dentists prefer placing composite instead of amalgam fillings whenever possible.
Generally, composite fillings are used to fill a carious lesion involving highly visible areas (such as the central incisors or any other teeth that can be seen when smiling) or when conservation of tooth structure is a top priority.
Composite resin fillings require a clean and dry surface to bond correctly with the tooth, so cavities in areas that are harder to keep totally dry during the filling procedure may require a less moisture-sensitive filling. The use of a rubber dam is highly recommended.
Glass Ionomer Cement
See main article Glass ionomer cement
These fillings are a mixture of glass and an organic acid. Although they are tooth-colored, glass ionomers vary in translucency. Although glass ionomers can be used to achieve an aesthetic result, their aesthetic potential does not measure up to that provided by composite resins.
The cavity preparation of a glass ionomer filling is the same as a composite resin; it is considered a fairly conservative procedure as the bare minimum of tooth structure should be removed.
Conventional glass ionomers are chemically set via an acid-base reaction. Upon mixing of the material components, there is no light cure needed to harden the material once placed in the cavity preparation. After the initial set, glass ionomers still need time to fully set and harden.
Glass ionomers do have their advantages over composite resins:
1. They are not subject to shrinkage and microleakage, as the bonding mechanism is an acid-base reaction and not a polymerization reaction.
2. Glass ionomers contain and release fluoride, which is important to preventing carious lesions. Furthermore, as glass ionomers release their fluoride, they can be "recharged" by the use of fluoride-containing toothpaste. Hence, they can be used as a treatment modality for patients who are at high risk for caries. Newer formulations of glass ionomers that contain light-cured resins can achieve a greater aesthetic result, but do not release fluoride as well as conventional glass ionomers.
Glass ionomers are about as expensive as composite resin. The fillings do not wear as well as composite resin fillings. Still, they are generally considered good materials to use for root caries and for sealants.
Resin modified Glass-Ionomer Cement (Compomer)
A combination of glass-ionomer and composite resin, these fillings are a mixture of glass, an organic acid, and resin polymer that harden when light cured. (The light activates a catalyst in the cement that causes it to cure in seconds.) The cost is similar to composite resin. It holds up better than glass ionomer, but not as well as composite resin, and is not recommended for biting surfaces of adult teeth.
In general, resin modified glass-ionomer cements can achieve a better aesthetic result than conventional glass ionomers, but not as good as pure composites.
Indirect Restorative materials
Porcelain (ceramic)
Porcelain fillings are hard, but can cause wear on opposing teeth. They are brittle and are not always recommended for molar fillings.
Gold
Gold fillings have excellent durability, wear well, and do not cause excessive wear to the opposing teeth, but they do conduct heat and cold, which can be irritating. There are two categories of gold fillings, cast gold fillings (gold inlays and onlays) made with 14 or 18 kt gold, and gold foil made with pure 24 kt gold that is burnished layer by layer. For years, they have been considered the benchmark of restorative dental materials. Recent advances in dental porcelains and consumer focus on aesthetic results have caused demand for gold fillings to drop in favor of advanced composites and porcelain veneers and crowns. Gold fillings are usually quite expensive, although they do last a very long time. It is not uncommon for a gold crown to last 30 years in a patient's mouth.
Other historical fillings
Lead fillings were used in the 1700s, but became unpopular in the 1800s because of their softness. This was before lead poisoning was understood.
According to American Civil War-era dental handbooks from the mid-1800s, since the early 1800s metallic fillings had been used, made of lead, gold, tin, platinum, silver, aluminum, or amalgam. A pellet was rolled slightly larger than the cavity, condensed into place with instruments, then shaped and polished in the patient's mouth. The filling was usually left "high", with final condensation — "tamping down" — occurring while the patient chewed food. Gold foil was the most popular and preferred filling material during the Civil War. Tin and amalgam were also popular due to lower cost, but were held in lower regard.
One survey of dental practices in the mid-1800s catalogued dental fillings found in the remains of seven Confederate soldiers from the U.S. Civil War; they were made of:
* Gold foil: Preferred because of its durability and safety.
* Platinum: Was rarely used because it was too hard, inflexible and difficult to form into foil.
* Aluminum: A material which failed because of its lack of malleability but has been added to some amalgams.
* Tin and iron: Believed to have been a very popular filling material during the Civil War. Tin foil was recommended when a cheaper material than gold was requested by the patient, however tin wore down rapidly and even if it could be replaced cheaply and quickly, there was a concern, specifically from Harris, that it would oxidise in the mouth and thus cause a recurrence of caries. Due to the blackening, tin was only recommended for posterior teeth.
* Thorium: Radioactivity was unknown at that time, and the dentist probably thought he was working with tin.
* Lead and tungsten mixture, probably coming from shotgun pellets. Lead was rarely used in the 19th century, it is soft and quickly worn down by mastication, and had known harmful health effects.
* Amalgam: The most popular amalgam was a mixture of silver, tin and mercury. According to the authors of the article " It set very hard and lasted for many years, the major contradiction being that it oxidized in the mouth, turning teeth black. Also the mercury contained in the amalgam was thought at that time to be harmful." as explained in the pre-eminent dental textbook of that century, The Principles and Practice of Dental Surgery by Chapin A. Harris A.M., M.D., D.D.S..[3]
Replacement fillings
Fillings have a finite lifespan: an average of 12.8 years for amalgam and 7.8 years for composite resins.[4] Fillings fail because of changes in the filling, tooth or the bond between them.
Amalgam fillings expand with age, possibly cracking the tooth and requiring repair and filling replacement. Composite fillings shrink with age and may pull away from the tooth allowing leakage. As chewing applies considerable pressure on the tooth, the filling may crack, allowing seepage and eventual decay in the tooth underneath.
The tooth itself may be weakened by the filling and crack under the pressure of chewing. That will require further repairs to the tooth and replacement of the filling.
If fillings leak or if the original bond is inadequate, the bond may fail even if the filling and tooth are otherwise unchanged.
Infra Red Lamp
An infrared heater is a body with a higher temperature which transfers energy to a body with a lower temperature through electromagnetic radiation.
Depending on the temperature of the emitting body, the wavelength of the infrared radiation ranges from 780 nm to 1 mm. The relationship between temperature and wavelength is expressed by the Stefan-Boltzmann Law. No contact or medium between the two bodies is needed for the energy transfer. A rough classification of infrared heaters is connected to wavelength bands of major emission of the energy: short wave or near infrared for the range from 780 nm to 1400 nm, these emitters are also named bright because still some visible light with glare is emitted; medium infrared for the range between 1400 nm and 3000 nm; far infrared or dark emitters for everything above 3000 nm.
Elements of infrared heaters
For practical purposes, most infrared heaters are constructed by either using the emission of a flame (usually soot or a heated matrix) or an electrically heated filament as the emitting body. If an electrically operated infrared heater (infrared lamp) is used, the filament is usually protected by a heat-resistant quartz glass tube. Depending on the filament temperature, a filling of the quartz tube with inert gas (e.g. halogen) may be required to prevent filament degradation. These emitters use the same materials and principle as a light bulb.
The most common filament material used for electrical infrared heaters is tungsten wire, which is coiled to provide more surface area. Low temperature alternatives for tungsten are carbon , or alloys of iron, chromium and aluminium (brand name ‘kanthal’). While carbon filaments are more fickle to produce, they heat up much quicker than a comparable medium-wave heater based on a FeCrAl filament.
Industrial infrared heaters sometimes use a gold coating on the quartz tube that reflects the infrared radiation and directs it towards the product to be heated. Consequently the infrared radiation impinging on the product is virtually doubled. Gold is used because of its oxidation resistance and very high IR reflectivity of approximately 95 %
Types of infrared heaters
Infrared heaters are commonly used in infrared modules (or emitter banks) combining several heaters to achieve larger heated areas.
Infrared heaters are usually classified by the wavelength they emit. Near infrared (NIR) or short-wave infrared heaters operate at high filament temperatures above 1800 °C and when arranged in a field reach high power densities of some 100s of kW/m². Their peak wavelength is well below the absorption spectrum for water, making them unsuitable for many drying applications. They are well suited for heating of silica where a deep penetration is needed.
Medium-wave and carbon (CIR) infrared heaters operate at filament temperatures of around 1000 °C. They reach maximum power densities of up to 60 kW/m² (medium-wave) and 150 kW/m² (CIR).
Efficiency of infrared heaters
Theoretically, the efficiency of an infrared heater is 100% as it converts nearly all electrical energy into heat in the filament. The filament then emits its heat by infrared radiation that is directly or via a reflector impinging on the product to be heated. Some energy is lost due to conduction or convection.
For practical applications, the efficiency of the infrared heater depends on matching the emitted wavelength and the absorption spectrum of the material to be heated.
For example, the absorption spectrum for water has its peak at around 3000 nm. This means that emission from medium-wave or carbon infrared heaters are much better absorbed by water and water-based coatings than NIR or short-wave infrared radiation.
The same is true for many plastics like PVC or polyethylene. Their peak absorption is around 3500 nm. On the other hand, some metals absorb only in the short-wave range and show a strong reflectivity in the medium and far infrared. This makes a careful selection of the right infrared heater type important for energy efficiency in the heating process.
Applications of infrared heaters
IR heaters are used in industrial manufacturing processes including curing of coatings; heating of plastic prior to forming; plastic welding; processing glass; cooking and browning food. They are used when high temperatures are required, fast responses or temperature gradients are needed or products need to be heated in certain areas in a targeted way. Their application is difficult for objects with undercuts.
They are also used to provide warmth to suckling animals whose mother cannot or will not provide them with natural warmth as well as to captive animals in zoos or veterinary clinics, especially for lizards and other reptiles, and tropical animals such as birds.
IR heaters are used in low-temperature infrared saunas.
Readmore »»
Elements of infrared heaters
For practical purposes, most infrared heaters are constructed by either using the emission of a flame (usually soot or a heated matrix) or an electrically heated filament as the emitting body. If an electrically operated infrared heater (infrared lamp) is used, the filament is usually protected by a heat-resistant quartz glass tube. Depending on the filament temperature, a filling of the quartz tube with inert gas (e.g. halogen) may be required to prevent filament degradation. These emitters use the same materials and principle as a light bulb.
The most common filament material used for electrical infrared heaters is tungsten wire, which is coiled to provide more surface area. Low temperature alternatives for tungsten are carbon , or alloys of iron, chromium and aluminium (brand name ‘kanthal’). While carbon filaments are more fickle to produce, they heat up much quicker than a comparable medium-wave heater based on a FeCrAl filament.
Industrial infrared heaters sometimes use a gold coating on the quartz tube that reflects the infrared radiation and directs it towards the product to be heated. Consequently the infrared radiation impinging on the product is virtually doubled. Gold is used because of its oxidation resistance and very high IR reflectivity of approximately 95 %
Types of infrared heaters
Infrared heaters are commonly used in infrared modules (or emitter banks) combining several heaters to achieve larger heated areas.
Infrared heaters are usually classified by the wavelength they emit. Near infrared (NIR) or short-wave infrared heaters operate at high filament temperatures above 1800 °C and when arranged in a field reach high power densities of some 100s of kW/m². Their peak wavelength is well below the absorption spectrum for water, making them unsuitable for many drying applications. They are well suited for heating of silica where a deep penetration is needed.
Medium-wave and carbon (CIR) infrared heaters operate at filament temperatures of around 1000 °C. They reach maximum power densities of up to 60 kW/m² (medium-wave) and 150 kW/m² (CIR).
Efficiency of infrared heaters
Theoretically, the efficiency of an infrared heater is 100% as it converts nearly all electrical energy into heat in the filament. The filament then emits its heat by infrared radiation that is directly or via a reflector impinging on the product to be heated. Some energy is lost due to conduction or convection.
For practical applications, the efficiency of the infrared heater depends on matching the emitted wavelength and the absorption spectrum of the material to be heated.
For example, the absorption spectrum for water has its peak at around 3000 nm. This means that emission from medium-wave or carbon infrared heaters are much better absorbed by water and water-based coatings than NIR or short-wave infrared radiation.
The same is true for many plastics like PVC or polyethylene. Their peak absorption is around 3500 nm. On the other hand, some metals absorb only in the short-wave range and show a strong reflectivity in the medium and far infrared. This makes a careful selection of the right infrared heater type important for energy efficiency in the heating process.
Applications of infrared heaters
IR heaters are used in industrial manufacturing processes including curing of coatings; heating of plastic prior to forming; plastic welding; processing glass; cooking and browning food. They are used when high temperatures are required, fast responses or temperature gradients are needed or products need to be heated in certain areas in a targeted way. Their application is difficult for objects with undercuts.
They are also used to provide warmth to suckling animals whose mother cannot or will not provide them with natural warmth as well as to captive animals in zoos or veterinary clinics, especially for lizards and other reptiles, and tropical animals such as birds.
IR heaters are used in low-temperature infrared saunas.
Diathermy
Dielectric heating (also known as electronic heating, RF heating, high-frequency heating) is
the phenomenon in which radiowave or microwave electromagnetic radiation heats a dielectric material, especially as caused by dipole rotation.
Mechanisms
There are two principal mechanisms by which a non-conductive material can be warmed in an EM field:
1. Electrical conduction: current flow in the material induced by the oscillating electric field generates heat by ohmic losses in the material.
2. Dipole rotation: Molecular rotation occurs in materials containing polar molecules having an electrical dipole moment, which will align themselves in the field by rotating; as the field alternates, the molecules reverse direction and accelerate the motion of individual molecules or atoms. Heat is a form of energy possessed by a substance by virtue of the vibrational movement, i.e. kinetic energy, of its molecules or atoms.
Dipole rotation is the mechanism normally referred to as dielectric heating, and is most widely observable in the microwave oven where it operates most efficiently on liquid water, and much less so on fats, sugars, and frozen water. This is caused by fats and sugars being far less polar than water molecules, and are thus less affected by the forces generated by the alternating electromagnetic fields. Meanwhile, frozen water molecules are fixed in place in a crystal lattice, and cannot freely rotate and absorb heat from molecular friction. Outside of cooking, the effect can be used to heat solids, liquids, or gases (see states of matter).
Communication microwave frequencies penetrate semi-solid substances like meat, and living tissue to a distance proportional to its power density. Some environmentalists are concerned that the widespread adoption of microwave-emitting mobile phones could harm human and animal health through dielectric heating.
Dielectric heating power
For dielectric heating the generated power density per volume is calculated by
p = \omega \cdot \varepsilon_r'' \cdot \varepsilon_0 \cdot E^2,
where ? is the angular frequency, er'' is the imaginary part of the complex relative permittivity, e0 is the permittivity of free space and E the electric field strength. The imaginary part of the complex relative permittivity is a measure for the ability of dielectric material to convert radio frequency electromagnetic field energy into heat.
Surgical uses
Main article: Electrosurgery
Surgical diathermy is usually better known as electrosurgery, and is also referred to occasionally as electrocautery. Electrosurgery involves the use of high frequency alternating current in surgery as either a cutting modality, or else to cauterize small blood vessels to stop bleeding. This technique induces localized tissue burning and damage, the zone of which is controlled by the frequency and power of the device.[1]
Heating uses
In the natural sciences, the term diathermy means "electrically induced heat" and is commonly used for muscle relaxation. It is also a method of heating tissue electromagnetically or ultrasonically for therapeutic purposes in medicine. [2]
Ultrasonic diathermy refers to heating of tissues by ultrasound for the purpose of therapeutic deep heating. No tissue is ordinarily damaged hence it is generally used in biomedical applications. [3][4]
Electric diathermy uses high frequency alternating electric or magnetic fields, sometimes with no electrode or device contact to the skin, to induce gentle deep tissue heating by induction or dipole rotation. No tissue is ordinarily damaged. [3][5]
Readmore »»
Mechanisms
There are two principal mechanisms by which a non-conductive material can be warmed in an EM field:
1. Electrical conduction: current flow in the material induced by the oscillating electric field generates heat by ohmic losses in the material.
2. Dipole rotation: Molecular rotation occurs in materials containing polar molecules having an electrical dipole moment, which will align themselves in the field by rotating; as the field alternates, the molecules reverse direction and accelerate the motion of individual molecules or atoms. Heat is a form of energy possessed by a substance by virtue of the vibrational movement, i.e. kinetic energy, of its molecules or atoms.
Dipole rotation is the mechanism normally referred to as dielectric heating, and is most widely observable in the microwave oven where it operates most efficiently on liquid water, and much less so on fats, sugars, and frozen water. This is caused by fats and sugars being far less polar than water molecules, and are thus less affected by the forces generated by the alternating electromagnetic fields. Meanwhile, frozen water molecules are fixed in place in a crystal lattice, and cannot freely rotate and absorb heat from molecular friction. Outside of cooking, the effect can be used to heat solids, liquids, or gases (see states of matter).
Communication microwave frequencies penetrate semi-solid substances like meat, and living tissue to a distance proportional to its power density. Some environmentalists are concerned that the widespread adoption of microwave-emitting mobile phones could harm human and animal health through dielectric heating.
Dielectric heating power
For dielectric heating the generated power density per volume is calculated by
p = \omega \cdot \varepsilon_r'' \cdot \varepsilon_0 \cdot E^2,
where ? is the angular frequency, er'' is the imaginary part of the complex relative permittivity, e0 is the permittivity of free space and E the electric field strength. The imaginary part of the complex relative permittivity is a measure for the ability of dielectric material to convert radio frequency electromagnetic field energy into heat.
Surgical uses
Main article: Electrosurgery
Surgical diathermy is usually better known as electrosurgery, and is also referred to occasionally as electrocautery. Electrosurgery involves the use of high frequency alternating current in surgery as either a cutting modality, or else to cauterize small blood vessels to stop bleeding. This technique induces localized tissue burning and damage, the zone of which is controlled by the frequency and power of the device.[1]
Heating uses
In the natural sciences, the term diathermy means "electrically induced heat" and is commonly used for muscle relaxation. It is also a method of heating tissue electromagnetically or ultrasonically for therapeutic purposes in medicine. [2]
Ultrasonic diathermy refers to heating of tissues by ultrasound for the purpose of therapeutic deep heating. No tissue is ordinarily damaged hence it is generally used in biomedical applications. [3][4]
Electric diathermy uses high frequency alternating electric or magnetic fields, sometimes with no electrode or device contact to the skin, to induce gentle deep tissue heating by induction or dipole rotation. No tissue is ordinarily damaged. [3][5]
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