• In dentistry, The material commonly called "composite" is made of an acrylic matrix called BIS-GMA mixed with a finely ground glass particle filler.  The acrylic will harden with the addition of a catalyst, similar to the way fiber-glass hardens.  In the case of light cured composites, the catalyst is already mixed into the paste, but does not become active until illuminated with a strong light.  To ensure bonding between the filler and the matrix, the filler particles are coated with a silane-coupling agent that contain a methacrylic group able to co-polymerize with the matrix-forming dimethacrylate monomers and functional groups able to interact with the filler.

• Dental amalgam is also a composite, although it is not customary to refer to it as such.  It is made up of finely ground silver/tin metal powder mixed with mercury.  The mercury dissolves the outside layers of the metal powder particles to form a matrix of silver-tin-mercury which hardens around the unreacted metal powder particles to form the finished amalgam composite.  For much more on dental amalgam, please click here.

• Dental cements are all composite materials made from different  powders mixed with different liquids.  The liquid partially dissolves the powder particles and forms a matrix which becomes hard enough to act as a "glue" and is used to cement Crowns and Posts.  All non metallic composite filling materials are really just more highly filled versions of their respective cements.

• Porcelain is not generally thought of as a composite material, but it is in fact composed of a glass matrix filled with crystalline particles.  While ceramics are an extremely important part of dentistry, very few dental professionals really understand them.  For this reason, I have written a Beginners course in dental ceramics to help fill this void.

What is Bonding, and how is it done?

Prior to the age of bonding, dental restorations (fillings, crowns, onlays etc.) had to be attached to teeth mechanically.  This is still done in the case of most fillings by the use of undercuts placed inside the cavity preparation (the "hole" in the tooth). The filling material is condensed into the cavity preparation so that it flows into the undercuts.  When hardened, the filling will not be able to dislodge because it is larger at the bottom of the hole than it is at the top.  When placing a cast restoration such as a crown or an inlay, there can be no undercuts.  Otherwise, the casting would not be able to seat.  The vertical walls of the preparation are made nearly parallel, usually slightly tapered.  The space between the restoration and the tooth is filled with a waterproof cement such as zinc phosphate which hardens and "locks" the restoration onto or into the tooth.   The cement flows into the tiny imperfections in the sides of both the preparation and the restoration and acts as a "lock and key" to keep the restoration from sliding out or off the prepared tooth.  

Bonding is a different process entirely.  Restorations that are bonded "stick" to the tooth without the aid of undercuts or "lock and key" cementation.  There are four types of bonding used in dentistry today.  

Dental Cements and the composite restorations derived from them

Interestingly, all dental cements, and all tooth colored filling materials are made of combinations of only two different powders ( top row), and four different liquids (left column) .  In most cases, the chemical combination of the various powders with the various liquids creates a  material which begins as a paste and "sets" as a hard cement.  Most of these materials are water soluble during the setting phase, but become waterproof after they become hard.  

Types of Non metal Composite material

Zinc phosphate cement

Zinc phosphate cement is one of the oldest and most reliable dental materials.  It has been used for at least two hundred years.  It is still used for cementing cast metal crowns and onlays. It is made by mixing a strong solution (37%) of phosphoric acid with zinc oxide powder.  The zinc oxide powder partially dissolves in the acid creating zinc phosphate which when dry is a very hard, waterproof matrix which bonds unreacted zinc oxide particles together.   Mixing and cementing with this material is something of an art since it must be mixed slowly or else it will harden too quickly, and the work must be kept dry until the cement is set or else it will dissolve in saliva or water.  Once set, it is still one of the most reliable and most durable cements for luting (cementing) cast metal crowns and onlays on teeth.  It is also used to cement posts in teeth and was used until quite recently as a base under amalgam fillings.  (A base is a layer of material placed under a filling to protect the nerve from hot and cold while the overlying filling is in service.  Some bases can also be useful as a method of desensitizing the nerve.)  

Zinc oxide has an added benefit since the acidity of the phosphoric acid etches the enamel on the tooth creating the irregular surface seen in the micrograph above.  The cement flows into these irregularities to create a tight mechanical seal with the tooth itself.  It also flows into irregularities in the structure of the casting to form a "lock and key" type of bond between the tooth and casting thus locking it in place.  With the advent of newer cements with a quicker working time and less demanding technique, zinc phosphate is used less and less today.  Note that zinc oxide is an opaque white powder.  While it can be manufactured to be any color, the set material remains perfectly opaque.  For this reason, and the fact that it lacks wear resistance, zinc oxide is not esthetic or tough enough to be used as a "tooth colored" filling restorative.

Polycarboxylate cement

Polycarboxylate cement is a newer innovation than zinc phosphate cement.  In this case, zinc oxide powder is mixed with polyacrylic acid. Sometimes the polyacrylic acid is freeze dried into a powder and mixed with the zinc oxide powder, in which case the powder is mixed with distilled water.  As with zinc phosphate, the zinc oxide dissolves and creates a matrix which eventually becomes quite waterproof, and though not nearly as strong a cement as zinc phosphate, it is much easier to work with, sets much more quickly and is less irritating to the nerve of the tooth.  As with zinc phosphate, the zinc oxide remains opaque and the color of this material is not easily controlled.  It is rarely used as a restorative filling material.  Like zinc phosphate, this cement is somewhat technique sensitive in that it too must be kept dry until it is completely set.

Silicate  and Glass Ionomer Cements

Silicate cement was probably the very first tooth colored filling material (if you discount whalebone). Glass Ionomer restoratives came later.  However, in order to understand silicate cement, and, indeed, in order to understand the characteristics of most modern composites, it is very important to understand the composition and chemistry of the glass powder that gives them their special characteristics.  

Glass is composed of silica (silicone dioxide) which is essentially quartz.  Silica is the chief component in ordinary sand.  The melting temperature of quartz is very high, but it was discovered early in human civilization that the addition of certain metallic oxides could serve to lower the melting point of the glass quite a bit.  These additional components, when added to sand in order to lower the melting temperature are called "fluxes".  When the glass mixture melts, it becomes a liquid with the consistency of syrup on a very cold day.  Glass does not have a specific melting temperature, and when it cools, it remains a "supercooled" liquid (think of a hard candy, like a lollipop), however contrary to mythology, it does not continue to flow at normal temperatures.  A third component of glass is a stabilizer.  Stabilizers make the glass strong and water resistant. Calcium carbonate, (limestone) is a stabilizer. Without a stabilizer, water and humidity attack and dissolve glass. Glass lacking a stabilizer is often called "waterglass" since it can dissolve in water.

• When lead is used as the stabilizer, the resulting glass has superior clarity and durability, and will ring like a bell when tapped. It is also fairly insoluble, even in acidic solutions.  Lead is NOT used in dental glass.  The FDA (US food and drug administration) has recommended that lead modified glass not be used to store liquids as small amounts of lead have been known to leach out of the glass and into the liquid.  Historically, lead "crystal" has been used for years in the manufacture of fine tableware including drinking glasses and wine canisters (Reference Waterford crystal).  Lead is not used to flux or stabilize any dental glass manufactured in North America or Europe.
• Boron oxide is, like silicone, a glass former.  When added to silicone glass at a minimum of 5% by weight, the glass becomes a borosilicate.  Glass fortified in this way is resistant to mechanical and thermal shock and is used to make baking pans (Pyrex), laboratory ware and sealed beam headlights.
• Alumina (aluminum oxide) is found combined with silicone in naturally occurring glasses called feldspars.  It is used in molecular form to toughen the glass and and is also used as a crystalline structure dispersed throughout the  glass that acts as a sort of framework or skeleton.  This "framework" stiffens the glass during firing and makes it less likely to slump.  The inclusion of crystalline structures transforms the glass into porcelain which is much tougher and less prone to fracture than the same glass without such a matrix.  Alumina is a major component in ordinary clay and is present in nearly all the ceramic products you buy such as the plates and cups in your dinnerware and your mother's bone china.  It is generally added to dental porcelain in the form of aluminum oxide.
• The addition of trace metals can give color to the glass. Cobalt imparts a blue color, while gold imparts red and copper a green color.  (These metals are added as oxides, and they generally have fluxing qualities, but they are added in such small amounts that they are not considered fluxes for purposes of calculating glass formulas.)
• The addition of zirconium and titanium oxides add opacity to the glass.  These oxides form a crystalline structure within the otherwise translucent glass, and this diffuses light as it penetrates, creating a milky or pure white appearance depending on the amount of zirconium or titanium oxides used.
• Fluxes are oxides of alkaline metals such as sodium, potassium, lithium, boron and lead.  They serve to dissolve the silica, a bit like water dissolves sugar.  This is important, since glass is composed of silicone dioxide which has a very high melting temperature. ( Pure quartz melts at 1713 degreed centigrade.  The addition of 25 % sodium oxide can lower the melting temperature to 793 degrees centigrade.)  The most common fluxes used in ceramics are sodium and potassium oxides, but there is a long list of fluxes, each one with its own set of characteristics and uses.

Alumino-Fluoro-Silicate glass

The glass powder that is used in the production of both Silicate cement and Glass Ionomer cement is made from a glass made with Sodium Fluoride and stabilized with minimal alumina.  It is technically known as Alumino-Fluoro-Silicate glass.  This glass is ground into a very fine powder.  While this glass is stabilized to make it insoluble in water, it is formulated to remain partially soluble in very highly acidic solutions.  (It is not soluble in saliva or in any food or liquid that can be consumed by mouth.)  By the use of various trace metals, zirconium, and other components, the glass can be fabricated to match the various colors and opacities of tooth structure.  The major characteristic of this type of glass, however is its ability to partially dissolve and form a hard, waterproof matrix when mixed with either of the two types of acids shown in the table above.  When the powder to liquid ratio is varied correctly, a stiff paste results.  This paste can then be used to fill cavities, and the paste will set in time to form a very hard and insoluble solid.  The hardness, durability and appearance of the resulting restoration is largely dependent on the nature of the chemistry of the matrix formed when the glass particles begin to dissolve in the acidic solution. 

Restorations and cements made with alumino-fluoro-silicate glass have a number of advantages and disadvantages:

• Alumino-fluoro-silicate glass cements and restorations bond chemically with both enamel and dentin (and also metalic structures).
  • This means that they can be applied directly to clean tooth structure without etching or bonding or even cutting retentive undercuts.
  • These materials will also chemically bond to metallic substructures such as gold and base metal crowns and bridges, so they can be used to anchor esthetic facings made of resin composite to these structures.
• Alumino-fluoro-silicate glass cements will slowly release fluoride into the adjacent tooth structure.  This converts hydroxyapatite into fluoroapetite, thus strengthening the tooth structure and making it more resistant to decay.
• The major disadvantages of restorations  and cements made from unmodified alumino-fluoro-silicate glass are:
  • The materials are very water soluble during the setting phases, and if they are allowed to get wet during placement, they can leach out allowing the final restoration to leak.
  • They are also not especially resistant to abrasion, and are not suitable as restorations on occlusal or stress bearing areas.

Silicate Cement

Silicate cement is made by mixing a powder made of Alumino-Fluoro-Silicate glass  with a 37% solution of phosphoric acid.  The acid partially dissolves the glass, chemically combining with it, thus creating a very hard and brittle matrix.  A fluid mixture of this cement can serve the same purpose as the zinc phosphate cement described above, however, its main use in dentistry has been as a tooth colored filling material.  While the matrix is very hard, its brittleness and lack of wear resistance limits its use as a restorative in stress bearing areas.  Until the advent of resin composites, silicates were the only tooth colored filling material available, and the only alternative to silver amalgam as a simple (non gold) permanent filling material.  Its use was limited to front teeth, or areas of decay on non stress bearing surfaces of  back teeth.  

Its largest single advantage, other than its color, is that the fluoride from the glass, (which is also a component of the matrix material due to the chemical reactions involved in mixing the powder with the liquid), tended to prevent further decay around the margins of the filling. (In fact, it is a characteristic of all the formulations using an Al-Fl-Si glass/acid combination that the finished restoration continues to leach small amounts of fluoride into the surrounding tooth structure throughout its life.  This is true of glass ionomer restorations as well.)   Its major disadvantage is its appearance.  Real teeth are somewhat translucent.  Silicate cements tend to be lacking in this characteristic.  In addition, the glass particles are prone to dislodging from the surface of the filling leaving a rough surface which is prone to staining.  The brittleness of the matrix is another esthetic difficulty since it causes surface crazing and marginal chipping as the restoration ages and creating more potential places for stains to lodge.  

Glass Ionomer  (polyalkenoate cement)

Glass Ionomer cements and restoratives (filling materials) are a fairly recent advent in dentistry.  While Silicate cements have been around for years, Glass Ionomer had to await the invention of poly-acrylic acid.  The mixture of poly-acrylic acid with Alumino-Fluoro-Silicate glass causes a partial dissolving of the glass particles.  The poly-acrylic acid chemically combines with the dissolved glass components and produces a hard matrix material similar to that in silicate cement.  (This is essentially an acid-base reaction resulting in the formation of a "metallic polyalkenoate salt" which precipitates and begins to gel until the cement sets hard.)  The characteristics of this matrix material, however, are strikingly different than the characteristics of the matrix found in silicate cements. Unlike silicates, the matrix is reasonably translucent allowing the color of the glass particles to dominate the esthetics.  It is also much less brittle than the matrix of Silicate cement making it a bit less prone to fracturing over time.  Since the filler is a glass, its esthetics can be precisely controlled.  The less brittle matrix means that the margins and surface of the restoration are less prone to chipping and crazing so there is much less staining with Glass Ionomer restorations than there is with silicates.   As a restorative, glass ionomers can be used in all esthetically sensitive areas with no reservations.  Of all the composite restoratives, glass ionomers are some of the prettiest restorations available.  

On the plus side, these restorations not only look good, but they bond to tooth structure quite well.   Bonding between the cement and dental hard tissues is achieved through an ionic exchange at the interface.  Polyalkenoate chains enter the molecular surface of enamel and dentin, replacing phosphate ions. Calcium ions are displaced equally with the phosphate ions so as to maintain electrical equilibrium.  This leads to the development of an ion-enriched layer of cement that is firmly attached to the tooth.  Glass ionomer restorations, like silicates also leach fluoride into the tooth structure throughout the life of the restoration and thus tend to reduce the likelihood of recurrent decay around the margins.  For an excellent detailed technical explanation of the chemistry of glass ionomer, click on this link to the Canadian Dental Association review of glass ionomers.

On the negative side, the matrix material is much less hard than the matrix of silicate cement, so the restorations wear faster than silicates.  They also lack fracture resistance.  Glass Ionomers are excellent fillings on the front surfaces of front teeth, but should not be used to rebuild top edges of these teeth due to their inherent weakness.  They are also used extensively in dentistry as  luting agents ("dental glue" for cementing crowns).  The material is very sensitive to water contamination during placement, and poor technique on the part of the dentist (or poor cooperation on the part of the patient) can shorten the lifespan of the resulting restoration considerably.  Most dentists have switched to using a version of glass ionomer mixed with acrylic resin known as a resin modified glass ionomer for cementing cast metal restorations.  The major uses of glass ionomer cements today are as bases under resin composite restorations and as luting agents for cementing crowns and bridges which have metallic substructures.  

Resin-glass composites (filled resins)

The most widely used tooth colored filling materials in use today are the resin (plastic) glass reinforced composites.  These restoratives, like the composites discussed above, are composed of a powdered filler material (in this case glass particles) in a hard matrix which binds them together (in this case acrylic).  Unlike the glass ionomer and silicate restoratives discussed above the composition of the hard, plastic matrix does not depend upon a chemical reaction between an acid and the glass particles.  This means that the glass used in resin based composites are not formulated to be soluble in acidic solutions.  Like everything else, this has some advantages, and a few disadvantages.  

The hard matrix is composed of a refined form of acrylic known as BIS-GMA.  The glass particles are mixed with the acrylic and then when the dentist is ready to place the restoration in the tooth he or she mixes a catalyst into the paste and this causes the acrylic to harden around the glass particles.  Thus the material resembles a refined version of fiber glass or auto body putty.  As an alternative, the catalyst may already be mixed into the paste, but it is not activated until the dentist shines a very bright light on it, causing it to harden.   This procedure is known as light curing.

The acrylic resin has certain characteristics which make it unsuitable as a restorative material if used by itself without the glass filler particles.  The unfilled resin is prone to abrasive wear, but its major disadvantage is that the material tends to shrink while it is setting.  This would  create large spaces between the filling and the walls of the cavity preparation in the tooth, or in combination with the bonding process, would cause intolerable stresses on the tooth and could possibly even break the tooth. The addition of substantial amounts of rigid glass filler prevents most of the shrinkage associated with the resin.  The glass particles are also much more wear resistant than unfilled resin, and if the particles are of irregular shape, they are less likely to dislodge from the resin matrix under stress. Thus the glass filler solves the durability problem as well.

The fact that the glass particles do not have to react with the matrix allows the manufacturer a great deal of leeway in the manufacture the glass powder.  He can flux and stabilize the glass with materials that give it characteristics like better wear, workability and esthetic qualities than he could achieve if he were constrained by the need to manufacture the glass  according to solubility specifications.  The glass can be formulated with virtually unlimited variations for esthetics.  Special formulations allow for particles of differing size for different restorative situations.  The particles may also have different shapes which allow for an attachment between adjacent particles thus strengthening the material.  Particle size and shape may be varied to allow for differing consistencies without compromising strength or wear characteristics.  He can also vary the qualities of the acrylic matrix independently of the filler particles.  

One disadvantage to standard resin systems is that unlike with Al-Fl-Si  glass/acid mixtures, there is no mechanism for fluoride fluxed into the glass to enter the resin matrix, and thus no way for fluoride to leach into the tooth structure offering a measure of decay resistance to the margins of the cavity preparation.  This problem has been overcome to a certain extent with the introduction of the compomers, and also by advances in the composition of the unfilled resin matrix itself.

A second disadvantage is that resin composites do not bond to tooth structure unless the tooth is acid-etched and a layer of thin plastic bonding resin is placed on the prepared surface first.  Al-Fl-Si  glass/acid mixtures chemically bond with tooth structure without the need for etching or special resin bonding agents. 

Even with these disadvantages, however, the advantages of resin composites are impressive.  By decoupling the chemical link between the glass filler particles and the surrounding matrix, the resulting flexibility has created huge developmental possibilities for manufacturers.  The evolution of dental composites is so advanced, that the industry is now working on a sixth generation of materials, and resin/glass composites have even begun to replace the ever popular silver amalgam as the inexpensive restoration of choice for back teeth.  

Types of resin composites

• Macrofill Composites---This was the first type of resin composite marketed for filling front teeth.  As the name implies, the particles in a macrofill are fairly large.   Crystalline quartz was ground into a fine powder containing particles 8 to 12 microns in diameter.   As mentioned above, the acrylic matrix in a composite tends to shrink on setting.  Excessive shrinkage in a filling material is undesirable because it would either leave a gap between the tooth surface and the filling material, or, if well bonded, would cause cracks in the tooth structure as the filling contracts during setting.  The inclusion of glass particles reduces this problem because they reduce the volume of acrylic, and act as a mechanical "skeletal structure" within the composite to help maintain the original volume of the filling.  The advantage of large particle size is that more of them can be incorporated into the mixture without making it too stiff to work with.  Macrofills are 70% to 80% glass by weight.  Unfortunately, macrofill composites have two undesirable qualities:

   

  • Due to large particle size, macrofills are not very polishable.  The relatively soft acrylic polymer polishes below the level of the glass particles, which constantly pop out of the surface leaving holes in their place.  This leads to a surface which, on a microscopic level, looks like a series of craters interspersed with boulders.
  • Large particles are relatively easily dislodged from the surface of the restoration during function exposing the relatively soft acrylic polymer which wears away exposing more filler particles which again pop out ad infinitum.  This tendency to abrade away makes macrofils unsuitable for posterior restorations.

  The first macrofill appeared on the market in the mid 1960's.  Most older dentists affectionately remember it by its brand name, Adaptic.  Adaptic had the additional disadvantage of containing no radiopaque materials which made it hard to distinguish from decay on x-rays.

• Microfill composites---Microfill composites use particles of very small size as a filler, about .04-.5 microns in diameter.  The very small end of this range is called a colloidal silica and is produced by "burning" silica compounds in an oxygen and hydrogen atmosphere to form macromolecular structures which fall into this size range.  This type of composite was invented to overcome the esthetic liabilities of the macrofils.  Microfill composites polish beautifully and can be formulated to be quite translucent.

  Unfortunately, the smaller the particle size, the fewer of them you can stuff into the composite because it becomes too stiff to work with.  A smaller particle has a relatively greater surface area in relationship to its volume than a bigger one.  In order to include many small particles in a composite mixture, their total surface area increases. As friction is a function of involved surface area, the increased surface increases internal friction and makes the composite so stiff that it cannot be manipulated.  According to Phillips Science of Dental Materials, "Colloidal silica particles, because of their extremely small size, have extremely large surface areas ranging from 50 to 400 square meters per gram."

  Therefore, due to its relatively low filler content,  this type of composite is weaker than composites with larger particle size, and has a relatively greater shrinkage during setting.  Microfills are only 35 to 50 percent by weight filler particles.  Microfils are used for small fillings in front teeth.  They are also used for direct veneers on front teeth because of their superior polishability.

  Microfil composites have  three main disadvantages.

  • Due to the relatively low density of filler particles, microfils are not as strong as composites with larger particle size, especially on the incisal edges of front teeth where the bulk of material is likely to be fairly small.
  • Also due to low density of filler particles, microfils are more prone to shrinkage while setting, and this limits their use in large, bulky fillings.
  • Due to the relatively high level of acrylic matrix material, microfills tend to be quite translucent which gives them an overall tendency to cast a slightly gray hue. 

  In order to overcome these limitations, it used to be common practice to use a layer of microfil composite over a bulk of macrofil in order to correct the hue problem and increase the strength of the structure to be built with it.  The microfil's purpose in this case is to lend the restoration a more polishable finish, and a translucent enamel-like appearance.  The purpose of the underlying macrofil is to give the restoration strength and reduce shrinkage stresses.

  Microfill composites are not generally used for posterior fillings because of the relatively unfilled nature of the material.  The relatively large amount of acrylic matrix wears too much when subjected to the stresses of grinding and chewing.

   

• Hybrid composites--- Hybrids contain a range of particle sizes ranging from 0.6 to 1 micrometers.  Developed in the late 1980's, these composites achieve between 70 to 75 percent by weight of filler particles.  The first generation hybrids achieved excellent wear characteristics which made them acceptable as posterior filling materials.  They also had fair polishability.  The second generation of hybrids achieved greater polishability and superior color optics by using uniformly cut small filler particles between the larger particles, as well as resin hardeners which help to maintain a surface polish during prolonged function.  Hybrids also have unique color reflecting characteristics which gives them a chameleon-like appearance.  In other words, these materials are able to emit their own color as well as absorb color from the surrounding and underlying tooth structure.  Hybrid composites are today the workhorse of the modern dentist.  They are used in nearly all anterior restorations, and are becoming commonplace in posterior restorations as well.

• Microhybrid composites---Microhybrids are similar to regular hybrids except that they employ microfil particles  (very fine colloidal silica particles, approx 0.04 microns) to fill in between the larger particles.  The extremely small filler particles lend superior polishability and allow for finer color characterization, while the composite, as a whole, remains about 70% -75% filled.  This formulation comes closest to the surface characteristics of microfill composites while maintaining the durability and strength of standard hybrids.  Microhybrids are formulated to be layered, and some of the shades are opaque which mask the gray of the more translucent shades.  Microhybrids are stiffer than standard hybrids, and do not slump, so they are often more appropriate for rebuilding large areas of a tooth freehand.  On the downside, they do not flow as easily as standard hybrids, and it can be difficult to get them to flow into marginal areas and tight corners.  The availability of opaque shades allow for better masking of the gray color that is visible when microfill composites are used to close diastema (spaces between the teeth).  Microhybrids can also be used for posterior restorations.

• Flowable composites---This composite restorative is formulated with a range of particle sizes about the same as hybrid composites.  The amount of filler is reduced and the amount of unfilled resin matrix material is increased.  This makes for a very loose mix.  It is delivered into a cavity using a syringe.  It flows freely over the inside surface of the cavity preparation.  This material has made it possible to fill small cavities in the tops of teeth without a shot since the area of decay is often small enough to be removed with little or no sensation in the tooth, and the flowable composite will bond even if there are no undercuts in the cavity preparation.  Flowable composites are often used to seal the dentin of a tooth prior to placing the filling material.  Due to the low level of filler particles, flowable composites are more prone to shrinkage, so they are generally not used by themselves to fill large cavities.

• Resin (Composite) Cements---When formulated as loose, sticky, chemically cured substances (i.e. with a separate catalyst that is manually mixed into the base at the time of use), filled resins make remarkably strong cements for crowns, veneers, onlays, posts, Maryland bridges, orthodontic brackets and other bonded appliances. Since both porcelain and tooth structure can be etched with acids, the resin component can flow into the microscopic irregularities in the appliances to be cemented as well as the irregularities etched into the tooth structure.  This etched bond is, by itself, quite strong, however the presence of the filler particles adds a second "lock and key" type of mechanism to help cement the appliance as well.   

Resin modified glass ionomers

Resin modified glass ionomers are glass ionomer cements that contain a small quantity of a polymerizable resin component.  These  materials have most of the advantages of glass ionomer materials with the added advantage of water insolubility while setting. These materials are always dispensed in two component systems and begin hardening only when both components are mixed together.  The resins included in some systems have dual curing capability, which means that they will cure chemically once the pastes are mixed, but the curing can be accelerated by the use of high intensity light.  The ability to light cure the excess material reduces chair time. 

• Resin modified glass ionomer cements
  • These are a real success story in dentistry.  Resin modified glass ionomer cements have become the standard material used to cement metal and zirconia based crowns and bridges onto prepared teeth.  They reduce post operative sensitivity and reduce the likelihood of cement washout.  They chemically bond to both the metal and the tooth structure.  They have much less shrinkage on setting than resin based composites.  They are also easy to use and simple to mix, unlike zinc phosphate cement which was the industry standard up until the introduction of these cements.
• Resin modified glass ionomer restoratives
  • These are used mostly as bases under composite resin restorations.  They lack the ability to resist occlusal wear, but their major virtue is that they shrink very little while setting and thus reduce post operative sensitivity while reducing compressive stresses on the tooth.  They also release fluoride into the tooth structure.  They are also useful for filling cavities around the gum line.  In this capacity they leach fluoride into the tooth throughout their service life thus reducing the likelihood of recurrent decay.

The Compomers (polyacid-modified resin composites)

A compomer is really a modified composite resin.  These materials have two main constituents: A resin modified with dimethacrylate monomer(s) with two carboxylic groups present in their structure, and a filler that is similar to the ion-leachable glass present in glass ionomer cements. The filler particles are only partially silanated to help the adhesion of the resin to the glass particles, while at the same time allowing some of the soluble fluoride in the glass to leach out into the tooth structure.  When first marketed, it was claimed that the carboxylic groups in the resin would allow adhesion to tooth structure without the acid etch bonding technique, similar to glass ionomer cements.  This turned out to be a false assertion.  Even so, compomers are still popular with dentists for filling deciduous (baby) teeth, and, due to their high degree of translucency, they are highly esthetic when used for the repair of cervical (gum line) caries.  They confer a degree of fluoride release into the tooth, although less than that found in glass ionomer cements.  Thus, at least in the short term, they prevent recurrent decay while allaying parents' concern about the presence of mercury in standard amalgam fillings.  They do not have the surface durability of standard composite resins, but will wear quite well for the life of a deciduous tooth.  Unlike glass ionomer restorations, they do NOT adhere to tooth structure without an acid etch bonding technique.   They are esthetically pleasing and seem to resist recurrent decay for several months after placement when used to fill cavities near the gum line.

• Paste compomer restorative (filling) material; These materials are excellent tooth colored filling materials when used on front teeth in non stress bearing areas, such as for filling cavities at the gum line, or in larger restorations if they are fully supported by natural tooth structure and do not involve incisal or occlusal surfaces.  They are especially good on the buccal or labial (front) surfaces of teeth where esthetics is extra important.  They are often used to cover exposed, sensitive root structure on both front and back teeth.  

In spite of the fact that they are less wear resistant than regular composites, some dentists use light activated compomers to  fill baby teeth due to their extended fluoride release, and also to allay parents' fears about the mercury in amalgam fillings.  The baby teeth generally exfoliate (fall out) before the wear becomes a problem.  Compomers are also useful in geriatric dentistry since oral hygiene is often poor in elderly patients, and they frequently suffer xerostomia (dry mouth).  The combination of poor oral hygiene and dry mouth causes rampant decay in these patients, and the constant release of fluoride at the tooth/restorative junction can be helpful to prevent recurrent decay.

• Flowable compomers; These are like the paste compomer restorative, but they contain much more of the unfilled resin.  They are used in the same fashion as flowable composites, except they are rarely used in stress bearing areas such as the occlusal surfaces of adult teeth. 

A note on radiopacity of dental materials

X-rays are an essential part of dental diagnosis, and it is very important that any material that remains implanted in any part of the patient's body, including his teeth, be radiographically distinguishable from natural structures or disease processes.  In other words, any material or device implanted in teeth or in any other part of the body must be visible on an x-ray.  Materials like amalgam, gold and titanium (for implants or posts) are made of metal and are naturally radiopaque (ie. they block x-rays and cast a white shadow on s-ray film).

Materials like restorative composites, porcelain, or various dental cements are not inherently radiopaque and without modification of their composition, would not be visible on an x-ray film except as a dark spot if deposited in bone or tooth structure.  Unfortunately, decay in teeth shows up as a dark area on an x-ray film, and in the early days of composite technology, before the addition of radiopacifiers, it was often difficult to distinguish between a composite filling or an area of decay in a tooth when looking at an x-ray.   The addition of zirconium dioxide, barium oxide or  Ytterbium oxide to any radiolucent (the oposite of radiopaque) material will impart the property of radiopacity.  These three oxides are chosen for their compatibility with the chemistry of composites.  Note that Barium Sulfate is used as a "milkshake" or enema  when taking medical x-rays for the observation of the gastro-intestinal tract. 

The addition of radiopacifiers is especially important in the production of dental cements used to lute crowns and bridges.  Even though the cement will spend its lifetime under the crown, excess cement will be forced out from between the crown and the tooth during placement, and often end up between the teeth or under the gums where it cannot be seen by direct observation.  When this happens, it can cause inflammation of the gums and even eventual loss of the tooth.  As long as the cement is visible on the x-ray, it will reveal the presence of the cement so that it can be removed.