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,
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
All non metallic composite filling
materials are really just more highly filled versions of their respective
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
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.
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
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.
to see an entire page devoted to the composition and
manufacture of cast metal dental alloys. This page is
meant for dental professionals and materials scientists and
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.
Acid etch enamel conditioning
this technique, a 10% solution of phosphoric acid is placed on the
portions of the tooth and left in place for fifteen seconds. When
it is washed off, the formerly shiny enamel surface now looks like it is
chalky, or frosted. Under a microscope, the surface looks like a
ragged landscape of jagged mountains and valleys (see micrograph to the
right). These microscopic irregularities are then filled with a
liquid acrylic plastic which hardens in place. Since the filling
material is composed of the same sort of plastic, mixed with
glass particles (see
filled resins below) it will
bond onto the plastic which becomes mechanically adhered to the conditioned
enamel. Click the image to learn more about the structure of
micrograph on the left shows what dentin looks like when it is sliced
perpendicularly to the
tubule openings are clearly
visible, but the hard material between them is still fairly smooth and
will not bond to a layer of liquid plastic in the same way as it does
to etched enamel. Etching the dentin dissolves a small amount of
the hard dentin material around the tubules allowing the strands of
collagen that permeate the dentin to project beyond the cut surface, and
partially opening up the
(image to the right). An aqueous solution of
2-hydroxyethyl methacrylate (HEMA)--a hydophylic (water soluble)
polymer (plastic)--is applied to the conditioned dentin. This
material flows into the tubules and between the exposed collagen fibers. This acts as a bridge between the otherwise hydophylic collagen fibers and a subsequent layer of hydrophobic (water
insoluble) resin, allowing the resin to thoroughly infiltrate between
the collagen fibers. Once the resin hardens, it serves as the basis
of dentinal bonding. Click either image to learn
more about the structure of dentin.
Certain materials such as
and polycarboxylate cements may be applied directly to unconditioned
enamel and dentin. They are applied in a liquid form, and this
liquid is fairly acidic. Metallic polyalkenoate salts combine with the hydroxyapatite by
replacing phosphate ions. The carboxylic groups of the
polyalkenoic chains can chelate (chemically combine with) the calcium of
the hydroxyapatite to bond the cement to both dentin and enamel. This cross linking of restorative
material and tooth structure gives excellent chemical bonding
The bonding of a dental amalgam to a tooth involves any
or all three of the above mechanisms to bond a
resin cement to the tooth structure and a mechanical mechanism to
bind the amalgam to the resin. The enamel and dentin are
conditioned with 10% phosphoric acid, HEMA is applied to the dentin for
dentinal bonding, and a layer of very loose filled resin is applied over
the tooth structure. Dental amalgam is condensed into the
tooth while the resin is still unset. This causes tags of amalgam
and filled resin to intermingle at the interface, and when both
materials set, they are securely mechanically locked
together. Thus the amalgam is locked to the resin, and the resin
is bonded to the tooth.
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
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
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
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
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
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
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
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
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.
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.
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
- This means that they can be applied directly to
clean tooth structure without etching or bonding or even cutting
- 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
- 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
Silicate cement is made by mixing a powder made of
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
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
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
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
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
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
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
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
- 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
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
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.
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
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
|For all practical purposes, patients rarely notice a
difference in the appearance of a restoration done with hybrid
composite versus one done with microhybrids. The decision
of which type of composite to use on any given restoration is
made by the dentist on the basis of practical considerations.
Thus patients should not be overly concerned with the
particulars of the materials used. His/her only
considerations should be the skill of the dentist and the
quality of care.
See my page on dental practices
for more information concerning this important point.
- 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.
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
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.
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
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
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
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
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.