Dental composite resins are types of synthetic resins which are used in dentistry as restorative
material or adhesives. Synthetic resins evolved as restorative materials since they were insoluble,
aesthetic, insensitive to dehydration, easy to manipulate and reasonably inexpensive.
Composite resins are most commonly composed of Bis-GMA and other dimethacrylate monomers,
a filler material such as silica and in most current applications, a photoinitiator. Dimethylglyoxime
is also commonly added to achieve certain physical properties such as flow ability.
Further tailoring of physical properties is achieved by formulating unique concentrations
of each constituent. Many studies have compared the longevity of composite restorations to
the longevity of silver-mercury amalgam restorations. Depending on the skill of the dentist, patient
characteristics and the type and location of damage, composite restorations can have
similar longevity to amalgam restorations. In comparison to amalgam, the aesthetics of
composite restorations are far superior.
History of use Initially, composite restorations in dentistry
were very prone to leakage and breakage due to weak compressive strength. In the 1990s
and 2000s, composites were greatly improved and have a compression strength sufficient
for use in posterior teeth. Method and Clinical Application
Today's composite resins have low polymerization shrinkage and low coefficients of thermal
shrinkage, which allows them to be placed in bulk while maintaining good adaptation
to cavity walls. The placement of composite requires meticulous attention to procedure
or it may fail prematurely. The tooth must be kept perfectly dry during placement or
the resin will likely fail to adhere to the tooth. Composites are placed while still in
a soft, dough-like state, but when exposed to light of a certain blue wavelength, they
polymerize and harden into the solid filling. It is challenging to harden all of the composite,
since the light often does not penetrate more than 2–3 mm into the composite. If too
thick an amount of composite is placed in the tooth, the composite will remain partially
soft, and this soft unpolymerized composite could ultimately lead to leaching of free
monomers with potential toxicity and/or leakage of the bonded joint leading to recurring dental
pathology. The dentist should place composite in a deep filling in numerous increments,
curing each 2–3 mm section fully before adding the next. In addition, the clinician
must be careful to adjust the bite of the composite filling, which can be tricky to
do. If the filling is too high, even by a subtle amount, that could lead to chewing
sensitivity on the tooth. A properly placed composite is comfortable, aesthetically pleasing,
strong and durable, and could last 10 years or more.
The most desirable finish surface for a composite resin can be provided by aluminum oxide disks.
Classically, Class III composite preparations were required to have retention points placed
entirely in dentin. A syringe was used for placing composite resin because the possibility
of trapping air in a restoration was minimized. Modern techniques vary, but conventional wisdom
states that because there have been great increases in bonding strength due to the use
of dentin primers in the late 1990s, physical retention is not needed except for the most
extreme of cases. Primers allow the dentin's collagen fibers to be "sandwiched" into the
resin, resulting in a superior physical and chemical bond of the filling to the tooth.
Indeed, composite usage was highly controversial in the dental field until primer technology
was standardized in the mid to late 1990s. The enamel margin of a composite resin preparation
should be beveled in order to improve aesthetics and expose the ends of the enamel rods for
acid attack. The correct technique of enamel etching prior to placement of a composite
resin restoration includes etching with 30%-50% phosphoric acid and rinsing thoroughly with
water and drying with air only. In preparing a cavity for restoration with composite resin
combined with an acid etch technique, all enamel cavosurface angles should be obtuse
angles. Contraindications for composite include varnish and zinc oxide-eugenol. Composite
resins for Class II restorations were not indicated because of excessive occlusal wear
in the 1980s and early 1990s. Modern bonding techniques and the increasing unpopularity
of amalgam filling material have made composites more attractive for Class II restorations.
Opinions vary, but composite is regarded as having adequate longevity and wear characteristics
to be used for permanent Class II restorations. Whether composite materials last as long or
has the leakage and sensitivity properties when compared to Class II amalgam restorations
was described as a matter of debate in 2008. Composition
As with other composite materials, a dental composite typically consists of a resin-based
oligomer matrix, such as a bisphenol A-glycidyl methacrylate or urethane dimethacrylate, and
an inorganic filler such as silicon dioxide. Compositions vary widely, with proprietary
mixes of resins forming the matrix, as well as engineered filler glasses and glass ceramics.
The filler gives the composite wear resistance and translucency. A coupling agent such as
silane is used to enhance the bond between these two components. An initiator package,
phenylpropanedione or lucirin) begins the polymerization reaction of the resins when
external energy is applied. A catalyst package can control its speed.
Advantages Advantages of composites:
Aesthetics: The main advantage of a direct dental composite over traditional materials
such as amalgam is improved aesthetics. Composites can be in a wide range of tooth colors allowing
near invisible restoration of teeth. Composite fillings can be closely matched to the color
of existing teeth. Bonding to tooth structure: Composite fillings
chemically bond to tooth structure. This strengthens the tooth's structure and restores its original
physical integrity. The discovery of acid etching of teeth to allow a micromechanical
bond to the tooth allows good adhesion of the restoration to the tooth. Very high bond
strengths to tooth structure, both enamel and dentin, can be achieved with the current
generation of dentin bonding agents. Tooth-sparing preparation: The fact that composite
fillings are glued to the tooth means that unlike amalgam fillings, there is no need
for the dentist to create retentive features destroying healthy tooth. Unlike amalgam,
which just fills a hole and relies on the geometry of the hole to retain the filling,
composite materials are bonded to the tooth. In order to achieve the necessary geometry
to retain an amalgam filling, the dentist may need to drill out a significant amount
of healthy tooth material. In the case of a composite restoration, the geometry of the
hole is less important because a composite filling bonds to the tooth. Therefore less
healthy tooth needs to be removed for a composite restoration.
Less-costly and more conservative alternative to dental crowns: In some situations, a composite
restoration may be offered as a less-expensive alternative to a dental crown, which can be
a very expensive treatment. Installation of a dental crown usually requires removal of
significant healthy tooth material so the crown can fit over or into the natural tooth.
Composite restoration conserves more of the natural tooth.
Alternative to tooth removal: Because a composite restoration bonds to the tooth and can restore
the original physical integrity of a damaged or decayed tooth, in some cases composite
restoration can preserve a tooth that might not be salvageable with amalgam restoration.
For example, depending on the location and extent of decay, it might not be possible
to create a void of the geometry necessary to retain an amalgam filling.
Versatility: Composite fillings can be used to repair chipped, broken or worn teeth which
would not be repairable using amalgam fillings. Repairability: In many cases of minor damage
to a composite filling, the damage can be easily repaired by adding additional composite.
An amalgam filling might require complete replacement.
Reduced quantity of mercury released to the environment: Composites avoid mercury environmental
contamination associated with dentistry. When amalgam fillings are drilled for height adjustment,
repair or replacement, some mercury-containing amalgam is inevitably washed down drains.
When amalgam fillings are prepared by dentists, improperly disposed excess material may enter
landfills or be incinerated. Cremation of bodies containing amalgam fillings releases
mercury into the environment. Reduced mercury exposure for dentists: Preparing
new amalgam fillings and drilling into existing amalgam fillings exposes dentists to mercury
vapor. Use of composite fillings avoids this risk, unless the procedure also involves removing
an existing amalgam filling. A review article found studies indicating that dental work
involving mercury may be an occupational hazard with respect to reproductive processes, glioblastoma,
renal function changes, allergies and immunotoxicological effects.
Lack of corrosion: Although corrosion is no longer a major problem with amalgam fillings,
resin composites do not corrode at all. Disadvantages
Composite shrinkage and secondary caries: In the past, composite resins suffered significant
shrinkage during curing, which led to inferior bonding interface. Shrinkage permits microleakage,
which, if not caught early, can cause secondary caries, the most significant dental disadvantage
of composite restoration. In a study of 1,748 restorations, risk of secondary caries in
the composite group was 3.5 times risk of secondary caries in the amalgam group. Good
dental hygiene and regular checkups can mitigate this disadvantage. Most current microhybrid
and nanohybrid composites have a polymerization shrinkage that ranges from 2% to 3.5%. Composite
shrinkage can be reduced by altering the molecular and bulk composition of the resin. In the
field of dental restorative materials, reduction of composite shrinkage has been achieved with
some success. Among the newest materials, silorane resin exhibits lower polymerization
shrinkage, compared to the dimethacrylates. Durability: In some situations, composite
fillings may not last as long as amalgam fillings under the pressure of chewing, particularly
if used for large cavities. Chipping: Composite materials can chip off
the tooth. Skill and training required: Successful outcomes
in direct composite fillings is related to the skills of the practitioner and technique
of placement,. For example, a rubber dam is rated to be important to achieve low fracture
rates and a longevity similar to amalgam in the more demanding proximal Class II cavities.
Need to keep working area in mouth completely dry: The prepared tooth must be completely
dry when the resin material is being applied and cured. Posterior teeth are difficult to
keep dry. Keeping the prepared tooth completely dry can also be difficult for any work involving
treatment of cavities below the gumline. Time and expense: Due to the sometimes complicated
application procedures and the need to keep the prepared tooth absolutely dry, composite
restorations may take up to 20 minutes longer than equivalent amalgam restorations. Longer
time in the dental chair may test the patience of children, making the procedure more difficult
for the dentist. Due to the longer time involved, the fee charged by a dentist for a composite
restoration may be higher than for an amalgam restoration.
Limited insurance coverage: Some dental insurance plans may provide reimbursement for composite
restoration only on front teeth where amalgam restorations would be particularly objectionable
on cosmetic grounds. Thus, patients may be required to pay the entire charge for composite
restorations on posterior teeth. For example one dental insurer states that most of their
plans will pay for resin fillings only "on the teeth where their cosmetic benefit is
critical: the six front teeth and on the facial surfaces of the next two teeth." Even if charges
are paid by private insurance or government programs, the higher cost is incorporated
in dental insurance premiums or tax rates. Direct dental composites
Direct dental composites are placed by the dentist in a clinical setting. Polymerization
is accomplished typically with a hand held curing light that emits specific wavelengths
keyed to the initiator and catalyst packages involved. When using a curing light, the light
should be held as close to the resin surface as possible, a shield should be placed between
the light tip and the operator's eyes. Curing time should be increased for darker resin
shades. Light cured resins provide denser restorations than self-cured resins because
no mixing is required that might introduce air bubble porosity.
Direct dental composites can be used for: Filling cavity preparations
Filling gaps between teeth using a shell-like veneer or
Minor reshaping of teeth Partial crowns on single teeth
Indirect dental composites Indirect composite is cured outside the mouth,
in a processing unit that is capable of delivering higher intensities and levels of energy than
handheld lights can. Indirect composites can have higher filler levels, are cured for longer
times and curing shrinkage can be handled in a better way. As a result, they are less
prone to shrinkage stress and marginal gaps and have higher levels and depths of cure
than direct composites. For example, an entire crown can be cured in a single process cycle
in an extra-oral curing unit, compared to a millimeter layer of a filling.
As a result, full crowns and even bridges can be fabricated with these systems.
Indirect dental composites can be used for: Filling cavities in teeth, as fillings, inlays
and/or onlays Filling gaps between teeth using a shell-like
veneer or Reshaping of teeth
Full or partial crowns on single teeth Bridges spanning 2-3 teeth
A stronger, tougher and more durable product is expected in principle. But in the case
of inlays, not all clinical long-term-studies detect this advantage in clinical practice.
Longevity and clinical performance Direct composite vs amalgam
Clinical survival of composite restorations placed in posterior teeth are in the range
of amalgam restorations, with some studies seeing a slightly lower or slightly higher
survival time compared to amalgam restorations. Improvements in composite technology and application
technique make composites a very good alternative to amalgam, while use in large restorations
and in cusp capping situations is still debated. According to a 2012 review article by Demarco
et al. covering 34 relevant clinical studies, "90% of the studies indicated that annual
failure rates between 1% and 3% can be achieved with Class I and II posterior [rear tooth]
composite restorations depending on the definition of failure, and on several factors such as
tooth type and location, operator [dentist], and socioeconomic, demographic, and behavioral
elements." This compares to a 3% mean annual failure rate reported in a 2004 review article
by Manhart et al. for amalgam restorations in posterior stress-bearing cavities.
The Demarco review found that the main reasons cited for failure of posterior composite restorations
are secondary caries, fracture, and patient behavior, notably bruxism Causes of failure
for amalgam restorations reported in the Manhart et al.review also include secondary caries,
fracture, as well as cervical overhang and marginal ditching. The Demarco et al. review
of composite restoration studies noted that patient factors affect longevity of restorations:
Compared to patients with generally good dental health, patients with poorer dental health
experience higher rates of failure of composite restorations due to subsequent decay. Socioeconomic
factors also play a role: "People who had always lived in the poorest stratus [sic][stratum?]
of the population had more restoration failures than those who lived in the richest layer."
The definition of failure applied in clinical studies may affect the reported statistics.
Demarco et al note: "Failed restorations or restorations presenting small defects are
routinely treated by replacement by most clinicians. Because of this, for many years, the replacement
of defective restorations has been reported as the most common treatment in general dental
practice..." Demarco et al observe that when both repaired and replaced restorations were
classified as failures in one study, the Annual Failure Rate was 1.9%. However, when repaired
restorations were reclassified as successes instead of failures, the AFR decreased to
0.7%. Reclassifying repairable minor defects as successes rather than failures is justifiable:
"When a restoration is replaced, a significant amount of sound tooth structure is removed
and the preparation [i.e. hole] is enlarged". Applying the narrower definition of failure
would improve the reported longevity of composite restorations: Composite restorations can often
be easily repaired or extended without drilling out and replacing the entire filling. Resin
composites will adhere to the tooth and to undamaged prior composite material. In contrast,
amalgam fillings are held in place by the shape of the void being filled rather than
by adhesion. This means that it is often necessary to drill out and replace an entire amalgam
restoration rather than add to the remaining amalgam.
Direct vs indirect composites It might be expected that the costlier indirect
technique leads to a higher clinical performance, however this is not seen in all studies. A
study conducted over the course of 11 years reports similar failure rates of direct composite
fillings and indirect composite inlays. Another study concludes that although there is a lower
failure rate of composite inlays it would be insignificant and anyway too small to justify
the additional effort of the indirect technique. Also in the case of ceramic inlays a significantly
higher survival rate compared to composite direct fillings can not be detected.
In general, a clear superiority of tooth colored inlays over composite direct fillings could
not be established by current review literature. See also
Dental restorative materials References