A course in Dental Composites page 9 (Composite Characteristics)-DoctorSpiller.com

A course in composites pages 1234567891011

Composite Characteristics

The criteria dentists use to evaluate the composites

The ability to retain a high polish has apparently become the major selling point in advertising campaigns for different brands of modern dental composites, but as more and more dentists switch from amalgam to posterior composites, they discover after several years that the ability to retain a high polish is far down on the list of properties they desire in their composites.

Truth be told, a large majority of production dentists do not spend much time polishing their composite restorations, preferring instead to finish them with a fine diamond or a carbide bur.  Beyond that, the newest composites are made with glass particles no larger than micro size, and the differences between their abilities to retain a high polish are marginal to say the least.

Dentists really compare the properties of composites on the basis of the following characteristics.

  • The unfilled resin (plastic) matrix of all composites shrinks as much as 3% during setting.
  • The presence of inorganic filler particles throughout the composite structure reduces the volume of the resin and thus reduces shrinkage of the restorative.
  • The higher the volume of the inorganic filler, the lower the shrinkage. Once again, the microhybrids and nanohybrids have the highest percentage of inorganic filler particles, and thus the lowest percentage of polymerization shrinkage.
  • Values of contraction for Bis GMA and Urethane dimethacrylate are typically 1.5 to 3 %.  By itself, BisGMA has relatively low shrinkage, but this is increased by the addition of the TEGDMA diluents.  Since the Urethane dimethacrylate resins do not require the addition of a diluent, they have slightly lower shrinkage values than the Bis GMA formulations.

    The ability to minimize polymerization volumetric contraction

  • The unfilled resin (plastic) matrix of all composites shrinks as much as 3% during setting.
  • The presence of inorganic filler particles throughout the composite structure reduces the volume of the resin and thus reduces shrinkage of the restorative.
  • The higher the volume of the inorganic filler, the lower the shrinkage. Once again, the microhybrids and nanohybrids have the highest percentage of inorganic filler particles, and thus the lowest percentage of polymerization shrinkage.
  • Values of contraction for Bis GMA and Urethane dimethacrylate are typically 1.5 to 3 %.  By itself, BisGMA has relatively low shrinkage, but this is increased by the addition of the TEGDMA diluents.  Since the Urethane dimethacrylate resins do not require the addition of a diluent, they have slightly lower shrinkage values than the Bis GMA formulations.

The defects produced by polymerization contraction are the following:

  • Open margins and/or white lines around margins
  • Debonding and open margins
  • Enamel cracking, especially when using strong bonding agents and acid etching techniques
  • Cuspal deflection, especially in well bonded restorations
  • Marginal staining
  • Secondary caries, especially in patients using a lot of sugar
    • Note that microleakage is less dependent on the degree of shrinkage than on the effectiveness of the bonding technique. Studies show that the degree of microleakage varied from bonding agent to bonding agent, but not the composite restorative used.
  • Post-op sensitivity:

This is due both to cuspal deflection and the tendency of the composite to shrink toward the light source resulting in the composite debonding form the pulpal floor.  This often leads to a small space into which water may enter, either by way of leakage, or by drawing fluid from the open dentinal tubules.  This micro layer of water between the pulpal floor and the restoration  accounts for much of the temperature and pressure sensitivity a patient may experience after a large cavity is bulk filled with composite.

This is one of the major reasons that glass ionomer is often used as a base under composite restorations.  Glass ionomer is much less prone to polymerization shrinkage than composite, and bonds quite well to the floor and walls of a cavity preparation. (Note: Most resin modified glass ionomers have nearly the same polymerization contraction as resin-glass composites and offer little benefit as bases under composite restorations.)

On the other hand, a dentist can avoid this problem without using a glass ionomer base by using a rather this simple technique:

  1. Etch all surfaces.  Place and cure bonding agent(s)
  2. Dry off any excess water using a stream of air
  3. Place sufficient flowable composite into prep and then using a disposable brush, brush the unset flowable material over the preparation until a very thin layer covers all parts of the prep, over any enamel as well as all dentinal surfaces.  Cure this layer.
  4. Place additional thin puddles (a fraction of a millimeter thick) of flowable composite onto the pulpal floor, and especially into deeper areas of the prep.  Cure this layer
  5. fill the preparation in steps placing bulk composite against the vertical walls of the prep and then backfilling the central areas.

Even with the greater polymerization shrinkage seen in flowable composites, the very thin layers used in this technique reduce the stresses on the pulpal floor to negligible levels.

According to Clinicians Report March 2010, Vol 3, Issue 3, the measured shrinkage (measured after five minutes of light curing) of the twelve most popular nanohybrid composite restoratives varies between 0.9% and 2.8%, while the shrinkage of the nine most popular flowable nanos varies between 3.0% and 6.0%.

The following two tables list actual shrinkage percentages of selected posterior composite restoratives and flowables (Clinicians report, March 2010, Volume 3, Issue 3):

compositeshrinkagetable

flowableshrinkagetableThe following table* lists the Maximum Polymerization Shrinkage Stress for eleven popular NanoHybrid and Microfill composite restoratives (all shade A3, stress values approximate).  Lower values are better: [Note: The remaining tables on this page were composed from the ADA Professional Product Review, Spring 2010, Volume 5, issue 2]

shrinkagestresstable

Depth of cure:

Depth of cure is extremely important to most dentists since it affects the length of time it takes to finish a large restoration.  It also affects the long term properties of the restoration since all mechanical and thermal properties are degraded when the composite fails to reach a minimum of 80% of ideal cure hardness.

The depth of cure depends on several factors:

  • The opacity of the filler particles
  • The density of the filler particles
  • The shade of the composite, darker shades having less depth of cure than lighter shades

According to Clinicians Report March 2010, Vol 3, Issue 3, the depth of cure of the latest crop of micro and nano composites varies widely, between 1.9 mm and 4 mm.

The following table lists the Total depth of cure for eleven of the most popular NanoHybrid and Microfill (Nanofill) composite restoratives (all shade A3, Depth of cure values approximate)  Higher values are better.

depthofcuretable

Workability:

Dentists want a composite that is not too sticky.  They want a composite that sticks to the tooth, but not their instruments!  If the composite is too sticky, it is difficult to be sure that it will not pull away from the walls or floor of the cavity preparation when the packing instrument is removed.

They want a composite restorative that flows easily enough to form a shaped bulk fairly easily, especially on anterior teeth, but does not slump too much.

They want a composite with the maximum depth of cure.  Dentists that begin using a composite based on other criteria may soon discover that the length of time it takes to place them is too great because of the number of increments they need to use, especially for darker or more opaque shades.

The best composites for these qualities are probably the macrofills (very few dentists still use these because of their poor wear characteristics) followed (in approximate order) by the hybrids, the microhybrids and the nanohybrids.  True nanofills are not used in dentistry because of their inherent stiffness, however microfilled composites are sometimes marked as nanofilled.

The ability to resist wear:

When it comes to wear resistance, the only things that really matter are the density of the filler particles, and the size of the particles.  The more densely packed, the less the wear, and the smaller the particle size, the better.

Hybrids do exceptionally well on density but less well on particle size.  The more recently developed ones resist wear quite well because they are highly filled.  These are still some of the most wear-resistant composites on the market.

Agglomerated nanofills (microfills) do better on particle size but less well on density of particles.  They generally exhibit poor working characteristics and shallow depth of cure, but they have the best wear characteristics compared to any of the other categories. In my experience, nanofills are somewhat more prone to fracture than other types of composites.

Nanohybrids, the newest addition to the composite pantheon  (75% to 82% filled by weight, containing agglomerated nanofil particles interspersed with micro and nano sized individual particles).  They have good working characteristics, and wear resistance nearly as good as the microfills.  They also cure to a greater depth than the agglomerated nanofils and are less prone to fracture in unsupported areas.

From the point of view of occlusal wear, amalgam beats all composites hands down.  In a clean mouth with minimal sugar exposure, a well placed (posterior) amalgam can last 20 or 30 years showing minimal wear.  In the same mouth, even the most wear resistant composite placed carefully may wear considerably within 5 to 10 years, especially if the patient bruxes or habitually eats very abrasive foods.

This does not imply that amalgam is superior in other respects to a properly placed posterior composite restoration.  The superior wear characteristics of amalgam may, in fact, mask its deficiencies such as marginal leakage and occult areas of recurrent caries.

The ability of the composite to match the coefficient of thermal expansion of dentin:

  • The reason this is a desirable characteristic is that the tooth and the restoration expand and contract at different rates when the patient eats or drinks hot and cold foods.  The larger the mismatch, the greater the likelihood that there will be percolation of fluids down the margins.  This leakage can result in staining at the margins of the filling, or even in caries if the foods that cause the expansion/contraction contain a heavy concentration of sugar.
  • If the bond between the tooth structure and the restoration is extremely strong, the constant expansion and contraction of the restoration can theoretically place stress on the tooth structure resulting in cracking of the enamel or sensitivity of the pulp to hot and cold stimuli.
  • In fact, manufacturers have about reached the limit of improvement of this property in the latest iteration of hybrids and nanohybrids.  The only way to reduce the coefficient of thermal expansion of a composite is to increase the density of the filler particles, and until a breakthrough in technology creates another class of composite entirely, no major improvements in this property can be expected.
  • At this time, the highest density I have seen is 90% by weight for one of the hybrids.
  • No composite can match the coefficient of thermal expansion of dentin since the coefficient of thermal expansion of dentin is 9 ppm/°C,while the coefficient of the most highly filled nanohybrid composite is approximately 30 ppm/°C. The agglomerated nanofills rate worse at 60  ppm/°C.   (Dental amalgam is rated at 25 ppm/°C, and unfilled acrylic rates at 90 ppm/°C.)

Mechanical properties of dental composites

The other mechanical properties of dental composites are all related directly the the density of the filler particles in the mix.  The higher the density, the better the properties.

The following table lists the flexural strength for eleven of the most popular NanoHybrid and Microfill composite restoratives (all shade A3, strength values approximate) Flexural strength measures the ability to resist fracture when subjected to bending.  It predicts a restoration’s ability to resist occlusal load without cracking.  higher values are better:

strengthtableThe following table lists the fracture toughness for eleven of the most popular NanoHybrid and Microfilled composite restoratives (all shade A3, toughness values approximate). fracture toughness measures the ability to resist crack propagation.  Higher values are better.

toughnesstable

* ADA Professional Product Review Spring 2010, Volume 5, issue 2

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