Table of Contents
- 1 Types of resin composites
Types of resin composites
These were the first type of resin composite marketed in the 1960’s 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 1 to 50 microns (µM) in diameter. (A micrometer, also called a micron, is a millionth of a meter, or a thousandth of a millimeter. An average grain of salt is about 60 microns.) The 1µM size is critical, since particles larger than this are visible to the naked eye. Particles 1µM and larger are called macro particles, while those smaller than 1µM are called micro particles
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. Furthermore, any filling made from resin alone would wear very rapidly in service.
The inclusion of the glass particles reduces these problems 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 when it sets. The advantage of large particle size is that large numbers of particles can be incorporated into the paste without making it too stiff to manipulate. Macrofills are 70% to 80% glass by weight, 60% to 65% by volume. Unfortunately, macrofill composites have two undesirable qualities:
Due to large particle size, macrofills are not very polishable. As a result, they feel rough and are prone to accumulation of plaque and stain. The relatively soft acrylic polymer tends to wear 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. This type of surface is prone to staining.
But wear is the major disadvantage of macrofilled composites. The constant loss of the glass particles exposes more and more of the soft plastic matrix to the abrasive forces encountered in the mouth, and the restoration slowly wears away over time.
However, the large particle size has one major advantage over small particle size. You can pack them more tightly into the resin matrix without the paste becoming too thick for the dentist to handle. This becomes more difficult to accomplish with small particle size. This is explained in detail below. More glass in the mixture reduces setting shrinkage.
A composite restoration wears exclusively because the glass particles are slowly dislodged from the surface leaving more of the soft resin matrix exposed to wear factors. If there were a way to keep the particles in place forever, the restoration would never wear down. In theory, the less acrylic and the more glass a composite contains, the better. An ideal composite filling would contain only glass, and no acrylic at all. This, of course, is impossible, since the resin is the material used to glue the silica particles together. It is also the component that gives the unpolymerized material the handling characteristics that allow the dentist to work with it in the first place.
The tendency for large glass particles to dislodge from the surface of macrofilled restorations makes them unsuitable for posterior restorations, since the occlusal (top) surfaces of the back teeth receive a lot of abrasive challenges. Any filling that wears excessively would allow the bite to change, and the teeth will move over time. In persons who brux (grind their teeth), this could cause a collapsed bite and contribute to Temperomandibular Joint Dysfunction (TMJ, or TMD).
The first macrofills appeared on the market in the mid 1960’s. Most older dentists affectionately remember them by their brand names, Adaptic and Concise. both of these products had the additional disadvantage of containing no radiopaque materials which made it difficult to distinguish from decay on x-rays.
Hybrids contain a range of particle sizes. First formulated in the 1980’s, they include about 75% conventional size particles (1-3 micron) and about 8% sub micron size (.02-.04micron) (Pictured to the right). They do not retain a high polish for long, due to the tendency of the largest particles to pop out of the surface, but they retain their easy working characteristics due to the high percentage of larger particle sizes. They are also much more resistant to wear than the older macros because of the smaller size of the particles overall, and because of the presence of the submicron particles, which are more difficult to dislodge than the larger particles. Also, they can be filled to a much higher density with glass particles than those composites containing only micro sized particles. The larger particles are necessary to keep the consistency of the paste from becoming too stiff, while the relatively small percentage of sub micron size particles take up the space between the larger particles. The highest particle density attained with hybrids is 90% by weight. Because of the high particle density, hybrids were the first composites that were promoted for posterior use, and they remain one of the most wear resistant posterior composite types on the market. The brands most familiar to dentists are Prisma APH, Herculite, Alert and P-50 (by 3-M).
MicroHybrids were the next step in hybrid evolution. They use up to three distinct particle sizes for more efficiency, and a much smaller size range of larger particles (0.6 -0.7 microns) than the older hybrids . The microhybrids achieve greater polishability but suffer from lower particle density due to the small size of the largest particles in the mix. (The reason for the this is explained below.) They also achieve 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. Microhybrids also have unique color reflecting characteristics which gives them a chameleon-like appearance. Their working characteristics are about as good as the hybrids, and their superior esthetics make them especially useful for anterior restorations. Unlike the hybrids, microhybrids are not generally recommended for posterior fillings owing to their lower particle density. However, like the hybrids and macros, their mechanical properties make them strong enough for rebuilding incisal edges on anterior teeth, and a few, such as Herculite XRV are even marketed for posterior use. Their particle size and esthetic qualities make them especially attractive for any anterior restoration. The brands dentists are most familiar with include Prisma TPH, Herculite XRV, Tetric Ceram, and Charisma Microfilled and Nanofilled composites—In dentistry, microfillers are particles that are smaller than 1 micron, while nanofillers are particles that are smaller than 0.1 micron. In reality, most of the older microfilled composites use particles that vary between .04 and .2 micron, while nanofilled composites are those that contain filler particles no larger than 0.1 micron (more generally .04-.05 micron).
Thus nanofilled composites are technically just a category of microfilled composites, although the term “nano” has come to imply the newer agglomerated microfill composites (defined below). The smallest nano particles are in a form called a colloidal silica, which is produced by burning silica compounds such as SiCl4 in an oxygen atmosphere to form spherical macromolecular structures which fall into this size range.
The characteristics of micro sized particles
A smaller particle has a relatively greater surface area in relationship to its volume than a larger one. A cube has a surface area equal to the sum of the area of its six sides. If the cube is cut in half, the two pieces together have a total surface area equal to the original cube plus the area of the two new sides created when the original cube was cut. As you continue to cut it up into smaller and smaller pieces, you continually add new surfaces to the original area of the cube. While the volume of the material you end up with is the same as the volume of the original cube, the surface area keeps expanding with each new segment created.
This fact gives micro particle sizes a major disadvantage when compared to macro sized particles. Since friction is a function of involved surface area, the increased surface area of micro particles also increases internal friction and a large volume of them included in the paste makes the composite so stiff that it becomes very difficult for the dentist to manipulate. 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.” Macrofilled composites are much easier for the dentist to handle than micros filled to the same density.
On the other hand, greater surface to volume ratio gives micro particles one major advantage over macro particles. The greater surface area, combined with the smaller volume of micro sized particles, makes them more difficult to dislodge from the plastic matrix. Furthermore, when a micro sized particle does pop out, it leaves a smaller crater behind, and affects the surface characteristics of the restoration less than the larger crater that a macro sized particle would leave behind. In other words, the more microsized particles the composite contains, the more resistant the finished composite is to wear in the mouth.
These dueling facts bring us back to square one. Macro composites can be filled to a very high degree without becoming too stiff for the dentist to work with, have minimal shrinkage, have good mechanical properties and are fine for anterior teeth, but they do not wear or polish well. Certainly, they are unsuitable for posterior applications where wear is a factor. On the other hand, highly filled micro filled composites would not only look great and resist shrinkage, but they would wear very well in any area of the mouth because of the better retention of their particles. Unfortunately, any composite that contains a very high percentage of disbursed micro and nano sized quartz particles would be so stiff that it would be impossible for the dentist to handle.
First formulated in the late 1970’s, microfilled composites were (and still are) filled to a maximum of 38% by weight, 25% by volume. Even though the particles are tiny, and thus retain better in the plastic matrix, the low density of glass particles in the micros gives them poor mechanical properties, including poor flexural, yield and tensile strength. Low fill density makes them them wear almost as badly as the macros, so they are not suitable for posterior restorations. Even so, their superior esthetic qualities keep them popular. They are used mostly to veneer over the larger particle sized macrofilled or hybrid restorations in anterior teeth to make them more polishable.
Manufacturers came up with a solution to the low density of particles in microfilled composite paste by pre-polymerizing the micro filled composite before putting it into the paste form distributed to dentists. This pre-polymerized composite can be fabricated to 80-90% by weight of nano sized glass particles using industrial machines. After the compressed composite polymerizes, it is then milled into a powder with particle sizes between 10 and 20 microns (in the range of the particle sizes in conventional macrofilled composites). This composite powder, made up of complex particles (called agglomerated microfiller or prepolymerized filler (PPF) or nanoclusters) is then mixed with resin to make the composite paste that is sold to the dentist. The final composite may contain between 70% to 80% glass particles by weight.
In other words, the dentist is supplied with a composite that is well filled with nano sized particles, yet due to the large size of the agglomerated nanoclusters is not so stiff that that it is difficult to place without leaving voids. The paste flows easily and has good wear characteristics due to the tiny size and high density of the filler particles. The particle fill-density is 72%-77% (about 61% to 64% by volume). This means that modern microfills (sometimes referred to as nanofills to differentiate them from the older, less filled microfills) wear quite well and are suitable for restorations on the occlusal surfaces of the posterior teeth.
Even though the agglomerated microfiller particles have sizes in the same range as the silica particles in the older macro composites, they do not give the final composite paste the same handling characteristics found in traditional macrofills. Each agglomerated particle is, after all, manufactured with the same plastic that is found in the liquid resin, and there is a great deal of resin between the particles. (Remember that about a third of the volume of the microfilled composite paste is resin, even when the density of the glass is 70% to 80% by weight.)
This changes the flow characteristics of the paste making it more difficult to work with than macrofilled or microhybrid composites. The major problem with microfilled composites is that they tend to be sticky, and to slump while the dentist places them. Their main advantage is their superior ability to resist wear during service and to polish to a high shine. While the viscosity of these nano composites can be adjusted by varying the size and density of the agglomerated nanoclusters, this does little to remove the sticky consistency, and only slows down the slumping.
Microfilled composites also tend to be more opaque than the other forms of resin-glass composites. This fact, along with the poorer handling characteristics make them less attractive for anterior buildups in spite of their inherent strength. The quintessential agglomerated microfilled composite is the original form of Heliomolar.
Nanohybrid composites are the newest addition to the pantheon of composite filling materials. They are becoming popular among dentists because they are advertised to have superior esthetic and wear characteristics, high polishability, and superior handling characteristics. They are marketed as universal composites because their handling and esthetic qualities make them suitable for anterior buildups, while their agglomerated nanoclusters interspersed with micro sized particles gives them very acceptable wear characteristics.
The compressive and diametral strengths, and the fracture resistance of the nanohybrid composites are equivalent to or higher than those of the other composites (hybrids, microhybrids and microfill). Nanohybrid composites show mechanical properties at least as good as those of universal hybrids and are suitable for both posterior applications (thanks to their surface hardness) and their excellent esthetic qualities and inherent strength.
These composites have three different types of filler particles:
- Prepolymerized, finely milled agglomerated nanoclusters
- Larger (sub-micron sized) glass or silica particles in the range of 0.4 micron
- individual nano-sized particles (.05 micron)
The large, agglomerated nanoclusters supply the composite with densely packed nano sized particles giving the set composite an extremely wear resistant surface, while at the same time keeping the unset composite paste fluid and easy to work with.
The micro sized particles fill in the spaces between the large agglomerated particles. The individual nano particles fill in the spaces between the micro particles.
While plain agglomerated nanofils are sticky and tend to slump, nanohybrids use the smaller, intervening particles to “take up the slack” between the larger particles and give the paste handling and esthetic characteristics that make them quite acceptable for anterior, as well as for posterior restorations. These composites have the wear characteristics of the tiny size of the particles in the agglomerated nanoclusters, as well as working characteristics and esthetics similar to the microhybrids.
The current brands of nanohybrids on the market include, Filtek LS, Filtek Supreme Plus, Esthetx HD, Grandio, Herculite Ultra, Premise, Renamel Microfill, Tetric EvoCeram, TPH3, Estelite Sigma Quick, Kalore, N’Durance, Reflexions XLS, and Hereaus Venus. For a comparison of many of these brands, please see Clinicians Report March 2010, Vol 3, Issue 3.
Older flowable composites are formulated with a range of particle sizes between 1 and 2 microns. The amount of filler is reduced (in the range of 50% by weight) and the amount of unfilled resin matrix material is increased. Newer brands are essentially “dilute” forms of nanofilled and nanohybrid composites at varying filler density to keep the mix flowable. Some brands contain Al-Fl-Si glass particles and release fluoride to the adjacent tooth structure. They are delivered into a cavity using a syringe.
Flowable composites, as their name implies, flow freely over the inside surface of the cavity preparation. This material has made it possible to fill small cavities on occlusal surfaces (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 and wear, so they are generally not used in bulk to fill large cavities.
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.