Table of Contents
- 1 Resin-glass composites (filled resins) The Basics
The most widely used tooth colored filling materials in use today are the resin (plastic)-glass reinforced composites. These restoratives, like the composites discussed on the previous page, are composed of:
- A powdered filler material (in this case glass or quartz particles)
- A hard plastic resin matrix which binds them together. The most frequently used plastic resin is a form of acrylic known as bisphenol-A glycidyl methacrylate, commonly referred to as BIS-GMA. This material is in a viscous liquid form until it is cured either by the addition of a peroxide catalyst or by applying a light source to a pre-catalyzed form of BIS-GMA.
Prior to the late 1980s, the glass particles were pre-mixed with the acrylic liquid into a paste. When the dentist was ready to place the restoration in the tooth, he or she mixed a catalyst into the paste and this causes the acrylic to harden around the glass particles. Thus the material resembled a refined version of fiber glass or auto body putty.
As an alternative, the catalyst may be pre-mixed into the paste by the manufacturer, 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 earliest plastics were actually invented in the early 19th century. They included vulcanized rubber (1837), gutta percha (1843), shellac (The history of shellac goes back 3000 years. It was used, mixed with slate dust, to make phonograph records until vinyl was perfected in 1949 and used to make 45’s.), polystyrene (1839–Today known mainly for its use as Styrofoam), rayon (1884) and cellophane (1908). Acrylic was first invented in 1931, but was unsuitable as a dental restoration because the polymerization of the monomer into solid plastic resulted in a total volumetric shrinkage of almost 25%. Later improvements, including using larger molecules and particles of prepolymerized plastic reduced this shrinkage to workable dimensions.
- The unfilled resin is prone to abrasive wear. Within a year or two of placement, an unfilled resin restoration may wear catastrophically, depending on the abrasive challenges it faces in the mouth.
- Unfilled resin shrinks on the order of as much as 3% while it is setting.
- If used by itself, without the glass filler particles, the resin would shrink away from the walls of the cavity preparation.
- If used without bonding techniques, this would create large spaces between the filling and the tooth, and the filling would always leak.
- If used with modern bonding techniques, this can lead to cracks in the adjacent cavity preparation walls.
If bonding techniques are used, the curing shrinkage of the unfilled resin would cause intolerable stresses on the tooth, drawing the edges of the cavity preparation together and causing fractures in the structure of the tooth itself.
The addition of substantial amounts of rigid glass filler prevents most of the shrinkage associated with the resin.
All the resins used in composite materials are based in methacrylate monomers. Early formulations used simple methyl methacrylate, but most resins now use dimethacrylates because they undergo less contraction on setting and have a more highly crosslinked three dimensional structure .
- Bis GMA
- Urethane Dimethacrylate
- Tri-ethylene glyycol dimethacrylate
The third resin, tri-ethylene glycol dimethacrylate (TEGDMA) is a co-monomer used to control the viscosity of the Bis GMA which by itself has the consistency of thick corn syrup and is difficult to mix with the filler.
Other matrix components include an initiator (e.g., benzoyl peroxide for chemical activation or camphoroquinone for visible light activation), coinitiators, polymerization inhibitors (to extend working time and storage stability), and various pigments.
The setting contraction of Bis GMA and urethane dimethacrylate is considerably smaller than that of unfilled plain (methyl methacrylate) acrylic resins because the dimethacrylate monomer and co-monomer molecules are are larger. The larger monomer molecules affect the three dimensional structure of the polymer. Values of contraction for Bis GMA and Urethane dimethacrylate are typically 1.5 to 3 % as opposed to 6% for methyl methacrylate acrylic polymer resin. By itself, BisGMA has relatively low shrinkage, but this is increased by the addition of the TEGDMA diluents (technical term for dilutent). Since the Urethane dimethacrylate resins do not require the addition of a diluent, they have slightly lower shrinkage values than the Bis GMA formulations.
A third resin system called silorane, is obtained from the reaction of oxirane and siloxane molecules. The advantage to this system is that it produces less shrinkage than either Bis GMA or urethane dimethacrylate based systems. This system reduces shrinkage by opening the oxirane ring during the polymerization process. Filtek™ LS (3M ESPE,) is a silorane-based composite material. The major disadvantage of silorane based systems is that system-specific bonding agent must be used to achieve the same bond strengths as are obtained using the standard systems.
Unlike the glass ionomer and silicate restoratives discussed on the previous pages, the composition of the hard, plastic matrix in resin-glass composites does not depend upon a chemical reaction between an acid and the glass particles. This means that the particles used in resin based composites do not need to be soluble in acidic solutions. It also means that unlike the glass particles in the silicates and glass ionomer, there is no inherent bond between the glass particles and the surrounding matrix.
In order to increase the retention of the filler particles in the resin matrix, manufacturers coat the filler particles with a coupling agent. The most commonly used agent is a silicone-containing molecule called γmethaacrylopropyltrimethoxysilane. It has the following chemical formula:
As the last seven letters of the name implies, this molecule is in a broad class of molecules called silanes. Silanes are double sided molecules with a silicone molecule in the center, one or more oxygen atoms one one side of the silicone, and an organic radical on the other. The oxygenated silicone side adheres to the glass particles, and the organic radical adheres to the dimethacrylate resin, strengthening the bond between the glass and the resin by adding a chemical, as well as a mechanical bond between them. Read more about silanes by clicking here.
The composition of the glass
The fact that the glass particles do not have to react chemically 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 aesthetic 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 aesthetics. Different formulations allow for particles of differing size for different restorative situations. Particle size and shape may be varied to allow for differing consistencies, with huge ramifications for strength and wear characteristics.
Unlike 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.
Also, unlike Al-Fl-Si glass/acid restoratives, resin composites do not bond to tooth structure unless the tooth is acid-etched and a thin layer of 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 without inherent bonding characteristics and fluoride release, 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 seventh 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.
The shape of the glass particles in dental composites
The trend in dental composites has been to achieve the greatest density of inorganic filler with the smallest particle size possible. The smallest particle sizes used in dental composites are sub-micron sized, in the range of .02-.04 microns (20 to 40 nanometers). These are fabricated in a furnace in which silicone tetrachloride is “burned” in an oxygen atmosphere to produce silica particles in this size range. The shape of these particles tends naturally to be spherical.
Numerous manufacturers have tried variously shaped larger sized particles in an effort to reinforce their composites, however, they have not met with very much success as of yet. Studies comparing shapes and sizes of various filler particles have shown that the composites containing the smallest sized spherical particles exhibit the maximum mechanical strength and maximum wear resistance. This is because spherical particles are easier to incorporate into a resin mix and fill more space, leaving less resin. Also, the tiny volume compared to the large surface area of the spherical sub-micron sized particles makes them much more difficult to dislodge from the surface of the restoration than any larger, oddly shaped particle.
A number of different shapes have been tried, from spiny snowballs to long fibers. Unfortunately, none of them has been successful for various reasons. Some negatively impacted the working characteristics of the composite, some reduced the depth of cure and none seemed to enhance the wear rate, polishability or other surface characteristics of their respective composites.