Composite restorative materials Dr. Waseem Bahjat Mushtaha Specialized in prosthodontics

Composite restorative materials The term of composite material may be defined as a compound of two or more distinctly different materials with properties that are superior or intermediate to those of individual constituents. Examples of natural composite materials are tooth enamel and dentin. In enamel, enamelin represent the organic matrix, where as in dentin the matrix consist of collagen. In both of these “composite” the filler particles consist of hydroxyapatite crystals. The difference in the properties of these two tissues is associated in part with difference in the matrix: filler ratio.

Constituents of composites 1) Principal monomers. 2)Diluent monomers 3) Inorganic fillers 4) Silane coupling agent 5) Polymerization inhibitors. 6) initiator/activator components 7) Ultraviolet stabilizers. 8) Optical modifiers

1) Principal (higher molecular weight) monomers. Many composites are based on an aromatic dimethacrylate system, the monomer being the reaction product of bisphenol-A and glycidyl methacrylate, often called (BIS-GMA) or Bowen’s resin. This highly viscous monomer can undergo free radical addition polymerization to give a rigid cross-linked polymer. Some products use alternative monomer which are described as urethan dimethacrylates (UEDMA). The properties of composites based on these latter monomers are in general similar to those of materials containing Bowen’s resin.

2)Diluent(lower molecular weight) monomers These monomers added to the composite formulation due to: 1) To reduce the viscosity of the material. 2) Enable proper blending with the inorganic constituents. 3) To facilitate clinical manipulation.

The monomer of choice may be: a) Monofunctional monomer, ethylene glycol methyl methacrylate (EGDMA). b) Difunctional monomers, ethylene glycol dimethacrylate (TEGDMA). Greater quantities of diluent monomer caused: 1) Lower viscosity. (advantage) 2) Greater shrinkage on polymerization (disadvantage).

N.B. Difunctional monomers are usually uses rather than monofunctional monomers due to: 1) They have less shrinkage on polymerization 2) They give a more cross-linked structure, which is harder and stronger, and has a lower coefficient of thermal expansion. 3) They give polymers with lower water absorption.

3) Inorganic fillers a) Incorporation of filler particles into a resin matrix will improves the properties of the matrix material (if the filler particles are well bonded to the matrix): 1) Improvement in mechanical properties (compressive strength, modulus of elasticity and hardness)

2) Reduction in coefficient of thermal expansion 3) Contribution to the aesthetics; glass particles are able to modify the optical appearance and match the color of the surrounding tooth material. 4) Reduction in the shrinkage on setting. 5) Less heat released in polymerization 6) The composite is radio-opaque if barium or strontium glasses are used. N.B. (it contributes the composite easier to polish)

b) Filler particles are most commonly produced by grinding or milling quartz or glasses to produce particles ranging in size 0.1 to 100µm. Silica particles of colloidal size (approximately 0.04), referred to collectively as the microfiller, are obtained pyrolytic process. During pyrolytic process, the silicon atoms are present in low molecular-weight compounds, such as SiCl4, that are typically polymerized by burning SiCl4 in an O2 and H2 atmosphere. During this process, macromolecules consisting of SiO2 are formed, these particles called pyrogenic (born in fire) silica particles. These macromolecules are of colloidal size and constitute the filler particles.

c) Composites often are classified on the basis of the average size of the major filler component. In addition to the filler volume level, the size, size distribution, index or refraction, radiopacity, and hardness are also important factors in determining the properties and the clinical application of the resulting composites.

d) To incorporate a maximum amount of filler into a resin matrix, a distribution of particle size is necessary. It is obvious that if a single particle size is used, even with close packing, a space will exist between particles. Smaller particles can fill these spaces and, by extending this process, a continuous distribution of particles can afford maximum filler loading. Most composites also contain some colloidal silica. Inorganic filler particles generally account for between 30 and 70 vol% or 50 to 85 wt% of the composite.

The amount of filler that can be incorporated in to the resin matrix generally is affected by the relative filler surface area. Colloidal silica particles have large total areas, thus, even small amount of filler particles have a large total surface area that can form polar bonds with monomer molecules and thicken the resin. Microfillers, because of their large surface area, are frequently added to composite formulations in amounts of less than 5 wt% to modify the past viscosity, thereby reducing the risk for sedimentation of the ground particles. The microfillers also enhance filler packing. So the microfilled composites, colloidal silica is the only inorganic filler.

e) To ensure acceptable aesthetics of a composite restoration, the translucency of the filler must be similar to that tooth structure. To ensure acceptable translucency the index of refraction of the filler much closely match that of the resin. For BIS -GMA and TEGDMA, the refractive indices are about 1.55 and 1.46, respectively, and a mixture of the two components in equal proportions a refractive index of about 1.5. most of the glasses and quartz that are used for fillers have refractive indices of approximately 1.5, which is adequate to achieve sufficient translucency.

f) Quartz has been used extensively as a filler, particularly in the first generation of composites. It has the advantage of being chemically inert but its also extremely hard, making it difficult to grind into fine particles. Thus, quartz containing composites are more difficult to polish and may cause more abrasion of opposing teeth or restorations.

4) Silane coupling agents It is important, for reinforcement of the polymer by the filler to occur, that the two constituents should be bonded together. To achieve this, the filler is usually treated with a vinyl silane compound.

5) Polymerization inhibitors Since dimethacrylate monomer will polymerize on the storage, an inhibitor is necessary. Hydroquinone has been widely used but was responsible for causing discoloration of the material so atypical inhibitor is butylated hydroxytoludene that used in concentration of 0.01 wt%.

6) initiator/activator components 1) Chemical activation. Benzoyl peroxide initiator and tertiary amine activators, or sulphanic acid type initiators may be employed. Of the tertiary amines, N,N-dimethyl-p-toluidine was used as activator, but now N,N- dihydroxymethyl-p-toluidine is widely used.

2) Ultraviolet. activation: composites containing benzoin methyl ether were developed. On application of u.v. light of appropriate wavelength, energy is absorbed and free radicals are generated to initiate polymerization. This system has now been superseded by visible light curing due to: a) Limited depth of polymerization b) Layering techniques attempting to overcome c) Potential harmful effects such as skin cancer and eye damage.

3) Visible light activation: composites have been developed which contain an α- diketone and an amine. On application of visible light of wavelength 460-485 nm, free radical are generated. Visible light-cured materials are now very widely used .

7) Ultraviolet stabilizers To prevent discoloration with age of composites, compounds are incorporated which improves color stability. (example 2-hydroxy-4- methoxybenzophenone.

8) Optical modifiers To match the appearance of teeth, dental composites must have visual coloration (shading) and translucency that can simulate tooth structure. Shading is a achieved by adding different pigments. These pigments often consist of different metal oxides that are added in minute amounts. Example titanium dioxide and aluminum oxide (0.001 to 0.007 wt%)

Classification of resin-based composites 1) Traditional composition (macrofill). 2) Microfilled composite. 3) Small particle-filled composite. 4) Hybrid composite.

1) Traditional composition (macrofill). This was the first type of resin composite marketed 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 8 to 12 microns in diameter.   As mentioned 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.  The inclusion of glass particles reduces this problem 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.  The advantage of large particle size is that more of them can be incorporated into the mixture without making it too stiff to work with.  Macrofills are 70% to 80% glass by weight

Unfortunately, macrofill composites have two undesirable qualities:   1) Due to large particle size, macrofills are not very polishable.  The relatively soft acrylic polymer polishes 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. 2) Large particles are relatively easily dislodged from the surface of the restoration during function exposing the relatively soft acrylic polymer which wears away exposing more filler particles which again pop out ad infinitum.  This tendency to abrade away makes macrofils unsuitable for posterior restorations

The first macrofill appeared on the market in the mid 1960's The first macrofill appeared on the market in the mid 1960's.  Most older dentists affectionately remember it by its brand name, Adaptic.  Adaptic had the additional disadvantage of containing no radiopaque materials which made it hard to distinguish from decay on x-rays (composites using quartz as a filler are radiolucent. Their radiopacity is less than that of dentin).  

2) Microfilled composite 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 macrofills.  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 particle size, and has a relatively greater shrinkage during setting.  Microfills are only 35 to 50 percent by weight filler particles.  Microfills are used for small fillings in front teeth.  They are also used for direct veneer on front teeth because of their superior polishability.

Microfill composites have  three main disadvantages. 1) Due to the relatively low density of filler particles, microfills 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. 2) Also due to low density of filler particles, microfills are more prone to shrinkage while setting, and this limits their use in large, bulky fillings. 3) 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. 

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

3) Small particle-filled composites Inorganic fillers are ground to a size smaller than those used in traditional composites. The average filler size of these materials range from 1 to 5µm, but the distribution of sizes is fairly broad. This broad particle-size distribution facilitates a high filler loading, and small particle-filled composites generally contain more inorganic filler (80 wt% and 60 to 65 vol%) than traditional composites. This is particularly true of those designated for posterior restorations.

Some small particle-filled composites use quartz particles as fillers, but most incorporate glasses that contain heavy metals. The matrix resin of theses materials is similar to that of traditional and microfilled composite materials. The filler consist of silan-coated ground particles. Colloidal silica is usually added in amounts of about 5 wt% to adjust the paste viscosity.

This category of composites exhibits the most superior physical and mechanical properties The compressive strength and elastic modulus of small particle-filled composites exceed those of both traditional and microfilled composites. The tensile strength of small particle-filled composites is double that of the microfilled materials and 1.5 times greater than that of the traditional composites. The coefficient of thermal expansion is less than that of other composites. Wear resistance is improved. Polymerization shrinkage is less than of traditional resins

Those materials filled with glass-containing heavy metals are radiopacity is an important property for materials used for restoration of posterior teeth to facilitate the diagnosis of recurrent caries.

Clinical consideration of small particle-filled composites: Due to improved strength of these composites and higher filler loading, they are indicated for applications in which large stresses and abrasion might be encountered, such as in Class I and Class II sites. The smooth surfaces are not as good as microfilled materials (disadvantage)

4) Hybrid composite 1) There are two kinds of filler particles in the hybrid composites. Most modern hybrid filler consist of colloidal silica and ground particles of glasses containing heavy metals constituting a filler content of approximately 75 to 80 wt%. The glasses have an average particle size of about 0.6 to 1.0 µm. In atypical size distribution, 75% of the ground particles are smaller than 1.0 µm.

Colloidal silica represent 10 to 20 wt% of total filler content Colloidal silica represent 10 to 20 wt% of total filler content. In this instance, the microfillers also contribute significantly to the properties. The smaller filler particles, as well as the greater amount of microfillers, increase the surface area. Thus, the overall filler loading is not as high as it is for some of the small particle-filled composites. 2) The hybrid composite is evident polish compared with that for the traditional and small particle-filled composites.

3) Physical and mechanical properties for these systems generally range between those of the traditional and small particle-filled composites. The properties superior to those of the microfilled composites. 4) Because the ground particles contain heavy metal species, they have radiopacities greater than of the enamel.

Clinical consideration of hybrid composites Because of their surface smoothness and reasonably good strength, these composites are widely used for anterior restorations, including Class IV sites. Although the mechanical properties generally are somewhat inferior to those of small particle-filled composites, the hybrid composites are widely employed for stress-bearing restorations.

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..

Light sources The following component of light-cure device: 1) A quartz-halogen bulb. 2) A transformer and control circuitry. 3) Appropriate light filters 4) A switch 5) A timer device 6) A curing tip: this is vary from around 5mm in diameter up to 7 mm or more for curing larger surfaces as in posterior composites.

Chemically activated materials a) The two pastes should be mixed in the correct proportion (equal volumes). b) Contamination of one paste by the other should be avoided. c) As far as possible, avoid incorporation of air during mixing. d) During mixing of some products, tints can be added to permit color matching between composite and tooth. e) The mixed materials should be placed in the cavity without delay.

Light cured materials a) In general, most commercially available light sources will polymerize most light curing materials. b) Under-curing must be avoided at all costs. This gives a material with a hard outer “skin” and soft material at the base of the cavity. c) Under-curing may result if the light source is not sufficiently close to the surface of the material being polymerized.

d) Over-curing is not harmful d) Over-curing is not harmful. This may be a wise precaution if using a light with a material from a different manufacture. e) Darker shades of material absorb more light so require longer curing times. f) In some instances, may begin to polymerize if exposed to strong ambient light.

Finishing procedures a) If surface finishing is required, this probably best achieved by: 1) Contouring the material with a diamond stone or tungsten carbide bur. 2) Polishing with a composite finishing system comprising mildly abrasive pasts and discs. b) Surface glazes are supplied with some materials (there are unfilled polymers). why?

Applications 1) Anterior restoration. 2) Posterior restoration : alternative to dental amalgam. 3) Used as core build-up materials. 4) Luting of resin-bonded bridge system. 5) Inlay materials. 6) Polymeric crown and bridge materials 7)Laminate veneer system.

Repair of composite Composites may be repaired by placing new material over the old composite. This is a useful procedure for correcting defects or altering contours on existing restorations. The procedure for adding new material differs depending on whether the restoration is freshly polymerized or an older restoration.

When a restoration has just been placed and polymerized, it may still have an oxygen- inhibited layer of resin on the surface. Additions can be made directly to this layer, because this represents, in essence, an excellent boning substrate. Even after the restoration has been polished, a defect such as porosity can still be repaired by adding more material. A restoration than has just been cured and polished may still have more than 50% of unreacted methacrylate groups to copolymerize with the newly added material .

As the restoration ages, fewer and fewer unreacted methacrylate group remain, and greater cross-linking reduces the ability of fresh monomer to penetrate in to the matrix. The strength of the bond between the original material and the added resin decreases in direct proportion to the time that has elapsed between polymerization and addition of the new resin. In addition, polished surfaces expose filler surfaces that are free from silan coating. Thus, the filler surface area does not chemically bond to the new composite layer, so the strength of repaired composite is less than half the strength of the original material.

Critique of composites 1) Biological consideration: pulp irritation, plaque can accumulate on a rough composite surface (less of this problem How?). 2) Bonding. Composite are not adhesive to enamel and dentin (How can solve this problem?). 3) solubility: very low. 4) Mechanical properties: generally good (compared between all types of composites?). 5) Aesthetics : though initial aesthetics of composites can be good, the resin may discolor over a period time. And the accumulation of plaque can cause discolaration.

6) Thermal properties: A- Composites have less thermal expansion than unfilled resins. Microfine filled material with high inorganic filler content have lower coefficients of thermal expansion. (why?) B- Composites are good thermal insulators. 7) Dimensional change on setting: this is comparatively small for polymers prepared from difunctional monomers, and which heavily filled.

8) With some composites it is difficult to obtain a smooth surface finish by abrading and polishing techniques. Also, abrasive wear in service roughens the material, because the polymer phase wear more rapidly than harder ceramic material. However , materials with microfine fillers can take and retain a smooth surface finish. 9) Most composite are radio-opaque.