Microfilled composites and their properties
These materials are characterized by a large total surface area of filler particles. Consequently, a significant volume of organic matrix is required for binding, leading to two main consequences: reduced mechanical strength and, conversely, the ability to easily polish the restoration to a mirror-like finish.
Positive characteristics:
- Excellent polishability and long-lasting gloss retention.
- High color stability and outstanding aesthetics.
- Low abrasive wear.
Negative characteristics:
- Lack of radiopacity.
- Insufficient mechanical strength for high-stress areas.
- High coefficient of thermal expansion.
These materials offer a wide shade palette (opaque dentin shades, enamel shades, cervical shades, incisal edge shades, and bleached shades), making it possible to create restorations visually indistinguishable from natural tooth tissues.
Indications for use:
- Restoration of Class III and V cavities.
- Correction of non-carious lesions (cervical wedge-shaped defects, hypoplasia, enamel erosions).
- Aesthetic restoration of Class IV cavities.
Material example: Filtek A110 by 3M ESPE.
Inhomogeneous microfilled materials
This is a special subgroup of standard microfilled composites. They are produced using the following technology: the organic matrix and inorganic component are polymerized together and then ground down to particles of 20–30 µm.
These materials contain fine silica particles and prepolymerizers (pre-polymerized particles of the same material produced industrially). The filler loading reaches 75–80%. Structurally, the material consists of an organic matrix with uniformly distributed silica microparticles and prepolymerized filler.
They offer higher strength compared to homogeneous analogs, high wear resistance, and the ability to be easily polished to a dry shine while maintaining good aesthetic properties.
Group representatives: Filtek A110 (3M), Helio Progress (Vivadent), Silux Plus (3M).
Hybrid composite materials
Hybrids were developed to improve microfilled composites by incorporating large, small, and ultrafine particles. This combined the advantages of different fractions.
Influence of particle size on properties:
- Large and medium particles increase strength and wear resistance, improve marginal adaptation, and bring the coefficient of thermal expansion closer to that of natural tooth structures.
- Ultrafine particles enhance aesthetics and polishability, and reduce polymerization shrinkage.
Classification of hybrid composites:
- Macrohybrids: contain microparticles (less than 1 µm) and macroparticles sized 0.04–0.1 (8–12) µm. They are fairly strong, radiopaque, and have acceptable aesthetics, though their surface finish is inferior to microfils. They may also undergo discoloration and wear opposing teeth (examples: Evicrol Molar, Prismafil, Polofil).
- Microhybrids: particle size ranges from 0.04–0.1 to 1–5 µm. They exhibit good physical, mechanical, and aesthetic properties, as well as high color stability. Drawbacks include 3–5% shrinkage, insufficient dimensional stability, and procedural complexity (require incremental placement). Indicated for all 5 cavity classes, veneer fabrication, and porcelain crown repair (examples: Tetric, Herculite XRV, Valux Plus).
Highly filled and nanofilled composites
Highly filled (maximally filled) materials contain 80–90% filler through a combination of macro-, mini-, and microparticles. They feature a modified matrix that reduces shrinkage to 1.7–2%, eliminating the need for directional polymerization techniques. They exhibit the "chameleon effect" (optically blending with surrounding tissues) and are used for Class I–V cavities. Examples: Arabesk TOP, Filtek Z250, Tetric Ceram.
Nanocomposites incorporate particles at the nanometer scale (1 nm = 1/1,000,000,000 of a meter). The first such material, Filtek™ Supreme (3M ESPE), was introduced in 2002. It is based on a silica-zirconia filler with spherical particles ranging from 5 to 75 nm in size. Some of these nanomers are aggregated into specialized structural complexes—nanoclusters—providing outstanding aesthetic and mechanical performance.
Briefly: answers to topical questions
What are the classifications of dental amalgams?
Dental amalgams are classified based on the number of constituent metals and the primary base metal of the alloy.
According to the number of metals:
- Simple amalgams — consist of two components.
- Complex amalgams — contain three or more components.
According to the base metal:
- Silver amalgam — silver is the primary element of the alloy.
- Copper amalgam — copper serves as the base metal.
What are the absolute contraindications for composite restorations?
Absolute contraindications for direct composite restorations involve three main factors.
The following conditions are distinguished:
- Allergic reaction — patient hypersensitivity to components of the filling material.
- Inability to isolate — clinical conditions preventing effective moisture control of the tooth.
- Presence of a pacemaker — the patient has an implanted cardiac pacemaker.
What is the placement technique for microhybrid composites in a cavity?
The placement technique for microhybrid composites involves incremental layering and directional polymerization.
The following methods may be used:
- Directional polymerization technique — the material is placed in small oblique or triangular increments; the light curing direction is chosen considering the composite's shrinkage vector.
- Horizontal incremental layering technique — the material is placed in flat increments parallel to the cavity floor, progressing from the pulpal floor to the occlusal surface.
In layered restorations, a microhybrid composite can serve as a superficial enamel layer 1–1.5 mm thick.
Class and exam questions
Why do microfilled composites exhibit low mechanical strength?
Due to the large total surface area of fine filler particles, a high volume of organic matrix is required for binding, which reduces the overall mechanical strength of the material.
What are prepolymerized fillers in inhomogeneous composites?
These are industrially pre-polymerized microfilled particles that are subsequently ground down to 20–30 µm and incorporated into a new composite formulation.
What is the main advantage of homogeneous composites over microhybrid ones?
Highly filled composites exhibit very low polymerization shrinkage (1.7–2% vs. 3–5% in microhybrids), avoiding the need for complex directional polymerization protocols.
What are nanoclusters?
These are specialized complexes within the structure of nanofilled composites (e.g., Filtek™ Supreme) formed by aggregated nanomers (particles ranging from 5 to 75 nm in size).
What else to study on this topic
- Directed polymerization technique for microhybrid composites
- Principles of incremental placement of composite materials
- Mechanisms underlying the "chameleon effect" in highly filled composites
- Impact of the coefficient of thermal expansion on restoration longevity
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