2023 State-of-the-Art in Resin-Based Composites and Future Trends

The most-frequently used monomers that compose a resin matrix for RBCs have been bisphenol A glycerolate dimethacrylate (bis-GMA), triethylene glycol dimethacrylate (TEGDMA), and urethane dimethacrylate (UDMA).² Although monomers have remained largely unchanged, significant modifications in RBCs have been seen with regard to the type, size, and distribution of the inorganic fillers. Fillers are mainly composed of radiopaque glass, but other types include alkaline glasses, metal oxides, zirconia, and hydroxyapatite.

Silane, a coupling agent, coats the inorganic filler particles, allowing their union with the resinous matrix. The result is a bifunctional molecule with a methacrylate residue on one side and a silane residue on the other capable of creating a covalent bond between the two phases.³ The binding of the filler particles to the resin matrix by a coupling agent is pivotal to improved mechanical and physical properties. It allows the load, under intense stress, to be transferred to the filler, thus avoiding a separation of the two phases.

Besides these two basic elements, RBCs also consist of initiators of the polymerization reaction, inhibitors (to avoid spontaneous polymerization), and pigments, with the latter allowing for the provision of shade, translucency, and radiopacity to the composite material.⁴

Historical Overview and Classification

During the 1960s and 1970s, macrofilled composites were introduced as an alternative to silicates. They were characterized by an average filler size of 10 μm to 50 μm. Initially, macrofilled composites were self-curing. Shortly thereafter, with the use of ultraviolet photopolymerization, they became curable with visible light. After the 1970s, the size of inorganic filler particles began to progressively decrease,¹ which has led to the creation of new categories of composites, namely microfilled, microhybrid, and, later, nanofilled and nanohybrids.

The advent of nanotechnology has led to a significant improvement in the quality of dental composites. Because of the reduced size of fillers and, consequently, increased filler content, better esthetics and mechanical performance can be obtained.⁵ Nanofilled composites comprise both nanomers (5 nm to 75 nm) and nanoclusters. Nanoclusters are composed by agglomerated nanomers, and a mix of silica oxides and zirconia is added to increase the handling of the composite, making it suitable for both anterior and posterior restorations.⁶ Nanohybrid composites are hybrid RBCs associated with nanofillers dispersed in prepolymerized resin. They are characterized by good handling and resistance to wear and also show superior esthetics and high polishing properties.

The long-term success of restorations is also related to surface quality. Rough surfaces (high Ra values) could cause plaque accumulation, allowing bacterial proliferation and eventually secondary caries development. Because of the smaller filler size, roughness values of nanofilled and nanohybrid materials can be greatly reduced, thereby facilitating less bacterial adhesion. Further advantages of nanofilled and nanohybrid RBCs include excellent esthetics, high glossiness, color stability, and good handling. Generally, roughness is inversely proportional to the gloss of the restoration. Compared with nanohybrids and nanofillers, microhybrid composites are characterized by lower gloss and higher surface roughness values.⁷

Composite materials can be classified according to filler size and material consistency. A classification based on filler size is presented in Table 1. 

Regarding consistency, composites can be classified in three categories: universal, packable, and flowable.^1,8 Universal composites have medium consistency and can be placed and modeled using spatulas and carvers. One advantage of these composites is that their esthetic and mechanical characteristics allow for excellent results in direct restorations in both anterior and posterior teeth. Packable composites are stiffer and, therefore, require placement with a plugger. Flowable composites are characterized by low viscosity due to not only their lower amount of filler load, but also the presence of modifying agents such as surfactants.⁹

In the past decade, a new category, bulk-fill composite, was introduced. This type of material is characterized by low stress during the material's shrinkage.¹⁰ Two types of consistency are available in bulk-fill materials: low viscosity and high viscosity.^11,12 The low-viscosity version needs to be "capped" with a conventional composite to protect against occlusal forces, while high-viscosity bulk-fill composites can be placed without using another material for occlusal coverage.

Future Direction of RBCs

The two most common causes of failure of large restorations in posterior teeth are fracture and secondary caries.¹³ Several approaches have been developed to address both of these issues, including the development of composites with improved mechanical performance and bioactive surfaces with contact-active properties.

Fiber-Reinforced RBCs

Because RBCs are increasingly used for extensive restorative treatments, materials with high fracture toughness and low crack propagation are in high demand. One of several approaches to fulfill this requirement is to add fibers to the RBC. Glass fibers have been used in the reinforcement of dental polymers for more than 30 years. The efficacy of RBC fiber reinforcement depends on many variables, such as the resins used and the quantity, length, shape, orientation, and conditioning of the fibers.^14-16

For fibers to behave as an effective polymer reinforcement, stress transfer between the polymer matrix and fibers is essential.¹⁷ Fiber-reinforced composites, therefore, have been introduced as a dental restorative material with the aim of broadening the indication of direct RBCs to large posterior cavities. They have also been proposed as a substitute for dentin to mimic its stress-absorbing property.¹⁸

However, the actual clinical effectiveness of fiber-reinforced RBCs has yet to be proven since there is a lack of clinical studies. Garoushi et al reported in an in vitro study improved mechanical properties for fiber-reinforced RBCs versus conventional RBCs.¹⁹ Conversely, in another in vitro study, Barreto et al noted no differences between fiber-reinforced RBCs and conventional RBCs in fatigue resistance and fracture strength of extensively restored premolars.²⁰

RBCs With Antibacterial Properties

Nanotechnology has opened the door for the development of dental materials with antibacterial properties using contact-active technology. Antimicrobial agents are incorporated within the matrix of the material without being released. Surfaces that effectively kill bacteria without releasing a biocide represent a trend toward permanent sterile materials. The development challenge lies in the fact that biofilms are known to be able to colonize all types of surfaces, both natural and artificial, over time.²¹

The beneficial characteristic of the active biomaterial is that it could inactivate bacteria directly on contact, thus avoiding the release of compounds that could be toxic even for mammalian cells besides bacteria. An active contact surface can be obtained by copolymerizing biocidal compounds within the red blood cell matrix or by chemical grafting of antimicrobial polymers or vectors to obtain a coating for the dental material. Silver-based coatings are also increasingly gaining popularity. Silver has been added to RBCs due to its antibacterial effect as well as its biocompatibility.²² It is able to interfere with the enzymatic activities of Streptococcus mutans and prevent its proliferation. The development of these types of "smart" materials that are able to control biofilm represents the future of restorative procedures.²³

Repairing Composites

Composite repair of small fractures, chips, and defects has been recognized as a desirable procedure but is not often advocated because it can be considered a compromise approach.²⁴ Regardless, this procedure has gained popularity as it allows for extended longevity of a patient's restoration. Repair can be carried out either immediately (contact point augmentation or shape modification) or on more aged composite restorations (in the case of secondary caries or fracture).^25-27

Dieckmann et al reported that the age of the repaired composite is pivotal in composite-composite bond strength, with immediate repair showing the highest values. Nonetheless, the researchers suggest that aluminate (AlO₂) sandblasting or use of diamond burs (regardless of the grain) and silane application together with an unfilled bonding resin may provide clinically acceptable bond values on aged composites.²⁸ Therefore, although hygroscopic/stress expansion and hydrolytic degradation at the resin-filler interface characterize aged composites, their repair could be considered a reliable procedure.

Conclusions

Although dentistry is far from having perfect restorative materials, significant improvements have been made in RBCs. Expectations are that further RBC development will include improved mechanical properties, reduced shrinkage and shrinkage stress, remineralization capability, and the inclusion of biofilm control compounds.

About the Authors

Gaetano Paolone, DDS
Instructor in conservative dentistry, Vita-Salute
San Raffaele University, Milan, Italy; Private Practice,
Rome, Italy

Carlotta Diana
Dental Student, Vita-Salute San Raffaele University, Milan, Italy

Giuseppe Cantatore, DDS
Professor, Vita-Salute San Raffaele University,
Milan, Italy