The Continuous Evolution of Composites for Direct Restorations

 The term "composite material" refers to the union of at least two distinct components, insoluble in each other. The resulting product is a material with different, often better, characteristics than the single components. Three main ingredients of dental composites are: the organic resin matrix, inorganic filler particles that are distributed in the resin matrix, and the silane coupling agent used to coat these particles for chemical bonding to the resin matrix. Dental composites also need other components, such as initiators for the polymerization reaction, inhibitors to avoid spontaneous polymerization, and pigments to create a tooth-matching color range.

Although countless composite restorations are placed each year worldwide, there is a discrepancy in affordability and accessibility to composite-based dental care globally. The World Health Organization (WHO) has reported that amalgam restorations are still prevalent in most low- and middle-income countries.¹ The high cost of amalgam alternatives has been identified as the most critical factor for this finding. Clearly, the material properties are not among these concerns, although it is still necessary to continue to develop new resources and reduce the cost of tooth-colored materials.

The era of biomimetic materials for use in direct dental restorations began in 1954 when unfilled methyl methacrylate resins and silicate cements were the only esthetic direct filling options available. Subsequently, adhesive epoxy resins were offered. Their ability to bind together a maximum volume of very small, fused silica particles was investigated. The time-consuming hardening of epoxy formulations led to the synthesis of bisphenol A-glycidyl methacrylate (bis-GMA) in 1956 by Bowen.

US patents for dental filling materials came along in 1962 and 1965.^2,3 Commercial dental composites, initially indicated only for anterior teeth restorations, were introduced in the mid-1960s.⁴ Composite development over the subsequent five decades can be broadly divided into three main periods: (1) Between the mid-1960s and late 1970s the first macrofilled composites appeared, starting with self-cured and culminating with ultraviolet (UV) light-cured and visible light-cured materials. (2) From the late 1970s to the mid-2000s the filler particle dimensions continued to decrease, leading to microfilled, nanofilled, and then nanohybrid composites. (3) From the 2000s to 2010, low-shrinkage, self-adhesive, and bulk-fill composites were developed.

Based on filler size distribution-Historically, dental resin composites have been labeled according to the filler size distribution and size regimes. Traditionally, glass particles with an average diameter of 20 μm to 50 μm were used. Thanks to technological advancements in grinding, "fine" and "ultra-fine" fillers were obtained reaching dimensions of approximately 0.5 μm to 1 μm.

Most materials produced thereafter became "hybrids," incorporating both so-called nano- and micron-sized particles.⁵ This classification, however, fails to inform on or allow for the prediction of material performance. Practitioners can only base their choice on "seeing" the color or "feeling" the handling of a resin composite. Interestingly, firmness and viscosity are closely related to intrinsic characteristics, such as the filler content or resin composition. Since the classification based on filler size distribution is too broad, using handling to classify materials would be preferable.

Based on consistency-Composites can also be classified based on their consistency as "universal," "flowable," or "packable." Universal composites can be molded and applied using spatulas. Flowables instead have low viscosity and, therefore, can be introduced directly inside the cavity through a syringe. Their viscosity is due to either the low quantity of filler or the presence of modifying agents, such as surfactants. These increase the composite's fluidity without reducing the amount of filler, which could lead to an inevitable decrease of the flowable material's mechanical properties.

Packable composites are more compact; their malleability is considerably lower. This consistency is not due to an abundance of filler, but rather is a result of modifications in the distribution of the filler's size. A further cause of their pronounced consistency would be the addition of other forms of filler, such as fibers. Furthermore, their mechanical features allow efficient restorations, particularly in posterior sectors that are exposed to occlusal loads.

Based on mechanical behavior-These many variations in filler size, morphology, amount, volume, distribution, and chemical composition have created a large variety of composite categories, and practitioners today may often be confused about the choice of proper restorative material.

For comparative investigations of material behavior, a 2009 study evaluated measurements obtained under identical test conditions. The study, therefore, aimed to compare 72 frequently used materials from several composite categories, including hybrid, nanohybrid, microfilled, packable, and flowable composites, as well as compomers and flowable compomers in terms of their mechanical behavior.⁶ The evaluation considered flexural strength (FS), flexural modulus (FM), compressive strength (CS), and diametric tensile strength (DTS). The filler volume proved to have the highest impact on the measured properties, with a maximum FS and FM at a level of about 60%, whereas such correlation was not measured for DTS or CS. A good association existed only between FS and FM, which also depends on filler volume, whereas DTS and CS were less sensitive.

The limited depth of cure (DOC) associated with conventional composite materials has precluded the use of thick layers. The stratification technique has often been utilized to reduce polymerization-linked shrinkage. Low-shrinkage composites were introduced to address this issue, but these materials were based on a novel monomer technology that required a specific bonding system, resulting in the composites being less practical to use.

Meanwhile, the demand for an effective amalgam alternative has continued, due in part to the global mercury "phase-down" program instituted by the WHO in 2013.⁷ Also, filling a cavity in bulk offers such advantages as saving time and virtually eliminating the presence of voids and/or contamination between layers.

A new composite called "reinforced glass-ionomer cement," like amalgam, can be placed in bulk with no need for a separate adhesive. However, because it is a non-UV-curing composite, working time is decreased, thus significantly complicating controlled restoration placement. A dual-curing and self-adhering urethane dimethacrylate (UDMA)-based material, this novel alkasite composite contains alkaline glass fillers, and with its ability to release substantial levels of fluoride it has been proposed for bulk placement in retentive preparations without requiring an adhesive.

An increasing number of self-adhering bulk-fill composites are being developed. Several criteria must be met for a composite to be truly suitable for bulk filling. Besides having increased DOC and overcoming the shrinkage issues, the composite should have appropriate wear and fracture resistance. It seems unlikely that all of these characteristics can be delivered by one ideal material, and compromises would seem to be inevitable. This is because several key properties are influenced by the same variable.

Undoubtedly, improved DOC is the key advantage in this new class of composites. Most bulk-fill composites still contain camphorquinone as the primary photoinitiator and a tertiary amine as co-initiator. DOC is chiefly limited by the attenuation of the curing light used.⁸ Conventional dental composites are applied in increments of 2 mm thickness to allow sufficient light penetration and photopolymerization.^9,10 This approach, however, is inconvenient and time-consuming, especially for deep posterior cavities. The development of bulk-fill composites has allowed the placement of a single layer of up to 4 mm to 5 mm.¹¹ The improved DOC is usually achieved through greater translucency of the material, increased photoinitiator content, or an additional photoinitiator type.¹²

A study published in 2021 reported on the genotoxic and cytotoxic effects associated with the release of unbound particles of bis-GMA and/or triethylene glycol dimethacrylate (TEGDMA).¹³ Various adverse effects have been correlated with the use of these materials. In particular, several in vitro studies demonstrated that bis-GMA may stimulate the production of prostaglandin E2 (PGE2), cyclooxygenase-2 (COX2) expression, and the pro-inflammatory activation of interleukin (IL)-1β, IL-6, and nitric oxide.^13-15 Consequently, manufacturers have shown interest in switching to bis-GMA-free composite materials to minimize the cytotoxic potential of restorative products.

Composites may have different blending effects (also known as color assimilation, color induction, or the von Bezold effect). These effects involve color shifting toward the surrounding hard dental tissues, resulting in a more negligible color difference when observing the restorative material and surrounding tissue together as compared to separately.^16,17 From a clinical standpoint, this property, defined as "color adjustment potential" (CAP),^18,19 can help reduce the number of shades needed and can compensate for color mismatches.

Many composites with high CAP have been recently introduced to the market. CAP can be visually assessed (CAP-V) or assessed with color measuring instruments (CAP-I).²⁰ Every composite is defined by a pigment color and a structural color. The pigment color is obtained from selective reflections of specific wavelengths. Structural colors depend on the material's crystalline structure, which can selectively reflect certain bands of wavelengths of light or color.²⁰ Structural colors rather than pigments, therefore, characterize contemporary mimetic composites. Manufacturers achieve this behavior by controlling the size and shape of the fillers.

The continuous evolution of composite systems has greatly contributed to the improvement of restorative and esthetic treatments. Shrinkage stress management, blending effects, and improved mechanical properties have yielded increasingly better-performing materials for everyday restorations.

The author thanks Mr. Mauro Mandurino for his valuable contribution to the writing of this article.

Gaetano Paolone, DDS
Adjunct Professor, Restorative Dentistry,
Università Vita Salute San Raffaele, Milan, Italy; Private Practice, Rome, Italy