Materials for Direct Restorations

Since 2003, the ADA Council on Scientific Affairs has classified materials for direct restorations into four categories: amalgam, resin-based composites, glass ionomer, and resin-modified glass ionomer.¹ Unlike indirect restorations, direct restorations are generally placed directly onto tooth structure within a single visit and do not require laboratory preparation. With the introduction of CAD/CAM systems, the line dividing direct from indirect restorative materials has been blurred, as this technology allows the in-office production of ceramic restorations. This Oral Health Topic will generally follow the 2003 classification (see Table 1, below),¹ but with the addition of more recently developed materials. More information on CAD/CAM materials may be found on our Indirect Restorative Material Oral Health Topic page. Adhesives and bonding agents, consisting of the same components as direct restoratives, may also be regarded as direct materials. Similarly, direct materials are used as pit-and-fissure sealants, but are preventive in nature rather than restorative. ANSI/ADA Standard No. 39 specifies requirements for pit and fissure sealants; please see our Oral Health Topics page on Dental Sealants for more information; while this Oral Health Topic focuses on direct materials in restorative applications.

Table 1. 2003 CSA classification and comparison of direct restorative dental materials.

Amalgam is an alloy that contains mercury, and has been used in restorative dentistry for over 150 years.^1-3 Dental amalgam is safe and effective,³ and especially suitable for heavy-load-bearing teeth, although concerns about esthetics may limit its contemporary usage, particularly in anterior teeth.^{1, 4} Properties, composition and dental and biocompatibility considerations are covered in the Oral Health Topic page on Amalgam as a Restorative Material.

Generally made up of an organic polymerizable resin matrix, inorganic filler material and a coupling agent,^{1, 4-6} resin composite restorative materials were developed as an esthetic alternative to amalgam by providing a tooth-colored filling. Durability, strength, and cost issues have prevented composites from entirely replacing the need for amalgam fillings, despite efforts to scale-back the use of mercury-based amalgam driven largely by environmental concerns. Many resin materials need to be light-cured, and adhesion to tooth structure requires “intermediary agents containing highly reactive chemicals”⁷ as well as relatively expensive curing devices which require protective equipment.⁴

Composites are a heterogeneous group and are often classified by resin type, particle filler size or curing type,^{4, 8, 9} although these classifications are nonexclusive and often overlap.⁴ The evolution of resin composite materials has involved modifications in curing type, filler particles, and resin composition.⁵ Increased depth of cure and reduction of shrinkage and curing time have driven contemporary research in composite technology.^{4, 10, 11}

Resin Matrix

Highly-viscous bisphenol A glycidyl methacrylate (BisGMA) began to be incorporated as a matrix following its patenting in the early 1960s,¹⁰ and urethane dimethacrylate in the 1970s,^{4, 12} while other monomers (such as triethylene glycol methacrylate, or TEGDMA) were eventually added to reduce the viscosity and allow easier manipulation and a wider variety of filler materials.^{4, 8, 12, 13} Methacrylate polymerization naturally results in shrinkage⁵ and thus may lead to marginal gaps and weakening of the bond between the composite and the tooth structure.^{4, 11} More recently, alternatives to methacrylate-based resins have been developed to avoid polymerization shrinkage, including the replacement of TEGDMA with BisEMA,¹⁴ and an epoxy known as silorane (from the combination of siloxane and oxirane) has been introduced by 3M ESPE.^{6, 15, 16}

Filler Particle Classifications

In the 1950s, silica was introduced as a filler to polymer matrix composites as a reinforcing material, although issues with shrinkage and bonding persisted.^{8, 17} Polishability and wear-resistance were problems with “traditional” or macro-filled composites throughout the 1970s, when mid-and micro-filled composites began to appear.^{4, 18} Micro-fillers (containing particles between 0.01 and 0.1 µm^{4, 9, 18}) were developed for better polishability but the lower filler load resulted in weak mechanical properties,⁵ until hybrids were developed in the in the 1980s, which combined macro- with micro-fillers allowing better mechanical properties along with polishablitiy.^{4, 5, 18} and nano-fillers (5-10 nm)⁷ in the 2000s.^{5, 18} Hybrids contain both nanoparticles (1 to 100 nm)⁷ and microparticles (≤1 µm), allowing a highly polished surface and gloss.^{4, 19}

Composites are traditionally incrementally filled (in stages of around 2mm), but bulk fill composites have allowed a single-step fill of 4mm or more.⁶ Bulk filled composites eliminate a step and provide an alternative to amalgam fillings,²⁰ although according to this ADA ACE Panel Report, among responding dentists, the incremental technique is preferred for posterior composite restorations, who tend to be concerned with inadequate cure of depth and polymerization shrinkage.

Composites can further be classified according to filler density. Decreasing the density of the fill results in lower viscosity and improvement in manipulation characteristics including ease and uniformity of flow and adaptation to cavity structure; these flowable resins, however, have and increased susceptibility to wear and polymerization shrinkage compared to higher-density, or packable, composites.^{4, 12} Packable composites are suitable for large posterior restorations; flowable composites are typically used as a liner or base, or limited to low-stress-bearing posterior restorations.²¹

Curing type

Traditional composites were self-cured, meaning they required hand-mixing of the matrix and monomer along with the filler particles.^{4, 5} Light-curing was introduced in the 1970s, providing better predictability and stability of the preparation.⁵ Early light-curing required UV light which not only was a hazard to eyes and oral mucosa, but also only provided a shallow depth of cure for the composite.⁵ A safer and more effective alternative to UV-cured composites was developed by the late 1970s, utilizing visible light on a camphorquinone initiator.^{4, 5}

Light curing, however, still has limitations on light penetration, and light-cured composites must be placed incrementally when they exceed a depth of around 2 mm.⁴Dual-cure resins have been developed for deeper applications where light penetration may be limited, and combines light-curing with the chemical reaction between benzoyl peroxide and an aromatic tertiary amine, resulting in more rapid polymeriztion.^{4, 10}

Properties of Resin Composites

ADA Specification No.27 stipulates requirements for resin-based filling materials, and includes standards and testing requirements, with shade, color stability, flexural strength, and radiopacity among them.^{22, 23} Table 2, below, lists common properties of types of composite resin materials.

Table 2. Some properties of Composite Materials.^{4, 6}

Glass ionomers, a technically incorrect but commonly used term for glass polyalkenoate cements as dental materials, were first successfully developed for dentistry in the early 1970s by combining glass powder with polyacrylic acid as a biocompatible adhesive.^{4, 24, 25} Primarily intended as a cement or cavity liner, and generally for non-load bearing fillings, glass ionomers have evolved to contain additional components to improve mechanical properties and water solubility.^{4, 24} Modern glass ionomers consist of an alumino-silicate glass powder and a polymeric acid.^{4, 12, 26<}Importantly, glass ionomers are particularly efficient at fluoride recharging, although the degree to which this provides protection against future caries development is debated.^{4, 27, 28}Their relatively low stress-bearing properties make them unsuitable for areas of high stress;¹² they are also more susceptible to abrasive wear than resin composites.^{4, 12} As pit and fissure sealants, glass ionomers benefit from a “short and simple application procedure” due to “lower susceptibility to moisture” compared to resin composites, in addition to their role as fluoride reservoirs, all of which are especially helpful in a pediatric dental environment.²⁹

Traditionally, glass ionomers have been classified into four types, according to application. Classification of the types varies according to silicate particle size and powder composition; larger particle sizes are intended for restorative purposes, and finer powders as cements (see Table 3).^{4, 12} Type I consists of luting and bonding cements, for use with crowns, bridges, and orthodontic appliances.^{4, 12, 26} Restorative glass ionomers are subdivided into Type IIa, which are more aesthetic (better tooth-color), and Type IIb, with better radiopacity.^{4, 26}Type III consists of lining and base cements.^{4, 26} Film thickness and P/L ratio (the powder-to-liquid ratio by weight or volume) influence classification and application of glass ionomers; smaller glass particles (~15 μm) serve as cements while larger particles (~50 μm) serves as restoratives. ADA/ANSI Standard No. 96 specifies requirements for dental cements,^{4, 30} including a maximum film thickness for luting cements of 25 μm; compressive strength a minimum of 50 MPa for luting, base and lining cements and 100 MPa for restorations.³⁰

Modified Glass Ionomers and Bioactive Materials

Glass ionomers may be modified with the addition of other materials, including methacrylate-based monomers (resins) or metals to further enhance specific mechanical properties.^{4, 25, 31} Sometimes referred to as type IV, metal-reinforced glass ionomers have added silver alloy or silver sintered to glass (cermet) intended to increase stress load capacity and fracture toughness.^{4, 12} By the 1980s hybrid (resin-modified) glass ionomers were developed which introduced the advantages of resin composite aesthetics and curing properties, and improved strength and polishability.^{4, 6, 12} These resin-modified glass ionomers decrease setting time and, like resin composite restorations, can be light- or self-cured.¹ Resin-modified glass ionomers can be used as reasonably successful alternatives to preformed metal crowns in pediatric patients.^{32, 33}

Development of glass-ionomer-based materials is changing rapidly, and many are currently being marketed as bioactive materials, along with some ceramic-based materials.^{6, 34} These restorations provide additional remineralization capabilities based on the addition of specific glasses that promote tissue integration. More information may be found on our ACE Panel Report on Bioactive Materials.

Table 3. Properties of Glass Ionomer cements.^{4, 6, 12}

Amalgam fillings have been the gold standard for direct materials in terms of durability and longevity, but composite restorations have often been shown in recent systematic reviews to perform as well if not better, but over shorter time spans due to study limitations.  A 2020 systematic review³⁵ published in JADA showed an Annual Failure Rate of 2.19 for large posterior composite restorations, similar to previous studies (see Table 4), while a 2012 study found that the “overall success rate of composite resin restorations was about 90% after 10 [years], which was not different from that of amalgam" and "hybrid and microfilled composites that were placed with the enamel-etching technique and rubber-dam showed the best overall performance.”³⁶ Similarly, for posterior composite restorations, “[t]he cumulative survival rates were 91.7% (6 y), 81.6% (12 y), and 71.4% (29 y).”³⁷ Highest failure rates have been found among glass ionomers and sandwich [resin and glass ionomer] restorations.³⁵ Despite the increases in longevity of newer composite dental materials, amalgam restorations remain the gold standard, and may last indefinitely, with estimated survivability of 55 to 70 years.⁴ A randomized control trial published in JADA in 2007 of 1748 restorations found a 3.5 times greater risk of secondary caries in composite over amalgam restorations, contributing to a higher rate of failure among composites.³⁸ Similar to other studies, this RCT found an overall survival rate of 85.5% for composites and 94.4% for amalgams.³⁸ See Table 4, below.

Table 4. Survival Probabilities and Annual Failure Rates.

Patient Exposure

Resin adhesives and resin-ceramic composite materials often contain methacrylate monomers, bisphenol A glycidal methacrylate (Bis-GMA), bisphenol A dimethacrylate (bis-DMA), urethane dimethacrylate (UDMA), and triethylene glycol dimethacrylate (TEGDMA), that beyond potentially causing localized allergic reactions and irritation, are known to be cytotoxic, genotoxic, and mutagenic.^{4, 42-44} During a restoration, these monomers are polymerized during the light- or chemical-activation process and ‘sealed’ by the dentine barrier; however, none of these polymers polymerize to 100% completion and a number of studies indicate that some leakage may occur.^{4, 45-47}

Bis-GMA and bis-DMA may leak the estrogenic toxin bisphenol A (BPA) in small quantities from degradation by salivary components or incomplete curing of the composite.^{45, 47-50} BPA has been associated with a number of systemic health conditions, including breast cancer, heart disease, and developmental and reproductive disorders,^{4, 45, 48} but the amounts that may be released from dental restorations is considered minor, particularly when compared to typical background exposure to BPA in the environment and in food products.^{45, 48, 50}The ADA and the World Dental Federation (FDI) have both released statements concerning BPA in dental materials; while the ADA finds that “the low-level of BPA exposure that may result from dental sealants and composites poses no known health threat,”⁵¹ the FDI encourages more research, but also “strongly discourages the use of BPA in the manufacture of dental materials.”⁵² View our Oral Health Topics page on Bisphenol A and the the policy statement from the FDI.

Occupational Exposure

Monomer exposure can be a risk to dental personnel, especially since resin monomers are known to permeate latex and vinyl gloves, and exposure risk is increased with uncured resins.^{4, 46} Localized reactions are largely in the form of contact dermatitis and eczema. Proper instrumentation should be used when handling uncured resin materials.^{4, 46}

Scientific Assessment of Dental Restorative Materials (Trans.2003:387)

Resolved, that although the safety and efficacy of dental restorative materials has been extensively researched, the Association, consistent with its Research Agenda, will continue to actively promote such research to ensure that the profession and the public have the most current, scientifically valid information on which to make choices about dental treatment requiring restorative materials, and be it further

Resolved, that the Association use its existing communications vehicles to educate opinion leaders and policy makers about the scientific methods used to assess the safety and efficacy of dental restorative materials, and be it further

Resolved, that the Association continue to promptly inform the public and the profession of any new scientific information that contributes significantly to the current understanding of dental restorative materials.

American Dental Association
Adopted 2003; Reviewed 2017

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ADA ACE Panel Reports:

ADA MouthHealthy: Dental Filling Options
Composite resins
Dental amalgam
Gold fillings
Bisphenol A (BPA)
Bridges
Crowns
Sealants
Veneers