Dental Resin Composites And Their Role In Minimally Invasive Operative Dentistry

(Taken from Banerjee A. A Clinical Guide to Advanced Minimum Intervention Restorative Dentistry. Elsevier, 2024; in press - ISBN: 9780443109713)

Introduction to dental composite filling materials

Modern restorative dental composite filling materials used to restore teeth using minimally invasive operative techniques, can be classified in terms of their:

• Retention mechanism – how they are secured in the tooth
• Constituent chemistry  - inert, bio-interactive and/or bio-active
• Clinical properties - aesthetics, strength, wear, handling characteristics.

As part of the “golden triangle” for the clinician to achieve an optimal bio-interactive clinical restoration of a patient’s tooth, it is essential that:

• The chemistry of materials are considered in conjunction with
• The histological tooth tissue to which they will adhere / interact with and with
• Their intricacies in clinical handling and placement techniques.

Dental resin composites are aesthetic (tooth-coloured), plastic adhesive restorative (filling) materials that consist of co-polymerised methacrylate-based resin chains including inert filler particles (conferring strength and wear resistance) and requiring a separate adhesive (bonding agent) to bond them to either enamel or dentine, respectively. Resin composites have been developed by the dental industry over the past 50+ years, after the introduction of the acid-etch technique (Buonocore, 1955) and methacrylate monomers (Bowen’s resin – Bis-GMA (1971).

Dental Composite Restoratives:

Composite resin matrix

The uncured resin composite consists of a mixture of several different types of resin dimethacrylate monomers, most of which are hydrophobic (water-repelling) in nature. The resin monomer chain length affects certain properties of a resin composite:

• Viscosity (or flowability): this is important in order to minimise voids trapped within the uncured resin composite during its clinical placement and packing within the depths of a cavity (the stiffer the consistency, the greater the risk of trapping air voids). The shorter the uncured monomer chain lengths (and lower the molecular weight), the less is the overall viscosity of the material. 
• Volumetric polymerisation shrinkage and stress: the setting process of a resin composite material is a light-activated, free-radical addition polymerisation chain reaction. The shorter the linear dimethacrylate monomer chains, the more of them need to join per unit volume with a greater amount of inter-molecular space closure resulting in the relatively increased shrinkage exhibited towards the centre of the restoration (ranging from <1 – 5 % vol depending upon the precise formulation and curing conditions). The polymerised resin is highly crosslinked but only 35 – 80% of these chains polymerise to transform monomers into polymers. As a result, shrinkage stresses develop throughout the different stages of the chemical setting transformation. During the photo-polymerisation process, usually of 10-20 seconds duration dependent on the material and light source, the resin composite is transformed from a fluid paste to a glassy, hard state. 

Reducing the polymerisation shrinkage can be afforded by:

• Modifying the material’s formulation, such as the resin matrix chemical composition or the filler particle content
• Applying incremental layering techniques clinically for placing direct conventional resin composites
• Using a layered or sandwich restoration technique to provide a base layer in a deeper cavity that might better relieve the internal stresses of the overall restoration
• Using bulk-fill resin composites with larger increments
• Modifying light-activation protocols

Flowable Composites:

Composite filler particles (Figure 1a,b)

Inert, dispersed filler particles are made from silica, quartz, barium or strontium glass derivatives (introducing radio-opacity, useful for material identification on dental radiographs). These are included within and bound to the resin.

Over the past 40 years, manufacturers have attempted to pack in various sizes / shapes / formulations of particle in increasing numbers per unit volume in an effort to affect the clinical properties of resin composites. The more space taken up by irregular and spherical-shaped filler particles, the less space there is for native resin monomers, leading to a reduction in overall shrinkage on light-curing.

Conversely, the more filler particles loaded into the resin composite, the more viscous it becomes so there is a fine line between balancing the filler load and its resin content, depending on the specific properties required of the material. The filler particles confer several properties on the cured resin composite material:

• Wear resistance: increasing the density of larger, harder and more irregular shaped filler particles will tend to increase the wear resistance of the resin composite as less resin matrix will be exposed at the restoration surface.

• Surface polish: the smaller, softer and more spherical-shaped the filler particles, the more polishable the resin composite (but less wear resistant).

• Aesthetics: the finer the filler particles, the more aesthetic the resin composite as the optical properties can be made to match more accurately that of enamel. Modern resin composites can have numerous shades (using pigments and various degrees of opacity). On the other hand, some rely on the ‘chameleon’ effect where a relatively translucent resin composite can encompass several tooth shades, blending into the surrounding natural colours effectively by transmitting colour from them. The size, shape, amount and type of filler particle can affect light scattering / propagation through the cured resin composite material, so affecting its aesthetic properties as well as light transmission during photo-curing.

• Physical properties, e.g. compressive and shear strength, elasticity: the correct balance of filler particle : resin content is required to ensure optimal physical properties, attempting to match those of natural tooth structure where possible.

Figure 1a Shows the size range of filler particles and their distribution within the resin matrix of modern dental resin composites.

1b diagrams showing four different types of filler particle morphology found in modern resin composites. These may be used in different combinations within the resin matrix. The colour-coded particles relate to the filler particle size scale in Fig 1a.

Both figures taken from Banerjee A. A Clinical Guide to Advanced Minimum Intervention Restorative Dentistry. Elsevier, 2024.

Composite polymerisation (setting) reaction

The setting (curing) process in resin composites is either light-activated (470 nm wavelength visible blue light) or dual-cured, that is both light and chemically activated. 

Some issues related to the setting reaction of resins include:

• Degree of monomer conversion (35 - 80%): not all the free monomer polymerises on curing, leaving some to potentially leach out of the cured resin composite so contributing to the long-term degradation of the material in terms of moisture ingress and possible sensitisation issues with some patients, especially in deep cavities closer to the pulp, where the uncured monomers might cause direct irritation to pulp cells with a concomitant inflammatory cellular reaction

• Shrinkage on curing (<1 - 5%): shrinkage occurs during polymerisation towards the centre of the restoration bulk and light source, so causing the resin composite to pull away from cavity margins so increasing the risk of marginal leakage at the tooth-restoration interface

• Shrinkage stress at the resin composite–tooth interface (3–8 MPa): the stresses generated by the polymerisation process can be high at cavity margins, the bulk of the material and the structural compliance of the cavity walls). This can lead to debonding and / or tooth cracking / fracture of the weakened marginal tissues

• Air-inhibited surface layer of uncured resin: the free radical addition polymerisation reaction is inhibited by air. After curing an increment of resin composite, a glossy film of uncured resin is retained on its surface and the monomers in this layer are used to provide adherence for the next increment placed upon it. In this way, resin composites can be added to incrementally within a cavity to build up the final restoration, as an additive process

• Depth of cure: conventional resin composites can be cured up to a depth of 2 mm (depending on the translucency of the material). A thicker section will not cure sufficiently at its base (the 470 nm light does not penetrate sufficiently at those depths to permit activation of the polymerisation reaction) – the resulting restoration may then be described as having an uncured or ‘soggy bottom’. Note, conventional resin composites cure most efficiently closest to the 470 nm light source. This ideally should be placed as close as possible to the uncured resin composite which should be built up within deeper cavities, in smaller <2mm thick, angled increments.

• ‘Bulk fill’ resin composites: these materials are available, described by manufacturers as a “dentine replacement” for the restoration of deep cavities. They can exhibit a relatively low viscosity, ‘flowable’ consistency so that they achieve good adaptation to the deep cavity floor and walls, almost “self-levelling” within the cavity.  Mechanical properties, such as toughness, may be compromised to a degree to achieve this effect, but this may be an acceptable compromise when used in bulk, within the depths of a large cavity, especially if overlaid with a more wear-resistant material.  Their depth of cure is achieved by the use alternative monomer combinations, creation of a relatively translucent material, often achieved with large filler particles, so allowing better light transmission through the material and hence improved monomer conversion at deeper levels within the material.  There are also more viscous, heavily filled versions of these materials, which can be placed in bulk, with better handling and adaptation characteristics at contact points and the development of occlusal anatomy, but there they be compromises also such as achieving adequate adaptation on the cavity floor (leading to an increased risk of voids). 

• Light- / photo-curing: Effective photo-polymerisation is a significant contributing factor for the quality and long-term clinical success of light-activated resin-based composite restorations.  The changes in light transmission throughout polymerisation of a light-curable resin composite are complex and are significantly affected by the constituents and chemistry of the material and the light-curing unit (LCU) itself and a suitably matched range of curing wavelengths. 

The tooth–resin composite interface

Hydrophobic resin composites adhere to inherently dry enamel micro-mechanically and the crystalline ultrastructure of enamel permits this to happen successfully, reliably and strongly. Three stages are required to develop the bond between resin composite and enamel after the enamel has been prepared mechanically during cavity preparation:

1. Acid etching (be aware of the alternative US term – ‘conditioning’) – this is the process of placing 37% orthophosphoric acid etch gel onto the prepared enamel surface for 20 seconds. This:
  - dissolves surface contaminants (saliva, proteinaceous substances) and the smear layer (a tenacious surface layer of organic and inorganic cutting debris <20 µm thick) and increases surface area for bonding.

  - produces micro-irregularities / micro-porosities in the prismatic enamel surface. This microscopic surface roughness will provide micro-mechanical retention for the flowable resin to penetrate and lock into.

2. After washing the acid etch gel off and drying the enamel thoroughly, a layer of fluid, unfilled hydrophobic bonding resin (that is essentially the resin without filler particles) was traditionally placed onto the etched enamel surface. This has now been superceded by modern dental adhesives (see below). This bond can flow easily into the micro-undercuts / porosities created by the etching process and it is then photo-polymerised. Note, the chemistry of the unfilled fluid bonding resin is not precisely the same as that for a dentine bonding agent.

3. The final resin composite can then be placed directly on the layer of photo-cured bonding resin, using the air-inhibited layer, in small angled increments (to lessen the effects of polymerisation shrinkage) and photo-cured to form the final contoured restoration 

The fundamental clinical problem to overcome when adhering resin composites to dentine, is to achieve intimate compatibility of two immiscible substrates: dentine is hydrophilic (likes water and is innately wet) whereas methacrylate resins are hydrophobic (water-repelling). Therefore, a dentine bonding agent (DBA) or dental adhesive is required as a coupling agent between the two. DBAs have three generic components:

• Acid etch: the first stage of the process, which has the following effects on dentine:

  -       Dissolves dentine smear layer created by the cavity preparation process 

  -       Helps to unblock and widen the dentine tubule orifices

  -       Demineralises the dentine surface so exposing the network of collagen in the dentine matrix

• Primer: a bi-functional coupling molecule (e.g. hydrophilic HEMA – hydroxyethylmethacrylate) which has one functional group that is hydrophilic (“water-liking”), so is compatible with the moist collagen in dentine and another that is hydrophobic (“resin-liking”), so is compatible with the bond / resin composite. The primer is carried into the moist collagen fibrillar network using a solvent (acetone or alcohol-based), displacing the water molecules from the collagen and permitting the primer to enter into the micro- and nano-spaces created around the collagen fibrils after the etching process.  
• Bond: this can be considered simplistically as the resin component of the composite without the filler particles. A mixture of low viscosity monomers capable of penetrating the spaces not occupied by the primer monomers, it can be photo-cured in the conventional way and then finally, the resin composite placed incrementally (chemically cross-linking to the monomers in the air-inhibited layer of the bond) and the restoration completed.

Bulk Fill Materials:

Developments in composite technology

The world of dental biomaterials is constantly evolving and by the time this article is published new materials will be available to be used clinically! Some of the ongoing challenges / developments are discussed below.

Reducing shrinkage stress / strain 

Manufacturers can alleviate the problem of significant volumetric shrinkage (often leading to increased marginal stress and / or material strain) by producing low-shrink materials, or materials that impart little strain on the tooth during their setting and maturation process. Historically, one such system used siloxane-oxirane (silorane) chemistry, as opposed to using conventional methacrylates. In this system, the monomers, instead of being linear, were ring-opening so, during the cationic polymerisation process, less shrinkage occurred (0.9%). This chemistry required a separate bonding agent, which was very hydrophobic, so minimising the water transition through the adhesive.

Another system is a dimer-acid nano-hybrid composite (based on dimer dicarbamate dimethacrylates), which exhibits polymerisation-induced phase separation (expansion) on curing so reducing some of the effects of polymerisation shrinkage.  Whilst these low shrink materials have been shown to be effective, commercially it seems, dentists are still using conventional resin composites, perhaps indicating that the clinical manifestations of shrinkage stress can be mitigated by careful operative handling.  Conversely, the ‘bulk fill’ material developments indicate that low stress / low viscosity composites that simplify restoration placement can become popular. Clinical studies are now showing favourable results with these materials and they are gaining in popularity. 

Dentine bonding agents / dental adhesives

With the advent of low-shrink composites, the importance of direct bond strengths may lessen and more emphasis placed on the ionic interaction between the materials and tooth structure. Medicinal ions could be transferred from the adhesive to help remineralise the enamel or dentine or act as anti-bacterial agents. Enzyme-inhibitor molecules have been introduced in an attempt to reduce enzymatic MMP-induced breakdown of collagen in the hybrid zone. Time will tell regarding their clinical efficacy and uptake by the profession.

‘Self-adhesive composites’

The ultimate development, a simple to use, self-adhesive composite hybrid restorative material, both self- and light-cured, has been developed and marketed at the time of publication (Surefil one™, Dentsply Sirona). This classification of material uses a modified polyacid system that promotes bonding to tooth structure while also acting as a copolymerising crosslinker between the covalent and ionic structural network in the set material.  Even though laboratory testing data indicate at least an equivalent performance of this material compared to other clinical restorative materials (in terms of in-vitro mechanical strength, dimensional stability, wear resistance, marginal integrity and adhesive performance to sound enamel and dentine), further long term clinical evaluation is required in the form of randomised controlled clinical trials to ascertain if this in-vitro data can be extrapolated to the clinical environment. In this area of materials research, the boundary between resin composite and glass-ionomer chemistry is becoming more blurred.

Want to learn more about Advanced Minimum Intervention Restorative Dentistry? Take a look at Professor Avijit Banerjee's Book - A Clinical Guide to Advanced Minimum Intervention Restorative Dentistry here.

Also find out more about his distance-learning MSc course in Advanced Minimum Intervention Restorative Dentistry, open to practising dentists & dental therapists, at Kings' College London, here.

About The Author:

Professor Avijit Banerjee

BDS MSc PhD (Lond) LDS FDS (Rest Dent) FDSRCS (Eng) FCGDent FHEA FICD

In 2022, he was awarded the prestigious William H Bowen Caries Research Distinguished Scientist Award from the International Association of Dental Research (IADR) in recognition of his global scientific and clinical research impact in this discipline over the last 25 yrs.

He has been appointed to the UK National Institute of Health Research (NIHR) Clinical Research Network, as Oral & Dental Health Speciality & Industry Lead for South London where he is responsible for the development of primary care clinical trials and their professional / participant recruitment programmes. He is setting up a primary care research network in South London, where oral healthcare practice teams are trained and enabled to carry out dental & oral healthcare research. He acts as an international cariology R&D KOL for many international dental industry partners, including GC Europe / UK, 3M Oral Healthcare, Septodont France / UK, Dentsply Sirona, Pulpdent, Colgate and Oral B.

Avijit is primary author of the new edition, “A Clinical Guide to Advanced Minimum Intervention Restorative Dentistry” (Elsevier, 2024), the successor to “Pickard’s Guide to Minimally Invasive Operative Dentistry” (10th edition; OUP, 2015), a definitive and globally respected text in its field, amongst other book editorships (Minimally Invasive Esthetics, Elsevier (2015), Odell’s Problem Solving in Dentistry, 4th ed, Elsevier (2020)) and 12 further book chapter contributions (including, amongst others, caries management in The Principles of Endodontics 3rd ed, 2019).  He is an editor-in-chief of Oral Health & Preventive Dentistry (Quintessence Ltd), Associate Editor of the British Dental Journal and a senior editorial board member of  Dental Update, International Journal of Adhesion & Adhesives and the Primary Dental Journal.

He is a senior executive member of the British Dental Association (BDA) Health & Science Committee (2012 - ), contributing to scientific policy direction at the BDA all whilst maintaining wet-fingered specialist clinical practice in Restorative Dentistry, Prosthodontics & Periodontics. He was President of the BDA Metropolitan Branch London Section (2018-19) and currently holds an Hon. Consultant Advisor post to the Office of the Chief Dental Officer, England (2020 - ). In this role he inputs into national NHS oral & dental care policy direction / advice to the UK government’s department of health and social care, focusing on the prevention of dental caries in the most vulnerable, high needs patient groups. He wrote the UK clinical caries management guidelines for general oral healthcare practice teams in 2020, for the post-lockdown recovery of clinical practice. He also chairs the Career Pathways Programme Board at the new UK College of General Dentistry, helping to develop an integrated system for career development for all oral healthcare team members in primary care. He is the elected UK council member of the European Federation of Conservative Dentistry (EFCD) and  co-chairs the UK chapter of the Alliance for a Cavity Free Future (ACFF), all these national / international roles enabling him to co-author international clinical guidelines on dental caries management and influence their clinical implementation globally, aiming to help implement / translate his cariology & operative dentistry research into primary care clinical practice in the UK and abroad.

Associated Links & Further Reading

• https://kclpure.kcl.ac.uk/portal/en/persons/avijit-banerjee
• https://www.kcl.ac.uk/people/avijit-banerjee
• https://orcid.org/0000-0003-0091-7348
• https://www.researchgate.net/profile/Avijit_Banerjee
• https://www.linkedin.com/in/avijit-banerjee-0b17518/
• http://www.kcl.ac.uk/dentistry/study/distance/AMID/AMIDStructure.aspx
• https://youtu.be/-W8NuD3MIxM
• https://youtu.be/wOyn3PdhjxU
• https://youtu.be/gBS4P3sxWqA
• https://youtu.be/L3VYMq7R4F
• https://www.youtube.com/watch?v=UCk6KGf8zto&ab_channel=IADR
• https://dentalblog.3m.com/dental/selective-caries-removal-take-it-or-leave-it-2/
• https://lnkd.in/eRwzhWui