Resin Polymers Based Tooth Coloured Filling Dental Materials

Polymer science: research advances, practical applications and educational aspects (A. Méndez-Vilas; A. Solano, Eds.) ________________________________...
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Polymer science: research advances, practical applications and educational aspects (A. Méndez-Vilas; A. Solano, Eds.) _______________________________________________________________________________________________

Resin Polymers Based Tooth Coloured Filling Dental Materials M. S. Zafar1,*, S. Liaqat2, S. Najeeb3, Z. Khurshid4, M. Alrahabi1 and S. Zohaib5 1

Department of Restorative Dentistry, College of Dentistry, Taibah University, Medina Munawarah, Saudi Arabia Biomaterials and Tissue Engineering Division, UCL Eastman Dental Institute, London, UK, WC1X 8LD 3 Department of Restorative Dentistry, Al-Farabi College, Riyadh, Saudi Arabia 4 Department of Prosthodontics, School of Dentistry King Faisal University, Hofuf , Saudi Arabia 5 Department of Biomedical Engineering, College of Engineering, King Faisal University, Hofuf , Saudi Arabia 2

Polymers based resin composite materials have gained popularity in recent years for a range of dental restorative applications. These materials are tooth couloured and have excellent aesthetic proprties. The incorporation of nanotachnology approaches to these materials has further improved their physical and mechanical properties hence expanding their clinical applications. This chapter described the chemistry and key properties of resin polymers based materiall being used in dentistry. In addition, approach for clinical restorations using these materials has been highlighted. Keywords: Polymer composites; Dental resins; Tooth filling materials

1. Introduction The aim of this chapter is to compile history, chemistry and other properties of resin polymers in dentistry. Today, teeth can be filled with gold, porcelain and amalgam; or tooth-coloured resin based materials such as glass-ionomer cements and polymer composites [1]. A range of factors such as extent of the decay, filling materials cost, patients' insurance coverage, and dentist's recommendation contribute in defining the choice of restoratives. Polymer based composites were commercially introduced in the mid 1960s. Initially it was indicated for restoration of anterior teeth. Since then dental composites have gone through marked improvement in its mechanical and physical properties, durability, wear resistance, and manipulative qualities. More recently, research has been carried out on re-mineralising, anti-bacterial, and self-adhesive properties of dental composites. Dental resin composites are widely used instead of conventional amalgam. Today, they are the most commonly available materials in dentistry as they are used for a number of clinical applications in dental clinics. It can be used as a filling material, as a luting agent, sealant, and in indirect restorations. Dental polymer composites mainly have three major components: inorganic fillers, an organic polymer matrix, and a coupling agent. The fillers can be glass or other reinforcing fillers. The matrix is mainly formed from high molecular weight monomers such as urethane di-methacrylate (UDMA), and bisphenol A-glycidyl methacrylate (Bis-GMA) see figure-1 [2].

Fig.1 Chemical structures of Bis-GMA, UDMA, TEGDMA and Bis-EMA6.

Filler particles are added to gain certein benefits such as increased strength, reduced polymerisation shrinkage and heat generation. A silane coupling agent is added to augment the bond between these two components and to aid filler distribution [3]. An initiator and activator are usually added to begin and later control the polymerisation process when

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external energy (light) is applied. The choice of suitable monomers for resin composite formulation intensely influences the viscosity, reactivity, and polymerization shrinkage, mechanical properties, water uptake and swelling behaviour. The properties of dental composites are significantly affected by the fillers employed. According to the nature and the particle size of the filler the dental composites have been classified into four main groups, traditional composites, micro-filled composites, hybrid composites, and small particle hybrid composites [3].

2. Composition of resin composites Dental composites are generally composed of a matrix monomer and filler [4, 5]. Bis-GMA and UDMA are some of the common monomers used. Silica is a commonly used filler. Additionally, silane coupling agents are added to strengthen the bonding between the matrix and fillers and dilutant comonomers such as TEGMA are added to improve the handling properties. Fillers are responsible for providing the mechanical properties to the material. Glass and/or ceramic fillers may be added to further improve the optical properties and wear resistance. Most resin composites are lightactivated which means they primarily polymerise upon exposure to a particular wavelength of light. Hence, photoactivating system is present in most modern dental composites. Camphorquinone, tertiary amine, is a commonly used photo-initiator [6]. PPD (1-phenyl-1,2-propanedione), Lucirin TPO (monoacylphos- phine oxide) Irgacure 819 (bisacylphosphine oxide) are some other photo-initiator systems that are used [7]. Reducing the filler size and number has a number of effects on the properties of resin composites [8]. Reducing filler and number tends to make the resin composites more flowable, a property desirable for injectable universal composites. Surfactants may be added to flowable composites to minmize polymerization shrinkage. These flowable composites are generally used to restore areas to tooth difficult to be restored with conventional composites. Packable composites usually contain fillers in a variety of sizes to maximize the mechanical properties of the resin composites and are applied and adapted much akin to dental amalgam to restore high stress-bearing areas. Conventional resin composites used to contain fillers as large as ∼50µm and possessed considerable strength. However, their main disadvantage was a lack of polishibility owing to their high surface roughness. To address these issues, ‘microfilled’ resin composites were introduced. These resin composites contain particle sizes in the range of 40nm. Although microfilled composites possess a much higher polishibility than conventional composites, they have the disadvantage of being weak and wearprone. To overcome these drawbacks, the large fillers are combined with nano-sized fillers to produce midfilled composites. Further modifications of resin composites gave rise to minifill composites (microhybrids) containing a combination of fillers in the average size range of 0.6 – 1 µm in combination with 40 nm fillers. Microhybrids are popularly known as universal composites owing to their high polishibility as well as possessing adequate mechanical properties for posterior restorations. Nanocomposites contain fillers in the size range of 5–100 nm and are highly polisihable. Nanocomposites and microhybrids may be combined to produce ‘nanohybrids’. More recently, a significant amount of research has focused on producing bioactive resin composites [1, 9, 10]. For example, incorporation of nanosized hydroxyapatite crystals not improve the mechanical properties of resin composites but may also have a potentially odontogenic effect to promote remineralization [11]. The mechanical properties of such materials may be further improved by silanization [12]. Newer polymers such as polyetheretherketone (PEEK) hold potential to be used as indirect esthetic restorative materials. But their clinical efficacy is yet to be proven [9, 13].

3. Properties of composites 3.1 Physical properties A physical property is any measurable property that describes the state of a physical system. In composites the changes in physical properties are under influence of external factors such as light, temperature, pressure, or force. 3.1.1 Optical properties of Composites Composite resins have been introduced into dentistry to reduce the weak properties of acrylic resins [14]. Resin composites are used in the complex oral environment where they are likely to interact with a variety of reagents such as saliva, crevicular fluid, bacterials toxins food components and beverages [15]. These chemicals causes surface staining of materials, and effects its colour and aesthetics . Colour determination in dentistry can be instrumental and visual. Usinf instrumental colorimetry can be used to eliminate any subjective errors during shade matching and colour assessments [16]. Dental composites are primarily planned to resolve an aesthetic rather than a functional problem [17]. Similarly at other times aesthetic requirements are added to mechanical ones. To ensure excellent aesthetics it is necessary for composite materials to maintain intrinsic colour stability[15] and a resistance to surface staining [18].

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3.1.2 Depth of cure Resin composites are cured upon activatio of polumerisation reaction in respose to lasers [19] or blue coloured light [20]. Light is scattered by composites, and can penetrate only to a small distance. The depth of cure is governed by how far light can penetrate into the material. It is important because it determines the thickness of layers of composite that the clinician can place. The depth of cure of composite depends on a number of factors. This includes monomer chemistry, (its size and composition), fillers addition, colour, translucency, and initiator systems [20]. The depth of cure is measured through a number of techniques. Some of the commonly employed methods are micro-hardness measurements, measurement using FTIR spectroscopy, micro-Raman technique, optical microscopy, and scraping test (ISO 4049) [20]. Each method has its own short comings, and does not address the entire process completely. Adequate depth of cure is very important for clinical success of the restoration. Unsatisfactory cure at the deeper parts of restoration will result in inadequate bonding of composite to tooth structure in deep cavities [21]. There will be loss of marginal seal and adaptation, which will result in micro-leakage, sensitivity, and recurrent caries. 3.1.3 Shelf life The shelf life is defined as the time period from the date of manufacturing, till the material retains its prescribed physical and mechanical properties [22]. Material properties changed over three to four years may not be necessarily clinically noticeable, but may impact the performance of restoration in long term [22]. Utilization of expired dental materials is not recommended, as the properties of the composite materials are affected by aging [22]. The shelf life of composites varies with the type of resin used and the manufacturer. Normally avoiding light and heat exposure can extend shelf life of composite restorations. Similarly, refrigerating the material can also extend shelf life. The average shelf life is 2 to 3 years if stored properly [23]. 3.2 Mechanical Properties Although composites take advantage of selected properties of each constituent material, the physical and mechanical properties of the composites are different from those of the separate phases. 3.2.1 Biaxial flexural strength and modulus The resin based filling materials ideally should have sufficient mechanical properties to withstand the stresses generated due to masticatory forces. The composite materials in the restored tooth cavity are under the influence of constant compressive, tensile, and shear stresses [24]. Flexural stress combines all these stresses. Therefore, determining the flexural strength of dental composites is a suitable way to assess the mechanical performance of filling materials. The common mode of failure of dental composite is brittle fracture [25]. This can be due to the propagation of the preexisting cracks under various stresses. These cracks can also be generated by the incorporation of air bubbles during material mixing / placement, or may be due to the polishing procedure [26]. The micro-structural imperfections could also lead to cracking in the material [26, 27]. Biaxial flexural strength, and modulus testing is considered more accurate and advantageous over three point, tensile, and compressive test [28, 29]. Usually circular discs are prepared of the desired composite, and placed on a knife edge ring support. In biaxial testing the maximum loading occurs in the central loading area, thus eradicating any edge effect [28]. 3.2.2 Toughness and resilience Toughness is the measure of the deformation energy (elastic and plastic) needed to fracture a material. Toughness is calculated by the area under the elastic and plastic portions of a stress strain curve. On the other hand resilience is the amount of energy necessary to deform a material to its proportional limit. Resilience is calculated by the area under the elastic portion of stress strain curve. Both toughness and resilience helps in estimating bulk-material-breakdown properties [30]. In oral cavity composite materials are under constant stresses. Materials with high toughness and resilience can with stand high stresses, and results in materials to with stand brittle fractures[31, 32]. 3.2.3 Wear behaviour In spite of great progression of dental materials in the last years, wear of composite materials is still a major concern. Several components of composite resins directly affect and limit their wear resistance. Composite size, shape and amount of fillers, their mechanical, chemical and adhesion properties all affects the wear properties [33]. A number of wear mechanisms are described in the literature: adhesive, abrasive, impact, fatigue, and so on [34]. Adhesive wear occurs between two materials when the surfaces adhere and the shear action results in detachment of fragments of one material and attachment to the other one [34]. Abrasive wear occurs due to sliding movements of a harder surface slides over a softer surface and damage it by plastic deformation or fracture. Impact wear takes place when two surfaces collide with each other while having large relative velocities in the direction normal to their interface [34]. Fatigue wear results from the repeated sliding or rolling of one surface over the other [31, 32].

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3.2.4 Bond strength (adhesion) of resin composites Adhesion to dentine depends on a number of factors and usually involves bonding through different mechanisms [35]. Adhesion of a material to a substrate can be mechanical, chemical, physical, or a combination of all of these [26]. Mechanical adhesion is the simplest form of adhesion. It can result from the surface roughness, and irregularities. These irregularities are sites for mechanical interlocking of the material with the dentine structure. In mechanical adhesion there is no molecular interaction between the material and substrate [26]. The second type of adhesion is physical adhesion. This type of bonding occurs when polar molecules are in close proximity, so they develop a dipole interaction. Usually the bonding forces are very small. This type of bond is weaker than the covalent and ionic bond. Another feature of this type of adhesion is that bonding is very fast, and reversible [26]. Chemical adhesion occurs when a molecule in the material surface chemically (ionic, or covalent) interacts with the substrate to form a strong bond. This type of adhesion is very strong as it involves molecular interactions [26]. Bond strengths can be calculated using a variety of tests [36]. The commonly used tests to assess the adhesion of a material include shear, tensile, push out, micro-shear, and micro-tensile [37, 38]. Each test has its own advantages and disadvantages. 3.3 Chemical Properties Chemical changes taking place due to addition of various fillers and monomers can affect various properties of dental composites. Some of the properties are highlighted below. 3.3.1 Water sorption Addition of calcium phosphates and other leachable components to composites can cause changes in their hygroscopic properties [39]. These materials have the ability to absorb water molecules into the composite. In this process water molecules diffuse into the composite, and gradually occupy the free volume. This will cause plasticisation of the matrix phase and expansion [39]. Some of the water molecules get bound inside the composite matrix, and allows absorption of more water molecules. These bound water molecules have the potential to disturb the inter-chain hydrogen bonding. This will result in expansion and plasticisation of the material, and may affect long term mechanical and wear properties [40, 41]. The water sorption can be beneficial to an extent, if it compensates for the polymerisation shrinkage during the polymerisation reaction [42]. The amount of expansion varies from one material to another which can have high clinical impact especially inside closed cavities. It can also cause serious problems eg tooth cracking if over expansion occurs. Research has shown that such over-compensation can cause cusp fractures, and internal cracks in materials [43]. Therefore balance between shrinkage and expansion is important for the clinical success. 3.3.2 Drug release Recently Drug/medical device products have been studied extensively. Use of drug delivery and combination products have been used clinically to deliver drug locally in cardiovascular diseases, cancer, orthopaedics, dental applications, and diabetes [44]. The common drug added to dental materials is chlorhexidine [45, 46]. Use of chlorhexidine, and other drugs in dental composites has also been previously documented in the literature [46, 47]. Chlorhexidine was added into various experimental dental composites due its low minimum inhibitory concentrations against oral bacteria and ability to inhibit metalloproteinases (MMPs) [46]. Resin materials discharging chlorhexidine in the initial stage maydecrease the need for extensive tissue removal [48]. The release of antimicrobial drugs has gained particular attention recently. As compared to systemic drug delivery, a key feature in local drug delivery is that high doses of drugs can be targeted to specific sites in quick time, without any serious systemic drug toxicity [49]. Different drugs show varying levels of release, which depends on the local environment. Some materials release high doses of antimicrobials very quickly, but the activity lasts only for a short term. On the other hand some materials are slow drugreleasing systems, and take time to reach a level of effectiveness. The drugs released must act before bacteria develop any protective extracellular matrix [49]. Similarly the role of fluoride released from the dental restorative materials is well accepted for preventing tooth decay [50-52] 3.3.3 Degree of conversion Review of the literature suggests the use of various different direct and indirect methods for the determination of degree of conversion of dental composites [53]. The most commonly used indirect methods include micro-hardness, use of differential thermal calorimetry (DTC), and differential scanning calorimetry (DSC) [54]. These techniques measure the relative rate of polymerisation, rather than the absolute degree of monomer conversion. FTIR and Raman are the commonly used direct methods for determination of degree of conversion. These methods are simple to use, and directly quantify the amount of un-reacted monomers. In this study monomer conversion was assessed using FTIR. The extent of conversion can be defined as the percentage of carbon-carbon double bonds converted into single bonds. One of the reason that causes the monomer to be un-reacted is its low mobility. Rapid polymerisation after light curing results in the development of a complex polymer network which leads to reduced movement of some of the monomer

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molecules [55]. Insufficient levels of monomer conversion can cause initiator leaching, less biocompatibility, reduced mechanical and wear properties, and colour instability [56]. On the other hand high monomer conversion means high polymerisation shrinkage and heat generation. 3.3.4 Polymerisation shrinkage and Heat generation Polymerisation shrinkage is considered an important property for the long term success of the restoration. Many factors are responsible for polymerisation shrinkage in a dental composite after restoration. These include restorative procedure, light intensity, cavity design, polymerisation characteristics, type of monomers used, and filler loading [33]. The observed magnitude of polymerisation shrinkage can also depend on the methods used for its measurement [57]. Furthermore the results obtained can vary between operators. Comparison of the published work on shrinkage can therefore be difficult. Some of the commonly used methods include: use of mercury dilatometer, optical methods, bonded disk method, gas pycnometer, and wall-to-wall shrinkage [58]. Each method has its own short comings, and there is no single method that explains all the issues that surrounds polymerisation shrinkage. Shrinkage is generally proportional to heat generation and are both proportional to the number of polymerising methacrylate groups [59, 60]. After polymerisation the molecules are more closely packed together, which leads to bulk contraction of the composite, and a reduction in volume [61]. The shrinkage and heat generation is directly proportional to the number of double bonds converting. High polymerisation shrinkage and heat generation is usually associated with low molecular weight monomers [5]. Current composite materials shows a polymerisation shrinkage in the region of 1-4 % by volume [59, 62]. 3.4 Biological Properties Materials biological properties are considered very important in the longevity of the restoration in oral cavity. A number of tests are used to assess the biocompatibility of the material in the oral environment. 3.4.1 Biocompatibility The majority of bulk and diluent monomers used in composites (Bis-GMA, TEGDMA, and UDMA, among others) are cytotoxic in vitro [63]. The biocompatibility of a cured composite depends on the extent of release of these individual monomers from the polymer matrix. Although composites may release some levels of un-polymerised monomers into surrounding oral cavity for weeks after curing, there is significant disagreement about the biological affects of these constituents. Inside the cavity a dentine barrier distinctly reduces the ability of components to leach into pulpal tissues [64]. On the other hand, composite used as a direct pulp-capping agents carries a higher risk for adverse biological responses, because no dentine barrier exists in that case. The International Organization for Standardization (ISO) standard for testing of toxicity of dental materials requires the testing of composite materials after storing in several aqueous and organic media, followed by testing of the eluants for biocompatibility [64]. The tissue that poses higher risk includes to be the mucosa in close, long-term contact with composites. Finally, there has been some studies about the components used in composite organic matrix that act as xenoestrogens [64]. Studies have shown that bisphenol A is estrogenic in vitro tests [64]. However, its estrogenicity from cured commercial composites has not been demonstrated yet.

4. Advantages during cavity preparation for resin composite restorations The tooth preparation for a composite restorations has the major benefit of saving the tooth structure and includes the following essential steps: I. II. III.

Removing the caries fault and old restorative material, The enamel margins should be prepared of 90 degrees or greater (>90 degrees usually preferable) Creating 90-degree (or butt joint) cavosurface margins on root surfaces.

In addition, the cavity preparation for composite restoration differs from amalgam restorations by the following: I. II. III. IV. V.

Minimum outline extension and conservative as possible (adjacent areas at-risk [grooves or pits] may be “sealed” rather than restored) No need to uniform axial or pulpal wall or both (varying depth) Enamel bevel Increase the surface area for bonding by roughening tooth preparation walls (by Acid etching) Possible use of a diamond stone (to increase the roughness of the tooth preparation walls).

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The composite restorative material retained within the tooth by micromechanical bonding of the composite to the roughened, etched, and primed enamel and dentin [35]. In some cases, a dentinal retention groove may be prepared to enhance the retention form. Composite restoration tooth preparation design vary depending on several factors. The required cavity preparation usually is decided by shape, size and location of the lesion and whatever extensions are necessary to provide access for vision and instrumentation. There are five designs of tooth preparations for composite restorations these designs are [65]: a. b. c. d. e.

Conventional design Bevelled conventional design Modified design Box design Slot preparation.

4.1. Conventional design Conventional are those typical for amalgam restorations. The Conventional tooth preparations indications indicated for: (1) preparations located on root surfaces (nonenamel areas) and (2) moderate to large Class I or II restorations. This design facilitates a better seal between the composite and the dentin or cementum surfaces and enhances retention of the composite material in the tooth. An inverted cone diamond stone or carbide bur is used to prepare the tooth, resulting in a preparation design similar to that for amalgam, but usually smaller in width and extensions and without prepared secondary retention form. The cavosurface angle in areas on the preparation periphery can be more flared (obtuse) than 90 degrees. The occlusal cavosurface angle is obtuse, yet provides for occlusally converging walls. Because conservation of tooth structure is important, Class I or II conventional composite preparations should be prepared with as little faciolingual extension as possible and should not routinely be extended into all pits and fissures on the occlusal surface where sealants may be otherwise indicated. Likewise, the boxlike form increases the negative effects of the Cfactor. 4.2. Beveled conventional design Beveled conventional tooth preparations are similar to conventional preparations in that the outline form has external boxlike walls, but with some beveled enamel margins. The outline form same as in conventional preparation. The beveled conventional preparation design indicated for a composite restoration used to replace an existing restoration (usually amalgam) with conventional tooth preparation design. This design is most typical for Classes III, IV, and V restorations. Some accessible enamel margins may be beveled and acid etched.to facilitate better marginal sealing and bonding, The key advantage of bevelling enamel is that enamel rods are exposed and available effectively for etching compared to non bevelled margins . Also increasing the etched surface area will increase the strength of bonding between enamel and resin, which increases retention of the restoration and reduces marginal leakage and marginal discoloration. Incorporation of a cavosurface bevel may enable the restoration to blend more esthetically with the coloration of the surrounding tooth structure. Bevels also are not placed on proximal margins if such beveling results in excessive extension of the cavosurface margins. The beveled conventional preparation is not commonly used for posterior restorations. 4.3. Modified design In modified tooth preparations design, there are no specified wall configuration or specified pulpal or axial depths. In contrast to conventional preparations, modified preparations are not prepared to a uniform dentinal depth. The extension of the margins and the depth of a modified tooth preparation are dictated solely by the extent (laterally) and the depth of the carious lesion or other defects. The objectives of this preparation design are to remove the fault as conservatively as possible and to rely on the composite bond to tooth structure to retain the restoration in the tooth. Modified tooth preparations conserve more tooth structure because retention is obtained primarily by micromechanical adhesion to the surrounding enamel and underlying dentin, rather than by preparation of retention grooves or coves in dentin. Often, the preparation appears to have been “scooped out "rather than having the distinct internal line angles characteristic of a conventional preparation design. Modified preparations are indicated primarily for the initial restoration of smaller, cavitated, carious lesions usually surrounded by enamel and for correcting enamel defects. They can be successful for larger restorations as well, however. For the restoration of large carious lesions, wider bevels or flares and retention grooves, coves, or locks may be indicated in addition to the retention afforded by the adhesive procedures. 4.4. Box design Box-Only design is indicated when only the proximal surface is faulty, with no lesions present on the occlusal surface. A proximal box is prepared with an inverted cone or round diamond stone or bur held parallel to the long axis of the tooth crown. The instrument is extended through the marginal ridge in a gingival direction. The initial proximal axial

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depth is prepared 0.2 mm inside the dentinoenamel junction. The form of the box depends on which shape is used— more boxlike with the inverted, more scooped with the round. The facial, lingual, and gingival extensions are dictated by the fault or caries. Caries excavation in a pulpal direction is done with a round bur or spoon excavator. Neither beveling nor secondary retention is usually indicated. 4.5. Facial or Lingual Slot In this design, a lesion is detected on the proximal surface, and the access to the lesion can be obtained from either a facial or a lingual direction, rather than through the marginal ridge from an occlusal direction. Usually a small round diamond stone or bur is used to gain access to the lesion. The instrument is oriented at the correct occlusogingival height, and the entry is made with the instrument as close to the adjacent tooth as possible, preserving as much of the facial or lingual surface as possible. The preparation is extended occlusogingivally and faciolingually enough to remove the lesion. The initial axial depth is 0.2mm inside the dentinoenamel junction. The occlusal, facial, and gingival cavosurface margins are 90 degrees or greater. Caries excavation in a pulpal direction is done with a round bur or spoon excavator. This preparation is similar to a Class III preparation for an anterior tooth .

5. Recent developments and future expectations With the development of organised dental profession in 19th century, the profession of operative dentistry was considered to be the entire clinical practise. With further development and new scientific discoveries during 20th century the need for more complex patient treatment was recognized. The concepts of tooth tissue regenration has developed remarkably in the recent years[66] Today the scope of operative dentistry involves around restoration of lost tooth morphology, function, and aesthetics, while still maintaining the integrity of surrounding soft and hard tissues [67]. The knowledge of dental materials and surrounding tissues is essential in restoring a lost tooth structure. Composite is considered a material of choice for both anterior and posterior restorations [59]. Increase in the demand of composite restorations with enhanced mechanical, chemical, and biological properties have left significant room for advancements in these areas [67]. More focus is given to improve polymerization shrinkage, micro leakage, thermal expansion mismatch, abrasion, fracture toughness, bonding, wear resistance, aesthetics, and mechanical properties [59, 67]. Composite dental restorations with time continues to grow significantly in organic phase, filler loading, and curing methodologies. TEGDMA enhances the molecular mobility in the polymerization process and decreases the viscosity of the bulk monomers [67]. In order to attempt the properties if existing resin composited, a number of midifications have been made. The inorganic fillers are added to improve mechanical and physical properties including abrasion resistance, dimensional and thermal stability. The common fillers added to composite formulations includes quartz, colloidal silica, and silica glass containing barium, strontium, and zirconium [67]. A number of other fillers are added recently including calcium phosphates, and anti bacterial agents for specific properties into the composites. Addition of fillers decreases the polymerisation shrinkage, and increase the mechanical properties. Current dental composites have adequate mechanical properties for usage in all areas of the mouth [67]. The most interesting development recently are the universal self-adhesive flowable composites. These naterials are very similar to conventional methacrylate systems, however contain acidic monomers such as glycerolphosphate dimethacrylate (GPDM) [67], and 4-Methacryloxyethyl trimellitic anhydride (4-META) [33]. These monomers have the ability to generate adhesion through mechanical interlocking and possibly chemical interactions. In the recent years, researchers have explored bioactive materials that can reverse the carious tissue degradation and remineralize the tissues [67]. A few of calcium phosphate-based materials have the capability to repair carious tissues [68]. More recently anti bacterial components e.g chlorhexidine have been added into the composite formulations [46]. These components will additionally add the anti bacterial properties to the dental composites. Additinally, the anti bacterial components will combat against seconary caries beneath the restoration. The future goal of composite research focuses on the developing materials with low polymerization shrinkage and stress, releasing fluoride to combat cariogenic bacterials and better mechanical and physical properties.

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