**5. Direct composite restorative materials**

A generalized definition of a composite is a multiphase material that exhibits the properties of both phases where the phases are complimentary, resulting in a material with enhanced properties. The first tooth-colored composite was silicate cement, which was introduced in 1870s. This composite formulation was based on alumino-fluro-silicate glasses and phosphoric acid. The dispersed phase was residual glass particles, and the matrix phase was the aluminum phosphate salt formed from the partial acid dissolution of the glass particles; however, these were brittle, required mechanical retention, and had an average longevity of only a few years. of only a few years. The first polymeric tooth-colored composite used in dentistry was based on poly (methylmethacrylate). This material was developed in the 1940s, and consisted of a poly (methylmethacrylate) powder, methyl methacrylate monomer, benzoyl peroxide, and n,n-dimethlyparatoluidine. These materials could be classified as composites, because upon mixing, the polymer powder formed a dispersed phase and the monomer polymerized to form the continuous phase. The polymerization was initiated at room temperature, using the redox initiator combination of benzoyl peroxide and n,ndimethlyparatoluidine. Although these materials were initially esthetic, they were plagued with a variety of problems, including poor color stability, high polymerization shrinkage, a lack of bonding to tooth structure, and a large coefficient of thermal expansion (CTE). The first polymer matrix composite incorporating silica fillers was

There has been a considerable amount of work done to evaluate the success or otherwise of DBAs in clinical studies. However, one of the great problems has been that many of the DBAs have been considerably changed or a new material introduced by the time these studies are completed or published. Many of the studies have also been performed on NCCL, which means the outcomes cannot really be applied to restorations in other parts of the mouth because NCCL dentine is usually sclerosed and therefore different from that of an intracoronal cavity. However, these outcomes will provide some indication as to whether the DBA is able to achieve a durable bond under very harsh conditions. Since the early materials were introduced, the retention rates of the DBAs to sclerosed cervical dentine have

With regard to clinical studies on posterior teeth restored with a DBA, there is still little evidence available. It would seem though, that clinical studies of resin composite restorations are showing evidence that when placed in the correct manner and the patient

When it comes to the use of DBAs, it is important to follow the manufacturers' directions carefully. Overetching can create a situation where there will potentially be a region of poorly or uninfiltrated dentine. This zone may be susceptible to acid or enzyme attack from

In the case of the self-etching priming materials, this is not believed to be a problem. However, the converse problem may occur: as mentioned, the dentine or smear layer may neutralize the etching primer if the primer has a relatively high pH. The anecdotal evidence would seem to indicate that gentle agitation of these solutions may assist with the etching.

A generalized definition of a composite is a multiphase material that exhibits the properties of both phases where the phases are complimentary, resulting in a material with enhanced properties. The first tooth-colored composite was silicate cement, which was introduced in 1870s. This composite formulation was based on alumino-fluro-silicate glasses and phosphoric acid. The dispersed phase was residual glass particles, and the matrix phase was the aluminum phosphate salt formed from the partial acid dissolution of the glass particles; however, these were brittle, required mechanical retention, and had an average longevity of only a few years. of only a few years. The first polymeric tooth-colored composite used in dentistry was based on poly (methylmethacrylate). This material was developed in the 1940s, and consisted of a poly (methylmethacrylate) powder, methyl methacrylate monomer, benzoyl peroxide, and n,n-dimethlyparatoluidine. These materials could be classified as composites, because upon mixing, the polymer powder formed a dispersed phase and the monomer polymerized to form the continuous phase. The polymerization was initiated at room temperature, using the redox initiator combination of benzoyl peroxide and n,ndimethlyparatoluidine. Although these materials were initially esthetic, they were plagued with a variety of problems, including poor color stability, high polymerization shrinkage, a lack of bonding to tooth structure, and a large coefficient of thermal expansion (CTE). The first polymer matrix composite incorporating silica fillers was

steadily improved to extent that retention rates are little different from GICs [79].

has a low caries rate, restoration survival is approaching that of amalgam [30].

oral bacteria, hence leading to bond failure [82].

**5. Direct composite restorative materials** 

However, there are no research data to support this [35, 79].

**4.4 Clinical studies** 

introduced in the 1950s. These composites had improved mechanical properties and good esthetics; they did not bond to tooth structure, and still exhibited significant polymerization shrinkage [10, 46, 55, 58].

One way to address the polymerization shrinkage problem is to use high molecular weight monomers. In 1962 Bowen [11], while at the National Bureau of Standards, synthesized an acrylated epoxy using glycidylmethacrylate and Bisphenol A epoxy for use as a matrix for dental composite. The resulting monomer, called Bis-GMA or Bowen's resin, possessed the viscosity of honey, and therefore limited the amount of filler particles that could be incorporated. Subsequent experiments incorporated triethylene glycol dimethacrylate (TEGDMA) as a diluent to reduce the viscosity. This monomer combination worked well, and has become one of the most widely used matrix monomer combinations for dental composites to date. The structures of Bis-GMA and TEGDMA are shown in Figs. 3 and 4, respectively.

Fig. 3. The chemical structure of Bis-GMA, a resin invented by Ray Bowen. It also is referred to as Bowen's resin.

Fig. 4. TEGDMA. The chemical structure of triethyleneglycol dimethacrylate (TEGDMA, which is also abbreviated TEDMA and TEGMA). The structure of methyl methacrylate (MMA) is shown for comparison.

Filling Materials for the Caries 351

One approach to reduce polymerization shrinkage and contraction stress is through the development of low-shrinkage or expanding monomer systems. These resin systems are based on ring-opening polymerization reactions that do not shrink to the extent of conventional vinyl polymerization resins. Monomers based on spiro-ortho carbonate have been prepared and evaluated in composite formulations. Although the composites formulated using these monomers did show less polymerization shrinkage, the property improvements were only incremental, and probably not significant enough to be realized

One problem that has not been addressed is the large difference between the Coefficient of Thermal Expansion (CTE) of resin composites and tooth structure. The CTE of tooth structure ranges from 9 to 11 ppm/\_C, compared with 28 to 50 ppm/\_C for dental composite restoratives [4]. The differential expansion and contraction of composites cause additional stress at the margin of the restoration that contributes to fatigue failure of the bond between the composite and tooth structure. Currently the only way to lower the CTE

The reinforcing phase in direct dental restoratives is based on glass or ceramic particles. Incorporation of these inorganic particles imparts improved strength and wear properties, decreased CTE, and reduced polymerization shrinkage. In addition, incorporation of heavy metals into the filler provides radiopacity. The initial composite fillers were limited in size because of the limited ability to grind and sieve quartz, glass, borosilicate, or ceramic particles. The particle size range was from 0.1 to 100 mm. Smaller particles have been prepared through hydrolysis or precipitation to produce what is termed fumed or pyrolitic

The most recent process to form particles is through sol-gel chemistry, which uses silicate precursors that are polymerized to form particles ranging from nm to mm dimensions. This sol-gel process can be used to form almost mono dispersed particle sizes, which can be a significant advantage because different particle sizes can be produced and blended to optimize the packing efficiency and filler loading of the composite. In addition, the ability to produce submicron size particles allows the production of nanocomposites in which the particles approach the size of the polymer matrix molecules. Theoretically, nanocomposites have the potential to exhibit excellent mechanical and physical properties at higher filler

The majority of current dental composites are cured using visible light ranging from 450 to 475 nm. Light sources include quartz halogen, laser, plasma arc, and most recently, light emitting diodes (LED). The minimum energy required for adequate curing is 300 mW/cm2. Newer lights have incorporated curing modes that step or ramp up the light intensity with time. These modes were added in an attempt to control the polymerization shrinkage and reduce the polymerization contraction stress. Although these lights have shown some promise, the clinical effectiveness of these controlled polymerization techniques is unknown. All of the lights used for curing composite increase the temperature of the

silica. The particle sizes obtained from this process range from 0.06 to 0.1 mm 6.

clinically [10, 25, 55, 57].

**5.2 Composite fillers** 

loadings [25, 55, 80].

**5.3 Curing of dental composites** 

of composites is to increase the filler loading.

Both of these monomers contain two reactive double bonds, and when polymerized, form covalent bonds between the polymer chains known as cross-links. Cross-linking improved the properties of the matrix phase, and the composite produced had improved mechanical and physical properties. Additional composite formulations have been prepared using various diluent monomers such as methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA), and an additional high molecular weight monomer based on a urethane dimethacrylate (UDMA). The chemical structure for UDMA is illustrated in Fig. 5 [10, 25, 46, 55, 58, 75].

Fig. 5. Urethane dimethacrylate. The chemical structure of popular difunctional urethane resins. R = a number of carbon compounds that can be used to lengthen or alter the properties of the monomer. Nitrogen in the form of NH–R–NH is the urethane component.

#### **5.1 Composite resin chemistry**

To reduce polymerization shrinkage and increase mechanical and physical properties requires the use of high molecular weight monomers that have the ability to cross-link. The high molecular weight reduces the volume change during polymerization. Cross-linking forms covalent bonds between the polymer chains, resulting in a dramatic increase in modulus and reduction in solubility [57]. Bowen's resin is the reaction product between Bisphenol A and glycidyl dimethacrylate. A lower molecular weight monomer such triethylene glycol dimethacrylate (TEGDMA) or EDMA is added to reduce the viscosity and allow increased filler loadings to be used. These monomers are also multifunctional and increase the number of cross-linking reactions during setting of resin matrix. These lower viscosity monomers may comprise 10% to 50% of a composite's composition.

One of the most significant problems with current monomers used for direct composite restorative materials is the shrinkage that occurs during polymerization. Currently, all commercial dental composites are based on vinyl monomers polymerized using free radical initiators. Conversion of these monomers results in a decrease in distance between the molecules, from a Van der Waals gap to the distance of a covalent bond. Although this distance is very small for a single monomer, the distance change over a long polymer chain is significant. Inclusion of filler reduces the volume of resin and its volume change, but the amount of filler incorporation is approaching the maximum theoretical packing fraction of 74 volume % for close-packed structures. The amount of shrinkage is controlled by the volume of resin, its composition, and the degree of conversion. Current commercial dental composites have a volumetric shrinkage ranging from 1.6 to 8 volume%. The contraction stress developed at the margin of the restoration can be sufficient to overcome the bond strength of the bonding system, resulting in a contraction gap. The contraction gap can lead to microleakage and all its associated problems (eg, secondary caries and pain) [55, 57].

Both of these monomers contain two reactive double bonds, and when polymerized, form covalent bonds between the polymer chains known as cross-links. Cross-linking improved the properties of the matrix phase, and the composite produced had improved mechanical and physical properties. Additional composite formulations have been prepared using various diluent monomers such as methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA), and an additional high molecular weight monomer based on a urethane dimethacrylate (UDMA). The chemical structure for UDMA is illustrated in Fig. 5

Fig. 5. Urethane dimethacrylate. The chemical structure of popular difunctional urethane resins. R = a number of carbon compounds that can be used to lengthen or alter the

properties of the monomer. Nitrogen in the form of NH–R–NH is the urethane component.

To reduce polymerization shrinkage and increase mechanical and physical properties requires the use of high molecular weight monomers that have the ability to cross-link. The high molecular weight reduces the volume change during polymerization. Cross-linking forms covalent bonds between the polymer chains, resulting in a dramatic increase in modulus and reduction in solubility [57]. Bowen's resin is the reaction product between Bisphenol A and glycidyl dimethacrylate. A lower molecular weight monomer such triethylene glycol dimethacrylate (TEGDMA) or EDMA is added to reduce the viscosity and allow increased filler loadings to be used. These monomers are also multifunctional and increase the number of cross-linking reactions during setting of resin matrix. These lower

One of the most significant problems with current monomers used for direct composite restorative materials is the shrinkage that occurs during polymerization. Currently, all commercial dental composites are based on vinyl monomers polymerized using free radical initiators. Conversion of these monomers results in a decrease in distance between the molecules, from a Van der Waals gap to the distance of a covalent bond. Although this distance is very small for a single monomer, the distance change over a long polymer chain is significant. Inclusion of filler reduces the volume of resin and its volume change, but the amount of filler incorporation is approaching the maximum theoretical packing fraction of 74 volume % for close-packed structures. The amount of shrinkage is controlled by the volume of resin, its composition, and the degree of conversion. Current commercial dental composites have a volumetric shrinkage ranging from 1.6 to 8 volume%. The contraction stress developed at the margin of the restoration can be sufficient to overcome the bond strength of the bonding system, resulting in a contraction gap. The contraction gap can lead to microleakage and all its associated problems (eg, secondary caries and pain) [55, 57].

viscosity monomers may comprise 10% to 50% of a composite's composition.

[10, 25, 46, 55, 58, 75].

**5.1 Composite resin chemistry** 

One approach to reduce polymerization shrinkage and contraction stress is through the development of low-shrinkage or expanding monomer systems. These resin systems are based on ring-opening polymerization reactions that do not shrink to the extent of conventional vinyl polymerization resins. Monomers based on spiro-ortho carbonate have been prepared and evaluated in composite formulations. Although the composites formulated using these monomers did show less polymerization shrinkage, the property improvements were only incremental, and probably not significant enough to be realized clinically [10, 25, 55, 57].

One problem that has not been addressed is the large difference between the Coefficient of Thermal Expansion (CTE) of resin composites and tooth structure. The CTE of tooth structure ranges from 9 to 11 ppm/\_C, compared with 28 to 50 ppm/\_C for dental composite restoratives [4]. The differential expansion and contraction of composites cause additional stress at the margin of the restoration that contributes to fatigue failure of the bond between the composite and tooth structure. Currently the only way to lower the CTE of composites is to increase the filler loading.

### **5.2 Composite fillers**

The reinforcing phase in direct dental restoratives is based on glass or ceramic particles. Incorporation of these inorganic particles imparts improved strength and wear properties, decreased CTE, and reduced polymerization shrinkage. In addition, incorporation of heavy metals into the filler provides radiopacity. The initial composite fillers were limited in size because of the limited ability to grind and sieve quartz, glass, borosilicate, or ceramic particles. The particle size range was from 0.1 to 100 mm. Smaller particles have been prepared through hydrolysis or precipitation to produce what is termed fumed or pyrolitic silica. The particle sizes obtained from this process range from 0.06 to 0.1 mm 6.

The most recent process to form particles is through sol-gel chemistry, which uses silicate precursors that are polymerized to form particles ranging from nm to mm dimensions. This sol-gel process can be used to form almost mono dispersed particle sizes, which can be a significant advantage because different particle sizes can be produced and blended to optimize the packing efficiency and filler loading of the composite. In addition, the ability to produce submicron size particles allows the production of nanocomposites in which the particles approach the size of the polymer matrix molecules. Theoretically, nanocomposites have the potential to exhibit excellent mechanical and physical properties at higher filler loadings [25, 55, 80].

#### **5.3 Curing of dental composites**

The majority of current dental composites are cured using visible light ranging from 450 to 475 nm. Light sources include quartz halogen, laser, plasma arc, and most recently, light emitting diodes (LED). The minimum energy required for adequate curing is 300 mW/cm2. Newer lights have incorporated curing modes that step or ramp up the light intensity with time. These modes were added in an attempt to control the polymerization shrinkage and reduce the polymerization contraction stress. Although these lights have shown some promise, the clinical effectiveness of these controlled polymerization techniques is unknown. All of the lights used for curing composite increase the temperature of the

Filling Materials for the Caries 353

possessed outstanding polishability and esthetics. These composites incorporate particles ranging from 0.04 μm to 0.4 μm. The early versions of microfilled resins were limited in the amount of filler that could be incorporated, because of the high surface area-to-volume ratio of the filler that caused large viscosity increases in the formulation. These composites only contained 35 to 67 weight % and 20 to 59 volume % glass fillers. One way to increase the volume of small particles is through the use of prepolymerized particles. In this process, submicron-sized particles are mixed with monomers such a Bis-GMA and TEGDMA at elevated temperatures. The mixture is then cured at elevated temperature and pressure, using benzoyl peroxide as an initiator. After polymerization, the material is chilled and ground to form particles having a size range of 1 to 200 μm. The prepolymerized particles allow higher filler loadings to be obtained with smaller particles; however, the prepolymerized particles cannot be bonded to the matrix phase using silane coupling agents. Interfacial bonding requires diffusion of the matrix monomers into the particles, with subsequent polymerization to provide micromechanical interlocks. Some investigators have suggested that the lack of interfacial bonding in these systems may contribute to

Therefore microfilled composites have a lower elastic modulus and lower fracture strength than materials that contain higher concentrations of filler. The prepolymerized particles allow the filler content to be maximized and polymerization shrinkage to be minimized, however, while making these composites highly polishable and possessing the ability to maintain a smooth surface during clinical wear. Because of these properties, microfilled composite resins are indicated for Class V restorations, non–stress-bearing Class III restorations, and small Class I restorations. They are also indicated for direct composite resin veneers if the patient does not demonstrate any parafunctional habits, such as bruxism. Because of their lower fracture strength and potential for marginal breakdown, microfills are generally contraindicated for posterior load bearing restorations such as Class

The majority of resin composites in clinical use today are categorized in the general term of ''hybrid composites.'' This broad category includes traditional hybrids, micro-, and nanohybrids. The ''hybrid'' moniker implies a resin composite blend containing submicron inorganic filler particles (.04 μm) and small particles (1 μm–4 μm). The combination of various sizes of filler particles corresponds to an improvement in physical properties as well as acceptable levels of polishability. These improvements in wear resistance and fracture strength, along with good polishability, make hybrids the material of choice for Class III and Class IV restorations. In addition, practitioners have used these traditional hybrids in posterior load-bearing surfaces such as Class I and Class II restorations because of their

Recent improvements in filler technology by manufacturers have allowed blends of both submicron particles (0.04 μm) and small particles (0.1 μm–1.0 μm) to be incorporated into a composite formulation. These materials are classified as micro-hybrid composites. The mixture of smaller particles distinguishes microhybrids from traditional hybrids and allows for a finer polish, along with improved handling. The desirable combination of strength and

failure [55, 57, 59].

II and large Class I restorations [55, 71].

improved strength and wear resistance.

**5.4.2 Hybrid composites** 

composite to varying extents, which can actually increase the degree of conversion; however, high-intensity light sources may cause sufficient temperature increases to result in damage to the pulp [7, 25, 55].

#### **5.4 Composite classification: properties and applications**

Composites generally are classified with respect to the components, amounts, properties of their filler or matrix phases, or by their handling properties. The most common classification method is based on filler content (weight or volume percent), filler particle size, and method of filler addition. Composites also could be defined on the basis of the matrix composition (BIS-GMA or UDMA) or polymerization method (self-curing, ultraviolet light-curing, visible light-curing, dual curing, or staged curing), but these do not communicate as much information about the properties. One of the most often used classification systems is based upon filler particle sizes. That system is extended here to include the particle size by order of magnitude, acknowledging mixed ranges of particle sizes, and distinguishing procured composite pieces as special filler. Composite filler particles are called *macrofillers* in the range of 10 to 100 μm, *midifillers* from 1 to 10 μm, *minifillers* from 0.1 to 1 μm, and *microfillers* 0.01 to 0.1 μm. Very large individual filler particles, called *megafillers*, also have been used in special circumstances. New ultrasmall fillers are being used that are from 0.005 to 0.01 μm in diameter and are called *nanofillers*. Accordingly, composites are classified by particle size as *megafill, macrofill, midifill, minifill, microfill,* and *nanofill*. Composites with mixed ranges of particle sizes are called hybrids, and the largest particle size range is used to define the hybrid type (e.g., minifill hybrid) because microfillers are normally the second part of the mixture. If the composite simply consists of filler and uncured matrix material, it is classified as homogeneous. If it includes procured composite or other unusual filler, it is called heterogeneous. If it includes novel filler modifications in addition to conventional fillers, then it is called modified, such as fiber-modified homogeneous. Another consequence of advances in the control of filler particle size, particle size distribution, particle morphology, and monomer technology has been the introduction of composites with specific handling characteristics. These include *flowable composites* and *packable composites*. *Flowable composites* are a class of low-viscosity materials that possess particle sizes and particles size distrubutions similar to those of hybrid composites, but with reduced filler content, which allows the increased amount of resin to decrease the viscosity of the mixture. *Packable composites*, also referred to as *condensable composites*, were developed in a direct effort to produce a composite with handling characteristics similar to amalgam, thus the moniker of "packable" or "condensable". These "amalgam alternatives" are intended primarily for Class I and Class II restorations. For posterior composite restorations, it is also possible to place one or two large glass inserts (0.5-to 2-mm particles) into composites at points of occlusal contact or high wear. These pieces of glass are referred to as *inserts* (or *megafillers*). Although they have demonstrated improved wear resistance to contact area wear, the techniques are more complicated and do not totally eliminate contact frea area wear. Furthermore, the bonding of the composite to the insert is questionable [7, 16, 55].

#### **5.4.1 Microfilled composites**

Microfilled composites were introduced to the market from the late 1970s to the early 1980s. Microfilled composites were developed to provide the dental profession with a material that

composite to varying extents, which can actually increase the degree of conversion; however, high-intensity light sources may cause sufficient temperature increases to result in

Composites generally are classified with respect to the components, amounts, properties of their filler or matrix phases, or by their handling properties. The most common classification method is based on filler content (weight or volume percent), filler particle size, and method of filler addition. Composites also could be defined on the basis of the matrix composition (BIS-GMA or UDMA) or polymerization method (self-curing, ultraviolet light-curing, visible light-curing, dual curing, or staged curing), but these do not communicate as much information about the properties. One of the most often used classification systems is based upon filler particle sizes. That system is extended here to include the particle size by order of magnitude, acknowledging mixed ranges of particle sizes, and distinguishing procured composite pieces as special filler. Composite filler particles are called *macrofillers* in the range of 10 to 100 μm, *midifillers* from 1 to 10 μm, *minifillers* from 0.1 to 1 μm, and *microfillers* 0.01 to 0.1 μm. Very large individual filler particles, called *megafillers*, also have been used in special circumstances. New ultrasmall fillers are being used that are from 0.005 to 0.01 μm in diameter and are called *nanofillers*. Accordingly, composites are classified by particle size as *megafill, macrofill, midifill, minifill, microfill,* and *nanofill*. Composites with mixed ranges of particle sizes are called hybrids, and the largest particle size range is used to define the hybrid type (e.g., minifill hybrid) because microfillers are normally the second part of the mixture. If the composite simply consists of filler and uncured matrix material, it is classified as homogeneous. If it includes procured composite or other unusual filler, it is called heterogeneous. If it includes novel filler modifications in addition to conventional fillers, then it is called modified, such as fiber-modified homogeneous. Another consequence of advances in the control of filler particle size, particle size distribution, particle morphology, and monomer technology has been the introduction of composites with specific handling characteristics. These include *flowable composites* and *packable composites*. *Flowable composites* are a class of low-viscosity materials that possess particle sizes and particles size distrubutions similar to those of hybrid composites, but with reduced filler content, which allows the increased amount of resin to decrease the viscosity of the mixture. *Packable composites*, also referred to as *condensable composites*, were developed in a direct effort to produce a composite with handling characteristics similar to amalgam, thus the moniker of "packable" or "condensable". These "amalgam alternatives" are intended primarily for Class I and Class II restorations. For posterior composite restorations, it is also possible to place one or two large glass inserts (0.5-to 2-mm particles) into composites at points of occlusal contact or high wear. These pieces of glass are referred to as *inserts* (or *megafillers*). Although they have demonstrated improved wear resistance to contact area wear, the techniques are more complicated and do not totally eliminate contact frea area wear. Furthermore, the bonding of the composite to the insert is questionable [7, 16, 55].

Microfilled composites were introduced to the market from the late 1970s to the early 1980s. Microfilled composites were developed to provide the dental profession with a material that

damage to the pulp [7, 25, 55].

**5.4.1 Microfilled composites** 

**5.4 Composite classification: properties and applications** 

possessed outstanding polishability and esthetics. These composites incorporate particles ranging from 0.04 μm to 0.4 μm. The early versions of microfilled resins were limited in the amount of filler that could be incorporated, because of the high surface area-to-volume ratio of the filler that caused large viscosity increases in the formulation. These composites only contained 35 to 67 weight % and 20 to 59 volume % glass fillers. One way to increase the volume of small particles is through the use of prepolymerized particles. In this process, submicron-sized particles are mixed with monomers such a Bis-GMA and TEGDMA at elevated temperatures. The mixture is then cured at elevated temperature and pressure, using benzoyl peroxide as an initiator. After polymerization, the material is chilled and ground to form particles having a size range of 1 to 200 μm. The prepolymerized particles allow higher filler loadings to be obtained with smaller particles; however, the prepolymerized particles cannot be bonded to the matrix phase using silane coupling agents. Interfacial bonding requires diffusion of the matrix monomers into the particles, with subsequent polymerization to provide micromechanical interlocks. Some investigators have suggested that the lack of interfacial bonding in these systems may contribute to failure [55, 57, 59].

Therefore microfilled composites have a lower elastic modulus and lower fracture strength than materials that contain higher concentrations of filler. The prepolymerized particles allow the filler content to be maximized and polymerization shrinkage to be minimized, however, while making these composites highly polishable and possessing the ability to maintain a smooth surface during clinical wear. Because of these properties, microfilled composite resins are indicated for Class V restorations, non–stress-bearing Class III restorations, and small Class I restorations. They are also indicated for direct composite resin veneers if the patient does not demonstrate any parafunctional habits, such as bruxism. Because of their lower fracture strength and potential for marginal breakdown, microfills are generally contraindicated for posterior load bearing restorations such as Class II and large Class I restorations [55, 71].

#### **5.4.2 Hybrid composites**

The majority of resin composites in clinical use today are categorized in the general term of ''hybrid composites.'' This broad category includes traditional hybrids, micro-, and nanohybrids. The ''hybrid'' moniker implies a resin composite blend containing submicron inorganic filler particles (.04 μm) and small particles (1 μm–4 μm). The combination of various sizes of filler particles corresponds to an improvement in physical properties as well as acceptable levels of polishability. These improvements in wear resistance and fracture strength, along with good polishability, make hybrids the material of choice for Class III and Class IV restorations. In addition, practitioners have used these traditional hybrids in posterior load-bearing surfaces such as Class I and Class II restorations because of their improved strength and wear resistance.

Recent improvements in filler technology by manufacturers have allowed blends of both submicron particles (0.04 μm) and small particles (0.1 μm–1.0 μm) to be incorporated into a composite formulation. These materials are classified as micro-hybrid composites. The mixture of smaller particles distinguishes microhybrids from traditional hybrids and allows for a finer polish, along with improved handling. The desirable combination of strength and

Filling Materials for the Caries 355

Changes in restorative treatment patterns, the introduction of new and improved restorative materials and techniques, effective preventive programs, enhanced dental care, and growing interest in caries-free teeth have greatly influenced the longevity of dental restorations; however, failure of restorations is a major problem in a practice treating primarily permanent teeth. Studies show that 60% of all operative work done is attributed to the replacement of restorations [44]. Composites have improved since their introduction, and their survival rates are improving. Clinical studies to evaluate the latest composite technologies have not been published; therefore most of the survival data are on older

In the 1970s, degradation or wear was considered the main reason for failure of composite restorations. Improvements in filler technology and formulation of composite materials have resulted in new reasons for replacement. Twenty years later, studies revealed secondary caries to be the new cause of failure. The main factors responsible for the change in reasons for replacement include improved clinical technique based on more adequate teaching of posterior composites at dental schools, and on gained experience through trial and error of clinicians in practice [55]. Advancements in composite properties and adhesive

In comparison of survival probability between amalgam and composite, a time period involving 3, 4, 5, and 7 years was considered [54]. In permanent teeth, the following values were measured: 3 year, 97.2% (amalgam) to 90% (composite); 4 year, 96.6% (amalgam) to 85.6% (composite); 5 year, 95.4% (amalgam) to 78.2% (composite); and 7 year, 94.5%

In summary, longevity of composite restorations depends upon factors involving the materials, the patient, and the dentist. The request for these esthetic, tooth-colored restorations will continue to increase, and patients must be educated about the expected life of these restorations as well as their advantages and disadvantages, so they can make an

[1] Adusei, G.O.; Deb, S. & Nicholson, J.W. (2004). The role of the ionomer glass component

[2] Akitt, J.W.; Greenwood, N.N. & Lester, G.D. (1971). Nuclear magnetic resonance and

[3] Alves, J.B. & Brandao, P.R. (2002). Atraumatic restorative treatment: Clinical, ultrastructural and chemical analysis*. Caries Research,* Vol. 36, pp. 430-436. [4] Anusavice, K.J. & Brantley, W.A. (2003). Physical properties of dental materials. In:

[5] Atin, T.; Buchalla, W., Keilbassa, A.M., et al. (1995). Curing shrinkage and volumetric

*Material Science & Material Medicine,* Vol. 15, pp. 751-754.

in polyacid-modified composite resin dental restorative materials. *Journal of* 

Raman studies of aluminium complexes formed in aqueous solutions of aluminium salts containing phosphoric acids and fluoride ions. *Journal of Chemical Social* 

*Phillip's science of dental materials,* (11th edn). Anusavice, K.J. (Ed.). pp. 41-71, WB

changes of resin-modified glass ionomer restorative materials. *Dental Materials,* Vol.

composite compositions.

technology also contributed to these changes.

(amalgam) to 67.4% (composite).

**6. References** 

informed decision on a treatment option.

*Association,* Vol. 14, pp. 2450-2457.

Saunders, Philladelphia.

11, pp.359–362.

surface smoothness offers the clinician flexibility for use in posterior stress-bearing areas as well as anterior esthetic areas. Although microhybrids offer superior strength, their polishability is not better than a traditional microfilled composite resin. The trend in the newer microhybrid materials is to maximize filler loading and minimize filler size. The latest version of microfilled hybrids has used nanofiller technology to formulate what have been referred to as nanohybrid composite resins. Nanohybrids contain nanometer-sized filler particles (.005–.0l microns) throughout the resin matrix, in combination with a more conventional type filler technology. Nanohybrids may be classified as the first truly universal composite resin with handling properties and polishability of a microfilled composite, and the strength and wear resistance of a traditional hybrid. These nanohybrids can be used in any situation similar to the microhybrids, with possibly a slight improvement in polishability because of the smaller particle size [55, 74].

#### **5.4.3 Packable composites**

Packable or condensable composites were developed to provide a composite that handled more like amalgam. This marketing ploy by dental product manufacturers was an attempt to increase the use of composites by older dentists who were not trained in their use in dental school, and younger dentists who were looking for a more user-friendly material. Packable composites have a higher viscosity and are less ''sticky'' than other composite restoratives. The viscosity increase is obtained through changes in the particle size distribution and incorporation of fibers [81]. These composites were introduced to the market as amalgam substitutes, as practitioners searched for the ideal esthetic material with handling properties similar to amalgam. Another desire was to find a material that would establish adequate proximal contacts more easily than traditional hybrid composites. Claims of improved handling properties and better adaptation to the matrix band in Class II restorations have piqued the interest of many clinicians. The dental professions have referred to these materials as ''packable composites'' instead of ''condensable,'' because of their greater viscosity and decreased stickiness compared with conventional hybrid composites. When initially placed, these materials were more viscous than traditional hybrid composites; however, after placement the viscosity decreased as the temperature of the material equilibrated with the temperature of the oral cavity. Although the ''packable composites'' showed improved handling properties for restoring Class I and II preparations, they have not fully solved the problem of achieving adequate interproximal contacts. Because packable composites do not have substantially better mechanical properties than hybrid composites, they would not be expected to perform better clinically [86]. In addition, because of the development of improved placement instruments and matrix systems to achieve better interproximal contacts, the need for packable or condensable materials has decreased, resulting in a decreased market share. In summary, the mechanical properties of the packable composites are not significantly better than other hybrid formulations, and there have not been sufficient long-term clinical studies to determine how these materials will perform long-term in the oral cavity. Their use as a direct dental restorative may be limited [18, 55, 57].

#### **5.5 Clinical survive probability of composites**

Composites are monitored in clinical studies by using United States Public Health Service (USPHS) categories [61] of interest: color matching, interfacial staining, secondary caries, anatomic form (wear), and marginal integrity [7].

surface smoothness offers the clinician flexibility for use in posterior stress-bearing areas as well as anterior esthetic areas. Although microhybrids offer superior strength, their polishability is not better than a traditional microfilled composite resin. The trend in the newer microhybrid materials is to maximize filler loading and minimize filler size. The latest version of microfilled hybrids has used nanofiller technology to formulate what have been referred to as nanohybrid composite resins. Nanohybrids contain nanometer-sized filler particles (.005–.0l microns) throughout the resin matrix, in combination with a more conventional type filler technology. Nanohybrids may be classified as the first truly universal composite resin with handling properties and polishability of a microfilled composite, and the strength and wear resistance of a traditional hybrid. These nanohybrids can be used in any situation similar to the microhybrids, with possibly a slight improvement

Packable or condensable composites were developed to provide a composite that handled more like amalgam. This marketing ploy by dental product manufacturers was an attempt to increase the use of composites by older dentists who were not trained in their use in dental school, and younger dentists who were looking for a more user-friendly material. Packable composites have a higher viscosity and are less ''sticky'' than other composite restoratives. The viscosity increase is obtained through changes in the particle size distribution and incorporation of fibers [81]. These composites were introduced to the market as amalgam substitutes, as practitioners searched for the ideal esthetic material with handling properties similar to amalgam. Another desire was to find a material that would establish adequate proximal contacts more easily than traditional hybrid composites. Claims of improved handling properties and better adaptation to the matrix band in Class II restorations have piqued the interest of many clinicians. The dental professions have referred to these materials as ''packable composites'' instead of ''condensable,'' because of their greater viscosity and decreased stickiness compared with conventional hybrid composites. When initially placed, these materials were more viscous than traditional hybrid composites; however, after placement the viscosity decreased as the temperature of the material equilibrated with the temperature of the oral cavity. Although the ''packable composites'' showed improved handling properties for restoring Class I and II preparations, they have not fully solved the problem of achieving adequate interproximal contacts. Because packable composites do not have substantially better mechanical properties than hybrid composites, they would not be expected to perform better clinically [86]. In addition, because of the development of improved placement instruments and matrix systems to achieve better interproximal contacts, the need for packable or condensable materials has decreased, resulting in a decreased market share. In summary, the mechanical properties of the packable composites are not significantly better than other hybrid formulations, and there have not been sufficient long-term clinical studies to determine how these materials will perform long-term in the oral cavity. Their use as a direct

Composites are monitored in clinical studies by using United States Public Health Service (USPHS) categories [61] of interest: color matching, interfacial staining, secondary caries,

in polishability because of the smaller particle size [55, 74].

**5.4.3 Packable composites** 

dental restorative may be limited [18, 55, 57].

**5.5 Clinical survive probability of composites** 

anatomic form (wear), and marginal integrity [7].

Changes in restorative treatment patterns, the introduction of new and improved restorative materials and techniques, effective preventive programs, enhanced dental care, and growing interest in caries-free teeth have greatly influenced the longevity of dental restorations; however, failure of restorations is a major problem in a practice treating primarily permanent teeth. Studies show that 60% of all operative work done is attributed to the replacement of restorations [44]. Composites have improved since their introduction, and their survival rates are improving. Clinical studies to evaluate the latest composite technologies have not been published; therefore most of the survival data are on older composite compositions.

In the 1970s, degradation or wear was considered the main reason for failure of composite restorations. Improvements in filler technology and formulation of composite materials have resulted in new reasons for replacement. Twenty years later, studies revealed secondary caries to be the new cause of failure. The main factors responsible for the change in reasons for replacement include improved clinical technique based on more adequate teaching of posterior composites at dental schools, and on gained experience through trial and error of clinicians in practice [55]. Advancements in composite properties and adhesive technology also contributed to these changes.

In comparison of survival probability between amalgam and composite, a time period involving 3, 4, 5, and 7 years was considered [54]. In permanent teeth, the following values were measured: 3 year, 97.2% (amalgam) to 90% (composite); 4 year, 96.6% (amalgam) to 85.6% (composite); 5 year, 95.4% (amalgam) to 78.2% (composite); and 7 year, 94.5% (amalgam) to 67.4% (composite).

In summary, longevity of composite restorations depends upon factors involving the materials, the patient, and the dentist. The request for these esthetic, tooth-colored restorations will continue to increase, and patients must be educated about the expected life of these restorations as well as their advantages and disadvantages, so they can make an informed decision on a treatment option.
