Restorative Resins in Dentistry

• Restorative resins are used in dentistry to restore teeth

History of Restorative Resins

• In the 20th century, silicates were the only tooth-colored aesthetic material available
• Acrylic resins replaced silicates in the 1940s due to their improved aesthetics and insolubility in oral fluids
• Acrylic resins were also cheaper and easier to manipulate than silicates
• Excessive thermal expansion and contraction of acrylic resins leads to stresses
• The addition of quartz helped solve the thermal expansion and contraction issue
• Early composites based on PMMA were not successful
• A major advancement was achieved with the introduction of bis-GMA by Dr. Ray I. Bowen in the 1950s

Uses of Composite Restorative Materials

• Restoration of anterior and posterior teeth
• Veneering of metal crowns and bridges
• Building up cores
• Cementation of orthodontic brackets, Maryland bridges, ceramic crowns, inlays, onlays, and laminates
• Application as pit and fissure sealants
• Repair of chipped porcelain restorations

Types of Restorative Resins

• Classified based on curing mechanism:
• Chemically activated
• Light-activated
• Classified based on size of filler:
• Conventional
• 8-12 um
• Small particle
• 1-3 um
• Microfilled
• 0.04-0.4 um
• Hybrid
• 0.6-1.0 um

Dental Composites

• Dental composites are highly crosslinked polymeric materials
• Reinforced by a dispersion of glass, crystalline, or resin filler particles or short fibers
• Filler particles are bound to the matrix using silane coupling agents
• Composition includes:
• Resin
• Filler particles
• Coupling agents
• An activator-initiator system is required to transform the resin into a hard, durable restoration

Benefits of Fillers

• Reinforcement of the matrix resin resulting in increased hardness, strength, and decreased wear
• Reduction in polymerization shrinkage
• Reduction in thermal expansion and contraction
• Improved workability by increasing viscosity
• Reduction in water sorption, softening, and staining
• Increased radiopacity

Important Factors for Fillers

• Amount of filler added
• Size of particles and their distribution
• Index of refraction
• Radiopacity
• Hardness

Types of Fillers

• Ground quartz:
• Makes restoration difficult to polish
• Causes abrasion of opposing teeth and restorations
• Colloidal silica:
• Used in microfilled composites
• Thickens the resin
• Glasses of ceramic containing heavy metals:
• Provide radiopacity
• Contains barium

Coupling Agent

• Bonds filler particles to resin
• Functions:
• Improves physical and mechanical properties
• Prevents water from penetrating the resin-filler surface
• 3-methoxy-propyl-trimethoxy silane is commonly used

Inhibitors

• Added to resin to minimize or prevent spontaneous or accidental polymerization of monomers
• Butylated hydroxytoluene (BHT) is a typical inhibitor, used in a concentration of 0.01% wt

Optical Modifiers

• Dental composites must have visual shading and translucency for a natural appearance
• Shading is achieved by adding pigments, usually metal oxide particles
• All optical modifiers affect light transmission through a composite
• Darker shades and greater opacities decrease the depth of light curing
• Titanium dioxide and aluminum oxide are most commonly used

Polymerization Mechanism

• Chemically activated
• Light-activated

Chemically Activated Composite System

• Utilizes a two-paste system:
• Base paste containing a benzoyl peroxide initiator
• Catalyst paste containing a tertiary amine activator (N,N-dimethyl-p-toludine)

Light Activated Composite Resins

• Earliest systems involved UV light activation, which had limitations:
• Limited penetration of light into resin
• Lack of penetration through tooth structure

Visible Light Activated System

• Uses a single paste system
• Contains:
• Photoinitiator - Camphoroquinone
• Amine accelerator - diethyl-amino-ethyl-methacrylate

Types of Lamps Used for Curing

• LED lamps:
• Emit radiation only in the blue part of the visible spectrum, between 440 and 480 nm
• Use a solid-state, electronic process
• QTH lamps:
• Have a quartz bulb with a tungsten filament
• Irradiate both LTV and white light that has to be filtered to remove heat
• Emit wavelengths in the violet-blue range (400 to 500 nm)
• PAC lamps:
• Uses a xenon gas that is ionized to produce a plasma
• High-intensity white light is filtered to remove heat and allow blue light (400 to 500 nm) to be emitted
• Argon laser lamps:
• Have the highest intensity and emit at a single wavelength
• Lamps currently available emit 490 nm

Depth of Cure and Exposure Time

• Light absorption and scattering in resin composites reduces the power density and degree of conversion (DC) with depth of penetration

• Intensity can be reduced by a factor of 10 to 100 in a 2-mm thick layer of composite

• This reduces monomer conversion to an acceptable level

• The curing depth is practically limited to 2–3 mm

• Light attenuation varies depending on opacity, filler size, filler concentration, and pigment shade

• Darker shades require longer curing times

• When polymerizing resin through tooth structure, exposure time should be increased by a factor of 2–3 to compensate for reduction in light intensity

• Light intensity can decrease based on quality and age of light source, orientation of light tip, distance between light tip and restoration, and contamination

• Chemically cured composites can be used with reliable results as luting agents under metallic restorations

Dual Curing

• Overcomes issues associated with light curing by combining chemical and light curing components in same resin
• Air inhibition and porosity are common problems with dual-cure resins
• Extra-oral heat or light promotes a higher level of cure

Reduction of Residual Stresses

• Involves two approaches:
• Reduction in volume contraction by altering the chemistry of resin system
• Clinical techniques designed to offset the effects of polymerization shrinkage

High Intensity Curing

• High-intensity lamps reduce chair time
• Short exposure times cause accelerated rates of curing, which leads to substantial residual stress build-up

Class of Composite, Particle Size and Clinical Use

• Traditional (large particle) composite:
  • Particle size: 1-50µm
  • Clinical use: High stress areas
• Hybrid (large particle) composite:
  • Particle size: 1, 1-20 µm, 2, 0.04 µm silica
  • Clinical use: High stress areas requiring improved polishability Cl (1/2/3/4)
• Hybrid (midifiller) composite:
  • Particle size: 1, 0.1-10 µm glass, 2, 0.04 µm silica
  • Clinical use: High stress areas requiring improved polishability Cl (3,4)
• Hybrid (minifiller/SPF) composite:
  • Particle size: 1, 0.1-2 µm glass, 2, 0.04 µm silica
  • Clinical use: Moderate stress areas requiring optimal polishability Cl (3,4)
• Packable hybrid composite:
  • Particle size: Midifiller /minifiller hybrid, but with lower filler
  • Clinical use: Situations where improved condensability is needed Cl(1,2)
• Flowable hybrid composite:
  • Particle size: Midifiller hybrid, but with finer particle size
  • Clinical use: Situations where improved flow is needed Cl(2)
• Homogenous microfill composite:
  • Particle size: 0.04 µm silica
  • Clinical use: Low stress and subgingival areas that require high polish and luster
• Heterogenous microfill composite:
  • Particle size: 1, 0.04 µm silica
  • Clinical use: Low stress and subgingival

Conventional / Traditional / Macrofilled Composite

• Composition: Ground quartz is the most commonly used filler
• Particle size: 8-12 µm
• Filler loading: 70-80 weight% or 50-60 vol%

Properties of Composites

• Compressive strength: Four to five times greater than unfilled resins (250-300 MPa)
• Tensile strength: Double that of unfilled acrylic resins (50-65 MPa)
• Elastic modulus: Four to six times greater (8-15 GPa)
• Hardness: Considerably greater (55 KHN) than that of unfilled resins
• Coefficient of thermal expansion: High filler-resin ratio reduces the CTE significantly

Esthetics of Composites

• Polishing results in a rough surface
• Selective wear of softer resin matrix occurs
• Composites exhibit a tendency to stain
• Radiopacity: Composites using quartz as filler are radiolucent, possess radiopacity less than dentin

Clinical Considerations for Composites

• Polishing is difficult
• Poor resistance to occlusal wear
• Tendency to discolor
• Rough surface tends to stain
• Inferior for posterior restorations

Microfilled Composites

• Developed to overcome surface roughness of conventional composites
• Composition:
• Smoother surface is due to the incorporation of microfiller
• Colloidal silica is used as the microfiller
• Filler particles are 200–300 times smaller than the average particle in traditional composites
• Filler particles consist of pulverized composite filler particles

Properties of Microfilled Composites

• Inferior physical and mechanical properties compared to traditional composites

• Resin constitutes 40–80% of the restorative material

• Increased surface smoothness is observed

• Areas of proximal contact exhibit tooth drifting

• Compressive Strength: 250-350 MPa

• Tensile Strength: 30-50 MPa (Lowest among composites)

• Hardness: 25-30 KHN

• Thermal Expansion Coefficient: Highest among composite resins

Clinical Consideration for Microfilled Composites

• Choice of restoration for anterior teeth
• Greater potential for fracture in class 4 and class 2 restorations
• Chipping occurs at margins

Small Particle Composite

• Introduced to improve surface smoothness, physical and mechanical properties of conventional composites
• Composition:
• Smaller size fillers
• Colloidal silica (5 wt%) to adjust paste viscosity
• Heavy metal glasses
• Filler content: 65-70 vol% or 80-90%

Properties of Small Particle Composites

• Display the best physical and mechanical properties
• Compressive strength: Highest compressive strength (350-400 MPa)
• Tensile strength: Double that of microfilled and 1.5 times greater than traditional composites (75-90 MPa)
• Hardness: Similar to conventional composites (50–60 KHN)
• Thermal expansion coefficient: Twice that of tooth structure
• Better surface smoothness than conventional composites due to small, highly packed fillers
• Composites containing heavy metal glasses are radio-opaque

Clinical Consideration for Small Particle Composites

• Used in stress-bearing areas like class 4 and class 2 restorations
• Choice of resin for aesthetic restoration of anterior teeth
• Used for restoring subgingival areas

Hybrid Composite

• Developed to obtain even better surface smoothness than small particle composites
• Composition:
• Two types of fillers
• Colloidal silica (10–20 wt%)
• Heavy metal glasses constituting 75
• Average particle size of 0.4-1.0 µm

Properties of Hybrid Composites

• Range between conventional and small particle composites
• Superior to microfilled composites
• Compressive strength: Slightly less than that of small particle composite (300–350 MPa)
• Tensile strength: Comparable to small particle composites (70–90 MPa)
• Hardness: Similar to small particle composites (50–60 KHN)
• Competitive with microfilled composites for anterior restoration
• Presence of heavy metal glasses makes the hybrid more radio-opaque than enamel

Clinical Considerations for Hybrid Composites

• Used for anterior restorations, including class 4, due to its smooth surface and good strength
• Widely employed for stress-bearing restorations

Flowable Composites

• Modification of SPF and Hybrid composites
• Feature reduced filler level
• Clinically used for:
• Class 1 restorations in gingival areas
• Class 2 posterior restorations where access is difficult

Posterior Restorations

• Amalgam is still a choice
• Composites (except flowable types) are used due to mercury toxicity and increased esthetic demand
• Require conservative cavity preparation and Meticulous manipulation technique

Packable Composites

• Introduced in the 1990s
• Feature elongated fibrous filler particles (about 100µm)
• Are time-consuming
• Inferior to amalgam in strength

Problems with Composites for Posterior Restorations

• Marginal leakage
• Time-consuming
• Composites wear faster than amalgam, especially in Class 5 restorations

Indications for Composites

• Esthetics
• Allergy to mercury
• Need to minimize thermal conduction

Indirect Posterior Composites

• Polymerized outside the oral cavity and luted with resin cement to overcome wear and leakage
• Used for fabrication of inlays and onlays
• Approaches for resin inlay construction:
• Use of both direct and indirect fabrication systems
• Application of heat, light, pressure, or combination
• Combined use of hybrid and microfilled composites

Uses of Composites for Resin Veneers

• Polymerized by visible light in violet-blue range or by heat and pressure
• Used for:
• Veneers for masking tooth discoloration
• Performed laminate veneers
• Advantages
• Ease of fabrication
• Predictable intra-oral reparability
• Less wear of opposing teeth or restorations
• Disadvantages
• Leakage of oral fluids
• Staining below veneers
• Susceptibility to wear during tooth brushing

Insertion

• Chemically Activated Resins
• Correct proportions dispensed
• Rapid spatulation with plastic instrument for 30 sec
• Avoid metal instruments
• Insert with syringe or plastic instrument
• Cavity slightly overfilled
• Matrix strip placed to apply pressure and to avoid air inhibition
• Light Activated Resins
• Single component pastes
• Controlled working time
• Hardens rapidly exposed to curing light
• Limited depth of cure
• Incremental build up
• High intensity light used
• Exposure not less than 40 – 60 sec
• Resin thickness not greater than 2.0-2.5mm

Acid Etch Technique

• Improves marginal seal between resin and enamel
• Mode of action:
• Creates microporosities by discrete etching of enamel
• Increases surface area
• Improves resin wetting of tooth surface
• Resin tags formed upon polymerization
• Acid used:
• 37% phosphoric acid

Dentin Bonding Agents

• Come as kit with primers/conditioners and bonding liquid
• Primers/conditioners used to:
• Remove smear layer and open dentinal tubules
• Provide modest etching of inter-tubular dentin

First Generation Dentin Bonding Agents

• Use glycerophosphoric acid dimethacrylate
• Disadvantage: low bond strength

Second Generation Dentin Bonding Agents

• Developed as adhesive agents for composites
• Have 3 times more bond strength than first generation
• Disadvantage: short term adhesion and bonds hydrolysed eventually
• Examples: Prisma, Universal bond, Mirage bond

Third Generation Dentin Bonding Agents

• Have bond strengths comparable to that of resin to etched enamel
• Complex use requiring 2-3 application steps
• Examples: Tenure, Scotch Bond 2, Prisma

Fourth Generation Dentin Bonding Agents

• Consist of an all-bond 2 system
• Includes 2 primers (NPG-GMA and BPDM)
• Utilize an unfilled resin adhesive (40%BIS-GMA, 30%UDMA, 30%HEMA)
• Bond composite to dentin and most surfaces, like enamel, casting alloys, amalgam, porcelain, and composite

Fifth Generation Dentin Bonding Agents

• Most recent product
• Simpler to use, with one step application
• Examples: 3M Single Bond, Prime and Bond (Dentsply)

Indications for Use of Dentin Bonding Agents

• Bonding composite to tooth structure
• Bonding composite to porcelain and various metals like amalgam, base metal, and noble metal alloys
• Desensitization of exposed dentin or root surfaces
• Bonding of porcelain veneers

Biocompatibility of Composites

• Relatively biocompatible
• Inadequately cured composites serve as reservoir
• Shrinkage of composite can cause marginal leakage and secondary caries
• Bisphenol A precursor of bis-GMA (Xenoestrogen), might cause reproductive anomalies

Survival Probability of Composites

• Judged on longterm clinical trials Survival Rates
• Composites after 7 years: 67.4%
• Amalgam: 94.5%
• Glass ionomer after 5 years: 64%
• Glass ionomer/composites avoided in class II restora

Recent Advancements in Restorative Resins

• Reducing filler particle size from micron level to nanometer level has changed:
• Distribution of filler particles in matrix
• Charge carriers transport between particles
• Conductivity of filler particles themselves Advantages of Nano-Composites:
• High adhesion of nanoparticles to polymer matrix enhancing strength of nanocomposites
• Small nanoparticle size ensures small pore size in case of matrix exfoliation, increasing strength
• Small amount of nanoparticles enhances adhesion of polymer to different substrates
• Optically more transparent compared to conventional composites

Summary

• Amalgam continues to be the best posterior restorative material due to:
• Ease of use
• Low cost
• Wear resistance
• Freedom from shrinkage during setting
• High survival probabilities