Dental Composites: UDMA, Fillers, and Properties

• Direct esthetic restorative materials can be placed directly inside the patient's mouth, with no need for impressions or lab steps
• These materials should have a good, natural appearance

Ideal Requirements for Restorative Materials

• Good esthetics require matching tooth color, translucency, refractive index, and smooth surface
• The material should resist staining and discoloration
• Biocompatibility requires non-irritation of the pulp or gingiva
• The material should not dissolve in oral fluids
• Adequate mechanical properties require strength, modulus of elasticity, and hardness/abrasion resistance
• Matching thermal expansion and contraction coefficients is important
• Minimal dimensional changes are desired
• Avoiding microleakage, recurrent caries, and sensitivity is important
• Bonding to enamel and dentine is needed
• Radio-opacity is needed for evaluation of marginal overhang and detection of recurrent caries

History of Dental Restorative Materials

• 1873: Silicate cements were developed and became popular in 1904
• 1948: The first PMMA acrylic resins were introduced
• 1951: Dimethacrylate was developed
• 1962: Ray Bowen developed Bis-GMA, a large, hydrophobic dimethacrylate monomer
• 1963: Dennis Smith developed polyelectrolyte cement, leading to polycarboxylate adhesive cements
• 1974: Wilson and Kent developed the first glass ionomer cement

Silicate Cements

• Composition includes alumina-silica glass powder mixed with phosphoric acid
• A main advantage is fluoride release
• Disadvantages include poor biological properties, irritation to the pulp by the acid, heat generation, and the presence of toxic arsenic

PMMA Acrylic Resins

• Advantages include initial good esthetics
• Disadvantages include high polymerization shrinkage, irritation, and loss of esthetics over time

Composite Resin Structures

• Composite structures are defined as a combination of two or more chemically different materials/phases
• They have a distinct interface separating the components with properties unachievable by the components alone
• Dental composite filling materials are also called composites, filled resins, composite resins, composite restorative materials, resin-based composites, and filled composites

Composition of Dental Composites

• Dental composites contain a matrix, fillers, a coupling agent, pigments, inhibitors, and an initiator system

Organic Matrix Role and Composition

• The role of the organic matrix includes holding of different components (dispersion medium), giving the composite plasticity, and facilitating polymerization reaction
• It is composed of one or more types of dimethacrylate oligo-polymers (oligomers)
• Each type has a role in modifying the properties of the final matrix
• The matrix usually includes Bis-GMA, Bis-EMA, UDMA, and TEG-DMA

Bis-GMA

• Bis-GMA is a long, bifunctional monomer that allows cross-linking by addition polymerization
• It is the most commonly used monomer in commercial products
• It has a shrinkage of 6% and is very viscous

Bis-EMA

• Bis-GMA is sometimes substituted with Bis-EMA (ethoxylated bisphenol A methacrylate)
• It has lower viscosity than Bis-GMA
• Bis-EMA can accommodate more filler loading

UDMA

• UDMA has a lower molecular weight compared to Bis-GMA
• It has less viscosity
• UDMA has slightly improved toughness, and greater susceptibility to visible light curing

TEG-DMA

• TEG-DMA exhibits very high shrinkage
• It has lower viscosity
• It acts as a diluent, allowing more filler loading, increased flexibility and decreased brittleness

Role and Types of Fillers

• Fillers improve mechanical properties, including strength, hardness, and fracture toughness
• Fillers decrease polymerization shrinkage, thermal expansion and contraction, and water sorption
• Fillers are responsible for providing viscosity and radiopacity
• Some fillers release fluoride
• Glass fillers contribute to good esthetics
• More fillers lead to enhanced mechanical properties

Filler Loading and Size

• Filler size affects polishability and esthetics
• Quantity impacts mechanical properties
• Large fillers result in higher mechanical properties but make it more difficult to polish
• Small sized fillers are better suited for anterior applications due to esthetic qualities

Classification of Dental Resin Composites

• Macrofill (1960): 10-50 μm particle size, 78% filler by weight, difficult to polish, surface roughness may attract plaque
• Fine Particle (1970): 0.4-5 μm particle size, 70-86% filler by weight, good mechanical properties and better finishing/polishing
• Microfilled (1980): 0.01-0.1 μm particle size, 25-63% filler by weight, easy to obtain and maintain smooth surface, poor mechanical properties/wear resistance, greater setting shrinkage

Hybrid Composites (1990)

• Hybrid composites have 0.04 & 1-5 μm particle size and 77-88% filler by weight
• These were developed to obtain the benefits of both fine and micro filled particles.

Nanofilled Composites

• Nanofilled composites include nanomers and nanoclusters (2000): 2-75 nm particle size, 78.5% filler by weight
• These have high translucency, good mechanical properties, and less polymerization shrinkage

Increasing Filler Loading

• Hybrid composites blend two types of fillers for high filler loading and good handling
• These have fine particles of average size 2-4 µm and 5-15% microfine particles (silica, 0.04-0.2 µm, or nanofillers, 2-75 nm), and a filler loading of 77-84% by weight
• Nano composites use a mixture of nano-sized particles and nanoclusters to reduce the space between filler particles
• Prepolymerized fillers add high concentrations of inorganic microfillers to a resin monomer under high heat and pressure

Nano Composites Advantages

• Reduced polymerization contraction
• Enhanced mechanical characteristic
• Improved esthetics due to small filler size (smaller than wavelength of light)
• Perfect surface finish

Coupling Agent

• A coupling agent (Silane) bonds to adsorbed humidity on fillers
• It acts as a surfactant Bonding oligomers

Initiator System

• Initiators are activated by energy to create free radicals that initiate polymerization
• The methods of energy include heat, chemicals, ultraviolet light, and visible light
• Self-cure systems utilize benzoyl peroxide (BPO) as the initiator and aromatic tertiary amine as the activator, supplied as two pastes, can be bulk filled but has color changes
• Light-curing systems use a photo sensitizer (α-Diketone) and aliphatic tertiary amine (DMAEMA), single paste in light-proof syringe, better esthetics and controlled working time but requires incremental packing

Depth of Cure

• Depth of cure depends on light intensity, wavelength range and curing time
• It is also impacted by distance between light source and resin composite surface

Inhibitors Role and Types

• Added inhibitors prevent polymerization during storage and control working time
• Hydroquinone is common, but has poor color stability and causes discoloration, so monoethyl ether of hydroquinone is used instead
• Eugenol also inhibits polymerization
• Oxygen can inhibit polymerization and is useful in the oxygen inhibited layer that helps in chemical bonding between composite increments

Pigments Role

• Pigments are inorganic oxides
• They are added in small amounts to provide shades that match the majority of tooth shades
• Ultraviolet stabilizers improve color stability
• Fluorescing agents provide fluorescence like natural tooth structure

Properties of Composites - Working & Setting Times

• Light-cured composites have an "on demand" setting
• Chemically-cured composites range from 3-5 minutes with working time from 1-1.5 minutes

Mechanical Properties of Composites

• These rely mainly on the filler loading, percentage, distribution, size/type of fillers and bonding between fillers/matrix
• Higher filler loading increases strength, stiffness, and toughness

Polymerization Shrinkage & Stress

• Consequences include marginal loss and microleakage
• Reduction can be achieved through the incremental technique, indirect composites (inlays or onlays), low-shrinkage composites, low-elastic modulus cavity liner, or slow curing rate

Thermal Properties of Composites

• The higher the filler loading, the lower the coefficient of thermal expansion and contraction
• The coefficient of thermal expansion and contraction of dental composites are slightly higher than that of enamel and dentin

Water Sorption

• The higher the filler loading, the lower the resin matrix and the lower the water sorption
• Excessive water sorption may lead to degradation at the filler/matrix interface due to hydrolysis of the bond between fillers and the matrix.
• Solubility of composite resins relates to the leaching of residual monomers
• Inadequate light-curing leads to greater solubility

Esthetic Properties of Composites

• These depend on the wide range of shades and translucency, excellent polish/polish retention, minimal wear, color stability, and natural fluorescence

Biological Properties and Biocompatibility

• Bis-GMA, TEGDMA, and UDMA have a cytotoxic effect in pure form
• Biocompatibility depends on composite type, cure method, efficiency of cure, and remaining dentin thickness

Manipulation of Composite

• For anterior composite (Esthetic), use Microfilled, Nanofilled, and Microhybrid
• For posterior composite (strength), use Nanofilled and Midihybrid

Etching and Bonding

• Use 37% phosphoric acid, enamel should be etched for 30 seconds and dentine for 15 (optional)
• Rinse for equal time as etching
• Avoid excessive dryness of dentine after rinsing
• Apply bonding agent and light cure

Applying Composite

• Light-cured composites should be applied in layers (incremental technique)
• This is done to ensure proper light penetration and polymerization and control shrinkage stresses
• Light-cure every increment for 20-40 seconds

Finishing & Polishing

• Finishing removes excess material
• Polishing makes the surface smooth and shiny

Applications of Resin Composites

• General applications include anterior restorations and anterior veneers, posterior restorations, core build-ups, cementation of indirect esthetic restorations and orthodontic brackets, and pit and fissure sealant

Modifications of Resin Composites

• Modifications include alterations of physical/mechanical properties, biologic properties, and mode of manipulation

Alteration of Physical and Mechanical Properties:

• Alteration of filler size, filler type (prepolymerized filler particles, ceromer, fiber reinforced composite), and matrix type

Alteration of Filler Type:

• Ceromer (ceramic optimized polymer): 75-80 wt.% inorganic, ceramic fillers with a submicron size of 0.04-1 micron
• These can be used for direct or indirect restorations
• Fiber Reinforced composite: Glass fibers are the most commonly used in dental composites due to low cost and chemical composition and refractive index that are nearly similar to that of the silica fillers

Alteration of the Matrix:

• Ormocers (organically modified ceramics): These consist of inorganic-organic copolymers, including a ceramic polysiloxane matrix with 75-80 wt% filler loading
• They combine high mechanical properties and reduced shrinkage
• Low-shrinkage matrix systems:

Ring opening silorane monomer

• In 2007, silorane (3M ESPE) was introduced
• Polymerization occurs by ring opening, therefore minimal shrinkage occurs
• Is hydrophobic and requires special type of bonding agent

High molecular weight resins

• DX 511, introduced in 2009, is a high MW resin
• It has a long rigid molecular core and flexible arms in the structure
• The long rigid core prevents monomer deformation/reduces polymerization shrinkage
• Flexible arms allow dissipation of the shrinkage stresses

Stress decreasing modulators

• SDR bulk fill composite matrix was introduced in 2009
• SDR contains conventional resin matrix and stress decreasing modulators that decrease polymerization shrinkage stresses

Alteration of Biologic Properties

• This includes fluoride releasing composites, antibacterial composites (under research) and bioactive re-mineralizing composites
• In the early 1990s, Compomer was introduced trying to combine esthetics of resin composite with adhesion and fluoride release of glass ionomer
• Compomer is composed of methacrylate resin (UDMA) as traditional composite modified with polymerizable dicarboxylic acid and glass ionomer is added in form of glass filler embedded in the resin matrix
• Giomer: is formed by a pre-reaction of fluoroaluminosilicate glass and polyacrylic acid producing glass ionomer with a stable surface modified hydrogel layer and hydrophobic resin does not allow sufficient fluoride release and recharge like glass ionomer

Antibacterial composites (under research)

• The antimicrobial strategies can be divided into stationary non-released additives (its effect is mainly by contact inhibition) and soluble released agent
• Examples on antibacterial components are MDPB quaternary ammonium compounds and silver nanoparticles
• Bioactive re-mineralizing composite: stimulate remineralization of tooth structure by releasing calcium, fluoride, and phosphates

Alteration of Mode of Manipulation

• This includes flowable composites, bulk fill composites, and self-adhesive composite
• Flowable Composites: light-cured composite with low viscosity that can be injected directly into the cavity
• The low viscosity in the early types of flowable composites relied on low filler loading with increased amount of diluents
• Properties: Good flow and adaptation, Low modulus of elasticity, Higher polymerization shrinkage, Low wear resistance and strength
• Applications: include cavity lining under composite restorations, low stress-bearing restorations, paediatric restorations and class V cervical restorations

Bulk Fill Composite

• In 2010, a new type of composite that allows filling the cavity with thicker increment 4-5 mm was introduced
• Flowable is used as bulk filling liner to be covered with outer layer of conventional composite.
• Viscous/packable can be used to fill the entire cavity with increased thickness of increments
• Technologies behind bulk fill include enhanced polymerization and depth of cure
• Enhanced translucency and modified more active photoinitiators as Lucirin are used
• Reduced polymerization shrinkage and shrinkage stresses is achieved with low shrinkage matrix and stress decreasing modulators that decrease stresses.

Self Adhesive Flowable Composite

• The bonding efficiency of these materials are much less than using conventional separate bonding steps

Common Light Curing Units

• Quartz-tungsten-halogen (QTH) lamps: have a quartz bulb with a tungsten filament
• They have several drawbacks including heat generation, bulb intensity diminishing with use, and need for calibration
• Light emitting diodes (LED lamps): LEDs have been developed to overcome the drawbacks of QTH lamps
• LEDs require low wattage, are powered by battery, generate no heat, and are quiet because a cooling fan not needed
• LEDs are replacing QTH lamps in practice
• Note that compatibility of the light cured and the photoinitiators present in the composite should be confirmed
• Some light cures specially LED have narrow wavelength range that is not compatible with some photoinitiators other than camphorquinone which is specially present in lighter shades of composites