Dental Materials Flashcards
What are the disadvantages of light curing composites?
Light sensitive during application
Limited depth of cure (2mm) that requires curing in increments (increased placement time)
Retina damage from curing light
Polymerisation shrinkage
Define the following terms:
(i) Degree of cure
(ii) Depth of cure
How much of the monomer is converted to polymer (via free radical polymerisation - in case of composites) - double bonds react for composites (heat cure greatest)
The depth at which the monomer/resin is converted to polymer, above which no conversion occurs e.g. composites have a limited depth of cure ~ 2mm after which results in unreacted resin at base of cavity “soggy bottom”
Give FOUR factors that affect depth of cure
How close light is to restoration - aim for light curing unit to be as close as possible without touching tooth
Light tip contamination - must not be contaminated
Type of composite i.e. shade (darker shade, lower depth of cure)
Curing time - follow manufacturers guidelines
Size of incremental layers i.e. size of restoration
List THREE light curing systems and give an advantage and disadvantage of each
- Quartz-Tungsten Halogen
Adv - Band width (400-500 nm - most photoinitiator react in this range)
Dis - cooling of unit critical (bulb life decreases) - Light emitting diode (LED)
Adv- cordless, slimline, low wattage, less lateral heat production (cf halogen), long-lasting light source
Dis- Narrow emission spectrum (460-480 nm) - Plasma-arc (PAC)
Rapid cure (1-3 s)
Expensive - Argon Laser
Adv - High energy, highest intensity
Very expensive
List THREE advantages of LED curing system over Quartz-Halogen
Less lateral heat production
Cordless & slimline with rechargable battery utilising low wattage
Long-lasting light source and ultra-energy efficient
Define oxygen inhibition and TWO ways to prevent it
Air and resin interface results in a sticky resin surface
Oxygen inhibits cure of resin surface
To overcome: overfill, use clear matrix strip and apply bond
What acid is used in acid-etching and list THREE of its functions
30-50% phosphoric acid
Raises surface energy of enamel to improve wettability by removing pellicle layer and contaminants
Increased surface roughness of enamel by dissolving HAP prsisms, increasing bonding area
Opens inner prism areas for interlocking tag formation
Allows for micromechanical retention
Why is it difficult to bond to dentine?
Dentine is hydrophilic, and most restoratives e.g. composites are hydrophobic - therefore require use of a coupling agent e.g. HEMA (hydrophilic & hydrophobic ends)
Cut dentine results a wet surface covered by a smear layer - composed of denatured collagen, debris & bacteria - a contaminant making bonding more difficult and must be removed
Describe dentine primer and its function
Bonds hydrophilic dentine to hydophobic resin, acts as coupling agent
Bi-functional molecule with hydrophilic and hydrophobic ends e.g. HEMA (hydroxylethylmetacrylate) with central R (CH2-CH2) spacer molecule necessary for flexibility of coupling agent
Hydrophilic (polar) end - OH bonds to collagen of demineralised dentine & via molecular entanglement
Hydrophobic end bonds to CH3 of resin
Describe the smear layer
When dentine is cut it is covered by the “smear layer”
Composed of denatured collagen and debris covering dentine surface - weakly bound to dentine (0.5-5 um thick)
It is contaminated with bacteria (caries) and cutting debris
Describe the function of inorganic filler and give TWO examples
Used to reinforce particles and/or fibres dispersed in matrix
Examples: Quartz, colloidal silica
Increase compressive strength (elastic modulus & hardness) and wear resistance
Reduces polymerisation shrinkage
Reduces TEC
Why is a coupling agent important for composites?
Coupling agents such as silicone coupling agent (y-MPTS) - bonds hydrophobic organic resin to hydrophilic inorganic filler (reaction with filler OH condensation reaction)
Improves wear resistance of final restoration - improves mechanical properties
