General Flashcards
3 types of forces applied to materials
- compressive
- tensile
- shear
Stages of material selection effected by material characteristics
- selection (adequate)
- preparation required (surface on/cavity design)
- Placement (curing/increments)
- Performance (fit/longevity)
- Patient expectations (aesthetics/durability)
5 restorative materials
- Amalgam
- Composite
- Glass ionomer cements
- Compomers
- Porcelain
4 impression materials
- Impression compound (non-elastic)
- Impression paste (non elastic)
- Hydrocolloids (alginate)
- Elastomers (polysulphides, polyethers, silicones)
3 properties of any material
- chemical
- physical
- mechanical
3 outcomes of forces applied to materials
- stretch/compression
- deform (temp/permanent)
- fracture (failure)
What is stress to a material
Force/area where force applied = pascal (Pa) - changed to giga pascals (GPa)
What is strain to a material
Change in length / original length = strain (ratio of change in length)
Explain the proportionate limit (stress strain graph)
Where stress and strain changes become non linear (leads to fracture - fracture stress)
Opposites of each mechanical property of materials (I.e hard and soft)
- Strong and weak
- rigid and flexible
- ductile and brittle
Explain the elastic modulus of a material
Lower gradient of stress strain graph means a lower elastic modulus
causes of failure to a material (7)
- fracturing (large force causing catastrophic destruction to structure)
- impact (large force causing fracturing)
- fatigue (repetitive small stresses causing fracture)
- permanent deformation (no fracturing, when releasing stress > elastic limit)
- abrasion (surface removed by grinding)
- lack of hardness (allows to be indented)
- de-bond (forces enough to break bond)
physical properties of materials
- viscosity
- thermal conductivity
- thermal expansion
- density
- radiodensity
chemical properties of materials
- setting mechanism
- setting time
- corrosive potential
explain the acid etch technique
- 35% phosphoric acid acts on Ca2+ of hydroxyapatite
- roughens surface by decalcifying causing exposure of collagen fibres for more molecular entanglement and removes smear layer increasing surface energy
why is it hard to bond to dentine
- more organic (water puddles from tubules but material hydrophobic)
- low surface energy
- inconsistent structure (soft then hard)
- smear layer
describe technique to bond to dentine
- etching = removes smear layer exposing tubules and roughens surface
- wetting agent used to increase surface energy (bonding agent covers tubules)
describe the 3 types of bonding (normally all used together)
- mechanical (physical interlocking of structures i.e. enamel and some dentine with composite)
- chemical adhesion (ionic bonds with mineralised part but mostly covalent bonds with organic regions
- van der walls adhesion (dipole interaction between bonding agent and substrate)
describe critical surface energy (why bonding agent used on dentine?)
liquid must have lower surface energy than surface placed on to flow and stick
- as dentine wet has lower surface energy than composite so bonding agent fixes this
what is a surface wetting agent
bonding agent (as increases surface energy allowing materials to stick)
describe the action of a wetting agent and what its 4 property requirements
meshes with acid cured surface via molecular entanglement
1&2. ability to flow and low viscosity
3. ability to make good contact with dentine
4. adhesion to substrate via van der walls, chemical, mechanical or all of the above
describe what the smear layer is and how we deal with it
- layer of debris on dentine contaminated with bacteria
- either remove by etch [total etch] or incorporate [self etch] via bonding agent penetrating it (can allow leaking thus hydrolysis)
two types of etching
- total etch
2. self etch
explain what total etch is
- 3 components
1. dentine conditioner
- removes smear layer (decalcifying) while hardly demineralising HA
2. primer (bonding agent) = double headed molecule, one phobic one phylic
3. adhesive = resin that penetrates now hydrophobic tubules and has molecular entanglement with tubules and collagen fibres = forms hybrid layer (hydrophobic with camphorquinone)
give an example of a primer
HEMA (hydroxy ethyl methacrylate)
- Methacrylate head = hydrophobic
- hydroxyl head polar bonds to similar regions of HA
Photoinitiator put in materials for curing
camphorquinone
issues with total etch
- can over etch meaning collagen fibres collapse or too deep and etch as so primer cant reach mineralised dentine
- too wet means is diluted and too dry means surface too smooth and only tubules as means collagen fibres collapse
describe what self etch is
- solution contains etch, primer and adhesive in one (or etch separate)
- incorporates smear layer into hybrid later via dissolving it
- good as less technique sensitive and less chance of post op sensitivity due to no drying stage = no breakdown of dentine architecture
- bad as not as strong bonding and lower pH may not etch as well
- ## uses bifunctional molecule (acidic methacrylate monomers (etch and infiltrate dentine with hydrophilic end and while usual polymerising at hydrophobic end)
describe what self etch is
- solution contains etch, primer and adhesive in one (or etch separate)
- incorporates smear layer into bonding
- good as less technique sensitive and less chance of post op sensitivity due to no drying stage = no breakdown of dentine architecture
- bad as not as strong bonding
- uses bifunctional molecule (acidic methacrylate monomers
what primer and solvent are used in a primer and adhesive mix for self etch
Bis-GMA and alcohol or acetone
draw backs to strong self etch compared to mild
breaks down too much HA so not as good ionic bonding and liable to allow breakdown of collagen fibres
blacks cavity classification
class i = pit and fissure caries ii = posterior approximal caries iii = anterior approximal caries iv = anterior approximal carries effecting incisal angle v = caries effecting cervical surfaces vi = carries effecting cusp tips
