Crowns Flashcards
The Principles of Crown prep
- Preservation of tooth tissue
- Retention and Resistance
- Stuctural Durability
- Marginal Integrity
- Preservation of periodontium
- Aesthetics
- Preservation of tooth tissue - how
Avoid weakening tooth structure and damage to the pulp.
Under prep - Poor aesthetics and occlusal and periodontal consequences
Over prep - Tooth strength and pulp comprimised.
- Retention and resistance definitions
Retention - prevents removal of restoration along PoI or long axis of tooth prep
Resistance - prevents dislodgement of restoration in apical of oblique direction. Prevents movement of restoration under occlusal forces.
How can retention and resistance be achieved
Taper - 6 degrees for opposing walls
length of walls - longer means < tipping displacement
grooves and slots
Improve retention by reducing numbers of PoI’s
How can structural durability be achieved.
Occlusal reduction, axial reduction, functional cusp bevel.
How can marginal integrity be achieved?
Finish line configuration - knife edge, bevel, chamfer, shoulder and bevelled shoulder.
How can preservation of periodontium be achieved.
margins: smooth and fully exposed - cleaning
Placed where dentist can finish and patient clean
Placed at gingival margin
Be aware of BIOLOGICAL WIDTH (2mm)
Reduction values for metal (gold veneer) Crown
Axial - 0.5mm,
Occlusal F cusp - 1.5mm, NF cusp - 0.5mm
Finish line - CHAMFER 0.5mm
Reduction values for Traditional ceramic porcelain crowns
Axial - 1mm
Occlusal - F cusp - 1.5mm, NF cusp - 1mm
Finish line - SHOULDER 1mm
Reduction values for MCC
Axial -1.3mm
Occlusal - F cusp - 1.8mm NF Cusp - 1.3mm
Finish line CHAMFER 0.5MM IF METAL
SHOULDER - 0.9MM PORCELAIN, 0.4MM METAL.
Reduction values for Alumina, Zirconia crowns
Axial -1.5mm
Occlusal - F cusp - 2mm NF cusp - 1.5mm
Finish line - CHAMFER 1-1.5mm
Significance of buccal prep
2 PLANES - avoid pulp horn
What is kaolin
hydrated aluminosilicate
What do dental ceramics consit of
- Little kaolin
- feldspar
- quartz
- metal oxide
- glass
What is feldspar and its purpose
Acts as a flux (binder)
- lowers the fusion and softening temperature of glass
Conventional dental ceramics - powder, how does this happen
Powder made by heating to >1000 degrees then:
-RAPIDLY COOLING, create crazing/cracks.
thiis frit is then milled, a binder added then mixed with distilled water to form restoration
What is leucite and its purpose
Potassium Aluminium silicate
-forms glass phase of ceramic,
Gives thermal and physical properties.
what happens during the fabrication of a crown
Heating -> sintering
occurs just above glass transistion temperature.
-this is when glass becomes single mass (coalesce)
-during this material shinks by 20% (need to allow for this during construction)
Properties of conventional ceramics
- aesthetics
- abrasion resistance
- chemically stable
- biocompatible
- thermal props
- dimenionally stable
Aesthetics of ceramics
- best of any restorative material
- stable colour, smooth surface
- Optical properties - reflectance, translucency, opacity, transparency, opalescence
Chemical stability
- no reaction to other materials
- not pick up stains
- good biocompatibility
- unaffected by pH
Thermal properties
similar to tooth
- TEC similar to dentine
- LOW thermal diffusivity
Dimensional stability
Very stable
-Fabrication shrinkage (20% during firing/sintering)
Mechanical properties
- high compressive strength
- high hardness
- tensile strength = LOW
- flexural strength = LOW
- # toughness = LOW
- Static fatigue - gradual decrease in strength over time (Si-O hydrolysis)
- surface microcracks ->slow crack growth
