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
Where should feldspathic crowns be used
LOW STRESS AREAS
- ANTERIOR
Alumina core - use and 2 named examples
ANTERIOR
- good aesthetics
- > alumina means >strength (flex 120MPa)
- INCERAM
- PROCERA
INCERAM - 85% Alumina
what technique is used to form this
SLIP CASTING
- ceramic core formed on refractory model, alumina slurry applied.
- heated to 1120 degrees for 20 hrs (below glass transition temp)
- PARTIAL SINTERING OCCURS
- porous core produced, infiltrate with lanthanum glass
HIGH strength - >400MPa
PROCERA -99% core - better/worse strength, what other property is better in this type of ceramic
BETTER - >700Mpa
better translucency
ZIRCONIA CORE - purpose/how can it be produced.
What temperature can it be sintered at?
-What other material is used with it in dentistry
CAD-CAM.
>1600 degrees
- YTTRIA STABLISED ZIRCONIA
How does Yttria stabilised zirconia stop crack propagation
What can they be used for
properties
Yttria is a tetragonal crystal structure
- if a crack begins when the stress at a crack tip reaches critical level, the crystal structure transforms into a monoclinic structure.
- this causes expansion and closes up crack tip
- used for bridge framework
strong/hard/tough
Fabrication steps of zirconia core
- impression taken, scan cast digitally
- create virtual bridge substructure, minimum thickness of connectors created.
- raw zirconia milled
- cut framework heated to 850 degrees, 20% shrinkage occurs
- veneered with feldspathic porcelain prior to final restoration
When would a crown be advised for a patient:
- little tooth tissue - weakened tooth
- fracture
- cuspal coverage ie Endo
- improve aesthetics
- indicated by RPD design
contraind to crown placement
- active perio disease
- active caries
- lack of tooth for prep
- destuctive prep
- unable to provide post/core
- unfavourable occlusion
MCC measurements for anterior/posterior
- labial
- palatal
- incisal
- taper
- labial: 1.4mm (0.9mm porcelain / 0.5mm metal) SHOULDER
- palatal: 0.5mm CHAMFER / 1.5mm chamfer
- incisal: 2-2.5mm
- 2-7 degrees
All-Ceramic measurements for anterior/posterior
- labial
- palatal
- incisal/occlusal
- taper
- labial: 1.5-2mm SHOULDER
- palatal:1.5-2mm CHAMFER
- incisal/occlusal: 2.5mm
- taper: 2-7 degrees
