Why your peers choose zirconia over PFM Flashcards
Pros of zirconia crowns (9)
- superior strength, durability and aesthetics to PFM or FGC
- minimal tooth reduction
- metal free* therefore prevents darkening around gingival margin and metal margin exposure due to gingival recession
- translucence can transmit colour of adjacent teeth (matching)
- can be cemented OR bonded
- highly biocompatible (promotes healthy tissue response)
- smooth surface reduces plaque accumulation
- rarely cracks but if there is a crack it wont propagate
- zirconia suitable for metal allergies or those who prefer metal free restos
*technically zirconium oxide - a metal oxide
Cons of zirconia crowns (2)
few if any
- hardness: was a concern about possible degradation/wear of opposing and adjacent teeth* but if monitored properly and highly polished you can reduce risk of damaging other teeth
- initially unaesthetic as they could only make white substructures but new materials are preshaded and fabricated to provide natural looking restos, now have HT zirc
- difficult to remove or do endo
*when eating teeth move and inteproximal can get worn, eventually causing open contact
3 types of zirconia restos
- solid (monolithic)
- PFZ (layered - emax)
- Zirconia HT-high translucent
What is the main issue with PFM that zirconia crowns will not experience?
metal is not always compatible with ceramic and can fracture off (difficult to restore without removal - use of opaque composites doesn’t always yield satisfactory results)
bottom image shows - abfraction causing bending of P which debonds from M on crown in bruxer where lateral excursion C-C with no group function (very similar to NCCLs in natural teeth)
Indications/contraindications of monolithic zirconia, why?
- indicated in an area that requires minimal clearance or
- severe bruxism
- posterior bridges (4-5 units)
- discoloured tooth preps
- endo treated teeth
pure zirconia has the highest flexural strength (1200MPa) and lifelike translucency and opalescence
C: may be less indicated in highly aesthetic areas as highly opaque e.g. anterior bridges
How are monolithic zirconia crowns fabricated?
designed and milled via CAD/CAM - fabricated from 100% monolithic medical grade zirconia (virtually unbreakable-even severe bruxers wont #) sintered for over 6hrs at 1500
Characteristics of PFZ (4)
slightly more aesthetic than monolithic and used prior to the creation of Zirconia HT in aesthetic areas
not as strong as solid zirconia but porcelain is designed to bond with zirconia’s substructure - therefore chipping/fracturing is rare
if crown requires max strength (heavy bit or limited occlusal clearance) then monolithic is better
more expensive than solid zirconia (extra lab work)
Flexural strength zirconia HT vs solid zirconia and other materials
solid - 500-8000MPa
HT - 590-720 MPa
but still stronger than PFM and 3x stronger than Emax
What are Crystalite and Opalite?
Crystalite: monolithic super translucent zirconia - as translucent as lithium disilicate
Opalite: monolithic zirconia
Advantages of Zirconia HT. Indications and contraindications.
Superb aesthetics - natural, vibrant translucency (can reflect and absorb light similar to natural tooth while maintaining strength.
Requires less occlusal prep and thinner wall to give higher translucency
I: simple implant, anterior and posterior crowns, up to 3 unit bridges
C: alot of force and bruxers
Advantages of Zirconia HT. Indications and contraindications.
Superb aesthetics - natural, vibrant translucency (can reflect and absorb light similar to natural tooth while maintaining strength.
Requires less occlusal prep and thinner wall to give higher translucency
I: simple implant, anterior and posterior crowns, up to 3 unit bridges
C: alot of force and bruxers
What are the ideal crown margins for Zirconia?
shoulder (better) and chamfer
feather edge contraindicated as it can fracture in hand during placement (0.1mm thick at margin), always ask your lab tech first
Dimensions of anterior vs posterior zirconia crowns.
A:
- 1.5mm incisal reduction
- 1mm wall
- clear and visible circumferential chamfer w reduction of atleast 0.5 to ideally 1mm at gingival margin
- incisal edges rounded
P:
- 1.5-2mm occlusal reduction
- 1mm wall
- 0.5-1mm circumferential chamfer
- round sharp points
bevel not recommended in either
Factors that can render a prep unacceptable for a crown (5)
- undercuts
- gutter prep (can form with round tip burs)
- 90 degree shoulder (made with fissure bur at right angle)
- parallel walls
- sharp incisal/occlusal edges
- differing paths of insertion
What to do if adjusting zirconia
- use appropriate diamond bur on slow speed
- use as little pressure as possible (pressure = heat = #)
- use water to cool
- polish!
Describe the difference in fracture mechanism between monolithic zirconia and PFZ crowns
MZ: a cone crack, where the material cracks in a divergent direction from the prep
PFZ: radial cracking in which the interface between the substructure and superficial layer split upwards
(image - outer aspect of tree trunk is the interfere; tip of pyramid is the occlusal aspect)
Monolithic vs PFZ
(extra)
What is the problem with plane line articulators? (2)
when is it acceptable to use them? (1)
Articular axis far from the physiological axis → posterior restorations end up high and need adjustment upon insertion
Lateral jaw movement cannot be stimulated
-ok to use for single posterior crowns w canine guidance
