Fixed prosthodontics and Lost-wax casting process Flashcards
what is fixed prosthodontics?
branch of prosthodontics concerned with the replacement and/or restoration of teeth by artificial substitutes that aren’t removable from the mouth
what is retention?
prevents removal of the restoration along the path of insertion/ long axis of tooth preparation
what is resistance?
prevents dislodgement of the restoration by forces directed in an apical/oblique directions and prevents any movement of restoration under occlusal forces
what surfaces do extracoronal restorations use for retention?
Use opposing external surfaces for retention
what surfaces do intracoronal restorations use for retention?
use opposing internal surface for retention
how does extracoronal restorations have retention?
provided by external surface (such as buccal and lingual walls) of the crown
how does intracoronal restorations have retention?
provided by resisting displacement by wedge retention
what 5 factors affect retention and resistance?
tapering, freedom of displacement, length, substitution of internal features and path of insertion
what is tapering?
the more parallel the opposing walls of a preparation, the greater the retention
what is the optimum angle of tapering?
16 degrees - 8 degree inclination on each side
what are the 7 types of margins/finish lines?
feather edge, chisel, chamfer, bevel, shoulder, sloped shoulder and beveled shoulder
what surface should the margin of the restoration be closely adapted to?
cavosurface finish line of the prepared tooth
what is an impression?
a negative reproduction of anatomical parts including prepared teeth, ridge areas and other parts of the dental arch
what is a die?
a positive reproduction of the prepared, it is sectioned, trimmed and ditched
what are the 7 criteria’s for die materials?
should reproduce surface detail accurately
material should dimensionally accurate
strong and resistance to abrasion
easy to section and trim
compatible with separating agent
colour that contrasts with wax
easily wettable by wax
what is the expansion of type IV die stone?
0.1% or less
what is the expansion of type V die stone?
as much as 0.3%
why does type V die stone expand more?
to compensate for relatively large solidification shrinkage of base metal alloys
what are some advantages of gypsum?
inexpensive, easy to use, produces consistent results
what is one disadvantage to gypsum?
it has poor abrasion resistance
what is die hardener used for?
reduces the die’s susceptibility to abrasion during wax carving
what is die spacers used for?
provides relief space for dental cement
what are some advantages of polymeric die materials?
stronger than gypsum, abrasive resistance and provides good detail production with silicone and polyether
what are some disadvantages of polymeric die materials?
expensive, epoxy die materials may shrink, not suitable for some impression materials
what are electroplated dies?
formed by electroforming, when areas are electroplated with fine powder silver or graphite to make them conduct electricity, causing a layer of pure metal to be deposited on the impression
what are some advantages of electroplated dies?
excellent abrasion resistance, moderately high strength, able to restore friction on worn telescopic crowns
what are some disadvantages of electroplated dies?
labor intensive, time-consuming, can be toxic (use of cyanide in Ag plating), costly equipment, not suitable for all types of impression materials (only silicone and polyether)
what are the 5 requirements for a working cast?
there should be an anatomical part and base
base should be strong/rigid to provide stable platform
anatomical part should be sectioned - 10-15mm thick exclusive of teeth
anatomical parts should be able to be removed and replaced on base
sectioned parts should retain dimensional relationship
why is a solid “check” cast important?
dies and sections can become less accurate with repeated removal, use of solid check model ensures fit of restoration at final stage to verify accuracy of the restoration.
what is dipping wax?
reinforces the wax copings in crown and bridge work by providing basis for precision
what are the requirements of wax patterns?
accurately fit the die
no thinner than 0.5mm
occlusion must be correct
wax coping should be allowed to cool before removal from die
frequent removal will increase warpage/distortion
marginal shape should be precise.
why are proximal contacts important?
help stabilise the teeth within the arch
what is the emergence profile?
part of the axial contour that extends from base of gingival sulcus past margin of gingiva and extends to height of contour
what are embrasures?
the spillway spaces formed from curvatures adjacent to contact areas
what are the functional cusps of maxillary teeth?
lingual (palatal) cusps
what are the functional cusps of mandibular teeth?
buccal cusps
what are the non-functional cusps of maxillary teeth?
buccal cusps
what are the non-functional cusps of mandibular teeth?
lingual cusps
why are provisional restorations used?
used to protect prepared tooth during time between preparation of tooth and placement of definitive restoration
what should a provisional restoration have?
pulpal protection
good marginal fit and seal
stability
function
strength and retention
good aesthetics
what are the three components in structural durability?
occlusal reduction, functional cusp bevel and axial reduction
what is occlusal reduction?
provides adequate bulk of metal and strength to the restoration
what is a functional cusp bevel?
wide bevel on functional cusp provides space for adequate material in an area of heavy occlusal contact.
what can happen if there is no functional cusp bevel?
thin area or perforations
over contouring
poor occlusion
what is axial reduction?
provides adequate thickness of the restorative material
what can occur if theres inadequate axial reduction?
the restoration will have thin walls subject to distortion
what are some advantages of complete cast crowns?
high strength restoration
high retentive qualities
adequate resistance
option to modify form to support RPD’s
what are some disadvantages of complete cast crowns?
aesthetics
removal of large amount of tooth structure
adverse effect on tissues
what are the indications for a complete cast crown?
extensive destruction from caries or trauma
endodontically treated teeth to compensate for loss of tooth structure
need for maximum retention and strength
provide contours to receive RPD
correction of occlusal plane
what is a sprue?
a channel in investment where molten alloy flows to reach hollow mould formed after burnout of wax patterns
how can you minimize distortion during waxing up?
apply in small increments
should not stand uninvested for more than 30 minutes
store at room temp
avoid unnecessary removal from die
avoid inclusions in pattern
avoid exposure to extreme temperatures
what are the mechanisms of investment mould expansion?
setting expansion
hygroscopic expansion
wax pattern expansion
thermal expansion
what is setting expansion?
result of normal crystal growth
what is hygroscopic expansion?
when investment is set in presence of water
what is wax pattern expansion?
when investment is still fluid and wax is warmed above temperature that it was formed at
What are high-noble alloys?
when at 40% weight is gold
