lecture 4 Flashcards
parts of a crown prep:
-axial wall
-interproximal wall
-finish line (chamfer finish line)
-occlusal anatomy
the design of the prep finish line dictates the shape and bulk of
restorative material in the marginal area of the restoration
a full coverage restoration can survive in the biologic environment of the oral cavity only if the
margins are closely adapted to the surface finish of the prep
types of finish line designs:
-knife edge
-chamfer (light and heavy)
-shoulder
-beveled shoulder
used primarily for full metal crowns only
especially gold crowns: gold’s malleability it can be burnished, adapted, and polishd to a precise fit with this margin design
knife edge (margin design)
disadvantages of knife chamfer
-difficult to see finish line in an impression or scan
-metal can be thin and is susceptible to distortion under heavy occlusal load
-often leads to an over-bulking of crown material to compensate for thinness along margin
-easy to miss creating distinct end of finish line and instead axial wall never terminates
for knife edge chamfer, to compensate for thin marginal area, labs will:
since margin thin and susceptible to distortion
overbulk crown at margin
indications for knife edge margins
tipped, molar, possible to be conservative in abnormal area
most conservative prep design
knife edge
(use a flame or needle shaped diamond bur)
light chamfer
metal crowns
zirconia crowns
heavy chamfer
-ceramic crowns
-zirconia crowns
-metal-ceramic crowns
use what bur to create chamfer
round ended diamond bur
axial depth more than ____bur, can create lipped edges of the margin which leave unsupported enamel likely to chip or break leaving an open and exposed margin
1/2 the bur
advantages of chamfer:
-well suited to full metal crowns and zirconia crowns
-ease of seating crown and placing of crown
-ease of detecting margin on impression or scan
-if full metal, chamfer is easy to wax and cast
-provides adequate thickness of restoration material at margin area
heavy chamfer good for:
chamfer good for:
heavy chamfer:
-high strength porcelain crowns, buccal of metal ceramic crowns
chamfer:
-full metal crowns, palatal/lingual of MCC’d, resin bonded crowns
light chamfer is ___mm deep into axial wall
0.3-0.5mm
light chamfer produces ____degree margin with rounded internal angle
30-45 degrees
chamfer combines knife edge burnish-ability with bulk of material for all what type of crowns
metal crowns
heavy chamfer provides room for ____ceramic margin.
internal rounded angle decreases material stress in this area
how deep?
what bur use?
thicker ceramic margin
1-1.2mm deep
large round ended diamond bur
for shoulder margin, want close to a _____degree angle.
used primarily for what kind of crowns:
what bur used:
90 degree (rounded angle)
metal ceramic crowns and all ceramic crowns where thickness of material is important [decreases stress on restoration]
flat end diamond bur used
some shoulder finish lines can include this at 3-45 degree angle which is often used for onlays and inlays, metal ceramic crowns where there is metal collar
bevels
-allows for adaptability of metal to ensure marginal fit
rough margin=
plaque accumulation = recurrent decay
significant roughness of prep finish line can prevent
close adaptation of crown margin
deep chamfer and rounded angles means=
crowns last longer
the finish line designs vary based on:
- restoration material
- location in mouth
- desired results
similar to chamfer finish line but comes with 90 degree cavosurface angle with large radius rounded internal angle.
best finishes for ceramic crown
heavy chamfer
crown prep concerns:
-pulp becomes irritate and inflamed during crown preparation
-rise in pulp temp by 10 degrees significantly increases the change of pulpal necrosis
to prevent pulpal issues with prep, must do what:
-use lots of water
-sharp bars
-light pressure
-high speed
-diamond burs
margin design instrumentation
preparation:
finishing:
caries removal:
preparation:
high speed handpiece
air/water spray
diamond burs
finishing:
slow speed handpiece
fine diamond burs (possibly carbide burs)
hand instruments too
caries removal:
slow speed with 2,4,6 round bur
spoon excavators
these burs remove enamel and dentin by grinding it away
diamond burs (leave rougher surface)
these burs remove tooth by shearing it away
carbide burs (smoother)
what burs are most efficient and 2-3 times faster?
diamond burs
these burs cut precision prep features with smooth surfaces
often used to place extra retention retention features in a crown prep such as seating groove
carbide burs
hand instruments can also be used to help smooth finish lines
chisel
hatchet
hoe
margin trimmer
advantages and disadvantages of full metal crowns
adv:
-gentle to opposing teeth
-can be thin
-conservative preps
-recurrent decays less likely
-very durable
-if missing contact, can add material
-long lasting
disadv:
esthetics
what margin design for full metal crown prep
can use knife edge, but usually a light chamfer is recommended .3-.5mm
[reduce until half a bur is in axial wall]
in full metal crown prep,
axial walls are reduced to create a tapered wall between ____ and ___degrees but can have TOC up to:
axial walls can be reduced as little as ___mm, but can be reduced more in needed
6-10 degrees
but can have up to 20 degrees
as little as .5mm
full metal crown prep, occlusal reduction depth of
1-1.5mm
need to do this to prevent pressure points and possible fracturing of a restorative material
ling angles and corners need to be rounded
use fine grit diamond bur
metal free
hard and tough
less translucent than natural teeth
tooth-colored ceramic like material
zirconia
