Margin Design Flashcards
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 line of the prep
what are the types of finish lines
-knife edge
- chamfer- heavy and light
- shoulder
- beveled shoulder
when is the knife edge used and what burs are used
- full metal crowns only
- gold crowns
- most conservative prep
- use flame or needle shaped bur
what are the disadvantages to knife edge
- difficult to see finish line in an impression or scan
- metal can be thin and is suscpetible 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
when is knife edge indicated
- tipped molar to be conservative in abnormal area
- young patients
-metal restoration - MOD onlay
- inaccessible areas
- when finish line extends to cementum
- lingual surface of mandibular posterior teeth
- pin ledge 3/4 crown
when is light chamfer used
- metal crowns
- zirconia crowns
when is heavy chamfer used
- ceramic crowns
- zirconia crowns
- metal ceramic crowns
what bur do you use to make chamfer
round ended diamond
what are advantages to chamfer
- well suited to full metal crowns and zirconia crowns
- ease of seating crown and placing 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
describe light chamfer
- 0.3-0.5mm deep into axial wall
- produces 30-45 degree margin with rounded internal angle
- chamfer combines knife edge burnish-ability with bulk of material for all metal crowns
describe the heavy chamfer
- provides room for thicker ceramic margin
- internal rounded angle decreases material stresses in this area
- 1-1.2mm deep
- use large round ended diamond bur
what margins can work with any crown
heavy chamfer or shoulder
describe a shoulder and what burs used
- close to a 90 degree angle
- flat end diamond burs
what does a modified shoulder have
a rounded angle
when is shoulder used
metal ceramic (PFM or PFZ) and all ceramic crowns
what does the 90 degree angle at the axial wall and finish line of the shoulder do
decrease stress on restoration material
describe shoulder with bevel and when it is used
- finish lines can have bevel at a 30-45 degree angle
- often used for inlays and onlays
- can be used for metal ceramic crowns where there is a metal collar
- bevel allows for adaptability of metal to ensure marginal fit
significant roughness of preparation finish line can prevent :
close adaptation of crown marginug
rough margin = ______ = ______
plaque accumulation; recurrent decay
what does the finish line design vary based on
restoration material, location in the mouth and desired results
what are advantages to knife edge
-conservative
- ideal for marginal adaptatoin
- useful in metal and gold crowns
- considered an ideal finish line
what are advantages and disadvantages to shoulder
- A: less distortion of crown margins, provides adequate bulk, good crown contours, can attain good esthetics
- D: marginal adaptation can be affected, more tooth structure needs to be removed
when is beveled shoulder indicated
gingival finish line on the proximal box of inlays and onlays, occlusal shoulder of onlays and mandibular 3/4 crowns, the finish line for extremely short walls, the facial margin of posterior metal ceramic crowns with a supragingival margin
are are advantages and disadvantages to shoulder with bevel
- A: to get proper marginal adaptation, the bulk of the material in border
-D: more tooth structure is removed, less conservative approach, prep should be extended apically
describe the chamfer
a finish line design for a tooth prep in which the gingival aspect meets the external axial surface at an obtuse angle
when is a chamfer indicated
cast metal restorations or lingual margin of metal ceramic restorations
what are advantages and disadvantages of chamfer
- A: conservative, good marginal adaptation, provides bulk to the restoration
- D: technique sensitive and any defect in fabrication can lead to unsupported tip of the crown
describe the heavy chamfer
similar to chamfer finish line but comes with a 90 degree cavosurface angle with a large radius rounded internal angle, in metal restorations a bevel is added to the finish line
what are advantages and disadvantages to a heavy chamfer
-advantages: best finish line for a ceramic crown
- disadvantages: technique sensitive, the little discrepancy can lead to the formation of a lip or unsupported fragile enamel
what do you use for caries removal
- slow speed with 2,4,6 round bur
- spoon excavators
how do diamond burs remove enamel and dentin
grinding
how do carbide burs remove tooth
shearing
how much faster are diamond burs than carbide burs
2-3 times
what are the advantages of full metal crowns
gold and many metal crowns are soft metals
- gentle to opposing teeth
- metal is strong so material can be thin
- allows for very conservative preps
- metals have some antibacterial properties making recurrent decay less likely
- most gold and full metal crowns are much more gentle on gingiva
- metal crowns are durable even if worn through or with endo access
- if contact is missing you can add it on
what are disadvantages to full fetal crowns
esthetics
what is the margin design for full metal crown
- light chamfer 0.3-0.5mm
-can use knife edge - reduce until half bur is in the axial wall then follow around the contours of the tooth to create chamfer
describe the axial reduction on full metal crown
- reduced to create taper between 6-10 degrees but can have TOC up to 20 degrees
- axial walls can be reduced as little as 0.5mm
describe occlusal reduction on full metal crown
- uniform occlusal reduction depth of 1.0-1.5mm following depths and pathways of developmental grooves
describe zirconia
- tooth colored ceramic like material
- considered metal free
- has propertieis similar to metal
- can be very hard and tought
- is less translucent than natural teeth
- allow for conservative preps bc of strength
- they wear opposing teeth less
describe the reduction in the zirconia esthetic crowns
- 1.25mm ideal reduction
- chamfer or modified shoulder
describe the reduction in the zirconia full strength
- 1.0mm ideal reduction
- chamfer or shoulder margin preffered, knife edge is ok
the more esthetic your zirconia is, the ____ reduction you will need
more
