Principles of cavity preparation (amalgam 2) Flashcards
what components of cavity preparation fall under stage 1 initial preparation steps :
=outline form
=primary resistance form
=primary retention form
=convenience form
what is the function of good resistance form ?
it enables booth the tooth structure and the restoration to withstand stresses without fracture
which type of force on restored tooth causes more destruction ?
lateral force
brittle materials (bulk for strength)
-amalgam
-composite
-ceramic\
weak against tensile forces
as opposed to brittle materials , ductile materials such as gold have some advantages such as:
- can be finished into thin margins as there is no need for bulk
-used to protect and support weak tooth structure
what is the correct width of a cavity prepared for amalgam?
1/4 intercuspal distance
-not more in order to preserve as much of the cusp as possible –>stronger tooth
-also to decrease surface area of restoration , making it less likely to fracture
shape of amalgam cavity should be ____ in order to optimize resistance form
box shaped with flat pulpal and gingival floor(also helps in retention form )–helps neutralize stresses at the cavity seat
uneven surface in a prepared cavity will result in:
stress concentration in the deepest areas in the cavity , resulting in fracture of tooth structure .
forces concentrated on elevated areas will cause fracture of the restoration itself
minimum occlusal thickness of amalgam :
-ceramic:
-cast metal:
-composite:
-amalgam:1.5mm
-cermaic: 2mm
cast metal : 1-2mm
composite : variable
what is undermined enamel and what should be done with it ?
undermined enamel is enamel with no dentine support ( carious dentine ) and it is to be removed
other than having smooth surfaces , how else may a dentist limit stress concentration when restoring a tooth ?
by roundening the sharp line angles
what is the only line angle , that if left sharp , has the capability to break the amalgam restoration itself and not the tooth structure.
the axio-pulpal line angle
(should be beveled or rounded)
what is the function of cusp capping ?
decreasing the height of the cusp in relation to its width in order to have better resistance form
when using amalgam , cusp capping is called___
cusp tipping (make sure not to have any thin surfaces and to do it uniformly)
the cavo-surface angle for the following restorations
-amalgam :
-gold inlays :
amalgam : 90 degrees
gold inlay: 135
gingival cavosurface line angle should be :
slightly beveled , to removed any undermined enamel that is destined to crack
primary retention form is resistance against tipping or lifting
factors affecting retention form :(not obvious ones)
-thickness of restoration
-amount of remaining tooth structure
-total surface of restoration exposed to masticatory forces
main retention mechanism of amalgam restorations ?
converging cavity walls , mechanical undercut retention
frictional wall retention (parallel walls having friction with the restoration and hence holding it in place) is most utilized by which type of restoration?
cast gold restoration
gripping action of dentin is heavily relied on when using which type of restoration ?
gold foil
which tooth is the occlusal dovetail most useful in ?
maxillary premolar
how does the box form of a cavity affect resistance and retention ?
resistance:
neutralizes stresses at the cavity seat
retention :
-frictional retention due to parallel walls
how does adding bulk to the restoration affect retention and resistance ?
retention:augments retention by more friction
resistance: prevent fracture of brittle restoration
cast gold metal restoration CSA:
diverging (feature of convenience form)
stage II of cavity preparation includes:
-removal of any remaining infected dentin or old restoration ( so prepare the normal form of the cavity first and THEN proceed )
- this is done because it allows for optimal visibilitywhen removing caries and permits immediate placement of a base and the restoration
why must an amalgam cavity extend deeper to DEJ ?
-to avoid cutting exactly at the DEJ which is a very sensitive area
-to be able to detect the spread of caries in dentine laterally
-to make sure that all remaining enamel is supported by sound dentine and there is no undermined enamel \
-provides bulk for restoration
-retention
round bur is used on lowspeed hand piece for caries removal (or excavator )
motion of excavator when removing caries:
directed laterally parallel to pulp horns from periphery to center with scooping motion
what is the minimum ammount of remaining dentin thickness that makes it not necessary to do pulp capping ?
2mm
what is the difference between varnishes , liners , and bases ?
both varnishes and liners provide a barrier to protect remaining dentin and pulp and provide galvanic insulation
the difference is varnish can be applies on all of the cavity walls while liners (is used in shallow cavities with dentin thickness being more than 2mm) are placed on the deepest portion of the ccavity
bases are used in cavities with little remaining dentin (0.5-2 mm). they provide thermal , galvanic , chemical , and mechanical protection to the pulp
which pulp protecting material provides thermal insulation?
bases
secondary retention and resistance forms :
mechanical features: retention lock , grooves , coves , skirts , pins , slots , bevelled enamel margins
treatment of prepared walls: etching priming m bonding
what is the difference between proximal axial grooves and proximal retention locks ?
both are modifications done to increase retention but proximal axial grooves enters enamel (extends up to CSA) and has no undercut while proximal retention locks are only in dentine and have an undercut
skirts improve both resistance and retention -they are done mostly in enamel
-non vital pulp will not provide gripping action of dentin due to dehydration
-enamel must be bevelled in case of using high strength restorative material
NOYs rule for keeping strong enamel wall at margins :
-enamel must rest on sound dentine
-enamel rods which formes CSA must have their inner ends resting on sound enamel
-outer ends of enamel rods must be covered by restorative material
best fluid for cavity toileting ?
water spray
Correct anatomic form renders teeth more___
self cleansing
what is meant by static forces , dynamic forces and cyclic forces
static:centric
dynamic:eccentric
cyclic:functional
The routine cavity depth
●1. Pulpal depth: ☞ 0.2 – 0.5 mm beyond the DEJ. ●2. Axial depth: ☞ 0.5 – 0.8 mm beyond DEJ
how do you differentiate between affected and infected dentin?
affected dentine is affected by acute caries , has a yellow color and is soft and is removed in flakes. infected dentin is infected by chronic caries , has a dark brown color and is harder(removed in debris)
secondary resistance and retention forms are required in ______
complex and compound tooth preparations
requirements of enamel wall:
●The enamel walls and margins should be finished → smooth, free from short, loose, friable or undermined enamel rods.
●The enamel wall must take the same inclination of enamel rods.
●The enamel wall must have an inclination suitable with the physical properties of the restorative material. ●The enamel wall must be beveled in case of using high strength restorative material.
short bevel:
long bevel:
full bevel:
counter bevel:
short bevel: part of enamel
long bevel: all of enamel , up to DEJ
full bevel: enamel and dentin
counter : against enamel rods of cusp
what are coves and what are slots ?
coves: small conical depressions prepared unhealthy dentin to provide additional retention
slots : retention grooves whose length is in a horizontal plane . prepared in gingival floor axial to DEJ
WHAT ARE skirts ?
thin extensions of facial and lingual proximal margins of cast metal only . they increase the total surface are of the preparation. they are usually prepared entirely in enamel. they increase both resistance and retention forms
pin dimensions
0.5mm in dentin with 1-2mm depth and 0.5-1 mm width
