Textbook Ch. 09: Principles of Tooth Preps Flashcards
What are the 5 Principles of Sound Preparation Design?
Preservation of Tooth Structure Retention and Resistance Structural durability Marginal integrity Preservation of the periodontium
How much tooth must be removed for an MOD onlay
1.5 mm
Why is retention and resistance so important?
Because there are no biologically compatible adhesives strong enough to withstand the forces of mastication by chemical adhesion alone
What does Retention prevent
Displacement of the crown via the path of insertion
What does Resistance prevent
Dislodgement of the crown by apical or oblique forces
And any movement under occlusal forces
What is the essential element of retention?
Two opposing vertical surfaces
Do the vertical surfaces have to be internal walls?
No, they can be external
What happens to retention as the taper angle increases?
It decreases
Why can’t the taper be perfectly perpendicular and parallel?
It is an unrealistic expectation
In the occlusal direction, interior vertical walls must __________ and exterior walls must __________ . (Diverge/Converge)
Interior = Diverge Exterior = Converge
What is considered the optimal range for taper angles
2.5-6.5 degrees
When taper angles raise above this threshold, stress concentration increases sharply
20 degrees
What is the minimum taper angle estimated to be required in order to prevent any undercuts?
12 degrees
How can you improve retention in regards to the number of paths by which a crown can be removed?
Retention improves by limiting the number of paths, ideally there should only be one way on and off
How does occlusogingival length relate to retention?
Longer preps have more surface area by which to provide retention
What must be true about the length of the crown in order to provide solid retention?
It must be great enough to interfere with the arc of the casting pivoting about a point on the margin on the opposite side of the preparation.
What are three internal retentive features
Box
Groove
Pinhole
This is an imaginary line along which the restoration will be placed onto or removed from the prep
Path of Insertion
What is the best distance to survey a tooth for POI?
from 12 inches away with one eye closed
Why is POI surveying even more crucial for FPD?
Because the POIs of all teeth involved must be parallel
What are the two directions in which the POI must be considered
Faciolingually
Mesiodistally
Faciolingually, the POI is parallel to the long axis, what about mesiodistally?
It must be parallel to the contacts of the adjacent teeth
What is the required occlusal clearance for gold alloys?
1.5 mm
What is the occlusal clearance for:
metal-ceramic crowns
2.0 mm
