Principles of Tooth Preparations Flashcards
A crown preparation is an ___ surgical procedure
irreversible
What are the 7 principles of tooth design and restoration design?
- preservation of tooth structure
- retention form
- resistance form
- added preparation features
- marginal integrity
- structural durability of the restoration
- preservation of the periodontium
In consideration of preservation of tooth structure are biological factors such as:
(what are other things you must consider in addition to the structure you are working on)
- dental pulp
- gingival tissues
- adjacent teeth
When considering preservation of tooth structure, if not careful, injury can occur such as:
- mechanical damage
- temperature damage
- chemical damage
- bacterial damage
Pulpal integrity: Any time a bur touches a tooth, the pulp is:
stimulated to respond
Pulpal integrity: A crown preparation _____ the pulp
“traumatizes”
Pulpal integrity: most pulps respond with:
a healthy healing response
Pulpal integrity: some pulps respond by:
dying (pulpal necrosis)
Pulpal integrity: some pulps remain ___ with lingering pulpal inflammation
hypersensitive
Why can the pulp become/remain hypersensitive or necrotic?
- decreased thickness of dentin remaining between prepared walls & pulp
- heat transferred to tooth during prep
When considering pulpal integrity, what is “vital core”?
leave at minimum 1mm of dentin surrounding pulp
How much MINIMUM of dentin should be surrounding the pulp?
1mm
If there are risks like killing the pulp, why do we prepare teeth for crowns?
Crowns are placed for many reasons such as:
- Esthetics
- Prevention of catastrophic fracturing
- Protection of Root Canal Teeth
- Make tooth stronger
What must be in balance when considering preservation of tooth structure?
(1&2 vs. 1&2)
- unnecessary destruction of sound tooth structure
- preservation of structural integrity of the remaining tooth structure
VERSUS
- mechanical requirements the patient needs
- esthetic requirements the patient desires
When considering preservation of tooth structure:
When preparing a tooth for a crown in a conservative manner, the operator allows for a:
morphological reduction
Preservation of the basic primary tooth anatomy within the preparation (axial wall and occlusal anatomy):
morphological reduction
Morphological reduction leaves the ____ of residual tooth structure surrounding the pulp
maximal thickness
What type of features does morphological reduction leave in the tooth preparations?
- anti-rotational features
- retentive features
- resistive features
As we create our crown preparation, we need to form the tooth utilizing geometric form criteria we call:
- retention
- resistance
A seating groove in a crown preparation allows for:
retention & resistance
Axial reduction in a crown preparation allows for:
retention & resistance
Prevention of removal along the path of insertion:
retention
Retention usually occurs:
in patients chewing sticky foods
Created by two opposing vertical surfaces with converging surfaces held to within certain parameters:
retention
Retention is created by two opposing ___ surfaces with ___ surfaces held to within certain parameters
vertical surfaces; converging surfaces
____ is also created using cements with offer frictional resistance to dislodgment.
retention
Retention is also created using ___ which offer ____ to dislodgment.
cements; frictional resistance
Retention is also created using cements which offer frictional resistance to dislodgment. this is a ____ aiding in retention.
shear stress
____ is also created through mechanical factors such as dental cements.
retention
Retention is also created through ____ factors such as dental cements
mechanical factors
Retention: Dental cements hold through _____ of projections of cement into irregularities of the surfaces being joined.
mechanical interlocking
What are the 2 ways to check resistance?
Wall height & TOC
What factors contribute to retention? (3)
- two opposing vertical surfaces with converging surfaces held to within certain parameters
- cement- offers frictional resistance
- cement- offers mechanical interlocking
Prevention of restoration being dislodged by apical or oblique forces:
resistance
Resistance is the prevention of the restoration being dislodged by ___ or ___ forces
apical or oblique
The apical or oblique forces that prevent the the restoration from being dislodged to ensure resistance are typically through ___ forces, but also to ___ forces which are not along the path of insertion
occlusal; lateral
The greatest determining factor in a crown’s dislodgment:
resistance
____ is built into a preparation by forming walls to block anticipated movement due to leverage & rotation (either vertically or horizontally)
resistance
How is resistance built into a preparation?
by forming walls to block anticipated movement due to leverage & rotation
Resistance is build into a preparation by forming walls to block anticipated movement due to ___ & ____
leverage & rotation
What factors can affect the resistance form of a tooth preparation?
- total occlusal convergence (TOC)
- preparation wall length
- tooth width after preparation
- geometric forms
- surface area of the tooth preparation
TOC=
total occlusal convergence
The geometric forms that can affect the resistance form of a tooth preparation limit:
the paths of insertion, grooves, boxes, vertical planes & pins & morphologic reduction
When considering factors that affect the resistance form of a tooth preparation, what category does morphological reduction fall into?
geometric forms
What is NOT a factor that affects the resistance form of a tooth preparation?
A) geometric form
B) total occlusal convergence
C) Surface area of the adjacent teeth
D) Tooth width after preparation
C- surface of adjacent teeth
should be surface area of the tooth prep
Defined as the angle of convergence of taper between two opposing walls of a tooth preparation:
total occlusal convergence
Total occlusal convergence is the:
angle of convergence of taper between two opposing walls of a tooth preparation
Total occlusal convergence is the angle of convergence of taper between _____ of a tooth preparation
two opposing walls
Defined as the angle of convergence of taper of one wall of a preparation in relationship to the long axis of the preparation:
inclination
Inclination is:
the angle of convergence of taper of one wall of a preparation in relationship to the long axis of the preparation
Inclination is the angle of convergence of taper of ____ of a preparation in relationship to the long axis of the preparation
one wall
Ideal TOC:
6 degrees
What can be used to accomplish a 2 to 3 degree converging angle to a tooth wall if the shank is held parallel to the intended path of insertion?
This overall contributes to:
tapered diamond bur
TOC
A tapered diamond bur can be used to accomplish a ____ to a tooth wall if the shank of the bur is held parallel to the intended path of insertion.
This overall contributes to:
2 to 3 degrees
TOC
Two opposing surfaces, each with 3 degrees of taper, add together for:
an overall TOC of 6 degrees
The angle formed between two opposing walls:
Total Occlusal Convergence (TOC)
What is required to seat a crown?
some taper
What feature of the crown preparation is most under operator control?
TOC
If on an exam, we ask you where to put a margin the answer is always:
supragingival (however in clinic this is not always the correct answer)
As occlusal convergence increases, ____ decreases
retention
What happens to retention as occlusal convergence increases?
retention decreases
Maximum crown retention is around ___ degrees of TOC
5 degrees
What happens when TOC is 20 degrees?
retention decreases to ~ 1/5
Describe the relationship between convergence & retention:
convergence is inversely proportional to retention
Based on evidence from clinical studies:
The average TOC of prepared teeth ends up in the range of:
10-20 degrees
Based on evidence from clinical studies:
Molar tooth preps tend to have ___ TOC than premolars or anterior teeth
greater
Based on evidence from clinical studies:
Mandibular tooth preps tend to have ___ TOC than maxillary tooth preps
greater
Based on evidence from clinical studies:
What causes the TOC to be greater than ideal?
likely due to challenges in access and visibility
____ preps are generally shorter with higher occlusal forces
posterior preps
____ preps require more angled TOC than ___ teeth to facilitate crown seating.
posterior; anterior
Summary on TOC:
- Ideal TOC angle = ____ degrees
- Acceptable TOC angle= ____ degrees
- If a prep TOC angle is more than ____ degrees, the preparation would benefit from further modifications to the prep design.
- 6-10 degrees
- 10-20 degrees
- 20 degrees
Wall length =
height
The greater the height of the tooth preparation wall, the greater the ____ of the restoration
retention
Increased height= increases the _____ = increased ____
area of cementation; retention
How do you measure the tooth wall height?
measured from incisal/ occlusal surface to the margin
What is being measured in these images?
wall height
Decreased wall length (height) creates a:
disproportionate decrease in resistance
For every 1mm increase in wall height, the retention and resistance form increases up to:
10%
The wall height should be greater than the:
tipping arc of displacement
The shorter the preparation, the ____ the taper MUST be
The taller the preparation, the ____ the taper MAY be
less; greater
A narrow tooth with a smaller diameter preparation can have ____ resistance than a wider, larger diameter tooth when they both have the same wall height.
greater
A narrow tooth with a smaller diameter preparation can have greater resistance than a wider, larger diameter tooth when they both have the same wall height.
This is because the small tooth has a:
shorter rotational radius for the Arc of Rotation (tipping arc)
For ideal and adequate resistance and retention, the height/width ratio should be greater than ____ for all teeth.
(This also implies the TOC is less than 20 degrees)
0.4
If a molar is on average 10mm facial-lingually, and the minimal ratio is 0.4, then ____ of wall height is ideal and adequate
4mm (10x 0.4)
Wall height ideals when preparation is within TOC of ___ degrees
10-20 degrees
Ideal wall height:
Molars:
Incisors & premolars:
molars = 4mm
incisors & premolars= 3mm
How is resistance form assessed? (3)
- Measure TOC between opposing walls
- Measure we all height parallel with long axis (3mm for incisors and premolars & 4 mm for molars)
- Measure tooth width ratio (Occlusal: facial/lingual should be atleast 0.4)
If you measure the tooth width ratio, occlusal: Facial/Lingual should be at least:
0.4
If the tooth preparation does not have the resistance form you desire, what modifications can you make to the prep to increase the resistance? (3)
- Decrease the TOC with axial re-shaping of prep walls
- Addition of grooves/box forms (these must be parallel to the path of insertion)
- Increase wall height
- Decrease the TOC with axial re-shaping of prep walls
- Addition of grooves/box forms
- Increase wall height
What do all of these contribute to?
Modifications to increase resistance
How might you decrease the TOC in order to increase resistance?
By axial re-shaping of prep walls
When increasing resistance by the addition of grooves/box forms, these must be:
parallel to the path of insertion
If you want to increase resistance by increasing wall height how might you accomplish this? (4)
- Lower margin
- Add wall height with core build up
- Crown lengthening surgery
- Orthodontic extrusion
- Lower margin
- Add wall height with core build up
- Crown lengthening surgery
- Orthodontic extrusion
These are all mechanisms to:
increase wall height (which would increase resistance)
Resistance can be improved by adding grooves which function to:
limit the path of withdrawal to only one
True or False: There are times when leaving a crown preparation less than ideal will actually increase the resistance and retention form:
true
There are times when leaving a crown preparation ____ will actually increase the resistance and retention form
less than ideal
What can you do if your TOC is not ideal (more than 20 degrees) ore you wall height is less than ideal?
geometric box forms can be added to your preparation
Geometric box forms include: (3)
- box
- groove
- pin hole
Box, groove, and pin holes (geometric box forms) are examples of ____ retention
auxillary
Most common to use ___ for secondary retention
grooves
Using grooves as a secondary retentive feature allows for: (2)
- increases surface area
- limit the path of draw
Retentive features should ideally be cut no deeper than ____ axially and ____ occlusally
1.5mm axially; 2mm occlusally
These are examples of ____.
These function as _____.
grooves; secondary retention feature
Grooves or box forms must have a definite wall perpendicular to the:
direction of the displacing force
____ is increased with increasing surface area
retention
Increasing surface area helps with retention but its not as vitally important as ____ and the ____
TOC parameters; height/width ratio
What makes a good margin? How do you define high marginal integrity? (3)
- fits as closely as possible to minimize cement film width (25 microns)
- Sufficient strength to withstand forces of mastication
- Located where dentist can finish and inspect and patient can clean
Sufficient strength to withstand forces of mastication describes:
high marginal integrity
What can be seen in the following image?
a good margin
Crown margin should fit as closely as possible to:
minimize cement width (25 microns)
Margins should be closely adapted to the ___ of the preparation
finish line
If margins are NOT closely adapted to the finish line of the preparation, what might occur? (3)
- microleakage
- recurrent caries
- possible de-cementation of restoration
What is wrong with the following image?
margin is not closely adapted to the finish line of prep
What might the crowns in this image lead to?
- microleakage
- recurrent caries
- possible de-cementation of restoration
What are the 3 requirements of the tooth preparation finish line?
- needs to be conservative of tooth structure
- readily identifiable on tooth, impression and die
- provides sufficient bulk for restorative material
The size of you finish line determines: (2)
- The bulk of material at the margin
- The fit of the restoration
Label the following types of finish lines (margins) (left to right)
- feather-edge
- knife-edge
- chamfer
- bevel
- shoulder
- beveled shoulder
- adequate occlusal/ incisal reduction and clearance without excessive removal
- uniform thickness of restoration material
- parallels the major planes of the tooth
These are all factors of:
morphological reduction
Label the following images regarding morphological reduction:
A) correct
B) flat (inadequate)
C) excessively flat
The morphological reduction should parallel the ____ of the tooth
major planes of the tooth
When considering morphological reduction, you should have adequate occlusal/incisal reduction and clearance without:
excessive removal
When considering morphological reduction, there should be no ___
pulpal involvement
Preparation features that contribute to structural durability include: (3)
- occlusal/incisal reduction
- functional cusp bevel
- axial reduction
- creates a space for an adequate bulk of material with normal contours of the tooth:
axial reduction
Not enough axial reduction leads to ___ which causes ____.
thin restoration walls; difficulty to finish and possible loss of strength
Too much axial reduction leads to ___ which causes ____.
thick over-contoured walls; plaque traps and periodontal inflammation
Label the axial reduction in the following image:
A) ideal
B) too thin
C) too thick
What happens if our preparation does not leave room for structural durability? (3)
- fracture of restoration
- grind through porcelain to adjust occlusion
- crown worn through due to thin material
Our crown preparations and final restorations should promote periodontal health by: (3)
- placement of margin for cleanse-ability
- marginal smoothness and crown adaptation
- axial contours and emergence profile
Margin guidelines:
- place margin where dentist can ___ & ___
- place margin where patient can ___
- place margin where margin can be recorded during ___
- place margin on ___ when possible
- place margin ___ when possible
- make margin as ____ as possible
- finish & evaluate
- clean
- impression taking
- enamel
- supra-gingival
- smooth
Although a margin guideline is to “place margin supra-gingival when possible”. There are many factors that come into play when deciding where to place your crown preparation margin. For lab, we are asking you to place it ideally ____.
0.5mm supra-gingival
