Principles of Tooth Preparation Flashcards
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
a crown preparation is an ______ surgical procedure
irreversible
what biologic factors should we consider
- dental pulp
- gingival tissues
- adjacent teeth
what injuries can occur
- mechanical damage
- temperature damage
- chemical damage
- bacterial damage
any time a bur touches a tooth, the pulp is _____
stimulated to respond
a crown prep ____ the pulp
traumatizes
how do pulps respond
- most respond with a healthy healing response
- some respond by dying - necrosis
- some remain hypersensitive with lingering pulpal inflammation
why is pulp responded in every crown prep
-decreased thickness of dentin remaining between prepared walls and pulp
- heat transferred to tooth during preparation
what is the vital core
a minimum of 1mm dentin surrounding pulp
what are reasons crowns are placed
-esthetics
- prevention of catastrophic fracturing
- protection of root canal teeth
- make tooth stronger
what must be balanced
- unnecessary destruction of sound tooth structure
- preservation of the structural integrity of the remaining tooth strucure
VS - mechanical requirements the patient needs
- esthetic requirements the patient desires
what is morphological reduction
- preservaton of the basic primary tooth anatomy within the preparation (axial wall and occlusal anatomy)
- leaves the maximum thickness of residual tooth structure surrounding the pulp
- leaves anti- rotational features retentive and resistive features in the tooth preps
what is retention
prevention of removal along the path of insertion
what is retention created by
two opposing vertical surfaces with converging surfaces held to within certain parameters
what does cement do for retention
offer frictional resistance to dislodgement
- a shear stress aiding in retention
what mechanical factors provide retention
- dental cements hold through mechanical interlocking of projections of cement into irregularities of the surfaces being joined
what is resistance
- prevention of restoration being dislodged by apical or oblique forces
- through occlusal forces and lateral forces not along the path of insertion
what is the greatest determining factor in a crowns dislodgement
resistance
resistance is built into a preparation by forming walls to block anticipated movement due to:
- leverage
- rotation (vertically or horizontally)
what factors can affect the resistance form of a tooth prep
- total occlusal convergence (TOC)
- preparation wall length
- tooth width after preparation
- geometric forms
- surface area of the tooth prep
how do geometric forms affect resistance
- limit the paths of insertion
- grooves, boxes, vertical planes, pins
- morphological reduction
what is total occlusal convergence
- the angle of convergence of taper between two opposing walls of a tooth prep
what is inclincation
the angle of convergence of taper of one wall of a preparation in relationship to the long axis of the preparation
what is the ideal TOC
6 degrees
what convergence will a tapered diamond make if the shank of the bur is held parallel to the intended path of insertion
2 or 3 degrees
two opposing surfaces each with 3 degrees taper, add together for an overall TOC of ____
6 degrees
what is the crown prep feature most under operator control
TOC
as occlusal convergence increases, retention _____
degreases
what is the max crown retention
around 5 degrees
approximately _____ retention when TOC is 20 degrees
1/5
what is the average TOC of prepared teeth
10-20 degrees
molar tooth preps have ____ TOC than premolars or anterior teeth
greater
mandibular tooth preps have ____ TOC than maxillary tooth preps
TOC
why do mandibular and molar preps have larger TOC
challenges in access and visibility
posterior preps are generally _____ with _____ occlusal forces
shorter; higher
posterior preps require _______ TOC than anterior teeth to facilitate ______
more angled TOC; crown seating
the greater the height of the tooth preparation wall, the _____ the retention of the restoration
greater
why does increased height increase retention
increases the area of cementation
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 prep, the____ the taper ____ be
less’;MUST
the taller the prep, the ____ the taper _____ be
greater; MAY
a narrow tooth with a smaller diameter prep can have ________resistance than a wider, larger diameter tooth when they both have the same wall height
greater
why does a narrow tooth with a smaller diameter prep can have greater resistance than a wider, larger diameter tooth when they both have the same wall height
small tooth has a shorter rotational radius for the arc of rotation (tipping arc)
ideal and adequate resistance and retention , the height/width ration should be greater than ____ for all teeth
0.4
what are the wall height ideals when prep is within TOC for molars? incisors and premolars?
-molars: 4 mm
- incisors and premolars: 3 mm
how is resistance form assesses
- measure TOC between two opposing walls
- measure wall height parallel with long axis
- measure tooth width ratio
what modifications can you make to the prep to increase the resistance
- decrease the TOC with axial re-shaping of prep walls
- addition of grooves/box forms
- increase wall height
how can you increase wall height
- lower margin
- add wall height with core build up
- crown lengthening surgery
- orthodontic extrustion
what can you do if your TOC is not ideal or your wall height is less than ideal
- add geometric forms such as a box, groove, pin hole
grooves or box forms must have a _______
definite wall perpendicular to the direction of the displacing force
describe the surface area of tooth preps
- retention is increased with increased surface area
- increasing surface area helps with retention but it is not as important as TOC parameters and the height/width ratio
how do you define high marginal integrity
- 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
what happens if the margins are not as closely adapated to the finish line of the prep
- microleakage
- recurrent caries
- possible de- cementation of restoration
tooth prep finish line:
- needs to be conservative of tooth structure
- readily identifiable on tooth, impression and die
- provides sufficient bulk of restorative material
the size of your finish line determines:
- the bulk of material at the margin
- the fit of the restoration
morphological reduction:
adequate occlusal/incisal reduction and clearance without excessive removal
- uniform thickness of restoration material
- parallels the major planes of the tooth
- no pulpal involvement
what are prep features that contribute to structural durability
- occlusal/ incisal reduction
- functional cusp bevel
- axial reduction
what does axial reduction do
creates space for an adequate bulk of material within normal contours of the tooth
what happens when there is not enough axial reduction
thin restoration walls- difficult to finish and possible loss of strength
what happens when there is too much axial reduction
thick over contoured walls- plaque trap and periodontal inflammation
what happens if our prep does not leave room for structural durability
- fracture of restoration
- grind through porcelain to adjust occlusion
- crown worn through due to thin material
our crown prep and final restorations should promote periodontal healthy by:
- placement of margin for cleanse ability
- marginal smoothness and crown adaptation
- axial contours and emergence profile
what are the margin guidlines
- place margin where dentist can finish and evaluate
- place margin where patient can clean
- place margin where margin can be recorded during impression taking
- place margin on enamel when possible
- place margin supra gingival when possible
- make margin as smooth as possible
what is important to keep in mind in preservation of tooth structure
- pulpal integrity
- morphological reduction
what is important to keep in mind in resistance and retention
-TOC
- wall length
- tooth width
- geometric forms
- surface area
what is important to keep in mind in marginal integrity
finish line
- margin
what are the types of finish lines
- feather edge
- knife edge
- chamfer
- bevel
- shoulder
- beveled shoulder
