lecture 3- principles of tooth preparations Flashcards
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
crown preparation is an _____surgical procedure
irreversible
in consideration of preservation of tooth structure for crowns are biologic factors such as:
dental pulp
gingival tissues
adjacent teeth
in not careful in preservation of tooth structure when prepping crown, injury can occur such as
-mechanical damage
-temperature damage
-chemical damage
-bacterial damage
any time a bur touches a tooth, the pulp is:
crown prep traumatizes the:
stimulated to respond
(pulpal integrity)
pulp
some pulps respond with healthy healing, some by dying (pulpal necrosis), some hypersensitive with lingering pulpal inflammation.
why?
decreased thickness of dentin remaining between prepared walls and pulp
-heat transferred to tooth during prep
level at minimum 1mm of dentin surrounding pulp
vital core
why prepare teeth for crowns if risks?
esthetics, prevention of catastrophic fracturing, protection of root canal teeth, make tooth stronger
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:
max thickness of residucal tooth structure surrounding the pulp
and
anti-rotational features, retentive and resistive features in the tooth preparation
as we create our crown prep, we need to form the tooth utilizing geometric form criteria we call
retention and resistance
prevention of removal along the path of insertion
(usually fails in patients chewing sticky foods)
retention
retention is created by:
-two opposing vertical surfaces with converging surfaces held to within certain parameters
-also with using cements which offer frictional resistance to dislodgement. this is a shear stress aiding in retention
prevention of restoration being dislodged by apical or oblique forces
-typically through occlusal forces
resistance
aimed to oppose lateral forces which are not along path of insertion
resistance
greatest determining factor in a crown’s dislodgement
resistance
how is resistance built into prep
by forming walls to block anticipated movement due to :
1. leverage
2. rotation (either vertically or horizontally)
what factors can affect the resistance from of a tooth preparation?
- TOC- total occlusal convergence
- prep wall length
- tooth width after preparation
- geometric forms [these limit the paths of insertion-grooves, boxes, vertical planes, pins; morphological reduction]
- surface area of the tooth prep
defined as the angle of convergence of taper between two opposing walls of a tooth prep
[resistance form]
Total Occlusal Convergence
the angle of convergence of taper of one wall of a prep in relationship to the long axis of the preparation
inclination of TOC
ideal TOC
6 degrees
-two opposing surfaces, each with 3 degrees taper, add together for an overall toc of 6 degrees
average toc ends up 10-20 degrees
what is the crown preparation feature most under operator control
TOC- total occlusal convergence
as occlusal convergence increases, retention:
decreases
max crown retention is around ____degrees of TOC
about 1/5 the retention when toc is ____ degrees
5 degrees
20 degrees
which teeth have greater TOC
molar preps have greater TOC than anterior and premolars
mandibular preps have greater TOC than maxillary
(due to visibility)
what teeth require more angulation and what are generally shorter?
posterior preps require more angled TOC to facilitate crown seating
posterior preps generally shorter with higher occlusal forces
the greater the height of the tooth prep wall, the ____the retention of the restoration
greater
increased wall height=
increased area of cementation =
increased retention
decreased wall length (height) creates a _______in resistance
disproportionate decrease
the wall height should be greater than the
tipping arc of displacement
shorter the prep, the (more/less) the taper MUST be
taller the prep, the (more/less) taper MAY be
shorter= less
taller= more
why would a narrow tooth with smaller diameter prep have greater resistance than wider, larger diameter tooth when both have same wall height
because small tooth has shorter rotiational 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
0.4
(implies TOC is less than 20 degrees)
wall height ideals when prep is within 10-20 degrees
molars 4mm
incisors and premolars 3mm
if the tooth prep does not have the resistance form you desire, 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 (these must be parallel to path of insertion)
- increase wall height:
-lower margin
-add wall heigh with core build up (possibly pins)
-crown lengthening surgery
-orthodontic extrusion
secondary retentive features:
-most common to use grooves for secondary retention
-increase surface area
-establish parallelism on opposing intra-coronal tooth surfaces
-limits the path of draw to one
-shorten the Arc of rotation, therefore, increasing resistance
retentive features should ideally be cut no deeper than _____mm axially and ___mm occlusally
1.5mm axially
2mm occlusally
grooves or box forms must have a definite wall _____ to direction of displacing force
perpendicular
retention is (increased/decreased) with surface area
increased
increasing surface area helps with retention, but its not as vitally important as
TOC parameters and height/width ratio
what makes good margin?
-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
if margins are not closely adapted to finish line of 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 to restorative material
size of your finish line determines:
bulk of material at margin
and
fit of the restoration
what kind of finish lines (margins) do you want for crown
chamfer or shoudler
(bevel for metal)
(feather-edge or knife-edge for gold)
what morphological reduction will create structural durability
-adequate occlusal / incisal reduction and clearance without excessive removal (do not want flat)
-uniform thickness of restoration material
-parallels the major planes of the tooth
-no pulpal involvement
preparation features that contribute to structural durability
-occlusal / incisal reduction
-functional cusp bevel
-axial reduction
creates space for an adequate bulk of material within normal contours of the tooth
axial reduction
if not enough reduction is done axially, then will have what type of walls? which would be a problem, why?
thin walls which are difficult to finish and possible loss of strength
if too much reduction done axially, then will have what type of walls? and this is a problem, why?
thick over contoured walls
-plaque traps and periodontal inflammation
what happens if our preparation 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 health by:
-placement of margin for cleanse-ability
-marginal smoothness and crown adaptation
-axial contours and emergence profile
margin guidlines:
- place where dentist can finish and evaluate
- place where pts can clean
- place on enamel when possible
- place supra-gingival when possible (0.5mm supragingival ideally)
- make margins as smooth as possible
