lecture 3- principles of tooth preparations

Question 1

7 principles of tooth design and restoration design

Answer 1

1. preservation of tooth structure
2. retention form
3. resistance form
4. added preparation features
5. marginal integrity
6. structural durability of the restoration
7. preservation of the periodontium

Question 2

crown preparation is an _____surgical procedure

Answer 2

irreversible

Question 3

in consideration of preservation of tooth structure for crowns are biologic factors such as:

Answer 3

dental pulp
gingival tissues
adjacent teeth

Question 4

in not careful in preservation of tooth structure when prepping crown, injury can occur such as

Answer 4

-mechanical damage
-temperature damage
-chemical damage
-bacterial damage

Question 5

any time a bur touches a tooth, the pulp is:
crown prep traumatizes the:

Answer 5

stimulated to respond
(pulpal integrity)

pulp

Question 6

some pulps respond with healthy healing, some by dying (pulpal necrosis), some hypersensitive with lingering pulpal inflammation.

why?

Answer 6

decreased thickness of dentin remaining between prepared walls and pulp
-heat transferred to tooth during prep

Question 7

level at minimum 1mm of dentin surrounding pulp

Answer 7

vital core

Question 8

why prepare teeth for crowns if risks?

Answer 8

esthetics, prevention of catastrophic fracturing, protection of root canal teeth, make tooth stronger

Question 9

when preparing a tooth for a crown in a conservative manner, the operator allows for a

Answer 9

morphological reduction

Question 10

preservation of the basic primary tooth anatomy within the preparation (axial wall and occlusal anatomy)

Answer 10

morphological reduction

Question 11

morphological reduction leaves:

Answer 11

max thickness of residucal tooth structure surrounding the pulp
and
anti-rotational features, retentive and resistive features in the tooth preparation

Question 12

as we create our crown prep, we need to form the tooth utilizing geometric form criteria we call

Answer 12

retention and resistance

Question 13

prevention of removal along the path of insertion
(usually fails in patients chewing sticky foods)

Answer 13

retention

Question 14

retention is created by:

Answer 14

-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

Question 15

prevention of restoration being dislodged by apical or oblique forces
-typically through occlusal forces

Answer 15

resistance

Question 16

aimed to oppose lateral forces which are not along path of insertion

Answer 16

resistance

Question 17

greatest determining factor in a crown’s dislodgement

Answer 17

resistance

Question 18

how is resistance built into prep

Answer 18

by forming walls to block anticipated movement due to :
1. leverage
2. rotation (either vertically or horizontally)

Question 19

what factors can affect the resistance from of a tooth preparation?

Answer 19

1. TOC- total occlusal convergence
2. prep wall length
3. tooth width after preparation
4. geometric forms [these limit the paths of insertion-grooves, boxes, vertical planes, pins; morphological reduction]
5. surface area of the tooth prep

Question 20

defined as the angle of convergence of taper between two opposing walls of a tooth prep

[resistance form]

Answer 20

Total Occlusal Convergence

Question 21

the angle of convergence of taper of one wall of a prep in relationship to the long axis of the preparation

Answer 21

inclination of TOC

Question 22

ideal TOC

Answer 22

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

Question 23

what is the crown preparation feature most under operator control

Answer 23

TOC- total occlusal convergence

Question 24

as occlusal convergence increases, retention:

Answer 24

decreases

Question 25

max crown retention is around ____degrees of TOC about 1/5 the retention when toc is ____ degrees

Answer 25

5 degrees 20 degrees

Question 26

which teeth have greater TOC

Answer 26

molar preps have greater TOC than anterior and premolars mandibular preps have greater TOC than maxillary (due to visibility)

Question 27

what teeth require more angulation and what are generally shorter?

Answer 27

posterior preps require more angled TOC to facilitate crown seating posterior preps generally shorter with higher occlusal forces

Question 28

the greater the height of the tooth prep wall, the ____the retention of the restoration

Answer 28

greater

Question 29

increased wall height=

Answer 29

increased area of cementation = increased retention

Question 30

decreased wall length (height) creates a _______in resistance

Answer 30

disproportionate decrease

Question 31

the wall height should be greater than the

Answer 31

tipping arc of displacement

Question 32

shorter the prep, the (more/less) the taper MUST be taller the prep, the (more/less) taper MAY be

Answer 32

shorter= less taller= more

Question 33

why would a narrow tooth with smaller diameter prep have greater resistance than wider, larger diameter tooth when both have same wall height

Answer 33

because small tooth has shorter rotiational radius for the Arc of Rotation (tipping arc)

Question 34

for ideal and adequate resistance and retention, the height/width ratio should be greater than ___ for all teeth

Answer 34

0.4 (implies TOC is less than 20 degrees)

Question 35

wall height ideals when prep is within 10-20 degrees

Answer 35

molars 4mm incisors and premolars 3mm

Question 36

if the tooth prep does not have the resistance form you desire, what modifications can you make to the prep to increase the resistance?

Answer 36

1. decrease the TOC with axial re-shaping of prep walls 2. addition of grooves / box forms (these must be parallel to path of insertion) 3. increase wall height: -lower margin -add wall heigh with core build up (possibly pins) -crown lengthening surgery -orthodontic extrusion

Question 37

secondary retentive features:

Answer 37

-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

Question 38

retentive features should ideally be cut no deeper than _____mm axially and ___mm occlusally

Answer 38

1.5mm axially 2mm occlusally

Question 39

grooves or box forms must have a definite wall _____ to direction of displacing force

Answer 39

perpendicular

Question 40

retention is (increased/decreased) with surface area

Answer 40

increased

Question 41

increasing surface area helps with retention, but its not as vitally important as

Answer 41

TOC parameters and height/width ratio

Question 42

what makes good margin?

Answer 42

-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

Question 43

if margins are not closely adapted to finish line of prep:

Answer 43

-microleakage -recurrent caries -possible de-cementation of restoration

Question 44

tooth prep finish line:

Answer 44

-needs to be conservative of tooth structure -readily identifiable on tooth, impression and die -provides sufficient bulk to restorative material

Question 45

size of your finish line determines:

Answer 45

bulk of material at margin and fit of the restoration

Question 46

what kind of finish lines (margins) do you want for crown

Answer 46

chamfer or shoudler (bevel for metal) (feather-edge or knife-edge for gold)

Question 47

what morphological reduction will create structural durability

Answer 47

-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

Question 48

preparation features that contribute to structural durability

Answer 48

-occlusal / incisal reduction -functional cusp bevel -axial reduction

Question 49

creates space for an adequate bulk of material within normal contours of the tooth

Answer 49

axial reduction

Question 50

if not enough reduction is done axially, then will have what type of walls? which would be a problem, why?

Answer 50

thin walls which are difficult to finish and possible loss of strength

Question 51

if too much reduction done axially, then will have what type of walls? and this is a problem, why?

Answer 51

thick over contoured walls -plaque traps and periodontal inflammation

Question 52

what happens if our preparation does not leave room for structural durability?

Answer 52

-fracture of restoration -grind through porcelain to adjust occlusion -crown worn through due to thin material

Question 53

our crown prep and final restorations should promote periodontal health by:

Answer 53

-placement of margin for cleanse-ability -marginal smoothness and crown adaptation -axial contours and emergence profile

Question 54

margin guidlines:

Answer 54

1. place where dentist can finish and evaluate 2. place where pts can clean 3. place on enamel when possible 4. place supra-gingival when possible (0.5mm supragingival ideally) 5. make margins as smooth as possible