Principles of Tooth Preparations Flashcards

1
Q

What are the principles of tooth design and restoration design?

A
  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
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2
Q

A crown preparation is an __________ surgical procedure

A

irreversible

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3
Q

What are the biological factors to consider for a crown prep?

A

-Dental Pulp
-Gingival Tissues
-Adjacent teeth

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4
Q

What kind of injuries can occur from a crown prep?

A

-Mechanical damage
-Temperature damage
-Chemical damage
-Bacterial damage

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5
Q

What is important to know about the pulp during a crown prep?

A

-Any time a bur touches a tooth, the pulp is
stimulated to respond.
-A crown preparation “traumatizes” the pulp

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6
Q

What are the responses of the pulp to a crown prep?

A

-Most pulps respond with a healthy healing response
-Some pulps respond by dying (pulpal necrosis)
-Some remain hypersensitive with lingering pulpal inflammation

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7
Q

Why can the pulp be damaged during a crown prep?

A

-Decreased thickness of dentin remaining between prepared walls and pulp
-Heat transferred to tooth during preparation

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8
Q

Leave at minimum ____mm of dentin surrounding pulp/vital core

A

1mm

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9
Q

If there are risks like killing the pulp, why do we prepare teeth for crowns?

A

-Esthetics
-Prevention of catastrophic fracturing
-Protection of Root Canal Teeth
-Make tooth stronger

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10
Q

What must we balance for a crown prep?

A

-Unnecessary destruction of sound tooth structure
-Preservation of the structural integrity of the remaining tooth structure
VERSUS
-Mechanical requirements the patient needs
-Esthetic requirements the patient desires

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11
Q

When preparing a tooth for a crown in a conservative manner, the operator allows for a ___________ Reduction.

A

Morphological

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12
Q

What is morphological reduction?

A

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

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13
Q

What does morphological reduction of a crown prep allow for?

A

-Leaves the maximum thickness of residual tooth structure surrounding the pulp
-Also, leaves anti-rotational features, retentive and resistive features in the tooth preparations

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14
Q

As we create our crown preparation, we need to form the tooth utilizing geometric form criteria we call:

A

Retention
Resistance

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15
Q

What is retention?

A

Prevention of removal along the path of insertion

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16
Q

When does retention of a crown usually fail?

A

patient eating sticky foods

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17
Q

How is retention created?

A

-two opposing vertical surfaces with converging surfaces held to within certain parameters
- cements which offer frictional resistance to dislodgement (shear stress aiding in retention)
- mechanical factors such as Dental cements hold through mechanical interlocking of projections of cement into irregularities

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18
Q

What is resistance?

A

-Prevention of restoration being dislodged by apical or oblique forces

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19
Q

What is the reason for issues with resistance?

A

occlusal forces

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20
Q

_________ is the greatest determining factor in a crown’s dislodgment

A

Resistance

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21
Q

-Resistance is also aimed to oppose lateral forces which are not along the…

A

path of insertion

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22
Q

How is resistance build into a prep?

A

forming walls to block anticipated movement due to:
-Leverage
-Rotation (either vertically or horizontally)

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23
Q

What factors can affect the Resistance form of a tooth preparation?

A

-Total Occlusal Convergence (TOC)
-Preparation wall length
-Tooth Width after preparation
-Geometric forms (Grooves, Boxes, Vertical Planes, Pins, Morphological reduction)
-Surface area of the tooth preparation

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24
Q

What is total occlusal convergence?

A

the angle of convergence of taper between two opposing walls of a tooth preparation

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25
Q

What is inclination?

A

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

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26
Q

What is the ideal total occlusal convergence (TOC)?

A

6 degrees

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27
Q

A tapered diamond bur will place a ______ degree converging angle to a tooth wall if the shank of the bur is held parallel to the intended path of insertion

A

2 or 3

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28
Q

Two opposing surfaces, each with 3 degree taper, addtogether for an overall TOC of ___ degrees

A

6

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29
Q

______ is the crown preparation feature most under operator control

A

TOC (total occlusal convergence)

30
Q

Some _______ is required to seat crown

A

taper

31
Q

Maximum crown retention is around ___ degrees of TOC

A

5

32
Q

Approximately 1/5 the retention when TOC is ____ degrees

A

20

33
Q

Convergence is inversely proportional to ________

A

retention

34
Q

The average TOC of prepared teeth ends up in the range of _____ degrees

A

10-20

35
Q

Molar tooth preps tend to have _________ TOC than premolars or anterior teeth

A

greater

36
Q

Mandibular tooth preps tend to have _______ TOC than maxillary tooth preps

A

greater

37
Q

Posterior preps are generally __________ (height) with _________ occlusal forces

A

shorter
higher

38
Q

Posterior preps require ______ angled TOC than anterior teeth to facilitate crown seating.

A

more

39
Q

Acceptable TOC angle ______ degrees

A

10 – 20

40
Q

The greater the height of the tooth preparation wall, the ________ the retention of the restoration

A

greater

41
Q

Increased height = Increases the area of cementation = Increased __________

A

retention

42
Q

For every 1mm increase in wall height, the retention and resistance form increases up to _____%

A

10%

43
Q

The wall height should be greater than the…

A

tipping arc of displacement

44
Q

The shorter the preparation, the _______ the taper MUST be

A

less

45
Q

The taller the preparation, the ______ the taper MAY be

A

greater

46
Q

What has greater resistance:
- A narrow tooth with a smaller diameter preparation
- A wider, larger diameter tooth

A

A narrow tooth with a smaller diameter preparation

  • This is because the small tooth has a shorter rotational radius for the Arc of Rotation (tipping arc)
47
Q

Ideal and adequate Resistance and Retention, the height/width ratio should be greater than ____ for all teeth

A

0.4

48
Q

If a molar is on average 10mm Facial-Lingually, and the minimal ratio is 0.4, then what should the wall height be?

A

10 X 0.4 = 4mm of wall height is ideal and adequate

49
Q

What is the wall height ideals when preparation is within TOC 10-20 degrees for molars, incisors, and premolars?

A

-Molars 4mm
-Incisors and premolars 3mm

50
Q

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

A

-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

51
Q

What are ways to increase the crown prep wall height?

A

-lower margin
-add wall height with core Build Up (possibly including pins)
-Crown lengthening surgery
-Orthodontic extrusion

52
Q

T/F: There are times when leaving a crown preparation less than ideal will actually increase the resistance and retention form

A

True

53
Q

What can you do if your TOC is not ideal (more than 20 degrees) or your wall height is less than ideal (molar 4mm, premolar and incisor 3mm)?

A

Geometric Forms can be added to your preparation
- Box
- Groove
- Pin Hole

54
Q

What are secondary retentive features for a crown prep?

A

-Most common to use Grooves for secondary retention.
-Increases surface area
-Established parallelism on opposing intra-coronal tooth surfaces
-Limits the path of draw to one
-Shorten the Arc of rotation, therefore, increasing resistance.

55
Q

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

A

1.5mm axially
2.0 mm occlusally

56
Q

Grooves or Box forms must have a definite wall _________ to the direction of the displacing force

A

perpendicular

57
Q

Retention is increased with increased _______

A

surface area

58
Q

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

A

TOC parameters and the Height/Width ratio

59
Q

What is a good margin?

A

-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

60
Q

What happens if margins are not closely adapted to the finish line of the preparation?

A
  • Microleakage
  • Recurrent Caries
  • Possible de-cementation of restoration
61
Q

The size of your finish line determines:

A

-The bulk of material at the margin
-The fit of the restoration

62
Q

Tooth Preparation Finish line should be…

A

-Needs to be conservative of tooth structure
-Readily identifiable on tooth, impression and die
-Provides sufficient bulk for restorative material

63
Q

What are the types of finish lines (margins)?

A
64
Q

What is important about morpholgical reduction?

A

-Adequate occlusal / incisal reduction and clearance without excessive removal
-Uniform thickness of restoration material
-Parallels the major planes of the tooth
-No pulpal involvement

65
Q

What are the preparation features that contribute to Structural Durability?

A

-Occlusal / Incisal Reduction
-Functional Cusp Bevel
-Axial Reduction

66
Q

What is axial reduction?

A

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

67
Q

What if you have not enough axial reduction?

A

– Thin restoration walls – difficult to finish and possible loss of strength

68
Q

What if you have too much axial reduction?

A

Thick over contoured walls – plaque trap and periodontal inflammation

69
Q

What happens if our preparation does not leave room for Structural Durability?

A
  • Fracture of restoration
  • Grind through porcelain to adjust occlusion
  • Crown worn through due to thin material
70
Q

Our crown preparation and final restorations should promote periodontal health by:

A

-Placement of margin for cleanse-ability
-Marginal smoothness and crown adaptation
-Axial contours and emergence profile

71
Q

What are the main margin guidelines?

A
  • 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*** (depends)
  • Make margin as smooth as possible
72
Q
A