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Fixed Prostodontics > Principles Of Tooth Preparations > Flashcards

Principles Of Tooth Preparations Flashcards

1
Q

5 Principles of Tooth Preparation and

Restoration Design

A

1) Preservation of tooth structure
2) Retention and resistance form
3) Structural durability of the restoration
4) Marginal integrity
5) Preservation of the periodontium

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2
Q

1) Preservation of Tooth Structure

3

A
  • Irreversible surgical procedure
  • Biologic
  • Causes of Injury
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3
Q

• Biologic

3

A
  • Adjacent teeth
  • Gingival Tissues
  • Dental Pulp
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4
Q

• Causes of Injury

4

A
  • Mechanical
  • Temperature
  • Chemical
  • Bacterial
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5
Q

• Pulpal Insult

2

A

Trauma from tooth preparation

• Decreased thickness of dentin protection

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6
Q

• Decreased thickness of dentin protection

A

• Remaining dentin thickness (between
prepared walls and pulp) key to its
protective nature

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7
Q

• Decreased thickness of dentin protection leads to (2)

A

→ Hypersensitivity

→ Pulpal inflammation and necrosis

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8
Q

Vital core

A

• A guide as to how much tooth structure can be removed w/o harming the
pulp

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9
Q

vital core depth (2)

A
  • Pulp + 1mm minimum surrounding dentin

* Retentive features cut no deeper than 1.5mm (axial) - 2mm (central fossa)

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10
Q

• Tooth flexure / fracture –

A

undermining

cusps

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11
Q

• Cuspal Coverage:

A

reinforcement and

protection of remaining tooth structure

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12
Q

• Must balance:

A 
ØUnnecessary destruction of sound tooth
structure
ØPreservation of the structural integrity
of the remaining tooth
Vs.
ØMechanical requirements
ØEsthetic requirements
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13
Q

• Morphologic reduction:

3

A 
• Preserves the basic primary axial
and occlusal tooth anatomy within
the preparation.
• Maximal thickness of residual tooth
structure surrounding the pulp is
retained.
• Anti-rotational effects for increased
resistance form.
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14
Q

Preparations must be designed

to resist

A

dislodging forces.

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15
Q 
The -- form of the
preparation is the most important
of the factors under operator
control which will determine if a
restoration will remain cemented in
place.
A

geometric

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16
Q

Retention Form

• Prevents

A

removal along path of insertion
• Occurs with sticky foods
• (least common problem)

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17
Q

Retention Form

• Provided for by:

A

• Two opposing vertical surfaces with minimal

convergence

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18
Q

• Cement offers frictional resistance –

A

shear stress.

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19
Q

• Mechanical Factors:

A

Dental Cements hold
through the mechanical interlocking of
projections of cement into irregularities of the
surfaces being joined.

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20
Q

Weakest -

Strongest –

A

tension

compression

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21
Q

Resistance Form
• Prevents restoration
dislodgement by

A

apical

or oblique forces.

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22
Q

apical

or oblique forces ex

A

• Occlusal forces

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23
Q 
Resistance to ---
forces, and not
retention along the
path of placement, is
the greatest
determining factor in a
crown’s resistance to
dislodgement.
A

lateral

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24
Q 
• Resistance form is built into a
preparation by forming walls to
block anticipated movement due
to:
(2)
A

• Leverage
• Rotation around a vertical or
horizontal axis

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25
Q

Factors Affecting Resistance Form
(“Freedom from Displacement”)
(5)

A
  1. Total Occlusal Convergence ( Taper)
  2. Preparation Wall Length (OC or IC Dimension)
  3. Tooth Width (FL Dimension) - (Ratio of wall length : tooth
    width)
  4. Geometric Form (Limited Path of Insertion)
    i. Grooves, boxes, vertical planes, pins
    a. (Supplemental / internal prep features)
    ii. Morphologic reduction / pyramidal shape of prep
  5. Surface Area
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26
Q

• Inclination:

A 
relationship of one wall
of a preparation to the long axis of
the preparation (“Taper”)
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27
Q

• Angle of convergence or total

occlusal convergence:

A

the
relationship of taper between two
opposing walls of a preparation.

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28
Q

• Historically, ideal TOC is

A

6o

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29
Q

Total Occlusal Convergence (Parallelism)

3

A

• Maximum R&R form obtained as opposing axial walls approach
parallelism.
• Some taper (total occlusal convergence) is required.
• Feature most under operator control.

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30
Q

As occlusal convergence increases, — decreases.

A

retention

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31
Q

• Maximum retention @ —; significant decrease > — TOC

A

5o

20o

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32
Q

For every 5 degree increase in the total convergence angle, the retention and
resistance form may decrease up to —%

A

50

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33
Q

What is an acceptable total occlusal

convergence?

A

•Guidelines should be based on the amount of
convergence required for adequate resistance
form and also on clinically achievable angles.

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34
Q

Average TOC of prepared teeth is in the range

of

A

10-20 degrees.

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35
Q

• — tooth preps have > TOC than (2) preps.

A

Molar

premolars and anterior

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36
Q

• Mandibular tooth preps have — TOC than maxillary tooth preps.

A

>

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37
Q
  • Mandibular tooth preps have > TOC than maxillary tooth preps.
  • Due to (2)
A

access and visibility

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38
Q

However, posterior preparations generally

require — TOC than anterior preparations.

A

more

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39
Q

• Posterior preparations are generally

40
Q

• Posterior occlusal forces are generally —.

41
Q

• Ideal TOC angle is:

A

6 to 10 degrees.

42
Q

• Clinically acceptable TOC angle:

A

10 to 20 degrees.

43
Q

• If the TOC angle is > 20 degrees, the preparation requires —.

A

modification

44
Q

2) Length

Tooth preparation height measured from the

A

incisal/occlusal surface to

the margin.

45
Q

• Greater height of the crown better the — of the restoration.

46
Q

• Increase height =

A

increases the are of cementation = Good retention

47
Q

Wall length (height of the preparation) should be greater than the

A

tipping arc of displacement, to prevent displacement of the

restoration.

48
Q

• “The preparation with — walls (a) interferes with the tipping
displacement of the restoration better than the short preparation
(b).”

49
Q

Decreased wall length causes

a

A

disproportionate decrease

in the resistance

50
Q

As the preparation length is
increased the retention and
resistance is —.

51
Q

For every 1 mm increase in preparation length,
the retention and resistance form increases up
to —%.

52
Q

• Wall length must interfere with

A

arc of rotation.

53
Q

• The shorter the preparation the — the taper must be.
• The longer the preparation, the greater the taper may
be.

54
Q

• The longer the preparation, the — the taper may

be.

55
Q

Height-Base Ratio – Arc of Rotation
• A narrow (smaller diameter) tooth can
have

A

greater resistance than a wider (larger
diameter) tooth with same wall length
• Shorter rotational radius for the arc of rotation
resists displacement

56
Q

For adequate R&R form, the
height:base ratio should be
greater than

A

0.4 for all teeth
(TOC = 10
-20 degrees).

57
Q 
Because of the small
diameter, the tangent line of
this narrow preparation falls
-- on the wall opposite the
axis of rotation, resulting in
a -- resisting area (A).
Preparation A is narrower
than preparation B, but its
height and taper are the
same. Because of the much
greater radius of the arc of
rotation its resisting area is
smaller than that of the
narrower preparation.
58
Q 
• If a molar is on average
10mm F
-L dimension and it
is determined that a 4mm
wall length is minimal for
resistance, the ratio of the
wall length to the F
-L
dimension would be
59
Q

For adequate R&R form the height / base ratio

should be greater than — for all teeth.

60
Q
  • Incisors and Premolars: – mm minimum height.
  • Molars: — mm minimum height
  • (TOC = 10-20 degrees!)
61
Q

Boxes, Grooves and Pin holes:

3

A
  • Used when R&R form of primary features is inadequate.
  • Establishes parallelism on opposing Intracoronal tooth surfaces.
  • 2o Auxiliary Retentive Features limits the paths of withdrawal to ONE
62
Q

2o Auxiliary Retentive Features shorten the

A

rotational radius

improving resistance to dislodgement

63
Q

Proximal grooves on short — dies provide resistance to F-L horizontal displacement.

64
Q

• Grooves and box forms must have a definite wall —

to the direction of the displacing force.

A

perpendicular

65
Q 
• --- component of
occlusal force has a rotating
effect on a crown in a horizontal
plane.
• Walls must be created to
block the movement
A

Horizontal

66
Q

• Preservation of — line angles

in preparation

67
Q

— planes (perpendicular

to arc of rotation)

68
Q

Pyramidal vs. Conical Form

Pyramidal preparations have

A

increased resistance form

morphologic reduction

69
Q

Pyramidal vs. Conical Form

Preserve

A

facioproximal and linguoproximal corners (line angles)

70
Q

Surface Area:

3

A 
• Retention is increased with
increased surface area in sliding
contact.
• The greater the surface area,
the greater the retention
(especially if consider longer
walls).
•Not as important as TOC, and
Height/Base ratio.
71
Q

How is resistance form assessed?

A
  • Measure TOC between 2 opposing walls (convergence guide)
  • (6o)10o – 20o (maximum)
  • Measure wall length parallel with long axis
  • 3mm minimum OC dimension (4mm – molars)
  • Measure tooth width (FL dimension) and wall height
  • OC:FL ratio should be at least 0.4
  • (4mm wall length / 10mm wide molar = 0.4)
72
Q

What modifications can be made to increase
resistance form?
(3)

A

• Decrease TOC with axial wall re-shaping.
• Addition of grooves / small boxes (parallel
to path of insertion).
• Increase wall length

73
Q

• Increase wall length (4)

A 
• Place margin at a more apical position
-Possibly subgingival
• Pin-retained core
• (Crown lengthening surgery)
• (Orthodontic extrusion)
74
Q

How can inadequate Retention / Resistance

Form be improved? (2)

A

• Reducing the TOC from 20° to 8° in the cervical 2 mm of the
axial walls significantly increased the resistance form.
• Limiting the paths of withdrawal to ONE path with the use of
grooves parallel to the intended path of withdrawal.

75
Q

• Limiting the paths of withdrawal to ONE path with the use of
grooves parallel to the intended path of withdrawal.

A

• Grooves must not be over-tapered in relation to the path of
withdrawal.

76
Q

Preparation modifications for over-converged tooth (2)

A

• Amalgam restorations removed; intracoronal walls
prepared for minimally convergent isthmus, boxes and
grooves.
• Axial walls prepared for minimal convergence in cervical
1/3; forming a shoulder with bevel.

77
Q

STRUCTURAL DURABILITY OF THE RESTORATION

2

A

• Casting must be rigid enough not to flex.
• Sufficient tooth structure must be removed
to create space for an adequate bulk of
restorative material without over-contouring
the restoration

78
Q

Preparation Features that contribute to
Structural Durability:
(3)

A
  1. Occlusal / Incisal Reduction
  2. Functional Cusp Bevel
  3. Axial Reduction
79
Q

Inadequate Occlusal Reduction / Clearance (3)

A

• Thin area → hole in casting with wear
• Fractured ceramic
• Occlusal “adjustment” to opaque porcelain or metal on
metal-ceramic crown

80
Q

• Morphologic Occlusal Reduction provides:

3

A

• Adequate occlusal/incisal clearance without excessive
removal of tooth structure.
• Uniform restoration thickness
• Parallels the major planes of the tooth (not flat)

81
Q
  • Morphologic Functional Cusp Bevel Reduction:

* Allows for

A

adequate thickness of restorative material
• without undue sacrifice of tooth structure
• without over-convergence

82
Q

Axial Reduction: creates space for an

A

adequate bulk of metal

within the normal axial contours of the tooth.

83
Q

Inadequate axial reduction →

A

(B) thin walls → weak or difficult to cast and finish
(C) bulky contours → plaque retention and periodontal
inflammation (poor emergence profile)

84
Q

Requirements for successful restoration margins:

3

A
  1. Fit as closely as possible to minimize cement film width (25µ)
  2. Sufficient strength to withstand forces of mastication (geometry of prep)
  3. Located where dentist can finish and inspect, and patient can clean
85
Q

4) Marginal Integrity
Considerations:
(2)

A
  • Marginal Geometry

* Margin Location

86
Q

The margin should fit as closely

as possible to minimize

A 
cement
film width (25 µ).
87
Q

Margins must be closely

adapted to the

A 
cavosurface
finish line of the preparation.
• Microleakage and recurrent
caries
• De-cementation of restoration
88
Q

Marginal Geometry – Cervical Finish Line

A

• Cross-sectional configuration

89
Q

Marginal Geometry – Cervical Finish Line
• Requirements:
(4)

A
  • Conservation of tooth structure
  • Readily identifiable on tooth, impression and die
  • Provide sufficient bulk of restorative material
  • Relative ease of tooth preparation
90
Q

Marginal Geometry – Cervical Finish Line
• The design of the preparation finish line will dictate:
(2)

A
  • The bulk of restorative material at the margin.

* The restoration fit.

91
Q

Marginal Geometry – Margin Types

4

A 
•Knife Edge
•Chamfer
•Shoulder
-Radial (Modified) Shoulder
•Bevel (Beveled Shoulder)
92
Q

Maintain periodontal health:

3

A
  • Margin location
  • Marginal adaptation and smoothness
  • Axial contours, emergence profiles
93
Q

• Location must preserve the optimal health and prognosis
of the periodontium.
(5)

A
  • Placed where the dentist can finish/evaluate.
  • Placed where the patient can clean.
  • Placed where they can be recorded during impression making.
  • Placed on enamel whenever possible.
  • Supragingival placement is optimal when possible.
94
Q

Margin Location

• — as possible

95
Q

5 Principles of Tooth Preparation and

Restoration Design

A

1) Preservation of tooth structure
2) Retention and resistance form
3) Structural durability of the restoration
4) Marginal integrity
5) Preservation of the periodontium

Fixed Prostodontics (19 decks)

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