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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
Factors Affecting Resistance Form (“Freedom from Displacement”) (5)
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
26
• Inclination:
``` relationship of one wall of a preparation to the long axis of the preparation (“Taper”) ```
27
• Angle of convergence or total | occlusal convergence:
the relationship of taper between two opposing walls of a preparation.
28
• Historically, ideal TOC is
6o
29
Total Occlusal Convergence (Parallelism) | 3
• Maximum R&R form obtained as opposing axial walls approach parallelism. • Some taper (total occlusal convergence) is required. • Feature most under operator control.
30
As occlusal convergence increases, --- decreases.
retention
31
• Maximum retention @ ---; significant decrease > --- TOC
5o | 20o
32
For every 5 degree increase in the total convergence angle, the retention and resistance form may decrease up to ---%
50
33
What is an acceptable total occlusal | convergence?
•Guidelines should be based on the amount of convergence required for adequate resistance form and also on clinically achievable angles.
34
Average TOC of prepared teeth is in the range | of
10-20 degrees.
35
• --- tooth preps have > TOC than (2) preps.
Molar | premolars and anterior
36
• Mandibular tooth preps have --- TOC than maxillary tooth preps.
>
37
* Mandibular tooth preps have > TOC than maxillary tooth preps. * Due to (2)
access and visibility
38
However, posterior preparations generally | require --- TOC than anterior preparations.
more
39
• Posterior preparations are generally
shorter.
40
• Posterior occlusal forces are generally ---.
higher
41
• Ideal TOC angle is:
6 to 10 degrees.
42
• Clinically acceptable TOC angle:
10 to 20 degrees.
43
• If the TOC angle is > 20 degrees, the preparation requires ---.
modification
44
2) Length | Tooth preparation height measured from the
incisal/occlusal surface to | the margin.
45
• Greater height of the crown better the --- of the restoration.
retention
46
• Increase height =
increases the are of cementation = Good retention
47
Wall length (height of the preparation) should be greater than the
tipping arc of displacement, to prevent displacement of the | restoration.
48
• “The preparation with --- walls (a) interferes with the tipping displacement of the restoration better than the short preparation (b).”
longer
49
Decreased wall length causes | a
disproportionate decrease | in the resistance
50
As the preparation length is increased the retention and resistance is ---.
increased
51
For every 1 mm increase in preparation length, the retention and resistance form increases up to ---%.
10
52
• Wall length must interfere with
arc of rotation.
53
• The shorter the preparation the --- the taper must be. • The longer the preparation, the greater the taper may be.
less
54
• The longer the preparation, the --- the taper may | be.
greater
55
Height-Base Ratio – Arc of Rotation • A narrow (smaller diameter) tooth can have
greater resistance than a wider (larger diameter) tooth with same wall length • Shorter rotational radius for the arc of rotation resists displacement
56
For adequate R&R form, the height:base ratio should be greater than
0.4 for all teeth (TOC = 10 -20 degrees).
57
``` 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. ```
low | large
58
``` • 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 ```
0.4.
59
For adequate R&R form the height / base ratio | should be greater than --- for all teeth.
0.4
60
* Incisors and Premolars: -- mm minimum height. * Molars: --- mm minimum height * (TOC = 10-20 degrees!)
3 | 4
61
Boxes, Grooves and Pin holes: | 3
* 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
2o Auxiliary Retentive Features shorten the
rotational radius | improving resistance to dislodgement
63
Proximal grooves on short --- dies provide resistance to F-L horizontal displacement.
15o
64
• Grooves and box forms must have a definite wall --- | to the direction of the displacing force.
perpendicular
65
``` • --- component of occlusal force has a rotating effect on a crown in a horizontal plane. • Walls must be created to block the movement ```
Horizontal
66
• Preservation of --- line angles | in preparation
axial
67
--- planes (perpendicular | to arc of rotation)
Vertical
68
Pyramidal vs. Conical Form | Pyramidal preparations have
increased resistance form | morphologic reduction
69
Pyramidal vs. Conical Form | Preserve
facioproximal and linguoproximal corners (line angles)
70
Surface Area: | 3
``` • 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
How is resistance form assessed?
* 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
What modifications can be made to increase resistance form? (3)
• Decrease TOC with axial wall re-shaping. • Addition of grooves / small boxes (parallel to path of insertion). • Increase wall length
73
• Increase wall length (4)
``` • Place margin at a more apical position -Possibly subgingival • Pin-retained core • (Crown lengthening surgery) • (Orthodontic extrusion) ```
74
How can inadequate Retention / Resistance | Form be improved? (2)
• 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
• Limiting the paths of withdrawal to ONE path with the use of grooves parallel to the intended path of withdrawal.
• Grooves must not be over-tapered in relation to the path of withdrawal.
76
Preparation modifications for over-converged tooth (2)
• 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
STRUCTURAL DURABILITY OF THE RESTORATION | 2
• 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
Preparation Features that contribute to Structural Durability: (3)
1. Occlusal / Incisal Reduction 2. Functional Cusp Bevel 3. Axial Reduction
79
Inadequate Occlusal Reduction / Clearance (3)
• Thin area → hole in casting with wear • Fractured ceramic • Occlusal “adjustment” to opaque porcelain or metal on metal-ceramic crown
80
• Morphologic Occlusal Reduction provides: | 3
• Adequate occlusal/incisal clearance without excessive removal of tooth structure. • Uniform restoration thickness • Parallels the major planes of the tooth (not flat)
81
* Morphologic Functional Cusp Bevel Reduction: | * Allows for
adequate thickness of restorative material • without undue sacrifice of tooth structure • without over-convergence
82
Axial Reduction: creates space for an
adequate bulk of metal | within the normal axial contours of the tooth.
83
Inadequate axial reduction →
(B) thin walls → weak or difficult to cast and finish (C) bulky contours → plaque retention and periodontal inflammation (poor emergence profile)
84
Requirements for successful restoration margins: | 3
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
4) Marginal Integrity Considerations: (2)
* Marginal Geometry | * Margin Location
86
The margin should fit as closely | as possible to minimize
``` cement film width (25 µ). ```
87
Margins must be closely | adapted to the
``` cavosurface finish line of the preparation. • Microleakage and recurrent caries • De-cementation of restoration ```
88
Marginal Geometry – Cervical Finish Line
• Cross-sectional configuration
89
Marginal Geometry – Cervical Finish Line • Requirements: (4)
* Conservation of tooth structure * Readily identifiable on tooth, impression and die * Provide sufficient bulk of restorative material * Relative ease of tooth preparation
90
Marginal Geometry – Cervical Finish Line • The design of the preparation finish line will dictate: (2)
* The bulk of restorative material at the margin. | * The restoration fit.
91
Marginal Geometry – Margin Types | 4
``` •Knife Edge •Chamfer •Shoulder -Radial (Modified) Shoulder •Bevel (Beveled Shoulder) ```
92
Maintain periodontal health: | 3
* Margin location * Marginal adaptation and smoothness * Axial contours, emergence profiles
93
• Location must preserve the optimal health and prognosis of the periodontium. (5)
* 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
Margin Location | • --- as possible
Smooth
95
5 Principles of Tooth Preparation and | Restoration Design
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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