Flashcards in Le4.PrinciplesOfToothPreps Deck (27)
Loading flashcards...
1
Why might under reduced tooth preps result in gingival inflammation?
Under reduced tooth preps generally lead to overcontoured restorations (b/c adequate porcelain thickness is necessary for strength), which encroach on the gingiva at the margins and cause inflammation
2
What is prefered supra- or subgingival margins? Why?
Supragingival margin preps are prefered b/c
-Easier to prepare w/ minimal tissue damage
-Usually in enamel vs dentin or cementum
-Easier for pt to keep plaque free
-Impressions more easily made (less soft tissue damage)
-Restorations more easily evaluated in future apts
-Subgingival margins are major cause of Periodontal Disease resulting from poor fit & irritation --> Inflammation
3
What are the indications for a chamfer bur? (2)
1. Cast metal crowns
2. Lingual margin of metal-ceramic crowns
4
What is the major advantage of putting a bevel on a prep for a cast metal crown?
Allows the cast metal margin to be burnished against the prepared tooth structure. Should result in decreased marginal discrepancies.
5
What's are two disadvantages of a bevel?
1. Extends prep apically
2. If internal diameter of crown is slightly larger than prep beveling actually increases marginal discrepancy vs chamfer or shoulder
6
When is a bevel indivated? (2)
1. Cast metal restorations
2. lingual margin of metal-ceramic crowns
7
When are shoulder preps indicated? (2)
1. Facial margin of metal-ceramic crowns
2. complete ceramic crowns
8
Is a chamfer or shoulder margin design more conservative?
Chamfer
9
When is a shoulder with a bevel indicated? (1)
Facial margin of metal-ceramic crowns
10
Why should a feather edge or "shoulderless" prep be avoided?
Because does not provide adequate bulk at the margins --> fracture of restoration at the margins
11
What is the 1 time a feather edge margin design is indicated for?
Periodontically involved teeth where diminishing diameter of root surface is encountered
12
Has research found one margin design superior to others?
NO
13
At Penn Dental Medicine, what finish line is indicated when preparing a Cast Metal restoration?
Circumferential internally rounded shoulder with bevel
14
At Penn Dental Medicine, what finish line is indicated when preparing a Porcelain fused to Metal restoration?
Circumferential internally rounded shoulder
15
At Penn Dental Medicine, what finish line is indicated when preparing an All Ceramic restoration?
Circumferential internally rounded shoulder
16
Path of insertion for crown on posterior tooth in normal alignment should parallel what reference plane?
Long axis of tooth
17
For a tipped tooth, the path of insertion should be perpendicular to what reference plane?
Occlusal plane
18
The angle of convergence is formed how?
The convergence of two opposing walls of a tooth prep as viewed in a given plane = Taper = Angle of Convergence
19
How much taper is recommended for a tooth preparation?
3 degrees on each surface, resulting in 6 degrees of total taper (=convergence angle). If bur is kept parallel to occlusal surface it should automatically form 3 degree taper.
20
What luting agent (cement) have studies indicated is the most retentive?
ADHESIVE RESINS > Glass ionomer > Polycarboxylate Zn Phosphate > ZOE
21
Resistance form of a prep is a function of relationship b/t what 3 factors?
1. Axial wall taper (should be 6 degrees)
2. Preparation diameter (want smaller diameter)
3. Preparation height (want greater height)
22
Why does resistance decreases as diameter increases, why? Note: Opposite of retention form which increased with increasing diameter due to increased surface area
Crown on narrower tooth has a shorter radius of rotation resulting in a lower tangent line and larger resisting area
23
How do proximal grooves or boxes in axial walls increase restention?
By limiting rotational movement (tipping) of crown
24
Preventing deformation depends on what two factors?
1. Adequate tooth reduction to allow for adequate bulk of material
2. Margin design, again, to allow for bulk of material at margin (i.e. NO feathered margins).
25
Functional cusps require a more or less reduction than non-functional cusps?
MORE reduction for increased bulk of material and thus increased strength
26
For a Porcelain Fused to Metal prep what are the following values?
1. Functional Cusp reduction?
2. Non-functional cusp reduction?
3. Facial axial wall reduction?
4. Proximal axial wall reduction?
5. Lingual axial wall reduction?
6. Functional cusp bevel?
7. Non-functional cusp bevel?
8. Shoulder?
1. 2.0 mm
2. 1.5 mm
3. 1.5 mm (Need to over reduce facial and interproximal
4. 1.3 mm to compensate for porcelain)
5. 0.75-1.0 mm
6. 2.0 mm
7. 1.0 mm
8. 0.5 mm supragingivally
27
