Le4.PrinciplesOfToothPreps

Question 1

Why might under reduced tooth preps result in gingival inflammation?

Answer 1

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

Question 2

What is prefered supra- or subgingival margins? Why?

Answer 2

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

Question 3

What are the indications for a chamfer bur? (2)

Answer 3

1. Cast metal crowns

2. Lingual margin of metal-ceramic crowns

Question 4

What is the major advantage of putting a bevel on a prep for a cast metal crown?

Answer 4

Allows the cast metal margin to be burnished against the prepared tooth structure. Should result in decreased marginal discrepancies.

Question 5

What’s are two disadvantages of a bevel?

Answer 5

1. Extends prep apically
2. If internal diameter of crown is slightly larger than prep beveling actually increases marginal discrepancy vs chamfer or shoulder

Question 6

When is a bevel indivated? (2)

Answer 6

1. Cast metal restorations

2. lingual margin of metal-ceramic crowns

Question 7

When are shoulder preps indicated? (2)

Answer 7

1. Facial margin of metal-ceramic crowns

2. complete ceramic crowns

Question 8

Is a chamfer or shoulder margin design more conservative?

Answer 8

Chamfer

Question 9

When is a shoulder with a bevel indicated? (1)

Answer 9

Facial margin of metal-ceramic crowns

Question 10

Why should a feather edge or “shoulderless” prep be avoided?

Answer 10

Because does not provide adequate bulk at the margins –> fracture of restoration at the margins

Question 11

What is the 1 time a feather edge margin design is indicated for?

Answer 11

Periodontically involved teeth where diminishing diameter of root surface is encountered

Question 12

Has research found one margin design superior to others?

Answer 12

NO

Question 13

At Penn Dental Medicine, what finish line is indicated when preparing a Cast Metal restoration?

Answer 13

Circumferential internally rounded shoulder with bevel

Question 14

At Penn Dental Medicine, what finish line is indicated when preparing a Porcelain fused to Metal restoration?

Answer 14

Circumferential internally rounded shoulder

Question 15

At Penn Dental Medicine, what finish line is indicated when preparing an All Ceramic restoration?

Answer 15

Circumferential internally rounded shoulder

Question 16

Path of insertion for crown on posterior tooth in normal alignment should parallel what reference plane?

Answer 16

Long axis of tooth

Question 17

For a tipped tooth, the path of insertion should be perpendicular to what reference plane?

Answer 17

Occlusal plane

Question 18

The angle of convergence is formed how?

Answer 18

The convergence of two opposing walls of a tooth prep as viewed in a given plane = Taper = Angle of Convergence

Question 19

How much taper is recommended for a tooth preparation?

Answer 19

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.

Question 20

What luting agent (cement) have studies indicated is the most retentive?

Answer 20

ADHESIVE RESINS > Glass ionomer > Polycarboxylate Zn Phosphate > ZOE

Question 21

Resistance form of a prep is a function of relationship b/t what 3 factors?

Answer 21

1. Axial wall taper (should be 6 degrees)
2. Preparation diameter (want smaller diameter)
3. Preparation height (want greater height)

Question 22

Why does resistance decreases as diameter increases, why? Note: Opposite of retention form which increased with increasing diameter due to increased surface area

Answer 22

Crown on narrower tooth has a shorter radius of rotation resulting in a lower tangent line and larger resisting area

Question 23

How do proximal grooves or boxes in axial walls increase restention?

Answer 23

By limiting rotational movement (tipping) of crown

Question 24

Preventing deformation depends on what two factors?

Answer 24

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).

Question 25

Functional cusps require a more or less reduction than non-functional cusps?

Answer 25

MORE reduction for increased bulk of material and thus increased strength

Question 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?

Answer 26

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

Question 27

For a Porcelain Full-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. Bevel? 9. Circumferential shoulder w/ bevel?

Answer 27

1. 1.5 mm 2. 1.0 mm 3. 0.75-1.0 mm (after bevel) 4. 0.75-1.0 mm (after bevel) 5. 0.75-1.0 mm (after bevel) 6. 2.0 mm 7. 1.0 mm 8. 0.5-1.0 mm 9. 0.5 mm supragingival