Cavity Preparation For Cast Metal Restorations Question And Answers

Cast Metal Restorations Important Notes

1. Pickling

• Pickling is the process of cleaning the gold casting with 50% warm HCl
• Used to remove surface oxides from casting
• It is not a routine procedure
• Used only when indicated

2. Casting defects

• Distortion
• Surface roughness and irregularities
• Discoloration
• Porosity
  • Solidification defects
  • Trapped gases
  • Residual air

3. Types of casting machines

• Centrifugal casting machine
  • Uses centrifugal force to accelerate the flow of molten metal into the mold space
• Air pressure casting machine
  • Compressed gas is used to force the molten alloy into the mold

4. Types of cast metal alloys

5. Types of die materials used

• Gypsum products
• Electroformed dies
• Epoxy resins
• Divestment

6. Properties of base metal alloys

• Low density
• Low percentage elongation
• High fusing temperature
• High hardness
• Tarnish and corrosion resistance

7. Retention forms in cast restorations

• Grooves
• Bevel
• Internal box
• External box
• Pins, slots, skirt, collar

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• Cusp capping
• Reciprocal retention

8. Onlay

• They are partly intracoronal and partly extra-coronal types of restoration
• It has cuspal protection
• It is mainly indicated when the width of the lesion exceeds one-third of the intercuspal distance
• Features of cavity preparation:
  • Capping of functional cusps
  • Cuspal reduction 1.5 mm
  • The bevel used is a hollow ground long bevel in the intracoronal portion and a counter bevel in the extra coronal portion
  • Table:
    • It is a transitional area between the intracoronal and extra-coronal portion
    • It should be flat
    • Relieved from opposing cusps by 1.5 mm
    • There should be 3 tables prepared for each cusp in different directions
    • It provides retention and resistance form
  • Shoeing of nonfunctional cusps
    • Provides retention and resistance form over the nonfunctional cusp

9. Composition of wax

10. Principles for correct spring

• Use of proper gauge of sprue
• Use of proper length of sprue
• Should be attached to the bulkiest portion of the wax pattern
• Attach it to the wax pattern with little heat to avoid distortion

11. Uses of casting ring liner

• Permits unrestricted expansion of the investment
• Acts as a cushion between rapidly cooling metal ring and the more slowly cooling investment during the casting and crystallization of gold

12. Objectives of burnout

• Complete removal of wax pattern
• Elimination of excess water from the investment
• Production of thermal expansion

Cast Metal Restorations Long Essays

Question 1. Classify casting defects describe mesioocclusal cavity preparation for gold inlay on a mandibular first molar.
(or)
Describe class 2 mesioocclusal cavity preparation for gold inlay on mandibular 1 molar tooth.

Answer:

Casting Defects:

• Distortion
• Surface roughness and irregularities
• Discoloration
• Porosity
  • Solidification defects
    • Localized shrinkage
    • Micro
  • Trapped gases
    • Pinhole
    • Gas inclusion
    • Subsurface
  • Residual air
    • Back pressure

Class 2 Inlay Cavity Preparation

1. Initial tooth preparation

Occlusal outline form:

• Anesthetize and isolate the tooth
• Penetrate the tooth with no.271 bur from the mesial surface
• Extend upto central fissure to uninvolved marginal ridge
• Maintain a uniform pulpal depth of 1.5mm
• Preserve distal marginal ridge
• Extend up to the smooth areas of the buccal and lingual slopes of the cusps of a tooth.

Proximal box preparation:

• Mesial ditch cut is given using the same bur
• Width of cut 0.8mm, 0.5 mm in dentin, and 0.3 mm in enamel
• Extend it faciolingually
• Extend gingivally
• Provide 0.5mm of clearance
• Break the contact using cuts over facial and lingual walls using No.271 bur

Resistance and Retention form:

• Flat pulpal floor
• Box-shaped preparation
• The extra thickness of gold
• Occlusal step
• Dovetail

2. Final preparation:

1. Removal of remaining caries, old restorative material

• Removal of soft caries and infected dentin with a spoon excavator

2. Pulp protection

• Use of pulp protective materials

3. Secondary resistance and retention forms

• Retention grooves were placed in the facial axial and in coaxial line angles using 169L carbide bur
• Bevels
• Gingival bevel 45° to the preparation
• Include one-half the width of the gingival wall
• Occlusal bevel At occlusal 1/3rd of adjacent occlusal wall
• Removes unsupported enamel

4. Inspecting, Cleaning, and Rinsing

• Clean the prepared tooth with air/water spray
• Dry it with a cotton pellet
• Inspect the angles and margins

Question 3. Define Inlay Describe the Indications, Contraindications, advantages, and disadvantages of cast gold restoration.
Answer:

Definition Of Inlay:

• An inlay is an indirect intracoronal restoration which is fabricated extra orally and cemented in the prepared tooth

Indications:

• Extensive proximal caries
• Patients with good oral hygiene and low caries index
• Postendodontic restoration
• Teeth with extensive restoration
• To maintain proper tooth contact and contour
• Presence of cast metal restoration
• Abutment teeth
• Teeth with heavy occlusal forces and attrition

Contraindications:

• Aesthetic consideration
• Patients with high caries index
• Young patients
• Periodontally weak teeth
• Extensive caries involving facial and lingual and multiple surfaces
• Low economic status
• Presence of dissimilar restoration
• Extensive occlusal wear facets

Advantages:

• Better reproduction of details
• Better wear resistant
• Biocompatible
• Strengthens remaining tooth structure
• Fewer chances of voids
• Easy to polish
• Less chair time

Disadvantages:

• Need of temporary restoration
• Expensive
• Technique sensitive
• Difficult to repair
• Weak tooth bonding
• Unesthetic

Question 4.  Give the difference in cavity preparation between amalgam and gold inlay.

Answer:

Differences in cavity preparation between amalgam and gold inlay:

Question 5 Define retention form. How it is achieved in cast restoration.

Answer:

Retention Form:

• It is that form of cavity that resists the displacement of restoration from tipping and lifting forces

Retention In Cast Restorations:

1. Grooves:

Resist lateral displacement of restoration

• Internal grooves
  • Indicated in a shallow and small cavity
  • Contraindicated in a deep cavity
• External grooves
  • Indicated in extra coronal preparation
  • Prevents dislocation
  • May be prepared in stepped form

2. Reverse Bevel:

• Indication Class 1, 2, 3 restoration
• Presence of sufficient gingival floor
• Placed over the gingival floor with an incline gingivally and axially

3. Internal Box:

• Indicated in the presence of sufficient dentin
• Contraindicated in class IV and V preparation
• Have vertical walls prepared in dentin
• Increases retention
• It should be 2 mm in dimension

4. External box:

• Box with an opening to the axial surface of the tooth
• The peripheral portion can be flared/beveled

5. Pins:

• Types of pins used Cemented, threaded, parallel, cast, and wrought

6. Slot:

• Indicated in the shallow cavity, restricted occlusal anatomy
• Have a depth of 23mm

7. Skirt:

• Indicated in missing wall

8. Collar:

• Depth 1.52mm, surface extension
• Indication Grossly carious teeth
• Short teeth

9. Cusp Capping:

• Provide sufficient height of cusp

10. Reciprocal Retention:

• Placement of retention made at every end of the preparation

Question 6. Discuss the causes of casting defects and their prevention.

Answer:

The causes of casting defects and their prevention:

• Casting defects can be prevented by following various steps systematically
• Types of casting defects along with their causes and prevention are as follows

Cast Metal Restorations Short Essays

Question 1. Bevel and flares.
or
Circumferential Tie.

Answer:

Circumferential Tie:

• It refers to the design of cavosurface margin of an inlay tooth preparation

Significance:

• The cavosurface margin is the weakest part of the restoration
• To strengthen it, it is designed by incorporating bevels and flares

1. Bevels:

• It is the inclination that one surface makes with another when not at right angles

Types:

• Partial Bevel
  • Involving less than 2/3rd of enamel
  • Use of type 1 casting alloys
• Short Bevel
  • Involves full thickness of enamel
  • Use of type 1 and 2 alloys
• Long Bevel
  • Involves full enamel and half dentin
  • Use in type 1, 2, and 2 alloys
• Full Bevel
  • Involves complete enamel and dentin
  • Hollow ground Bevel
  • Concave in shape
• Counter Bevel
  • In cases of cusp capping

1. Flares: They are concave/flat peripheral portions of the facial/lingual proximal walls

Question 2. Die materials.

Answer:

Die materials:

Properties:

• Compatible with impression materials
• Have smooth surface
• Have adequate strength
• Easy to fabricate
• Have contrasting color
• Reproduce accurate details

Materials:

Question 3. Direct Wax Pattern.

Answer:

Direct Wax Pattern:

• Prepared in the oral cavity
• Done using matrix band or without it

Technique:

Isolate the tooth
↓
Apply band and retainer
↓
Soften inlay wax
↓
Compress over the prepared tooth
↓
Hold it with finger pressure til it sets
↓
Burnish it
↓
Remove band and retainer
↓
Check for centric occlusion
↓
Check for high points
↓
Smoothen it
↓
Attach sprue former and reservoir
↓
Remove the wax pattern

Advantages:

• Fewer discrepancies
• Less laboratory work

Disadvantages:

• Require more skill
• Requires more chair side time

Question 4. Indirect Wax Pattern.

Answer:

Indirect Wax Pattern:

• Prepared outside the oral cavity
• Use of type 11 inlay wax

Steps:

Lubricate the die
↓
Adapt inlay wax over it
↓
Carve the wax
↓
Attach sprue former
↓
Remove the wax pattern

Advantages:

• Less chair time
• Finishing and polishing
• This can be done on a die

Disadvantages:

• More laboratory work
• Errors due to inadequate casting

Question 5. Sprue.

Answer:

Sprue:

• Provides a channel so that molten metal flows into mold space after the wax pattern has been eliminated.

Types:

• Wax
• Plastic
• Metal

Functions:

• Provides a channel for the flow of molten alloy
• Provides reservoir to compensate for shrinkage
• Provides a channel for wax elimination

Requirements:

• Diameter Greater than the thickest part of the wax pattern
• Attachment to the thickest part of the wax pattern
• Sprue length such that the end of the wax pattern is l/8th to l/4th inch away from the open end of the casting ring
• Direction 45 to the bulkiest portion of the wax
• ReservoirTo compensate shrinkage

Question 6. Casting machines.

Answer:

Casting machines:

Types:

1. Centrifugal Casting Machine:

• Cheapest
• Used for small casting
• Uses centrifugal force to accelerate the flow of mol¬ten metal into the mold space

Steps:

Heat the ring at 1200°F for 15 minutes
↓
Move the arm of casting machine 23 turns clockwise and
lock it
↓
Heat the gold alloy
↓
Place the ring in the casting machine
↓
Release the lock of the arm
↓
This forces molten gold into a mold
↓
Recover the ring from the casting machine and cool it
↓
Removes the casting and cleans it

2. Air pressure Casting machine:

• Compressed air/gases like carbon dioxide or nitrogen are used to force the molten alloy into the mold.

Question 7. CADCAM for Inlay.

Answer:

CADCAM for Inlay:

• CADCAM indicates Computer Aided Design Computer Aided Machine
• In it, the whole casting design is fabricated
• This design is fed up with the computer
• The whole design is scanned
• It is then fabricated into the final prosthesis
• The computer is connected to a machine that processes the prosthesis
• Thus, the machining process of the prosthesis according to the design fabricated by the computer

Question 8. Types of Cast Gold Alloys.

Answer:

Types of Cast Gold Alloys:

Type 1 Soft

• Used for fabrication of small inlays
• Low strength
• Easily burnished
• Gold content7583%

Type 2 Medium

• Used for fabrication of inlays and onlays subjected to moderate stresses
• Cannot be heat treated
• Gold content7075%

Type 3Hard

• Used for high-stresses
• High strength
• Can be heat-treated
• Gold content6570%

Type 4 Extra Hard:

• Used for crowns, bridges, and removable partial dentures
• Increased hardness
• Can be heat-treated
• Gold content 60%

Question 9. Indications and contraindications of Onlay

Answer:

Indications and contraindications of Onlay:

Only:

• It is a combination of intracoronal and extra-coronal cast restoration which covers one or more cusps

Indications:

• Extensive weakened tooth due to caries or large restoration
• As post-endodontic restoration
• Presence of other cast restorations in adjacent or opposing teeth
• When the mesiodistal diameter of the tooth needs to be extended
• To correct the occlusal plane
• As an abutment for RPD

Contraindications:

• Patients with high caries rate
• Young patients
• Short clinical crown height

Cast Metal Restorations Short Answers

Question 1. Reverse bevel.

Answer:

Reverse bevel:

• A reverse bevel is a bevel of generous width that is prepared on the facial margin of a reduced cusp with a flame-shaped, fine grit diamond instrument
• Its width should extend beyond any occlusal contact with opposing teeth
• It should be at an angle that results in a 30-degree marginal metal
• It should be wide enough so that the cave surface margin is beyond any contact with the opposing dentition

Question 2. Sprue former.

Answer:

Sprue former:

• A sprue former is made of wax, plastic, or metal
• Thickness is in proportion to the wax pattern

Functions

• To form a mount for the wax pattern
• To create a channel for the elimination of wax during burnout
• Forms channel for entry of metal which compensates for allo£ shrinkage during solidification

Question 3. Porosities in casting.

Answer:

Porosities in casting:

Types:

1. Caused by solidification shrinkage

• Localized shrinkage porosity
• Suck back porosity
• Microporosity

2. Caused by gas

• Pinhole porosity
• Gas inclusion
• Subsurface porosity

3. Caused by air entrapment:

• Back pressure porosity

Prevention

• Use of correct sprue thickness
• Correct placement of sprue
• Use of reservoir
• Use of adequate casting forces
• Use of porous investment and proper vents
• Place the pattern away from the end of the ring

Question 4. Inlay Taper.

Answer:

Inlay Taper:

• Ideally, a tooth preparation should have slight diverg¬ing walls from gingival to occlusal surface

Value:

• Optimal taper25° per wall
• For short longitudinal walls 2° taper
• For high longitudinal walls Increased but not more than 10°
• Preparation should never have one side with more taper than the other
• For shallow preparation Axis of the taper is parallel to the long axis of the tooth
• For class 5 Axis of the taper is perpendicular to the long axis of the tooth.

Question 5. Hot spot porosity.

Answer:

Hot spot porosity:

• It is localized shrinkage porosity
• Occurs due to shrinkage of molten alloy when alloy solidifies from a molten state
• Cause Sprue former directed at 90°
• Prevention Direct the sprue former at 45°

Question 6. Back pressure porosity.

Answer:

Back pressure porosity:

• This is caused by inadequate venting of the mold
• If the bulk of the investment is too great the escape of air becomes difficult causing increased pressure in the mold
• The gold will solidify before the mold is filled resulting in a porous casting with rounded short margins
• Avoided by
  • Using adequate casting forces
  • Use of investment of adequate porosity
  • Place the pattern not more than 68 mm from the end of the ring

Question 7. Subsurface porosity.

Answer:

Subsurface porosity:

• It is porosity caused by gas

Cause:

• Simultaneous nucleation of solid grains and gas bubbles

Prevention:

• Controlling the flow of molten alloy

Question 8. Electroformed dies.

Answer:

Electroformed dies:

Advantages:

• Dimensional accurate
• Hard, abrasion-resistant
• Imparts a smooth surface
• Cheap
• Better marginal definition
• Does not absorb oil or water
• Prevent cuspal wear

Disadvantages:

• Difficult to trim
• Silver bath health hazard
• Noncompatible with impression material
• Color contrast is not good
• or adaptation to wax
• The pattern tends to lift from margins

Question 9. Onlay.

Answer:

Only:

• It is a combination of intracoronal and extra-coronal cast restoration which covers one or more cusp

Tooth Preparation:

• Occlusal Outline form:
  • Isolate the tooth
  • Start preparation with 271 bur
  • Maintain a pulpal depth of 2 mm
  • Reduce the cusps
  • Occlusal divergence 3°-5°

Proximal Box Preparation:

• Extend the preparation both mesially and distally

Question 10. Inlay Wax.

Answer:

Inlay Wax:

Properties:

• Plasticity
• Solid below approximately 40°C

Types:

• Type 1
• Type 2

Composition:

• Paraffin wax- 40 60%
• Ceresin -10%
• Gum dammar- 1%
• Carnauba- 25%
• Coloring agents

Question 11. Localized shrinkage porosity

Answer:

Localized shrinkage porosity:

• Occurs when cooling is incorrect
• The sprue freezes before the rest of the casting
• It results in large irregular voids near the sprue casting interface

Prevention:

• Sprue former should be directed at 45 degrees
• Placement of reservoir

Cast Metal Restorations Viva Voce

1. Sprue former diameter is between 8018 gauge
2. Types of sprue former wax, metal, and resin
3. Occlusal bevel in inlay3045°
4. Gingival bevel in inlay 30°
5. The sprue is placed at a 45° angle to the wax pattern
6. Wax interocclusal records help check occlusal clearance
7. Binder present in investment provides strength
8. Casting ring liner helps in permitting the expansion of mold
9. Sprue should be positioned in the thickest portion of the wax pattern
10. There should be a minimum gap of 1/8 1/4th inch between the ends of the casting ring and wax pattern
11. Back pressure porosity is caused by the entrapped air in the mold that does not escape through the pores of the investment
12. Type 1 or type B wax is used for the direct technique of wax pattern construction
13. Type C is used for indirect technique