ICP L14: Modification of cavity form for different materials

Question 1

What determines cavity design

Answer 1

• structure and properties of dental tissue
• extent and nature of dental disease
• properties of proposed restorative material

Question 2

How are cavities modified

Answer 2

Macroscopically
= using bur, air rotor, slow handpiece to change the physical shape

Microscopically
= chemically by conditioning the tissue

Question 3

What is retention

Answer 3

Property of a cavity/restoration to resist displacement in the direction of its insertion

Question 4

What is resistance

Answer 4

Property of a cavity/preparation that prevents displacement of a restoration in apical or oblique directions

Question 5

What are the principals of cavity modification that have to be counted for

Answer 5

1. Cavo-surface angle
2. Internal line angles (where two internal surfaces meet)
3. Internal point angle (where three internal surfaces meet)
4. External walls
5. Internal walls

Question 6

What are direct restorations

Answer 6

Those done inside the patients mouth

• plastics
• mouldable
• various physical properties
• complex restorations

Question 7

What are indirect restorations

Answer 7

Fabricated in laboratory and luted at chair side

- these cannot be changed in shape by the dentist

Question 8

What macroscopic modification is made for all cavities

Answer 8

Rounded internal line angles; this is to reduce internal stresses and the risk of crack propagation

Question 9

What cavity modifications are made for amalgam restorations

Answer 9

1. Rounding internal line angles
2. Cavity undercuts, grooves, slots, flat surfaces; these give retention and prevent further displacement and flat surfaces improve resistance

Question 10

What cavity modifications are made for composites

Answer 10

1. Rounding internal line angles
2. Enamel margin bevel to remove unsupported enamel, increase SA for bond/seal
3. Enamel acid etch to remove smear layer and improve micro-mechanical retention by demineralising prisms

Question 11

What cavity modifications are made for GICs

Answer 11

1. Rounding internal line angles
2. Enamel margin bevel to remove unsupported enamel, increase SA for bond/seal
3. Dentine conditioner to remove smear layer and prepare surface for Ca2+ chemical adhesion

Question 12

Why is the ideal Cavo-serface angle 90 for amalgam restorations

Answer 12

Because angles much greater or lower means that there will be a thin section of amalgam (brittle) and this will fracture and break off causing food collection in this area and caries development

Question 13

Why is bevelling used in anterior teeth that are going to have a composite restoration but not in posterior teeth

Answer 13

Because anteriorly it is better for aesthetics as it will blend the enamel, dentine and composite resin in making it look less obvious due to the diffuse margin

Question 14

Outline how cavity modifications are made in the enamel for composites

Answer 14

Selective demineralisation of enamel prisms via etching to provide a rough surface

Question 15

Outline how cavity modifications are made in the dentine for composites

Answer 15

The dentinal tubules are etched to remove the smear plug and the overall smear layer on the dentine surface by demineralising the intertubular and intratubular dentine and exposing the collagen - this will allow resin to flow into these areas

Question 16

Outline the sequence of cavity modification for composites

Answer 16

Acid etch, primer, bond

Question 17

Outline how cavity modifications are made for GICs and why they aren’t commonly used

Answer 17

Dentine conditioners remove the smear layer and prepare the surface for Ca2+ bonding - this isn’t always used because clinically the results are good without this stage as ion exchange between the tooth minerals and cement occurs regardless and this creates the chemical bond