Key concepts of cavity design

Question 1

2 reasons why we drill holes in teeth

Answer 1

To remove caries

To help make a restoration

Question 2

Why is it important to remove caries?

Answer 2

Debride an infected tissue - remove bacteria

Question 3

Why make a restoration? (3)

Answer 3

Restorations restore function and reduce pain
A well-formed cavity aids retention of restoration
Restoration may protect damaged residual tooth structure

Question 4

Why not cut a cavity? (5)

Answer 4

Natural tooth tissue is finite (ageing population expecting to keep teeth)
No restorative material is adequate to truly replace it
Rebuilding tooth is complex & time-consuming
Cavity weakens a tooth
Replacing a restoration always enlarges cavity

Question 5

What is the main difference in cavity design now and in the past?

Answer 5

Past: remove all bacteria
Now: remove most bacteria

Question 6

What are the basic objectives of cavity design? (5)

Answer 6

Remove all bacteria whilst removing minimal tooth tissue
Stop marginal fracture of tooth or material
Resist displacement of material
Retain material in tooth
Prevent further decay

Question 7

Which approach did dentists take to cavity design 1890s-1980s?

Answer 7

Mechanical approach

Question 8

4 properties of amalgam as restorative material

Answer 8

Does not bond to tooth tissue (fair marginal seal)
Relatively rigid
Not antibacterial
Brittle

Question 9

What is the cavosurface angle?

Answer 9

Angle of tooth structure formed by junction of prepared cavity wall and external surface of tooth

Question 10

What happens if the cavosurface angle is less than 90 degrees?

Answer 10

Marginal enamel prisms are unsupported = unsupported enamel

Question 11

What happens when the cavosurface angle is more than 90 degrees?

Answer 11

Incomplete enamel prisms supported by dentine and not exposed to direct pressure

Question 12

What is more likely to not be displaced in an amalgam restoration: a hemospherical cavity or parallel-sided cavity?

Answer 12

Parallel-sided cavity could only be removed by destruction of the filling or tooth

Question 13

Why do we undercut dentinal walls?

Answer 13

Retention for simple restoration

Question 14

What do we use to position margins and where do we position them?

Answer 14

To allow placement of matrix

To place margins in cleansable areas

Question 15

What was distinct about the shape of cavities from 1890s - 1980s?

Answer 15

All looked the same - not px / case dependent

Question 16

Name 3 instrument types used 1890s-1980s

Answer 16

Burs (diamond and TC)
Excavators
Chisels, marginal trimmers

Question 17

What approach did dentists take to cavity design in 1980s - 2000s?

Answer 17

Preservation of tooth structure

Question 18

What key principles about cavity design changed in 1980s? (4)

Answer 18

Margins must be caries free
Follow caries rather than rigid design principles
Use properties of contemporary materials
Use prevention to reduce caries

Question 19

How did amalgam restorations change in 1980s? (3)

Answer 19

Smaller cavities (modern high copper alloys stronger)
Rounded internal angles
Bonding (enhance marginal seal and increase retention)

Question 20

Give an advantage and disadvantage of GIC

Answer 20

+ Good bond to dentine (retained without mechanical features, accept dished cavitites)
- Mechanically weak (wear and fracture)

Question 21

What is ART?

Answer 21

Atraumatic restorative treatment is where caries is removed with hand instruments only, and unsupported enamel removed. Restored with GIC. Came in in 1980s-2000s

Question 22

Is ART a good technique to use?

Answer 22

Good in developing countries with no electricity

Morally dubious - not completely removing caries, just removing it from ADJ

Question 23

How well does composite bond to enamel and dentine? When did it come into use?

Answer 23

Excellent bond to enamel (marginal seal established, retention)
Fair bond to dentine (retention)
Cutting retentive features not usually needed
1980s - 2000s

Question 24

When did fissure sealants come into use? Why advantageous?

Answer 24

1980s- 2000s.

Prevention - no cavity prep

Question 25

Why does a cavity for composite look like? (3)

Answer 25

Outline dictated by spread at ADJ
No minimum depth
Retention provided largely by adhesion

Question 26

What is tunnel preparation? Is it still used?

Answer 26

Conservative approach to cavity design, where handpiece is used to drill diagonally to caries, and cavity is filled using matrix band
Very difficult access makes clearing caries at ADJ difficult & time-consuming so rarely used

Question 27

What was the opinion towards unsupported enamel 1980s - 2000s?

Answer 27

Less of a concern but must be considered in view of occlusal loading
Support with GIC base optional
Bond with composite

Question 28

What is the basis of stepwise excavation? How has it changed in its use from 1980s - now?

Answer 28

Bacteria beneath restoration become quiescent and dentine remineralises
Used to re-enter 6 months later, now not needed - bacteria shouldn’t come back

Question 29

Process of stepwise excavation

Answer 29

Prepare ADJ
Prepare dentine pulpally but leave some softened caries
Restore with GIC
Re-enter 6 months later and restore

Question 30

How did instrumentation change 1980s - 2000s?

Answer 30

Burs rounded & sometimes smaller, containing microfine diamond
Air abrasion, laser, chemomechanical (minimal invasive)

Question 31

What approach have dentists taken towards cavity design since 2000?

Answer 31

Minimal intervention
Do we need to excavate? Good marginal seal and caries arrested over soft infected dentine if sound marginal seal at 10 years
Reduces risk of exposure of pulp and should arrest lesion progression

Question 32

Advantages of minimal intervention (5)

Answer 32

Ensure true cause of caries is addressed
Selective caries removal to not expose pulp
Minimises iatrogenic (operator’s fault) damage
Sound margins at ADJ with enamel
Can use magnification

Question 33

What instrumentation has been used for cavity design since 2000? (3)

Answer 33

Burs (round & pear-shaped diamond; round rosehead in slow handpiece)
Hand excavator
Air abrasion