ICP L13: Restoration of cavities with different materials

Question 1

Outline the process of tooth fracturing in previously restored teeth

Answer 1

1. Deflection of cusp is weakened by the restoration
2. Open interface between restoration and tooth (bond failure)
3. Microleakage
4. Recurrent caries
5. Fracture

Question 2

How does fracture resistance of a restored tooth compare to an unrestored tooth

Answer 2

It is lower because high stresses occur at the tooth-restoration interface and this can cause crack propagation and fatigue failure on mastication

Question 3

How can the shape of a restoration be optimised to prevent fracture

Answer 3

• minimise reduction of tooth tissue
• decide the correct geometry of the cavity
• keep stress levels low to avoid fracture of the restored unit

Question 4

Why do composites, GICs, porcelains and gold not need retentive cavity designs

Answer 4

Because they either use bonding agents or chemical adhesion to the tooth so they are not reliant on undercuts and sharp angles like dental amalgams

Question 5

Why do dental amalgams need a retentive cavity

Answer 5

Because they aren’t adhered to the tooth and so rely on mechanical retention

Question 6

What are the limitations of dental amalgams

Answer 6

• brittle in thin sections
• low tensile strength
• susceptible to creep
• breaks under bending force
• they are non-conservative so weaken the tooth
• the sharp angles give high stress concentration

Question 7

What are the benefits of using composites

Answer 7

1. Light cured
2. Etching provides micro mechanical retention
3. Good creep resistance and compressive strength
4. Excellent aesthetics

Question 8

What are the limitations of using composites

Answer 8

1. Polymerisation shrinkage
2. Dentine bonding is weaker than bonding to acid-etched enamel as there are both organic and inorganic phases
3. There is high wear in contact areas

Question 9

What are the benefits of using GICs

Answer 9

1. Natural adherence to tooth
2. No shrinkage
3. Fluoride release and uptake
4. Ultra conservative restoration
5. Aesthetics

Question 10

What are the limitations of using GICs

Answer 10

1. Low strength and high wear at early stage
2. Needs protection from dehydration
3. Low diametral tensile strength
4. Low tensile strength
5. Unable to withstand high tensile loads
6. Restoration must be supported by tooth tissue

Question 11

What is the issue with dental materials being homogeneous and isotropic

Answer 11

This means they are not dynamic structures that can adapt to the changing loads and so there will be compromised stress distribution; this is different to the original enamel and dentine which are both anisotropic

Question 12

What are the methods for stress analysis

Answer 12

1. Photoelasticity

2. Finite element analysis

Question 13

What is photo elasticity stress analysis

Answer 13

An experimental technique based on bifringence
- ray of light passing through bifringent material has two refractive indices and the magnitude of each is directly related to the state of stresses at that point within the material

Question 14

What is finite element stress analysis

Answer 14

Computer modelling by producing a mesh complex shape

Question 15

Where is the stress location with bonded restorations e.g. composties

Answer 15

Higher stresses at the cusp region causing cusp flexion

Question 16

Where is the stress location with non-bonded restorations e.g. amalgams

Answer 16

Stress is concentrated around the sharp corners of the cavity design

Question 17

Where will cracks occur in bonded restorations

Answer 17

Within the enamel in contact to the opposing tooth as it will undergo cuspal flexion

Question 18

Where will cracks occur in non-bonded restorations

Answer 18

Internal line angles and EDJ where the stress in concentrated

Question 19

What can minimise crack formation

Answer 19

Having perfect contact between the tooth and restoration ensuring that stress will be uniform and minimised along the interface