Compound Structures Flashcards
What is a tooth to a bioengineer?
Highly sophisticated functional microstructure not yet fully understood
Damaged structure = cannot be properly repaired with today’s technology
What is a tooth to a dentist?
Block of enamel sitting on dentine
Can be filled
What is the elastic modulus, fracture strength and compressive strength for enamel?
Elastic module (high = brittle) = 85GPa Fracture strength = 10MPa Compressive strength = 400MPa
What is the elastic modulus, fracture strength and compressive strength for dentine?
Elastic module = 15GPa
Fracture strength = 50MPa
Compressive strength = 300MPa
What to restore?
Structure Form and colour/shade Resist functional/parafunctional forces Resist wear and degradation Resist static and dynamic fatigue
Mechanical properties of enamel?
Hard
Brittle
Crystalline = translucency
Mechanical properties of dentine?
Porous
Tough
Softer
Absorbs load from enamel
Mechanical properties of PDL?
Visco-elastic
Active suspension system = will harden or soften depending on the situation BUT fails in bruxism and epilepsy = large loads of teeth, exceeds structural strength
How to restore the compound structure of teeth?
Using compound systems made up of components with desirable properties
Make bio-mechanical design considerations
Use effective and reliable adhesive interface assembly
What are the relevant properties of restorative materials?
Elastic modulus Compressive strength Flexural strength Fracture toughness Coefficient of thermal expansion Wear behaviour Fatigue behaviour
Coefficient thermal expansion of enamel and dentine?
10ppm degrees C
Coefficient thermal expansion of composite? Relevance of this?
50ppm degrees C= pushes tooth apart = enamel fractures
Define a composite material
Single entity containing 2 or more constituent phases
What does composite structure of composite filler alter the properties and behaviour of the material?
Reduces polymerisation shrinkage
Limits fracture propagation
Increases wear resistance
Improves optical properties
Why are compound structures needed?
To restore compound structures
Compensate for inadequate properties in otherwise useful materials
Utilise materials with different properties
Create a system which utilises the most desirable property
Types of compound systems?
Enamel bonded to dentine Composite bonded to ionomers Beta quartz inserts in composite Porcelain bonded to metal Porcelain bonded to alumina Glass infiltrated ceramic crowns Porcelain acrylic metal dentures
How to join materials in a compound system?
Specific adhesion
Micromechanical adhesion
Mechanical interlocking
Joining materials in a compound system?
Moulding around base materials Casting against 2nd material Welding Soldering Adhesives
Interactions between materials in a compound system?
Desired benefits of the compound structure
Unavoidable drawbacks of materials interactions
What are the useful interactions between materials in a compound system?
Complementary properties - aesthetic vs strength
Mechanical protection
Adhesion
What are the unwanted interactions between materials in a compound system?
Electromechanical corrosion
Mismatched thermal expansion interface damage
Unstable dimensional form
What are the constitutional forms used in restorative dentistry?
Block form e.g. restoration (filling)
Shell form e.g. crown
Laminate form
How to achieve success with compound systems?
Match the restorative system to the remaining substrate by means of:
1. Appropriate choice of compound system
2. Appropriate choice of restorative design construct
Compensate for inadequacies of materials by design construct
Effective and reliable adhesive assembly
Features of gold shell crown?
Ductile and easily deformed
360 degree wrap prevents deformation
Features of adhesive porcelain crown?
Brittle and easily fractured if deformed
Adequate bulk prevents deformation
What to take into account to achieve an effective and predictable adhesive assembly?
Strength of bond to restorative substrate
Strength of bond to tooth
Durability of the chemistry
Cohesive strength of the component materials
Resistance of the lute to surface degradation and wear
Desirable properties of an enamel replacement?
High elastic modulus Tooth coloured Reliable bonding High strength Resistant to mechanical fatigue
Desirable properties of a dentine replacement?
Low elastic modulus Adhesive Resilient to absorb shocks F releasing Resistant to degradation
