Compound systems Flashcards
Enamel and dentine
- elastic modulus
- fracture strength
- compressive strength
85 GPa / 15 GPa
10 MPa / 50 MPa
400 MPa / 300 MPa
Teeth as compound structures
Enamel is hard and brittle with low tensile strength (like glass)
-provides external hardness and rigidity to masticate and not wear down
Dentine is soft and tough, less brittle and able to absorb loads (like wood)
-supports enamel and absorbs masticatory loads that are subsequently transmitted and distributed through root mass and alveolar bone via PDL
PDL is a pro-prioceptive active suspension system that will absorb loads and retain tooth in bone with element of ‘cushioning’ (does not exist in implants - ankylosed to bone)
Alveolar bone
What is a composite material?
A single entity containing 2 or more constituent phases
e.g. dental composite restoratives
What is a compound system?
Making materials work with each other
A Functional Unit comprising
two or more constituent materials
e.g. Porcelain-bonded to metal crowns
The need for compound structure
Need to restore compound structure
Need to restore form and colour/ shade
Need to resist functional/ parafunctional forces
Need to resist wear and degradation
Need to resist static and dynamic fatigue
–>to compensate for inadequate properties in otherwise useful materials
–>to utilise materials with different properties
–>to create a system which utilises the most desirable property of each consituent material
Relevant properties of restorative materials
Elastic modulus Compressive strength Flexural strength Fracture toughness Coefficient of thermal expansion Hygroscopic expansion Wear behaviour Fatigue behaviour
Properites of composite fillers
Composite structure alters the properties and behaviour of the material Reduces polymerisation shrinkage Limits fracture propagation Increases wear resistance Improves optical properties
Examples of compound systems
Enamel bonded to dentine Composites bonded to ionomers Beta quartz inserts in composites Porcelain bonded to metal Porcelain bonded to alumina Glass infiltrated ceramic crowns Porcelain-acrylic-metal dentures
Methods of joining materials in a compound system
Specific adhesion Micromechanical adhesion Mechanical interlocking Moulding around base material Casting against second material Welding Soldering Adhesives
Useful interactions between materials in a compound system
Complementary properties
–e.g. Aesthetics vs. Strength
Mechanical Protection
Adhesion
Unwanted interactions between materials in a compound system
Electromechanical corrosion
Mismatched thermal expansion interface damage
Unstable dimensional form
Constructional forms used in restorative dentistry
Block form: a solid object
-intra-coronal restorations such as a block of amalgam, GIC, resin composite, ceramic inlay, gold inlay
Shell form: an outer full coverage
-any form of extra-coronal partial or full-coverage crown
Laminate form - a thin material that replaces labial or palatal surfaces and obtains its strength by nature of bonding to underlying enamel / dentine
Coefficient of thermal expansion
- tooth
- composte
Tooth: 10ppm/ degree C
Composite: 50ppm/ degree C
Ideal properties of dentine replacements
Low elastic modulus -matching dentine Adhesive Resilient to absorb shocks Fluoride-releasing Resistant to degradation
Ideal properties of enamel replacements
High elastic modulus -matching enamel Tooth-coloured Reliable bonding mechanism High strength Abrasion matched to enamel Resistant to mechanical fatigue Resistant to static fatigue
