Dental Material Properties: Stress, Strain, and Biocompatibility
Understanding Dental Material Properties
What is Stress?
Stress is the internal reaction of an object to an external force that attempts to deform it. It has equal intensity but acts in the opposite direction to the external force. Stress can be caused by:
- Traction: Pulling the object
- Compression: Compressing the object
- Shear: Compressing the object, but not along the same line
What Can Happen to a Filling?
A filling can:
- Break
- Fall out
- Wear down
- Become distorted
- Remain intact
What is Strain? What is Recoverable Deformation?
Strain is the change in length of an object when stress is applied. Recoverable deformation occurs when the object returns to its original shape after the stress is removed.
Fractures of an Object
Fractures occur when the object has low resistance compared to the applied stress.
Malleability vs. Ductility
Malleability is the property of a material that allows it to undergo large compressive strains without breaking when cold, such as being compressed into plates.
Ductility is the property of a material that allows it to undergo large tensile strains without breaking when cold, such as being stretched into wire.
What is Resiliency?
Resiliency is the maximum amount of energy a deformed object can withstand and still recover its original shape after the stress is removed.
What is Tenacity?
Tenacity is the energy required to deform a material until it breaks.
Forces That Can Break a Material
Tensile, compressive, and shear forces can break a material.
How Does a Tooth Break?
A tooth typically breaks due to compression or shear force.
What Does Hardness Tell Us?
Hardness measures the resistance of a material to being scratched or penetrated.
Fatigue
A material experiences fatigue when subjected to intermittent stress over a long period. These stresses cause micro-cracks that eventually lead to fracture.
Thermal Conductivity
Thermal conductivity is the amount of heat (in calories per second) that passes through a body when a temperature difference of 1°C is applied.
Specific Heat
Specific heat is the amount of heat needed to raise the temperature of a material by 1°C.
Coefficient of Thermal Expansion
The coefficient of thermal expansion is the fractional increase in length for each 1°C increase in temperature.
Importance of Thermal Expansion
Thermal expansion is important because expansion and contraction in the mouth can cause leaks in fillings or larger restorations, which can directly affect the patient’s dentin.
Biocompatibility
Biocompatible materials do not cause adverse reactions, even if they trigger some bodily responses.
Elastic vs. Inelastic Impression Materials
Elastic impression materials, especially hydrocolloids and elastomers, are currently more widely used.
Hydrocolloids: Mixtures/Solutions
A hydrocolloid is a colloidal suspension, a mix between a solution and a suspension. Some particles dissolve completely in water, while others are mixed but do not dissolve. Hydrocolloids can be shaped and re-shaped between SOL and GEL states.
Difference Between Solution and Suspension
Solution: Liquid (solvent) + Powder/Solid (solute), e.g., water with salt.
Suspension: Liquid (solvent) + Powder/Solid (no solute), e.g., water with soil (mud).
Reversible and Irreversible Hydrocolloids
Reversible (Agar): Complex handling but good print results, comes in two tubes in gel form.
Irreversible (Alginate): More convenient and widely used than agar, which began to be used for transport during World War II in the 1940s.
Difference Between SOL and GEL
SOL: A low-viscosity liquid with polysaccharide chains placed randomly (when we mix alginate).
GEL: A more viscous material with elastic properties (alginate when fabricated).