Basics in dental material ; Physical properties as DIMENTIONAL STABILITY , DENSITY , Solubility of dental material l MCQs in dental material for dental students
PHYSICAL PROPERTIES OF DENTAL MATERIAL
1. DIMENTIONAL STABILITY of dental biomaterial :
- The degree to which a material maintains its original
dimensions. Many materials change shape when they set or harden.
- Impression materials should not change dimensions when set. In addition, dental materials should have no dimensional changes when set.
- Amalgam is a filling material for posterior teeth, it may sometimes change shape permanently as a result of a heavy biting force.
- This is bad property; on the other hand, the investment materials that forms the gold for dental casting should expand for a certain amount to compensate for the contraction of the molten metal after it is cooled from the molten stage.
2. DENSITY of dental material :
- The density, or more precisely, the volumetric
mass density, of a substance is its mass per unit volume.
3. ADHESION, COHESION AND BONDING
- When the molecules of one substrate adhere or are attracted to molecules of the other substrate, the force of attraction is called adhesion when unlike molecules are attracted and cohesion when the molecules involved are of the same kind.
- Bonding is applying an intermediate layer to connect substrates of different materials.
Metamerism:
- Two objects that are matched in color under one light source but are not matched under other light sources form metameric pair.
Surroundings:
- Colors of wall, lips or clothes of the patient modify the type of light reaching the object. Avoid colored and inclined walls, Cover patient’s clothes if so bright. Remove lipstick and heavy makeup.
4. Solubility of dental material:
- Is a chemical property referring to the ability for a given substance, the solute, to dissolve in a solvent.
- Restorative materials should not dissolve in the mouth, and if it is dissolves, it should not release toxic substance.
5. Absorption of fluids of dental biomaterial:
- Some materials will absorb water or other fluids. If it is too much or continued for long time, this will result in serious dimensional changes and the material will be unhygienic. On the other hand, some materials like acrylic water for a day and stop after that, so it is acceptable.
6. Electrical activity of metal dental material :
- When a metal is in contact with a fluid electrolyte, the chemical potential causes enough ions to dissolve to form a saturated solution and produce an equal number of free electrons.
- The loss of electrons by a metal is known as oxidation. It is the ability of metal to ionize by losing electrons.
- If there is a high difference in the electrode potentials of metals in contact with the same solution like gold and aluminum an electric cell may develop and the patient may feel discomfort.
- Dentistry uses a wide variety of metals for restorations and prosthetic devices. A galvanic couple is a corrosive cell that is developed when two different metals are separated by electrolytes.
- In essence, the electrochemical action produced by the reaction of two dissimilar metals.
- The presence of metallic restorations in the mouth may cause a phenomenon called galvanic action, or galvanism. This results from a difference in potential between dissimilar fillings in opposing or adjacent teeth.
- These fillings, in conjunction with saliva or bone fluids such as electrolytes, make up an electric cell.
- This cell short-circuited, and if the flow of current occurs through the pulp, the patient experiences pain and the more anodic restoration may corrode, like gold with amalgam.
Tarnish
- Is a surface discoloration on a metal or a slight loss or alteration of the surface finish or luster.
Corrosion
- Is a process whereby deterioration of a metal is caused by reaction with its environment.
- Galvanic Corrosion is an electrochemical process and is
dependent on the ability to conduct electrical current, either by means of free
electrons in metals or via ions in solution.
7. Tissue reaction and Biocompatibility:
- Biocompatibility:
Ability to be in contact with a living system without producing an adverse
effect.
- Some restorative materials are damaging to the living tissues, which are in contact with, like silicate filling, and zinc phosphate cement, which is acid and may kill the dental pulp unless a protective lining is used.
- Dental materials should not show any allergic reaction to the tissues and also should not provide good culture for the growth of bacteria and candida infection, like soft lining materials.
8. Mechanical Properties:
- All mechanical properties are measures of the resistance of a material to deformation, crack growth, or fracture under an applied force or pressure and the induced stress.
- An important factor in the design of a dental prosthesis is strength, a mechanical property of a material, which ensures that the prosthesis serves its intended functions effectively and safely over extended periods.
- Knowledge of mechanical properties will allow you to differentiate the potential causes of clinical failures that may be attributed to material deficiencies, design features, dentist errors, technician errors, or patient factors such as diet, biting force magnitude, and force orientation.
- All
mechanical properties are measures of the resistance of a material to deformation, crack
growth, or fracture under an applied force or pressure and the induced stress.
Stress:
- Is the force applied per unit area (F/A) produced on the material in response to externally applied force.
- Stress
Units: Kg/Cm2, pound/ inch2, Newton/ m2.
- Strain: is
the change in dimension per unit dimension caused by externally applied forces.
It is a unit less quantity.
Types of Stress
- Every stress is accompanied by a strain of the same typ. There are different types of stresses according to the direction of the applied force.
A- Tensile stress:
- It is the force per unit area produced in the material in response to externally applied force, which tends to stretch or elongate the material. Tensile stress is usually accompanied by tensile strain.
B- Compressive stress:
- It is the force per unit area induced in the material in response to externally applied force, which tends to compress or shorten the material; it is usually accompanied by compressive strain.
- Investment materials, restorative materials & models should have high compressive strength.
C- Shear stress:
- It is the force per unit area induced in the material in response to externally applied force, one part of the force is in one direction while the other is in the opposite direction.
- Shear force is the force, which cause tearing a paper or card. If one part of the crown is in occlusion while the rest is not, shear stress will develop. It is accompanied by shear strain.
- Usually three types of stress occur at the same time, if a piece of metal is being bend it will exhibit tensile stress on the outer surface, compressive on the inner & shear in the middle.
Torsion stress:
- Results from twisting a body.
Stress strain curve
- The application of an external force, producing a stress within a material, results in a change in dimension or strain within the body.
- There is a linear relationship between stress and strain up to the point P. Further increases in stress cause proportionally greater increases in strain until the material fractures at point T.
Proportional limit:
- is the greatest stress produced in material where the stress is directly proportional to the strain. It’s unit is the same of the stress.
Ultimate point:
- it is the greatest force per unit area applied on the material that causes fracture.
Young’s Modulus of Elasticity:
- It’s the constant of the proportionality which equals to any stress value divided by its corresponding strain value, the unit is the same of that of stress , Y= stress/ strain
Flexibility:
- the ability of
a material to deform elastically and return to its original
shape when the applied stress is removed.
Stiffness:
- Stiffness is the rigidity of an object, the extent to which it resists deformation in response to an applied force.
Elastic limit:
- The greatest stress to which the material can be subjected such that it will return to its original shape and dimension when the stress is removed.
- If the strength is increased beyond the elastic limit or the proportional limit the material will deform and if we remove, the stress the material will not return to its original dimension is called plastic deformation.
Ductility:
- It is the ability of the material to withstand permanent deformation under tensile stress without fracture; it depends on plasticity and tensile strength.
- It is the ability of the material to be drawn into a fine wire.
MALLIABILITY:
- It is the ability of the material to withstand permanent deformation under compressive stress without fracture. It is the ability of the material to be drawn into a sheet.
TOUGHNESS:
- It is the total work or energy required to break the material. It is the total area under the stress – strain curve. It requires strength and plasticity.
BRITTLENESS:
- Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a snapping sound. A material is brittle if, when subjected to stress, it breaks without significant plastic deformation.
RESILIANCE:
- The amount of energy absorbed by a structure when it is stressed within the proportional limit.
- The ability of a substance or object to spring back into shape freeing the absorbed energy.
HARDNESS:
- It is the resistance of the material to deformation caused by penetrating or starching the surface.
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