NEET MDS Lessons
Dental Materials
PHYSICAL PROPERTIES OF MATERIALS
Definite and precise terms are used to describe the physical properties of dental materials.
a. Hardness. Hardness is the measure of the resistance of a metal to indentation or scratching. It is an indication of the strength and wearability of an alloy or metal.
b. Ductility. Ductility is the measure of the capacity of a metal to be stretched or drawn by a pulling or tensile force without fracturing. This property permits a metal to be drawn into a thin wire.
c. Malleability. Malleability is the measure of the capacity of a metal to be extended in all directions by a compressive force, such as rolling or hammering. This property permits a metal to be shaped into a thin sheet or plate.
d. Flexibility and Elasticity. These terms differ in their technical definition but they are very closely related. Flexibility is the characteristic of a metal, which allows it to deform temporarily. The elasticity of a metal is used when it returns to its original shape when the load or force is removed.
e. Fatigue. Fatigue is the property of a metal to tire and to fracture after repeated stressing at loads below its proportional limit.
f. Structure (Crystalline or Grain Structure). Metals are crystalline and many of their physical properties depend largely upon the size and arrangement of their minute crystals called grains.
(1) Grain size. The size of the grains in a solidified metal depends upon the number of nuclei of crystallization present and the rate of crystal growth. In the practical sense, the faster a molten is cooled to solidification, the greater will be the number of nuclei and the smaller will be the grain size. Generally speaking, small grains arranged in an orderly fashion give the most desirable properties.
(2) Grain shape. The shape of the grains is also formed at the time of crystallization. If the metal is poured or forced into a mold before cooling, the grains will be in a flattened state. Metal formed by this method is known as cast metal. If the metal is shaped by rolling, bending, or twisting, the grains are elongated and the metal becomes a wrought wire.
g. Crushing Strength. Crushing strength is the amount of resistance of a material to fracture under compression.
h. Thermal Conductivity. Thermal conductivity is defined as the ability of a material to transmit heat or cold. A low thermal conductivity is desired in restorative materials used on the tooth whereas a high thermal conductivity is desirable where the material covers soft tissue.
Manipulation
Selection
o Microfilled composites or hybrids for anterior class III, IV, V
o Hybrids or midifills for posterior class I, II, III, V
Conditioning of enamel and / or dentin
Do not apply fluorides before etching.-->Acid-etch --> Rinse for 20 seconds with water --> Air-dry etched area for 20 seconds but do not desiccate or dehydrate --> Apply bonding agent and polymerize
Mixing (if required)--> mix two pastes for 20 to 30 seconds
o Self-cured composite-working time is 60 to 120 seconds after mixing
o Light-cured composite-working time is unlimited (used for most anterior and some posterior composite restorations)
o Dual-cured composite-working time is > 10 minutes
o Two-stage cured composite-working time is >5 minutes
Placement
use plastic instrument or syringe --> Light curing --> Cure incrementally in <2 mm thick layers. Use matrix strip where possible to produce smooth surface and contour composite .Postcure to improve hardness
Manipulation
1. Selection-based on strength for models, casts, or dies
2. Mixing
(1)Proportion the water and powder
(2) Sift powder into water in rubber mixing bowl
(3) Use stiff blade spatula to mix mass on side of bowl
(4) Complete mixing in 60 seconds
3. Placement
(1) Use vibration to remove air bubbles acquired through mixing
(2) Use vibration during placement to help mixture wet and flow into the impression
Applications/Use
- Load -bearing restorations for posterior teeth (class I, II)
- Pinned restorations
- Buildups or cores for cast restorations
- Retrograde canal filling material
(1) Alloy. An alloy is a solid mixture of two or more metals. It is possible to produce a material in which the desirable properties of each constituent are retained or even enhanced, while the less desirable properties are reduced or eliminated.
(2) Amalgam. When one of the metals in an alloy mixture is mercury, an amalgam is formed. A dental amalgam is a combination of mercury with a specially prepared silver alloy, which is used as a restorative material.
(3) Mercury. Mercury is a silver-white, poisonous, metallic element that is liquid at room temperature
Structure of gypsum products
Components
a. Powder (calcium sulfate hemihydrate = CaSO4½H2O)
b. Water (for reaction with powder and dispersing powder)
Investment Techniques
Single step investing technique:
The investing procedure is carried out in one step either by brush technique or by vacuum technique.
a). Brush technique:
The accurate water-powder ratio is mixed under vacuum. A brush is then used to paint the wax pattern with mix then the casting ring is applied over the crucible former and the ring is filled under vibration until it is completely filled.
b). vacuum technique:
• The mix in first hand spatulated, and then with the crucible former and pattern is place, then ring is attached to the mixing bowl.
• The vacuum hose is then attached to the assembly. The bowel is inverted and the ring is filled under vacuum and vibration
Two-step investing technique:
The investing procedure is carried out in two steps:
• First, the wax pattern is painted with a thick mix andis left till complete setting, the set investment block(first cost) is immersed in water for about tenminutes . the casting ring is then applied over the crucible former and filled with the properly mixedinvestment (second coat) till the ring is completely filled and the mix is left to set.The two-step investing technique is recommendedwhenever greater amount of expansion is required. Thistechnique also minimizes the distortion of the waxpattern and provides castings with smoother surfaces.
• The investment is allowed to set for the recommendedtime (usually one-hour) then the crucible former is removed. If a metal sprue former is used, it is removedby heating over a flame to loosen it from the wax pattern. Any loose particles of investment should beblown off with compressed air should be placed in a humidor if stored overnight.
Introduction
The science of dental materials involves a study of the composition and properties of materials and the way in which they interact with the environment in which they are placed
Selection of Dental materials
The process of materials selection should ideally follow a logical sequence involving
(1) analysis of the problem,
(2) consideration of requirements,
(3) consideration of available materials and their properties, leading to
(4) choice of material.
Evaluation of the success or failure of a material may be used to influence future decisions on materials selection.