NEET MDS Lessons
Dental Materials
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
COMPOSITE RESINS
Applications / Use
- Anterior restorations for aesthetics (class III, IV, V, cervical erosion abrasion lesions)
- Low-stress posterior restorations (small class I, II)
- Veneers
- Cores for cast restorations
- Cements for porcelain restorations
- Cements for acid-etched Maryland bridges
- Repair systems for composites or porcelains
Polymerization--reaction of small molecules (monomers) into very large molecules (polymers)
Cross-linking-tying together of polymer molecules by chemical reaction between the molecules to produce a continuous three-dimensional network
Mechanical properties
1. Resolution of forces
Uniaxial (one-dimensional) forces-compression, tension, and shear
Complex forces-torsion, flexion. And diametral
2. Normalization of forces and deformatations
Stress
Applied force (or material’s resistance to force) per unit area
Stress-force/area (MN/m2)
Strain
Change in length per unit of length because of force
Strain-(L- Lo)/(Lo); dimensionless units
3. Stress-strain diagrams
Plot of stress (vertical) versus strain (horizontal)
- Allows convenient comparison of materials
- Different curves for compression, tension, and shear
- Curves depend on rate of testing and temperature
4. Analysis of curves
- Elastic behavior
- Initial response to stress is elastic strain
- Elastic modulus-slope of first part of curve and represents stiffness of material or the resistance to deformation under force
- Elastic limit (proportional limit)- stress above which the material no longer behaves totally elastically
- Yield strength-stress that is an estimate of the elastic limit at 0.002 permanent strain
- Hardness-value on a relative scale that estimates the elastic limit in terms of a material’s resistance to indentation (Knoop hardness scale, Diamond pyramid, Brinnell, Rockwell hardness scale, Shore A hardness scale, Mohs hardness scale
- Resilience-area under the stress strain curve up to the elastic limit (and it estimates the total elastic energy that can be absorbed before the onset of plastic deformation)
- Elastic and plastic behavior
- Beyond the stress level of the elastic limit, there is a combination of elastic and plastic strain
- Ultimate strength-highest stress reached before fracture; the ultimate compressive strength is greater than the ultimate shear strength and the ultimate tensile strength
- Elongation (percent elongation)- percent change in length up to the point of fracture = strain x 100%
- Brittle materials-<5% elongation at fracture
- Ductile materials->5% elongation at fracture
- Toughness-area under the stress strain curve up to the point of fracture (it estimates the total energy absorbed up to fracture)
- Time-dependent behavior
the faster a stress is applied, the more likely a material is to store the energy elastically and not plastically
- Creep-strain relaxation
- Stress relaxation
Impression Material
|
Materials |
Type |
Reaction |
Composition |
Manipulation |
Initial setting time
|
|
Plaster |
Rigid |
Chemical |
Calcuim sulfate hemihydrate, water |
Mix P/L in bowl |
3-5 min
|
|
Compound |
Rigid |
Physical |
Resins, wax, stearic acid, and fillers |
Soften by heating
|
Variable (sets on cooling) |
|
Zinc oxide-eugonel |
Rigid |
Chemical |
Zinc oxide powder, oils, eugenol, and resin |
Mix pastes on pad
|
3-5 min
|
|
Agar-agar |
Flexible |
Physical |
12-15% agar, borax, potassium sulfate, and 85% water |
Mix P/L in bowl
|
Variable (sets on cooling)
|
|
alginate |
Flexible |
Chemical |
Sodium alginate, calcium sulfate, retarders, and 85% water |
Mix P/L in bowl
|
4-5 min
|
|
Polysulfide |
Flexible |
Chemical |
Low MW mercaptan polymer, fillers, lead dioxide, copper hydroxide, or peroxides |
Mix pastes on pad
|
5-7 min
|
|
Silicone |
Flexible |
Chemical |
Hydroxyl functional dimethyl siloxane, fillers, tin octoate, and orthoethyl silicate |
Mix pastes on pad
|
4.5 min
|
|
Polyether |
Flexible |
Chemical |
Aromatic sulfonic acid ester and polyether with ethylene imine groups |
Mix pastes on pad
|
2-4 min
|
|
Polyvinyl siloxane |
Flexible |
Chemical |
Vinyl silicone, filler, chloroplatinic acid, low MW silicone, and filler |
Mix putty or use two-component mixing gun
|
4-5 min
|
Solution Liners (Varnishes)
Applications
o Enamel and dentin lining for amalgam restorations
o Enamel and dentin lining for cast restorations that are used with non adhesive cements
o Coating over materials that are moisture sensitive during setting
Components of copal resin varnish
o 90% solvent mixture (e.g., chloroform, acetone, and alcohol)
o 10% dissolved copal resin
Reaction
Varnish sets physically by drying Solvent loss occurs in 5 to 15 seconds (a film forms the same way as drying fingernail polish)
Manipulation
Apply thin coat over dentin. enamel. And margins of the cavity preparation Dry lightly with air for 5 seconds Apply a second thin coat Final thickness is 1 to 5 µ.m
Properties
o Physical
Electrically insulating barrier that prevents shocks. Too thin to be thermally insulating. Decreases degree of percolation attributable to thermal expansion
o Chemical
Forms temporary barrier that prevents microleakage into dentinal tubules until secondary dentin formation occurs. Decreases initial tendency for electrochemical corrosion
o Mechanical
Very weak and brittle film that has limited lifetime
Film adheres to smear layer