NEET MDS Lessons
Dental Materials
Zinc Phoshate Cement
Uses. Zinc phosphate cement is used both as an intermediate base and as a cementing medium.
(1) Intermediate base. A thick mix is used under permanent metallic restoration. This layer of cement protects the pulp from sudden temperature changes that may be transmitted by the metallic restoration.
(2) Cementing medium. Zinc phosphate cement is used to permanently cement crowns, inlays, and fixed partial dentures upon the remaining tooth structure. A creamy mix of cement is used to seat the restoration or appliance completely into place. The cementing medium does not cement two objects together. Instead, the cement holds the objects together by mechanical interlocking, filling the space between the irregularities of the tooth preparation and the cemented restoration
c. Chemical Composition.
(1) Powder. primary ingredients - zinc oxide and magnesium oxide.
(2) Liquid. Phosphoric acid and water in the ratio of two parts acid to one part water. The solution may also contain aluminum phosphate and zinc phosphate Liquids exposed in open bottles will absorb moisture from the air in high humidity. The liquids will lose moisture if humidity is low. Water gain hastens setting; water loss lengthens setting time.
PROPERTIES OF ZINC PHOSPHATE CEMENT
a. Advantages. Some advantages of zinc phosphate cement as a cementing medium are:
o Inconspicuous appearance.
o Speed and ease of usage.
o Sufficient flow to form a thin layer for the cementing of closely adapted crowns, fixed partial dentures, and inlays.
o Low thermal conductivity beneath a metallic restoration.
b. Disadvantages. Some disadvantages of zinc phosphate cement as a cementing medium are:
o Low crushing strength that varies between 12,000 and 19,000 psi.
o Slight solubility in mouth fluids.
o Opaque material not suitable for visible surfaces.
c. Strength. The ratio of powder to liquid increases the strength of phosphate cements to a certain point. For this reason, the dental specialist must use as thick a mix as practical for the work being performed.
SETTING REACTIONS OF ZINC PHOSPHATE CEMENT
a. Chemical Reaction. The chemical reaction that takes place between the powder and liquid of setting phosphate cement produces heat. The amount of heat produced depends upon the rate of reaction, the size of the mix, and the amount of heat extracted by the mixing slab.
b. Powder to Liquid Ratio. The less powder used in ratio to the liquid, the longer the cement will take to harden. Good technique minimizes the rise in temperature and acidity of the setting cement that can injure the pulp. Generally, for increased strength, decreased shrinkage, and resistance to solubility, it is advisable to blend as much powder as possible to reach the desired consistencies.
c. Setting Time. The setting time of zinc phosphate cement is normally between 5 and 9 minutes.
Lower the temperature of the glass mixing slab to between 65° and 75° F (18° to 24° C), if the glass mixing slab is not already cooled below the temperature at which moisture will condense on it. → Blend the powder slowly. → Mix the powder over a large area of the cool slab. → Use a longer mixing time, within optimum limits.
Precautions. The following precautions should be observed.
o Prevent loss or gain of moisture in liquid cement by keeping bottles tightly stoppered.
o Dispense drops only when ready to mix.
o Use a cool, dry glass slab (65° to 75° F).
o Use the same brand of powder and liquid.
o Add increments of powder slowly.
o Use the maximum amount of powder to obtain the desired consistency.
(To incorporate the most powder, the material should be mixed with a moderate circular motion over a large area of the slab, turning the spatula often.)
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
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
Waxes
Many different waxes are used in dentistry. The composition, form, and color of each wax are designed to facilitate its use and to produce the best possible results.
Applications
o Making impressions
o Registering of tooth or soft tissue positions
o Creating restorative patterns for lab fabrication
o Aiding in laboratory procedures
Classification
a. Pattern waxes-inlay, casting, and baseplate waxes
b. Impression waxes-corrective and biteplate waxes
c. Processing waxes-boxing, utility, and sticky waxes
Types
1) Inlay wax-used to create a pattern for inlay, onlay or crown for subsequent investing and casting in a metal alloy.
2) Casting wax-used to create a pattern for metallic framework for a removable partial denture
3) Baseplate wax-used to establish the vertical dimension. plane of occlusion. and initial arch form of a complete denture
4) Corrective impression wax-used to form a registry pattern of soft tissues on an impression
5) Bite registration wax-used to form a registry pattern for the occlusion of opposing models or casts
6) Boxing wax-used to form a box around an impression before pouring a model or cast
7) Utility wax -soft pliable adhesive wax for modifying appliances, such as alginate impression trays
8) Sticky wax-sticky when melted and used to temporarily adhere pieces of metal or resin in laboratory procedures
Components
a. Base waxes-hydrocarbon (paraffin) ester waxes
b. Modifier waxes-carnauba, ceresin, bees wax, rosin, gum dammar, or microcrystalline waxes
c. Additives-colorants
Reaction-waxes are thermoplastic
Properties
Physical
a. High coefficients of thermal expansion and contraction
b. Insulators and so, cool unevenly; should be waxed in increments to allow heat dissipation
Chemical
a. Degrade prematurely if overheated
b. Designed to degrade into CO2and H2Oduring burnout
Mechanical-stiffness, hardness, and strength depend on modifier waxes used
ZINC OXIDE AND EUGENOL
This material is used for many dental purposes ranging from temporary restorative material to pulp capping. The material is composed of a powder that is basically zinc oxide and a liquid that is called eugenol.
Chemical Composition.
The powder must contain between 70 and 100 percent zinc oxide. The manufacturer may add hydrogenated resins to increase strength and zinc acetate to hasten the set.
Eugenol is usually derived from oil of cloves. The oil of cloves contains more eugenol (82 percent) Eugenol is an obtundent (pain-relieving agent). It is a clear liquid that gradually changes to amber when exposed to light.
Physical Properties.
This material relieves pain, makes tissue less sensitive to pain, is slightly antiseptic, and is low in thermal conductivity. It provides a good marginal seal when placed in tooth cavities. The crushing strength (compression strength) of pure zinc oxide and eugenol is about 2,000 psi, which is low in comparison to other cements. The addition of hydrogenated resin increases the crushing strength to 5,000 psi.
CLINICAL USES OF ZINC OXIDE AND EUGENOL
Treatment Restoration. It helps prevent pulpal irritation in carious teeth, lost restorations, advanced caries, or pulpitis. This dental material also exerts a palliative effect on the pulp.
Temporary Cementing Medium. Zinc oxide and eugenol is used as a temporary cementing medium for crowns, inlays, and fixed partial dentures.
Intermediate Base. Zinc oxide and eugenol is used as an intermediate base. This material provides insulation between metallic restorations and vital tooth structure. Because of the low crushing strength, its use is sometimes contraindicated.
Surgical Packing or Dressing. The surgical dressing applied and adapted over the gingival area after a gingivectomy. This dressing protects the area and makes the tissue less sensitive.
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.
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