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- L_o)/(L_o); 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

Finishing and Polishing

Remove oxygen-inhibited layer .Use stones or carbide burs for gross reduction.Use highly fluted carbide burs or special diamonds for fine reduction.Use aluminum oxide strips or disks for finishing. Use fine aluminum oxide finishing pastes. Microfills develop smoothest finish because of small size of filler particles
 

I . Procedure for single casting :

A 2.5 mm sprue former is recommended
for molar crowns 2.0 mm for premolars & partial coverage crowns .

II . Procedure for multiple casting :

Each unit is joined to a runner bar .

A single sprue feeds the runner bar

4 . SPRUE FORMER DIRECTION
Sprue Should be directed away from the delicate parts of the pattern
It should not be at right angles to a flat surface .(leads to turbulance  porosity .)
Ideal angulation is 45 degrees .

5 . SPRUE FORMER LENGTH

Depends on the length of casting ring .. Length of the Sprue former should be such that it keeps the wax pattern about 6 to 8 mm away from the casting ring. Sprue former should be no longer than 2 cm. The pattern should be placed as close to the centre of the ring as possible.

Significance

Short Sprue Length:

The gases cannot be adequately vented to permit the molten alloy to fill the ring completelyleading to Back Pressure Porosity.

Long Sprue Length:

Fracture of investment, as mold will not withstand the impact force of the entering molten alloy.

Top of wax should be adjusted for :

6 mm for gypsum bonded investments .

3 -4 mm for phosphate bonded investments .
TYPES OF SPRUES

I . - Wax . II . Solid

- Plastic . Hollow
- Metal .

Spruing Technique:

Direct Spruing:

The flow of the molten metal is straight(direct) from the casting crucible to pattern area in the ring. Even with the ball reservoir, the Spruing method is still direct. A basic weakness of direct Spruing is the potential for suck-back porosity at the junction of restoration and the Sprue.

Indirect Spruing:

Molten alloy does not flow directly from the casting crucible into the pattern area, instead the alloy takes a circuitous (indirect) route. The connector (or runner) bar is often used to which the wax pattern Sprue formers area attached. Indirect Spruing offers advantages such as greater reliability & predictability in casting plus enhanced control of solidification shrinkage .The Connector bar is often referred to as a “reservoir .

Armamentarium :
1 . Sprue
2 . Sticky wax
3 . Rubber crucible former
4 . Casting ring 
5 . Pattern cleaner 
6 . Scalpel blade & Forceps 
7 . Bunsen burner

COMPOSITE RESINS

Types

• Amount of filler-25% to 65% volume, 45% to 85% weight
• Filler particle size (diameter in microns)
  • Macrofill 10 to 100 µm (traditional composites)
  • Midi fill- 1 to 10 µm(small particle composites)
  • Minifill— 0.l to 1 µm
  • Microfill-: 0.01 to  0.1 µm (fine particle composites)
  • Hybrid--blend (usually or  microfill and midifill or minifill and microfill)
• Polymerization method
  • Auto-cured (self-cured)
  • Visible light cured
  • Dual cured
  • Staged cure
• Matrix chemistry
  • BIS-GMA type
  • Urethane dimethacrylate (UDM or UDMA) type
  • TEGDMA-diluent monomer to reduce  viscosity

Pit-and-Fissure Dental Sealants

Applications/Use

Occlusal surfaces of newly erupted posterior teeth
Labial surfaces of anterior teeth with fissures
Occlusal surfaces of teeth in older patients with reduced saliva flow (because low saliva increases the susceptibility to caries)

Types

Polymerization method

Self-curing (amine accelerated)
Light curing (light accelerated)

Filler content

Unfilled-most systems are unfilled because filler tends to interfere with wear away from self-cleaning occlusal areas(sealants are designed to wear away, except where there is no self-cleaning action a common misconception is that sealants should be wear resistant)

Components

Monomer-BIS-GMA with TEGDM diluent to facilitate flow into pits and fissures prior to cure
Initiator-benzoyl peroxide (in self-cured) and diketone (in light cured)
Accelerator-amine (In light cured)
Opaque filler-I % titanium dioxide. or other colorant to make the material detectable on tooth surfaces
Reinforcing filler-generally not added because wear resistance is not required within pits and fissures

Reaction-free radical reaction 

Manipulation

Preparation

Clean pits and fissures of organic debris. Do not apply fluoride before etching because it will tend to make enamel more acid resistant. Etch occlusal surfaces, pits, and fissures for 30 seconds (gel) or 60 seconds (liquid) with 37% phosphoric acid . Wash occlusal surfaces for 20 seconds. Dry etched area for 20 seconds with clean air spray. Apply sealant and polymerize

Mixing or dispensing

Self-cured-mix equal amounts of liquids in Dappen dish for 5 seconds with brush applicator. Light cured-dispense from syringe tips 
Placement

-pits, fissures, and occlusal surfaces  --> Allow 60 seconds for self-cured materials to set. 

Finishing

Remove unpolymerized and excess material .Examine hardness of sealant. Make occlusal adjustments where necessary in sealant; some sealant materials are self-adjusting

Properties

Physical

Wetting-low-viscosity sealants wet acid etched tooth structure the best

Mechanical

Wear resistance should not be too great because sealant should be able to wear off of  self-cleaning areas of tooth
Be careful to protect sealants during polishing procedures with air abrading units to prevent sealant loss

Clinical efficacy

Effectiveness is 100% if retained in pits and fissures .Requires routine clinical evaluation for resealing of areas of sealant loss attributable to poor retention .
Sealants resist effects of topical fluorides