Majed Sharayha
Posts : 130
Reputation : 5
Join date : 2011-08-23
Age : 31
 |  Subject: Sheet 1, Dr. Mohammad Rabab'a, 1522012  Sat Feb 18, 2012 5:33 am | |
| Download Link-------------------> Click here------------------------------------------------------------------- -The restorative materials are :
* Direct vs. Indirect
* Temporary vs. Permanent
* Fixed vs. Removable
-We are going to talk about the amalgam ( حشوات الفضة \ الرصاص)
- We use the composite fillings in the anterior teeth ( الحشوات التجميلية \ البيضاء)
- We can use the minerals as dental materials :
* Precious Alloys ( gold, platen )
* Non-Precious Alloys ( cheaper, and have mechanical advantages over the precious alloys )
* Acrylics, Polymers
- Main categories of dental materials :
* Polymers ( اللدائن ,يستخدم في الأطقم )
* Ceramists
* Metals and Metals Alloys ( strongest, handle the environment in the oral cavity)
- To make an indirect crown we must take an impression to have the similar structure of the original tissue.
- To select a dental material, we use an evidence based approach.
- The properties of Dental Materials : Mechanical, Surface, Mixing, Light/Color, Handling, … etc.
- Mechanical Properties : Flexibility, Stress, Expansion and Deflation ( imp. Because we need a material that matches the original tissue).
- Remember that between the enamel and the dentine there is interloping in the dentinoenamel junction and this interloping appear during the formation of these layers.
- Also there are chemical interactions by the Hydroxyabatite.
- The ideal replacement of a tooth structure is by a dental material that carries the same properties.
- The major disadvantage of Amalgam is that it doesn’t interact with enamel or dentine at a micromechanical level. “While composite does”
- The ant. Teeth have an occlusal force about 150 N, while the post. Teeth about 500 N “increased in the cusp/fossa area”
- Dentures reduce the occlusal forces by 15%
- Bulk properties :
1) Stress.
“the material isn’t usually regular, but we describe it this way to facilitate its studying”
* The stress is the force applied on an area “ pressure “ (N/m²) ( Pascal )
* Types : a. Axial : * compressive
* tensile
b. Non-Axial : * shear
* torsion
* bending
* The compression is 10% harder than the tensile stress, because in compression we are approximating the nuclei toward each others at a submicroscopic level, which will cause repulsive forces.
* The Apical Foramen is a very minute tube. So during the root canal treatment, we use a file of a specific dental material, and we apply torsion and bending forces on it. [The type of this material will determine wither this file will break or not. We can use the K. file “stainless steel” or the nickel titanium “which is better because of its super elasticity”]
* An example of the tensile stress is the occlusion pattern of the ant. Teeth, when the lowers push the uppers toward outside.
* Most of the occlusal forces are in many directions, compressive, tensile or shear.
* The response of the material to the stress depends on its type, in the case of tensile stress it will elongate, while in the case of compression it will squeeze.
2) Strain.
* Deformation/ original length (unit less)
* One of the most important experiments is when the scientists check the change in the length of the material when a force is applied to it. “This change could be an elongation to a limit or a fracture “
* The stress-strain curve: a. engineering “which we will discuss “
b. true “more complicated”
* The engineering stress-strain curve is hyperbolic
* From the curve:
- Point A is the proportional limit (proportional relation between the stress and the strain)
- point B is the elastic limit (we call a material an elastic when a force is applied on it, and in response it elongates without exceeding the elastic limit and will return back to its original length )
- The plastic deformation occur when a material exceed the elastic limit and can’t get back to its original length.
- The yield strength is 0.2% of the plastic deformation and it’s easier to measure at the lab.
- The elastic modulus is the stress / strain (Pascal)
we measure it within the elastic range to check if we have any change in the volume in addition to the linear change.
- The Ductility : we use this property to make wires
- The Malleability : we use it to make sheets “ in the golden fillings “
- Point C is the ultimate strength.
- Point D is the Fracture point .
- The area under the curve is the Toughness ( which is the resilient of a material to fracture)
The Quiz :
X: stiff ( no elastic deformation at a low strain )
Ductile ( it show elastic deformation)
Strong and Tough ( because the area under its curve is large)
Y : Flexible ( to the elastic limit)
Brittle ( it will fracture immediately )
Z : Flexible and Ductile.
Done by :Haneen Kharoub | |
|
