leen al-hunaiti 2019-03-05آ material increases, cushioning effect becomes better but its...
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-Porosity means that we can find pores; either on the surface or under
the surface of the final result of the denture.
-There are different types of porosity (different reasons contributed in making these different types of porosity):
1) Contraction Porosity: Caused by: a) Polymerization Shrinkage. (refer to sheet #6 page 21)
b) Insufficient pressure when packing. (It is under our control).
-As we know, when we make the mould in the dough stage we put it in a
flask and we apply enough(sufficient) pressure to ensure that the dough
spreads in all the mould sample).
c) Insufficient amount of acrylic dough.
d) Resin should be packed in the dough stage (the good time). (for
example, if the resin was packed in earlier stages, pressure will lose its
effect, so contraction porosity will happen).
-Today’s lecture is the 2nd part of the previous one.
-Some information were excluded because the doctor mentioned them in the previous lecture.
-The last page of the sheet summarizes the most important things in the lecture.
-All slides are included. Good luck!
2) Gaseous Porosity: (Remember that the boiling temperature of the monomer + exothermic reaction heat= 100.8 ͦ ) . If
we exceeded this boiling point, volatilization of the monomer occurs.
(monomer converts to gas due to the high temperature).
3) Granular Porosity:
This type of porosity happened
a) Due to loss of monomer while resin mix is left to stand until dough stage is reached (since monomer is considered volatile). Remember that
the container must be well-closed until reaching the dough stage.
(Dough becomes dry if the container was not well- closed, and this leads
to granular porosity.)
b) If we didn’t put enough amount of the monomer (used improper ratio of powder and liquid), so if there wasn’t enough liquid, the mix is going
to be dry and denture surface will appear opaque and blotchy.
-As we know from the previous lecture, flask is put in a 74 ͦ water bath
in addition to the exothermic reaction’s temperature (very hot) . So after
8 hours (one complete cycle), when we take out the flask we shouldn’t
put it in a very cold area; because there will be internal stresses and
therefore crease lines, warpage, distortion, and crazing (tiny surface
*Internal stresses can be minimized by:
1-Slowly cooling the flask.
2- Using acrylic rather than porcelain teeth to ensure compatible
shrinkage. (We don’t use porcelain teeth because their properties differ
from the properties of our teeth).
This slide wasn’t mentioned
by the doctor.
⁂Properties of Resin:
1) Biocompatibility: In general, all the materials that we use in the oral cavity are biocompatible. But remember: Biocompatibility deceases
if residual monomers increase (because these materials may leach in the
patient’s mouth, and some people may be allergic to them).
*For people who are allergic to such materials, we should use
alternatives like: light cured resin because it doesn’t contain the
monomer in its composition. Other materials like: Polycarbonate, Vinyl
acrylics may be used as alternatives.
2) Dimensional Stability and Accuracy: it depends on the way of processing; if we respected the ratios, decreased the internal
spaces, etc. the dimensional stability will increase.
*Impression should match the final denture. (if changes happened while
converting the impression into the denture, the fitting of the denture will
change, thus it won’t be suitable for the patient’s teeth).
*retention means: resistance to displacement in occlusal direction (the fit of the material isn’t changed).
1) Creep (to flow under pressure): It is a problem, and it is minimized by crosslinking agents. (so here is one of the advantages of crosslinking agents)
2) Poor/low impact strength of the acrylic: If the denture fell down, it might fracture (it has no resiliency), so patients are advised
to clean the denture at half-filled basin in order not to fracture.
3) Poor resistance to fatigue: -Happens because of the high & continuous load on the denture.
-May result from wearing ill-fitting denture.
4) Vinyl Acrylics are tougher than PMMA and permit larger deformation before fracture.
5) Vinyl Acrylics have higher impact strength compared to PMMA.
6) Vinyl Acrylics have higher abrasion resistance. (Abrasion resistance is lower for pour type acrylics).
**Vinyl Acrylics have better properties in general but they are not
commonly used because they are expensive.
7) In some cases, patients commonly fracture their
dentures, so the solutions are:
a) Using “High Impact Resistant Resins” which contain rubber
toughening agent (Decrease cracks, but may lower flexural modulus and
lead to fatigue due to excessive flexure).
b) Incorporation of fibers to produce fiber reinforced resin:
1-Carbon Fibers. 2-Aramid Fibers. 3-Glass Fibers.
*Carbon fibers: Poor esthetics is the biggest problem in these fibers.
(they darken the color of the denture).
-Need surface treatment: means that the bond with the denture base must
*Patients complain they don’t enjoy the taste of hot drinks; because of
low thermal conductivity.
*Coefficient of thermal expansion is high: it means that small change in
temperature reflects big change in the dimension.
(This range isn’t critical in the oral cavity.)
-Patients are instructed to wash the denture after each meal. Each
denture has some sort of roughness, and it contains small pores (may be
invisible). With bad oral hygiene, remnants of food and many other
causes, Candida albicans may occur in the oral cavity.
-Patient must know where to put the denture, not any bleaching solution
is suitable for the denture; because they could affect the mechanical
properties of the denture (weaken the denture) and change the color of
the denture (blotchy/ white).
(remember: hot water affects the
*This is a cross section of a
denture, notice the white, blotchy
appearance of the denture. (because
of the solution)
⁂Artificial Teeth: 1-Porcelain. 2-Acrylic.
*Acrylic teeth have different sizes, different colors, …
*Tooth’s shape: 1-Neck (gingival part) near the base
(not highly cross-linked; to give flexibility in order to
bond to the denture’s base)
2-Middle part. 3-Incisal part (or Occlusal part).
(highly cross-linked; because it should be very strong to
(all of the previous parts are “acrylic”.)
(It should resist
wear, and allow
(Must be durable)
.during firing shrinkagethan required molds are used to allow for Larger-
-Small pins are added to the base for retention to the denture base.
(because porcelain cannot chemically bond to the base of denture, it
bonds by a mechanical method).
-Mechanical retention to denture base
-Difficult to adjust
-Produce clicking sound
-Wear opposing natural teeth
-Transmit greater forces to supporting tissue
-Produced in reusable molds using dough technique or injection
-Made from highly cross-linked resin to resist crazing
-Pigments added to simulate natural teeth color
-Base and core made from lightly cross-linked resin to allow softening
when in contact with monomer from denture base
)advantage(Most important to denture base material Chemical bond -
-Easy to grind during occlusal adjustment. (porcelain is hard to grind)
-Do not wear natural, artificial opposing teeth (advantage #2)
-Easily re-polished (same as grinding)
-Compatible with denture base material (strong chemical bond)
-Stain over time (While in porcelain: less staining)
*Which of the 2 previous types would you expect to do less
resorption (be kinder to the tissue)?
Acrylic type, because it is lighter, because of less wear of the opposing
teeth; so the load that is transformed to the ridge is lesser compared to the porcelain.
*The left teeth
teeth (because we used
a mechanical mean);
we can identify the
small metal pins
(where the tooth will
attach to the base).
⁂Maxillofacial Materials: (Ex: defects in eye/ nose/ear processes)
Materials used: o PMMA: hard and stiff. o P