leen al-hunaiti 2019-03-05آ  material increases, cushioning effect becomes better but its...

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  • 7

    Leen Al-Hunaiti

    Dr.Nisreen Alsalem

  • ⁂ Porosity:

    -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.

  • ⁂Processing Strains:

    -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

    defects) occur.

    *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).

    ⁂Mechanical Properties:

  • 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

    be improved.

    *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.)

  • ⁂Chemical Properties:

    -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

    mechanical properties)

    *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

    resist wear.)

    (all of the previous parts are “acrylic”.)

    (Statically Acceptable).

    (It should resist

    wear, and allow

    adjustment)

    (Must be durable)

  • 1-Porcelain Teeth:

    .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).

    -Better esthetics

    -Biocompatible

    -Mechanical retention to denture base

    -Difficult to adjust

    -Produce clicking sound

    -Wear opposing natural teeth

    -Transmit greater forces to supporting tissue

    -Heavy

    2-Acrylic Teeth:

    -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)

    -Easily wear

  • *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

    represent porcelain

    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