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 Polyurethane. o Room temperature vulcanized silicons: good physical

and mechanical properties, similar to addition silicon.

Fabrication: similar to denture construction: impression, master cast is poured, wax pattern, try in, investment and flasking.

Properties: tensile strength is important. (for

example, if the patient’s nose was replaced, there will

be a gap between the nose & the face if there was no

tensile strength.

⁂Denture Lining Materials: (New important

subject)

*Remember: One of the uses of the acrylic resin is relining the

denture.

*Divided into:

1)Hard reline materials

2)Soft reline materials:

a-Semi-permanent soft lining material

b-Temporary soft lining material

c-Tissue conditioners

1-Hard reline materials:

Criteria for using it to reline dentures: Poor retention and stability (denture is moving from its place)

Loss of vertical dimension (wear) Degradation of the denture base (denture base

contains pores) For older patients for whom getting use to a new

denture base would be difficult (it is difficult for old patients to get used to a new denture from scratch)

Lack of denture extension into mucobuccal fold areas (important for facial support)

*Materials used in hard relining:

1-Heat cure resin (needs more time), it is done in the lab.

2-Cold cure resin, chairside (used in urgent cases). Disadvantages of cold cure resin:

Poor taste

Poor color stability

Exothermic reaction

Lack of control over amount of denture removed & thickness of reline

*Important note (before we continue): If the softness of the

material increases, cushioning effect becomes better but its life-span will

be shorter; (very soft means that it has high amount of lubricantthe

material will be rapidly leachingthe material becomes harder, and if

the material became hard, we should replace it) and that’s why the

material’s life span is short.

-On the other side, if the softness was moderate , the lubricant will be

less slow leaching of the material the material becomes durable.

**Note: when the material becomes hard (rigid), we know that it must be replaced.

2- Soft reline materials: (they differ from each other in the level

of softness)

Tissue conditioners (softest) > Temporary soft lining material >

Semi-Permanent.

*Semi-permanent material is long lasting and has the less softness.

*Tissue conditioners are used for short period of time and they have

a quick effect (they also have less serviceability).

a) Temporary soft lining materials.

Similar to tissue conditioners in application and composition (tissue conditioners are used for short period of time, and they need continuous replacements)

Not as soft as tissue conditioners but remain soft for up to a month or two. (if the patient has a trauma, we use the temporary soft lining material)

Slightly viscoelastic

Uses: When frequent replacement of conditioner is not possible. To ascertain if permanent liners would be beneficial. To improve fit of denture until a new denture is made.

b) Permanent soft liners: (Done for patients who

have trauma/ severe ridge resorption/ patients who are

always complaining of pain).

*Need more time, highly expensive.

*When: In cases of discomfort and soreness from an

otherwise satisfactory denture. Lasts for 6 months (in some

cases, it might last for 1 year).

*This discomfort is usually associated with mandible due to

small surface area, possibility of sharp, thin resorbed ridge

*Soft liners absorb some of the masticatory forces. (cushioning

effect).

*these points were not

mentioned by the doctor.

*Plasticizers: act as lubricants for polymer chains and make it easier

for them to slide over one another, so material can deform easily.

(provide softness for the material).

-Question: If you had a patient who has Candidal infection,

(before you make a new denture you should treat this

infection. Which type of material would you use? Tissue

conditioner. (we need it for a short time because we are going to

replace the whole denture).

⁂Available materials (for soft relines): (they

differ in the amount of plasticizers)

*tissue

conditioner: high

plasticizer.

*temporary:

moderate

amount of

plasticizer.

*permanent: low

amount of

plasticizer.

(durable)

(Semi-Permanent)

*memorize this table: (summary for what we’ve discussed)

c) Tissue Conditioners:

Usually needed in cases of tissue injury such as inflammation or ulceration, functional impression

Tissue conditioners: soft materials applied to fitting surface of denture to allow better stress distribution

1. Composition: PEMA+ ethyl alcohol solvent+ plasticizer (has high plasticizer so it is soft).

2. Needs to be replaced every few days due to leaching out of solvent and plasticizer

*have reasonable price.

*needed for short period of time.

Requirements:

soft and resilient, a degree of permanent deformation for functional impression purposes.

Disadvantages:

-Need for frequent replacement. (Here we start thinking about using another soft material)

-Prone to microorganism colonization

-Prone to damage by denture cleansers (material is soft, and if cleanser is added the denture might fracture), so patient should be instructed to use plain soap and water.

Manipulation: mixed (powder and liquid) and freshly applied to fitting surface then seated. Should be inspected after 2-3 days (short time)

Properties: Initially viscoelastic and soft Plasticizers leach out so the material become harder with

time (when the material becomes rigid, it should be replaced)

Provide a cushioning effect Non-irritant due to absence of acrylic monomers

⁂Denture Repair (one of the uses of acrylic)

1-Cold- cured acrylic repair material:

Broken pieces are fixed by sticky wax.

-Firstly, you should exactly determine the place of the broken pieces, and

double check that you are capable of fixing it.

Then,1) we make roughening for the area of fracture, 2) we add the

monomer (liquid) on the surface, 3) we mix the liquid and powder, 4) we

stick it to the place of fracture, and finally 5) we get the final result (fixed

denture).

*Fractures found in the middle of the dentures cannot be repaired.

2-Light-cured repair materials: (light cured is

an indicator for allergy)

⁂ The most important things in today’s lecture:

1- Types of porosities (page 1&2)

2- The main difference between gaseous porosity and

granular porosity is the time when the monomer was

lost.

(*Gaseous porosity: loss of monomer happens during

. heating-overflasking” because of the “

*Granular porosity: loss of monomer happens after

mixing before “flasking” while using an open-

container.)

3- The alternatives that we use for allergic patients.

(page 5).

4-Dimensional Stability (page 5).

5- Vinyl Acrylics (page 6)

6- Carbon, Aramid, and Glass fibers (page 7) (all of

them give high impact strength)

7-Porcelain and Acrylic teeth properties.

8- The important note in page 13.

9-Acrylic has the best bond with the denture’s base.