(9) Dental Base Resins 2

8/2/2019 (9) Dental Base Resins 2

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Manipulation4. Processing: porosity, processing strains.common mistakes can be happened while you are making a denture. The most common one is

porosities, and the problem in these is that they are unstable and they cause weakness of

the denture. And there are many types of porosities and each one has a specific reason.

Porosity: caused by,

1. Polymerization shrinkage (contraction porosity)

2. Volatilization of monomer (gaseous porosity), which is caused bythe evaporation of the monomer, which happens when boil the water muchhigher than 100, because the evaporation temp. of the monomer is 100.3 .

3. And usually this happened in the thick portion of the denture, which is the

palatal portion. Since these areas are thick, the temp. raised quickly and cause

evaporation. So, during processing, temp. should be raised slowly.

4. Granular porosity, due to loss of monomer while resin mixis left to stand until dough stage is reached. Also if theresin mix is dry and its because of2 reasons: 1- adding too much powderwhen its mixing with the monomer, so the mix will be dry. 2- the mix it correct

but when we dont cover it after we finish mixing, therell be evaporation and

itll be dry also.


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1. Contraction porosity: occurs due to monomer

contraction (shrinkage) by 20% during processing.

Processing involves a raise in temperature to

initiate polymerization at first as its put in a boiling waterand then temperature raised due to theexothermic reaction

During this, resin flows (under pressure) into

spaces created by curing contraction and itll be lessthat what we want. SO, excess resin is important tomaintain this pressure.


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Once resin becomes rigid, thermal contractionmay occur (change from curing temperature toroom temperature). Curing temperature for cold

cure resin is lower than heat cure resin. Insufficient amounts of resin packed in the flask

may lead to voids or porosity. Also resin should bepacked in the DOUGH stage. Prior to that the

resin would flow too rapidly and pressure is lost


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Porosity

2. Gaseous porosity:

1. Caused by a rise in the resin temperatureduring curing above 100C (> boiling

temperature of resin)2. Gaseous monomer forms and causes gaseous

porosity

3. This is avoided by allowing a slow and

controlled rise in temperature


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Processing strains: Internal strains occur during processing of resin, and

thats because the pressure and raising of the temp. that you cause while working.

These stresses and strains shouldnt be allowed to relax completely, because If its

allowed, warpage, distortion and crazing (tiny

surface defects) occur. Some are relieved as the material flows but thermal

contraction strains may remain. This can be minimized by:

Slowly cooling flask (cooling and warming)

Using acrylic rather than porcelain teeth to ensurecompatible shrinking . Porcelain and acrylic resin has differentcoefficient of contraction and expansion. Now, this difference can cause stress

inside the material if we use porcelain teeth. But if we use the acrylic one, the

coefficient will be similar; so, therell be no stress created inside the material.


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Properties of resin

Biocompatibility:

High, however, allergy may occur due toleachable components mainly the monomer and

benzoic acid. Allergy is mainly associated with cold cure resin

due to high residual monomer

As a replacement, denture bases maybe

constructed from polycarbonate


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Properties continue,

Dimensional stability and accuracy:They are dimensionally stable if the patient takes care of them (putting them in a humid

environment) and if our processing was correct according to the temp. and pressure.

Otherwise, theyll loose water and the shape might change.

What is the difference between retention andstability?

It is important for the denture to be retainedintraorally. Why?

Accurate fit to ensure good adhesion (large surfacearea) and cohesion (accurate fit)

To ensure good peripheral seal (all of these things we take themnext year)


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Properties continue,

Mechanical properties: one of the properties of the resin material is low impact state,which mean, if the patient accidentally drops the denture, itll easily break.

Creep is a problem,which is changes in shape and its susceptible to distortion. Itsminimized by cross-linking agents

Dentures are prone to fracture Commonly, midline of upper

denture

Mainly caused by:

Trauma, leading to cracks then failure. So, if there are defects, pores, bubblesor tiny fractures inside the denture, the denture will be weaker.

Poor quality processing: lack of bonding between resin and teeth

Crazes

Tensile strength 50 MPaElastic modulus Low

Flexuralmodulus

2200-2500MPa


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Mechanical properties continue,

Solution to patients who commonly fracture dentures:

High impact resistant resin (contain rubber tougheningagent), decrease crack, but the problem is that it may lowerflexural modulus and lead to fatigue due to excessiveflexure. And finally it might be broken.

Incorporation of fibers to produce fiber reinforced resin: Carbon fibers: difficult to handle, poor esthetics

Aramid fibers: lack of bonding with rein

Ultra high molecular weight polyethylene fibers, UHMPE: lowdensity, neutral color, biocompatible, bonds to resin but processing

is time consuming Glass fibers: most promising, incorporated as short fibers or loose

form.

Sometimes, they add sheets of these fibers to make the denture stronger (specially the

palatal area). The most common place that they put them is the midline (even upper

or lower midlines), because the midline is easier to be broken.


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Physical propertiesThermal conductivity Very low, thats why patients always

burn their mouths, disadvantage:Isolates tissue from

temperature sensation

Coefficient of thermalexpansion (CTE)

High, if teeth are fromporcelain, differentialexpansion loose teeth

Water sorption & solubility Absorb water 1-2% wt.

slowlyand we can lose it, so we should

put it in a humid environment.Insoluble in oral fluids


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Denture lining materialAfter we make the denture and the patient used it; then after few years, it contacts telling that

the denture is loosed or its not with the vertical dimension of the face. So, wrinkles willappear in the face and also the ability of eating will be affected as well.

Sometimes, theres nothing wrong with the denture but still the patient is complaining from pain;

then he/she just has to get used to it.

Divided into:

Permanent hard reline materials

Semi-permanent soft liners

Tissue conditioners/temporary liners


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Hard reline materials:

Criteria for using it to reline dentures:

Poor retention and stability

Loss of vertical dimension

Degradation of the denture base (destroyed for some reasons)

For older patients for home getting use to a newdenture base would be difficult (they can do relining to make it fitbetter)

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


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Materials used: its composition is similar to the resin material. Sometimes it uses ethyl instead of methyl.

Ethyl is less irritant than methyl. So, they are different than each other; so, if the patient irritate

from one of them, we should give him the other one.

Heat cure resin, in the lab.

Cold cure resin, chairside. (can be used in the clinic immediately)Disadvantages:

Poor taste

Poor color stability Exothermic reaction, it can cause irritation, so, we shouldnt keep it in the

patients mouth all the time; we place it for sometime and then we finish the setting

out side.

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


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Type I Type II (


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Semi-permanent soft liners

When is it used: In cases of discomfort andsoreness from an otherwise satisfactorydenture. Lasts for 6 months maximum.

This discomfort is usually associated with themandible due to small surface area,possibility of sharp, thin resorbed ridge

Soft liner with absorb some of the masticatoryforces


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Desirable qualities in materials used: Rubbery

Resilient

Low elastic modulus

Some materials (polymers) are naturallyrubbery. Others can be modified by addingplasticisers

Plasticisers: act as lubricants for polymer

chains and make it easier for them to slideover one another, so material can deformeasily. In other words, it adds some elasticity to the material. And if they leach out(lost), the material will be brittle.


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Semi-permanent reline materials:

Silicon rubber: polydimethyl siloxane polymer+filler to achieve correct consistency

The material solidifies by cross-linking rather

than polymerization since its already a polymer An adhesive is needed to bond silicon to denture

because they are from another material (not like the acrylic liners). E.g.:

Alkyl-silane coupling agent

Silicon polymer dissolved in solvent

Disadvantages: weak bond, encourage Candidaalbicansgrowth (susceptible for fungal infection)


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Semi-permanent liners

Acrylic soft liners: can bind to denture base

1. Leachable plasticizer systems, composition:

Powder: mix of PMMA & PEMA

Liquid: MMA with 25-50% plasticizer (dibutylphalate)

Disadvantages and recommendations: plasticizerleaches out so I becomes stiff. Avoid using high

temperature and strong bleaches

2. Polymerisable plasticizer systems, advantage:

resist dissolution. Hard at room temperature,softens in the mouth


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Relative merits of soft liners


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Tissue conditioners/temporary soft linersIf the patient feel a pain due to excess material or a hot one, we can use this type of liners to

relief it. It has to be removed every 2 or 3 days because it has plasticizers.

Usually needed in cases of tissue injurysuch as inflammation or ulceration.

1. Tissue conditioners: soft material applied to

fitting surface of denture to allow betterstress distribution

1. Composition: PEMA+ ethyl alcohol solvent+plasticizer.

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


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Disadvantages:

Need for frequent replacement

Prone to microorganism colonization

Prone to damage by denture cleansers, so patientshould be instructed to use plain soap and water


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Acrylic teeth

Advantages compared toceramic teeth: Tough

Bond to denture base material

Easy to grind during occlusaladjustment

Do not wear natural, artificialopposing teeth

Easily repolished

Disadvantages: Soft and easily wear

Stain over time

Constructed in layers tosimulate natural color

Gingival portion ismade from minimally

cross-linked resin toensure good bondingwith denture base

Construction considerations


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References

Introduction to dental materials. Chapter 3.2

Dental materials, clinical applications fordental assistants and dental hygienists.

Chapter 13


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(9) Dental Base Resins 2