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Dental Base Resins

Scripters’ notes: The record for this script was unclear but I tried my best to write

everything mentioned in the lecture, so I apologize in advance if I missed anything

in the lecture.

Many thanks for Elite batch’s dental material scripters, without their scripts I

wouldn’t have been able to finish it.

All slides are included.

Denture construction:

Denture construction requires a lot

of steps like taking an impression

followed by pouring the impression

in the lab, making an extension,

making another impression, teeth

setting etc.

Dental uses:

Complete dentures: for patients who have lost all of their teeth

Partial dentures: for patients who have lost only a certain number of teeth (whether upper or lower teeth)

Artificial teeth that are attached to complete or partial dentures

Denture repair

Denture teeth

Soft liners: a soft material that is applied underneath the denture ex: sometimes when dentures are placed in the patient’s mouth for the first time , the patient may feel some pain or sourness because some part of the denture may be sharp or irritant , so the dentist applies this soft liner to act as a cushion so when the

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patient bites or uses the denture during eating it will not be irritant to the soft tissues

Construction of customized trays

Soft tissue replication on cast metal framework

This is how the set material looks like, so it is similar to the color of the gingiva (pink in color) it is supplied as:

1- as a dough material 2- as powder and liquid (have to be mixed)

Composition: they all have the same components in general , the slight differences in composition is the reason why some materials are cold cured and others are heat cured or light cured , and therefore manipulation will be slightly different between them Multiple types of acrylic resins are available. Heat cured: need temperature to start the reaction and eventually set

Cold cures: set at room temperature as soon as the components are mixed, the reaction starts and in a certain time the material will set and become hard

Light cured: they need to be subjected to a certain wavelength of light from a machine to set and become hard

Pour and cure resins: poured into moulds to make dentures.

Injection moulding: injected into moulds to make dentures. The last two are not commonly used because:

1- they are highly flowable and that makes them weak (main cause) 2- their reaction will not be 100% finished and that will leave uncured

components which can cause irritation to patients

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Requirements of denture base polymers To function properly as a denture, as a tray, as an artificial tooth

1- Physical properties: Appearance: because it’s replacing teeth and surrounding structures it needs to have a good esthetic appearance.

Glass transition temperature: it refers to the temperature at which the material starts to become soft and flexible OR the temperature at which the material is transferred gradually from being hard to soft Ex: if the patient washes the denture in boiling water, this high temperature will soften the material and cause it to dissolve

Note: the material needs to have ability to resist distortion

Dimensional stability: the material shouldn’t change in dimension; otherwise it will not fit in the patient’s mouth.

Specific gravity value: it needs to have low specific gravity value, so it should not be too heavy. Example: if an upper denture is heavy, by gravity it might fall and retention will be very difficult inside the patient’s mouth, as soon as we start speaking or eating, it will easily

Thermal conductivity: the property of a material to conduct heat

If the patient is wearing a denture which has low thermal conductivity (good insulator), when he drinks something hot, he will not feel the hot temperature in the soft palate or gingiva but when it reaches his throat, the high temperature is going to burn his tissues. In the beginning, the denture prevented him from feeling the high temperature, so he needs to start feeling the high temperature at the anterior part of the oral cavity

Remember: the complete dentures cover everything, the entire area back to the beginning of the soft palate.

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Conclusion: low thermal conductivity is essential to prevent burning in the soft tissues when eating or drinking hot food and drinks.

Radiopacity: it means that when we take a radiograph, the material appears in white color so we can see it. Importance: if a very small part of the denture was broken (partial denture for one tooth) or accidentally swollen by the patient → If it falls into the esophagus and stomach , it’s not that dangerous because it will come out eventually but if it went into the lungs , it is a dangerous issue so we have to take an x-Ray to see where the denture has fallen and if the material is not radiopaque, I can’t see it on the radiograph Conclusion: having a good radiopacity is good in such incidents.

2- Mechanical properties:

Elastic limit Flexural strength Fatigue life and fatigue limit: common proper in dentures, they will be fixed and subjected to forces in multiple directions inside the oral cavity, and dentures may be broken due to fatigue.

Impact strength: one of the main complaints of patients when they come back to the clinic after giving them their complete dentures is the fact that when they are cleaning it, they may drop it by mistake and it will be broken because it has low impact strength and because in case it falls, it will fall in the water and it will not hit a hard surface so it will not break.

Abrasion resistance

→As a dentist, you always advice the patients not to use an abrasive material

when cleaning the denture, and some of them do. So the surface of the denture

material needs to have good abrasion resistance to prevent damage and roughing

of the surface

● If the surface of the denture becomes rough while it’s in contact with soft

tissues like buccal or labial mucosa, it will cause irritation and the patient can’t

wear it.

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3- Chemical properties:

Inert: it should not react actively with components all around Remember: the oral

cavity is an active environment (saliva, varying pH etc)

Insoluble: they shouldn’t be soluble inside the oral cavity. *if it has high solubility,

it will become weak

Low absorption: it should not absorb high quantities of fluids all around it

because it will expand and its dimension will change so it will become weak.

4- Biological properties:

Toxicity and irritation before and after mixing: it should not be toxic or irritant.

5- Biological properties: 1- Cost of material and processing

2- Repair should be easy

3- Long shelf life

Heat cured resins (consists of liquid & Powder):

1- As a powder: Beads or granules of PMMA (PolyMethyl MethAcrylate) Initiator: benzoyl peroxide: a component which will initiate the setting reaction. Pigments: organic dyes, e.g.: ferric oxide, cadmium sulfide. Opacifiers: titanium/zinc oxide. Also, barium or radiopaque glass fillers (up to 20%) which may decrease strength and change appearance. → used to make it radiopaque → if the material becomes too radiopaque, it will not be esthetically appealing, the color of the denture will not be normal , so opacifires quantities should be balanced

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Plasticiser: dibutyl phthalate: help manipulate the material after mixing it

Synthetic fibers: nylon/acrylic →in the soft tissues there are very small blood vessels, these fibers are present to mimic the appearance of blood vessels that are found in soft tissues →mainly used for esthetic purposes.

2- As a liquid:

Liquid: clear, colorless, high vapor pressure at room temperature so you can smell it, it has a strong odor. Methyl methacrylate monomer, boiling point of 100.3°C. ● Why is that important? → usually when heat cured, acrylic resin is used and placed in hot water to raise the material in order for the material to set, we should not raise the temperature above 100 °C, otherwise this monomer will evaporate and make voids in the denture and cause it to become weak

Crosslinking agent: ethylene glycoldimethacrylate, a material important to add strength

Inhibitor: hydroquinone, it will inhibit the reaction between crosslinking agents and the monomers → we don’t want any reaction to happen before the mixing of the powder and liquid and that’s what the inhibitors do.

Store in a dark container to prevent occurrence of free radicals, otherwise it will evaporate and become too viscous.

The material of choice:

Acrylic resin (polymethyl methacrylate): Desirable qualities:

1- Good esthetics 2- Cheap 3- Easy to process

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Disadvantages: 1- Not strong enough: especially impact strength. 2- Susceptible to distortion: especially if they are not processed correctly and

also if they aren’t used correctly by the patient. 3- Low thermal conductivity

4- Radiolucent: we can’t see them in x-rays.

Polymerization and cross-linking of acrylic resins: Polymer: large long-chain molecules formed by chemically joining together smaller molecules called monomers Co-polymer: is formed when two or more different types of monomers join together. It is a term that is used to tell you that you have more than one type of monomer in your material. Cross-linked polymers: the joining of adjacent long-chain polymers by bonding of short chains along their sides

→ When side chains of adjacent polymers are joined by weak bonds, the polymers are easily manipulated, bent, or stretched → When side chains of adjacent polymers are highly charged, the bond is stronger, and the cross-linked polymers are stronger and stiffer.

Polymerization reactions: There are two types of polymerization: addition and condensation Addition polymerization: It is the most common form of polymerization for dental materials

It occurs in four stages: activation, initiation (or reduction), propagation, and termination.

Polymerization:

A. Addition polymerization: 1- Activation: by producing free radicals activators: ● it depends on the type of the material:

1. Tertiary amine : if it’s cold cured 2. Heat : if it’s heat cured

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2- Initiation: when an initiator such as benzoyl peroxide is decomposed and free radicals are produced. 3- Propagation: occurs when free radicals break the bond between carbon atoms in monomer and adjacent monomers bond together. 4- Termination: occurs when monomer units are used up, or free radical is tied up by a reaction.

B. Condensation polymerization: Usually more than one type of monomer is used. The reaction produces by-products such as water, hydrogen or alcohol.

● doesn’t occur in acrylic resin material.

Acrylic resins polymerization reaction: The reaction is an addition polymerization reaction to convert MMA (methyl methacrylate) to PMMA (polymethyl methacrylate). Explanation: as soon as activation occurs, benzoyl peroxide will start to break down and it will release highly active components, called free radicals which can cause oxidation So we need these highly active components so it can start joining the monomers together to make a polymer. The reaction continues, more polymerization occurs, and the reaction stops:

1- When there are no free radicals left

2- or when all the monomers have been polymerized (termination)

Mixing and curing of heat-cure resin: Powder: liquid ratio affects the workability of the mix and dimensional stability (3:1) Liquid is poured into a container then powder is added gradually. Too little monomer → the mix will be dry and this will lead to porosity in the final denture due to voids and bubble incorporation during mixing and that’s what is called a granular porosity. Too much monomer → the mix will be runny and excess amounts of residual monomers will be left uncured and that will lead to irritation

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Granular porosity → associated with too little monomer in the mix → leads to weakening in the final denture.

●usually when the correct ratio between powder and liquid is reached, the technician covers the mixing bowl and waits for a while until he actually can take the material and manipulate it. ● If he doesn’t cover the bowl that contains the mix, the monomers will evaporate and if it evaporates, the mix will result in granular porosity. Granular porosity can result from two things:

1- either evaporation of the monomer due to the fact that the technician didn’t cover the mixing bowel

2- or the amount of the monomer incorporated with the powder is too little from the beginning

MMA monomer: 1- Volatile

2- Flammable

Precautions: 1- Kept in a sealed container 2- Dark container away from light source or heat source 3- No contact should be allowed with the polymer beads (carry benzoyl

peroxide initiator)

Setting stages: After mixing, we can’t immediately take the mix, manipulate it and place it in the flask because the setting goes through stages: Sandy stage: The first stage seen when the powder “polymer” and liquid “monomer” are mixed together and the mixture looks like sand and you can’t manipulate it. Sticky or stringy stage: It can’t be manipulated.

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Dough stage: happens in less than 40 minutes The material is cohesive “not sticky” and it can be manipulated and shaped. Dough stage period is the:

1- working time: time to pack acrylic resins in the moulds. 2- doughing time: time taken to reach the doughing stage.

Rubbery stage: If I wait too long before manipulating the mix in the dough stage, it’s going to become rubbery, if I try to shape it or stretch it, it will go to its original shape (it will be elastic)

Stiff stage: (the final stage) It’s not completely set, but it’s too stiff and too rigid to be manipulated Importance of pressure applied when flasking. The mix should be loaded in the flask or mould during the dough stage to make the denture. Then the flask is closed and pressure is applied so that the material is evenly spread all over. If the material was in the sandy stage or sticky stage, it is too runny and you will not be able to apply pressure on it. Note: whenever we are using these materials, an excess amount of material is used because during processing and due to the setting reaction, there will be some polymerization shrinkage, so we always need to place extra dough (to compensate shrinkage)

Flasking and Heat curing:

Curing, important considerations: Benzoyl peroxide decomposes at 65 degrees (Exothermic reaction)

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Danger of gaseous porosity. How to avoid? Curing should be to high degree to prevent residua monomer. Why? Long cycle: 8 hours or longer in 74°C water Short cycle: heat at 74°C for 1.5 hours then boil for an additional hour. Suggested cycles for curing in your text book:

1- 7 hours at 70°C then 3 hours at 100 °C 2- Bring to a boil within 1 hour the boil for 1 hour

There are many suggested cycles but the idea is that you need a high temperature up to 100 °C in order to allow all the monomers to be converted to polymers so that few or no monomers are left behind “to avoid irritation” After we finish processing, cooling is needed cooling needs to be very slow so you should not place it in cold water or put it in the fridge, we usually leave it on the bench and allow it to cool down gradually. By this, distortion in the denture will be prevented If it was cooled quickly, this will create stress inside the material and it will be distorted and it may not fit in the patient’s mouth.

1- We start with this model which has the wax denture. Remember: we start with wax and transfer it to the final material “acrylic”. 2- We place the model or cast in this flask “mould” and all around is plaster.

3- It is closed and packed in hot water.

4- The wax will be melted away.

5- We open the flask again and use the mixed acrylic dough and place it where the wax used to be.

6- Close up again and put it in the long or short cycle, then it will be set.

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Processing strains: a. Internal strains occur during processing of resin.

b. If allowed to relax, warpage, distortion and crazing (tiny surface

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

strains may remain. This can be minimized by:

i. Slowly cooling flask: Sometimes if there is a stress created within the material due to manipulation , it can lead to defects on the surface of the denture which will lead to weakening of the denture , so again cooling has to be slow

ii. Using acrylic rather than porcelain teeth to ensure compatible shrinking:

Another factor that is important to prevent stress is the type of teeth that we use in these dentures

There are two types of artificial teeth: 1- Acrylic resin teeth: similar material to the denture material

2- Teeth that are made from porcelain: porcelain is more esthetic , more biocompatible , looks more natural , but it’s from a different material than acryl. If I use porcelain in teeth, and the denture base material is acrylic , as soon as processing starts and high temperature is applied ,these two materials will expand and then contract at a different range which might cause de-bonding between them and will also create stress in the denture base Because: these two components have different coefficients of thermal expansion That’s why it’s recommended to use the acrylic teeth on the acrylic denture because: they both expand and contract at a similar range, so there will be no de-bonding or stress created. This minimizes the stress or strain within the denture base material during processing and also decreases the chance of de-bonding and those teeth won’t become loose in the acrylic base material compared to porcelain teeth.

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Cold cure resin:

Composition: identical to heat cure resin except polymerization is initiated by tertiary amine (e.g. sulfinic acid or dimethyl-p-toluidine) which is an acceleratoractivator. Note: you don’t have to memorize the name, just know it’s a tertiary amine so as soon as you mix the liquid with the powder, the tertiary amine will activate benzyl peroxide and the reaction will start and free radicals will be released.

Working time is less than heat cured acrylic, and viscosity rapidly increases. Explanation: because polymerization immediately starts, so we need to be quick using this material. Usually it isn’t used to make dentures because: 1- it’s weaker than heat cured materials 2-working time is less, so you don’t have enough time to manipulate it properly. 3-there is no heat used to induce polymerization and that may leave residual monomers uncured and that can cause irritation to the patient.

Cold vs. heat cure:

Smaller polymer beads are used in cold cure resin to ease dissolution in monomer. Cold curing method results in a polymer with a lower molecular weight leading to:

1- Lower strength

2- Residual uncured monomer, so, higher solubility

3- Lower color stability, more prone to yellowing (may become yellowish in color after sometime)

Cold Cure Resins

Cold cure resins used for making special trays

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Manipulation, processing of cold cure resin:

Allowed to cure at room temperature or in warm water. It starts to polymerize soon after mixing (2.5 hours). Packing consistency is reached in 5 minutes and working time is short. (Danger of raised bite) and has lower dimensional changes due to less residual stress.

Pour and cure resin (pourable):

Same composition, smaller bead size. Mixed and poured into a mould of

hydrocolloid. Excellent reproduction of details but inferior in other aspects

(strength, residual MMA, distortion)

Uses: 1- In Custom made trays 2- In repairing broken dentures: when

they use it for denture repair, they put it in a pressure pot, but it can also set in room temperature.

3- rarely used to make partial or complete dentures because: A. They are highly flowable (more

liquid): and because of that there is a higher chance to have MMA monomer remaining, and that will cause irritation to the patient.

B. They are not very strong

C. they can be distorted easily

Fluid Resins

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Light cure resin (blue light): Used as denture hard relining material, repair material, construction of trays. Composition:

1- UDMA 2- Colloidal silica to control flow 3- Photoinitiator

4- Activator

Since its light cured, it has to have a special component that is sensitive to light and that’s what is called a photoinitiator. Camphoquinone: is the most common photoinitiator. → so as soon as you subject it to a certain wavelength of light from a unit called a light curing unit, it will start to set and become hard within a short period of time There is no need for temperature or mixing and it is only one component like a sheet that is slightly flexible. It can be adapted on the model or cast; we cut the excess and light cure it.

Injection moulding: (the material has high flowability) Acrylic is forced into a mould

Sprue hole, vent hole

Curing is occurring furthest from the holes. What is the benefit?

Materials used: 1- Cakes

2- Premixed

Light Cured Resins

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Disadvantages: (due to its disadvantages, it is not commonly used) 1- low strength 2- residual monomers 3- needs special equipment

Structure of the set materials: Complex: Mechanical properties are affected by how strong the bond is between the 2 phases. When mixing powder and liquid: PMMA beads are readily soluble in MMA, why? Degree of interpenetrating network formed depends on: Larger beads are more difficult to be penetrated by monomer Notes about the previous figure: What happens when the material starts to set? Not all the particles become polymerized and reactend, some of them remain unpolymerized and they are surrounded by a matrix of the reacted material In every material “mostly”, when you mix the two components together, not every single part of the material becomes reactend, we will have a matrix of reacted particles and within the matrix, unreacted particles may be found. Usually, the beads in the powder are large, so as soon as you mix them with the

monomer, it can’t penetrate the particle completely, but in cold cured materials,

the particles are very small and they are easily penetrated by the monomer and

easily dissolved.

Unreacted PMMA

Unreacted PMMA Unreacted PMMA

Matrix

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Note: the mechanical properties will be affected by how strong the two

components are attached together (the matrix and unreacted components)

Properties of resin:

Biocompatibility: High, however, allergy may occur due to leachable components, mainly the monomer and benzoic acid. (Some patients may show allergy from the monomer) Allergy is mainly associated with cold cure resin due to high residual monomer after setting (soln:terminal boil for 2-3 hrs) , so there are alternatives to be used if the patient is allergic.

As a replacement, denture bases maybe constructed from polycarbonate, vinyl acrylics, light activated acrylic and the pigments maybe toxic (it can be slightly toxic to the patient). How to know if the patient is allergic? As soon as he wears the denture, he will feel pain, sensitivity, irritation, or redness in the mucosa.

Mechanical properties: Soft, weak, flexible, unless denture constructed in proper thickness (but not too thick, why?) Creep is a problem, minimized by crosslinking agents, more common in heat-cured resin and dentures are prone to fracture due to poor impact strength. Poor resistance to fatigue (fatigue may result from wearing ill-fitting dentures) Design of denture includes notch design to accommodate frenum, resin material are notch sensitive. It can also be distorted if the patient doesn’t use the proper way to clean them (like soaking the denture in boiling water)

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So all of this depends on how accurate the dentist gives the patient the

instructions.

It does suffer from some disadvantages: 1- You might be tempted to make it thicker, but thickening the denture material will be uncomfortable to the patient. 2- if you take a look inside the oral cavity, you will notice that there are structures like muscle attachments, frenum (lingual,buccal,labial frenum), so when we make a denture for the patient, we need to make some space for these structures, otherwise, as soon as the patient starts to speak or move his muscles, it’s going to interfere with the denture and the denture may fall down, or the frenum and muscle attachments will be injured. That’s why we make spaces in the denture for muscle and frenum attachments, these areas look like a notch in the denture, and this notch may be a weak site , it may cause weakening of the denture.

Crazes, surface cracks may result from: 1- Dentures drying out (absorption and loss of water cycle) If the patient takes the denture out, he has to place it in a moist environment because if it dries out it will be distorted (we need to tell the patient how to store and maintain the denture). 2- Using porcelain teeth During denture repair when monomer contacts cured acrylic resin.

Modified acrylic materials Manufacturers try to modify the components of the denture to improve the impact strength to minimize fracture if the patient drops the denture accidently and the solutions are: Solution to patients who commonly fracture dentures (area of active research):

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High impact resistant resin (contain rubber toughening agent)(Greater cost)

Incorporation of fibers to produce fiber reinforced resin:

1- Carbon fibers: difficult to handle, poor esthetics, need surface treatment “they are not used commonly”

2- Aramid fibers: fine filaments, readily wetted by MMA, no special surface

treatment. (Commercially: Kevlar)

good option because they bond very well to acrylic resin material 3- Glass fibers: incorporated as short fibers or loose form, need care

regarding surface treatment and fiber orientation.

good option because they can lead to high strength and they are highly esthetic

Opacity: incorporating bromine additives

Physical Properties: Thermal conductivity:

Low: isolates tissue from temperature

sensation , inclusion of sapphire whiskers increases conductivity

Coefficient of thermal

expansion:

High , affects fit of denture

Heat distortion temperature (Tg 105 C for heat cure , 90 C cold

cure):

Resins deform above their glass transition temperature , distortion temperature

Water sorption and solubility:

Absorb water 1-2% wt slowly .insoluble in

oral fluids .soluble in ketones , esters, alcohol causes crazing

Specific gravity:

Low , so better retention

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Chemical properties:

Candida albicans may colonize denture surface and can be prevented by proper cleaning. Denture whitening, denture looks bleached. Maybe caused by a combination of:

1- Using hot water to soak the denture (common reason) 2- Contact with solvents such as acetone 3- Improper networking between polymer beads and matrix phase

Denture Stomatitis: It is an inflammation that has the same shape of the denture on the palate, residual ridge, buccal and labial areas. It affects the areas underneath the denture and it is mainly caused by candida albicans, common in elderly people. Remember: we all have candida albicans normally inside our bodies but because we have normal flora and good immunity, it doesn’t affect us. As soon as normal flora is disrupted and the immunity is lowered, in other words “as we get older”, candida albicans can be active and lead to denture stomatitis. Another problem is denture bleaching (whitening) Caused by combination of factors: 1- using hot water 2-Acetone 3- Using acrylic material with poor network between matrix and beads (mismatch in refractive index)

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Alternative polymers (if the patient is allergic): 1- Polycarbonate: processed by injection moulding (processed at high

temperatures) 2- Vinyl polymers (low Tg temperature)

Problems (disadvantages): ● too flexible or too soft (easily distorted) Moving on to artificial teeth; two types are available. They can either be made from porcelain (dental ceramic) or from acrylic dental material. In general, certain requirements are favorable in artificial teeth. These include (Specification No. 15)*:

Good appearance

Good bonding with denture base

Resistance to abrasion, distortion and crazing

Strength and toughness

Biocompatible

Allow adjustment when checking occlusion Specification No. 15: Refers to certain standards that are issued by organizations like the ADA and ISO whom are responsible for setting standards for different materials.

Acrylic teeth:

Produced in reusable molds using injection or the dough technique and they can be available in several shapes.

Made from rigid, highly cross-linked resin to resist crazing.

Pigments are added to replicate natural teeth colors.

Base and core of teeth are made from lightly cross-linked resin (but middle and incisor teeth are made from rigid highly cross linked) to allow softening and sticking to the denture (Chemical attachment).

Porcelain teeth:

Larger than required molds are used to allow shrinkage during fitting: Since porcelain shrinks as soon as it sets; the molds need to be slightly larger than the actual size of the tooth.

Small pins are added to the base of the tooth for retention to the denture base (Mechanical attachment).

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Properties:

Instructions to patients on denture care:

Liquid soap, mild hand soap, nonabrasive denture cleanser should be used and denture and teeth should be cleaned with a brush

Brushed or rinsed after each meal and before soaking in denture soak

Denture tablets that available in pharmacies can be used to make a denture soak to remove debris and saved dentures from fungal and bacterial growth.

Calculus deposits can be softened by soaking denture in white vinegar diluted with water 1:1 and dentures should store in a moist environment

Dentures should be soaked overnight in commercial or homemade soaks

Undiluted bleach should be avoided since it fades color and corrode metal parts of partial dentures

Tissue bearing surface should be cleaned with soft brushes to avoid abrasion

Best Regards

Hummam Rihawi

Acrylic teeth:

Chemical bond to denture base material

Easy to grind during occlusal adjustment

Do not wear natural, artificial opposing teeth

Easily repolished

Compatible with denture base material (CTE)

Stain over time

Easily wore

Porcelain teeth:

Better esthetics

Biocompatible

Mechanical retention to denture base

Difficult to adjust because they have a hard surface

Produce clicking sound when hitting opposing teeth

Wear opposing natural teeth

Transmit greater forces to supporting tissue

Heavy

More expensive