Dentures: Function and Properties

Function of Dentures

• Dentures support artificial teeth and gums, maintaining their proper position during eating and speaking.
• Denture bases are made from acrylic polymers or metal alloys.

Dentures

• The denture base rests on soft tissues, supporting artificial teeth.
• Artificial teeth are attached to the denture base.

Properties of an Ideal Denture Base Material

• Exhibits good aesthetics, mimicking a natural appearance.
• Possesses high elasticity, hardness, toughness, impact, and fatigue strength.
• Maintains good dimensional stability.
• Is light, with low specific gravity and density.
• Lacks odour, taste, or any toxic substances.
• Remains insoluble in oral fluids and resistant to abrasion.
• Shows good retention to various materials like polymers, porcelain, and metal.
• Simplifies repair and manipulation.
• Has a long-lasting shelf life.
• Resists bacterial growth.
• Has high thermal conductivity/diffusivity and diffusivity, with high Tg and a low CTE similar to artificial teeth.
• Is radiopaque for easy identification on radiographs.
• Cleans easily.
• Is inexpensive to use as a denture-base material.

Materials Used for Denture Bases

• Cellulose had several issues due to camphor leakage, causing blistering, staining, and colour loss.
• Phenol-formaldehyde was hard to process and lost colour.
• Vinyl resins exhibited low fracture resistance and fatigue strength.
• Vulcanite had poor aesthetic qualities.
• Polymethylmethacrylate (PMMA/Acrylic) is the new material of choice, offering aesthetic quality, affordability, and easy processing.

Composition and Structure of Acrylic Resin

• Acrylic resin dentures are formed from PMMA through free radical addition polymerization.
• Resins are either heat-cured or cold-cured.
• A powder-liquid mix is used, which when mixed and heated forms a solid. This method allows for processing by dough technique, decreases polymerization shrinkage, and reduces the heat of reaction.
• The powder typically consists of PMMA beads or granules, benzoyl peroxide (initiator), pigments/dyes, titanium/zinc oxides (opacifiers), dibutyl phthalate (plasticiser), or nylon/acrylic (synthetic fibre).
• The liquid consists of methyl methacrylate monomer, hydroquinone (inhibitor), and diethylene glycol dimethacrylate (cross-linking agent).
• Pre-polymerized PMMA powder beads are combined with MMA monomers to form a sandy mix, dissolving PMMA beads and leaching out the initiator (benzoyl peroxide). Upon further reaction it becomes a dough, then packed in flasks, slow cured to maximize polymerization.
• The setting reaction is a free radical addition polymerization. There are specific working, dough, and setting times, with the mix being placed in a flask before setting.
• MMA is a low viscosity liquid with a boiling point at 100 degrees. Temperatures should not be rapidly increased, or monomer evaporation will create pores.

Manipulation of Denture Base Material

• The polymer-to-monomer ratio is 3:1. Excess monomer results in shrinkage, while excess polymer results in granular porosity due to inadequate polymerization.
• Adequate mixing is required to prevent air bubbles.

Methods of Curing

• Polymerization of PMMA is exothermic.

Heat

• Gradual temperature increase is necessary, to prevent monomer evaporation.
• Gradual cooling of the flask, is essential to avoid warpage and crack formation in compression moulding.
• Gaseous porosity formation can occur if the reaction is exothermic, temperature should not rise too high before polymerization.

Chemical

• The powder contains an initiator benzoyl peroxide. The liquid component which consists a tertiary amine reacts with the benzoyl peroxide creating free radicals.
• If the temperature is too low, the reaction slows.

Light

• Not ideal for PMMA due to its ineffective curing of thick bases.
• UDMA and camphorquinone are present.

Microwave

• Polymerization occursvia microwave radiation.
• Curing occurs in just 3 minutes in a typical microwave.

Properties of PMMA Denture Base

• Includes dimensional stability, mechanical properties, crazes, monomer properties, thermal conductivity, CTE, water sorption and solubility, and biocompatibility.

Dimensional Stability

• Important for denture retention and occlusal direction.
• Adhesion to mucosa is through a thin film of saliva. A peripheral seal is also important.
• An issue for the elderly due to altered saliva production, solved by denture fixatives.

Mechanical Properties

• A tensile strength of 50 MPa.
• Low elastic modulus.
• High flexural modulus.
• Low fracture toughness and impact strength.
• Denture failures occur through flexural fatigue and production of crazes from processing errors or high deformation. Fracture is also more common in those who are prone to fracturing their dentures, and this can be improved with butadiene styrene, however it will lower the flexural modulus.
• Creep, a problem with acrylic resins, as they are viscoelastic materials.

Crazes

• Result from restrictions on dimensional changes, leading to internal strains, with foggy/hazy appearance.
• Internal strain relaxation causes warpage and crazing and at this stage it isn't yet a crack.
• Prolonged, externally applied loads may cause brittle fractures over time.
• Heat, differential contraction, or solvents can form crazes.

Monomer Properties

• High molecular weight (HMW) prepolymers (PMMA beads) are used to give better mechanical properties.
• Residual monomers should be kept low due to them being irritants and acting as plasticizers.

Thermal Conductivity

• Low thermal conductivity and diffusivity in PMMA make it a thermal insulator.
• Problematic because it isolates oral soft tissues.
• Repeated cycles of drying and wetting cause tensile stresses that create crazing, which occurs due to mismatch in denture base and artificial tooth.

CTE

• High CTE in PMMA.
• Is not an issue, however can cause teeth to be lost, however this has a solution.
• The heat resistance is low, dentures can soften and warp from boiling.

Water Sorption and Solubility

• PMMA's polar nature leads to water absorption and consequently it effects fatigue life, although this compensates for shrinkage.
• It's virtually insoluble in oral fluids.

Biocompatibility

• PMMA as a material is highly biocompatible.
• Some patients show allergic reactions due to leached monomers or benzoic acid, especially those with cold-cure resin dentures and delayed hypersensitivity.

Advantages of PMMA

• Exhibits excellent aesthetics and ease of processing with low density.

Disadvantages of PMMA

• It is low strength, prone to distortion, low thermal conductivity and radiolucent.

Processing Irregularities

Gaseous Porosity

• It occurs if heat goes above the boiling point, its effects can be prevented by using long cycles.

Granular Porosity

• It occurs due to an access of the polymer.

Contraction Porosity

• It occurs due to under-packing as acrylics.
• A lack of pressure, in the clamp during curing cycles will also lead to porosity.

Use of Staineless Steel/Metal

• It has better durability, and is more resistant to breakage, than other materials
• Used as a denture base, for RPDs.
• Has adequate thermal conductivity, it's corrosion resistant, which allows a good thermal response to temperature change, they are also both light and thin.

Denture-Lining Materials

• Permanent hard and Semi-permanent soft liners for dentures.
• Use of denture lining are used in temporary situations or for tissue conditions.
• It involves an additional lining, so as to better support oral soft tissues.
• Relining occurs due to either lack of retention, collapse of bone, degradation, and elderly wearing dentures who are unable to wear dentures.

Hard Reline Materials

Life of Denture are affected by bone resorption

• Bone resorption is more common mandibular.
• Materials are either cold or heat cured acrylic resins, at chair side or lab in heat cured acrylic for relining.
• There are two different types of these for cold-cured, type I and II.
• Type I, is more irritant.
• Types are less irritating for a patient reducing Tg in turns increases possibility of dimensional instaibility, but relining has a closed mouth technique.

Type I

Composed of PMMA and Benzoyl Peroxide, and its liquid consists of monomers, and amines

Type II

Its powder consists of PEMA with benzoyl peroxide, and its liquid is butyl-methacrylate and amine.

Soft Lining Materials

Temporary vs Permanent

• Materials are able to act as shock absorbers, and have a cushioning effect upon dynamic loading, but is not very long lasting so needs replaced.
• They used when conditions, do not allow material to be long lasting.

Materials for Amalgam

• Amalgam itself, is a blend of materials including mercury which has issues so in turn more materials are used.
• History*
• First found in china
• Green Vardinan studied it, then now 100 million are used per year.
• Mercury is found in amalgam due to its ability to dissolve any material, or alloy when compounded.
• In turn, as a corally, due to mercury it's imposible to have an amalgam mercury free.

Compostition of Amalgam

• Amalgam is based on silver and tin/mercury, copper increases both strength an hardness.
• Zinc, in turn decreases oxidation of the matieral.

Traditional vs new

• There are powder with a liquid system and, powders have triple distilation in its component.
• Powders also contain intermetallic compound as Silver and Tin, known as gamma phase.
• As stated it's either Lathe cut , or spherical cut and therefore needs produced.

Manufacturing

• Heated in atmosphere to gain Silver and tin, after melting silver, and other compounds oxydase easily then is used as.
• In order not for any issues with either material it needs Zinc, as then an oxide free alloys occur less expansions.

Alloys and Preperations

• Alloys needs power form as power react with and increases surface area.
• They alloys are all milted into liquid, therefore prep for powder can occur.
• Through ether Lathe or Spheres there are way to manipulate the power form.

Methods

Either Lathe or Spherical:

• Both methods have an end go aol of a homogenous liquid.
• Spheres have low viscosity and they heat metallic ingots to 420 degrees, the liquid can can be sprayed this lowers its surface tension.
• It will turn inot a set of particels and with spheres it allows it to harden on the surface with consistence sizes.

Morphology Summary

• spheres are easy to carve.
• lathe is small area.
• Mixed is good area.
• spheres requires mercury contact for low level issues.
• strong and easy carving is a good point.
• good carving skills needed, more merucy to remove for carving and for final carving.

Reactions

• As the mercury floods surfaces the outer surfaces the gamme change its phases, it causes initial shrinkage.
• The cores become then bridged phase where cores then undergo, silver to combine the alloy.
• tin slowly reacts to gain gamma the matrix set amalgam is then created with both pahses.
• Gamma releases the phases into the amalgam then the allloys completely become gamma with mercury.

Structures and time

• Core has gamma particles and matrix.
• Amalgam overall is stronger then metal that has all.
• 2 is a weak aspect, removing any aspects the reduces phase changes.
• with this it can set very hard with mercury.

Other compounds

• alloys, that increase tensile strength, the allloys can increase if that occurs with alloys and condensation.

Dimensional change

1 dissolves with initial contractions 2 there increase volume the in material 3 the mix of water it increases the expansion by marginal gaps, it can then crack the tooth as in the restoration.

Thernal Properties

• Metals create and creates thermal issues, as sensitivity.
• Thermal and expansion mismatches creates pumping in tubs.

Amalgams

• They Multi phase metals

corrosion

• It releases mercury and the process of corrosions means to combine or in-turns damage the restoration.

Creep

• When material load with time happens it becomes the most prone
• deformation, then increases and is in a creep and the edges then have a hard time to stop deformation

HI-copper phases and Amalgams

• Phase 2 with high-copper, phases to create a high strength and decreased the risks.
• Phase for Cu sn5 to use it to make a copper enhanced alloy, to test, the final restoration will benefit.

Steps and Variables

• To put in in effect for variables and to test, the final restoration will benefit from them.
• alloy has a ratio and this can mean.
• the excess Hg (mercury) can be eliminated through the best condensation.

Trituration/Contamination

• Need for mixing and a safe and sealed restoration.
• It's important to reduce to be less.
• If done wrong, overfilling causes issue to then be better filled.
• The less you've of it , then that's better.

Clin Phases

• Cavity prep then pulp protection. 1 matrix wedge is in line. 2 selections are in line 3 condense well. 4 burmish. 5 the wedge comes off. 6 polishing and finishing is complete.

Condesation

• Its the main goal that makes merucy to over fill and then carve off.
• No voids to create post for issues and sensitivity.
• And also by maximizing adaptions.

Condesation/Surface

• surface increases is the area of contact the and this lowers for the sphere amount of pressure.
• important to prevent and not break.
• to help with strength, lathe is harder to fill, sphere is easier is.

High alloy Phases

It gets easier for is a metal with is more in that part