Partial Dentures 7 Flashcards
What is the denture base
The denture base is that part of the denture that the artificial teeth are connected to. It may also make up the major connector for a denture in the case of all acrylic dentures. (It is essentially the part of the denture that serves as replacement for the gingivae and alveolar support for the teeth, and it therefore has an aesthetic role as well as a functional role, keeping the denture strong and rigid).
What are the requirements of denture base materials
Firstly, the material should be able to bond to the artificial teeth. It would be embarrassing and pointless if the artificial teeth were not securely held.
Like all materials used in the mouth it should be biocompatible.
For the aesthetics to be acceptable, the material should be able to be highly polished, translucent and have a good colour to simulate the appearance of the oral soft tissues.
Porosity is a term used to describe bubbles that may occur in a material during polymerisation. These may affect the properties of the material as well as the appearance and we will be looking at this problem in more depth later
Not all of the monomer ever gets fully converted to a polymer during polymerisation, and the monomer that is left over (residual monomer) being of small molecular weight compared to the large polymer chains may act as a plasticiser, and more importantly may leech out of the material while in the mouth, which could be a problem if it is toxic.
As a reminder of what strength and modulus refer to, when looking at the stress / strain graph, modulus refers to the gradient of the line, such that a steeper line would be a higher modulus. Strength refers to the amount of strain placed on a material before it breaks.
what is resilience defined by
Resilience is defined as the amount of energy absorbed by a material up until the point at which it undergoes permanent deformation.
what is toughness
Toughness is the amount of energy absorbed by a material until it breaks. Neither resilience or toughness are included in the standard requirements, however, they are important properties.
What are the desirable properties of denture based materials
Looking back at the standards however, we can see there are several properties that are not included that would be desirable as well as resilience and toughness. Dentures are frequently dropped, so a high impact strength is desirable, and since dentures frequently break then it would be an advantage if they could be repaired rather than replaced. As dentures frequently cover large surfaces intra-orally then it may be nice if they could conduct heat to give the patient more awareness of what they are about to swallow. Talking of swallowing, small dentures or parts of dentures sometimes are swallowed, or inhaled, and so it would be an advantage if they were made of a material that could be identified on a radiograph
What is the historical perspective of denture based polymers
Historically, denture bases have been made from all kinds of materials, including bone, wood and antler. Here are some examples of some of the more successful historical materials. Vulcanite is a highly cross-linked rubber – similar to the material that goes to make car tyres. This doesn’t sound promising, but Vulcanite dentures were the most popular dentures from the 1850s to the 1940s. Bakelite dentures were briefly introduced when Bakelite, as one of the first truly plastic mouldable materials was invented in the early 1900s. Despite looking better than Vulcanite they had poor mechanical properties and so didn’t last long.
When was PMMA developed
Poly methyl methacrylate was introduced as a denture base material in 1935. Soon denture teeth were made from it also, replacing the porcelain teeth that had been popularised at the time of Vulcanite. The chemistry and properties of PMMA has changed very little since the 1940s, but just how good a denture base material is it?
What is the chemistry of PMMA
PMMA undergoes addition polymerisation. This involves 4 stages – activation of an initiator – in which a chemical is made to produce a free radical. Initiation is where the free radical starts the polymerisation process by connecting with a monomer molecule. As one monomer attaches to another the reaction is said to propagate, and finally if the growing chain meets another and joins with it, or it doesn’t encounter any more monomer molecules, the reaction terminates.
What does tenure base resin usually contain and how do they iniate chain growth
How is PMMA polymerised
With denture base materials, the most common form of activation is using heat, the initiator molecule is typically benzoyl peroxide, which in the presence of heat breaks down to form a free radical. This is similar tohow camphorquinone breaks down in the presence of blue light when we are looking at composite materials.
The free radical attacks double bonds on the monomer molecule, opening them up and exposing a site for reaction. Once on molecule joins with another, another free radical is produced and the reaction continues.
How does shrinkage occur with PMMA
As the monomer molecules join together to form polymer chains, the space between them is reduced. This results in material shrinkage, which for pure MMA monomer is about 21% by volume
fill mould with liquid and set it turning it to polymer
Polymers occupy less volume than monomers that create them
How do we minimise dimensional changes
To overcome this problem, we rarely use neat MMA but rather mix it with PMMA beads that are already polymerised. As these don’t undergo any further shrinkage, but take up room from monomer molecules, the net effect is the creation of a material that undergoes only about 6% by volume. Much less than before, but still enough to be a potential problem when we are looking at uses that require a high degree of accuracy
In a heat cured acrylic resin what components are needed
name the steps of molecular level of pMMA
Molecular level
PMMA +MMA-
Mix and apply heat and pressure
Breaking down benzoyl peroxide as new polymer formed
But interact with PMMA beads
Activation - bring together
Initiatory- appy what an pressure
Propagate before we polymer
Eventually it sets
Joined together at molecular level
What is the setting reaction
What steps do we take in processing
Dentures are made using the lost wax technique, in which a wax version of what we want to create is used to make a mould. The wax is then boiled out leaving a space where it was
What does dough moulding involve
PMMA/MMA dough is then put into the mould and the 2 halves of the mould squeezed together and held there under pressure
What does processing do
The polymerisation is then activated by heating the mould up in a vessel of boiling water. This is typically done overnight and may take 8 hours or more to fully complete the polymerisation
What are the processing problems
Porosity is a problem that can occur if processing of PMMA is not carried out correctly
When does gaseous porosity occur
Gaseous porosity occurs when MMA liquid boils in the middle of the polymerisation process. MMA liquid has a boiling point of 100.3 ºC and as such shouldn’t be reached by the heat source of boiling water. However, the polymerisation reaction is exothermic and temperatures of 170 ºC can be reached in the mould. It usually occurs in the deepest parts of the denture as the surface is nearest the heat source and hardens before the temperature gets high enough.
To prevent gaseous porosity from occurring, the MMA must be heated very slowly, so that it has fully solidified before the boiling point is reached. For this reason, the denture processing technique involves at least 4 hours of slow warming up.
What is contraction porosity
Contraction porosity occurs due to the 6% shrinkage that is inherent in PMMA production. To avoid it happening, a little extra PMMA dough is added to the mould before heating, and the mould is kept under very high pressure throughout the procedure to prevent bubbles from growing in size.
What is granular porosity
Granular porosity is more rare and is due to the dough becoming dry before processing
What are the advantages of PMMA
PMMA is a very good denture base material, so far nothing better has been developed. Here is a list of it’sadvantages.
Tg is the glass transition temperature and may need some explanation
What are the thermal properties of polymers
The glass transition temperature is the temperature above which a solid polymer becomes soft and rubbery. Think of wax that starts off solid and hard, but goes soft when heated before it melts. The temperature at which it goes soft is the glass transition temperature. In the mouth liquids as hot as 70 ºC are frequently taken in, and so a denture base material must have a Tg higher than this.
What are the factors affecting Tg
If we look at the Tgs of the methacrylate family we can see that only PMMA has a Tg higher than 70 ºC the other materials would go soft. This may be an advantage of we wanted a material needed to be soft at mouth temperature such as a sports mouth guard, but as far as dentures go, only PMMA is satisfactory
What are the disadvantages of PMMA
What are the problems with PMMA
The number of broken dentures that occur can be a big drain on resources for the NHS
What sort of dentures break the most
What are the upper complete dentures
Upper complete dentures more commonly fracture in the midline.
It is thought that this type of fracture due to stress occurring in the denture as the denture flexes during function. These stress can be concentrated into small areas and so initiate cracks forming but features such as fraenal notches and diastema. Therefore factors contributing to fracture are increased flexing, and if this is accompanied by any factor that concentrates the stress to a small area then fracture is likely
What can be used to improve properties of denture base polymers
Several ways of improving this failing of PMMA have been attempted. More recently injection moulding has proved to be successful, but this is expensive and high tech compared to the dough moulding method we have looked at
What are metal strengtheners
Metal bars and meshes are often added by technicians, but there is little evidence that they actually strengthena denture and may in fact act as a stress concentrator
What are co polymers
Graft co-polymer of butadiene styrene is the best solution yet to improving the properties of pmma and creates what is known as “high impact acrylic”, one example of this is the commercial product Lucitone 199.
The toughness is greatly increased, however the modulus is reduced in the process and the dentures are more flexible
Why are plastic acrylic teeth used in dentures
*Mixing - pre set proportions in capsules, reproducibleresults
*dimensions -see later
*working time: time for mixing and manipulation:
*e.g. Impressions gather put in tray hold in mouth and seat clean
*e.g composite dispense pack shape cure
*acrylic mix, put on cast, pack, remove remnants, heat cure
Describe other teeth materials
Other materials have been recently tried including polycarbonates and nylons, both of which have not really taken off, however, nylon flexible partial dentures are becoming more popular amongst the dental profession, but it is the opinion that flexible materials may contribute to increased forces on abutment teeth and underlying bone increasing the rate of bone resorption
In summary, PMMA is the best material we have despite it’s shortcomings. It can have great aesthetics and has adequate physical properties– just don’t drop them!
Describe 3 curing methods
Describe alternative materials such as polyamide
What is the best material for denture bases
PMMA
great aesthetics
adequate physical properties
