Dental materials sheet # 2 - shadi jarrar

by **Shadi Jarrar** 24/10/2011, 11:58 pm

http://www.4shared.com/document/uCiTlBaM/dental_materials__2_by_shadi.html

Denture lining materials

First keep in mind that after extraction the tissues keep changing especially in the first few months and after the first year of extraction, bone resorption takes place (but also keep in mind that bone resorption slows with time). Also, soft tissues lining this bone undergoes changes.

Because of these changes, the dentures lose their proper fit on mouth arches with time, and this in turn reduces adhesion and cohesion forces needed to make the denture retentive.. this is why we need to replace the old fitting surfaces of the dentures with new ones.. and this is what we call relining.

But it want to replace the whole denture base keeping only the teeth in their position.. this is called rebasing “replace the whole base”

We choose relining (and not rebasing) if we are satisfied in denture surfaces other than fitting surface.. meaning that the occlusal and polished surfaces of the denture should be perfect. If they are not, we choose rebasing.

So, relining is : a process in which a film of a plastic is added to the denture in order to obtain an improved fit to the denture bearing mucosa, and this is done by one of two ways:

1) Indirect method -laboratory : we use the denture as a tray , take an impression and send it to the lab. so they reflask it, remove the impression material and cure the relining material “this will be discussed later in details in this sheet”

2) Direct method -chairside : we use a material at the chairside, we put it on the fitting surface and this material will be your new fitting surface.

The impression we get by the indirect method could be anatomical or functional- the later means to get impression under certain loads.

Relining materials could be hard or soft.

Hard relining materials could be permanent or temporary:

The permanent reline could be heat, chemically or light activated acrylic, and the relining process is carried out either by flasking or curing in a light chamber depending on the type of he material. In most times the relining material has the same composition as the original denture base material itself.

Properties of relining material:

1) They should have good chemical bond to the original denture.

2) Satisfactory strength of the lining material.

3) There shouldn’t be warpage “dimensional change” in the denture due to relining procedure.

4) Relining should take as short time as possible.

Relining could be used with complete or partial dentures.

Now .. in more details we will talk about lab and chairside techniques :

1) laboratory technique: we use the dentures as treys to get impressions of the patients’ jaws. Ask the patient here to close on his teeth in order not to change the vertical dimensions while getting the impression. Then, you send the impression to the lab where they flask it and use three pours inside the flask.. after the investment material sets they remove the impression material “after heating” .. that way, there would be spaces between the denture base and the cast, they fill these spaces with acrylic “that will undergo compression molding technique” ,cure and deflask it to get the ultimate fitting surface.

The liners used here are permanent.

In order not to distort the original denture, heating process should be controlled within 74-77^oC. If we still worry about distortion of the denture, we can use chemically activated resin “despite the disadvantages of the chemically activated resin”

Finally, the denture should be finished and polished. J

2) Chairside technique: we use chemically activated resin -in most cases- on the fitting surface of the denture. It takes 6-15 minutes to set..

· The problem with this material is that it produces heat because it’s exothermic reaction, plus the fact that the residual monomer here is much more(cytotoxicity).. which can burn the underlining tissues. Discoloration of this material is another problem due to tertiary amides used as activators here.

· Also, porosity of some materials results in bad odors because they can absorb water house candida.

· This material used here is considered as a temporary impression material because it exhibits a short life span.

· If the denture was removed before the impression material sets completely, the denture will distort and won’t fit well.

· Some dentures exhibit weak bond to the denture specially that they are of different compositions.

Light activated material has a slightly different composition, so we use a bonding agent. This material needs 30-45 minutes to set.

In rebasing, only occlusal dimension and teeth positions are preserved while the whole remaining denture is replaced. Mainly, an impression is taken to be sent to the lab where they make a mold using investment material .. the remaining steps resemble those for processing denture base for any denture. ”the dr. said that without explanation”

If the occlusal relation is not good, the denture should be replaced (not rebased).

Tissue conditioners”a type of denture base liners”

They are soft elastomers that are used to treat irritated mucosa surrounding the denture. They stay in the denture for few days so you can add some medications to them as they are soft materials.

They are mixed at the chairside, placed inside the denture and set inside the mouth. They confine the anatomy of the residual ridges . they lose their viscosity easily as the plasticizers will evaporate. So that they are used in a short period of time.

So for example, if we have a patient with irritated mucosa because of poor-fitting denture, se apply conditioners to treat fungal or bacterial infections. They can be applied for 3-4 days.

Mainly they are composed of a powder that contains: polyethyle acrylate (not PMMA) and a liquid that contains: aromatic easter+ ethyle alcohol(up to 30% of the mixtue)+plasticizers that keep the material flowy.[note here that the liquid does not contain monomers, so no polymerization takes reaction.. that is why the material stays elastic. Plus that the plasticizers have large molecular weight which inhibits further enlargement of the polymer chains and the chains keep passing along each others enabling the material to have changes in it’s shape giving the material it’s soft consistency.

But as the plasticizer is lost the material become hard.. here you should change the material again.

Why does the conditioner have this importance?

- It’s viscose behavior which allows it the adaptation to the irritated tissues.

- The viscoelastic and elastic behavior which cushion the occlusal forces. The viscoelastic behavior is affected by the molecular weight of the powder and liquid of the conditioners.

But these properties make the material less durable so it has short period of time.

Soft relining materials could be long-term or short-term. Long term materials are not permanent; they can not last more than one year. “permanent materials like PMMA”

The requirements of long-term soft liners:

- They should have bond strength to the denture base.

- Good dimensional stability during and after processing.

- Ideally they should have permanent softness, but actually none of them exhibits this behavior.

- Low water sorption.

- Good color stability.

- Easy processing.

- Biocompatibility.

When do we need to have a soft denture lining materials for a long period of time?

-Mainly when we have a soft fragile mucosa or a sharp underling bony mass that causes irritation to the mucosa.. so they absorb occlusal forces.

- In case of severe undercut and we can not go through surgery because the patient is medically compromised. Here we need an elastic material to engage the undercut.

-In case of acquired or congenital defects in the palate such as cleft palate.. the part which will cover that defect will be soft material.

So the soft lining materials are mostly acrylic but other types are available as well. The acrylic material can be heat or chemically activated. The chemically activated materials are used for period to increase the fit of the denture to the oral mucosa until they can be permanently relined. But the problem with them is that cleaning them is difficult and they absorb water and alcohol will be lost which will cause deboning from the denture.

Chemically activated soft materials are mixed chairside. Setting takes few minutes.

It’s compositions can be :

Powder: polyethyle methacrylate and peroxide initiator.

Liquids:aromatic easters, ethanol, aromatic easters and tertiary amides.

Or

Powder: polyethyle methacrylate, peroxide initiator and plasticizers like ethyle glycolate.

Liquids: methyle glycolate and tertiary amides.

Or

they can be none acrylic .. made of silicone

the biocompatibility is on edges, because the plasticizers can develop premalignant lesions.(still uncertain)

they could be acrylic based.. they are more durable and better than those used at chairside and they are long term materials (but not permanent). They tend to be removed away from the denture and to have porosity… mainly used for patients with chronic soreness caused by bruxism.

Because soft relining materials are elastic and soft they are difficult to be finished.. they also lose their elasticity after a period of time can not be used as soft materials anymore. Storage of water and bacterial and fungal infections is –again- another problem in these materials. Medications can be added to these liners to resist infections.. but some patients can abuse these drugs.

Bonding of the material differs from a material to another.. some materials have better bonding properties like “poly phostazine (??)” materials while silicone has a poor bonging strength.

Regarding viscoelasticity , silicone is more elastic and maintain a stable consistency over the time longer than the acrylic materials and they have better oral hygiene.

The amount of plasticizers released from soft liners in 3-4 times more than the environmental exposures of these materials .. which – again- can be pre malignant.

Heat activated soft reliners:

Powder : acrylic polymers and copoymers.

Liquid : acrylic polymers and plasticizers.

Glass transitional temperature is a temperature below which the material becomes hard. If we use a material with low glass transitional temperature it will be flowy inside the mouth.

So in order to reduce the amount of plasticizer used we can use materials with low glass transitional temperature.(such as higher molecular weight methacrylates)

Other heat activated soft liners include vinyl resin based materials, polyvinyl and polyvinyl acetate.

Silicone rubber soft liners.. their problem that with time they lose their adhesion to denture base. They can be heat or chemically activated. Heat activated is composed of a single component paste system while the chemically activated type is composed of two compartments. Heat activated is processed using compression molding technique either for original denture production or by remodeling.

Silicone rubber soft liners adhere poorly to the denture. They undergo significant volume changes and loss and gain of water. Even they have low glass transition temperature, they still contain leach plasticizers which cause irritation to the tissues and cause the material to become harder faster.

Several adhesives we use to improve the adhesion of the liners to the denture bases dissolve the liner material.

Also, other materials other than acrylic and silicon are available.. (like polyphoshozine material).

Soft liners can be difficult to clean mechanically which causes bad odors and taste and can complicate with candida like candida albicans .. so that, they are not always a good choice.

The End