Polymers Used in Prosthetic Dentistry-Dental Materials Lecture Note

POLYMERS

• Polymer is a molecule that is made up of many units.
• Oligomer is a short polymer composed of two, three, or four mer( mer = unit) units.
• Monomers  (mono= single) are the molecules that unite to form a polymer, and the process by which this occurs is termed polymerization.If monomers of two  or more different                   
• types are joined, copolymers are formed.Copolymers  may be either random(mers do not appear in specific order) or block (large numbers of one type of mer appear arranged in sequence).Atoms along the length of any polymer are joined through strong, primary covalent bonds.
• There are three basic spatial structures of polymers: linear, branched, and cross- linked.
• Linear and branched molecules are discrete but are bonded to one another through weak, physical bonds.Upon heating, the weak bonds break and the ability of the chains to then slide past one another results in a softened material.Upon cooling, the bonds reform and hardening occurs.Materials that are able to undergo this process are termed thermoplastic (polyvinylacrylics, poly(methyl methacrylate)).
• Cross- linking results in the  formation of a network structure of covalently bonded atoms;primary linkages occur between chains, and the polymer actually becomes a single giant macromolecule.The spatial structure that allows chain sliding upon haeting is not present in cross- linked  materials.cross- linked polymers therefore do not undergo softening upon heating and are termed thermosets (sylicones, cross-linked poly(methyl methacrylate), bisphenol A-diacrylate,cis-polyisoprene).
• Longer chains and higher molecular  weight result in the polymer‘s increased strength, hardness, stiffness, resistance to creep along with increased brittleness.Small plasticizer molecules, when added to a stiff uncross-linked polymer, reduce its rigidity.When small molecules surround large ones, the large molecules are able to move more easily.A plasticizer therefore lowers the glass-transition temperature of the polymer,so a material that is normally rigid at a particular temperature may become more flexible.The glass-transition temperature is the temperature at which a polymer ceases to be glassy and brittle and becomes rubberlike.

Polymerization

• There are two types of polymerization reactions: additional polymerization, in which no by- product is formed, and condensation polymerization, in which low molecular weight by- product such as water or alcohol is formed.Polymerization has four stages: activation, initiation, propagation, termination.Reaction may be accelerated by light, heat, radiation or small amount of peroxides.Free radical additional polymerization is used for the synthesis of polymers.The free radicals are produced by reactive agents called initiators.The most popular initiator is benzoyl peroxide.Activation-involves decomposition of the peroxide initiatior using special conditions.Initiation-involves production of free radicals, which will encourage a polymer chain to begin growing.Activator allows polymerization to occur at low temperature(aromatic tertiary amines).
• Free- radical molecules have chemical groups with unshared electrones.In chemically activated systems, free radicals are produced by the reaction of an organic poroxide initiator and amine accelerator.In light- activated systems, the  scission of camphorquinone results in the production of two molecules with one production, the free radicals attack the double bonds of available monomer molecules, resulting in the shift of the unshered electron to the end of the monomer and the formation of activated monomer molecules.Propagation- Activated monomers attack the double bonds of additional available monomers, resulting the rapid addition of monomer molecules to the free radical.This stage continues as the chain grows and length.Termination-it is posible for the propagation to continue until the supply of monomer molecules is exhausted.These reactions produce dead polymer chains which are not capable of further additions.Small amounts of inhibitors, such as hydroquinone, may be added to the monomer to increase storage life.Hydroquinones react with free radicals, thereby decreasing the rate of initiation.

Classification of denture base, liners and tissue conditioners:

Denture base:

I. Heat –cured (PMMA )

1. conventional            

• Unfilled
• Re-inforced (Carbon,Polyfiber)

    2.high impact

II. Autopolymerized(PMMA)

III.Injection molded  1.  PMMA polycarbonate nylon

Soft liners:   

I. Acrylic

II.silicone      a)  room temperature vulcanizing (RTV) heat cured

Tissue conditioners-

plasticized acrylics

Denture base polymers

The polymeric denture base can consist of either a simple stiff base on which the teeth are arranged, or a sandwich of stiff base and a resilient liner to provide greater retention and comfort.When the tissue underlying a loose denture is traumatized due to the constant motion of the hard plastic over the mucosa, a viscoelastic gel known as a tissue conditioner can be molded onto the fitting surface of the denture in situ so the tissue can heal and an accurate impression of the untraumatized fitting surface can be taken prior to making a new, better-fitting denture.

Requirements of denture base polymers:

• Physical properties: good esthetic, thermal conductivity, dimensional stability,“light“, radiopaque.
• Mechanical properties: high value of modulus of elastisity,sufficient flexural strength,sufficient abrasion resistance.
• Chemical properties: materials should be chemically inert, insoluble in oral fluids, should not absorb water.
• Biological properties: should not be harmful to the technician, doctors and patients;should be non- toxic and non- irritant to the patients, should not be able to sustain the growth  of bacteria or fungi.

Heat-cured acrylic

It consists of powder and liquid.The major component of powder is  polymer (beads of polymethylmethacrylate), pigments(pink pigment-cadmium salts) and initiator-benzoyl peroxide ~0.5%.The liquid: monomer (methylmethacrylate), cross- linking agent(improves the physical properties of the set material), inhibitor(prolongs the shelf life of the liquid components).The inhibitor, which is usually hydroquinone, works by reacting with radicals formed within the liquid to form stabilized radicals which are not capable of initiating polymerization.The ratio of powder to liquid is  important since it controls the workability of the mix as well as the dimensional change on setting.A powder/liquid ratio by weight is 2.5:1.The mixture should be lefted for few minutes, and the mixing vessel should be closed to prevent evaporation of monomer.There are few stages of this material: „sandy“ consistency, after the short period of  time it becomes „sticky“,which forms strings of material.The „dough“ stage.The material can be moulded like plasticine and does not stick to the mixing vessel.The material is packed in the mould at this stage.Later can be „rubber“ and  „hard“ stages.

The dough is packed into a two-part gypsum mould.The excess of dough is removed, then the flask is closed again using the  pressure and the heat.The polymerization reaction itself is exothermic, so if the rate of reaction is too high, it cal lead to porosity.Under the pressure the dough flows into every part of mould and you can avoid of porosity.There are few kinds of porosity: granular porosity-it can be then there is insufficient amount of monomer to bind all the polymer beads together,evaporation of monomer.Contraction porosity-the use of insufficient dough to create an excess in the mould or the application of insufficient pressure during curing can lead to this kind of porosity.Gaseous porosity-it can be then the  temperature of dough during polymerization is rised significantly above 100.3˚C, the monomer at this temperature will boil and will produce spherical voids.

High- impact acrylic

High-impact acrylic denture base is also made by the hear- cured dough method.Impact resistance arises from the incorporation of a rubber phase into the beads during their suspension polymerization.

Autopolymerizing denture base (cold curing resines)

The autopolymerizing denture base is chemically similar to the heat- cured denture base except that a reducing agent is added to the monomer.The reducing agent is usually a tertiary aromatic amine, although barbituric acid derivates.The reducing agent reacts with the benzoyl peroxide at room temperature to produce peroxy free radicals, which initiate the polymerization of the monomer in the denture base.Autopolymerizing materials are used for repairing and relining of dentures, because their mechanical properties are weak,  and there is high residual free- momomer content.

Injection- molded plastic

The injection- molded plastics have the advantage of consistent molecular weight, but the disadvantage of capital equipment costs, and difficulties associated with attachment of teeth to the denture base.The plastics still offered for the use as injection-molded denture base acrylic are polycarbonate and nylon.They represent very small fraction of the market, although they offer a real alternative to metal dentures for patients sensitized to conventional methacrylate or to nickel or cobalt.

The technician has little leeway when using injection-molded plastics.The mold should be dry to prevent the generation of steem during molding.Patience is required to ensure the melt has reached the right temparature and cools sufficiently after  molding.Inadequate spruing will lead to underfilled molds, as can underheating the melt;overheating the melt can cause explosions, especially when polycarbonate is injected into moist molds.

Injection moldings rely almost totally on mechanical forces to retain the teeth.Low melt temperatures will cause strong forces to be  put  on the teeth  during the injection phase and  may dislodge some molars, even from plaster  molds.Depolymerization or oxidation from overheating the melt can result in porosity, loss of strength, color changes, and increased fouling.

Light- activated materials

This material consists of a urethane dymethacrylate matrix with an acrylic copolymer and has a microfine silica filler.It is supplied in premixed sheet or rope form.A base plate is made by adapting the material to a cast and polymerizing in a light chamber at 400 to 500nm(blue light).Teeth are added to the base with additional material followed by a second light exposure.The system eliminates the need for flasks, wax, boil-out tanks, packing presses, and heat processing units required for the construction of the conventional dentures.Light- activated materials contain no methylmetacrylate monomer, they may be considered for use in those patients who have demonstrated a sensitivity.polymerization shrinkage is  smaller than conventional systems.

A comparison of denture base materials

1.Heat cured         

Advantages:   

• good apperance
• high glass-transition temp.
• Easy fabrication
• Low capital costs
• Good surface finish

Disadvantages: 

• free monomer content or formaldehyde can cause
• sensitation
• low impact strength
• fatique life too short
• radiolucency

2.Heat cured, rubber       

Advantages: 

• improved impact strength
• reinferced

Disadvantages: 

• reduced stiffness

3.Heat cured, fiber           

Advantages:   

• high stiffness
• reinforced                                             
• very high impact strength
• good fatique life
• polypropylene fibers make good
• translucency
• good surface finish

Disadvantages:  

• carbon and Kevlar fibers make
• poor color
• poor surface

4.Autocured                     

Advantages:     

• easy to deflask
• dimensional accuracy
• capable of flexural strength than heat cured

Disadvantage:  

• no cheaper over long term
• increased creep
• increased free-monomer content
• color instability
• reduces stiffness
• tooth adhesion failure

5.Injection molded           

Advantages:    

• dimensional accuracy
• low free-monomer content
• polycarbonate and nylon make
• good impact strength

Disadvantages: 

• high capital costs
• difficult mold design problems
• less craze resistance
• less creep resistance

6.light activated              

Advantages:     

• no methylmethacrylate monomer
• decreased polymerization shinkage
• possible improved fit compared to
• conventional materials
• requires little equipment
• time savings

Disadvantage:   

• decreased elastic modulus

Denture lining materials

Permanent soft lining materials

Permanent soft lining materials are resilient polymers used to replace the fitting surface of a hard plastic denture, either because the patient cannot tolerate a hard fitting surface or to improve retention of the denture.Because the  lining is soft, its dimensional stability is important, as are its  durability and resistance to fouling.However, because by definition soft lining materials are above their glass-transition temperature when in the mouth, such physical phenomena as water absorbtion, osmotic presence of soluble components, and biodegradability play a greater role in the clinical success of a liner than  they do in the glassy polymers used as denture bases.

Acrylic soft liners

The acrylics consist of either highly plasticized intrinsically glassy polymers or soft acrylics that have a natural glass-transition temperature (softening temp.) at least 25˚C less than that  of the muoth.The plasticizer used to soften the acrylic can either be unbound to the acrylic and hence free to diffuse out during use, resulting in a loss of resilence, or it can be reacted into the cured matrix of the acrilic.

Silicone soft liners

The silicones used as soft liners can be devided into two types: room temperature vulcanizing (RTV) and heat cured.The resilence of silicones makes them at first  seem to be the ideal soft materials.However, silicones have poor tear strength, no intrinsic adhesion to acrylic denture base, and, if not properly cured, a tendency to osmotic pressure effects.RTV silicones‘ geatest drawback is their lack of adhesion, which is especially a problem around the edges of the attachment between acrylic and silicone.Heat cured silicones have in their formulation a silicone methacrylate than can polymerize into curing denture base and into the heat- cured addition silicone.The RTV silicones use a condensation cross-linking system based on organo-tin derivates such as those used in impression rubbers.Their degree of cross-linking is lower and their serviceability is low as a result, with frequent reports in the literature of swelling and buckling during use and excessive sensitivity to  denture cleansers.The rupture strength of some RTV silicones is known to deteriorate cosiderably when exposed to waterfor long periods.The heat-cured silicones achieve a greater degree of cross-linking and have much longer clinical lifetime.

Temporal soft lining materials

These materials are similar to tissue conditioners, but they are not so soft  as conditioners immediately affter setting but they retain their softness for longer, taking up to a month or two to harden.These materials are viscoelastic.Some kinds of cleansers can cause surface degradation and pitting of material.Temporal soft lining material can be used improving the fit of ill-fitting denture until such a time as a new denture will be maden.These materials will go hard, when this occurs, the surface is rough and increases the risk of trauma.In this state the base can be colonized by Candida,it leads to stimatitis.

Tissue conditioners

Tissue conditioners are soft denture lining material which may be applied to the fitting surface of a denture, to the dentures of patients who have undergo surgery.,they are useful when a tooth is being added to a denture( very shortly after extraction).The material consists of powder (polymer beads-polyethylmethacrylate) and liquid (solvent-ethyl alcohol; + plasticizer- butylphthalyl butylglycolate).

A comparison of soft liners and tissue conditioners

1. Soft liners:

a)   Acrylic      

Advantages : 

• high peel strength to acrylic denture base
• high rupture strength
• some can be polished if cooled
• reasonable resistance to damage by denture cleansers

Disadvantages:  

• poor resilience
• loses plasticizer in time  
• some buckle in water

b)  Silicone(RTV)   

Advantage:    

• resilience

Disadvantage: 

• low tear strength
• low bond strength to dentures
• attacked by cleaners
• buckle in water
• poor abrasion resistance

c)  Silicone heat cured   

Advantages:  

• resilience
• adequate bond strength to acrylic
• more esistant to aqueous environment and
• cleanser than RTV

Disadvantages: 

• low tear strength
• poor abrasion resistance

2. Tissue conditioners      

Advantages:  

• viscoelastic properties almost ideal
• can be applied chairside
• denture fit well
• can record freeway space

Disadvantages: 

• low cohesive strength
• affected by cleanser
• alcogol can sting inflamated mucosa.

Gypsum Products used in Dentistry

Introduction

Gypsum products used in dentistry are based on calcium sulphate hemihydrate (CaSO4.2H20)

The current ISO Standard for Dental Gypsum Products identifies five types of material as follow:

• Type 1 Dental plaster, impression
• Type 2 Dental plaster, model
• Type 3 Dental stone, die, model
• Type 4 Dental stone, die, high strength, low expansion
• Type 5 Dental stone, die, high strength, high expansion

Composition

Dental plasters(plaster of Paris): Dental plaster is produced by a process known as calcinations.

Plaster remains the most important model material for many dental restorations. As the precision of fit of restorations and orthodontic appliances depends on the quality of the dental plaster/stone, they must comply with the high quality demanded in dental technology.

Dental stone

Manipulation and setting characteristics

Plaster and stone are mixed with water to produce a workable mix. Hydration of the hemihydrate then occurs producing the gypsum model or die.

Water/powder ratios for gypsum model and die materials.

	Water (ml)	Powder(g)	W/P ratio (ml/g)
Plaster	50-60	100	0.55
Stone	20-35	100	0.30
Theoretical ratio	18.6	100	0.186

For hand mixing a clean, scratch free rubber or plastic bowl having a top diameter of about 130 mm is normally recommended.

Flexible Mixing Bowls

The material should be used as soon as possible after mixing since its viscosity increases to the stage where the material is unworkable within a few minutes.

            The setting reaction is exothermic, the maximum temperature beibg reached during the stage when final hardening occurs.

Control of setting times:

Factor which control the settings of gypsum products can be divided into those controlled by manufactures and those controlled by the operator.

The manufacture can control the concentration of nucleating agents in the hemihydrated powder

Operator controls temperature and W/P ratio and mixing time.

Properties of the set material

The strength gypsum depends, primary, on the porosity of the set material and the time for which the materials is allowed to dry out after settings.

Since stone is always mixed with at a lower W/P ratio than plaster it is less porous and consequently much stronger and harder. (When porous increased, strength decreased).

The ability of dental gypsum products to reproduce surface details of hard or soft tissues either directly or from impression is central to their suitability as model or die materials. This ability is judged by measuring the extent to which accurately machined lines in a block of stainless steel can be reproduced in a sample of the material.

Applications

When strength , hardness and accuracy are required dental stones are normally used in preference to dental plaster.

The stone materials are less likely to be damaged during the playing down .

Thus , these materials are used when any work is to be carried out on the model or die as would be the case when constructing a denture on a model or a cast alloy crown on a die.

                When mechanical properties and accuracy are not of primary importance the cheaper dental plaster is used.

Advantages and disadvantages

Advantages

Inexpensive and easy to use

Accuracy and dimensional  stability

Disadvantages:

Mechanical properties are not ideal.

Basic Dental Materials Filling materials, Denture Materials, Crown and Bridge materials and Bonded Restorations

Filling Materials

Composite Fillings

These materials consist of tiny glass particles suspended in a resin (plastic) matrix. The size and amount of the glass particles give the composite filling materials different characteristics, such as strength and polishability. Composites are used in areas where the fillings are relatively small and there is plenty of tooth structure for support.

Composite fillings are less durable than amalgam if the filling is large, but comparable in durability if the filling is small to average size. Composite fillings in back teeth are significantly more difficult and time-consuming to place than amalgam fillings, therefore more expensive. Composite materials are most commonly placed directly into the tooth (like amalgam fillings), but can also be prefabricated and bonded into place indirectly (like a crown).

Amalgam

Amalgam is the most commonly used material for back teeth. It contains approximately 50% mercury and varying amounts of silver (30%), tin, zinc, and copper. It is the least costly and least time-consuming to place. It does not hold its shape over time, corrodes easily, and is expected to last 5-10 years. The controversy is that it contains mercury, a known neurotoxin (poison to the nervous system).

Galloy

Galloy is a brand new material containing silver, tin, copper, indium, and gallium. It is meant to be mercury-free alternative to amalgam. Why is the American Dental Association developing & patenting this substitute for mercury amalgam if mercury amalgam is safe?

Direct Composite

A direct composite is a special plastic material that bonds to tooth structure, is tooth colored, is more easily repairable, and requires less tooth structure to be trimmed away than any other material. It is expected to last 5-7 years, although small to moderate size fillings may last longer. Research has shown that it reinforces the tooth and makes it stronger. Cost and time to perform is about 50-75% more than amalgam. Composites are a petrochemical derivative and, as such, are a possible problem for the environmentally sensitive.

Indirect Composite Inlay/Onlay

This type of restoration is used when ideal fit and durability is desired, which is seldom achieved with a direct composite filling. Cost is approximately 2-3 times that of an amalgam filling and takes two visits.

Porcelain Inlay/Onlay

Dental ceramics (sometimes referred to as dental porcelains) have come a long way! Until a few years ago, these materials were relatively weak (that’s why they required support from a metal substructure) and abrasive (causing wear on the opposing teeth). Today there are many different types of ceramic systems: Feldspathic, Leucite-reinforced, Polymer-reinforced, Zirconium-based–each with unique properties. From rebuilding broken teeth to replacing missing teeth (even in the back of the mouth), there is a ceramic to do the job. However, they are more difficult to use than conventionally cemented (non-bonded) crowns.

Picking the right ceramic for the job, proper tooth preparation, quality laboratory work, and meticulous cementation technique are all needed for a successful tooth restoration. It costs about the same as an indirect composite inlay/onlay and takes two visits. Most ceramic and resin-based materials contain metals in the form of oxides (such as aluminum) or even heavy metals (such as cobalt, barium or cadmium). These are usually added to give the amterials strength and improve their appearance. Sometimes they are added to make the restoration show up on x-rays. The number of materials that do not contain any of these products is very limited. However, the advantage of being oxide-free is lost when these are bonded to the tooth using an oxide-containing luting agent.

Gold Inlay/Onlay

Because of gold’s long history, it is the standard against which other materials are judged. This type of restoration is used when maximum strength is desired and appearance is not a factor. Gold is almost never used in its pure form; rather gold is used as an alloy with other metal elements. It costs approximately three to four times more than an amalgam and takes 2 visits. There are many formulations of gold, varying from 1% to 99%. The other metals are added in order to give the gold strength and the ability to bond to porcelain (in the case of porcelain veneer fused to cover a gold crown). The most commonly added metals are palladium, silver, copper, and platinum.

The composition and amount of each metal in the alloy determines whether it is classified as a “high noble,” “noble,” or “base” metal. “Noble” metals are defined as gold, platinum and palladium. The most expensive gold alloys are “high noble” and they are defined as hving at least 60% noble metals and at least 40% gold. An alloy can still be called “noble” if it has at least 25% noble metal content. The cheapest materials fail even that test and are called “base” alloys–they have less than 25% noble metals. It is especially important for patients with metal sensitivities to avoid the base alloys since these usually contain toxic metals such as nickel and chromium;. But even the high noble materials can be incompatible for patients and even toxic; palladium, for example, is toxic.

Titanium Inlay/Onlay

Titanium is used when a gold alloy is not biocompatible; otherwise, the benefits, cost, and time to perform are the same as for a gold alloy, even though it is not a precious metal. It takes two visits.

Crown and Bridge Materials

Gold Alloy

A gold alloy is used when maximum strength is desired and appearance is not a factor. There are many formulations of gold, varying from 1% to 99%.

Titanium

Titanium is used when maximum strength is desired, appearance is not a factor, and a gold alloy is not biocompatible. There are different purities of titanium, with grade-1 being the purest. This is used in joint replacement, dental implants, and bone pins. Cost is the same as for gold alloy.

Non-Precious Alloy

Non-precious alloys are used when maximum strength is desired, appearance is not a factor, but cost is most important. Since it does not contain any gold, cost is less. There are two basic formulations, one that contains nickel and one that is nickel-free. The controversial issue is that nickel, beryllium, cobalt, chromium, and palladium may cause immune problems and/or toxicity.

Porcelain

Porcelain is used when appearance and wear resistance is the most important factor. It is much more fragile than metal and may break easily. Porcelain alone is not normally recommended for bridges.

Indirect Composite

Indirect composites are used when appearance is an important factor but when the risk of porcelain fractures and wearing down the other teeth is to be avoided. These are not quite as wear-resistant or esthetic as porcelain but very acceptable for normal situations.

Zirconium Oxide

One of the most difficult areas in dentistry today is the restoration of dental structures with biocompatible materials that are strong enough to withstand the forces of chewing (500-1000lbs pressure on molar teeth). Recent technology from Germany now offers a material that has overcome most of the pitfalls of present day products. Patients now have a choice of a material that is esthetic, strong, pure, biocompatible and capable of being used for single and long span dental bridgework. That material is called Zirconium oxide.

Zirconium oxide has the following superior characteristics that make it the most ideal material available:

* Excellent biological compatibility: absolutely bio-inert.

* Outstanding physical and mechanical qualities:

* Hardness (Vickers) 1200 HV

* Compressive Strength 2000 MPa

* Bending Strength 1000 MPa

* Modulus of Elasticity 210 GPa

* Tensile Strength 7 Mpavm

* Wear characteristics (Ring on disc) <0.002 mm 3/h

* Absolute corrosion resistance: Ringer’s solution 370C <0.01mg/ m2x24h

* Very small particle size: <0.6ym

* No glass phase for particle binding

* Extremely high density

* Porosity: 0%

* Purity (Zr/Hf/Y): 99.9%

* Translucence of the framework material makes excellent cosmetic results possible

* Equivalent fit to precision gold castings: edge opening 20-50 ym. Precludes the need to use adhesive cements.

* Zirconium oxide is manufactured and optimized industrially so that the material qualities remain unchanged through the complete pro duction chain.

* Optimal material for crowns: tasteless, radiopaque, no pulp irrita tion because there is no need to use adhesive cements and minimal invasive preparation by dentist.

Zirconium oxide forms the core of each crown and provides the cross-link that bridges the gap of missing teeth. The precision fit of the Zirconium core is derived from computer guided Swiss lathes that cut the form out of a solid Zirconium oxide block. The cutting instructions are obtained from a laser beam that reads 120 points per millimeter from the anatomy of a model of the prepared teeth. Once formed, new synthetic porcelain (99.9% pure) is baked on to the Zirconium core and then shaped like a tooth. Because of the extreme accuracy of the crown fit, the crowns can be cemented with biocompatible dental luting material. This avoids the use of an invasive procedure of etching the tooth with acid and injuring the pulp or nerve of the tooth. This latter procedure often times results in the pulp dying and necessitating root canal therapy.

Advantages of ET zirconium high performance ceramic compared with other full ceramics

Zirconium oxide ceramic primarily stands out due to its high crack resistance. Crack resistance is the resistance with which the material counteracts the spreading of cracks. If a material is stressed, it usually comes to excessively high tension within a defect area (pores, surface deficiencies, cavities) or it cracks. While with metals under high tension in the area of cracks, plastic deformation appears and the top of the tension can be reduced by rounding the cracks; in ceramics due to missing plastic deformation possibility the cracks continue to grow. The unusual feature of zirconium oxide ceramic in comparison with other ceramics is that at the appearance of a high-tension area a transformation of the crystal structure can take place. This process is also accompanied by a volume expansion. By this volume increase it builds wedges in the crack and therefore it reduces the continuation of the crack. While the critical tensile strength (KIC) e.g. of Dicor, Vita Mark II and Empress is in the area of 1-2.5 Mpam-1/2, zirconium oxide shows values in the range of 10 Mpam-1/2. Even In-Ceram (glass infiltrated Al203 ceramic) and Procera aluminum oxide (pure Al203 ceramic) show values less then 5 Mpam-1/2.

In connection with the tensile strength there also stands the characteristic of bending strengths. While conventional glass ceramics show results of 100-200 Mpa and aluminum oxide ceramics lie in the area of 400-600 Mpa, zirconium oxide reaches a bending strength of over 1000 Mpa.

Because of the high tensile strengths exhibited in test results, it is now possible to fabricate posterior bridges with zirconium oxide. Further decisive advantages of zirconium oxide are its high resistance to corrosion; stability to hydrolysis and its high biocompatibility in comparison with other ceramics makes this material ideal for restorative dentistry.

In medicine, zirconium oxide is being used more and more as the material of choice especially for hip prosthesis. For years there has existed substantial clinical tests and examinations which confirm the high quality of zirconium oxide.

Denture Materials

Dentures are usually made from acrylic, stainless steel, and chromium-cobalt, but can be made of nylon, a gold alloy, or titanium. Most pink-colored acrylics and vinyls contain cadmium, which is considered toxic and/or immune reactive. The alternative is to use cadmium-free pink or clear materials. Metals are used to increase rigidity and increase retention of the prosthesis in the mouth during function. If metals are not used, the opposite is true, which is not desirable from a functional perspective.

Bonding

Since all direct fillings (composites) and most indirect restorations (inlays, onlays, and crowns) being placed today use a process called bonding, it’s good to have a basic knowledge of how this process works. While there are a myriad of variations in this process, here are the basic steps:

Step 1–Prepare the tooth surface using a mild acid solution. This creates a “honeycomb” in the top layer of tooth.

Step 2–Paint a liquid resin-bonding agent on the tooth. It flows and “locks” into the honeycomb created in Step 1 (technically forming a hybrid layer that is part-tooth and part dental-resin). This layer is “cured” (hardened using a photo-chemical reaction) with a visible light source.

Step 3–Place luting cement if an indirect restoration (e.g., crown) is being used. Essentially, this material is a more liquid form of white filling material. It bonds to both the tooth and the pre-fabricated ceramic restoration and fills the gaps between them. The surface of the first layer of cured bonding agent is highly reactive and is easily bonded to with today’s composite filling materials.

Bonded Restorations

Since the 1960s, alloy-porcelain combinations, known to the dentist as bonded restorations have been available. These porcelain-covered metal castings combine the strength of a metallic superstructure with the aesthetic appearance of dental porcelain, creating the illusion that the restorations are real teeth. Alloys have been developed to which dental porcelains form durable retentive bonds, and many of these are now based on nickel-chromium. These metal frameworks are so rigid that they can be bonded via composites to the backs of acid etched teeth, thus eliminating the need for cutting down sound teeth, figure 1. Just as etching dental enamel creates retentive ‘chasms’, these nickel-chromium alloys can be electrolytically etched to produce features that allow the formation of mechanical bonds with resin-based composite cements.

Internal view of a dental bridge bonded via a resin-based cement to the backs of acid-etched teeth.

The oxides that form on these alloys can also be used to promote chemical links to cements via bifunctional primers, thus eliminating the challenge of producing a uniformly etched surface.

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Comparison of Indirect Restorative Dental Materials-Dental Materials Lecture note

Indirect restorations are generally done in two or more dental visits.  The first visit involves cutting the tooth down to a size and shape in which it will be able to support and retain a custom manufactured restoration.  An impression, which records the shape of the preparation, down to the most minute detail is taken and sent to the laboratory where the restoration is fabricated.  The patient leaves the office with a temporary restoration generally made of plastic and is given an appointment for an insert date several weeks later after the lab has finished making the restoration.  The final restoration is inserted using a cement to adhere it to the tooth preparation on the second visit.  (Note: In some practices, these restorations are fabricated in-office using a computer assisted design technique, in which case the patient may receive the finished restoration at the same visit.)

Comparison of Indirect Restorative Dental Materials

Factors	All-Porcelain (ceramic)	Porcelain Fused To Metal	Gold Alloys (high nobel)	Base Metal Alloys (non-nobel)
General Description	Porcelain, ceramic or glasslike fillings and crowns	Porcelain is fused to an underlying metal structure to provide strength to a filling, crown or bridge.	Alloy of gold, copper and other metals resulting in a strong, effective filling, crown or bridge.	Alloys of non-noble metals with silver appearance resulting in high strength crowns and bridges.
Principal Uses	Inlays, onlays, crowns and aesthetic veneers	Crowns and fixed bridges.	Inlays, onlays, crowns and fixed bridges	Crowns, fixed bridges and partial dentures.
Leakage and Recurrent Decay	Sealing ability depends on materials, underlying tooth structure and procedure used for placement.	The commonly used methods used for placement provide a good seal against leakage.  The incidence of recurrent decayy is similar to other restorative procedures.
Durability	Brittle material, may fracture under heavy biting loads. Strength depends greatly on quality of bond to underlying tooth structure.	Very strong and durable.	High corrosion resistance prevents tarnishing; high strength and toughness resist fracture and wear.
Cavity Preparation Considerations	Because strength depends on adequate porcelain thickness, it requires more aggressive tooth reduction during preparation.	Including both porcelain and metal creates a stronger restoration than porcelain alone; moderately aggressive tooth reduction is required.	The relative high strength of metals in thin sections requires the least amount of healthy tooth structure removal.
Clinical Considerations	These are multiple step procedures requiring highly accurate clinical and laboratory processing. Most restorations require multiple appointments and laboratory fabrication.
Resistance to Wear	Highly resistant to wear, but porcelain can rapidly wear opposing teeth if its surface becomes rough.	Highly resistant to wear, but porcelain can rapidly wear opposing teeth if its surface becomes rough.	Resistant to wear and gentle to opposing teeth.
Resistance to Fracture	Prone to fracture when placed under tension or on impact.	Porcelain is prone to impact Highly resistant to fracture. fracture; the metal has high strength.	Highly resistant to fracture.
Biocompatibility	Well tolerated.	Well tolerated, but some patients may show allergenic sensitivity to base metals.	Well tolerated.	Well tolerated, but some patients may show allergenic sensitivity to base metals.
Post-Placement Sensitivity	Sensitivity, if present, is usually not material specific.
	Low thermal conductivity reduces the likelihood of discomfort from hot and cold.	High thermal conductivity may result in early post-placement discomfort from hot and cold.
Esthetics	Color and translucency mimic natural tooth appearance.	Porcelain can mimic natural tooth appearance, but metal limits translucency.	Metal colors do not mimic natural teeth.
Relative Cost to patient	Higher; requires at least two Patient office visits and laboratory services.
Average Number of Visits to Complete	Minimum of two; matching esthetics of teeth may require more visits.	Minimum of two; matching esthetics of teeth may require more visits.	Minimum of two.